Low maintenance and built-in redundancy - Grand Island, Nebraska
As a critical part of a $21.5 million upgrade to its wastewater treatment facilities, the City of Grand Island, Nebraska, has invested in an advanced grit removal system for high-performance grit removal from the wastewater treatment plant (WWTP) headworks.
Designing for built-in redundancy and achieving grit particle removal performance down to 90 microns were important factors in designing the optimum solution for the plant operators, replacing aging and inefficient equipment. An Advanced Grit Management™ solution from Hydro International was selected and proved its capability with challenging first-flush conditions experienced within days of being commissioned.
“The existing wastewater infrastructure at Grand Island was around 50 years old,” said Dr. Jue Zhao, PE, WWTP Operations Engineer, City of Grand Island. “The City had many issues with treatment plant and equipment, and we were faced with a growing population and rising flow volumes. As a consequence, the City decided to invest $21.5 million, including construction and consulting service, over a five year WWTP refurbishment project built to meet our future needs.”
A key section of the WWTP headworks, the pre-treatment grit removal facilities were undersized and performing poorly. During peak flow events, grit was being flushed through the aerated grit chambers into the adjacent primary clarifiers and depositing there. As part of the overall wastewater treatment utilities upgrade and based on discussions with the City, engineer Black & Veatch selected Hydro International’s HeadCell® grit removal technology, with associated Hydro International Grit Snail® and SlurryCup™ grit washing and collection equipment, to remove grit down to 90 microns ensuring reliable and effective protection to downstream treatment equipment and processes.
“We were first approached in 2012 to conduct a study to consider the wastewater handling and treatment needs of the City well into the 21st century, taking account of matching population growth and regulation changes,” commented Nathan White, PE, Black & Veatch Engineering Manager. “This resulted in recommendations for upgrading the major sanitary interceptor sewers, as well as the main treatment plant.”
Grit removal technology comparison
“Grit removal at the headworks was one of the priority areas," said White. "A major concern in any treatment plant is to provide continuous treatment through redundancy and the existing plant with its grit basins was inadequate; indeed much of the equipment such as valves and pumps were corroded and barely functioning, so a complete replacement was required.
“We evaluated two alternative types of grit removal to meet the new maximum 40 mgd (million gallons per day) flow capacity, a mechanical vortex based design and Hydro’s HeadCell / SlurryCup / Grit Snail treatment train. The specification for each of the two treatment technologies was to have a grit removal plant that removed 95% of grit down to 90 microns at the average design flow of 13 mgd per treatment train, and 95% grit down to 150 microns at max peak hourly flow capacity per treatment train of 30 mgd.
“Overall capital and installation costs were very similar. In consultation with the City’s Public Works engineers we made a visit to a nearby plant at Lincoln, Nebraska already using HeadCell treatment technology. The City engineers were impressed with the collected grit quality, and we went with the Hydro design because of its high grit removal performance combined with low mechanical equipment needs which offer much less maintenance costs in the future.”
Replanning tired design
Grand Island, the fourth largest city in Nebraska with over 50,000 inhabitants, sits on the Platte River, which flows east out of Colorado and Wyoming into the Missouri at the Iowa border. The water table is high, and pumping is necessary to maintain flow in pipelines and across the treatment plant. The plains topography comprises low, rolling hills and the soil is wind-blown and silty/sandy in composition, which inevitably ends up being washed by surface water drainage into the sewers and the treatment plant. As well as domestic and city sewage, the industrial wastewater includes a major meat processing plant, with all waters treated by the WWTP before discharge to the Platte through the Wood River.
The existing plant was sized for 35 mgd hydraulic capacity, but its age meant that its capacity was restricted to around 27 mgd. Although the original 1965 grit treatment building structure was in good condition, the equipment and facilities such as the Parshall flume for flow measurement were in poor condition and undersized, the bar screens required replacement and the wet well concrete at the base of the building was in need of repair. Additionally, the existing facilities had no provisions for odor control. Hydrogen sulfide gas accumulates in the long, level pipe runs characteristic of the topography and must be handled.
“As a result of all these factors, the project team decided to start with the new headworks facilities consisting of pre-treatment and a grit removal building,” said Nathan White. “There was sufficient building space to achieve the ideal configuration for a plant with parallel treatment trains, providing the redundancy capacity we desired. This made the planning, construction and installation of the equipment very straightforward.”
Black & Veatch worked in depth with the Hydro International team on the design, to optimize the layout with each treatment train as a true mirror image of the other. After the initial mechanical bar screening, the set up on each side comprises a 12 ft HeadCell unit with 10 separation trays and a dedicated pump. The treatment is completed by SlurryCup grit washing and classification and Grit Snail dewatering units, which dewater the grit for disposal at not less than 60% total solids and a maximum organics content of 15%. A dedicated control panel serves each and allows full automation and integration with the WWTP SCADA system for remote monitoring and control, including automatic start-up when incoming flows exceed set capacities.
“After grit treatment, the wastewater exits via the primary clarifiers and flow distribution structure to downstream processes including activated sludge process and digestion,” said Nathan White. “Achieving a linear series of pipe runs, especially in the inflow to the HeadCell units has allowed us to reduce turbulent flow, maximize flow rates and possible settlement of suspended particles, and thus minimize any maintenance requirements in the pipework and downstream equipment including abrasion damage.”
Coping with extreme grit loads
The built-in redundancy is designed to maximize the City’s return on the investment in the pre-treatment and grit removal facilities, which has an anticipated design life of 50 years. Predicted peak day flows of 25 mgd per treatment train are well within the plant design capacity with 20% of additional capacity. It is anticipated that only hourly peak flows would reach the equivalent of the 60 mgd capacity when both treatment trains would be engaged; 99% of the time only one treatment train would be required to be operational.
Construction started in July 2013, the new plant was put into commission in early March 2015, running with ground water to test the equipment, then with an initial loading from the sewers to fully test the new pumping station in parallel with the old equipment. After just two days, the grit removal system was put under an extreme test due to the very tight construction schedule, and came through with excellent results.
Nathan White explained: “The replacement North Intercept sewer had been completed a year before and the old pipeline was due to be taken out of service. We took the decision to divert the sewer through the new grit removal plant as the commissioning had proved problem-free.
“A year’s worth of construction trash, sediment, grease and grit was washed through with the raw sewage and the Hydro equipment coped with the influx without any problems. It was possibly the worst conditions the plant will face and we ran both treatment trains for a considerable time to catch the accumulated material.
“While we have not made any comparative survey of the rates of grit removal, before and after, we are continuing to optimize the plant, with a great deal of helpful service from the Hydro team. Regular site visits and reports from site operatives have shown that grit removal is much improved from before and the equipment maintains its efficient performance.”
Dr. Jue Zhao confirmed the progress with HeadCell: “I worked in consultancy before I joined the City project team, in 2012, and have prior experience with HeadCell. The grit removal is already performing much better than previously, with some fine tuning ongoing with Hydro’s help.”