Biogas and landfill gas
What is landfill gas? Why is biogas formed? What is the difference between biogas and natural gas? How long has biogas been produced by man? How much biogas is produced these days? How many biogas plants are there in the world? How much of the European gas demand can be covered by biogas? Why is the volume of biogas often reported as number of Nm3? What is the energy content of biogas? Biogas produktion
What type of materials can be used for biogas production?
What type of material gives the best gas yield? Is animal manure a good substrate? What digester systems are applied today? What is the difference between one stage and two stage fermentation? When Should I choose one or the other? Can I use different raw materials in my digester or does it have to be the same all the time? I have heard that biogas plants always smell. Can the smell be reduced? Can I build a biogas plant in my remote one family house and produce the energy needed? Biogas upgrading Digestate
What is digestate?
How can I store the digestate and in what form? Will the liquid digestate still produce gas during storage and if so how do I deal with it? Is it possible to recycle the digestate to the farmland? Can I sell the digestate? Which bits of legislation apply to the application of digestate to farmland? Can digestate from biogas production really replace mineral fertilizers? What are the nutritional properties of digestate? How much digestate can I apply to land? Are there timesof year when digestate cannot be spread? Does the use of digestate on my land bring a risk of importing pathogens onto my farm? How will I find out what is in the digestate? Will the digestate smell when it is applied? How about environmental effects – emissions of ammonia and nitrous oxide and leaching of nitrate? Are there forbidden raw materials if digestate has to be usesd to farmland? Is there difficult ones to manage? Environment
What are the environmental benefits of producing biogas?
Isn’t methane a green house gas just like carbon dioxide? I have heard that there are methane losses from the biogas production plants. Doesn’t that reduce the environmental benefits with biogas? Do biogas plants cause a lot of extra transport to and from the digester? Isn’t it more efficient to produce other bio-fuels like ethanol or biodiesel than produce biogas? Economy Biogas utilization Grid injection Biogas as a vehicle fuel
What are the benefits of using biogas as a vehicle fuel?
How does a gas car function compared to a regular car? How far can you drive with a car fuelled with biogas? Where do I find filling stations that sell gas? How do you fuel a gas car? Who produces gas vehicles that run on upgraded biogas? Alternative uses of substrate More information Is the question you're looking for missing? Please email us and we might add your question!
Biogas and landfill gas
What is biogas?
Biogas is a product of a biological degradation of organic substances under exclusion of oxygen. The bacterial degradation process is called anaerobic digestion. Biogas typically contains between 50 and 70% methane, 20 to 45% carbon dioxide and trace gases. The high methane content makes biogas an excellent source of renewable energy to replace natural gas and other fossil fuels. Many different types of anaerobic digestion systems are available and the technology is used on farms (to treat manure and other agricultural residues), by industry (to treat food and beverage or pulp and paper wastes), by municipalities (to treat municipal sludge and the organic fraction of the MSW), or at plants using combinations of various substrates. What is landfill gas? Landfill gas is the result of the decomposition of organic waste in landfills and is very similar to biogas. The methane content is usually lower, i.e. around 45% due to the air infiltration while landfill gas is removed from the landfill by underpressure. Like biogas, landfill gas can be used to displace fossil fuels. Why is biogas formed? Anaerobic digestion is a process that occurs in nature. It involves bacteria that require an environment that is void of oxygen. Converting organic waste into biogas can be simplified into a four step process. The first step is called hydrolysis where the complex substances are broken down by enzymes into mostly soluble compunds that can be uptaken by bacteria. Step two, the acidification, involves a group of anaerobic bacteria – referred to as the acid formers – that produces organic acids as an intermediate product. In the third step acetate and hydrogen is formed. The fourth step involves a group of bacteria – known as the methane formers – that produce methane from acetate or hydrogen. What is the difference between biogas and natural gas? Natural gas is a fossil fuel made up of methane (70-90%), ethane, propane, butane, pentane, carbon dioxide, nitrogen, hydrogen and hydrogen sulfide. Unlike biogas, it is not renewable. In the table below the content in Danish natural gas is compared to the content of landfill gas and biogas produced in a digester.
