Construction Manual Final - Solar CITIES
Farmer’s Friend Biodigester ModelConstruction ManualNational Biodigester ProgrammeCambodiaNovember 2005Updated December 2006
Table of Contents1.INTRODUCTION.32.RESPONSIBILITIES OF A MASON .33.SIZES OF BIODIGESTER.34.COMPONENTS OF A BIODIGESTER.45.SELECTION OF PLANT SIZE .46.CONSTRUCTION MATERIALS AND APPLIANCES.57.SELECTION OF CONSTRUCTION SITE .88.CONSTRUCTION OF BIODIGESTER.98.18.28.38.48.58.68.78.89.LAY OUT OF BIODIGESTER AND DIGGING OF PIT.9CONCRETING IN FOUNDATION AND CONSTRUCTION OF CENTRAL PILLAR.11CONSTRUCTION OF DIGESTER AND GAS-HOLDER .12PLASTERING OF DIGESTER AND GAS-HOLDER .15CONSTRUCTION OF TURRET, MANHOLE AND OUTLET TANK .16CONSTRUCTION OF INLET .18FITTING OF PIPELINE AND APPLIANCES .19CONSTRUCTION OF COMPOST PITS .20FINISHING WORKS AND INSTRUCTIONS TO USERS .2110. CONCLUSION .21ANNEX: Drawings and Cost & Quantity Estimation of Biodigesters
1.IntroductionA biodigester is a structure constructed under the ground, made with cement, brick/stone, sandand pipes & appliances to decompose organic material and produce biogas to supplementconventional fuel sources and bio-slurry to apply as organic manure in the farms. By feeding therecommended amount of cattle dung/pig manure and water every day in the digester, clean gasis produced. This fuel is used mainly for cooking and lighting purposes where as the digestedslurry is used in vegetable gardens and agricultural fields.The success or failure of any biodigester depends mainly upon the appropriateness of the design,suitability of site for construction and quality of construction works including quality ofconstruction materials and workmanship involved during construction. This booklet will providethe knowledge on methods for selecting appropriate size and site for construction as well assteps of construction works related to the Farmer’s Friend (2005) model of biodigester.2.Responsibilities of a MasonThe mason’s role is vital in successful installation of biodigesters. The following are some of themajor responsibilities of a mason: Select proper size of bio-digester based upon the availability of feeding materials Ensure that the quality standards of construction materials and appliances are properlycomplied with. Follow strictly the design and drawing as provided to them during construction of biodigesters. Comply with the Construction Manuals while installing the biodigesters. Provide necessary information on benefits of biodigester to the users and motivate themfor biodigester installation Provide the users with minimum requirement of knowledge and skill to operate variouscomponents of bio-digester Ensure timely completion of the work Report progress and difficulties, if any to supervisors regularly Do not allow untrained masons to take lead responsibly in constructing biodigester Work as extension worker and promoter of the technology in their areas of influences Provide regular follow-up and after-sales services to the users to ensure trouble-freefunctioning of completed plantsThis manual is prepared to assist the masons to successfully carry out their anticipated roles inconstructing quality biodigesters.3.Sizes of BiodigesterFarmer’s Friend Biodigesters are fixed dome design plants. 4, 6, 8 and 10 m3 biodigesters ofFarmer’s Friend Biodigester models are eligible for obtaining subsidy from the Government ofCambodia under the National Biodigester Programme. No other sizes and designs will beeligible to receive subsidy under the programme. The following table presents some facts aboutthese 4 sizes biogas plants.
