Filamentous Bulking | Activated Sludge Process | Wastewater Treatment Plant

Filamentous Bulking

Solids Separation Problems

By recycling biological solids, the activated sludge process can achieve a high SRT for system stability and efficiency and a low HRT for system economy. In this manner, the microorganisms can be grown to a high concentration and maintained in a manner that is somewhat independent of the liquid detention time. Of course, this is only possible if good solids separation and settling occurs.

One class of problems is related to physical problems with the design of the secondary clarifier. These include such things as improper inlet design, weir placement and differential settling (that is uneven settling of the entire basin). Similarly hydraulic overloading could cause a washout of biological solids. One may identify these types of problems with lab settling cylinder tests.

There are numerous solids separation problems in the activated sludge process which are related to the microorganisms.

• Dispersed Floc and Dispersed Growth (microstructure)

The choice of SRT value influences the settling characteristics of the biological solids. This situation is referred as the microstructure of the floc or its basic structure due to bacterial aggregation and flocculation. The condition known as dispersed growth occurs at low SRTs. The microbes are spread as individual organisms or tiny clusters rather than flocs. This is due to the physiological state of the microorganisms in that at low SRTs, the microbes are growing at a very fast rate and at these high growth rates, the organisms do not form polymers which cause the flocs to form. Thus good settling does not occur.

A condition known as dispersed floc can occur at high SRTs. This is a function of the physiological state of the microorganisms in that at high SRTs the microbes are in a low growth rate and actually start to break up the good flocs previously formed. This results in poor floc structure, and poor settling.

• Other Problems (slime, blanket rising, foaming)

We'll look at challenges that aren't caused by filamentous organisms or have nothing to do with microstructure characteristics in this section (dispersed growth and dispersed floc). When an excessive amount of exocellular slime (polymer) is released, the solids become slimy and jelly-like. As a result, the settling is poor. This often occurs during periods of nutrient deficiency and may be corrected by adding nutrients.

Pin floc is the result of weak small flocs which have low filament levels. These flocs are small and compact. The larger one will settle but the smaller ones don't, leaving behind a cloudy turbid effluent.

Blanket rising is a condition caused by denitrification in the secondary clarifier. The nitrogen gas bubbles formed attach to the sludge flocs and cause them to float. This usually the results of sludge remaining too long in the clarifier. This can only occur in a nitrifying plant since nitrate must first be formed. Thus the SRT and dissolved oxygen concentration are related to this problem.

Foaming is caused by non-degradable surfactants. These surface active substances travel undegraded through the aeration basin, creating foaming and causing significant volumes of particles to float to the surface. Another type of foaming is caused by the filamentous organisms Nocardia and Microthrix. Exocellular lipids and surfactants produced by these organisms are non-degradable and cause flocs to float. It may be due to high SRT, high MLSS concentration, high rates of aeration, or high concentrations of hydrocarbons in the influent. These foams are quite persistent and difficult to break mechanically.

Toxic substances can result in a condition known as curdled floc. This is caused by the interaction of positively charged heavy metals with negatively charged surface of bacterial cells. This is obviously related to the presence of toxic heavy metals in the wastewater.

Excessive shearing action by mechanical mixing can cause a "pin-point" type of floc.

• Filamentous Bulking

Filamentous organisms are those which grow in the form of very long rods, many times longer than wide. They may be effective degraders of BOD, but they form flocs which do not settle well or compact well.

The macrostructure is due to the network of filamentous organisms within the floc. The filamentous network provides a structural frame for the floc- forming bacteria and provide strength for the floc. In the first case, filamentous bulking is demonstrated to be caused by an overabundance of filamentous organisms, whereas in the third case, poor settling, pin-point floc, is demonstrated to be caused by a lack of filamentous organisms. Nonbulking is depicted in the second scenario as a state with a good balance of filamentous and nonfilamentous organisms. This approach suggests that some filamentous organisms are necessary to form a good backbone for a strong, large floc.

(Case 2). Excessive filaments, on the other hand, obstruct settling and compaction. (Case 1). Excessive filaments cause a diffuse floc structure or may result in briding between flocs because filaments frequently grow beyond the boundaries of a single floc.

• Summary of Solids Separation Problems

As can be seen, the activated sludge process has a number of solids separation issues that are related to microorganisms (rather than to physical aspects such as hydraulics and basin design). These problems are related to the type of floc formed in the aeration basin.

In summary, there are nine basic types of flocs including:

1. an ideal floc containing floc-forming bacteria and filamentous bacteria that grow in balance,
2. a filamentous floc in which the predominant filamentous bacteria cause inter-floc bridging,
3. a filamentous floc in which the predominant filamentous bacteria produce a diffused floc,
4. a viscous floc formed by the release of non-degradable surfactants and lipids by Nocardia during periods of nutritional deprivation, 
5. a floating floc created by the release of non-degradable surfactants and lipids by Nocardia.
6. a gas-entrained floc resulting from the entrapment of insoluble nitrogen gas released during denitrification.
7. The interaction of floc-forming bacteria with relatively large quantities of hazardous chemicals such as multivalent cations results in a curdled floc.
8. a pin-point floc in which filamentous bacteria are absent, and
9. a floc that is scattered and contains small clusters of bacteria but lacks real floc particles.

The type of floc formed is determined by the environment in the activated sludge aeration basin. Various stimulatory conditions can cause deleterious situations which results in poor floc structure which do not settle or compact well and then subsequent loss of biological solids. These include:

1. Environmental changes which promote the rapid and excessive growth of filamentous bacteria
2. Excessive amounts of exocellular lipids, surfactants, or slime are produced as a result of environmental changes.
3. Denitrification-friendly operational conditions
4. The presence of relatively high concentrations of toxic substances, and
5. The presence of excessive shearing action within the aeration basin.