The experience of SS disposal in Germany, the USA, France, Finland, and several other countries indicates that if there is an effective sludge treatment technology and control over its use, most of the SS (up to 60%) can be used as a fertilizer in agriculture, in urban landscaping, as well as for inland reclamation, reforestation, and other works. The transformation of SS into a new substance or product that meets quality protocols, along with a new level of processing, will provide an opportunity to obtain scarce critical raw materials from secondary raw materials, leading to the successful management of SS waste. Ongoing research and development in the field of SS waste management shows that SS recycling can be used to produce valuable organic substances and plant nutrients that can be used as fertilizers [23,24,25] or additives to improve soil quality and to increase the fertility and productivity of agricultural land.

In the Regulation on the European Waste list (waste code 19 08 05), municipal sludge from WWTP is assessed as safe. However, an exception is described in the article of the Waste Framework Directive 2008/98/EC [26], stating that the sludge is safe only: if the competent authority of an EU Member State considers that the relevant evidence is sufficient to assign the code ANH to this type of waste. Once recovered, the SS must be processed to ensure economical and safe transport and disposal. Depending on the type of plant, primary and secondary sludges are treated separately or together. Co-treatment options include thickening, stabilizing, dewatering, and drying the sludge. Additional and well-established methods for controlling SS include lime treatment, anaerobic digestion (AD), and composting with other organic matter. The final disposal stage is the distribution of sludge over the soil surface, incineration, and disposal. In Europe, there are technologies for extracting phosphorus and, in rare cases, nitrogen. Removal of waste for agricultural purposes usually takes place in the area adjacent to the plant to minimize transport costs.

The SS treatment methods are stabilization (composting, aerobic digestion, treatment with lime, etc.) and dewatering (air drying, vacuum filtering, centrifuging, etc.) [2]. The main objective of SS treatment is to produce biosolids that are safe for agricultural use (i.e., reduction or elimination of pathogens and reduction of fermentability of the final biosolids).

AD is the most important SS treatment method. The SS is thickened, digested, and dewatered before it is used for other applications. A useful product of AD is the biogas, consisting of 60–70% CH₄, 30–40% CO₂ and small quantities of H₂, N₂, H₂S and H₂O, which is used for energy production. AD consists of a series of chemical reactions that lead to the decomposition of organic materials in the absence of oxygen [2]. Co-digestion is the simultaneous digestion of a homogenous mixture of two or more substrates. In the case of sludge digesters, this is a very common practice, with the addition of organic waste from the food industry or households as a co-substrate.

Although AD has been known for over a century, it has only been studied in depth for various commercial applications in the last three decades [27]. The use of AD for organic fertilizers is challenging due to the long duration of microbial reactions, usually 20–40 days. Each stage of decomposition involves a different consortium of microorganisms, belonging to both bacterial and Archean domains, with varying needs for growth. The physicochemical characteristics of the co-substrates added to the AD bioreactors at the treatment plant significantly influence the co-fermentation and the quality of the digestate. The co-digestion process increases biogas production, as there is a higher degree of volatile solids decomposition [28], resulting in an 82% increase in CH₄ yield, and a 29.5% removal of volatile solids compared to the treatment of SS alone. Digestate obtained after AD requires additional treatment before disposal. The solid fraction obtained after digestion consists of partially decomposed organic matter and a high content of NH₄⁺-N and minerals [29]. Regardless of the type of food waste, NH₄⁺-N levels in 2540–7200 mg/kg of dry matter and humidity of 70–96% have been reported in different countries [29]. The content of 60–70% NH₃ generates emissions and phytotoxic effect on plants [30]. Therefore, further stabilization through the composting process is required to improve the properties of food SS into a beneficial soil change in an environmentally sustainable manner.

Ting and Lee [31] reported H₂ and CH₄ production from SS using a strain of clostridium. In this work, the authors investigated the formation of H₂ and CH₄ during the AD of pre-treated SS (acidified, alkalized, and frozen/thawed). Yang and Wang [32] provided a critical review of AD sludge handling. H₂ production during sludge fermentation was observed. The primary consumers of H₂ are homoacetogenic bacteria, sulfate-reducing bacteria, and hydrogenotrophic methanogens. Homoacetogenic bacteria are strictly anaerobic and can catalyze acetate formation from H₂ and CO₂, such as Clostridium thermoaceticum and Clostridium aceticum [33,34]. A biochemical reaction can be described by the following equation:

Sulfate-reducing bacteria can use H₂ to form sulfides in the presence of sulfate [35], even at extremely low H₂ concentrations (0.02 ppm). The following equation describes the biochemical reaction:

Hydrogenotrophic methanogens can use H₂ and CO₂ to form CH₄ [36]. For example, a biochemical reaction is described as the following equation:

Other H₂ consumption pathways are also observed in the production of propionic, valeric, and caproic acids, and the detailed reactions can be described as follows [37]:

AD also contributes to a “short carbon cycle” by removing carbon from the atmosphere. Carbon from organic residues for biogas production continues to be reused in the digestate, returning it to the soil [38]. Biogas and biomethane production as renewable fuel sources is justified in terms of reducing greenhouse gas emissions (CO₂ and CH₄) in agriculture and animal husbandry, as well as for improving the climate by 2050 [39,40,41]. Europe is the largest producer of biogas and biomethane globally [41]. Globally, more than 1000 M tons of biomethane could be produced by 2040, with average costs reduced by 15% compared to prices in 2018. Thus, digestate as a biofertilizer will increase shortly, contributing to agriculture and lessening the demand for the carbon-intensive production of mineral fertilizers.

