Organic Waste Recycling

366 Organic waste recycling: technology and management

Example 7.2

Design a water hyacinth treatment system to treat the following wastewater.
Wastewater characteristics:

BOD 5 = 200 mg/L

COD = 300 mg/L

Flow Rate = 100 m^3 /day

Choose organic loading = 25 kg BOD/ (ha-day) (From Table 7.9)

Area required = 100 x 200 x 10-3/ (25)

= 0.8 ha

If two basins are to be constructed,

Area of individual basin = 0.4 ha

Length: width = 4: 1

Width = (0.4 x 10^4 /4)0.5 = 31.6 m

Choose width = 32 m

Length = 125 m

Choose depth = 1.2 m (Root depth = 0.9 m; bottom layer = 0.3 m

Check with data in Table 7.9

Hydraulic loading rate = 100/(2 x 0.4)

= 125 m^3 /(ha-day)

Hydraulic retention time = 4000 x 1.2 x 2 / (100)

= 96 days

The design is acceptable.

Example 7.3

Estimate the yield of duckweeds from an aquatic treatment system treating
domestic sewage.

Area of ponds = 1 ha (or 10^4 m^2 )

Doubling time of duckweed = 10 days

Choose stocking density = 400 g/m^2 (wet weight)

Note:

• Stocking density reported in the literature varies from 200 g/m^2 to
  700 g/m^2


• Lower initial density may cause algal blooms that could suppress
  the growth of duckweed.


• Harvesting interval has to be chosen such that the growth of
  duckweed is not reduced due to overcrowding.