where rA,S is the removal rate per unit area of carrier
(unit for example kg COD/(m 2 · d)),
A2• is the overall area of the carrier (for example m 2 ).
5.6.2 Biofilters with recycle
Biofilters
(5.51)
Most biofilter plants have recycle which is normally made by recycling directly over
the filter (Fig 5.13). The recycle ratio, R, is defined as
(5.52)
The purpose of recycling is to ensure a suitable flow of water through the filter. This
is important for most types of biofilter plants. Also, the recycle lowers the influent
concentration to the biofilter and can therefore affect the process, both through the
order of reaction and as to whether oxygen is limiting or not.
The equation for the removal of substrate will be the same as for biofilters without
recycle, but the reaction term will be influenced by the recycle.
0 1
01 2 v;x.~~2.s
0 3
•
0 4
c 1
c 1.2
c3
::J
c4
s1
s 1.2
s3
s4
x1
X1.2
0 6 C 6 S 6 X 6
~
x4
0 5C5SsXs
Fig5.13 Biofilter with recycle.
5.7. Concepts and definitions
The concepts and definitions for biofilters are in most cases identical to those used
in activated sludge plants.
Treatment efficiency is defined as (see Fig 5.13)
(5.53)
Similar to the treatment efficiency in activated sludge plants, the treatment efficiency includes the effect of the secondary settling.
Recycle ratio is defined in Expression (5.52).
Volumetric loading rate is defined as
(5.54)
165