Optimization and design of laundry greywater treatment process with multiple electrodes electrocoagulation

Abstract

It is becoming increasingly important to relook at the impact that greywater has on the environment and the opportunities that it can provide for water re-use. This is particularly true for South Africa which has an average sewer blockage rate that far supersedes international averages. Population growth, and the resulting increase in the rate of inflow or infiltration often results in wastewater treatment plants being stretched beyond their design capacity. To cope with current and future wastewater treatment demands, developments of alternative efficient and ecologically safe wastewater treatment methods are an absolute imperative. Treatment of greywater with electrocoagulation-flocculation (ECF) techniques is one of the promising technologies. However, ECF applications seem to be heuristically approached, perhaps due to the complexity and multitude of ECF process control variables and a lack of understanding of their interactions. These variables such as, electrode types; effluent types; voltage; current density; ECF, reactor volume; mixing speeds; and many others, may not only have a single effect on output but multiple inter-variable effects that need multi-parameter factor analysis. Design of Experiment (DoE) and Response Surface Methodology (RSM) are statistical techniques that can be employed to investigate the interaction of factors on the treatment efficiency of the ECF. In this research, the ECF efficacy was investigated by varying electrode types (iron and aluminum, alternatively as anodes) in 800 mL and 2000 mL reactors to treat laundry greywater (LGW) under varying operating conditions of initial pH (3.5 to 8.5), voltage (10 to 15 V) and mixing speed (0 to 500 rpm) that were carefully planned with DoE. The optimum operating conditions were found to be at initial pH of 4.5 to 5.75; applied voltage of 10V to 13.5 V; at mixing speeds between 125 and 350 rpm. Aluminium electrodes favoured effluents with more chlorides in LGW than the iron electrodes, therefore the addition of chlorides may improve iron ECF. In this study, the results revealed that the volume of the effluent can be increased to values 2000 mL while achieving favorable treatment efficiency. This suggests that the ECF method can be scaled-up for the treatment of LGW with high TSS, pH, turbidity, and color removal efficiency.