Activated Sludge (ASI)

Your Viscous Bulking Primer

When we consider the importance of secondary settling we should always remember that if this part of the activated sludge process fails, then the whole process fails. With that point in mind we will focus in on one of the things that can go wrong in the clarification process -viscous bulking, a world-wide problem caused by the over-supply of the glue which holds the flocs together.

Activated sludge flocs consist of aggregates of microbes, filamentous organisms, organic and inorganic particles held together by exocellular polymers produced by the bacteria. The process of flocculation or bioflocculation causes changes in floc characteristics, supernatant turbidity and variations in settling and dewatering properties. While the flocculation process is critically important, it is not well understood.

Microbial extracellular polymeric substances (EPS)are the key components for the aggregation of microorganisms in biofilms, flocs and sludge. They are composed of polysaccharides, proteins, nucleic acids, lipids and other biological macromolecules. EPS provide a highly hydrated gel matrix in which microbial cells can establish stable synergistic communities.

Activated sludge flocs are held together by EPS and polyvalent cations which form a 3-dimensional matrix. The EPS outer layer on many bacteria (glycocalx or capsule) allows them to adhere to surfaces (produce biofilms) as well as providing a diffusion barrier for solutes and nutrients. (Fig. 1) This film also offers protection from predation, freezing, heat, drying, pH changes, biocides, detergents, loss of nutrients, and heavy metal toxicity.

Viscous bulking: Too much of a good thing?

Viscous bulking is a major operational problem in wastewater treatment systems. It is the result of excessive EPS production by microorganisms, causing poor compaction and settling of biomass in secondary clarifiers, increased effluent BOD and poor sludge dewaterability with all of its associated problems.

One group of organisms historically associated with EPS production in activated sludge systems is Zoogloea ramigera. This organism is often blamed for causing the excessive "zoogloeal" or slime growth which causes viscous bulking, but it would appear that the term "zoogloeal" is often used to generically describe the overproduction of EPS by any organism not just Zoogloea ramigera.

Zoogloea ramigera : Saint or Sinner?

It is easy to imagine how an unusually high occurrence of this organism would hinder the ultimate settling of the sludge. Zoogloea ramigera (Figs. 2 and 3) is considered to be the classic floc forming organism, providing the glue which starts off the flocculation process by sticking together the bits, living, dead and inert which makes up the operationally critical activated sludge floc.

This organism is easily recognized in activated sludge by its "fingers" which appear to exude from the sludge or existing independently. An amorphous or globular form has also been reported in paper mill activated sludge.

Z. ramigera forms flocs by the intertwining of extracellular fibrillar strands contained within a Zoogloeal matrix. The name Zoogloea means "animal glue," and it is this sticky matrix that makes Z. ramigera unique. Zoogloea ramigera has a significant amount which contains more than 90% water and only 1-2% of EPS.

What's in a name?

Although Zoogloea species have often been blamed for slime production in activated sludge, there appears to be little to support the association of a specific strain of Zoogloea with episodes of viscous bulking or poor dewaterability. The terms "zoogloea", "zoogloeal" (and variants "zooglea" and "zoogleal") are often used to describe the generic overproduction of EPS by any organism, in the way that the term "Nocardia" is used to describe the nature of a foaming incident.

I recently found the following entry in an on-line glossary . . . "Zooglea - (Water Quality) - A jelly-like matrix developed by bacteria which is abundant in activated sludge flocs and trickling filter slimes."

The proliferation of names for overgrowth of EPS does not help (e.g. slime, slime bulking, viscous bulking, viscous sludge, zoogloeal or zoogleal bulking, non-filamentous bulking and turbid growth), but it is indicative of the breadth and seriousness of the problem. In recently reported studies, members of the genus Thauera have been shown to cause viscous problems originally blamed on Zoogloea spp.

Zoogleal type clusters have been reportedly been involved in poor sludge compaction and dewatering, particularly during plant startup and have been involved in episodes of viscous bulking. However, they are also typically observed in treatment systems using selectors and are not necessarily associated with sludge quality problems.

Possible conditions for Zoogloeal overgrowth

Growth of Z. ramigera is reported to be favored in highly loaded plants, particularly those with significant amounts of simple or easily degraded carbohydrates, VFA or RBCOD in the wastewater. Other reports associate specific operating conditions of both high and low F/M, high MCRT, long HRT and nutrient deficiency.

Overgrowth of Z. ramigera would be detrimental of the process requiring manipulation the above parameters to fix the problem. Anoxic basins or selectors are reported to control growth, but interestingly, excessive anoxic retention has been reported to encourage growth of Z. ramigera.

Nutrient deficiency: Prepare to be slimed!

In my experience nutrient deficiency in activated sludge is only encountered in the treatment of industrial (non municipal) wastes. Nutrient deficiency should not occur in activated sludge plants treating predominately municipal waste, but there are reports to the contrary. Nutrient deficiency may cause the production of viscous foam, and impart a jelly-like consistency to the activated sludge which interferes with sludge compaction. This is referred to as "slime bulking".

I have seen slime bulking associated with nutrient deficient moving bed biofilm reactors (MBBR) and activated sludge plants treating paper mill waste. Microscopic examination revealed long jelly-like fibrils which were attached to the biomass on the supports in conjunction with filamentous bacteria.

Slime bulking is caused by the growth of floc-forming organisms which under conditions of a unbalanced nutrient supply or nutrient deficiency, produce substantial amounts of EPS slime, particularly in the presence of an excess of easily degraded substrates. These products may be released from the cell and because of their surface active properties, cause foaming.

EPS can be identified and visualised by the india ink test (negative staining technique). (Fig. 4) These slimes are also reportedly structurally weak and may be cleaved and dispersed due to shear in pumps and mechanical aerators.

Nutrient deficiency may also select for certain filaments e.g. Thiothrix, Types 021N, 0041 and 0675, leading to filamentous bulking. Types 0041 and 0675 exhibit a Neisser positive staining slime coat (capsular material) under nutrient deficiency which may assist in identification.

Operational Considerations: The Fix.

The solution invariably involves addition of the limiting nutrient, such as ammonia or urea to provide nitrogen, or phosphoric acid to provide phosphorus as is often done in paper mill treatment. There is usually enough nutrient if the ammonia-N plus nitrate-N in filtered (0.45 um) effluent is greater than 1-2 mg/L and the soluble orthophosphate is greater than 0.5-1 mg/L. However, in cases where RBCOD is available, higher N and P concentrations may be necessary to achieve this effluent. My own approach to this problem has always been add the deficient nutrient until it appears at the levels mentioned above and the difficulty should resolve itself.

Conversely, non-filamentous bulking or turbid growth has been treated by the addition of synthetic cationic polymers or minerals (usually divalent or trivalent cations). These compounds help bridge small and diffuse flocs, thereby increasing density and improving the settling characteristics of the sludge blanket in the clarifier.