Vacuum toilets produce little diluted black water which is transported by the vacuum system to a bio-gas reactor in which black water is treated anaerobically together with bio-waste from kitchens. A well managed anaerobic digester should produce 1 m³ gas/m³ volume. The biogas mixture is about 70 % methane and 30 % carbon dioxide (Doelle, 1998). The methane can be used as a source of renewable energy and can be used to produce electricity as well as cooking and lighting gas. The sludge, which is rich in nutrients and organic matter can be used after sanitisation in agriculture as a fertiliser. Vacuum toilet-biogas plant systems have been applied in following projects:

Settlement Flintenbreite, Luebeck, Germany: An integrated sanitation concept with vacuum toilets, vacuum sewers and a bio-gas plant developed by OtterWasser GmbH has been implemented in the settlement Flintenbreite, peri-urban area of the city of Luebeck (figure 22). The settlement was planned for 350 inhabitants and is connected to a bio-gas reactor for black water treatment (Otterpohl et al., 2001). Vacuum toilets and vacuum pipes are used for black water collection and transportation. The little diluted black water is transported to the bio-gas reactor in which black water is treated together with bio-waste from kitchens. The bio-gas is used to operate the heat and power generator. After the treatment the hygienic end product is used as fertiliser.

black water mixed with shredded bio-waste is sanitised by heating the feed to 55 °C for 10 hours. The energy is further used by the digester that is operated mesophilic at around 37 °C. The relatively small amount of water added to the black water keeps the volumes small enough for transportation. There is a 2-weeks-storage tank for the collection of the digester effluent. Bio-gas is stored in the same tank within a balloon that gives more flexibility in operation. The fertiliser will be pumped off by a truck and transported to a farm that has a seasonal storage tank for 8 months.

Solar Passive Building „Wohnen & Arbeiten“, Freiburg-Vauban, Germany: about 40 inhabitants in the 4-storey-building „Wohnen & Arbeiten“, are connected to the sanitation system as in Flintenbreite (Panesar and Lange, 2003). black water is collected with vacuum toilets and transported to the bio-gas plant by vacuum sewers. In the biogas plant, organic wastes are added to the black water with the help of a feeding device. The biogas plant is connected to the internal gas system of the house; it provides cooking gas for the households. The sludge is stored in a storage tank for the fertiliser. At the beginning grey water was treated with an aerated sand filter, but due to technical problems, it was later replaced by a membrane-filter-module.

Figure 22: Vacuum -bio-gas system, greywater bio-filter and rainwater infiltration (Otterpohl, 2001)

Norwegian experience with water saving toilet - storage tank - thermophilic aerobic reactor: in Norway, unlike in the projects in Germany, a different type of toilet is used for black water collection (Skjelhagen, 1999). The toilet does not need water for transportation, only about 0.2 l per flush for cleaning the toilet. The black water is stored under the toilet bowl until it contains 20 - 25 l and is flushed by gravity to a storage tank that is located not farther than 10 m from the toilet. grey water is treated in a compact treatment plant consisting of sludge collecting filter bag, filter and UV. The organic waste is stored in a closed sub-surface tank in which bio-mass produces acids that lowers pH below 5. As a result the waste remains conserved. There is no production of gases.

After a year of storage, the black water, grey water sludge and semi-liquid organic waste are pumped by a truck-tanker and transported to a pre-storage tank. The tank content is sanitised and stabilised in a thermophilic aerobic reactor with a processing temperature of 55 - 60 °C and kept in a post-storage tank before application in agriculture. No energy is added for heating the bio-mass.

Also, at the agricultural university of Norway 24 student apartments have been connected to a recycling system based on aerobic sanitisation of black water with organic household waste,collected using a vacuum toilet system and rendering an odourless and sanitised fertiliser slurry (Jenssen, 2001).

Picture from Tan Lap village, close to Hanoi (figure 23) shows an anaerobic digester for decomposing the manure of at least 5 pigs and black water of one family. The biogas is used for cooking and light, the effluent is used as fertilizer. The biogas can be transported via PVC-tubes to houses up to 100 meters away. It can be directed by valves to the gas stove or the gas lamp in the kitchen. The methane produced displaces the use of firewood (estimated at 2500 kg per family per year, for which families spend between $ 5 and $ 10 per month). The plants also improve sanitation and promote cleaner air.

Figure 23: Biogas production and use in Vietnam