Design, Construction and Maintenance of a Biogas Generator | Anaerobic Digestion

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Biogas generators can be used at household or community level to produce usable fuel and fertilisers from human and animal waste. This document covers many of the technical aspects of designing, constructing and maintaining a biogas generator however does not explore the cultural, social and education facets of the project. These should be researched separately with reference made to the case studies within this document.
  OXFAM Technical Briefs  –   Repairing, cleaning and disinfecting hand dug wells 1 Design, construction and maintenance of a biogas generator Biogas generators can be used at household or community level to produce usable fuel and fertilisers from human and animal waste. This document covers many of the technical aspects of designing, constructing and maintaining a biogas generator however does not explore the cultural, social and education facets of the project. These should be researched separately with reference made to the case studies within this document. Introduction Biogas generators extract by-products from organic waste (including human and animal excreta, food stuffs, etc) which can be used to replace traditional fuels and fertilisers. Biogas generators produce 2 useful products: 1.Biogas  –  biogas is a natural gas which can be useddirectly as a fuel for cooking and heating or usedto run a converted generator for electricityproduction.2.Fertiliser  –  digested sludge from the bottom of thebiogas generator and over-flow effluent water canbe used as a fertiliser for cropsThe benefits of biogas generators are explicitly listed below and should be made clear when suggesting the construction of the biogas generator to users in order to improve speed and likelihood of acceptance: 1.Biogas generators provide a safe and cleaner wayof storing excreta and subsequently bring aboutrelated advantages linked to safe sanitation2.Biogas generators provide free fuel for cooking,heating and lighting3.Biogas generators provide fertiliser for crops4.Biogas requires far less time and effort to collectthan other fuels (e.g. wood)5.Biogas reduces the need for wood and thereforereduces deforestation and the burden on women of collecting wood6.Biogas creates no smoke and therefore reduceshealth problems caused by burning other fuelsindoors7.Biogas is environmentally friendly and does notrelease as many greenhouse gases when burnedcompared to other fuels8.Dangerous bacteria in faeces are killed duringdigestion in the biogas generatorBiogas has an energy density of 6kWh/m 3 . 1m 3  of biogas has the approximate equivalent energy of some common fuels as shown in Table 1  .   Table 1: Approximate equivalent energy [1]     Application Equivalent Cooking 10kg cow dung 5kg wood 2kg charcoal Electricity Can generate 1.25kWh Biogas production Literature states that the biogas production rate of human excreta is 0.02-0.07m 3  /kg/day however the data varies greatly and is dependent on many variables (diet, food intake, water intake, climate, etc). Similar variance is apparent in human waste production data. Literature suggests that an average adult can be expected to produce 1-1.3kg of urine and 0.2-0.4kg of faeces per day [7]  (if local figures are available then use these instead). GTZ suggest the following production rates of biogas from wastes of different animals per day in warm climates (in addition to human excreta other organic waste such as cattle dung can be added to the generator to increase biogas production). See Table 2  . Table 2: Approximate biogas production rates of different waste Source Biogas per day (m 3 /day) [5]   1kg Cattle dung 0.04 1kg Pig dung 0.06 1kg Chicken droppings 0.07 1kg Human excreta 0.02-0.07 The amount of biogas produced in a human excreta fed plant can be increased with the addition of animal manure, however care must be taken to ensure the correct solid:liquid (and also carbon:nitrogen) ratio is applied. Water should be mixed with the manure (in a solid:liquid ratio of 1:1) in a mixing tank to create a slurry prior to being added to the digester.  OXFAM Technical Briefs  –   Repairing, cleaning and disinfecting hand dug wells 2 Biogas use  Appliances connected to the biogas distribution pipes can extract gas from the gas holder when required. Cooking  –  Stoves specifically designed for biogas are available which make the best use of the fuel  –  a much higher fuel:air ratio is required with biogas burners compared to butane burners (1:6 by vol. compared to 1:30) [5] . These are available from a number of manufacturers for between US $15-$30 (e.g. Rupak Enterprises, India) [9] or can be made more cheaply locally (see Case Study II  ).  