Upgrading secondary wastewater plant effluent by modified coagulation and flocculation, for water reuse in irrigation

In this study, the feasibility of using coagulation, flocculation, and sedimentation (CF-S) for advanced treatment of secondary effluent released from the Yazd Intermittent Cycle Extended Aeration System was investigated. Four coagulants including ferric chloride (FeCl3), polyaluminum chloride (PAC), ferrous sulfate (FeSo(4)), and potassiumferrate (K(2)FeSo(4)) along with Gflog C-150 as flocculant polymer were used. In this study, returned chemical sludge was considered as a modification. Preliminary CF-S processes showed that FeSO4 and K2FeO4 had low removal efficiencies. Thus, these two coagulants were abandoned and CF-S processes were continued only with PAC and FeCl3 coagulants which had higher efficiencies in the removal of biological oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), and turbidity. Removal efficiency was higher when half of the chemical producing sludge was returned as compared with using both coagulants simultaneously along with 2 mg L-1 of C-150 as flocculant. In the optimum dosage, when half of PAC and FeCl3 sludge were returned, the volume of produced sludge was reduced by 40% and 28%, respectively, as compared without returned sludge. For the PAC coagulant in the optimum dosage with half of the sludge returned, all 2012 EPA standards of irrigation were met for both 'processed and non-processed type' agricultural crops.