Clean Tech 101

Anaerobic Digestion: Theory & Application

Anaerobic digestion (AD) is the process by which organic materials in an enclosed vessel are decomposed by micro-organisms. Biogas is produced during anaerobic digestion and it comprises mainly of carbon dioxide and methane. AD systems are most commonly known as biogas systems.

Based on the system design, it is possible to combust biogas to run a generator producing heat and electricity. This can be burned as a fuel in a furnace or boiler, cleaned, and used as a replacement for natural gas.

The products of anaerobic digestion are:

  • Biogas
  • Digestate
  • Fibre
  • Separated liquid

What is Anerobic Digestion - Run Time - 2:20mins

Theory of Anaerobic Digestion

The AD process is constructed in an airtight container called a digester. Any material that needs to be processed can be shredded to increase the surface area, which is then made available to the microbes in the digester to improve the digestion speed. Below is a run down of the AD process:

  • Hydrolysis takes place through complex organic molecules which are disintegrated into amino acids, simple sugars and fatty acids by adding hydroxyl groups.
  • Acidogenic bacteria further break it down into volatile fatty acids producing carbon dioxide, ammonia and hydrogen sulfide as byproducts. This is process is called acidogenesis.
  • Then, in acetogenesis, the simple molecules obtained from acidogeneis are further digested by bacteria known as acetogens to produce hydrogen, carbon dioxide and acetic acid.
  • In methanogenesis, carbon dioxide and water are produced by bacteria known as methanogens. The pH level must be between 5.5 and 8.5 with the temperature between 30 and 60°C, so that digestion rates are maintained.
Image Credits: surreycc.gov.uk

Types of Anaerobic Digestion Systems

There are two general AD system configurations, which are:

  • Completely Mixed – This includes a large tank whereby, fresh materials are mixed with semi-digested material. These systems are suited for manure or other agri-food inputs with low dry matter content. By re-circulating the liquid effluent, material with high dry matter content will work in completely mixed systems.
  • Plug Flow – This system includes long channels through which manure, and other inputs, move as a plug. Systems such as this are suited for thicker materials.

Temperature Ranges

The major temperature ranges for AD systems are:

Thermophylic (50–60°C) – Operates at high temperatures and the micro-organisms quickly decompose organic matter and produce large biogas volumes. The rapid breakdown implies that digester volume is smaller than that in other systems. More insulation is needed for maintaining the right temperature range and higher energy consumption may occur while heating the system.

Mesophylic (35–40°C) –These systems require a longer treatment time to ensure that lower temperature microorganisms decompose organic matter. Normally these systems are stronger while temperature changes are considered. Mid-sized and small agri-food systems will operate typically in this temperature range. Certain AD systems are designed specifically for the concentration of the solid content to bring down the average overall retention time required in a mesophylic system.

Psychrophylic (15–25°C) – There are AD systems in Manitoba and Quebec that operate at this temperature range. These are easy to manage and stable; however, longer retention times are needed for achieving equivalent pathogen removal and gas production.

Scale of AD Systems

Three common options for the scale of AD systems are:

Farm-Based Systems

Farm-based systems are designed specifically for the manure of one farm, for a number of nearby smaller farms or for using energy crops from local fields. Lower cost components may be used and a lower control level or complexity is involved.

Food Processing Systems

AD systems that are built at a food processing site may be similar in features and design to a farm-based system or may be designed for the removal of organic matter from wastewater. Food processing systems are scaled so as to meet the heating requirements of the facility or manage byproducts obtained on- site or from a number of food processing facilities.

Centralized Systems

Material from a large number of food processing plants and farms are brought to a centralized facility that operates, based on a high biosecurity hauling process.

Materials like source-separated organics are mostly added for boosting gas production. The digestate is transferred immediately to remote fields, stored, to enable the convenient handling of land application. In several instances, heat from a centralized AD system is used close to a commercial facility or for supplying electricity to homes.

Challenges of Anaerobic Digestion of Manure

The operation and control of AD systems may be quite complicated. Strategic management techniques include the following:

  • Mixing primarily fresh organic material which is less than a week old
  • Adding material which has already been cooled in storage thus increasing the heating requirements
  • Optimizing production so it is consistent, enabling sufficient biogas to be generated
  • Installing and managing an interconnected group of systems to handle tank heating, hydrogen sulphide reduction, methane transfer, material flow, electrical production, heat production, surplus heat management and interconnection with the electrical grid.

Suitable Input Materials

A number of organic materials can be digested in AD systems, which include:

Manure

This is animal feed that was not digested fully. It contains considerable amount of energy that can be harvested in an anaerobic digester. The rules that need to be considered include the following:

  • Digestion of cattle and dairy manure is commonly used
  • Digestion of swine or poultry manure may provide added challenges especially due to their high nitrogen levels and for optimizing the blend other materials may be added.
  • Other inorganic materials or sand will settle out in the digester and should be considered in the design. A number of digesters will need shutdown and removal of built-up materials after being used for 10 years.
  • AD systems are not effective with manure that is diluted.
  • Solid manure can be used but non-digestible material and floating material will cause problems in system operation.

Food Byproducts

Most food byproducts breakdown rapidly in the digester. While using food byproducts the following rules must be followed:

  • Optimize the carbon nitrogen ratio especially for materials having high protein levels.
  • The recipe in the AD processor needs to be tailored based on the different food byproducts
  • When food byproducts come from a range of sources there may be minimal certainty about the quality or consistency of material when compared to material from one consistent source.

Energy Crops

Energy crops like haylage, corn silage and grasses require storage on-site and they all follow specific rules.

Adding solid energy crops to liquid systems require specially designed solid input devices that prevent liquids or gas from escaping.

Adding energy crops may result in a floating or swimming layer, which if dried out forms a crust that may take up digester capacity. In case insufficient mixing is done, the crust may pose a serious headache to digester operators.

On-Farm Mixing of Off-Farm Source Material

Mixing manure with off-farm source material may increase the production of biogas. Due to high carbon content of these materials, based on the quality and quantity of feedstock, biogas production can be tripled or doubled.

Conclusion

Anaerobic digestion of recycling biosolids and sewage sludges has been highly successful in the past and presently the growing interest in this field is primarily to ensure that the biodegradability of materials going to the landfill is significantly reduced.

One must remember that AD is a multi-option sophisticated process that provides options for improved recycling of nutrients and organics and also has the potential for the reclamation of a considerable proportion of the inherent energy of organic waste.

Sources and Further Reading

Kris Walker

Written by

Kris Walker

Kris has a BA(hons) in Media & Performance from the University of Salford. Aside from overseeing the editorial and video teams, Kris can be found in far flung corners of the world capturing the story behind the science on behalf of our clients. Outside of work, Kris is finally seeing a return on 25 years of hurt supporting Manchester City.

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