Biogas Utilisation

The simplest way to utilise the biogas produced from sewage sludge is to burn it within boilers to provide the digester with process heat.   This is however, not always the most economical way of utilising the biogas.   Combined heat and power (CHP) units and micro-turbines can be used to produce electrical power and heat.   These forms of energy can be used on site, offsetting the requirement to purchase electricity from the grid, while also claiming Renewable Obligation Certificates (ROCs) which are currently worth 4p per kWhr.  

Biogas is produced naturally from the anaerobic digestion of organic matter. The composition of the biogas depends on the characteristics of the feedstock, and in this case is about 60% methane (CH 4 ) and 40% carbon dioxide (CO 2 ) with traces of hydrogen sulphide (H 2 S).   Biogas produced from sewage sludge will often contain siloxanes.  

Siloxanes are a family of man made organo - silicon compounds containing silicon, oxygen and methyl groups. They are frequently used in the manufacturing of pharmaceutical, and industrial products, of which the residues end up in Waste Water Treatment Works. In anaerobic digesters low molecular weight siloxanes volatilise into the gas and therefore siloxanes frequently found in landfill gas and sewage sludge derived biogas include Hexamethycycotrisiloxane (D3), Octamethycyclotrasiloxane (D4) and Decamethylcycopentasiloxane (D5). Siloxanes do not affect the anaerobic digestion process or the digester itself, however, during the combustion of the biogas they are converted into silicates, micro-crystalline quartz and silicon dioxide (SiO 2 ). These can become deposited on the engine abrading the surfaces, and may eventually cause damage to the engine (Schweigkofler et al., 2001).