Anaerobic digestion

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).

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.

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

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".

Biogas Plus

Digester technology provider

General information
Company:	Biogas Plus	Webpage:	https://www.biogasplus.nl
Location:	Venray (NL)	Business-Model:
TRL:	9	Patent:
Technology name:	Compact Plus	Technology category:
Technology and process details
Reactor:	Complete mix digester	Heating:
Atmosphere:		Pressure:	bar
Capacity:	18.000 tons (input), 320.000 Nm3 green gas/year (output).	Temperature:	°C
Catalyst:		Other:
Feedstock and product details
Feedstock:	Animal manure	Product:	Green gas
Pre-treatments:		Post-treatments:	Biogas upgrading (integrated)

Planet Biogas

Digester technology provider

General information
Company:	PlanET Biogas Group GmbH	Webpage:	https://www.planet-biogas.com
Location:	Vreden (DE)	Business-Model:
TRL:	9	Patent:
Technology name:	PlanET	Technology category:
Technology and process details
Reactor:	Complete mix digester (modular)	Heating:
Atmosphere:		Pressure:	bar
Capacity:	kg·h-1	Temperature:	°C
Catalyst:		Other:
Feedstock and product details
Feedstock:	Animal manure, biogenic waste materials	Product:	Green gas, heat & electricity
Pre-treatments:		Post-treatments:	Biogas upgrading (integrated)