Introduction

Sewage sludge is a nutrient-rich semi-solid residue produced during the wastewater treatment process. Different volumes and types of sludge are formed depending on the content of the wastewater and the sludge treatment method used. Figure 1 displays a typical wastewater treatment process, which may involve screening, filtration, primary treatment, and physical, biological, and chemical secondary treatment. Table 1 shows sewage sludge's average mineral and organic composition by type and region. The residues (suspended particles, deposits, sediments, and viscous masses) left over after primary and secondary treatment that include more solids are referred to as sewage sludge (SS). SS comprises of organic molecules of live cells, cell fragments, extracellular matrix, carbon polymers or monomers, nitrogen ammonium or amino acids, dissolved and suspended elements, and inorganic elements. Moreover, sewage sludge may include pathogens and harmful compounds such as heavy metals, which may pollute soil, surface water, and groundwater. In this regard, a recent study assessed the ecological and human health risks of heavy metals in sewage sludge samples collected from 22 different cities' wastewater treatment plants (WWTP) covering seven geographical regions of Turkey and found that the site-specific overall total cancer risk (TCR) for adults was within the acceptable range (10−6–10–4). However, the risk value for children (1.6 × 10–4) was slightly higher. Similarly, adult participants' Hazard Index values were lower than the reference value of one, but children's values were greater (up to 2.52). Lead (Pb), arsenic (As), and chromium (Cr) were the critical metals contributing to carcinogenic hazards in children and adults (Yakamercan et al. 2021). Many factors influence the organic content and load of SS, including plant layout and location, population served, and general or specific activities performed (domestic, tourism, agricultural); finally, the nature and content of industrial effluents released into sewers may change depending on the activities of these industries and seasonal variations in wastewater treatment (Bolesta et al. 2022). The organic content and load of SS comprise of many components such as humic, fulvic, and lignocellulosic compounds (Zhao et al. 2023), as well as polysaccharides (sugars, cellulose, lignin) (Palmieri et al. 2019; Wiśniowska and Kowalczyk 2022), nitrogen (proteins and amino acids), inorganic nitrogen (NH4+, NO2−, NO3−) (Thant et al. 2021; Xie et al. 2022), organic and inorganic phosphorus (PO43−) (Figueiredo et al. 2021; Han et al. 2019; Huang et al. 2017; Lee et al. 2018; Paltrinieri et al. 2019; Zhang et al. 2022a, b, c, d), fats (lipids and fatty acids) (Owusu-Agyeman et al. 2020; Shao et al. 2022), and metals (Man et al. 2020; Penido et al. 2019; Zhang et al. 2021a, b, c, d), A substantial amount of sewage sludge organic load is also made up of dead or alive microorganisms (bacteria, viruses, protozoa, and others) (Robledo-Mahón et al. 2020). Metals are also found in sewage sludge in generally insoluble forms, such as sulfites, oxides, hydroxides, silicates, phosphate, insoluble carbonates and salts (Zhang et al. 2021a, b, c, d).

Fig. 1
figure 1

Illustration of a conventional wastewater treatment plant and points where phosphorus can be extracted and recycled from municipal wastewater, including efficiencies: sludge liquor [Secondary treated effluent (1), Process water (2), Sewage sludge (Sewage sludge (3), thickened (4), dewatered (5), dewatered (6)); solid phase (sewage sludge ash (7. a, 7. b, 7. c))] (adapted from Montag 2008)

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Table 1 Typical composition of sewage sludge by type and region
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As many treatment facilities reach capacity, the challenge for sewage sludge management is identifying cost-effective, creative, and ecologically responsible solutions. SS as a raw material to create value-added products is a long-term, sustainable development plan for SS management, and this study aims to highlight the present condition of various sewage applications for energy and resource recovery to contribute to sustainable development and industrial ecology based on waste feedstocks. This entails innovating toward greener and more sustainable products derived from SS and addressing the toxicity and persistence issues inherited from sewage sludge. This vision can be achieved by introducing new chemical designs such as new stabilization processes (including liming and digestion), fermentation (Co) for the conversion into Volatile Fatty Acids (VFA), conditioning, concentration into biomass (for further biological or physicochemical treatment), nutrient and phosphorus recovery procedures, and innovations to ensure that chemicals, materials, products, and energy are obtained from sewage sludge to achieve industrial ecology and broader sustainable development goals.

