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Review

Future Directions of Sustainable Resource Utilization of Residual Sewage Sludge: A Review

by
Weicheng Zheng
1,
Yuchao Shao
2,*,
Shulin Qin
1 and
Zhongquan Wang
1
1
Hangzhou Research Institute of China Coal Technology & Engineering Group, Hangzhou 311201, China
2
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(16), 6710; https://doi.org/10.3390/su16166710
Submission received: 4 July 2024 / Revised: 29 July 2024 / Accepted: 1 August 2024 / Published: 6 August 2024
(This article belongs to the Topic Biomass Transformation: Sustainable Development)
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Abstract

:
With the simultaneous increase in wastewater generation and wastewater treatment rates in China, the annual production of residual sewage sludge (RSS) has been steadily rising, exceeding 70 million tons with an 80% moisture content. The sustainable resource utilization of RSS will be the predominant disposal method instead of sanitary landfilling in China. This review aimed to systematically analyze the major sustainable resource utilization technologies for RSS. Firstly, the basic characteristics of RSS in China were analyzed. A comparative analysis was conducted to assess the advantages, disadvantages, and applicability of three primary sustainable resource utilization technologies for RSS: building materials, energy utilization, and phosphorus recovery, aiming to provide clear insights for the development of future strategies. The research findings revealed that no single method can economically and environmentally address all challenges in the utilization of RSS resources. It is necessary to adopt appropriate resource utilization technologies according to the characteristics of RSS from wastewater treatment, achieving integration of pollution control and resource utilization. This review can provide scientific guidance for future sustainable utilization of RSS resources.

1. Introduction

As the global issue of energy resource shortages becomes increasingly severe, the resource utilization of waste has become an urgent imperative [1,2,3,4]. Residual sewage sludge (RSS) is generated during the biological treatment of wastewater. It consists of a complex and heterogeneous substance primarily composed of water, organic (e.g., pathogenic bacteria, parasites) constituents, and inorganic (e.g., inorganic particles, heavy metals, salts) constituents, with extracellular polymeric substance as its framework [5,6,7]. The main elements in RSS are C, N, and P (nutrients), but these can act as pollutants if released into the environment [8]. This is problematic as there are already other pollutants in the RSS (i.e., pathogenic bacteria, heavy metals). Therefore, RSS is both a pollutant and a potential resource. However, the resources within RSS cannot be directly utilized and require specific technological means to achieve resource utilization. As a result, sludge resource utilization technologies have garnered the attention of many researchers and continue to evolve [9].
Currently, the activated sludge process and its variations are the most widely used technologies in the field of wastewater treatment [10,11]. The activated sludge process transfers pollutants from wastewater into RSS, inevitably leading to the generation of substantial amounts of RSS. In 2020, it was estimated that the wastewater treatment capacity of China’s sewage treatment plants reached 190 million cubic meters per day; the production of wet sludge with an 80% moisture content in cities was approximately 66.64 million tons (data were collected from the Ministry of Housing and Urban-Rural Development, China). It is projected that the wet RSS production will reach 90 million tons by 2025. In recent years, with increased environmental inspections in China, the construction of water pollution prevention and control facilities has entered a rapid development phase. Currently, the treatment rate for municipal sewage and industrial wastewater has reached as high as 97.89% reported by the Ministry of Housing and Urban-Rural Development, China. However, the disposal of RSS has not received adequate attention in various regions. This situation has resulted in a lag in the construction of sludge disposal facilities compared to wastewater treatment facilities. The most widely used sludge disposal method in China is sanitary landfilling [12,13,14], which not only consumes a significant amount of public resources but also wastes the valuable resources contained within the sludge. The resource utilization of RSS in China has significant potential for development.
This article begins by describing the basic characteristics of RSS from wastewater treatment plants in China and systematically elaborates on the research progress of major resource utilization technologies for RSS (i.e., building materials, energy utilization, and phosphorus recovery). It analyzes the advantages and disadvantages of each technology, aiming to provide a reference for further research on sludge sustainable resource utilization and disposal.

2. Chemical Properties of Residual Sewage Sludge

Residual sewage sludge (RSS) originates from urban sewage treatment plants and industrial wastewater treatment plants. The chemical components of RSS from both sources are essentially the same, but the contents of each compound are different. For instance, industrial RSS usually contains more recalcitrant organic compounds, heavy metals, oils, and other specific pollutants present in industrial wastewater. A research group conducted investigations on the chemical compounds of RSS produced by 90 sewage treatment plants from 25 cities in China, and it was discovered that there were significant differences in the composition of different types of RSS [15]. The representative indicators of RSS, including nutritional content (nitrogen, phosphorus, and potassium) and heavy metal concentrations, are shown in Table 1 (data are sourced from reference [15]). The maximum and minimum values of these indicators were significantly different. For example, the concentration of Cr in some RSS could reach as high as 2960 mg/kg, while the minimum value was only 9.6 mg/kg; the organic matter in RSS was in the range of 13.4% to 51.4%. The high heavy metal contents in RSS are caused by the discharge of heavy metals from specific industrial activities such as metallurgy, chemical manufacturing, and electronics production. Additionally, the organic matter in industrially-derived RSS is typically lower than in domestically-derived RSS.
The chemical composition of RSS is different from sludge obtained in different operating stages of municipal wastewater plants. It can be observed that the total solid content in RSS was low, while the volatile solid and protein content was relatively high, compared to primary sludge and digested sludge (Table 2; data are sourced from reference [15,16]). By consolidating the statistical findings presented in Table 1, it becomes evident that the organic matter content in RSS generated across various regions or from diverse wastewater sources consistently exceeds 50%, with nutrient elements such as nitrogen and phosphorus consistently surpassing 2%. This underscores a notable degree of resource potential within RSS. However, RSS also contains significant heavy metal components that cannot be ignored. The concentration of heavy metals in RSS is closely related to the composition characteristics of the treated wastewater. Therefore, different resource utilization methods should be adopted for RSS having different characteristics.