How long has biogas been produced by man? Anaerobic digestion is a naturally occurring process. For instance swamp gas, a form of biogas, is produced from the anaerobic degradation of wetland vegetation that has settled to the bottom. Biogas was first used in an engineered plant on a lepper station in Bombay 1859. The first European plant for the treatment of sewage was built 1895 in Exeter, England. The biogas was used to fuel street lamps in town. Source: http://www.adelaide.edu.au/biogas/history/ How much biogas is produced these days? The estimated biogas production in the European Union for 2007 can be seen in the table below. The table shows amounts of exploited biogas, and not biogas burnt in flares. Biogas production in European Union 2007 (in ktoe) (Source Biogas Barometer - July 2008):
2Decentralised agricultural plants, municipal solid waste methanisation plants, centralised codigestion plants. 3Estimated based on: "Produktion och användning av biogas år 2006", ER 2008:02. Biogas production the most recent years can be seen in the figure below: 
Source: Biogas Baromter, 2004-2008. How many biogas plants are there in the world? The number and type of plants is increasing and it is difficult to know the exact figure. Biogas is being produced from manure, sewage sludge, biowastes, landfills, manure as well as from energy crops and from industrial and municipal wastewaters. It has for example been reported that there are 3 to 4 million family size biogas plants in India. In Europe the largest number of biogas plants is in Germany with about 3750 agricultural biogas plants in 2007. How much of the European gas demand can be covered by biogas? Estimations on biogas production potential seem to vary e.g. depending on the consideration of various raw materials and also on the state of the art of technologies. Major amounts of methane can be obtained from industrial and municipal wastes, sludges, manures, industrial wastewaters and from energy crops. The natural gas consumption in Europe was ca 530 billion m3 in 2006. It has been reported that all the presently imported natural gas in EU could be replaced with biogas. It must, however, be noted that the economical and environmental feasibility of certain technology and raw materials is dependent on several factors and also on alternative options. Why is the volume of biogas often reported as number of Nm3? Nm3 refers to volume of gas under normalized conditions, i.e. at 0°C and 1 atm. This is used to make the volumes standards easier to compare because the volume of the gas is dependent on the gas pressure and temperature. E:g. 100 Nm3 gas would be equal to 113 m3 at 35°C. What is the energy content of biogas? The energy content of biogas and landfill gas is dependent on its content of methane. The energy content for biogas with a methane content of 65 %, and for biogas upgraded to 97 % methane (See also question: "Why is biogas sometimes upgraded") can be seen in the table below, as well as the energy content of some other fuels:
Biogas production
What type of materials can be used for biogas production?
Biogas can be produced from various agricultural, industrial and municipal organic sources. Typical agricultural feedstocks are pig and cow slurry, chicken manure, various harvest residues (leaves) and specially cultivated energy crops (maize, grass, cereals). Individual degradation rates vary significantly with material composition, e.g. carbohydrate-, protein- and fat content (See also question: "What type of material gives the best yield?"). Desirable industrial feedstocks include highly loaded wastewaters from the food, fermentation, pharmaceutical and biochemical industries (e.g.fermentation slops, fruit water, whey) as well as solid wastes (e.g. fruit and vegetable residues, press cakes, stomach contents). Food residues from large kitchens, spent oils, fats and biogenic wastes from municipalities are also valued due to their high gas yield. Typical unwanted organic materials are wood wastes, garden wastes and bones due to their low degradability. When selecting wastes for digestion, the total solids content, the percentage of volatile solids, the C/N-ratio and the biodegradability have to be carefully considered. Organic materials which contain inhibitory components such as pesticides, antibiotics or disinfections should not be used. Heavy metals do not usually inhibit the digestion process but they can prevent the application of the digestate as organic fertilizer on agricultural land. See also Potential of Co-digestion and Animal by-products and anaerobic digestion. What type of material gives the best gas yield? The biogas yield depends on the volatile solids (VS) content, the composition of the organic material and the bioavailability. Fats provide the highest biogas yield, but unsaturated long chain fats have rather poor bioavailability, sometimes followed by inhibition. Short chain fats and carbohydrates show the fastest conversion rates but the gas yield is considerably lower. The following table shows the specific gas yields and the methane content of the basic classes of substrate compounds.
