Effect of mechanical cleaning with granular material on the permeability of submerged membranes in the MBR process.

The research on fouling reduction and permeability loss in membrane bioreactors (MBRs) was carried out at two MBR pilot plants with synthetic and real wastewater. On the one hand, the effect of mechanical cleaning with an abrasive granular material on the performance of a submerged MBR process was tested. Additionally, scanning electron microscopy (SEM) measurements and integrity tests were conducted to check whether the membrane material was damaged by the granulate.The results indicate that the fouling layer formation was significantly reduced by abrasion using the granular material. This technique allowed a long-term operation of more than 600 days at a flux up to 40 L/(m2 h) without chemical cleaning of the membranes. Moreover, it was demonstrated that the membrane bioreactor (MBR) with granulate could be operated with more than 20% higher flux compared to a conventional MBR operation. SEM images and integrity tests showed that in consequence of abrasive cleaning, the granular material left brush marks on the membrane surface, however, the membrane function was not affected.In a parallel experimental set up, the impact of the operationally defined "truly soluble fraction" <0.04 microm from wastewater and activated sludge on the ultrafiltration membrane fouling characteristics was investigated. It was shown that the permeability loss was caused predominantly by the colloidal fraction >0.04 microm rather than by the dissolved fraction of wastewater and activated sludge.

Enhanced membrane bioreactor process without chemical cleaning.

In membrane bioreactors (MBR) for wastewater treatment, the separation of activated sludge and treated water takes place by membrane filtration. Due to the small footprint and superior effluent quality, the number of membrane bioreactors used in wastewater treatment is rapidly increasing. A major challenge in this process is the fouling of the membranes which results in permeability decrease and the demand of chemical cleaning procedures. With the objective of a chemical-free process, the removal of the fouling layer by continuous physical abrasion was investigated. Therefore, particles (granules) were added to the activated sludge in order to realise a continuous abrasion of the fouling layer. During operation for more than 8 months, the membranes showed no decrease in permeability. Fluxes up to 40 L/(m(2) h) were achieved. An online turbidity measurement was installed for the effluent control and showed no change during this test period. For comparison, a reference (standard MBR process without granules) was operated which demonstrated permeability loss at lower fluxes and required chemical cleaning. Altogether with this process an operation at higher fluxes and no use of cleaning chemicals will increase the cost efficiency of the MBR-process.