High-solid anaerobic digestion of sewage sludge: achievements and perspectives

Abstract

High-solid anaerobic digestion (HS-AD) has been applied extensively during the last few decades for treating various organic wastes, such as agricultural wastes, organic fractions of municipal solid wastes, and kitchen wastes. However, the application of HS-AD to the processing of sewage sludge (SS) remains limited, which is largely attributable to its poor process stability and performance. Extensive research has been conducted to attempt to surmount these limitations. In this review, the main factors affecting process stability and performance in the HS-AD of SS are comprehensively reviewed, and the improved methods in current use, such as HS sludge pre-treatment and anaerobic co-digestion with other organic wastes, are summarised. Besides, this paper also discusses the characteristics of substance transformation in the HS-AD of SS with and without thermal pre-treatment. Research has shown that the HS effect is due to the presence of high concentrations of substances that may inhibit the function of anaerobic microorganisms, and that it also results in poor mass transfer, a low diffusion coefficient, and high viscosity. Finally, knowledge gaps in the current research on HS-AD of SS are identified. Based on these, it proposes that future efforts should be devoted to standardising the definition of HS sludge, revealing the law of migration and transformation of pollutants, describing the metabolic pathways by which specific substances are degraded, and establishing accurate mathematical models. Moreover, developing green sludge dewatering agents, obtaining high value-added products, and revealing effects of the above two on HS-AD of SS can also be considered in future.

Emerging Trends and Prospects for Municipal Wastewater Management in China

“…A high content and biodegradability of organic constituents in SS is key to successful implementation of advanced AD anaerobic digestion plus landscape/land application. Unfortunately, as discussed above, in many areas of China the organic content in SS has been found to be much lower (usually volatile solid (VS) < 55% of total solid (TS)) than that in developed countries . In such instances, SS contains a high proportion of grit (14–56.8% TS, with an average value of about 35% TS) , and multivalent metals (4–11.7% TS). , Such high concentrations of these inorganic constituents in SS can negatively influence the biodegradability of the organic constituents in SS .…”

“…In such cases, use of conventional AD systems to treat SS is not always feasible. For example, in 2019, 70 of China’s 5200 WWTPs incorporated the conventional AD systems but only 20 of these were operated routinely …”

Hydrothermal Liquefaction of Biomass as One of the Most Promising Alternatives for the Synthesis of Advanced Liquid Biofuels: A Review

“…The objective is to transform the most degradable fraction of organic matter into biogas, a mixture of gases made up mainly of methane, carbon dioxide, and other gases to a lesser extent (CO, N 2 , H 2 , water vapor, H 2 S) [ 46 ]. The matter resulting from this process, which is called biomethanization digestate, can be considered a waste or can be treated again to obtain another type of biofuel [ 52 , 53 , 54 ].…”

High-Solid Anaerobic Digestion: Reviewing Strategies for Increasing Reactor Performance

“…However, the increase in solid content results in several negative effects, leading to imbalances in the process. Xu et al [21] reviewed the performance of HS-AD of sewage sludge, also considering 6% total solid content as a barrier for defining HS-AD. They indicated that the main limitation when operating under these conditions was associated with process instability due to mass transfer limitation problems, high viscosity, and the accumulation of inhibitory compounds.…”