Water Treatment with Aluminum Sulfate and Tanin-Based Biocoagulant in an Oil Refinery: The Technical, Environmental, and Economic Performance

Abstract

Water extracted from natural sources often requires treatment to meet the quality standards necessary for industrial use, involving physico-chemical processes such as coagulation, flocculation, and sedimentation. Inorganic coagulants, such as aluminum sulfate, are commonly used, although they generate a sludge with residual aluminum, classified as hazardous waste. Given this, biocoagulants, such as natural tannin-based polymers, have emerged as a promising alternative. Thus, the objective of this study was to evaluate the environmental performance of water treatment and sludge disposal at an industrial water treatment plant (WTP) of an oil refinery located in Brazil using aluminum sulfate and biocoagulant. The WTP of this study is located in the state of Bahia, Brazil, and is supplied by a surface water body, the Paraguaçu River—Lago de Pedra do Cavalo—which comes from a semi-arid region, and a lake called Catu Korea. The environmental analysis was carried out using the life cycle assessment (LCA) method, using the methodological framework recommended in ISO 14044, followed by economic analysis and circular economy analysis. The inventory used in the analyses contains field data, company records, related literature, and ecoinvent database version 3.3. The impact assessment considered the ILCD 2011 Midpoint+ method package, the AWARE method, and the cumulative energy demand (CED) method in SimaPro 8.4 software. The comparative results showed the greatest impacts in the Energy Demand, Water Footprint, Eutrophication, and Land Use categories for the biocoagulant scenario, in contrast to the Human Toxicity, Acidification, Ecotoxicity, Particulate Matter, Carbon Footprint, and Abiotic Depletion categories for aluminum sulfate. The economic analysis showed that 65% of the operational costs for material and energy inputs in water treatment are due to the use of electricity, and the water pumping stage is the biggest contributor to this consumption. Even though the price of the biocoagulant was identified as eight times that of aluminum sulfate, the water treatment cost with the biocoagulant was 21% higher compared to that with aluminum sulphate. In this regard, circular economy propositions for sludge valorization are discussed for use, recycling, or proper disposal. Thus, the environmental and economic analysis in this study offers insights into eco-efficiency promotion in water treatment and sludge management.

1. Introduction

1.1. Industrial Water Treatment

Sustainable water and effluent treatments are essential tools for reducing the environmental impact of industrial activities. The partial or complete replacement of chemical coagulants, such as aluminum sulfate, with bio-based coagulants can reduce the environmental footprint of water and effluent treatment due to greater biodegradability and non-toxicity [1]. According to the Portal do Tratamento de Água [2], in recent years, industrial water treatment has undergone many advances, such as the use of more sustainable coagulating agents, and has become more complex with the advent of new technologies, such as remote access to water treatment, the use of plant extracts with coagulating properties, and better methods for the extraction of coagulant compounds from bio-based sources. These changes are due to market demands that increasingly seek more products aligned with environmental issues, making the market more demanding and competitive.

The tannin-based biocoagulant presents greater efficiency and minimal environmental impact [1]. As a result, organizations are adopting continuous improvement strategies in the management of water and effluent treatment processes. In this context, it is necessary to understand the changes in environmental, social, and economic impacts on business to support decision-making, so that it can remain on the competitive front.

The clarified water treatment steps normally include coagulation, flocculation, decantation, filtration, disinfection, pH correction, and fluoridation [3]. Coagulation is a mechanism used to remove colloidal particles that cause turbidity, color, taste, and odor in water [4]. Coagulants are responsible for the union of smaller dispersed particles that form larger and more stable particles.

Coagulation is the first stage of water clarification treatment (Figure 1). The most commonly used coagulants are aluminum compounds (aluminum sulfate, aluminum chloride, and sodium aluminate) and iron compounds (ferric sulfate, ferrous sulfate, and ferric chloride). Then the flocculation step takes place in a tank called the flocculation chamber, where it receives the water with the coagulated particles and forms the flocs [4].

The sedimentation stage consists of a slow-flow tank, allowing the floc to accumulate at the bottom. The detention time and the depth of the tank directly influence the decanted floc rate. The decanted flocs are deposited as sediments at the bottom of the decanter to be removed in the form of sludge and destined for treatment, followed by use or disposal. The amount of sludge varies from 3% to 5% of the total volume of treated water [5]. Subsequently, the filtration step removes microorganisms and the remaining colloidal particles.

The most common filtration technology is the rapid sand filter. The disinfection step uses a disinfectant (e.g., chlorine) to reduce the load of various pathogenic microorganisms such as bacteria and viruses remaining in the water and the risk of microbiological contamination of its use. Figure 1 shows the stages of industrial water supply.

The coagulants used in water treatment are classified into inorganic and organic [6]. Aluminum sulfate is an inorganic coagulant that is widely used due to its proven technical performance, cost-effectiveness, simple operation, and availability [7]. However, aluminum sulfate contains toxic components that cause irritation in the mucous membranes such as the eyes and respiratory tract and pollutes water bodies when disposed of improperly [8].

Thus, one of the main disadvantages of aluminum sulfate is WTP sludge contamination, which presents difficulties for simple sludge handling by companies due to its high volume and high humidity [9]. In this regard, biocoagulant use as a substitute for metal salts can alleviate the environmental disadvantages of metal salt coagulants, such as water toxicity, due to its biodegradability and abundant supply.

The use of aluminum-based inorganic coagulants in water treatment contributes to increasing the concentration of residual aluminum in treated water and in the generated sludge [8]. Aluminum is toxic to the ecosystem quality [1]. According to Pratibha and Fahad [4], the use of organic coagulants is an alternative to replace the use of inorganic coagulants for water treatment, mainly due to the characteristics of biodegradability, low toxicity, and low sludge production.

