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ANAEROBIC DIGESTION



Anaerobic digestion (AD) is a natural process that occurs in swamps, peat bogs, even landfills. It is a series of process whereby micro-organisms mainly bacteria breaks down organic or biodegradable material in the absence of oxygen (anaerobic) to produce biogas - a mixture of of gases mostly methane and carbon dioxide, with a small amount hydrogen and trace hydrogen sulfide and a stabilized residue know as digestate.


The Environmental Permitting Regulation (England and Wales) (Amendment) Regulation 2012 defines Anaerobic Digestion to mean the mesophilic and thermophilic biological decomposition and stabilisation of biodegradable material which-


(a) is carried on under controlled anaerobic conditions,

(b) produces a methane-rich gas mixture, and

(c) results in stable sanitised material that can be applied to land for the benefit of agriculture or to improve the soil structure or nutrients in land.


AD has been adapted by the waste industry in the treatment of organic wastes including organic industrial wastes (food processing for example) and the biodegradable portion of source-segregated or mechanically separated MSW. Anaerobic digestion is also widely used as a source of renewable energy. With increased focus on climate change mitigation, the re-use of waste as a resource and new technological approaches which have lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom, Germany and Denmark. The process produces a biogas, consisting of methane, carbon dioxide and other traces gases. This biogas can be used directly as cooking fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The use of biogas as a fuel helps to replace fossil fuels. So where the biogas and digestates are utilized, i.e. the biogas is burnt to generate power and the digestate is applied to land improve soil structure, then AD can be said to be both a waste recovery and recycling operation.


AD chemistry and biology

The process that takes place in an Anaerobic digester is similar to that in a landfill, hence it's description as a controlled landfill reactor. The same processes occur except for the initial aerobic stage in a landfill. In AD, the aerobic stage is deliberately and completely excluded. Any aerobic treatment of the final digestate produced takes place outside the AD reactor.

There are four key biological and chemical stages of anaerobic digestion:


Hydrolysis

Acidogenesis

Acetogenesis

Methanogenesis


The digestion process begins with bacterial hydrolysis of the input materials to break down insoluble organic polymers, such as carbohydrates, and make them available for other bacteria. The enzyme produced by the bacteria breaks down the cellulose, lipids and proteins into simple sugars, fatty acids and amino acids respectively.


Acidogenic bacteria further converts the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. The process of acidogenesis is similar to the way milk sours. The gas produced during this phase typically contains 80% methane and 20% hydrogen. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. At the same time, other bacterial communities convert more complex carbohydrates directly into acetic acid. Finally, methanogens convert these products, mainly the acetic acid to methane and carbon dioxide. The methanogens are very active during this phase and they further convert carbon dioxide and hydrogen (generated during the hydrolysis of lipids) to generate more methane. The final gas composition is made up of approximately 66% methane and 33% carbon dioxide. Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8. The remaining, indigestible material the microbes cannot use and any dead bacterial remains constitute the digestate.


A simplified generic chemical equation for the overall processes outlined above is as follows:


C6H12O6 → 3CO2 + 3CH4


AD Technology The digester is designed to reproduce the conditions in a landfill (excluding initial aerobic phase) but over a shorter period of time (two to four weeks). The feedstock is mixed with water and the resultant slurry is fed into the reactor where it is stirred continuously to ensure good mixing. The feedstock is also heated to maintain optimum temperature. As material is added to the reactor, an equivalent volume is removed. Any gas formed is collected at the top of the reactor and is withdrawn for storage. The gas is usually burnt on-site in a gas engine to generate power. Heat can also be recovered (to heat the feed-stock) from the exhaust gas of the engine used to burn the gas. This is discussed later.

Configuration

Anaerobic digesters can be operated using a number of different process configurations:

Batch or continuousTemperature: Mesophilic or thermophilicSolids content: High solids or low solidsComplexity: Single stage or multistage

Batch or continuous

Anaerobic digestion can be performed as a batch process or a continuous process.

In a batch system biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process.


To start the anaerobic digestion process, the batch processing mixed with already processed material. In a typical scenario, biogas production will be formed with a normal distribution pattern over time. Operator can use this fact to determine when they believe the process of digestion of the organic matter has completed. There is a risk of odour generation if a batch reactor is opened and emptied before the process is well completed.


A more advanced type of batch approach has limited the odour issues by integrating anaerobic digestion with in-vessel composting. In this approach inoculation takes place through the use of recirculated degasified percolate whereby

excess water from a digestate is removed and returned to the reactor. After anaerobic digestion has completed, the biomass is kept in the reactor which is then used for in-vessel composting before it is opened.


As the batch digestion is simple and requires less equipment and lower levels of design work, it is typically a cheaper form of digestion. Using more than one batch reactor at a plant can ensure constant production of biogas. In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow; first in – first out). Here, the end products are constantly or periodically removed, resulting in constant production of biogas. A single or multiple digesters in sequence may be used. Examples of this form of anaerobic digestion include continuous stirred-tank reactors, upflow anaerobic sludge blankets, expanded granular sludge beds and internal circulation reactors.


