TM 5-814-3/AFM 88-11, Volume III
Temperature correction = 0.46 (from 20EC to 10EC);
pH correction = 0.50 (pH 8.5 to 7.0);
' 774 mg/L.
MLVSS concentration =
This concentration is low; the sludge age would be too short to give a stable performance. To obtain
a stabilized sludge:
Use 3200 MLVSS,
MLVSS = 3200 ' 4000 mg/L.
Check sludge age (with negligible sludge wasting):
' 16 days.
This sludge age is considered adequate.
h. Check size of final clarifier with solids loading.
Spiraflow clarifier allows 30 lb solids/sq ft-day.
(441 gpd/sg ft)(8.34)(4000)
Actual solids loading =
' 29.4 lb/sq ft&day < 30.
i. Adjustable oxidation ditch weir. A weir on each oxidation ditch is provided and sized so that the
head differential over the weir between the maximum and minimum flow rates is less than 1.25 in.
Maximum flow rate = Qpeak + maximum recirculation rate;
Minimum flow rate = Qmin + recirculation rate.
For average daily flows between 200,000 gal/day to 1 mgd, the length (L) of the overflow weir is:
3.5 Qavg gpm
' 11.9, use 12 ft.
j. Calculate the return sludge flow rate, QR:
The equipment manufacturer recommends the following:
Minimum pumping capacity = 0.25Qavg = 0.25347.2 gpm = 87 gpm.
Maximum pumping capacity = 1.0Qavg = 1.0347.2 gpm = 347 gpm.
2 pumps, each with 44 to 174 gpm capacity.
k. Calculate amount of sludge for disposal. The amount of sludge wasted is the same as in the vertical-
shaft aerator example since the influent and effluent characteristics and the MLVSS concentrations are
Px = 683.2 lb/day of VSS from 2 oxidation ditches.
Total sludge 933.4 lb/day dry solids from 2 oxidation. ditches.
2 pumps, each with a capacity of 120 gpm for a 52-min/day wasting schedule as in the vertical-shaft
Sizing of sludge drying beds.
1 250 8.34
BOD population equivalent =
' 12,265 persons;
Drying bed area = 12,265 1 sq ft/cap = 12,265 sq ft.
Use 10 drying beds, each at 35 ft 35 ft.
Total area = 12,250 sq ft.