Table- 1: Quantity of Feeding RequiredSNCapacity ofplant (M3) *Freshrequired(M3)day ** (Kg)dungeveryWater requiredevery day (litre)140.8 - 1.620-4020-40261.6 - 2.440-6040-60382.4 - 3.260-8060-804103.2 - 4.080-10080-100***4.DailygasproductionCapacity of plant means the volume of digester and gas storage domeAverage retention time: 40 daysComponents of a BiodigesterThere are 6 main parts of the biodigester: inlet (mixing chamber) for cattle dung fed plant andmaturation pond for pig manure, digester (digestion chamber), gas holder (storage chamber),outlet (displacement chamber) and gas conveyance system and slurry compost pit(s). The mix ofdung and water (mixed in inlet or mixing chamber) or pig pig manure flushed from stable passesthrough the inlet pipe to the digester. The mixer produces gas through digestion process in thedigester and the produced gas is stored in the gas holder (top of dome). The digested slurrypasses out from digester to outlet tank (displacement chamber) and flows out to the compost pitsthrough overflow opening in the outlet tank. The gas is then supplied to the kitchen through thepipe line. The Farmer’s Friend Biodigester generally consists in detail of:1. Inlet (Mixing Tank)2. Inlet Pipe(s) separate for cattle dung/pig manure and latrine3. Digester4. Gas Holder (dome)5. Manhole6. Outlet (Displacement Chamber) and overflow opening7. Main Gas Pipe and Turret8. Main Gas Valve9. Pipeline10. Water Outlet (Water Trap or Drain)11. Gas Tap12. Gas Stove with rubber hose pipe13. Gas Lamp (Optional)14. Slurry pit(s)5.Selection of Plant SizeThe size and dimensions of the biodigesters have been decided based upon 40 days retentiontime and 50% gas storage. This means that the fresh feeding fed into the digester should remaininside it for at least 40 days before it comes out through outlet. Likewise, the plant should beable to store 50% the gas produced in 24 hours. Therefore the size of the biodigester has to beselected based upon the daily available quantity of feeding materials.
Before deciding the size of biodigester to be installed, all the dung available from cattle or thepig manure has to be collected to know how much feeding material is available every day. Thefollowing table shows the capacity of biodigesters to be decided based upon the availability offeeding material (dung or pig manure).Quantity of feeding material availabledaily (kg)Recommendedof Plant (cum)SizeQuantity of Fuel woodsaved per day (kg)20-4044 to 841-6068 to 1261-80812 to 16More than 801016 to 20If the plant is not fed properly, gas production will be less than the theoretical expectation. If gasproduction is less, the gas collected in the gasholder will not have sufficient pressure to push thedigested slurry to the outlet. In such case, the slurry level will be raised and reach to the gasholder instead of flowing to outlet. When the main gas valve is opened in this situation, theslurry also passes to the pipeline together with the gas. Therefore, if there is not enough quantityof feeding material available as per the prescribed rate, bigger size of biodigester should not beinstalled. Underfed and bigger plants will just increase the cost of installation and also createproblem in operation. The important point to be considered while deciding the size ofbiodigester is that the basis for selecting size is the availability of dung not the family sizeor gas demand. If the farmer has higher number cattle or pigs then only the size isdetermined by the gas demand which is usually taken to be 0.33 cum gas per person perday.6.Construction Materials and AppliancesIf the construction materials to be used for the construction of biodigester are not of goodquality, the biodigester will not function properly even if the design is correct and workmanshipinvolved in construction is excellent. The plant will never be of high quality if inferior quality ofconstruction materials is used. In order to select these materials of best quality, required qualitystandards and specifications of these materials are briefly described below:CementCement should be high quality Portland cement from a brand with a good reputation. It must befresh, free from lumps and stored in dry place. Cement with lumps should be used forconstruction. Bags of cement should not be stacked directly on the floor or against the walls.Wooden planks have to be placed on the floor to protect cement from dampness. Cement bagsshould be stalked at least 20 cm away from any walls.SandSand should be clean and should not contain soil or other materials. Dirty sand will have verynegative effect on the strength of the structure. If sand contains more than 3% impurities, it mustbe washed. The quantity of impurities especially the mud, in the sand can be determined by asimple ‘bottle test’. A small quantity of sand is put into a transparent bottle and water is pouredinto it. The bottle is shacked vigorously for a while. The bottle is then left stationary to allow the
