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 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.