Organic Waste Recycling

232 Organic waste recycling: technology and management

Example 5.1

A municipality in Southeast Asia produces wastewater with the following
characteristics:
Flow rate = 2000 m^3 /day
BOD 5 = 100 g/m^3
NH 3 -N = 20 g/m^3

If this wastewater is to be applied to an HRAP to produce algae density (Ca)
of 200 g/m^3 , determine the following:

a) suitable dimensions (depth x width x length) of the HRAP and the

algal productivity to be obtained, assuming the optimum HRT is 5

days, Ii is 10,000 ft-candle and Į = 10-3.

b) if it is desired to double the algal productivity from this wastewater,

what measures can be done to achieve this objective?

a) From Equation 5.2,

z = ln (10,000)/(200 × 10-3) = 46.05 cm

Volume of HRAP = 2000 × 5 = 10,000 m^3

Surface area of HRAP = 10,000/0.46 = 21,739 m^2

To allow for sludge layer and freeboard, the recommended dimensions
(depth × width × length) are 0.8 × 100 × 200 m^3.
Note: To provide plug-flow and mixing conditions, baffles need to be placed
in the HRAP similar to that shown in Figure 5.5.
The algal productivity is 2000 × 200 = 400 kg/day

b) To double the algal productivity, Ca has to be 400 g/m^3 and the value of z
determined from Equation 5.2 is 23 cm. With the same wastewater flow rate of
2,000 m^3 /day, the land area of the HRAP is double of (a), or the HRAP
dimensions (depth × width × length) are 0.4 × 100 × 400 m^3.
Note: to provide sufficient nutrients for algal growth, NH 4 -N concentration in
the wastewater needs to be increased to 40 g/m^3.

Hydraulic retention time (HRT)

The optimum HRT for HRAP should be such that most nutrients are converted
into algal cells. From theoretical analysis, Oswald and Gotaas (1955) derived
the following relationship: