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The use of "selectors" for the control of filamentous bacteria within an activated sludge wastewater treatment system.

The question continues to arise from time to time as to what is a selector, how does it work, and where one can be acquired.

First, some background on activated sludge wastewater treatment processes, how they work and why.

An activated sludge wastewater treatment process is a suspended growth type system (the bacteria are suspended in the water media free of attachment to any fixed surfaces). It is in reality a "high tech" pond. The principal differences between a simple stabilization pond and an activated sludge system are;

1. An activated sludge system keeps all of the solids (bacteria) in the aeration basin mixed and in suspension (versus a pond where only a small portion of the bacteria are in suspension, and the rest have settled out to the bottom).

2. In an activated sludge type system, the bacteria are collected at the effluent end of the aeration basin in a secondary clarifier for compaction and recycle back to the front end of the aeration basin to reseed the process (in a stabilization pond, minimal reseeding occurs). As a result of this reseeding (via the return activated sludge line, or RAS), a mixed liquor concentration in the range of 1500 to 10,000 mg/l can be maintained. This can provide a lot of biological degradation in a very small space (versus the pond with an average mixed liquor concentration usually in the range of 25 to 35 mg/l, therefore, requiring a lot of acreage to treat the same amount of biodegradable wastes).

Within a pond system, any and all microbiological organisms will live in the biomass. However, by engineering the activated sludge system to provide a very specific F/M ratio (food to microorganism ratio = number of diners in the cafeteria for the amount of food to be served), and sludge age (how long a specific solid particle, i.e.: bacteria, stays in the system) an activated sludge system provides an environment that is specifically well suited for floc forming bacteria as opposed to free swimming type bacteria or filamentous bacteria (long strings or chains of bacteria cells). This has been done because you not only must convert a soluble waste in the wastewater to a solid (a bacteria cell), but also must have a solid which is capable of settling out on a timely basis.

The activated sludge processes usually work very well unless a stress occurs (too little food for the numbers of diners available, i.e.: low F/M ratio; insufficient ammonia or ortho-phosphate nutrient available to support the required growth and reproduction of the floc forming bacteria; too little dissolved oxygen to support the respiration needs of the floc forming bacteria, septicity, etc.). When these stresses occur, the filamentous bacteria, with their higher surface area (each cell has its own surface area versus the floc formers where only the cells on the outside surface of the floc have a surface area) can compensate for their usually slower reproductive rate by being more competitive for these low level essential requirements, and gain a foothold in the biomass.

Filamentous bacteria are actually excellent BOD (biochemical oxygen demand) reducers, however; the do not settle very easily forming a bridge between floc (and within floc), they have a very high negative zeta potential (high charge which will require high dosages of polymer to counter), and hold a lot of water preventing good dewatering of the sludge. They can increase polymer consumption, increase solids handling costs and can cause bulking in the clarifiers or foaming in the aeration basins.

Complete mix type activated sludge systems (the aeration basin is mixed to the point where the water at any point within the basin is equal in BOD, mixed liquor concentration, etc.) should never be used for industries where the influent wastewater contains high concentrations of very soluble, non-complex type wastes. Within these systems, there is no competitive pressure to hold down the filamentous bacteria (versus a plug flow type system where the influent all enters the front end of the plant where the bacteria tend to remain in a feeding frenzy (the effluent exiting the aeration at the opposite end with low food concentration).

OK, so I have a filamentous problem. What are my alternatives to regain/maintain control?

1. Correct the stress in the system, and then hasten the recovery of the floc forming bacteria by adding a strong oxidizing chemical to the RAS line (if more than 2 exposures are available each day, i.e.: less than 12 hours hydraulic retention in the aeration basin) or for extended aeration systems (18 to 36 hours HRT) add directly to the aeration basin (to where excellent mixing and dispersion of the oxidizing agent can be assured). The chemical of usual choice here is chlorine (either as a gas or as a hypochlorite bleach solution), or hydrogen peroxide (H₂O₂). Reseeding with commercial bacteria can hasten the recovery time after chlorination if necessary.

2. Add a selector to the front end of the process.

Selectors - What are they?

A selector is a small tank or zone at the front end of the aeration basin where influent wastewater and return activated sludge mix prior to entering the main aeration basin. It is called a selector because it selects desirable, non-filamentous, bacteria.

Note: Selectors are NOT effective for controlling all types of filamentous bacteria!

Aerobic selectors are an engineered system based on favoring only those organisms which are capable of high soluble substrate uptake rates along with the capability to store these substrates and then use polyphosphate hydrolysis for energy generation. Most facultative floc forming bacteria in an activated sludge system can do this, however, many filamentous organisms cannot.

