Aquatic weeds and their utilization 355From Equation 7.7,
k = 0.07 + 1.45 (0.048) = 0.14 day-1
From Figure 7.15, for the 10% BOD 5 remaining and d = 0.2, the value of k.t
is 4.
Therefore the value of t is 4/0.14 or 28.6 days.
Chose a “t” value of 30 days, the pond area is 500 × 30/1.5 = 10,000 m^2. The
dimension of this water hyacinth pond is 200 × 50 × 1.5 m^3 (length × width ×
depth). Note: a freeboard of about 0.5 m should be added to the depth of this
pond.
Check for BOD 5 loading rate = 500 m^3 /d × 200 g/m^3 × 1/10,000 m^2
= 100 kg/(ha-day)
Note: the value of organic loading rate determined from this example is
within the range shown in Figure 7.15 and should result in about 90% BOD 5
removal. If, due to evapotranspiration, the effluent flow rate value is much less
than 500 m^3 /day, average between the influent and effluent flow rate should be
used in calculating the pond dimensions.
Hydraulic loading rate
Hydraulic loading rate, expressed in units of m^3 / (m^2 -day), is the volume of
wastewater applied per day per unit surface area of the aquatic system. Most
likely, the use of hydraulic loading rate stems from its popular use in land
application systems. Typically, aquatic systems are operated in a continuous
flow manner and as a result hydraulic loading or dosing rate is not a pertinent
design parameter.
Hydraulic application rate
Hydraulic application rate refers to the volumetric flow rate of wastewater
applied to the aquatic system per unit cross sectional area of the reactor
assuming the wastewater is applied uniformly to the system. The unit for this
parameter is m^3 / (m^2 -day), which reduces to an expression of fluid velocity.
Hydraulic application rate has not been used widely, but offers a much better
unit of comparison for system performance data than the aforementioned
parameters for several reasons. One reason hydraulic application rate should be
a better design parameter is because hydraulic application rate is a gauge of the
fluid velocity that is thought to have a significant effect on removal mechanisms
operative in aquatic systems. In addition, hydraulic application rate can be
compared directly from system to system.