- Determine the required VSS reduction
The sludge age required for winter conditions is obtained from Fig. 3 using the minimum
winter temperature and required VSS reduction.
To achieve a 40 percent VSS reduction in the winter, the degree-days required from
Fig. 3 is 475
0
Gd. Therefore, the required sludge age is 475°Gd/15°C = 31.7 days. During
the summer, when the liquid temperature is 3O
0
C, the degree-days required is (3O
0
C)
(31.7 d) = 951
0
Gd. From Fig. 3, the VSS reduction will be 46 percent.
The total mass of solids processed by the digester will be 3,933 Ib/d (1785.6 kg/d)
which is the total mass of solids wasted from the treatment facility—See Design of a
Complete-Mix Activated Sludge Reactor, step 3. The total mass of VSS input to the di-
gester is:
(0.8)(3,933 Ib/d) = 3146 Ib/d (1428.3 kg/d).Therefore, during the winter:
- VSS reduction = (3,146 lb/d)(0.40) = 1258 Ib VSS reduced/d (571.1 kg/d)
- Digested (stabilized) sludge leaving the digester = 3933 Ib/d - 1258 Ib/d = 2675 Ib/d
(1214.5 kg/d).
- Compute the volume of digested sludge
The volume of digested sludge is:
V= ^
(p)(s.g.X% solids)where V= Sludge volume (ft
3
) (m
3
)
W 8 = Weight of sludge (Ib) (kg)
p = density of water (62.4 lb/ft
3
) (994.6 kg/m
3
)
s.g. = specific gravity of digested sludge (assume s.g. = 1.03)
% solids = percent solids expressed as a decimal (incoming sludge: 5.0%)
Therefore, the volume of the digested sludge is:V=
(62.41b/
6
ft3)(5T3)(0.05) =832ft3/d = 6223gal/day(23.6L/d)During the summer:- VSS reduction = (3,146 lb/d)(0.46) = 1447 Ib VSS reduced/d (656.9 kg/d)
- Digested (stabilized) sludge leaving the digester = 3933 Ib/d - 1447 Ib/d = 2486 Ib/d
(1128.6 kg/d). - Volume of digested sludge:
V = (6
2 .41b(^2) / 4
ft3)
(^6) (i^)(0.05) - ?74 ftVd - (^579) ° (^8) ^(2L9 L/d)
- Find the oxygen and air requirements
The oxygen required to destroy the VSS is approximately 2.3 Ib O 2 /lb VSS (kg/kg) de-
stroyed. Therefore, the oxygen requirements for winter conditions are: