The influence of iron-based co-precipitants on activated sludge biomass

European Community legislation will increase the requirement for efficient phosphorus removal. The impact of chemical phosphorus removal by co-precipitation has only been considered from an effluent quality point of view. This paper considers the direct impact of two iron-based chemicals-iron (III) chloride and iron (II) sulphate-on an activated sludge pilot plant. Average chemical oxygen demand (COD) removals were 84.0% for iron (III) and 78.6% for iron (II). Nitrification appeared to be inhibited by the addition of high doses of iron (III). Ammonia removals of just 17.3% were observed compared with 88.7% for iron (LI) addition. Suspended solids removal was poor (average removals = 69.5%, for iron (II) and 48.0% for iron (III)) during the last few months of the trial due to problems with filamentous bacteria and denitrification in the sedimentation tanks. The oxygen uptake of the biomass dosed with iron (II) (0.0121 mg O-2 mg biomass(-1) h(-1)) was on average 32.4% higher than that of iron (III); the latter averaged 0.0086 mg O-2 mg biomass(-1) h(-1). Species diversity index (S-T) was Similar for both chemicals; S-T = 24.7 with iron (II) addition and S-T = 22.1 with iron (III). There was a difference in the flee characteristics of the biomass. The flocs produced with iron (II) were larger and more dense than those with iron (III).