One of the important treatment methods for the use of SS is composting or co-composting with other suitable biodegradable wastes and additives [42,43]. SS composting makes it possible to drastically reduce their volume, and such a technological process is not associated with significant emissions of hazardous substances, compared to incineration, and is acceptable from an environmental point of view. In recent years, compost production has gained wide popularity as a rational method of biowaste disposal due to its environmental friendliness, technological simplicity, and low cost [44,45,46]. In the process of composting, SS undergoes physical and chemical transformations with the formation of a stable humified final product, which ensures its disinfection and turns it into valuable components. However, the method is effective only at a specific moisture content of SS, and dehydration is required. This method makes it possible to obtain an inert and neutralized final product due to its (1) low cost and simple processing, (2) low energy consumption, and (3) environmental friendliness. During composting, there is a decrease in both organic contaminants and the bioavailability of heavy metals [47,48]. Composting improves sanitary and hygienic indicators due to the death of pathogenic microorganisms, helminth eggs, and fly larvae. Compost disinfection is effectively carried out by adding lime and calcite. The resulting product can be used to improve the structure and fertility of the soil. In this context, composting can be seen as the preferred strategy for the disposal of SS. However, the possible presence of heavy metal ions limits its application due to the impact on human health and environmental pollution [49,50]. Sludge composting is primarily considered an effective treatment method to overcome the problems associated with pathogenic microorganisms and other contaminants in the sludge, and prevents uncontrolled fermentation of organic components [51,52,53]. However, the latter can (1) interfere with plant growth, (2) cause inconvenience, and (3) be a source of CH₄ emissions [54,55,56].

Cao et al. [57] studied composting additives and fillers to investigate the aerobic properties of the composting process for food waste and residual sludge. Residual sludge, food waste, and bagasse mass ratios of 1:1:1, 2:1:1, and 4:1:1 were used. During composting, changes in nitrogen and phosphorus content in nutrients, microbiota, and metabolic function were observed. It is advantageous to maintain a high reactor temperature when the amount of residual sludge is relatively large and the proportion of residual sludge in the reactor is higher than the content of the original protease. The analysis of the microbial community showed that the 1:1:1 mass ratio promotes the growth and reproduction of beneficial bacteria and the reduction of the number of pathogenic bacteria in the reactor. On the other hand, a reactor with a high proportion of residual sludge encourages the growth of phosphorus-releasing bacteria. The conversion of ammonia and pentose phosphate in a heap of 1:1:1 mass ratio proved to be more favorable for glycolysis and to promoted the pentose phosphate cycle of the heap [57]. Co-composting is an efficient solid waste management method that helps to manage various solid wastes (i.e., poultry waste, animal waste, food waste, agricultural waste, municipal solid waste, industrial sludge, sewage, etc.) and turn them into valuable products, which can increase soil fertility by acting as a soil conditioner. This simple process has the lowest operating costs and is an alternative to expensive fertilizers used to improve the soil. The co-composting process can promote the formation of heavy metal complexes, whose mobility and availability tend to decrease with decreasing toxicity. Biodegradable waste is successfully co-composted with various types of organic waste to produce a higher-quality product. To enhance aeration, achieve the required moisture content, and control the ratio of carbon to nitrogen, the composting of SS is carried out with fillers, such as solid household waste, peat, sawdust, foliage, plant tops, straw, ground bark, and a portion of the finished compost. Optimal conditions for implementing the process are created when the humidity of the mixture of the sediment with the filler is 60–65%, the ratio of carbon to nitrogen is 20–30:1, and the pH of the fermented mass is between 5.5 and 8. The intensity of the process and quality of the resulting compost depend on the conditions created for the vital activity of the microorganisms [58]. Using biochar in compost production helps to reduce the production of gaseous pollutants and to improve the compost quality. In addition, when combined with sensitivity analysis results, emissions of biogenic air pollutants from compost and biochar production, as well as essential issues contributing to undesirable environmental impacts, were identified [59]. Wang et al. [60] reported the use of four different phosphates, as well as a mixture of ferrous sulfate and potassium monophosphate for SS composting. The addition of phosphates contributed to an increase in the temperature and the decomposition of organic matter, as well as to the conservation of nitrogen. Moreover, ferrous sulfate and phosphate show a synergistic effect in reducing nitrogen losses. The content of total phosphorus and mobile phosphorus in the compost with the addition of 1% phosphate was 40.9% and 66.1% higher than in the compost with the control treatment. Sequential extraction with the addition of calcium magnesium phosphate (CMP) makes it possible to reduce the mobility of Cd, Zn, and Cu by 24.2%, 1.7%, and 18.8%, respectively. However, in most cases, lead mobility increased. Seed germination control showed that the compost of all processing methods is favorable for agricultural use. The germination index of treated CMP with monopotassium phosphate (MKP) was 99.9 ± 11.8%, which was the highest among all treatments [60]. Thus, the introduction of phosphate additives provided an increase in the temperature and decomposition of organic matter in the SS during composting. All treatments with phosphate additives showed higher nutrient content and maximum nitrogen retention, as well as an increase in total phosphorus and available phosphorus in the product.

One of the promising developing methods for SS treatment is pyrolysis. The pyrolysis method consists of the irreversible chemical change of waste under the influence of temperature, without access to oxygen. Such processing plants require large areas, expensive equipment and, as a result, significant investments; therefore, the processing of sludge by pyrolysis, similar to incineration, is challenging to implement on a large scale [61]. The advantages of the method include the ease of storage and transportation of the resulting products and the fact that the equipment has low power requirements. Compared to direct waste incineration, the advantage of this method lies in its effectiveness in preventing environmental pollution, that is, the release of harmful chemicals into the air. The products resulting from pyrolysis have a high density, drastically reducing the volume of residues subjected to underground storage. The material resulting from pyrolysis exhibit reduced sludge toxicity and can be when used in agriculture [62].