Defeating the Warren County "grit monster"
Few wastewater treatment plants (WWTPs) get to experience the disaster of what actually happens if a plant has absolutely no grit removal in operation at the headworks. At Warren County, in Ohio, over a five year period, engineers discovered the hard way the impact of grit on downstream processes – and the eventual struggle and costs necessary to remove it.
When aeration basins were finally cleaned in 2013, 2,400 cubic yards or 1,386 tons had been removed and dewatered in a total of 128 geotextile bags. The story of Warren County, and its battle against what came to be called the 'grit monster’, presents salutary lessons for engineers, owners and operators of every wastewater treatment facility.
In the 1960s, there was no grit removal from the influent water to the Warren County plant, but the water flow rates were low - in the 100,000 gallons per day (GPD) range - and operational regimes were relatively relaxed; the grit load was thought not to be very significant. In the 1980s, an aerated grit tank was installed, using a chain and bucket removal system, effectively removing only the heavy solids.
"It was an unpopular, very smelly maintenance task, so I guess it wasn’t done very well," says Warren County Water & Sewer Department Deputy Sanitary Engineer David A. Walling, "Back then we could dispose of the load without encountering odor issues off site, too."
The increasing grit loading resulted in the deployment of the first proper grit management equipment purchased for the treatment plant in 1990. The system was needed to reduce grit loading on the Vertical Loop Reactors (VLR), a modified activated sludge process for biological nutrient removal. In the late 1990s, a second VLR was added to the plant and the system expanded working to a design capacity of 7.2 MGD each. In 2006, flooding problems at the headworks became so troublesome that the grit collection system was effectively bypassed and the grit problem ignored.
A grit monster emerges
“It was what happened in the 2000s that really brought the grit problem to a crisis and showed up what we had effectively been overlooking until that point,” explains Walling. “Also, we hadn’t really understood what grit in a waste water plant actually was. We originally considered it was a bunch of heavy solids with a particle size of over 250 microns that gets washed in with first flush events. Of course, the heavy component is a large part of it by weight, but you also have to factor in other material like fine abrasive particles, inorganics and organics, floating trash, eggshells, coffee grounds. There are seeds, bone fragments and other organic food waste particles of all sizes. Oils and fats solidify around particles and change the way the grit settles out. Domestic kitchen garbage disposal systems are one example of a grit generator, which makes management and removal of grit a much more complex, as you have to target all these different types of grit and more. It means they have a wide range of specific gravities, shape and surface area, and particles do not settle like you would necessarily expect.”
With a building boom in the 1990s came an unanticipated problem: plumbers’ PVC knockouts from new housing installations passed easily through the 6 mm headworks screen protecting the headworks, jamming the grit removal underflow system. The two inch underdrain outlet and valve clogged, resulting in flooding of the concrete chamber and the pre-treatment building. The electric motors of the grit slurry pumps were ruined several times, requiring replacement.
By 2005, the grit problem on the site was well recognized and investment plans began to be drawn up for a major system upgrade. By this stage, on-line processing volume had reduced by almost 20%, from around 7.2 MGD to 5.8 MGD. Typical of the difficulties was the situation of the coarse bubble aerator tank, which was so clogged up with grit that it was ineffective.
In 2006, the operators gave up on the maintenance battle and flooding problems and the grit collection system was bypassed altogether, while maintenance teams held out for the new system to be agreed and implemented. A new design for the headworks plant to tackle the grit problem was intended take two years to complete. This included plans for on-line expansion to accommodate population growth, including new residential house building, and associated commercial and industrial development - but the plans were delayed for years by the 2008 financial crisis.
Removing five years of grit
With the upgrade finally approved in 2012, the plant had to begin the task of tackling five years of grit accumulation without any removal system. The contractor appointed to remove grit from VLR basins 1 and 2 estimated there were 1,200 cubic yards of grit which would take a few months to remove.
