Design of Grit Chamber with Example

Grit chambers are long narrow tanks that are designed to slow down the flow so that solids such as sand, soil, inorganic matter will settle down.

Design Aspects of Grit Chamber

These are the design aspects of Grit chamber, which needs to be satisfied during the design of grit chamber

Cleaning Interval of Grit Chamber

Cleaning Interval is of grit chamber depends on the the method used to clean the chamber i.e. hand cleaning, hydraulic cleaning, machine cleaning. Also the cleaning period will also be dependent on the local conditions like what kind of waste will be available there?, intensity and frequency of rainfall which will result in silt collection in the chamber.

Generally the cleaning interval of grit chamber must be one or two weeks.

Dimensions of Grit Chamber

The minimum depth of grit chamber must be 300mm, and the ratio between the depth and length should be 1:16.

Detention Time of Grit Chamber

It is the average time for which water remains in the Grit chamber before moving on to further treatment. Detention Time of Grit chamber is about 30 to 90seconds but for design purposes it is generally taken as 60seconds.

Velocity of Flow of Sewage in Grit Chamber

Grit chambers are designed to settle down the heavy inorganic particles by slowing down the velocity of flow here in order to slow down the velocity the area of chamber is increased. The velocity of flow should not be too high that the particles will not get sufficient time to settle down and if the velocity of flow becomes too slow then suspended organic matter will also settle down.

So to maintain the required velocity in the grit chamber the chambers can be divided into compartments, called proportional weir. Setting velocity by Stocke’s law depending on Reynold’s number can be given as

V = {{ g(ρ_s - ρ)d^2 } \over 18 μ} \\ \\  V= { g \over 18 } * {{(S_s - 1)} \over {γ}} * d \\ 

Where, S_s = Specific Gravity = (ρ_s / ρ)

γ = Kinematic Viscosity = (μ/ρ)

When the setting particles are greater than 1mm then nature of setting particles is termed as turbulent and in such case setting velocity can be termed by newton’s equation as

V = \sqrt{ 3.3 \, g \, {{ ρ_s - ρ } \over ρ} \, d}

Generally, when grit particles lie between 0.1mm to 1mm then the zone of settling is called as Transition Zone. And where the settling velocity is given by Hazen’s equation

V = 418 (g-1)d [{{3t + 70} \over 100}]

where d is in between 0.1mm to 1mm.

Also Read : Grit Chamber-Types, Working Principal | In Sewage Treatment Plant

Design of Grit Chamber Example

Question:

Design a rectangular grit chamber for a flow rate of 20mld. Specific gravity= 2.65 and size to be removed is 0.2mm and above.

• Average temperature: 20°C
• Flow-through velocity: 0.23 m/sec
• Width of grit chamber: 1.2m
• Freeboard: 0.3m
• Space for sludge accumulation: 0.25m

Answer:

here we have to calculate H and L, we have given B then if we calculate Area then we can have H. So for A we have Q = AV_h where we have given Q and V_h And to calculate L, we have L = V_h V_s , So we have V_h and need to calculate V_s.

Step 1: Settling Velocity (V_s)

V_s = d(3T + 70)

Here, d = diameter of grit particle= 0.2mm = 0.0002m, T= temperature

V_s = 0.0002(3×20+70)

V_s= 0.026m/sec

Step 2: Area of Grit Chamber (A)

We can calculate area of chamber from discharge

Q = AV_h

Q = 20mld = 20 x 10⁶ letres/day = 20 x 10³ m³/day

Q = 0.23 m³/day

now, from Q we will calculate A

Q = AV_h  \\  A= Q/V_h \\ A= (0.23/0.23)=1m^2

As we have calculated the area and we have width of chamber given then we can calculate height

Step 3: Dimensions of Grit Chamber

• If one of the quantities (B or H) is given then you can find another one.
• If any quantity is not given then take height 1m.

A= B*H \\ we \, have \, B=1.2m \\ then \, H=1/1.2 \\ H= 0.83m

To calculate length we have, L = V_h x Detention Time

So we need calculate detention time first

Step 4: Calculation of Detention Time

Detention \, Time = { Depth \over Settling \ velocity} \\  Detention \, Time = {0.83\over 0.026 } = 32sec

But when T= 20°C take, detention time = 40sec

Step 5: Calculation of Length

L= V_h \ V_s = 0.23 * 40 = 9.2m

Length of grit chamber is increased by 10 to 50% to allow non-uniformity in flow and settling of particles. It is not mandatory.

Solution:

Overall Depth = 0.83 + 0.3(Free Board) = 0.86m

Therefore dimensions of grit chamber are L x B x H = 9.2 x 1.2 x 0.86 m

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