The TV show “Mythbusters” has made a name for itself proving and debunking popular myths in a swashbuckling style that still manages to incorporate real science. In one episode, the scientists went to a snow-covered mountain and tried to trigger an avalanche by yodeling. In another, they badgered certain plants in a greenhouse with loud rock music and soothed others with soft classical music to see which environment promoted growth.
The wastewater business, like other industries, has its share of myths; some are widely endorsed and perhaps even true, while others are accepted due to a lack of widely circulated evidence to the contrary. The purpose of this column is to debunk what is considered to be one of wastewater’s enduring myths: The idea that grit removal tanks need to be sized to remove very fine, as well as coarse, grit.
Grit Must Go
Grit, of course, is a menace to the wastewater treatment process. It grinds away at mechanical equipment and accumulates downstream. Grit accumulation in primary and secondary process basins reduces the plant’s treatment capacity, diminishes performance and increases operational maintenance costs.
Grit must go. But how much material must be removed and at what cost? And what factors should determine the sizing of grit chambers?
The argument to incorporate bigger grit chambers capable of removing fine sediment (typically less than 100 microns) stems from the idea that bigger grit particles behave like smaller ones when they attract oils and organics as they move through the system. In other words, a 100-micron grit particle with lighter materials attached to it has a lower specific gravity and settling velocity, making it settle in a tank at a slower rate, comparable to a 60-micron particle. Therefore, the argument goes, chambers need to be sized to remove 60-micron particles to capture the actual 100-micron grit particles that do the true damage.
There are many problems with this argument. First, evidence shows that grit particles do not, in fact, settle at appreciably different rates when covered with oils and organics. By sizing the chamber according to sand-equivalent size, you capture the same grit particles you need to capture, along with all the attached organics. This is not good. You also capture finer sediment, which is a waste of resources.
Admittedly, some field studies have shown that the specific gravity of grit could drop to as low as 1.3 from 2.65 when organics get attached to grit; however, what is usually overlooked is the mass of organic material required to drop the specific gravity from 2.65 or greater to 1.3. Calculations based on Stokes’ Law have shown that this increase in mass compensates for the decrease in specific gravity, resulting in a composite solid “grit” that has settling characteristics similar to clean grit with a specific gravity of 2.65.
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:
Particle size. The bulk of material arriving at a treatment plant is larger than 100 microns (greater than 90%) and, in some cases, 150 microns. There is no need to size to remove finer material when very little arrives in the waste stream. In fact, an individual grain with a particle size of 100 microns or less is not visible to the naked eye.
Cost. The size of a grit chamber can increase substantially when it is sized to remove very fine material. As explained above, the quantities of very 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 to 5% of grit.
Organics. When separators are sized to remove very fine grit, a high 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. The cost and handling of the separation process can be significant.
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 (less than 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 dumped in landfills without any further treatment. If metals and nutrients are trapped within the grit and dumped, they could leach into groundwater sources and cause problems for human and aquatic life. It is best to remove these pollutants through clarification, chemical and biological processes.
Sizing Grit Chambers
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 cutoff grit size, which typically should be 100 microns or more. 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 sof the grit.
The subject of grit chamber sizing will, no doubt, be debated for some time. Studies will be done, papers will be delivered and books will be written. Perhaps even “Mythbusters” will be interested. You never know.