Alternatively simple modifications can be made to butane/propane stoves (by expanding the gas jet cross-section by 2-4 times or modifying fuel:air ratio by adjustment if possible) to obtain the desired compact and slightly blueish flame [5] . Figure 1   shows the relative ideal diameters of the biogas jet and the mixing pipe. Figure 2   shows a biogas stove in use. Figure 1: Biogas burner dimensions [4]   Figure 2: Biogas stove [17] Lighting    –  Biogas can be used in gas lamps to provide lighting which can be used, for example, to light the toilets. With an efficient biogas lamp, a lighting intensity comparable to that of an electric bulb of 25-75W can be produced. Biogas lamps can be purchased from a number of manufacturers (in China, India and Brazil) from around US $6 [10] . References to a popular Brazilian design  –   the ‘Jackwall’ lamp (c. USD $10) have been made in a number of internet articles however suppliers/technical information on this particular model cannot be found. Biogas mantle lamps such as the one shown in Figure 3   (source: Sustainable Sanitation (Flickr), Kabul) are relatively common. Figure 3: Biogas lamp  Alternatively normal gas lamps can be modified for efficient biogas use (however they will never be as fuel efficient as kerosene ones). Again the gas jet should be widened so that the diameter of the jet is approximately 1/6 that of the mixing pipe (see Figure 1  ). The flame should be adjusted to be the same size as the mantle, with a uniform brightness and a steady sputtering murmur. Electricity generation    –  For the simplest, smallest scale electricity generation it is possible to run adapted diesel engines on a mixture of biogas and diesel. GTZ suggest that a largely unmodified diesel engine can be used with only the addition of a gas/air mixing chamber (50%-100% of the cylinder size) situated on the air intake. Engines can run without stuttering (stuttering signifies a too high biogas:diesel ratio) on a biogas:diesel ratio of up to 4:1 (80% biogas, 20% diesel). This can be achieved simply by tuning down the amount of diesel entering the engine.  According to GTZ 1.5m 3  of biogas with 0.14l of diesel can produce 1kWh of electricity.  Although large scale electricity generation in developed countries (especially Germany) is common, it is still relatively rare in developing countries due to the required quality (and therefore processing) of the gas (levels of CO 2 , H 2 S and water vapour need to be reduced). A publication by GTZ (‘Engines for Biogas’) describes in detail suitable engine modifications and design (see Further Reading  ). Some approximate biogas consumption rates are shown in Table 3  . Table 3: Approximate biogas consumption rates [1]     Application Volume biogas (m 3 ) Consumption per person per meal 0.15-0.30 Boil 1 litre water 0.03-0.04 Cook 0.5kg rice 0.12-0.14 Cook 0.5kg vegetables 0.16-0.19 Gas lamp lighting (1 hour) 0.07-0.20 Biogas mechanics Figure 4: Schematic of biogas reaction [1]    Stage 1  –   Hydrolysis Bacteria decompose long chains of complex carbohydrates and proteins in the biomass into smaller molecules. Stage 2  –   Acidification  Acid-producing bacteria convert the smaller molecules produced in the first step into acetic acid (CH 3 COOH), hydrogen (H 2 ) and carbon dioxide (CO 2 ).  OXFAM Technical Briefs  –   Repairing, cleaning and disinfecting hand dug wells 3 Stage 3  –   Methane formation (Anaerobic) Methane-producing bacteria convert the acetic acid (CH 3 COOH), hydrogen (H 2 ) and carbon dioxide (CO 2 ) into methane (CH 4 ) and carbon dioxide (CO 2 ). This mixture of gas is known as biogas. Table 4: Typical biogas composition [2]    Compound Symbol Presence Methane CH 4  50%-70% Carbon dioxide CO 2  30%-40% Hydrogen H 2  5%-10% Nitrogen N 2  1%-2% Other gases H 2 O, H 2 S Trace Process parameters There are a number of parameters which effect the production of biogas which should be kept at an optimum level for human excreta biogas generators: 1. Substrate temperature    –  Digestion works bestat around 36°c. Expected approximate gas returnof 1kg of human waste over 60 days is as follows: o 0.43m 3  biogas at 35°c o 0.3m 3  biogas at 25°c o Unsatisfactory under 15°c Areas where