Livestock Research for Rural Development 11 (2) 1999

Citation of this paper

Low cost biodigesters for zero grazing smallholder dairy farmers in Tanzania

Innocent Rutamu

Tanga Dairy Development Programme,
PO Box 1474, Tanga, Tanzania


This report presents the findings of a research project sponsored by African Development Foundation (ADF)  and carried out in Tanga, Muheza, West and East Usambara parts of Tanzania between the months of August 1994 and January 1996.

Forty-six biodigesters were installed in selected villages. The gas produced was of low pressure, of right composition and safe to use. It was also observed that differences in attitude and animal feeds in our operation zones did not significantly affect gas production and composition. The biodigesters are to a certain extent fragile and there is a need for protection against perforation and sunlight to increase durability. The gas produced if well used could save up to more than 50% of cooking energy needs.

It was observed that, on average, 5 hours of  household labor were saved per day because of the  introduction of  Biogas. The use of biogas alleviates the work load of women and makes it possible for men to participate in providing for energy needs at homestead level; a role which is otherwise traditionally women's. It was estimated that the cost of investment could be recovered in a period in the range of 9 to 18 months, according to the relative prices of materials and of charcoal. Furthermore, the use of low-cost biogas reduces the cutting of highly valuable hard wood and hence protects the environment.

Keywords: biodigesters, biogas, women,


In many parts of Tanzania, deforestation has become a major problem. In some areas like the Usambara mountains in the north eastern Tanzania, the Government has intervened by legislating against grazing animals. To this end, zero grazing of cattle is encouraged by the Tanga Smallholder Dairy Development Programme (TSDDP) and about 4000 families in Tanga Region own at least a pair of dairy cattle. Although this positively affects the environment and provides additional income, it significantly increases the workload for those who provide fodder for cattle and clean the stalls. As is often the case, this labour is usually performed by the women of the household, who are already burdened with daily tasks, one of the most taxing of which is the collection of fuel wood. However, if the cattle manure was fed into a biodigester, and the resulting biogas was used for cooking, there would be three positive results:

Although biogas technology has existed in Tanzania for over a decade, this option is not feasible for the target population at the present time. With a population of over 25 million, about 200 biogas units had been built by 1991. The main reason for the small number of installations is the fact that the conventional units being built in the country are large and expensive, costing approximately US $ 1400 for one unit. Furthermore, repair and maintenance require highly skilled labour and most component parts, constructed mainly from concrete and steel, are far out of the financial reach of a smallholder with two cattle. There was, however, another type of biodigester which had not yet been widely tested or used in Tanzania in place of the conventional biodigesters: a low-cost, simply constructed, plastic biodigester being used successfully in Colombia (Botero and Preston 1987) and Vietnam (Bui Xuan An et al 1997). The type of plastic needed is locally manufactured in Tanzania, maintenance and repair are simple, cheap, and do not require skilled labour and the cost of construction is under US$ 50.

The recorded history of biogas technology in Tanzania goes as far back as 1982. This is when the sole authority in biogas, the Centre for Agricultural Mechanization and Rural Technology (CAMARTEC) started putting up a few biogas units in some villages in Arusha region. Until 1989 only 200 units had been installed all over the country (Sasse et al 1991)

Over the period, CAMARTEC had also been involved in training technicians in biogas plant construction. Despite all the efforts the technology did not diffuse to the rural poor communities. Some reasons for poor diffusion of biogas technology in general have been suggested to include: high installation and maintenance costs ( Moutik and Srinivasta 1975; Sathianathan 1975; Moutik 1982), lack of technical base for maintenance and repair (Roy 1981) and organizational difficulties (Fulford 1988).