Value addition products from sewage sludge

Phosphorus

Phosphorus is derived from limited and non-renewable phosphate rock mining resources. According to recent projections, mineral reserves would meet world demand for the foreseeable future. Most of these resources, however, are concentrated in a few countries, notably Morocco, China, and the United States, which have 70% of the world's P mineral resource reserves. This results in significant reliance on imports and long-term instability for countries with high P consumption. More research into the potential for phosphorus recovery in municipal wastewater is now required, as natural phosphate from mine extraction has been added to the list of essential raw materials, and the use of fertilizers has increased significantly in recent years to boost agricultural productivity (Xu et al. 2018; Yu et al. 2021a, b). Phosphorus is found in complex forms or is bonded to other metals in wastewater treatment plant streams. Municipal wastewater contains substantial phosphorus from metabolic waste (urine, feces). Phosphates (P-PO4) and orthophosphates comprise 60–85% of total phosphorus (inorganic and organic P)—a typical phosphorus cycle is shown in Fig. 2. Polyphosphates are inorganic phosphorus, while phospho-lipids or polynucleotides are organic phosphorus. Sewage sludge is one of the most significant secondary phosphate sources, and phosphorus recovery is an essential component of the long-term objective of transforming sewage sludge treatment facilities into sources of chemicals and energy (Wang et al. 2022a, b, c, d; Zhang et al. 2019a, b). The average P contents in raw sewage sludge per type and selected countries are shown in Table 2. Table 3 provides an overview of various phosphorus recovery technologies, encompassing their efficacy, costs, technological maturity, and limitations.

Fig. 2
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A typical phosphorus life cycle

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Table 2 Country- and type-specific classifications of the average phosphorus concentration in sewage sludge
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Table 3 An overview of various phosphorus recovery technologies, encompassing their efficacy, costs, technological maturity, and limitations
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There are three kinds of phosphorus (P) recovery technologies from wastewater: direct recovery from wastewater, recovery from sewage sludge (SS), and recovery from sewage sludge ashes (SSA) after incineration. Using crystallization, precipitation, or biological processes such as algae and microorganisms, accomplishes direct wastewater recovery. Wet chemical or thermochemical procedures often recover phosphorus from solid sludge or ash. Figure 1 depicts a typical WWTP with all potential P recovery streams included. P may be soluble in the sedimentation process effluent, biological sludge stream, digested sludge, and centrates stream (also called reject water stream). Organic P is present in activated sludge; part of this P is freed as orthophosphates during anaerobic digestion; hence, streams from anaerobic digestion operations are excellent areas for integrating P recovery systems.

The most prevalent technique for extracting P from wastewater is chemical precipitation using metal salts such as iron or aluminum sulfates or chlorides. Xia et al. (2020) and Zheng et al. (2020) identified magnesium ammonium phosphate hexahydrate (NH4MgPO4H2O), also known as struvite, and a group of calcium orthophosphates (Ca-P), including hydroxyapatite (Ca5(PO4)3OH), as the most feasible compounds for direct crystallization or precipitation from wastewater, respectively. Because they cannot extract P that is chemically and physiologically bonded in sewage sludge, the recovery potential of these methods is approximately 30%.

Wet chemical digestion/bioleaching of sewage sludge

Various mineral and organic acids' potential to extract P from sewage sludge or ash has been investigated, i.e., hydrochloric acid (Pérez et al. 2021; Semerci et al. 2021), sulfuric acid (Boniardi et al. 2021; Phuong et al. 2022), nitric acid (Becker et al. 2019, Fang et al. 2018), and organic acids, including oxalic acid and phosphoric acid. These acids have been investigated as leaching agents for the treatment of SSA with the view of reducing the need to remove excess ions like sulfate, chloride, and nitrate from the leachate solution. Solubilizing P from a solid surface is the underlying principle of P recovery using acidic wet chemical techniques through bioleaching or chemical leaching processes. When pH values are below 2, more than 80% of P from sewage sludge or SSA may dissolve. To produce a P dissolution rate between 66.5 and 99.4% for each kilogram of ISSA, roughly 0.3–0.68 kg of strong acids is needed (Semerci et al. 2019). The P content of SSA ranges between 5 and 11% by dry weight (Boniardi et al. 2021). Since its working principle is relatively simple and does not need costly equipment, wet chemical extraction by acid leaching is a favorable approach and one of the most established ways to recover phosphorus from SSA. In this process, the pH value of sewage sludge or sludge ash is decreased by applying acids, allowing the existing phosphorus compounds (biologically and chemically bound) to dissolve. Sulphuric, hydrochloric, or phosphoric acid, as well as CO2, are utilized for this purpose. The degree of dissolution is determined by the pH and, consequently, by the acids used.

