The largest city in Alberta and the fourth largest in Canada with over one million people, Calgary is situated on the confluence of the Bow and Elbow Rivers. Warm dry summers produce plenty of wind-blown dust and sand and 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. While much of Calgary's sewer system is sanitary sewer, significant portions of the old sewers are combined stormwater and sanitary systems. The collected wastewater leads to high concentrations of grit in the raw sewage as well as a large quantity of additional grit carried in highway runoff.
Bonnybrook is the largest of Calgary's three wastewater treatment plants (WWTP). As part of a major upgrade to serve the city's growing population, a new system had to be built. It had to be done within the limited space available and still cope with the unique characteristics of the influent wastewater at Bonnybrook, including wide variations in flow caused by dry summers and peak winter thaws. The City of Calgary decided to invest in a high-performance Advanced Grit Management® system from Hydro International to remove large amounts of grit and improve the operating efficiency of the WWTP. It installed the HeadCell grit removal, classifying and dewatering system to protect downstream processes at the Bonnybrook WWTP from abrasion and grit deposition and so reduce maintenance and operating costs. The new system is designed to treat a peak flow rate of 1,390 megalitres/day (368 MGD).
Bonnybrook's grit challenge
Zorica Knezevic, senior project engineer, City of Calgary Water Resources, said: “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. For example, we had noted that up to 20% of the digesters' tank volume was taken by settled grit with the previous technology. 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 annually.”
Consultant engineering firm, Stantec, was hired to design the new headworks and pre-qualify the suppliers. Ryan Roberts, vice president of water at Stantec, said: “The Bonnybrook WWTP project is a complete replacement of the headworks facility. Other, separate projects are in hand to upgrade other plant facilities, including many new processes.” For the headworks project, Stantec included maintenance costs in the lifecycle evaluation of technology options and project justification. The project scope includes 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 passed on much grit to accumulate in downstream processes, grit characterization had already been assessed by the City in a separate exercise,” said Roberts. “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 the plant influent grit gradation and settling velocity as well evaluating the performance of the existing aerated grit chamber system. The study found that the existing grit system was removing only 26–29% of the influent grit. It also identified the reason; virtually all of the influent grit had a settling velocity lower than a 212 micron sphere of silica sand, a conventional design point, for which the original plant was designed. Pat Herrick, regional sales manager for Hydro International, added: “Grit characterization to determine the size distribution and settling velocity profile of the influent grit is essential when designing a new grit removal system for a plant of this size.” He continued: “By having the grit characterization data available, Stantec was able to work with the City to determine the grit removal system performance requirements that formed part of the system specification. These requirements were part of a pre-selection Request for Quotation that was written by Stantec.” The Request for Proposals package included the native grit characterization data, 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 along with other performance and operational features, together with a proven track record at other regional installations with similar performance requirements. The grit removal system was pre-selected by the City. As a part of pre-selection requirements, Hydro was under obligation to provide equipment shop drawings, on which Stantec based its development of the tender package for the general contractors. During the design phase, CFD 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 disposed of to landfill. It has to pass the standard ‘paint filter’ test – 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 (368 MGD) from screened sewage. At the normal flow design maximum of 418 ML/d (110 MGD) the units will remove 95% of all grit of 75 microns (SG 2.65) and larger from screened sewage. The first sewage began to flow through the system in late August 2014. Commissioning included successful performance testing of the Hydro grit removal system by an independent third party. Zorica Knezevic concluded: “After commissioning, the Bonnybrook grit removal plant has continued to perform well.”