426 Organic waste reuse and recycling: technology and management
shift. In this case, the entire area would receive the design daily flow during 8-
hour application period. Storage facilities would be required to hold wastewater
during the 16 hr non-operating period.
Example 8.4
Determine the area required for land treatment of wastewater using the overland
flow method.
The following information is given: screened raw wastewater is to be used;
the flow (Q) is 3,000 m^3 /day; the influent BOD 5 (Co) is 200 mg/L; and the
required effluent BOD 5 (C) is 20 mg/L.
The necessary design calculations are as follows:
Compute the required removal ratio, (C - 5)/Co, from Equation 8.8:
(C - 5)/Co = (20 - 5)/200 = 0.07
Select an application rate, q, in valid range of the model:
q = 0.25 m^3 / (hr-m)
Determine required value of slope length, z, using Figure 8.9:
z = 37 m
Select application period, Pd.
Pd = 8 hr/day
Compute q for area calculation
q = (0.25 m^3 /(hr-m)/1.5
q = 0.17 m^3 /(hr-m)
Compute required total area from Equation 8.14. Assume 7 days/wk
application frequency.
Area = (Q) (z)/ (q) (Pd)
Area = (3,000 m^3 /day) (37 m)/(0.17 m^3 /(hr-m))(8 hr/day)
= 81,620 m^2 ≈ 8.2 ha
Comparison of current design based on hydraulic loading
Assume the hydraulic loading = 3 cm/day (Figure 8.1)
Land area required is given by:Area = 3.65 Q/Lh (8.14)
Where,
Area = Wetted land area, ha;
Q = Design flow, m^3 /day; and
Lh = Hydraulic loading, cm/yr.