Lignin is not degradable under anaerobic process conditions and can prevent the degradation of carbohydrates, proteins and fats in plant materials. The gas yield that can be achieved in a biogas plant is also influenced by the hydraulic retention time, the fermentation temperature and inhibitory effect of substrate components and intermediate products of the digestion process, e.g. ammonia, volatile fatty acids or hydrogen sulfide. In the table below methane yield from some typical substrates can be seen:
In the table below the methane yield from some energy crops can be seen:
Source: Åke Nordberg, Biogas from crops, JTI. Presentation at "10th Anniversary for Biogas in Trollhättan". May 31st 2006. Is animal manure a good substrate? Animal manure is a good substrate for biogas production because it contains all micro- and macro-nutrients which are necessary for an efficient methane production. The high buffer capacity of manure results in a stabilization of the process conditions and makes the digestion process more robust against an accumulation of volatile acids. Gas yields of manure are relatively low and it is therefore important that the dry matter content is reasonably high. For pig and cow slurries the dry-matter content varies widely in the range of 3-12 %. Dry matter contents below 5 % make the application of digester systems often uneconomic because large reactor volumes are necessary and the specific gas yield per m3 of manure is low. Chicken manure is characterized by high dry matter content but the high ammonia content can lead to inhibitory effects during digestion. Therefore chicken manure should be treated together with other substrates low in ammonia in order to prevent an inhibition. If the chickens are kept in open feedlots the manure is contaminated with sand which can cause operational problems by formation of bottom layers in the digester. Solid manure with typical dry matter contents between 10 to 30 % often needs a pretreatment of the inhomogeneous material because the high straw content can result in a clogging of pumps and pipes. Additional operational problems, like scum layer formation, can occur and hardly degradable bedding materials may be enriched in the fermenter. What digester systems are applied today? Biogas is produced in gas tight heated digestion tanks which are operated mostly at mesophilic (32-42°C) or thermophilic (50-57°C) temperatures. A wide variety of digester systems have been developed for slurries and solid substrates. For liquid substrates continuous flow tank fermenters is most often applied. The substrate is pumped several times per day into the digester, displacing an equal volume of digested material. The substrate is usually mixed intermittently in time intervals ranging from several times daily to several times per hour. Most often applied are completely mixed vertical stirred tank reactors with adjustable propeller mixer. For lower treatment capacities and as a first stage of multi-stage biogas plants horizontal plug-flow digesters with a horizontal paddle stirrer are applied. Mixing is necessary to avoid scum and bottom layers, to achieve an even temperature throughout the digester und to improve the release of gas bubbles trapped in the substrate. For solid materials special garage type digester systems are applied which are operated discontinuously with percolation of process water. This type is mainly applied in agriculture and for source separated waste. Only few continuously operated dry fermentation systems are available. They are applied in agriculture, for source separated waste and for municipal solid waste Depending on the fermenter design the gas is stored in the top of the digester or in an external gas storage tank. In the continuously operated flow digester the hydraulic retention time of the substrate has to be longer than the doubling time of the bacteria to prevent wash-out. Organic rich industrial waste water is usually degraded rather fast because all the compunds are in solution. Hydraulic retention time of the digester is therefore short (in the range of a few hours to a couple of days). To avoid wash-out in these cases system like bio-filters, UASB, EGSB and contact reactors can be applied. These systems are withholding the bacterial mass. What is the difference between one stage and two stage fermentation? When Should I choose one or the other? Anaerobic digestion is a multistep-process (See also question: "Why is biogas formed?"). When hydrolysis and acidification is allowed to take place in a separate tank, the anaerobic treatment is commonly called two-stage process. The separation into two stages allows adapting the process conditions on the specific requirements of the microorganisms. The pH-optimum for the hydrolytic bacteria is between 5.0 and 6.0 and for the methanogenic bacteria between 7.3 and 7.8. The two-stage process is advantageous if hydrolysis is the rate limiting step. In the case of solid substrates it permits the operation of a high rate anaerobic methanogenesis of the liquefied solids. A real two-stage process is only possible with a few easy degradable industrial waste waters. In most cases the hydrolysis reactor is often used also as an equalization tank if different substrates are used for biogas production. A complete separation of hydrolysis and methanogenesis cannot be achieved in full-scale plants, because methane is always formed in the hydrolysis digester. Can I use different raw materials in my digester or does it have to be the same all the time? The anaerobic digestion process has the advantage that a lot of different substrates can be used for biogas production. The digester can be operated by a mixture of different raw materials but a rapid change in the substrate composition should be avoided. Biogas formation is a complex process of different consortia of micro-organisms which fulfill different roles in the overall process. If the substrate properties are suddenly changed the interdependence of the bacteria is disturbed and the microbial population needs some time for adapting to the new conditions. A fast change of the substrate composition can result in an accumulation of intermediates such as volatile fatty acids and alcohols causing process instability. Any change of the raw materials should be done slowly and well controlled in order to have time for an acclimatization of the bacteria. I have heard that biogas plants always smell. Can the smell be reduced? The odours from biogas plant may vary greatly depending on the type of application and the raw materials as well as on the operation of the plat. The odours may origin from the reception of the raw materials, from the pre-storage or post-storage of the plant or from insufficient combustion of the biogas. In well designed and operated plants odours are rare. In plants in operation, odours can be reduced by covering all tanks and preventing leakages as well as through implementation of new, well isolated tanks, or through implementing and improving treatment processes for the off gases. Can I build a biogas plant in my remote one family house and produce the energy needed? This depends on your energy consumption. In most cases a family's energy consumption is higher than the energy that could be produced from its own wastes and wastewaters. Further, the capital investment would be high. On the other hand, as the energy prices increases, and we develop more energy efficient technologies, it might be possible to cover reasonable ratio of your energy consumption through anaerobic digestion. Some skillful and motivated individuals have already built their own biogas plants which operate on their own wastes as well as additional material, (e.g. crops). In these cases a reasonable amount of biogas for heating and cooking purposes is produced. There is a major interest to develop this kind technologies and concepts as part of sustainable societies. However, it should be pointed out that this is more common in the developing nations and is likely to face many restrictions in developed nations. Biogas upgrading
Why is biogas sometimes upgraded?
