Wastewater Challenges

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.

These include:

  • Sufficient Mixing and Retention time to allow “food” transfer from the wastewater to the microorganisms,
  • Proper Nutrient Balance of carbon, nitrogen, and phosphorus (100-5-1) to aid in the conversion of food to new cells and byproducts, plus
  • Appropriate Flocculation (formation of loosely aggregated particles into masses) and solids removal.

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:

  • Excessive Sludge Aging – the metabolism of older sludge slows and food intake decreases reducing the robustness of the process. This can result in deterioration of the sludge solids quality. As older bacteria die off, they add to the BOD and nutrients in the wastewater stream. This can have the impact of increasing solids and total suspended solids (TSS) contained in the final plant effluent.

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.

  • Filamentous Bulking – a problem with certain bacteria strains, this can be caused by low dissolved oxygen (DO) concentration in the aeration basin or by the F/M ratio dropping too low. Understanding the microbiology of WWTP operations is extremely valuable for establishing a baseline in order to gain advanced warning of changing conditions in the plant.
  • Odor Control Issues – can be exacerbated if the increased oxygen demands generated by accelerated biological activity cannot be satisfied consistently. Dead spots where water can stagnate without sufficient oxygen, concentrations of ammonia, phosphates, or sulfur compounds can all be sources of undesirable odors. Maintaining good water movement or supplementing the water stream with chemical additives can reduce the risk of odor problems.

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.

  • Biological Balance – During the peak of summer’s heat, good floc formation is essential to efficient WWTP operation. Certain conditions can cause microorganisms to die off. Understanding the microbiology of a specific plant in advance can help in planning for seasonal upset conditions.
  • Algae and Weeds – Algae and weed growth in and near water treatment and storage basins and ponds can impact weirs and filter bed recirculation by affecting the media surface or plugging within the media as plants die and decay.
  • Energy – The density of mixed liquor suspended solids (MLSS) is another area where balance must be maintained. Energy costs can escalate while maintaining sufficient DO for the working bacteria when MLSS concentration gets higher than desirable.
  • Excess Capacity Issues – Beyond what happens in a wastewater plant’s biological treatment processes, seasonal thunderstorms bring their own assortment of problems. Seasonal storms are always a threat to overloading a combined sewer system, quickly resulting in combined sewer overflows (CSOs). The U.S. EPA offers a range of guidance documents that support the EPA’s Combined Sewer Overflow (CSO) Control Policy.

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