Nutrient removal in a cold-region wastewater stabilization pond: Importance of ammonia volatilization

Nitrogen (N), phosphorus (P), and carbon (C) flux through a three pond wastewater stabilization system (WWSP) was measured over the Course of a year in a cold weather region (Minnesota) with 4 months of ice cover. The system was surprisingly efficient at N removal (averaging 80%) primarily through volatilization of un-ionized ammonia during the late spring when the pH was above 8 and ammonia levels were still high. P removal was less efficient, with over 50% of influent P leaving the system in the effluent. Soluble carbonaceous biochemical oxygen demand (CBOD) removal was > 90%, with most of the removal occurring in the primary pond. Algal carbon requirements were met by a combination of CO2 released by bacterial oxidation of influent organic matter and inorganic carbon in the influent. Photosynthesis provided much of the oxygen needed for CBOD removal in the primary pond, and the onset of aerobic conditions was nearly coincident with the highest rates of ammonia volatilization. Bacterial respiration in the sediment returned > 80% of the sedimented C back to the water column long term. These results demonstrate the importance of ammonia volatilization as a N sink in WWSPs that experience ice cover setting up conditions for both high primary production and high ammonia levels in spring.