The improper disposal of these sludges into bodies of water can impact water quality by introducing aluminum residues and other components that may alter the chemical composition of the water, harming aquatic fauna and flora [10]. Additionally, the aluminum released into the water can be toxic to aquatic organisms, having adverse effects on the life cycles of different species and impacting aquatic biodiversity [4]. Contributing to water acidification, especially in acidic water ecosystems, also poses a challenge, as many species do not tolerate acidic environments [4].

Inadequate sludge management can lead to the accumulation of aluminum residues in the soil, affecting its quality and fertility [1]. To mitigate these impacts, it is essential to adopt waste management practices, efficient sludge treatment, continuous monitoring of water quality, and the exploration of more sustainable alternatives in industrial water treatment. This aligns with environmental regulations to preserve ecosystems and water resources [2].

Despite the disadvantages of aluminum sulfate in industrial water treatment processes, such as contaminated sludge formation and potential acidification, these issues can be mitigated. The use of more precise dosing technologies and automated control systems significantly reduces sludge formation, ensuring operational and economic efficiency. Careful adjustment of pH and strategic combination with other coagulants help minimize the risk of acidification [1].

Investing in ongoing research to enhance coagulant formulations and exploring more sustainable alternatives reflects the industry’s commitment to addressing challenges associated with aluminum sulfate, seeking solutions that optimize its use and minimize its impact on industrial water treatment processes.

The effective management of sludge generated in the industrial water treatment process emerges as a fundamental component in the pursuit of sustainability and operational efficiency. In an industry witnessing a notable annual growth rate of 8–10%, with a global volume of treated water reaching approximately 400 billion cubic meters per year [2], attention to waste management, particularly the resulting sludge, becomes crucial.

The percentage of industrial water reuse, currently at 20% [2], underscores the growing importance of water recycling in industrial processes. In this context, sludge management is not only an environmental necessity but also a vital strategy to optimize water resources and reduce the water footprint of industrial operations. Sludge, often a complex mixture of suspended solids and organic materials, demands innovative approaches to minimize its environmental impact.

The substantial investment in research and development, approximately US $1.5 billion annually [2], reflects the industry’s commitment to continuous innovation. This investment not only propels the development of advanced water treatment technologies but also aims to find effective solutions for handling generated waste, including sludge. The development of efficient methods for the treatment and disposal of sludge has become a priority, aiming to minimize negative environmental impacts.

According to Nath et al. [6], the use of biocoagulants increases savings in the use of other WTP inputs, such as chlorine, fluorine, and lime, in addition to reducing sludge treatment costs. Pratibha and Fahad [4] used a biocoagulant produced from Moringa oleifera seeds and verified its potential in water treatment as an alternative to aluminum sulfate. Nath et al. [6] states that biocoagulants are renewable and biodegradable, with economic, social, and environmental advantages.

1.2. Tannin-Based Biocuagulant

According to Abujazar et al. [11], the use of biocoagulants derived from vegetable tannins has been shown to be efficient in water treatment with low-toxicity sludge. Therefore, the increased use of biocoagulants to replace aluminum sulfate has been applied to water treatment. Thus, water treatment with biocoagulants should be investigated in different contexts to identify its technical, environmental, and economic performance [12].

An innovative product used in water treatment is the tannin-based biocoagulant produced from the cultivation of Acacia mearnsii, the so-called black acacia, originally from Australia [5]. Tannins are secondary plant metabolites that occur in plant leaves, bark, and fruits [12]. Black acacia was introduced in Brazil in 1918, and its large-scale commercial cultivation began in the 1930s [13]. Initially used to produce tannin from the bark used in tanneries and as firewood for energy generation, the use of Acacia wood has been expanding over the years, finding application in civil construction and the cellulose industry, thus expanding its production chain [13].

In addition to its economic role, the cultivation of black acacia in the southern region of Brazil assumes significant social relevance, being the main source of income for more than 35,000 families, according to a survey carried out in 2015 by the Associação Gaúcha de Empresas Florestais (AGEFLOR) [13]. These families depend on resources from the sale of products derived from acacia, involving activities such as seedling production, planting, harvesting, and transportation. In 2016, a planted forest area in Rio Grande do Sul totaled 780,000 hectares, of which 89,000 hectares were acacia, representing 11% of the state production and 100% of national production, according to data from AGEFLOR [13]. The geographic distribution of black wattle forest plantations in Brazil, in the state of Rio Grande do Sul, is shown in Figure 2.

The biocoagulant is widely used in water treatment plants (WTPs) for its high efficiency and non-hazardous sludge. Studies with tannin-based biocoagulants applied to freshwater treatment reveal that its composition, which is related to the plant from which it was extracted and the tannin modification degree, affects its water treatment effectiveness [4].

When comparing biocoagulants based on tannins with aluminum sulfate in water treatment, the biocoagulant presented the following advantages: a reduction in the amount of metallic salt applied in the treatment, shorter time in the coagulation process, and better quality of the generated sludge, presenting a higher removal efficiency in the filtration process [4].

WTP sludge has been discarded into waterways without any treatment; however, this practice has been questioned by environmental agencies due to risks to public health and aquatic life. According to the Portal do Tratamento de Água in Brazil in 2023 [2], it is estimated that the production of WTP sludge in large Brazilian cities is approximately 90 tonnes (t) per day on a dry basis. There is a large variability in the amount of sludge generated over the year at WTPs. Normally, the greatest production of sludge occurs during the rainy season, consequently requiring the application of greater quantities of coagulants [2].

According to ABNT [14] and Brasil [15], the sludge from WTPs is classified as solid waste and should be treated for recovery or disposal in landfills. The sludge generated in WTPs can be collected and directed to other industries, generating industrial symbiosis. For instance, sludge generated in WTPs can be used in processes such as cement and brick manufacturing, compost production, grass and tree cultivation, and land disposal for soil recovery. In addition, non-toxic WTP sludge can be released into sewage collection networks or directly into sewage treatment plants. In this sense, the challenge is to find ecologically safe and economically viable ways for WTP sludge management.

