Organic Waste Recycling

Aquatic weeds and their utilization 367

To compensate for other factors, choose the harvesting interval to be once
every 10 days, and to maintain continuous duckweed growth, only 50 % of the
total duckweeds or 50 % of the pond area are to be harvested.
Amount of duckweeds to be harvested each time = (400 x 10^4 g)/2
= 2 tons
Monthly harvest = 2 x 30 /(20)
= 6 tons wet mass
If duckweeds contain 95% water, dry mass = 6 x 0.05
Amount of duckweeds = 0.3 tons dry mass

The yield of duckweeds is 0.3 tons dry mass / (ha-month).

Example 7.4

Wastewater from a food-processing factory has the following characteristics:

Flow rate = 100 m^3 /day

COD = 300 mg/L

BOD 5 = 200 mg/L

NH 4 + - N = 10 mg/L

PO 4 3- - P = 5 mg/L

The factory plans to convert this wastewater to produce protein biomass
using the existing pond system that has an area of 1.2 ha. The available
alternatives are to grow either water hyacinth or duckweeds. Determine a
suitable weed option that will give more financial return annually.
The following information are given:

Water hyacinth
Average stock density = 2 kg/m^2
Area doubling time = 10 days
Protein content = 17% by weight (dry)
Harvesting is done by manually (no addition cost)
Market price of protein = 1 Baht/kg

Duckweeds
Average stock density = 0.7 kg/m^2
Area doubling time = 15 days
Protein content = 20% by weight (dry)
Harvesting cost = 100 Baht/ton dry wt.
Market price of protein = 3 Baht/kg