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One filter isn't enough so drainage work gets 16

By Jackie R. Broach
Coastal Observer

A drainage project at the intersection of the North Causeway and Highway 17 has been revised to better protect Pawleys Creek from pollution, according to the county’s capital projects manager.

The project replaced a drainage pipe under the highway and is putting concrete pipe in a ditch that runs along the North Causeway.

Plans originally called for a filter known as a Smart Sponge to be placed in a concrete vault near Pawleys Creek. The sponge is designed to trap pollutants and kill bacteria in stormwater.

New plans call for the use of multiple sponges that will be installed throughout the system, allowing water to be filtered more efficiently and more effectively, said Donald Corinna, the capital projects manager.

“The long and short of it is when we design these things, we design them for a certain peak event,” Corinna said. “With this, we were designing for a 25-year storm event.”

The manufacturer of the sponge determined that in a 25-year storm too much water would move too fast through the 48-inch pipe for the single filter to handle.

“We started looking at options and the engineer came up with a better system,” Corinna said.

Filters will be placed in a stainless steel basket at each of the 16 catch basins throughout the system.

“That way the water is immediately filtered and we just have to worry about the street flow into each catch basin,” he said. “We’re using the same amount of sponge material and filtering the same volume of water, just in 16 different places. We think it will probably work better this way.”

The town of Pawleys Island had raised concerns about the project increasing pollution in Pawleys Creek. Those were put to rest by the county’s decision to use the Smart Sponge filter.

Mayor Bill Otis said he had some initial concerns about why the project’s plans changed and spoke with Corinna about them this week.

“I want to be sure it’s not being done because somebody misfigured the quantity of water going through the system, and he tells me that’s not the case,” Otis said.

Based on his conversation with Corinna, he said he feels comfortable with the changes and will follow up on the matter.

“I think at this point what I’m going to ask for is that we get assurances from the county on this concern, because we’ve been involved with it and we want to make sure that we’re all on the same page,” he said.

A side benefit of the altered plan is that it’s expected to bring down the cost of the project as the county no longer has to build a concrete vault for the sponge material.

The county budgeted $89,300 for installation of the sponge and vault, but without the vault, the cost should now fall somewhere between $50,000 and $60,000, Corinna said.

This project is the first to be funded through the county stormwater fees that took effect last year. Total cost of the project is $618,394.

Corinna said he isn’t sure what effect the changes might have on the cost of operations and maintenance. The sponges have to be cleaned regularly to remain effective, much like the filters in heating and air conditioning systems, but a maintenance schedule hasn’t been set.

However, the sponges should be easier to reach and maintain in the catch basins than in the vault, he said.

While the savings are welcome, Corinna said, “that’s not why we did this. It was purely to enhance the efficiency of the system and the quality of the water.”

Pawleys Creek has been closed to shellfishing for decades because of high bacteria levels. Smart Sponges are supposed to kill about 97 percent of bacteria in water.

The sponges should be installed in early December. The project is due for completion in February or March, but is running ahead of schedule.

“The contractor is doing a fine job,” Corinna said. “He’s about 35 to 40 percent done, so we might finish early.”

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