The impact of residual coagulant on downstream treatment processes

A series of jar tests were undertaken to optimise for suspended solids (SS) and phosphorus removal from raw wastewater. The residual metal concentration in the settled wastewater from the jar test experiments and the residual concentration from the optimum doses plus two higher doses were selected for investigation. The identified levels of residual metal were fed into a four lane activated sludge pilot plant to investigate the impact of metal concentration on (i) activated sludge performance and (ii) sludge production and characteristics. Optimum pre-precipitation studies showed residual ion concentrations of 1.68 and 3.46 mg l-1 for Fe(III) and Al(III) respectively. At these levels %P removal increased by approximately 25 and 60% respectively. NH3 removal decreased by approximately 20 and 34% in the activated sludge treatment process. Chemically dosed biomass had a significantly lower oxygen uptake rate than the control which was accompanied by a reduction in VSS; 10% for Fe(III) and 17% for Al(III). Changes in sludge characteristics were also observed. Chemical sludge had a greater settleability but a lower dewaterability than biological sludge. Sludge floc morphology was characterised which showed chemical flocs to be consistently smaller and visually denser than biological sludge flocs. The work presented in this paper considers the impact of residual iron and aluminium coagulants on downstream treatment processes.