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Passive house »Wohnen & Arbeiten«

Walter-Gropius-Strasse 22 • D-79100 Freiburg • Germany • Vox: +49 761 4568330
Email: post(at) • Web:
The Idea 
What is a Passivehouse? 
Why an alternative sanitation concept?
Energy Concept 
Research Outcomes 
The Cost 
Co-generation Unit 
Sanitation Concept 
Biogas Plant 
Grey Water Facility 
Residents, Owners 
Guided Tours 

Why an Alternative Sanitation Concept?

  Vacuum toilet
  Click on the picture to enlarge: 1024 x 768
  Biogas reactor
  Vacuum pump
  Gas stove with biogas
  Gasherd mit Biogas

Today, every reasonably educated citizen of the world knows that we have an energy problem, which every person in the world must collectively try to solve because the available fossil energy resorces -crude oil before others- will soon be depleted. However, only few people are aware of the fact that there is an even bigger challenge in sight, behind the energy problem: Exhaustion of other resources. A severalfold of the needed energy even for our lavish lifestyles can easily be obtained from the Sun. But how is it with other resources such as phosphorus or drinking water?

Let's take phosphorus as an example. In most of the phosphorus reserves on earth, phosphorus is not in its pure stage. Phosphorus usually exists in compounds, and appears in the nature as phosphates. Phosphorus is used by people to make fertilizers (plants need phosphate to grow). We, as humans then eat the plants (or the animals which have eaten the plants), so phosphorus accumulates in small amounts in our skeleton. However, most of the phosphorus we take in is excreted. The excreted phosphate, after being diluted, ends up either in rivers and seas (where it contributes to over-fertilisation) or must, with high expenses, be treated in sewage treatment plants, where it is precipitated using complex chemical methods. In both cases, the phosphorus is not economically available to the people anymore: In the sea, the dilution is too high and the refuse from the sewage treatment plants are contaminated with heavy metals like lead and other materials. As long as this is the case, we need to find more phosphorus to use as fertilizer in agriculture.

Phosphorus is not a rare material like gold or platinum, but 80% of the available phosphorus worldwide is mixed with cadmium. Cadmium is strongly toxic; so only the remaining 20% of the available phosphorus in the world can be used by people. Assuming that the phosphorus consumption in the world stays constant, the phosphorus reservoirs will suffice for another 60 to 160 years, depending on which expert you ask the question to. That means: If we keep on acting like we have been so far, we will not have much to eat in 200 years the latest.

We all know today that there is an energy problem but when are honest, we also know the most important solution: The Sun, which will for sure keep on shining for more than 200 years. It seems to be quite different for the other resources: If we all "consumed" phosporus, it would not indeed disappear, but rather end up in the seas and the hazardous waste sites and not have further economical value to humans. Our "western" canal systems and sewage treatment plants were good advances in the past, however, now they are 100 years old. They devour serious expenses for operation and maintenance. There is 100 years over again for us to replace the system with a circulation system. Our installation is one of the few possible technical solutions to this problem.

Our research installation today is too expensive to reproduce in other houses economically. However, if the installation worked, it would be a waste-water free house: The black water (from toilets) would go from the biogas plant to the fields and is used for fertilization there, the grey water (the remaining sewage) is treated on site in a membrane filter and then used for flushing the toilets and for watering the gardens in the summer. On one side, there is the cost of our expensive installation, and on the other side there is the "eliminated need" for a waste water canal network (we do not need it anymore!). Many say that our installation can be produced in a more cost effective way, then both alternatives would have the same price.

Due to financial problems of one of our partners the biogas reactor part of our installation never went into production. Hoewever, scientific results have been obtained by other projects (e.g. in Lübeck) so today there is no need to start it here.

Thus, our "KombiVak"- System is a good solution for the waste water problem, at least in the cities.

The sanitation concept, which is implemented through the "Wohnen und Arbeiten" consists of three basic parts:

  1. The Vacuum toilets make sure that only 0,5 to 1 liter of water is used every time the toilet is flushed, instead of 5 to 9, like in conventional toilets. in this system, excrements and urine are sucked off into a separated, low-pressure pipe system. One can find comparable systems since long time ago in trains (ICE-fast trains in Germany-), planes and ships.

  2. The Biogas plant ferments the delivered excrements and urine (so called black water) in a reactor at 37° C and produces biogas. Biogas consists of 40% methane and 60% corbondioxide. The carbondioxide is passive in the combustion, it only helps by making sure methane is not as explosive as it is in its pure stage. One can use biogas for heating or for cooking. We have decided to use it for cooking, since the biogas is available throughout the year. Moreover, the amount of biogas produced from the black water alone is not adequate to make up a substantial percent of the energy needed for heating the Passivehouse.

    However: Due to financial problems of one of our partners, the biogas reactor part unfortunately never went into production. Today, results have been obtained from other similar installations (e.g. in Lübeck) so there is no scientific need to start the biogas reactor

  3. With the Grey water filter it is relatively easier to re-clean the grey water: Because the water used in the shower, kitchen and the washing machines (the so-called grey water) has neither excrements or urine in it, there is no acute danger of infection. Using a reed purification plant, a ventilated sand filter or in our case, a membrane filter, the water can not indeed be purified enough to be drinkwater again, but would be clean enough to be used for flushing and for garden irrigation.

All three components have different uses:

  1. The vacuum toilets save water. With the price of water in Freiburg being over 3 € per cubic meter, also money. On the other hand, the savings unfortunately are not effectual enough to finance the vacuum toilet technology.
    With the drink and waste water prices as high as 8 €, the vacuum toilets may actually be profitable in the eastern states in Germany.

  2. Biogas is a form of energy. A biogas plant generates more energy (in the form of biogas) than total energy that the pumps in the plant use up and the heat energy needed to keep the reactor at 37° Celsius. However, the total expenditure would not indeed be a satisfactory justification for using this plant. The actual reasoning lies in the closure of the nutrition cycle like described above.

  3. The Grey water filter does not only supply us with costless flushing water for the toilets and for irrigation of the gardens, but also helps, along with the biogas plant, to make our house a wastewater-free house. When one compares the total expenditure of the systems of purification plants and the decentralized biogas plants, the biogas plant is certainly more expensive. However, with biogas plants, one does not need a sewage network anymore: Grey water and rain the water can be used and purified on site, and the purified black water is used on the fields as fertilizer.

    The membrane filter had been in use and functioning well until about 2007. We decided to stop it regardless because the filtering consumess more electricity per cubic metre than our local water company has to use for delivering fresh water. In Freiburg, there is no water scarcity so it is better to save electricity.
    Often, we receive the question: "Do you also use the rain water?" For our house, the answer is no, because using the rain water does not make ecological sense in Freiburg: The water of the Black Forest luckily comes with its own pressure to the management of the city; the local water distributor Badenova needs only to spend 0,1 kWh pump energy per cubic meter of water in order to bring the water to the houses. Using rain water would however require 1 kWh / m3, which is ten times more energy. There is also the ecological investments on the rain water containers (often made out of concrete), because it also costs energy to build and to install the rain water containers.
  Schematical Demonstration of the Material Flow
in an Ecological Sanitation Concept
  Stoffkreislauf im KombiVak-System
Next Page: The energy concept of "Wohnen & Arbeiten"
Translation: Selin Devranoglu 2005