Fine grit removal saves Niagara’s aeration process from smothering
A high-performance, fine grit removal solution from Hydro Intl. has allowed the aeration process at a North American wastewater treatment plant to breathe again.
The Niagara County Sewer District #1 and Niagara County Wastewater Treatment Plant (WWTP) are located on the Niagara River in western New York. The river drains four of the Great Lakes and represents possibly the largest flow of freshwater in the world. There are many towns and cities that discharge into this waterway, and the Great Lakes themselves have only recently had their pollution load brought under control, so stringent efforts are made to maintain standards.
Grit composition at the Niagara County WWTP is characterized by extra fine, slow-settling particles. Undetected for eight years, it was choking the efficient operation of the plant.
In particular, Niagara was experiencing deterioration in its aeration process: Submerged fine air diffusers were fouling due to the accumulation of fine grit particles in the aeration tanks.
Having seen a successful grit removal system in operation at a nearby plant in Buffalo, N.Y., Niagara installed a Hydro Intl. SlurryCup and Grit Snail system, which reduced tank cleaning times and costs and enabled the biological digestion processes to operate efficiently.
“We have a different and more difficult grit problem than most WWTPs,” said John Timkey, chief operator for Niagara. “In my long experience in other treatment plants, Niagara is unusual in that all our incoming sewage is from sanitary sewers. There are no combined sewers bringing in coarser particle grit from road washings and surface water.
“As a result, most of our grit is very fine and slow-settling, and the current headworks was never designed for removing fine grit down to 75μ or so. We don’t have a primary clarifier unit, for example, and raw sewage is pumped straight from grit removal to the aeration tanks.
“The old aeration system, installed in the late 1970s, mechanically agitated the top couple of feet of the 14-ft deep tanks and, as a result, grit settled steadily in the bottom of the tank. However, the accumulation was tolerated for many years and was only recognized as a problem when the plant upgraded to the submerged fine air diffuser system.”
Aeration Turning Inefficient
In 2000, Niagara County upgraded its aeration system to fine bubble diffusers. The original surface aerator had not been particularly effective in aerating the whole water column, and it could flood in high flow or ice up in winter. The project objectives also were to reduce energy costs and improve the efficiency of the aerobic process by having a slow stream of fine bubbles distributed across the whole tank to allow more interaction with the microbial agents, oxygen and organics.
“In 2000, before my time at Niagara, the four 866,000-gal, 14-ft deep aeration tanks were cleaned out in turn and the fine bubble diffusers installed,” Timkey said. “The diffusers are seated about a foot off the tank floor. The process means there is a great density of diffusers to create the fine bubbles and much more pipework across the tank floors, which also meant it was more difficult to clean.
“That’s probably when grit dynamics changed. The bubble-churned water agitated the slowly settling sediments, keeping more solids in suspension.Some of it probably ended downstream, but the tanks were steadily accumulating settled fine grit.”
The aeration process worked well for a few years, but then the energy and process efficiency savings started to diminish.
“This was 2008, around the time that I joined Niagara County,” Timkey said. “Although no one had kept detailed records of energy use and the decline in air transfer was gradual over the eight-year period, we observed bubble flow diminished with many dead spots.”
“We decided to have a look at what was happening and drained down a tank,” Timkey said. “We couldn’t see many of the diffusers; they were smothered under 3 or 4 ft of fine sediment mixed in with clumps of fibers. The incoming raw water hadn’t changed—it was the process upgrade that had highlighted a pre-existing problem.
“A contractor was brought in initially to clear the tanks. It was a very onerous task, as it was important to avoid damage to the air pipes on the tank floor. For one tank this cost $12,000—a price that could not really be justified.
“In the years that followed, we decided to give the job to the seasonal workers who were working here during the summer. Between two and four of them at any one time would be detailed to wash a tank with a 2-in. fire hose at 40 lb pressure, washing layers of grit down to the end where a pump would pick it up.
“Accumulated grit and sediment were pumped into a baffled channel, then dumped into a dewatering pit and eventually filled into steel containers for transport to landfill.
“It took all summer to clean out two tanks, which we did every year for five years,” Timkey said. “This cost us perhaps $5,000 to $6,000 per tank, and we were employing local labor, not some long distance contractor.
“At the time, we did not have any provision in the budget to reconfigure the headworks, so we had to have a more pragmatic solution for making the aeration tanks efficient. We looked at the installation of a mechanical belt press to dewater the removed slurry to reduce disposal costs. However, it turned out to be expensive to run with heavy mechanical maintenance requirements.”
Finding a Solution
Timkey had heard that the nearby Bird Island WWTP in Buffalo, N.Y., had a similar problem with the upgrading of its aeration basins. “Bird Island was, in fact, a little different; they had combined sewers incoming with a range of grit sizes partly derived from surface water drainage, but their headworks was fairly successful in removing the coarse grit component. When they upgraded to fine bubble aeration, they realized how much the finer grit was affecting them,” Timkey said.
At Bird Island, the solution devised was to regularly pressure-wash drained tanks with fire hoses to fluidize the grit and move it to one end of the tank, then pump the resulting slurry to a skid-mounted SlurryCup and Grit Snail system. This reduces the volatile organic solids contents and discharges a clean dry grit product that is suitable for landfill.
“We realized that this could also be a viable solution for us,” Timkey said. “Our four basins conveniently are in a close formation with a drainage point, so we could install the SlurryCup and Grit Snail in one place where the four corners of the basins meet. At Bird Island they have a skid-mounted system, which they have to move around the 16 tanks.”
The SlurryCup removes grit and fine abrasives as small as 75 μ, with minimal organic solids. Its high-energy vortex operates as a centrifugal solids separator and classifier with secondary washing. The solids separation occurs within the unit as a result of centrifugal forces exceeding fluid drag forces. Classification and separation of particles according to size occurs within the boundary layer. Once the particles are captured in the boundary layer and swept to the center, the SlurryCup hydraulic valve uses rinse water for secondary washing to separate attached organics. This allows the SlurryCup to remove and wash more than 90% of grit as small as 75 μ in diameter in both headwork and sludge-degritting applications.
The Grit Snail captures fine grit by providing sufficient clarifier area to retain 75 μ particles. A slow-moving, cleated belt gently lifts grit from the clarifier pool without re-suspending captured fine grit particles, which would allow them to escape with the clarifier overflow. The combined SlurryCup and Grit Snail degritting system delivers clean, dry grit with 60% total solids and less than 20% volatile solids.
“Now we run the SlurryCup while cleaning the basins, which can take up to three weeks (five days a week for six to seven hours per day),” Timkey said. “A large proportion of the dewatered grit is removed and stored in 5 cu yd containers for removal to the landfill.
The system only needs to be operated in spring through fall and is put away in winter. The degritted effluent is recycled back into the headworks for treatment. We remove annually between 10 and 15 tons of grit down to 75 μ dewatered to around 60%, with less than 20% organics.
“After nearly three years of operation, the aeration tank floors remain clean, and we certainly get better process results from aeration. Also, the seasonal workers prefer grass cutting or painting to hosing sewage.
“It is difficult to put a quantity on energy savings, but the energy input remains at an even level, not climbing up to push air through the diffusers against the sediment. An additional advantage is the SlurryCup uses no power and is self-cleansing in action, so operating costs are low. We are not convinced that, even if we did upgrade the headworks to remove fine grit, we would get better value than from this solution.”