Design of Grit Chamber -Environment Civ105- 15

Term Paper CIV 105 ENVIRONMENTAL SCIENCE & ENGINEERING -II Topic: - Design of Grit Chamber

Submitted To Mr. Kiran Kumar

Submitted By Atish Kumar Roll No.:- 15 Class: - Diploma Civil Semester: - 5th

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Contents 1) Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 3 2) Types Of Grit Chamber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3) Principle of Working of Grit Chamber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4) Types of Velocity Control Devices . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 6 5) Design of Grit Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . 7 6) Method Of Cleaning . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7) Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

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Introduction Grit chambers:Grit chambers are basin to remove the inorganic particles such as sand, gravel and mineral matter to prevent damage to the pumps, and to prevent their accumulation in sludge digestors. Grit Chamber is usually constructed for this purpose, as this is needed to protect major mechanical equipment from wear and tear including damage. Other than that, it also ensures that formation of deposit especially in pipelines can be avoided and overall this can save the time that otherwise would be spent to clean the accumulation of waste.

Grit chambers

Types Of Grit Chamber:1) Horizontal Flow Type :The design of a horizontal flow type is such that as wastewater flows through in horizontal direction at a certain velocity (0.3m/s) particles will start to settle at the channel before reaching the outlet point. Normally it is constructed with consideration put in mind to remove particles that would otherwise be trapped on a 0.21 diameter mesh size. Once the grit settles to the bottom of the chamber, it can then be removed out from the system, using conveyor with buckets or plows.

Application :a) This chamber have horizontal straight line flow velocity, which is kept constant by over varying discharge b) Simple operation in treatment process

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2) Aerated Grit Chamber :An Aerated Grit Chamber offers a low-cost, echanically simple way to remove grit from a wastewater stream. Air is introduced into an eductor tube which is located near the bottom of a grit chamber thereby creating a toroidal flow pattern. The continuous rising flow deflects off an energy-recovery baffle at the liquid surface. This flow pattern causes the grit to settle to the bottom of the chamber while keeping lighter organic material in suspension to be processed further downstream. Since the driving force is a blower, this type of grit removal has no moving parts below the surface which means that maintenance is significantly reduced. Once the grit has settled, either a recessed-impeller grit pump or, more commonly, an air-lift pump is used to remove the grit slurry and send it on for dewatering. The air-lift pump option is particularly advantageous since the air supply needed for operation of the aerated grit chamber is already in place

Application:a) Extremely simple mechanical design b) No moving parts below the water surface c) Can use the blower air for air – lift pumping as well d) Possible septic condition of the plant influent may be alleviated through pre-preparation in the grit chamber

Aerated Grit Chamber

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3) Vortex grit chamber:The vortex grit separator efficiently removes grit from the treatment plant influent stream. A vortex grit separator usually follows screening equipment. The screened influent enters tangentially and flows around the upper chamber. Adjustable, rotating paddles augment the spiraling flow to create a mechanically induced vortex which settles the grit, transports it to the center opening of the fixed floor plate for collection in the lower chamber, and lifts and returns the lighter organic particles to the main flow. The grit solids are removed from the lower chamber by an air lift or recessed impeller pump for further washing and dewatering.

Application:a) b) c) d)

Efficient grit removal over variable flow rate Rugged precision bearing drive Compact design reduce civil work expense Low energy consumption Simple and inexpensive maintenance requirement

Vortex grit chamber

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Principle of Working of Grit Chamber: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 are scoured from the bottom.

Types of Velocity Control Devices :1. A sutro weir in a channel of rectangular cross section, with free fall downstream of the channel. 2. A parabolic shaped channel with a rectangular weir. 3. A rectangular shaped channel with a parshall flume at the end which would also help easy flow measurement.

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Design of Grit Chamber :1) Velocity of Flow Velocity of wastewater flow must be 0.3 m/sec. This will permit the deposition of the bulk of heavier mineral solids in wastewater while most organic matter remains in suspension. A velocity of flow in the range 0.15 - 0.3m/sec. is generally recommended. In order to keep the velocity within the desirable limits, it is usually necessary to provide two or more channels to manage fluctuations in sewage flow.

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 106m2/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 v = [0.707(Ss-1)d1.6 u-0.6]0.714

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2) Period Of Detention:One minute (volume of the grit chamber/flow rate) is the detention time normally employed. Since sedimentation of granular solids is dependent to a large extent upon the surface area of the aerated grit chamber, their width could be kept large. A length to width ratio of 8 to 1 may be used limiting the effective depth to about 2 m.detention time of grit chamber varies 4-6 minutes

Design criteria:Depth range Length Bidth Peak flow rate Average flow rate Detention time Volume Detention time calculation

3 - 5m 10 – 20m 1.5 × depth 1.3 m3/s 0.5 m3/s 4 – 6 min Peak flow rate × Detention time Volume /average flow rate

3) Method Of Cleaning:Grit chambers can be cleaned in two ways:i.

Mechanically 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.

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Manually cleaned: - Manually cleaned grit chambers should be cleaned at least once a week. The simplest method of cleaning is by means of shovel.

4) Grit Storage:Storage space for grit may be provided throughout the length of the aerated grit chamber or by means of one or more pits deeper than the remainder of the basins. Concentration of grit is also useful for cleaning purposes. Channel may be provided with a frequency of cleaning of 15 days.

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Reference     

http://nptel.iitm.ac.in/courses/Webcourse-contents/IITKANPUR/wasteWater/Lecture%2022.htm#Grit Chambers http://pars-treatment.com/news/view.asp?newsID=114 http://www.westech-inc.com/product.cfm?ItemID=46&Cat=10 http://www.hitachi-pt.com/products/si/water/activity/sewage.html http://ezinearticles.com/?Aerated-Grit-Chamber---Perfect-Grit-Chamber-Design-ForWastewater-Treatment&id=2523523

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