sand particles to settle down. The particles of sand are heavier than that of silt and clay, so itsettles faster where as the mud particles settle slower. After 30 minutes, the layer of mud versessand inside the bottle is measured without shaking the bottle. If the depth of mud is more than3% of the total depth, than it can be concluded that the sand contains too much mud. If thishappens, the sand should be washed before use. Coarse and granular sand are best forconcreting, however, fine sand has to be used for plastering and mortar works.GravelsSize of gravel should not be very big neither very small. It should not be bigger than 25% of thethickness of the concrete product where it is used. The thickness of concrete layer in thefoundation and that of outlet slabs is not more than 7.5 cm (3”), therefore the maximum size ofgravels should be 2 cm or ¼ size of the size of thickness of concrete layer. Gravel should beclean, hard and of angular shape. If it is dirty, it has to be washed properly before use.WaterWater is mainly required for making the cement mortar for masonry works, concreting worksand plastering. It is also used to soak bricks before using it. Besides, it is required for cleaning orwashing construction materials if they are dirty. The water from ponds or cannel may be dirty soit is better not to use it. Dirty water will have an adverse effect on the strength of structure.Water from water tap or well or any other sources that supply clean water has to be used.BricksBrick plays a very important role in construction especially for Farmer’s Friend model ofbiodigesters. Bricks should be of high quality (no.1), usually the best quality available in thelocal market. The bricks should be well burnt, straight, regular in shape & sizes and should nothave cracks or broken-parts. High quality bricks make a clear metallic sound when hitting themto each other. Such bricks should be able to bear a pressure of 120 Kg per square centimetre.Before use, bricks must be soaked for few minutes in clean water. Wet brick will not absorbwater (moisture) from the mortar which is needed for setting properly.Acrylic Emulsion PaintIt is used to make the gas holder (dome) of biodigester air-tight. Paint of this type should meetquality standard and they must be approved from concerned quality control authority.Mild Steel BarsMS bars are used to construct the covers of outlet tank and water drain chamber. It should meetthe engineering standard generally adopted. For plants of 4, 6 and 8 cum, MS rods of 8 mmdiameter and for plant of 10 cum capacity 10 mm diameter is recommended. MS bar should befree from heavy rust.Main Gas PipeGas stored in the gas holder is conveyed to the pipeline through this pipe which is placed in thetopmost portion of the dome. The joint of reduction elbow with this pipe should be perfect andgas tight otherwise gas leakage from this joint can not be stopped easily. Therefore it isrecommended that the reduction elbow has to be fitted in a workshop to ensure perfect airtightness of the joint. The gas pipe should be properly galvanised and approved by concerned
quality control authority. This pipe should be made up of light quality iron and MS rod has to bewelded at one end to embed it with the concrete during installation. The length of this pipeshould be at least 60 cm.Main gas valveIt controls the flow of biogas in the pipeline from the gas holder. It is opened when gas is to beused and closed after each use. If substandard quality of main gas valve is used, there is alwaysrisk of gas leakage. This valve should be of high quality and approved by the concerned qualitycontrol authorities.Pipes and fittingsThe pipe to be used to convey gas from gas holder to the point of application should conform toquality specification as per Cambodian standard. Light quality Galvanised Iron pipe is bestsuited for this purpose; however, high quality PVC pie could also be used. The pipe should be ofhalf inch diameter. For length of more than m, ¾” diameter pipe has to be used. If GI pipe isto be used, a six meter pipe should weigh at least 6 kg.The fittings used in the pipeline of a biogas plants are socket, elbow, tee and nipples. Thesefitting should meet the required quality standards. The weight of 100 pieces of each GI fittingshould be 13 kg, 9 kg and 6 kg for tee, elbow and socket respectively.Water OutletIt drains the water condensed inside the pipeline when biogas comes in contact with the coolpipe. This is an important component of biogas plant and therefore, its quality should carefullybe controlled. It should be easy to operate and threads in it should be perfect. It should beensured that the hole in the screw nut is bored properly and is located at the right place. Thethickness of the nylon washer has to be 4mm and either a 4 cm long handle pin or a properlyknurled opener should be used. This appliance should be approved by the concerned authorities.Gas TapGas tap is used for regulating flow