BATCH CULTURE and CONTUINUOUS REACTORS
1. Batch culture is where biomass is added to a reactor containing the
substrate consisting of a carbon source, an energy source and
nutrients. The cells grow using these and at some time later, one of
these is exhausted. This is referred to as the “limiting substrate” – all
other substrates are considered “in excess”
2. If the carbon and energy source runs out, then the biomass stops
growing and the remaining excess nutrients are not uptaken (unless
they are an energy source as well). If the nutrient runs out first, then
(after this) the remaining excess carbon and energy source may
continue to be utilised (at a slower arte than when the biomass is
growing). Thus further products may be formed. Since there are no
nutrients remaining , there can be no further biomass growth. These
two differing profiles are important in wastewater treatment.
3. Batch culture consists of six phase the first two of which are the lag
phases, periods of time where the cell adjusts to its new environment
and is not yet capable of growing at it maximum possible rate (µ MAX).
Following growth at the maximum rate, there is a very rapid decline
form the maximum growth rate to zero over a very short time period.
Because of this short time, the cell is unable to adjust its
macromolecular composition, enzyme levels etc, since at one time
there is excess substrate and then, a few moments later, there is
substrate limitation and then, a few moments later no substrate
remaining. This situation is unlike continuous culture, where we are
able to sustain biomass at any specific growth arte for 0 to µ MAX
indefinitely by an appropriate feeding policy. Consequently, these
cells come to a steady state , in which there macromolecular
composition and their enzymes levels have been adjusted to best suit
the specific growth rate at which they are growing.
4. In batch culture, microorganisms go through a number of growth
phases. Initially, the specific growth rate is less than the maximum
specific growth rate possible and the growth rate increases as the
microorganism adapts to the growth environment. At some point during
the process, the microorganism will grow at the maximum specific
growth arte until the limiting substrate becomes limiting. At this point,
the specific growth rate rapidly declines from the maximum specific
rate to zero. This process occurs in a very short time and ,
consequently, the microorganism is unable to make any significant
adjustments to enzyme levels, pH and osmotic gradients,
macromolecular composition etc.
5. In continuous culture, on the other hand, microorganisms are placed in
an environment where the feed rate to the system and from the system
is fixed. Thus, microorganisms experience a constant, and steady
supply of limiting substrate and nutrients. Consequently, they can (over
time) adjust their enzyme levels, pH and osmotic gradients,
macromolecular composition etc. to achieve an “optimal growth”. This
situation is generally referred to as “steady state” and may take up to
10-20 generations to achieve.
6. When a mass balance for biomass and limiting substrate is undertaken
in a continuous culture, there are three major outcomes:
a. The specific growth rate is equal to the feed rate divided by the
reactor volume. This is also referred to as the dilution rate.
D = F/V = µ
b. The biomass concentration in the outlet stream is equal to the
biomass from substrate yield times the difference between the feed
concentration and outlet concentration of the limiting substrate:
X = yXS * (S0 - S)
Since the outlet concentration of limiting substrate is generally far
less than the feed concentration, this reduces to:
X  yXS * So
c. The concentration of biomass in the reactor is the same as the
concentration of biomass in the effluent.
7. Continuous systems are, consequently, not very effective in the case
of slow growing organisms. Under such conditions, either the federate
must be very small (meaning that the throughput of waste is very small)
or the reactor volume must be very large (which is expensive).
8. To overcome this problem, cell recycle is introduced. Under this
condition, the biomass density in the reactor becomes very large and,
since the feed concentration of limiting substrate remains the same,
each cell gets less and less of this limiting substrate. Consequently,
the specific growth rate reduces rapidly. Such systems are also
referred to a “high biomass density systems” or “high density, low
growth rate systems”
9. The importance of low growth rate systems lies in the fact that specific
(limiting) substrate uptake is described by the relationship:
QS = μ + 
Where  is the “growth associated” specific substrate uptake rate
(mmol substrate / g biomass) and  is the “non-growth associated”
specific substrate uptake rate (mmol substrate / g biomass / h). “Nongrowth associated” processes include the maintance of concentration
and osmotiv gradients, DNA repair and other cellular process requiring
ATP in the absence of growth. As the growth rate becomes lower and
lower, the “non-growth associated” specific substrate uptake rate is a
larger proportion of the total specific substrate uptake. In addition, for
the case where the limiting substrate is used to generate energy via
catabolism, under lower and lower specific growth rates, the proportion
of the total specific substrate uptake that is channeled through the
catabolic process is increased. This has major consequences for
wastewater treatment, since it means that at low growth rates the
amount of sludge production (anabolism) will be less than under higher
growth rates.
10. For this reason high density systems are widely used.
11. When a mass balance for biomass and limiting substrate is undertaken
in a continuous culture with biomass recycle, there are three major
outcomes:
d. The specific growth rate is no longer equal to the feed rate divided
by the reactor volume. Hence, feed rates higher than the maximum
feed rates possible in continuous systems are possible.
The biomass concentration in the outlet stream is no longer equal
to the biomass from substrate yield times the difference between
the feed concentration and outlet concentration of the limiting
substrate.
e. The concentration of biomass in the effluent is not the same as the
concentration of biomass in the reactor.
12. Sequencing Batch Reactors of Fill/Draw Reactors are semi batch
reactors that are able to accumulate biomass to a high concentration
by repeated cycles of supply if new growth medium (filling), allowing
growth, settling, take off of spent liquor, and then REPEATING supply
of new growth medium (filling), allowing growth, settling, take off of
spent liquor. This is achieved by using a CSTR and having the reactor
stirrer being able to be turned off and on and having both the influent
and effluent pumps being also to also be turned on and off. When
aeration to the tank is included and there is the provision for this to be
turned on or off, then the reactor can be operated in an aerobic
operation, anaerobic operation and an anoxic operation. Such
operational modes make this reactor very versatile and, it is for this
reason, that it is gaining in popularity and application. Significant
aeration savings can be obtained by operating a number of SBR’s in
parallel since the volume aerated can be significantly reduced by
having the aeration cyclers of individual reactors staggered over the
full operational cycle time.