Free nitrous acid pretreatment of wasted activated sludge to exploit internal carbon source for enhanced denitrification

• Free nitrous acid (FNA) pretreatment of waste activated sludge (WAS) was studied.
• FNA pretreatment of WAS enhanced internal carbon source exploitation.
• Denitrification performance and sludge reduction improved after FNA pretreatment.
• FNA pretreatment also relieved greenhouse gas N2O production in denitrification.

Using internal carbon source contained in waste activated sludge (WAS) is beneficial for nitrogen removal from wastewater with low carbon/nitrogen ratio, but it is usually limited by sludge disintegration. This study presented a novel strategy based on free nitrous acid (FNA) pretreatment to intensify the release of organic matters from WAS for enhanced denitrification. During FNA pretreatment, soluble chemical oxygen demand (SCOD) production kept increasing when FNA increased from 0 to 2.04 mg HNO2-N/L. Compared with untreated WAS, the internal carbon source production increased by 50% in a simultaneous fermentation and denitrification reactor fed with WAS pretreated by FNA for 24 h at 2.04 mg HNO2-N/L. This also increased denitrification efficiency by 76% and sludge reduction by 87.5%. More importantly, greenhouse gas nitrous oxide production in denitrification was alleviated since more electrons could be provided by FNA pretreated WAS.