Biogasdigesters take organic material such as animal dung into an air-tight tank,where bacteria break down the material and release biogas – a mixture of mainlymethane with some carbon dioxide. The biogas can be burned as fuel for cookingor other purposes such as to run a lamp, which consumes about 0.23 m3 of gasper hour to provide light equivalent to a 40 W light bulb. Most biogas plantusers do not have a lamp due to the additional gas demand it creates, and thesolid residue can be used as organic compost.

In thecase of cow dung, the cow dung is thoroughly mixed with water in the ratio of1:1 and put in a sealed container (various containers depending on type). Thecontainers are filled, leaving 10 cm at the top. The containers are fitted withslurry inlet and gas outlet pipes (Figures 2.4 to 2.9). The slurry is left toferment, producing the methane. The methane is allowed to collect in the deadend of the container or in the gas reservoir if provided. After seven days thetap to stoves or any other device connected to the unit can now be turned onwhen required for use. The slurry is poured out after a 30- day period and thecontainer filled again. In some cases, a slurry output is fitted at the bottomof the tank.

Bio-digestersare of various types. The common plants consist of an underground brick builtvessel with a ground-level inlet for new cow manure mixed with an equal amountof water (slurry) and outlets for gas and residue. Building a bio-digesterbegins with the excavation hole measuring 4 m in diameter, about 2 m deep atthe bottom of which a concrete floor is cast. An outer wall is built up usingbricks, with each circular row of bricks gradually leaning inwards to form adome. A gas pipe made of galvanised steel is held in place in the centre bybrick and mortar, and takes the gas to the biogas stove in the kitchen througha high density polyethylene (HDPE) pipe. The cow slurry is collected in aninlet tank at ground level and flows under gravity into the digester vessel. Asthe slurry flows in, an equal volume of digested residue is displaced into aground-level reservoir at the outlet. All materials used are available locally,except for the gas burners and HDPE piping which come from elsewhere in shopssituated in nearby towns (Figure 2.4). The digesters use mesophilic bacteria,which operate best at temperatures between 28°C and 36°C. It is important thatthe bacteria have sufficient time to break down the slurry, and the size ofdigester is chosen so that the slurry is held for an average retention time of40 days.

Plantscan be constructed in different sizes to serve different market sectorsincluding domestic (individual households) and institutions (schools, hostels,hospitals, hotels). Most domestic plants have a volume of 1, 2 and 3 m3,corresponding to manure from 50 to 100 kg per day and 2–6 cows, and produceabout 1 m3 of biogas per day, with a maximum input of 5 kg solids and 20 lwater per day (Figures 2.4 to 2.9). The typical plant consists of the digestervessel and gas collection chamber floating over a pre-cast digester tank. In asimple form, the digester is usually prefabricated from ferro-cement and sunkinto the ground. The gas holder is constructed from fibreglass reinforcedplastic (FRP), weighed down with concrete to increase the gas pressure. It isadvised to use waterproof cement in the construction of the digester tank(Figure 2.5).

A simplemodel of the portable digesters that stand on the surface, to allowconstruction where excavation is impossible or undesirable—for example, in theextreme clay soils that may cause the dome to crack easily—can be seen inFigures 2.6, 2.7 and 2.8. Cow dung is simply mixed with water from the slurryin a bucket and poured into the plant inlet, and no additional water is needed.On-site construction takes only about four hours (instead of four days) andsemi-skilled or unskilled masons can easily install the units.

In allcases biogas gradually collects in the gas holder/chamber as the wastedecomposes, and a pipe is used to take it to a special biogas stove in thekitchen. A valve is used to open and shut the flow, and a regulator varies theflame. The effluent from the plants is virtually odourless and has a highcontent of nitrogen, phosphorous and potassium, so it can be used as a gardenfertiliser.

Types of biogas unit in use

Figure2.4: Example of a common type of biodigester. 

Figure 2.5: Example of a typical domesticplant.

 Figure2.6: Portable steel iron type.

Figure 2.7: Portable steel iron type withpressure device.

Figure 2.8: Portable plastic drum/barrel type. 

Figure 2.9: Simple but non-portable digester.

The problem the technology or innovationis aimed to address

§  Alternative source of fossil energy torescue deforestation

§  Take-up of methane (green gas) to reducegreen gas effects

§  Reduce carbon dioxide from theatmosphere (carbon sink)

§  Facilitate use of manure in crop fieldsto improve soil fertility and structure for increased production

Justification

Thetechnology/innovation is considered appropriate for promotion as best practiceto respond to effects of climate change and variability in arid and semi-aridareas for five major reasons:

§  User and environment friendly

§  Cost-effective alternative source offossil energy

§  Reduces methane going into atmosphere

§  Reduces deforestation hence maintainingor even increasing forests (carbon sink)

§  Facilitates use of manure in crop fieldsfor improved crop production and hence livelihood.

Where tested in semi-arid or arid areas,the technology/ innovation has been adopted and utilised by beneficiaries

Thereare more than 1010 units currently in use in various areas of Tanzania coveringthe semi-arid areas of Southern, Central, Northern and Western Zones whichcover a large number of districts. Most of the regions are semi-arid, occupiedby agropastoralists, and there is very limited forest cover for firewood.Simple small-scale bio-digester units are used in these areas and the commonfarmer can easily install and maintain them, even with very limited watersupply.

Scaling-up approaches

Small-scalefarmers and non-farmers who obtain cow dung from the farmers in the arid zone,schools and other institutes as alternative source of firewood. The use ofbiogas was first introduced in the humid areas where there is a tremendous needto save the forests and labour for firewood. This technology was then diffusedto the arid zones through awareness creation by politicians and technocratshighlighting the advantages of using the gas. Farmer visits and study tours aswell as on-station demonstrations were used as a learning approach for thefarmers.

