Hierarchical Approach to the Management of Drinking Water Sludge Generated from Alum-Based Treatment Processes

Abstract

The management of drinking water treatment plant (DWTP) sludge is challenging for water treatment facilities. Previous studies reported mainly on handling sludge through landfilling, release into water bodies, discharge into wastewater treatment plants, onsite disposal, and incineration methods for the treatment of sludge. The limitations of these sludge-handling methods are well documented. This article focuses on the hierarchical approach as an alternative and comprehensive method for handling DWTP sludge. The core of hierarchical management streamlines the minimization of the generated DWTP sludge; treatment of DWTP sludge to reduce toxicity; changing of the physicochemical form of DWTP sludge; and finally, the reuse, recycling, and recovery of DWTP sludge. The premise is to achieve zero landfilling of DWTP sludge, establish a circular economy, generate job opportunities, and preserve the environment. Thus, this study also proposes two main technologies, which are gravity-based sludge separators for fractionating the sludge and photocatalytic membrane reactors (PMRs) as a technology for the treating and/or recovery of nutrients and minerals from DWTP sludge. Until the chemical deductive or minus approach becomes a reality in water treatment, the use of PMRs and gravity-based sludge separators will enhance the management of DWTP sludge when incorporated into the hierarchical approach.

1. Introduction

The water treatment process generally involves five main stages, namely coagulation, flocculation, sedimentation, filtration, and disinfection [1]. During the coagulation and flocculation stages, different chemicals are added to cause impurities in the raw water to coagulate, agglomerate, and settle at the bottom of the tank due to their increased density. Alum [Al₂(SO₄)₃·14H₂O], ferric chloride [FeCl₃(H₂O)_x], and polyelectrolytes are some of the coagulants used for the removal of suspended and dissolved substances from raw water during the treatment of drinking water [2,3]. The waste product produced by these treatment processes (coagulation and flocculation) is called drinking water treatment plant (DWTP) sludge or water treatment residuals (WTR) [4,5,6,7,8,9]. The content of DWTP sludge is composed of organic and inorganic compounds materials from the source water and the chemicals added during the treatment process [5,7,10]. Understanding the sources of sludge, the raw water, as well as the treatment process, is of paramount importance, as these affect the final characteristics of the sludge produced [11,12,13].

In a conventional drinking water treatment process, the sludge is dried and landfilled. However, good waste management practices do not promote the release of toxic waste into the environment, as the sludge may contain potentially toxic metals that could leach into the environment [10,14]. In addition, DWTP sludge may also be characterized by other toxic materials that arise from anthropogenic processes that happen along the river basin, including mining, farming, and other industrial processes which waste may end up reaching the treatment plant and end up in DWTP sludge [10,14]. Moreover, during flocculation and coagulation, 60–99% of cyanobacterial cells from the intake water are removed from the treated water and may result in the buildup of both cyanobacterial cells and cyanotoxins in the sludge [14].

Typically, DWTP sludge is transported from the treatment plant to another location for thickening and drying. Once the sludge is dried, it is then landfilled. Water treatment plants consequently spend a lot of money on acquiring land for landfilling purposes [13]. Since land is a finite resource, the use of land for landfilling purposes is not environmentally sustainable, nor is it economically viable. There is, therefore, a need to come up with alternative ways of dealing with the sludge [12]. The increase in the demand for clean drinking water coupled with the increase in population and urbanization suggests that even more DWTP sludge will be produced in the future [15]. Moreover, as the content of the source water continues to become complex due to anthropogenic processes happening in the river basin, including mining and farming, the sludge produced is anticipated to be toxic and contain high levels of metals, fertilizer, and other materials that are potentially toxic and require treatment [15,16].

The cost of acquiring land for landfilling purposes and the environmental cost associated with the leaching of toxic substances from the sludge into the soil and underground water make it prudent to develop alternative methods for dealing with this waste. In South Africa, a water treatment utility spent more than 15 million USD to acquire land that could be used for landfilling purposes [13]. The current approach is clearly not sustainable. Moreover, a survey revealed that worldwide expenditure on inorganic coagulants for water and wastewater treatment was approximately 1.37 billion USD in 2018, and the amount increased to 1.84 billion US by 2023 [17]. In 2015, the UK water industry consumed over 0.33 million tons of coagulants and produced around 0.18 million tons of water treatment residues, with associated costs of approximately 40 million EUR and 8 million EUR, respectively [17]. Europe and the United States generate 100,000 and over 2 million tons of solid residuals annually, while global sludge production reaches about 10,000 tons per day. This shows that there is a need to develop a circular economy in DWTP sludge management that promotes the recovery and recycling of DWTP sludge. This can reduce costs and help generate revenue from the waste.

Currently, the hierarchical approach to waste management is proving to be the most efficient method for handling regular waste. This approach puts emphasis on reducing the volume and/or toxicity of waste, though it has not yet been embraced for the management of DWTP sludge. The hierarchical approach to waste management prioritizes the reduction of the amount and toxicity of the generated waste [6,18,19]. These waste management alternatives can then be followed by reuse, recycling, and recovery. This review proposes and discusses ways that this approach can be adapted for managing DWTP sludge. It seeks to present an overview of the current research in the reduction, reuse, recycling, and recovery of DWTP sludge or its components. A proposal for the adoption of sludge separation techniques and photocatalytic membrane reactors (PMRs) is also presented and discussed. These two proposed techniques aim to separate the sludge into its different constituents and then reduce its volume and toxicity through advanced oxidation processes, such as photodegradation of the pollutants and photoreduction of any positive species, such as metal ions, found in DWTP sludge. This review recommends the adoption of a comprehensive set of technologies that can help achieve the reuse, recycling, and recovery of DWTP sludge for water and wastewater treatment, land application, and brick manufacturing and the recovery of alum and metals amongst others to achieve zero landfilling. When selecting the technologies, the reduction of generated waste should be prioritized; recycling, reuse, and recovery should then follow, and disposal should only be used as a last resort.

While some DWTPs use polymers or iron-based coagulants, this review will focus on alum-based coagulation treatment plants. This review seeks to outline the contents of DWTP sludge with the target to provide insights into alternative approaches that can be adopted to reduce, reuse, recycle, and/or recover the content of DWTP sludge produced by conventional DWTPs. Finally, the review will identify possible barriers to the adoption of such approaches and recommend legislative reforms or capacity building to overcome the barriers.

2. Methodology

The approach employed in the literature review involves a two-step process: Firstly, it entails locating relevant literature through a thorough search in databases while adhering to specific criteria for selecting pieces of literature; secondly, it involves analyzing the chosen literature by extracting information using a predefined set of questions. An extensive literature survey was conducted in Scopus and Google Scholar. It included papers from as early as 1971 to the present. Our search utilized specific keywords such as “recycling”, “reuse”, “recovery”, and “hierarchical approach to DWTP sludge management”. To streamline and identify a relevant and manageable collection of studies from the gathered literature, the most relevant, most recent, and most influential literature in the field was selected to form part of this review. The results from a Scopus analysis showed that most of the works from as early as 1971 remain relevant today and are still highly cited (Figure 1a). This is mainly because there is little work done worldwide on the reuse, recycling, and recovery of DWTP sludge. However, even more importantly, there was zero work found that focused on the adoption and streamlining of the hierarchical approach in DWTP sludge management. Figure 1b shows that America is the leading country when it comes to research outputs on the recycling, reuse, and recovery of DWTP sludge, with only 202 publications on Scopus. Most of the work has been done in developed economies. This review seeks to generate knowledge on the technology and approaches that are affordable to implement and can be adopted by developing economies.

3. Characteristics of Drinking Water Treatment Plant Sludge

Most water treatment utilities use conventional treatment processes that can be categorized as pre-treatment, treatment, and post-treatment. Most water treatment processes typically include coagulation, flocculation, sedimentation, and filtration, as depicted by the simplified schematic representation in Figure 2. These techniques separate liquids from solids and lead to the generation of DWTP sludge. During coagulation, ferric chloride, alum, or polyelectrolytes are added to promote the coagulation of suspended solids. The suspended solids agglomerate in bigger lumps with a higher density and start to settle at the bottom of the coagulation and sedimentation tanks. During the sedimentation and gravity filtration stages, the solid is continuously removed in the form of sludge by gravity-based sedimentation and filtration systems. The content of DWTP sludge depends on the processes used during water treatment, and it is important to first discuss the various water treatment processes to trace the origin of each component of DWTP sludge. For this review, the contribution of the quality of the raw feedwater and chemicals used in the treatment process will be assessed to determine the extent to which they affect the quality of the generated DWTP sludge.

Sludge can be obtained through different methods in filtration and pre-filtration processes. These methods are commonly employed in water treatment and industrial settings to separate solids from liquids. In the filtration process, a porous medium or filter media is utilized to separate solids from liquids [12]. As the liquid flows through the filter, solid particles or suspended matter become trapped, forming a layer of accumulated solids known as the filter cake, residuals, or sludge. The sludge obtained from the filtration process can be subjected to additional treatment to remove excess water, making it more manageable for disposal or further treatment. Techniques like mechanical dewatering, such as using centrifuges or filter presses, can be implemented to decrease the moisture content of the sludge.

Pre-filtration serves as a preliminary step before the main filtration process. Its purpose is to remove larger particles or debris that may obstruct or impede the effectiveness of the primary filtration system. Pre-filtration methods can involve techniques like screening, sedimentation, or the use of settling tanks. During pre-filtration, the collected solids or debris settle at the bottom of tanks or are trapped by screens. These accumulated solids can be periodically removed, resulting in the generation of sludge.

Similar to sludge acquired from the filtration process, pre-filtration sludge can undergo further treatment, such as dewatering, to decrease its water content. Overall, filtration and pre-filtration processes in water treatment and industrial applications yield sludge as a byproduct. The characteristics of the obtained sludge are influenced by the nature of the process, the composition of the treated liquid or wastewater, and the subsequent treatment steps applied to the sludge.

