Nutrient removal in wastewater treatment high rate algal ponds with carbon dioxide addition

The influence of CO₂ addition to high rate algal ponds (HRAPs) on nitrogen removal was investigated using two pilot-scale HRAPs operated with different hydraulic retention times (HRT: 4 and 8 days), and was compared to the nitrogen removal by the 8-day HRT pond before CO₂ addition was installed. Nitrogen balances were calculated by partitioning total nitrogen into organic and inorganic nitrogen (NH⁺₄-N and NO⁻₃-N), and by separation of the organic nitrogen into particulate (PON) and dissolved organic nitrogen (DON). PON was further divided into algal organic nitrogen (AON) and bacteria organic nitrogen (BON) to investigate nitrogen mass flow in the HRAPs. This research shows that the proportion of algae in the algal/bacterial biomass in the longer 8-day HRT HRAP_8d (55.6%) was appreciably lower than that in the shorter 4-day HRT HRAP4d (80.5%) when CO₂ was added to control the maximum pH to <8.0 during the summer. Higher bacterial biomass in the longer 8-day HRT HRAP corresponded with higher nitrification rates, indicating that the longer 8-day HRT in the summer was detrimental for two reasons: lower algal productivity and increased nitrogen loss through nitrification/denitrification. Overall nitrogen removal of ~60% in the HRAPs with CO₂ addition was mainly achieved by algal assimilation followed by sedimentation in the settling unit.