Allentown's drinking water and sewer service provider is moving ahead with a billing change that will increase most residents' bills by about $13 a month.

The Lehigh County Authority Board of Directors on Monday unanimously approved billing its customers in the city monthly, rather than quarterly.

The change is due to take effect "as soon as possible," according to a statement from the authority.

Authority officials say monthly billing, resulting in the additional cost, is possible under terms of their lease signed in 2013 to operate the city's water and sewer system.

"This action is necessary to increase revenues as required to cover high debt service costs associated with the lease, in which LCA paid a sum of $220 million to the city to address its growing pension liability at the time the agreement was finalized," the authority stated. "LCA delayed implementation of the monthly rates for a period of five years in order to gain necessary operating experience with the system and to understand long-term needs of the aging water and sewer system."

About 10 percent of city customers already pay their bills monthly, according to the authority. Officials there are looking into expanding monthly bills to their suburban Allentown customers, as well.

The increased cost of $13.39 per month is a fixed charge, meaning customers can't conserve water to try to avoid paying it.

For a residential customer using 6,000 gallons of water per quarter, the new $38.38 monthly bill would be 54 percent more for three months of bills than the current quarterly bill of $74.61.

Allentown Mayor Ray O'Connell at a public meeting Thursday on the change called it "unconscionable and a clear violation of the lease."

"The city is prepared to take any and all legal remedies to block the implementation of this rate increase and protect its residents," O'Connell said in a statement provided by his office.

The authority says it remains open to discussions with the city on amending the lease to "lessen the burden on fixed-income households."

However, "this seems unlikely to occur in the near term, given the city's recent public statements threatening legal action against the LCA," the authority states.

"We want customers to know this was not a decision the board of directors entered into lightly," authority Executive Director Liesel Gross says in the statement. "We appreciate everyone who took the time to share their input with us over the past few weeks, and we will continue to reflect on the input provided as we make future decisions. Additionally, we will continue to be open to negotiating other possibilities for rate-making with the City of Allentown."

In a related development Monday, the authority board authorized its staff to file for arbitration in a dispute with the city over "the accuracy of data provided by the city" ahead of the signing of the 50-year lease. That information concerns operating costs and revenue capacity on the wastewater side of the operation, according to the authority.

Kurt Bresswein may be reached at kbresswein@lehighvalleylive.com. Follow him on Twitter @KurtBresswein and Facebook. Find lehighvalleylive.com on Facebook.