Anaerobic digestion

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Anaerobic digestion is a process through which micro-organisms break down organic matter, such as animal manure, wastewater biosolids, and food wastes, in the absence of oxygen. Anaerobic digestion intended for biogas production takes place in a sealed tank (called an anaerobic digester), which is designed and constructed in various shapes and sizes specific to the site and feedstock conditions. These sealed vessels contain complex microbial communities that break down the waste and produce biogas and digestate (i.e., the solid and liquid material end-products of the process). The biogas can be used as a source of energy. The remaining digestate can be used as a fertiliser, or it can be post-treated according to its intended use, e.g. by drying or composting to use it as a soil improvement agent.

Feedstock

Multiple organic materials can be combined in one digester, a practice called co-digestion. Co-digested materials include, amongst others, manure, food waste, energy crops, crop residues, and fats, oils, and greases (FOG) from restaurant grease traps.

Pre-treatment

Biomass is first separated from impurities as stones and glass. An agitator provides a good mixing between different biomass types to avoid strong changes in composition. The feed is a stirrable mixture and the dry matter content may be a maximum of 15-20% of the slurry. Co-substrates are often reduced in size by shredding before they are fed in order to make the contact surface of the biomass as large as possible.

For residual flows from the food industry, crop residues and manure, thermal and chemical pre-treatments are mainly applied. The most important effects of thermal pre-treatment are: reducing particle size, increasing solubility and improve the biodegradability. Additional advantages of thermal pre-treatment are: (1) higher loading of the digester is possible, (2) lower viscosity of the treated material which results in lower energy input for mixing the digester, (3) improved dewaterability of digestate and (4) sanitised product.

Process and technologies

Process

There are three basic anaerobic digestion processes, namely psychrophilic, mesophilic, and thermophilic, which take place over different temperature ranges. Psychrophilic digestion is a low temperature (<20°C) process. Mesophilic digestion takes place between 20 and 45°C, which can take a month or two to complete, and thermophilic digestion between 45 and 65°C, which is faster, but its micro-organisms are more sensitive. The majority of the agricultural biogas plants are operated at mesophilic temperatures. Thermophilic temperatures are applied mainly in large-scale centralised biogas plants with co-digestion. The process of anaerobic digestion takes place through four successive stages: hdyrolysis, fermentation, acetogenesis, and methanogenesis. In the hydrolysis step, the feedstock is broken down into soluble substrates (e.g., sugar and amino acids) by enzymes. Fermentation involves the conversion of sugar, amino acids, and fatty acids into ammonia, organic acids, hydrogen (H₂) and CO₂. In the acetogenesis step, volatile fatty acids are broken down into acetic acids, CO₂ and H₂. Finally, methanogenesis step converts acetate, formaldeyde, and H₂ to CH₄ and water.

Simplified scheme of pathways in anaerobic digestion (not own work)

Usually, the produced biogas must be dried and drained for condense water and biological or chemical cleaned for H₂S, NH₃ and trace elements. Further upgrading of the biogas to increase the CH₄ content could be realized by membrane separation of CO₂ and pressurising the biogas.

Product

Anaerobic digestion produces two valuable outputs, namely biogas and digestate. Biogas is composed of methane (CH₄), which is the primary component of natural gas, at a relatively high percentage (50 to 75%), carbon dioxide (CO₂), hydrogen sulfide (H₂S), water vapor, and trace amounts of other gases. The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems. Biogas can also be purified by removing the inert or low-value constituents (CO₂, water, H₂S, etc.) to generate renewable natural gas (RNG). This can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or processed further to generate alternative transportation fuel or other advanced biochemicals and bioproducts.

The digestate can be used in many beneficial applications provided that is is appropriately treated post processing. This could be in form of animal bedding, nutreint-rich fertilizer, organic-rich compost, or as soil amendment.

Technology providers

Technology comparison

Company name	Country	Technology category	Technology name	TRL	Capacity [kg/h]	Processable mass [kg]	Temperature [°C]	Feedstock: Food waste	Feedstock: Garden & park waste	Product: biogas	Product: Renewable natrual gas (RNG)
Company 1	[Country HQ location]	[Technology category (if different sub-categories are defined this has to be specified here, the available categories can be found on each technology page under the chapter Process and technologies)]	[Technology name (the "branded name" or the usual naming from company side)]	[4-9]	[numeric value]			●	●	●	●
Company 2	[Country HQ location]	[(if different sub-categories are defined this has to be specified here, the available categories can be found on each technology page under the chapter Process and technologies)]	[Technology name (the "branded name" or the usual naming from company side)]	[4-9]	[numeric value]			●	●	●	●

Biogas Plus

Anaerobic digestion provider

General information
Company:	Biogas Plus
Country:	Venray (NL)
Contact:
Webpage:	https://www.biogasplus.nl
Technology and process details
Technology name:	Compact Plus	Technology category:	Conversion (Biochemical processes and technologies)
TRL:	9	Capacity:	18.000 tons (input), 320.000 Nm3 green gas/year (output). kg·h-1
Atmosphere:		Catalyst:
Pressure:	bar	Reactor:	Complete mix digester
		Other:
Feedstock and product details
Feedstock:	Animal Manure	Product:	Green gas

CCS

CCS Enegie-advies together with Greenmac developed the Bio-UP technology which is able to upgrade the produced biogas into green gas. CCS offers the Bio-UP technology via lease contracts or turn-key. The Bio-UP is a proven concept which already operates at "melkveeproefbedrijf De Marke".

Planet Biogas

Anaerobic digestion provider

General information
Company:	PlanEt Biogas Group GmbH
Country:	Vreden (DE)
Contact:
Webpage:	https://www.planet-biogas.com
Technology and process details
Technology name:	PlanET	Technology category:	Conversion (Biochemical processes and technologies)
TRL:	9	Capacity:	kg·h-1
Atmosphere:		Catalyst:
Pressure:	bar	Reactor:	Complete mix digester (modular)
		Other:
Feedstock and product details
Feedstock:	Animal manure, biogenic waste materials	Product:	Green gas, heat & electricity

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