of gas to the gas stove. Care should be taken to install gas tapof high quality. It has been often complained by the users that this taps are becomingproblematic with gas leakage through them. It is important that the ‘o’ ring is placed properlyand is greased thoroughly and regularly. The gas tap should not be too tight or loose to operate.The taps to be used in biodigesters should be approved by concerned quality control authority.Rubber Hose PipeIt is used to convey gas from the gas tap to the stove. This pipe should be made up of highquality neoprene rubber and should not develop cracks when folded. It should have 15 mm outerand 9 mm inner diameters. The minimum wall thickness of the pipe should be 2.5 mm.Gas StoveGas stoves can be found with single and double burners. In general a single burner gas stoveused for household purpose consumes 350 to 400 litre of gas per hour. The efficiency of gasstove is very important for the successful functioning of the biodigester. The stove should be ofgood quality and strong enough to firmly rest in ground. The primary air intake should be easily
adjustable and the holes should be properly placed. The jet and pipe leading to the burner shouldbe straight and aligned properly. The holes in the burner cap should be evenly spread across it.Gas LampGas lamp is another important appliances used in biodigesters. Often users complain about themalfunctioning of these lamps. These lamps should be of high quality with efficiency more than60%. Usually, a biogas lamp consumes 150 to 175 litres of biogas per hour. Lamps to be used inbiodigesters have to be approved by the concerned quality control authority.7.Selection of Construction SiteSelection of construction sites are mainly governed by the following factors: The site should facilitate easy construction works. The selected site should be such that the construction cost is minimised. The selected site should ensure easy operation and maintenance activities like feeding ofPlant, Use of main gas valve, Composing and use of slurry, Checking of gas leakage,Draining condensed water from pipeline etc. The site should guarantee plant safetyBased upon the above mentioned factors, it is recommended to select plant location based uponthe following considerations. Please note that it will not be possible to meet all the requirementsas stated below, however, it should be ensured that as many as possible points are considers. For effective functioning of biodigesters, right temperature (20-35 C) has to bemaintained inside the digester. Therefore it is better to avoid damp and cool place –Sunny site is preferable. The area to construct plant should have even surface. The site should be in slightly higher elevation than the surrounding. This helps inavoiding water logging. This also ensures free flow of slurry from overflow outlet to thecomposting pit. To make plant easier to operate and avoid wastage of raw materials, especially thedung/pig manure, plant must be as close as possible to the cattle shed or pig sty. To mix dung and water or flush pig manure to the digester, considerable quantity ofwater is required. If water source is far, the burden of fetching water becomes more.However, the well or ground water source should be at least 10 meter away from thebiodigester especially the slurry pit to avoid the ground water pollution. If longer gas pipe is used the cost will be increased as the conveyance system becomescostly. Furthermore, longer pipeline increases the risk of gas leakage. The main gasvalve which is fitted just above the gas holder should be opened and closed before andafter the use of biogas. Therefore the plant should be as near to the point of applicationas possible. The edge of plant should be at least 2 meter away from the foundation of house or anystructure.
There should be enough space for compost-pit(s) as these are integral parts of thebiodigester. The site should be at sufficient distance from trees to avoid damage of bio-digester fromroots. Type of soil should have enough bearing capacity to avoid the possibility of sinking ofstructure. When space is a problem, the pig-sty can be constructed on top of the plant after properbackfilling.8.Construction of BiodigesterWhen suitable sites and construction materials and appliances to meet the quality standards areselected, the construction work of biodigester has to be started. The following are the steps inconstructing biodigester: Lay-out of plant and digging of the pitFixing the curvature and laying of the foundationConstruction of a central masonry block and collarConst
6. Construction Materials and Appliances If the construction materials to be used for the construction of biodigester are not of good quality, the biodigester will not function properly even if the design is correct and workmanship involved in construction is excellent. The plant will never be of high quality if inferior quality of
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