Organic Waste Recycling

352 Organic waste recycling: technology and management

difficult in aquatic treatment systems because of complex flow patterns and
volume displacement attributable to the plants. Generally, hydraulic retention
times reported in the literature are based on the assumption of their ideal
complete mix or plug flow hydraulics, neither of which are valid assumptions.
In addition, no attempt has been made to account for plant volume displacement
in the hydraulic retention times reported in the literature. Very little data are
available for hydraulic characterization of existing aquatic systems. In summary,
while hydraulic retention time has been used widely as an aquatic system design
parameter, little insight into how the hydraulic conditions affect the efficiency of
pollutant removal mechanisms can be derived from its usage.
Based on performance data of water hyacinth ponds used to treat domestic
wastewaters on San Diego, California, U.S.A. and elsewhere, it was suggested
that if the length/width ratios of the ponds are not great (probably less than 3/1),
a first order complete mix reaction may be applied (WEF 2001) as shown in
Equation 7.4.

C kt

C

t

e

+

=

1

1

0

(7.4)

Where:

C 0 = Influent BOD 5 concentration, mg/L

Ce = Effluent BOD 5 concentration, mg/L

kt = first order BOD 5 removal rate constant at temperature ToC, day-1

T = Liquid temperature,^0 C

k 20 = first order BOD 5 removal rate constant at 20o C, day-1

kT = k 20 (1.06)T-20

t = hydraulic retention time, days

The estimated k 20 value is 1.95 day-1

To describe the process kinetics of water hyacinth ponds in more details,
Polprasert and Khatiwada (1998) developed an integrated kinetic model which
encompasses the effects of both the suspended and biofilm bacteria and the
hydraulic dispersion number on the BOD 5 removal efficiency. Based on a
dispersed flow model, an equation to predict BOD 5 removal efficiency is shown
in Equation 7.5.

a d a d

d

e

a e a e

ae

C

C

/ 2

1

/ 2

1

1 / 2

1

0

( 1 )^1 ( 1 )^1

2

− −

+ − −

= (7.5)