Evaluation of the Use of Sewage Sludge Biochar as a Soil Amendment—A Review

Abstract

In recent decades, minimization and recycling/reuse policies were introduced to reduce the quantities of generated waste and for alternative waste recovery. Organic wastes represent 46% of total global solid waste. Possible uses of organic wastes include using it as fertilizer and amendment for soil, for energy recovery and for the production of chemical substances. Sewage sludge disposal and reuse are identified as future problems concerning waste. The total amount of sludge generated in the entire world has increased dramatically, and this tendency is expected to increase significantly in the years to come. In most developed countries, special attention is given to sewage sludge treatment in order to improve the quality and safety of using it on the ground surface. Sewage sludge pyrolysis is considered an acceptable method, from an economic and ecological perspective, for the beneficial reuse of sewage sludge. This method has many advantages because, during the pyrolysis process, the sludge volume is reduced by 80%, pathogenic agents and hazardous compounds from sewage sludge are eliminated, metals are immobilized in solid residue and organic and inorganic fractions are immobilized in a stabilized form of pyrolytic residues (biochar). The biochar generated by sewage sludge pyrolysis does not contain pathogenic agents and is rich in carbon and nutrients.

1. Introduction

By 2050, the world’s population is expected to reach over 9 billion and, in the meantime, it is estimated that the demand for electric energy will double, and the water and food demand will increase by approximately 60% [1]. Meeting these demands entails the exploitation of limited natural resources, and the quantity of global waste will rise in direct proportion with the increase in population and consumption rates [1,2]. Waste could be recycled or reused in order to be transformed into a resource that can be used and not only dumped [3,4]. Until now, landfilling was considered one of the most frequently used options for waste disposal, but landfills can be a costly option if we consider the cost of environmental pollution and resource depletion [2]. For pollution prevention and the optimal use of limited resources, it is necessary to develop conservation methods and waste recycling methods in the European Union [5].

Over the last 20 years, minimization and recycling/reuse policies were introduced for the reduction of generated waste quantities and for the alternative use of waste management strategies in order to reduce environmental impact [6]. The European Commission considers that “if waste is likely to become a resource that will be reintroduced in economy as raw material, then higher priority needs to be given to reuse and recycling” [7]. Not only does waste recycling enhance resource efficiency, but it also reduces environmental pollution [2]. The new strategies of waste management need to comply with Eco-innovation trends in order to meet the main goal of the European Commission: “reduce, reuse, recycle” [8].

Increasing waste quantities cause environmental degradation, especially concerning the pollution of soil, water and air because of unsustainable methods of waste disposal and management [2]. Recovering waste material by means of upcycling (reusing waste in such a way as to create a product of higher quality or value than the original) and recycling techniques has direct advantages (primarily raw materials and secondary raw materials) as well as indirect benefits (energy, water and gas) [2].

Organic wastes are secondary products of agricultural, industrial or municipal activities and are named “waste” because they are not primary products, and still, the objective is to make “wastes” a source that can be reused [4]. The generation of organic wastes continues to rise, and this becomes a global problem because, if the manipulation of these wastes is not performed correctly, a large number of organic wastes can deteriorate the quality of air, water and soil [1,9].

Organic wastes represent 46% of total global solid waste and can be managed and rendered valuable through various methods, thus reducing the depositing of waste in landfills [10]. Possible uses of organic wastes include their use as fertilizers and amendments for soils, energy recovery (heat, liquid fuels, electricity) and the production of chemical substances (volatile organic acids, ammonium products, alcohols) [4].

An important recycling alternative is the application of organic wastes on the surface of agricultural fields as soil fertilizer and amendment [3,4].

The use of untreated organic waste in agriculture can produce some positive aspects such as: improving soil texture; pH correction; organic compounds; recycling valuable nutrients from waste and increasing the number of nutrients in the soil; reducing the need for chemical fertilizers [3,4,5,6]. Some negative aspects can also be mentioned, such as: the pollution of surface and ground water with nitrogen, phosphorus and pathogens; air emissions of ammonia, nitrogen dioxide, methane, hydrogen sulphide and odors; the accumulation of phosphorus, copper, zinc, sodium and salts in the soil [2,7].

Organic wastes represent a potential source of organic matter and nutritious elements, such as nitrogen, phosphorus and potassium, and can be used for improving soil quality and for increasing crop yield [11]. Organic wastes for use in agriculture as fertilizers can generally be classified into three categories: organic waste of animal origin (animal manure), green wastes and composts based on vegetal sources and urban waste (sewage sludge and organic household waste) [5]. The use of various organic wastes in agriculture depends on more factors, including the characteristics of wastes, such as the contents of nutrients and heavy metals, the smell generated by wastes, transport costs, benefits for agriculture and aspects regarding regulations [4]. The importance of these factors can vary depending on the type of organic waste, but many of the considerations regarding the use of organic wastes are similar to most organic wastes [4]. Possible accumulations of persistent contaminants and their potential transfer to crops have determined a major concern with the use of wastes in agriculture [12]. The uncontrolled use of large quantities of waste for the elimination of potentially harmful substances in the environment is not recommended [13]. Possible effects on the environment that may occur following the application of organic wastes to the surface of agricultural soils are [4]:

• Air emissions of ammonia (NH₃), nitrous oxide (N₂O), methane (CH₄), hydrogen sulphide (H₂S), odorants, etc.
• Surface water pollution by nitrogen (N), phosphorous (P), organic matter (influencing the oxygen level), pathogenic agents and water silting.
• Groundwater pollution by nitrogen (N), phosphorous (P) and pathogenic agents.
• Accumulation of phosphorous (P), copper (Cu), zinc (Zn), sodium (Na) and salt.
• Accumulation of heavy metals, phenolic and polycyclic aromatic hydrocarbons compounds that can reduce soil fertility and change others [13].

Organic wastes used as soil amendment/fertilizer can be processed by means of various methods, which can add agronomic, economic and ecological value [5].

The use of organic wastes as soil amendment represents an important option as it enhances the storage of organic carbon in the soil, recycles valuable nutritious substances from wastes (for example, N, P, K), increases the number of nutrients and improves the soil texture, the water storage capacity, the cation-exchange capacity, the electrical conductivity, the pH, the microbial population, the organic matter in the soil and reduces the necessary quantity of chemical fertilizers [3,14,15,16].

Among the organic amendments used in agriculture, we can mention: biosolids, animal manure, municipal organic wastes, compost from various organic wastes, crop residues, marine algae, etc. [17]. An important waste flow is made up of animal wastes (manure) and wastes from urban sources (sewage sludge), which are similar with regard to the high water and nutrients content (N and P) and can be used as organic fertilizers [5,18].

Taking into account the global demands and the rising cost of inorganic (chemical) fertilizers, the use of amendments/organic fertilizers is growing rapidly, and their presence on the surface of agricultural fields is increasing in many countries [17]. A wide range of organic amendments are available on the market, which testifies to the explosive growth of ecological agricultural systems and the importance of these amendments for improving soil quality [17].

