Digestate is the material remaining after the anaerobic digestion (decomposition under low oxygen conditions) of a biodegradable feedstock. Anaerobic digestion produces two main products: digestate and biogas. Digestate is produced both by acidogenesis and methanogenesis and each has different characteristics. These characteristics stem from the original feedstock source as well as the processes themselves.
Anaerobic digestion is a versatile process that can use many different types of feedstocks. Example of feedstocks can be from:
These are just some of the different sources that anaerobic digestate can come from. The chemical make-up of the digestate produced can vary depending on what feedstock is used. Sewage sludge and animal manure generally have the majority of its energy contents consumed due to the original energy source (food) being digested inside the person or animal first. This allows sewage sludge and animal manure to be good candidates for co-digestion together with other feedstocks to produce a better digestate for agricultural purposes as well as increased biogas production. [2]
During this stage, the acidifying bacteria convert water-soluble chemical substances, including products of hydrolysis, to short-chain organic acids, such as formic, acetic, propionic, butyric, and pentanoic, alcohols, such as methanol and ethanol, aldehydes, carbon dioxide, and hydrogen. Ammonia and hydrogen sulfide are other products of acidogenesis. This bacteria operate within a pH range from 4.0 to 8.5. This process can also lower pH inside the biodigester over time causing the microbes to not to be able to function. For this reason pH must be carefully monitored. [2]
Since acidogenesis is early in the process of anaerobic digestion, most of the organic matter has not been fully degraded leaving a digestate that is fibrous and consists of structural plant matter including lignin and cellulose. Thus, it is often referred to as solid digestate. Acidogenic digestate has high moisture retention properties. The digestate may also contain minerals (primarily phosphorus) and remnants of bacteria.
Methanogenesis is the last stage of anaerobic digestion. During this phase methanogenic Archaea produce methane from the substrates generated during acetogenesis. These substrates are mainly acetate and hydrogen. Methanogenesis can also occur using another metabolism based on the cooperation of fermenting bacteria and methanogens archaea, the syntrophic methanogenic pathway. During syntrophic methanogens bacteria belonging mainly to the Clostridia class oxidize acetate into hydrogen and CO2, which are successively exploited by hydrogenotrophic Archaea for the methanogens. The methanogenic microbes are fairly sensitive to pH changes and prefer a range from 5.0-8.5 depending on the species. [2] This is why in some biodigesters the chambers for the different anaerobic digestions stages will be separated for optimal biogas production.
By this point most of the organic matter has broken down leaving behind the Methanogenic digestate known as a sludge (sometimes called a liquor or liquid digestate). The sludge is high in nutrients such as ammoniums and potassium. The other byproduct of this step is methane, which is often collected and used as a fuel source.
This is when the fibrous digestate (solid fraction) of the acidogenic digestate is combined with the liquor digestate (liquid fraction) of the methanogenic digestate to create the whole digestate. This combination of the two digestates consists as a sludge form. The liquid fraction constitutes up to 90% of the digestate by volume, contains 2–6% dry matter, particles <1.2 mm in size, and most of the soluble nitrogen and potassium, while the solid fraction retains most of the digestate phosphorus, and contains dry matter content ˃ 15%. [3]
Combining the two into a whole digestate allows for increased availability of a wide array of nutrients that can be useful for agricultural activities. Some anaerobic biodigesters will only have one digestion chamber allowing these two digitates to mix together on their own without further intervention.
The major parameters to assess digestate quality when being used for agricultural applications include pH, nutrients, total solids (TS), volatile solids (VS), and total carbon (TC). This quality depends on feedstock and type of anaerobic digester system. [3] Generally the ammonia content of the digestate accounts for approximately 60-80% of the total nitrogen content, but for a feedstock like kitchen food waste it can be as high as 99%. Digestate has also been reported to have a higher phosphorus and potassium concentration than that of composts. The average P to K ratio is about 1:3. All this together makes digestate a potentially viable source for agricultural soil amendments of certain crops. [4]
The primary use of digestate is as a soil conditioner. [5] Acidogenic digestate provides moisture retention and organic content for soils. This organic material can break down further, aerobically in soil. Methanogenic digestate provides nutrients for plant growth. It can also be used to protect soils against erosion.
Acidogenic digestate can also be used as an environmentally friendly filler to give structure to composite plastics.
Growth trials on digestate originating from mixed waste have showed healthy growth results for crops. [6] Digestate can also be used in intensive greenhouse cultivation of plants, e.g., in digeponics.
