City officials soon will take a fresh look at the water and sewer fees paid by Lawrence residents — with an eye toward financing a $64 million wastewater treatment plant on the Wakarusa River by the end of the decade.

Officials don’t yet know how much rates will increase in coming years. But they said higher rates would be necessary to maintain the city’s existing utility system and expand it to be ready for 20,000 expected new residents south of the river by 2025.

“In order to be prepared for the growth that’s projected … it’s going to require some cost increase for the customer,” Assistant City Manager Debbie Van Saun said Thursday.

The last utility rate study was performed in 1999 and suggested water and sewer rate increases for every year until 2004. City Hall stuck to the fee schedule laid out in the report; the average family’s monthly wastewater bill, for example, will increase from $28.16 this year to $29.85 in 2004.

“They’ve been followed fairly closely over time,” said Roger Coffey, the city utilities director.

The exception to the rule: the Lawrence City Commission imposed “system development charges” lower than the report recommended. Those charges are paid by builders to expand water and sewer service into new areas of the city.

The 1999 report recommended that developers be charged $445 for a new 5/8-inch water main by 2004; commissioners set the fee at $420 instead. And the report suggested a $735 charge for new sewer meters by 2004; that fee actually will be $520.

The new report will guide water and wastewater fees from 2005 through 2009. The Lawrence City Commission is expected to soon give approval to the Wakarusa River plant, with construction slated for 2007.

“We have the majority of this (plant cost) coming during the time that’s covered by the rate study,” Van Saun said.

“We hope, of course, that we don’t have huge increases,” Coffey said. “Our desire is to have water and wastewater fees that people can afford.”

Van Saun said the study would be performed by an outside consultant, with work beginning by the end of this year. The report, she said, should be complete by the middle of next year.