Capturing influent organic substrate for endogenous denitrification to enhance nitrogen removal in low C/N ratio municipal wastewater

Abstract

Intending to reduce aerobic oxidation of carbon sources and leave more carbon resources for denitrification to improve nitrogen removal in low C/N ratio wastewater, batch tests were conducted by allowing sludge to directly capture carbon sources from the influent for feeding denitrifier. The capture effect of three pretreatments and capture modes, i.e., (a) oxic famine (Ofam) pretreatment and then anoxic feast (Afea) mode (Ofam-Afea); (b) anoxic famine (Afam) pretreatment and then oxic feast (Ofea) mode (Afam-Ofea); (c) Afam pretreatment then aerobic feeding of treated sludge in continuous stirred tank reactor (CSTR) with side-stream push-flow reaction (PFR) zone (Afam with side-stream PFR), were investigated. The storage of organic substances and accumulation of intracellular polymers by the sludge in Ofam-Afea mode was compared with that of the sludge in Afam-Ofea mode. The results show that the total capture efficiency (TCE) of the sludge in the Ofam-Afea mode was around 60 %, the amount of glycogen (Gly) remained essentially unchanged, and the intracellular polyhydroxyalkanoates (PHA) accumulated significantly without apparent “stagnation” for carbon source switching. In Afam-Ofea mode, the TCE increased to 60 %–70 %. The amount of Gly increased by 31.2 %–50.0 %, PHA accumulated considerably, and high denitrification efficiency was achieved, whereas denitrification stopped dramatically with the stage of carbon sources switching. The Afam with side-stream PFR allowed high-speed denitrification and hydrolytic fermentation, eliminating the “stagnation” of switching carbon sources. The optimal VPFR/VCSTR values were 1–1.5. A shorter side-stream time was equivalent to CSTR, whereas a longer side-stream time caused the sludge to synthesize fewer intracellular polymers.