Elevated temperatures affect the transformation of organic matter, which is critical in any wastewater treatment process. The two basic processes influenced by temperature in optimized activated sludge systems are transformation of organic matter into a suspended biomass, and the subsequent separation of that biomass from the flow. Favorable conditions for growth are necessary to an activated sludge process.
It is important that the number of active microorganisms is balanced against the amount of “food” or biochemical oxygen demand (BOD) available to those microorganisms in activated sludge treatment. Temperature changes can impact the balance of that process, and thereby have an adverse effect on the efficiency of wastewater treatment.
In a conventional activated sludge process, the food-to-microorganism ratio (F/M ratio) is typically between 0.25 and 0.45, depending on the facility and temperature. In instances of extended aeration activated sludge, the ratio is generally between 0.05 and 0.15.
This F/M Activated Sludge Calculator allows one to compare the relationships among various factors in the process. The complexity and interconnection of those factors can be seen in this training program download from Michigan’s Department of Environmental Quality.
Mesophilic bacteria is the most common type of bacteria used in activated-sludge biological treatment plants. Its biological activity and reproduction rates increase rapidly as temperatures rise to 95oF (35oC) — though it can be hampered by extremely hotter temperatures. Rising temperatures can also decrease BOD reduction efficiency, increase the concentration of suspended solids, and lead to a variety of other problems:
During the height of summer, lowering the mean cell retention time (MCRT) or sludge retention time (SRT) can help prevent excessive sludge aging. Adjusting the sludge wasting rates can also help compensate by shifting the proper amount of return activated sludge (RAS) from the secondary clarifier back into the primary aeration tank. This 30-minute settling test can be used to monitor how much of a returned sludge flow rate is appropriate to keep the mixed liquor suspended solids (MLSS) at an optimum rate for re-seeding the raw wastewater inflow to the primary treatment aeration tank.
Coping With Other Summertime Concerns: Summer conditions can also create problems related to the cost and labor involved in keeping WWTPs operating efficiently and in compliance.
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