3. Resource Utilization Technologies for Residual Sewage Sludge Disposal

The composition of RSS can be divided into inorganic matter (e.g., calcium, silicon, aluminum, and iron) and organic matter (e.g., carbon, nitrogen, and phosphorus). Different components in RSS can be targeted for resource utilization using various technologies, including sludge building material utilization for recovering inorganic matter, sludge energy utilization for recovering organic matter, sludge phosphorus recovery for reclaiming phosphorus resources, and sludge carbonization for recovering carbon elements (Figure 1).

3.1. Residual Sewage Sludge for Building Material Utilization

RSS contains elements such as calcium, silicon, aluminum, and iron, which are similar to the components of building materials. Therefore, RSS has the potential for utilization in building materials. The Chinese government has been promoting policies and regulations to encourage the sustainable utilization of resources and the development of circular economy practices, providing a supportive regulatory environment for the application of RSS in construction. Currently, there are three main directions for the utilization of RSS in building materials: brick production [17], cement production [18], and ceramic granule production [19] (Figure 2).

3.1.1. Sludge Brick-Making

Brick factories typically use clay as the primary raw material for brick production. The major components of clay are SiO2, Al2O3, and small amounts of calcium, magnesium, iron, carbonate minerals, etc. [20]. These substances, at sintering temperatures of around 1000 °C, react with aluminum, calcium, magnesium, iron, sodium, and silicon to form new silicate materials. Easily fusible materials create a liquid phase that binds the remaining solid solution (eutexia), solid solution, and new minerals during cooling, forming stable crystals. The inorganic composition of RSS is similar to that of clay. The SiO2, Al2O3, and CaO contents in sewage sludge and sludge incineration ash are shown in Figure 3 [21]. In 1987, scholars used dried urban RSS mixed with clay to produce qualified sintered bricks [22]. These researchers were pioneers in RSS brick-making technology, and their early work primarily focused on exploring the feasibility of the technology. In 2003, researchers conducted a systematic study of RSS brick-making [23]. The results indicated that key factors affecting brick quality were the sludge-to-clay ratio and firing temperature. Increasing RSS content reduced the brick’s shrinkage rate, water absorption rate, and compressive strength. They also conducted tests on the leaching of toxins from the RSS, revealing low levels of metal leaching.
Another method of making bricks from RSS is by using RSS ash produced after incineration. In developed countries such as Europe, the United States, and Japan, incineration is a common method for RSS disposal, and the main components of RSS ash are SiO2 and Al2O3, which are similar to the components of clay [21]. To utilize the abundant ash resources generated from RSS incineration, RSS ash is used in brick-making. A research group found that adding RSS ash to clay can lead to a decrease in the compressive strength of bricks, but it increases the water absorption rate [22]. When 10% of RSS ash was added, the compressive strength of bricks only decreased by 1.72%, and the water absorption rate increased from 0.03% to 0.07%. When the addition amount was increased to 50%, the compressive strength decreased by 20.4%, and the water absorption rate increased to 1.7% [22]. Another study attempted to make bricks entirely from RSS ash [24]. This required controlling the particle size of the RSS ash to below 20 μm, pressing it using a high pressure of 98 Mpa, and firing it at temperatures above 1000 °C. However, due to its good water absorption and high calcium content, the bricks tended to develop moss on the surface and turn white due to leaching of calcium carbonate [24]. These issues could only be addressed by increasing the firing temperature and applying chemical coatings, which resulted in no economic advantage compared to regular bricks. Some researchers have also studied the production of unfired bricks using RSS. For example, a group used construction waste, RSS, and fly ash as raw materials, with quality proportions of 60%, 2%, and 15%, respectively, along with 5% desulfurized gypsum as an activator [25]. After the shaping, curing, and aging processes, unfired bricks were produced having a compressive strength of 13.75 Mpa. Although this method has the limitation of the RSS disposal capacity, if the bricks were successful, then scaling production would still assist in overall RSS utilization. One method is not going to solve RSS utilization, but a combination of methods that are proven to work will.