The demand for fertilizers derived from organic wastes depends on numerous factors, such as [19,20]: the lucrativeness of these fertilizers compared to chemical fertilizers; costs and quality (agronomic value, contents of pollutants); the existence of a market or a demand for these wastes; the existence of technical requirements, legislation and applicable standards for wastes; competition with other organic and/or chemical fertilizers; the lack of adverse effects on the environment or on human health resulting from using wastes; uncertainty regarding their positive effect on crops.

The present article aims to describe the methods used for the management of sewage sludge in agriculture and to evidence the most economically efficient waste quantities reduction and environmentally friendly products with large scale applications. The main purpose is to present sewage sludge biochar as a potential fertilizer by evidencing the positive and negative aspects of different applications and the transformation of sewage sludge into biochar.

2. Sewage Sludge

2.1. Sewage Sludge Management

In a wastewater treatment plant, depending on the treatment stage, more types of sludge are generated [7]:

• Primary sludge contains between 2–9% solids; the rest of 90% is water and is generated during primary treatment (filtering, grit removal, flotation, precipitation, sedimentation) when heavy solids, grease and oils are separated from raw wastewater [7,21].
• Secondary sludge (activated sludge) is produced during biological treatment when microorganisms decompose the biodegradable organic contents from wastewater [7,21]. The total concentration of solids ranges between 0.8–3.3%, depending on the type of biological treatment process used, the rest being water [7]; the organic part from the activated sludge contains: carbon 50–55%, oxygen 25–30%, nitrogen 10–15%, hydrogen 6–10%, phosphorous 1–3% and sulphur 0.5–1.5% [7].
• Tertiary sludge is obtained in advanced wastewater treatment stages for improving effluent quality before it is discharged into the environment (river, lake, sea, land, etc.) when the removal of nutrients is necessary (nitrogen and phosphorous) [7,21].
• In many cases, chemical sludge is obtained from a chemical process implemented in the sewage treatment plant, involving the dosage of a suitable coagulant upstream of the primary sedimentation in order to reduce the organic loading for the next treatment [7]. Additionally, in some wastewater treatment plants, some compounds such as alumina or iron salts are introduced into the treated wastewater to precipitate phosphorus [22,23,24]. This approach is also considered a chemical treatment. The resulting sludge is often mixed with secondary sludge and is impossible to separate as a chemical sludge.

The disposal and reuse of sewage sludge is identified as a future problem concerning wastes [25]. The total amount of sludge generated worldwide has increased dramatically, and this trend is expected to rise significantly in the years to come [26]. In the European Union, important amounts of sewage sludge are produced, approximately 11 million tons (dry/dewatered quantity) per year [27]. Germany produces the largest amount of sludge, followed by the United Kingdom, France, Spain and Italy [8]. In comparison with Spain and Italy, they produce more than 500,000 t DM annually. Additionally, it is estimated that these five countries generate almost 75% of the sewage sludge amount from Europe [8]. The continuous increase in the amount of sewage sludge, its storage, use and disposal represent a major environmental problem [26,27,28,29]. Sewage sludge can damage the environment when stored directly on the soil surface because of its fermentation capacity and the presence of hazardous substances, both organic and inorganic, including pathogenic organisms and heavy metals [27]. The storage of sewage sludge in landfills influences leaching production and CO₂ emission directly in the air [8]. Sewage sludge involves adequate and accepted management from an environmental viewpoint because of the large quantities of wastes generated, in a context in which the management and disposal of sludge make up some of the most complex problems in sewage treatment plants [27,30]. The management of this waste is still a challenge, especially for developing countries, mainly because of deficiencies in clear regulations, the lack of a methodology for selecting an adequate sludge management system and the high investment and modernization costs for old sewage treatment plants [31]. Because of the increasing contents of toxic pollutants, in many countries, sewage sludge is included in the category of hazardous wastes, as they contain a high organic load, chemical pollutants, including heavy metals, pesticides, hazardous organic compounds, pathogenic bacteria, viruses and other pathogenic agents [32,33]. Some amounts of sludge are recycled during wastewater treatment installation to optimize operations, yet large quantities of sewage sludge must still be disposed of and managed [8]. The following stages make up the sewage sludge treatment process: compaction, stabilization, conditioning, dewatering, hygienisation and drying, but all these stages are not compulsory in all sewage treatment plants [34,35]. Restrictive regulations regarding wastewater treatment before discharge and the rising costs for sludge disposal pose two interdependent problems because sludge cannot be removed in the traditional manner, and sludge treatment techniques have led to an increase in infrastructure and in costs for supplementary treatment [36]. If adequate sludge treatment and disposal methods are used, environmental problems such as foul smell or waste volume increase can be avoided, and the energy obtained in the sludge treatment processes can be reused [37]. Sludge disposal methods can have significant effects on the environment, such as risks for public health and the possibility of atmospheric, soil and water resources contamination; hence, its adequate treatment, its controlled disposal and its correct management are, in general, of great importance [31,38]. Conventional methods used for sewage sludge management are no longer viable because of strict regulations, the lack of available space and the increasing environmental and health problems caused by the presence of pathogenic agents, pharmaceutical substances, hormones, heavy metals and persistent organic pollutants [34]. Owing to stricter environmental regulations, traditional sludge disposal methods are being replaced by new treatment methods [39].

According to European regulations, the methods involving sludge storage are now replaced by methods that stabilize and recycle wastes, their objective being, among others, that of promoting pro-ecological sludge management [13]. Guided by the three pillars of sustainability (economy, environment, society), the recovery of resources creates economic opportunities, limits the reintegration of heavy metals and contaminants in the environment when the solids are removed and reduces concerns regarding the smells emanated by sludges [36]. Lately, various modern technologies have been introduced, which increase sludge reusability rates and reduce the number of noxious substances from this organic waste [6,40]. The choice of a sewage sludge management method or technique from the available options should be based on the environmental effect and the reduced potential environmental impact of the technology used [6]. These modern methods can recover potentially hazardous wastes by processing and using them in agriculture, in various industrial sectors and for the production of thermal and electrical energy [8,13].

2.2. Sewage Sludge Treatment and Recycling

The objectives of modern technologies for sludge treatment are: reducing weight and volume, destroying pathogenic microorganisms, removing foul/offensive smells and reducing volatile solid contents for safer disposal [34,41]. The number and types of technological processes of sludge treatment not only depend on the nature and amount of processed sludge but also depend on the sewage sludge’s final management method [28]. The treatment of this waste changes the sludge’s physical and chemical properties; therefore, it can affect the quality and feasibility of the final product obtained from sewage sludge [42].

The major sewage sludge disposal strategies include:

• Organic recycling—associated with the potential of sewage sludge for fertilization (using sludge in agriculture, restoring degraded soils, composting sludge for the production of fertilizers, mechanical and biological treatment) [8].
• Energy and material recycling—associated with using the fuel and other minerals resulting from the thermal processing of sludge (incineration, pyrolysis, gasification, co-incineration in concrete plants and in the power sector [8].

