Handbook of Civil Engineering Calculations

Therefore, the volume of thickened waste activated sludge exiting the gravity belt thick-

ener is 11,502 gal/day (43.5 m

3

/d) at 5.0 percent solids.

4. Determine the solids capture
   The solids capture is determined using the following:

(^0) Solids Capture ,., _ /n/xF v (%) Solids = ——: in Feed - Solids : in x Filtrate /1/W (100%)w»/\
' Solids in Feed
v
'
Using values defined previously:
Solids Capture <%) = 5506 Ib/d - [(75,458 &No> lb/gal)(1.0)(0.009)]
x (100%) = 89.7%
Since only 89.7 percent of the solids entering the gravity belt thickener are captured,
the thickener will actually be required to operate (360 min/d)/(0.897) = 400 min/d (6
hours 40 minutes per day) five days per week in order to waste the 3,933 Ib/d (1785.6
kg/d) of dry solids required. This implies that the actual volume of thickened sludge will
be (11,502 gal/day)/(0.897) = 12,823 gal/day (48.5 m^3 /d). The actual filtrate flow rate will
be (75,458 gal/day)/(0.897) = 84,123 gal/day (318.4 m^3 /d).
The thickened sludge is generally pumped immediately to sludge storage tanks or
sludge digestion facilities. If the thickener is operated 6.67 h/d, the thickened sludge
pumps (used to pump the thickened sludge to downstream processes) will be sized based
on the following thickened sludge flow rate:
=^12 (6.67 h/d)(60^f^7 mm/h)X - &^32 «•»,P.02L/s) v '
Hydraulic (thickened sludge) throughput for a gravity belt thickener ranges from 25
(1.6 L/s) to 100 (6.3 L/s) gal/min per meter of belt width. The filtrate flow of 84,123
gal/day is generally returned to the head of the wastewater treatment facility for repro-
cessing.

DESIGN OF AN AEROBIC DIGESTER

An aerobic digester is to be designed to treat the waste sludge produced by an activated
sludge wastewater treatment facility. The input waste sludge will be 12,823 gal/day (48.5
L/d) (input 5 d/wk only) of thickened waste activated sludge at 5.0 percent solids—See
Thickening of a Waste Activated Sludge Using a Gravity Belt Thickener. Assume the fol-
lowing apply:

1. The minimum liquid temperature in the winter is 15
   0
   C (59
   0
   F), and the maximum liq-
   uid temperature in the summer is 3O
   0
   C (86
   0
   F).


2. The system must achieve a 40 percent Volatile Suspended Solids (VSS) reduction in
   the winter.


3. Sludge concentration in the digester is 70 percent of the incoming thickened sludge
   concentration.


4. The volatile fraction of digester suspended solids is 0.8.