3.1.2. Cement Produced from Sludge

Cement is one of the most important materials in urban construction. The global cement production in 2022 was approximately 4.31 billion tons, with China’s capacity reaching 2.36 billion tons, accounting for a staggering 55% share (data were collect from the National Bureau of Statistics, Beijing, China). Cement kilns used in cement production are characterized by ultra-high temperatures (>1200 °C), large capacity, and prolonged residence times for flue gases [26]. In the early 1990s, developed countries in Europe and North America proposed a technical route for the co-disposal of waste in cement kilns. This involves the environmentally friendly and resourceful treatment of waste materials that meet the combustion conditions in cement kilns. Lime production primarily requires raw materials rich in calcium and aluminum, such as limestone and clay [27]. RSS can provide SiO2, Al2O3, and CaO materials as well as fuel for cement production. Therefore, using RSS to produce cement not only achieves complete resource utilization of the RSS but also addresses the increasing volume of RSS by incorporating it into the vast cement industry market.
It is critical to control the sludge content in the sludge-infused cement preparation process. A research group studied the impact of substituting different levels of RSS ash for cement on the heat of hydration and the 7-day compressive strength of cement mortar samples [28]. The results showed that RSS ash in cement affected the hydration process, reducing the rate of heat generation during hydration, particularly in the early setting period. The maximum reduction in the 7-day compressive strength of cement mortar samples was 20% when the RSS ash content was increased. In the process of using RSS to produce cement, the migration of heavy metals in RSS has also received considerable attention. Heavy metals in RSS can be divided into two categories: easily volatile or semi-volatile heavy metals like mercury, lead, and cadmium, which can be trapped by the waste gas treatment system or released into the atmosphere along with flue gases, and non-volatile heavy metals like chromium and nickel, which can enter the solid phase. Researchers studied the physicochemical interactions between textile wastewater sludge and portland cement and analyzed changes in the leachability of metals [29]. It was found that the incorporation of textile wastewater sludge into cement mortar resulted in the formation of more weak interfaces and increased porosity. However, leaching tests showed that all toxic metals were stabilized, and the leaching rate of metals from portland cement-textile wastewater sludge mortar was very low, posing little to no environmental risk [29]. Another group finely ground powder from heavy metal-containing sludge incineration and added it to portland cement [23]. The concentrations of leachable heavy metals in cementitious materials with different addition levels were lower than the limits specified in Chinese standard GB 30760—2014 (the technical specifications for the co-disposal of solid waste in cement kilns) [30]. They also investigated the leaching of heavy metals in cement specimens with different water-to-cement ratios and ages. The leaching rate of chromium in specimens with a water-to-cement ratio of 0.5 and 40% addition of sludge ash powder reached 34.2%, and the leaching concentration of all heavy metals was 0.11 mg/L, indicating a potential risk of exceeding the limits [31]. In conclusion, using RSS for cement production is a technically feasible, environmentally friendly, and economically viable direction for RSS recycling. However, careful consideration is needed when selecting this technology to treat RSS from specific industries with higher heavy metal content.

3.1.3. Sludge Ceramsite Production

Ceramsite is an artificial lightweight aggregate made from various inorganic minerals such as clay, shale, slate, coal gangue, and fly ash. It is produced through molding and calcination, possessing advantages like porosity, lightweightness, high water absorption, fire resistance, freezing resistance, and corrosion resistance [32]. Ceramsite finds wide applications in construction materials, cultivation substrates, water treatment filters, land reclamation, and many other fields [32]. The traditional raw materials for ceramsite production, such as fly ash and clay, mainly consist of SiO2 and Al2O3, which are also the major inorganic components of sludge [21]. Therefore, utilizing sludge for ceramsite production is feasible in terms of material composition. The preparation process of sludge ceramsite is shown in Figure 4. The sludge is dewatered, dried, molded, and calcined to produce sludge granules [33]. Studies have shown that ceramsite exhibits better compressive strength and more densified surfaces when the mass fraction of SiO2 is between 30% and 45% [34]. However, the mass fraction of SiO2 in urban sludge in China is significantly lower than this range [35]. This leads to lower strength and increased fragility of ceramsite produced solely from sludge. Therefore, when using sludge for ceramsite production, it is often necessary to add other raw materials to optimize the material composition and obtain high-quality qualified ceramsite [36]. Clay and fly ash are the most commonly used auxiliary materials when using sludge for ceramsite production. Their addition can promote ceramsite formation, increase strength, and improve overall ceramsite quality. This is because the calcium in these additives can encourage the formation of more calcium feldspar during the firing process. Additionally, the higher content of Al2O3 can facilitate ceramsite bonding, reduce surface cracks in ceramsite, and lower its apparent density. Silicon, aluminum, and iron in these materials can serve as the framework for ceramsite [34,37,38]. In the preparation process of ceramsite, the most significant factors affecting ceramsite production are the calcination (preheating and sintering) processes. If a relatively long preheating at a low temperature is carried out, it allows for the sufficient decomposition of organic matter in the sludge, which is beneficial for the bloating process of ceramsite, thereby reducing the apparent density of the ceramsite. The sintering temperature is crucial in ceramsite production. When sintered at a low temperature (below 1000 °C), the resulting ceramsite has more surface cracks and higher apparent density. However, when sintered at a high temperature (above 1200 °C), a larger amount of liquid phase is generated inside the ceramsite, with lower viscosity and higher fluidity. The enclosed gas is prone to escape, and the internal pores of the ceramsite are filled by the liquid phase, leading to an increase in apparent density [39,40,41].
In conclusion, utilizing RSS along with certain other inorganic materials to produce ceramsite is promising for RSS utilization. It not only reduces sludge volume but also substitutes for some of the raw materials and fuel used in ceramsite production, achieving the resource utilization of sludge. Moreover, the leaching of heavy metals such as copper, lead, and cadmium in ceramsite is much lower than national standards [42]. In recent years, research on sludge ceramsite has focused more on ceramsite modification. The main objectives of modification are to increase specific surface area, reaction sites, and adsorption capacity. This is often achieved through methods such as metal compound modification, magnetic modification, charge modification, and alkali modification, to obtain ceramsite with enhanced application value [43,44,45].