The stabilization and reduction of heavy metal mobility and the availability of organic pollutants, as well as the destruction of the pathogenic agent, have caused the intensification and diversification of sewage sludge treatment technologies over the last decade [40]. Treatment methods for this waste change sludge properties and can affect the quality of the final product, so the choice of the optimal sludge treatment method can have ecological, economic and social consequences [42]. There are various treatment methods, such as anaerobic digestion, composting, stabilization with alkaline material or with some other chemical substances, thermal treatments, pyrolysis, incineration, etc. [43,44]. The selection of the most adequate method for reusing sewage sludge should consider economic and environmental aspects [44].

2.2.1. Stabilization of Sludge by Drying in Layers

Sludge disinfection and stabilization processes by drying in layers are designed for the removal of pathogenic bacterial flora and the drying/dewatering of sludge [13]. Drying is a relatively simple technological procedure which can constitute the final treatment stage for sludge that is intended for agricultural purposes or which is applied before using other thermal sludge treatments [13].

2.2.2. Composting

Composting is a commonly used method for sewage sludge treatment, which can cause biodegradation of organic matter and lead to the formation of substances that affect heavy metal mobility [45]. Volatile organic compounds (VOCs), ammonia (NH3) and hydrogen sulfide (H₂S) are the major components emitted by sewage sludge composting installations and are generally associated with foul smells and health risks [46]. Numerous studies have shown that these organic compounds, such as volatile organic sulphur compounds, volatile fatty acids, alcohols, aldehydes, ketones, terpenes and aromatics, are emitted during sewage sludge composting [46,47]. Volatile organic compounds are the major precursors that accelerate the depletion of ozone in the convective boundary layer, thus causing environmental problems such as global warming, photochemical smog and the creation of tropospheric ozone [46].

2.2.3. Vermicomposting

This is a bio-oxidation and stabilization process of organic matter resulting from the interaction of microorganisms and earthworms [27]. Microorganisms are mostly responsible for the degradation of organic material, even though earthworms stimulate microorganisms due to the alteration of the subsoil through feeding, aeration and excretion, which leads to the accelerated mineralization of organic matter and the improvement of nutrient availability for plants [28]. Toxic compounds are accumulated in the bodies of worms, which are eliminated from the end product at the end of the process [13].

2.2.4. Anaerobic Digestion

Anaerobic digestion is frequently used in large sewage treatment plants in order to transform sewage sludge into biogas (for example, methane and hydrogen) or added-value products (for example, organic acids) [47]. During this process, the development of pathogenic fauna is limited and the organic carbon contents in the sludge are reduced along with the carbon to nitrogen ratio (C/N) [13]. After fermentation, the sludge can be used in agriculture and for the recovery of degraded soils, and the biogas with a high methane content can be used as fuel and burnt in gas turbines, thus recovering energy [1,13,48].

2.2.5. Pyrolysis

Pyrolysis is the process by means which organic substances are thermally decomposed in the absence of oxygen or in conditions of limited oxygen, at temperatures between 300 and 900 °C [13,40]. The main purpose of the thermal processing of sewage sludge is to use the energy stored in sludge and, at the same, to minimize the effect on the environment in order to respect the strictest standards [40].

During pyrolysis, lignin, cellulose, hemicellulose, grease and starch from the raw material are thermally divided, forming three major products: biogas, bio-oil and biochar [49]. It should be noted that the proportion of the three phases depends on temperature, retention time, pressure, turbulence and effluent characteristics [43]. The three phases are:

• The gaseous fraction, which contains mainly hydrogen, methane, carbon dioxide and carbon monoxide, together with some other gases which are quantitatively irrelevant [40,50].
• The liquid fraction, consisting of tar and/or oil, which contains such substances as acetic acid, acetone and methanol [40].
• The solid fraction (biochar) which mainly consists of carbon and is most commonly pure carbon with small quantities of inert material [40].

Biochar, bio-oil and biogas yield depends on the pyrolysis method used, as well as on the pyrolysis conditions [51]. Pyrolysis gas can also be used as fuel, while the liquid product (the oil) can be used as raw material for the chemical industry and even as fuel [40].

2.2.6. Co-Pyrolysis

Co-pyrolysis biochar, produced from the pyrolysis of biomass with the addition of another biomass or non-biomass precursor, is the potential to overcome the limitation of pristine biochar and achieve superior performance for heavy metal adsorption and immobilization. Consequently, several researchers attempted to use the co-pyrolysis biochar of lignocellulosic biomass with the addition of another biomass or non-biomass precursor for the absorption and immobilization of heavy metals [52].

For example, coal or biomass are mixed with sewage sludges and paper waste for pyrolysis or co-pyrolysis. The pyrolysis of mixed biomasses or feedstocks is called co-pyrolysis. This process both enhances product yield and improves product properties [53]. Therefore, biochar from the co-pyrolysis of sewage sludge and lignocellulosic biomass is a potential adsorbent for ammonium removal from water [54].

2.2.7. Wet Oxidation

Sewage sludge wet oxidation takes place in an aqueous phase at temperatures between 150–330 °C and pressure of 1–22 Mpa using pure or atmospheric oxygen [55]. During this process, the organic content of sewage sludge is thermally degraded, hydrolyzed, oxidized and transformed into carbon dioxide, water and nitrogen [40]. The entire process takes place under two different conditions: the first one is carried out in subcritical conditions below 374 °C and a pressure of 10 Mpa, and the second one is performed under supercritical conditions under 374 °C and a pressure of 21.8 MPa [40].

2.2.8. Gasification

Gasification is the thermal process which implies the division of dry sludge into ash and combustible gases, commonly at temperatures of approximately 1000 °C, in an oxygen-reduced atmosphere [55]. Gasification transforms dry sludge into combustible gases, known as “syngas” (mainly composed of H₂, CO, CO₂ and CH₄), under partial oxidation at high temperatures of 700–1000 °C [56]. Compared to incineration, due to the fact that it is a net process of chemical reduction, gasification can prevent the occurrence of some problems, including the need for supplementary fuel, emissions of sulphur oxides, nitrogen oxides, heavy metals and ash, as well as the potential production of chlorine, dibenzodioxin and dibenzofuran [40]. During the gasification process, sewage sludge undergoes a complex physical and chemical alteration, starting with drying or dewatering, followed by pyrolysis and thermal decomposition [40].

3. Sewage Sludge Biochar

Sewage sludge used as soil amendment/fertilizer must be treated in order to reduce water contents, foul smell and the number of pathogenic organisms [42]. In most developed countries, special attention is given to the adequate treatment of sewage sludge in order to improve the quality and safety of using it on the soil surface [32]. For example, the study [57] evidenced the valorization of sewage sludge biochar into biochar and studied the effect of the use of biochar with soil (soil nutrients, microbial abundance and plant biomass enhancement).

The usefulness of fertilizers from sewage sludge is determined, first of all, by the content of organic matter, nitrogen, phosphorus, potassium and the presence of contaminants which limit its application [35]. There is a current tendency towards the production of organic fertilizers or organic minerals from sewage sludge [35].

Owing to existing legislation that sets limits to storing wastes and their application on land surfaces, many researchers sought other methods for reusing and recycling sludge [7].

