As a critical part of a $21.5 million upgrade to its wastewater treatment facilities, the Nebraska city of Grand Island installed a high-performance grit removal system at the treatment plant headworks.

The system is designed for redundancy and will remove 95 percent of grit 90 microns and larger. The Advanced Grit Management solution from Hydro International proved its capability in challenging first-flush conditions within days of commissioning.

“Our wastewater infrastructure was about 50 years old,” recalls Dr. Jue Zhao, P.E., treatment plant operations engineer. “The city had many issues with the plant and equipment, and we were faced with a growing population and rising flows. As a consequence, the city decided to invest $21.5 million in a five-year refurbishment project built to meet our future needs.”

Comparing technologies

The pretreatment grit removal facilities were undersized and performing poorly. During peak flows, grit was being flushed through the aerated grit chambers into the primary clarifiers and depositing there. Based on discussions with city officials, the engineering firm Black & Veatch selected HeadCell grit removal technology (Hydro International), with that company’s grit washing and collection equipment, to remove grit 90 microns and larger and reliably protect downstream equipment and processes.

“We were approached in 2012 to study the city’s wastewater handling and treatment needs well into the 21st century, taking account of matching population growth and regulation changes,” says Nathan White, P.E., Black & Veatch engineering manager. “This led to recommendations for upgrading the major sanitary interceptor sewers, as well as the main treatment plant.

“Grit removal at the headworks was one of the priority areas. The existing plant with its grit basins was inadequate. Much of the equipment, including valves and pumps, was corroded and barely functioning, so a complete replacement was required.

“We evaluated two types of grit removal to meet the new maximum 40 mgd flow capacity: a mechanical vortex-based design and the HeadCell/SlurryCup/Grit Snail system from Hydro.

The specification for each was to remove 95 percent of grit 90 microns and larger at the average design flow of 13 mgd per treatment train, and 95 percent of grit 150 microns and larger at the maximum peak hourly flow capacity of 30 mgd per treatment train.

“Overall capital and installation costs were very similar. In consultation with the city’s Public Works engineers, we visited the plant at Lincoln, Nebraska, already using HeadCell technology. City engineers were impressed with the collected grit quality. We went with the Hydro design because of its high grit removal performance combined with low mechanical equipment needs, which offer much less maintenance cost in the future.”

Need for renewal

Grand Island, the fourth-largest city in Nebraska (population 50,000) sits on the Platte River. The water table is high, and pumping is necessary to maintain flow in pipelines and across the treatment plant, which discharges to the Wood River (a Platte tributary).

The plains topography is composed of low, rolling hills, and the soil is wind-blown and silty/sandy. Particles are inevitably washed by surface runoff into the sewers and the treatment plant. Industrial dischargers include a major meat processing plant.

The existing plant was sized for 35 mgd hydraulic capacity, but age limited its capacity to 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 building base needed repair. The existing facilities had no provisions for odor control.

Redundancy plan

“As a result of all these factors, the project team decided to start with the new headworks, consisting of pretreatment and a grit removal building,” says White. “There was sufficient building space to achieve the ideal configuration for a plant with parallel treatment trains, providing the redundancy we desired. This made the planning, construction and installation of the equipment very straightforward.”

Black & Veatch worked closely with the Hydro team to optimize the layout. Each treatment train is a mirror image of the other. After mechanical bar screening, the setup on each side consists of a 12-foot 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 to not less than 60 percent total solids and a maximum organics content of 15 percent. A dedicated control panel serves each train and allows full automation and integration with the plant SCADA system for remote monitoring and control, including automatic startup 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 treatment and biosolids digestion,” says White. “Achieving a linear series of pipe runs, especially in the inflow to the HeadCell units, allowed us to reduce turbulent flow, maximize flow rates and possible settlement of suspended particles, and thus minimize maintenance in the pipework and downstream equipment, including abrasion damage.”

Extreme loads

The built-in redundancy helps maximize return on investment in the pretreatment and grit removal facilities, which have a design life of 50 years. Predicted peak-day flows of 25 mgd per train are well within the plant design, leaving 20 percent spare capacity. Officials expect that only hourly peak flows will reach the equivalent of 60 mgd when both trains would be engaged; 99 percent of the time only one treatment train would need to operate.

Construction started in July 2013, and the new plant was commissioned in early March 2015, first running with groundwater to test the equipment, then with an initial loading from the sewers. After just two days, the grit removal system was put under an extreme test and came through with excellent results.

“The replacement North Intercept sewer had been completed a year before, and the old pipeline was due to be taken out of service,” White says. “We decided 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.”

Marking progress

“While we have not made any comparative survey of the rates of grit removal before and after, we continue to optimize the plant, with a great deal of helpful service from the Hydro team. Regular site visits and reports from site operators have shown that grit removal is much improved from before, and the equipment maintains its efficient performance.”

The city’s Zhao confirmed progress with the grit system: “I worked in consultancy before I joined the city project team in 2012 and had prior experience with HeadCell. The grit removal is already performing much better than previously, with some ongoing fine-tuning with Hydro’s help.”

About the author

Marcia Sherony (wastewaterinquiry@hydro-int.com) is national sales
manager for the Water and Wastewater Division of Hydro International, which specializes in high-performance equipment and systems for removal of grit, sugar sand, abrasives and fixed solids.