Aquatic weeds and their utilization 353Where:
Ce, Co, and t = as defined in Equation (7.4)a 1 = 1 + 4 k.t.d (7.6)
d = dispersion number, which is 0 at plug flow and infinite at complete
mix condition
k = kfs + kfb .as (7.7)
kfs = first-order rate constant for suspended bacteria, day-1
kfb = first-order rate constant for biofilm bacteria, day-1
as = specific surface area of water hyacinth ponds, m^2 /m^3
as = 1/z + 2/w + 2/l + Rs/z (7.8)
z = pond depth, m
w = pond width, m
l = pond length, m
Rs = effective root surface area per unit pond area of surface area, m^2 /m^3
For a pond with relatively large width and depth,
as = (1+Rs)/z (7.9)To use Equation (7.5) is predicting BOD 5 removal efficiency; the values of
the above parameters have to be determined experimentally or obtained from the
literature. Figure 7.13 determines the applicability of Equation (7.5) in
predicting the effluent BOD 5 concentration of a water hyacinth pond treating a
domestic wastewater and showing the significance of the biofilm bacteria
attached on the root surfaces in BOD 5 removal. To assist in the calculation, a
diagram based on Equation (7.5) and showing the relationship among BOD 5
removal efficiency, k.tand d is given in Figure 7.14. Example 7.1 demonstrates
the application of Figure 7.14 in the design of a water hyacinth pond for
wastewater treatment.
Example 7.1
A water hyacinth pond is designed to treat a wastewater whose flow rate is 500
m^3 /day and BOD 5 concentration is 200 g/m^3. Determine the appropriate
dimensions of this pond so that at least 90% BOD 5 removal is achieved. The
following information is given; d = 0.2, T = 20oC,z = 1.5 m, Rs = 1.18 m^2 /m^3 ,
kfs= 0.07 day-1,kfb =0.048 day-1.
From Equation 7.9,
as = (1+1.18)/1.5 = 1.45 m^2 /m^3