Thecritical and essential factors for successful promotion and adoption of thetechnology/innovation comprised: raising awareness about the importance of thetechnology on the environment as a cheaper, user-friendly source of energy;making available the necessary components for fully functioning units such ascookers, bulbs and the necessary connections; advocacy to the various users;and increasing end-users’ capacity to construct and maintain the units forsustainability.

Challengesencountered with respect to further dissemination of the technology/innovation, adoption and scaling-up/out were: 1) availability of accessories toensure complete units (connections, stoves, appropriate gas tubes; 2) farmers’ability to construct and maintain their units without outside intervention; 3)water availability in arid areas; and 4) adding value to the slurry for safeuse as manure.

Recommendationsfor addressing the challenges consist of involving private manufacturers in thedesign of the units and quality of required materials; training farmers in the constructionand maintenance of units, and producing manuals for each type; adopting waterharvesting methods and borehole technologies; and conducting research on valueaddition to the manure.

Lessonslearnt about the best ways to ensure widespread adoption of the technologiesconsisted of introducing the technology along with other innovations such assimple stoves, dairy animals, use of manure as a source of nutrients for fieldcrops, simple water borehole construction and an economical approach to maintainingthe water.

§  One of the most important benefits thatbiogas brings to villagers is saving of time. About three hours per day used togather wood and to cook (work done mostly by women) can be released for incomegeneration and other purposes. It takes about 75% less time to cook with biogasthan with wood, so parents have time to engage in income-generation activities.

§  Women can earn money by selling halftheir compost, and still have enough for their own land. Having their ownsource of income has enabled women to acquire small loans.

§  Cooking with biogas instead of woodbrings significant health benefits, especially for women. The reduction insmoke and soot reduces the incidence of respiratory complaints, eye problemsand headaches.

§  The cookware is easier to clean and thehouse does not become so dirty. The technology is therefore gender friendly asit releases women from the burden of searching for firewood, provides a simpleand efficient source of manure and decreases the family expenditure on energy,which in most local families falls to women.

§  Biogas units are used to convert animalwaste into a mixture of methane and carbon dioxide that can be used forlighting and cooking, and the digested residues applied as organic fertilisersin crop fields; this reduces the total amount of green gas going into theatmosphere.

§  There is a possibility of claimingcarbon credits under the Clean Development Mechanism set up by UNFCCC.

Name and contact of the organisation:

Ministryof Livestock Development and Fisheries,

P. O.Box 1952

Dar esSalaam, Tanzania;

Email:drd@ud.co.tz; utafitimifugo@ uahoo.com;

Tel:+225 22 2863358;

Fax:+225 22 2863967;

Mobile:784 482404

Name and contact of presenter:

Dr LyimoHurbert Naftal,

P. O.Box 1952;

Dar esSalaam, Tanzania;

Email: hlnlyimo@yahoo.com;

Tel:+225 784 482404;

Mobile:+225784482404170

Name and contact of key scientist:

Prof PLekule,

P. O.Box 30000

SokoineUniversity of Agriculture, Morogoro, Tanzania;

Email: lekule@yahoo.com;

Name and contact of key partners:

HeiferInternational Tanzania,

P. O.Box 1648,

Arusha,Tanzania;

Email: www.ango-tz.org;

BaptistChurch,

PrivateBag, Singida, Tanzania;

CARMATECTanzania,

P. O.Box 6122, Arusha, Tanzania;

Telephone:+255 27 2553214

§  Biogas units are used to convert animalwaste into a mixture of methane and carbon dioxide that can be used forlighting and cooking, and the digested residues applied as organic fertilisersin crop fields; this reduces the total amount of green gas going into the atmosphere.

§  There is a possibility of claimingcarbon credits under the Clean Development Mechanism set up by UNFCCC.

§  Biogas facilitates easy disposal ofanimal manure, leading to the village environment becoming cleaner.

§  Biogas brings significant educationalbenefits for children, as there is more time for them to study.

§  The demand for organic farming increasesthe chance of more income from using the organic fertiliser from biogas.

§  Without biogas, a rural family uses anaverage of 3.5 tonnes/year of firewood on a traditional stove. Firewood isbecoming scarce because of pressure for land, and a biogas digester replacesthe use of wood. A study for a clean development mechanism (CDM) projectestimated that in general, an average biogas digester saves 4 tonnes/year ofCO2 from the replacement of unsustainable firewood.

§  Biogas programmes have created work andprovided training to local people including masons and maintenance technicianswho might otherwise migrate to the cities in search of employment.

The economy of biogas, training andsupport

Atypical plant measuring 3 m3 costs about £190. The amount varies betweendistricts. To encourage the farmer, the project can pay up to 60% of the totalcost with the remaining 40% being paid in kind by the farmer through theprovision of sand, gravel and bricks for the construction, participating in theconstruction and providing food for the construction workers and theirsupervisors.

When acustomer orders a biogas plant, a technician coordinates the work, arranges formaterials to be delivered, organises training of the owners and checks thequality of the construction. The supervisor for that area will help the newuser to become familiar with the plant operation and gives each user amaintenance manual.

The useof trained, local installation staff is essential to ensure the long-termreliability of digesters. The project may engage supervisors and masons fromthe local area to do the installation. New recruits are often unemployed youthfrom families with biogas plants, so they come with practical experience ofplant operation. They initially help the trained masons in their work untilthey have sufficient experience to work independently. Both men and women aretrained as maintenance technicians. Use of local construction workers buildslocal knowledge of the operation and maintenance of biogas plants.