Storm Events

Many treatment facilities experience increased influent flows during storm events due to the inflow of storm water from streets and low lying areas. Storm flows often sweep excessive dirt and silt into the plant. This can result in a rapid increase in the MLSS concentration in the aeration basin, but the concentration of the MLVSS will remain the same. Although the dirt and silt contributes to the total amount of MLSS, it is generally all inert matter that does not help to metabolize the organic waste load. This problem is often accompanied with high TSS in the effluent, but no elevation in the effluent BOD. Solids increase drastically but the percent of volatile solids may drop to 50%. Although this inert matter must be wasted out of the system, it must be performed in a controlled manner to prevent too drastic of a decrease in the microorganism population.

Temperature Changes

Temperature changes affect activated sludge systems in a variety of ways. The activity of microorganisms slows with colder temperatures. Because of this, operators often have to adjust to cold conditions by increasing the overall mass of microorganisms in the aeration basin just to accomplish the same level of treatment. Filamentous bacteria tend to take advantage of temperature changes that change the growth rate of other organisms. At the onset of winter, filamentous bacteria are less affected by the cooling temperatures and therefore experience less of a growth reduction that other organisms. At the onset of summer, they are prone to experience a growth spurt much sooner than the other organisms. For this reason, many plants experience filamentous bacteria related problems during seasonal temperature changes in the spring and fall.
The ability to dissolve oxygen is also affected by temperature. Water will dissolve more oxygen when it is cold than when it is warm. This means that the aeration system will be working its hardest during the hot summer months. During the cold months of the winter, it may be necessary to reduce the amount of aeration. (This is also a good way to save energy).

Toxic Discharges

All plant operators should be alert for the possibility of toxic dumps, accidental spills (particularly the midnight variety), storms, or other collection system activity that may change the influent flow or waste characteristics. Toxic discharges can be introduced into the sewer intentionally and unintentionally. The variety of potential toxic substances is enormous. Operators have only a little control over toxins once they enter the treatment plant. Occasionally, there will be a spare basin available where the toxic influent can be directed in order to prevent the total die-off of the plant (provided the spill is identified early). If the organisms are killed off, preparations should be made as soon as possible to re-seed the aeration basin with healthy organisms from another plant. Be sure that the toxic substance has been completely removed or neutralized before re-seeding. Some toxic discharges occur out of the blue, while others happen on a regular basis.

Operators can work with their industrial discharges to prevent large toxic discharges from damaging the plant. Often, industrial dischargers can be convinced to release strong or toxic wastes at a low discharge rate rather than all at once. Certain industries such as canneries create seasonal problems, which the operator should prepare for in advance. Septage is one category of waste that many treatment facilities struggle from. Septic tanks are common, but dedicated septage receiving stations are not. This means that septage tank sludge is often dumped into wastewater facilities for treatment. Depending on the facility size and condition, septic waste can cause great harm to activated sludge systems. Of particular concern is that septic discharges encourage the growth of filamentous bacteria, such as type 021N, that are responsible for serious filamentous bulking problems.