Joint influence of hydraulic load and hydraulic retention time on oilfields wastewater contaminant removal dynamics in free water surface flow constructed wetland

Abstract

Constructed wetlands have been proposed to address the frequency and magnitude of oil and gas-related environmental contamination. The effect of co-variation of hydraulic load and hydraulic retention time on the dynamics of contaminant removal efficiency of heterogeneous plant species was assessed using ordinary least squares regression. The results showed that hydraulic load (HL), hydraulic retention time (HRT) and plant species jointly explain 87%, 79%, 83%, 85% and 66% of the total variance in removal efficiency of conductivity, TDS, BOD, COD, and total coliform bacteria, respectively. The models also explain 86%, 80% and 81% of the variations in removal efficiency of oil and grease, total phosphorus, and nitrate. More than 90% of the explained variance of total coliform removal efficiency is jointly attributable to hydraulic load and retention time. Hydraulic load of 1000 L and retention time of 72 h (1000 L 72 h) recorded optimum removal efficiency for TDS and conductivity. Optimum removal efficiency for BOD, COD and total coliform bacteria were achieved at HL and HRT of 1000 L 72 h, 1000 L 48 h and 1250 L 24 h, respectively. Alternanthera philoxeroides recorded the highest removal efficiency for oil and grease, conductivity and TDS, whereas Ruellia simplex recorded the highest removal efficiency for COD. Typha latifolia had the highest removal efficiency for total phosphorus and nitrate. Plant species suppressed the relationship between HL and HRT (1250 L 48 h, 1500 L 72 h, 1750 L 48 h and 2000 L 48 h) and removal efficiency for conductivity. Similarly, plant species suppressed the relationship between 1000 L 48 h and 1750 48 h and removal efficiency for TDS. These relationships underscore the complex dynamics between optimal contaminant removal efficiency and required hydraulic load, hydraulic retention time and plant species.