District residents and businesses will pay an average 13 percent more for monthly water and sewer service starting Oct. 1.

Under higher rates approved July 2 by DC Water’s board of directors, an average household of four to six people will be charged $96.53 a month, up about $11 from the current average bill.

That includes a new fixed “water system replacement fee” that will be used to repair and replace the aging drinking water system, including pipes that date back a century or more. DC Water says the median age of its pipes is 79 years, with some in the ground since before the Civil War.

The fee, which is based on meter size, will amount to $6.30 to $9.67 per month for most household meters, DC Water officials said. Customers with larger meters, such as high-rise office buildings and large hotels, will be charged $1,292 to $5,785 for the new monthly fee, a spokesman said.

DC Water is following an industry trend toward charging fixed fees to provide revenue dedicated to repairing and replacing aging infrastructure. Officials for the Washington Suburban Sanitary Commission (WSSC) instituted a similar fee July 1 for its customers in Montgomery and Prince George’s counties.

Utility officials say they can’t keep up with needed repairs to their aging systems by depending on usage-based charges because federally mandated low-flow toilets, shower heads and faucets aimed at water conservation have led to people using less water. Meanwhile, their costs to repair and replace aging water and sewer systems — some of it required by federal court settlements — have risen, along with their fixed operating costs such as electricity, chemicals and labor.

George S. Hawkins, DC Water’s general manager, said the utility is “very conscious” of the fact that its rates have doubled in the past six years, as the utility has stepped up maintenance on its aging system.

“The scale of the work has increased, so our rates have also increased,” Hawkins said Tuesday.

As part of the new billing plan, customers will pay less for the first 400 cubic feet of water, or about 3,000 gallons, used, both out of faucets and in toilets and showers. About 44 percent of residential customers use that amount or less, DC Water says. Those lower rates will encourage conservation and benefit people who live alone, such as seniors on fixed incomes, Hawkins said.

He said customers who get financial assistance under DC Water’s Customer Assistance Program (CAP) for lower-income residents won’t be charged the new infrastructure fee.