1.3. Brazilian Semi-Arid

Water plays a crucial role in industrial processes, being an essential resource for various manufacturing activities. In many regions, especially those characterized by the arid climate of a semi-arid zone, rivers play a vital role as the primary source of this precious liquid. The challenge lies not only in the limited availability of water in these rivers but also in the urgent need to implement efficient strategies for its use. Rivers that originate in a semi-arid region often face cycles of water scarcity, becoming susceptible to extreme climatic variations. The increasing demand for water in industrial sectors can intensify the pressure on these already vulnerable water resources. Therefore, it is imperative to adopt a responsible and sustainable approach to water management in industrial processes in these areas.

To ensure the long-term viability and preservation of these rivers, it is crucial to implement water-efficient practices in industries. This may include the adoption of advanced technologies that minimize waste, reuse water at different stages of the industrial process, and promote recycling. Additionally, awareness of the importance of water conservation should be integrated into both industrial operations and local communities.

Collaboration between the industrial sector, regulatory bodies, and local communities is essential for the development and implementation of effective water management policies. Conservation strategies should be customized to meet the specific characteristics of the semi-arid region, considering not only industrial needs but also environmental and social balance.

In summary, water from rivers originating in a semi-arid region is a valuable resource that requires attention and immediate action to ensure its sustainability. The efficient use of water in industrial processes is a shared responsibility that not only ensures the continuity of industrial operations but also preserves the health of local water ecosystems, promoting sustainable and equitable development.

According to the Instituto Nacional do Semiarido-INSA in 2023 [16], the Brazilian semi-arid region extends across the nine states of the Brazilian Northeast region and also the North region of Minas Gerais state as shown in Figure 3. In total, it occupies 12% of the country’s area and is home to around 27 million inhabitants (Figure 3) in urban (62%) and rural (38%) areas, making it one of the most populous semi-arid areas worldwide.

The Brazilian semi-arid region is mainly characterized by irregular rainfall and high rates of evapotranspiration, which contribute to water scarcity. In this region, the supply of water for multiple uses is below demand. During a period of prolonged drought, the situation worsens, reducing the water supply of 1262 municipalities with a negative impact on economic activities.

Responsible for a 21% share of the Gross Domestic Product (GDP), the industrial sector consumes 10% of the water collected for use in Brazil [16]. These numbers show the industry’s efficiency in consuming this resource, which is essential for the economic development of countries and human survival. For this reason, it is necessary to invest in initiatives for reducing water pollution from WTP sludge disposal, along with water reuse, rainwater use, and water use efficiency.

1.4. Life Cycle Assessment in Industrial Water Treatment

Life Cycle Assessment (LCA) has been widely employed in studies related to water issues, providing a method to estimate and reduce environmental burdens associated with the life cycle of products or processes. The versatility of LCA stands out as a guiding tool in industrial decision-making, enabling the comparison of technologies, scenario creation, and the identification of performance improvements in processes.

LCA use in the water sector dates back to the 1990s in Europe, with studies such as Friedrich [17] and Guanais et al. [18], respectively, evaluating the environmental and energy burdens of an urban water treatment unit. This methodology is applicable in the water supply sector to analyze various cases, including comparing treatment technologies, identifying critical points in treatment and distribution systems, and assessing the final disposal of sludge. Even though there is a prevalence of comparative studies among new technologies in water supply, the relevance of LCA should be directed to CE practices such as sludge energetic and material recovery [19].

In this context, the objective of this work is to evaluate the technical, environmental, and economic performance of the water treatment of a petrochemical plant using different coagulants: aluminum sulfate and tannin-based biocoagulant.

2. Materials and Methods

The WTP of this study is located in the state of Bahia, Brazil, and is supplied by a surface water body, the Paraguaçu River—Lago de Pedra do Cavalo—which comes from the semi-arid region, and a lake called Catu Korea. The WTP is of the conventional type, consisting of water abstraction in a surface water body, followed by a raw water pipeline, treatment, storage, and distribution.

Water collection is performed by 3 sets of motor pumps with a flow rate of 194 L s⁻¹, which pump raw water to the WTP where treatment takes place through a conventional process of coagulation, flocculation, decantation, filtration, disinfection, and pH correction, along with sludge disposal. The WTP’s annual water consumption is 8,418,360 m³ on average. The water losses in the raw water collection system are around 18% and in the treatment and distribution system together, they are 48%. The water supply system operates 24 h a day. A search of data published in the literature regarding LCA of water treatment was carried out using the keywords (water treatment; sludge management; life cycle assessment; aluminum sulfate; biocoagulant) defined in this study in RStudio software 4.0.4, from the CAPES/MEC Periodicals Portal database and the SCOPUS database as a search criteria. The Bibliometric Map was generated to visualize the density of research trends over 20 years (2003–2023).

LCA is a tool used to compile and evaluate the aspects and potential environmental impacts of the product throughout its life cycle. LCA assesses the environmental impacts of the product from the extraction of raw materials to its final disposal, also called “cradle to grave”. The LCA studies are standardized by ISO 14.040 and 14.044 norms [20]. LCA supports opportunity identification to improve the environmental performance of products, assisting decision makers in environmental optimization and adding value to the product [14].

The application of LCA in this study followed the methodological framework recommended in ISO 14044, 2006 [20], according to the four standardized phases. The reference flow was 1 m³ of water treated with aluminum sulfate or tannin-based biocoagulant. For every 1 m³ of treated water, 0.02 m³ of sludge is generated. The foreground inventory of inputs and outputs of black wattle cultivation for 1 planted hectare is presented in

Table 1. Foreground inventory for cultivation of Acacia mearnsii in 1 hectare to produce 85.8 t of wood, bark, and branches.