3.2. Residual Sewage Sludge Utilization for Energy

The solid phase of RSS contains a very high proportion of organic components, which has the potential for energy utilization. However, RSS contains a large amount of moisture, making it difficult to be directly utilized for energy. In order to develop more green energy, China continues to explore new technologies and methods to provide more options and possibilities for RSS resource utilization and has already made significant progress in RSS energy utilization research. Various RSS energy utilization technologies have emerged, including common methods such as anaerobic fermentation [46,47] and co-incineration of RSS [48,49].

3.2.1. Sludge Anaerobic Fermentation

Anaerobic fermentation of RSS is the process of utilizing anaerobic microorganisms in sludge to hydrolyze and convert organic substances such as proteins and polysaccharides into small water-soluble organic molecules [50]. These molecules are then further converted into methane, carbon dioxide, and other products by specific anaerobic microorganisms. This process not only reduces the volume of sludge but also recovers methane, achieving RSS reduction and resource utilization. The biochemical reactions involved in the fermentation process are illustrated in Figure 5 [51]. The anaerobic fermentation of RSS is typically divided into three stages: hydrolysis, acidogenesis, and methanogenesis. Among these stages, hydrolysis is considered the limiting factor for the overall fermentation rate because the rate of hydrolysis of extracellular polymeric substances and cell membranes is relatively slow. Therefore, pretreatment of RSS before anaerobic fermentation is often conducted to enhance the release and degradation of organic matter in sludge, thereby expediting the fermentation process. Common pretreatment techniques include hydrothermal treatment and acid/alkali treatment [52,53,54,55,56]. Hydrothermal treatment initially aimed to improve the dewaterability of sludge, but it was later found that this process increased both the rate of anaerobic fermentation and methanogenesis. A previous study has indicated the optimal temperature for sludge hydrothermal treatment falls within the range of 140 to 160 °C [57]. Temperatures higher than this range can significantly increase the production of recalcitrant compounds, leading to the inhibition of the fermentation process. Another study investigated the influence of low-temperature hydrothermal treatment on anaerobic digestion of RSS [58]. The researchers performed hydrothermal treatment of RSS at 180 °C for 30 min, resulting in methane production increasing from 142.7 mL CH4/g-VS to 343.1 mL CH4/g-VS compared to the control group. Because of its remarkable enhancement in biogas production rates, the “hydrothermal pretreatment + anaerobic fermentation” process has gained widespread application [59].
The main influencing factors in the anaerobic fermentation process of RSS include the C/N ratio, pH, temperature, and sludge retention time. Depending on the fermentation temperature, fermentation is typically categorized as low-temperature fermentation (4~20 °C), mesophilic fermentation (20~45 °C), or thermophilic fermentation (45~60 °C) [60]. Temperature affects the activity of different microorganisms, resulting in variations in fermentation products. Researchers demonstrated that when the anaerobic fermentation temperature was increased from 35 °C to 55 °C, the proportion of acetic acid in volatile fatty acids in the fermentation broth decreased from 85.6% to 65.8%, while the proportion of propionic acid increased from approximately 0% to 15.5% [61]. Similar to the mechanism of temperature influence, changes in pH also affect the dominant microbial community during fermentation [62]. The optimal pH for anaerobic digestion is typically in the range of 6.8 to 7.4, but organic acids are generated during fermentation. Therefore, anaerobic sludge digestion systems are usually operated to maintain a pH around 8. The initial C/N ratio of sludge is generally around 12, while the optimal C/N ratio for anaerobic fermentation of sludge falls between 20 and 30 [63]. A C/N ratio that is too low can result in excessive ammonia production, inhibiting fermentation, while a too-high C/N ratio can lead to a decrease in the nitrogen content of the sludge system, affecting microbial growth [63]. Therefore, in practical applications, organic matter such as kitchen waste [64] or other organic materials [65] is often added for co-fermentation to adjust to the appropriate C/N ratio and enhance biogas production.
In summary, anaerobic fermentation of RSS for biogas production is a promising method for RSS utilization. The key challenge in the application of this technology lies in controlling fermentation parameters to achieve efficient biogas production. Additionally, attention should be paid to substances in the sludge such as ammonia, sulfides, metal ions, hydrogen, and fatty acids [66], as they can have inhibitory effects on the anaerobic fermentation process.