3.1. DWTP Sludge Characteristics

DWTP sludge constitutes a mixture of solids like sand and organic substances. It also contains precipitated solids and other pollutants that have been removed from the source water [5,7,8]. Between 20 and 60% of DWTP sludge by weight is mainly due to the chemicals added during the treatment process. DWTP sludge accounts for 90% or more of the total waste produced by the treatment system. The system also produces odor, which accounts for the remaining 10% of waste. DWTP sludge also contains microorganisms from the soil and other sources [20,21,22]. The exact quantities of the solids, minerals, nutrients, and microorganisms in DWTP sludge vary from treatment plant to treatment plant and tend to vary from season to season (Table 1) [11]. The ferric chloride and Al-based coagulants carry traces of metals and other impurities that end up in DWTP sludge [23]. Yet, the polymer-based coagulants tend to contain pollutants such as allyl chloride, which are highly carcinogenic and tumorous. The sludge may also contain a range of metals and non-metals from the raw water, as shown in Table 1.

In general, organic compounds from actinomycetes produce an earthy or woody odor, while other compounds emit a musty odor. A study by Zaitlina and Watson (2006) showed that there is a correlation between musty odors and the concentration of actinomycete spores. The study also showed that Streptomyces griseoluteus also causes odors in the air and DWTP sludge [24]. Raw water contains organic matter that may contribute to the odor associated with DWTP sludge [25]. DWTP sludge also contains Microcystis aeruginosa, Dolichospermum circinale, Oscillatoria sp., and Raphidiopsis raciborskii, which remain viable in stored sludge for 2–12 days. During this storage period, the cells may lyse, resulting in the release of cyanotoxins. However, most of the solid waste called DWTP sludge is produced at the sedimentation and filtration stages of the DWT process and is due to human activities, the treatment process, and the contribution of the source water [26].

DWTP sludge includes byproducts of disinfection, suspended solids, trace contaminants, and naturally occurring substances. The disinfection byproducts are and can typically include polyaromatichydrocarbons (PAHs), trihalomethanes (THMs), and halo-acetic acids (HAAs) [24,26,27]. These are formed when disinfectants react with organic matter present in the water and may pose potential health risks if present at elevated concentrations. PAHs cause kidney damage and jaundice. PAHs may also cause the breakdown of red blood cells and inflammation of the skin. Moreover, exposure to THMs can cause colon cancer and reproductive and birth defects in humans. Long-term exposure to HAAs causes kidney and liver problems [24,26,27].

Table 1. Quality of DWTP sludge, and description of the source of the component of DWTP sludge.

Content of Sludge	Quantity in DWTP Sludge	Source	Reference
Aluminum and Iron	100 mg/L	Treatment chemicals (alum and ferric chloride used as coagulants) and source water	[1,28]
Arsenic	2–10 ug/L	Source water (from rocks and soil. Also found in industrial effluent and industrial sources (such as wood preservatives)	[10,29]
Calcium and sodium	50 mg/L	Treatment Chemicals	[11,28]
Manganese and Potassium	50 mg/L	Treatment chemicals (occurs as traces in alum and iron oxide used in coagulation)	[12,25]
Fluoride	10–20 ug/L	Source water	[11,25]

Table 1. Quality of DWTP sludge, and description of the source of the component of DWTP sludge.

Content of Sludge	Quantity in DWTP Sludge	Source	Reference
Aluminum and Iron	100 mg/L	Treatment chemicals (alum and ferric chloride used as coagulants) and source water	[1,28]
Arsenic	2–10 ug/L	Source water (from rocks and soil. Also found in industrial effluent and industrial sources (such as wood preservatives)	[10,29]
Calcium and sodium	50 mg/L	Treatment Chemicals	[11,28]
Manganese and Potassium	50 mg/L	Treatment chemicals (occurs as traces in alum and iron oxide used in coagulation)	[12,25]
Fluoride	10–20 ug/L	Source water	[11,25]

The DWTP contains ions like aluminum, iron, arsenic, fluoride, and iron. Table 1 shows that, in DWTP sludge, there are elevated concentrations of aluminum and iron. Aluminum is a neurotoxin and causes oxidative stress in the brain, liver, and kidney. Overdose from aluminum can cause oxidative stress and proximal muscle weakness. Exposure to high concentrations of iron causes liver toxicity. Prolonged exposure to arsenic causes cell injury and interference with the cellular respiratory system. DWTP sludge has also been reported to contain elevated amounts of fluoride, which can cause osteosclerosis and bone deformities [1,10,11,25,28,29].

DWTP sludge is reported to be highly hydrophobic and contains phosphates, nitrates, and perchlorates. These qualities make it desirable to reuse, recover, or recycle DWTP sludge. This is viable, because an average DWTP produces between 700 and 1000 tons of sludge per day. In other cases, large treatment plants can treat up to 3600 ML/d of raw water and produce 1300 tons of dried sludge per day [25]. This demonstrates that there is enough sludge produced to make the recycling, reusing, and recovery of minerals and nutrients from sludge profitable.

The chemical composition of DWTP sludge is important for determining the potential for resource recovery, recycling, and reuse of the sludge. High concentrations of organic matter, heavy metals, or other contaminants may require additional treatment steps before the reuse, recycling, or recovery of metals or nutrients from DWTP sludge, which can be costly. Moreover, the presence of pathogens or harmful microorganisms in the sludge may make it less desirable to recycle or reuse. On the other hand, certain components of sludge, like phosphorus, nitrogen, aluminum, and others, can be recovered and reused as valuable resources. Recovering valuable resources like phosphorus from sludge can reduce the dependency on non-renewable resources.

Evaluating the characteristics of sludge also involves considering technological advancements in sludge treatment and disposal. In this review, the use of buoyancy-based sludge separation techniques and photocatalytic membrane reactors for the fractionation, treatment, and recovery of components of the sludge promises to have positive outcomes by promoting sustainable DWTP sludge management. Moreover, there is still a need for intensive research on the components of DWTP sludge. There is still a lot of work that needs to be done to determine the physical, chemical, and biological properties of sludge.

3.2. Conventional Sludge Treatments or Management Methods

3.2.1. Landfilling

Landfilling is mainly used because it is easy to set up and run. However, landfilling has negative long-term consequences on the environment, because the content of the sludge leaching into the groundwater may end up in rivers and other water bodies [5]. To meet their obligation to water treatment plants, they invest millions of dollars in constructing drying beds and engineered landfills to meet the guidelines provided by the legislative frameworks on effluents from DWTPs. In South Africa, the National Waste Act 36 of 1998 (NWA 1998) gives strict conditions for permits to dispose of waste DWTP sludge generated during water treatment [30]. This is because DWTP sludge is categorized as hazardous waste in South Africa; as such, water treatment facilities are required to treat sludge as waste. Elsewhere, in the United States of America and the United Kingdom, DWTP sludge is not characterized as hazardous waste; however, there are still concerns over the presence of heavy metals in the sludge [25]. Due to the high costs of acquiring land and constructing engineered landfills, companies in South Africa, the United States of America (USA), and the United Kingdom (UK) are looking for alternatives to landfilling. This is more important, because land is a finite resource and landfilling is not viable or sustainable in the long term [10].

3.2.2. Release into Water Bodies

Other common approaches include the release of DWTP sludge into rivers. This practice remains in many undeveloped countries where legislation is not available for the regulation of such waste, or in some cases, the legislation is there but it is not enforced [8]. The direct disposal of DWTP sludge into water bodies has adverse effects on the environment, and the long-term effects are detrimental to the environment since DWTP sludge is toxic and may contain a variety of pollutants, including heavy metals like arsenic and mercury. The aluminum in the alum used for treatment purposes tends to accumulate in the environment and leads to aluminum poisoning in aquatic animals and, later, to humans due to the bioaccumulation of non-biodegradable pollutants. High aluminum and ferric concentrations cause acidification of water and are toxic to the fish in water bodies [5]. In humans, aluminum poisoning causes Alzheimer’s disease, which is a disease that falters cognitive development in human beings. The same is true also for other heavy metals that are found in DWTP sludge, like As and Hg, which become detrimental to human health as they bioaccumulate in human bodies [31]. Moreover, polyelectrolytes used during coagulation and sludge thickening contain pollutants such as allylchloride, which can be carcinogenic and can cause the formation of tumors in the stomach when ingested by living organisms such as fish. This shows that the release of DWTP sludge into water bodies is not an environmentally friendly practice, as it has a negative influence on the aquatic life (flora and fauna) in water bodies.

3.2.3. Discharge into Wastewater Treatment Plant (WWTP) Systems

The other option is to discharge DWTP sludge into WWTPs; the implication is that there will be no need for thickening and drying of the sludge. The release of DWTP sludge into WWTP transfers the waste from the DWTP to be treated together with WWTP sludge. This is common practice in many countries, particularly in the UK and the USA [32,33,34]. The release of alum-containing DWTP sludge into WWTP systems has been reported to have benefits, as this improves the treatment process by promoting the removal of phosphorus, nitrogen, and other organic materials [34,35,36,37,38]. This makes the process more economical, considering that the discharge of DWTP sludge into WWTP is compliant with environmental laws that regulate effluents. However, the disadvantage of this method is that it increases the load or amount of waste being treated at the WWTP. Moreover, the introduction of DWTP sludge into the WWTP may limit the possible uses for the WWTP sludge, and the DWTP sludge may also contain toxic metals and organic waste that may render the WWTP non-compliant with laws stipulating the permitted quality of effluents from WWTPs.

3.2.4. Onsite Disposal

Some treatment plants use onsite disposal as a DWTP sludge management approach. However, even though this method has low operational costs, its environmental cost is not yet fully understood and would require constant monitoring. Since DWTP sludge is characterized as hazardous waste, as is the case in South Africa, the regulations do not allow for its release into water bodies or land [12]. In most countries, including South Africa, the UK, and the USA, the law states that producers of waste need to have a waste management license before managing their waste. Also, the law stipulates the quality of the allowable effluent that can be released into any environment [39]. Moreover, environmental law also needs to stipulate that it is a requirement that each water treatment plant ensures that they document the quality of the sludge to determine whether the sludge can be disposed of onsite or not. In America, the EPA does not characterize DWTP as hazardous waste; however, in South Africa, DWTP is considered to be hazardous, and therefore, the National Waste Act of 2008 prohibits the direct release of DWTP sludge into the environment [30].

