Municipal Sludge Treatment Research Articles

Sustainable Utilization of Dewatering Sludge for the Development of Reinforcement Grouting Materials in Downhole Applications

The mining and processing of coal resources generate substantial coal-based solid wastes, such as coal gangue and slag, which pose environmental challenges, occupy land, and are difficult to manage. However, utilizing these wastes for the stabilization and solidification (S/S) of municipal sludge containing chromium (Cr) and nickel (Ni) offers an effective solution for mitigating environmental and groundwater pollution while enabling sustainable waste treatment and resource utilization. This study applied an alkali-activated coal gangue–S95 granulated blast furnace slag-based binder (CGS) to the S/S treatment of municipal sludge. The effects of the liquid-to-solid ratio, alkali activator dosage, sludge content, and incineration on compressive strength and the leaching of Cr and Ni were analyzed. The results showed that compressive strength decreased with increases in the sludge content and liquid-to-solid ratio, while incinerated sludge (ESA) samples exhibited better strength than raw sludge (ES). Incineration decomposed the calcite (CaCO3) into CaO, which facilitated the oxidation of Cr(III) to Cr(VI) and increased Cr leaching in the ESA. However, the ESA samples demonstrated superior heavy metal stabilization, as CGS reduced Cr(VI) to Cr(III) and immobilized it through the formation of chromite phases. Using ESA as a binder in CGS provides a safe, efficient approach for resource recovery and heavy metal stabilization, offering a novel solution for the environmental management and utilization of coal-based solid wastes.

Strength characteristics and prediction model of solidified municipal sludge reinforced by dredged silt combined with sodium silicate and polyurethane

The treatment and resource utilization of municipal sludge and dredged silt have been rendered urgent by the acceleration of urbanization and stricter environmental protection demands. An effective solution was developed to address the challenges of poor mechanical properties and the difficulty in directly using cement-based materials for municipal sludge treatment. The utilization of dredged silt with high water content served as the foundational skeleton material. Appropriate proportion (0.5:1.0) of sodium silicate, which accelerates cement hardening, and polyurethane, which facilitates chemical bond cooperation, was combined to form SP material, partially replacing cement. Unconfined compressive strength (UCS) tests were subsequently conducted on the solidified municipal sludge. These tests aimed to investigate the influence of the dredged silt mixing ratio, initial water content of dredged silt, and the SP mixing ratio on the strength of the solidified municipal sludge. Furthermore, a strength prediction model was established for solidified municipal sludge, taking into consideration the mixing ratios of dredged silt and SP. The research findings indicate that dredged silt can serve as a skeleton structure for solidified municipal sludge. The UCS of the solidified municipal sludge increases with the increase in the dredged silt mixing ratio, and reaches a maximum value at mixing ratio of 1.0. SP materials can partially replace cement, the appropriate proportion of SP and cement can synergistically improve the strength of solidified municipal sludge, and the optimal SP mixing ratio is 50%. Furthermore, the strength prediction model constructed with independent variables such as dredged silt mixing ratio, curing age, water-to-cement ratio, and SP mixing ratio demonstrates better predictability for the strength development of solidified municipal sludge.

Optimization of Cl and heavy metal removal during heat treatment of municipal solid waste incineration fly ash and municipal sludge after co-washing.

Heat treatment, known for its detoxification and volume reduction characteristics, is a promising technology for the management of municipal solid waste incineration fly ash (MFA) and municipal sludge (MS). This paper uses the solid residue from MFA and MS after co-washing as the raw material to study the melting properties, phase transformations, changes in Cl content, heavy metal removal efficiency, and leaching toxicity. The results indicated that co-processing of MFA and MS can effectively reduce the melting temperature. The migration of Cl elements was influenced by the CaO/(SiO2 + Al2O3) ratio and the formation of Cl-containing phases. During the heat treatment process, Cr and Ni remained relatively stable, while Zn, Pb, Cu, and Cd were affected by Cl migration. Chlorine can promote the volatilization and escape of these heavy metals, but as Cl became fixed within Ca10(SiO4)3(SO4)3Cl2 and Ca5(PO4)3Cl, the removal efficiency decreased. To achieve the highest removal rates for these elements, the addition of MFA should be limited to no more than 20%, and the CaO/(SiO2 + Al2O3) ratio should be below 2.9. After heat treatment, the leaching concentrations of Zn, Pb, Cu, Cr, Cd, and Ni met applicable standards. The findings of this study could provide a method for the treatment of MFA and MS after co-washing and can serve as guidance for heat treatment processes.

