Process Control

Microscopic examinations of activated sludge can help to assess the condition of the biomass in an aeration basin and the settleability of the sludge. It can also aid in the identification of filamentous bacteria that may cause problems in wastewater treatment plants.

• Floc Particles: Sizes and Shapes
• Dispersed Growth
• Slime Bulking
• Toxicity
• Filamentous Bulking & Foaming

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Additional technical assistance with wastewater treatment plant operations is available from the DNR. Contact Jack Saltes in Madison at (608) 264-8954. For additional help from the DNR, you might also try the Laboratory Certification Home Page.

Floc Particles: Sizes and Shapes

When floc particles first develop in the activated sludge process, that is, at a relatively young sludge age, the particles are small and spherical. Because filamentous organism do not develop or elongate at relatively young sludge ages, the floc-forming bacteria can only "stick" or flocculate to each other in order to withstand shearing action.

Bacterial flocculation and the absence of filamentous organisms result in spherical floc particles.

As the sludge age increases and the short filamentous organism within the floc particles began to elongate, the floc forming bacteria now flocculate along the lengths of the filamentous organisms.

These organism provide increased resistance to shearing action and permit a significant increase in the number of floc-forming bacteria in the floc particles. The presence of long filamentous organisms results in a change in the size and shape of floc particles.

The floc particles increase in size to medium and large and change from spherical to irregular.

Factors Interrupting Floc Formation

• Young sludge age (< 3 days)
• Toxicity (heavy metals etc.)
• Slug discharge
• Lack of active and abundant ciliated protozoan population
• Excessive shearing
• Excessive surfactant

Dispersed Growth

Dispersed growth is a population of bacteria that is suspended in the liquid portion of the mixed liquor. These bacteria are still growing rapidly and have not begin to flocculate. Most dispersed growth is bacterial.

Only a little dispersed growth should be present in a properly operating activated sludge process.

Ciliated protozoa play an important role in the removal of dispersed growth. Dispersed growth is also removed from the bulk medium by its adsorption to the surface of floc particles.

A significant amount of dispersed growth is present at the start-up of an activated sludge process. A lot of food is available, and the bacteria are very active and are multiplying rapidly. The presence of significant or excessive dispersed growth within the mixed liquor can also be due to the interruption of proper floc formation.

For more information on the interruption of floc formation see, Floc Particles, Sizes & Shapes.

Slime Bulking

Often in industrial and municipal activated sludge processes a nutrient deficiency may occur. The nutrients that are usually deficient in these processes are either nitrogen or phosphorus. This deficiency results in the production of nutrient deficient floc particles, loss of settleability, and, possibly a billowy white or greasy gray foam on the surface of the aeration tank.

During a nutrient deficiency, the bacteria within the floc particles remove soluble BOD from the wastewater. However, when nitrogen or phosphorus is deficient, the soluble BOD is not degraded but it is stored within the floc particles as an exocellular polymer-like material. This slimy material interferes with settling and may cause foam upon aeration.

Operational Considerations

The solution usually involves addition of the limiting nutrient, such as ammonia to provide nitrogen, or phosphoric acid to provide phosphorus. There is usually enough nutrient if the ammonia plus nitrate in filtered (0.45 um) effluent is greater than 1 mg/L and the soluble orthophosphate is greater than 0.5 mg/L. However, in cases where easily degradable, soluble BOD is available, higher N and P concentrations may be necessary.

Toxicity

Toxicity assessment is one of the most valuable applications of microscopic observation of microorganisms in activate sludge. The higher life forms, particularly the ciliates and the rotifers, are generally the first to be impacted by toxic materials and may serve in essence as an in-plant biomonitoring test for toxicants or other adverse stresses.

The first noticeable sign of toxicity or stress is usually the slowing or stopping of cilia movement for these organisms and small flagellates and ciliates begin to predominate. This is an indication of the break up of the floc and an over abundance of free bacteria used by these organism as a food source.

Indications of toxicity upset include:

• Loss of the higher life forms in the activated sludge (these are the most toxic sensitive microbial components).
• A dispersed activated sludge biomass with poor floc formation and pin floc.
• Unusually low oxygen use, caused by poor biomass growth.
• Poor BOD removal.

What can you expect to see under the microscope if toxic conditions exist?

• There will be a sudden increase in flagellates. This is sometimes called a flagellate "bloom".
• The of protozoa and higher life forms will begin to die off
• Break-up of floc, sometimes accompanied by foaming
• Loss of BOD removal
• Filamentous bulking upon process recovery. Filamentous bacteria are very often the first to recover after a toxic upset.

Toxic wastes generally do not favor filaments directly (except in the case of H2S), rather upset conditions allow filaments to grow.

Microscopic examination of activated sludge can diagnose toxicity, however, this is usually "after the fact". A better method of toxicity detection is the use of oxygen uptake rate testing to detect toxicity early and to find the source.

Filamentous Bulking and Foaming

Significant Foam Producing Filaments

To determine if aeration tank or clarifier foam is due to the growth of foam- producing filamentous organisms, a sample of fresh foam should be spread thinly and evenly over a clean microscope slide, and the slide stained by Gram staining.

Microthrix parvicella and Nocardia spp each stain Gram positive (staining purple or dark blue). Microthrix is a long thin filament while Nocardia is a short branched filament. So, if you have foam and slide of mixed liquor stains Gram positive, you can easily determine which filament is responsible.

Type 1863 stains Gram negative (staining pink). Type 1863 is a long filament which looks like a dashed line.

If the foam is not due to foam-producing filamentous organisms, it may be due to the presence of a nutrient deficiency. To determine if a nutrient deficiency is the cause of foam production, a representative sample of mixed liquor should be treated with India ink and examined under phase contrast microscopy for the presence of nutrient deficient floc particles.

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