There is a need for a more innovative initiative for the handling of DWTP sludge. The use of the hierarchical approach has shown promising results when it comes to handling general waste.

3.2.5. Incineration Methods

The incineration of DWTP sludge is a waste management method that involves burning the sludge at high temperatures to reduce its volume and convert it into ash. This process has been adopted by some water treatment facilities as a means of disposing of the sludge generated during the treatment process. The advantages of this method include that it remarkably reduces DWTP sludge, which reduces the volumes of waste that need to be landfilled [40]. Moreover, since incineration happens at high temperatures, pathogens and other harmful microorganisms are also destroyed during incineration. Incineration can produce energy in the form of heat, which can be harnessed for electricity generation or used to offset fuel consumption in other processes.

However, the disadvantage of the incineration of DWTP sludge is that it releases various pollutants into the environment, including heavy metals, volatile organic compounds (VOCs), and greenhouse gases. These emissions can contribute to air pollution and climate change. Also, the incineration process produces a lot of ash, which requires proper disposal, depending on its composition [12]. If the ash contains residual contaminants, its disposal can be challenging, hazardous, and costly [41]. Furthermore, the construction and operation of incinerators are costly and not viable for most developing countries.

While the incineration of DWTP sludge offers volume reduction and pathogen elimination benefits, its disadvantages and potential environmental and health effects warrant careful consideration. Moreover, the costs associated with this technology are not viable for most developing economies. Water treatment facilities should therefore explore alternative waste management methods. Striking a balance between waste reduction, resource recovery, and environmental protection is crucial for sustainable and responsible DWTP sludge management.

4. Hierarchical Approach to the Management of Drinking Water Treatment Plant Sludge

No single waste management approach is sufficient for managing the wide range of materials found in DWTP sludge. The hierarchical approach provides a combined approach that emphasizes the reduction in the volumes of waste from the source and the reduction in toxicity of the waste. The hierarchical approach also promotes waste reuse, recycling, and recovery. The hierarchical approach is promoted worldwide and can help limit the amount of DWTP sludge that ends up in a landfill. The hierarchical approach can be summarized into four main priority areas:

• minimization of the generated DWTP sludge;
• treatment of DWTP sludge to reduce toxicity;
• changing of the physical or chemical form of DWTP sludge;
• reuse, recycling, and recovery of DWTP sludge.

4.1. Minimization of the Generated DWTP Sludge

The priority for waste management should ideally minimize the amount of waste produced. In most instances, when designing a DWTP and during the process optimization, little focus is directed to planning for the byproducts of the treatment system [13]. However, regulations and the increase in environmental and health concerns have made it important to consider DWTP sludge during the design and process optimization of any DWTP [41]. The following section will detail how reducing the volumes of coagulants, choosing cleaner source water, and using dewatering systems can have an effect in reducing the volume of produced sludge.

4.1.1. Mass Balances

The amount of DWTP sludge produced by water treatment plant (WTP) processes depends on two main things, namely the source water and the added chemicals like alum (Figure 3). A combination of the two items forms the content of the resultant DWTP sludge.

In conventional coagulation treatment plants, chemicals are added to facilitate coagulation, and no other ingredient is added. It is possible to then calculate the amount of sludge that will be produced using mass balances [Equation (1)].

$${\mathit{M}}_{\mathit{i}\mathit{n}}={\mathit{M}}_{\mathit{o}\mathit{u}\mathit{t}}+{\mathit{M}}_{\mathit{A}\mathit{c}\mathit{c}\mathit{u}\mathit{m}\mathit{u}\mathit{l}\mathit{a}\mathit{t}\mathit{i}\mathit{o}\mathit{n}}$$

(1)

where M_in is the total incoming mass due to added chemicals (such as coagulants and slaked lime) and the contribution by influent suspended solids from the raw water, M_out is the total mass that leaves the treatment plant, and M_Accumulation is the mass that remains with the treatment system [13]. For this design calculation, we can assume a steady-state process and unsteady-state configuration. ${\mathit{M}}_{\mathit{A}\mathit{c}\mathit{c}\mathit{u}\mathit{m}\mathit{u}\mathit{l}\mathit{a}\mathit{t}\mathit{i}\mathit{o}\mathit{n}}$ is highly negligible or equal to zero, and therefore, under steady-state conditions, Equation (1) can be simplified to Equation (2) as follows:

$${\mathit{M}}_{\mathit{i}\mathit{n}}={\mathit{M}}_{\mathit{o}\mathit{u}\mathit{t}}$$

(2)

In short, this can be written as

$${\mathit{Q}\mathit{C}}_{\mathbf{1}-\mathit{i}\mathit{n}}+{\mathit{Q}\mathit{C}}_{\mathbf{2}-\mathit{i}\mathit{n}}={{\mathit{Q}\mathit{C}}_{\mathit{o}\mathit{u}\mathit{t}}+\mathit{M}}_{\mathit{s}\mathit{l}\mathit{u}\mathit{d}\mathit{g}\mathit{e}}$$

(3)

where ${\mathit{M}}_{\mathit{s}\mathit{l}\mathit{u}\mathit{d}\mathit{g}\mathit{e}}$ is the mass of sludge generated, Q is the flow rate, ${\mathit{C}}_{\mathbf{1}-\mathit{i}\mathit{n}}$ is the alum concentration, ${\mathit{C}}_{\mathbf{2}-\mathit{i}\mathit{n}}$ is the concentration due to the incoming suspended solids from the raw water, and ${\mathit{C}}_{\mathit{o}\mathit{u}\mathit{t}}$ is the concentration of the output. To determine the amount of sludge produced by the system, Equation (3) can be rewritten as follows:

$${\mathit{M}}_{\mathit{s}\mathit{l}\mathit{u}\mathit{d}\mathit{g}\mathit{e}}={\mathit{Q}(\mathit{C}}_{\mathbf{1}-\mathit{i}\mathit{n}}+{\mathit{C}}_{\mathbf{2}-\mathit{i}\mathit{n}}-{\mathit{C}}_{\mathit{o}\mathit{u}\mathit{t}})$$

(4)

Equation (4) shows that an increase in the coagulant concentration (${\mathit{C}}_{\mathbf{1}-\mathit{i}\mathit{n}}$) and an increase in suspended solids in the source water will lead to an increase in the amount of sludge produced. Moreover, the equation shows that an increase in the efficiency of the treatment system leads to higher amounts of sludge that will be produced, while an increase in the flow rate of the system will lead to an increase in the amount of sludge that will be produced by the treatment plant [42].

When all the factors in Equation (4) are considered, there are only three main quantities that can be regulated during the treatment process to reduce the amount of sludge produced, namely the choice of source water, flow rate, and alum concentration. However, since the flow rate is regulated by demand, only the coagulant concentration and the choice of source water can be regulated during treatment. The use of cleaner water sources such as underground water has been reported to require little or no chemical treatment, such as the addition of coagulants. Therefore, where possible, the use of underground water as a source of raw water should be prioritized, and groundwater can be used to supplement it in cases where the available volumes of underground water are not sufficient. Moreover, the coagulants should be added only as needed to avoid generating sludge that is mainly concentrated with the coagulants used in treatment processes.

4.1.2. Conditioning of DWTP Sludge

Mechanical dewatering is an example of a sludge conditioning technique that reduces the volume of sludge by removing water from the sludge. Mechanical dewatering is reported to reduce the volume of sludge, as it increases the solid concentration and reduces the volume [26]. The water in DWTP sludge can be categorized into free and bound water. Moderate mechanical strain is able to remove the water from DWTP sludge to obtain a dry product with a lower volume. The advantage of this is that it reduces transport, storage, and landfill costs, and it also reduces the limitation of energy in cases where DWTP sludge is going to be incinerated. However, the mechanical methods of dewatering sludge do not consider the atmospheric condition of the area, which can greatly affect the dewatering of the sludge [24]. Mechanical methods such as mechanical sludge compression fail to remove the bound water from DWTP sludge.

Yuqi and Yili proposed the use of electro-osmotic dewatering. This method is useful for removing water from solid–liquid mixtures that can be compressed, and the method also considers the atmospheric properties of the area [43]. It is particularly effective for sludge containing colloidal particles, which is difficult to dewater using conventional mechanical techniques such as compressing the sludge [44]. Additionally, electro-dewatering has several advantages, such as pathogen removal, lower energy and transportation costs, and the prevention of filter fouling.

The appropriate methods for conditioning and removing water from DWTP sludge also depend on the region. In warm regions, direct electrical compression is used on untreated DWTP sludge. In cold regions, a combination of natural freezing and thawing, along with compression or electrical compression, is recommended. The freezing and thawing process can reduce the duration and energy consumption of electro-osmotic dewatering, but it will not decrease the final water content [24]. Electro-osmotic treatment can increase the rate of dewatering and improve the efficiency, especially for untreated WTRs (Figure 4). Freezing and thawing treatment can greatly enhance the dewaterability and settleability of DWTP sludge. Structural changes in the sludge occur after freezing and thawing, such as an increase in floc size and improved floc strength [2].

4.1.3. Sludge Separator for Fractionating Sludge into Its Different Constituents

This work proposes the development of new technologies for the processing and treatment of DWTP sludge. The first of these technologies is the sludge separator. This technology uses buoyancy to separate the constituents of the sludge. Buoyancy is a fundamental principle in fluid mechanics that describes the upward force exerted on an object immersed in a fluid [45]. This force is equal to the weight of the fluid displaced by the object and is responsible for objects floating or sinking in a fluid medium. Therefore, buoyancy defines the ability of each constituent of DWTP sludge to float or sink in water and will depend on its density or how tight its particles are packed per unit volume. The theory suggests that, when the sludge is poured into a separation tank and allowed to settle using gravity, the most dense constituents will settle at the bottom, whilst the least dense constituents will float at the top [46].

