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 the alum sludge, an inescapable by-product of the production of drinking water 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 pollutants and 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 acted as P adsorbent can secure the P removal. Meanwhile, high removals of ammoniacal-nitrogen and organic matters can also be obtained due to the active bacteria attached growth on the alum sludge surface. The results show that average removal efficiencies of 65.4 ± 12.3% for COD, 67.8 ± 9.2% for 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 pollutants feedback through reject water recycling in a MWWTP.