3.2.2. Sludge Co-Incineration

Burning RSS and coal together, at temperatures exceeding a thousand degrees Celsius, can completely oxidize and decompose the organic matter present in the RSS [67]. Additionally, burning the organic matter in the RSS can generate heat, making it an efficient and convenient method for the harmless and resourceful disposal of RSS [67].
When co-burning RSS and coal, there are specific requirements regarding the moisture content of the RSS. The moisture content significantly impacts the heating value of the RSS and can have adverse effects on boiler combustion efficiency and stability if it is too high. High moisture content can also lead to reduced system temperatures and blockages in the feeding system. Even after mechanical dewatering, the moisture content of RSS can still be around 80%. Further reducing the moisture content through techniques like sludge drying consumes a considerable amount of energy and can affect boiler flue gas emissions. Therefore, many researchers have investigated the impact of different moisture contents of RSS and coal blends on boilers [68,69,70]. Studies have examined the influence of RSS blending ratios and RSS moisture content on 100 MW coal-fired power plant boilers [68]. They found that as the blending ratio increased, the heat flux on the furnace wall in the primary combustion zone significantly decreased. Beyond a 10% blending rate, it had a noticeable impact on the combustion stability within the furnace. When 20% RSS was co-burned, the average heat flux decreased by approximately 17%, as shown in Figure 6. A group researched the combustion of coal powder and RSS in a tangentially fired coal-fired boiler [69]. They found that RSS with a moisture content of 56%, when co-burned at a blending rate exceeding 20%, significantly affected fuel characteristics, reducing ignition performance and increasing nitrogen oxide emissions. Co-burning RSS also has a significant impact on excessive flue gas emissions. Some researchers discovered that when the RSS mixing ratio was lower than 20%, the emission of gaseous pollutants such as SO2, NOx, and dioxin increased linearly with the sludge mixing ratio [70]. However, it could also meet the national standard for the pollution control of municipal solid waste incineration in China. Another group studied the combustion characteristics of brown coal mixed with RSS in a circulating fluidized bed and found that when brown coal was co-burned with RSS, CO and hydrocarbon emissions were lower compared to burning brown coal alone; SO2 emissions decreased with an increase in the RSS percentage in the mixture, while there was a slight increase in NOx emissions [71]. Researchers also conducted co-combustion of coal and RSS in a circulating fluidized bed boiler [72]. The results indicated that during the co-burning process, the CO volume fraction in the flue gas increased with an increase in the sludge co-burning ratio. In contrast, the NO volume fraction significantly decreased, and there was a slight increase in the SO2 volume fraction. When the RSS co-burning ratio increased from 0% to 100%, the carbon content in the fly ash increased from 8.09% to 28.26%, and the combustion efficiency decreased from 99.23% to 87.76%. Compared to coal combustion alone, co-burning RSS reduced the fly ash melting temperature [72]. The different research studies mentioned above show that co-burning RSS has varying effects on the emissions of nitrogen oxides and sulfur compounds in exhaust gases, depending on the characteristics and proportion of the sludge used.
Different types of boilers have varying fuel requirements. In China, coal powder boilers, which have high fuel quality requirements, are the most widely used in power plants. Therefore, the practical application of co-burning RSS is limited, and RSS needs to be dried before being mixed and co-burned with coal powder. In contrast, circulating fluidized bed boilers have a significant advantage in fuel adaptability. They are commonly used for co-burning RSS because they can handle sludge with relatively high moisture content. Additionally, their furnace temperature is lower, allowing for better control of the formation of nitrogen oxides and dioxins and other pollutants [73]. Therefore, circulating fluidized bed boilers are more suitable for co-burning sludge. In summary, co-burning RSS effectively addresses the challenge of degrading toxic and harmful substances in RSS while fully utilizing the organic matter’s heat value. It achieves both the harmless disposal and resource utilization of RSS, making it a relatively straightforward and feasible technology for RSS utilization. However, co-burning RSS requires the selection of circulating fluidized bed boilers and imposes higher demands on flue gas treatment.