The rate of settling of each constituent of sludge will be determined by the following equations:

$${\mathit{F}}_{\downarrow }=\mathit{m}\times \mathit{g}$$

(5)

where ${\mathit{F}}_{\downarrow }$ is the weight or downward force acting on the sludge constituent due to gravity, m is the mass of the constituent, and g is gravity. This force is acting against the upward force called buoyancy (${\mathit{F}}_{\uparrow }$):

$${\mathit{F}}_{\uparrow }=\mathit{g}\times \mathit{\rho }\times \mathit{V}$$

(6)

where V is the volume, and $\mathit{\rho }$ is the density. When these two forces are added, we obtain the final weight of each component of the sludge in the water (${\mathit{F}}_{\mathit{T}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$):

$${\mathit{F}}_{\mathit{T}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}={\mathit{F}}_{\downarrow }-{\mathit{F}}_{\uparrow }$$

(7)

From Equation (3), we can see that the components of the sludge with different densities will settle at different levels or layers. This is desirable and can make it easier to treat, recycle, reuse, and recover different components of the sludge by first fractionating it into its constituents. Figure 5a shows a schematic design of the proposed sludge separator.

The advantage of buoyancy is that it is a predictable principle that is well established and scalable. Buoyancy is used in ship manufacturing, submarine desires, and other cutting-edge technology. This shows that the technology developed using these principles can also be scaled to industrial-scaled water treatment processes without losing the efficiency of the design or technology.

However, this principle also has some limitations. Buoyancy assumes that the medium of the fluid is homogenous and remains homogenous at any given point. Fluid properties vary, especially in large bodies of water, leading to deviations from the ideal buoyancy behavior. Another assumption made by the principle of buoyancy is that the fluid has no viscosity. However, in real-life situations, fluids are viscous, and the viscosity can influence the movement of objects in the fluid and alter the buoyancy effect. The principle of buoyancy does not account for object shapes or interactions of the objects in the fluid with the fluid itself.

Preliminary studies in a beaker suggest that the use of buoyancy for the separation of DWTP sludge is a promising solution for separating the sludge into multiple layers that can then be further characterized to determine whether they need treatment, recovery, or reuse (Figure 5b). In the future, this technology will contribute immensely towards the adoption of the hierarchical approach to minimize the volume and toxicity of DWTP sludge.

4.1.4. Design of DWTP

The efficiency of a DWTP in treating raw water is influenced by its design. DWTPs consist of flocculation and sedimentation tanks, among other systems, which come in various shapes and sizes (Figure 6) [12,13]. The configuration of these tanks plays a crucial role in their effectiveness and can impact the amount of treatment chemicals required to achieve the desired water quality for drinking purposes. Efficient settling and flocculation tanks require fewer treatment chemicals compared to less efficient ones to treat the same volume of water and ensure its suitability for drinking.

DWTP sludge is primarily produced during the sedimentation and filtration stages of the treatment. Coagulating chemicals such as lime, alum, and activated silica are added to the raw water. These coagulants are first dosed into the raw water, which then proceeds into a flocculation tank. Three main tank configurations are commonly used: rectangular, cylindrical, and spiral flocculation tanks. The efficiency of each tank design/configuration directly affects the rate of sludge production.

Rectangular tanks are widely preferred and have diverse applications. Flow occurs horizontally along the length of the tank, and baffle walls may be installed to prevent short-circuiting (Figure 6a). Rectangular tanks have lower maintenance costs and are suitable for large-capacity plants. Circular tanks, on the other hand, are better suited for sedimentation involving a continuous vertical flow. The influent flows through a central pipe and then undergoes a radial flow (Figure 6b). These tanks are equipped with mechanical sludge scrapers to collect and transport the sludge through a sludge pipe at the bottom. Circular tanks are more expensive than rectangular tanks but offer higher clarification efficiency [12,13].

In South Africa, one of the major water suppliers utilizes a spiral flocculation tank, which they designed and patented (Figure 6c) [12,13]. Spiral tanks are based on the observation that horizontal flow-style tanks are more efficient than vertical flow-style tanks. Although horizontal flow tanks may require more land space, they are more adaptable to changes in the raw water quality, quantity, and turbidity. This is because horizontal tanks have a longer residence time and greater sludge storage capacity due to their larger surface area. Submersible pumps on a traveling bridge (Figure 6d) are employed to remove the sludge in this system. It has been reported that this system promotes optimal coagulation and flocculation. Other DWTPs use conventional rectangular or cylindrical tanks for flocculation.

4.1.5. Choice of Treatment System (Additive vs. Deductive Treatment Systems)

Conventional water treatment plants are based on an additive approach. In this approach, chemicals like alum are added to the water to facilitate the removal of dissolved and suspended impurities from the raw water. The added chemicals lead to an increased amount of sludge later due to the contribution of the mass of the added chemicals [27]. Moreover, the added chemicals lead to the formation of more complex water and sludge matrices that may need further treatment before reuse or recycling or disposal. However, in a deductive treatment system, the impurities in the drinking water are removed without adding any chemicals. This leads to less sludge generation and produces less complex sludge that can easily be reused. The deductive approach includes the use of ultrafiltration, nanofiltration, advanced oxidation, and ion exchange, amongst others.

Currently, there are three main minus or deductive treatment techniques, which are bank filtration, biofiltration, and membrane filtration. Bank filtration is a natural means of water treatment that uses geochemistry, biology, and hydrology to minimize the turbidity, organic contents, microorganisms, and natural organic matter found in the source water. This is widely used in the USA, where water from rivers and lakes is allowed to flow into aquifers, where it undergoes filtration, ion exchange, and adsorption. These processes can remove pharmaceuticals, pesticides, and some emerging pollutants. Biofiltration involves the use of porous biofilms for the treatment of water. Biofilms are widely used for stabilizing water to prevent bacterial growth that causes a deterioration in water quality. The disadvantage of biofilms is that they require a very long contact time to be effective.

Membranes have also been used for filtration, adsorption, and dialysis to promote a deductive or minus approach to treating water. Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis have been reported for the removal of ions, organic matter, and pathogens. The use of membranes can substitute the coagulation, flocculation, and disinfection steps in water treatment by just introducing one potent membrane.

4.2. Reuse of DWTP Sludge

In the systematic hierarchical approach, reuse refers to the utilization of DWTP sludge in its original state without undergoing any treatment or modification of its chemical composition. It involves repurposing the sludge without extracting or altering any specific components of the DWTP sludge [32]. The reuse of DWTP sludge for various purposes has been extensively documented, including its application in water and wastewater treatment processes, soil conditioning, and numerous other uses [33,47,48]. This section will examine the reuse of DWTP sludge specifically for the removal of microbes through physical straining, electrostatic attraction, biofilm formation, and inactivation.

4.2.1. Reuse of DWTP Sludge for Removal of Microbes from Water and Wastewater Treatment Systems

Knowledge has been generated on mechanisms like attachments, adsorption, electrostatic interaction, and inactivation by which the sludge removes bacteria, viruses, and other inorganic and organic materials from the water or wastewater (Figure 7). Acid/base-modified DWTP sludge has been reported to achieve 0.92–1.68 log removal of E. coli [49]. However, the coating of DWTP sludge in metal has shown better stability compared to the acid/base-modified DWTP sludge. Different findings show that DWTP sludge that contains metals has a lower solubility, which gives rise to its stability [50]. The metal-containing DWTP sludge was found to be effective for the removal of bacteria through electrostatic attractions [51,52].

Other researchers have reported that DWTP sludge can be reused for bacterial removal, virus removal, and the trapping of metal ions (Figure 7) [53,54]. The removal mechanisms are largely controlled by interactions between the sludge and microbes, thus making the sludge’s physicochemical properties, such as pore size, important in this regard.

Figure 7. Techniques for microbe removal reproduced with permission from [54].

Figure 7. Techniques for microbe removal reproduced with permission from [54].

When DWTP sludge is used as a biofilter, physical straining contributes immensely towards the removal of pathogens, especially for stormwater treatment. The formation of biofilms and extended retention times in the biofilter lead to reduced porosity and enhance the straining of pathogens, leading to their removal [51].

Due to the large surge area of DWTP sludge, it is possible to remove E. coli by surface diffusion on the filter media [49]. DWTP sludge is also highly porous, and the mesopores (2–50 nm) are the ones responsible for trapping the bacteria, depending on the size of the bacterial cell. To achieve this, the pore sizes of the DWTP sludge biofilter must be two to five times larger than the cell size of the microbe [55].

Previous work has shown that bacterial depositions occur largely on hydrophobic surfaces, and DWTP sludge presents the perfect surface for such a deposition to occur [56]. The findings show that the high cell surface hydrophobicity of most microbes is the main reason why they tend to attach themselves to non-polar surfaces and organic material [57]. Further, it was demonstrated that there is a relation between surface-free energy and the hydrophobic nature of DWTP sludge, since the sludge has low surface energy. This translates to high hydrophobia and high attraction to microbes, which attach themselves to the sludge and get removed from water systems [56,57].

DWTP sludge can be reused to form a biofilter that can attract microorganisms using attractive van der Waals interactions. The biofilters can also use repulsive electrostatic interactions to reject or repel other microorganisms. Attractive interactions between the microorganisms and the biofilters are due to positively charged biofilter surfaces and negatively charged virus cells [58,59,60]. The presence of metals on the surface of the biofilter promotes electrostatic interactions between the bacteria and DWTP sludge biofilter, and the bacteria tend to be trapped in the biofilter and are removed from the water or wastewater [60]. Holistically, the interaction can be described as adsorption when the microbes are adsorbed onto the surface of the biofilter made from DWTP sludge [50].

DWTP sludge formed by alum-based treatment processes can be used to formulate biofilters that exhibit polymer-like qualities during hydrolysis. These filters adsorb onto the virus and other microbes [61]. Figure 8 shows the mechanisms for the removal of E. coli from stormwater runoff using a bioretention column that contains alum [62]. Moreover, the biofilm also achieves the removal of pathogens by mechanical trapping [63]. Mechanical trapping is largely influenced by the type of the microorganism being trapped [58].

