Anaerobic digestion disposal of sewage sludge pyrolysis liquid in cow dung matrix and the enhancing effect of sewage sludge char

Abstract Sewage sludge pyrolysis liquid (SSPL) is a mixture of various toxic organic compounds and moisture, its disposal or valorization are crucial for the sustainability of any pyrolysis plant of sewage sludge (SS). As SSPL showed strong inhibition on anaerobic digestion (AD), in order to fulfill its clean disposal, the study investigates its AD in cow dung matrix under thermophilic condition with sewage sludge char (SSC) addition. Three SSCs produced at different temperatures and two biochars were added into the AD system for comparison. Batch experiments (55 °C, 26 days) reveal that addition of SSCs/biochars could increase cumulative methane yield by 57.5–128.7%/120.7–133.7% compared with the group without char addition. Among three SSCs, the one produced at 550 °C (550SSC) shows the best effect and has similar promotion effects to those of conventional biochars. The mechanism involved is explained by the following facts: SSCs/biochars addition alleviated microbial inhibition effect of SSPL by adsorbing the toxic organic compounds in the liquid at the early stage, hosting microbes to grow, improving the reaction rate and stabilizing the pH of AD system at around 8, finally helping to obtain an earlier occurrence of daily methane yield peak and contributing to higher methane yields. The rich trace elements contained in SSCs, especially in 550SSC, help to promote the growth of microbes and contribute to the decomposition of toxic organic compounds. GC-MS analyses of digestates reveal that SSPL can be well degraded with help of SSCs/biochars addition after AD process. The obtained results suggest that AD of SSPL in cow dung matrix with 550SSC addition is a reliable and economy alternative for its treatment and valorization.