3.2.3. Residual Sewage Sludge Carbonization

RSS carbonization mainly involves two pathways: pyrolytic carbonization to form biochar and hydrothermal carbonization to produce hydrochar [74,75]. Pyrolytic carbonization refers to the process in which RSS is heated in an oxygen-free environment, causing organic materials to undergo decomposition, resulting in residues (biochar), bio-oil, and a mixture of gases. On the other hand, RSS hydrothermal carbonization refers to a series of reactions that occur when RSS is subjected to water, a temperature of approximately 150 °C to 300 °C, and saturated vapor pressure. These reactions involve hydrolysis, decarboxylation, condensation, and aromatization processes.
RSS pyrolytic carbonization originated in developed countries such as Japan, the United States, and Europe in the 1980s [76]. After several decades of technological iterations, it has become mature. China introduced this technology from Japan in 2005. However, at that time, there was a lack of awareness regarding RSS treatment and disposal, and the imported equipment was expensive, making it challenging to promote its application. During the sludge pyrolytic carbonization process, the decomposition of organic materials generates combustible gases, and the heat produced during combustion can be used to dry the RSS. Thus, the entire carbonization process requires only a small amount of additional energy to maintain the reaction. Carbonization is typically classified into low-, medium-, and high-temperature carbonization, and their technical differences and characteristics are outlined in Table 3. The advantages of RSS pyrolytic carbonization technology include lower energy consumption compared to traditional drying and incineration methods, pollution-free combustion of pyrolysis gas, solidification of heavy metals with no leaching risk, and high utilization and value of the carbonized products [76,77].
RSS hydrothermal carbonization is a novel approach for RSS resource utilization. It carbonizes sludge in water, eliminating the need for dewatering pretreatment, which significantly reduces the cost of RSS preprocessing. Additionally, it substantially decreases the generation of greenhouse gases during the pyrolysis process, making it a highly promising RSS utilization technology [77]. Elemental analysis revealed that 80% of the carbon elements and 40% of the nitrogen and sulfur remained in the hydrochar, while the dehydration and decarboxylation reactions of organic compounds during the hydrothermal reaction resulted in H/C and O/C ratios decreasing to 1.53 and 0.39, respectively [78]. With increasing carbonization time, the oxygen-containing functional groups in RSS-derived hydrochar decreased, and its aromatic structures increased. The organic composition of RSS and the hydrothermal conversion process are highly complex. A summary of the migration and transformation pathways of major organic compounds in sludge during hydrothermal treatment is shown in Figure 7 [79,80,81,82]. The key factors influencing the calorific value of hydrochar are reaction temperature, time, and product moisture content [83]. As the reaction temperature and time increase, the calorific value increases due to the higher carbon content in the products, with the moisture content of the final product having a more significant impact on its calorific value.
Due to its higher calorific value, the products of RSS carbonization can be used as fuel, and RSS-derived hydrochar also possesses a relatively high surface area, making it suitable as an adsorbent material [84,85,86]. Currently, many researchers have conducted modifications on RSS-derived hydrochar, leading to the development of a variety of applications. These include adsorption of specific pollutants in wastewater, heavy metal adsorption, catalysis, soil improvement, and enhancement of fermentation processes [87,88,89,90]. With ongoing research into carbonization reactions and the continuous expansion of its application scenarios, RSS carbonization technology holds prospects for wide-ranging applications.

3.3. Phosphorus Recovery from Residual Sewage Sludge

Phosphorus is an essential element required for the metabolic processes of both animals and plants, making it crucial for modern agricultural production. Phosphorus primarily exists in nature in the form of phosphate minerals, with approximately 90% of all globally mined phosphorus ores being used for phosphorus fertilizer production. Therefore, phosphorus is a strategic resource, and researchers have focused their attention on phosphorus resources in wastewater [91]. Through the action of phosphorus-accumulating bacteria, about 90% of the phosphorus in wastewater ends up in sludge, making sludge an important secondary source of phosphorus. Figure 8 presents common processes for recovering phosphorus from sludge and its thermal treatment products [92].
RSS has a high moisture content, and phosphorus is mainly present in the form of organic phosphorus. The phosphorus content per unit mass of RSS is very low, making direct extraction of phosphorus from raw RSS highly challenging. However, after RSS incineration, the mass and volume of the RSS are greatly reduced, and the concentration of phosphorus in the RSS ash is significantly increased. Studies have shown that RSS ash can contain around 80% of the phosphorus from the original sludge [93]. Therefore, phosphorus is typically recovered from the ash fraction of RSS incineration. During the incineration process, the incineration temperature has a significant impact on the content and form of phosphorus in RSS ash [94]. When the incineration temperature is above 850 °C, the phosphorus exists in the form of volatile oxides and is carried away by the flue gas as it cools, leading to a substantial reduction in phosphorus elements remaining in the bottom ash [94]. Additionally, numerous studies have indicated that the proportion of bioavailable phosphorus in incineration ash produced at temperatures above 700 °C is relatively low [95,96]. Therefore, strict control of the incineration temperature and the amount of fly ash produced during the incineration process is necessary.
The recovery of phosphorus from RSS ash mainly involves wet chemical and thermal chemical methods. The wet chemical method involves adding acid to the RSS ash to dissolve the phosphorus. A research team achieved a phosphorus leaching rate of 94.6% from RSS ash and 91.1% from dried RSS using 0.4 mol/L hydrochloric acid (75 mL/g) and 0.6 mol/L hydrochloric acid (25 mL/g) at room temperature [97]. However, heavy metals may also leach out during this process [98]. Subsequently, separation of the heavy metals and phosphorus from the phosphorus-rich leachate is required. Continuous precipitation methods, known for their ease of operation and cost advantages, are widely used. In continuous precipitation, the pH of the phosphorus-rich leachate is adjusted by adding an alkaline substance, causing the phosphorus to precipitate in the form of aluminum-phosphorus, while the heavy metals remain in the solution [99]. Thermochemical methods are another important means of phosphorus recovery. Chlorides (e.g., CaCl2/MgCl2) are added to RSS ash, and at high temperatures, chloride ions form volatile compounds with heavy metals such as cadmium, copper, lead, tin, zinc, which are separated from the RSS ash along with the flue gas [100]. Most of the remaining phosphorus in the ash fraction is in forms like Ca-P and Mg-P, which can be directly used in phosphate fertilizer production and are readily absorbed by plants [101]. Additionally, chloride incorporation can enhance the bioavailability of phosphorus. For instance, CaCl2 doping can convert AlPO4 into Ca2PO4Cl, which has higher biological availability [100]. However, thermochemical methods require higher energy consumption and larger investments, limiting their practical application.
Recovering phosphorus resources from RSS ash is an economically viable RSS resource utilization technology. However, this method is most effective when the RSS is incinerated separately, and the resulting ash fraction has a higher phosphorus content. It also requires specific incineration temperature conditions. Therefore, in practice, only a portion of RSS incineration ashes are suitable for phosphorus recovery.