Matsushita et al. have also reported the inactivation of microbes and viruses in DWTP sludge biofilms [61]. However, the proposed mechanisms for the inactivation differ and are inconsistent. Li and co-workers suggest that the presence of metals on the surface of DWTP sludge could alter the surface charge of the biofilm and give rise to the electrostatic attraction, which attracts active and infectious microbes [62]. However, some works have shown that microbes that are attracted to the surface of DWTP sludge that contains metals are disintegrated because of damage to their cell structure [64]. This requires further investigation. Generally, the reuse of DWTP sludge in water and wastewater treatment and the transfer of DWTP sludge into WWTP systems show a lot of promise. Table 2 shows other possible options for reusing DWTP sludge.

Table 2. Reuse of DWTP sludge in water and wastewater treatment processes.

Reuse of DWTP Sludge	Methods	Advantage	References
Coagulant	Reuse as a coagulant in water and wastewater treatment	Less sludge requires landfilling. More economical than landfilling.	[65,66]
Adsorbent	Reused for recovery of minerals from WWTPs	Can be added to WWTP to recover P, Cr, Au, As, and other elements	[67,68,69]
Substrate	Used in wetlands for removal of excess Phosphorus	Assistive remediation. Removal of excess phosphorus	[63,70,71,72,73,74,75]
Biofilter	Used for treating stormwater runoff	Evidence of pathogen removal	[76,77,78]

Table 2. Reuse of DWTP sludge in water and wastewater treatment processes.

Reuse of DWTP Sludge	Methods	Advantage	References
Coagulant	Reuse as a coagulant in water and wastewater treatment	Less sludge requires landfilling. More economical than landfilling.	[65,66]
Adsorbent	Reused for recovery of minerals from WWTPs	Can be added to WWTP to recover P, Cr, Au, As, and other elements	[67,68,69]
Substrate	Used in wetlands for removal of excess Phosphorus	Assistive remediation. Removal of excess phosphorus	[63,70,71,72,73,74,75]
Biofilter	Used for treating stormwater runoff	Evidence of pathogen removal	[76,77,78]

The main disadvantage with most of these applications is that DWTP sludge contains heavy metals, and these metals may leach from the biofilters or when it is used as a coagulant, adsorbent, or as substrate.

4.2.2. Reuse of DWTP Sludge for the Removal of Cation Substances in Water and Wastewater Systems

DWTP sludge can remove anions, including P, Se, N, As, ClO⁴⁻, and others, through adsorption (Table 3). DWTP sludge can therefore be used for land remediation where the soil is contaminated by heavy metals. DWTP sludge from aluminum-based treatment processes has been found in the removal of selenium (Se). In the environment, Se bioaccumulates, leading to dysfunctional reproductive and nervous systems. It has been reported to cause the death of fish, plants, and insects and the deformation of human bodies when in high concentrations [79]. The ability of DWTP sludge to remove Se was tested by reacting Se with Al-based DWTP sludge across a range of pH found on soil and in water bodies. The results showed that DWTP sludge adsorbed between 1400 mg and 2100 mg/kg of selenium. Characterization of the used sludge showed that Se was on the surface of DWTP sludge compared to inside the DWTP sludge’s inner sphere. However, at pH 8, research suggests that inner sphere complexation happens. More research demonstrated that the oxidation of Se was irreversible after adsorption onto DWTP sludge, making Se more stable [10]. This shows promising results, suggesting that DWTP sludge from alum-based treatment processes can be reused for the removal of Se from water bodies.

Arsenic (As) in the environment may originate from industrial processes, natural deposits, and agricultural runoffs. The consumption of As can cause paralysis, blindness, vomiting, cancer, and other disorders [25]. Therefore, the removal of As from the environment is very important to avoid these effects. Makris et al. (2006) reported the use of Al-based DWTP sludge for As (III) and As (V) sorption [29]. The findings of their study showed that Al-based DWTP sludge was capable of removing As (V) (about 14,000 mg/kg of Al-based DWTP sludge), and yet, the efficiency was slightly lower for As (III) (about 8000 mg/kg of Al-based DWTP sludge) [80].

A separate study sought to determine the effect of Al-based DWTP sludge on the bioaccessibility of As (V) and its phytoavailability in soil [29]. The soil that was used had arsenic emanating from the application of fertilizers and pesticides. When phosphorus-containing fertilizers were applied to the same soil, the As was released from the soil into water bodies, and the phosphorus was adsorbed in place of the As. This leads to an increase in the concentration of As in water bodies, especially in groundwater [81]. However, it was found that, when Al-based DWTP sludge was applied to the soil, it stabilized the As and prevented it from being released into the groundwater. Moreover, the Al-based DWTP sludge reduced the bioaccessibility. The results suggest that Al-based DWTP sludge can be employed for the remediation of As-containing soils even if phosphorus is added to the soil.

When As was added to livestock and poultry feed, it improved the growth and reduced parasitic and bacterial diseases [29]. Therefore, most food for livestock now contains As. Makris et al. (2006) evaluated the ability of Al-based DWTP sludge for the degradation of As. Ratios of 2.5, 5, 10, and 15% Al-based DWTP sludge by weight were evaluated [29]. It was found that 5, 10, and 15% Al-based DWTP sludge had lower As concentrations. The findings suggested that, if DWTP sludge could be mixed with the poultry litter, this could limit the amount of As that leaches into the environment.

Table 3. Reuse of DWTP sludge for the removal of ions.

Ion Removed Using DWTP	Method of Removal	Amount Removal	Reference
Selenium Se	Adsorption	1400 mg to 2100 mg/kg	[11,25,79]
Arsenic As (III) and As (V)	Adsorption	8000 to 14,000 mg/kg of Al-based DWTP sludge	[25,29]
Perchlorate ClO4−	Immobilization	65%	[82]
Aluminum Al (III)	Adsorption	Up to 2780 mg/kg	[28,83,84]
Phosphates [PO4]3⁻	Adsorption	83%	[28,83,85]
Nitrates, Ammonia NO3−, NH3	Adsorption	experiments and field tests often exceed 80 or 90%	[38,86]

Table 3. Reuse of DWTP sludge for the removal of ions.

Ion Removed Using DWTP	Method of Removal	Amount Removal	Reference
Selenium Se	Adsorption	1400 mg to 2100 mg/kg	[11,25,79]
Arsenic As (III) and As (V)	Adsorption	8000 to 14,000 mg/kg of Al-based DWTP sludge	[25,29]
Perchlorate ClO4−	Immobilization	65%	[82]
Aluminum Al (III)	Adsorption	Up to 2780 mg/kg	[28,83,84]
Phosphates [PO4]3⁻	Adsorption	83%	[28,83,85]
Nitrates, Ammonia NO3−, NH3	Adsorption	experiments and field tests often exceed 80 or 90%	[38,86]

4.2.3. Reuse of DWTP Sludge for the Removal of Anionic Substances in Water and Wastewater Systems

DWTP sludge can also be used for the removal of anionic species. Some anionic species like perchlorate (ClO⁴⁻) have very bad effects on human health. Perchlorate causes the thyroid to reject the uptake of iodine, and this may lead to hypothyroidism. This can have very bad impacts on children and fetuses, which need the thyroid hormone to grow [25]. In adults, the effects can be reversed. An extended lack of thyroid hormone leads to poor brain development [29]. Studies have been done to investigate the removal of perchlorates from the environment using Al-based DWTP sludge, and the finding showed that DWTP sludge removed about 65% of the ClO⁴⁻. Furthermore, the results showed that the ClO⁴⁻ was immobilized on the Al-based DWTP sludge. Therefore, DWTP sludge can be used for anion removal in general and ClO⁴⁻ removal from the environment by adsorption [29]. This clearly shows that DWTP sludge can be used for the removal of anionic species from the soil during land applications or agricultural applications.

4.2.4. Land-Based Applications of DWTP Sludge

The use of DWTP sludge in land-based applications for curing laden soils or for conditioning soils was reported by Babatunde et al. [11]. During the curing of laden soils, DWTP sludge is added to the soil to remove excess ions like phosphorus, which can have negative consequences when in excess in the soil. On the other hand, DWTP sludge can also be added to soils with low phosphorus concentrations to trap the applied phosphorus so that it does not get lost by leaching into the groundwater or other water bodies. This helps improve the soil quality and nutrient content. However, the leaching of Al from the Al-based DWTP sludge remains a concern that needs to be addressed [83]. The risk of leaching of the Al increases with a decrease in the pH of the water, and at pH < 5.2, Al starts leaching due to the increased solubility. Figure 9 shows the five mechanisms by which phosphorus can be removed from the laden soils using DWTP sludge. The mechanisms include ion exchange, ligand exchange, hydrogen bonding, surface precipitation, and internal diffusion.

However, under normal circumstances, the soil is at a neutral pH, and the Al is not expected to leach. Plant toxicity due to Al leaching still requires further investigation, as some research suggest that it may occur even at a neutral pH [85]. However, other researchers believe plant toxicity due to Al is not possible, since the pH of the soils is way above 5.2, which is critical for the solubility of Al [87]. Radioactivity, the presence of heavy metals and toxic pesticides and fertilizers, has also been a source of concern when considering DWTP sludge for agricultural applications in soil conditioning and curing [78]. Table 4 presents the advantages of using DWTP sludge for conditioning and curing. However, the stability assessment of the leaching of metals during long-term application would be a critical issue that needs to be addressed, especially for application in soil conditioning and curing.

Table 4. Reuse of DWTP sludge in agriculture and other land-based uses.

Ways for Reusing Sludge	Methods	Advantage	References
Soil conditioning	Mixed with soil to improve soil texture, water, and nutrient retention	Less sludge requires landfilling. More economical than landfilling.	[84,85,88,89]
Curing of laden soils	Removal of excess phosphorus	Less costly. Phosphorus can be recovered from DWTP sludge at a later stage.	[90,91,92,93,94,95,96,97,98,99]

Table 4. Reuse of DWTP sludge in agriculture and other land-based uses.

