Temperature Modeling and Control for Biological Wastewater Treatment Design

Temperature modeling during preliminary engineering tasks of biological wastewater treatment system design ensures engineers evaluate and account for the cooling and/or heating requirements necessary for proper operation of a treatment system. This paper presents methods and practices for modeling and controlling temperature, case studies of several industrial biological wastewater systems, and a presentation of equipment used for cooling or heating wastewater. The model presented has been used in the design of biological reactors including nitrification / denitrification systems. An equilibrium model was developed in the 1970's ( Comprehensive Temperature Model For Aerated Biological Systems, Argaman and Adams, 1977) to predict average monthly temperatures of in-ground basins for the biological wastewater treatment reactor in both winter and summer conditions. This model accounted for heat sources from solar radiation, biochemical reactions, and mechanical inputs, and heat losses from evaporation and conduction through walls and water surfaces. The original model considered aerated basins with mechanical surface aeration or submerged diffused aeration. With industrial wastewater treatment processes becoming more complex, the model has been adapted and expanded to be part of a more holistic approach to process design. These processes typically include equalization, activated sludge (both anoxic and aerobic), secondary clarification and possibly tertiary polishing. The optimal temperature range in biological oxidation is narrow thus requiring tighter control by designers. By considering upstream and recycle processes in the entire wastewater treatment system, a more accurate estimation of wastewater temperatures can be determined. This allows designers to consider cooling and/or heating options earlier in the evaluation process to ensure costs for all equipment are included. The temperature of industrial wastewater streams can be highly variable depending on the manufacturing process and site location. This paper includes a case study of a conventional activated sludge plant as well as a chemical facility with a complex nitrification / denitrification system with both the predictive and actual temperature data analysis. In addition, there will be a discussion of both heating and cooling requirements at two other facilities in the Chemical Process Industries. The types of cooling equipment available for use to the CPI industry will also be provided with their advantages and disadvantages.