The personal benefit of a biogas plant to the owner depends on how energy and fertilizer requirements were previously met.  The more energy had to be bought (charcoal, wood, and kerosene), the greater the benefit. The distance for water collection and the position of the cow-shed in relation to the inlet of the biodigester are also important considerations. The national benefit on savings to the environment (against deforestation) is an added advantage (Sasse 1988). The benefit in terms of fertilizer depends on how the slurry is used: if used while fresh, the benefits are higher with extra nitrogen / kg manure (Subramanian 1978). Other benefits were not easy to quantify (Sasse et al 1991); examples being pathogens inactivated in the fermentation processes (eg. typhoid, paratyphoid, cholera and dysentery bacteria). Sasse (1988) reported that bookworms and bilharzia parasites are killed in three weeks of retention time. However this benefit is more obvious in cases where the biodigester is also connected to the latrine.

Relief to women and children achieved through use of biogas is evoked in two ways. First that since in traditional African societies women are the ones who cook food for the family and hence have to be in the kitchen, if the use of biogas removes the smoke nuisance usually accompanied by use of firewood, then the women will be the first people to enjoy the difference (Subramanian 1978). Secondly, it has been reported that men participate more in household duties as the work becomes easier (Scheinman 1984). In the event of biogas introduction men may contribute to the energy acquisition either by financing the installation of the plant (cash contribution) or by physically getting involved in the installation of the unit. In this way a responsibility which was formerly put on women's shoulders is partly carried by men. In cases where women had to walk long distances in search of firewood, use of biogas would be an obvious relief.

Biogas plants are generally considered expensive, so ideas for reducing costs without reducing reliability or effectiveness need to be tested (Fulford 1988). It is advised by Sasse et al (1991) and Fulford (1988) that during the research period, an extension programme should be available to find out reasons for any plant failure. An effective biogas research and promotion requires skills of many people. Moutik and Srinivasta (1975) and Fulford (1988) suggest a presence of a sociologist or anthropologist to train and motivate people in respective areas.

Material and methods

Research hypothesis and objective

The research hypothesis was that, simple, efficient, low cost biodigesters could be constructed by smallholder farmers who practice zero grazing of cattle using only locally produced materials. Then this could lead to:

The objective of this research was to investigate the possibility of replacing firewood by biogas through the use of an affordable plastic biodigester.

Design and procedure

The research began with a survey in the randomly-selected villages where the research took place. Seventy two farmers were interviewed.  Survey questions focused on:

The survey elicited information on how the use of biogas would affect the livelihood of the farmers compared to the status quo in terms of economics, labor needs, socio-cultural values, and its influence on agriculture and environmental protection.

Village and District workshops

The aim of the workshops was to clarify and crosscheck field observations made during the survey, and to incorporate the ideas and priorities of the farmers into the research. Implementation of the research was discussed with farmers, including their contribution to the project. During the installation stages, technicians trained the farmers and other household members in maintenance, operation and general repair. Village extension workers were also trained. Other involved parties such as forestry and community development extension workers also took part. At these workshops the researcher was able to get an accurate picture of how the population felt about the technology, and to assess the level of demand for the biodigesters. In total, 2 district and 4 village meetings were conducted drawing participants from 12 villages.

Finally, a two-day workshop was held and attended by institutions responsible for environmental protection in the area and policy makers, to discuss the findings of the study and suggest ways through which these institutions could disseminate the technology. These institutions included the Centre for Agricultural Mechanization and Rural Technology (CAMARTEC), Soil Erosion Control and Agro-forestry Project, (SECAP), Tanga Smallholder Dairy Development Programme (TSDDP) and the Village Development Project (VDP).

Installation of biodigesters and measurements

A plastic biodigester prototype was field-tested in different ecological zones, looking at efficiency, maintenance requirement and durability. Biodigester temperature, gas flow, gas pressure, pH, efficiency, gas composition and gas production were measured along with difference in retention time and efficiency. The effect of environmental temperature and type of fodder on the efficiency of the biodigester units was examined. Maintenance requirements under different conditions were also studied. Analysis on the costs and benefits to the farm household of having such a unit was done. This took into consideration previous firewood costs and consumption in comparison to biogas daily production and use. The impact of biogas use in relation to the workload of women and children was studied. The men's involvement in contributing to biogas use, either financially or physically was examined.