Anoxic and anaerobic selectors are based on either the ability of the desired microbe to denitrify or to have the ability to store and hydrolyze intracellular polyphosphate.

Therefore; selectors are not consistently effective against all types of filamentous organisms. Types of filaments in which selectors are NOT EFFECTIVE would include;

1. Filaments which have the capability for high substrate uptake and storage capacity (such as Microthrix parvicella).

2. Filaments which grow on either complex organic matter or particulate material whose concentration is not influenced in a selector system (Types 0675, 0041 or 0092).

3. Filaments which can denitrify (Type 0092)

Summary: Types M. parvicella, Types 0675, 0041 and 0092 are not effectively controlled by selectors since they all prefer slowly biodegradable substrates! Use of a selector may make the filamentous condition worse!

Filamentous bacteria types for which selectors have been successful include;

Sphaerotilus natans ( loves low D.O., short MCRT)

Type 1701 (a low DO, short MCRT)

Type 021N^* (a short MCRT, low F/M, septic waste, low nutrients)

Thiothrix spp^* (a short MCRT, low nutrient, high sulfides)

Nostocoida limicola (a long MCRT, low F/M)

Haliscomenobacter hydrossis (a short MCRT, low F/M, low nutrient, low D.O.)

Type 1851 (a long MCRT)

Nocardia spp.^** (a long MCRT, low F/M)

Note: All of the above thrive a highly soluble, readily biodegradable organics., and can be controlled by aerobic, anoxic or anaerobic selectors.

^{* Selectors not effective if stress caused by low nutrient
availability.
^{** Selectors not consistently effective, but are promising.}}

Selector Design;

Using an anoxic selector as an example (because it takes into account both kinetic - ability to rapidly take up and store substrate, and metabolic - denitrification , aspects of selector performance.

Size: Large enough so that enough time is provided for denitrification to remove the

desired amount of soluble organic matter in the selector with a high F/M.

Generally, the required soluble COD limit on the selector effluent (to the aeration

basin) should be less than 60 mg/l to limit filamentous growth.

Sufficient NO³ -N available from the RAS to remove the desired COD in the

selector. Sanitary wastes require approximately 4.0 mg/l NO³ -N per mg/l of

soluble COD to be removed.

The hydraulic retention in the selector is always based on the rate-limiting step

(usually the NO³ -N availability). If the selector is too small, viscous bulking can

develop (slime) due to too rapid a substrate uptake in the aeration basin (DO

uptakes greater than 100 mg/l/hr).

In summary,

Aerobic selectors rely solely on kinetic (uptake) mechanism.

Anoxic selectors rely on both uptake rate and on ability to denitrify. An anoxic zone provides the necessary conditions for nitrate reduction and phosphorus removal by "luxury uptake".

Anaerobic selectors are void of both dissolved oxygen and NO³ . They use starved intracellular polyphosphate for energy to store soluble COD and release it for energy requirements. For anaerobic selectors, the RAS must contain 4% to 6% P. Typically, most anaerobic selectors are a three-or-four stage reactor equipped with submersible mixers to maintain biosolids in suspension. Return activated sludge (RAS) is discharged to the first stage selector, while raw influent is directed to the second stage. By staggering the RAS and raw wastewater influent location, the volatile fatty acids and soluble BOD, which promote phosphorous release, are not consumed during RAS denitrification. These are many times used when Biological Nutrient removal is required and as an added benefit, the anaerobic selector inhibits the growth of filamentous bacteria that cause bulking sludge.

Selectors are NOT universally successful in reducing filamentous populations in activated sludge systems.

Where do I get a selector?

To install a selector in a wastewater treatment plant, you will have to contract with a consulting engineering company. Sometimes they are constructed as a separate tank prior to the aeration basin, sometimes they are designed to utilize the front end of the existing aeration basin (using curtains, etc. to separate off a compartment. Most are constructed with multiple segments (usually 3 compartments) to provide flexibility for changes in loading, etc. All design engineers familiar with activated sludge wastewater treatment systems will be familiar with selectors, although most experience to date has been in the municipal sector - not industrial.

There are no package unit manufactures available from which to purchase a pre-fabricated system.

One last note.

Within the municipal sector, filamentous control has been achieved by oxygen starving an aeration basin for say up to 8 hours. This concept utilizes the same principals as the anoxic selector. This technique has not been totally successful, and to my knowledge, no name champion has taken on this process as the way of the future. The biggest drawback is that you loose wastewater treatment in that basin while the aeration is turned off (OK for a lot of municipalities this is not a big deal, since they have adequate redundancy - surplus capacity in other basins - to use this procedure without loosing treatment). If additional tanks exist or an equalization basin is prior to the system, moving back the feedpoint of the RAS line may just be a minor process change to make.

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