Developing a novel lightweight substrate for constructed treatment wetland: The idea and the reality

The cost-effective and environmentally friendly substrates play an important role in functioning constructed wetlands (CWs). This study innovatively composes aluminum sludge and polyurethane under the action of a catalyst to develop a novel lightweight substrate (Al-NLS), which is expected to be used as a main substrate in CWs. The driving force of it lies in reducing construction costs of CWs in practice, while offering a flexible and considerably efficient substrate in CW. The bulk density of the resultant Al-NLS was apparently reduced by about 80 % compared with common substrate of gravel. The static adsorption experiments showed that the adsorption behavior of Al-NLS on phosphorus (P) was in accordance with Langmuir isotherm (with maximum P adsorption capacity of 1.12 mg/g,) and quasi-second-order kinetic models. The use of Al-NLS as a single stage wetland substrate to treat real domestic wastewater has favorable removal efficiencies of COD (65.9-74.3 %), TP (86.9-98.6 %), NH4+-N (61.1-52.2 %) and TN (35.4-45.6 %). The mechanisms of P removal onto Al-NLS were examined by various physiochemical characterizations of Al-NLS and raw alum sludge, which revealed that P adsorption was via forming Hydroxy-Al by ion exchange. In addition, high throughput sequencing analysis revealed that Al-NLS had positive impact on the richness and diversity of microbial community for pollutants removal, while the abundance of key functional bacteria such as proteobacteria and bacteroidota was higher in the longer retention time of CW (HRT = 2 days). These results indicated that Al-NLS could be used as a new alternative substrate in CWs technology.