The resilience of constructed wetlands treating greywater: the effect of operating conditions and seasonal temperature decline

Abstract

The use of constructed wetlands to remove pollutants from greywater is a viable solution, but seasonal variability of microbial activity and persistence of some of the organic compounds may impact their treatment efficiency. These shortcomings could be overcome using reactive media like manganese oxides (MnOx) that, as strong oxidants, extend the set of abiotic reactions. The reactivity of MnOx can be altered by the presence of dissolved oxygen, presence of vegetation and ammonium, among many other factors. In this study, constructed wetlands filled with commercial filtering material consisting of MnOx (specifically pyrolusite) or sand were used to treat greywater in an outdoor batch experiment exposed to natural seasonality. The effect of five variables (controlled ones: the presence of MnOx, plants, ammonium, the use of aeration, and uncontrolled: ambient temperature) on the removal of organic pollutants was evaluated. The four selected organic pollutants were: 5-methylbenzotriazole (5MBTR), metoprolol (MTP), bisphenol S (BPS), and diclofenac (DCF). The removal efficiency of DCF in the presence of MnOx was higher than 99% and unaffected by the temperature. The removal was confirmed to be associated with abiotic oxidation of DCF by MnOx. In contrast, sand-filled treatments outcompeted MnOx in removing 5MBTR, MTP, and BPS. The lower removal of 5MBTR, BPS, and MTP in the treatments with MnOx was probably due to organic carbon deficiency. This hypothesis was partially verified by the enantioselective analysis of MTP and the identification of its transformation product (MTP acid, MTPA) in both sand and MnOx treatments, indicating similar biotic removal mechanisms despite contrasting removal efficiencies (by 46–78%). The removal of the studied compounds by biodegradation correlated strongly with the determined electrophilicity index (EI), and the reactivity with MnOx with ionization potential (IP). The unaerated treatments with sand and plants were found the most effective and provided the greatest treatment resilience. In these treatments, the highest removals of the organic pollutants were 65%, 78%, 95% and 80% for 5MBTR, BPS, MTP and DCF, respectively.