4. The Comparison of Resource Utilization Technologies for Residual Sewage Sludge

RSS utilization technologies each have their own advantages and limitations (Table 4). Brick and cement production can comprehensively utilize RSS, but they are constrained by blending ratios and metal leaching issues, requiring pretreatment and potentially affecting product quality. Lightweight aggregate production is widely applied but also faces limitations in strength and blending ratios. In terms of energy utilization, anaerobic digestion and co-combustion technologies can convert RSS into energy, but they come with time costs, space requirements, and secondary pollution issues. Pyrolysis carbonization and hydrothermal carbonization technologies offer economic potential and the advantage of not requiring pretreatment, but the former requires high investment and solutions for exhaust gas issues, while the latter faces challenges of equipment corrosion and moderate investment costs. Phosphorus recovery technologies, through wet chemical and thermochemical methods, can achieve resource recovery but are affected by incineration temperatures, raw material limitations, and heavy metal volatilization issues. Overall, when choosing RSS utilization technologies, it is essential to comprehensively consider their characteristics, environmental standards, economic benefits, and operational feasibility. Future research will focus on improving efficiency, reducing costs, and minimizing environmental impacts.

5. Conclusions and Perspectives

This review systematically analyzed the fundamental characteristics of RSS and the sustainable resource utilization technologies available for it. The review found that although RSS contains a significant amount of organic matter and nutrients, its high moisture content and heavy metal concentrations limit its direct use. Three main sustainable resource utilization technologies—building material utilization, energy utilization, and phosphorus recovery—each have their own advantages and limitations. Selecting the appropriate technology requires considering the characteristics of the RSS and the application context to achieve an integration of pollution control and resource utilization.
Future research should focus on technological innovation and policy support to improve the efficiency and economic viability of RSS utilization. Other widely studied RSS treatment technologies, such as fermentation for carbon source production, can also be considered. Interdisciplinary collaboration and life-cycle assessment will be crucial, along with enhancing public awareness and international cooperation to share best practices. Additionally, risk management and environmental safety must be integral throughout the development and application of RSS utilization technologies to ensure their sustainability.

