Nutrient removal and nitrogen balances in 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 (4 and 8 days). Nitrogen balances were calculated by partitioning total nitrogen into organic and inorganic nitrogen (NH4 + -N and NO3 - -N), and to further separate the organic nitrogen components to particulate and dissolved organic nitrogen. Particulate organic nitrogen was also separated into algae organic nitrogen and bacteria organic nitrogen to investigate nitrogen concentration flow in the HRAPs. This research shows that the proportion of algae in the algal/bacterial biomass in the longer 8-day hydraulic retention time (HRT) HRAP8d (55.6%) was appreciably lower than that in the shorter 4-day HRT HRAP4d (80.5%) when CO2 was added during the summer. Increased bacterial biomass in the HRAPs in which the maximum pH was controlled to <8.0 by CO2 addition stimulated bacterial nitrification, indicating that biological nitrification could be an important nitrogen conversion pathway when CO2 added to the HRAPs. Overall nitrogen removal of ~60% in the HRAPs with CO2 addition was mainly achieved by algae/bacteria assimilation followed by solid sedimentation in a settling unit.