To what content the chemical characteristics of sludge extracellular polymeric substances affect anaerobic digestion efficiency: A review

Anaerobic digestion (AD) has traditionally been regarded as one of the most low-carbon approaches in waste activated sludge (WAS) disposal throughout the world. To continuously improve the operational efficiency of AD systems, tremendous pretreatments have been explored to extract the sludge extracellular polymeric substances (EPS), aiming to enhance the release of sludge organics into supernatant. Recent studies have found that extracting EPS could effectively promote AD, mainly via the promotion of the sludge hydrolysis and the substrates release, as well as the enhancement of the interaction between substrates and methanogens. However, to what content and how the EPS extraction would affect the performance of AD systems, for WAS treatment, was seldom reported. Here, the differences in chemical compositions and structures of various sludge EPS layers were first summarized. Then, the effect and mechanism of the extraction of different EPS layers on AD systems were emphatically analyzed. Finally, the dynamic equilibrium between EPS extraction and methane yields, as well as the utilization of EPS extraction sludge as carbon sources were summarized for further practical engineering application. It was found that the contribution of extracting different EPS layers to AD performance exhibited an increasing trend: S-EPS<LB-EPS<TB-EPS. Specifically, extracting TB-EPS significantly enhanced the release of exchangeable Ca and organic-binding Fe, which improved the hydrolysis of sludge organics (6–7 times higher than S-EPS) and led to a notable increase in AD efficiency, with methane production rising by 14–35 %. These findings contribute to a deeper understanding the effect of extracting different EPS layers on methane production and provide insights for designing precise sludge pretreatment strategies.