Trees, trees and more trees proved to be a natural solution for meeting tightened effluent ammonia limits at the City of Woodburn (Ore.) Wastewater Treatment Plant.

Poplar trees — 41,000 of them — were planted in 1999 on 84 acres next to the 3.3 mgd (design) activated sludge plant. Flow averages 2.0 mgd from May through October, and during those months the poplars provide tertiary treatment with beneficial reuse for 1.0 mgd.

“It works very well,” says Curtis Stultz, plant supervisor, who was part of a brainstorming session with city staff and consultant CH2M HILL that produced the idea. “We were just trying to find an alternative to adding tankage and other equipment to meet the stricter standards.”

Proof of concept

The success of an earlier 7-acre pilot project to prove the concept led to the 84-acre commitment. Three varieties of fast-growing poplars were planted as 8-foot-tall whips or 8-inch cuttings. “The varieties were chosen so that if we got hit with a tree disease or a fungus, we wouldn’t lose the whole plantation,” says Stultz.

The trees were planted on an engineered grid designed to provide full visibility from end to end in each row. The plantation abuts the MacLaren Youth Correctional Facility, and the grid layout satisfied concerns about an escapee running through the poplar forest.

Treatment plant effluent is discharged to the Pudding River or diverted through a 12-inch pipe to three distribution manifolds at the plantation. From there, water is distributed to different sections of the plantation through 4-inch pipe. Each day, Craig Prosser, operator III, checks laboratory effluent samples before deciding which of the 10 hp and 50 hp pumps he will use to control the irrigation rate. Rotating-head sprinklers micro-spray the poplars at 0.5 gpm during summer.

Regular harvesting

The original plan was for the trees to be a moneymaker. A regular harvest and replacement program was laid out, but Stultz isn’t certain it is paying off: “I just haven’t taken the time to calculate how we are doing profit-wise.”

Recently, 12,500 trees harvested from 24 acres produced 1,700 tons of wood chips, which were shipped to a Georgia-Pacific mill for use in making corrugated boxes. The harvested acreage was restored, and new plantings are scheduled for this year to complete a seven- to 12-year cycle. Liquid and cake biosolids are applied to the poplars at standard agronomic rates. Unused biosolids are applied to 1,000 acres on a nearby farm.

A study to expand the Woodburn treatment plant is underway between the Oregon State University and CH2M HILL. The study aims to document that present application rates are under-watering the trees and that therefore no breakthrough to groundwater is occurring. The results could allow the city to reduce the acreage required for poplars or wetland development by allowing higher loading rates.

Reaching out

Stultz says that while regulatory and space limitations dictate closing the tree plantation to the public, the plant team is involved in community outreach. For example, Larry Arendt, pretreatment coordinator, makes classroom presentations to fourth- and fifth-grade classes about the water cycle, watersheds, surface water pollution, water conservation and wastewater treatment. His presentation is based on the River Ranger Project, an interactive program promoted by Clean Water Services, an Oregon water resource management utility.  

Each year the plant supports the local Oregon Earth Day celebration, either through sponsorship or by staffing a booth. Plant tours are offered for local students and occasionally they have hosted dignitaries from around the world, including the agricultural minister of China. A representative from Russia was interested in using the cottony fluff produced by the poplars. “You never know who is going to call you on the phone to inquire,” says Stultz.

He adds, “Ours was one of the first treatment plants to use poplars for treatment. I think what we are doing is rather good because we’re able to decrease the amount of ammonia discharged into our receiving stream, while using the water for something that is good. We didn’t have to put as many tanks in the ground or spend as much on chemical additions to treat the wastewater. We are letting nature take its course. At the same time, we improved the landscape and aesthetics of the plant.”