| Hydro International Eutek SlurryCup and Eutek Grit Snail grit washing and dewatering systems. |
The new system showed far better removal efficiency, which improved both clarifier and digester operation. The primary clarifier can now be used to thicken the sludge since the grit has been removed. This helps the digester, as less overall sludge volume allows for a longer solids residence time, which has improved volatile solids destruction.
The WPCP treats sanitary flow from the metropolitan area. Forty permitted industrial users make the plant influent high in biological oxygen demand (BOD) and ammonia.
When a grit removal upgrade was planned, the operator wanted to base the design of the new grit system on data. The city and its engineer hired an independent testing company to characterize its grit. Analyses of grit entering the plant and following the Mechanically-Induced Vortex (MIV) grit system were performed.
As a result, particle size and settling velocity could be used to select the correct target particle size for the new 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, so the influent grit gradation was relatively coarse. But as much as 43 percent of the influent grit settled at the same velocity as a 106-micron sphere of clean sand.
As is the case in virtually all wastewater treatment plants, the grit particles settled much more slowly than their physical size would indicate. Grit is typically an irregularly-shaped particle, which reduces its settling velocity. Additionally, grit is composed not only of silica sand but often of other components such as gravel, concrete, minerals, and other materials with specific gravities less than 2.65 -- the conventional design assumption.