Nevertheless, the degree of P re-dissolution also enhances the re-dissolution of heavy metals in sewage sludge or ash. Particles are separated before the pH of the solution is increased again. The pH is raised (as in crystallization and precipitation processes) by adding lime or caustic soda. The P may then be precipitated or crystallized. Several procedures are utilized to avoid generating high levels of heavy metals in the product, such as complexing agents like citric acid, nanofiltration, solvent extraction, or ion exchangers (Shiba and Ntuli 2017). Phosphorus in the fly ash from ISSA incineration is present primarily as AP, such as CaP2O6 and Ca3(PO4)2, with high bioavailability. The Ca, Cl, and Mg compounds in sewage sludge, such as CaO and KCl, may offer additional reaction sites for the conversion of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP), such as Ca2P2O7, Ca5(PO4)3Cl, Ca4Mg5(PO6), and Mg3(PO2). These processes' by-products, like those from crystallization and precipitation, can be used as fertilizer. Up to 90% of P may be recovered from SSA using the wet-chemical digestion method (Liu et al. 2021a, b), i.e., in a more recent study to dissolve P, digested and non-digested sludge from five wastewater treatment facilities were acidified with H2SO4. 60–100 mmol o-P was liberated per added mol H2SO4 for the five examined sludge types. The maximum P release was seen when the pH was reduced to 2 (all studies included exposing the sludge to acid for 1 h). Since CO2 stripping increases acid consumption, more acid was needed for digest sludge. Even so, digested sludge may release up to 80% more P than non-digested sludge (25%) (Quist-Jensen et al. 2018). Although acid extraction might be a potentially successful method of recovering P, metal co-solution often impedes P recovery, and acid extraction cannot recover P bound to extracellular polymeric substances (EPS). To that end, a targeted clean extraction method based on acidic cation exchange resin (ACER) was recently developed to enhance P release from EPS and eliminate metal interference. The findings demonstrated that a low dose ACER could efficiently extract EPS-bound P and P-precipitates, and the P concentration of the extract surpassed 50% of total phosphorus (TP). In the ACER extract, the TP release efficiency rose by 13%-23% compared to acid extraction, while the dissolved metal level was reduced by more than 90%. This was due to ACER's acidity and metal capture. Eventually, more than 90% of Ortho-P in the extract was recovered as calcium phosphate (Ding et al. 2022). Phosphorus bioleaching from sewage sludge by biogenic sulfuric acid, using Acidithiobacillus thiooxidans, 5.0% (w/v) sulfur supplementation, and 1.0% (w/v) solid–liquid ratio, was also investigated recently. Although bioleaching requires more time than chemical leaching, resulting in a greater yield of P. Bioleaching can be an efficient alternative if the leaching time can be shortened (Lee et al. 2020).

Thermochemical incineration/metallurgical treatment

Thermal treatment by incineration is often employed as a sewage sludge disposal technique in various countries, making ISSA in P recovery particularly enticing (Fang et al. 2021a, b). P in the forms of Ca–P, Al–P, and Fe–P abounds in ISSA, and breaking these metal-P linkages with acid may aid in extracting P from the precursors (Fang et al. 2021a, b). Moreover, in this regard, additives are frequently employed to enhance the bioavailability of P during the thermochemical treatment of sewage sludge. Additionally, CaO additive may assist in stabilizing P in SSA, since it has been shown that incineration of sewage sludge with CaO facilitated the transition of non-apathic inorganic P to AP, and NAIP, like AlPO4, interacted with CaO to form Ca2P2O7 and Ca3(PO4)2 (Zhao et al. 2019a, b). The thermochemical treatment does not eliminate P from the matrix (sewage sludge, SSA) but somewhat changes it into an accessible form from the matrix. High temperatures (ranging from 500 to 1000 °C depending on the method) are used to heat sewage sludge or SSA for this purpose, and organic compounds are degraded consequently and may be removed via the gas stream (Galey et al. 2022; Herzel et al. 2022; Hu et al. 2021). Mineral salts (chlorides, sulfates) are frequently added to remove heavy metals. The heavy metal chlorides generated may then be separated utilizing the gas phase. Phosphorus recovery potentials of up to 90% are theoretically attainable in this process.

