Biomedical Engineering Reference
In-Depth Information
Figure6.7 Rotatingbiologicalcontactor
Membrane 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 perme-
able 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 bio-activity 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 com-
munity are able to be adequately enhanced. This is of particular relevance to
xenobiotics and the more recalcitrant components of wastewaters, as their bio-
logical breakdown 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 avail-
able oxygen, though the high organic loading and efficient intra-system microbial
conservation combine to make the hydraulic retention time entirely independent
of the solids themselves. Since the membrane allows gaseous transport while
constraining the biological phase, there is provision within the reactor for bubble-
less aeration and oxygenation consequently can take place over a relatively large
surface area, thereby improving the efficiency of this process. In addition, the
membrane itself may become an attachment zone for biofilm formation.
Thus, the membrane bioreactor can offer a greater degradation capacity for
persistent chemicals, making possible the biological
removal of benzene,
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