Counter-Flow Leach Bed (CFLB) Pre-digester

The use of leach beds for digestion is an area of active research. A leach bed is a container of biomass through which liquid flows in order to dissolve digestible materials. Many leach bed system use liquor that is recycled from the effluent of the leach bed itself, or from a second digester through which the leachate from the leach bed flows. The recycled liquor contains active microbes that start the process of digesting the biomass in the leach bed (pre-digestion), which then continues in the second digester to generate biogas.   

However, there are issues with the flow of liquid through the leach bed. The usual method of feeding liquor to the leach bed is to spray it over the top of the biomass, so it flows downwards through the biomass and can be removed from the base. The flow through the biomass can be uneven. Filters are used at the bottom, to support the biomass load. These can get clogged by biomass, as it digests to form much smaller particles.

An alternative approach feeds the liquor from the base of the digester and removes it from further up the container (counter-flow). Tests are being done on this approach in two ways:

  1. In a small pilot plant, made from plastic barrels in an insulated shed.
  2. In Birmingham City University, Bioenergy Research Unit, as a laboratory test programme.   

This research will allow the design and running of the system to be optimised.

Plastic barrels of small pilot system

Once the CFLB predigester system has been optimised, systems to process food and other biomass residues will be built on a full-scale. One of the first of such systems will be installed at CMC Vellore in India. This may be followed by others. CMC Vellore has an effective solid waste management centre.

One application of AD systems with CFLB predigesters will be for residues from food processing operations. Examples could include coffee berry pulp from both wet and dry coffee processing. Other examples could include residues from fruit drying and juicing.

Another application is to process food and other biomass wastes from institutions such as hospitals, colleges, hostels and even hotels.  

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