It should be noted that in the case of thermochemical phosphate products, important constituents for fertilizer (for example, nitrogen and sulfur) escape during the process (Kwapinski et al. 2021). Recent works have also shown that extracting P from inorganic contaminants such as trace metal elements (TME) from sewage sludge may be accomplished using thermochemical treatment with Cl-donors such as MgCl2, CaCl2, or NaCl. Adding chloride compounds promotes the fixation and production of bioavailable P species in the solid phase, while TME vaporization occurs in the gas phase through chlorination processes (Galey et al. 2022; Xia et al. 2020).

Metallurgical processes, in which sewage sludge or sewage ash is heated to temperatures ranging from approximately 1500 °C to almost 2000 °C with or without previous treatment, may be used as well to process both dry sewage sludge and SSA (Schnell et al. 2020; Sichler et al. 2022; Smol 2019). Consequently, not only are heavy metals removed in the reactor (by introducing chlorides and sulfates to the gas phase), but an iron alloy may also be recovered as a viable product (Timilsina 2022). The P is absorbed into the resulting slag or separated through the gas stream, depending on the technique. Depending on the process, the results are P-containing slag, elemental P, or thermal phosphoric acid. The efficient use of dried sewage sludge as a feed source is a considerable advantage. In theory, metallurgical processes make up to 90% of recovery potentials possible (Schönberg et al. 2018).

Biological processes

In biological P recovery methods, polyphosphate-accumulating organisms (PAO) accumulate P above their usual metabolic needs in biological P recovery techniques (Anders et al. 2023; Chu et al. 2022). This is accomplished by cycling the PAO between anaerobic and aerobic conditions. PAO consumes carbon sources such as volatile fatty acids (VFA) and stores them as polyhydroxyalkanoates (PHA) under anaerobic circumstances (Zhang et al. 2022a, b, c, d). They break down the polyphosphate to create the necessary energy, generating orthophosphate as a by-product. PAO oxidizes PHA under aerobic conditions to produce energy for cell development and orthophosphate uptake (Saoudi et al. 2022). Since the amount of orthophosphate consumed exceeds that produced under anaerobic conditions, P accumulates in the biomass. Several studies, however, have demonstrated that P recovery has a detrimental impact on the stable operation of the Enhanced Biological Phosphorus Removal (EBPR) process owing to the reduction in polyphosphate (poly-P) generated by P recovery and the upgrowth of glycogen accumulating organisms (GAOs) (Zhu et al. 2021). The phosphate kinase (ppk) and phosphate hydrolase (ppx) functional genes are unique to PAOs, and their absence or presence determines whether bacteria remove P. ppx is an enzyme that converts polyphosphate (poly-P) to P. In contrast, ppk is a polyphosphate kinase that converts P to poly-P. (Ni et al. 2022). Since the biological process converts 30–50% of COD, 20–30% of total nitrogen, and 90% of total P into sludge, it has emerged as a cornerstone for WWTP resource recovery and one of the most promising platforms for sewage sludge treatment (Wu et al. 2020). A recent example is the AAO-SBSPR (anaerobic-anoxic-oxygen/sequencing batch side stream P recovery) system developed for P recovery and nutrient removal based on the simultaneous, anoxic/aerobic volume ratio adjustment to increase denitrifying P uptake activity. At an SRT of 35 days, the results indicate that up to 59.8% and 75.2% of P was continually recovered from natural and synthetic wastewater, respectively. P recovery was lowered from 0.059 to 0.033 mg P/mg VSS, resulting in a poorer P recovery efficiency in the system fed with natural wastewater due to its lower influent P load and P concentration in activated sludge. The examination of kinetic and stoichiometric parameters indicated that the activities of polyphosphate-accumulating organisms (PAO) in P recovery systems were maintained under conditions of high P recovery efficiency, and more glycogen was degraded to provide energy for acetate absorption. The AAO-SBSPR system fed with natural wastewater exhibited the highest average TP removal efficiency, at 94% (Zhu et al. 2021). EBPR in a continuous high-rate anaerobic/aerobic system (A-stage EBPR) was also recently investigated. It was discovered that P and COD removal were successful at SRTs of 6, 5, and 4 days while treating actual wastewater, with a further drop to 3 days resulting in biomass washout. An SRT of 4 days provided the best steady-state operating conditions, with high P (94.5%) and COD (96.3%) removal percentages and no detectable nitrification (Zhang et al. 2021a, b, c, d).

