Turning waste into power with anaerobic digestion

Economic, social and environmental factors dictate the necessity to manage waste to a new level. Wastewater plants, landfills, farms and food plants are all the owners of biomass that can be digested to produce biogas and turned into power.

Biogas can be burned directly in boilers for heat recovery and used for manufacturing, heating or cooling; this is the simplest method of direct biogas utilisation on site. Alternatively, it can be burned in engines to generate electric power (cogeneration). Combining hot water recovery with electricity generation, biogas can provide an overall conversion efficiency of 65-85%.

Apart from the main purpose of capturing biogas and using its energy, there are other advantages, such as:

• Reducing emissions of landfill gas into atmosphere
• Saving space required to store/utilise the waste
• Removing odours and increasing liveability of the surrounding areas
• Using a by-product to produce fertiliser
• Attracting government funding for capital expenditure

There are three types of reactors used for anaerobic digestion of the biomass: covered lagoons, plug flow and complete mix reactors. Covered lagoons, though being the cheapest method, provide a very low rate of conversion; they are also very smelly and require a lot of space. The plug flow method is only used for dairy manure as it requires a higher presence of solids. The material requires 20-30 days to convert the biomass.

Complete mix digesters, though implying higher initial investments, have significant advantages over the other types of reactors:

• High yield of biogas
• Minimum footprint
• Faster return of the investment
• Greater bioenergy conversion
• Reduction in solid waste
• Better quality of biosolids
• Pacification of volatile organic compounds

There are several mixing technologies used in the digesters, falling into two categories - liquid (sludge) mixing and gas mixing methods. The gas mixing diffuser method has numerous advantages over the liquid mixing, such as:

• Compact and highly reliable
• Energy efficient
• Can be retrofitted
• No moving parts within the digester
• No need for decommissioning for routine servicing
• Heat from compression of gas increases overall efficiency
• Sizing and selection of equipment is based on digester design and floor geometry, allowing selection of the most suitable option
• Roof-mounted manifold or ground-level installation are possible

Anaerobic digester systems in Australia are coming back into use due to the further understanding of their benefits and the development of technologies. There are several organisations providing grants for construction of biogas plants and installation of digester systems, such as Low Carbon Australia and the Australian government’s Clean Technology Investment Program.