the atmospheric temperature falls below 15°c seasonally should not be considered for unheated biogas generators (see Figure 5  ). Within limits, low temperatures can be compensated for with a longer retention time. Figure 5: Areas highlighted can be considered for biogas 2. Hydraulic retention time (HRT)  –  The averageamount of time that the liquid part of the slurryshall be in the digester for. This should be longenough to reduce many of the pathogens and toallow the maximum amount of gas to be extractedhowever will increase the digester volume. TheHRT should be based on a compromise of pathogen removal time and digester size.Commonly the HRT lies in the range 60-100 days if no wastewater sewage outlet is available howevercan be reduced to as little as 5 days [8] in the presence of a sewage outlet. A summary of pathogen inactivation times (within the sludge) is shown in Table 5  : 3. Solid retention time (SRT)  –  The sludge (moresolid part of substrate) should be stored for arelatively long retention time (anywhere between 1and 5 years depending on digester design, wastecontent, digester size, etc) before being removedfrom the generator and pre-treated for theappropriate time prior to use as fertiliser (e.g. hightemperature composting with added soil components, see Table 7  ). Over this period of time the maximum amount of gas will have been produced and the number of pathogens in the waste should have decreased considerably. The solid retention time is very much variable and the ideal is generally found by experiment and experience. Table 5: Pathogen reduction times [3]    Little research can be found as to the deactivation time of Vibrio Cholera however one research paper has been written in which a 2 stage anaerobic digester is used [20] . 4. pH    –  The pH of the slurry should not drop below 6.2(this will have a toxic effect on methane-producingbacteria). A healthy digestion process is indicated by aneutral pH (7.0) [6] .5.  Agitation/mixing  –  Mixing of the slurry can increasegas production by ensuring an even distribution of bacteria and fresh substrate. The large scale industrialgenerators are often fitted with motor-driven rotatingpaddles whilst smaller agricultural ones are mixed withlong poles by hand. There is little information on theoptimum frequency of mixing in human waste feddigesters but GTZ suggest the gas productionincreases dramatically when mixing is undertaken(slowly and perhaps once a day or once a week).Different time intervals between mixes should be triedto identify the optimum level for any specificgenerator.6. Solids content    –  Generally a slurry with asolids:liquids (faeces:urine) ratio of 1:1 should beaimed for. A Total Solids (TS) content of between 7-11% is ideal  –  since the actual liquid content of faecesis quite high faeces and urine should be added inapproximately equal amounts to achieve this.7. Inhibiting factors  –  The presence of heavy metals,antibiotics and detergents in the slurry can inhibit thebiogas production process. Any addition of these tothe digester should be avoided. Anything which is notbiodegradable  should not be added to the digestersince it will take up valuable space and could lead to ablockage. It would seem that anal cleansing material(including water and paper) can be added to thedigester [19]  whilst keeping in mind that the ideal solid:liquid ratio of 1:1 should be adhered to where possible. 8. Carbon:Nitrogen ratio  –  The carbon:nitrogen ratioof the inlet waste should be in the region 9-25:1 forefficient biogas production (the methane producingbacteria work well with this ratio). Degradable food,agricultural and animal waste can be mixed to thecorrect solid:liquid ratio (1:1) in an influent collecting Pathogens & parasitic ova Thermophilic fermentation (53°c -55°c) Mesophilic fermentation (35°c -37°c)  Ambient fermentation (8°c -25°c) Days Fatality (100%) Days Fatality (100%) Days Fatality (100%) Salmonella 1-2 100 7 100 44 100 Shigella 1 100 5 100 30 100 Poliviruses 9 100 E-Coli titre 2 10 -1 - 10 -2 21 10 -4 40-60 10 -4 -10 -5   Schistosoma ova Several hours 100 7 100 7-22 100 Hookworm ova 1 100 10 100 30 90  Ascaris ova 2 100 36 98.8 100 53  OXFAM Technical Briefs  –   Repairing, cleaning and disinfecting hand dug wells 4 tank prior to addition to the main digester. Different types of waste can be added to alter the C/N ratio to reach an ideal. Approximate values for C/N ratios and TS values are shown in Table 6  . Table 6: Approximate C:N ratios of wastes [13]    Type of waste C:N ratio Cow dung 16-25:1 Pig dung 6-14:1 Chicken droppings (fresh) 5-9:1 Sheep/goat droppings 30-33:1 Human excreta 6-10:1 Fresh grass 12-15:1  Vegetable residue 12-30:1 Case studies show that biogas generators run on purely human waste can be effective (see Case Study I  ) however gas production can be increased by aiming for a higher C:N ratio. The requirement to do this depends on the primary reason of implementation. 