Organic Waste Recycling

374 Organic waste recycling: technology and management

Solution:

Based on Equation 7.11, determine ‘t’ values.

For FWS: 10/200 = e-0.678*t

tFWS = 4.4 days

For SF: 10/200 = e-1.104*t

tSF = 2.7 days

Surface area (A) = (t*Q)/(d* Ȗ)

AFWS = (4.4*760) / (0.7*0.75) = 6370 m^2

ASF = (2.7*760) / (0.5*0.3) = 13680m^2

To check for hydraulic flow of SF, choose 2 SF units, each with a surface
area of 7,000 m^2 or length × width of 100 × 70 m^2. If depth of SF is 1 m, the Ac
value is 70 × 1 = 70m^2.
The value of Q/Ac is 380/70 = 5.43 m/day, less than 8.6 m/day.

Note: AFWS and ASF are surface areas of FWS and SF constructed wetlands,
respectively. Although the SF constructed wetland has higher k value than the
FWS constructed wetland, due to the low porosity (Ȗ) of the bed media, it
requires more land area than that of FWS constructed wetland.
Because of current interest in the utilization of constructed wetlands in
wastewater treatment/recycling, more comprehensive design models with
respect to BOD 5 , nutrient and faecal coliform removal that will encompass, not
only temperature but also other important parameters, should be developed.
Polprasert et. al. 1998 applied a dispersed flow model incorporating biofilm
activity (similar to Equation 7.5) to describe organic removal efficiency of a
FWS constructed wetlands with satisfactory results. They also found the biofilm
activity to be several times more significant in organic removal than the
suspended bacterial activity in the FWS constructed wetland bed.
From their surveys of more than 20 constructed wetlands sites in the USA,
Reed and Brown (1992) did not observe the need of annual harvesting of the
common emergent vegetation, but recommended that the vegetative detritus
present in the wetland beds be cleaned out on some extended schedule.
However, because vegetative plants obviously grow faster in the tropics than in
temperate climates, it would be useful to study the need and frequency of plant
harvesting for constructed wetlands located in tropical areas in accordance with
the area doubling time, as discussed in section 7.4. When subjected to frequent
harvesting, Sawaittayothin and Polprasert (2006) found N uptake by cattail
plants to be 80% of the total N input when they were cultured in a SF