Removing Fine Grit from Large Flows
The Littleton-Englewood Wastewater Treatment Plant (WWTP), just south of Denver, is the third-largest publicly owned treatment works in the state of Colorado. It receives sewage from the city of Littleton, the city of Englewood and 20 connector districts in a 128-sq-mile service area. The plant discharges effluent to the South Platte River, a major watershed in the Denver metro region that serves as a source of drinking water for downstream communities and accommodates recreational activities ranging from kayaking and swimming to panning for gold.
The plant has been in constant upgrade mode since it started operating in 1977. Early improvements have added capacity, upgraded the plant’s solids handling operation, improved secondary treatment processes, expanded its administration building and in 1991, added a nitrification process to remove high levels of ammonia from the effluent.
“We’ve been in construction pretty much nonstop for a long time,” said Greg Farmer, plant process specialist. The plant’s latest set of improvements was driven largely by continued efforts to stay ahead of capacity requirements and the need for a denitrification process to remove nitrates created by the systems designed to remove ammonia.
But the plant upgrades could not stop there. The plant’s grit-removal equipment had been underperforming for years, and the inevitable grit upgrade had to be delayed while other process upgrades were being finished. With the plant’s capacity about to increase—from average monthly flows of 36 million gal per day (mgd) to 50 mgd, with peak hour flows of 100 mgd—staff decided to address the grit concerns before increased flows created more of an issue.
“The grit system we had just wasn’t getting the job done,” Farmer said. “We were seeing signs that grit was passing through the plant and there was the subject of wear and tear on our machinery. It was time to deal with the problem once and for all.”
The Littleton-Englewood WWTP had in place a pair of large settling tanks that slowed the wastewater velocity to the point where grit would settle out. A chain mechanism pulled back a percentage of the coarse grit, but according to Farmer, the medium and fine grit would pass right through. He said the existing equipment had moving parts that broke down often submerged underwater. Every time machines broke down, the grit operation would have to be paused for several days, leading to 10 or more days per year when grit of all sizes would pass through the plant.
Farmer’s staff chose to implement an upgraded grit-removal system supplied by Eutek Systems, now a division of wastewater equipment supplier Hydro Intl. Two complete HeadCell systems were implemented, each consisting of one HeadCell settleable solids concentrator unit, one SlurryCup grit separation and classification unit and one Grit Snail grit-dewatering escalator.
Farmer said he saw a demonstration of the HeadCell solution several years ago at WEFTEC, the industry’s national conference, and started asking questions. He looked at grit-removal systems as far away as Germany and Sweden but found the total three-process system to be the best fit for the Littleton-Englewood WWTP.
The HeadCell is a modular, multiple-tray settleable solids concentrator that removes coarse and fine grit with minimal headloss. The high-efficiency flow distribution header evenly distributes influent over multiple stacked conical trays. The multiple stacked trays greatly increase surface area, allowing 100 mgd to be treated in a 16-by-32-ft footprint.
Tangential feed establishes a vortex flow pattern where solids settle into a boundary layer on each tray and are swept down to the center underflow collection chamber. It captures very fine particles due to the large surface area and short settling distances. Evenly split flow eliminates thermal short-circuiting, which reduces the performance of conventional grit basins. These settled solids are continuously pumped to the grit-washing system, which discharges a concentrated grit stream to the grit-dewatering system.
The grit separation and classification unit washes organics from grit captured by a grit separation system—in this case a HeadCell settleable solids concentrator. It minimizes organic content to reduce objectionable odor and makes the grit suitable for landfill disposal. The separation and classification unit uses a combination of an open free vortex-type flow regime and boundary layer effects to capture, classify and remove fine grit, sugar sand, snail shells and other high-density fixed solids from grit slurries and primary and secondary sludge.
Grit and fine abrasives are then discharged into the Grit Snail clarifier, where they settle onto a slow-moving escalator belt. The captured solids are gently lifted out of the clarifier pool at 1 to 5 ft per minute. Degritted water flows out of the large clarifier via an overflow weir, while the dewatered, low-organic grit is discharged into a disposal container for landfilling.
Together, at a peak day wet-weather flow of 80 mgd with less than 12 in. of headloss, the systems at Littleton-Englewood WWTP remove 95% of all grit down to 150 microns in size. Shortly after the upgraded grit-removal systems went online in mid-2008, Farmer was seeing positive qualitative results that he expects will be backed up by quantitative studies.
“Looking at the grit, it does look better than what we were getting before,” Farmer said. “It’s blacker. We’re getting less organics, which is good because there is less of an odor going out with the trucks. The way the system works, the separation allows grit to settle and the lighter organics to pass back to the plant, where they can be treated rather than landfilled.”
The grit-dewatering escalator is pulling fine grit out of the grit underflow—something the old machinery was incapable of doing, Farmer said. This will reduce wear and tear on the plant’s mechanical equipment because fine grit is no longer able to escape into the degritted effluent that returns to the plant. Other benefits Farmer has been seeing right away are fewer breakdowns and less water getting taken away in trucks hauling grit to nearby landfills.
“Under the old system, water would get in the truck and it would drip,” Farmer said. “If you were driving along behind one of those, you could get sprayed. People will appreciate this change quite a bit.”