Sewage sludge pyrolysis is considered an acceptable method from economic and ecological perspectives for the beneficial reuse of sewage sludge [38]. This method has many advantages because, during pyrolysis, sludge volume is reduced by 80%, pathogenic agents and hazardous compounds from sewage sludge are removed, metals are immobilized in a solid residue, thus reducing their leaching, and organic and inorganic fractions are immobilized in a stabilized form of pyrolytic residues (biochar) [29,40,58,59]. The biochar obtained through the pyrolysis of sewage sludge does not contain pathogenic agents and has a rich content of carbon and nutrients [60].

3.1. Types of Pyrolysis

Slow and fast pyrolysis are the most frequently used processes for producing biochar, which is intended for use as soil amendment [61]. Slow pyrolysis is a thermal process which occurs at high temperatures (300–700 °C), high pressures, long residence times (from hours to days) and slow heating rates (0.01–2 °C s⁻¹), producing almost equal quantities/yields of solid, gaseous and liquid products [61,62]. Fast pyrolysis is also a process of organic conversion characterized by short residence time (<2 s), fast heating rates (>2 °C s⁻¹) and high temperatures (500–1000 °C) [62]. Fast pyrolysis produces a large quantity of bio-oil (75%), biogas (13%) and biochar (12%) [61,62].

3.2. Factors Influencing Biochar Quality

The parameters of the pyrolysis process, such as temperature, residence/retention time, heating rate and the size of the raw material particle, can affect the qualitative and quantitative characteristics of biochar and its interactions with the medium in which it is applied [63,64].

3.2.1. Raw Material

The raw material from which biochar is produced, as well as the parameters of the pyrolysis process, influence the content of heavy metals and polycyclic aromatic hydrocarbons in biochar [65]. The elementary composition of biochar varies depending on what the raw material biochar is produced with and the characteristics of the method employed for obtaining it [63]. For example, sewage sludge has a high nutrient content resulting in high nutrient biochar compared to wood-based biochar, which has a low nutrient content. The chemical composition of biochar produced at low temperatures is similar to that of the biomass used for pyrolysis, while the properties of biochar obtained at very high temperatures differ from the used raw material [66].

3.2.2. Temperature

The temperature is a very important thermodynamic parameter which influences structural and physicochemical biochar characteristics [67]. Biochar obtained at high temperatures (500–1000 °C) occupies a larger area and microporosity, while a lower pyrolysis temperature causes lower biochar adsorption capacity [62]. The concentration of elements such as Ca, K and P and the ratio of C:N and C:O in biochar increase together with a rise in temperature [61,66].

During pyrolysis, the rise in temperature leads to a rise in biochar pH, an increase in the surface area and higher hydrophobicity, whereas low pyrolysis temperature (<500 °C) facilitates partial carbonization, thus generating biochar with smaller size pores, reduced surface area and higher content of O [49].

3.2.3. Retention Time

As far as the duration of the pyrolysis process is concerned, fast pyrolysis has a reduced or negligible effect on the ratio of C:O in biochar compared to slow pyrolysis, but a considerable increase in surface area was reported in the case of slow pyrolysis [66].

3.2.4. Heating Rates

Heating rates have an important effect on biochar structure, as well as on bio-oil and biogas production [61]. Faster heating rates positively impact the production of bio-oil, providing high yields of bio-oil (75%) from biomass with non-condensable gasses (13%) and solid biochars (12%), whereas slow heating rates create favorable conditions for biochar production [62].

3.3. The Use of Sewage Sludge Biochar as a Soil Amendment

Transforming sewage sludge into biochar can be an efficient method for the disposal of this waste compared to other methods currently used, such as storing/landfilling, incineration or direct use in agriculture [59].

A carbon-rich product, biochar is of interest due to its multiple functions, such as isolating (sequestration of) carbon in the soil, reducing greenhouse gas emissions, improving soil quality and providing a remedy for environmental damage [68]. Biochar is cheap and can be obtained from a variety of wastes using thermochemical procedures with extended applications in a cost-effective manner [69].

Biochar has high porosity, wide microstructure, high nutrient content and efficient water storage capacity and is used as soil amendment, whereas charcoal is mainly used for producing heat [70].

Previous research showed that following the conversion of sewage sludge into biochar, a part of the contaminants from the soil are removed, and the bioavailability and mobility of heavy metals are reduced [59].

Given that biochar quality and performance vary significantly depending on the raw material and pyrolysis conditions, future progress in developing biochar is expected to focus on the purposes of biochar use [49]. Biochar composition represents an important indicator for determining its potential use [71].

Biochar can have different properties, such as: physical properties (for example, bulk density, surface) and chemical properties (for example, pH, cation-exchange capacity, conductivity, mole ratio, concentration of various elements/nutrients (for example, carbon, oxygen, nitrogen, potassium, phosphorus, calcium, magnesium, etc.) and contaminants) [61,62,68,72,73,74].

The use of biochar in the soil is a valuable agricultural practice (Figure 1), which improves soil physical and chemical properties, reduces greenhouse gas emissions, enhances the efficiency of using nutrients, increases crop productivity, reduces losses of nutrients through leaching, alters nutrient content and availability, remedies contaminated soils, reduces soil erosion and enhances soil structure and the efficiency of using fertilizers [43,62,68,72,75,76,77].

3.3.1. Heating Rates Biochar Influence on Soil Physical Properties

The presence of biochar in soil has a significant effect on soil’s physical properties, influencing texture, depth, porosity, structure, particle size distribution, surface area, bulk density, water retention capacity, porosity and reaction to temperature variations [62,72,81].

The porous structure of biochar particles improves water retention and nutrient retention in soil, as well as microbial accumulation [62]. Biochar porous structure is composed of numerous aromatic compounds and other functional groups, which enable nutrient infiltration in the soil and serve as shelter (refuge) for soil microbes [62]. Additionally, biochar particles have a large internal surface and pores, which can be important for biological processes [75,82]. For example, aerobic microorganisms live on the surface of soil aggregates, while denitrifying bacteria and semi-aquatic species live inside moist areas of the soil [75,82]. Biochar pores serve as a habitat and refuge for microorganisms in the soil, including bacteria (with sizes between 0.3 and 3 µm), fungi (2–80 µm) and protozoa (7–30 µm) [61].

Biochar macropores (>200 nm) have the correct size to host bacteria; biochar also contains micropores (<2 nm) and mesopores (2–50 nm), which may store the water and dissolved substances necessary for microbial metabolism [61]. Micropores are responsible for adsorption surface and high capacity, whereas mesopores are important for liquid–solid adsorption; micropores are necessary for aeration, hydrologic processes, root displacement and soil structure [62]. Pore fraction and size depend on the temperature used for obtaining biochar because high temperatures increase water and organic matter volatilization, forming larger pores [61]. From this perspective, the biochar particle can be compared to a soil aggregate because biochar can provide similar functions, such as organic matter protection, habitat for soil fauna, humidity retention and nutrient retention in the soil [75,82]. Additionally, pore surface and volume can change when reaching the soil by pore closing due to absorption of organic and mineral substances (materials) or, on the contrary, by volatile matter mineralization, which can lead to pore blockage [75,82]. An example of the presence of macro- and micro-pores on the surface of biochar prepared from rice straw biochar is presented in Figure 2 [83].