Acids, proteins, and enzymes

Several enzymes (e.g., lipases, dehydrogenase, glycosidase, peroxidase, and aminopeptidases) can be recovered from activated sewage sludge because they include vital biochemical groups (carbohydrates, proteins, and lipids) (Raheem et al. 2018). Protein and amino acid content in activated sewage sludge may also be recovered, and the retrieved protein has been demonstrated to be of good quality for animal feed production (Zhu et al. 2022). Thermal hydrolysis (TH) and wet oxidation (WO) of sewage sludge have been shown to improve protein and amino acid recovery (Malhotra and Garg 2021). Alkaline thermal hydrolysis (ATH) to produce liquid fertilizer comprising nitrogen-containing plant-growth-promoting nutrients (N-PGPN) and plant-growth-promoting bio-stimulants (N-PGPB) from sewage sludge (Tang et al. 2022) have been recently investigated. VFA are important anaerobic digestion intermediates with various applications. VFA produced by the acidogenic digestion of sewage sludge are particularly intriguing bio-products that have the potential to help wastewater treatment facilities achieve carbon neutrality. Co-digestion of sewage sludge and organic waste (OW) for VFA production has proved to be helpful in resource recovery and offering sustainable and creative waste management. As a result, the influence of substrate proportions on VFA formation from the co-digestion of primary sewage sludge and different OW has been studied. The production of VFA can be increased over time, and the products can change from acetic acid to caproic acid. These VFAs can then be used in wastewater treatment plants to close the loop by replacing fossil-based carbon sources like methanol in biological nutrient removal. This could make WWTP chemically self-sufficient plants (Liu et al. 2018). Recent studies have also looked into the viability of paper waste (PW)/sewage sludge (SS), sewage sludge (SS)/cheese whey, orange peel waste (OPW)/sewage sludge (SS), primary sludge (PS) and waste activated sludge (WAS)/lipids, and sewage sludge/macroalgae co-fermentation for the production of VFA (Luo et al. 2022; Iglesias-Iglesias et al. 2020; Shao et al. 2022; Yin and Wang 2022a). Overall, co-fermentation increases bioavailable organic matter and functional microorganisms, as well as metabolic activity, i.e., the conversion of ethanol to acetyl-CoA and the circular fatty acid biosynthesis pathway, which improves VFA production efficiency (Shao et al. 2022). Additionally, producing medium chain carboxylates (MCC) from SS is a low-cost and environmentally friendly way of obtaining value-added chemicals. Table 4 provides an overview of acids, proteins, and enzymes obtained from SS, together with their respective production methods, characteristics, and activities.

Table 4 Overview of acids, proteins, and enzymes obtained from sewage sludge, together with their respective methods of production, characteristics, and activities
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Sewage sludge derived biochar/hydrochar/catalysts

