Evaluating the role of iron dosing in sludge reduction in a sequencing batch reactor integrated with an anaerobic side-stream reactor

Abstract

Anaerobic side-stream reactors (ASSRs) can reduce sludge production in activated sludge systems, although the responsible mechanisms remain uncertain. This study evaluated the role of iron dosing in sludge reduction in a sequencing batch reactor (SBR) integrated with an ASSR. A control SBR (SBR.C) and an SBR.M integrated with an ASSR were operated in parallel, with iron concentrations of 1 and 5 mg Fe/L applied sequentially. Increasing iron from 1 to 5 mg Fe/L increased the observed sludge yield from 0.401 to 0.523 g VSS/g COD in SBR.C and from 0.209 to 0.275 g VSS/g COD in SBR.M. However, the relative reduction in observed sludge yield remained approximately 47% at both iron concentrations. The higher iron dose was also associated with lower soluble COD and nutrient concentrations in the ASSR, consistent with greater retention of organic matter and nutrients within the sludge matrix. At 5 mg Fe/L, mass balances showed that direct sludge degradation within the ASSR accounted for only a limited fraction of the lower sludge production. Sludge from SBR.M showed higher substrate uptake and a lower apparent biomass yield, while the additional COD uptake was not accompanied by proportional net polyhydroxyalkanoates or glycogen accumulation. These findings indicate that iron primarily modified organic matter retention within the sludge matrix, whereas the lower sludge production in SBR.M was more associated with altered carbon utilization, in which increased substrate uptake was not accompanied by proportional net biomass formation.