Slow Specific Control Measures

Slow, specific control measures for undesired filamentous growth consists of the following steps:

• Identify the undesired filamentous bacteria.

• Identify the operational conditions responsible for undesired filamentous growth.

• Adjust the operational condition to control the undesired filamentous growth. SELECTORS

There are three selectors that commonly are used to control undesired filamentous growth. These selectors are anoxic, anaerobic, and F/M (Figure 15.5). The selectors have operational conditions that "select' or promote the growth of floc bacteria. Selectors are located upstream of the aeration tanks, and the RAS that contains the filamentous organisms is discharged to the selector.

Anoxic and anaerobic selectors

Anoxic and anaerobic selectors

Return activated sludge (RAS)

F/M selector

Return activated sludge (RAS) FIGURE 15.5 Selectors.

An anoxic selector contains nitrate (NO3-) and/or nitrite (NO2-) and little or no free molecular oxygen. Some or all of the primary clarifier effluent that contains substrate or soluble cBOD is fed to the selector. In the presence of soluble cBOD, free molecular oxygen is quickly removed and an anoxic condition is established. The selector retention time is 1-2 hours; by recycling the RAS through the selector on a continuous basis, several filamentous bacteria that cannot use nitrate or nitrite can be controlled. These filamentous bacteria include Haliscomenobacter hydrossis, Nocardioforms, Nosticoda limicola, Sphaerotilus natans, type 1701, and type 021N.

An anaerobic selector is similar to an anoxic selector with the following exceptions: (1) the selector contains very little or no nitrate and/or nitrite and free molecular oxygen and (2) the selector has an anaerobic condition that permits the biological uptake of phosphorus. An anaerobic selector is capable of controlling aerobic filamentous bacteria and filamentous bacteria that are not capable of biological phosphorus uptake. Under anaerobic conditions, filamentous bacteria are not able to take up soluble substrate at a rate comparable to floc-forming, poly-P bacteria. The poly-P bacteria take up phosphorus under an aerobic condition (aeration tank immediately following the anaerobic selector) and use the stored phosphorus (energy) to outcompete filamentous bacteria for soluble substrate in the anaerobic selector.

An F/M selector has a retention time of only 5-15 minutes, and all of the primary effluent must be fed to the selector. Due to the short retention time in the selector, the selector may have an aerobic, anoxic, or anaerobic condition. Most F/M selectors have an aerobic condition.

An F/M selector establishes a substrate gradient. The gradient can be established by using an F/M gradient in the same tank or across several tanks. The F/M selector gives a substrate advantage to floc bacteria, because they absorb most of the soluble substrate at the influent end of the selector. An F/M selector can be used to control the undesired growth of low F/M filamentous bacteria.

Part V

Fermentation and Methane Production

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