September 4:2007
9/11/07
5:49 PM
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Wastewater Treatment Table 1 Settling velocities for grit particles showing that increased size for grit with attached organics compensates for lower specific gravity. Grit size* (SG 2.65)
Settling velocity (cm/s)
Composite grit size (SG 1.3)
Settling velocity (cm/s)
70 90 100 150 200
0.36 0.58 0.70 1.41 2.22
182 234 260 390 520
0.40 0.61 0.73 1.38 2.07
*Grit size is in microns. Temperature is 15oC
Figure 2 Particle size gradations for grit across the US.
0.70 cm/s. If organic material attaches to the grit and the specific gravity drops to 1.3, the composite “grit� will have a size of 260 microns and a settling velocity of 0.73 cm/s. These results show that there is very little difference between the settling velocities and, therefore, sizing the separator based on the
not provide optimal removal. Optimal removal should take out the bulk of free draining grit with very little organic material attached to the grit. This should exclude the very fine sediment for the reasons explained below: 1. Particle size: The bulk of material arriving at a treatment plant is larger than
There is adequate data available showing that sizing to remove very fine material does not provide optimal removal. clean grit will meet the treatment goals and not result in grit accumulating in clarifiers and digesters downstream of the grit removal system. Whereas some publications have suggested the need to remove very fine sediment (typically below 100 microns) during the grit removal process, there is adequate data available showing that sizing to remove very fine material does 38 | September 2007
100 microns (>90 percent) and in some cases 150 microns. There is no need to size to remove finer material when very little arrives in the waste stream (see Figure 2). In fact, an individual grain with a particle size of 100 microns or below is not visible to the naked eye. 2. Cost: The size of a grit chamber can increase substantially when it is sized to remove very fine material. The quanti-
ties of fine particles are so small that this extra cost associated in removing it is not justified. For instance, it takes almost double the time for a 75 micron particle to settle compared to a 100 micron particle. In effect, that means doubling the size of the separator to remove an additional 3-5 percent of grit. 3. Organics: When separators are sized to remove very fine grit, a large volume of organics is removed with the grit (because of similar settling velocities). To separate the grit from the organics requires putting in more equipment to separate the grit from the organics. The cost and handling of the separation process can be significant. 4. Pollutants of interest: Research of sediment from wet weather flows has shown that nutrients, hydrocarbons and metals are usually attached to very fine sediment. If fine sediment (<100 microns) is removed during grit removal, it defeats the purpose of downstream processes incorporated into the treatment plant to treat these pollutants. Grit removed in chambers is typically landfilled without any further treatment. If metals and nutrients are trapped within the grit and landfilled, they could leach into groundwater sources. It is best to remove these pollutants through clarification, chemical and biological processes. A good washing and classification system is crucial to ensure that captured grit is free draining with low moisture and organic content. If the performance of the classification system is worse than the performance of the grit chamber, the overall performance of the grit system is compromised, and grit re-suspension will occur, leading to grit accumulation in clarifiers and downstream basins. Conclusion Grit chambers should be sized based on the gradation of grit entering a treatment plant. Sizing based on the physical size and typical grit specific gravity is adequate to remove the intended cut-off grit size, which typically should be 100 microns or above. This is because sub-100 micron material is in the realm of powder rather than grit. Even though organics and oils reduce the specific gravity of grit, there is a concurrent increase in mass and as such, no significant change in settling characteristics of the grit. Robert Andoh is Director of Innovation, with Hydro International. Hydro International is represented in Ontario by ACG Technology. For more information E-mail: greg@acgtechnology.com
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