Assessing performance of local materials for the treatment of dry weather flows in open drains: Results of semi-controlled field experiment research in Bangalore, India

Abstract

The efficacy of two low-cost locally available aggregate filter materials (gravel and terracotta rubble) were tested in a semi-controlled pilot setup deployed to treat dry weather flows in an open stormwater drain in Bangalore, India. The treatment setups were operated at a similar hydraulic loading rate (HLR) of 265 mm/day using dry weather flows (DWF) in open stormwater drains from surrounding land uses for a period of 14 months. Significant variability in the quality dry weather flows was observed during the testing period. The average COD, BOD5, TN and TP levels of 1528 mg/l, 439 mg/l, 28.6 mg/l and 9.8 mg/l respectively were observed corresponding to high strength wastewater. Differences in porosity resulted in different hydraulic retention times (HRT), with 1.21 days and 1.52 days for the gravel and terracotta setup. While no significant difference was observed in the quality of effluent from both setups, the KBOD5 estimated for gravel setup (0.89) was significantly higher than the terracotta setup (0.70). Similar BOD5 mass removed per day (MMR) efficiency of 78% was observed for both setups. Data suggests the usefulness of gravel material in treating inflows rich in organic matter. Terracotta setup provided higher MRR reduction of 48%, 24% and 43% for DCOD, TP and TN, whereas higher MRR reduction of 70% and 65% for TSS and COD was observed in gravel setup. Neither materials were effective at removing Fecal Coliform (FC), particularly in the later stages of the study. Ammonium-N levels showed periodic increases in both setups, which could be attributed to low HRT leading to incomplete nitrification-denitrification processes. While outflows did not meet discharge standards, specified by India's Central Pollution Control Board (CPCB), the results suggest that a significant reduction in common contaminants can be achieved informing future interventions and modelling studies promoting sustainable urban water management in the megacity context. Study shows that an average BOD5 and COD mass loading rate (MLR) of 28 g/m2/day and 138 g/m2/day respectively should be targeted as a baseline condition in inflows to meet national discharge standards set by the CPCB (30 mg/l and 250 mg/l respectively).