Milwaukee will have to guarantee more than just a low price in its last-minute offer to sell Lake Michigan water to Waukesha and wrestle the deal away from Oak Creek, Waukesha officials said.

Waukesha and Oak Creek are continuing to negotiate a final contract for the lakeshore community to sell Waukesha no more than an average of 8.2 million gallons a day of lake water by midcentury. That is the maximum volume set by the eight Great Lakes states last year in approving Waukesha's request for a lake water supply.

The total cost of the project was estimated at $207 million in 2012. A revised financial plan to be completed later this year is expected to boost the final price to an estimated $300 million, Waukesha Water Utility General Manager Dan Duchniak said.

Waukesha's water rates would more than double in future years to pay for the lake water supply, officials said.

Embracing a Milwaukee offer could carry significant additional costs and it has not been determined if those will be fully offset by savings that would come with switching suppliers, Duchniak said.

One costly consideration is that Milwaukee adds a phosphate compound to its drinking water to reduce leaching of toxic lead from household pipes.

Oak Creek does not add phosphorus and Waukesha uses a different chemical, sodium silicate, for corrosion control in household pipes, Duchniak said.

Accepting Milwaukee's offer would require Waukesha to remove the phosphorus in compliance with strict federal standards before treated wastewater could be returned to Lake Michigan. Cost of the removal is not known at this time.

Even so, Waukesha is taking a close look at the Milwaukee offer, Mayor Shawn Reilly said.

"We've worked with Oak Creek for years, but my responsibility is to our ratepayers in Waukesha," he said.

Water utility staff is asking Milwaukee for more details to clarify that offer, and Reilly said he does not expect the analysis to be completed until early September. At that time, the utility will recommend either the city council sign a final deal with Oak Creek or accept the Milwaukee offer and begin negotiations with a new partner, Reilly said.

In looking at Milwaukee's offer, Waukesha at the outset would give up an estimated $60 million in savings that it would gain with an Oak Creek supply by building two sets of pipelines in a common right-of-way for much of the distance between those cities, said Duchniak.

One pipeline and a set of pumping stations would bring the water from Oak Creek to Waukesha. The other pipeline and set of pumping stations would carry fully treated wastewater from Waukesha to the Root River in Franklin.

No common corridor is possible with a Milwaukee water deal because the Great Lakes states' approval last year of a Lake Michigan supply requires the wastewater discharge to go to the Root River.

Although the lake water pipe between Milwaukee and Waukesha would be a shorter distance than the water pipe between Oak Creek and Waukesha, it has not been determined if those cost savings equal the savings from a common right-of-way, Duchniak said.

Milwaukee Water Works Superintendent Jennifer Gonda confirmed she is providing Waukesha with technical information on pumping capacity, costs of service and water chemistry.

"We have a reliable system that has redundant water treatment and excess capacity," Gonda said. "We can provide high quality water at a competitive price."

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On June 21 of last year, delegates for the governors of the eight Great Lakes states unanimously approved Waukesha's request for a Lake Michigan water supply with conditions.

One condition of the states' approval requires Waukesha to return 100% of the volume of water purchased from a supplier to the lake as fully treated wastewater. A pipeline and pumping stations to be built by Waukesha would discharge that flow to the Root River, a lake tributary at S. 60th St. in Franklin.

Related:Great Lakes governors make the right decision on Waukesha

Milwaukee's late offer will not delay completion of the lake water supply project, Waukesha officials said.

Construction of pipelines and pumping stations is expected to start in 2019 or 2020 and be completed by 2022. The city will be able to tap into its new supply in 2023. At that time, Waukesha will stop using its 10 groundwater wells, including seven deep wells that draw radium-contaminated water out of a sandstone aquifer.

Looming over the city's planning efforts like a dark shadow is a possible legal challenge to the states' approval of the lake water diversion to Waukesha under terms of a 2008 federal law known as the Great Lakes protection compact.

August 2 is the deadline for the Great Lakes and St. Lawrence Cities Initiative to decide on filing a lawsuit in federal court to block the diversion. The binational group representing mayors of Great Lakes cities in the United States and Canada opposes the project.