reduction (grinding) is beneficial in continuous vertical systems, as it accelerates digestion, while batch systems avoid grinding and instead require structure (e.g. yard waste) to reduce compaction of the stacked pile. continuous vertical dry digesters have a smaller footprint due to the shorter effective retention time and vertical design. wet digesters can be designed to operate in either a high-solids content, with a total suspended solids (tss) concentration greater than ~20%, or a low-solids concentration less than ~15%. high solids (wet) digesters process a thick slurry that requires more energy input to move and process the feedstock. the thickness of the material may also lead to associated problems with abrasion. high solids digesters will typically have a lower land requirement due to the lower volumes associated with the moisture. high solids digesters also require correction of conventional performance calculations (e.g. gas production, retention time, kinetics, etc.) originally based on very dilute sewage digestion concepts, since larger fractions of the feedstock mass are potentially convertible to biogas. low solids (wet) digesters can transport material through the system using standard pumps that require significantly lower energy input. low solids digesters require a larger amount of land than high solids due to the increased volumes associated with the increased liquid-to-feedstock ratio of the digesters. there are benefits associated with operation in a liquid environment, as it enables more thorough circulation of materials and contact between the bacteria and their food. this enables the bacteria to more readily access the substances on which they are feeding, and increases the rate of gas production. complexity digestion systems can be configured with different levels of complexity. in a single-stage digestion system (one-stage), all of the biological reactions occur within a single, sealed reactor or holding tank. using a single stage reduces construction costs, but results in less control of the reactions occurring within the system. acidogenic bacteria, through the production of acids, reduce the ph of the tank. methanogenic archaea, as outlined earlier, operate in a strictly defined ph range. therefore, the biological reactions of the different species in a single-stage reactor can be in direct competition with each other. another one-stage reaction system is an anaerobic lagoon. these lagoons are pond-like, earthen basins used for the treatment and long-term storage of manures. here the anaerobic reactions are contained within the natural anaerobic sludge contained in the pool. in a two-stage digestion system (multistage), different digestion vessels are optimised to bring maximum control over the bacterial communities living within the digesters. acidogenic bacteria produce organic acids and more quickly grow and reproduce than methanogenic archaea. methanogenic archaea require stable ph and temperature to optimise their performance. under typical circumstances, hydrolysis, acetogenesis, and acidogenesis occur within the first reaction vessel. the organic material is then heated to the required operational temperature (either mesophilic or thermophilic) prior to being pumped into a methanogenic reactor. the initial hydrolysis or acidogenesis tanks prior to the methanogenic reactor can provide a buffer to the rate at which feedstock is added. some european countries require a degree of elevated heat treatment to kill harmful bacteria in the input waste. in this instance, there may be a pasteurisation or sterilisation stage prior to digestion or between the two digestion tanks. notably, it is not possible to completely isolate the different reaction phases, and often some biogas is produced in the hydrolysis or acidogenesis tanks. residence time the residence time in a digester varies with the amount and type of feed material, and with the configuration of the digestion system. in a typical two-stage mesophilic digestion, residence time varies between 15 and 40 days, while for a single-stage thermophilic digestion, residence times is normally faster and takes around 14 days. the plug-flow nature of some of these systems will mean the full degradation of the material may not have been realised in this timescale. in this event, digestate exiting the system will be darker in colour and will typically have more odour. in the case of an upflow anaerobic sludge blanket digestion (uasb), hydraulic residence times can be as short as 1 hour to 1 day, and solid retention times can be up to 90 days. in this manner, a uasb system is able to separate solids and hydraulic retention times with the use of a sludge blanket. continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents, enabling the bacteria and the food to be in contact. they also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks. inhibition left: farm-based maize silage digester located near neumünster in germany, 2007 - the green, inflatable biogas holder is shown on top of the digester. right: two-stage, low solids, uasb digestion component of a mechanical biological treatment system near tel aviv; the process water is seen in balance tank and sequencing batch reactor, 2005. the anaerobic digestion process can be inhibited by several compounds, affecting one or more of the bacterial groups responsible for the different organic matter degradation steps. the degree of the inhibition depends, among other factors, on the concentration of the inhibitor in the digester. potential inhibitors are ammonia, sulfide, light metal ions (na, k, mg, ca, al), heavy metals, some organics (chlorophenols, halogenated aliphatics, n-substituted aromatics, long chain fatty acids), etc. total ammonia nitrogen (tan) has been shown to inhibit production of methane. furthermore, it destabilises the microbial community, impacting the synthesis of acetic acid. acetic acid is one of the driving forces in methane production. at an excess of 5000 mg/l tan, ph adjustment is needed to keep the reaction stable. a tan concentration above 1700- 1800 mg/l inhibits methane production and yield decreases at greater tan concentrations. high tan concentrations cause the reaction to turn acidic and lead to a domino effect of inhibition. total ammonia nitrogen is the combination of free ammonia and ionized ammonia. tan is produced through degrading material high in nitrogen, typically proteins and will naturally build in anaerobic digestion. this is depending on the organic feed stock fed to the system. in typical wastewater treatment practices, tan reduction is done with via nitrification. nitrification is an aerobic process where tan is consumed by aerobic heterotrophic bacteria. these