Anaerobic digestion

A natural biological process :

Anaerobic digestion is a natural biological process in which organic matter or waste is transformed into biogas. This phenomena exists in nature (gas in swamps, cows digestion gases, termites digestion gases, ...). It was discovered in 1776 by an Italian scientist, Volta, and was later explained by Lavoisier at the end of the 18th century. To provide waste and effluents treatment and produce renewable energy, this natural process is harnessed, boosted 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 constituants 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, WWTP using anaerobic digestion produce 5 to 10 times less sludge than an equivalent aerobic WWTP.

Implementations of Anaerobic Digestion are very diverse:

Implementation of the anaerobic digestion process can very basic: the most significant example is the program launched by Mao Zedong in the 1970's, which allowed 11 million rural China households 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 subsistance 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 burried in landfills undergoes the anaerobic digestion fermentation process and generates biogas. This biogas is captured and burned in co-generation engines to produce electricity and heat

Infinitely Stirred AD Tanks (ISAT) :

• volume and olfactive nuisance reduction of the sludge produced by aerobic waste water treatment plants
• energy production from the organic fraction of urban household waste
• production of energy from agricultural by-products and waste; outputs are biogas and sludge, which has useful land fertilizing properties and reduced olfactive nuisance, making it fit for spreading on agricultural land within the constraints of the local environment agencies regulations
• energy production from selected crops specifically grown for anaerobic digestion processing

All the above application are implemented in an Infinitely Stirred Anaerobic Digestion 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" name) using mixers or propellers; products remain in the AD tank for 25 to 50 days. In the best cases, approximately 75% of the inbound organic matter is transformed into biogas.

Anaerobic Digestion for Waste Water Treatment :

• removal of the organic matter or pollution contained in liquid effluents and by-products, generated by either the food and drinks industry or by urban communities.

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 maximised 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 to treat 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). The treated effluent can be directly compatible with the local regulatory constraints imposed for disposal in the environment (river, aquipherous, …). In some instances, it is necessary to implement a polishing stage to remove nitrogen (nitrates) and phosphorous (phosphates) in order to comply with the standards for disposal in the environment. The organic matter removal efficiency of anaerobic digestion plants designed for waste water treatment ranges between 70% and up to 98% in the better designed reactors.

Fostering and controlling the anaerobic digestion process :

During the anaerobic digestion process, a number of different bacteria and biological micro-organisms are activated. 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, ...). Anaerobic processes differ depending mainly on :

1. the nature of the effluents and by-products to be treated
2. their organic matter content
3. the anaerobic digestion temperature retained (see next section)
4. the mechanism used to facilitate the contact between the bacteria and the organic matter
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 before feeding to the AD tank can be implemented, such as degrilling, grease separation by aero-flottation, ...
At the outlet of the AD tank, the effluent can undergo further treatment steps, where applicable, using aerobic or physico-chemical technologies; the objective is to remove the mineral content (nitrogen, phosphorous, potassium, ...) of the effluent , as minerals are not (or almost) affected by anaerobic digestion, in order to meet the local effluent discharge regulatory constraints.

Advantages of anaerobic digestion for waste water treatment

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)