Aged landfill leachate enhances anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) is considered as a sustainable pathway to recover energy from organic wastes, but the digestive efficiency for waste activated sludge (WAS) is not as expected due to the limitations in WAS hydrolysis. This study proposes an effective strategy to simultaneously treat WAS and landfill leachate, aiming to promote WAS hydrolysis and enhance organics converting to methane. The effects of landfill leachate on the four stages (i. e., solubilization, hydrolysis, acidogenesis, and methanogenesis) of AD of WAS, as well as the effect mechanisms were investigated. Results showed that adding appropriate amounts of landfill leachate could promote the steps of solubilization, hydrolysis and acidogenesis of WAS, but had no-effect on methanogenesis. The hydrolysis and acidogenesis efficiency in the leachate added digesters were 2.0%-8.4% and 35.2%-72.7% higher than the control digester. Mechanism studies indicated that humic acid (HA) contained in the leachate was conducive to the processes of both hydrolysis and acidogenesis, but detrimental to the methanogenesis. Effects of heavy metals (HMs) on AD of WAS was also dose-dependent. Digestive performance was inhibited by excessive HMs but promoted by moderate dosages. Humic acid and metal ions tend to interact to form complexes, and thus relieve their each inhibition effects. It is also found that the stability of sludge flocs was reduced by the leachate through reducing both apparent activation energy (AAE) and median particle size (MPS) of the sludge. Microbial community and diversity results revealed that the relative abundance of microbes responsible for hydrolysis and acidogenesis increased when landfill leachate was present. This research provides a more technically and economically feasible approach to co-treating and co-utilizing WAS and landfill leachate.

Automated summary

Anaerobic digestion (AD) is a sustainable method for recovering energy from organic wastes, but the efficiency for waste activated sludge (WAS) is limited due to WAS hydrolysis constraints. This study proposes a strategy to treat WAS and landfill leachate simultaneously, aiming to enhance WAS hydrolysis and organic conversion to methane. The effects of landfill leachate on the different stages of AD of WAS were investigated, showing that appropriate amounts of leachate can promote solubilization, hydrolysis, and acidogenesis, but not methanogenesis. Mechanism studies revealed the beneficial effects of humic acid in leachate on hydrolysis and acidogenesis, and the dose-dependent effects of heavy metals on the AD process. Additionally, microbial community analysis showed an increase in microbes responsible for hydrolysis and acidogenesis with the presence of landfill leachate.