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The Anaerobic Digestion Process

The Anaerobic Digestion Process – An Introduction

The Anaerobic Digestion Process is the increasingly attractive waste and biofuel crop treatment process in which both pollution control and energy recovery can be achieved. The result is known as renewable energy or alternative energy. Its use is becoming popular now that energy prices are high and rising, because it is a proven operation if adequately regulated, with the ability to produce a valuable by-product, a flammable gas of mostly methane.

Image shows an anaerobic digestion plant process steps incuded all in one tank.
CC BY-NC by chesbayprogram

This gas can be utilized to generate power and is a uniquely sustainable process because although carbon in the form of carbon dioxide is eventually emitted during combustion the Anaerobic Digestion Process in effect avoids the production of any net increase into the atmosphere.

Many household (MSW) agricultural and industrial wastes are candidates well suited for the anaerobic digestion process because they contain high levels of easily biodegradable organic materials.

The Need for AD Process Efficiency

Problems such as low methane yield and process instability are often encountered in anaerobic digestion, preventing this technique from being widely applied. A wide variety of inhibitory substances are the primary cause of anaerobic digester upset or failure since they are present in substantial concentrations in wastes. Considerable research efforts have been made to identify the mechanism and the controlling factors of inhibition.

The inhibitors commonly present in anaerobic digesters include ammonia, sulfide, light metal ions, heavy metals, and organics. Due to the difference in anaerobic inocula, waste composition, and experimental methods and conditions, literature results on inhibition caused by specific toxicants vary widely. Co-digestion with other waste, adaptation of microorganisms to inhibitory substances, and incorporation of methods to remove or counteract toxicants before anaerobic digestion can significantly improve the waste treatment efficiency.

This review provides a detailed summary of the research conducted on the inhibition of anaerobic processes.

Anaerobic Digestion Process Steps

The best way to understand the AD process is to teach yourself the biochemistry. To help you with that we made the following video. Watch the video, and you should appreciate far better the complex set of biochemical reactions, facilitated by the micro-organisms which produce the biogas:

Another interesting quote about the anaerobic digestion process from says that:

Anaerobic digestion (AD) is a biological process in which biodegradable organic matters are decomposed by bacteria creating solid and gaseous byproducts [1]. The biogas byproduct consists of methane (CH4), carbon dioxide (CO2), and other trace amount of gases.

There are three main sources of organic waste all of which are suitable for anaerobic digestion. These three sources are food waste, industrial waste, and animal waste. Food waste like scraps left on the dinner plate, may come from households, restaurants, and breweries.

Pharmaceuticals and paper manufactures are some examples of industrial waste. Lastly animal waste is in abundant supply at commercial farms and sewage plants.

Increasingly finding renewable fuel sources has become an issue as fossil fuels are a limited resource. One solution to producing renewable fuel sources is methane production via the anaerobic digestion process.

The beginning of the process takes places inside of the digester, which is basically a tank sealed to prevent gaseous oxygen from entering. An anaerobic digester in concept is very similar to the human digestive system.

There are two kinds of bacteria that are required for anaerobic digesters to function properly.

The first type, fermenting bacteria, feed off organic materials and release organic acids. From these organic acids the second type of bacteria called methanogenic are produced.

The methanogenic bacteria also feed on the organic matter and create methane as a by product.

The anaerobic digestion process is the most widely used technique nowadays for the treatment of wastewater sludge.

The anaerobic digestion process

Finally, we quote, in the text below:

The following aspects are some benefits to anaerobic digestion. The organic make-up of the sludge is substantially lessened by transformation into gaseous outputs. The awful odor given off by the sludge is gotten rid of and the last stage of digested sludge has an individualistic smell of tar. Fats, oils and greases are also digested during this procedure turning them into water and carbon dioxide. Additionally, there is a substantial decline in the amount of disease causing bacteria and an apparent chemical transformation after the digestion process.

The liquid portion consists of elevated amounts of ammonia because of the digestion of organic nitrogen which is comprised of protein. This adjusts the broken down sludge liquid to possibly be adequate for usage in the area of agriculture. The gas produced from this biological process is a combination of carbon dioxide and methane and can be utilized for anaerobic digester heating or to produce energy.

Image is the anaerobic digestion process steps thumbnailThe anaerobic digestion process is a natural process which takes place in the absence of oxygen. Organic material is digested by bacteria in a closed reactor vessel and biogas is produced. This controlled digestion process is normally accelerated by increasing the reactor temperature into the mesophilic range (normally between 30-37°C), or into the thermophilic range (normally between 55-65°C). The decomposition of organic material consists of the following basic processes (Dohanyos et al., 2000).

Since the anaerobic digestion process is usually carried out in a single reactor vessel the processes described above run concurrently. Biogas consists of 45-85 % methane (CH4) and 15-45 % carbon dioxide (CO2), with the exact proportions depending on the production conditions and processing techniques. In addition, hydrogen sulphide (H2S), ammonia (NH3) and nitrogen gas (N2) may be present in small amounts.

Biogas is normally saturated with water vapour. …The energy value is expressed in joule (J) or watt hour (Wh). Pure methane has an energy value of 9.81 kWh/Nm3 (9810 Wh/Nm3). The energy value of biogas varies between 4.5 and 8.5 kWh/m3, depending on the relative amounts of methane, carbon dioxide and other gases present. Both methane and carbon dioxide are odourless.

If raw biogas smells, it is usually due to the presence of sulphur compounds. Biogas may ignite at concentrations of about 5-20 % in air, depending on the methane concentration. Methane is lighter than air, whereas carbon dioxide is heavier. This is considered to be advantageous from a safety point of view, since methane easily rises and is quickly diluted by the air.

Biological decomposition of organic substances in conditions without oxygen (anaerobic conditions) is a complex process whose efficiency depends on the operation and interaction of different types of bacteria. Each type of bacteria is responsible for implementing a single step and exactly the entire process. So an organic substrate may have the role of raw material required for a type of bacteria growth, while may be a contaminant for another type.


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