Biogas composition varies depending on how the biogas was produced and which substrates were used (See question: "What is the difference between biogas and natural gas?". Besides methane and carbon dioxide, biogas may contains other substances such as water, hydrogen sulphide, oxygen, nitrogen, ammonia, siloxanes and particles. Water and hydrogen sulphide have to be removed for most applications. In order to increase the calorific value of the gas to grid standard, carbon dioxide is also removed. Upgraded biogas can then replace natural gas. Which techniques are available for upgrading of biogas? If biogas is intended to be fed into the natural gas grid or to be used as a vehicle fuel, then removal of CO2 and other minor contaminants is required. Biogas upgrading and cleaning are usually performed in two steps, where the first step is removal of the minor compounds (e.g. hydrogen suplhide) and the second step is the separation of the CO2 from the methane. The most common technologies for biogas upgrading are the water scrubber technology and the PSA (Pressure Swing Absorption) technology. Water scrubber In a water scrubber, the biogas is introduced to a stream of water in a pressurised column. Carbon dioxide, sulphur hydrogen and ammonia are dissolved in the water phase and thus removed from the gas phase. Methane, on the other hand, does not dissolvable as easily in the water and largely remains as a gas. After the pressurized column, the water enters a flash tank where the pressure is decreased. By lowering the pressure, any dissolved methane is released and is feed back into the unscrubbed biogas flow entering the scrubber column. The water, that now contains dissolved carbon dioxide enters a desorption tower where carbon dioxide is released from the water phase by further lowering the pressure. The water can be recycled to the scrubber tower again. Alternatively, fresh water can be used thereby eliminating the need for the desorption tower. Scrubber with organic solvents The process of scrubbing biogas with an organic solvent like polyethylene glycol is similair to the water scrubber. The difference is that carbon dioxide and hydrogen sulphide are far more soluble in organic solvents that in water, and hence smaller upgrading plants can be built with the same capacity. Selexol and Genosorb are trademarks for chemicals used for this purpose. The carbon dioxide can also be absorbed in a chemical solution containing amines and the technique is then referred to as amin scrubber. The chemical selectivity binds carbon dioxide when the biogas flow meets the liquid in a column. The liquid is then re-generated by heating to release carbon dioxide. One advantage with the amin scrubber compared to the water scrubber is that it can take place at low pressure. 
Biogas upgrading plant in Gothenburg, Sweden. Pressure Swing Adsorption (PSA) During pressure swing adsorption (PSA), methane and carbon dioxide are separated from each other by their differences in size and physical properties. Carbon dioxide, oxygen, nitrogen and water are adsorbed on activated carbon or zeolites under increased pressure, while methane flows unaffected through the column. When the activated carbon becomes saturated with carbon dioxide it is regenerated by lowering the pressure which will release carbon dioxide. The column works in different phases in which the pressure is changing and thus the name of the technique. In the first phase the carbon dioxide is adsorbed on the material under high pressure, then the material is re-generated by lowering the pressure in steps. In the last of these steps usually a slight underpressure is applied. The pressure is lowered in several steps since the gas released first contains methane which is fed back into the biogas stream that enters the adsorption column. The gas released when the pressure is further lowered contains mainly carbon dioxide. Membrane A traditional technology for biogas upgrading is gas permeation through membranes, simultaneously achieving CO2 separation, an efficient removal of water, ammonia and considerable reduction of H2S. In recent years the method has been improved quite a bit. The relative dry, compressed gas flows through the membrane, where CO2, water and part of the H2S is separated from the methane. CO2, water and other unwanted components permeate more rapidly through the membrane than methane, thus leaving the membrane housing as permeate. See also: Biogas Upgrading to Vehicle Fuel Standrad and Grid Injection. Digestate
What is digestate?
Digestate is a by-product of the anaerobic digestion (AD) process, representing the treated/digested substrate. Some characteristics of digestate are shown in the table below: Nutrient distribution in digestate, compared with cattle and pig slurries.
Digestate is more homogenous, compared to raw pig or cattle slurry, with an improved N-P balance. It has a declared content of plant nutrients, allowing accurate dosage and integration in fertilisation plans of farms. Digestate contains more inorganic nitrogen, which is more accessible to the plants, than untreated slurry. 
How can I store the digestate and in what form? Digestate can be temporarily stored in specially built storage containers. Legislations in many European countries require up to nine months storage capacity for digestate (as well as for untreated animal manure and slurry), in order to ensure optimal and efficient utilisation as a fertiliser and to avoid application during the winter season. Digestate can be stored in concrete tanks, covered by natural or artificial floating layers or membranes, or in lagoon ponds. 
Storage tanks covered with natural floating layer. Source: Danish Biogas Association An important way of preventing emissions and leakage is to store and handle digestate properly. Experience in Denmark shows that placing an artificial floating cover on digestate storage tanks can reduce ammonia volatilization from 20% to less than 2%. 
