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As published in the November 2009 issue of Pollution Engineering…

Midwest WWTP Goes Wet

By replacing its dry-polymer operation with liquid-polymer technology, a small Nebraska wastewater treatment plant maximizes its sludge-processing capabilities.

wastewater treatment

When you are the man in charge of a wastewater treatment plant that daily treats 1.2 million to 1.4 million gallons of water, services a city with 13,000 residents, and have received eight awards for excellence in operation for its efforts, you have a certain reputation to uphold.

The facility, which processes the sludge on Monday, Wednesday and Friday every week, now has a 55-gallon drum of liquid polymer that is pumped directly into the blending system on an
as-needed basis.

So, last fall when Jack Robbins was surveying operations at the 25-year-old wastewater treatment facility in Beatrice, Neb., where he was Superintendent of Water Pollution Control for the Board of Public Works, he did not really like what he saw in the sludge processing area.

Specifically, he saw a dry polymer make-down feed system that had been installed in 1981, one that required mixing dry polymer in one tank before it was transferred to another tank prior to mixing with the incoming wastewater. Polymer was integral to this operation because it caused the suspended solid waste particles to clump and precipitate as sludge, which was then filtered out of the wastewater for disposal. Through the years, this system, while state-of-the-art nearly 30 years ago, had become a drain on the Water Pollution Control department because of its inefficient operation, the rising cost of dry polymer, and safety concerns associated with handling the polymer.

Robbins knew there had to be a better way to process the solid wastes, both for the treatment plant and its employees, as well as the residents of Beatrice, which is located on the Big Blue River in the state’s southeastern corner, some 40 miles south of Lincoln and 100 miles south of Omaha. Working with Tony Bilek, a Partner at Mc2 Inc., an Omaha-based manufacturer’s representative for water and wastewater equipment, Robbins began the search for a more efficient method of polymer handling.

“They had two big aging tanks, two big mixers, a day tank, two large metering pumps – it was just a big eyesore, it took up a lot of space and it was kind of dirty and cumbersome to work with,” said Bilek. “Jack asked what was out there, something more simple.”

The answer turned out to be as simple as the difference between hot to cold, young to old, or in this case, dry to wet.

The liquid polymer mixing system takes up much less floor space and reduced the polymer usage significantly. This not only reduced overall cost for the plant but improved safety for the worker’s environment.

Getting wet

The facility’s daily throughput of 1.4 million gallons of water was considered small in the wastewater treatment business, so the plant was not a huge user of dry polymer. So Bilek suggested a switch to a liquid system.

“When I was looking over the [liquid system], I saw that it would eliminate the polymer mixing and we also wouldn’t need the tank for the sludge mixing either,” said Robbins. “We also wouldn’t have to make the polymer anymore. When you were mixing polymer, the heat and humidity would cause problems and occasionally you’d get spillage, which is a safety hazard.”

Convinced he had the answer, Robbins put the wheels in motion for a wet polymer system from Fluid Dynamics, a division of Neptune Chemical Pump Co., Lansdale, Pa. The system was installed in March, with nary a misstep or hang-up since day one.

“The start-up went great,” said Bilek. “We primed the pump, got the water going and had it optimized in an hour. It’s a pretty simple system, there’s not a lot of sophistication to it, and that’s the way Jack wanted it.”

A liquid polymer mix system such as the dynaBLEND mixer shown above saved the Beatrice WWTP money and improved worker safety.

Compared to the old dry-polymer system, which required a 30-foot by 30-foot square of floor space, the wet system measured 2 feet by 2 feet (and 5-foot eight inches tall), which eliminated much of the clutter in the sludge-processing area and made it safer for the plant’s employees.

“This is a much smaller footprint,” said Robbins. “The polymer tank we had before was six feet in diameter and four feet high and the mixing tank was six feet high and three feet in diameter – and now they’re all gone. We also had a storage bin for the dry polymer.”

Instead, the facility, which processes the sludge on Monday, Wednesday and Friday every week, now has a 55-gallon drum of liquid polymer that is pumped directly into the blending system on an as-needed basis. Robbins also reported an economical benefit, since the facility is using less polymer.

“We’re using pretty close to half of the polymer that we used before,” he said. “We’ve gone through one 55-gallon drum in two months. We’d probably go through 400 to 500 pounds of dry polymer in the same time.”

Efficiency is key

A facility operator at a wastewater treatment plant that has been in operation for more than 25 years should look for ways to make its operations more efficient, both in terms of service and cost for equipment, utilities, chemicals and man hours. When taking stock of the old dry-polymer system that had been in use in Beatrice, Robbins knew there had to be a better way.

“There’s less cleanup time, there’s no more time needed to make the polymer, it’s just a matter of pumping it into the system and away you go. It’s saved us not only the cost of the dry polymer, but also in man-hours because we don’t have to clean and maintain it like we had to with the old one,” said Robbins.

Inside the Wet Polymer System

liquid polymer The key to the company’s high-energy, non-mechanical liquid-polymer activation and blending technology, is its corporate-named HydroAction Technology, which produces in excess of six times the mixing energy per unit volume than a comparable-sized mixer. There are three stages in the mixer’s operation:

Stage 1: A pressure drop across a variable-orifice water-control valve produces a high-velocity water jet. This water jet, which travels at approximately 70 feet per second, is aimed directly at, and impinges on, the polymer as it enters the mixing chamber. At this point, the only point where high energy exists in the mixing chamber, the polymer is coiled up and not susceptible to damage.

Stage 2: In the system’s concentric mixing chambers, the newly blended polymer re-circulates multiple times for additional exposure to non-damaging turbulence, completing the blending process. This recirculation ensures that polymer solution is present directly after the point of neat, concentrated polymer injection for an ideal activation and blending environment.

Stage 3: The mixing energy then naturally diminishes in the system’s concentric chambers, while the flow path through the concentric chambers prevents polymer from short-circuiting the process.

This three-stage mode of operation induces high mixing energy without the use of mechanical impellers, which could cause polymer damage and gelling. Preventing polymer gelling and damage maximizes polymer investment by reducing polymer use.