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pre-treatment methods

introduction pre-treatment

Especially hardly biodegradable substrates such as cellulose are challenging in anaerobic digestion. The rate of hydrolysis of the substrate determines the velocity of anaerobic digestion as the complex polymeric structures have to be made accessible for microorganisms. Different pre-treatment technologies have been developed in recent years with the aim to increase the availability of the lignocellulosic part of biomass. Some of these technologies do not increase the biogas yield but the degradation rate.

aims of pre-treatment methods

  • Enhancement of bioavailability
  • Increase of specific surface
  • Decrease of degree of polymerisation
  • Acceleration of hydrolysis
  •  De-crystallisation of cellulose
  • Degradation of hemicellulose and lignin

For achieving an increased biogas yield from pre-treated substrates, these aims need to be reached.
Requirements for successful pre-treatment
Production of easily bioaccessible monomeric sugars

  • Avoidance of degradation or loss of carbon
  • Avoidance of production of inhibitors
  • Achieving a positive energy footprint of the chosen method

Different types of pre-treatment

  • Enzymatic
  • Mechanic
  • Thermal
  • Chemical

Enzymatic pre-treatment

Cellulases are enzymes with the ability to cut long cellulose chains into smaller structures. The three most important cellulases are:

  • β-1,4-endoglucanases: attack regions with low crystalline structure and thus the end of the chains are exposed
  • β1,4exoglucanases or cellubiohydralases: are able to separate single cellubiose units from the cellulose chain
  • β-glucosidases or cellubiases: are able to separate the disaccharide cellubiose into its two monomeric glucose molecules

Due to complexity and heterogenity of plants, a suitable and diverse spectrum of enzymes is necessary to digest cellulose rich materials.
The efficiency of enzymatic pre-treatment is enhanced if lignin content and hemicellulose content are low. Since enzymes can be adsorbed by lignin, it is very inhibitory for enzyme activity. The pore volume of the substrate in comparison to enzyme size plays an important role but the pore volume is affected adversely if there is a high content of hemicellulose.

Mechanic pre-treatment

The aims of mechanical pre-treatment are the reduction of particle size of the substrate and the destruction of the crystalline structure. Thus the accessible surface is increased and the degree of polymerisation is decreased. These factors lead to a better hydrolysis and a lower digestion time. The mechanical methods are chipping, grinding and milling. The most important parameter for mechanical pre-treatment is the particle size which influences the energy demand significantly.

Thermal pre-treatment

The thermal influence leads to fractioning of lignocellulose. At temperatures of 150 – 180 °C hemicellulose and then lignin are fractionised because of an exothermal reaction. At temperatures above 160 °C phenolic and heterocyclic compounds like vanillin, vanillin alcohol, furfural and 5-hydroxymethylfurfural which might act inhibitory in anaerobic digestion are produced.
Pre-treatments using water comprise Steam Explosion and Liquid Hot Water. At high temperatures and pressure the substrate is exposed to steam. After a suitable retention time the steam is released abruptly. The method Liquid Hot Water uses water instead of steam.

Chemical pre-treatment

Another possibility is to expose the substrate to acids or bases. Monosaccharides are relatively stable in a pH-range of 3 to 7 if no amin components are present. Beyond these pH-boarders a lot of chemical reactions may proceed. The structure of the hemicellulose backbone and the side chains determine the stability at thermal, acidic and alkaline conditions. Especially the use of strong acids enhances the possibility of inhibitors being built up. The danger of occurrence of corrosion and production of toxic effects on microorganisms rises.

Thermo-chemical pre-treatment

In thermo-chemical pre-treatment the substrate is exposed to a catalyst in presence of high temperature. The catalyst accelerates the reaction of breaking down the hemicellulose. Thus lower temperatures can be applied. Under thermal conditions carbon hydrates are very reactive. The occurrence of chemical reactions like enolisation or Maillard reaction is dependent on the pH and the reactants. Monosaccharides and amin-components interact and as a result thermal by-products are built which may have inhibitory effects on the microorganisms of the anaerobic digestion.