Forty-six biodigesters were installed in phases in selected villages. Twelve units were installed first. Since experience has shown that farmers do not value handouts, particularly new ones, they paid 50% of installation cost as indication of their commitment. This money was used to help install the next two sets of 12 units and the funds from those units was used to set up the final ten biodigester units. These units were tested over a period of five months, which included both dry and rainy season conditions when demand for fuelwood and fodder availability were different.

Results and discussion

Survey results
Family size

The average household size was found to be five persons.

Economic activities

All families depended on subsistence agriculture with dairy farming as one of the main cash generators. Twelve percent had other small business activities such as tailoring, pottery and selling of vegetables or fruits. The major means of transport was a bicycle (5% of households had bicycles). The average distance from the nearest shopping centre was 2 km (12% on both extremes were eliminated).

Water availability

The mean distance from water source to the house was 500m. Since at least 60 litres of water are required for a cow/day as well as 60 litres to put into the biodigester, areas with serious water problems were not given priority in the survey. Water is available in most villages throughout the year: (These areas are endowed with mountain streams). For this reason it was concluded that water is generally available for daily activities. It was found out that in 86% of all homesteads, mothers and daughters are responsible for water collection. Installation of biodigesters in areas with water problems would result in increasing the burden rather than relief to women in the households.


The average number of livestock units was 2.5. Zero grazing is practiced mainly because of the following external forces:

Exposure to biogas technology

Reasons expressed for not having biodigesters:

Regarding the cost of the biodigester:

Ninety one percent of all interviewees hoped to save time in fuelwood collection and thought that troubles faced in fetching fuel wood during rainy season could be avoided.

Other Sources of Energy e.g Fuelwood

It was generally observed that:

Socio-economic characteristics

Basing on the results of the survey, women's role as daily managers in the household becomes clear. Some initial findings of a VDP gender survey presented during the dissemination workshop supported our findings. Also based on the interviews in 4 villages in Tanga Region it was found that women spend 69% of their time on housework, as opposed to 31% for men. Out of this 69%, approximately 34% was spent collecting firewood. For some households this worked out to be an average of 5 to 8 hours per day. This average was explained as an aggregate which included both the work of wives and daughters. Ninety five percent of all fuel requirements came from charcoal or firewood.

A second issue for scrutiny is the population's interest and awareness on biodigesters. It was found that 56% of those interviewed had heard of biodigesters and that 61% of those interviewed felt that they could meet the initial purchase costs.

Biodigesters require roughly 60 litres of water per day, which is equivalent to the daily water needs of dairy cattle. The average household walks more than 500 meters in order to fetch water. This implies that water is a prerequisite for owning a biogas plant, otherwise instead of lessening women's labor it will worsen it, or else the biodigester will remain dormant because of lack of water. This will be waste of time and money. A second implication is on the part of replicability of the technology to other areas. It should be remembered that one other important pre-requisite in the case of Tanzania is cattle, especially zero-grazed cattle. This was almost taken for granted because of the present big pool of zero grazing farmers in Tanzania. If 25% of these farmers were to put up a low-cost biodigester, this would mean more than 10,000 units, which is a great service to our environment.  No wonder this technology has still a great potential not only in Tanzania but also in other countries where zero-grazing of cattle is practiced.

When biodigesters were introduced family labor was reduced by roughly 30-40%. In a few cases men became more active in the kitchen. For example, men who refused to use firewood for cooking would wake up in the morning and cook their tea using the more convenient biogas stove. The table below summarizes some of the effects on household labour.

Table 1: Workload before and after biogas introduction
Activity Hours per day required to complete the activity before the introduction of Biogas Hours required to complete activity after the introduction of  Biogas  Hours of  household labor per day saved because of the  introduction of  Biogas

Collection of dung

Feeding biodigester

Collecting firewood

Cooking & cleaning

Slurry transport











(- 0.1)

(- 0.5)




TOTAL 8.6 3.6 5.0

Some comments are in order. First, much of the time saved is in cooking and cleaning. Part of this is because less supervision is needed in using biogas. Second, most families didn't pay for firewood. Instead they collected it from forests which typically (in Muheza District, for example) were 1-2 kilometers away. Third, some of the extra time needed in collection of dung could be more accurately called organization and sorting of dung. There was no saving in the work for  fodder collection.

