Sustainable treatment scheme for in-situ remediation of contaminated drains using engineered natural systems

Ensuring water security in resource-constrained, densely populated regions is a significant challenge globally. Due to insufficient treatment infrastructure, untreated sewage discharge into drainage channels is prevalent, especially in developing countries. This leads to the pollution of already dwindling water bodies and threatens future water availability. In this context, in-situ treatment within drains using nature-based systems is an attractive option. This study evaluates microbial bioremediation and phytoremediation as engineered natural solutions for in-stream treatment of municipal wastewater. A three-stage treatment system consisting of anoxic biofilm, aerobic biofilm, and hydroponic floating wetlands was adopted. Each stage was optimized for operational parameters through batch and continuous flow studies. The anoxic biofilm system using autoclaved aerated concrete (AAC) as the attachment media, at an optimized hydraulic retention time (HRT) of 2 h, showed the best performance with respect to COD removal. Comparable COD removal was observed in both externally aerated and non-aerated aerobic biofilm systems with coir fibre at 6 h HRT. However, aerated system outperformed non-aerated system at low HRTs. The hydroponic system with Canna indica effectively removed residual ammonia-N with an HRT of 2 h. The sequential continuous flow studies employing the optimized conditions showed significant removals of COD (86%) and ammonia-N (97.6%). The results highlight that locally available materials having a high specific surface area can be used as biofilm supports for COD removal, and floating wetlands employing indigenous macrophytes can be an ideal choice for in-situ nutrient removal. The Life Cycle Assessment (LCA) showed that the developed system did not have direct significant impacts on freshwater eco-toxicity and eutrophication. The proposed hybrid treatment system can be implemented as modular units without major drainage modifications or energy-intensive operations. The study, therefore, finds potential application in densely populated settlements in low-income countries where systematic sewage treatment options remain inadequate.