Nearly 100,000 hybrid poplar trees on 180 acres near the Missoula Wastewater Treatment Plant are thriving on up to 20 percent of the effluent from the 12 mgd design/8 mgd average advanced treatment facility.

“That’s a million gallons a day that isn’t discharged into the Clark Fork River,” says Starr Sullivan, superintendent of the Wastewater Division for this Montana city of 69,000.

The trees were planted by hand in May 2014 as 10-inch whips, with only an inch or two poking out of the soil. By that September they were 7 feet to 10 feet tall. When they reach maturity by 2027, the trees will be harvested and sold for lumber, more than offsetting the lifecycle cost of their care.

Cutting down nutrients

The planting grew out of a Voluntary Nutrient Reduction Plan (VNRP) developed in 1998 by four key-point dischargers into the Clark Fork, headwater of pristine trout streams. The cities of Missoula, Butte and Deer Lodge, along with the Montana Department of Environmental Quality (MDEQ), the U.S. EPA and a now-defunct paper mill, set a goal to restore the beneficial use of effluent and eliminate nuisance algae growth in the river. The VNRP called for each discharger to take site-specific, voluntary measures.

The Missoula treatment plant team engaged several engineering consultants to study alternatives, including irrigation reuse, rapid infiltration and wetland treatment. Before trying those options, the team chose in-plant treatment in the form of a biological nutrient removal process, part of a major plant upgrade in 2004 (TPO, February 2014).

The plant also launched a pilot project to experiment with irrigation reuse, despite land application regulations that were more stringent at the time. The pilot had 330 trees of local varieties planted on 1.6 acres of facility property. The city hired a student from the University of Montana to plant and maintain the trees.

Going full-scale

The MDEQ designated the project in the discharge permit as a second outfall. That meant effluent had to meet set limits instead of being applied to the trees at agronomic rates. The pilot proved successful, and in 2011 the state changed the land application rules, enabling the city to proceed with the field-scale project.

The city leased farmland next to the plant through Hybrid Energy Group, a consulting firm also contracted to manage the planting project. Irrigation components were installed, and the site was cleared and prepared for planting. 

Equipment included valves from Nelson Irrigation and an Allen-Bradley programmable logic controller (Rockwell Automation) for zone timing and flow monitoring. For manifolds and distribution laterals, nearly 15,000 feet of 3- to 10-inch PVC pipe was buried up to 36 inches deep, depending on the terrain. Some 750 1-inch flexible risers were installed to feed more than 500,000 feet of 3/4-inch drip tubing (John Deere Landscapes) with pressure-compensated emitters spaced on 30-inch centers.

A submersible pump (Franklin Electric) sends effluent nearly a mile from a discharge basin at the plant. Six sand filters (Fresno Valves & Castings) treat the flow before distribution for irrigation. Control is achieved by manually setting the pump; a magnetic flowmeter (Badger Meter) monitors the flow.    

Tall, tall trees

An 8-foot-tall wildlife fence surrounds poplar woods. “It’s not for security, but to keep the deer out,” says Sullivan. The trees will grow to about 70 feet tall at maturity and produce a 24-foot sawlog nearly 10 inches in diameter at the top. Most will be harvested for lumber and sold. Pruned limbs and other debris will be used as an additive by EKO Compost, a private firm next to the plant that receives biosolids on a conveyor belt.   

The long-term benefit of the poplar tree program can be measured by the reduction of phosphorus and nitrogen introduced into the river. “Even though our nutrient discharge is already low, if we were going to reduce it another 20 percent by mechanical means, it would cost five to 10 times as much,” Sullivan says. “It’s a great solution for small communities that can’t afford $10 million for a mechanical plant.”