Sewage Treatment by Kolkata’s Natural Wetland System

The metropolis of Kolkata stands uniquely positioned to implement a natural sewage treatment paradigm through the utilization of waste stabilization ponds, specifically within the East Kolkata Wetlands (EKW). These shallow oxidation ponds harness solar irradiation and algae bacteria symbiotic processes to effectively treat incoming sewage. Concurrently, nutrient-rich effluents are assimilated through fish production, converting available nutrients into protein—a hallmark of nature-based treatment. A portion of raw sewage is used to cultivate a chunk of vegetables before treatment in fish ponds, and the reclaimed water after treatment is used for vegetable and paddy cultivation downstream. This investigation explains the delineation of a sewage flow system to EKW, a Ramsar-designated site. Substantively, it offers quantitative insights into the sewage volumes and quality undergoing treatment. The sewage flow is higher in the winter months (909.07 MLD) compared to the summer months (709.34 MLD). In general, the sewage from the Kolkata city flowing to the EKW is moderately polluted. Extensive scrutiny of sewage from pond inlets and outlets serves as a quantitative metric for evaluating treatment efficacy. EKW efficiently treats the sewage, demonstrating 59.1% Biological Oxygen Demand (BOD) removal and a 99.28% reduction in fecal coliform. The natural treatment system excels in removing ammoniacal nitrogen (80.38%) and phosphate (90%). The treated water’s quality along the EKW boundary, culminating at the Kulti Gong River discharge point, was systematically assessed. Analytical findings indicate that all measured concentrations in the treated water adhere to prescribed inland surface water discharge standards prescribed by the Central Pollution Control Board, India, barring a marginal elevation in BOD during winter. Evidently, the EKW system adeptly manages substantial sewage volumes, fostering efficient treatment while concurrently facilitating resource recovery through fish production, yielding economic dividends. Despite its substantial land footprint, preserving this inherently sustainable wastewater management paradigm is imperative.