Additionally, both solid and liquid digestates have been shown to be of use in hydroponic crop production. Multiple studies have shown that digestate can produce similar or higher yields across multiple crops when compared to standard growing practices used in hydroponics and soilless substrate growing. [7] [8] [9]
Application of digestate has been shown to inhibit plant diseases and induction of resistance. Digestate application has a direct effect on soil-borne diseases, and an indirect effect by stimulation of biological activity.
Digestate is technically not compost although it is similar to it in physical and chemical characteristics. Compost is produced by aerobic digestion-decomposition by aerobes. This includes fungi and bacteria which are able to break down the lignin and cellulose to a greater extent.
Treatment, for example by ultrasonication, has shown to enhance solubilization of digestate as measured by increased levels of soluble chemical oxygen demand (sCOD), soluble total organic carbon (sTOC), and soluble total nitrogen (sTN) released into the solution. [10]
The standard of digestate produced by anaerobic digestion can be assessed on three criteria, chemical, biological and physical aspects. Chemical quality needs to be considered in terms of heavy metals and other inorganic contaminant, persistent organic compounds and the content of macro-elements such as nitrogen, phosphorus and potassium. Depending on their source, biowastes can contain pathogens, which can lead to the spreading of human, animal or plant diseases if not appropriately managed.
The physical standards of composts includes mainly appearance and odor factors. Whilst physical contamination does not present a problem with regards to human, plant or animal health, contamination (in the form of plastics, metals and ceramics) can cause a negative public perception. Even if the compost is of high quality and all standards are met, a negative public perception of waste-based composts still exists. The presence of visible contaminants reminds users of this.
Quality control of the feedstock is the most important way of ensuring a quality end product. The content and quality of waste arriving on-site should be characterised as thoroughly as possible prior to being supplied.
In the UK the Publicly Available Specification (called PAS110) governs the definition of digestate derived from the anaerobic digestion of source-segregated biodegradable materials. [11] The specification ensures all digested materials are of consistent quality and fit for purpose. If a biogas plant meets the standard, its digestate will be regarded as having been fully recovered and to have ceased to be waste, and it can be sold with the name "bio-fertiliser". [12]
Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical, and biological properties. It is commonly prepared by decomposing plant and food waste, recycling organic materials, and manure. The resulting mixture is rich in plant nutrients and beneficial organisms, such as bacteria, protozoa, nematodes, and fungi. Compost improves soil fertility in gardens, landscaping, horticulture, urban agriculture, and organic farming, reducing dependency on commercial chemical fertilizers. The benefits of compost include providing nutrients to crops as fertilizer, acting as a soil conditioner, increasing the humus or humic acid contents of the soil, and introducing beneficial microbes that help to suppress pathogens in the soil and reduce soil-borne diseases.
Biogas is a gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, wastewater, and food waste. Biogas is produced by anaerobic digestion with anaerobic organisms or methanogens inside an anaerobic digester, biodigester or a bioreactor. The gas composition is primarily methane and carbon dioxide and may have small amounts of hydrogen sulfide, moisture and siloxanes. The methane can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel; it can be used in fuel cells and for heating purpose, such as in cooking. It can also be used in a gas engine to convert the energy in the gas into electricity and heat.
Vermicompost (vermi-compost) is the product of the decomposition process using various species of worms, usually red wigglers, white worms, and other earthworms, to create a mixture of decomposing vegetable or food waste, bedding materials, and vermicast. This process is called vermicomposting, with the rearing of worms for this purpose is called vermiculture.
Biosolids are solid organic matter recovered from a sewage treatment process and used as fertilizer. In the past, it was common for farmers to use animal manure to improve their soil fertility. In the 1920s, the farming community began also to use sewage sludge from local wastewater treatment plants. Scientific research over many years has confirmed that these biosolids contain similar nutrients to those in animal manures. Biosolids that are used as fertilizer in farming are usually treated to help to prevent disease-causing pathogens from spreading to the public. Some sewage sludge can not qualify as biosolids due to persistent, bioaccumulative and toxic chemicals, radionuclides, and heavy metals at levels sufficient to contaminate soil and water when applied to land.
Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.
Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or irrigation water. Agricultural wastewater treatment is required for continuous confined animal operations like milk and egg production. It may be performed in plants using mechanized treatment units similar to those used for industrial wastewater. Where land is available for ponds, settling basins and facultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles. Animal slurries are usually treated by containment in anaerobic lagoons before disposal by spray or trickle application to grassland. Constructed wetlands are sometimes used to facilitate treatment of animal wastes.
A mechanical biological treatment (MBT) system is a type of waste processing facility that combines a sorting facility with a form of biological treatment such as composting or anaerobic digestion. MBT plants are designed to process mixed household waste as well as commercial and industrial wastes.
Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge treatment is focused on reducing sludge weight and volume to reduce transportation and disposal costs, and on reducing potential health risks of disposal options. Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermophilic digestion, composting, or incineration. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options. Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.
Green waste, also known as "biological waste", is any organic waste that can be composted. It is most usually composed of refuse from gardens such as grass clippings or leaves, and domestic or industrial kitchen wastes. Green waste does not include things such as dried leaves, pine straw, or hay. Such materials are rich in carbon and considered "brown wastes," while green wastes contain high concentrations of nitrogen. Green waste can be used to increase the efficiency of many composting operations and can be added to soil to sustain local nutrient cycling.
Biodegradable waste includes any organic matter in waste which can be broken down into carbon dioxide, water, methane, compost, humus, and simple organic molecules by micro-organisms and other living things by composting, aerobic digestion, anaerobic digestion or similar processes. It mainly includes kitchen waste, ash, soil, dung and other plant matter. In waste management, it also includes some inorganic materials which can be decomposed by bacteria. Such materials include gypsum and its products such as plasterboard and other simple sulfates which can be decomposed by sulfate reducing bacteria to yield hydrogen sulfide in anaerobic land-fill conditions.
The following article is a comparison of aerobic and anaerobic digestion. In both aerobic and anaerobic systems the growing and reproducing microorganisms within them require a source of elemental oxygen to survive.
Dark fermentation is the fermentative conversion of organic substrate to biohydrogen. It is a complex process manifested by diverse groups of bacteria, involving a series of biochemical reactions using three steps similar to anaerobic conversion. Dark fermentation differs from photofermentation in that it proceeds without the presence of light.
Agricultural pollution refers to biotic and abiotic byproducts of farming practices that result in contamination or degradation of the environment and surrounding ecosystems, and/or cause injury to humans and their economic interests. The pollution may come from a variety of sources, ranging from point source water pollution to more diffuse, landscape-level causes, also known as non-point source pollution and air pollution. Once in the environment these pollutants can have both direct effects in surrounding ecosystems, i.e. killing local wildlife or contaminating drinking water, and downstream effects such as dead zones caused by agricultural runoff is concentrated in large water bodies.
Manure is organic matter that is used as organic fertilizer in agriculture. Most manure consists of animal feces; other sources include compost and green manure. Manures contribute to the fertility of soil by adding organic matter and nutrients, such as nitrogen, that are utilised by bacteria, fungi and other organisms in the soil. Higher organisms then feed on the fungi and bacteria in a chain of life that comprises the soil food web.
Source-separated organics (SSO) is the system by which waste generators segregate compostable materials from other waste streams at the source for separate collection.
Sharad P Kale is a scientist known for developing a biogas plant based on biodegradable waste resource (Nisargruna). He is the head of Technology Transfer and Collaboration at Bhabha Atomic Research Centre (BARC). On 26 January 2013, the Government of India honoured him with the Padma Shri Award in the Discipline of Science and Engineering.
Reuse of human excreta is the safe, beneficial use of treated human excreta after applying suitable treatment steps and risk management approaches that are customized for the intended reuse application. Beneficial uses of the treated excreta may focus on using the plant-available nutrients that are contained in the treated excreta. They may also make use of the organic matter and energy contained in the excreta. To a lesser extent, reuse of the excreta's water content might also take place, although this is better known as water reclamation from municipal wastewater. The intended reuse applications for the nutrient content may include: soil conditioner or fertilizer in agriculture or horticultural activities. Other reuse applications, which focus more on the organic matter content of the excreta, include use as a fuel source or as an energy source in the form of biogas.
Anaerobic membrane bioreactor or AnMBR is the name of a technology utilized in wastewater treatment. It is a technology in membrane filtration for biomass retention. AnMBR works by using a membrane bioreactor (MBR) in a anaerobic environment. Anaerobic bacteria and archaea convert organic materials into carbon dioxide (CO2) and methane (CH4). The sewage is filtered and separated by membranes leaving the effluent and sludge apart. The produced biogas can later be combusted to generate heat or electricity. It can also be upgraded (purified) into Renewable natural gas of household quality. AnMBR is considered to be a sustainable alternative for sewage treatment because the energy that can be generated by methane combustion can exceed the energy required for maintaining the process.
Digeponics (pronounced die-jeh-ponics, as in digestion) is a method of agriculture which integrates the products of anaerobic digestion, including CO2 and digestate, with greenhouse cultivation of vegetables.
Lutispora saccharofermentans, is an anaerobic bacteria. Lutispora saccharofermentans was first isolated from methanogenic enrichment cultures derived from a material collected from a lab-scale methanogenic landfill bioreactor.
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