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GIZ Endev (German International Cooperation – Energizing Development Program) has positive experience with the use of plastic tube digesters (PTDs) in several South American countries and therefore has supported efforts to test the technology in Rwanda as well. Within that context, a limited survey was carried out in November 2010 in Kenya where these digesters have been installed and used for a number of years. The article below provides the main findings of this field survey and also draws on the results of another study performed independently by Kenyan researchers in 2009. Both studies show that hundreds of plastic tube digesters have been installed in the country over the last decade and that over 80% are in operating condition.
GIZ has been working since 2007 with the Government of Rwanda and SNV (Netherlands Development Organisation) in the development of a market for household biogas digesters in the country. To date, uptake has been much lower than expected and the initial investment in the masonry biogas digesters (from $1200 to $1400 for a typical 6m3 unit) was found to be one of the mayor hurdles. GIZ EnDev has had some success with plastic tube digestersin their program in South America and was interested to learn from the experience in Kenya with the possibility to use the technology in the Rwanda biogas program. It therefore commissioned a survey of PTDs in November 2010. A total of 39 PTDs were visited by a Kenyan consultant, out of these over 80% were found to be in operation (some since 2004). While the GIZ was being carried out, it was discovered that another team of researchers, Matiri and Kiruiro, conducted a study in 2009 on 60 digesters. This article summarizes the main findings of both studies.
Diffusion of Plastic Tube Digesters (PTDs) in Kenya
In Kenya, the diffusion of biogas technology started in the late 1950s and was initially slow, mainly because of its high construction costs and the poor performance of many installations.
Plastic tube digesters were introduced in Kenya in the early 1990s. Their low prize makes them an attractive alternative to the masonry biogas digesters. Several programs promoted this technology but adaptation was slow. Most PTDs failed, in large part due to the low quality of the digester bags. In 2005, the Kenya Agricultural Research Institute-Embu (KARI-Embu) reintroduced PTDs. At the same time some private companies had also started to produce and install these plastic digesters. The GIZ survey estimates that by the end of 2010 around 300 PTDs have been constructed. Matiri and Kiruiro report even higher numbers: around 300 digesters were in use in 2007 and over 600 by 2009. KARI-Embu as well as Catholic Diocese and the Kirinyaga Ecosystem Project installed the majority of these systems. According to their report, all the digesters they visited were in good working condition. Furthermore, the majority of farmers in Kenya have the potential to use PTD technology in terms of sufficient land size (one and half acres) and number of cows (three zero-crazing animals).
The GTZ commissioned survey found that there are at least three companies involved in the installation of PTDs in Kenya:
- "Pioneer Technologies Limited",
- "Modeline Electrical and Mechanical Engineers", and
- "Biens Limited". Pioneer Technologies, a local plastics company, is regarded as the company that started to distribute PTDs on a large scale in Kenya. T he company claims to have installed over 300 digesters from 2006 to 2010. However, the company's database is incomplete, as in the earlier years not much effort went into maintaining records.
In 2002, the first generation of plastic tube digester was basically made out of two Polythene layers. The latest generation PTD was developed by Pioneer and is sold by the other companies as well. The digester bag material is composed of a double layer; 300 micron UV treated Polythene reinforced with fiberglass. The thickness and the UV treatment increased the digesters' potential lifespan, which according to the supplier is estimated to be up to 15 years. The companies are distributing two digester sizes of 6 and 16 m3, costing about $500 and $800 respectively. This includes the stove, gas pipes, installation, as well as transportation to most parts of Kenya but excludes the cost of digging the trench.
Un-inflated digester bag after installation
PTD in use with gas pipe and inlet in the back
Plastic Tube Digester (PTD) Performance
Findings indicate that most of the users are quite satisfied with their PTD. The GIZ authorized survey covered 35 digesters; another 4 were discovered to be decommissioned for various reasons. It found that out of these, 29 (or about 80%) were in good working condition. Six digesters were not operating, because either the digester (four households) burst after animals/children stepped on it or in another case the pipe to the stove was disconnected. The remaining digester was out of use due to a thick layer of scum that drastically reduced its gas production. Matiri and Kiruiro found in their survey even better results as all 60 digesters visited were in use.
Both studies confirm: PTD users receive similar benefits as the ones provided by masonry digesters. Households use less charcoal and firewood and therefore save time (especially for women) and money. Most digesters produce enough gas to cook nearly all of the family’s meals (about five hours per day). Owners also mentioned improvements in sanitation and air quality due to the better disposal of dung.
Three central problems specifically related to the PTD technology itself can be identified: insufficient gas production, low gas pressure and the vulnerability of the digester bag. The first two issues are connected. Temperature variations inside the digester lead to low gas production and pressure in the morning and evening hours. Many households place some weight on top of the digester bag to increase gas pressure. The usage of a gas storage bag, as practiced in the South American GIZ EnDev program, is also an inexpensive way to store more gas and increase pressure. The low gas pressure does not allow the use of a biogas lamp but with the now available Solar LED lights, this should no longer be an issue.
The main reason for households to abandon their PTD is the digester bag’s vulnerability to piercing or bursting. In order to avoid this, the usage of a fence is recommended. One owner had fixed the bag with a bicycle repair kit.
Further research is necessary regarding the optimization of the retention time and the stabilization of the temperature.
PTDs have certain weaknesses compared to the masonry models. But the fact that these plastic tube digesters are successfully operating in some areas of Kenya (mostly without subsidy support) shows that the technology should not be discarded. Instead, it should be followed more closely to see where and under which conditions it can be an alternative option for households in Kenya and other countries in the region, who are interested to invest in biogas.
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