Biochar/hydrochar is the solid residue of thermally processed biomass. Biochar has recently been used instead of activated carbon as a low-cost and environmentally friendly adsorbent for heavy metal removal (Inyang et al. 2016), hydrogen synthesis (Bhavani et al. 2022), CO2 capture (Cao et al. 2022), and methylene blue removal (Cao et al. 2022). Biochar-based catalysts have received much interest. For example, the conditioning of SS biochar with MnCl2 and NH4OH activation has dramatically improved their catalytic performance (Mian et al. 2019b). According to Wang et al. (2020), sulfurized biochar generated from sewage sludge (SSB) performed better in peroxymonosulfate (PMS) and peroxydisulfate (PDS) activation for bisphenol A (BPA) elimination. Mian et al. (2020) also showed that nitrogen-doped sludge biochar may be created in a single pyrolysis phase and used as peroxymonosulfate activation (PMS) catalysts. Peng et al. (2020) synthesized a porous carbon with Fe and N produced from sewage sludge that was used as a substrate for Pt nanoparticles in methanol electrooxidation. Yang et al. (2020) used a simple hydrothermal approach to manufacture sewage sludge-activated carbon-based CoFe2O4 (CoFe2O4-SAC) nanocomposites, which were then used to degrade the antibiotic norfloxacin (NOR) by heterogeneously activating peroxymonosulfate (PMS). The carbonization of raw sewage sludge generates several metal phase structures and surface functional groups that catalyze several organic pollutants degrading processes, including Fenton-like reaction, ozonation, H2O2/persulfate activation, and photoreaction (Mian et al. 2019a). Cost-effective catalysts were recently developed via SS pyrolysis by transforming it into hierarchically porous carbon material with oxygen reduction reaction (ORR) catalytic activity to replace precious metal-based catalysts in air–cathode microbial fuel cells for practical wastewater treatment and energy generation. It was also shown that Co-doping Mn and N species on the carbonized sewage sludge matrix improved the ORR catalytic performance in air–cathode microbial fuel cells to levels comparable to the prevalent Pt/C catalyst (MFC). Highest power density of MFC with Mn-N/SC air–cathode was 1120 mW/m2, equivalent to power density of commercially available MFC with air–cathode and Pt/C catalyst (1240 mW/m2) (Huang et al. 2019).

Construction materials

Due to the rising need for concrete in buildings, the annual demand for concrete is anticipated to exceed 18 billion by 2050 (Liu et al. 2020a, b, c). The use of construction materials accounts for 10% of global CO2 emissions, with cement accounting for 85% of these emissions (Shanmugapriya 2022). Though recent studies indicate that adding sludge to concrete may diminish its mechanical strength, one potential use would be making low-strength mass concrete, which might be used as a filler for roads with moderate traffic (Vilakazi et al. 2023).

Additionally, SSA can be used in building materials after removing phosphorus. Recent studies have also shown that SSA may be used as a supplementary cementitious material in mortar or concrete to replace cement partially. However, the source of the sewage sludge used and the method of manufacture significantly affect the fundamental properties of SSA, resulting in various and often conflicting impacts on the mortar's mechanical quality and durability (Liang et al. 2022). Additional research has also demonstrated that municipal sewage sludge (MSS) may be used as thermal insulation bricks due to the porous structure formed by the evaporation of large quantities of organics at high temperatures (Yang et al. 2021).

Biopolymers, microplastics, cellulose/other materials

Wastewater microplastics (MP) are synthetic polymers with dimensions smaller than 5 mm from various sources, including clothing synthetic fibers, the polymer manufacturing and processing sectors, and personal care items. They can absorb chronic organic pollutants and essential metals from their surroundings. These compounds, which might be created due to biota digestion or environmental degradation, can endanger human health and ecosystems (Rolsky et al. 2020). Wastewater treatment facilities (WWTP) absorb MP from industry, landfill, residential wastewater, and rainwater (Mahon et al. 2019), and it has been demonstrated that sewage sludge is a primary environmental source of MP pollution (Li et al. 2018). Extraction protocols for MP in sludge vary and frequently involve a combination of techniques, such as mixing the sludge with a high-density solvent, such as sodium chloride or zinc chloride, for buoyancy separation, followed by capture via sieves or vacuum filtration (Rolsky et al. 2020). Currently, the only methods for removing MP and trace contaminants are either prohibitively expensive or ineffective (flocculation and precipitation) or necessitate the addition of chemical reagents (depth filtration, microfiltration, and ultrafiltration). The presence of extracellular polymeric molecules in sewage sludge will likely hamper MP removal. Cellulose is the primary ingredient of total solids in MSS, and it may be recovered using chemical or biological processes (Rhein et al. 2022). Crystalline cellulose has a variety of applications in biology, pharmacology, mechanical engineering, and other domains. Rotating belt filters were evaluated in a recent study to maximize cellulose recovery from sewage sludge for future valorization in building applications. The value of recovered cellulosic material reached 26.6 gm3 at a maximum solid removal of 74%. Table 5 provides an overview of value-added compounds derived from sewage sludge, including their application, performances and attributes.

