The Effect of Free Nitrous Acid Pretreatment on The Anaerobic Digestibility of Thickened Waste Activated Sludge

Pretreatment methods have been applied to difficult to degrade organic waste to enhance anaerobic digestion. Many of these methods are characterized by intensive costs and/or negative environmental impacts. Compared with other methods, free nitrous acid (FNA) is inexpensive since it can be produced within a wastewater treatment plant (WWTP) and transforms to nitrogen gas during subsequent chemical reactions, posing little or no threat to the environment. The main goal of this study was to evaluate the effect of FNA pretreatment on the biodegradability of thickened waste activated sludge (TWAS) via methane production. FNA concentrations ranging between 0.4 – 2.8 mg N/L were investigated at a constant pH of 5.5 and contact time of 24 hours using biochemical methane potential test and semi-continuous reactors. The pretreatment showed an increase in the concentrations of soluble chemical oxygen demand (sCOD) by about 2.5 times. The BMP test treated with 1.4 mg N/L FNA showed the highest methane yield of 30% over the untreated test. However, in the semi-continuous reactors, the reactor that was fed with 0.7 mg N FNA/L yielded the maximum methane production which was 20% higher than the untreated reactor.