Input	Amount	Unit	GSD2
Pesticide	2.75	kg	1.05
Nitrogen fertilizer, N	7.00	kg	1.05
Phosphate fertilizer, P2O5	4.20 × 101	kg	1.05
Potassium fertilizer, K2O	2.10 × 101	kg	1.05
Fertilizer, inert material	7.00 × 101	kg	1.50
Energy (mechanization)	1.26 × 103	MJ	1.05
Seedlings, Acacia mearnsii	2.50 × 103	p	1.50
Carbon Dioxide, Photosynthesis	9.78 × 101	t	1.50
Output	Amount	Unit	GSD2
Air Emissions
Dinitrogen Monoxide	2.60 × 10−1	kg	1.50
Ammonia	3.40 × 10−1	kg	1.20
Nitrogen Oxides	5.00 × 10−2	kg	1.40
Emissions to Water
Nitrate	5.30 × 101	kg	1.50
Phosphorus	2.79 × 101	kg	1.50
Emissions to groundwater
Cadmium	3.78 × 10−5	kg	1.80
Chrome	1.86 × 10−2	kg	1.80
Copper	2.47 × 10−3	kg	1.80
Lead	6.52 × 10−5	kg	1.80
Zinc	9.00 × 10−3	kg	1.80
Emissions to surface water
Cadmium	1.69 × 10−3	kg	1.80
Chrome	1.97 × 10−1	kg	1.80
Copper	1.54 × 10−1	kg	1.80
Lead	1.97 × 10−2	kg	1.80
Nickel	1.01 × 10−1	kg	1.80
Zinc	6.11 × 10−2	kg	1.80
Emissions to the ground
Cadmium	8.58 × 10−4	kg	1.50
Chrome	−1.63 × 10−1	kg	1.50
Copper	−1.02 × 10−1	kg	1.50
Lead	1.32 × 10−4	kg	1.50
Nickel	−4.11 × 10−2	kg	1.50
Zinc	1.48 × 10−2	kg	1.50

GSD2: Square geometric standard deviation.

. The foreground inventory of tannin-based biocoagulant production, containing data obtained in the field from the company records in Brazil, located in the state of Rio Grande do Sul, is presented in

Table 2. Foreground inventory for 1 kg of tannin-based biocoagulant production.

Input	Amount	Unit	GSD2
Bark, Acacia mearnsii	2.94 × 101	kg	1.07
Water	8.70 × 10−1	m3	1.05
Electricity	6.3 × 10−1	kWh	1.5
Methanol	2.57	kg	1.05
Output	Amount	Unit	GSD2
Air emissions
Methanol	2.57	kg	1.50
Water	7.00 × 10−2	m3	1.05
Particulate Matter	7.00 × 10−2	kg	3.00
Total organic carbon	7.00 × 10−3	kg	1.05
Volatile organic compounds	3.00 × 10−3		1.50
Carbon dioxide, Biogenic	3.62 × 101	kg	1.05
Sulfur dioxide	4.00 × 10−2	kg	1.05
Carbon monoxide	1.80 × 101	kg	1.05
Emissions for Treatment
Wastewater	8.00 × 10−1	m3	1.50

GSD2: Square geometric standard deviation.

. Emissions were estimated based on the model proposed by (NEMECEK AND SCHNETZER, 2012) [21]. To calculate potential emissions from burning black wattle bark, we used the USEPA (2002) [22] emission factors.

The foreground inventory for water captured, treated, and distributed was obtained from the water operational control reports of the oil refinery water supply system and is presented in

Table 3. Foreground inventory for 1 m³ of treated water.

Stage Input	Amount for aluminum sulphate sreatment	Amount for biocoagulant sreatment	Unit	GSD2
Uptake
Chlorine	1.2 × 10−2	1.2 × 10−2	kg	1.05
Water	2.4	2.4	m3	1.05
Electricity	6.7 × 10−4	6.7 × 10−4	kWh	1.07
Treatment
Coagulant	2.6 × 10−2	3.3 × 10−2	kg	1.05
Water	2.0	2.0	m3	1.05
Sodium Carbonate	6.0 × 10−2	6.0 × 10−2	kg	1.05
Polyelectrolyte	1.3 × 10−3	0	kg	1.05
Electricity	1.9 × 10−1	1.9 × 10−1	kWh	1.07
Chlorine	1.2 × 10−2	1.2 × 10−2	kg	1.05
Infrastructure	3.07 × 10−9	3.07 × 10−9	unit	3.28
Output
Materials	Amount for aluminum sulfate treatment	Amount for biocoagulant treatment	Unit	GSD2
Sludge emission into water
Sludge	2.00 × 10−2	2.00 × 10−2	m3	1.50
Aluminum	5.48 × 10−3	0	kg	5.00
Sulfur	1.00 × 10−2	0	kg	1.5
Zinc	1.55 × 10−3	0	kg	5.00
Cadmium	2.58 × 10−2	0	kg	5.00
Nickel	3.73 × 10−4	0	kg	5.00
Manganese	1.00 × 10−3	0	kg	5.00
Copper	3.00 × 10−5	0	kg	5.00
COD, Chemical Oxygen Demand	0	5.00 × 10−3	kg	1.50
Phosphorus	5.00 × 10−3	0	kg	1.50
Nitrogen	3.00 × 10−5	0	kg	1.70
Chrome	0	0	kg	5.00
Lead	4.01 × 10−4	0	kg	5.00
Arsenic	3.00 × 10−4	0	kg	5.00

GSD2: Square geometric standard deviation.

. The chemical inputs used in the treatment stage for the aluminum sulfate scenario were collected from spreadsheets obtained in the field. Electricity consumption data were obtained through the annual electricity consumption reports of the water supply system.

The system boundary was delimited from the capture of raw water until the distribution of treated water, which includes the following aspects: electricity consumption, use of chemical inputs, and transportation. The background inventories of the material and energy suppliers were obtained from the ecoinvent^® database [23], version 3.3 with cut-off criteria (alloc rec) with market datasets (which include transport estimates) in SimaPro^® software version 8.4. [24] is one of the most used software worldwide in LCA studies applied to water treatment, in addition to presenting several methods for evaluating life cycle impacts and inventory databases.

