Anaerobic digestion (AD) has a rich history dating back centuries. From early experiments to modern applications, AD has evolved into a vital technology for sustainable energy production and waste management. Today, AD plays a crucial role in reducing reliance on fossil fuels and promoting a circular economy.
There has been an interest in the manufacturing of gas by the decomposition of organic materials as early as the 17th Century. Modern founders of Chemistry documented the discovery of methane gas when disturbing sediments in streams and lakes. In the early 19th Century methane was proven to be present in cattle manure. In the middle of that century the first anaerobic digester was built in a leper colony in India and in England, 1895 the technology was developed further to generate gas from a septic tank to provide gas lighting. Shortly after, the first dual purpose tank for sediment and sludge treatment was built in Hampton, London.
After the First World War anaerobic lagoons were starting to be replaced by closed tank systems for wastewater treatment and shortly after in the 1930s proper research began into the technology. Petroleum production had increased with the advent of WWI, slowing the production of biofuels, however, fuel shortages in WWII restored interest in anaerobic digestion but dwindled again after the war. The Petroleum Crisis of the 1970’s brought interest back in the technology and it has continued to grow steadily since.
In the late 20th early 21st Century anaerobic digesters are commonly found near or alongside farms to reduce nitrogen from manure and to reduce the cost of sludge disposal. Agricultural AD for energy production has been expanding in Germany with nearly 9,000 anaerobic digesters in the mid half of the 2010s. Around the same time there were over 250 in the UK with double that planned by the end of the decade. While in the US there were just 200 across 34 States.
Policy to increase generation has been a significant factor in growth of anaerobic digestion technology. Feed In Tariffs (FIT) where introduced in Germany providing compensation for investment in renewable energy generation. Although plants increased markets were unsure of the potential. The government intervened and amended the FIT multiple times and the industry has continued relatively rapid growth.
With the requirement for cleaner energy sources, the sustainability movement, circular economy, the reduction of food waste, soil degradation and the byproducts of anaerobic digestion, the sector is set for increasingly rapid growth.
Pasteurisation is a prerequisite for processing food waste. It can occur before or after the anaerobic digestion process. The materials are loaded into pasteurisation tanks and heated to 70 degrees for an hour to destroy pathogens like E-Coli and Salmonella. This needs to be completed to PAS110 accreditation standards.
The digestion process starts with the break down of the organic materials by bacteria and water (Hydrolosis). Carbohydrates are subsequently broken down for other bacteria. Further reaction occurs when fatty acids – sugar and amino acids are broken down to convert the input organic materials to carbon dioxide, hydrogen, acetic acid, ammonia and organic acids (Acidogenisis). Then Methanogens, a type of bacteria (Archaea) convert these products to Methane and Carbon Dioxide (Methanogenisis).
There are 4 key stages to the anaerobic digestion process hydrolosis, acidogenesis, acetogenesis and methanogenesis.
Anaerobic Digestion plants can be designed and engineered to optimise production in a range of configurations. They are categorised into batch or continuous production, mesophilic or thermophilic temperature conditions, high or low solids and single or multi-stage process.
Batch processing tends to require a less complex design but requires more infrastructure and larger volume of digesters. Greater heat energy is required in a thermophilic system (48-60c) in comparison to mesophilic digestion (20-42c). Thermophilic digestion has a shorter time to production and higher methane content. Low solids content can contain around 15% solids, anything above that is considered high solids content and know as dry digestion. In single stage processing the 4 stages happen in one tank or digester. A multiple stage processes separates the Hydrolosis and Methanogenesis stages.
In batch processing materials are added to the digester then sealed for the duration of the process. It requires some adding of bacteria to start the process, like growing a culture in a petri dish. This produces a normalised pattern of biogas yield over time. Odour control in batch processing can be more difficult so is sometimes integrated with in-vessel composting.
In continuous processing, organic materials are continually added to the digester. This results in continual production of biogas. Single or multiple digesters can be used. There are several types of reactor used according to the resource materials that the anaerobic digestion plant was designed for.