9. Location  –    To increase the temperature of thesubstrate, sunny locations (i.e. away from trees orshade) are good locations for biogas generators. As a precaution for (unintended) leaching,generators should be situated at least 30m awayfrom any water source or stream. No permanentstructures or through ways should be build on theground above a generator.10. Fertiliser output    –  Per day an average adult willexcrete around 10-12g of nitrogen, 2g of phospherous and 3g of potassium  –  sludge/slurryfrom the generator has excellent potential for useas agricultural fertiliser.Since many biogas generators run on a continuouscycle it is necessary to post-treat any sludge priorto use as fertiliser (retention times in continuousgenerators cannot be guaranteed). A number of post-treatment options are suggested in Table 7  . Table 7: Post treatment techniques for faecal sludge [3]    Post-treatment Conditions Effect Advantage Disadvantage Composting The higher the temperature the shorter the required time. WHO recommends min 1 week at over 50°c for faecal matter. May give good hygienic quality (temperature and time dependant) Low-tech. May degrade organic pollutants Labour intensive. Risk of re-growth. Leaching water effluent. Further anaerobic digestion for liquid effluents By using a process adapted to high ammonia content (8g/l) at a pH close to 8 it is possible to have a sanitising mesophillic process. May give good hygienic quality (temperature and time dependant) Low-tech. Mesophillic treat degrades organic pollutants Risk of re-growth and methan emissions.  Ammonia treatment  Ammonia added as (aqueous or granulated) urea. Treatment of Efficient for inactivation of bacteria, parasites and some viruses. pH and Low-tech.  Ammonia recycled as fertiliser. (Low) risk of re-growth. 0.5%NH 3  for one week or 2% urea for 2 weeks at temperatures above 10°c. uncharged NH3 dependant. Land application May be spread-on, subsurface drainage or worked-in.  Advised to be worked-in and not used on fruit or vegetable crops which are likely to be eaten raw. Only apply weeks before planting/seeding or before winter If worked-in then more rapid reduction in enteroviruses than indicator bacteria. Needs storage capacity. Survival of pathogens for 2 months in soil, grass and silage shown (in laboratory) and for up to 1 year on soil and bio-solids (spread-on, sub-surface drainages) Sludge drying bed For sludge which is not directly used, partially dig up the ground and pile up the excavated soil to earthen bunds.  Alternatively concrete drying beds can be built. May give good hygienic quality (temperature and time dependant) Low-tech. Perimeter bunds will help in keeping surface run-off from entering sludge drying beds. Low risk for re-growth. Should be rainwater dilution protected and not applicable in monsoon areas. High loss of nutrients Sludge separation and drying [4]   Since sludge has a total solids (TS) content of 2-10% one of the easiest methods of post-processing sludge for use as fertiliser or disposal is sludge separation and drying. Drying beds can be constructed using concrete and slow aggregate filters or by excavation of soil. BORDA suggest that the following design of sludge drying beds may be suitable: Figure 6: Example design of a sludge drying bed For hygiene and odour reasons only digested sludge (odourless) should be separated and dried in open beds.  Any layer of sludge for drying should not exceed 20cm in depth and even this may take several weeks to dry. Drying beds should be roofed in places of frequent rain. Wastewater (if collected by a drainage system) can be distributed as liquid manure whilst the moist sludge (left on the aggregate filter) is left to dry or composted (see sludge/slurry composting box). It should be noted that if sludge is left to dry fully (without composting) a high loss in nutrients (up to 50% nitrogen) is observed. Figure 7: Sludge drying beds constructed by University of Chiang Mei and GTZ in Thailand [4]
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