Is There A Cure For Wastewater's Summertime Blues?

As we approach the hottest months of the year, wastewater treatment plant (WWTP) operators face a range of challenges arising from elevated summer temperatures. With all due respect to Eddie Cochran, this article outlines some of the cures — or at least preventive steps and countermeasures — for wastewater treatment’s “summertime blues.”

The Origins Of The Problem

Unlike other industrial processes, wastewater treatment is as much a function of biological balance as it is one of physical movement and flow. Because biological and chemical reactions are impacted by changes in temperature, wastewater treatment processes can change drastically throughout the summer months.

How elevated temperatures affect the two basic processes of optimized activated sludge systems — transformation of organic matter into a suspended biomass, and eventual separation of that biomass from the flow — is critical to the success of any wastewater treatment process. The activated sludge process depends on favorable conditions for growth, including:

• adequate mixing and retention time to allow for transfer of “food” from the wastewater to the microorganisms,
• the conversion of food to new cells and byproducts, aided by a proper nutrient balance of carbon, nitrogen, and phosphorus (100-5-1), plus
• appropriate flocculation and solids removal.

In activated sludge treatment, the number of active microorganisms needs to be balanced against the amount of “food” — i.e., biochemical oxygen demand (BOD) — available to those microorganisms. Without proper compensation, temperature changes that impact the balance of that process can have a detrimental effect on wastewater treatment efficiency.

The food-to-microorganism ratio (F/M ratio) in a conventional activated sludge process is typically between 0.25 and 0.45, depending on the facility and temperature. For extended aeration activated sludge, that ratio is typically between 0.05 and 0.15. This F/M Activated Sludge Calculator provides an opportunity to compare the relationships among various factors in the process. The complexity and interconnection of those factors can be appreciated in this training program download from Michigan’s Department of Environmental Quality.

Interrelated Heat-Induced Summer Problems

Biological activity and reproduction rates of mesophilic bacteria, the most common type of bacteria used in activated-sludge biological treatment plants, increases rapidly as temperatures warm up to 95^oF (35^oC) — although 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 can result in deteriorating sludge solids quality as the older sludge reduces the robustness of the process through slow metabolism and decreased food intake. 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.  
  
  Lowering the mean cell retention time (MCRT) or sludge retention time (SRT) during the height of summer helps 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. A 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 can be 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 most 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

Other ripple effects of summer conditions can also generate problems related to the cost and labor involved in keeping WWTPs operating efficiently and in compliance.

• Biological Balance. Good floc formation is essential to efficient WWTP operation during the peak of summer’s heat. Be aware of conditions that can cause microorganisms to die off. This is another area where understanding the microbiology of a specific plant ahead of time can help in planning for seasonal upset conditions.
• Algae and Weeds. Stay on top of algae and weed growth in and near water treatment and storage basins and ponds. These can impact weirs and filter bed recirculation by affecting the media surface or plugging within the media as plants die and decay.
• Energy. Another area to maintain balance is in the density of mixed liquor suspended solids (MLSS). When MLSS concentration gets higher than desirable, energy costs can escalate in order to maintain sufficient DO for the working bacteria.
• Excess Capacity Issues. Regardless of what’s going on in a wastewater plant’s biological treatment processes, seasonal thunderstorms are always a threat to overload a combined sewer system and quickly generate combined sewer overflows (CSOs) that bring a whole different variety of problems. The U.S. EPA offers a range of guidance documents that support the EPA’s Combined Sewer Overflow (CSO) Control Policy.