Overcoming deep-dewatering challenges in food waste digestate with polyethylene oxide as an innovative conditioning agent

The effective treatment of food waste digestate is critical for reducing environmental pollution and mitigating carbon emissions, with deep dewatering playing a pivotal role. Conventional dewatering agents such as polyaluminum chloride (PAC) and polyacrylamide (PAM), commonly employed in municipal sludge treatment, exhibit limited efficacy when applied to food waste digestate due to the latter's high salinity and advanced fermentation stages. This study introduces polyethylene oxide (PEO) as a novel conditioning agent and investigates its dewatering performance in comparison to PAC and PAM, elucidating the underlying mechanism. PEO conditioning markedly improves deep-dewatering, reducing digestate moisture content from 93.11 % to 56.71 % and lowering specific resistance to filtration (SRF) by 90.3 %. In contrast, PAM, PAC, and their combination achieve moisture reductions to 81.18 %, 84.49 %, and 87.07 %, respectively, with significantly lower SRF improvements. PEO promotes the release of bound water by weakening solid-liquid binding energy, facilitating the transition of bound water to free water and enhancing overall water mobility. Moreover, compressibility coefficient analyses and X-ray computed tomography (X-CT) reveal that PEO treatment significantly increases filter cake porosity, with an effective porosity rate of 56.65 %, resulting in superior drainage performance. The enhanced dewatering efficiency of PEO stems from its ability to improve water permeability within the filter cake during compression, distinguishing its mechanism from traditional flocculation (PAM) and coagulation (PAC) approaches. This work highlights the potential of PEO as a highly effective solution for food waste digestate treatment in solid waste management, with its salt-resistant properties further extending its applicability to high-salinity waste streams.

A Review on Sludge Drying after High Pressure Filter Pressing

Sludge drying is a key step in municipal sludge treatment. After mechanical dewatering, the water content of sludge is usually between 50% and 70%, which needs further drying. The main drying technologies include thermal drying, solar drying, ultrasonic drying and microwave drying, among which thermal drying technology is the most mature. Research shows that low-temperature drying technology has obvious advantages in reducing the release of harmful substances, energy consumption and cost. In terms of technology application, domestic sludge drying process is mainly divided into direct drying and indirect drying. Direct drying directly evaporates water through high-temperature gas, while indirect drying makes use of heat conduction effect to contact with high-temperature media. At present, indirect drying technology is mainly used in China, such as paddle type, disc type and thin layer drying. In particular, the low-temperature heat pump drying technology excels in terms of safety, footprint and energy consumption, with its ability to operate at lower temperatures, reduce the release of hazardous gases, and achieve environmental and economic benefits in a closed-cycle mode. This technology is suitable for small and medium-sized wastewater treatment plants and shows clear advantages in cases of budgetary constraints.

Current Status and Trends of Municipal Sludge Treatment and Disposal in China

<p style="text-align:justify"><span style="font-size:10.5pt"><span style="line-height:200%"><span style="font-family:"Times New Roman",serif"><a name="_Hlk170823277"><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">      As urbanization accelerates, an increasingly significant problem of municipal sludge treatment and disposal has emerged in China. Sludge is both a waste and a resource, therefore, its reasonable and effective treatment and disposal are crucial for protecting the environment and promoting the recycling of resources. </span></span></a><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">This article reviewed the current status and technologies of sludge treatment and disposal, proposed the mainstream routes for sludge treatment and disposal in China, including anaerobic digestion-land application, drying incineration-construction material utilization, aerobic </span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">composting-land application, deep dewatering-emergency landfill. According to the comparative analysis of carbon emission, economic benefit and life cycle assessment, anaerobic digestion-land application </span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">was</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%"> considered the optimal technological route. It ha</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">s</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%"> extremely low carbon emissions</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%"> (</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">-44.43 kg·t<sup>-1</sup></span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">) (calculated as CO<sub>2</sub>/dry sludge)</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">, low net costs</span></span> <span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">($31.93/t)</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%"> and significant environmental benefits</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">, including </span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">SO<sub>2 </sub>(-1.9×10<sup>5 </sup>kg), electricity (-6.2×10<sup>8 </sup>kWh) and fuel (-4.6×10<sup>7</sup>MJ)</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">.</span></span> <span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">Based on this, it highlighted the issues in sludge treatment and disposal, such as vague policies, an incomplete management system, and inefficiency in the resource utilization of sludge. In response to these issues, suggestions were made to improve relevant policies, achieve more precise regulation, and establish an industrial chain for the resource utilization of sludge.</span></span> </span></span></span></p> <p style="text-indent:24.0pt; text-align:justify"><span style="font-size:10.5pt"><span style="line-height:200%"><span style="font-family:"Times New Roman",serif"><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">In summary, this article offer</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%">ed</span></span><span lang="EN-US" style="font-size:12.0pt"><span style="line-height:200%"> innovative insights into the optimal technological route for sludge treatment and disposal in China, while highlighting the practical engineering significance of addressing policy, management, and resource utilization challenges. Its recommendations have the potential to drive significant improvements in sludge management practices, contributing to a cleaner, greener, and more sustainable urban environment.</span></span></span></span></span></p>

