Rottebehaelter as a component of decentral systems has been increasingly used in Austria, Germany and Switzerland for domestic wastewater pre-treatment. Since its application is until now only limited to German speaking countries, there is no English word for it, but can be called pre-composting tank. However, in this work the term “Rottebehaelter� is used to make clear to which system it refers. It was first introduced by Dr. Joachim Niklas, OEKOTEC, and Mr. Peter Stolz, AREAL, in Germany. Today there are also other companies including Mall-Beton offering this system.

A Rottebehaelter consists usually of an underground monolithic concrete tank having two filter beds at its bottom or two filter bags that are hung side by side and used alternately in an interval of 6-12 months (figure 25) (Gajurel, 2003).

Figure 25: Rottebehaelter systems (left: filter bed system, right: filter bag system)

It is watertight and structurally sound in order to avoid entering of extraneous groundwater in it and leakage of the filtrate into the groundwater which would cause groundwater pollution. The top opening is covered by a prefabricated concrete slab and provided with ventilation so that air can enter. A shutter for changing the filter bag or emptying treated material, adding bulking agents such as straw, bark etc. into the retained materials, inspection and cleaning has been provided on the covering of the tank.

The influent is discharged into one of the two filter beds or filter bags retaining solid materials while draining the liquid. The principal role of the filter medium (filter bed or filter bag) is to cause a clean separation of particulate solids of the influent with no additional energy consumption. The filter medium is designed to recover a valuable solid product. Therefore, attempts are made to create a surface deposition of the solids in a recoverable form. When the influent is discharged into the filter bed or filter bag a filter cake is formed at the bottom. Its depth increases during the filtration due to deposition of solid material on its surface.

There is also another kind of Rottebehaelter, in which six filter bags are hung side by side. This is used only in Austria (figure 26 and 27). Unlike the previously described Rottebehaelter, two pipes with a slope of 5 % are clamped inside the Rottebehaelter, under which filter bags are hung side by side in their mountings. There are three filter bags for each pipe. The influent is alternately distributed to both pipes by a means of diversion. The influent is first diverted into one of two pipes where three empty filter bags are hung in such a way that when one is full, the influent flows to next filter bag and so on. The solid materials in the influent are retained while liquid passes through the filter bag and is collected at the bottom of the Rottebehaelter.

Figure 26: Rottebehaelter with several filter bags system (left: top view , right: cross-section)

Figure 27: Existing Rottebehaelter with several filter bags in Austria (Ambros et al., 1998)

The filtrate that is collected at the bottom of the Rottebehaelter is periodically pumped or piped (at a good gradient) for further treatment. Normally, the pre-treated filtrate in the Rottebehaelter is further treated in Constructed wetlands adjacent to the Rottebehaelter and then discharged into the water bodies nearby. For reuse tertiary treatment is needed. The solid materials, which are retained in the filter medium, are bio-degradable and during the filling phase and silence phase (the filled one is left in a silence phase for degradation) decomposition takes place. Because of the degradation, the volume of the material in the filter bag or filter bed will be reduced.

Compared to Rottebehaelter with a filter bed at its bottom, the filter bag system has some advantages. In the filter bag the outer part of the material is always in aerobic condition, because it has a continuing contact with air. Therefore, there is no or only little odour nuisance. Moreover, handling of treated material is not so difficult as in filter bed systems because material is collected in the filter bag which can be taken out easily. Regarding the handling, Rottebehaelter with several filter bags is simpler than other systems. Because the material is distributed into 6 filter bags instead of only two, each bag is 3 times lighter than in other systems. However, since the filter bag is attached to the influent pipe, there is no opening in the filter bag. Therefore, adding bulking agents like straw or bark, which is important for composting of retained materials, is not possible unless they are added through the toilets.