Biogas from Dry Digestion

Dry Digestion

A dry digestion plant consists of several bioreactor units where the organic waste materials or other biomass materials are digested in anaerobic conditions to produce biogas. The modular construction ensures process stability and therefore a stable supply of gas to the combined heat & power (CHP) unit. After processing, the digestate is turned into compost or used directly as a valuable, nutrient rich bio-fertiliser for agriculture & horticulture.

The technology of “dry digestion” is a single-stage batch anaerobic process where each bioreactor unit is loaded with a portion (batch) of fresh feedstock which is allowed to digest and then removed from the bioreactor. New feedstock is loaded and the process repeated.

Some Benefits of Dry Digestion:

• Low labour and operational costs
  
• Few moving parts in the biogas plant, thus reducing maintenance costs
  
• Modern, computer-controlled system allowing remote control and process monitoring/adjustment
  
• Low process energy consumption
  
• Good quality gas yields with low H2S, making desulphurisation generally unnecessary
  
• Possibility of modular expansion
  
• Use of front-end loaders to fill and empty the digesters, and therefore potential use of existing equipment
  
• Simple process that will tolerate a large variety of non-organic contaminants (sand, wood plastic etc.) in the substrate throughout the digestion process, these contaminants can be screened out after digestion
  
• Low storage cost of the digested substrates compared to liquid digestate
  
• Cheaper digestate transport costs

Plant Operation

The bioreactors, in which the process takes place, are each fully sealed reinforced concrete containers, they are sized to promote efficient processing of the feedstock and also to provide easy access for front-end loaders during filling and emptying. New feedstock is inoculated (mixed) with a proportion of processed substrate before being fed into the bioreactor and continued inoculation with bacterial biomass occurs during digestion, through controlled recirculation of percolation liquid, which is sprayed over the substrate from nozzles distributed across the ceiling of the bioreactor. Dry digestion needs no stirring or mixing of the substrate during digestion, the rate of the process is regulated by the application volume and temperature of the circulated percolation liquid which is stored and conditioned in a separate tank. The temperature of the percolate is carefully controlled using process heat from the CHP unit. During digestion biogas is extracted from the bioreactors, dried and stored for use in the CHP unit/s to produce electrical and thermal energy.

Apart from the mechanical filling and emptying of the containers, the entire operation is computer controlled and automated. Key process parameters can be monitored and easily adjusted either at the plant or remotely in “real time”. Retention time in each bioreactor is dependent on the feedstock but is typically 25 days. As each bioreactor operates independently, the combined process is very stable and a digestion process failure is practically impossible.

Process Selection

Dry digestion technology can be selected where available feedstocks contain a higher percentage of organic dry matter than would be typical for use in a wet digestion process. One of the key advantages of dry digestion is that most biomass materials can be used with minimal or no pre-treatment. Possible feedstocks include residential and commercial food waste; solid municipal biowaste; green cuttings
and grass from landscape maintenance; energy crops (fresh or ensiled) and solid animal manure, with high straw content.

BAL® Dry Digestion Plant - Seven Bioreactors