Troubleshooting a Tertiary Clarifier Problem

Find out how the staff at La Plata Wastewater Treatment Plant solved a tertiary clarifier problem during an enhanced nutrient removal upgrade

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Editor's Note: This article is part of a profile on the La Plata (Maryland) Wastewater Treatment Plant, which will be featured in the February 2016 issue of Treatment Plant Operator. Make sure you don't miss out by subscribing to the print version of TPO. We'll send a copy right to your doorstep.

At the height of a 2011 enhanced nutrient removal upgrade at the La Plata (Maryland) Wastewater Treatment Plant, things went terribly wrong in the tertiary clarifier. Operators had turned on the rapid-mix tank, then had started adding alum and polymer to bind phosphorus to the aluminum. Rather than phosphorus precipitating from the solution, its numbers rose.

Adding more alum increased the sludge blanket in the clarifier. Instead of floating 15 feet above the bottom of the tank, solids settled, then were agitated by influent entering from the bottom. Suspended solids flowed over the weirs to the denitrification sand filters (Parkson Corp.).

“We couldn’t get the headloss down in the filters,” says Robert Stahl, director of operations. “Whenever we had high flows, we almost had to open the bypass because the headloss shot so high.”

Operators believed adding alum to the tertiary clarifier was the problem. Before the upgrade, pumps had injected it into each bioreactor’s clarifier. At the Maryland Rural Water Association conference, vendors confirmed the operators’ suspicions. Resuming the original feed method cleared up the tertiary clarifier.

“We had to issue a change order to reinstate the alum lines to the reactors,” says Stahl. “Suddenly, phosphorus went from way over to way under and hasn’t been a problem since.”

Meanwhile, a Parkson representative diagnosed the new sand filters as totally clogged. The only solution was to remove and clean the sand, a two-week process.

Operators used air lifts to pump the sand from Filter 1 down a trough and into a dump truck. When the dump box was full, they tilted it to dewater the sand. Water drained down a trough designed by the operators and was then pumped to the influent wet well. After the crew stockpiled the sand in a retaining area, they power-washed and vacuumed the basin.

“We built two screens, then pumped the sand from Filter 2 through the screens and into Filter 1,” says Stahl. “After putting Filter 1 back into operation, we pumped and screened the sand from Filter 3 into Filter 2. Then we used a conveyor to move the sand from the retaining area to Filter 3. When we finished, you could hear a collective sigh of relief.”


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