Disinfection due to filtration is already applied in the field of drinking water and landfill leachate treatment. The different types of membrane filtration are listed in table 5.

Table 5 Membrane technologies in wastewater treatment (MUNLV, 2003)

In wastewater treatment micro and ultrafiltration is mainly applied for separation of suspended solids, respective disinfection. A porosity of less than 0.2 µm (ultrafiltration) is required to remove pathogens totally (ATV 1998). Although viruses may be even smaller, they are also removed because they are located on particles.

Membranes can be used as a separate final step after biological treatment or as integrated unit in an intensive technology like activated sludge reactor. The possible applications within the activated sludge process are shown in figure 8.

Figure 8: Implementation of membrane technologies in activated sludge systems

Firstly the filtration can be applied as last unit for tertiary treatment. Secondly the membrane bioreactor has been developed in the last years that is characterised by a combination of activated sludge process and membrane filtration. In this case, the filtration unit replaces the sedimentation unit. The activated sludge process can be operated with higher biomass concentrations than the conventional activated sludge process. Therefore the space needed for this technique is much smaller.

Advantages of filtration are the following:

• Pure physical treatment

• No chemical agents necessary

• No unwanted by-products

• Good efficiency

• Can be combined with activated sludge process (membrane bioreactor)

As disadvantages are known:

• High investment and running costs, especially energy demand

• Clogging due to fouling and biofouling on the membranes which requires the use of

chemicals

• Membranes must be replaced from time to time

Figure 9: Membrane modules by Zeenon, Germany

Membranes represent a very promising technology that still needs development to cope with biofouling effects.

Figure 10: Cross flow filtration

Figure 11: Vibrating membranes with cross flow filtration

Figure 12: Vacuum rotation membrane by Huber, Germany

New technologies work with vibrating or rotating membranes to avoid fouling like shown in Figure 13 and 14.

Reverse osmosis is the best technology to produce clean water from wastewater since it removes even salts, heavy metals and pharmaceutical residues.

Figure 13: Principle of reverse osmosis

Figure 14: Treatment scheme of reverse osmosis

Like shown in Figure 14 a high driving power is needed for reverse osmosis, see also table 5. That is why it requires high energy costs. A practical example is presented in the next chapter.