Digesters are generally either continuous vertical plug flow or batch tunnel horizontal digesters. Continuous vertical plug flow digesters are upright, cylindrical tanks where feedstock is continuously fed into the top of the digester. Batch tunnel digesters, the feedstock is deposited in tunnel-like chambers with a gas-tight door. Contaminant removal depends on the nature of the waste streams (feedstock) being processed and the required quality of the digestate. Size reduction (grinding) is beneficial in continuous vertical systems, as it accelerates digestion. Batch systems avoid grinding and instead require structure (e.g. yard waste) to reduce compaction of the stacked pile.
The two conventional optimal operational temperatures for anaerobic digestion are Mesophilic digestion, which takes place between 30c to 38c. At ambient temperatures between 20c and 45c. Thermophilic digestion takes place between 49c to 57c. Temperatures can be increased to 70c.
Mesophilic systems are considered to be more stable than Thermophilic systems. This is due to the number of Mesophiles, which are more tolerant to environmental conditions. The majority of digesters globally are mesophilic systems.
Thermophilic systems are considered less stable with higher energy inputs yet higher biogas yields in less time. Higher temperatures deliver faster reaction rates and pathogen reduction of materials. This is necessary where Animal By-Products legislation governs the input of this type of materials and use of digestate back to agriculture.
The most important aspect of Anaerobic Digestion is the organic Feedstock. Many types of organic materials can be used in the anaerobic digestion process. As biogas is the main objective of production the more digestible the better the biogas yield.
Feedstocks can include organic wate materials such as paper, grass, food, sewage and animal waste. Wastes with high wood content do not degrade due to lignin and require pretreatment before further degradation. Digesters can also be fed with energy crops as codigestion or cofermentation usually located on-farm.
Anaerobic Digesters were originally designed for use with sewage sludge and manure. However, this is not the most digestible type of material with the greatest calorific value. This type of organic material has already had much of the energy taken from it by the animals that produced it. So, in order to produce higher yields in this type of digestion there are often multiple feedstocks mixed with manure such as grass, corn, animal by products, grease, fats, oils and household food waste, creating a greater calorific value and therefore higher gas yield.
Moisture levels in feedstock also have an impact on production. Drier materials are more suited to horizontal or tunnel chambers, which have a very low rate of water discharge. The wetter the material the more readily it can be pumped however, this takes up area and volume relative to gas produced. High solids can be used to dilute wetter feedstocks. A carbon to nitrogen ratio level also needs to be assessed as this can have an impact on gas production.
If the feedstock has significant levels of material contamination such as glass, plastic or metals then this has to removed. Contaminants to feedstock can seriously affect machinery and equipment used to process the materials. There are also a number of legislative requirements in relation to the levels of contaminant in the feedstock as once processed goes back to land as digestate. The higher the level of contaminant the greater pretreatment is required increasing costs of the digestion process. If the feedstock load is highly contaminated then the feedstock maybe incinerated.
As organic and food waste material poses such a threat to climate due to its potential methane leakage in landfill if not disposed of properly an opportunity has arisen for the digestion of prepackaged, spoiled, mislabelled, expired or recalled food resource. Depackaging equipment is used to extract the organic materials from its packaging. A range of equipment can hydraulically and mechanically separate the material from the packaging whether, glass, metal or plastic. In general a hopper and conveyor move the material into screw press and or shredder. Packaging removed with magnets and blowers can be reused for recycling or recovery.
Three different types of solids of feedstock are associated with Anaerobic Digestion. Dry High Solids, Wet Pumpable High Solids and Wet Low Solids.
Dry High Solids digesters are designed to process materials with a solids content between 25-40%. These types of digesters are designed to process materials without the addition of water. Wet Solids Digesters can operate with a High Solids (Dry) content greater than 20% or a Low Solids (Wet) content of less than 15%.
High Solids Wet Digesters process a thick slurry. They require more energy to input and process the feedstock. They also require monitoring of performance calculations such as the production of biogas.
Low Solids Wet Digesters require less energy input than Dry High Solids. Benefits include more thorough monitoring of performance calculations and greater bacterial contact with materials increasing the rate of gas production.
The time it takes to produce biogas and digestate varies greatly according to feedstock and anaerobic digestion method. For example, in a two-stage mesophilic system the average time can vary between 15-40 days. A single stage thermophilic system averages at approximately 14 days. The continuous “plug flow” method of materials input may result in the incomplete breakdown of feedstock and so the output digestate may become darker and have a stronger odour.
