Filaments and Bacteria...
Volume - 5...

Inert Material and Polymer Addition...

There exist several methods of chemical addition to enhance activated sludge settling. Most used are synthetic, high molecular weight, cationic polymers alone or in combination with an anionic polymer that serve to overcome the physical effects of filaments on sludge settling. These are usually added to the MLSS as it leaves the aeration basin or to the secondary clarifier centerwell. Use of polymer does not significantly increase waste sludge production but can be quite expensive, up to $450. per million gallons treated. A polymer supply company should be consulted for selection of a polymer. Jar testing should be performed at your plant to determine the type of polymer needed and its dosage, which are quite plant specific. In some instances, inorganic coagulants/precipitants such as lime or ferric chloride can be beneficial. These produce a voluminous precipitate that sweeps down the activated sludge, improving settling. Sludge production may be significantly increased if these are used. The weighting action of inert biological solids has also been used to aid sludge settling in activated sludge modifications such as the Hatfield or Kraus processes that recirculate anaerobic digester contents through the aeration basin. Clay and fiber addition have been used by some industries (e.g. papermills) to help sludge settling on a short-term basis.

Chlorination...

Two toxicants, chlorine and hydrogen peroxide, have been used successfully to control filaments. Chlorine is the most widely used as it is relatively inexpensive and available on-site at most plants, and only this will be discussed here. Chlorination for bulking control is widespread, used by more than 50% of plants. The goal of chlorination is to expose the activated sludge to sufficient chlorine to damage filaments extending from the floc surface while leaving organisms within the floc largely untouched. Filamentous and floc-forming bacteria do not appear to significantly differ in their chlorine susceptibility. Chlorine dosage is adjusted such that its concentration is lethal at the floc surface but is sublethal within the floc, due to chlorine consumption as it penetrates into the floc. This is analogous to "peeling an orange" and removing the filaments attached to its surface. Chlorination is not a cure-all for all activated sludge microbiological problems, as this will actually make problems worse if the problem is non-filamentous, e.g. slime bulking, zoogloeal bulking or poor floc development.

Chlorine can be applied from a chlorinator using chlorine gas feed or as a liquid hypochlorite. A separate chlorinator should be dedicated to bulking control and an independent rotameter and sampling point in this chlorine line is needed. The chlorine addition point is of most importance and should be at a point where the sludge is concentrated, raw wastes are at a minimum, and at a point of good mixing. Poor initial mixing results in the consumption of large amounts of chlorine without bulking control. Three common chlorine addition points are: (1) into the RAS stream at a point of turbulence (elbows in pipes; into the volute or discharge of RAS pumps; and into and below the liquid level in a riser tube of an airlift RAS pump); (2) directly into the final clarifier center well or feed channel; and (3) in an installed sidestream where the MLSS is pumped from and returned to the aeration basin. Chlorine addition to the RAS line(s) is the method of choice and most generally successful. Chlorine addition to the aeration basin does not work and only causes floc dispersion and system damage.

The two most important parameters are chlorine dosage and frequency of exposure of the activated sludge to chlorine. Chlorine dose is measured conveniently on the basis of sludge inventory in the plant (overall chlorine mass dose). Effective chlorine dosages usually are in the range 1-10 pounds chlorine/1,000 pounds MLVSS inventory/day (2-4 should work). Chlorine dosage should be started low and increased until effective. Most domestic waste plants can achieve a frequency of exposure of the activated sludge inventory to chlorine of three or greater times per day (the optimum) in the RAS line. The needed frequency is a function of the relative growth rates and efficiencies of kill of filamentous and floc-forming organisms. Success has been achieved at frequencies as low as one per day, however, not below this.

In plants with long aeration basin hydraulic residence times (usually industrial waste plants), the daily passage of solids through the RAS line is generally too low (<1/day) for successful bulking control using chlorine at this point. Here, most success has been achieved using multiple chlorine addition points such as the RAS line(s) and the final clarifier(s). Chlorination controls filament extension from the floc surface and merely reduces the symptoms of bulking. Filaments will regrow rapidly, often with a vengeance, after termination of chlorination since the cause of the bulking has not been addressed.

Signs of overchlorination are a turbid (milky) effluent, a significant increase in effluent TSS, a loss of the higher life forms (protozoa), and a reduction in BOD removal. It is normal to see a small increase in effluent suspended solids and BOD₅ when using chlorine for bulking control. Microscopic examination of the activated sludge during chlorination is recommended to control chlorine application. Chlorine effects on filaments include, in order: a loss of intracellular sulfur granules (in those filaments that have these); cell deformity and cytoplasm shrinkage; and finally filament lysis. For filaments that don't have a sheath, the sludge SVI usually declines within a few days of chlorine use, if the dosage and frequency requirements are met. For sheathed filaments, the sheath is not destroyed by chlorine. Here, sludge settleability remains poor until the sheaths are washed out of the system by sludge wasting, which requires 1-2 sludge ages. Chlorine use for sheathed filaments should be stopped when mostly empty sheaths remain (60-80% of filaments) and not continued until the SVI falls, which can result in overchlorination.

Activated Sludge Foaming...

Activated sludge foaming is caused mostly by two filaments: Nocardia spp. and Microthrix parvicella (there are other non-filament causes of foaming). Both of these filaments have three causes in combination: (1) high grease and oil; (2) longer sludge age; and (3) low oxygen conditions or septicity. Nocardia appears to be favored at higher aeration basin temperature and M. parvicella at lower temperature. Antifoam chemicals are not effective to control this type of foam, due to the physical interlocking of the filaments in the foam. RAS chlorination is of limited use in Nocardia foam control, but is more useful for M. parvicella. This is because Nocardia is found mostly within the floc, and the higher chlorine dosages needed to get at Nocardia may destroy the activated sludge floc. For Nocardia foams, surface spraying of a 50 mg/L chlorine solution can be effective. Both these filaments grow on grease and oil. Systems that lack primary clarification (the main grease and oil removal mechanism) appear to suffer more foaming problems. Communities with enforced grease and oil ordinances appear to suffer less from foaming problems. Also, treatment of septage, which contains substantial grease and oil, has been associated with foaming problems. There is some relationship of foaming by these filaments and low oxygen concentration and septicity in the system. Septicity appears to cause the breakdown of grease and fat to organic acids, which specifically favor these filaments. Successful foam control may need control of septicity and low oxygen conditions. The most used control method for foaming is to reduce the system MCRT. M. parvicella can usually be controlled by a MCRT reduction to between 8 and 10 days. Nocardia can often be controlled by MCRT reduction to <8 days, but this is variable and somewhat temperature dependent. Plants in warmer climates have had to reduce the MCRT to <3 days for Nocardia control. Many foams reach problem levels because they are not removed and buildup on the surface of aeration basins and final clarifiers. Needed are enlarged surface scum traps and forceful water sprays to carry this material out of the aeration basin or the clarifier. Foam should be removed entirely from the system and not recycled back into the plant (for example, into the headworks). Foam disposal into aerobic or anaerobic digesters can result in foaming there, so this should be avoided.

Summary...

Filamentous bulking and foaming problems require the identification of the causative filament(s). This information leads to specific remedies, appropriate for the filament(s) involved. Short term control methods are often used to quickly stop a bulking problem. However, the best approach is to investigate the long term control methods suitable for your plant to arrive at long-term, trouble free operation.