UF boost for biotreatment
15 Jan 2000
Biological treatment of industrial wastewaters generally includes conventional activated sludge treatment, biofilters or membrane techniques. However, Tony Robinson, sales engineering manager of Stork Protech, suggests membrane bioreactor (MBR) technology as a possible alternative.
Developed and applied by Wehrle Werk over the past few years in over 40 process plants, this combination of biotreatment with membrane filtration has been successful in eliminating many troublesome organics, such as aromatics and phenols. Compared with the conventional wisdom for biological treatment - a large tank for aerobic treatment, followed by a second equally large one for sludge settlement and concentration - the MBR system occupies little ground space. Its crossflow membrane system has a high flux rate, leading to reduced ultrafiltration area. But the secret of its success in dealing with difficult organics is, firstly, its operation at elevated pressure and temperature giving better oxygen concentration and, secondly, the extended age of the biomass achieved by recirculation from the membrane filters.
At steady state a small amount of sludge is bled off, normally at around 2 to 3 per cent solids, far less - and at much lower concentrations - than from conventional plants. The permeate from the UF units is particularly low in BOD and COD, with no suspended solids. It can then be recycled as process water.
