Water Journal : Water Journal April 2012
wastewater treatment technical features 138 APRIL 2012 water Introduction Traditionally it has been considered by the water industry that biogas production at wastewater treatment plants (WWTPs) is only cost effective at a large scale, unless the facilities have existing digestion and primary sedimentation facilities. The current perception is that smaller WWTPs -- typically less than 20 MLD in size -- are limited in their opportunities for beneficial biogas generation. This perception has arisen due to two key factors: • Smaller facilities have limited sludge volume, which typically means lower biogas production; and • The capital costs associated with installation of the new infrastructure required to generate biogas and produce power and heat for use as a renewable energy source are proportionally high, meaning these facilities are not cost-effective at the smaller scale. However, recent project experience has shown that these smaller WWTPs can also produce biogas cost-effectively. The secret to success? The introduction of non-sewage high-strength waste streams. These wastes can be co- digested with sewage sludge to boost biogas production. In some instances, these biogas generation facilities can also provide a new revenue stream through power production and tipping fees for the owning and operating company. The increased biogas yield, coupled with the income from tipping fees, can "tip" the scales, so to speak, which improves the business case for these facilities markedly. The other advantages of these facilities are the beneficial utilisation of the high-strength wastes in producing green energy, and a reduction in waste to landfill. Barriers to Co-Digestion Facilities So why aren't these types of facilities increasingly being installed throughout Australia? There are, in fact, a number of non-sewage waste anaerobic digestion and biogas generation facilities in operation throughout Australia that operate successfully. However, the co-digestion of sewage and non-sewage wastes is not as prevalent in the industry in Australia as it is in other countries, including the USA and Europe. Typical barriers to these projects have been footprint availability and the perceptions identified earlier, as well as a lack of local experience. Traditional thinking has led the owners and operators of these smaller WWTPs to believe that the WWTP should include primary sedimentation and digestion in the existing treatment process for these types of projects to be feasible and cost-effective. However, recent project experience has disproved the need for pre-existing primary sedimentation and digestion process units, and shown that the cost of retrofitting these WWTP facilities to include primary sedimentation at the smaller scale may actually outweigh the benefit obtained. It can be more cost-effective to install high-strength waste and waste-activated sludge co- digestion facilities at the smaller WWTP flows without the inclusion of primary sedimentation and primary sludge as a digester feed source. This results in a smaller footprint for the facility upgrades, as well as cost-effective and simpler implementation and integration with the existing facility in most cases. Of course, this needs to be evaluated on a case-by-case basis, and depends on the waste streams available and the biogas production efficiency of those wastes. Biogas Production So how do we generate biogas efficiently, and which waste streams are preferred? Biogas is a renewable energy source, and at WWTPs it is produced through anaerobic ("without oxygen") digestion of wastewater sludge. The biogas produced typically has a high methane and energy content (60%--70% of the energy content of natural gas), with energy content typically in the range of 22,000 kJ/m3 --24,000 kJ/m3. Biogas produced in small facilities can be used to produce heat and power through co-generation using a reciprocating engine or microturbine. The electricity produced can be utilised on- site to offset the site power requirements, while the heat can be used for digester and on-site building heating. Biogas can also be used to replace natural gas consumption in boilers. Anaerobic digestion has additional benefits, including reduction in the volume and mass of the solids residual (biosolids) produced, stabilisation and pathogen reduction for the biosolids which can allow for their beneficial reuse, and decreasing the environmental footprint of the WWTP. K Simmonds, J Kabouris Even small WWTPs can develop a positive business case RENEWABLE ENERGY GENERATION THROUGH CO-DIGESTION OF NON-SEWAGE WASTES Figure 1. Summary of the anaerobic digestion and biogas generation process.
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