Assessing catchment-scale performance of in-stream Provisional Green Infrastructure interventions for Dry Weather Flows

Abstract

Provisional Green Infrastructure (PGI) refers to a range of instream intervention strategies that are currently being developed and deployed by non-state actors in the context of Indian megacities to supplement inefficient operation of grey infrastructures while providing localized multi-functional benefits. This study addresses how variations in the location and configuration of PGI interventions at the watershed catchment scale impact their cumulative performance in terms of contaminant removal efficiency, cost, and public accessibility using a SWMM-based TOPSIS approach. We consider 4 PGI upscaling scenarios in a 5.4 sq. km. watershed in the periphery of Bangalore, India: S1) Downgradient dispersed S2) Downgradient centralized S3) Upgradient dispersed S4) Upgradient centralized. Additionally, S0 was employed as a ‘control’ scenario (in which no interventions are made), and S5 assumed the construction of a conventional wastewater treatment plant (WWTP). Total Suspended Solids (TSS) removal efficiencies ranged from 53% to 97% and performed better in upgradient locations. Total Phosphorus (TP) removal ranged from 59% to 90% and were less sensitive to catchment location, but were substantially improved in scenarios with dispersed configurations. Costs associated with PGI upscaling (4.2–6.3 million INR) were substantially lower than for S5 (37.5 million INR), and accessibility scores were considerably larger in upgradient PGI scenarios. Results suggest dispersed configurations of PGI interventions are optimal wherever such an approach is feasible. In cases where dispersed configurations prove unfeasible, centralized configurations should be placed preferentially in upgradient locations in the watershed, and in locations with higher pollutant concentrations and lower flow rates such as channels with shallower slopes. Authors emphasize PGI should not be viewed as a functional replacement for conventional wastewater treatment infrastructures, but rather as a supplementary stop-gap function in contexts where functional separation of wastewater and stormwater are unlikely to occur, or where existing large scale WWTP’s operate below their designed capacity.