IMPACT OF COVERING AERATION BASINS ON ACTIVATED SLUDGE SYSTEM PERFORMANCE WHILE COMPLYING WITH MON AND HON REQUIREMENTS

National Emissions Standards for Hazardous Air Pollutants (NESHAP) for miscellaneous organics (MON) and for hazardous organics (HON) (40 CFR Part 63) indicate that covering aeration basins may be considered as an operational alternative in industrial activated sludge plants (ASPs) to achieve compliance. The volatile organic compounds (VOC) Rule (40 CFR Part 60, Subpart YYY), scheduled for April 2004, will also target VOC emission reduction in ASPs by covering aeration basins. However, covering aeration basins may raise mixed liquor temperatures, adversely affecting ASP performance in terms of retarding kinetic rates and increasing effluent suspended solids concentration, thereby possibly affecting compliance with National Pollutant Discharge Elimination System (NPDES) permit limits.Predictive methods of estimating temperature increases for covered ASP aeration basins and linking them to the impact of such temperature increases on ASP performance would be helpful in evaluating the merits of such MON and HON compliance initiatives. The heat-balance model described in this paper is a tool that can be used to estimate such temperature changes.An example of applying this model is presented for a recently completed project for an ASP serving a synthetic organic chemical manufacturer. Bench-scale covered activated sludge reactors' simulation of the process over the temperature range 33 to 43°C confirmed that increased aeration basin temperatures resulted in lower kinetic rates and poorly settling solids. The model was used to estimate temperatures in the covered aeration basins for different conditions to select a strategy to prevent the temperature in the basins to rise above 38°C during summer operation. Model simulation results showed good agreement between measured (41.8°C) and modeled (40.8°C) basin temperatures, that diffused aeration had a relatively small cooling effect while COD removal had a significant heating effect when compared with the baseline value, cooling the compressed air had a minor effect when compared with the effect of cooling influent wastewater, and that the target basin temperature of 38°C could be reached by cooling the influent wastewater by approximately 4°C.