But removing the accumulated grit proved to be much more difficult that first anticipated. Access to the VLRs was difficult - getting equipment in without damaging the installation because of the covers and the tank positions. Various methods of removal were tried and failed: bucket and chain methods were abandoned in favor of pumping in liquid effluent to reagitate the deposits and remove them with mobile suction pumps. However, new and existing belt filters brought in for the dewatering process were extensively damaged by abrasion and debris.
Eventually it was decided to vacuum extract the fluidized grit with a 2,000 gallon ‘honey dipper’ vacuum suction pump and deposit the grit into very large geotextile bags, 3,000 gallons a time, in a temporary plastic-lined concrete tank. From here, the bags were left until no more liquid emerged, before the bags could be cut open and the contents trucked out and disposed of. It was a slow, dirty, smelly and unwelcome task. Each 45 ft x 15ft x 3ft bag at around 3,000 gallons capacity contained around 75 cubic yards of dewatered grit at an estimated 1,200 lbs per cubic yard.
By the end of the process in 2013, a total of 128 bags of grit had been collected, around 2,773 cubic yards or 1,386 tons of grit from two reactors.
"There was also a big environmental problem during the grit removal operation," says Walling. "The neighboring residents are only a hundred yards or so away in one direction; this caused heavy odors when the wind was blowing in the wrong direction. We had take special care to work with our neighbors and talk to them about what was going in."
Finally in 2012, with new pre-treatment screening now in place, two 12 ft HeadCell® advanced grit separation units were installed, each with associated SlurryCup® and Grit Snail® classification and dewatering equipment, as part of the Phase III plant expansion which had a 12 MGD daily design per train. With VLRs 1 and 2 offline for degritting, a third VLR was installed in 2012 and took the whole plant load. Protected by the HeadCell® grit removal system, after one year of operation there was virtually no accumulated grit in VLR 3.
The new HeadCell® grit removal system is capable of alternating operation between the parallel trains with redundant capacity and future proofing in mind. In practice, liquid is kept trickling over both the HeadCell units, maintaining optimum settling performance across the surfaces of the stacked trays. The units are housed in the grit management building at the headworks, so that odor control with a recirculating air scrubber is effective and economical.
New system very effective
"The stacked multi-tray design of HeadCell® is very effective in removing grit,” says Walling. “The compact dimensions meant it was simple to install in our headworks, and it turned out that, with the construction costs, there was little difference in total cost when compared to other grit management technology. Since 2012 we have had minimal grit accumulation in nearly three years, and no accumulation of foam. Although we haven’t done grit profiling and other comparative before and after tests, the massive improvement is plain to see."
Warren County WWTP is unusual in having an absolute measurement of the total grit volume from the aeration tanks alone to calculate a return on investment from investment in an efficient removal system. Walling estimates savings of over $190,000 would be made by removing the same amount of grit through the HeadCell® system as was removed in total from the VLRs, notwithstanding the other savings made from avoiding downstream equipment damage.
At the same time, the grit had a huge impact on the efficiency of the aeration process. It was estimated to result in a 20% reduction in treatment volume in the aerators, with efficiency further compromised by reduced anaerobic conditions, oils and grease accumulation, promotion of hydrophobic organisms and foaming issues and sludge settling due to filamentous growth.
"And we have learned a number of hard and expensive lessons, which we think every waste water treatment plant operator should take to heart before it’s too late. I can summarize them: grit is a 24/7, 365-day battle. If you don’t remove grit in the headworks building, grit costs a lot more dollars once it passes down the line. Avoid unnecessary wear and tear on equipment and pay for grit removal up front to keep hydraulic and treatment capacity and efficiency maximized."
The Warren County grit monster - in numbers
The cost of removing grit the hard way, after accumulation WWTP over 4 to 5 years
Contractor bid for removal: $74 per cubic yard
Actual grit removed: 2,773 cubic yards
74 x 2,773 = $205,202 (over four years)
The cost of removing grit with HeadCell®
Disposal cost: $3 per cubic yard
Actual grit removed: 2,800 cubic yards per year
3 x 2,800 = $8,400 per year (disposal only)
Savings over four years: $196,802