Seasonal in-situ aeration in constructed wetlands to facilitate nitrification

Throughout most of the year, Arcata California is home to approximately 18,000 residents who are served by an innovative wastewater treatment plant that utilizes constructed wetlands to treat a range of flows from 1.0 MGD to 13 MGD. The City of Arcata is required to renew its National Pollutant Discharge Elimination System (NPDES) permit through the North Coast Regional Water Quality Control Board (NCRWQCB), which will dictate the need to reduce nitrogen levels compared to current effluent values. This project explores facilitating the nitrification and denitrification process with an in-situ aeration system, which has the potential to reduce nitrogen levels to assumed requirements on the next NPDES permit issued by the NCRWQCB. The City of Arcata is required to comply with the renewed NPDES permit standards by January 27, 2017. The primary objective of this study is to determine the nitrogen removal efficiency of a small-scale in-situ aeration system for the Arcata Wastewater Treatment Facility (AWTF). The aquaphyte, a scaled down treatment marsh was built, planted, and aerated to observe the nitrification and denitrification process, which converts ammonia nitrogen in the wastewater to di-nitrogen in the atmosphere. Design parameters were altered to determine which variables have the greatest influence on the conversion of ammonia to nitrate (nitrification) and nitrate to di-nitrogen (denitrification). Aerating the aquaphyte proved to be an effective method to reduce ammonia concentrations. This experiment was scaled up to a larger system in Treatment Wetland - 4 (TW- 4) to determine if the 98% reduction of ammonia concentrations seen in the aquaphyte could be replicated. There were many variables that created a divergence of similitude between the Aquaphyte and TW–4, and the results observed in the aquaphyte did not translate to the aeration system installed in TW-4. Operating conditions in the aeration system and inflows into TW-4 were altered to find similar operating parameters as the aquaphyte. TW-4 has been operating as a treatment marsh for approximately 35 years and has developed a substantial volume of nitrogen in the form of biomass from plant growth and senescence cycles, accreted ammonia that has precipitated into the wetland soil, and settled algal solids. The accumulation of biomass was the primary difference between the aquaphyte and TW-4. Influent loading into both systems was pumped from the same source (Oxidation pond 2). Nonetheless, the oxygen that was transferred into TW-4 to facilitate nitrification appeared to be consumed by the carbonaceous oxygen demand that was created from the accumulation of biomass throughout its operation. Therefore, the larger scale TW-4 nitrification system experiment resulted in little to no ammonia reduction.