Digestate is the material remaining after the

anaerobic digestion 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 ferm ...

(decomposition under low oxygen conditions) of a

biodegradable Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...

feedstock A raw material, also known as a feedstock, unprocessed material, or primary commodity, is a basic material that is used to produce goods, finished goods, energy, or intermediate materials that are feedstock for future finished products. As feeds ...

. Anaerobic digestion produces two main products: digestate and

biogas Biogas is a mixture of gases, primarily consisting of methane, carbon dioxide and hydrogen sulphide, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste and food waste. It is a ...

. Digestate is produced both by acidogenesis and

methanogenesis Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...

and each has different characteristics. These characteristics stem from the original feedstock source as well as the processes themselves.

Digestate feedstock sources

Anaerobic digestion is a versatile process that can use many different types of feedstocks. Example of feedstocks can be from: *

Sewage sludge Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to s ...

s: Liquid sludge, untreated sewage sludge, composted sludge, and lime treated sludge. * Animal wastes: Animal fats, animal blood, food remains, stomach contents, rumen contents, animal carcasses, and poultry, fish, and livestock

manure 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 Soil fertility, fertility of soil by adding organic ma ...

. *

Energy crop Energy crops are low-cost and low-maintenance crops grown solely for energy production by combustion (not for food). The crops are processed into solid, liquid or gaseous fuels, such as pellets, bioethanol or biogas. The fuels are burned to ...

s: Usually corn, maize, millet, and clover. This can be whole crops used in co-digestion or as waste (stems and stalks) from harvesting of these crops. *

Municipal waste Municipal solid waste (MSW), commonly known as trash or garbage in the United States and rubbish in Britain, is a waste type consisting of everyday items that are discarded by the public. "Garbage" can also refer specifically to food waste, a ...

s: Food waste, coffee/tea filters, organic leftovers, bakery waste, and kitchen waste. * Agricultural wastes: Fruits, molasses, stems, plant straw, and

bagasse Bagasse ( ) is the dry pulpy fibrous material that remains after crushing sugarcane or sorghum stalks to extract their juice. It is used as a biofuel for the production of heat, energy, and electricity, and in the manufacture of pulp and building ...

(residue after crushing sugarcane or sorghum stalks). *

Industrial waste Industrial waste is the waste produced by industrial activity which includes any material that is rendered useless during a manufacturing process such as that of factories, mills, and mining operations. Types of industrial waste include dirt an ...

s: Food/beverage processing waste, dairy wastes, starch/sugar industries wastes, slaughterhouse wastes, and brewery wastes. 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.

Anaerobic digestion process

Anaerobic digestion of organic matter consists mainly of four phases: enzymatic

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...

(break down of large

polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

s to smaller molecules), acidogenesis (acid formation), acetogenesis (

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

production), and

(CH₄ production). Digestate is produced both by acidogenesis and methanogenesis and each has different characteristics.

Acidogenic digestate

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. 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 Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...

and

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...

. Thus, it is often referred to as solid digestate. Acidogenic digestate has high moisture retention properties. The digestate may also contain

minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed ...

(primarily phosphorus) and remnants of

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

Methanogenic digestate

Methanogenesis Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...

is the last stage of anaerobic digestion. During this phase methanogenic ''

Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...

'' 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 The Clostridia are a highly polyphyletic class of Bacillota, including ''Clostridium'' and other similar genera. They are distinguished from the Bacilli by lacking aerobic respiration. They are obligate anaerobes and oxygen is toxic to them. S ...

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. 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 Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

. The other byproduct of this step is

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...

, which is often collected and used as a fuel source.

Whole digestate

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%. 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.

Digestate characteristics

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. 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.

Uses

The primary use of digestate is as a

soil conditioner A soil conditioner is a product which is added to soil to improve the soil’s physical qualities, usually its fertility (ability to provide nutrition for plants) and sometimes its mechanics. In general usage, the term "soil conditioner" is often ...

. Acidogenic digestate provides moisture retention and organic content for soils. This organic material can break down further, aerobically in

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

. Methanogenic digestate provides nutrients for

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

growth. It can also be used to protect soils against erosion. Acidogenic digestate can also be used as an

environmentally friendly Environment friendly processes, or environmental-friendly processes (also referred to as eco-friendly, nature-friendly, and green), are sustainability and marketing terms referring to goods and services, laws, guidelines and policies that cl ...

filler to give structure to composite plastics. Growth trials on digestate originating from mixed waste have showed healthy growth results for crops. 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. Application of digestate has been shown to inhibit plant diseases and induction of resistance. Digestate application has a direct effect on soil-born diseases, and an indirect effect by stimulation of biological activity.

Digestate and compost

Digestate is technically not

compost 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, food waste, recycling organic materials and manure. The resulting ...

although it is similar to it in physical and chemical characteristics. Compost is produced by

aerobic digestion Aerobic digestion is a process in sewage treatment designed to reduce the volume of sewage sludge and make it suitable for subsequent use. More recently, technology has been developed that allows the treatment and reduction of other organic waste, ...

-decomposition by aerobes. This includes

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

and

which are able to break down the lignin and cellulose to a greater extent. Treatment, for example by

ultrasonication image:Sonicator.jpg, A sonicator at the Weizmann Institute of Science during sonicationSonication is the act of applying sound energy to agitate particles in a sample, for various purposes such as the extraction of multiple compounds from plants, m ...

, 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.

Standards for digestate

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 A Publicly Available Specification or PAS is a standardization document that closely resembles a formal standard in structure and format but which has a different development model. The objective of a Publicly Available Specification is to speed up ...

(called PAS110) governs the definition of digestate derived from the anaerobic digestion of source-segregated biodegradable materials. The specification ensures all digested materials are of consistent quality and fit for purpose. If a

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".

References

*Peng, Wei & Pivato, Alberto. (2019). Sustainable Management of Digestate from the Organic Fraction of Municipal Solid Waste and Food Waste Under the Concepts of Back to Earth Alternatives and Circular Economy. Waste and Biomass Valorization. 10. 10.1007/s12649-017-0071-2.

External links

* {{waste Anaerobic digestion Biodegradable waste management Biogas technology Mechanical biological treatment Soil improvers