Formulation of effluent quality index and its application to assess seasonal treatment efficacy of sewage treatment plants across Delhi-NCR

The sewage in Delhi-NCR was treated in 12 STPs and treated to comply with the KPIs established by the CPCB and NGT. The treated effluent isn't benchmarked against any index at present. An EQI, with seven categories, is proposed with a compliance-based aggregation of the SI scores. The 12 STPs were categorized as per the EQI for two sampling periods (monsoon and post-monsoon). The national capital region of Delhi discharges substantial untreated sewage into river Yamuna despite the presence of sewage treatment plants (STPs). Conventional assessment of STP effluents relies primarily on parameter-wise pass/fail reporting, which does not communicate overall effluent quality, the magnitude of regulatory exceedance, or the parameters responsible for non-compliance. This investigation developed an effluent quality index (EQI) integrating seven key performance indicators (KPIs) namely, biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), total phosphorus (TP), faecal coliform (FC), and pH, using linear and logarithmic interpolation on a seven-category scale (Good to Severe + , 0 to >500). Field sampling of 12 sequential batch reactor (SBR)-based STPs during monsoon and post-monsoon showed robust performance of SBRs with removal of COD (74–97%), TSS (86–98%), and BOD (86–96%), with predominant Central Pollution Control Board and National Green Tribunal compliance for organics, solids, and TN. However, TP was found to be non-compliant at five and 11 STPs during monsoon and post-monsoon, respectively. Only four (during monsoon) and one (during post-monsoon) STPs discharged fully compliant effluent. The EQI simultaneously quantified effluent quality, treatment magnitude, and ranked non-compliant KPIs by exceedance, enabling operators and regulators to prioritize targeted, cost-effective interventions.