Protecting Oil Boom City from Wastewater Grit Damage - Bismarck, North Dakota
Rapid population growth pressures have seen extensive investment in public utility services in the City of Bismarck, North Dakota’s state capital, including expansion and updating its waste water treatment plant. Essential for protecting this investment is the installation of Hydro International’s unique HeadCell® advanced grit removal system in the headworks, to ensure that long term maintenance and operational costs are minimized.
Located on the mighty Missouri river, Bismarck and North Dakota are both experiencing and anticipating a major population expansion due to a 21st century oil boom and associated industrial and domestic growth. North Dakota has had the USA’s biggest population growth since the 2010 census, estimated at 9.95% in four years, while still remaining 47th in terms of actual population numbers.
“The State has had to start replanning all its services to allow for this,” explains Bob French, Bismarck WWTP Supervisor. “For waste water treatment, a report was commissioned from Short Elliott Hendrickson to look at projected needs over the next 15 or 20 years, and what we would require in terms of plant upgrades. With Bismarck’s population of 60,000 plus and growing, the previous plant was being challenged hydraulically to meet the required flows and loadings. It also needed to be upgraded to modern treatment standards.”
Study identifies grit solution
“We were commissioned by the City of Bismarck to start a city-wide master Waste Water Treatment Planning Study in 2003,” states John E. Fisher, Principal and Senior Operations Planning Specialist for US-wide consulting engineers Short Elliott Hendrickson. “The scope was to look at all of the City’s systems and processes in line with projected future growth and future environmental regulations.
“It became obvious that an immediate priority needed to be given to improving the pre-treatment of waste waters, focussing on screening, grit removal and pumping. The City was aware of significant damage and disruption occurring through grit damage, particularly with pumping and downstream digester equipment and processes. The source of the problem was due to the fine soils and wind-blown sands found in the region of the Great Plains.
“In 2004 we recommended and instituted a pilot study which we conducted with the HeadCell unit and with the team from Hydro International; we didn’t specifically look at costs and savings because we had had previous experience with the effectiveness of the technology and its low maintenance requirements. Instead we focused on performance vs. benefit, and concluded that as the pre-treatment package with HeadCell removed 87% of all grit, with the smallest limit set down to 90 microns (0.0035 inch), the potential of refining the removal down to 50 microns or so to achieve closer to 100% grit removal, did not add significantly to benefit.
Bob French continues: “The report recommended that fine grit separation at the pre-treatment stage is an essential element in the new plant, and Hydro International’s HeadCell separation units, with SlurryCup® and GritSnail®, provided us with the solution.
“As a result of the $19 million investment by the City of Bismarck, construction of the new pre-treatment building started in 2006 as part of a general expansion and upgrade of the WWTP. Now in 2015 the headworks have been totally upgraded from when it was built in 1982.”
Plain problems overcome
Features of the City’s location on the Great Plains of North America which affect the design and operation of the WWTP are the flat topography and the variation in seasons between long dry spells and short, intense bursts of rain.
As a result of the terrain, pipeline runs are long and often require pumping stations. The combination of the flat runs and dwell points with the dry spells is accumulation of grit in the pipes, which are shifted in bursts either by jet pumping or by heavy rain fall and first flush events. Hydrogen sulfide build-up in the long runs of pipe are accommodated by the design of the treatment buildings where the gas is removed and neutralized as part of the headworks process.
Damage avoidance by design
The WWTP, particularly the pre-treatment plant at the headworks, had to be designed to accommodate the peaks of accumulated grit loading. It was critical to prevent damage to downstream treatment equipment and processes such as pumps and valves, reduce accumulation in basins and pipes and disruption to aeration and digesters from fine grit.
The previous plant had been designed for a maximum flow of 12 million gallons a day (MGD), with a 7.5 MGD average. This has now been doubled to 24 MGD as the peak design flow, allowing a certain amount of future proofing both in terms of population growth and of heavier rain events due to climate change.
Guaranteed grit removal
In the pre-treatment building, fine mesh screens (0.1 inch or 2500 microns) are backed up by twin HeadCell stacked tray grit separation units which provide separation down to 0.0035 inch (90 microns) particle sizes. As part of supplying the grit prevention and removal package, Hydro International’s Grit Management team conducted a study to characterize influent grit and provide a profile of particle size distribution and settling velocity. The study results enabled the HeadCell units to be designed to perform to a guaranteed 95% removal down to the 90 microns size.
Two 12 MGD HeadCell units are run in parallel during wet weather, but only one unit is required to be in operation during average flows. The complete grit removal system consists of SlurryCup grit washing units, grit pumps and GritSnail dewatering units. The dewatered grit is of sufficient quality for landfill, and the degritted water is returned to the treatment plant.
Performance improvement maintained
French adds: “Performance has been excellent so far, especially as it is running well within maximum design parameters; and reliability is also good, so maintenance requirements are low. Abrasion wear of downstream equipment has been reduced and the grit removal systems are offering good protection to other plant processes, including the clarifier, digestion, and boiler equipment, and projected trickling filter beds are at lower risk of expensive maintenance costs over their operational lifetime. The Hydro International equipment does what they said it would do.
“The plant design lends itself to redundancy with the parallel units, as we can keep the grit system in operation at all times by switching between individual trains of equipment during average flow. In contrast, the previous grit pre-treatment equipment which utilized a mechanically-induced vortex design was not sufficiently protecting the plant, nor was it keeping up with the population expansion.”
John Fisher concludes: “We regularly visit Bismarck WWTP as we have ongoing projects there, and am glad to report that the immediate significant improvement in grit removal through the pre-treatment plant at the headworks has been maintained.”