Efficiency of mechanical pre-treatment on European MBR plants

In order to ensure a stable and reliable operation of municipal MBR plants, an enhanced mechanical pretreatment of the raw wastewater is essential. Removal of hair, fibrous material and other contraries which can lead to operational problems at the membrane modules is of particular importance. As the number of full scale applications is continuously increasing, now appropriate investigations can be carried out to determine the removal efficiency of different units (screens, sieves) or different combination of units (multi staged system). First results of such investigations on two German MBR plants were already presented [F.-B. Frechen, W. Schier and M. Wett, Water Practice & Technology, Vol. 1, No. 3, IWA Publishing, 2006, doi: 10.2166/WPT.2006057.]. In 2006, DESEE executed further investigations on another three wwtps in The Netherlands, UK and Germany. This paper summarizes the results, thus updates the information available, discusses the present findings and gives an outlook on future developments.

Mechanical pre-treatment of wastewater is essential and crucial for the MBR process. There is still discussion ongoing on how to design the optimal pre-treatment system. This is mainly due to the fact that still today there is relatively poor knowledge about the ability of different pre-treatment units – mainly sieves – and entire pre-treatment systems to modify the raw wastewater in a way that avoids braid formation at the membrane modules. The results presented confirm that, applying smaller gap sizes, increasing removal efficiencies concerning SS are measured. Moreover, in view of future applications it can be expected that more and more two-dimensional gap geometries will be employed and with this, removal performance of sieve units will increase. The above described investigations are another step on the way to close this lack of knowledge, and they promote a view that regards not only nominal gap sizes but also takes into account aspects of one-dimensional and two-dimensional gap geometries, one-stage or two-stage pre-treatment and removal efficiencies of single units and entire pre-treatment systems.