Impact of inoculum on domestic wastewater treatment in high-rate ponds in pilot-scale: Assessment of organic matter and nutrients removal, biomass growth, and content

Discharging untreated wastewater deteriorates water bodies, but biological treatment methods can mitigate this by reducing organic matter, nutrients, and phosphorus. Systems using microalgae-bacteria consortia are promising nature-based solutions (NbS) that require no artificial aeration and can produce valuable products. This study, conducted on a pilot scale with domestic wastewater, involved two high-rate ponds (HRP1 and HRP2) with a depth of 0.30 m and lengths of 6.0 m. While HRP1 was not inoculated, HRP2 received activated sludge, and both were operated in two sequential batches followed by continuous operation with a four-day hydraulic detention time. After 20 days, biomass and system stabilization were primarily observed in HRP2, which achieved total nitrogen removal of 84.2 %, phosphorus removal of 63.7 %, and organic matter removal of 74.7 %. The inoculated pond showed increased stability and sedimentation capacity, with average volatile suspended solids of 152.75 ± 120.53 mg/L, a flocculation efficiency exceeding 90 %, and a sludge volume index of 19.83 ± 28.54 mL/g. Chlorophyll-a concentrations were higher in HRP2 (0.84 ± 0.48 mg/L), indicating higher algal biomass. Protein concentrations were 98.86 ± 47.78 mg/gVSS in HRP2, while carbohydrate concentrations averaged 67.87 ± 135.07 mg/gVSS, slightly superior for HRP2. Additionally, HRP2 demonstrated a lipid content of 23.03 %, attributed to a dominant Chlorella sp. presence. These findings underscore the need to optimize operational parameters and microbial consortia for effective biomass production and resource recovery in HRPs. Such low-cost NbS contribute to pollution control and align with the UN's sustainability objectives (SDGs) 3, 6, 11, 13, and 14.