Soil bulk density is an indicator of soil fertility and compactness because low bulk density is characterized by a larger number of pores, whereas high bulk density indicates hard soil with low porosity and weak soil structure [72]. High bulk density leads to poor ventilation, which can inhibit root growth, influencing the absorption of nutritious substances and water [72]. Soil bulk density decreases along with increasing biochar use since biochar reduces the density of soil volume and increases porosity in order to improve air permeability, thus optimizing soil structure; this further contributes to the growth of crops [72]. Biochar application to soil can modify soil density, with potential effects on the soil-water interaction, plant rooting forms and soil fauna [75,82]. This is due to the low density of biochar compared to some minerals and also because biochar contains macro- and micropores, which can retain air or water, thus significantly reducing the density of the entire biochar particle [75,82].

Other biochar physical properties which are important for microorganisms in the soil include its surface because a larger surface increases the chances of microbial colonization, and its black color attracts more heat and can thus accelerate microbial growth and enzymatic activity [61]. The specific surface of biochar is important because it contributes to the adsorption capacity of metal ions and organic compounds [84]. Biochar modifies the soil water retention capacity by altering soil porosity and compactness [72].

3.3.2. Biochar Influence on Soil Chemical Properties

The rapid beneficial effect following the application of biochar on nutrient availability largely owes to the availability of large quantities of K, P and Zn and a lower content of Ca and Cu in biochar [73]. Long-term benefits regarding nutrient availability include organic matter stabilization, slow nutrient release and enhanced retention of cations due to the increase in cation exchange capacity [73].

Biochar can increase total organic carbon in the soil, which, in turn, increases soil fertility [62]. Biochar contains high concentrations of N, P, Ca and K, which can be used as nutrients for the soil or microorganisms [71]. In contrast to other organic matter from the soil, biochar remains in the form of particles for long periods of time, even though the particle size is reduced as time goes by [85].

The pH value of biochar is an important property for using it in agriculture as soil amendment [71]. An increase in soil pH after the application of biochar owes to the alkaline pH of biochar, which varies depending on production temperature and raw material type [61]. Another factor leading to an increase in pH values in biochar modified soils is the presence of negatively charged phenolic groups, carboxyl and hydroxyl, which connect H⁺ ions to soil solution, thus reducing the concentration of H⁺ ions in the soil and increasing soil pH [61]. The positive impact of biochar on the soil by increasing pH is more obvious in acid soils that have low organic-matter levels [61].

Biochar is often zwitterionic (an organic ion having two contrary charges) and, subsequently, covers both positively and negatively charged surfaces [49]. Negatively charged functional groups are likely to attract cations and contribute to the increase of cation exchange capacity in soil [49].

Biochar obtained from sewage sludge has potential benefits as a soil amendment. In modified soils using sewage sludge biochar, increased levels of pH, total nitrogen, organic carbon and available nutritious substances were identified [86]. The most important influence of sewage sludge biochar on the soil properties is phosphorus availability [86]. Increases in nitrogen and sodium availability were also reported following the application of sewage sludge biochar to soils [86].

3.3.3. Biochar Influence on Plants

Biochar has agronomic value and can be used as a soil conditioning agent due to improving soil biological and physical properties, including increases in nutrient content in the soil and of water retention capacity, owing to its internal structure, thus reducing nutrient leaching and enhancing nutrient availability for plants, contributing to the growth of crops [62,72,81].

Biochar properties improve soil chemical and physical properties, which can directly impact plant growth, given that water and air availability and infiltration depth in root area are determined mainly by the physical composition of soil horizons [62,82]. The very different biochar properties stimulate plant root growth, and roots can even grow in biochar pores [82]. Each pore offers the space in which microorganisms can develop and increases the air and humidity amount and nutrient residence time, thus contributing to the increase of microbial activity and plant growth [63].

The effect of biochar on plant root growth depends on biochar properties and the conditions that can restrict root and shoot growth in various soil media [82]. The increase in soil porosity can improve soil conditions and contributes to plant growth [72]. Adding sewage sludge biochar to agricultural soils increases plant productivity due to biochar’s capacity to provide macro- and micro-nutrients to plants [86]. The increase in plant production following the addition of biochar obtained from sewage sludge was similar to the increase in yields obtained by applying chemical fertilizers [86].

Depending on the chemical composition of raw materials and pyrolysis conditions, biochar can contain fertilizing elements (N, P, K) [81]. The effect of biochar on crop productivity partially depends on the biochar application rate/dose [73]. Different elements, such as carbon, hydrogen, oxygen, nitrogen, potassium, magnesium, phosphorus, sulphur, boron, chlorine, copper, iron, manganese, molybdenum, zinc, cobalt, silicon and sodium are micro- and macronutrients which are essential for plant growth [87].

Increased nutrient availability for plants is the result of the direct contribution of nutrients provided by biochar, as well as increased nutrient retention and possible alterations of soil microbial dynamics [73]. Biochar contributes to the mineralization, fixation and transformation of organic nitrogen in soils, modifies organic nitrogen distribution and intensifies the nitrogen cycle from the soil to plants [72]. Mineral components such as Ca, Mg, K and P can enable biochar to operate as a direct source of nutrients, which further contributes to plant growth and, on the other hand, the high content of certain minerals, especially toxic heavy metals from biochar, could pose a risk to human health and the environment [68]. Heavy metal stabilization and availability in sewage sludge biochar is a major concern because of high concentrations of heavy metals [86]. It was observed that Pb, Ni, Cu, Zn and Cr remain in biochar because of their high boiling points, whereas Hg, As, Cd and Se are discharged during carbonization [86].

The concentration of heavy metals in biochar increases during pyrolysis, depending on the increase of pyrolysis temperature, and yet it was observed that general heavy metal availability decreases following pyrolysis [86]. Biochar has a unique property of connecting polar compounds’ surface charge functional groups, which assist heavy metal and agrochemical immobilization on biochar surface and limit their mobility in plants [49,62]. Adding sewage sludge biochar to soil decreased the availability of As, Cr, Co, Ni and Pb but increased the availability of Cd, Cu and Zn [86]. Even though Cu and Zn are necessary micronutrients for plant growth, they can have toxic effects if their concentration is high [86]. Certain studies have shown that, in spite of raising Cd, Cu and Zn availability, the concentration of these heavy metals in sludge biochar modified soil of cultivated crops has not exceeded recommended limits [86]. On the contrary, other studies mention that in the case of sewage sludge biochar use as soil amendment, the Cu concentrations in the plants grown beyond recommended limits [86].

The increase in agricultural productivity due to using biochar as soil amendment can be attributed to increases in soil fertility, soil pH, soil cation-exchange capacity, nutrient retention and soil microbial activity [86]. Biochar cation-exchange capacity indicates the adsorption capacity of cations such as NH⁴⁺ and Ca²⁺, which are essential nutritious substances for plants [84]. Cation-exchange capacity is used to describe soil fertility because almost all nutritious substances used by plants and microbes are absorbed in their ionic form [71]. Soil with high cation-exchange capacity has the ability to hold or bond nutritious cations for plants on the surface of biochar, humus and soil particles; therefore, nutrients are retained, their leaching is not allowed, and so they are available for absorption by plants [73]. Biochar can raise the soil cation- and anion-exchange capacity, enhancing soil properties by increasing pH, total contents of N and P and favoring root development [62]. In general, favorable reactions in crops after using biochar can be the result of raising soil pH, macro- and micronutrient availability and water retention capacity [73]. When biochar is added to, for instance, agricultural fields consisting of crops and weeds, for soil recovery, etc., it can cause alterations in species composition of plant communities and root biomass, leading to alterations of all rooting characteristics (for example, depth, length, form) [82].

