Making the Connection

The Lab Detective traces erroneous wastewater treatment plant influent BOD figures to the presence of heavy metals in the drinking water system.
Making the Connection
Interior of corroded copper pipe commonly used in older house plumbing.

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Operators at water and wastewater utilities sometimes forget how connected the two disciplines really are. If you’re licensed as both a drinking water and wastewater operator, you may find yourself wearing different hats throughout the day.

Running the water plant, checking chlorine residual readings, adjusting chemical feed rates, and maintaining equipment are all a part of the daily routine. Out in the distribution system, operators may find themselves responding to consumer complaints, flushing hydrants, looking for and repairing leaks, reading meters, collecting monthly samples, and completing state reports.

A dual-licensed operator might then don the wastewater hat. There, duties include running the treatment plant, collecting samples, cleaning clarifiers, running biosolids dewatering equipment, adjusting chemical dosages and maintaining equipment. An operator also may be in charge of checking the lift stations in the collection system and taking monthly samples for state reporting.

Noticing an issue

It is during those last duties where there may be a reason to put on the Lab Detective hat. While working as an operator of a water and wastewater utility, the Lab Detective found himself wondering about shortcomings in the wastewater plant. The facility was designed for nitrogen removal using the activated sludge process.

At times during the day, the plant’s effluent chlorine residual would drop rapidly, even though there was no decrease in chlorine feed and no problems with the chlorination equipment. Something was causing a significant chlorine demand during certain times of the day, usually in the early afternoon.

At an Activated Sludge Process Control and Troubleshooting course at a nearby training center, the detective heard the instructor describe a phenomenon called nitrite lock — a condition where a small amount of nitrite (usually only about 1 to 2 mg/L) makes its way all the way through the treatment processes to the chlorine contact tank.

Since one milligram of nitrite can consume about 5 milligrams of chlorine residual, measuring nitrite becomes significant when battling chlorine residual problems. The detective found this was indeed the root cause of the chlorine residual problems at his wastewater plant. But what was allowing nitrite to remain in solution? Why didn’t the nitrite get fully oxidized to nitrate during nitrification?

The detective found that several conditions were allowing the nitrite to persist into the chlorine contact basin. A leading cause was shortened detention time during the oxic (or aeration) period at peak flow conditions. More time was needed to allow complete nitrification while the highest flow of the day was seen inside the plant.

DO problems

Another shortcoming of this facility was the available oxygen output of the centrifugal blowers used for diffused aeration. During the daily peak flow, multiple blowers were needed to maintain adequate dissolved oxygen (DO) in the aeration system, especially during summer when the density of a cubic foot of air is less than in winter.

The detective often wondered why the design engineers hadn’t sized the blowers slightly larger.  What data had they used when calculating the system’s airflow requirement? Doing a little investigating at slow times during his shift, the detective found some of the original drawings, plans and criteria used for the treatment plant design.

An interesting figure was used for the organic loading calculation — an unusually low number was used for the annual average BOD loading. Since the plant was built quite a while before the detective began working there, he had to go into the “document dungeon” to find the monthly lab data and state reports that were used in the plant’s design.

Since the detective was a chief operator and was a part of all regulatory reporting for the water and wastewater system, he knew the current operating conditions of both sides. On the drinking water side, corrosion control measures had to be taken in certain parts of the distribution system due to elevated amounts of lead (Pb) and copper (Cu) in the sections of town with older homes.

Tracking the source

While the wastewater treatment plant was being upgraded, the distribution system lead and copper levels remained just below the action levels required by the state for corrosion control. A year or so after the wastewater plant expansion, the utility was required to institute a lead and copper control program in the areas of the water system most affected.

Many of the homes built in the 1970s had copper water lines joined with lead solder, and these heavy metals were slowly eroding. The detective found that the distribution system operators responded to many more reports of water leaks in these areas, as well.

Since the detective collected samples from the digesters and completed the yearly EPA 40 CFR Part 503 biosolids report, he knew what amounts of these same heavy metals were accumulating in the biosolids. Since lead and copper were just two of the metals tested for annually to meet state and federal regulations, it was easy to see the levels increasing over time. It would not be too long before the utility would need to investigate alternative methods of treating and managing the biosolids.

But what about those suspicious low BOD values entering the treatment plant? Could the increased amount of lead and copper interfere with the biology in the BOD analysis?

A look back through many years of lab results seemed to confirm the detective’s hypothesis. When side-by-side analyses of BOD, CBOD and COD were compared, the results fell in line with what appeared to be lead and copper interfering with the biological methods, while the COD analysis stayed constant. Monthly data that surrounded the time frame when corrosion control inhibitor was first added to the water system also confirmed the detective’s suspicion.

Referring to the latest edition of Standard Methods for the Examination of Water and Wastewater and discussing this possibility with the certified laboratory manager, the detective found that copper is indeed a source of interference with the biological test methods for BOD and CBOD. Standard Methods recommends avoiding the use of copper lines in laboratory-grade water systems and in distillation units, and also advises against storing reagent-grade water in vessels containing heavy metals.

Collaborative solutions

Copper in particular is a toxic heavy metal known to kill bacteria and algae in water systems, reservoirs, lakes and rivers. Copper sulfate pentahydrate, known as “blue stone”, has been used in many industries, including treatment of lumber to prevent insect degradation. Copper can also affect the results of biological tests like BOD, causing false low BOD levels.

The treatment plant superintendent, certified laboratory manager, utility engineer and Lab Detective concurred that the elevated level of copper in the drinking water, which becomes influent wastewater, was the cause of the reduced influent BOD values. Further, they concluded that these false values were part of the initial loading calculations used to size treatment plant unit processes — blower capacity, aeration tank size, and others.

The outcome of this story is that the treatment plant engineers, operators and lab personnel worked together to gather representative samples and obtain accurate lab results, which then were used to justify newer, more energy efficient centrifugal blowers with higher output than those previously installed. After the new blowers were installed, the facility began to achieve exceptional effluent quality at a reduced cost — a win-win situation!

Note: The author acknowledges Lisa Saupp of Aqua Pure Laboratory in Silver Springs, Fla., for her help in research for this article.

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


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