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.
Highlights
With the increasing global population and the growth of cities, the quantity of municipal wastewater has rapidly✉ Corresponding authorFront
These findings suggest that the highsolid Anaerobic digestion (AD) (HS-AD) of sewage sludge (SS) may need a special anaerobic digester with the enhancement of stirring impeller and mix system, which is different from the conventional anaerobic digester
This paper comprehensively reviews the current research on the main factors affecting process stability and performance, the improvement methods including pre-treatment and HS anaerobic co-digestion (HS-AcD) of SS and other organic wastes, and the characteristics of substance transformation
Summary
With the increasing global population and the growth of cities, the quantity of municipal wastewater has rapidly. Hidaka et al (2013) reported that the AD of SS containing 10% TS can be successfully achieved under mesophilic conditions, and highlighted that controlling total ammonia concentration renders the HS-AD of SS suitable for use in small facilities These findings were further confirmed by Liao et al (2014), who found that the HS-AD of SS significantly increased the volumetric biogas production rate and the treatment capability of digesters. The numbers of such publications have increased in the last 15 years, with more than twice as many publications containing the term “HS-AD” than the term “HS-AD of SS” in this period This indicates that HSAD has been mainly applied to the processing of non-SS organic wastes in the last 15 years, as confirmed by recent studies (Li et al, 2011; Fagbohungbe et al, 2015; André et al, 2018). The aims of this paper are to provide a comprehensive review of the research progress in the development of HS-AD systems for SS processing, to identify knowledge gaps, and to discuss the future directions of research to improve the HS-AD of SS
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