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Prepared by Zulkifli Ahmad
GRIT REMOVAL
Grit Sand, gravel, broken glass, egg shells, and other material having a settling velocity substantially greater than the organic material in wastewater
Grit removal -to protect mechanical equipment from abrasion and wear; reduce the formation of deposits in pipelines and channels; and reduce the frequency of digester cleaning that is required because of accumulated grit.
- to separate the grit from the organic material in the wastewater. This separation allows the organic material to be treated in subsequent processes.
Prepared by Zulkifli Ahmad
Theory
All particles are assumed to settle in accord with Newton’s equation :
Prepared by Zulkifli Ahmad
The dimensionless constant ( β) ranges from 0.04 to 0.06. The Darcy-Weisbach friction factor is taken to be in the range 0.02 to 0.03.
Prepared by Zulkifli Ahmad
In a horizontal-flow grit chamber, to assure removal of the grit and scour of organic matter that settles, three conditions must be met :
i) the overflow rate of the chamber must be equal to the settling velocity of the inert grit particle.
ii)the horizontal velocity must be less than the scour velocity of the inert particles.
iii) The horizontal velocity must be greater than the scour velocity of the organic particles.
Prepared by Zulkifli Ahmad
Types of grit removal systems:
horizontal-flow grit chambers detritus tanks aerated grit chambers vortex-flow grit chambers
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Capacity and Redundancy:Aerated grit chambers must be able to pass the extreme peak hydraulic flow rate with the largest unit out of service. In addition, the grit handling equipment (collection channel and mechanical equipment) must be able to handle the highest grit loading at the highest flow rate.
LocationTypically, the grit chamber is placed downstream of coarse screens or mechanical coarse solids reduction devices. They may be placed upstream of fine screens to protect the fine screen from excessive wear.
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Detention TimeA detention time of 2 to 5 minutes at peak hourly flow is sufficient to achieve greater than 95 percent removal of the the traditional design particle size (0.20 mm diameter, specific gravity of 2.65). Typically, the detention time is set at 3 minutes at peak hourly flow .Detention times ranging from 3.6 to 7.8 minutes at average flow conditions have been reported to achieve efficiencies ranging from 94 to 98.8 percent .Longer detention times improve grit removal and may be necessary to capture smaller grit particles
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Geometry. The shape of the grit chamber is designed to enhance the spiral roll of the wastewater as it passes through the chamber. There is no typical geometry. Chambers have depths of 2 to 5 m. The width to liquid depth ratio ranges from 1:1 to 5:1 with a typical value of 2:1. The length-to-width ratio ranges from 2.5:1 to 5:1 Lengths range from 7.5 to 27.5 m
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
BafflesFour types of baffles have been used in aerated grit chambers: inlet, outlet, intermediate, and longitudinal. The first three types of baffle are used to prevent short circuiting of the flow through the tank. The longitudinal baffle is used in conjunction with the air supply to control the roll pattern.
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Air Supply. The shape of the grit chamber is not the only key to good design; diffuser placement, air source, and adequate baffling all affect performance .Coarse bubble diffusers are recommended for supplying the air. They are typically placed 0.6 to 1 m above the bottom of the chamber.The air supply and control system should be able to provide air over the range 0.0019 to 0.0125 m 3 /s.m of tank length
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Velocities. The primary design criterion for the aerated grit chamberis the velocity of flow across the bottom of the tank.Based on theoretical calculations and experimental evidence, for efficient removal of grit the velocity across the bottom of the tank should be less than 0.15 m/s.Other experimental evidence is that a range of 0.03 to 0.40 m/s for the bottom velocity provides efficient removal
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Quantities and Characteristics of GritThe range in grit quantities varies from 0.004 to 0.037 m 3 /1000 m 3 of wastewater for separate sewersand from 0.004 to 0.18 m 3 /1000 m 3 for combined sewers.Grit solids content will vary from 35 to 80 percent with a volatile content of 1 to 55 percent. Grit from a properly operating aerated grit chamber should have a volatile content no greater than 10 percent The moisture and volatile content is influenced by the efficiency of washing.
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Grit Sump. The volume of the grit sump at the bottom of the grit chamber should be designed based on the anticipated maximum load, efficiency of collection, and grit removal frequency. The failure of grit removal systems often is not a function of the sedimentation of the particles, but rather the inability of the removal equipment to keep up with the load. As a result, settled grit is scoured out of the grit chamber.The sump side walls are set at a steep angle. Angles of 60 to 90 from the horizontal are shown in the literature.
Prepared by Zulkifli Ahmad
Aerated Grit Chamber Design Practice
Grit Removal Equipment. The four methods of removing grit from the sump are: inclined screw or tubular conveyors, chain and bucket elevators, clamshell buckets, and pumping. Currently, chain and bucket systems are seldom installed.
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