Database of Waste Management Technologies Life

Anaerobic Digestion (AD) technologies

 

Generic information

The process of Anaerobic Digestion (AD) employs specialised bacteria to break down organic waste, converting it into biogas, a mixture of carbon dioxide and methane, and a partially stabilised wet organic mixture (digestate) in the absence of oxygen. Digestion and fermentation processes have been observed and practised ever since prehistoric times, but their study and development on a scientific basis only could start after the discovery of micro-organisms and of enzymes secreted by bacteria on the one hand, the application of chemical engineering concepts in fermentation on the other hand.

In most cases complex populations develop that are capable of conducting consecutive processes capable to break down organic waste. The latter follow a sequence of hydrolysis, acidogenesis (formation of fatty acids), acetagenesis and eventually methanogenesis in a balanced, steady, and controllable fashion. These four consecutive steps normally proceed side by side, but the first and last step are sometimes singled out, since they may require specialised conditions, controls, and auxiliaries.

Figure 1

Figure 1: Steps of the anaerobic fermentation process

AD is ether a ‘wet’ process used for materials with moisture contents more than 85% or a ‘dry’ process used for materials with moisture contents less than 80%. Anaerobic processes require less energy input than aerobic composting and create much lower amounts of biologically produced heat. Additional heat may be required to maintain optimal temperatures but the biogas produced contains more energy than is required, therefore the process is a net producer of energy.

As the process progresses biodegradable material is converted into a combustible gas known as ‘biogas’ primarily consisting of a mixture of methane and carbon dioxide. Biogas can be burned for heat and/or electricity production.

The material remaining consists of a wet solid or liquid suspension of non-biodegradable materials; recalcitrant organics; microbes (biomass) and microbial remains; and decomposition by-products. This partially stabilised wet mixture is known as ‘digestate’. This wet mixture can be dewatered into its solid and liquid fractions. Sometimes these 2 fractions may both be referred to as ‘digestate’, but for clarity they will be referred to as digestate and liquor further on.

Anaerobic digestion (AD), also known as biogas technologies, are designed and engineered to control and optimise the biological digestion of biodegradable materials to produce methane gas for energy production.

The technologies are, by their nature, enclosed, using specifically designed vertical and/or horizontal vessels, interconnecting pipe-work, mixers, macerators and pumps.

AD processes last around two to three weeks depending on the ease and degree to which

materials are converted into biogas and the technology used. For example, for waste containing a larger amount of woody (high lignin content) material, longer residence times will be required to achieve the desired biogas production.

Figure 2

Figure 2: Principles of the anaerobic digestion plant

There are two main classifications of AD techniques: ‘wet’ and ‘dry’ (see table below). In essence, ‘wet’ AD systems process more liquid materials (c.85% moisture), whereas ‘dry’ AD processes are used to treat drier materials (c.80% moisture) ranging from thick slurry to a wet solid. Waste feedstock is mixed and macerated with a large proportion of process effluent and/or fresh water to prepare the waste; giving it the moisture and flow properties required.

Figure 3