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 Cmin is 6 mg/L and Cnat is 10 mg/L. Statistical analysis has shown a significant direct relation between ΔWFG,mef and WFG,ref reached 0.952 and a significant inverse relation with Cef -0.982. WFG,T has shown a significant direct relation with Cr 0.974. WFG,T- ref has shown a significant direct relation with Cr as 0.971 and WFG,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.