Adequate grit removal is particularly important to Calgary’s wastewater treatment, given the City’s location and topography. In the severe northern prairie winters, heavy ice and snow are frequently interrupted by warm Chinook winds that sweep up from the south as the Arctic fronts temporarily retreat. The resultant rapid thaws can leave the streets awash with runoff and sand used for winter road safety.
The City of Calgary, has installed the world’s largest HeadCell® grit removal, classifying and dewatering system from Hydro International. It will protect downstream processes at the Bonnybrook wastewater treatment plant from abrasion and grit deposition.
“High performance grit removal is important to achieve lower operation and maintenance costs and retain the specified capacity of downstream process units, such as bioreactors, fermenters
and digesters,” said Senior Project Engineer Zorica Knezevic, City of Calgary Water Resources. “For example, we had noted that, with the previous technology, up to 20% of the digesters’ tank volume was taken by settled grit. Wear and tear on equipment was also a factor, and removing grit was part of periodic maintenance. We estimated it at approximately 6,000 man hours
Climate-driven grit headaches
While much of Calgary’s sewer system is sanitary, significant portions in older areas are combined with stormwater. Collected wastewater leads to high concentrations of grit in raw sewage, as well as additional grit carried in highway runoff.
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The project scope included screening and grit removal; screening, washing and compacting; grit classification and dewatering; solids conveyance and storage; and flow distribution and measurement.
As the removal efficiency of the existing aerated grit tanks was low, and grit was accumulating in downstream processes, grit characterization had already been assessed by the City in a separate exercise. This was necessary to develop an accurate design basis for a cost-benefit evaluation of the various treatment alternatives considered.
The grit characterization study determined plant influent grit gradation and settling velocity, as well as evaluating the performance of the existing aerated grit chamber system.
It found that the existing system was removing only 26% – 29% of the influent grit. This was because virtually all of the influent grit had a settling velocity lower than a 212 micron sphere of silica sand. This is a conventional design point, for which the original plant was designed.
The request for proposals package included native grit characterization data and project performance requirements, as well as performance testing and penalty requirements. Hydro International was pre-selected based on its ability to remove grit particles as fine as 75 micron and other performance and operational features.
As a part of pre-selection requirements, Hydro had to provide equipment shop drawings. Stantec Engineering developed the tender package for the general contractors based on these. During the design phase, computational fluid dynamics analysis was performed to determine optimum influent channel design for the ten HeadCell units.
Big protection, small footprint
The complete headworks process includes new 6 mm bar screens, screenings washer/compactors, and new screenings conveyors. Each of the ten HeadCell units then removes and concentrates fine grit, which is pumped to a SlurryCup™ classifier unit that cleans the grit to minimize the associated organic material. Washed out organic material is returned to the treatment plant.
The washed grit slurry flows by gravity to a Grit Snail® dewatering unit to produce a dewatered grit, with an average of no more than 20% volatile solids by weight and greater than 60% total solids. There are five Grit Snail units, each with two SlurryCup units mounted on top. This configuration saves space and capital cost.
Washed grit is sent to landfill if it passes the standard “paint filter” test, which is an assessment of free-draining liquid from waste solids through standard filters.
Guaranteed grit removal rate
Each of the ten HeadCell units supplied to the upgraded Bonnybrook WWTP meets the peak flow specification of removing 95% of all grit at 150 micron and larger, at specific gravity (SG) 2.65, at a flow of 1390 Megalitres/day from screened sewage. At the normal flow design maximum of 418 ML/d the units will remove 95% of all grit of 75 microns (SG 2.65) and larger from screened sewage.
Sewage first began to flow through the system in late August 2014. Commissioning included successful performance testing of the Hydro International grit removal system by an independent third party.