1. DESIGN OF MECHANICAL DETRITOR TANK FOR GRIT REMOVAL
Details Value Units Formula
Average Flow 60 MLD
Peak Factor 2.25 (As per CPHEEO - give correct ref pg/table no. and estimate
population)
Peak Flow 135 MLD
Number Of Working Units 2 Nos. (Providing 100% standby units) Choose min. no. of units for techno-
economic optimisation. Give cpheeo ref pg no for 100% standby
requirement
Grit Size 0.00015 m
Specific Gravity 2.65
Settling Velocity of Grit particle 0.018 m/s Table 5.6,Pg.5-41 CPHEEO 1993
Surface Overflow Rate 959 m^3/m^2/day Table 5.6,Pg.5-41 CPHEEO 1993
Plan Area for Detritor 70.385818561001 m^2
Side Length of Square Detritor 8.4 m
Kc Constant 4 Range 3-4.5 CPHEEO Manual Page 5-42
Ss Specific Gravity 2.65
Acceleration due to gravity g 9.81 m/s^2
Diameter of Particle 15 mm
Critical Displacement Velocity of Grit 0.160929798359409 m/s Vc=(8k*(Ss-1)gd/f)^0.5
Detention time at avg flow 60 s Pg 5-42, Cl. 5.6.2.7.1.3 ,CPHEEO Manual
So, Volume of each detritor tank 20.8333333333333 m3
Liquid depth of tank required 0.3 m
Providing liquid depth 0.6 m Providing LD in the range of 0.6 - 0.9, to accommodate the Scrapper
mechanism - a manufacturing std., at the cost of excess detention time
Horizontal velocity of flow 0.155009920634921 m/s
Check for Vh < Vc: Hence Ok Vh should be less than Vc, to prevent Scouring or resuspension of grit
Actual detention time provided 121.92768 s
Grit Storage Space
Rotary Speed of scrapper 0.25 rpm
Time between scraping .e Duration of 1
rotation
4 mins (Retention time of settled Grit particles till next round of scrapping)
Grit Content 0.1 m^3/ML Grit content for domestic sewage varies from 0.05m3 to 0.15m3 per
ML
So, Flow of grit 0.00416666666666667m3/min
Quantity Of Grit required to be stored 0.0166666666666667m3 Volume = Discharge x Retention Time
Depth needed to hold grit 0.000118102796674225m Negligible. Can be accomodated in provided liquid depth.
Providing Free Board 0.5 m
Size of Detritor: 8.4 m L x 8.4 m W x 0.6 m LD + 0.5 m FB
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