Wastewater plant costs increasing

August 16 , 2007

The Cathlamet Town Council got an eye-opening recommendation for a new wastewater treatment plant Tuesday.

Engineers from Gray and Osborne, Inc., offered recommendations for the style of plant and the location, and they gave a sobering estimate for the costs.

When first proposed in 2003, a new sewage treatment plant was expected to cost $5-8 million; new estimates range from $10.5 million to $16 million, depending on the location and type of plant.

In the current construction market, the volatile cost of materials is pushing the cost up $500,000 to $700,000 per year, said Jay Swift of Gray and Osborne.

Also pushing up the costs are revised calculations of how much water infiltrates the system through cracked pipes and storm drains in wet winter weather. Observations made last winter show that the amount of water going through the system is three times higher that previously expected. The system has to be designed to handle the peak flow.

The engineers evaluated two types of plants on three possible sites.

They compared oxidation ditch systems with membrane bioreactor systems. The oxidation ditch circulates effluent in an oval channel with aerators and submerged brushes.

The membrane bioreactor system sends effluent into enclosed tanks; inside the tanks are membrane boxes; water is drawn out of the boxes, which filter solids for further treatment.

The engineers considered three sites--the town's current site at the mouth of Birnie Creek; a site on land owned by the Wahkiakum School District, and a site on log yard land owned by Hancock Forest Industries.

In all cases, membrane system plants are more expensive, the engineers said.

Among the sites, the school district site would be the least expensive for building, but the cost of sewer mains to convey wastewater to the plant and back to the discharge would make it higher overall than building on the existing site. Because of wet soil, piling would have to be driven at the existing site to support a new facility. The distance from town drives up the cost of the log yard site, the engineers said, and piling might be required there to support facilities,

Estimates for the oxidation ditch plants are, school site, $10.77 million in 2007 dollars; existing site, $10.53 million, and log yard site, $12.5 million.

Estimates for the membrane system are, school site, $14.2 million; existing site, $16.1 million, and log yard, $15.9 million.

"These are very conservative figures," engineer John Wilson said. "We've included a 25 percent contingency."

Overall, the engineers recommended using an oxidation ditch system sited on school district land. The site has lower construction costs; it is remote and removed from the town center, and it frees the existing site for some other use. Membrane plants also have a higher annual operating cost, they said.

The group discussed the possibility of replacing lines connecting mains to houses to reduce the infiltrating drainage water. The town has already taken several steps to evaluate and reduce infiltration, but it still occurs in stormy periods.

"Peak flow is important but not a huge percent of the cost of the project," commented engineer John Wilson.

Al Bollinger of the state Department of Ecology suggested the plant be planned to handle expected flows and projected growth; the town could go after infiltration over a long period of time.

The officials and engineers want to apply this fall for state funding to design the project.

Mayor Dick Swart said he would set up a meeting with funding agencies to present the project to them in order to gauge how much funding would be available for it.

The council is negotiating with the Wahkiakum School District board of directors over a purchase price for the school site; the engineers said they would draft more layouts for the site so that the council negotiators would know exactly how much land they needed.