Table 5 Overview of value-added compounds derived from sewage sludge, including their application, performances and attributes
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Energy from sewage sludge

Biodiesel, bio-oil, syngas

MSS has been investigated as a potential feedstock for bio-oil production, as shown in Table 6. Bio-oil can be produced from sewage sludge using rapid pyrolysis with operating characteristics distinct from slow pyrolysis, including a higher heating rate and a shorter residence time at temperatures ranging from 400 to 600 °C. Hydrocarbons, high-molecular-weight organic acids and carbonyl compounds, phenols, ketones, aromatic and aliphatic compounds, alcohols, acetic acid, nitrogenous chemicals, sulfur compounds, and water are the components found in bio-oil generated by sewage sludge pyrolysis (Gao et al. 2020). Sewage sludge produces bio-oil with a high molecular weight, making the oil sticky and unstable (Nazari et al. 2017). This is most likely due to improved polymeric processes, which result in higher concentrations of sewage sludge solids and bio-oil with a greater molecular weight. Recent research by Arazo et al. (2021) indicates that pyrolysis at 260 °C and 4 MPa in ethanol and a hybrid Ni/HZSM-5 catalyst may improve sewage sludge bio-oil. Catalytic upgrading may lower the relative quantities of acids, aldehydes, phenols, ketones, and nitrogenated chemicals in pyrolytic bio-oil.

Table 6 Summary of energy products and vectors obtained from sewage sludge, including their production methods, qualities and performances
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Dry sewage sludge transesterification without catalysis has also been studied to produce biodiesel. Results indicate that 380 °C is the ideal temperature for non-catalytic transesterification of sewage sludge to biodiesel with simultaneous removal of emerging pollutants such as microplastics and antimicrobials (Jung et al. 2022). Abdulhussein Alsaedi et al. (2022) investigated the extraction of lipid feedstock from sewage sludge for biodiesel production.

Gasification of activated sewage sludge in near-critical and supercritical water using different catalysts such as Ru/C, Raney-iron, Raney-cobalt, and others for syngas production has been recently investigated (Schmid et al. 2021). The results indicate that it is feasible to enhance syngas production by up to 11%, with heating values ranging from 4.84 to 5.11 MJ/Nm3 and cold gas efficiency (CGE) ranging from 33.91 to 36.15%. In 2020, Chen et al. used sorption-enhanced steam gasification to treat sewage sludge and make hydrogen-rich syngas in a fixed bed system. The primary goal was to determine this technique's best circumstances and sorbents. The sorbent was CaO, and the sol–gel process made numerous CaO based sorbents.

Biogas/biohydrogen

Sewage sludge has also been identified as a promising resource for hydrogen and biogas production using anaerobic digested sludge (Liu et al. 2019; Wang et al. 2022a, b, c, d; Xu et al. 2022a, b), even though exogenous chemicals in sewage sludge, including flocculants, heavy metals and antibiotics, may regrettably be damaging to anaerobic digested sludge systems. (Wang et al. 2022a, b, c, d). Because of its ability to meet the principles of waste reduction, resource recovery, and purification, thermal treatment has been recognized as one of the most promising methods for H2 generation from sewage sludge, and alkaline thermal treatment of sewage sludge led to higher H2 production and in-situ carbon capture (Zhao et al. 2019a, b). Recently, Liu et al. (2021a, b) used conventional and microwave pyrolysis at temperatures ranging from 600 to 900 °C to evaluate the effect of pyrolysis temperature on biogas production from sewage sludge. Conventional pyrolysis yields varied significantly as temperature increased, while microwave pyrolysis yields changed very little. Since microwave pyrolysis produced more aromatic chemicals in the tar, it was discovered that microwaves accelerated the hydrogen transfer process at higher temperatures. It was shown that sludge microwave pyrolysis is not an excellent way to make hydrogen-rich biogas because it can cause many side reactions that stop hydrogen production, especially the hydrogen transfer reaction. Combining and using SS and lignocellulosic biomass for energy may be a fundamental way to reduce pollution and resource use. Wang et al. (2022a, b, c, d) studied how to make solid char fuel (hydrochar) by co-hydrothermal carbonizing SS and pinewood sawdust (PS). By integrating hydrothermal carbonization and anaerobic digestion, more than 60% of the energy in SS and PS could be recovered. Gong et al. (2022) looked into hydrothermal pretreatment and supercritical water gasification to find a new way to treat dewatered sludge for hydrogen production.