Where “sludge blankets” are used in upflow sludge blanket digestion (UASB) the time can be as little as 1 hour or 1 day while the material can be kept in the reactor for 90 days. Continuous digesters have mechanical devices to mix the contents. This allows for greater contact between feedstock and bacteria for enhanced production.
A recent study conducted in Sweden showed that bacteria in the reactor changes in composition under pressure. The produced carbon dioxide dissolves in water than methane under pressure. Along with pH, salinity and temperature, pressure also has an effect on the quality and rate of methane output and therefore, High Pressure Anaerobic Digestion (HPAD) or Autogenerative High Pressure Digestion is becoming more common.
The composition of feedstock is a significant factor in determining the methane gas production rate and yield. There are a number of ways of verifying the composition and characteristics of the feedstock. Solids, materials and organic analysis are critical for digestion design and operation. Yield can be estimated from material composition and degradability. It is also necessary to determine carbon dioxide ratio between feedstock and gas phases, also requiring reactor temperature, pH level, composition and how the material degrades, potential gas produced from the feedstock(s) and the analysis of gas produced.
There are a number of elements that can be present in feedstock that have an adverse effect on bacteria and the optimum production of methane. Ammonia, light metal ions like Sodium, Potassium, Calcium, Aluminium and Magnesium. As well as heavy metals, some organics like chlorophenols and long chain fatty acids can negatively impact on the degradation process.
If there is an imbalance of ammonia that alters pH levels this can have an effect on methane production, destabilising the production of acetic acid. If a higher content of ammonia exists adjustment is required to keep the reaction stable.
Biogas is a primary production objective of Anaerobic Digestion. Biogas is created by the breakdown of organic waste resources feedstock by bacteria known as archaea. It’s composition is mainly methane CH4 (50-75%), carbon dioxide CO2 (25-50%), Nitrogen N2 (0-10%) and Hydrogen H2 (0-1%). There are also some trace elements that include Hydrogen Sulfide and water vapour.
The biogas may require treatment for use as a fuel known as “scrubbing”. A range of other cleaning and filtration processes may exist depending on the outcome of the digestion process from certain feedstocks. Hydrogen Sulfide is a good example of this as a toxic material and legislation the UK and US details that if high amounts then the gas has to be treated or additions to the digestion process have to be made.
Digestate from Anaerobic Digestion is the solid output from the input feedstock that now also contains mineral rich dead bacteria. Digestate is output in three forms, fibrous, liquid or sludge. In multiple stage AD systems digestate can come from different tanks. Digestate can also be further processed, common in single stage systems.
Acidogenic Digestate consists of lignin, cellulose and a variety of mineral components. It can be used to make fibreboard or in ethanol production. Methanogenic Digestate is rich in nutrients and can be used as an organic fertiliser. Levels of contaminants need to be assessed and there are numerous policies about distributing on land and levels of contaminants. A maturation or composting stage can sometimes be completed after digestion. This oxidises ammonia into nitrates enriching the digestate as a soil conditioner. Large-scale composting can often be found operating alongside Anaerobic Digestion.
Another output from the anaerobic digestion process is waste water. This is from the original feedstock and as a result of the bacterial breakdown of the feedstock under a certain temperature. The water can come from the dewatering process of the digestate and in many plants is filtered and reused in the application.
The water will usually have elevated levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Levels of this in the effluent can pollute and require measurement and monitoring. Further treatment of waste water is often needed and results in oxidation or reverse osmosis.
Methane in biogas is separated and can be burned to produce heat and electricity. This is often completed on site and used to heat the digester to optimum temperature. Gas and electricity from the digestion process can also be injected into the national power infrastructure. It is renewable and therefore qualifies for renewable energy subsidies. Biogas is not released into the atmosphere and does not contribute to climate warming.
Raw biogas from the anaerobic digestion process is enriched to produce biomethane as injected directly into the grid for use. Upgrading of methane to biomethane for direct injection also includes further “scrubbing” to remove hydrogen and carbon dioxide.
