Organic Waste Recycling

Aquatic weeds and their utilization 381

b) Assuming an optimum density of the water lettuce is 20 kg (wet

weight)/m^2 , water content of the water lettuce is 90%, and the area

doubling time of water lettuce is 20 days. Determine the monthly

productivity of the water lettuce from this wastewater treatment system.

Can this harvested water lettuce from this wastewater treatment system

be consumed by the astronauts and why? The WHO microbiological

guidelines for wastewater reuse is given in Table 2.28.

7.8 A university campus in central Asia produces wastewater with the following
characteristics:

Flow rate = 3,000 m^3 /day

BOD 5 = 150 g/m^3

Fecal coliforms = 10^6 no./100 mL

Temperature = 25°C

It is required that the above wastewater be treated to attain the effluent BOD 5
and fecal coliform standards of 20 g/m^3 and 10^3 no./100 mL, respectively,
suitable for discharging to the nearby canal.

a) Determine the dimension (length × width × depth) of a subsurface flow

(SF) constructed wetlands that needs to be constructed to achieved the

above requirements of BOD 5 and fecal coliform reduction. The following

information about SF constructed wetlands and cattail plants are :

Bed porosity = 0.3

Depth of cattail roots = 0.5 m

Evapotranspiration = 50% of influent wastewater

Removal of BOD 5 and fecal coliforms in SF constructed wetlands follow

Equation 7.11

b) Suppose the same SF constructed wetland in (a) is converted to become

free-water-surface (FWS) constructed wetland by raising the effluent

outlet pipe to be 10 cm above the constructed wetland bed. The bed

porosity of the FWS constructed wetlands is 0.7, while the value of k 25

for BOD 5 and fecal coliform removal are 0.7 and 1.6 day-1, respectively,

for use in Equation 7.11