Two 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. In may ways it provides a superior process design for biogas production, and yet it is seldom used in practice.
Why is that?
Unfortunately, due to the need for two reactor vessels, and increased pumping and control complexity, the initial investment cost is higher than for single stage anaerobic digesters, so AD facility designers seldom recommend 2-stage digester designs.
The Principles of “Traditional” Two Stage Anaerobic Digestion Design
The biochemistry of the anaerobic digestion reaction takes place in a four phase degradation process. It consists of hydrolysis, acidogenesis, acetogenesis and methanogenesis.
Biochemists have for a long while known that each stage can be identified through the presence of separate characteristic bacteria/ micro-organisms in each phase. Also, the gas composition produced differs between the hydrolysis stage and subsequent stages, and the initial stages take place more rapidly than the subsequent stage.
This means that by splitting up the biogas production process into two stages (tanks) in which the first is used for “acidification” (loosely speaking – the first 2 steps), and the second stage (tank) for acetogenesis and methanogensis (methane production) there are advantages, as follows.
- At a biochemical level the environmental conditions in each digester stage can each be optimised to suit the prevalent bacteria.
- The first stage reactor tank can be smaller, tending to reduce total reactor volume needed, with associated reductions in mixing/ stirring energy consumption
- The second reactor produces a better quality biogas (often referred to as biomethane – although not a biomethane quality high enough for natural gas grid injection).
How Traditional Two Stage Anaerobic Digestion Works
The substrate (feedstock/ water mixture) is fed into the first reactor where acidification takes place leading to hydrogen production and accumulation of acids and alcohol, which are the precursor for the acetogenic microorganisms in the second stage.
The (biochemical) hydrolysis phase is either carried out batch wise or continuously and is connected to the second methanogenic reactor. The ratio of both reactor volumes is defined by the different retention time and generation time of the bacteria. This requires smaller volumes in the hydrolysis and bigger in the methane reactor, because the retention time in hydrolysis is significantly lower.
For fibrous and high solid content enriched input substrates, mixing problems could occur in the hydrolysis phase and form a floating surface layer (scum or crust). This can result in incomplete hydrolysis of the substrate.
In order to prevent this condition, proper stirring technology has to be chosen in the design phase. Due to conversion of solid carbon into the liquid phase a liquefaction of the substrate occurs. via blogs.qub.ac.uk-atbest
The biogas produced in the first stage contains a high percentage of carbon dioxide, and some hydrogen. The second stage tank provides a higher quality biogas than a single stage AD plant. It is also reported that plant stability is better for two stage anaerobic digestion plants.
In this discussion it is important to be aware of the fact that the hydrolysis referred to here is “biochemical hydrolysis” and NOT the “Thermal Hydrolysis“ (high temperature and pressure hydrolysis) stage applied often to sewage sludges, as the first stage (e. g. Cambi Hydrolysis Process). “High temperature and pressure hydrolysis” is normally only used for the breakdown of biomass (such as sewage sludges) which would otherwise mostly pass right-through any biogas plant run at normally applied (mesophilic) temperatures
Efficient two-stage anaerobic digestion systems for the production of biogas (e.g. methane) come in two types.
Two Types of Two Stage Anaerobic Digestion
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.
In the other the raw incoming material is placed in a reactor, sealed from the air and encouraged to ferment as a solid, usually with added “innoculum” containing anaerobic organisms sprayed onto the solid pile in a vessel.
In both types of anaerobic digestion process the substrate is moved through two separate reactor tanks connected in series.
Once the initial fermentation “dry” 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 two stage anaerobic digester.
Whichever of these types is used, the sequence is the same.
As stated earlier, the four “biochemical stage sequence”, which is key to all anaerobic digestion processes is hydrolysis, followed by acidogenesis, acetogenesis, and methanogenesis.
Two key bacterial groups are at work during these stages, acetogens and methanogens.
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.
All the stages (process steps) are shown in our video below:
Transfer of effluent from the second reactor to the first reactor assists in maintaining a suitable pH in the high solids phase, in mixing there is an effect of seeding the first stage with methanogens.
In two stage reactors, where both are liquid phase based, methane may be 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.
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 optimise 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.