The impact categories assessed were carbon footprint, acidification, eutrophication, ecotoxicity, human toxicity, particulate matter, land use, and abiotic depletion with the ILCD 2011 Midpoint+ method package [25], energy demand with the cumulative energy demand (CED) method [26], and water footprint with the available water remaining (AWARE) method [27]. The uncertainty of the foreground inventory parameters was characterized by the square geometric standard deviation (GSD2) for basic and pedigree uncertainties [28] using the uncertainty factors of Goedkoop et al. [24].

The selection of methods employed in this study was based on their widespread use in the field of water treatment. Furthermore, the decision to apply more than one impact assessment method for comparison purposes was a way to assess their influence on the study. As highlighted by Weidema [28], various methods have been described in the literature, and these need to be tested and evaluated. In this context, the adoption of a diversified approach aims to provide more comprehensive insights into environmental impacts in water treatment, aligning with the ongoing need for validation and improvement of these methodologies.

In LCA studies, there are several uncertainties in the quality of the data and their results, in addition to a certain degree of subjectivity resulting from the need for judgment and discernment on the part of the experts responsible for conducting the study, as well as limitations in available scientific knowledge. For this reason, for the calculation of uncertainty in the results of the categories evaluated in this study, we used the Monte Carlo method with lognormal distribution for the foreground inventories, also considering the uncertainties contained in the background inventories of the ecoinvent database, for 10,000 cycles and a 95% confidence interval.

The economic analysis was carried out based on data provided by the oil refinery, water treatment data, daily water flow, pumping, and sludge generation. Biocoagulant production data were collected in the field during technical visits to biocoagulant-producing companies. Aluminum sulfate price data were obtained from a price search with the main national suppliers. Economic analysis is often associated with LCA studies due to its ability to provide a comprehensive and integrated view of both environmental and economic impacts throughout the life cycle of a product or process.

The incorporation of economic analyses in LCA studies is crucial for aiding sustainable decision-making and identifying improvement opportunities in both environmental and economic terms. Economic analysis can offer insights into resource efficiency, production costs, economic feasibility of alternatives, and overall performance optimization, thereby contributing to more informed and balanced strategies for sustainability.

For the purposes of reproducibility of this study, readers can model the data presented in the inventories (Table 1, Table 2 and Table 3), adopting the impact categories used in this work and with the support of SimaPro^® and the ecoinvent^® database.

3. Results and Discussion

3.1. Life Cycle Assessment

The Bibliometric Map generated based on a density visualization of research trends over 20 years (2003–2023) is shown in Figure 4.

According to the result presented in Figure 4, in 2003, the main focus of studies on WTP sludge treatment was the adsorption of phosphorus and other elements, while in 2013, adsorption mechanisms and the development of reuse strategies were the main focus. In 2023, energy recovery and carbon footprint reduction were the main focus of WTP sludge treatment. Balbinoti et al. [1] state that there is an effort by industries to minimize the impact generated by the treatment of industrial waters, especially regarding the search for the use of biocoagulants, such as tannin-based coagulants. Pratibha and Fahad [4] investigated Moringa oleifera as vegetable coagulants in the treatment of wastewater from the dairy industry and slaughterhouses, respectively, and reported 90% removal of color and turbidity, while obtaining a reduction of 64% in chemical oxygen demand. In summary, the search for best practices in the industrial water treatment process has intensified in the last 20 years, as shown in Figure 4.

The amount and properties of WTP sludge are specific to each location, which is dependent on different factors such as seasonal changes in raw water quality, treatment process technology, and chemical use [29]. The silica (SiO₂) accounts for a significant part of the sludge composition, which comes mainly from the raw water, followed by aluminum oxide (Al₂O₃)) and Iron oxide (Fe₂O₃) from the coagulants used in the WTP [30].

The proportions of (Al₂O₃) and (Fe₂O₃) are directly related to the dose and type of coagulants used in the WTP. Aluminum-based and iron-based coagulants are the most popular chemicals used for coagulation and flocculation processes [31]. The metallic coagulant’s popularity is due to their comparatively low cost, high availability, and high efficiency in removing water turbidity and color [32].

Aluminum sulphate [Al₂(SO₄)₃] and aluminum chloride (AlCl₃) are the most common metallic coagulants, which are used in water and wastewater treatment [33]. Aluminum content varies greatly among WTP sludges, which accounts for approximately 16% of the mass composition on average [34]. Other major components of WTP sludges are calcium (Ca), silicon dioxide (SiO₂²⁻), iron (Fe), chloride (Cl⁻), sulfate (SiO₄²⁻), and humic acids.

In addition, other oxides such as calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na₂O), potassium oxide (K₂O), and phosphate (P₂O₅), as well as some trace metals, can be found in the WTP sludge. The main physical characteristics and chemical composition of WTP sludge are summarized in

Table 4. Physical characteristics of water treatment plant sludge.

Parameter	Unit	Range	Source
pH	–	5.12–8.0	[35]
Total Organic Carbon, TOC	g kg−1	17–149	[36,37]
Surface area	m2 g−1	28.0–227	[38,39]
Electric conductivity, EC	dS m−1	0.22–1.66	[36,40]
Cation-exchange capacity	cmol kg−1	13.6–56.5	[36]

and

Table 5. Chemical composition of water treatment plant sludge in percentage (%) mass basis of the chemical composition.

SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O	K2O	P2O5	Remaining Components	Study
52.75	20.15	6.75	0.3	–	0.87	3.69	–	16.36	[41]
53.6	20.9	6.6	0.3	1.9	–	–	–	16.70	[42]
24.68	30.39	11.59	0.16	0.17	–	0.35	–	32.66	[19]
43.12	15.97	5.26	5.56	0.85	0.52	0.25	–	28.47	[43]
53.36	15.28	21.01	1.2	–	–	5.41	0.83	2.91	[44]
40.61	27.36	6.99	2.62	1.89	1.05	1.28	–	18.20	[45]
29.63	17.57	5.18	11.85	2.15	6.09	2.85	0.94	23.74	[46]
49.2	26.3	6.6	0.8	1	0.6	3.2	–	12.30	[47]
64.3	21.2	10.4	2.05	1.06	0.17	0.79	–	0.03	[48]
10.9	1.34	68.65	8.23	0.61	–	–	9.39	0.88	[49]
54.1	28.84	9.92	3.1	0.64	0.3	0.75	–	2.35	[50]
9.41	51.01	0.7	1.31	0.22	0.07	0.2	0.36	36.72	[51]
43.29	32.19	5.52	0.17	0.33	0.13	2.08		16.29	[52]
33.4	29.3	10.5	2.7	0.89	–	–	–	23.21	[53]
52.78	14.38	5.2	4.39	3.08	0.97	3.62	0.17	15.41	[30,54]

, respectively.

The adequate use of water in the Brazilian semi-arid region is a key water security strategy as around 27 million people use water from this region for domestic consumption (Figure 3). The physical-climatic conditions that prevail in the Brazilian semi-arid region require more commitment and greater rationality in the management of natural resources, mainly water. As a result, industries need to act more efficiently in water resource preservation and use. Figure 5 presents the water footprint and energy demand of 1 m³ of water treated in the industry with aluminum sulfate and a tannin-based biocoagulant.

The biocoagulant had a larger water footprint than aluminum sulfate due to the plant’s cultivation stage, which takes an average of seven years to reach the cutting size. As the cultivation of Acacia mearnsii does not have a mechanical irrigation system (only rainwater and groundwater are used), the water footprint includes the remaining water used in the product’s life cycle. Another contribution to the water footprint was the production of biocoagulant, which requires 0.87 m³ per kg of biocoagulant (Table 2). In this regard, to make the biocoagulant more ecological, it is necessary to rationalize the use of water in its production.

Table 6. Contribution of energy demand per energy source for 1 m³ of water treated with aluminum sulfate and biocoagulant.

Energy Source	Biocoagulant (MJ)	Aluminum Sulfate (MJ)
Non-renewable, fossil	12.65	4.79
Non-renewable, nuclear	1.71	1.64
Non-renewable biomass	0.04	0.04
Renewable biomass	0.42	1.16
Renewable, wind, solar, geothermal	0.06	0.06
Renewable water	3.15	2.92

presents the CED by energy source for water treatment with the biocoagulant and aluminum sulfate in the refinery’s water supply chain.

The energy demand shown in Figure 6 points to the advantage of treating industrial water with aluminum sulfate, with an impact 31% lower than the treatment using biocoagulant. Table 6 shows the high demand of non-renewable fossil energy for treatment with biocoagulant due to the large demand for fossil fuels to drive machines (chainsaws, tractors, cutting machines, and scrapers) in the Acacia mearnsii cultivation stage. Regarding the WTP, for every 50 m³ of water treated, 1 m³ of sludge is generated. Figure 6 presents a comparison of the environmental impacts resulting from water treatment with biocoagulant and aluminum sulfate.

In Figure 6, a greater impact on water treatment is noted with aluminum sulfate in several categories, except for eutrophication and land use. For the biocoagulant, the carbon footprint category was influenced mainly by the use of non-renewable energy; however, it is possible to observe that there is carbon dioxide sequestration from the atmosphere during the cultivation of Black Acacia, which is an environmental benefit. The carbon footprint of the water treatment with the biocoagulant is lower than that with aluminum sulfate.

For the acidification category, an environmental profile with less impact on the use of the biocoagulant is perceived. Regarding the delivery process of the aluminum sulfate coagulant, a large energy demand is noted for the use of transport and movement of machinery, making this contribution directly related to air emissions from the burning of diesel. Another factor to be considered is the demand for non-renewable energy in some stages of aluminum sulfate production, such as coal and natural gas, which intensified the impacts related to the use of aluminum sulfate. However, the cultivation of black wattle, as well as transport between the transformation processes, required a high use of diesel in its stages, however, in smaller quantities than aluminum sulfate.

For the eutrophication category, the use of biocoagulants has a greater potential impact. The impacts of this category are mainly related to the release of nutrients into the water, as well as emissions from the burning of fuel and the use of non-renewable energy of fossil origin. Furthermore, there is also a contribution attributed to the use of fertilizers in the Black Acacia cultivation stage, which includes long-term emissions into groundwater.

The human toxicity category presented a disadvantage for the use of aluminum sulfate in relation to the biocoagulant, as it is an inorganic product with high contaminant potential when it is released into the water bodies from sludge disposal. The difference between the human toxicity results was 93%, which shows that the use of aluminum sulfate in water treatment processes is an environmental concern. WTPs that use aluminum sulfate as a coagulating agent are quite widespread among companies worldwide. However, if there is no adequate destination for sludge generated through this process, the risk of contamination of nearby water sources is quite high. It is important to highlight that the disposal of sludge generated in water bodies causes a significant change in the physical and chemical properties of the aquatic ecosystems, which can significantly affect biological activity.

For the particulate matter category, the result of the environmental performance of sludge generated with aluminum sulfate presents the worst scenario in relation to the biocoagulant. It is worth mentioning that the particles suspended in the air due to these processes have different diameters, and for this category, only particles with a size smaller than or equivalent to 2.5 μm were considered due to their highest impact potential for respiratory system effects.

Water treatment with the biocoagulant has a greater environmental impact in terms of land use compared to that with aluminum sulfate. The cultivation of Black Acacia, which is the basic raw material for the manufacture of the biocoagulant, demands more land use in relation to the production of aluminum sulfate.

The aluminum sulfate scenario indicated that the greatest contribution within the category of abiotic depletion is the use of electricity, followed by the use of chemical agents involved in the water treatment process.

