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Anaerobic digestion is a sustainable biotechnology for treating food waste (FW) with biogas production driven by microorganisms. Generation of FW increased greatly in China, which are mainly treated with anaerobic digestion technology. A survey into nine full-scale mesophilic anaerobic FW digesters was conducted, together with four sewage sludge digesters as a comparison, to investigate the specific physicochemical features and the resulted microbiota. Results showed that FW digesters were characterized with high salt, volatile fatty acid and ammonia concentrations. A unique "core" microbial community indigenous inside FW digesters was identified, including 70 bacterial and 4 archaeal genus-level taxa, which was quite different from that in sludge digesters. Stable carbon isotope signature of biogas revealed that, hydrogenotrophic methanogenesis (HM) played a significant role in methane generation. The hydrogenotrophic Methanobacterium demonstrated strong positive correlations with the syntrophic acetate oxidizing bacteria (SAOB) Syntrophaceticus and other SAOB candidates (Aminobacterium , Gelria , DTU014, Alkaliphilus), forming a unique SAO-HM functional guild which particularly prevailed in digesters receiving household FW. Feedstock composition was the essential factor influencing microbial structure, followed by salinity, ammonia, and VFAs. The high ammonia concentration and prominent contribution of diverse SAO-HM players indicate that, tailored management strategy needs to be developed for treating household FW. [Display omitted] •Full-scale anaerobic digesters treating household food waste was studied. •A unique core microbial community identified for FW digesters included 74 genera. •Hydrogenotrophic methanogenesis is a key pathway identified with isotope signature. •Acetate-oxidizing syntrophs of known and candidate members were abundant. •Special FW features, high salinity and ammonia shaped the community structure. [ABSTRACT FROM AUTHOR]

The rapid progress of urbanization strategies in China has led to a significant increase in sewage sludge production. Sewage sludge contains organic compounds, pathogens, and heavy metals, making the effective utilization of its organic components a crucial concern in sludge treatment. Anaerobic digestion is widely recognized as a viable method for converting the organic components of sewage sludge into methane, volatile fatty acids (VFAs), and soil fertilizer. Co-digestion, which involves the simultaneous digestion of sewage sludge with other organic wastes, holds great promise for renewable methane production. This review provides an overview of anaerobic co-digestion, focusing on the production of methane through the utilization of sewage sludge and other organic wastes. The kinetics and process parameters involved in co-digestion are discussed, along with the mechanisms underlying the synergistic effect on methane production. These mechanisms include the modulation of the carbon-to-nitrogen (C/N) ratio, dilution of toxic compounds, and the bioaugmentation effect on enzymes and microbial communities. Various methods for improving co-digestion, such as pretreatment and additive techniques, are explored. Furthermore, the industrial application of co-digesting sewage sludge with other organic wastes is highlighted to showcase the commercial feasibility of this technology for methane production. • An assessment was conducted on the co-digestion of sewage sludge with other organic wastes. • Mechanism and improvement for the co-digestion process were discussed. • Industrial application demonstrated the commercialization feasibility of co-digestion technology. • Future challenges associated with co-digestion technology were identified. [ABSTRACT FROM AUTHOR]

• Sixteen sludge treatment plants in China were investigated and analyzed. • Complexity index and humic-like substances were proposed to evaluate the stabilization level. • The humic-like substance contents in the end-products were 157.8–461.1 mg/gVS. • Complexity index can predict the humification degree. The treatment, disposal and resource recovery of sewage sludge is a major bottleneck for the water and environmental remediation efforts in China. In this paper, sixteen sludge treatment plants using anaerobic digestion and aerobic composting as stabilization procedures were investigated and analysed. The organic degradation rates varied from 0.5% to 80.2% of different plants, showing the close relationship with raw sludge property and treatment process. The increment rate of humic-like substances ranged from 19% to 81% in different cases. It has been aware of that stabilization procedures coupled the degradation of simple organics (proteins, polysaccharides, lipids) with the synthesis of complex organics (humic-like substances). Therefore, an alternative methodology, considering the content of humic-like substances (no less than 150 mg/gVS) and the fluorescence complexity index (up to 5.0) in the end-products, was proposed to evaluate the stabilization level. Humic-like substances indicate the ecological value of the end-products. Fluorescence complexity index, combining the reduction of protein-like substances with the increment of humic-like substances, can predict the humification degree. The new criterion can be the supplementary of the current ones. [ABSTRACT FROM AUTHOR]