Ultrafiltration fractionation of potentially inhibitory substances of hydrothermal liquefaction aqueous phase derived from municipal sludge

Hydrothermal liquefaction (HTL) is a promising thermo-chemical technology for municipal sludge treatment due to its potential for biocrude oil recovery and minimizing biosolids management costs. However, the process generates a high volume of an aqueous byproduct that needs to be treated due to its high chemical oxygen demand (COD) and various organic and inorganic compounds. Although the aqueous phase is known to contain recalcitrant and potentially inhibitory substances that may affect its biological treatment, their molecular weight distribution (MwD) and its impact on anaerobic biodegradability are poorly understood. Ultrafiltration (UF) was conducted to fractionate HTL aqueous into different molecular weight (Mw) fractions using 300, 100, 10, and 1 kDa membranes. Mesophilic biochemical methane potential (BMP) assays were conducted to assess the anaerobic biodegradability of each fraction, and the first-order model was used to calculate the degradation kinetics of potential inhibitory compounds. The highest percentage of organics (65 %) was found in the Mw<1 kDa range, whereas the 10>Mw>1 kDa had the lowest percentage (8 %). There was no significant difference in the cumulative specific methane produced from various Mw fractions (p>0.05). The Mw<1 kDa fraction had the highest first-order specific methane production rate (0.53 day−1), whereas the unfiltered HTL had the lowest (0.38 day−1). Although UF fractionation increased the rate of anaerobic degradation of HTL aqueous for the Mw<1 kDa fraction, the observed methane potential was only 55 % of the theoretical value. This implies that 45 % of COD remains undegraded even after permeation through the lowest Mw cut-off membrane. Therefore, further characterization of HTL aqueous is needed for compounds with molecular weights below 1 kDa to fully understand the nature of inhibitory organics and their impact on anaerobic digestion. Furthermore, pretreatments utilizing techniques such as adsorption and advanced oxidation may be necessary to enhance the specific methane yields from various HTL aqueous fractions, thereby bringing them closer to the theoretical yield.

Comparative analysis of adding cotton straw and corn stover to improve the combustion performance of municipal sludge

To address issues of high water content and low calorific value during combustion of municipal sludge, we added water-absorbent, easy-to-burn agricultural waste to improve the overall combustion performance. Cotton straw or corn stover were added to the sludge and mixed at high-speed to compare their capacities for improving combustion performance. Scanning Electron Microscopy (SEM) revealed that cotton straw or corn stover attached to the surface of the municipal sludge particles after blending, while analysis of thermogravimetric curves and activation energies of the blends showed that combustion and exhaustion rates increased significantly when 40% cotton straw or corn stover were blended into the sludge. Using the quadrilateral cut-ring boiler as a prototype, the mix of sludge with cotton straw or corn stover was simulated, and FLUENT software was used to obtain the temperature and pollutant emissions of the boiler. Sludge blended with cotton straw or corn stover increased furnace temperature and reduced SO2 and NO emissions, while that with cotton straw burned at higher temperatures with lower SO2 and NO emissions. Overall, the CO content of sludge combustion was lower when blended with proportions of cotton straw or corn stover under 50%. The findings of this study lay a theoretical foundation for treatment of municipal sludge according to local conditions.

