The biology in biological treatment plants
Potential
activated
SIUd(je
orgamsm
No
?
Is
flushed
out
Dies
No
Dies of
cold/heat
Dies
of
starvation
No
Is
washed out
No
Dies
Activated
slud9e
orgamsm
Fig 3.2
Selection in an activated-sludge plnnt.
b. substrate
c. settling or
flocculation
characteristics
60
under the varying- however predominantly aerobic - conditions.
In the denitrifying plant, nitrate will in certain periods be the electron acceptor. In that case bacteria which can utilize both oxygen and
nitrate as electron acceptors will have favourable conditions.
The comments made in the previous section on selection in biofilters
also apply here. The organisms must be able to utilize the primary
and/ or secondary substrate. In respect of biological phosphorus
removal, Acinetobacter are selected by their ability to take up small
organic molecules (acetic acid, alcohols etc.) under anaerobic conditions. Other bacteria which are normally found in activated sludge
do not have this ability I 6 I.
Settling forrns an integral part of an activated sludge process. If an
organism is sufficiently large and heavy, it will settle by itself and
hence be retained in the plant. If the organism, however, is small and
light, it must join with other similar organisms to be able to return
to the reaction tank through a floc formation.