Environmental Engineering Reference
In-Depth Information
sludge rate. The results of this calculation can then be adjusted based
on sampling and visual observations to develop the optimum return
sludge rate.
Note:
The %SSV
60
must be converted to a decimal percent and total flow
rate, and wastewater flow and current return rate in million gallons per
day must be used.
⎡
Influent Flow (
MGD)
Current Return Flow (MGD)
⎤
Estimated Return Rate (MGD)
=
⎥
×
%SSV
6
⎢
0
+
⎣
⎦
•
60
is representative.
Assumes that the return rate, in percent equals %SSV
Assumes that %SSV
•
60
.
The actual return rate is normally set slightly higher to ensure that
organisms are returned to the aeration tank as quickly as possible. The
rate of return must be adequately controlled to prevent the following:
•
Aeration and settling hydraulic overloads
•
Low
MLSS levels in the aerator
•
Organic overloading of aeration
•
Solids loss due to excessive sludge blanket depth
Example 8.2
Problem:
The influent flow rate is 4.2 MGD and the current return acti-
vated sludge flow rate is 1.5 MGD. The SSV
60
is 38%. Based on this infor-
mation, what should be the return sludge rate in million gallons per day
(MGD)?
Solution:
Return Rate = (4.2 MGD + 1.5 MGD) × 0.38 = 2.2 MGD
8.14.3 sludge volume index
The sludge volume index (SVI) is a measure of the settling quality
(a quality indicator) of the activated sludge. As the SVI increases, the
sludge settles more slowly, does not compact as well, and is likely to
result in an increase in effluent suspended solids. As the SVI decreases,
the sludge becomes denser, settling is more rapid, and the sludge is
becoming older. SVI is the volume in milliliters occupied by 1 gram of
activated sludge. The settled sludge volume (mL/L) and the mixed liquor
suspended solids (mg/L) are required for this calculation:
SSV (mL)
×
(mg/L)
1000
Sludge Volume Index(SVI)
=
(8.8)
MLSS