Floating cover on digestate storage tanks reduces ammonia volatilization. Source: DIAS Will the liquid digestate still produce gas during storage and if so how do I deal with it? After AD treatment, emissions of methane (and of ammonia) from digestate are possible, but many years of experience with good agricultural practice proved that notable reductions of emissions, odours and nutrient leakage are possible. After leaving the digester, digestate can still produce certain amounts of methane and ammonia. Experience from Denmark shows that up to 20% of the total biogas production can take place outside the digester, at ambient temperature in storage tanks. The process is often called post-digestion and the amount of biogas produced depends on the AD process temperature and the retention time inside the digester. In order to prevent methane emissions from digestate and to collect the extra biogas production, storage tanks for digestate should always be covered with a gastight membrane for gas recovery. When digestate is transported to storage facilities out in the fields, these should also be covered with a natural floating layer, as a minimum, in order to reduce the risk of further emissions. 
Membrane covered storage tanks. Source: Danish Biogas Association Is it possible to recycle the digestate to the farmland? The standard use of digestate is as liquid fertiliser on agricultural land. For optimum utilisation of digestate as fertiliser, the same basic principles as those used for untreated slurry and manure, must be considered: Due to its higher homogeneity and flow properties, digestate penetrates into soil faster than raw slurry. Nevertheless, application of digestate as fertiliser involves the risk of nitrogen losses through ammonia emissions and nitrate leaking. N-efficiency will increase considerably and nutrient losses by leaching and evaporation will be minimised if digestate is used as fertiliser in conformity with good agricultural practice. For that, some simple rules should be respected: Some general measures are recommended, in order to produce digestate of good quality, suitable for safe recycling: Digestate can be used as a top-fertiliser on crops in full vegetation. This application offers little concern about loss of nitrogen as nitrate into ground water, since most of the nutrients are absorbed directly by the plants. Danish experience indicates that when digestate is applied as top-fertiliser, some of the nutrients are absorbed through the leaves. Example of national regulations of the nutrient loading on farmland (Nordberg, 1999)
Can I sell the digestate? Yes, - but you have to make sure you are following your country's rules. Check this with the national agricultural advisory services and/or the national Environmental Protection Agencies (EPA’s). It is necessary to perform chemical analysis of the digested manure. Average analyses are always best, based on good incremental samples. When you know the nutrient content in the digested manure (biogas slurry) you can make a sales calculation. After that you need to determine the actual nutrient prices of NPK according to the world market prices or to consult an agricultural advisory centre. Then the actual sale price of the digestate based on the nutrient content can be calculated. Which bits of legislation apply to the application of digestate to farmland? No general answer can be given to this question. Nearly all countries have their own rules. FOr instance some EU-countries have to abide the Nitrate Directive, environmental sensitive areas, water directives etc. whereas others do not. The best advice is to consult you country EPA authorities or their home page as well as the agricultural authority within your country. Can digestate from biogas production really replace mineral fertilizers? Yes, digestate can replace the use of mineral fertilisers. The fertiliser replacement value of the digestate depends upon its crop nutrient content. This in turn is a function of the quantity of crop nutrients in the material that is fed into the digester. If the feedstock, as for example cow slurry, contains 3-4 kg of total nitrogen (depending on its dry matter content) then there will be the same amount of total nitrogen present in the digestate. When digestate is used as a fertiliser it acts like any other organic liquid such as animal slurry. Its efficiency, as in the case of any fertiliser application, will depend on a number of factors such as the method of spreading, the time of year applied, weather conditions and the dry matter concentration. When spreading the digestate as indeed any other fertiliser, it is vital to ensure that the quantities of crop nutrients applied do not exceed crop requirements and that Codes of good Farming/Agricultural practice are followed together and that there is compliance with the EU Nitrates Directive (See below for further information) any /or other national regulations. What are the nutritional properties of digestate? The fertiliser value of the digestate varies from one plant to another depending upon the qualities and quantities of the individual inputs that are fed the digester. For example, an input of 100% cow slurry would go into the digester containing 3 kg of total nitrogen (see table below). It would leave the digester as digestate also with 3 kg of total nitrogen. Similarly the quantities of phosphate (P2O5) and potash (K2O) in the digestate reflect those that are present in the feedstocks that go into the plant. However a mixed feedstock with 70% dairy cow slurry, 20% grass and 10% blood would produce a digestate with a total N concentration of 5.2 kg (calculated from the table below). Indicative nutrient content of typical feedstocks (kg/fresh tonne):
