Design of biofilters
5.9 .4 Design of biofilters for dissolved organic matter
Today there are no design criteria for biofilters for the removal of organic matter,
based on the biofilm theory. In the first section of this chapter it was suggested how
a biofilter for the removal of organic matter from wastewater can be designed. The
design takes place in 4 stages. (The calculation procedure for stages I and II are
shown in Fig 5.9).
Stage 0: Determine the order of reaction in the biofilm itself. It is typically zero order.
Stage I: Calculate, using Expression (5.32), whether oxygen or organic matter is
potentially limiting.
Stage II: Calculate whether the biofilm is fully active (~;?: 1) or only partially active
(~ < 1 ). Often we will find that the biofilm is only partially active, that is, ~
<1.
Stage III: The removal in the filter can be calculated by means of a mass balance, for
example Expression (5.50), for an ideally mixed plant. The removal rate for
organic matter, rA,S. or oxygen, rA,O:b is calculated using one of the 4
expressions below.
For ~;?: 1 the consumption of substrate
rA,s= kovf,s· L
or if, instead, we look at the oxygen consumption
rA,o2= kovt,o2· L
kovf,s
where kovf,OZ = - -
Voz,s
For ~<1 and organic matter as a limiting factor
rA,S = (2 · Ds,2 · kovf,S .Sz) 112 = k112A,s· S2 112
For ~<1 and oxygen as a limiting factor
rA,o2= (2 · Do2,2 · kovt,o2· So2,il 112 = k112A,o2· So2.i 12
(5.61a)
(5.61b)
(5.61c)
(5.61d)
(5.61e)
A mass balance for organic matter over the plant with for example organic matter
as the limiting factor is then (with symbols from Fig 5.13):
(5.62)
In Table 5.5, half order rate constants for both removal of organic matter and
nitrogen are stated.
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