When the Numbers Don’t Add Up

An investigation of bulking sludge reveals the presence of filamentous bacteria and suggests a solution based on improving maintenance and operating practices.
When the Numbers Don’t Add Up
Dissolved oxygen readings are shown on a Hach LDO meter and a YSI 556 multimeter.

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The plant operator was at his wits' end. The sludge just wouldn't settle, no matter how he tried to control it. Solids wash-out was a regular event, causing the effluent to be out of compliance with state TSS, BOD and fecal coliform regulations.

To keep the solids from overflowing the clarifier weirs, the plant was wasting large amounts of solids to the aerobic digesters. The plant owner (who also owned the development the plant serves) was frustrated with the cost of hauling this excess sludge away. The lab detective learned of the operator's dilemma and scheduled a visit.

Looking at history

The operator gave a summary of the plant's recent history. It served a small planned development (mainly residential, about 25 percent commercial). A strip mall contained a locally owned restaurant, a few shops, and a dry cleaner. The activated sludge plant operated in the extended aeration mode, designed to treat 80,000 gpd, but treating about 45,000 gpd on average. The collection system was only a few miles long and almost free of inflow and infiltration.

The influent is received in a flow-equalization basin, then fed into the first of several large aeration tanks operating in series. The final aeration tank discharges to a flow splitter box before two secondary clarifiers. Effluent flowed from these clarifiers to a chlorine contact tank and then to a land-application spray field site nearby.

In troubleshooting a treatment plant, the lab detective has learned that accurate data is very important. He travels with his own field instruments that he maintains and calibrates. Included in the array of meters is a colorimeter for testing nutrients like ammonia, nitrite, nitrate and orthophosphorus, and a multi-probe meter for in-tank readings of pH, dissolved oxygen (DO), oxidation-reduction potential (ORP), temperature and conductivity. He also carries sample bottles for collecting samples of mixed liquor suspended solids (MLSS), foam or whatever else he might want to study later.

Comparing readings

When an operator describes current operating conditions, the lab detective notes the operator's data, the types of instruments used to get the data, the operator's use of the meters, and the care and maintenance of the instruments. (In one instance, a plant operator-trainee did not realize that new DO probes are shipped dry and that electrolyte must be added to the probe before the first use. This gave some interesting DO readings to say the least!)

Side-by-side comparisons of parameters like DO with field instruments showed that the operator's DO readings were in proper range (1 to 2 mg/L) for aerobic treatment, although slightly on the low side. ORP showed a very low millivolt reading, indicating more reduced chemical species present than oxidized species in the MLSS. Ammonia is one of the reduced chemical compounds that, when present in higher numbers than oxidizers (like DO), can cause ORP to be negative while the DO meter detects oxygen.

These are some of the pieces of data that do not always add up. One would believe that when there is measurable DO present, the ORP should be at least on the positive side of the millivolt scale. A quick check of the influent flow equalization (EQ) basin showed zero DO and -400 mV on the ORP meter, indicating a pretty septic raw wastewater. The odors coming from the EQ basin supported this theory.

With much of the data pointing to a low DO filament or a septicity filament — or both — causing the slow-settling sludge, the lab detective gathered some MLSS for microscopic analysis. Back at the lab, microscopic observation with a low-power objective revealed a very open and diffuse floc structure with excessive thin filaments extending from the floc.

He prepared and stained several smears using three staining procedures (Gram, Neisser and PHB), and subsequent results were all negative. The relative size of the filament, the location, the lack of attached growth and its staining reactions all pointed to a filament called Haliscomenobacter hydrossis, or H. hydrossis, as the cause of the slow-settling sludge.

One characteristic observed, but not common with H. hydrossis, is the formation of 'rosettes,' where the filament seems to radiate from a common central point and spread outward like a sunburst. Rosettes are commonly seen with other filaments like Thiothrix I and II, Types 0914 and 021N. Again, not all of the data added up.

Choosing the remedy

When deciding a path for correcting a plant's problems, data like this becomes important. Several potential remedies can be derived from the above discussion, but which is the best course of action? Which will bring the facility into compliance quickly in the most environmentally friendly and cost-effective way?

The lab detective decided to consult with a few other folks in the wastewater industry — people who specialize in filament identification and write manuals on such topics. He sent samples to Dr. David Jenkins of David Jenkins and Associates, and Dr. Michael Richard of Michael Richard Wastewater Microbiology LLC, two authors of the Manual on the Causes and Control of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems, Third Edition. This manual is a valuable reference tool in any lab detective's library.

Again, the diagnosis was debatable. Richard concluded that the filament appeared to be H. hydrossis, while Jenkins thought otherwise. Jenkins commented that it was an unknown filament that did not exhibit the characteristics of H. hydrossis.

So now what? The majority of evidence pointed to a bulking filament similar to but not definitively H. hydrossis. H. hydrossis can appear in activated sludge that is somewhat septic with low DO and elevated organic loading. Higher-than-normal organic loading can occur from septic influent wastewater, where fermentation occurs, producing volatile fatty acids that are readily metabolized by facultative anaerobic bacteria.

Other filaments were also present in the MLSS sample along with Spirochetes, bacteria with long, spiral-shaped cells. Spirochetes are commonly found in anaerobic environments and can be seen twisting and wiggling about under a higher-powered microscope.

Armed with this information, the lab detective recommended aeration of the influent equalization tank and increasing the available DO in the plant aeration tanks. It was also necessary to pull the diffusers and clean and repair the broken diffusers, since this had not been done in several years. He also recommended proper operation of the influent flow EQ basin.

Reducing shocks

Flow equalization basins are designed to absorb the shock of diurnal fluctuations of flow. EQ basin liquid levels fill during high-flow periods and draw down during low-flow periods, allowing a constant medium flow into the treatment system. This steady flow reduces the chance of hydraulic wash-out of the clarifiers. The sludge age can be slowly increased, producing a heavier MLSS with greater numbers of flocculating bacteria than filament bacteria.

An important note about knowing the filament you are dealing with: The lab detective recommended against using chlorine or another biocide in the MLSS to combat the filaments. Since the filament most likely causing the bulking sludge has a sheath, excessive chlorine may be needed to penetrate the sheaths before killing the bacteria inside, all the while killing other unsheathed bacteria needed for treatment and probably causing a worse out-of-compliance situation.

The detective discussed these findings with the plant owner and operator, who developed an action plan. It included immediate changes to the influent EQ basin's aeration and flow control and scheduled aeration diffuser maintenance. The plan also included collection system cleaning and process control training.

The owner relayed this information to the state regulatory agency. With the plan in place, the solids stopped washing out of the clarifiers, the effluent cleared up, and the regulators were happy. The filaments decreased but were not yet gone entirely.

About the author

Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.


Manual on the Causes and Control of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems. Third Edition. David Jenkins, Michael Richard and Glen Daigger. IWA Publishing, 2004.

Wastewater Microbiology: A Handbook for Operators. Toni Glymph, AWWA, 2005.

Special thanks to Dr. Michael Richard and Dr. David Jenkins for their input and assistance with this article.


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