As in the case of the original inputs, not all the nutrients are available for the uptake and growth of the next crop. It is worth noting though that unlike the original slurry where 50% is usually available for the next crop when applied under optimum conditions, after digestion the proportion that is available can increase to some 70-75%. The increase arises from chemical changes that take place during the digestion process. This illustrates how the individual feedstocks and the proportions of each that are fed to the digester can influence the nutrient content of the end product – the digestate or biofertiliser. In order to make maximum use of the digestate – in fact a biofertiliser- it needs to be applied at the correct time of the year when the crops are growing. How much digestate can I apply to land? There are three questions that you need to address: Once these questions have been answered then you will know how much of the nutrients that you need and are permitted to apply. The nutrient content of the digestate would be tested regularly for the biogas plant (and this should also apply to farmers who have their own plants) so that you will receive a status report/ quality assurance when you take delivery. There are supplementary questions that you must also address. Are there times of the years when digestate cannot be spread? Yes. There are limitations and each country is likely to have its own rules quite apart from any that are laid down on an international basis such as the conditions pertaining to the Nitrate Vulnerable Zones in EU member countries. The limitations are set to lessen the risk of surface run off of nutrients and percolation through under-drainage networks into ditches, streams and rivers and ultimately into lakes and coastal waters. For practical reasons, application of nutrients to land when the plants are not growing and therefore are unable to take them up is a waste of the money spent on spreading and of the monetary value of the nutrient itself. If wasted thus you will have to buy more mineral fertiliser and at to-days costs it will affect your balance sheet! Does the use of digestate on my land bring a risk of importing pathogens onto my farm? When the farm uses feedstocks produced on that farm there is no greater risk of importing pathogens than there was when there was no anaerobic digester. Indeed, the digestion process reduces the concentrations of pathogens very significantly relative to raw slurry or any other input. The degree of pathogen reduction depends on the number of days (hydraulic retention time) that the feedstock remains in the digester, the temperature inside the digester and also whether pasteurisation is employed. Pasteurisation is NOT required for farm-based feedstocks such as slurry and crops. Even if digestate is delivered to the farm from elsewhere the risk of importing pathogens is greatly reduced when compared with importing slurry. If feedstocks such as cooked food, stomach contents and blood from slaughter-houses, etc form part of the feed stock mix the biogas plant MUST be compliant with the EU Animal By-Products Regulation (EC Regulation No. 1774/2002). Such material must be pasteurised either pre- or post digestion. This leads to a major reduction in faecal indictor organisms. Furthermore, the eggs of gastrointestinal worms, modular and roundworms from pigs and cattle, tapeworms from cats and dogs and the larva of lungworms in cattle for example, can all be inactivated within just 8 days residence time in a farm digester working at 35-37°C. The residence time in such digester is usually at least between 18-21 days and in many cases even longer. How will I find out what is in the digestate? If a farm has its own plant the owner will know what feedstocks have been used and the source of any non - farm inputs that have been received under contract from food processors, etc. and this be must pasteurised if it contains animal by-products. If you want to know the exact composition of the nutrients in your digestate (biofertiliser) you should send a sample to a laboratory for analysis If a farmer wants to purchase or import digestate (biofertiliser) from a biogas plant elsewhere then it is likely/ desirable that he should know what feedstocks have been put into the digester the end product of which he would be applying to his land. Managers of co-digestion plants are bound by strict codes of practice set both by the EU and their respective governments. While they are not under any obligation to declare the actual feedstocks that they use to produce the biogas and digestate, they are bound by law to meet the EU and national regulations for handling animal by –products when such material has been included in the feedstock mix. Thus the digestate that is delivered to a farm should have a cover note that states that it is free from salmonella and contains less than 1,000 cells per /g of E.coli. It would also detail the nutrient content and chemical analysis of the digestate that is being delivered. Increasingly individual governments or trade associations already have developed or are developing publicly available quality assurance standards that offer a high level of biosecurity but what is more, are perceived to be acceptable standards by the supermarkets. Will the digestate smell when it is applied? Slightly but only for a short time. This will depend on the method of application and the weather conditions. However you and your neighbours will notice a very marked reduction in odour compared with normal slurry! The odour from the digestate is less pungent and will dissipate quickly after spreading especially if it is applied by injection, dribble bar, trailing hose or trailing shoe applicators or is incorporated quickly into the soil. The main reason for the reduced odour is that the organic fatty acids which give rise to the offensive smells associated with normally managed slurry are broken down during digestion and converted into biogas. Environment
How about environmental effects – emissions of ammonia and nitrous oxide and leaching of nitrate?