Ferrentino et al. (2020) studied how hydrothermal carbonization (HTC) and anaerobic digestion (AD) affect each other. Specifically, they looked at how recycling HTC liquor and hydrochar from digested sludge into the anaerobic digester increased biomethane production. Thermal hydrolysis pretreatment (THP) is another extensively used method for increasing AD. HAs are insoluble organics often found in sludge AD but seem to be significantly liberated during THP. Zhao et al. (2023) investigated the role of released HAs in methanogenesis. The findings showed that HA derived from thermally hydrolyzed sludge increased methanogenesis. Despite this, the effectiveness of HA extracted at different temperatures to promote methane synthesis varied greatly. It needed to be made clear how the ability of HA to increase methanogenesis was connected to their molecular weight, electrical activity, or the make-up of their electroactive groups.

Supercapacitor synthesis

Supercapacitors have received much interest because of their high-power density and extended cycle stability. Unfortunately, they have a few significant restrictions that limit their use. The cost of the electrode material is the most essential consideration. Porous carbons such as graphene, carbon nanotubes, activated carbon, and carbon nanofibers are being studied as electrode materials. It is commonly assumed that nitrogen-doped carbon-based materials may effectively modify the electrical structure of carbon-based materials, hence changing the surface's acid–base characteristics.

Additionally, the presence of nitrogen atoms allows for more electrons at nitrogen sites, enhancing electron transport and improving specific capacitance. The porous carbon structure and nitrogen doping work together to increase the electrochemical performance of supercapacitors. The nitrogen-containing functional groups make the material more hydrophilic and raise its Faraday pseudo-capacity (Lu et al. 2022). Supercapacitors are considered one of the most promising energy storage and supply technologies due to their extended life cycle, ultrahigh power density, and fast charge/discharge process (within seconds). Based on their energy storage technique, supercapacitors are divided into pseudo-capacitors and electrochemical double-layer capacitors (EDLC). Although they have a higher specific capacitance, pseudo-capacitors have limited practical applications owing to their weak rate capability and cycle stability. Because of electrostatic ionic charge build-up at the electrode/electrolyte double-layer interfaces, EDLC often shows outstanding charge/discharge cycle stability. Carbon-based porous materials have been identified as potential EDLC candidates, although their poor energy density may restrict their practical use. For EDLC electrodes, heteroatom-doped carbon materials with high surface areas and correct pore size distribution are recommended. Lin and colleagues improved the wettability and electroactivity of carbon materials by creating defects on the side surfaces of few-layered graphene (Zhang et al. 2018). Porous carbon derived from activated sewage sludge char has shown promise as a supercapacitor electrode material. When employing typical activation agents, such as KOH, the electrochemical performance is hampered by the high-ash concentration of the sludge and the absence of mesopores. Novel activation techniques have recently been applied to evaluate the production of hierarchical porous carbon from high-ash sewage sludge pyrochar. Tan et al. (2019) recently investigated the pyrolysis of sewage sludge to Cu-doped porous carbon for use in energy storage systems. The form, content, and pore structure of the resultant Cu-doped porous carbon may be readily adjusted by changing the flocculation capacity of Cu (II). Table 7 examines the cost estimates analysis of different sewage sludge treatment systems to determine their economic feasibility, an essential consideration for their future advancement and practical implementation.

Table 7 Overview of cost-effectiveness analyses performed on different sewage sludge treatment methods
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Conclusions

Communities worldwide must adapt and update their strategies for managing water resources due to the constraints imposed by climate change. Improving water quantity and quality while reducing energy consumption and carbon dioxide emissions related to public water and sanitation services is imperative. Municipal wastewater treatment plants are anticipated to broaden their scope beyond the control of wastewater contamination. In addition, they will be able to generate drinkable water, sustainable energy, recyclable plastics, bio-based polymers, essential minerals, and eco-friendly chemical compounds. These compounds have energy potential and can be used as raw materials to mitigate future shortages of raw materials and energy. This will allow the WWTP to comply with environmental regulations while making revenue by reusing on-site energy or selling chemicals, materials, or energy. Moreover, it will be pivotal in adjusting territories and water supplies to present and projected ecological shifts.