LIHU'E, Kaua'i — A new $8 million vacuum-based purification plant by Grove Farm Co. that opens next month will turn plantation ditch water into drinking water.

It's a unique solution to Kaua'i's water problems, and suitable because while most of the sugar plantations are gone, their irrigation systems in many areas survive.

"These islands are plumbed for sugar irrigation," said Kaua'i Water Department manager Ed Tschupp.

The new filtration system is needed, in part, because plantations no longer irrigate the fields over Lihu'e's aquifer. That water once seeped down to recharge groundwater supplies. In addition, engineers are finding that wells that once produced lots of water are producing less.

Grove Farm will sell water to the county to meet the needs of future urban development by the company and its sister firm, Lihu'e Land Co., both of which are owned by former AOL Time Warner chief Steve Case.

"For the past 10 years or more, Lihu'e has been under a restrictive metering policy that has stifled residential development," Tschupp said. Any new developer of more than a few lots was required to come up with the water to support the project, and for most small developers, that was impossible.

The Case companies control most of the land suitable for development in the region, and after several well-drilling ventures yielded disappointing results, they decided to put to use the old plantation irrigation system, fed by surface water. Ultimately, the new filtration plant's water is expected to be roughly competitive in cost with pumped water from wells, and in Lihu'e, where wells don't produce much, it may be cheaper than pumping, Tschupp said.

The plant is designed to produce up to 3 million gallons of drinking water per day, Tschupp said. A Grove Farm official said it is also designed to be expandable if demand increases.

The county is committed to buying 2 million gallons daily, which should cover roughly half the daily demand for the area from Puhi north to the Wailua Bridge. That region is now supplied by 2 million to 3 million gallons daily from a dozen or so Lihu'e-area wells, plus close to 1 million gallons a day from the overtaxed Kapa'a-Wailua water system.

Grove Farm settled on a filtration technology from Zenon Environmental that uses a patented vacuum-based system for creating drinkable water from ditch water. The key to the system is a membrane that allows only the smallest molecules to pass through.

The Maui County Department of Water Supply employs a related system at four of its plants, where pressure is used to push water through the membrane, which catches contaminants.

"Where surface water is available, it's a good technology to use and it can be cheaper than pumping," said Maui water director George Tengan.

The Zenon system being used on Kaua'i has thousands of hollow, spaghetti-like tubes called ZeeWeed that are placed in a vat of untreated water. The ZeeWeed tube walls have millions of holes so small that bacteria or parasites can't get through, only the smallest viruses. A vacuum system sucks the filtered water through the tubes.

Chlorination kills any remaining viruses, and would be used in any case to deal with possible contamination inside water pipes, Tschupp said.

Consumers probably won't notice too much of a change, but a couple of things may be different, he said.

The smell of chlorine may be more prevalent. The chemical will evaporate out if the water is left standing, for example, in a pitcher in the refrigerator, Tschupp said.

Surface water has fewer dissolved minerals than pumped groundwater, and this "softer" water will require less soap for washing.

The soft water may eat away at some of the minerals that have been deposited on the insides of water pipes, so there may be some sediment coming out of faucets for a while, but that's not a health issue.

"The water is safe to drink," he said.

Old pipes that are badly rusted could fail with the loss of the mineral linings.

"We may be replacing some piping," Tschupp said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.