Behaviour of Inorganic Coagulants in Secondary Effluents from a Conventional Wastewater Treatment Plant

Abstract

An experimental laboratory scale study was conducted to compare the effectiveness of three inorganic coagulants — aluminium sulphate, ferric chloride and polyaluminium chloride (PAC) — in reducing the turbidity of secondary effluents from a conventional wastewater treatment plant. For each inorganic coagulant, the dose and the pH of coagulation were optimized. The results enabled the selection of the most appropriate coagulant, its dose and the optimal operational conditions to obtain the required water quality (3–5 NTU) for re-use in agricultural drip irrigation systems. PAC was identified as the most suitable coagulant/flocculant for reducing the turbidity of this type of water. The optimal conditions corresponded to pH 6 and a dose of PAC of 20 mg/l, but the recommended conditions are 50 mg/l for pH 8 (the initial pH of the water) to obtain a turbidity reduction of 90%, which is independent of the initial turbidity of water (i.e. 5–32 NTU).