Design Criteria, Advantages and Disadvantages for Grit Removal
With respect to grit removal systems, grit is traditionally defined as particles larger than 0.21mm (0.008in) (65 mesh) and with a specific gravity of greater than 2.65 (U.S. EPA, 1987). Equipment design was traditionally based on removal of 95percent of these particles.
However, with the recent recognition that smaller particles must be removed to avoid damaging downstream processes, many modern grit removal designs are capable of removing up to 75 percent of 0.15 mm (0.006 in) (100 mesh) material.
Aerated Grit Chamber
Aerated grit chambers are typically designed to remove particles of 70 mesh (0.21 mm/0.008 in) or larger, with a detention period of two to five minutes at peak hourly flow. When waste-water flows into the grit chamber, particles settle to the bottom according to their size, specific gravity and the velocity of roll in the tank.
A velocity that is too high will result in lower grit removal efficiencies, while a velocity that is too low will result in increased removal of organic materials. Proper adjustment of air velocity will result in nearly 100 percent removal of the desired particle size and a well-washed grit.
Design considerations for aerated grit chambers include the following (WEF, 1998):Air rates typically range from 0.3 to 0.7 m3/m•min (3 to 8 ft3/ft•.min) of tank length.
A typical minimum hydraulic detention time at maximum instantaneous flow is two minutes.
Typical length-to-width ratio is 2.5:1 to 5:1.
Tank inlet and outlet are positioned so the flow is perpendicular to the spiral roll pattern.
Baffles are used to dissipate energy and minimise short circuiting.
Vortex-Type Grit Chamber
Two designs of vortex grit units exist: chambers with flat bottoms and a small opening to collect grit and chambers with a sloping bottom and a large opening into the grit hopper. Flow into a vortex-type grit system should be straight, smooth, and streamlined.
Read Also: Operation and Maintenance of Grit
The straight inlet channel length is typically seven times the width of the inlet channel, or 4.6 m (15 ft), whichever is greater. The ideal velocity range in the influent is typically 0.6 to 0.9 m/s (2 to 3 ft/s) at 40 to 80 percent of peak flow.
A minimum velocity of 0.15 m/s (0.5 ft/s) should be maintained at all time velocities will not carry grit into the grit chamber (WEF, 1998).
Horizontal Flow Grit Chamber
Horizontal flow grit chambers use proportional weirs or rectangular control sections to vary the depth of flow and keep the velocity of the flow stream at a constant 0.3 m/s (1 ft/s). The length of the grit chamber is governed by the settling velocity of the target grit particles and the flow control section-depth relationship.
An allowance for inlet and outlet turbulence is added. The cross sectional area of the channel is determined by the rate of flow and the number of channels. Allowances are made for grit storage and grit removal equipment. Table 4 lists design criteria for horizontal flow grit chambers.
Advantages of Grit Removal
Some advantages of aerated grit chambers include:
Aerated Grit Chamber
Consistent removal efficiency over a wide flow range.
A relatively low putrescible organic content may be removed with a well-controlled rate of aeration.
Performance of downstream units may be improved by using pre- aeration to reduce septic conditions in incoming waste-water.
Aerated grit chambers are versatile, allowing for chemical addition, mixing, pre-aeration, and flocculation.
Vortex-Type Grit Chamber
These systems remove a high percentage of fine grit, up to 73 percent of 140-mesh (0.11mm/0.004 in diameter) size.
Vortex grit removal systems have consistent removal efficiency over a wide flow range.
There are no submerged bearings or parts that require maintenance.
The footprint (horizontal dimension) of a vortex grit removal system is small relative to other grit removal systems, making it advantageous when space is an issue.
Head-loss through a vortex system is minimal, typically 6 mm (0.25 in). These systems are also energy efficient.
Horizontal Flow Grit Chamber
Horizontal flow grit chambers are flexible because they allow performance to be altered by adjusting the outlet flow control device. Construction is not complicated. Grit that does not require further classification may be removed with effective flow control.
Hydrocyclone
Hydrocyclones can remove both grit and suspended solids from waste- water. A hydrocyclone can potentially remove as many solids as a primary clarifier.
Disadvantages of Grit Removal Process
Grit removal systems increase the head loss through a waste-water treatment plant, which could be problematic if head loss is an issue. This could require additional pumping to compensate for the head loss.
The following paragraphs describe the specific disadvantages of different types of grit removal systems.
Aerated Grit Chamber
Potentially harmful volatile organics and odours may be released from the aerated grit chamber. Aerated grit chambers also require more power than other grit removal processes, and maintenance and control of the aeration system requires additional labour.
Vortex-Type Grit Chamber
Vortex grit removal systems are usually of a proprietary design, which makes modifications difficult.
Paddles tend to collect rags.
Vortex units usually require deep excavation due to their depth, increasing construction costs, especially if unrippable rock is present.
Read Also: Advantages and Disadvantages of Comminutor and Grinder
The grit sump tends to clog and requires high-pressure agitation using water or air to loosen grit compacted in the sump.
Horizontal Flow Grit Chamber
It is difficult to maintain a 0.3 m/s (1 ft/s) velocity over a wide range of flows.
The submerged chain, flight equipment, and bearings undergo excessive wear.
Channels without effective flow control will remove excessive amounts of organic material that require grit washing and classifying.
Head loss is excessive (typically 30 to 40 percent of flow depth).
High velocities may be generated at the channel bottom with the use of proportional weirs, leading to bottom scour.
