Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 138

themselves. Since the membrane allows gaseous transport while constraining the
biological phase, there is provision within the reactor for bubbleless 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 per-
sistent chemicals, making possible the biological removal of benzene, nitroben-
zene, dichloroaniline and polyaromatic hydrocarbons (PAHs), for example, which
represent a significant risk, both to the environment and human health, due to
their high toxicity. Removal efficiency for these substances can approach 99%.
The membrane bioreactor has proved its suitability as an efficient system for
degradation of recalcitrant compounds and significantly higher biomass con-
centrations and utilisation rates are routinely achieved than in corresponding
alternative treatment systems. In common with most operational, rather than
experimental, biological detoxification processes, not all of the contaminants
present in the effluent are typically completely converted into carbon dioxide
and water, a certain percentage being turned into metabolic byproducts instead,
though this can amount to less than 5% in a well-managed bioreactor system.
Part of this involves the gradual and controlled introduction of novel wastewater
elements, to ensure that acclimatisation is maximised and any potential tendency
for 'shock loadings' avoided. This is a clear example of the value of permitting
optimised microbial adaptation to the individual application.
These systems are, of course, more expensive than the conventional activated
sludge or trickling filters, but produce a much smaller quantity of excess sludge
for subsequent disposal of treatment. In addition, they produce an elevated COD
removal and would seem particularly well suited to use in small-scale plants
where the production of high-quality final effluent is a priority.
Cellulose Ion-Exchange Media
For effluents requiring a highly selective removal of high molecular weight pro-
teins, cellulose ion-exchange media provide an example of treatment involving
the use of isolated biologically derived materials. The ion exchange medium is
replenished with brine as required, and the proteins collected are removed in the
resulting saline solution, for subsequent coagulation and drying. This enables a
valuable material to be recovered, typically for use as an animal foodstuff, while
reducing the wastewater BOD by 90% or more.
Sludge Disposal
Many of the treatment processes described in this chapter give rise to primary or
secondary sludges. Typically, these byproducts require disposal and, like many