A promising approach of reject water treatment using a tidal flow constructed wetland system employing alum sludge as main substrate

This study examined a novel reuse of alum sludge, an inescapable by-product of drinking water treatment process when aluminium salt is added as a coagulant, as the main medium in a laboratory-scale multi-stage constructed wetland (CW) system for reject water treatment. Such reject water is a main concern in municipal wastewater treatment plant (MWWTP) for increasing the organic and nutrient loading. A ‘tidal flow’ strategy was employed to enhance the wetland aeration to stimulate organic matters (OM) and ammoniacal-nitrogen (N) oxidation while the ‘step feed’ operation was adopted to supply the necessary amount of carbon source for denitrification. The results reveal that alum sludge acting as P adsorbent can secure the P removal. Meanwhile, high removals of N and OM can also be obtained due to the active bacteria growth on the alum sludge surface. The results show that average removal efficiencies of 65.4 ± 12.3% for chemical oxygen demand (COD), 67.8 ± 9.2% for five-day biochemical oxygen demand (BOD₅), 33.6 ± 17.0% for N and 99.5 ± 0.49% for P can be achieved over a period of 190 days. This indicates that novel reuse of alum sludge as medium in CW system can provide a promising approach for reject water treatment. Therefore, it will significantly reduce the amount of pollutant feedback through reject water recycling in a MWWTP.