Table 1. Experiments using sewage sludge biochar/sewage sludge.

Experimental Conditions	Raw Materials/Application Rate	Effect on the Soil	Effect on Plants	Ref.
The experiment was conducted in a growth room (laboratory conditions) for a time span of 7 weeks. Study plant: rice.	Control soil. Soil + B (biochar)-fungus residues, (200 °C and 350 °C). Soil + B-sewage sludge, (200 and 350 °C). Soil + B-soybean straw, (200 and 350 °C). Soil + B-peanut shells (200 and 350 °C). Soil + B-rice straw, (200 and 350 °C). Application rate: 3%.	The various types of biochar modified the organic matter dissolved and the As and Cd bioaccumulation. The highest As concentration was recorded in the treatment with sewage sludge biochar (200 °C). The effect on the soil pH was minor.	The biochar from peanut shells and the biochar from sewage sludge increased considerably the number of seedlings/siblings. As and Cd bioaccumulation in rice plants was significantly reduced in the case of biochar types obtained at high pyrolysis temperatures.	[88]
The experiment was conducted in greenhouse conditions for a time span of 16 weeks. Study plant: cherry tomatoes.	Control soil. Soil + B-sewage sludge. Soil + sewage sludge. Application rate: 10 t/ha.	No data	No significant difference was recorded between the heights of plants. The dry quantity of tomatoes in the treatment with sewage sludge was larger. Trace/minor elements, with the exception of Sb and Sr, had lower concentrations in the tomatoes treated with biochar.	[89]
An experiment in plastic containers, in field (outdoors) conditions, for a time span of 49 days. Study plant: maize	Control soil. Soil + sewage sludge, 15 t/ha + AN (ammonium nitrate). Soil + B (biochar)-sewage. Sludge, 15 t/ha + AN. Soil + sewage sludge, 7.5 t/ha + B- from sewage sludge, 7.5 t/ha + AN. Soil + inorganic fertilizer, 300 kg/ha + AN.	Zn, Cu and Pb concentrations increased as a result of applying sewage sludge and biochar. The electrical conductivity (EC) of the soil increased, especially in the treatment with biochar + ammonium nitrate. The content of P, N, C, K and Na did not vary significantly.	The growth of maize and nutrient absorption were improved in the treatment with biochar and sewage sludge. Zn and Cu absorption was reduced in the case of treatment with sewage sludge biochar. Zn and Cu recorded the highest values in the treatment with sewage sludge + AN.	[90]
An experiment in field conditions, time span 24 months. Sunflower seeds were planted, which were harvested after 6 months. The plots were kept up to 24 months.	Control soil. Soil + B (biochar)-pinewood. Soil + B-paper from sewage sludge and wheat chaff. Soil + B-sewage sludge. Soil + B-vinewood. Soil + B-mixture of wood Splinters. Application rate 15 t/ha.	The sewage sludge biochar recorded the lowest values in C and the highest values in N. The sewage sludge biochar increased the pH level. EC recorded decreasing values throughout the 24 months.	No data	[91]
For this experiment, the samples (200 g) were incubated for 200 days at 28 ± 2 °C.	Control soil. Soil + sewage sludge. Soil + B-sewage sludge. An application rate of 8% and 4%.	At an application rate of 8%, the biochar increased the soil pH. EC increased as a result of the application of biochar and sewage sludge. Soil respiration increased in all treatments. Cu, Ni and Zn recorded decreased quantities in the biochar samples.	No data	[92]
An experiment was conducted in field conditions on a terrain contaminated with Cd for 4 months. Study plant: rice.	Control soil. Soil + B-sewage sludge. Application rate: 1.5 t/ha and 3 t/ha.	The pH of biochar modified soils increased in proportion to the application rate. The organic matter increased significantly at an application rate of 3 t/ha of biochar.	The biochar increased the biomass and the yield of rice plants. The Cd bioaccumulation in rice grains, in ramifications, roots and husk of plants was reduced in comparison with the results from the control option.	[93]
The experiment was conducted in greenhouse conditions in a time span of 60 days. Study plant: eucalyptus saplings.	Control soil. Soil + fertilizer. Soil + B-sewage sludge. Soil + B-sewage sludge + fertilizer. Soil + sewage sludge. Soil + sewage sludge + fertilizer. Application rate: 40 t/ha.	No data	There was no significant difference between results obtained in the treatment with sludge and those with biochar. Adding fertilizer in the sewage samples and in the ones with biochar did not influence significantly the growth of the eucalyptus saplings.	[94]
An experiment was conducted in an incubator in which the soil samples modified with different types of biochar were kept at 25 °C for 70 days.	Control soil. Soil + B-rice straw. Soil + B-wheat straw. Soil + B-maize straw. Soil + B-kitchen waste. Soil + B-sewage sludge. Soil + B-eucalyptus. Soil + B-Chinese silver grass. Soil + B- maize cob. Soil + B-poultry manure. Application rate 2%.	With the exception of the types of biochar obtained from kitchen waste, Chinese silver grass and poultry manure, all the other types of biochar increased the soil pH. Electrical conductivity and cation-exchange capacity increased in all samples.	No data	[95]
The experiment was conducted in greenhouse conditions for a time span of 60 days. Study plant: maize.	Control soil. Soil + B-sewage sludge (550–700 °C, for 3 h). Soil + B-sewage sludge (600 °C for 1 h). Application rate: 5, 10, 20, 60 t/ha.	The quantity of total N increased in accordance with the increase in the application dosage, and the total content of P recorded the highest values in the case of sewage sludge biochar produced at 600 °C for 1 h.	The biochar obtained at 600 °C did not have a negative effect on the growth of plants. The biochar produced at 550–700 °C inhibited the growth of plants, especially at 60 t/ha. Both types of biochar increased the P concentration in plants. The N in plants increased significantly in the case of the biochar produced at 550–700 °C.	[96]
The experiment was conducted in greenhouse conditions, in a time span of 8 weeks, using soil contaminated with hydrocarbons (PAHs). Study plant: lettuce.	Control soil. Soil + sewage sludge. Soil + B-sewage sludge. Application rate: 2%; 5% and 10%.	No data	The lettuce plant biomass increased in all samples modified with biochar. In the case of sewage sludge, we could observe an increase in the biomass only in the case of the application rate of 2%. Compared with the control soil, the biochar and the sewage sludge reduced the bioaccumulation of hydrocarbons (PAHs) in the lettuce.	[97]
An experiment conducted under laboratory conditions. The experimental conditions varied, including the quantity of sewage sludge biochar (1–5%) and soil temperature (4, 25 and 45 °C).	Soil contaminated with Pb, Cd and Ni. Soil (Pb, Cd, Ni) + B-sewage sludge, rate of: 1%, 2.5%, 5%. Soil contaminated with Cr. Soil (Cr) + B-sewage, rate of: 1%, 2.5%, 5%, 10%, 50%. Soil contaminated with As. Soil (As) + B-sewage sludge, application rate: 5%, 10%, 25%, 50%, 100%, 200%, 300%, 400% and 500%.	The variation of experimental conditions played an important role in metal stability. In comparison with the control soil, Fe, Pb, Cu, Zn, Cr, As concentrations decreased in accordance with the increase in the application rate of sewage sludge biochar.	No data	[98]
An experiment in greenhouse conditions, time span 15 weeks. Study plant: cucumber.	Control soil. Soil + sewage sludge. Soil + B-sewage sludge. Application rate: 2%, 5% and 10%.	The sewage sludge and the biochar reduced hydrocarbons availability, especially in the case of a 10% application rate of biochar. As, Cd, Cu, Pb and Zn increased their levels in accordance with the increase of the sewage sludge application rate. The biochar reduced levels of As, Pb, Cu and Zn, while Cd increased.	The cucumber biomass was higher in biochar modified samples. The hydrocarbons (PAHs) concentrations reached the lowest values in the case of biochar samples. As, Cd, Cu, Pb and Zn recorded high values at an application rate of sewage sludge of 10%. The biochar increased Cd but significantly reduced As, Cu, Pb and Zn in the cucumber.	[99]
An experiment in greenhouse conditions, conducted in a time span of 8 weeks, in which contaminated soil was used. Study plant: turnip.	Control soil. Soil + B-sewage sludge. Soil + B-soy beans. Soil + B-rice straw. Soil + B-peanut shell. Application rate: 2% and 5%.	The type of biochar determined increases in pH, EC, NH4 + -N, NO3 − N, C. All types of biochar decreased hydrocarbons (PAHs) concentrations. As, Cd, Cu, Pb, and Zn decreased in accordance with the increase in the application rate.	The turnip reached the highest values at an application rate of 2% of sewage sludge biochar. At a 5% rate, the turnip yield was reduced in the case of all types of biochar. All types of biochar reduced hydrocarbons (PAHs) bioaccumulation in the turnip. As, Cd, Cu, Pb and Zn bioaccumulation was significantly reduced only in the case of the 5% rate.	[100]
The experiment was conducted in greenhouse conditions, in a time span of 11 weeks. Study plant: thatching grass/Coolatai grass (Hyparrhenia hirta).	Control soil. Soil + sewage sludge. Soil + B-sewage sludge. Soil + sewage sludge + fertilizer. Soil + B-sewage sludge + fertilizer. Soil + fertilizer. Application rate sewage sludge and biochar: 10 t/ha.	No data	The largest quantity of grass was obtained in the samples modified with biochar + fertilizer. The mixture of biochar + fertilizer also increased some of the important chemical characteristics of the grass.	[101]
Hg contaminated soils were used, which were collected from 2 areas. The experiment was conducted in greenhouse conditions from the 18th of June until the 5th of October 2016. Study plant: rice.	Soil from area 1. Soil 1 + B-sewage sludge. Soil from area 2. Soil 2 + B-sewage sludge. Application rate: 5%.	The biochar increased the soil’s pH and the MeHg (methylmercury) concentration in the soil. The Hg concentration in both soils was not influenced by the addition of biochar in soils.	The Hg and MeHg concentrations from the rice plants decreased in the treatments with sewage sludge biochar.	[102]

