Handbook of Civil Engineering Calculations

Solids (Ib/d) = (4.0 Mgd + 5.0 Mgd)(4,500 mg/L) (8.34 Ib L/mg-Mgal)

= 337,770 Ib/d = 14,074 Ib/h (6389.6 kg/h)

Therefore, the solids loading on the settling tanks at design flow is:

Solids Loading =

l

*?™^ «1.18 Ib/ft

2

-h (5.77 kg/m

2

-h)

11,889 ft

which is within the solids loading rate design criteria stated above.
Related Calculations. Liquid depth in a circular settling tank is normally meas-
ured at the side wall. This is called the sidewater depth. The liquid depth is a factor in the
effectiveness of suspended solids removal and in the concentration of the return sludge.
Current design practice favors a minimum sidewater depth of 12 ft (3.66 m) for large cir-
cular settling tanks. However, depths of up to 20 ft (6.1 m) have been used. The advan-
tages of deeper tanks include greater flexibility in operation and a larger margin of safety
when changes in the activated sludge system occur.

THICKENING OFA WASTE-ACTIVATED

SLUDGE USING A GRA VITY BELT

THICKENER

A wastewater treatment facility produces 58,000 gal/day (219.5 m^3 /d) of waste activated
sludge containing 0.8 percent solids (8,000 mg/L). Design a gravity belt thickener instal-
lation to thicken sludge to 5.0 percent solids based on a normal operation of 6 h/d and 5
d/wk. Use a gravity belt thickener loading rate of 1,000 Ib/h (454 kg/h) per meter of belt
width. Calculate the number and size of gravity belt thickeners required, the volume of
thickened sludge cake, and the solids capture in percent.

Calculation Procedure:

1. Find the dry mass of sludge that must be processed
   Gravity belt thickening consists of a gravity belt that moves over rollers driven by a vari-
   able speed drive unit. The waste activated sludge is usually pumped from the bottom of a
   secondary settling tank, conditioned with polymer and fed into a feed/distribution box at
   one end. The box is used to distribute the sludge evenly across the width of the moving
   belt. The water drains through the belt as the sludge is carried toward the discharge end of
   the thickener. The sludge is ridged and furrowed by a series of plow blades placed along
   the travel of the belt, allowing the water released from the sludge to pass through the belt.
   After the thickened sludge is removed, the belt travels through a wash cycle.
   The 58,000 gal/day (219.5 m^3 /d) of waste activated sludge contains approximately
   3933 Ib/d (1785.6 kg/d) of dry solids: See Design of a Complete-Mix Activated Sludge
   Reactor, step 3—"Compute the Quantity of Sludge Wasted," and step 6—"Compute the
   WAS and RAS Requirements."
   Based on an operating schedule of 5 days per week and 6 hours per day, the dry mass
   of sludge that must be processed is:

Weekly Rate: (3,933 lb/d)(7 d/wk) = 27,531 Ib/wk (12,499 kg/wk)