The digestate usually contains higher concentrations of available nitrogen (ammonium) as compared with the feedstock. This together with the higher pH of the digestate relative to the inputs may lead to increased emissions of ammonia gas during storage and spreading. The overall emissions of ammonia gas arising from the stored digestate can be significantly higher than those for the storage of cattle and pig slurry unless the tanks are topped with a natural crust, floating layer of straw or leca pebbles (as used for cavity wall insulation). Some countries now require the stores to be fitted with fixed covers. Are there forbidden raw materials if digestate has to be usesd to farmland? Is there difficult ones to manage? This question implies that there is a need to ‘get rid of’ the digestate as a by-product of the digestion process when in fact it is a valuable fertiliser. Attention needs to be drawn to the fact that there are two broad categories of biogas plants – those that are built by individual or groups of farmers to serve their own needs with or without co-digestion and those built primarily for the management of industrial and municipal waste. The latter have to find an outlet for the digestate. Both will have to conform with EU and their respective national laws as to the inputs that are permitted. However, the former can decide what combinations of feedstocks that they wish to include and would avoid any material that could be or be perceived to have a detrimental effect on the quality of the digestate that they propose to use on their own land or to sell to other farmers in their vicinity. Anaerobic digestion plants that handle industrial and municipal wastes must ensure that any ‘difficult’ materials (eg. plastic, glass, etc) have been separated at source before the materials are accepted at the biogas plant or have been screened out before the material is fed into the digester. These issues would have been dealt with in the earliest stages and therefore by the time the digestate is ready for use as a fertiliser any problems would have been resolved. Digestate/biofertiliser with its quality assurances and high level of biosecurity is becoming an increasingly valuable product both from the financial and environmental perspectives. The digestate/biofertiliser can displace mineral fertiliser use and moreover avoid the CO2 emissions arising from its production, shipping and land transport. Any additional carbon dioxide produced during the transport of the digestate/ biofertiliser on the farm or back to the farms from a centralised plant that serves a group of farmers is relatively minor when compared with the amount that is produced during inorganic fertiliser production and use. What are the environmental benefits of producing biogas? There are several environmental benefits to biogas production and utilization, such as: - production of renewable energy from biomass - reduce CH4-emissions from manure - replacement of petrol or diesel when utilized as a vehicle fuel - production of a digestate that can be utilized as a bio-fertilizer Isn’t methane a green house gas just like carbon dioxide? Methane is indeed one of the known green house gases. Methane emission is for example generated by manure storage and via rumen fermentation by cattle. Methane emissions also occur in swamps and other anaerobic media. The contribution to the greenhouse effect of methane is stronger than the contribution of CO2. To compare the contribution of CH4 to the green house effect we use the GWP-factor (Global Warming Potential factor). CO2 has the GWP-factor of 1 and for CH4 the GWP factor is 21. So the contribution of a kg CH4 to the green house effect is as strong as 21 kg CO2. It is therefore important to minimize the emissions of methane in every step from biogas production to biogas utilization. I have heard that there are methane losses from the biogas production plants. Doesn’t that reduce the environmental benefits with biogas? A biogas production plant should be well covered to minimize the emission of CH4 from storage of the biomass before and after digestion, to achieve a maximum environmental benefits of the produced biogas. No plant can be 100% leak free, but the total methane losses should be limited to a few percent (1 or 2 %) to have a clear positive contribution when replacing fossil fuel with biogas. Calculations shows that the net emission reduction of biogas various between 50 – 150% of green house gas emissions in comparison with energy production with fossil fuels. Do biogas plants cause a lot of extra transport to and from the digester? Depending on the scale of the digester there could be a concentration of transportation in the neighbourhood especially during harvesting time when energy crops are used as a co-substrate. However, the overall effect is limited. It should be noted that when biogas is injected to the gas grid it is very effectively transported compared to petrol and diesel, and thus contributes to a decrease in transportation. Isn’t it more efficient to produce other bio-fuels like ethanol or biodiesel than produce biogas? Production of biogas is about three times more efficient in comparison with the production of other bio-fuels (for the first generation of bio-fuel technology). It is possible to combine bio-fuel production and biogas production from the residues of bio-ethanol production. The combination will improve the sustainability en especially the green house gas balance. Economy
How much does it cost to upgrade the biogas to grid quality or as a vehicle fuel?
Above all, the economy of scale is very important, i.e. the price is a function of the size of the installation. The treatment cost follows a hyperbolic curve. At 50 m3 per hour the cost is in the order of X UScts/kWh of biomethane and drops down to 10 UScts at 200m3/hr to level off at treatment capacities of more than 300 m3/hr at 5 to 8 UScts/kWh. Independent of the upgrading technology, the indicative treatment cost per volume of biomethane given by the providers are comparable. As long as no major market has been established, the competition remains weak. If a provider needs a project, prices are lower than if he is booked out. Biogas utilization
What can biogas be used for?