Ways for Reusing Sludge	Methods	Advantage	References
Soil conditioning	Mixed with soil to improve soil texture, water, and nutrient retention	Less sludge requires landfilling. More economical than landfilling.	[84,85,88,89]
Curing of laden soils	Removal of excess phosphorus	Less costly. Phosphorus can be recovered from DWTP sludge at a later stage.	[90,91,92,93,94,95,96,97,98,99]

4.3. Recycling of Drinking Water Treatment Plant Sludge

The recycling of DWTP sludge includes the repurposing of sludge to form a new substance. DWTP sludge can be repurposed mainly for the construction, civil engineering, and water treatment sectors. DWTP sludge has been reported to be useable for making construction materials, such as for brick manufacturing, cement manufacturing, and for the construction of landfills [89]. DWTP sludge can be recycled for cleaning, stormwater, and agricultural water to remove metals from the stormwater and agricultural water. In most cases, coagulants like alum and ferric chloride are mixed with silica and slaked lime to fast track coagulation and adjust pH to remove metals and even kill bacteria. The added silica and slaked lime make DWTP sludge a good candidate to be used in the construction industry. The following sections detail other recycling avenues for DWTP sludge in the construction sector.

4.3.1. Recycling of DWTP Sludge for Ceramic Material in the Construction Industry

DWTP sludge can be mixed with clay in varying proportions, depending on the physical and chemical properties of the DWTP sludge. The mixture of the clay and DWTP sludge can then be sintered to produce clay–sludge bricks [89]. When making bricks, their integrity and strength must never be compromised. The findings from one study showed that, if 20% or less of the DWTP sludge is mixed with 80% clay, the bricks retain their strength [100]. A mixture of DWTP sludge and rice husk ash was found to have high strength, even more than most bricks that are already on the market [26]. However, it was also found that, if the mixture contains lime, the brick will quickly lose its strength, because lime absorbs water into the mixture, and this weakens the tensile strength of the bricks in the long run. The 70% DWTP sludge: 30% rice ash mixture had the highest strength [89,100]. These results show that DWTP sludge can be used for the manufacture of bricks. Figure 10 shows the results of adding 0–10% DWTP sludge when making bricks. The results also show that an increase in the ratio of DWTP leads to an increase in the strength of the produced brick. This is expected up to a certain optimum ratio of DWTP sludge to clay.

Several studies have suggested that Al-based DWTP sludge can also be used for the manufacture of cement [31,40,89,101,102]. Spray-dried Al-based DWTP sludge was reported to be able to manufacture Portland cement [103]. The report investigated different conditions under which the Al-based DWTP sludge could be reused for the manufacture of cement. The report showed that the addition of Al-based DWTP sludge in the Portland manufacturing process leads to an increase in the strength of the material that was made from DWTP sludge-based cement [102]. The cement made from DWTP sludge was used for making lightweight paving and ceramics. The product was also highly permeable and adsorbed water [40]. The mixture of 80% Al-based DWTP sludge and 20% ash met the Japanese and Chinese Standards for masonry. The findings suggest that Al-based DWTP sludge can be used in place of clay and limestone when manufacturing cement.

The recycling of DWTP sludge has also been reported in cases where it was used as raw material during the manufacture of ceramics [104]. DWTP sludge is first dried and then mixed with clay at different ratios to form a slurry. The resultant ceramics are reported to be lightweight and environmentally friendly [105]. The ceramics also had compressive strength. A further experiment on DWTP sludge containing ceramics showed that they have water absorption that meets Taiwan’s standards for all lightweight ceramics [106].

Recycling DWTP sludge to make ceramic materials can be a promising approach for waste management and resource utilization. This practice involves incorporating sludge into the production of ceramic products, such as bricks, tiles, and pottery. Recycling DWTP sludge into ceramic materials reduces the volume of waste that would otherwise require disposal in landfills, thereby contributing to waste reduction and resource conservation. By utilizing DWTP sludge as a raw material, the demand for conventional raw materials (e.g., clay and silica) in ceramic production can be reduced, conserving natural resources. Incorporating sludge into ceramic production can lead to energy savings, as the organic matter in the sludge acts as fuel during the firing process. Recycling DWTP sludge into ceramics represents a form of waste valorization, where an otherwise waste material is turned into a valuable resource.

However, the chemical and physical properties of DWTP sludge can vary, depending on the source and treatment processes, which may affect its suitability for ceramic production. Incorporating sludge into ceramics requires careful processing and quality control to ensure the final products meet the desired standards and do not pose any health or environmental risks. DWTP sludge contains trace amounts of heavy metals and other contaminants, which, if not properly controlled, could leach into ceramic products and pose health hazards.

There may be regulatory requirements and standards to comply with when using DWTP sludge in ceramic production, which can add complexity to the recycling process. The firing process during ceramic production can release emissions, including particulate matter and greenhouse gases, which can contribute to air pollution and climate change. Thoroughly characterizing DWTP sludge before recycling is essential to understand its chemical composition and potential impacts on ceramic product quality and safety.

Implementing rigorous quality control measures during ceramic production is crucial to ensure the final products meet safety and performance standards. Recycling DWTP sludge to make other ceramic materials can offer waste reduction and resource conservation benefits, but it comes with challenges related to sludge characteristics, quality control, and potential environmental and health implications. Implementing sustainable practices, such as sludge characterization, quality assurance, and safe handling and disposal, can help mitigate these challenges and make the recycling process more environmentally and economically viable.

4.3.2. Recycling of DWTP Sludge for Water and Wastewater Treatment

The Al-based DWTP sludge can also be recycled for the purposes of application in drinking water treatment (DWT) processes and wastewater treatment processes (WWTPs). The Al-based DWTP sludge is mixed with a fresh coagulant, and the new mixture is then added in place of the fresh coagulant [107]. This finding shows that the recycled mixture had enhanced turbidity removal efficiency. The new mixture had better coagulation efficiency compared to a fresh coagulant, which helped reduce the amount of used coagulant [108]. This, therefore, confirmed that the recycling of DWTP sludge for DWT is a feasible exercise and that it may help reduce the cost of a treatment process (

Table 5. Recycling of DWTP sludge in the construction industry.

Reuse of Sludge in Construction	Methods	Advantage and Disadvantages	References
Cement manufacture	Aluminum-based DWTP sludge is used in place of clay	Less sludge requires landfilling More economical than landfilling	[109,110]
Brick making	DWTP sludge added to the mixture for brickmaking.	Forms lightweight bricks of adequate strength.	[111,112]
Construction of ceramics	DWTP sludge is used in place of lime and clay	Forms lightweight, porous ceramics	[113,114,115,116]

) [107].

Recycling DWTP sludge for water and wastewater treatment systems presents a promising approach to enhance efficiency, minimize waste generation, and promote sustainability. This method involves reutilizing the sludge generated during water treatment for various purposes within the treatment systems, leading to the recovery of valuable resources like phosphorus and organic matter, which can be reused as fertilizers or soil amendments in agriculture. Consequently, reusing the sludge also reduces the costs associated with waste disposal and raw material acquisition.

However, successful implementation of this recycling practice requires ensuring compatibility between the recycled sludge and the existing infrastructure and processes of the water treatment plant. This may necessitate modifications or upgrades to accommodate the intended use of the recycled sludge. Moreover, it is imperative to implement proper treatment and disinfection procedures to control the presence of pathogens and contaminants that may be present in the recycled sludge.

Inadequate treatment of the recycled sludge and its potential contaminants could adversely affect the water quality, posing risks to human health and the environment. Hence, careful attention must be paid to address these concerns effectively during the recycling process.

While recycling DWTP sludge for water treatment offers the potential for sustainability, resource recovery, and cost savings, it is not without challenges. These challenges primarily revolve around ensuring the quality of the sludge, adhering to regulatory requirements, and addressing possible environmental and health implications. To make the recycling process environmentally and economically viable while ensuring safe and effective water treatment, it is crucial to adopt sustainable practices such as comprehensive sludge characterization, rigorous quality control, and strict adherence to relevant regulations.

4.4. Recovery of Nutrients and Resources from Drinking Water Treatment Plant Sludge

Recovery is the extraction of nutrients, metals, energy, or other key constituents of DWTP sludge for reuse. Since Al-based DWTP sludge contains a variety of organic and inorganic matter, these substances can be extracted from DWTP sludge through processes like adsorption, filtration, electrostatics, and many other methods. This process can be used for the extraction of phosphorus, recovery of ammonia or nitrogen, and recovery of alum. Various approaches in which sludge is applied for the recovery of nutrients will be outlined in this section.

4.4.1. Extraction of Phosphorus from Al-Based DWTP Sludge

Phosphorus is a metal that occurs naturally in the Earth’s crust. Phosphorus is mined as a rock and is often used to produce inorganic fertilizers for agricultural applications. However, phosphorus rock is a finite resource, and the reserves are becoming depleted. DWTP sludge can trap phosphorus, and the phosphorus can accumulate in the DWTP to the point where the DWTP can be used as a source of phosphorus [117,118,119]. This could provide an alternative to the shrinking phosphorus reserves. A study on Al-based DWTP sludge showed that, if it is mixed with swine waste, it has the potential to provide plant-available phosphorus. The phosphorus-containing Al-based DWTP sludge also increased phosphatase activity, and this was attributed to the mineralization of the phosphorus in the Al-based DWTP by microbe action [120,121]. The results showed that the Al-based DWTP sludge can be used for the recovery of phosphorus via mineralization with microorganisms found in the soil. The plant’s available phosphorus can then be used as a fertilizer. The advantage of this method is that it reduces the reliance on the mined phosphate rock, and it also helps reduce the amount of sludge that is disposed of, improves soils, and contributes to the soil’s nutrient cycle [117].

