93 A batch settling test has been performed on an industrial

9.3 A batch settling test has been performed on an industrial wastewater having an initial suspended solids of 597 mg/C to develop criteria for the design of a primary clarifier. The test column was 5in (125 mm) in diameter and 8ft (2.44 m) high, and sampling ports were located 2, 4, 6, and 8 ft (0.61 1.22, 1.83, and 2.44 m) from the water surface in the column. The suspended solids remaining after the various sampling times are given in Table 9.11. The wastewater flow is 2.5 MGD (9460 m3/d) Determine: a. The design overflow rate and detention time if 65% percent of the suspended solids are to be re- moved. Use scale-up factors of 0.65 for the overflow rate and 1.75 for the detention time.

Solution

the overflow rate can be defined as total flow per unit area

area = (2*Pi*d*h)/2 i.e (2*3.14*0.125*2.44)/2 =0.9577m²

dischare or total flow = 9460*0.65(65% is to be removed)= 6149m³/d

overflow rate =6149/0.9577= 6420.59m/d

now detention time is = volume of tank / flow rate

volume= (Pi*d²*h)/4 = (3.14*0.125²*2.44)/4= 0.0299*1.75 = 0.0523m³

flow= 6149m³/d

detention time= 0.0523/6149 = 0.00008619days = 0.00008619*24*60*60 = 7.44 sec

b. if a circular clarifier is used volume will remain constant i.e 0.0523

it implies that (3.14*d²*2.44)/4 = 0.0523

from this d = 0.165m = 165mm

d. similarly the depth can be seen as (3.14*.125²*h)/4= 0.0523

h=4.26m