ven by the corresponding values of the influent, i.e. SI,in and XI,in respectively. These values are given in the Appendix.
Fig. 2 shows the steady-state value of the chemical oxygen demand as a function of the residence time. The COD decreases
steadily as the residence time is increased from the value at the first branch point, 0.19 days, towards the value at the
second branch point, 3.29 days. However, the decrease is only marginal for values of the residence time greater than
that at the second branch point. The dotted horizontal line gives CODtotal = 125 mg l−1
. Hence to satisfy the legislative
requirement the residence time must be chosen such that total COD is below this dotted line. The residence time at which
CODtotal = 125 mg l−1
is 1.67 days. Thus, in order to satisfy the relevant legislation for COD, the biological reactor must be
operated at a residence time higher than 1.67 days. At the second branch point BP2 the value of COD is 9.38 mg l−1
. Thus
operating the reactor at a residence time of 3.29 days ensures that the value of COD in the effluent is only around 7.5% of the
legislative requirement. If the residence time is doubled from the value at BP2 to 6.58 days, then the COD decreases only to
6.92 mg l−1
.