Steel-lined wastewater treatment pond saves money

DSM Chemicals was about to open a facility to recycle caprolactum from used carpeting, when management realized a new water treatment tank volume was needed: the answer: a site constructed steel tank

DSM Chemicals North America, of Augusta, Georgia, was about to open a new facility to recycle caprolactum from used carpeting when management realized that their 10-year-old wastewater treatment plant would not be adequate to handle the added load.

Caprolactum is the primary raw material used to make nylon 6 for carpeting, clothing, linings and other goods.

DSM has been manufacturing caprolactum since 1964.

Through the years, management responded to the growing demands by adding new plants in 1974 and again in 1976.Then came plans for the new recycling facility.

When it became clear that the existing facility could not handle the additional wastewater generated by the recycling operation, management moved quickly to come up with a solution.

"We needed to increase our treatment capacity very fast because we didn't want to block the recycling operation,." said Reese Bridges, maintenance manager for Augusta Service Co, the DSM service organization.

One option was to erect another storage tank.

Not only was the cost a deterrent to that option, time was running out.

A second option was to reactivate a 1.5 acre on-site lagoon which had been formerly used by the company for wastewater treatment.

The good news: it was big enough to do the job.

Two hundred and sixty feet in diameter and 14 feet deep, the pond had a capacity of 10 million gallons.

Moreover, all the ancillary pumps, piping and processing equipment were already there.

The bad news: the walls and bottom, lined with one-inch thick gunnite over a polyethylene liner, had weakened --- which was why the environmentally conscious company, rather than risk a leak, had taken the lagoon out of service years earlier.

Now, however, management decided to take a harder look at the lagoon option.

Erecting a new tank would cost anywhere from USD1 to USD1.5 million, they estimated.

"We figured it would be much more cost-effective, saving perhaps as much as USD1 million, to use the existing impoundment rather than build a new tank above the ground," explained Bridges.

To help them make a decision, DSM sought the opinion of Fisher Tank Company, builder of aboveground steel storage tanks.

Years before, when the lagoon was taken out of service, it had been Fisher that built the replacement facility: two 3-million gallon wastewater treatment tanks.

Management further decided that, if the lagoon alternative were feasible, DSM would take the "fail-safe" route: they insisted that the walls and bottom of the earthen impoundment be covered with a leak-proof steel lining.

Contract engineer Ron Liljegren and Operations Manager Jack Whiting journeyed from Fisher Tank to the Augusta site to size up the project.

That same day, over lunch, Liljegren and Whiting worked out a budgetary price and presented it to DSM.

By the end of the week, Fisher Tank got the go-ahead to proceed on the USD500,000 project.

Briefly, DSM had considered the option of placing another plastic liner on top of the gunnite.

"Most impoundments put in a plastic poly liner, and may or may not have gunnite over that," explained Bridges.

"It's unusual to employ a steel liner".

But the plastic liner option was quickly rejected.

"The plastic would have cost about 75% as much as a steel liner," said Dan Skedsvold, DSM Chemicals senior environmental specialist, "but we felt more comfortable with steel because of its reliability".

Within days, Liljegren, who served as project supervisor, mobilised two 10-man crews under foreman Tom Limperous.

Working in two 10-hour shifts, Fisher Tank personnel began lowering the heavy 8-foot by 20-foot carbon steel plates into the excavation and positioning them against the embankment walls.

Since specs called for a liner that was 100% steel to guard against any possible leakage, Fisher Tank personnel employed only continuous welding along the seams and vacuum.

The underlying gunnite, with its poly liner, remained in place, functioning as a secondary containment system in compliance with environmental regulations.

In the unlikely event of a leak, the secondary system would drain the water through the bottom.

For back-up protection, the entire liner was coated with an anti-corrosion epoxy.

Bridges gave high marks to the FTC performance.

"They met all our expectations and," he added, "they validated our decision to use the old impoundment.

What's more", he said, "We haven't had any leaks at all and no liner problems".

With the steel-lined lagoon on-line, DSM almost doubled its treatment capacity.

"When we only used the two tanks, " Skedsvold pointed out, "we had six million gallons of aeration capacity.

Converting the impoundment increased this by 4.5-million gallons, bringing the total at the site to over 10 million gallons.

The new facility, like the other two tanks, handles about 850,000 gallons of water each day.

Wastewater has a residence time of only one day throughout the plant before we discharge it into the river, absolutely clean.".

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