Volatile Fatty Acids
VFA's may be enhanced by retaining the primary sludge in the primary clarifiers as long as "is practical". This will allow the sludge to ferment, creating the VFA's. When a treatment facility tries to pump a 4 to 6% solids content sludge to the solids handling (ie: Primary digesters) processes, this usually occurs. In one facility, the standard operation is to pump close to a 5% solids primary sludge to it's primary digesters. At this same plant, during a pilot test of co-thickening primary sludge with waste activated sludge, the primary tanks were constantly pumping a very light primary sludge to its dissolved air floatation thickeners. This resulted in a dramatic decrease in its VFA's, which impacted the operation and maintenance of its anaerobic selector. VFA';s may also be created by having long detention times in the wastewater collection system, warm raw wastewater, and artificially creating them by pumping primary sludge to special "pre-fermentation tanks, where the primary sludge is aged, then elutriated, and the wash water with the VFA's in it, transferred to the selector influent to be used as a food source.

In brief then: biological phosphorus removal may be achieved by incorporating anaerobic and aerobic zones in the activated sludge process. If we place an anoxic zone in the process stream, the removal of Nitrogen can also be achieved.

There are MANY ways in which selectors may be utilized. Here are three modifications to the activated sludge process which is shown first:

Fig 1. Activated sludge without a selector

Fig 2. Activated sludge with an anoxic selector following a "single stage" nitrification process

Fig 3. Activated sludge with an anaerobic selector preceding a "single stage" nitrification process for "good settling activated sludge solids." (We have this at our plant.)

Fig 4 Activated sludge with an anaerobic selector preceding a "single stage" nitrification process, and an anoxic selector for denitrification. Note the additional "carbon source" required for preferred optimization in the operation of the anoxic selector.

For review, if needed, refer to a previous presentation on nitrification and denitrification.

Process Control for Activated Sludge & its Selectors
Oxidation Reduction Potential (ORP)
Alkalinity
pH
Dissolved oxygen (or lack of it)
Ammonia
Nitrates
Batch sequencing: Number /day & length of times of cycles

Treatment Goals for Activated Sludge & its Selectors
Reduce nitrates
Alkalinity recovery
Reduce energy consumption
Process stability

REFERENCES
WPCF, MOP 11, Vol II, WPCF, 1990
Design of Municipal Wastewater Treatment Plants, Vol II WEF manual of Practice No 8, WPCF, 1992
George Tchobanoglous & Franklin Burton, Wastewater Engineering, 3rd Edition, Metcalf and Eddy, Inc
Texas Water Utilities, Manual of Wastewater Treatment 6th Edition, Texas Water Utilities, 1991
Kenneth A. MacKichen & Mark J. Hammer, Hydrology & Quality of Water Resources, Whiley, 1981
Walter Weber, Physicochemical Processes For Water Quality Control, Whiley, 1972
Mark J. Hammer & Warren Viessman, Jr., Water Supply and Pollution Control, 5th Edition , HarperCollins, 1993

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