Credit: Photo: Pam Broviak
Adding sodium hydroxide for pH adjustment caused calcium carbonate to drop out of solution and accumulate in this elbow and the downstream pipe—a result expected by the operators based on prior chemical analysis but unavoidable due to design conditions specific to this plant.
When you visit a water plant, what image are you left with? Depending on your experience, you may envision a building pulsating with the noise of pumps and motors; a vast array of tanks filled with water; or a large room of pipes and computer screens with red, green, or yellow lights. When I enter a water plant I immediately think of a large chemistry experiment—an image etched into my mind after years of dealing with a small but complicated plant in Illinois.
Before the plant was built, the raw water, with 0.4 mg/L of manganese, was only chlorinated and fluoridated. Our city decided to build a plant to oxidize and remove the manganese in order to meet secondary drinking water regulations. I don't think at the time anyone involved realized the impact of that decision—the launching of a challenging and often frustrating experiment in water chemistry that is now in its 12th year.
I had just joined the city in 1993 as the plant went online and the operators discovered that the ozone treatment was not achieving complete removal of the manganese. In conjunction with the plant construction, the consultant had drilled a new well, developed the well, and installed the pumps and equipment—and never thought to test the water. It was only a few hundred feet from the old wells—how could the water be expected to be any different? This well had 3 mg/L of iron, while all other wells had negligible iron concentrations. The water quality from this well was never accounted for in the design, and all the ozone was used up in oxidation of the iron. To obtain complete removal of both the iron and manganese, the operators ended up having to dose chlorine ahead of the ozonators.
The next blow came a few months later when an employee at work complained that a roast she cooked for four hours never turned brown. Eventually the tavern owners were calling to complain their fried turtle wasn't cooking. After some difficulty, we finally managed to identify the problem—nitrates in the water. Based on advice from a chemist we consulted, we dosed the system with chlorine, flushed all the hydrants, and solved the “red meat” dilemma.
Then we moved on to address lead and copper problems that arose due to the new treatment system. In order to achieve compliance with the EPA's lead and copper rule, our chemist suggested we increase the pH. We knew this was risky due to the levels of calcium carbonate in our raw water, but the polyphosphate we were already adding only addressed the lead levels. Copper still remained high, and adding sodium hydroxide seemed like the best answer.
As we had expected, the operators recently learned that the calcium carbonate is indeed dropping out—the pipe elbow and first few feet of piping downstream of the injection point had accumulated enough deposition over the past few years to close the area of a 16-inch-diameter pipe to a 4-inch opening. The crew spent a day clearing the pipe and changing the elbow to a tee to allow for future maintenance. At least this time, the problem had been anticipated.
We continued to consult with a chemist to help address other minor issues over the years. The entire experience has convinced me that every water treatment plant design should be signed off by not only an engineer, but also a chemist. In our case, we learned to never consider a change in treatment without consulting a chemist in addition to the engineer and operator. Large cities may be fortunate to have a chemist on staff at their plant, but smaller cities would be better served if the EPA would require a chemist's signature in order to approve a treatment design.