Large amount of sludge has been a great trouble and raised significant concerns in China. This paper reviewed the current situation of sludge production, management, treatment and disposal in China. Total sludge production in China had an average annual growth of 13% from 2007 to 2013, and 6.25 million tons dry solids was produced in 2013. Per Capita sludge production in China is lower than that in developed countries. However, sludge management is poor in China. Administrative agents of sludge are not in accordance with each other. Laws and regulations of sludge management are incomplete and sometimes unrealistic. As to sludge treatment and disposal, many technical routes have been applied in China. Thickening, conditioning, and dewatering are three most used treatment methods, while application ratios of stabilization and drying are low in China. More than 80% of sludge is disposed by improper dumping in China. Regarding proper disposal, sanitary landfill is the commonest, followed by land application, incineration and building materials. According to the overall situation of China, “thickening-anaerobic digestion-dewatering-land application” is the priority technical route of sludge treatment and disposal. Good changes, current challenges and future perspectives of this technical route in China were analyzed and discussed in details. [ABSTRACT FROM AUTHOR]

• Extracellular protein and carbohydrate increased and declined by adding particles. • Secondary structure of protein and MW of EPS changed little by silica particles. • Degradation of protein and polysaccharide increased and declined as particles added. • Promotion in protein degradation mainly contributed to the increased VS conversion. • Extracellular organics amounts in unit VS mainly correlated to degradation of sludge. The influence of micron-sized silica particles on the distribution of organic matters in sludge and anaerobic degradation of macromolecular organic components (MOCs) in sludge was investigated. With the addition of micron-sized particles in the influent (VS/TS decreased gradually from 90.46 ± 0.21 % to 33.36 ± 0.17 %), the protein degradation percentage was significantly promoted while the polysaccharides degradation percentage was largely inhibited, resulting in the total MOCs degradation and methane production increasing firstly (with the promotion extent within 10 %) and then declining slightly, with the peak value at VS/TS of 56.03 ± 0.21 %. The shifted degradation percentage of protein and polysaccharides were caused by the significant changed distribution of organic matters in sludge. With the addition of micron-sized silica particles, the MW of EPS and secondary structure of protein in EPS changed little, which brought about little influence. While, the promoted extracellular protein content (so that the total protein content) and declined extracellular carbohydrates content (so that the total polysaccharides content), were found to be strongly correlated to the enhanced protein degradation and inhibited polysaccharides degradation. The results suggested that large amounts of grit in sludge might not be the main reason for lower degradability of sewage sludge in China. [ABSTRACT FROM AUTHOR]

