Phosphorus Recovery Potential from Decentralised Wastewater Treatment Systems in India: A Constrained Mass-Balance Study of Auroville

Decentralised Wastewater Treatment Systems (DEWATS) are widely implemented across India to address sanitation gaps in peri-urban and semi-urban regions. However, these systems are primarily designed for pollution control rather than resource recovery, resulting in the dissipation of valuable phosphorus into sludge and receiving environments. This study evaluates the phosphorus recovery potential of an operational DEWATS serving the Auroville Residential Zone, Tamil Nadu, using a constrained mass-balance approach suited to data-limited, low-energy sanitation systems. Influent and effluent monitoring data were combined with literature-derived partitioning coefficients to reconstruct internal phosphorus pathways and quantify recoverable fractions without intrusive internal sampling. Three magnesium-based recovery scenarios were assessed: pre-primary dosing, pre-secondary (vortex) dosing, and sludge-line processing. Results indicate that pre-primary dosing offers the highest theoretical phosphorus recovery but introduces substantial operational complexity. Pre-secondary dosing provides the most balanced outcome, achieving improved effluent phosphorus reduction and operational stability with modest recovery yields and minimal infrastructure modification. Sludge-line recovery generates a higher-purity product but requires significantly greater capital investment. The findings also highlight a distinct tropical advantage, where elevated temperatures enhance phosphorus bioavailability and recovery efficiency compared with temperate systems. Despite technical feasibility, current fertiliser subsidy structures limit economic competitiveness. The study concludes that nutrient recovery in Indian DEWATS should prioritise effluent quality improvement and system resilience over maximum recovery yields, and that targeted policy support is required to enable decentralised circular phosphorus management.