Technical Report

Pretreatment of feedstock for enhanced biogas production

February 2014

Anaerobic digestion (AD) is a well-established process for renewable energy production in which biomass (also referred to here as substrate or feedstock) is broken down and converted to biogas (a mixture of methane, carbon dioxide and traces of other gases) by microorganisms.

Commonly used substrates for biogas production include industrial waste such as dairy waste, agricultural waste such as fodder residue and manure, and energy crops such as maize (corn). The ability to make biogas out of many different substrates is one of the main advantages of anaerobic digestion over other processes like ethanol production. However, some substrates can be very slow to break down (so that biogas is produced) because:

• they contain chemicals that inhibit the growth and activity of the microorganisms,
• they create physical problems like floating, foaming or clumping, and block impellors and pipes in biogas plants, or
• their molecular structure is poorly accessible to microorganisms and their enzymes (for instance because of their highly crystalline structure or low surface area).

Sometimes all these problems occur at once. Pretreatment can be used to overcome some of these problems. This brochure mainly focuses on substrates with poorly accessible molecular structures (i.e. lignocellulosic substrates), which include many agricultural residues such as maize leaves, some industrial residues such as brewers’ spent grains, and some energy crops such as switchgrass. Some emerging biogas substrates also come under this category, such as oil palm empty fruit bunches (EFB). Pretreatment technologies that are used for other substrates such as sewage sludge, but not for lignocellulose, are covered briefly in section 8.

In biogas substrates, the main sources of methane are sugars and other small molecules. In plants (lignocellulosic substrates) these small molecules come from the breakdown of starch, cellulose and hemicellulose. While starch (α-1-4 linked D-glucose) is relatively easy and quick to break down biologically, cellulose (β-1-4 linked D-glucose) and hemicellulose (a polymer of various sugars and uronic acids) are used to maintain the structure of the plant, and are, by necessity, difficult and slow to break down. The breakdown of cellulose and hemicellulose is further complicated by the bonds between different cellulose chains (termed cellulose crystallinity) and by the presence of lignin, another polymer which slows down the breakdown process (see Figure 1). It is generally believed that lignin cannot be degraded by anaerobic bacteria, although this has been challenged (DeAngelis et al., 2011), and may even inhibit the degradation of other substances like cellulose. Pectin also affects breakdown, binding cellulose fibrils together and binding plant cells together (Carpita and Gibeaut, 1993). Breaking down this lignocellulose complex is the key to biogas production (Noike et al., 1985).

Various pretreatment technologies have been developed in recent years to increase the availability for AD of sugars and other small molecules in biogas substrates, particularly in lignocellulosic material. These pretreatment technologies aim to:

• make AD faster,
• potentially increase biogas yield,
• make use of new and/or locally available substrates, and
• prevent processing problems such as high electricity requirements for mixing or the formation of floating layers.

Many of these technologies have been developed by the wastewater treatment or bioethanol industries.

The aim of this brochure is to describe different pretreatment technologies and to discuss their positive and negative aspects with respect to different substrates for AD. Due to the wide range of different technologies and information from different providers, this brochure does not give detailed information about specific costs. There are many different types of pretreatment, and they can be divided up into the principles by which they function (Table 1).