Bacterial Growth : Batch vs Continuous

When naturally occurring bacteria grow on carbonaceous material under suitable conditions, they will usually multiply until one necessary growth factor becomes exhausted and becomes growth limiting. If no additional nutrients are added to supplement that (or those) depleted, then no further growth will take place. Growth in such a closed loop system is called batch culture and can be divided into four distinct phases - these are lag, exponential or logarithmic phase stationary phase and death phase.

The Lag PhaseThe lag phase is a period of non-bacterial production between introduction of bacteria to the growth medium and establishment of the maximum growth (exponential) rate. Its duration is variable and governed by a number of varying factors including previous cultural history i.e. have the bacteria been previously grown on this substrate and the physical growth conditions themselves.

Germination and GrowthOnce the bacteria germinate and growth commences, the rate of growth will rapidly increase up to a maximum exponential rate, which is species specific. It is during this exponential or logarithmic phase of growth that the bacteria are multiplying (doubling) at maximum rate and therefore producing the highest specific yield of biomass. Some bacteria can grow with a doubling time of 15 minutes i.e. every 15 minutes every cell divides into 2 daughter cells and so on. One can calculate that given an unrestricted nutrient availability, a single bacterial cell, with a doubling time of 15 minutes can multiply to produce an amount of bacterial biomass the equivalent of the weight of the earth every 24h! The reason why this does not happen is that the exponential phase of bacterial growth in nature is always very short lived, being limited by nutrient availability - Schledel, H.G. (1986)

Batch vs Continuous CultureWhen bio-augmenting a biological treatment plant with specifically tailored bacteria it is important to maximise the effect from the smallest quantity of base bacteria in order make the process of bio augmentation viable on cost grounds. In the past Cleveland Biotech has achieved this by growing up bacteria prior to use in its Mark I Activation unit. Although this amplifies the bacteria prior to addition it relies on BATCH CULTURE and therefore is subject to the non-bacterial yielding lag phase, the time it takes for the bacteria to germinate and start to grow and also the daily inconvenience of operator involvement to empty the batch into the plant. These factors restrict the amount of bacteria that can be produced in any 24h period, and because a synthetic substrate is used (not the effluent stream in question), the bacteria once produced are not acclimatised to the effluent and thus on application to the treatment plant there is a further lag in their growth.

As described above in batch culture the growth conditions undergo continual changes; the bacterial concentration rises as the substrate concentration diminishes. For many applications however it is desirable to grow the bacteria over extended periods during which the substrate concentration and other conditions remain constant and the cells grow at a constant, fully acclimatised exponential rate on the effluent in question.

Such a situation can be achieved by frequent transfer of the growing bacteria from a base inoculum to fresh growth medium. In practice this is achieved by constant addition of fresh effluent to the growing bacteria and concomitant withdrawal of equal volumes of the growing bacterial culture. This type of bacterial growth is referred to as CONTINUOUS CULTURE.

Under such growth conditions the rate of bacterial growth (or doubling of bacterial numbers) increases in direct proportion to the rate at which fresh nutrient (effluent) is added to the system. At the same time the yield of bacteria in unit time increases similarly up to the maximum growth rate of the bacteria.

Thus in continuous culture, a bacterial species with a doubling time of 2 hours, if fed with a complete volume change of fresh effluent (nutrients) every 2h, will produce 12x the amount of maximally growing bacteria compared with a similar batch growth system over the same period of time - Brooke, A. G. (1984)

The BacceleratorThe Baccelerator has been developed to exploit the latter type of microbial growth. By the automatic daily addition of site specifically selected bacteria, into the aerated growth chamber containing continuously fed influent waste water and a special biological accelerant, large, daily quantities of highly active, fully-acclimatised bacteria are produced and fed continuously into the inlet feed to the plant.

The effect of this continuous feed of bacteria is to boost the resident microbial activity of the effluent treatment plant and / or overcome any transient biological shocks, which may have impeded the natural microbial degradation processes within the plant.