Top Above Header

Dry Anaerobic Digestion

wet and dry AD options

The patented Linde single-stage dry anaerobic digestion process is an example of the dry digestion process, as we explain here.

October 2011 – Linde may no longer be trading or may have been bought out. If you know about Linde, use our contact form to update us please.

Linde states that it can be operated in thermophilic or mesophilic mode and has been developed for the anaerobic treatment of solid substrates with dry matter (TS) contents between 15 and 45%.

The digestion principle is based on a plug flow reactor principle, which has a horizontally orientation.

The most outstanding feature of this dry digester is that it is constructed as a horizontally arranged unit within a concrete tube which looks similar in scale and geometry to common aerobic in-vessel composting designs. This is reported to give it extreme “sturdiness”, which might also be described as “robustness” and “high reliability”, we assume.

a dry anaerobic digestor linde processdry-anaerobic-digestor-linde-process

Within the essentially plug flow reactor vessel, several electric motor driven agitators of are aligned externally with their drive shafts perpendicular to the side walls, and arranged in series along the length of the vessel.

This method of mixing/agitation provides for the net horizontal plug-flow movement, gradually toward the discharge end from the fed hopper/tank, and the use of these mixers is reported to prevent the formation of floating scum and settlement of material, with a high degree of reliability.

A sturdy conveyor frame is fixed to the digester bottom which will reliably transfer the sediments to the fermenter discharge.

All components such as feeding unit, agitator drive units, digestion residue discharge units and the gas system are easily accessible for maintenance and normally installed in individual housings.

The organic material treated is fed into the digester by a compact feeding unit. If required, the TS content in the input is adjusted as desired at the same time.

At the digester end the digestion residue is discharged from the reactor through a low-wear (and presumably low maintenance discharge system.

In addition to the treatment of bio-wastes, green wastes and organic industrial wastes this process can very well handle residual wastes and mixed refuse (otherwise nowadays known as BMW – Biological Municipal Waste) with high TS contents.

Process Features and Advantages

The following features and advantages over other systems are reported by Linde:-

  • high biogas production rate through large gas discharge area, low digester filling level and several agitators (low-speed units)
    low space requirement
  • use of small compact digesters because material needs not be diluted
  • low heat requirements and low wear through minimised material flow
  • no or very low process water consumption depending on material characteristics
  • low energy demand in material handling, conveying and fermentation through the use of low-speed units only and staggered operating times
  • flexible adjustment to fluctuating throughput rates through a variable filling level in the digester
  • high VSS degradation through quasi-continuous plug flow.
The information on this page was provided courtesy of Linde. July 2011 – 2014 – Linde website no longer available.

Recent Developments in the Dry Anaerobic Digestion Process

Dry anaerobic digestion until recently in reality actually meant “high solids” anaerobic digestion. The expression “dry” simply indicated that the water content was lower than for the traditional AD plants used for farm slurries/ manure, which have historically been “very low solids” systems. All these systems relied on water and pumping for conveyance. An essential requirement for good biogas yield within the shortest time is that the fermenting substrate be mixed to ensure that the methane producing (methanogenic) bacteria are exposed to the food they need throughout the tank. Those dry AD systems that don’t include mixing need longer residence periods than normal wet AD systems.

Some authors on this subject refer to dry AD as being “Solid-state anaerobic digestion (SS-AD)”.

The wet AD systems are continuous process flow systems. Dry AD systems may be either continuous or batch processes and may be operated in the temperature ranges of mesophilic or thermophilic AD systems.

However, during the period 2000 to 2010, truly “dry” (two-phase) anaerobic digestion systems have also been developed which resemble in-vessel composting plants, where the organic waste is transported in the buckets of front-end loading vehicles.  In such “dry” systems the use of water as the means of conveyance through the process is abandoned. Instead digestate is usually sprayed onto the substrate of organic matter and mechanical mixing using rotating blades may or may not be provided.

Water based systems can be thought of as within the following dry solids ranges:

  • Normal “wet” anaerobic digestion > 5% to 20% Total Solids (TS) content
    “Dry” anaerobic digestion systems (for example the Dranco system) 30% to 45% TS with the upper limit being the ability to find pumps capable of pumping these high and viscous mixtures. for example, at the start of the 2000s Dranco was said to be using concrete pumps which were designed for construction sites, because these were the most rugged pumps available in the early 2000s.
  • Solid materials handling in the recent “dry-handled” type anaerobic digestion systems, would have TSs substantially above 45% TS.
The solids contents referred to here are higher than much of the literature states elsewhere, due to recent improvements in pumping and mixing technology. Always check actual solids capabilities with the manufacturer of proposed equipment for any design work. 

SS-AD  is good for the organic fractions of municipal solid waste and subtsrates such as crop residues and bio-energy crops. Proponents of SS-AD suggest that smaller reactor capacity requirements are possible, and less energy is used for heating. In addition, and no there is no energy consumed for stirring. The digestate volume from SS-AD is lower which makes it much easier to dispose of when land application for fertilizer sales is ruled out by the presence of heavy metals and other unacceptable contaminants, than the effluent of liquid AD. But, as we have already hinted, SS-AD systems are also burdened by other disadvantages such as larger amounts of required liquid suitable for re-injection for innoculation of the substrate, and a much longer retention time.

The best known brands in “dry” anaerobic digestion are Dranco, Kompogas and Linde. However, Linde were not thought to be still trading in this product when this article was updated (November 2014).
SMARTFERM is a company which is a newcomer with a dry anaerobic digestion system for organic waste processing. It uses the term “dry fermentation” to describe its process, which involves the production of biogas from stacked solid organic waste in a non-continuous batch process.
The Viessmann Group of biogas companies offer dry (and wet) anaerobic digestion systems ranging from 50 kWel to 20 MWgas.

Anaerobic Digestion of Manure

2 Responses to Dry Anaerobic Digestion

  1. Calixto Villanueva December 29, 2014 at 7:03 pm #

    dry is best. Muchos Gracias for your blog. Really looking forward to read more. Fantastic.

  2. Shashi Kumar N K September 4, 2015 at 11:39 am #

    Dear Sir,
    We are looking for Dry Fermenter Technology for producing Biogas and Biogas upgradation system. The capacity will be around 200 Tonns of agro wastes per day which includes several Tonns of vegetable wastes, nonedible oil cakes and cow and animal refuse.
    Please let us know the available proven technology in this and what will be the area required for installing the complete system. We are also looking for a complete Turn-key supply of the systems .
    Looking forward for a reply at the earliest.
    Regards
    Shashi Kumar

Leave a Reply