Anaerobic Digestion

A natural biological process

Anaerobic digestion is a natural biological process in which organic matter is transformed into biogas. This phenomenon exists in nature (gas in swamps, cows digestion gases, etc) and occurs in landfill sites. It was discovered in 1776 by an Italian scientist, Volta, and was later explained by Lavoisier at the end of the 18th century. In order to treat the waste, and produce renewable energy, this natural process is harnessed,  fostered and controlled in closed reactors (tanks) containing no air (oxygen).

Specific bacteria breed inside the reactor and feed on the organic matter contained in the effluents. Carbon (C) and hydrogen (H), the two main constituents of organic matter, are transformed into a biological gas which contains 60 to 70% methane (CH4). This biogas can be used to substitute fossil fuels. Thanks to the transformation of the carbon contained in the waste into biogas, waste treatment plants using anaerobic digestion produce 5 to 10 times less sludge than an equivalent aerobic facility.

Different types of Anaerobic Digestion Processes

Implementation of an anaerobic digestion process can very basic, an example of which is the program launched by Mao Zedong in the 1970's, which allowed 11 million rural households in China to have biogas as their only source of energy. The biogas is produced by "mini-AD tanks" which process all animal, agricultural and human waste of the small, family-run subsistence farms.

In an industrial context, the AD process can be implemented in different ways, depending on the objectives.

Captation of biogas produced by landfills

• The organic fraction of waste buried in landfills undergoes an anaerobic digestion process since waste finds itself in absence of air as it piles up and thus generates biogas. This biogas is captured and burned or used in co-generation engines to produce electricity and heat. This also significantly reduces greenhouse gas (GHG) emissions.

Infinitely Stirred / Completely mixed AD Systems

• Volume and olfactive nuisance reduction of the sludge produced by aerobic waste water treatment plants
• Production of energy from the organic fraction of municipal solid waste (MSW)
• Treatment of sludge and organic waste from the industrial, commercial and institutional sector (ICI) in order to reduce the volume of sludge and waste and to produce renewable energy (biogas) used on-site.
• Production of energy from agricultural by-products and waste; outputs are biogas and sludge, which has good fertilizing properties and reduced olfactive nuisance, making it fit for spreading on agricultural land within the constraints of the local environment agencies regulations
• Production of energy from selected crops (called energy crops) specifically grown for anaerobic digestion processing

All of the above applications are implemented in an infinitely stirred / completely mixed tank in which products to be digested are fed in semi-liquid or sludge form. The content of the tank is stirred continuously (hence the "infinitely stirred" or completely mixed name) using different types of mixing means; products remain in the AD tank for 25 to 50 days. In general, 50 to 60% of the inbound organic matter is transformed into biogas, 75% in the best cases.

Anaerobic Digestion for Waste Water Treatment

• Removal of the organic matter or pollution contained in liquid effluents generated by either agri-food businesses or other industries and production of renewable energy (biogas), which can replace the fossil fuels they consume.

Anaerobic digestion plants for waste water treatment are designed to process only liquid effluents; the contact between the organic matter, which is dissolved in the liquid, and the bacteria is maximized using pumps (recirculation). The bacteria are either fixed on a supporting material (which can be static or mobile), or grouped in colonies which form small granules (this is known as "granular sludge"). The residence time of the effluent inside the reactor ranges from a few hours to a few days. At the outlet of the digester, the treated liquid is freed of most of the organic matter (very different from the semi-solid sludge at the outlet of infinitely stirred digesters). Anaerobic digestion plants designed for waste water treatment generally transform 90% or more of the organic matter into biogas, as high as 98% in fully optimized systems.

Fostering and controlling the anaerobic digestion process

During the anaerobic digestion process, a number of different bacteria and biological micro-organisms are activated (see http://www.ecocorp.com/other_pages/anaerobicdigestion.htm). Critical environmental parameters such as pH and temperature are monitored and regulated, in order to maximise the bacteria breeding rate, the digestion of organic matter and the production of biogas.

The anaerobic digestion process is selected and the tank is sized depending on the organic waste to be treated and the target performance of the system (waste removal rate, retention time in the tank, etc.) Anaerobic processes differ depending mainly on: (1) the nature of the effluents and residues to be treated, (2) their organic content, (3) the the temperature of the anaerobic digestion used (see next section), (4) the mechanism used to facilitate the contact between the bacteria and the organic matter and (5) the consumption (or absence of consumption) of chemical reactants to control the process.

In order to facilitate the digestion process by bacteria, pre-treatments can be implemented before feeding to the AD tank, such as degrilling, grease separation by aero-flottation, etc.
At the outlet of the AD tank, the effluent can undergo further treatments, where applicable, using aerobic or physico-chemical technologies; in order to treat among other things the mineral content of the effluent (nitrogen, phosphorous, etc. ) and meet the local effluent discharge regulatory constraints.

The treatment of organically loaded effluents using anaerobic digestion offers the following advantages, in comparison with aerobic digestion processes :

• Very low organic sludge production: 5 to 10 times less in terms of dry matter content
• Lower energy (usually electricity) consumption: : 5 to 10 times less energy required by the treatment system
• Reduction of pathogenic agents
• Odors removal : almost complete elimination of odors
• Profits and/or savings related to the renewable energy production, in the form of biogas
• Reduction of the Global Warming gases, linked to the use of the biogas : "short cycle" CO2 is emitted by the combustion of biogas (meaning the CO2 released was captured by the biological cycles a few months before, and thus does not increase the inventory of CO2 in the biosphere), instead of "long cycle CO2" emitted by the combustion of fossil fuels.
• Excellent flexibility to seasonality in effluent production (example : wine industry where AD is only activated during the crop season)