Top Above Header

Two Stage Anaerobic Digestion

Anaerobic digestion achieved in 2 stagesTwo stage anaerobic digestion can provide both biogas plant process stability and the high rates of biogas production which come with a high degree of mass conversion.

Efficient two-stage anaerobic digestion systems for the production of biogas (e.g. methane) come in two types. The distinction between the two types is that in one, water is added to make a slurry and the solids and liquid is mixed pumped, and digested through two separate reactor tanks connected in series. In the other the raw incoming material is placed in a reactor, sealed from the air and encouraged to ferment as solid, usually with added “innoculum” containing anaerobic organisms sprayed onto the solid pile in a vessel. Once the initial fermentation stage is complete, water is then added and a traditional wet-process methane producing second-stage takes place. This second type is known as a high solids phase and a low solids phase.

Whichever of these types is used, the sequence is the same. The four stages key of anaerobic digestion are hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Two key bacterial groups are at work during these stages, acetogens and methanogens.

This video can also be viewed on the YouTube website at 2 Stage Anaerobic Digestion.

During the first stage, hydrolysis, acidogenesis, acetogenesis occur. During the second stage, biogas is produced in a second reactor by a methanogen-rich anaerobic culture which has in-effect been cultured in the low solids medium. Transfer of effluent from the second reactor to the first reactor assists in maintaining a suitable pH in the high solids phase, in mixing te is an effect of seeding the first stage with methanogens. In two stage reactors where both are liquid phase based, methane maybe produced from both reactors and recovered.

 Overall Energy Recovery Can be Best from 2 Stage Biogas Plants

Researchers point out that the supremacy of two-stage on traditional one-stage anaerobic digestion (AD), in terms of overall energy recovery (ER) from biomass has frequently been proved. However, the process conditions ensuring this result, as well as the reasons for higher efficiency, are usually considered to be proprietary secrets and are unclear.

It is due to the different growth rates, preferences and requirements of the microorganism species involved in each step, that physical separation in two independently performing reactors is an option to which enable specific conditions for the microbiology at each stage to be set to optimize output. The first stage reactor is often called the hydrolytic reactor, of a two stage AD system.

Anaerobic digestion (AD) is the biological degradation of organic material in the absence of air. An anaerobic digester is a man-made system that harnesses this natural process to treat waste, produce biogas that can be converted to heat and electricity and anaerobic digestate, a soil improving material. Anaerobic Digestion (AD) is the preferred stabilisation process for the treatment of wastewater sludges and organic wastes. The process provides volume and mass reduction and delivers valuable renewable energy with biogas production.

A company called Arrowbio is one of the best known internationally, for using two stage anaerobic digestion within its process designs.

Anaerobic Digestion of Sludge

3 Responses to Two Stage Anaerobic Digestion

  1. Walter Hunt January 12, 2015 at 5:49 am #

    How popular are two stage digesters? I did not see many companies selling them.

  2. Erney October 6, 2017 at 9:56 pm #

    Why limit your discussion to just the wet/wet anaerobic digestion two stage AD Plant design options? There is also the possibility of two stages dry/wet. The GICON Process is a two-stage dry-wet anaerobic digestion process operated in batch mode. Take a look at ‘s digester design.

  3. biogasman October 10, 2017 at 10:13 pm #

    Two stage digesters cost more to build than the single stage plants which are by far the most common. A new technology option is to add a pre-treatment stage which can pay for itself because it removes ammonia and phosphorus, allowing high ammonia feedstocks to be used and for an additional product profit stream. See

Leave a Reply