Solving The Riddle Of High CBOD Readings

A plant team in Alabama leaves no stone unturned in finding the cause of apparently elevated weekly average CBOD levels.
Solving The Riddle Of High CBOD Readings
Plant team members who solved the CBOD riddle are, from left, Damon Chambers, operator; Andrew Douglas, project manager; and Houston Black, operator.

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Since June 2012, the 0.995 mgd Hanceville (Alabama) Wastewater Treatment Plant had met all NPDES discharge permit limits set by the state Department of Environmental Management.

However, August 2014 brought two violations: one for CBOD weekly average concentration, and one for CBOD monthly average. It took a few weeks, but the plant team finally identified the cause of the violations. The source of the problem was surprising — and the plant’s effluent most likely was never truly in violation.

A simple change 

The trouble started on July 4, when the silicone pump tubing on the effluent sampler split during the night, and a 24-hour composite sample was not pulled. Plant personnel replaced the pump tubing and suction line with new silicone pump tubing and a new vinyl suction line. Until that point, most effluent CBOD values had ranged from 1.0 mg/L to 4.6 mg/L, versus a weekly average limit of 14.2 mg/L and a monthly average limit of 13.0 mg/L.

Around the second week of July, the CBOD weekly average went up to 6.0 mg/L; it rose to 11.6 mg/L the following week. Clearly something was causing CBOD to elevate. The team’s first response was to increase sludge wasting to about 90 minutes per day. Normally, sludge was wasted for 30 minutes per day, maintaining the mixed liquor suspended solids (MLSS) concentration in both aeration basins at 2,500 to 3,000 mg/L.

However, the week after the wasting volume increase, the CBOD test result averaged 10.9 mg/L. Microbiological examination of the MLSS indicated that all was fine: There were plenty of stalked and free-swimming ciliates, as well as some rotifers, and there was no sign of a filamentous bacteria issue.

After wasting at higher rates continued for two weeks, the CBOD test result increased to 12.0 mg/L. The MLSS concentration had fallen to 1,000 mg/L, so wasting was cut back to the normal 30 minutes per day. The MLSS started climbing back toward normal; obviously, the MLSS level was not the problem.

Adjusting aeration

Next, the team took the No. 2 aeration basin offline and increased the blower output to the No. 1 aeration basin from 80 percent to 100 percent. The thought was that increased air to the MLSS would allow soluble CBOD to be oxidized more completely, and so the effluent CBOD would decrease. Instead, the weekly average CBOD result rose to 12.5 mg/L. 

Of course, the problem with troubleshooting CBOD issues is that it takes a week to see the results. With most other process control issues, operators can make a change and see results in a few hours or by the next day. However, with CBOD, an operator makes a change, takes samples and then has to wait five days before any results can be determined and analyzed.

Next, the team considered the lab as a possible cause of the problem. They exchanged all CBOD bottles for new ones, discarded all CBOD reagents and ordered new reagents. They also split samples with another lab. For the first subsequent week, the CBOD results were 12.3 mg/L for the Hanceville lab and 11.3 mg/L for the other lab — both still below the 13.0 mg/L monthly limit, but too close for comfort.  

Further, the team cleaned the effluent sampler tubing with bleach and jetted the discharge pipe where the sampler tubing was located — just in case a dead animal or something else in the discharge line was causing the high CBOD. The jetting crew reported no obstacles in the pipe.

Also that week, the team thoroughly cleaned the chlorine contact chamber of all debris and put it back online. They increased the feed rates of chlorine and sulfur dioxide to help burn up more CBOD in the effluent. Despite all that, the week’s CBOD increased to 13.4 mg/L.   

Digging deeper

As a next step, the operators began profile sampling throughout the plant. They collected grab samples at the chlorine contact chamber, the clarifiers and the final effluent cascade, and set them up for CBOD analysis. Next, they borrowed a portable composite sampler from another treatment plant and set it up at the secondary clarifiers.

The effluent CBOD reading for that week averaged 12.5 mg/L; however, the CBOD result from the portable unit at the clarifiers averaged 1.3 mg/L. This was puzzling. Next, the team set the portable sampler alongside the plant effluent sampler and had samples from both analyzed. The plant sampler CBOD result read 12.8 mg/L, while the portable sampler read 4.1 mg/L. 

Checking yet another variable, the team swapped the pump tubing on the portable sampler with the pump tubing on the plant sampler. The CBOD result on the plant sampler then dropped from 8.1 mg/L to 2.8 mg/L the next day. This showed that the problem was the new vinyl pump tubing that had been installed on the effluent sampler in July. After the vinyl tubing and strainer were changed out with new braided PVC tubing and a new strainer, the CBOD problem disappeared. Effluent CBOD results returned to the normal range of about 2.0 to 4.0 mg/L.

In my 34 years as a wastewater operator, I have never heard of new vinyl tubing giving high CBOD values, nor had my fellow operators or the effluent sampler sales representative. One theory is that the vinyl tubing was made overseas where manufacturing standards are lax. In any case, it appears the plastic tubing was contaminated with something that affected plant effluent CBOD readings.

Plant teams experiencing unexplained issues with CBOD or BOD levels might want to consider the sampler tubing as a possible cause of the problem.

About the author

Andrew J. Douglas is a project manager with ClearWater Solutions in Hanceville, Alabama.   


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