Water treatment using aluminum sulfate as a coagulating agent for industrial use is quite widespread among companies. If there is no adequate disposal for sludge generated from this process, the risk of contamination of nearby water sources is quite high [10]. The WTP analyzed does not have water source conservation programs, does not fully treat the sludge, and also does not have full control over the amount of waste generated. All sludge generated is sent to a lake at the industrial unit itself or recycled in the water treatment process.

3.2. Cost Analysis

The average operating costs of water treatment are presented in

Table 7. Operational costs of material and energy inputs in the water treatment with aluminum sulfate and biocoagulant for 504,000 m³/month water flow.

Stage	Material		Price (US$/kg)	Monthly Quantity (kg)		Monthly Cost (US$)
Water catchment	Inputs for Aluminum Sulfate treatment
	Pre-Chlorination (Chlorine)		0.67	6300		4221
	Inputs for Biocoagulant treatment
	Pre-Chlorination (Chlorine)		0.67	6300		4221
Water treatment	Inputs for Aluminum Sulfate treatment
	Aluminum sulfate		0.14	13,100		1834
	Sodium carbonate		0.30	30,000		9000
	Chlorine		0.67	6300		4221
	Polyelectrolyte		3.52	700		2464
	Inputs for Biocoagulant treatment
	Biocoagulant		1.13	17,000		19,210
	Chlorine		0.67	6300		4221
	Sodium carbonate		0.30	30,000		9000
	Electricity for bothevaluated scenarios
Stage	Equipment	Amount	Operating hours per day	Potency (kW)	Price (US$ kWh−1)	Monthly cost (US$)
Water catchment	Input
	Bombs J-5401	3	24	156	0.10	33,696
Water treatment	Input
	Bombs J-5202	2	24	46	0.10	6739
	Scraper Bridges	3	24	4.5	0.10	972
	Slow Mix	3	24	3.8	0.10	820
	Quick Mix	2	24	6.9	0.10	993

. These costs are based on internal reports from the oil refinery, as well as data from the aluminum sulfate and biocoagulant production sector. The costs do not include fixed costs for personnel, maintenance, and laboratory analyses due to the complexity of collecting these data and the confidentiality imposed by the refinery.

The costs associated with managing sludge generated in water treatment have significant implications for long-term economic viability. Sludge, a common byproduct of this process, requires careful consideration regarding collection, transportation, processing, and final disposal. The determination of economic viability is directly influenced by the costs associated with these stages. The choice of disposal techniques, such as landfills, controlled incineration, co-processing, or agricultural reuse, impacts operational costs and compliance with environmental standards.

Investing in advanced sludge treatment technologies, while potentially involving higher initial costs, can result in reduced operational expenses and the production of higher-quality sludge. Additionally, strategies that allow for resource recovery, such as biogas production through anaerobic digestion, can not only mitigate costs but also generate additional revenue.

Choosing waste management methods that balance operational efficiency, environmental compliance, and economic considerations contributes to long-term financial sustainability in water treatment. This approach not only meets regulatory requirements but also minimizes environmental impacts, ensuring effective and economical sludge management.

With the results shown in

Table 8. Comparison of the monthly cost of water treatment with aluminum sulfate and biocoagulant for a flow of 504,000 m³.

Input	Aluminum Sulfate Treatment (US$)	Biocoagulant Treatment (US$)
Chemicals	21,740	36,652
Electricity	43,221	43,221
Total Cost	64,961	79,873

, it was identified that more than 65% of treatment costs are due to the use of electricity and that the water pumping stage is the stage that contributes the most to this electricity consumption. Therefore, it is necessary to optimize the use of electricity within the water supply system to improve the environmental performance.

It is also shown in Table 8 that the price of aluminum sulfate is one-eighth that of the biocoagulant. However, as pointed out in the study, a major problem found in the use of aluminum sulfate as a coagulating agent in water treatment is the appropriate disposal of the sludge generated from metal-based coagulants, which is classified as hazardous waste and must be properly disposed of. However, this disposal has high costs associated with it, which often leads to improper disposal. The sludge generated from water treatment with the biocoagulant is not contaminated with heavy metals, which simplifies its handling for a circular economy practice.

From an economic point of view, aluminum sulfate, although widely used as a coagulant in water treatment, can present significant costs associated with its processing, transportation, and, mainly, the management of waste resulting from the treatment. This waste often involves specialized disposal methods, contributing to additional operational costs.

On the other hand, the biocoagulant, as a more sustainable alternative, may initially have slightly higher production and implementation costs, but offers long-term benefits. Its natural and biodegradable origin can reduce costs associated with waste management since the biocoagulant can be more easily integrated into environmental processes.

Therefore, the destination of the sludge generated from biocoagulants tends to be more economically viable than the sludge generated from the use of aluminum sulfate, in addition to being a source of income when valued in circular economy proposals (Figure 7).

3.3. Circular Economy

The industrial water supply presents challenges in its life cycle, which range from the water treatment process to the final disposal of the sludge generated in the WTP [2]. This management must involve innovation throughout the value chain to reduce waste generation, recycle waste, and eliminate waste disposal in landfills. Therefore, after diagnosing the environmental impact of the sludge generated at the WTP, a proposal for a circular economy in sludge management was made (Figure 8).

Innovative applications for using WTP sludge have been recognized for their notable performance in mitigating pollution, standing out as efficient adsorbents of polluting substances [55]. This approach was not only revealed as an effective strategy to face environmental challenges but also represents a practical and promising example of water sustainability, promoting harmony between environmental preservation and advanced water treatment processes. An effective toxin-reduction practice using WTP sludge is the agricultural application of the sludge as an organic fertilizer, those from coagulants with iron compounds [55]. Sludge contains nutrients such as phosphorus and potassium (Table 5) and organic matter that benefit agricultural soils.

