A continuous flow bioreactor is a well-stirred vessel containing
microorganisms (X) through which a substrate (S)
flows at a continuous rate.
The microorganisms grow through the consumption of the substrate,
producing more microorganisms and products.
The products will typically contain carbon dioxide, nitrogen,
water and other
species, including biological compounds, specific to the process
under consideration. The nature of these products is unimportant in this study.
Unused substrate, microorganisms,
and products flow out of the reactor. The use of a continuous flow
bioreactor to treat sewage or industrial wastewaters is known as the
activated sludge process.
One drawback associated with the activated sludge process is the
production of `sludge'.
Traditional methods for disposing of excess
sludge, which include incineration, the use of landfill sites and
dumping at sea, are becoming increasingly regulated in many countries
due to environmental concerns about the presence of potentially toxic
elements in the sewage sludge. Furthermore, a combination of
the limited amount of land available for landfill, particularly
in urban areas, with stringent legislation has seen the economic costs of using
landfill sites to increase sharply. It should be noted that incineration
does not eliminate the need for landfill sites as
a product of incineration is an ash containing high heavy materials content
and general toxicity.
Thus there is a pressing need, and growing interest,
in methods that reduce the volume and mass of excess sludge
produced as part of biological wastewater treatment processes.
A promising method to reduce excess sludge production is to
increase the biodegradability of the sludge by
disintegrating it within the reactor.
This approach works primarily by causing the disintegration of
bacterial cell walls.
Among the many techniques that have been
reported for application to the activated sludge process,
chemical treatments and ozone treatments have been the most
widely adopted commercially [Oh et al, 2007].
In processes involving ozonation
a part of the sludge is removed from the
reactor and treated with ozone in a sludge disintegrator. This ozonation
stage converts the live sludge into a mixture of soluble substrate
The liquidized sludge is then returned to the bioreactor as a feed solution
where the soluble substrate is biodegraded by live sludge.
These techniques have shown to lead to much lower levels of
MLSS (mixed liquor suspended solids).
A simple model is considered for a reactor cascade in which each reactor
may be connected to both a settling unit and a sludge disintegration
unit (SDU). The sludge disintegration unit is not modelled per se. Instead
sludge disintegration terms are added to a conventional activated sludge model.
These terms assume that the disintegrator unit destroys the biochemical
activity of the sludge, converting a fraction, α, directly into usable
substrate and the remainder, (1-α), into organic particulates. We obtain aqualitative understanding of the performance of the process
by finding the steady-state solutions of
the model and determining their stability.
For a specified mixed liquor suspended solids
(MLSS) content the values of the dimensionless
residence time and the sludge disintegration factor are determined
that ensure zero excess sludge production. We show that if the
sludge disintegration factor is sufficiently high then
the MLSS content is guaranteed to be below the target value
provided that the residence time is higher than the washout value.