The first step when planning an anaerobic digestion (AD) project is to complete a simple feedstock assessment. This involves evaluating potential farm feedstock availabilities (slurry, grass and energy crops) as well as residues from food processing and industries in the area. The second step is to analyse samples of these feedstocks to determine a number of parameters such as dry matter content, total chemical and biological oxygen demand, volatile fatty acids, etc, as well as their potential biogas yield.

Once the availability and the properties of feedstock are known, digester size and technology can be designed.

RED II

Feedstock largely determines the biogas yield as well as the amount of time it takes to extract the maximum gas yield.

However, feedstocks also have a big impact on meeting new sustainability criteria set out in the EU Renewable Energy Directive II (RED II), which ensures GHG emissions are reduced by 70% when using biogas compared to its fossil fuel equivalent. This requirement is likely to limit the volumes of purpose-grown energy crops used to produce biogas.

AD feedstocks including chicken manure, protein-rich food wastes, rendering and many other nitrogen-rich feedstocks are generally favourable from a sustainability perspective but can often bring many other challenges.

Dr Melanie Hecht of Schaumann BioEnergy said that nitrogen-rich feedstocks are generally high in biogas yield and are readily available, so they are attractive to AD plant owners.

However, excessive feeding of nitrogen-rich feedstocks can lead to problems including inhibiting microbial populations, foaming, reducing gas quality and yields and cause machinery and structural failure.

Ammonia inhibition

Ammonia (NH3) is produced during degradation of nitrogen-rich feedstocks and is a cytotoxic. This means it can inhibit bacteria and archaea involved in biogas production, reducing gas yields. The simple solution to this is to decrease feeding rates of high-nitrogen feedstocks, but this isn’t always possible. Decreasing temperature, diluting feedstock or using ammonia binding products can also work.

Hydrogen sulphide levels

Nitrogen-rich feedstocks produce biogas with higher levels of hydrogen sulphide (H2S). This can lead to corrosion of steel, concrete and timber and is also very toxic. Solutions here include blending air/O2 with the biogas, adding ferric products, using activated carbon filters or external gas scrubbers.

Struvite formation

Struvite (magnesium, ammonium, phosphate) is a mineral that can form in lines and in pumps and can clog system pipes and other equipment. The mineral is rock-hard, so removal is difficult. Solutions include insulating pipe systems to reduce temperature fluctuations (where struvite can form), diluting digester contents, using a binding agent or increasing the temperature of digester.

These issues can also be generate from the wrong concentrations of the feedstock such as grass and whole crop so careful planning must go into feedstock composition.