Without chemicals and without filtration, the 12 mgd design/8 mgd average wastewater treatment plant in Missoula, Mont., reliably reduces effluent phosphorus to 0.3 mg/L. A 2004 upgrade costing $18 million converted the existing conventional aeration tanks (1 million gallons total volume) to the Modified Johannesburg Process. Two new trains of bioreactors with a combined volume of 1.4 million gallons were added to the two existing trains, more than doubling the aeration tankage.

In the future, a major share of plant effluent will go to fertilize a tree plantation instead of being discharged to the Clark Fork River.

Out With Nutrients

An innovative process enables the high degree of phosphorus removal. The conventional-looking concrete aeration tanks have been divided into a plug-flow arrangement of seven baffled cells each. Return activated sludge (RAS) is mixed with primary effluent as it enters the first of seven inline biological treatment cells. In the first tank (pre-anoxic), bacteria remove the residual nitrate-nitrogen from the RAS so that the nitrate demand for BOD is satisfied.
Flow proceeds to an anaerobic tank where it is mixed with a flow rich in volatile fatty acids (VFA) from the modified anaerobic digester (fermenter). Here, bacteria take in VFA as an energy source. The third tank in the bioreactor is anoxic; flow from the ends of the aeration tanks is recycled to provide nitrate-nitrogen removal.

The fourth, fifth, sixth and seventh tanks are conventional plug-flow aeration tanks. Here, BOD is removed, ammonia-nitrogen is converted to nitrate-nitrogen, and bacteria use the VFAs that were fed into the anaerobic tank to reduce the soluble phosphorus concentration to levels as low as 0.01 mg/L.

Lab Monitoring

The Missoula Wastewater Division has an on-site laboratory that performs both process control and permit compliance testing. All analyses are done in-house except metals and toxic organics. The lab uses an auto-analyzer due to the large number of samples at low nutrient levels.

Gene Connell, treatment supervisor, compiles process data that staff members collect. Standard activated sludge process tests are performed daily. A complete phosphorus and nitrogen profile of each bioreactor cell is done weekly. The process data is compiled in a plant-developed Microsoft Access database. The results are plotted on graphs to display treatment efficiency. Process changes are made accordingly.

Because phosphorus is removed biologically, there is no need for the Missoula staff to add chemicals such as alum or ferric chloride. As a result, treatment is more sustainable. In addition, Missoula ratepayers save hundreds of dollars per day in chemical expenses. In the event that chemical addition becomes necessary, the plant has the ability to add ferric chloride to the splitter box before the secondary clarifiers. Starr Sullivan, wastewater division superintendent, observes, “Our staff members are definitely the ‘hope for the best, plan for the worst’ types.”

Careful oversight of the treatment process keeps effluent TSS at 3 mg/L, and the total effluent phosphorus averages 0.29 mg/L during the summer permit period. The facility also reduces nitrogen to an average 7 mg/L. To achieve similar phosphorus removal efficiency, most treatment facilities combine biological and chemical removal and use polishing filters. Missoula gets it done with neither chemicals nor filters.

Control Automation

The Missoula team continues to investigate other methods of nutrient removal in anticipation of lower regulatory discharge limits to the Clark Fork River. The team is working with the private company AlgEvolve by cooperating with on-site piloting of photoreactors that grow algae to remove phosphorus to very low levels while creating a usable end product and avoiding creation of undesirable chemical sludges. 

The facility’s SCADA system uses the Wonderware System Platform (Invensys) and Allen-Bradley (Rockwell Automation) programmable logic controllers (PLCs) on a fiber-optic network. All servers and switches have online redundancy. The system allows tight control with low staffing. The five plant operators work 10-hour, four-day shifts to provide seven-day-per-week coverage. They rotate on-call duties, and Specter’s Win-911 automated call-out system alerts them to problems after hours.

The latest major plant upgrade, completed in 2012, replaced the headworks building at a cost of $9 million. Three 100 hp submersible pumps (Sulzer Pumps Solutions) lift influent into the plant. In front of the pumps are 1/4-inch FlexRake bar screens (Duperon) that lift the screenings from the lower level channel to the main-floor washing and compacting equipment (Huber Technology). Grit is removed using PISTA Grit vortex and grit washing systems (Smith & Loveless). A system manufactured by AMBIO Biolfiltration keeps headworks odors from affecting the community.

After anaerobic digestion and dewatering with a centrifuge, biosolids are conveyed next door to a private composting facility. Since 1975, dewatered sludge has been processed using a system from EKO Compost (a company founded by a professor from the University of Montana in Missoula) that bags the composted biosolids for retail and wholesale customers. The compost operation uses community yard waste and wood waste as bulking agents.

Missoula uses UV disinfection (TrojanUV) in place of elemental chlorine. The operators have found UV much easier and safer to use, and the system eliminates one chemical influence on the river.

Watering Poplars

The city has piloted a beneficial reuse program, using a portion of plant effluent to irrigate a 1.6-acre hybrid poplar tree plot. Instead of discharging the small amounts of phosphorus, nitrogen and other constituents to the Clark Fork River, those constituents are taken up by trees.

Pilot project results are encouraging: Testing in and around the grove shows no significant negative effects on groundwater or soil, and the poplars are thriving. The trees have grown from 12-inch whips in 2009 to trees that now stand 13 feet tall. Encouraged by the success, city council members have authorized the lease, construction and long-term management of a 130-acre hybrid poplar tree farm near the facility. It will be planted in spring 2014.

This reuse of effluent complements Missoula’s biosolids practices. EKO Compost will use the tree pruning material for a compost bulking agent. The mature trees will be harvested and sold as saw logs. Planting the new forest and bringing it to maturity in 12 years will cost an estimated $1.3 million; the saw logs’ value is projected at $2 million.

Sullivan estimates that at maturity the 70,000 poplar trees will drink up one million gallons of effluent per day. “The expansion has the potential to remove nearly 20 percent of what our current discharge is to the river,” he says.

About The Author

Grant Weaver, P.E., an ABC Class IV wastewater operator, is president of The Water Planet Company, a wastewater treatment consultancy in New London, Conn. He can be reached at grantweaver@thewaterplanetcompany.com