Grit chambers are of two types: mechanically cleaned and manually cleaned. In mechanically cleaned grit chamber, scraper blades collect the grit settled on the floor of the grit chamber. The grit so collected is elevated to the ground level by several mechanisms such as bucket elevators, jet pump and air lift. The grit washing mechanisms are also of several designs most of which are agitation devices using either water or air to produce washing action. Manually cleaned grit chambers should be cleaned atleast once a week. The simplest method of cleaning is by means of shovel.
An aerated grit chamber consists of a standard spiral flow aeration tank provided with air diffusion tubes placed on one side of the tank. The grit particles tend to settle down to the bottom of the tank at rates dependant upon the particle size and the bottom velocity of roll of the spiral flow, which in turn depends on the rate of air diffusion through diffuser tubes and shape of aeration tank. The heavier particles settle down whereas the lighter organic particles are carried with roll of the spiral motion.
Grit chambers are nothing but like sedimentation tanks, designed to separate the intended heavier inorganic materials (specific gravity about 2.65) and to pass forward the lighter organic materials. Hence, the flow velocity should neither be too low as to cause the settling of lighter organic matter, nor should it be too high as not to cause the settlement of the silt and grit present in the sewage. This velocity is called "differential sedimentation and differential scouring velocity". The scouring velocity determines the optimum flow through velocity. This may be explained by the fact that the critical velocity of flow 'vc' beyond which particles of a certain size and density once settled, may be again introduced into the stream of flow. It should always be less than the scouring velocity of grit particles. The critical velocity of scour is given by Schield's formula:
V = 3 to 4.5 (g(Ss - 1)d)1/2
A horizontal velocity of flow of 15 to 30 cm/sec is used at peak flows. This same velocity is to be maintained at all fluctuation of flow to ensure that only organic solids and not the grit is scoured from the bottom.
The settling velocity of discrete particles can be determined using appropriate equation depending upon Reynolds number.
Stoke's law: v= g(Ss-1)d2
18u
Stoke's law holds good for Reynolds number,Re below 1.
Re=vd
u
For grit particles of specific gravity 2.65 and liquid temperature at 10°C, u=1.01 x 10-6m2/s. This corresponds to particles of size less than 0.1 mm.
Transition law: The design of grit chamber is based on removal of grit particles with minimum size of 0.15 mm and therefore Stoke's law is not applicable to determine the settling velocity of grit particles for design purposes.
v2 = 4g(rp-r)d
3 CDr
where, CD= drag coefficient Transition flow conditions hold good for Reynolds number,Re between 1 and 1000. In this range CD can be approximated by
CD= 18.5 = 18.5 Re0.6 (vd/u)0.6
Substituting the value of CD in settling velocity equation and simplifying, we get
Primary sedimentation in a municipal wastewater treatment plant is generally plain sedimentation without the use of chemicals. In treating certain industrial wastes chemically aided sedimentation may be involved. In either case, it constitutes flocculent settling, and the particles do not remain discrete as in the case of grit, but tend to agglomerate or coagulate during settling. Thus, their diameter keeps increasing and settlement proceeds at an over increasing velocity. Consequently, they trace a curved profile.
The settling tank design in such cases depends on both surface loading and detention time.
Long tube settling tests can be performed in order to estimate specific value of surface loading and detention time for desired efficiency of clarification for a given industrial wastewater using recommended methods of testing. Scale-up factors used in this case range from 1.25 to 1.75 for the overflow rate, and from 1.5 to 2.0 for detention time when converting laboratory results to the prototype design.
For primary settling tanks treating municipal or domestic sewage, laboratory tests are generally not necessary, and recommended design values given in table may be used. Using an appropriate value of surface loading from table, the required tank area is computed. Knowing the average depth, the detention time is then computed. Excessively high detention time (longer than 2.5 h) must be avoided especially in warm climates where anaerobicity can be quickly induced.
Design parameters for settling tank
Types of settling
Overflow rate m3m2/day
Solids loading kg/m2/day
Depth
Detention time
Average
Peak
Average
Peak
Primary settling only
25-30
50-60
-
-
2.5-3.5
2.0-2.5
Primary settling followed by secondary treatment
35-50
60-120
-
-
2.5-3.5
Primary settling with activated sludge return
25-35
50-60
-
-
3.5-4.5
-
Secondary settling for trickling filters
15-25
40-50
70-120
190
2.5-3.5
1.5-2.0
Secondary settling for activated sludge (excluding extended aeration)
