Pre-treatment of municipal MBR applications

Since the mid nineties the MBR technology has been introduced into municipal wastewater treatment in Europe. First wwtps went in operation performing a conventional mechanical treatment without any advanced mechanical pre-treatment unit. After a short operation period braids were hanging between the membranes and hence module sludging was observed. Thus, mechanical pre-treatment was upgraded introducing sieves primarily to reduce hairs and fibrous substances. Today, however, knowledge is still relatively poor concerning effectiveness and treatment efficiency of a large number of aggregates that had been brought to market, varying in gap size, gap geometry and others. This paper describes the fundamentals of sieving, gives an overview of mechanical pre-treatment on municipal MBR plants and finally presents the results of investigations quantifying and comparing the removal efficiency of different types of sieves.

Improvements could be achieved when using mesh or hole geometry in plant A and plant C and reducing the gap size in plant B. 5.5. Definition of a parameter for wastewater characterization concerning braid formation (braid formation capacity BFC) All results published so far have the disadvantage that the parameters determined are SS and some other standard parameters. This is due to the fact that a direct determination and quantification of hair and fibres is very difficult. But even after having solved the problem of a practical analytical method for hair and fibres quantification, one still lacks knowledge concerning what the determined amount of hair and fibres does in different parts of a wwtp and what amount of braid can be expected. Thus, a parameter is needed that is able to characterize different wastewaters (influent, after different sieves, etc.) concerning the likeliness to form braid under standard conditions. We call this the braid formation capacity BFC. At DESEE, work is underway to develop a standard method for a reliable and repeatable method to determine the likeliness of braid formation of different wastewaters. From this ongoing work, some first conclusions may be presented here: • Braid formation needs a basic amount of hair and fibres. If this amount is not reached, no braids are formed. • A linear relationship between SS concentra- Pre-treatment of wastewater prior to treatment in a MBR system is essential and crucial for the MBR process. There is still a discussion ongoing on how to design the optimal pre-treatment system. This is mainly due to the fact that today there is very poor knowledge about the ability of different pre-treatment units — mainly sieves — to modify the raw wastewater in a way that avoids braid formation in the MBR tanks. Until today, only the parameter of nominal gap size is regarded. In the future, it is necessary to discuss about removal efficiency instead of gap size only. Of course geometry plays an important role concerning efficiency as well as operational considerations. The removal efficiency up to now is determined by using the parameter suspended solids SS. In the future, this parameter which is not the core of the problem should be replaced by another parameter that characterizes different wastewaters concerning their ability and likeliness of forming braids. At DESEE, work is ongoing to find a suitable method to describe the braid formation capacity BFC, that would allow to quantify the efficiency of different pre-treatment systems really achievement oriented, regarding braid formation.