Blue–green infrastructure of Urban Ponds: nature-based algal harvesting for greenhouse gas mitigation and bioenergy recovery

ABSTRACT

Urban ponds are widely recognised as high-emission hotspots of greenhouse gases (GHGs), mainly in the form of methane. Whereas hyper-eutrophication also simultaneously presents an opportunity to harness algal biomass for substantial energy recovery. The present study addresses this dual challenge and opportunity by studying Hauz Khas Pond, a 15-acre hyper-eutrophic urban pond in South Delhi, India. This pond receives a continuous inflow of treated effluent to maintain water levels in the pond.

Comprehensive long-term monitoring of nutrient dynamics, water quality, and biomass generation revealed persistent hyper-eutrophic conditions with TSI of 197.6 ± 10.7 with minor seasonal fluctuations. Continuous nutrient loading ((PO₄³⁻: 4–8 mg L⁻¹, NO₃-N: 1.9-3.13 mg/L),and shallow depth (1-2.5m), is causing high algal productivity and benthic methanogenesis leading to high methane emissions (~1.7 times freshwater systems). Although biomass assessment revealed average standing algal biomass in pond of approximately 183 tonnes and 43% of which is excess eutrophic biomass (approx. 80 tonnes) and can be harnessed for energy recovery without affecting ecological health of aquatic life in pond. The harvested algal biomass was characterized using biochemical methane potential assays, which demonstrated competitive methane yields under anaerobic digestion. This recoverable fraction alone holds methane generation potential of about 20000 m³ equivalent 0.37 m³m^-2. This finding indicates the possibility of an in situ energy recovery system. Since India has sufficient solar energy availability Power-to-Gas technology is further being proposed to enhance the methane percentage upto 95%.This technology involves injecting renewable hydrogen into the anaerobic digestion process, which upgrades the biogas produced to pipeline- grade methane.

By combining nutrient management, continuous harvesting, and integrating renewable energy, this nature-based algal harvesting approach can achieve controlled emissions while enhancing urban water quality. Our research redefines eutrophic urban lakes as multifunctional blue-green infrastructure that seamlessly integrate sustainable water management, climate mitigation, and circular bioenergy recovery in rapidly urbanizing regions.

Keywords: Bioenergy recovery; Blue–green infrastructure; Circular bioeconomy; Nature-based solutions; Urban eutrophic lakes; Methane emissions; Algal biomass harvesting; Anaerobic digestion