Biomedical Engineering Reference
In-Depth Information
Q
,
S
0
(1 -
y
s
)
Q
X
,
S
(1 +
R
)
Q
Waste water
Effluent
S
e
,
X
e
V, X, S
Settling tank
or Separator
Air
RQ
S
R
,
X
R
(
R + y
s
)
Q
S
R
=
c
Rs
S
X
R
=
Activated sludge reactor
or Oxidation ponds
c
R
X
y
s
Q, S
R
,
X
R
Excess sludge
FIGURE 16.25
Schematic diagram of a typical wastewater processing unit.
Fig. 16.26
. Organisms living in soil and aquatic environments actively participate in carbon
and nitrogen cycles. For example, certain organisms fix atmospheric CO
2
to form carbohy-
drates, while others degrade carbohydrates and release CO
2
into the atmosphere. Similarly,
some organisms fix atmospheric nitrogen (N
2
) to form ammonium and proteins, while others
convert ammonium into nitrite and nitrate (nitrification), and others reduce nitrate into atmo-
spheric nitrogen (denitrification). Sulfur-oxidizing organisms convert reduced sulfur
compounds (sulfur and sulfide) into sulfate, and sulfate-reducing organisms reduce sulfate
into hydrogen sulfide. These interactions among different species take place in natural
systems in a more complicated manner. The sustainability of the ecosystem maintained by
a balanced action of all the organisms involved in completing the cycles.
16.7. SUMMARY
One of the important aspects of reactor design is to seek desirable stable steady-state reac-
tion conditions as the reactor set point. Stable steady state is a subset of sustainable states that
the system operates at a single valued point. While a sustainable system can be weakly stable,
exhibiting limit cycle or oscillation between bound and nonzero values, industrial bio-
processes are preferably to be strictly stable. Only stable steady state can be operated such
that the bioprocess system will not runaway when fluctuations are encountered. As
such the quality of the product can be maintained. A real bioprocess system is operated
such that the output fluctuations are small and it gradually returns to the desired set point
when a fluctuation is expired. Therefore, bioprocess system stability is an important topic
for bioprocess system (reactor) design and operations.
Stability of a reactor operation is usually associated with the existence of MSS. MSS exist if
the rate law is not monotonic or the rate could be lower at very high-feed reactant concentra-
tions (than that at lower reactant concentrations). When back mixing is strong (for example,
CSTR or chemostat), multiple states appears. Therefore, MSS could exist for nonisothermal
exothermic reactions (of any kinetics) and isothermal substrate inhibited systems in
a CSTR. Feed conditions also affect whether MSS exits or not.
Catalyst stability or rather instability is another important issue. Catalyst deactivation is
affected by the reaction mixture, temperature, and flow conditions.
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