Enhancement of the Wastewater Treatment Process of a PETRO System by Natural and Commercial Coagulants

Water pollution due to insufficient wastewater treatment is a global concern. In this paper, coagulation and flocculation as a tertiary polishing unit process were investigated to find a solution for a non-compliant wastewater treatment facility. The Palapye Pond Enhanced Treatment and Operation (PETRO) system has not been compliant for a long time with effluent characterised by high turbidity, Biological Oxygen Demand/Chemical Oxygen Demand (BOD/COD), Total Suspended Solids (TSS), Nitrates (NO3−), and Phosphates (PO43−) The effluent from the plant is released into the stream that drains into the nearby Lotsane dam, posing significant danger to the water quality of the dam. The main objective of the study was to investigate the effect of coagulation and flocculation processes at the tertiary stage of the wastewater treatment process. Response Surface Methodology (RSM), Central Composite Design (CCD) and Multi Response Surface (MRS) were used to optimise the coagulation process and generate regression models to predict the coagulation and flocculation. The performance was evaluated using turbidity, Colour, COD and TSS as response variables. Response surface analysis indicated that the experimental data could be adequately fitted to quadratic polynomial models. Under optimum conditions the removal efficiency for Al2(SO4)3·18H2O: 91.1% (turbidity), 88.2% (colour), 58.9% (COD), 83.0% (TSS); for FeCl3·6H2O: 93.2%, 88.7%, 63.8%, 91.3%; for Moringa: 91.8%, 85.4%, 56.6%, 83.7%. The optimal removals based on MRS for Al2(SO4)3.18H2O, FeCl3.6H2O and Moringa oleifera were 90.7%, 89.7%, 59.9% and 88.5%; 94.7%, 90.8%, 58.1% and 93.8%; 94.0%, 87.2%, 60.1% and 82.1% for turbidity, colour, COD and TSS respectively. This research has demonstrated that the coagulation/flocculation process, operating synergistically with pH-induced precipitation softening, can be incorporated as an enhancement to the secondary treatment stage of the wastewater treatment facility. At the optimal alkaline conditions (pH 12–12.6), the dominant mechanism is the precipitation of native hardness ions (Mg2+, Ca2+) as Mg(OH)2 and CaCO3, which enmesh colloidal particles, while the added coagulants play a refining role by enhancing floc structure and settling. The study introduces a comparative evaluation of three coagulants within a single RSM-CCD optimisation framework, employing desirability functions for multi-response optimisation.