The James B. Messerly Water Pollution Control Plant (WPCP) had a history of difficulty with grit removal.

The plant, in Augusta, Ga., (population 195,000), treats residential and significant industrial wastewater from the metropolitan area. Forty permitted industrial users make the plant influent high in BOD and ammonia. Food, paper, textile and chemical plant waters add organic and inorganic chemicals to incoming flow.

In the 1980s, the Messerly plant received an upgrade to its aeration basins and digesters along with a new grit removal system. Three mechanically induced vortex (MIV) units replaced an aerated grit basin; each was sized to handle 15 mgd average and 30 mgd peak flow at a time when total plant flow averaged 26 mgd and peaked at 60 mgd.

Even after updating the grit washing and dewatering system in 2004, the grit collection units performed below expectations. The plant added a stacked tray grit removal system with new washing and dewatering technology in 2010. These systems delivered the necessary grit removal efficiency and helped enhance downstream processes.

Design based on data

The latest grit removal system was installed as part of a larger plant upgrade. City representatives and their engineering consultant visited sites to observe the planned technology and also hired an independent testing company, Grit Solutions (now Black Dog Analytical), to characterize the plant’s grit.

The company analyzed grit entering the plant and downstream of the existing grit system. During the testing, 70 to 76 percent of the grit collected had a physical size of 212 microns and larger, and 45 to 50 percent was between 300 and 820 microns, meaning the influent grit gradation was relatively coarse. However, as much as 43 percent of the influent grit settled at the same velocity as 106-micron spheres of clean sand. The grit system was not effective in capturing this slow-settling material.

As in nearly all wastewater treatment plants, the grit particles settled much more slowly than their physical size would indicate. Grit particles typically have irregular shapes that reduce settling velocity. In addition, grit includes silica sand as well as components such as gravel, concrete particles, minerals and other materials with specific gravities less than 2.65, the conventional grit system design assumption.

Based on plant data, engineers decided that the new grit system should be designed to remove 95 percent of all grit particles 106 microns and larger in order to capture more than 70 percent of the incoming grit.

An added step

The plant installed three 12-foot-diameter, 12-tray Eutek HeadCell stacked tray grit removal units with Eutek SlurryCup and Eutek Grit Snail units for washing and dewatering (Hydro International). The system was designed to remove 95 percent of all grit 110 microns and larger at a peak flow of 38.6 mgd per unit.

The existing MIV system remained, and its effluent was pumped to the HeadCell system downstream. From the HeadCell separators, flow travels by gravity through the rest of the plant. Two of the three HeadCell units are typically in operation. Processing an average flow of 31 mgd and a peak flow of 46 mgd, the new system typically removes an additional 8 to 12 cubic yards of grit per week, or about 1.0 to 1.24 cubic feet per million gallons. That is almost 10 times the amount removed by the MIV system.

The new process operates by taking the effluent from the MIV system through the new HeadCell stacked tray system. Grit collected in the HeadCell is pumped to the SlurryCup, introduced tangentially at a controlled rate to establish a free vortex flow. The design of the vortex chamber effectively separates the grit to the perimeter, where it falls by gravity into the boundary layer at the bottom of the unit. The boundary layer sweeps the grit to the center for collection. Excess water and separated organic materials flow out of the unit for additional processing.

A vortex valve regulates the volume of washed grit slurry exiting the SlurryCup and introduces it into the Grit Snail dewatering unit. A slow-moving cleated rubber belt lifts settled grit from the clarifier, allowing it to drain as it is elevated to discharge. Clarified water is returned to the treatment plant.

Better downstream

At the Messerly plant, the system’s better grit removal efficiency improved both clarifier and digester operation. The primary clarifier now can be used to thicken sludge, since the grit has been removed. This helps the digester, as less overall sludge volume allows for a longer solids residence time, improving volatile solids destruction.

ESG Operations operates the plant for the City of Augusta. Percy Nolan, operations manager, states: “It is literally amazing how much grit the system is removing — as much as 16 cubic yards per week.”

About the author

Marcia Sherony is national sales manager for the Water and Wastewater Division of Hydro International, based in Hillsboro, Ore. She can be reached at 503/200-9855.