I am working on two stage dry anaerobic digestion of FOGO but struggling with the pumping of high TS effluent from first stage to second stage. Is there any expert who could guide me with his experience in high solids anaerobic digestion?
Ayesha Rafique
For high solids slurries in our high rate AD+™ plants, we use rotary lobe volumetric pumps such as https://www.vogelsang.info/int/products/pumps/rotary-lobe-pumps/
How Traditional Two Stage Anaerobic Digestion Works
Image showing two stage anaerobic digestion - Anaerobic digestion achieved in 2 stages.
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 precursors 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 times and generation times 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 the conversion of solid carbon into the liquid phase, a liquefaction of the substrate occurs.
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.
Comparison of 1 and 2-stage anaerobic digestion - the biogas plant schematics compared.
A Comparison of one and two-stage biogas 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.
On 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 (2nd) 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.
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 that enables 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 stabilization 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.
Hi Ayesha, it looks like we have similar research interests (two stage anaerobic digestion). Would you like to connect?
