Aerobes and Effluents 137Membrane Bioreactors
This system, instead of utilising conventional methods of gravity settlement,
achieves the desired biomass retention by means of a cross-flow filtration process,
as shown in Figure 6.8.
The development of effective methods of micro- and ultra-filtration has opened
up the potential for using membrane bioreactor technology on various forms of
domestic and industrial effluents. There are three general types of reactor systems
which have been developed, namely solid/liquid separation, gas permeable and
extractive systems. The membrane element allows the passage of small molecules,
but retains the total resident microbial population. As a result, the cumulative
overall bioactivity and the resultant speed of remediation is boosted, since not
only are micro-organisms no longer lost with wash-out flow, but also, conditions
for even the slowest-growing member species of the microbial community are
able to be adequately enhanced. This is of particular relevance to xenobiotics
and the more recalcitrant components of wastewaters, as their biological break-
down is often brought about by bacteria which themselves have a relatively long
establishment period within the population. The high biomass levels within the
bioreactor itself obviously necessitate abundant readily available oxygen, though
the high organic loading and efficient intrasystem microbial conservation com-
bine to make the hydraulic retention time entirely independent of the solids
Figure 6.8 Schematic membrane bioreactor