bacteria release nitrate and nitrite which are later converted to nitrogen gas through the denitrification process. hydrolysis and acidogenesis can also be impacted by tan concentration. in mesophilic conditions, inhibition for hydrolysis was found to occur at 5500 mg/l tan, while acidogenesis inhibition occurs at 6500 mg/l tan. feedstocks anaerobic lagoon and generators at the cal poly dairy, united states the most important initial issue when considering the application of anaerobic digestion systems is the feedstock to the process. almost any organic material can be processed with anaerobic digestion; however, if biogas production is the aim, the level of putrescibility is the key factor in its successful application. the more putrescible (digestible) the material, the higher the gas yields possible from the system. feedstocks can include biodegradable waste materials, such as waste paper, grass clippings, leftover food, sewage, and animal waste. woody wastes are the exception, because they are largely unaffected by digestion, as most anaerobes are unable to degrade lignin. xylophagous anaerobes (lignin consumers) or high temperature pretreatment, such as pyrolysis, can be used to break lignin down. anaerobic digesters can also be fed with specially grown energy crops, such as silage, for dedicated biogas production. in germany and continental europe, these facilities are referred to as "biogas" plants. a codigestion or cofermentation plant is typically an agricultural anaerobic digester that accepts two or more input materials for simultaneous digestion. the length of time required for anaerobic digestion depends on the chemical complexity of the material. material rich in easily digestible sugars breaks down quickly, whereas intact lignocellulosic material rich in cellulose and hemicellulose polymers can take much longer to break down. anaerobic microorganisms are generally unable to break down lignin, the recalcitrant aromatic component of biomass. anaerobic digesters were originally designed for operation using sewage sludge and manures. sewage and manure are not, however, the material with the most potential for anaerobic digestion, as the biodegradable material has already had much of the energy content taken out by the animals that produced it. therefore, many digesters operate with codigestion of two or more types of feedstock. for example, in a farm-based digester that uses dairy manure as the primary feedstock, the gas production may be significantly increased by adding a second feedstock, e.g., grass and corn (typical on-farm feedstock), or various organic byproducts, such as slaughterhouse waste, fats, oils and grease from restaurants, organic household waste, etc. (typical off-site feedstock). digesters processing dedicated energy crops can achieve high levels of degradation and biogas production. slurry-only systems are generally cheaper, but generate far less energy than those using crops, such as maize and grass silage; by using a modest amount of crop material (30%), an anaerobic digestion plant can increase energy output tenfold for only three times the capital cost, relative to a slurry-only system. moisture content a second consideration related to the feedstock is moisture content. drier, stackable substrates, such as food and yard waste, are suitable for digestion in tunnel-like chambers. tunnel-style systems typically have near-zero wastewater discharge, as well, so this style of system has advantages where the discharge of digester liquids are a liability. the wetter the material, the more suitable it will be to handling with standard pumps instead of energy-intensive concrete pumps and physical means of movement. also, the wetter the material, the more volume and area it takes up relative to the levels of gas produced. the moisture content of the target feedstock will also affect what type of system is applied to its treatment. to use a high-solids anaerobic digester for dilute feedstocks, bulking agents, such as compost, should be applied to increase the solids content of the input material. another key consideration is the carbon:nitrogen ratio of the input material. this ratio is the balance of food a microbe requires to grow; the optimal c:n ratio is 20–30:1. excess n can lead to ammonia inhibition of digestion. contamination the level of contamination of the feedstock material is a key consideration when using wet digestion or plug-flow digestion. if the feedstock to the digesters has significant levels of physical contaminants, such as plastic, glass, or metals, then processing to remove the contaminants will be required for the material to be used. if it is not removed, then the digesters can be blocked and will not function efficiently. this contamination issue does not occur with dry digestion or solid-state anaerobic digestion (ssad) plants, since ssad handles dry, stackable biomass with a high percentage of solids (40-60%) in gas-tight chambers called fermenter boxes. it is with this understanding that mechanical biological treatment plants are designed. the higher the level of pretreatment a feedstock requires, the more processing machinery will be required, and, hence, the project will have higher capital costs. national non-food crops centre. after sorting or screening to remove any physical contaminants from the feedstock, the material is often shredded, minced, and mechanically or hydraulically pulped to increase the surface area available to microbes in the digesters and, hence, increase the speed of digestion. the maceration of solids can be achieved by using a chopper pump to transfer the feedstock material into the airtight digester, where anaerobic treatment takes place. substrate composition substrate composition is a major factor in determining the methane yield and methane production rates from the digestion of biomass. techniques to determine the compositional characteristics of the feedstock are available, while parameters such as solids, elemental, and organic analyses are important for digester design and operation. methane yield can be estimated from the elemental composition of substrate along with an estimate of its degradability (the fraction of the substrate that is converted to biogas in a reactor). in order to predict biogas composition (the relative fractions of methane and carbon dioxide) it is necessary to estimate carbon dioxide partitioning between the aqueous and gas phases, which requires additional information (reactor temperature, ph, and substrate composition) and a chemical speciation model. direct measurements of biomethanation potential are also made using gas evolution or more recent gravimetric assays. applications schematic of an anaerobic digester as part of a