The controlled application of treated sludge to agricultural areas not only provides an adequate way of disposing of waste but also contributes to the reduction in pollutants in water bodies [21]. The nutrients present in the sludge support soil fertility, reducing the need for synthetic chemical fertilizers. This, in turn, decreases the emission of nutrients to nearby water bodies, which is a common source of water pollution. Furthermore, applying sludge to the soil can increase the water retention capacity and improve the soil structure, reducing the risk of erosion. However, it is essential to ensure adequate application and monitoring practices to avoid negative environmental impacts, ensuring the safety and effectiveness of this pollutant-reduction proposal [21].

Studies such as Akamatsu (2017) [55] investigated applications of sludge as a raw material for the manufacture of bricks and cement. WTP sludge, when treated properly, can be incorporated into various construction materials, offering economic and environmental benefits. A common application is the use of sludge as an additive in the production of bricks and blocks. Treated sludge can be mixed with clay and other components during the brickmaking process, which reduces the amount of waste going to landfills and also transmits improved properties to construction materials. Furthermore, the presence of minerals in WTP sludge strengthens bricks, making them more durable. This practice not only provides an environmentally friendly alternative to waste management but also reduces the demand for natural resources traditionally used in the production of construction materials. However, it is crucial to ensure that the sludge incorporation in products is carried out safely, meeting regulatory standards [55].

Using WTP sludge is a practice that aims to optimize resources, promote sustainability, and reduce environmental impact. A common approach involves sludge use as an integral part of advanced water treatment systems [21]. The main aspects of this practice are as follows:

• Enhanced Coagulation and Flocculation: Adequate coagulant dose in WTP based on monitoring solids concentration in water supports environmental and economic optimization water treatment.
• Bioremediation and Biological Filtration: WTP sludge can also be applied in bioremediation systems. Microorganisms present in sludge can contribute to the degradation of organic pollutants, promoting a more natural and sustainable approach to sludge management.
• Biogas Production: In some cases, WTP sludge can be used in the production of biogas through anaerobic processes. The biogas generated can be used as a source of renewable energy to power part of the operations of the water treatment system itself.
• Nutrient Recycling: WTP sludge often contains valuable nutrients such as phosphorus and potassium. Their reintroduction into the biomass production system can be planned to recover these nutrients, contributing to the overall efficiency of the water supply system.
• Dehydration and Thermal Drying: In some cases, WTP sludge may be subjected to dehydration and thermal drying processes to transform it into a more stable material suitable for use as fertilizer or in other processes outside of the water treatment sector.
• Coagulant recovery: WTP sludge coagulant reuse, generally metallic salts, contributes to resource efficiency, reduces operational costs, and minimizes the amount of waste generated.
• Use in Construction: Treated WTP sludge can be used in construction for the production of bricks, blocks, or landfill material. It is important to ensure that the sludge meets necessary safety and quality standards.
• Metal Recovery: In some cases, WTP sludge may contain valuable metals. Chemical or biological extraction processes can be applied to recover these metals, allowing for reuse or recycling.
• Composting: If the WTP sludge is organic, it can be composted to produce organic fertilizer. Sludge mixes with suitable composting material such as vegetable scraps, straw, or sawdust allow the composting process to take place. The end product can be used as fertilizer in agricultural areas.

Implementing these practices requires compliance with environmental regulations, water quality standards, and public health to ensure the safety and effectiveness of the WTP sludge valorization in the circular economy. Figure 8 shows the destination options of the sludge generated at WTPs through the Circular Economy (CE) model.

The Circular Economy (CE) has become a promoted model to be adopted in environmental management, mainly in the water supply and sewage management sectors [56]. Water utilities consider CE approaches to manage their resources, which aim to eliminate waste [56]. Geissdoerfer et al. [57] affirmed that the CE concept is an alternative option to the conventional “Take-Make-Dispose” concept of the linear economy model. An essential aspect of the CE is to keep resources within the economy as long as possible so that a product or its material descendants are continuously reused to create added value [56]. However, due to the scarcity of landfills and increasingly strict environmental regulations, discarding WTP sludge in landfills is becoming more expensive and less desirable [35,58]. As such, there is a pressing need to develop more sustainable sludge management practices, including material reuse, recycling, and recovery [56].

The choice of the reuse technique depends on the sludge composition, local regulations, environmental conditions, and acceptable practices in the specific region. It is crucial to conduct laboratory analyses to understand the sludge composition before deciding on the best approach for its use. Additionally, it is important to follow environmental and safety guidelines to ensure responsible use.

3.4. Limitations and Recommendations

This research should be continued by increasing the study system boundary by adding the use of water by industry and wastewater treatment stages. The literature data used for sludge composition should be modeled with data from laboratory analysis in further studies. Furthermore, LCA and economic analysis should be performed for the proposed CE scenarios to compare the environmental and economic performances among the evaluated scenarios.

4. Conclusions

The use of biocoagulants to replace aluminum sulfate in water treatment processes is supported by environmental advantages, although it may involve higher costs. By opting for biocoagulants such as plant seeds or organic polymers, it is possible to mitigate environmental impacts associated with the production and use of traditional inorganic coagulants. Furthermore, the choice of biocoagulants in water treatment minimizes the risk of aluminum contamination in treated water, addressing public health concerns. Biodegradability and less persistence in the environment are intrinsic characteristics of biocoagulants, giving them a significant environmental advantage compared to conventional inorganic options. This approach aligned with sustainable practices for water conservation is also favorable from a social point of view, reflecting a commitment to the preservation of natural ecosystems and community acceptance. The environmental awareness of more ecological practices supports the positive image of entities that adopt biocoagulants in their water treatment. Although the initial cost of biocoagulants may be higher, a comprehensive analysis must consider the long-term benefits to the environment and society. The decision to use biocoagulants stands out as an investment in the environmental health and reputation of organizations, promoting a balanced approach between economic aspects and environmental responsibility.