Overview - Recent Development
As a side activitiy of a forrestry project several fixed dome biogas digesters have been build during the late nineties.
BORDA (Bremen Overseas Research and Development Association), a German consultancy, has been working since 1989 with the non-governmental organisation LPTP (Lembaga Pengembangam Teknologi Pedesan = Institute for Rural Technology Development) on an Integrated Rural Development Project (IRDP) sponsored by the Federal Ministry for Economic Cooperation of Germany. The LPTP was founded by very involved students in 1979 who attach great importance to participation by the target group in each case. The central field of activity was in the development and dissemination of appropriate technologies. Biogas has only been part of the LPTP programme since BORDA entered the cooperation.
At the beginning of the eighties a number of biogas plants were installed in the project area through the agricultural extension service of the regional administration. None of the plants ever supplied energy for the operators because of serious defects in the plant system. Consequently, project staff were faced with scepticism and a lack of interest on the part of members of regional administration. LPTP, in the meantime, has shown that technically perfect biogas plants can be built and are useful for farmers. The former situation has changed drastically on the basis of this, so that LPTP, in cooperation with the regional administration, was able to arrange a national conference on biogas with over 100 participants in autumn 1992.
Types of Plant
After a survey of the project area, carried out by BORDA and the LPTP which established all data relevant to biogas, had shown the region to be basically suitable for biogas dissemination, the IRDP employed an engineer for this sector. In the late autumn of 1990 a BORDA engineer supervised the construction of the first three fixed-dome plants. In autumn 1992 there were 58 biogas plants, and by December a further 27 had been built including a large-scale plant of 93 m3 digester volume for a dry meat producer. Otherwise, standard types with a volume of digester of 6, 9, 13 and 18 m3 are offered. The large number of new plants can be attributed to the dissemination area being extended to include a community between Cepogo and Boyolali.
So far there has only been one case of gas leakage on a plant and this was immediately repaired by the project staff. Good quality and high gas yields (0.275 m3/m3 volume of digester per day) combined with an exemplary fertiliser advisory service mean that not only the biogas department, but also IRDP as a whole, has a good reputation in the region.
The CAMARTEC Type
The type of plant selected and adapted to Javanese conditions was the fixed-dome plant with a "weak-ring" developed further by CAMARTEC of Tanzania. As stabling and the use of organic fertilisers was already known, the principle propagated by CAMARTEC, the "biogas unit", could be consistently adhered to.
The Javanese version differs from the Tanzanian one in the following points:
- 12 cm wall thickness in the semi-spherical dome; as the bricks are only 4 - 5 cm thick it is difficult to lay them in one-quarter brick thickness.
- conical floor board; in addition to giving better static properties, the bowl-like floor serves as a water bath for the bricks.
- reinforced "strong-ring" to provide safety against the frequent, light earthquakes.
Since 1992 the "weak-ring" has been omitted as it is assumed that the change in cross-section below the "strong-ring" automatically functions as a predetermined breaking point.
When the cattle were fed on king-grass, there were frequently problems with scum in the plant. The high position of the pipe to the compensation chamber presumably made this problem worse. Since 1992, the digester and compensation chamber have been connected by a shaft running from the bottom of the digester up to under the "strong-ring". In some cases, older plants were converted by means of a by-pass shaft. Since then the problem seems to have been solved even if the scum itself cannot be avoided.
The interior plastering was carried out with the agent "Tricosal" in three layers of plaster, three intermediate brushing coats and a final coat of finishing plaster. The gas pipes consist of 1/2" PVC pipes but where these are longer than 50 m, 3/4" pipes are used. The water trap consists of a short U-pipe with a gas-tight plug which can be opened when necessary.