Modelling the effects of ferric salt dosing for chemical phosphorus removal on the settleability of activated sludge

•The effects of Fe3+ salt dosing on the settleability of activated sludge was modelled.•ZSV increased and SSVI decreased with increasing dose of Fe3+ up to 50 mgL-1.•New settleability models for ZSV and SSVI for Fe3+ dosed sludge were proposed•Proposed models correlated with experimental data from a full-scale ASP plant. There is a growing utilisation of ferric salts (Fe3+) for precipitation of phosphorus from wastewater due to tighter regulatory limits for wastewater effluent discharge. The effect of Fe3+dosing on activated sludge settleability indicators; zone settling velocity (ZSV) and stirred specific volume index (SSVI) were examined in laboratory-based batch settleability tests over a three years period. The experimental data revealed that ZSV increased with increasing dose of ferric salt as SSVI decreased with the highest changes obtained at a dose rate of 50 mgL-1. Fe3+ facilitated the agglomeration of the activated sludge flocs, thereby improving settleability. At >50 (mgL-1) of Fe3+, there was a slight breakdown in sludge settleability due mainly to surface charge reversals. The modelling of the settleability of ferric sludge in recent wastewater engineering practice for design and improvement purposes is presently based on conventional empirical sludge settleability models. This paper proposed a new activated sludge settleability model that describes the effects of Fe3+ dosing on the ZSV and SSVI of activated sludge. The exponential form of the Vesilind equation was optimised and validated to include ferric chemical dosing parameters. The proposed ferric settleability equations were found to effectively describe the settling characteristics of ferric sludge.