2.7.1 Design of Sludge Pit

Two categories of solids form sludge. The solids contained in the raw water which are a cause of turbidity in water and the solids of Aluminium Hydroxide. The volume of Sludge Pit depends on the rate of sludge generated. Following parameters and their typical values are given in the literature for the estimation of sludge produced from the use of Alum as coagulant:

Design Parameter
Range
Adopted
Kg of Dry Sludge / Kg of Coagulant (Alum)
Range
0.33 - 0.44
0.40
Turbidity Removal
Mg of TSS / NTU Removed
0.9 - 1.5
1.40
Sludge Solids Concentration (%)

5
Specific Gravity of Sludge

1.05
Ref: Water Treatment Principles & Design by MWH (P -1659 )

Estimation of Sludge Solids

The rate of sludge generation is estimated as follows:

Flow through each Clarifier = 11.67 Cusec
Turbidity of the incoming water = 100 NTU
Turbidity of the water leaving Clarifier = 40 NTU
Turbidity that forms part of the sludge & collected in Sludge Pit = 60 NTU

The relation between turbidity and the suspended solids is described by the following equation
TSS (mg / l) = 1.40 x NTU
= 84 mg / l
= 84 /1000
= 0.084 gm / l or 0.084 Kg / M3

Sludge generation rate from Turbidity causing Solids = 0.084 x 11.67 x 3600/ 35.28
= 100.0 Kg /Hour

Alum when added in raw water forms Aluminium Hydrooxide flocs which settle down on the floor of the Clarifier carrying turbidity causing solids with it. Sludge generation rate due to addition of alum has been worked on the basis of addition of 60 mg /l of Alum concentration in the raw water.

Hourly consumption of Alum = 60 x 11.67 x 3600 / 1000 x 35.28
= 71.45 Kg / Hr

Sludge generated due to Alum addition is calculated as follows:

Sludge production from Alum (Kg/Hr) = 0.40 x 71.45
= 28.60 Kg / Hr

Total Sludge (Kg / H) = 100.0 + 28.60
= 128.60 Kg / H

Estimation of Sludge Volume

Volume of Sludge shall be computed from the following expression:

V= Ws / ρw Ssl Ps
Where:

V = Volume of Sludge
Ws = Weight of Total Dry Solids
ρw = Density of water
Ssl = Specific Gravity of Sludge
Ps =Percentage Solids expressed as solids

Volume of Sludge = 128.60 / 1000 x 1.05 x .05
= 2.45 M3 / H
= 2.45 x 35.28
= 86.44 Cft / H

Volume of Sludge generated per day = 86.44 x 24 x 3 = 6223.70 Cft /D
(From 3 Clarifiers)

Total Flow Per Day = 11.67 x 3 x3600 x24
= 3024864 Cft
Volume of Sludge as (% of total flow) = 6223.70 x 100/ 3024864
= 0.20 %

Ref: Water Treatment Principles & Design by MWH (P -1646) states that Underflow from sedimentation tanks typically contain on the order of 0.1 to 0.3 percent of the plant flow but contains most of the solids removed.

Sizing of Sludge Pit

Two Sludge Pits shall be provided in each Clarifier. Sludge pit shall be sized to store one hour of the sludge generated and shall be cleaned every hour by opening valves installed on the Sludge Pipe connecting Sludge Pit with the outer manhole.

No of Sludge Pits = 2

Volume of Sludge Pit = 86.44 /2
= 43.22 Cft
Bottom width of the Sludge Pit = 2.0 Ft
Top Width of the Sludge Pit = 4.5 Ft
Average Width of the Sludge Pit = 3.25 Ft
Depth of Sludge Pit = 43.22 / 3.25 x3.25
= 4.0 Ft
Sludge Draw-Off Pipe

Water Treatment Principles & Design by MWH (P -814 ) recommends a minimum diameter of 6 “ for sludge draw - off pipe to prevent clogging. Sufficient head (16.5’) will be available for the quick removal of sludge from the sludge pit. It is therefore proposed to provide 3 Sludge Draw - Off pipes each of 6“ diameter in each clarifier from Sludge Pit to manhole. A Flow Control Valve along with actuator and timer shall be provided on the Sludge Draw - off for the automatic removal of sludge after a fixed interval of time. Time interval between the two sludge removals shall be fixed by the operator. Sludge Draw Off pipes shall be Polyethylene pipes.