Ultrahigh pressure filtration dewatering of municipal sludge conditioned by steel slag in combination with Fe2+/ sodium persulfate

Municipal sludge treatment in urban contexts faces significant challenges, especially when dealing with sludge characterized by exceptionally high water content and organic content. This study investigates the application of sodium persulfate oxidation activated by ferrous ions (Fe2+/SPS) in conjunction with waste steel slag as a conditioning agent for municipal sludge, capillary suction time and specific resistance filtration experiments were conducted on conditioned sewage sludge, along with dewatering tests under ultrahigh pressure conditions. Additionally, to investigate the potential underlying mechanisms, Scanning Electron Microscopy analysis, Zeta potential measurements, and comprehensive sample analyses were performed. The research reveals substantial improvements in sludge dewatering performance when steel slag is combined with Fe2+/SPS treatment. This combined treatment introduces additional ferrous ions into the sludge, generating radicals that disrupt the structure of extracellular polymeric substances and cells within the sludge. Steel slag also acts as a skeleton builder material, creating larger drainage pathways within the sludge cake under ultrahigh pressure, enhancing permeability, and accelerating moisture release. This combined treatment method holds promise for achieving deep sludge dewatering, under ultrahigh filtration pressure, with water content reduced to below 40%. The study not only highlights the advantages of using steel slag in combination with Fe2+/SPS treatment for sludge dewatering but also provides insights into the underlying mechanisms. This comprehensive treatment method reduces the excessive use of ferrous ions and costs while significantly enhancing dewatering performance, offering a promising solution for municipal sludge treatment and management.

Biodegradable Microplastics in Municipal Wastewater and Sludge Treatment Processes: A Review on Occurrence, Fate, and Effects

Biodegradable Microplastics in Municipal Wastewater and Sludge Treatment Processes: A Review on Occurrence, Fate, and Effects

Enrichment characteristics and environmental risk assessment of heavy metals in municipal sludge pyrolysis biochar

Pyrolysis treatment of municipal sludge is a promising sludge treatment technology. However, due to the presence of heavy metals in sludge, the practical application of sludge pyrolysis products, especially biochar, is greatly affected. In this paper, the enrichment characteristics of different heavy metals in municipal sludge and its pyrolysis biochar and their leaching characteristics in the environment were studied. The environmental risks of municipal sludge and biochar were also assessed. The experimental results revealed that pyrolysis would promote the enrichment of different heavy metals in sludge in pyrolysis biochar. And with higher pyrolysis temperature, the enrichment is stronger. But for metals with high volatility such as Hg, Cd, Se, increasing the pyrolysis temperature will reduce their enrichment. Pyrolysis can inhibit the heavy metals in sludge leaching, and the effect of pyrolysis temperature on different kinds of heavy metals is different. The leaching rates of Se, As and Cr first increased and then decreased as the temperature of pyrolysis increased. The leaching rate of Cd increased as the temperature of pyrolysis increased. Considering the leaching rate of each heavy metal, the leaching toxicity of heavy metals in pyrolysis biochar is the lowest at the pyrolysis temperature of 400 °C. The risk assessment of municipal sludge and biochar revealed that the most harmful heavy metals in the original sludge were Hg, Cd and Se, followed by Cu and Zn. Sludge pyrolysis into biochar can reduce the content of Hg, Cd and Se. The use of copper and zinc water pipes should be reduced for sewage pipe laying to decrease the contents of Cu and Zn in municipal sludge.

Dynamic Characteristics Analysis of Metallurgical Waste Heat Radiative Drying of Thin Layers of Sewage Sludge

The utilization of metallurgical waste heat for urban sludge drying and dewatering not only affects the subsequent cost of sludge treatment but also provides a pathway for the rational utilization of metallurgical waste heat. The influence of different experimental conditions on sludge drying characteristics, such as drying temperature and thickness, was analyzed based on metallurgical waste heat. Based on the analysis and evaluation of the drying kinetics parameters of commonly used drying mathematical models, a modified Midilli drying kinetic model is proposed. The kinetic parameters and effective diffusivity of sludge drying were analyzed in three stages of sludge drying: rising rate, constant rate, and falling rate. By utilizing the Arrhenius equation, the relationship between the effective diffusion coefficient and thermodynamic temperature is established, revealing the apparent activation energies for the three stages of urban sludge drying as 29.772 kJ·mol−1, 37.129 kJ·mol−1, and 39.202 kJ·mol−1, respectively. This is closely related to the migration, diffusion, and mass transfer resistance of sludge moisture, indicating that the thickness of sludge accumulation affects the drying time of sludge during the treatment of municipal sludge.