Author Contributions

Conceptualization, Y.S.; methodology, Y.S.; validation, W.Z.; investigation, W.Z.; data curation, W.Z.; writing—original draft preparation, W.Z.; writing—review and editing, W.Z., Y.S., S.Q. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Weicheng Zheng, Shulin Qin and Zhongquan Wang were employed by the company Hangzhou Research Institute of China Coal Technology & Engineering Group. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. The primary pathways for resource utilization of residual sewage sludge.
Figure 1. The primary pathways for resource utilization of residual sewage sludge.
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Figure 2. Utilization pathways of residual sewage sludge in construction materials.
Figure 2. Utilization pathways of residual sewage sludge in construction materials.
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Figure 3. Ternary plot of SiO2, Al2O3, and CaO contents for sewage sludge and incinerated sludge ash. PC = portland cement, GBFS = granulated blast furnace slag, PFA = pulverized fuel ash, MK = metakaolin, SF = silica fume. Reproduced from the work by [21] with permission from Elsevier, copyright 2020.
Figure 3. Ternary plot of SiO2, Al2O3, and CaO contents for sewage sludge and incinerated sludge ash. PC = portland cement, GBFS = granulated blast furnace slag, PFA = pulverized fuel ash, MK = metakaolin, SF = silica fume. Reproduced from the work by [21] with permission from Elsevier, copyright 2020.
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Figure 4. Preparation process of sludge ceramsite. Reproduced from the work by [33] with permission from Elsevier (Amsterdam, The Netherlands), copyright 2021.
Figure 4. Preparation process of sludge ceramsite. Reproduced from the work by [33] with permission from Elsevier (Amsterdam, The Netherlands), copyright 2021.
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Figure 5. Biochemical reactions during the sludge fermentation process. Reproduced from the work by [51] with permission from Elsevier, copyright 2018.
Figure 5. Biochemical reactions during the sludge fermentation process. Reproduced from the work by [51] with permission from Elsevier, copyright 2018.
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Figure 6. Heat flux distribution on the right water-wall (unit: kW/m2). Reproduced from the work by [68] with permission from Elsevier, copyright 2017.
Figure 6. Heat flux distribution on the right water-wall (unit: kW/m2). Reproduced from the work by [68] with permission from Elsevier, copyright 2017.
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Figure 7. Organic matter conversion pathway during hydrothermal carbonization of sludge. Reproduced from the work by [81] with permission from Elsevier, copyright 2021.
Figure 7. Organic matter conversion pathway during hydrothermal carbonization of sludge. Reproduced from the work by [81] with permission from Elsevier, copyright 2021.
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Figure 8. Summary of common process and methods for recovering and recycling P from sewage sludge. Reproduced from the work by [92] with permission from Elsevier, copyright 2022.
Figure 8. Summary of common process and methods for recovering and recycling P from sewage sludge. Reproduced from the work by [92] with permission from Elsevier, copyright 2022.
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Table 1. Analysis of nutrient and heavy metal contents in residual sewage sludge.
Table 1. Analysis of nutrient and heavy metal contents in residual sewage sludge.
IndicatorAverage ValueMaximum ValueMinimum Value
Organic matter (%)51.477.013.4
Total nitrogen (%)3.67.20.3
Total phosphorus (%)2.314.70.0
Total potassium (%)1.47.40.1
Cd (mg/kg)2.849.6/
Hg (mg/kg)5.713.60.0
Pb (mg/kg)93.0729.02.7
Cr (mg/kg)266.72960.09.6
As (mg/kg)29.0301.00.0
Ni (mg/kg)120.2615.42.7
Zn (mg/kg)1794.714,285.015.9
Cu (mg/kg)493.310,531.116.9
B (mg/kg)41.798.6/
Table 2. Typical composition of sludge solids in municipal wastewater plants.
Table 2. Typical composition of sludge solids in municipal wastewater plants.
Composition (wt%)Primary SludgeDigested SludgeResidual Sludge
RangeTypical ValueRangeTypical ValueRange
Total solids5.0~9.06.02~54.00.8~1.2
Volatile solids60~806530~604059~88
Oils and fats6.0~30/5~2018/
Protein20~302515~201832~41
Cellulose8.0~15108.0~1510/
Nitrogen1.5~4.02.51.6~3.03.02.4~5.0
Phosphorus0.8~2.81.61.5~4.02.52.8~11
Potassium0~1.00.40~3.01.00.5~0.7
Table 3. Characteristics of sludge pyrolysis carbonization.
Table 3. Characteristics of sludge pyrolysis carbonization.
CharacteristicsLow-Temperature CarbonizationMedium-Temperature CarbonizationHigh-Temperature Carbonization
Carbonization temperature (°F)≤600800~10001200~1800
Additional pressure//6~8 Mpa
Pretreatment requirementNo dryingDrying to reduce the sludge moisture content (<90%)Drying to reduce the sludge moisture content (<30%)
Operating odorMicro-odorOdorlessOdorless
Biochar calorific value (Kcal/kg)3600~49002600~43002000~3000
ApplicationWideLessSeldom
Table 4. The comparison of resource utilization technologies for residual sewage sludge (RSS) disposal.
Table 4. The comparison of resource utilization technologies for residual sewage sludge (RSS) disposal.
RSS Utilization MethodsAdvantagesRestrictions
Building material utilizationBrick-making
  • Full utilization
  • Low levels of metal leaching
➢
Limited amount of doped
➢
Pretreatment required
➢
Brick quality
Cement production
  • Inexpensive
  • Full utilization
➢
Limited amount of doped
➢
Low strength of cement mortar
➢
High levels of metal leaching
Ceramsite production
  • Ceramic particles have a wide range of applications
  • Low levels of metal leaching
➢
Low strength of ceramic granules
➢
Limited amount of doped
Utilization for energyAnaerobic fermentation
  • Low carbon
  • Energy production
➢
Long required time
➢
Large footprint
➢
Secondary pollution
Co-incineration
  • Fully decomposed
  • Energy utilization
  • Heavy metal stabilization (except mercury)
➢
Tail gas
➢
Limited amount of doped (or pretreatment required)
➢
Equipment requirements
Pyrolytic carbonization
  • Economic output
  • Full utilization
  • Heavy metal stabilization (except mercury)
➢
Pretreatment required
➢
High investment
➢
Difficulties in the operating process
➢
Tail gas
Hydrothermal carbonization
  • Process moderate
  • No preprocessing required
  • No exhaust
  • Economic benefits
➢
Equipment corrosion
➢
Medium investment
Phosphorus recovery (from ash fraction of RSS incineration)Wet chemical methods
  • Process is simple
  • Resource recovery
➢
Incineration temperature requirements
➢
Metal leaching
➢
Raw material restrictions
Thermal chemical methods
  • Resource recovery
  • Full utilization
➢
Tail gas
➢
Raw material restrictions
➢
Heavy metal volatilization
➢
High incineration temperatures
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Zheng, W.; Shao, Y.; Qin, S.; Wang, Z. Future Directions of Sustainable Resource Utilization of Residual Sewage Sludge: A Review. Sustainability 2024, 16, 6710. https://doi.org/10.3390/su16166710

AMA Style

Zheng W, Shao Y, Qin S, Wang Z. Future Directions of Sustainable Resource Utilization of Residual Sewage Sludge: A Review. Sustainability. 2024; 16(16):6710. https://doi.org/10.3390/su16166710

Chicago/Turabian Style

Zheng, Weicheng, Yuchao Shao, Shulin Qin, and Zhongquan Wang. 2024. "Future Directions of Sustainable Resource Utilization of Residual Sewage Sludge: A Review" Sustainability 16, no. 16: 6710. https://doi.org/10.3390/su16166710

APA Style

Zheng, W., Shao, Y., Qin, S., & Wang, Z. (2024). Future Directions of Sustainable Resource Utilization of Residual Sewage Sludge: A Review. Sustainability, 16(16), 6710. https://doi.org/10.3390/su16166710

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