4.4.2. Recovery of Ammonia/Nitrogen from DWTP Sludge

Ammonia is used in agriculture as a source of nitrogen for plant growth. However, ammonia is lost to the environment by eutrophication. Eutrophication is a process where excessive nutrients, primarily nitrogen and phosphorus, enter a water body and stimulate the growth of algae and other aquatic plants [122]. This can lead to the depletion of oxygen levels in the water, harm fish and other aquatic life, and create harmful algal blooms that produce toxins that can be harmful to human health. There are several pathways through which nitrogen and phosphorus enter water bodies, including agricultural runoff, urban stormwater runoff, wastewater discharges, and atmospheric deposition [99]. Fertilizer and manure application on farms can lead to excess nutrients running off into nearby water bodies, while urban development can result in increased impervious surfaces and stormwater runoff that carries nutrients into waterways [119,122].

Reducing nutrient inputs to water bodies is essential to combat eutrophication. This can be achieved through better land use practices, improved wastewater treatment, and reduced air pollution. By addressing the sources of excess nutrients, we can protect the water quality and ensure that our water resources remain healthy and sustainable. In industries, ammonia is produced via the Haber–Bosch process [122]. This process requires a lot of energy to occur. Since ammonia is used for agricultural applications, in time, ammonia finds its way into the DWTP. The recovery of the ammonia using DWTP sludge has been reported, and 55% to 65% recovery was achieved. The recovery of ammonia is desirable, because the process for its production not only uses a lot of energy but also releases greenhouse gases [99]. Laboratory and pilot-scale studies were carried out on bioretention boxes to demonstrate their ability to remove pollutants. Bioretention is a stormwater management practice that involves the use of engineered soil and vegetation to capture and treat stormwater runoff [119]. This process is designed to mimic the natural hydrological cycle and remove pollutants from stormwater before it enters water bodies. These studies confirmed that bioretention is an effective treatment method, with the ammonium recovery rates ranging from 60% to 80% [122]. Therefore, the recovery of ammonia using bioretention would reduce the release of greenhouse gases and save energy. This demonstrates that the recovery of ammonia and nitrogen from DWTP sludge should be prioritized. It has potential environmental and economic benefits.

4.4.3. Recovery of Alum from the Al-Based DWTP Sludge

Due to urbanization, population increase, and demand for treated water in general, the demand for alum for coagulation purposes in a treatment plant continues to increase. This, therefore, makes the recovery of alum from Al-based DWTP sludge a desirable exercise [123]. Since alum is highly soluble in acidic media, acidification has been reported to have positive results when it comes to converting the solid alum in DWTP sludge into liquid alum, which is more soluble. The recovered alum has been reported to be comparable to fresh alum. A comparison between the fresh alum and recovered alum shows that their turbidity removal is similar. Moreover, the recovered sludge has an improved dewatering capacity. The use of acidification for alum recovery is up to 70% efficient [44]. The efficiency of the recovery and the quality of the recovered alum depend on the pH at which DWTP sludge is being acidified, as the more acidic the pH, the higher the recovery efficiency and quality of the recovered alum [124]. This may be cost-effective, and it reduces the amount of sludge that requires landfilling. However, the acidity used for the recovery process may need further treatment.

On another hand, alum has also been recovered using alkaline solutions. The Al-based DWTP sludge can be treated with NaOH or Ca(OH)₂ at alkaline conditions to recover the alum. The more alkaline the pH, the better the aluminates recovery. The recovery starts to happen at a pH of about 11.8. The use of NaOH is more efficient than Ca(OH)₂ [124]. However, the main problem is that the use of pH for recovery is non-selective, and therefore, other impurities may also end up in the recovered material.

Unlike the pH-based methods for the recovery of alum, Donnan dialysis uses electrochemical potential difference for the recovery of alum on an ion exchange membrane, as shown in Figure 11. This process reduces the risk of impurities accumulating on the recovery alum, since it is more selective. Moreover, the Donnan membrane has a higher recovery efficiency than the pH-based methods for alum recovery [125,126].

Donnan dialysis is, however, a very slow process and usually requires up to 24 h run time to achieve optimal results [125]. The efficiency can be improved by increasing the membrane’s surface area, and this can lead to a decrease in the required retention time for the recovery. However, the limitation of the use of this technology is that the membranes require treatment and proper disposal after use. Moreover, the costs of formulating the membranes also remain relatively high [127]. It has been demonstrated that the use of Donnan membranes for the recovery of alum is more efficient than other processes, including acid digestion. Donnan dialysis allows for the recovery of water. However, the membranes are very prone to fouling, and sludge would be expected to clog the pores of the membrane during selective filtration or dialysis. [125].

4.4.4. Recovery and Treatment of Nutrients and Metals from DWTP Sludge

Another important technology that is being proposed for adoption in DWTP sludge management is the use of photocatalytic membrane reactors (PMRs) for the treatment and/or recovery of metals and nutrients from DWTP sludge. This technology is still at the prototyping stage. The design was conducted in two parts, namely basic process flow design and chemical engineering design. Process flow outlines the layout of the intended design or process, and chemical engineering outlines the chemical principles to be considered during design.

It is proposed that the PMR must be placed after the sludge separator that was proposed earlier or after any other type of clarifier to prolong the lifespan of the membranes (Figure 12). To further improve the lifespan of the membrane, a membrane can be added at the inlet of the photoreactor to remove any material that may cause fouling of the membranes [128].

Material balances can used to analyze the pollutant concentration in a control volume, which is the reactor, by accounting for material entering and leaving the system. There is no mass loss associated with the production of energy; hence, the mass and energy balances will be conserved separately. The material balances can be developed on the assumption that the flow conditions in the reactor are ideal.

The general equation for mass balance is given by Equation (8) [129,130]:

$$\mathit{I}\mathit{n}\mathit{p}\mathit{u}\mathit{t}=\mathit{O}\mathit{u}\mathit{t}\mathit{p}\mathit{u}\mathit{t}+\mathit{A}\mathit{c}\mathit{u}\mathit{m}\mathit{u}\mathit{l}\mathit{a}\mathit{t}\mathit{i}\mathit{o}\mathit{n}+\mathit{C}\mathit{o}\mathit{n}\mathit{s}\mathit{u}\mathit{m}\mathit{p}\mathit{t}\mathit{i}\mathit{o}\mathit{n}$$

(8)

For a continuous flow reactor, operated under a steady state and the concentration of substances, the volume (V) needed to reduce the mass/molar flow rate, F_AO to F_A1, is given by

$$\mathit{V}={\int }_{{\mathit{F}}_{\mathit{A}\mathbf{1}}}^{{\mathit{F}}_{\mathit{A}\mathit{O}}}{\displaystyle \frac{\mathit{d}{\mathit{F}}_{\mathit{A}}}{{-\mathit{r}}_{\mathit{A}}}}$$

(9)

where F_A is molar flow rate for substance A, and r_A is the rate of the reaction.

Using mass balance, it is possible to calculate the efficiency of the reactor by determining its ability to convert mass. Mass conversion and volume can be used to determine the retention rates. Equation (10) will be derived from the conversion and volume to calculate the residence (t):

$$\mathit{t}={\mathit{C}}_{\mathit{A}\mathit{O}}{\int }_{\mathbf{0}}^{{\mathit{X}}_{\mathit{A}}}{\displaystyle \frac{{\mathit{d}\mathit{X}}_{\mathit{A}}}{\left(-{\mathit{r}}_{\mathit{A}}\right)}}$$

(10)

where C_AO is the concentration of metals/nutrients, A the reactor inlet, and X is the conversion of A.

Moreover, Equation (8) can be used to calculate the desired reactor volume.

$${\displaystyle \frac{{\mathit{V}}_{\mathit{r}}}{{\mathit{F}}_{\mathit{A}\mathit{O}}}}={\int }_{\mathbf{0}}^{{\mathit{X}}_{\mathit{A}}}{\displaystyle \frac{{\mathit{d}\mathit{X}}_{\mathit{A}}}{-{\mathit{r}}_{\mathit{A}}}}$$

(11)

The membrane length can be determined using Equation (12).

$$\mathit{l}\mathit{e}\mathit{n}\mathit{g}\mathit{t}\mathit{h}={\displaystyle \frac{\mathbf{4}{\mathit{F}}_{\mathit{A}\mathit{O}}}{\mathit{\pi }{\mathit{D}}^{\mathbf{2}}}}{\int }_{\mathit{O}}^{\mathit{X}}{\displaystyle \frac{\mathbf{1}}{-{\mathit{r}}_{\mathit{A}}}}{\mathit{d}\mathit{X}}_{\mathit{A}}$$

(12)

where D is the membrane diameter.

Mechanical design involves selecting suitable construction materials and the positioning of the components to improve the operability and integrity of the PMR. Operating and design pressure, temperature, and flow rate will be optimized using Ergun Equation (13). It is important to estimate the pressure drop during reactor design, since a pressure drop adds to the running costs.

$$-{\displaystyle \frac{\mathit{d}\mathit{P}}{\mathit{d}\mathit{z}}}=\mathbf{150}\left[{\displaystyle \frac{{\left(\mathbf{1}-{\mathit{\epsilon }}_{\mathit{b}}\right)}^{\mathbf{2}}}{{\mathit{\epsilon }}_{\mathit{b}}^{\mathbf{3}}}}\right]{\displaystyle \frac{\mathit{\mu }\mathit{u}}{{\mathit{d}}^{\mathbf{2}}}}+\mathbf{1.75}\left[{\displaystyle \frac{\left(\mathbf{1}-{\mathit{\epsilon }}_{\mathit{b}}\right)}{{\mathit{\epsilon }}_{\mathit{b}}^{\mathbf{3}}}}\right]{\displaystyle \frac{\mathit{\rho }{\mathit{u}}^{\mathbf{2}}}{\mathit{d}}}$$

(13)

where z is the axial length, ${\epsilon }_b$ is the fractional bed porosity, $\mu$ is the fluid viscosity, u is the superficial velocity, d is the average diameter of the nanoparticle, $\rho$ is the density of the fluid, and P is the pressure.

