MOUNT PLEASANT — Homeowners’ water and sewer bills here may go up 7.5 percent this summer, continuing a trend of rising rates.

On average, Mount Pleasant Waterworks has increased its rates by 5.4 percent annually during the past five fiscal years.

An aging water and sewer system — and less consumption because of heavy rains and conservation measures — are driving the increases, Mount Pleasant Waterworks Manager Clay Duffie said.

“This is happening across the country,” he said. “In a lot of the country, what we have in the ground is 50 years old. In some cases, it’s more than 75 years old.”

The life spans of all those pipes vary. Last year, Mount Pleasant Waterworks had to spend more than $1 million to replace part of a large pipe along Simmons Street that was only 25 years old.

Replacing pipe is more costly than installing new pipe because the work must take place around other roads and utility lines.

“It’s gotten to the point now where half the cost is the pipe and half the cost is the restoration cost, putting the neighborhood back the way it was,” Duffie said. The utility system’s cost would be about $350 million to build but would cost more than $2 billion to replace, he added.

Meanwhile, demand is expected to fall this year by 2 percent, largely because of heavy rains and other conservation steps, such as more low-flush toilets.

“To make that up, you have to raise the rates by 2 percent,” he added.

Here’s how it will impact households: Those homes using 5,000 gallons of water will see their monthly bill rise by about $4.07, but a $3 special assessment for relocating utility lines on U.S. Highway 17 will come off. That means the net increase would be $1.07.

Even with that increase — and those of previous years — Mount Pleasant Waterworks still would have the lowest rates in Charleston County and the second lowest rates in the tri-county area, Duffie said.

The Mount Pleasant Waterworks commissioners will hold a public hearing on the increase at 5:30 p.m. June 3, and they could approve it on June 24.

Earlier this month, the utility’s customers noticed a redesigned bill that more clearly laid out the penalties for paying late.

But Barbara Powell of Mount Pleasant said she felt the late charges, which begin at $5 and escalate to $80 within 43 days, are “totally excessive.”

While she has her water and sewer bill paid each month through an automatic bank draft — and has avoided any late fees, she said, “I was just outraged when I saw it because I know a lot of people are struggling in our community,” she said.

Duffie said the utility wants everyone to pay on time, and about 60 percent of its customers have set up some sort of automatic payment.

He said the bills were redesigned “so there are no surprises about when the bill is due and when the charges are added.”

Reach Robert Behre at 937-5771.