Treatment of agricultural drainage water (ADW) in a down-flow hanging sponge (DHS) reactor has been investigated for a period of 1 y. The reactor was operated at different hydraulic retention times (HRTs) of 5.26, 2.63, and 1.5 h and corresponding organic loading rates (OLRs) of 2.01, 1.86, and 4.43 kg COD/m3 d, respectively. The results obtained indicated that, decreasing the HRT from 5.26 to 1.5 h negatively affected the chemical oxygen demand (COD) removal. Average COD percentage removal values dropped from 89.3 ± 5.4 to 72.2 ± 16% respectively. Total suspended solids (TSS) concentrations were reduced by 85.5 ± 8.3, 90.4 ± 2.1, and 91.4 ± 4.9 at HRTs of 5.26, 2.63, and 1.5 h, respectively. However, the nitrification efficiency was slightly decreased from 85.2 ± 17.4 to 82 ± 11.4% by decreasing the HRT from 5.26 to 1.5 h and increasing the OLR from 2.01 to 4.43 kg COD/m3 d, respectively. Likewise, reducing the HRT from 5.26 to 1.5 h exerted a slight negative effect on the removal of the total phosphorus (TP). Based on these results it is recommended to operate such a system at a HRT of 2.63 h and OLR not exceeding 2.0 kg COD/m3 d for providing an effluent quality complying with the standards regulating the use of treated wastewater for agricultural purposes.

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