Organic Waste Recycling

372 Organic waste recycling: technology and management

Figure 7.18 Typical cross-section of SF constructed wetlands (U.S. EPA 1988)

It is apparent from the k 20 values that the SF constructed wetlands are more
efficient in the removal of BOD 5 than the FWS constructed wetlands. This
superior efficiency is obviously due to better contact between the liquid
wastewater and the micro-organisms (mainly bacteria) present in the SF wetland
beds responsible for BOD 5 biodegradation, including the filtration and
adsorption mechanisms.
It is important to point out that the value of ‘t’ determined from the Equation
(7.11) is the time the liquid stays in the void spaces of the constructed wetland
beds. Determination of the total bed volume has to include the porosity
parameters (Ȗ) as follows:

Ȗ = Vv/Vt (7.12)

Vt = Q .t/Ȗ (7.13)

Where,

Vvand Vt = void volume and total bed volume, respectively, m^3.

Q = wastewater flow rate, m^3 /day

Ȗ = porosity, unitless

The values of Ȗ for some bed media are given in Table 7.13, which indicate
the larger the media diameter, the higher the Ȗ values. Due to the level of free
water maintained in the FWS constructed wetlands (10 – 60 cm), their Ȗ values
are in the range of 0.65 – 0.75 (WEF 2001).
Since the effect of evapotranspiration (section 7.4) can be significant during
the summer and windy periods, the effluent flow rate of a constructed wetland
may be much less than that of the influent flow rate. In this respect, the value of
Q used in the calculation of Vt (Equation 7.13) should be the average between
the influent and effluent flow rates. Depth of a constructed wetland should be

CATTAILS

SLOTTED PIPE FOR

WASTEWATER

DISTRIBUTION

INLET STONE

DISTRIBUTOR

SOIL OR

GRAVEL

EFFLUENT OUTLET

HEIGHT VARIABLE

WATERTIGHT MEMBRANE

RHIZOME NETWORK

SLOPE 1%