CHANGES IN INFLUENT FLOWS AND CHARACTERISTICS
Although it is typical for large municipal wastewater treatment plants to receive very stable hydraulic and organic loadings, other systems are faced with plant loadings that change on a day-to-day basis. Of all of the secondary treatment systems, activated sludge is particularly ill suited to deal with this problem. One of the reasons that this is true is that filamentous bacteria are always potentially ready to exploit changes in the system. Filamentous bacteria are very strong competitors and are usually able to exploit changing conditions, whereas all of the “good guy” type microbes suffer. For this reason, it is important that operators understand the changes that can occur to the influent characteristics and flow rate that can affect their plant. The following details the common problems encountered in this regard.
Some systems do not receive the same waste load everyday. A good example of this is a small package plant that serves a school. The hydraulic and organic loading in this situation is highly variable. On the weekends and during the summertime, there may be little or no flow to the treatment plant. However, just as the F:M ratio can be described in terms of the feeding rate for you pet dog, the problem of variable loading can be explained in these terms as well. Would your dog be happy to receive no food over the weekend or all through the summer? Probably not! The organisms in a activated sludge wastewater treatment facility are not different. In this situation, it may become necessary to “feed” the plant a substitute food source for the times when there is no influent flow. Coincidentally, many small treatment plant operators actually use commercially purchased dog food as a way of supplementing the loading to their systems. Rabbit food offers an even better alternative, because it does not contain the fats that are present in high amounts in dog food.
Because the hydraulic loadings may be received in short bursts as apposed to being spread evenly throughout the day, flow equalization is often necessary. This is not always taken into account by the system’s design engineer, and so the operator is left to make due.
Several recycle flow streams occur in treatment plants that must be accounted for if the loading to the system is to be accurately understood. Liquid from digesters, thickeners and dewatering processes are generally routed back to the head of the treatment plant. Depending on the source, these recycle streams can have very high TSS and BOD content and may have extreme levels of ammonia or nitrate. The liquid that is decanted out of anaerobic digesters can often have a BOD in excess of 1,000 mg/L and ammonia concentrations over 100 mg/L. When this waste is reintroduced into the treatment system, it adds a substantial load that must be considered.
The liquid that results from aerobic digester supernating can often be heavily laden with filamentous bacteria. Supernating under these conditions can actually result in the creation of a filamentous bacteria induced sludge bulking problem. Chlorination of the supernatant prior to feeding it back into the plant can be used to control this problem. The liquid stream from centrifuges and belt presses can be very high in TSS and BOD if the thickening/ dewatering process is not functioning properly. All of this must be considered by the operator when assessing the system’s loading.