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Turning waste into power with anaerobic digestion

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Waste management has risen to a new level, driven by economic, social and environmental factors. Biomass from wastewater plants, landfills, farms and food plants 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. 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%.

Advantages also include reducing emissions of landfill gas into atmosphere; saving space required to store/utilise the waste; removing odours and increasing liveability of the surrounding areas; utilising a by-product to produce fertiliser; and attracting government funding for capital expenditure.

Anaerobic digestion of the biomass can use three types of reactors - covered lagoons, plug flow and complete mix reactors. Though the cheapest, covered lagoons provide very low rate of conversion, are 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, and 20-30 days to convert the biomass. 

Complete mix digesters have significant advantages over the other types of reactors including high yield of biogas, minimum footprint, faster return of the investment, greater bioenergy conversion, reduction in solid waste, better quality of bio solids, and pacification of volatile organic compounds.

Mixing technologies used in the digesters fall into two categories – liquid (sludge) mixing and gas mixing methods. The gas mixing diffuser method has numerous advantages over liquid mixing, such as high reliability, energy efficiency, retrofit capability, no moving parts within the digester, no need for decommissioning for routine servicing, increased efficiency due to heat from compression of gas, sizing and selection of equipment based on digester design and floor geometry, and roof mounted manifold or ground level installation options.

Better understanding of anaerobic digester systems is making them popular in Australia again. 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.

Hurll Nu-Way offers a range of digester mixing systems, biogas storage systems, gas compressor packages and other equipment for waste and water treatment. 

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