Technology Outline

1) Basics of anaerobic digestion
2) Mesophilic and thermophilic anaerobic digestion
3) Products of anaerobic digestion
4) Components of anaerobic digestion systems
5) Types of anaerobic digestion systems
6) Applications of anaerobic digestion

1) Basics of anaerobic digestion

Anaerobic digestion is the process in which organic matters are decomposed by micro-organisms in the absence of air to produce biogas comprising mainly of carbon dioxide and methane. Anaerobic digestion involves biogas generation under a controlled environment in a specially designed facility. Under these conditions, the gas yield can reach its theoretical maximum and the digestion process can be shortened to be within days rather than years as in the the case of landfill gas formation. Besides, the residues can be used for the production of compost for agricultural applications.

Anaerobic digestion involves a combination of processes:

  • The first is hydrolysis, where complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups.
  • The second is acidogenesis where a further breakdown into simpler molecules occurs, producing ammonia, carbon dioxide and hydrogen sulfide as by-products.
  • The third is acetogenesis where the simple molecules from acidogenesis are further digested to produce carbon dioxide, hydrogen and mainly acetic acid, although higher-molecular-weight organic acids are also produced.
  • The fourth is methanogenesis where methane, carbon dioxide and water are produced.

2) Mesophilic and thermophilic anaerobic digestion

There are two conventional operational temperature levels - mesophilic (35 -40°C) and thermophilic (50 -60°C). The temperature difference affects the rate of decomposition, liquid and solid separation, pathogen control, thermal input, retention time of wastes, and the capital investment of a system.

Although the thermophilic process is more expensive to operate due to the higher temperature, it has following advantages:

  • faster rate of decomposition and liquid and solid separation;
  • sterilization of residues; and
  • higher methane yield

3) Products of anaerobic digestion

An anaerobic digestion plant yields the following.

  • Biogas: mainly made up of methane (50 -60%) and carbon dioxide (40 -50%), can be used for heat production or electricity production, or for both in the case of combined heat and power systems.
  • Liquid digestate: liquid-rich short-chain organic compounds that can be used as soil conditioner.
  • Fiber digestate: residues that can be used for the production of compost.

4) Components of anaerobic digestion systems

The design of an anaerobic digestion system varies depending on the substrate to be digested. Basically, an anaerobic digestion system is composed of the following components:

  • Pretreatment facility: preparation of the organic fraction of the substrate for anaerobic digestion. This may include receiving, sorting, separating and compressing of wastes.
  • Digester: It is the bio-reactor where organic matter is decomposed to produce biogas. It is an oxygen-free container that provides an anaerobic environment for the activities of methanogenic bacteria. Besides, temperature in the digester is maintained to optimize biogas production.
  • Biogas storage: Biogas generated can be stored in the digester or in a gas storage unit. The biogas is then piped to the power generation facility.

5) Types of anaerobic digestion systems

Anaerobic digestion systems can be categorized into the continuous process, batch process and semi-continuous process according to the way of flow of substrate in the digester.

Continuous process
In a continuous process, the substrate is added to and removed from the digester continuously. Since fresh substrate is added continuously, all reactions involved in biogas generation will occur at a fairly constant rate. This results in a fairly constant biogas production rate. Usually, two digesters are used in the continuous process and the substrates are digested in two stages. The advantage of this process is that the digesters can be used as storage devices.

Batch process
In the batch process the substrate is put in the digester and then the digester is sealed for the entire period without adding additional substrate until the decomposition process is near completion. Most of the digested substrate is then emptied and the digester is filled with new substrates, and then the digestion process starts again. In a batch process, the production of biogas is non-continuous. Gas production will peak at the middle of the process and will be low at the beginning and at the end of the process. Typically, in order to ensure a more steady supply of biogas, a number of batch digesters with substrates at different stages of anaerobic digestion are operated in parallel.

Semi-continuous process
In a semi-continuous process, the digester initially works under the batch process. A small fraction of the digester contents is then released. After the release process, fresh material is injected to make up the contents and the digestion process is allowed to run for a while until the next release, followed by subsequent injection again.

6) Applications of anaerobic digestion

Anaerobic digestion can be used to treat municipal solid wastes, sewage sludge, organic wastes derived from agricultural, industrial and commercial sources. In Hong Kong, anaerobic digestion is principally applied in sewage treatment process in the sewage treatment plants. Energy is only a by-product of the treatment process. Biogas generated is usually used for electricity generation to meet part of the on-site electricity requirements, and also for use in boilers or engine-driven blowers.

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