and

Table 2. Some studies evidenced the influences of sewage sludge biochar use on organisms or plants.

Experimental Conditions	Raw Materials/Application Rate	Effect on Seed/Test Organisms	Ref.
The test period was 28 days in laboratory conditions. Test organisms: Collembola (Folsomia candida) and Enchytraeid (Enchytreus crypticus).	Control soil. Soil + B (biochar)-poplar splinters, (500–550 °C). Soil + B-poplar splinters, (430–510 °C). Soil + B-sewage sludge (500–550 °C). Soil + B-pine splinters (600–900 °C). Soil + B-pine splinters (500–550 °C). Soil + B-pine splinters (440–480 °C). Application rates: 0%, 0.5%, 1.3%, 3.2%, 8%, 20% and 50%.	The biochar from pinewood (600–900 °C), in high concentrations, increased the mortality of Collembola adults. The biochar from poplar wood produced at 500–550 °C and 430–510 °C had a strong stimulating effect on the reproduction of Collembola but without a significant effect on the Enchytraeids. The sewage sludge biochar had a low effect in the case of both tested organisms.	[105]
Maize seeds (Zea mays) were germinated in Petri dishes on a thin, moist filter paper. All Petri dishes were incubated in the dark at 25 °C for 72 h.	B-sewage sludge (550–700 °C, for 3 h), application rates: 0 (control), 2.5; 5; 10; 20; 60; 100 t/ha. B-sewage sludge (600 °C, for 1 h), application rates: 0 (control), 2.5; 5; 10; 20; 60; 100 t/ha.	Almost no maize seed germinated in the samples of biochar produced at 550–700 °C. The high content of volatile matter and electrical conductivity of this biochar affected plant germination. In the case of biochar obtained at 600 °C, the number of germinated seeds, the plant stem length, the root length, and the quantity of dry biomass increased in accordance with the rise in application rates.	[96]
Four types of sewage sludge were used, collected from different sewage treatment plants situated in Poland that were subjected to pyrolysis. Test organisms: Garden cress (Lepidium sativum), bacteria (Vibrio fischeri) and crustaceans (Daphnia magna).	Sewage sludge (SSKN) from the sewage treatment plant in Koszalin (KN). B-sludge 500 °C, (BCKN). B-sludge 600 °C, (BCKN). B-sludge 700 °C, (BCKN). Sewage sludge (SSKZ) from Kalisz (KZ). B-sludge 500 °C, (BCKZ). B-sludge 600 °C, (BCKZ). B-sludge 700 °C, (BCKZ). Sewage sludge (SSCM) from Chełm (CM). B-sludge 500 °C, (BCCM). B-sludge 600 °C, (BCCM). B-sludge 700 °C, (BCCM). Sewage sludge (SSSI) from Suwałki (SI). B-sludge 500 °C, (BCSI). B-sludge 600 °C, (BCSI). B-sludge 700 °C, (BCSI).	The conversion of sewage sludge into biochar caused a reduction of root growth inhibition. In the case of bacteria testing, all types of sludge and two types of biochar, BCKN and BCCM, were obtained at 700 °C and recorded acute toxicity. The sludge pyrolysis determined a reduction of the toxicity of D. magna only for the types of biochar obtained at 500 °C. The highest increase in mortality for D. magna, both after 24 and after 48 h, was observed for BCKZ and BCSI biochar obtained at 700 °C.	[104]
The garden cress seeds were introduced in extracts for 24 h. Then, they were laid on moist paper in Petri dishes that were incubated at 25 °C ± 0.1 for 72 h.	Redistilled water (control variant). Sewage sludge (Krakow). B-sewage sludge (Krakow). Sewage sludge (Krzeszowice). B-sewage sludge (Krzeszowice). Sewage sludge (Słomniki). B-sewage sludge (Słomniki).	The content of the water-soluble forms of copper, cadmium, lead, and zinc was lower in all types of biochar in comparison with sewage sludge. Significant stimulation of the root growth in relation to the control variant was observed in the case of types of sewage sludge biochar produced in the sewage-treatment plants from Krakow and Krzeszowice.	[106]
The test was performed in a germination chamber for 14 days. Study plant: lettuce (Lactuca sativa) and perennial ryegrass (Lolium perenne).	Control soil, B-poplar splinters (500–550 °C), B-poplar splinters (430–510 °C), B-sewage sludge (500–550 °C), B-pine splinters (600–900 °C), B-pine splinters (500–550 °C), B-pine splinters (440–480 °C). Application rate: 0.4; 0.9; 2.1; 4.9; 11.3 and 26%.	Of all types of biochar, the biochar obtained from sewage sludge significantly stimulated plant growth. The biochar from poplar splinters (430–510 °C), the biochar from pine splinters (440–480 °C), and the biochar from poplar splinters (600–900 °C) had a negative effect on plant growth.	[107]

