Waste activated sludge disintegration with free nitrous acid – Comprehensive analysis of the process parameters

Current data about disintegration of waste activated sludge (WAS) with free nitrous acid (FNA) do not provide information about the efficiency of concentrations higher than 6.1 mgHNO₂-N/L and exposure times over 72 h. Moreover, current references do not represent disposable soluble chemical oxygen demand (SCOD) release as data on COD consumption for denitrification of residual nitrites are not presented. In this study, WAS from a large wastewater treatment plant (WWTP) was exposed to an FNA concentration ranging from 0.0 to 352.7 mgHNO₂-N/L for as long as 6 days to provide missing data. FNA concentrations higher than previously reported have been shown to release greater amounts of organic matter, even when the requirement for denitrification is taken into account. Therefore higher concentrations may be beneficial for WWTPs, especially when a drop in costs of sludge disposal is considered. Demand of organic compounds for nitrites denitrification can constitute up to 86% of total released SCOD and therefore this process has to be included. Besides, tests for specific biomethane potential (SBMP) showed that increasing FNA concentrations above 6.4 mgHNO₂-N/L did not benefit the growth of SBMP anymore, while FNA at least up to 3.7 mgHNO₂-N/L did not increase the methane potential compared to control.