In term of men's contributions, most husbands financed the purchase of materials to build their biodigester. Some 60% dug trenches and 45%, usually young boys, fed manure into the plant. This participation was not there when fuelwood was used.

Use of biodigester slurry

There was no efficient slurry use from the biodigesters. This was caused by the fact that cow shed construction did not take into consideration the pasture position as far as slope was concerned since the idea was not there before.

Technological and financial characteristics

As mentioned earlier, it was estimated that biodigesters could pay for themselves in anywhere between 5 to 16 months. The key assumption in this calculation was that biodigesters could be used instead of charcoal for 54% of the household cooking. Secondly, calculations depend on which price of charcoal is used and which investment costs are assumed. For example, it was generally agreed that a family would use one bag ("gunia" in Swahili, equivalent to roughly 28 kg) of charcoal per week. If a family used the same amount of charcoal but purchased it more often, in smaller units ("debes" in Swahili, a tin with capacity of 20 litres of water, or cups rather than "gunia") the price would be different. A "gunia" (in Tanga) was 2,000 Tsh (Tanzanian shillings) while "debes" could be either 2,500 or 3,000 Tsh. Secondly the investment cost was put at anywhere between 40,000 and 80,000 Tsh, depending also on subsidization and inflation.

In general the number of months needed to pay back the investment was calculated as:

cost of investment / savings per month                    


                cost of investment / 2.31 times price of a gunia of charcoal     

In other words it was assumed that the energy production of a biodigester was equal to 15.1 kg per week of charcoal (0.54 gunias of charcoal) or 64.7 kg per month (2.31 gunias) and that this energy substituted for the purchase of charcoal. The table below summarizes the results using various assumptions.

Table 2: Months needed to pay back biodigester under various price assumption.

Cost of one bag of charcoal
Cost of investment  (in Tsh*) 2,000 Tsh(*) 2,500 Tsh(*) 3,000 Tsh(*)


   8.7 months

13.0 months

17.3 months
  6.9 months

10.4 months

13.9 months
  5.8 months

  8.7 months

11.5 months

(*) Tsh:   Tanzanian shilling

Technical data

On average, gas production was 15 to 16 litres per day per kilogram of manure. The retention time (the time of residence of the substrate in the biodigester) was 16 days and the composition of the gas was found to be 65% methane, 32% carbon dioxide and 3% impurities.

With the aid of CAMARTEC other statistics were measured in Tanga Urban, Muheza, Korogwe and Lushoto Districts. In general, and as can be seen in the table below, there was very little variation amongst locations.

Table 3: Environmental and Technical statistics
Characteristic Tanga  Urban Muheza Korogwe Lushoto

Temperature inside biodigester (C)

Air temperature  (C)

Gas flow (litres/hour)

Gas pressure (mm)


Flame temperature  (C)

Ignition temperature  (C)

Efficiency (kWh/m of gas)

































According to CAMARTEC all these statistics are within reason. A one-degree temperature variation within the biodigesters was not a constraint and the optimal pH is within a range of 6.5 to 7. The pH values in Tanga Urban and Muheza were therefore a little low. Further investigation especially with the feed analysis may be important to find out reasons for differences in pH. The flame and ignition temperatures are important. If the flame temperature is too high cooking utensils could be ruined; if the ignition temperature is too low the gas could be accidentally ignited. Both temperatures presented no problems for biogas users.

Environmental conservation

In 1992, it was reported by the Bellerive Foundation that uncontrolled cultivation within and around the Usambaras is the main cause of environmental degradation. At this moment boundaries have been set around the forest beyond which cultivation is not allowed. Tree cutting, however, for fuelwood and charcoal making goes on unabated. The main concern is that hard wood tree species such as Sorindeia madagascariensis (Mpilipili), Newtonia buchananii (Mnyasa), Casearia battiscombei (Mkokoko) Khaya nyasica (Mondoro) Funtumia africana (Kiimboli) and Isobertina schefferi (Mbarika) are used for making charcoal and for fuelwood. It has been recorded that it takes on average 20 years for indigenous hard wood species to reach maturity, while tonnes of charcoal and fuelwood are burnt every day around the Usambaras.

