Effects of alkali, autoclaving, and Fe+ autoclaving pretreatment on anaerobic digestion performance of coking sludge from the perspective of sludge extracts and methane production

Pretreatment of activated sludge is an important step in increasing the reaction speed during anaerobic digestion by accelerating the hydrolysis process. It is necessary not only to analyze the changes in the general properties of the sludge before and after pretreatment but also to further analyze and evaluate the sludge structure and extracellular polymeric substances (EPS). In this study, the changes in coking sludge extracts after pretreatments with alkali, autoclaving, and Fe+ autoclaving were analyzed and compared using EPS heat extraction method. Moreover, the methane production potential of the pretreated coking sludge was investigated via biochemical methane potential (BMP) test. The results showed that after alkali, autoclaving, and Fe+ autoclaving, the concentration of protein and polysaccharide in the bound sludge extract accounted for approximately 40% and 28%, 62% and 51%, and 66% and 83% of the total protein and polysaccharide extracted from the sludge, respectively. In the experiment without pretreatment, there is no phenomenon of gas production from coking sludge. According to the BMP test results, Fe+ autoclaving pretreatment showed the highest methane production of 257 mL/gVSS. This study revealed that the analysis of sludge extracts was necessary in assessing the effects of anaerobic digestion pretreatment and methanogenic potential. Moreover, coking sludge showed higher methanogenic potential after Fe+ autoclaving pretreatment.