present some experimental methods mentioned by specialized literature regarding the use of sewage sludge biochar and sewage sludge as potential fertilizers.

3.4. The Toxicity of Sewage Sludge Biochar

Before the application of biosolids to the soil, an adequate ecotoxicological characterization is recommended because treated sewage sludge can contain heavy metals and organic compounds, which could pose risks for the soil and water [103]. According to ISO guides regarding soil quality, an adequate soil ecotoxicological evaluation involves the assessment of the soil’s capacity to function as a habitat for organisms and the assessment of its capacity to immobilize contaminants for the prevention of contaminating subterraneous waters and surface waters [103]. The determination of contaminants by means of chemical analyses is not sufficient for the estimation of possible risks following the use of sludge biochar in the soil as potential fertilizer [104]. Given that biochar is used especially in agriculture, it is also important to establish its toxicity to different groups or organisms [104]. Biochar may have both positive and negative effects on plants, fauna and microorganisms, and a consequence of biochar can be its direct effect on organisms because of its content of organic or inorganic pollutants [65]. It is advisable to use pyrolysis temperatures which are higher than 600 °C in order to obtain low toxicity levels, and not all types of biochar are suitable for agricultural applications [65].

In the case of biochar produced using the rapid pyrolysis method with lower nutrient content and higher porosity, it is indicated to be used for the degraded soil’s remediation. The biochar produced at a lower temperature (<500 °C) with a higher amount of nutrients can be used in large agriculture applications [61,62]. Additionally, it can be mentioned that not all soils have improved significantly, and not all crops react in the same way when biochar is applied as an amendment to the soil [70,75].

The excellent sorption capacity of biochar can deactivate agrochemicals substances such as pesticides and herbicides and absorb nutrients with bioavailability reduction [65,74].

In cases where biochar is added, for example, in agricultural fields containing crops and weeds, it can determine some modification of plants, for example, changes in root characteristics (depth, length, shape) [76,82].

Generally, biochar can pose ecotoxicity risks because certain plant species are sensitive to certain chemical elements (P/B/Cu/Na/Zn/Mn/Cl) and, in such cases, the phytotoxicity of biochar prevails and will vary depending on application rates/doses [68]. Additionally, the application of biological tests allows the analysis of possible interactions between different contaminants and constitutes the most important proof of the existence or absence of a toxic effect on organisms [104].

Table 2 present studies that evidenced the influence of sewage sludge biochar use on some test organisms or plants.

3.5. Future Directions

The recycling and reuse of organic waste is an area of global interest, as the amount of waste will increase due to population growth. Researchers aim to reduce the negative effect on the environment generated by the organic waste increase worldwide. The use of organic waste as soils amendments is a method of recycling and reuse of nutrients present in this waste category. It is recommended that organic waste be properly treated before utilization to reduce or eliminate the toxic components.

In proper conditions, the organic waste can be recycled and reused for crop fertilization, thus reducing the negative environmental impact of waste quantities. The identification of the optimal method for organic waste treatment must be performed by considering the economic and ecological aspects so that the final product does not create negative effects on the environment and have high production costs. Additionally, some purposes of organic waste future utilization can refer to the remediation and recovery of contaminated soils.

It is necessary to evaluate other methods used for the organic waste treatment and to identify the potential effects on the environment, soil, plants, etc., from the perspective of waste management issues.

Some comparative studies from the domain of organic waste utilization can reveal other perspectives related to the long term effects and also the interaction between organic waste and chemical substances used as soil improvers.

4. Conclusions

Organic waste, used in untreated form, can have a negative impact on the environment by contaminating water sources, soil, odors and greenhouse gas emissions with direct and indirect implications for human health.

The use of treated organic waste as fertilizer, in proper condition, can represent a sustainable solution to reduce the organic waste quantities.

The laboratory evaluation of the treated organic waste utilization as soil fertilizers can offer, in many cases, good direction and perspective for large scale applications.

The amount of sewage sludge is slowly increasing, which has led to the occurrence and development of numerous sewage sludge recycling and reuse methods. The identification of the optimal method of managing this waste must take into account reductions in the amount of this waste, the environmental impact, the costs and the final purpose of using the obtained end product.

The most efficient method of managing sewage sludge is pyrolysis because the process results in bio-oil, biogas, and biochar. All these final products can be valorized in different phases, for example, in the chemical industry, agriculture or can be used as fuel.

The most important parameter in the pyrolysis process is temperature, which determines the quality and properties of the end products. Pyrolysis removes smell, reduces volume, destroys pathogens, eliminates a part of the pollutants present in sludge and reduces heavy metal bioavailability. Sewage sludge biochar has the long-term capacity to improve the soil’s physical and chemical properties, which directly influence the growth and development of plants. The effect of sewage sludge biochar on agricultural soils and crops largely depends on the application rate/dose used and can replace the use of chemical fertilizers, which can have negative effects on the soil as long as chemical fertilizers generally improve only crop productivity.

It is important to determine the effect of sewage sludge biochar on soil properties, as well as if there is a negative impact on the soil and how it affects plant growth. It is also recommended to test these organic wastes’ toxicity because after conducting soil analyses, it is difficult to determine their effect on seed germination or on the organisms living in soils and which interact directly with the fertilizers added to the soil. High rates of sewage sludge decreased soil pH but increased soil electrical conductivity, as well as heavy metal concentrations and polycyclic aromatic hydrocarbons. Sewage sludge increased heavy metal concentrations in cultivated plants in proportion to rising application rates.

The evaluated studies shown that sewage sludge’s transformation into biochar reduces the bioavailability of heavy metals and toxic components from sludge.

Sewage sludge biochar, in contrast with sewage sludge, can have a reduced impact on soils and plants, confirming that the transformation of sewage sludge into biochar represents an efficient method of reusing this organic waste.

Untreated sewage sludge can have a toxic effect on seeds and organisms used in various experimental studies, but sewage sludge biochar positively stimulated the development of the tested organisms, as most studies revealed.