Optimization of a Vermifiltration process for the treatment of high strength domestic greywater in hot climate area: A Response Surface Methodology approach

Discharging untreated or partially treated greywater spreads diseases to humans and depletes dissolved oxygen in water, endangering aquatic life. Current greywater treatment methods often require high capital investment, large floor space, and significant energy, whereas vermifiltration is an ecologically safe, cost-effective technology that efficiently reduces high levels of organic matter in wastewater. The present study focuses on the modeling and optimization of COD removal of a vermifiltration system, using Response Surface Methodology. The vermifilter consists of sawdust, sand, and gravel as filter media, and Eudrilus Eugenia as worm species. Experiences were conducted at room temperatures (26 - 31°C). Key factors considered as influencing COD removal were hydraulic loading rate (HLR), initial COD, and earthworm density (EWD). All three factors significantly impacted COD removal, with notable cross effects. The model predicted a maximum COD removal of 91.51% for influent with 1087 mg/L COD, 178 earthworms, and 133 L/m²/day HLR, achieving a residual COD value of 92.29 mg/L, that meet the requirements for the WHO discharge guidelines. However, due to high variability of household greywater quality in the area, the system has been full-scale designed for the value of 2500 mg/L which corresponds according to the model, to 123L/m²/day HLR. The life cycle cost (LCC) of the treated water is therefore 0.083USdollars /m³. Earthworm's growth was satisfactory (17 - 52.5%) in most filters. Finally, results suggest that the model can be used to design field-scale vermifiltration systems with minimal variation.