North Dakota plant prepares for population growth & new regulations
Dickinson, N.D., has become one of the fastest-growing cities in the U.S. since the state’s oil boom. This population growth has changed the city’s dynamics and created challenges for the existing wastewater treatment facility (WWTF).
Wastewater discharged by domestic and industrial users in Dickinson was collected and conveyed to the 50-year-old WWTF, which was designed to treat an average daily flow of 2.23 million gal per day (mgd) through the city’s sanitary collection system. At the facility, wastewater was treated by a series of processes before discharging to the Heart River under to the city’s NPDES permit.
In 2009, the WWTF was nearing its designed hydraulic and organic loading capacities. The city retained Apex Eng. to recommend necessary improvements to the facility to meet future needs.
Dickinson had experienced a boom-and-bust cycle in the 1970s and 1980s, so city officials were hesitant to overbuild city infrastructure. By the time the facility plan was completed in 2010, it was evident the oil industry was not leaving the state any time soon.
In the 2010 plan, projections planned for a population of 26,180, which at the time seemed overly optimistic. “The recommendations in the 2010 facility plan to meet the design population included an expansion of the city’s existing aerated ponds and construction of rapid infiltration basins for effluent disposal,” said Karla Olson, project manager for planning, design and construction engineering for Apex.
By early 2011, the population was exploding at a rate no one had predicted. “Current projections predict a population range of 35,000 to 38,000 in the next five to 10 years, which meant that the pond system outlined in the 2010 plan no longer was feasible for Dickinson’s rapid growth,” Olson said. At the same time, the North Dakota Department of Health (NDDH) was evaluating its statewide strategy for nutrient limits, and the city soon would have more stringent limits for discharges to the Heart River. Apex recommended a mechanical treatment facility to provide flexibility for expansion and reliable treatment to meet stringent effluent limits.
The new 3.65-mgd water reclamation facility (WRF) was constructed on a new site southeast of the existing four-cell-lagoon facility and can serve a population of 35,000. Because the common collection point is 2.5 miles from the new treatment site, a new pump station and force main were built to transport influent from the city to the new WRF. The buildings were constructed for future build out of the facility. Piping for future expansion was extended from the buildings and tanks. The next phase will consist of another treatment train for an additional 17,500 people, bringing the final service population to approximately 70,000.
Meeting Effluent Limits
NDDH determined that new effluent limits for a continuous discharge to the Heart River would be more restrictive than the existing limits. Target biological oxygen demand and total suspended solids effluent limits would be approximately 10 mg/L, and the effluent NH3-N concentration would be less than 3 mg/L. In addition, NDDH’s total nitrogen and phosphorus limits would be implemented in the near future, and provisions for removing these parameters had to be made during the design of this facility, so the design took into account a total nitrogen limit of 10 mg/L and a phosphorus limit of 1 mg/L.
The selected biological treatment process is an integrated fixed-film activated sludge system, which combines the benefits of an attached growth technology with an activated sludge suspended-growth reactor. In this system, a mixed liquor suspended solids concentration is maintained, in addition to the fixed-film growth on the moving bed biofilm reactor media, via a return activated sludge line from downstream clarifiers. It can be utilized to remove phosphorus and denitrify through the incorporation of anaerobic and anoxic zones as well as internal recycle streams, which will enable biological nutrient removal to meet the total nitrogen and phosphorus effluent limits.
“The facility was designed to treat the most stringent limits to date in the state of North Dakota,” Olson said. “It was designed for biological nutrient removal to meet a total nitrogen and phosphorus limit.”
Reuse & Recycle
The city will be recycling treated wastewater effluent for industrial, agricultural and in-plant uses.
A diesel topping facility, Dakota Prairie Refinery, was planned approximately 7 miles west of Dickinson. The new facility would have a positive impact on the local and state economy. Limited water sources exist for industrial use, but Dickinson was able to provide treated effluent for the refinery process water.
The oil industry also requires a substantial amount of water for the well fracturing process. Currently, potable water is being purchased from the city of Dickinson for this purpose; however, the process does not require potable water quality and therefore, an alternative source can be utilized. “The [plan] to conserve potable water and provide a reliable water source to the oil industry presents an exciting opportunity for the city of Dickinson and the state of North Dakota,” Olson said.
Another planned use for the treated effluent will be agricultural irrigation. The city of Dickinson will discharge treated effluent to one of its existing holding ponds for this purpose. Effluent also will be used for fire protection, providing an additional measure to conserve potable water.