Effects of free nitrous acid and nitrite on two-phase anaerobic digestion of waste activated sludge: A preliminary study

Nitrite, a product of the nitritation of sewage or digestion liquid, has been used to pretreat waste activated sludge (WAS) before anaerobic digestion. In this study, the effects of free nitrous add (FNA) and nitrite on two-phase anaerobic sludge bioconversion were investigated. The experimental results indicated that both nitrite and FNA promoted sludge organic solubilization. Notably, nitrite promoted volatile fatty acids (VFAs) accumulation while FNA inhibited VFA accumulation in the first phase (acidogenic phase). In the second phase (methanogenic phase), neither nitrite nor FNA improved the net cumulative methane production from WAS. Although net cumulative methane production was not enhanced by the addition of nitrite or FNA, the volatile solids (VS) degradation rate was improved with nitrite addition in the two-phase anaerobic digestion process, indicating that nitrite is more favorable than FNA for the two-phase anaerobic digestion of WAS. It is expected that these findings can offer useful insights into future design of anaerobic digestion system with the treatment by the nitrite from digestion liquid. (C) 2018 Elsevier B.V. All rights reserved.