Development of a Biological Process Model for Optimizing Nutrient Removal at South Truckee Meadows Water Reclamation Facility
AuthorMettler, Mark A.
AdvisorMarchand, Eric A.
Civil and Environmental Engineering
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Due to increases in population, communities around the world have experienced a subsequent increase in wastewater production. In many cases either wastewater plant expansions are completed or new remediation facilities are constructed to manage the increased load and prevent a significant decline in effluent water quality. However, this may not always be economically feasible, necessary, or even recommended until the current facility's treatment processes have been optimized. This study sought to use computer modeling to optimize the treatment processes of Washoe County's South Truckee Meadows Water Reclamation Facility (STMWRF) to prevent seasonal algal blooms from occurring in the nearby storage reservoir. The first task in the project was to develop a BioWin computer model that could accurately simulate STMWRF's oxidation ditch treatment processes. Influent and effluent samples from three separate 24-hour sampling periods were collected, analyzed, and used to develop and calibrate the computer model. Upon completion of the calibration process, the model was able to predict the average concentrations of several key parameters within an error range of 5%. The second phase of the project used the model to perform a process optimization study for STMWRF. Preliminary data had indicated elevated concentrations of nitrate/nitrite and orthophosphate, and low concentrations of ammonia in the oxidation ditch effluent. Therefore, alternative treatment scenarios focused on increasing the amount of non-aeration occurring within a 24-hour period to enhance denitrification, phosphorus uptake by microorganisms, and potentially decrease energy consumption. Computer simulations were conducted to produce two alternative aeration schedules that would be capable of reducing effluent ammonia and nitrate/nitrite concentrations well below their respective discharge limits. The third phase of the study involved evaluating the efficiency of the oxidation ditch treatment processes under the proposed alternative aeration schedules. Initially, the aeration scheme employed at STMWRF included only four hours of non-aeration in a 24-hour period and none of these non-aeration times appeared to be strategically scheduled. The first and second alternative aeration schedules tested increased the duration of non-aeration to seven and nine hours, respectively. Additionally, influent ammonia loading and cost benefit analyses were conducted to determine the optimum times to shut down the air blowers. Sampling proved that both alternative aeration schedules had significantly improved nitrate/nitrite and orthophosphate removal processes without creating large increases in effluent ammonia concentrations. Average effluent ammonia and nitrate/nitrite concentrations under the second alternative aeration schedule were determined to 0.49 (mg NH_3-N)/L and 1.00 (mg NO_x-N)/L, respectively, which were well below the target discharge limits of 2 (mg NH_3-N)/L and 3-5 (mg NO_x-N)/L, respectively. As a result, it was recommended that this scheme be implemented permanently at STMWRF so its treatment processes can be considered optimized and Washoe County can proceed with future expansion projects.