Temperature

The two conventional operational temperature levels for anaerobic digesters are determined by the species of methanogens in the digesters:

Mesophilic digestion takes place optimally around 30 to 38 °C, or at ambient temperatures between 20 and 45 °C, where mesophiles are the primary microorganism present. Thermophilic digestion takes place optimally around 49 to 57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present.

Mesophilic species outnumber thermophiles, and they are also more tolerant to changes in environmental conditions than thermophiles. Mesophilic systems are, therefore, considered to be more stable than thermophilic digestion systems.


Though thermophilic digestion systems are considered to be less stable and the energy input is higher, more energy is removed from the organic matter. The increased temperatures facilitate faster reaction rates and, hence, faster gas yields. Higher temperatures also facilitates greater sterilization of the end digestate. This is an added benefit in countries where legislation, such as the Animal By-Products Regulations in the European Union, which requires end products to meet certain levels of reduction in the amount of bacteria in the output material.


Certain processes shred the waste finely and use a thermal pretreatment stage (hygienisation) to significantly enhance the gas output of the following standard mesophilic stage. The hygienisation process is also applied to reduce the pathogenic micro-organisms in the feedstock. A drawback of operating at thermophilic temperatures is that more heat energy input is required to achieve the correct operational temperatures, which may not be outweighed by the increase in the outputs of biogas from the systems. Therefore, it is important to consider an energy balance for these systems.


Solids content

There are three different operational parameters associated with the solids content of the feedstock to the digesters:


High solids (dry—stackable substrate)

High solids (wet—pumpable substrate)

Low solids (wet—pumpable substrate)


High solids (dry) digesters are designed to process materials with a solids content between 25 and 40%. Unlike wet digesters that process pumpable slurries, high solids (dry – stackable substrate) digesters are designed to process solid substrates without the addition of water. The primary styles of dry digesters are continuous vertical plug flow and batch tunnel horizontal digesters. Continuous vertical plug flow digesters are upright, cylindrical tanks where feedstock is continuously fed into the top of the digester, and flows downward by gravity during digestion. In batch tunnel digesters, the feedstock is deposited in tunnel-like chambers with a gas-tight door. Neither approach has mixing inside the digester. The amount of pretreatment, such as contaminant removal, depends both upon the nature of the waste streams being processed and the desired quality of the digestate. Size reduction is beneficial in continuous vertical systems, as it accelerates digestion, while batch systems avoid size reduction 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. Unlike low solids, high solid digesters will typically have a lower land requirement due to the lower volumes associated with the moisture.


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

There are two main configuration complexities: Single and Double-Stage Digestion Systems.

A singe-stage digestion system requires a single, sealed reactor or holding tank in which all of the biological reactions occur. 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. As Methanogenic bacteria require stable pH, 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. Odour may be an issue unless the lagoons are properly fitted with covers.


A two-stage digestion system (multistage) has different digestion vessels which are optimised to bring maximum control over the bacterial communities (mainly acidogenic and methanogenic bacterias) living within the digesters. Acidogenic bacteria produce organic acids (thereby affecting pH) and grow and reproduce quicker than methanogenic bacteria. Methanogens 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 processes employ pasteurisation or sterilisation stage prior to digestion or between the two digestion tanks to kill harmful bacteria in the input waste. 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, the configuration of the digestion system, and whether it be one-stage or two-stage.

In the case of single-stage thermophilic digestion, residence times may be in the region of 14 days, which, compared to mesophilic digestion, is relatively fast. 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 two-stage mesophilic digestion, residence time may vary between 15 and 40 days.

Feedstock

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 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. Xylophalgeous anaerobes (lignin consumers) or using high temperature pretreatment, such as pyrolysis, can be used to break down the lignin. 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.


Anaerobes can break down material with varying degrees of success from readily, in the case of short-chain hydrocarbons such as sugars, to over longer periods of time, in the case of cellulose. Anaerobic microorganisms are unable to break down long-chain woody molecules, such as lignin. 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.



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. Substrate compositionSubstrate 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.

Moisture content

A second consideration related to the feedstock is moisture content. Dryer, 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

Many components of MSW contain inorganic wastes which are not amenable to biodegradation. Pre-treatment of the waste load is required to remove the inorganic or non biodegradable components. If it is not removed, then the digesters can be blocked and will not function efficiently. It is with this understanding that mechanical biological treatment plants are designed. Mechanical is obviously quicker but the digestate may contain unwanted contaminants including plastics, metals, glasses and in some cases heavy metal contaminants. In such cases, the digestate has to be disposed of to landfill which defeats the whole purpose (see legislative definition of AD above).

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.