The growing amount of sewage sludge from wastewater treatment plant is an emerging challenge in China. The efficient anaerobic digestion of sludge filtrate generated from hydrothermally pretreated sewage sludge can promote the disposal of sewage sludge. Herein, a pilot-scale anaerobic baffled reactor (ABR) assisted by symbionts of short chain fatty acid-oxidation syntrophs (SFAS) and exoelectrogens was developed to improve its stability and efficiency for filtrate treatment. The results demonstrated that the symbionts of exoelectrogens and SFAS, which were enriched by introduction of electrodes in the ABR system, promoted the degradation of butyric, propionic and acetic acids. Therefore, the COD removal efficiency increased from 74.1% to 86.6% and the methane content increased from 81.5% to 92.2% with methane production rising from 241 to 282 mL/g COD removed. Furthermore, the economic evaluation indicated that the energy consumption of electrodes was 0.600 kWh/m3 of sludge filtrate, the net energy profited from increased methane was 2.344 kWh/m3 of sludge filtrate. These results confirmed that the ABR system assisted by symbionts of SFAS and exoelectrogens was feasible for treatment of sludge filtrate in terms of both technical and economic level through pilot-scale verification. Image 1 • Anaerobic digestion of sludge filtrate assisted by SFAS and exoelectrogens was assessed. • The COD removal efficiency reached 86.6% with methane yield of 282 mL/g COD removed. • The operating cost of MECs was $ 0.084/m3 of filtrate. • Applying ABR system assisted by SFAS and exoelectrogens was feasible for sludge treatment. [ABSTRACT FROM AUTHOR]

Handling excess sludge produced by wastewater treatment is a common problem worldwide. Due to limited space available in landfills, as well as difficulties involved in using excess sludge in agriculture, there is a need for alternative disposal methods. Although anaerobic digestion (AD) is widely used in processing sludge, only partial energy recovery from methane and sludge volume reduction can be achieved, resulting in a substantial amount of sludge remaining, which needs to be disposed of. Direct incineration after sludge drying is one possible option, a practice that is already in place in some cities in China. A comparison between direct incineration and conventional AD (with or without pretreatment by thermal hydrolysis) has to be made with respect to the energy balance and investment & operational (I & O) costs. This comparison reveals direct incineration to have the lowest energy deficit and I & O costs. Therefore, it is expected that direct incineration without AD will become the preferred sustainable approach to handling sludge. Image 1 • Landfills and agricultural use of excess sludge is becoming increasingly constrained. • Alternative sludge disposal routes must be sought. • Anaerobic digestion (AD) is not a final solution for excess sludge. • Direct incineration without AD has the lowest energy deficit and I & O costs. • Focus on more efficient dewatering instead of AD is recommended. [ABSTRACT FROM AUTHOR]

With ongoing economic development and population growth in China, large quantities of wastewater are discharged and substantial amounts of sewage sludge are generated, which is a potential biomass resource for methane production. This study used data from statistics yearbooks and literatures to evaluate the spatio-temporal distribution of sludge generation, potential and distribution of methane production from sludge, and the potential of greenhouse gas (GHG) emissions reduction (PGER) from sludge treatment and disposal in provinces in mainland China. The results showed that a total of 6.03 Mt of dry solids in 2015. The eastern part of the country produced more sludge than that the western part. The standard coal equivalence of sludge was 3.5 Mt, ranged from 0.7 to 399.8 Kt, among the 31 provinces of mainland China. Total sludge could produce 1.27 billion m3 of methane per year through anaerobic digestion (AD). The distribution of methane density exhibited that the east part was higher than the west part in China. The GHG emissions totaled 18.82 Mt CO 2 -eq as a baseline for the current sludge disposal methods without AD technology. The PGER was 10.77 Mt CO 2 -eq for the sludge disposal routes with AD technology. The sludge disposal for the project route of building material exhibited the highest PGER of 4.24 Mt CO 2 -eq among the disposal methods. As the least GHG emissions method, sludge treated by AD and the residue disposed for land using as fertilizer should be recommended. Our findings provide a sludge disposal reference for the government and industries. For further study, it is suggested to take the co-digestion technology into consideration for sewage sludge to produce methane, which would perform a higher methane production and GHG emissions reduction potential. Image 1 • A total of 6.03 Mt dry solids of sludge were generated in China in 2015. • Dry sludge with calorific value of 3.5 Mt was generated in 2015. • Methane production potential from sludge was estimated at 1.27 billion m3 in 2015. • Land application after anaerobic digestion can reduce GHG by 177.7 Kt CO 2 -eq. [ABSTRACT FROM AUTHOR]