Ammonia, pH and dissolved inorganic carbon supply drive whole pond metabolism in full-scale wastewater high rate algal ponds

Abstract

High rate algal ponds are an established cost-effective and efficient upgrade to conventional wastewater treatment ponds for the biological treatment of wastewaters. However, few studies have investigated ecosystem metabolism and its drivers in these ponds. Hourly, daily and seasonal changes in rates of gross primary production, net ecosystem production and ecosystem respiration were determined from automated dissolved oxygen, pH and temperature measurements in full-scale high rate algal ponds over a two-year period. The open water metabolism model was used to assess the dynamics of pond metabolism against a number of measured environmental variables. For the most part, net ecosystem production was positive, meaning that the pond was in net autotrophy, with microalgal photosynthetic production exceeding respiratory organic matter consumption. pH had a negative influence on ecosystem respiration, with respiration rates increasing as diurnal pH increased. Similarly, free ammonia concentration reduced gross primary production rates. Modelled CO2 addition to maintain pH 8, resulted in decreased ecosystem respiration and increased net ecosystem production, particularly in summer with a 50% increase in net ecosystem production. However, modelled CO2 addition did not influence net ecosystem production during winter. The use of the Datasondes coupled with the open water metabolism model provides a cost-effective tool for improved high rate algal pond management through the capture of high frequency metabolism parameters. Better understanding of how the high rate algal pond environment affects both photosynthesis and respiratory losses, will help improve real-time management of the pond, including ammonia and CO2 loading management, to improve effluent quality and biomass yields and reduce excess CO2 off-gassing to atmosphere.