Spread-over effect

Even at this early stage of low-cost biogas technology in Tanzania, there are signs for high rate of adoption. Some young men trained during the research period are introducing the technology to other areas. Lack of capital for starting up small revolving activities as far as putting up biodigesters and extension service to users remains the only limitation.

Conclusions and recommendations

Since the tubular plastic type of biodigester is affordable, simple to make and the construction materials are available in Tanzania, a strong dissemination is important to make the technology available to as many people as possible. A certain threshold number of biodigesters is necessary to justify costs of extension and follow-up before the technology has developed enough and users or else village technicians can solve problems related to repair and maintenance. To reach this, a group of technicians, who are provided with a small working capital to stock sufficient materials to build at least ten biodigesters, is needed. The biodigesters are built on a cash basis with a small margin to allow the construction to continue in a revolving manner. Each technician would be able to buy a bicycle used to move around to provide extension services to users.


This  research project was sponsored by the African Development Foundation (ADF) in Washington. Planning the research approach, developing questionnaires for the survey, organization of equipment and implementing the research required the combined efforts of several talented and helpful people. I am grateful to all farmers in Muheza and Korogwe who participated in the survey and their useful contributions in the Village and district meetings. Ms Evarista Kalalu of the Village Development Programme is thanked for her major role she played in the socio-economic survey. I am also grateful to my colleagues in different departments and organizations whose advice and contributions in Village and District meetings made this research possible. Mr. Simon and Professor Lekule of Sokoine University of Agriculture were very helpful in improving the biodigester prototype to fit in Tanzanian conditions. Mr Harold Ngowi and Mr Msafiri of the Centre for Agricultural Mechanization and Rural Technology are thanked for their assistance in the technical evaluation of the biodigester units. Ms Birgit Van Munster of the Tanga Smallholder Dairy Development Programme is thanked for her kind support in the early phase of research preparation. Ms Marie Elena John Smith is especially thanked for her constructive criticisms and advice in the research preparation and through out the research period. And lastly but not least,  my sincere appreciation goes to Dr Thomas Reginald Preston for his dedication towards poverty alleviation in the developing world and especially for introducing the idea of low cost biogas technology in Tanzania.


Botero R and Preston T R 1987  Low cost biodigester for production of fuel and fertilizer from animal excreta. ACP EEC Convention of Lome. Wageningen.

Bui Xuan A, Rodriguez L, Sarwatt S, Preston T and Dolberg F 1997. Installation and performance of low-cost polyethylene tube biodigesters on small-scale farms.  World Animal Review  88: 38-47. FAO

Fulford D 1988 Running a biogas programme. ITP New Yorkshire.

Moutik T K 1982 Biogas Energy in India, Academic Book Centre, Ahmedabad. India.

Moutik T K and Srinivasta UK 1975  Biogas Plants at Village level. Problems and Prospects in Gujurat, CMA, IIM, Ahmedabab. India.

Roy R 1981 Family and community biogas plants in Rural India. Appropriate technology, ITT Publications, London W.8.

Sasse L 1988  Biogas plants. Vieweg & Sohn. Wiesbaden.

Sasse L, Kellner C and Kimaro A 1991  Improved Biogas unit for Developing countries.  GATE Publication, Eschborn.

Sathianathan  M A 1975 Biogas Achievements and Challenges. Assoc. Voluntary Agencies for Rural Development. New Delhi, India.

Scheinman D 1984 A descriptive and analytical study of eight villages in Tanga region, and their capacity to support grade dairy cattle. Utrecht, The Netherlands.

Subramanian S K 1978  Biogas guides for house wives. Poona.

Received 10 January 1999

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