Toxic Shock

Here are ways to find out if your plant process has been affected by some kind of toxic waste stream — and some things you can do about it
Toxic Shock
Barren stalks with no organisms present.

The operators of the Sunbelt Wastewater Treatment Plant arrive for their day shift and notice an unusual odor around the grounds.

After a morning briefing with plant personnel about daily duties, the chief operator (Joe) and shift operator (Bob) head out to investigate. They find an unusual sight: foam billowing in the aeration tanks and a very noticeable sour, chemical-like odor.

The clarifiers also look odd: Clouds of solids are rising and extending throughout the water, and the normally clear supernatant now has a greenish, turbid appearance. Large, irregular floc particles can be seen going over the effluent weirs.


Comparing solutions

Joe and Bob agree the plant looks sick, but not on how to correct the problem. Bob thinks the aeration dissolved oxygen (DO) should be increased and the waste sludge rate should be greatly increased to wash the “infected” biomass from the system, followed by reseeding with mixed liquor suspended solids (MLSS) from another treatment plant. He says there is enough digester capacity to waste solids into, and enough ability to dewater and haul away the cake sludge.

Joe considers Bob’s ideas for a moment, and then recalls some training they recently had about activated sludge process control and troubleshooting. During the course, they learned about using various process tests to help determine effective solutions for situations like this.

Joe instructs Bob to collect some samples and meet him in the lab. There, Joe asks the technician to help with the additional process control tests he wants to run. During normal plant operation, the operators run settleability tests, a centrifuge spindown on the mixed liquor, and analysis for DO, pH, turbidity and total chlorine residual.

Today Joe asks for two more tests: a microscopic exam and an oxygen uptake rate (OUR) test. While out collecting samples for lab testing, Bob checks the aeration tank DO and finds a very high level — well over 5.0 mg/l. He reports this finding to Joe while bringing in the samples.


Valuable tests

The benefits of the OUR analysis and the microscopic exam as part of routine plant operation process control cannot be over-emphasized. These tests can be viewed as a health checkup for wastewater treatment facilities. Although every plant is different, each plant has its best operating set points. By gathering as much data as possible to learn how the plant runs at its best, an operator can make changes to bring the plant back into compliance when things seem to go wrong.

An OUR test measures the amount of oxygen (in mg/l) that a sample of MLSS uses per hour. A calculation is done to get the final result as milligrams of oxygen per liter per hour (mg O2/l/hr). When a treatment plant is running optimally, the amount of oxygen used by the bacteria to stabilize the wastewater is normally much higher at the inlet end of the aeration tank than at the outlet.

This demand for oxygen by healthy aerobic microorganisms is high where the food supply is the highest and the bacteria are actively consuming BOD. The oxygen demand lessens as the flow reaches the end of the aeration tanks before entering the clarifiers, by which point almost all of the BOD has been consumed.

However, when a treatment plant is upset, the OUR test may have different results. In the case of an abnormally high organic load entering the plant, the outlet OUR might be as high as the aeration inlet OUR, indicating that increased amounts of BOD are exiting the plant. More treatment time, more DO, or both may be required.

If the treatment plant has experienced a toxic batch of influent, as Joe suspects, the OUR test result would likely be very low at the aeration inlet and outlet. In this case, the aerobic bacteria are inhibited by the influent waste stream, or even dead, consuming very little oxygen. The DO test Bob ran while collecting samples confirms this theory.


A close look

A microscopic exam is another great way to do a checkup of the biomass. If done on a routine basis — say, once or twice a week — it can help operators predict a negative change in operation and make adjustments before effluent quality deteriorates.

Some of the protozoa we rely on to indicate the degree of treatment are very sensitive to toxic loads, which could be from heavy metals, high influent ammonia, pesticides, herbicides, formaldehyde, or many of the other thousands of chemicals we use in our households and commercial establishments.

The Sunbelt lab analyst sets up several wet-mount slides for the operators to view. Using some of today’s technology, the lab’s new microscope has a built-in LCD screen so multiple people can view the slides at the same time.

The observation is telltale: The stalked ciliates that normally dominate the mixed liquor are now missing. Remnants of stalk ciliate colonies are now seen as just the stalk itself — no individual animals living at the ends of the branches. An obvious lack of free-swimming ciliates, particularly the crawling species, and a lack of rotifers indicates something major has shocked the microlife.

The previous micro exam, done about one week ago, showed a very healthy, active and mixed population of all the above microorganisms. Today’s results show very little activity and small, dispersed floc particles with much ‘debris’ all around the floc. The operators also observe noticeable reduction of filament bacteria.


What to do?

Joe and Bob sit down to review the data they’ve collected. It appears that the treatment plant has suffered a toxic shock load that has killed most of the bacteria required for treatment. What to do next?

If this happens to your plant, the first thing to do is notify all applicable regulatory authorities. Depending on the severity of the shock load, the treatment plant could be in violation of its discharge permit. Notifying local, state and federal authorities of the abnormal plant operating condition is critical, and usually mandatory, especially if the effluent quality has been affected.

Next, an action plan should be agreed upon and implemented. Wastewater treatment facilities sometimes suffer from events like those described above, and it’s imperative to have a plan to get the plant into compliance as quickly as possible. In the case of a toxic shock, here are some suggestions that can help the facility recover:

1. Make sure the toxic stream is not still flowing into the plant. If it is, collect samples of the influent in various containers, plastic and glass. Contact your laboratory and describe what is happening. They will recommend sample amounts and preservatives if needed. Record the time, date, location and the sample collector’s name or initials when grabbing the samples.

2. If the influent appears normal, reduce the waste sludge flow rates or stop wasting altogether. The idea is to retain as many solids as possible and begin rebuilding the biomass.

3. Keep the return activated sludge rate high and maintain aeration DO levels at 1.0 to 2.0 mg/l.

4. Consider returning some waste sludge from an aerobic digester if possible to build up biomass. Alternatively, reseed with fresh return sludge from another treatment plant. Monitor the plant health and run process control tests to observe the recovery.

If your facility routinely suffers from frequent toxic influent waste streams, the source of the pollution should be located and stopped. A pretreatment program can help locate the culprit, and may be required by regulatory authorities. The deliberate dumping of chemicals toxic to treatment plants is a violation of the federal Clean Water Act and can easily contaminate receiving waters.


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


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.