A win-win situation for environment and economy: Analysis of maximizing benefits in municipal sludge treatment plants

The huge cost (economic cost and environmental cost) of treating municipal sludge has caused enormous pressure on China. How to balance economic cost and environmental benefits in China's sludge treatment industry is an important issue that the government urgently needs to address. This article selects 9 types of municipal sludge treatment plants (MSTP) that can represent different treatment methods and scales in China, and attempts to help all provinces in China find the optimal solution for maximizing sludge treatment benefits (economic benefits and environmental benefits) under different constraints (economic constraints and environmental constraints). Firstly, this paper analyzes the economic and environmental costs and return of 9 types of MSTP. Secondly, we use the unguided Super-SBM model to combine the costs and return of 9 types of MSTP to measure the benefits of each plant. It has been found that under ideal conditions, medium Incineration treatment (IT) and large building material (BM) utilization are the most beneficial options for sludge treatment in China. Finally, we use a simple linear programming model to establish a constrained linear programming model for the environment and economy, substituting provinces that are divided into four categories under different constraint conditions into the model, and finding the optimal ratio for each type of province to build MSTP, in order to maximize benefits. Research has found that after adding constraints, the future development trend of municipal sludge treatment in China will mainly focus on small BM utilization and medium LA. This research result has universal applicability. As long as the constraints of this article are met, any country or province can adjust the construction proportion of municipal sludge treatment plants according to the research results of this article.

Biogeochemical transformation of mercury driven by microbes involved in anaerobic digestion of municipal wastewater

Anaerobic digestion (AD) with municipal wastewater contained heavy metal mercury (Hg) highly affects the utilization of activated sludge, and poses severe threat to the health of human beings. However, the biogeochemical transformation of Hg during AD remains unclear. Here, we investigated the biogeochemical transformation and environmental characteristics of Hg and the variations of dominant microbes during AD. The results showed that Hg(II) methylation is dominant in the early stage of AD, while methylmercury (MeHg) demethylation dominates in the later stage. Dissolved total Hg (DTHg) in the effluent sludge decreased with time, while THg levels enhanced to varying degrees at the final stage. Sulfate significant inhibits MeHg formation, reduces bioavailability of Hg(II) by microbes and thus inhibits Hg(II) methylation. Microbial community analysis reveals that strains in Methanosarcina and Aminobacterium from the class of Methanomicrobia, rather than Deltaproteobacteria, may be directly related to Hg(II) methylation and MeHg demethylation. Overall, this research provide insights into the biogeochemical transformation of Hg in the anaerobic digestion of municipal wastewater treatment. This work is beneficial for scientific treatment of municipal wastewater and effluent sludge, thus reducing the risk of MeHg to human beings.

Comprehensive competitiveness assessment of four typical municipal sludge treatment routes in China based on environmental and techno-economic analysis

To find a sustainable and effective municipal sludge treatment route requires a systematic assessment of the comprehensive competitiveness of diverse sludge treatment routes. Four typical treatment routes in China including co-incineration in coal power plants (CIN), mono-incineration (IN), anaerobic digestion (AD) and pyrolysis (PY) were selected in this study. A novel assessment model integrating life cycle assessment (LCA), techno-economic analysis (TEA) with analytic hierarchy process (AHP)-Entropy method was established, and comprehensive competitiveness indicated by comprehensive index (CI) of the four routes was deeply evaluated. Results displayed CIN route (CI = 0.758) showed the best comprehensive performance for its best environmental and economic performance. This was followed by PY route (CI = 0.691) and AD route (CI = 0.570), indicating the enormous potential of sludge PY technology. IN route showed the worst comprehensive performance (CI = 0.186) owing to its high environmental impact and lowest economic benefit. It was noted that greenhouse gas emissions and severe toxic potential were the main environmental challenges faced by sludge treatment. Besides, result of sensitivity analysis revealed that the comprehensive competitiveness of diverse sludge treatment routes was improved with the increase of sludge organic content and sludge reception fee.

An Overview of Municipal Wastewater and Sludge Treatment Process

An Overview of Municipal Wastewater and Sludge Treatment Process

Effect of municipal sludge addition on the sinter process and sinter quality

ABSTRACT In order to analyse the feasibility of sintering for municipal sludge treatment, a sintering pot was used for the experiment. The results showed that the main effects of sludge addition on the sintering process and sinter quality come from the changes in the heat addition and moisture content of the granulated sinter. The addition of sludge could improve the granulation efficiency of the sinter mixture and the particle size composition of the sinter. When the proportion of sludge was 7%, the addition of sludge completely replaced the water required for mixing, and the overall sintering index was the best. In this case, after reducing fuel consumption by 10%, except for the content of P, the sinter quality and the sintering index could met the requirements of steel production. Finally, the use of fuel and raw materials with high P content needed to be reduced with the addition of sludge.

Regeneration of Activated Sludge into SiO2-Decorated Heteroatom-Doped Porous Carbon as Advanced Electrodes for Li-S Batteries.

