The food- and raw materials industry (primary sector of the economy) often has a substantial constant volume of an organic residue classified as a secondary product. This organic residue has a high potential as raw material for the production of energy.
Organic residues are since the old days transported to the agricultural sector as cattle feed. In Europe, diseases as BSE and others within this sector, are showing the vulnerability of transportation of this organic residue. Thence the production processes are directly influenced if something occurs in that direction.
The industry tried to solve this problem to introduce conventional digesting technology to produce energy from the organic residues. By introduction of this technology the outcome is that the digestate after digestion is transferred to the agricultural fields as fertiliser without the necessity of any form of digestate treatment.
However, transferring the digestate to the filed is an expensive operation due to the high transportation- and handling costs. Beside this exploitation issue the limitations in governmental documents (licenses, permits) are showing stricter limits and demands and finally the digestate should compete with primary manure discharge from farmers.
Environmental engineering's agency Colsen b.v. developed for this situation an alternative procedure with no attachments or interference’s with the agricultural sector (cattle feed/fodder). Colsen b.v. treats the organic residues in a mesophilic- or thermophilic digester installation and treats the digestate in the by Colsen b.v. developed digestate treatment installation.
This principle results in the treatment of the organic residues into biogas, green energy, compost, fertiliser and water consisting of a quality to discharge to the surrounding surface waters.
Fermentation represents a natural and anaerobic dissimilation process during which organic biomass is transformed. Certain bacterial strains are capable of converting organic material mainly into methane (CH4) and carbon dioxide (CO2). This anaerobic digestion process takes place in two different ranges on the temperature scale, i.e. from 30- 40°C (mesophilic range) and from 50-55°C (thermophilic range).
Thermophilic digestion is often applied for treatment of industrial organic waste products or manure. It is also possible to combine both waste streams, which is than called codigestion.
Via a unique mixing- and heating device applied by Colsen the digester can be operated efficiently under very stable conditions. The produced biogas is desulphurized (BIDOX) and can than be used for the production of heat and/or power in a CHP application.
The anaerobic digestion process consists of four subsequent biological steps: hydrolysis, acidification, acetogenesis and methanogenesis. During these steps, about 90% of the degradable biomass is converted into biogas (conversion yield depends on the origin of the organic matter). Via a tailor-made pre-treatment, a unique mixing system and the applied thermophilic conditions, equal biogas productions are obtained in hydraulic retention time of just 20 days. This compared to yields in mesophilic conditions and 30-50 days of retention time. This advantage pays off in significant lower investment costs for a thermophilic digestion system. The biogas yield depends greatly on the composition of the influent (each type of biomass is characterized by its own parameters).
The produced biogas contains about 55-65% CH4 and 30-35% CO2. Besides, the biogas consists of small amounts of NH3 and H2S and traces of N2 and H2.
The digestate is separated into a liquid and a solid fraction. The dewatered solid fraction equals about 10 % (w/w) of the influent volume. The liquid fraction can be further processed or purified based on customer needs. In general, post treatment aims at the reduction of nitrogen, phosphorous and COD from the digestate.
An additional advantage of thermophilic digestion is the fact that most pathogenic bacteria, viruses and germs are pasteurized.