Grey water footprint for evaluating Zefta wastewater treatment plant: a case study

The numbers of wastewater treatment plants (WWTP) in Egypt are increasing, yet the general level of pollution associated with wastewater discharge after treatment has not been evaluated. Grey water footprint (GWF) was used to assess the effluent discharges from Zefta WWTP. GWF, before and after treatment, was calculated and followed up to determine its impact on the receiving freshwater body. 150 samples were collected and analysed for BOD₅ to determine the optimum operating conditions. Averages values were DO = 2.2, SV₃₀ = 500, SVI  = 167, SA = 9.3 d, MLVSS = 2392 mg/L, f/m = 0.16, MLSS in RAS = 7922 mg/L, WAS = 140 m³/d and the HRT = 12 h. The removal efficiency of COD and TSS in the primary settling tank reached 39% and 69%, respectively. Average calculations of removal efficiency percentile reached 90-93%. Average freshwater quantities required to reduce pollutants in the receiving body stream were seasonally determined for Zefta WWTP as 5.3 × 10⁷ m³/year. The average influent BOD₅ was 376 mg/L, it was reduced to 47 mg/L in the effluent, wherever the C_min is 6 mg/L and C_nat is 10 mg/L. Statistical analysis has shown a significant direct relation between ΔWF_G,mef and WF_G,ref reached 0.952 and a significant inverse relation with C_ef -0.982. WF_G,T has shown a significant direct relation with C_r 0.974. WF_{G,T- ref} has shown a significant direct relation with C_r as 0.971 and WF_G,T as 0.803. It can be concluded that ΔWF is effective in evaluating the efficiency of wastewater treatment and its effect on the quality of receiving water bodies.