The regeneration of harmful activated sludge into an energy source is an important strategy for municipal sludge treatment and recycling. Herein, SiO2-modified N,S auto-doped porous carbon (NSC@SiO2) with high conductivity (70 S m-1) is successfully obtained through a simple calcination method of the activated sludge from wastewater treatment. Further, P-doped NSC@SiO2 (NSPC@SiO2) is designed to achieve a higher surface area (891 m2 g-1 vs 624 m2 g-1), a larger pore volume (0.87 cm3 g-1 vs 0.08 cm3 g-1), and more carbon defects. Due to its special structure, NSPC@SiO2 is used as a sulfur host of lithium-sulfur batteries. The results of polysulfide adsorption experiments, S 2p X-ray photoelectron spectra (XPS), Li2S nucleation experiments, polysulfide symmetric cells, measurement of the galvanostatic intermittent titration (GITT), polarization voltage difference, lithium-ion diffusion rate, and Tafel slope verified that NSPC@SiO2 greatly improved the adsorption capacity of polysulfides, lowered the barrier to Li2S formation and the internal resistances of cells, and accelerated Li+ ion diffusion and the reaction kinetics of polysulfide conversion, resulting in the excellent performance of polysulfide capture and superior rate performance and cyclic stability. By comparing NSPC@SiO2 with NSC@SiO2, a higher initial capacity (1377 mAh g-1 vs 1150 mAh g-1 at 0.1C), better rate capacity (912 mAh g-1 vs 719 mAh g-1 at 2C), and low capacity decay (0.094% per cycle within 200 cycles) are obtained. Our work provides direction for the treatment, disposal, and resource utilization of activated sludge.

Comparison of Carbon Emissions in Different Treatment and Disposal Process Routes of Municipal Sludge

With the introduction of the goal of carbon neutrality, the efficient resource recycling of municipal sludge has been given increasing attention. In order to scientifically evaluate the routes of sludge treatment and disposal from the perspective of carbon emissions, four typical routes were chosen for accounting the carbon emissions per ton for dry sludge (DS). Based on the Intergovernmental Panel on Climate Change (IPCC), combined with Chinese sludge characteristics, carbon emissions were divided into three types:the direct emissions, indirect emissions, and carbon offsets, and accounting boundaries were initiated at sludge thickening and ended at products or energies. The results showed that the total carbon emission of R4 (gravity thickening+thermal hydrolysis+anaerobic digestion+plate and frame filter pressing+transportation+land utilization) was 99.41 kg·t-1(calculated as CO2/DS, same below), which was the route with lowest carbon emissions. If the fugitive emission of CH4 from anaerobic digestion was avoided, the route (R4) could achieve carbon neutrality at this stage. Process units with larger carbon emissions should focus on optimization to reduce the carbon emissions, such as through thermal drying (1049.24 kg·t-1), deep dewatering (960.99 kg·t-1), sanitary landfill (786.24 kg·t-1), incineration (635.52 kg·t-1), aerobic composting (614.17 kg·t-1), and thermal hydrolysis (544.67 kg·t-1). The main carbon offsets were the incineration power generation (-1440.29 kg·t-1), CH4 collection of anaerobic digestion (-435.06 kg·t-1), land utilization (-415.83 kg·t-1), and building materials utilization (-169.75 kg·t-1). In summary, "anaerobic digestion and land utilization" has a great potential for carbon offsets, which should be advocated for as the widely used treatment.

Recent Research on Municipal Sludge as Soil Fertilizer in China: a Review.

Due to the annual increase in wastewater treatment in most Chinese cities, a major environmental issue has arisen: safe treatment, disposal, and recycling of municipal sludge. Municipal sludge has a high content of carbon and essential nutrients for plant growth; hence, it has gained interest among researchers as a soil fertilizer. This study discusses the potential usage of municipal sludge as soil fertilizer (indicators include nitrogen (N), phosphorus (P), and trace elements) along with its shortcomings and drawbacks (potentially toxic elements (PTEs), organic matter (OM), pathogens, etc.) as well as reviews the latest reports on the role of municipal sludge in land use. The use of municipal sludge as a soil fertilizer is a sustainable management practice and a single application of sludge does not harm the environment. However, repeated use of sludge may result in the accumulation of harmful chemicals and pathogens that can enter the food chain and endanger human health. Therefore, long-term field studies are needed to develop ways to eliminate these adverse effects and make municipal sludge available for agricultural use.