Biogas can be used for the same applications as natural gas. All gas appliances can be adjusted to the lower heating value of biogas. It is demonstrated that biogas can be applied for the production of heat, e.g. in hot water and steam boilers; for the production of electricity and heat in combined heat and power plants (CHP) in microturbines or in hot fuel cells (solid oxide fuel cells, molten carbonate fuel cells). Biogas can also be used as a vehichle fuel (see question: "What are the benefits of using biogas as a vehicle fuel?"). For most applications some sort of gas cleaning and upgrading is required. As a minimum requirement water vapour and hydrogen sulphide usually have to be reduced. What is the best way to use biogas? Best utilisation practices are site specific. However, the goal should be to choose the most energy efficient technology under consideration. Most biogas plants are equipped with a combined heat and power (CHP) unit. But unfortunately, only in a minority of the installations is the heat really used entirely. In industry, steam production is often the solution. For economic reasons it is extremely important to use all the energy - including heat. Grid injection
Can I inject my biogas into the grid?
Only a few countries like Sweden and The Netherlands have laws allowing the injection of biogas into the gas grid. Others, like switzerland and Austria, have private contracts between grid operators and producers. Some other countries do not allow biogas into the grid at all, as e.g France. However, the situation is changing fast. Germany will soon have one of the most progressive feed-in laws for gas and France will soon allow gas injection as well. The quality and energy content of natural gas varies considerably depending on where the gas is coming from and biogas precondition requirements vary depending on the local grid quality. An international gas industry working group (Marcogas) has therefore established common rules for gas injection. See also: Injection of Biogas into the natural gas grid in Laholm, Sweden. Biogas as a vehicle fuel
What are the benefits of using biogas as a vehicle fuel?
Biogas can be used in a number of applications including fuel for natural gas vehicles. The main environmental benefit is that fossil fuels like petrol and diesel can be replaced. Natural gas used as a vehicle fuel gives 20-30 % lower CO2 emissions. For biogas the reduction of green house gas emissions can be as much as 100 %. In fact, a reduction above 100 % can be achieved when biogas produced from manure is utilized as a vehicle fuel. Methane, which is a strong green house gas, is released into the atmosphere from manure in traditional manure storage. Biogas as a vehicle fuel can thus both decrease the leakage of methane from manure and decrease the emissions of fossil carbon dioxide. Another advantage is that vehicles running on upgraded biogas or natural gas have lower emissions of particles, NOx and SOx. 
How does a gas car function compared to a regular car? There’s no real difference in function. Natural gas vehicles are usually regular cars with the added feature of gas propulsion. By adding compressed gas storage containers (up to 200 bars overpressure), a pressure regulator and a fuel-injection system, and by adjusting the control soft ware, the car is prepared to use both gas and gasoline as automotive fuel. In the future, the engine technology will become more adapted, in order to better utilize the inherent potential of gaseous fuels regarding fuel efficiency. How far can you drive with a car fuelled with biogas? Most natural gas cars allow a driving distance of 200-400 km. These cars are also equipped with a petrol tank that gives an additional driving distance of 200-700 km, that can be used if you should run out of gas. Where do I find filling stations that sell gas? With these links you can find filling stations in several European countries: www.erdgasautos.at (Austria) www.cng.cz (Czechoslovakia) www.gazdefrance.fr (France) www.erdgasfahrzeuge.de (Germany) www.ngva.co.uk (Great Britain) www.guidametano.com (Italy) www.dutchfour.com (The Netherlands) www.fordonsgas.se (Sweden) www.erdgastanken.ch (Switzerland) How do you fuel a gas car? When filling at a gas refueling station, the gas is usually transferred through the hose very quickly, making the time spent on filling more or less comparable to liquid filling. The big difference is that the dispenser is connected to the fueling point in a gas-tight manner. Automatic safety measures in the dispenser and the car ensures that no gas will be transferred in any case of operator error, including the worst-case scenario of someone driving away without disconnecting the dispenser. 
Filling station for upgraded biogas in Östersund, Sweden. Who produces gas vehicles that run on upgraded biogas? Here are links to some companies that produces gas vehicles: Citroën Fiat Mercedes Opel Volkswagen Retrofitted cars are also available for other brands. Alternative uses of substrate
Wouldn’t it be easier to burn or compost the waste that is now used for biogas production?
There is no simple answer to this question. It would depend largely on the type of waste, availability of capital and desired end products. Combustion may be the most attractive choice for low moisture wastes – especially where there is a need for heat and/or electricity; and a limited land base to dispose of the end product; or the end product is not suitable for land application. Composting may be the option of choice where funding is limited and there are strong markets for the compost. Biogas production is likely the best choice when both energy production and land nutrients are desired. Decision on which option makes most sense should be made on a case by case basis. More information
Where do I find manufacturers that build biogas plants and upgrading plants? Here are some links to national networks and association that might be useful while searching for manufactures, as well to find further information: Austria ARGE Kompost & Biogas Canada Canadian Biomass Innovation Network Denmark Danish Biogas Association Germany German Biogas Association Sweden Swedish Gas Association Swedish Waste Management The Swedish Water & Wastewater Association Bioenergy Portal Switzerland Biogas Forum www.biomassenergie.ch USA The AgSTAR Program Suppliers of biogas plants can also be found on Plant suppliers. | |||||||||||||||||||||||||||||