The thickness of the PMR will be determined using hoop stress Formula (14).

$$\mathit{t}={\displaystyle \frac{\mathit{P}\mathit{R}}{\mathit{S}\mathit{E}-\mathbf{0.6}\mathit{P}}}+{\mathit{t}}_{\mathit{c}}$$

(14)

where t is the vessel wall thickness, the design pressure is P, the inside radius of the vessel is denoted by R, the maximum allowable stress for the steel is S, the joint efficiency is E, and the corrosion allowance is t_c.

The design pressure will be determined using Barlow’s Equation (15):

$$\mathit{P}={\displaystyle \frac{\mathbf{2}\mathit{T}\mathit{S}}{\mathit{D}}}$$

(15)

where the pressure is P, the wall thickness is t, the allowable stress is S, and the outside diameter is D.

Each of the PMR can treat/recover minerals or nutrients and pass the effluent onto the following PMR, which will also selectively treat/recover another component of the effluent. The pollutants can be fractionated based on their sizes, RED/OX potential, adsorption onto the photocatalytic membranes (PMs), and other characteristics that are determined by the nutrients’/minerals’ interaction with the PMs [130]. The photoreduced metals that precipitate can be removed by either filtration on membranes, depending on the size of the nutrient molecule, or the metal particle size and isoelectric potentials.

5. Economic, Social, and Environmental Impacts of Adopting the Hierarchical Approach in DWTP Sludge Management

The implementation of a hierarchical approach in managing sludge from drinking water treatment plants offers an opportunity for both economic and social advantages for these entities. Additionally, there are notable environmental benefits that a country can reap from this approach. For instance, DWTP sludge can be utilized for soil remediation, contributing to the restoration of contaminated soils [131]. Furthermore, by combining DWTP sludge with manure, the excess phosphorus and nitrogen runoff can be mitigated, preventing groundwater and river contamination [132,133,134]. Consequently, the recycling and repurposing of DWTP sludge as a construction resource signifies a shift in its perception from waste to valuable material, resulting in economic benefits.

The economic benefits of recovering coagulants from DWTP sludge are evident, as it reduces the demand for fresh alum. However, it is crucial to enhance the quality of the recovered alum to ensure its safety for reuse in DWTPs. Moreover, the recovery of phosphorus (P) from DWTP sludge and other waste materials provides an opportunity to utilize the sludge as an alternative to the limited phosphate rock currently mined. This recovery process is particularly advantageous, since it reduces the need for phosphate rock mining, which is associated with the release of acid mine drainage (AMD) and other harmful substances that have detrimental effects on the environment. Additionally, the recovery of ammonia presents an environmentally friendly alternative to the Haber–Bosch process for ammonia production [132]. By recovering these valuable resources, DWTPs can benefit economically, leading to reduced costs for consumables. The recycled coagulant, for instance, decreases the reliance on fresh coagulants for coagulation processes.

As communities increasingly recognize and value environmental protection, the adoption of the hierarchical approach and its principles by water treatment entities demonstrates their commitment to environmental sustainability, resulting in social benefits. This commitment can also inspire communities in the vicinity to adopt good environmental practices and establish businesses centered around the recycling, reuse, and recovery of nutrients and minerals from DWTP sludge.

5.1. Barriers to Hierarchical Approach for DWTP Sludge Management

The implementation of a hierarchical approach in managing DWTP sludge may encounter various barriers, including legislative obstacles, lack of suitable technology, and other challenges. A primary difficulty arises from the fact that DWTP sludge composition varies among different DWTPs. Consequently, any adopted or developed technology needs to be diverse and customized to meet the specific requirements of each DWTP.

Another obstacle lies in the processing of DWTP sludge before it can be reused. For instance, when recovering alum for reuse in the treatment process, the sludge must undergo thorough processing to eliminate any contaminants that may adhere to the recovered alum. These rigorous processing and quality assurance measures are essential to prevent the reintroduction of toxic substances into the treatment system. Therefore, considerable effort is required to ensure that the recovered alum meets the necessary standards.

Furthermore, the seasonal variability of raw water, which noticeably contributes to the content of DWTP sludge, poses another challenge. The inconsistent quality of the sludge makes it less favorable to utilize as an alternative source of alum, nutrients, or metals, since there is no guarantee of successfully recovering the desired materials.

Additionally, research on DWTP sludge and its potential in recycling, reuse, and recovery is relatively limited. This lack of information restricts its adoption and hinders the scaling up of laboratory experiments. Moreover, DWTPs are not primarily designed for profit generation. Consequently, the entities involved in DWTP management may lack the necessary funding to construct facilities that facilitate the reuse, recycling, or recovery of nutrients and metals from DWTP sludge. Furthermore, while these facilities may not yield short-term profitability, they offer notable environmental and social benefits.

A serious barrier to the implementation of the hierarchical approach in DWTP sludge management relates to legislation. In several countries, including South Africa, some regulations govern several aspects, including the treatment that alters the physical or chemical composition of the waste treatment residue (WTR) and its reuse, recycling, recovery, and disposal in land (NEMWA 2008). In South Africa, the National Environmental Waste Management Act (NEMWA) of 2008 specifies that a license is mandatory for DWTP sludge management [135].

Although many countries are advocating for the hierarchical approach to general waste management, there is still a need to adopt the hierarchical approach in DWTP sludge management. There is also a need to incentivize companies involved in DWTP sludge management to promote it as a viable business venture. Furthermore, countries should facilitate smoother trade processes for recycling, reuse, and recovery firms, thereby encouraging more entities to engage in waste management activities.

5.2. Opportunities in the Recovery of Nutrients from DWTP Sludge

There exists a considerable knowledge gap regarding research on the reuse, recycling, and recovery of nutrients from DWTP sludge. Additionally, there is a need to acquire a comprehensive understanding of how weather conditions or seasonal variations affect the composition of DWTP sludge in drinking water treatment plants. The available knowledge regarding the scalability and economic viability of different techniques for reuse, recycling, and recovery is limited. Thus, it is crucial to develop and scale up technologies such as the proposed sludge separator and the photocatalytic reactor that can enable the efficient recovery of nutrients and minerals from DWTP sludge. Moreover, the research conducted on DWTP sludge reuse should have the potential to influence policy, encouraging the adoption of reuse, recycling, and recovery practices by more recycling companies in the country [136,137]. This, in turn, can yield remarkable social and economic benefits for the organizations involved in water treatment, the communities they serve, and the nation.

Furthermore, while considerable research has been conducted on the use of membrane technology, photocatalysis, and their combination for the recovery of nutrients from wastewater treatment plant (WWTP) sludge, further exploration is required to understand their effectiveness in treating and recovering nutrients from DWTP sludge [137,138,139,140]. Others have proposed the use of peroxide amongst other processes for treating DWTP sludge [138].

6. Conclusions

It is crucial to “re-think” and “re-evaluate” the strategy for managing DWTP sludge and adopt a systematic hierarchical approach that aims for zero sludge landfilling. By implementing this approach, sludge can be reclassified as a valuable resource rather than mere waste. The hierarchical approach ensures the optimal utilization of all resources, minimizing waste. Currently, water treatment utilities in the country spend huge amounts of money on acquiring land for landfilling purposes. However, with the hierarchical approach, not only can land be conserved but it can also create employment opportunities and generate income from DWTP sludge.

Various methods and technologies have been proposed to reduce, reuse, recycle, and recover DWTP sludge. One important approach is the reduction of sludge generation by minimizing the use of chemicals and adding only necessary chemicals at the required concentrations for effective treatment. Additionally, water treatment utilities can invest in deductive water treatment approaches that rely on techniques such as adsorption, filtration, electrolysis, and advanced oxidation, eliminating the need for chemical additives. In cases where sludge generation is unavoidable and landfilling is necessary, proper drying and compression of the sludge can effectively control its volume and save land. Another technique used in the United States involves pumping DWTP sludge to wastewater treatment systems, reducing the burden at the DWTP but potentially increasing the amount of sludge handled at the wastewater treatment plant and necessitating additional treatment.

Even with efforts to reduce sludge at its source, there will still be residual sludge generated. Studies suggest that this sludge can be reused for microbial and pathogen removal through physical straining, surface diffusion, hydrophobic attraction, electrostatic interaction, and retention. It can also be used for removing cationic and anionic species from various water sources and aid in the recovery of phosphorus and nitrogen. DWTP sludge has shown promise for land applications, such as soil conditioning and remediation of contaminated soils, offering important economic and environmental benefits.

Another promising method is the recovery of phosphorus, ammonia/nitrogen, and alum from DWTP sludge. The sludge possesses properties that facilitate the trapping of nitrogen and phosphorus, which can subsequently be recovered for reuse in agriculture. This is particularly important, as mining these materials incurs high environmental and economic costs due to energy requirements. Recovering nitrogen and phosphorus from DWTP sludge proves more viable using cost-effective methods. Lastly, if reduction, reuse, and recovery have been applied to DWTP sludge, recycling becomes a viable option. Reports indicate that, due to its clay-like properties, DWTP sludge can be employed in cement manufacturing, brick making, and ceramics construction, acting as a substitute for clay and cement during the manufacturing process. Moreover, the proposed adoption of sludge separators and photocatalytic membrane reactors has the potential to treat and recover nutrients and metals from DWTP sludge. There is, however, still a need to continue doing more research on these technologies and their viability for upscaling.

When these methods are combined, it becomes possible to achieve zero landfilling of DWTP sludge, establishing a circular economy, generating employment, and preserving the environment. However, countries must review their regulations to create an enabling environment for companies engaged in DWTP sludge management. Additionally, there is a need to develop innovative technologies that allow water treatment entities to derive value from the reuse, recycling, and recovery of minerals and nutrients from DWTP sludge.

There is a need for an in-depth economic analysis for the adoption of the different technologies or approaches in DWTP management. Such a study can make it easier for policymakers to see the benefit of adopting the approach to the fiscus, the environment, and social cohesion that can be improved by the creation of sustainable jobs in waste recycling.