Water Journal : Water Journal April 2012
wastewater treatment water APRIL 2012 141 Some of the key environmental approval and planning considerations to consider for a co-digestion facility at an existing WWTP include: • Existing environmental approvals: - Will the buffer distances meet the Environmental Protection Authority (EPA) guidance for current treatment operations? - Will the discharge permits be met? • New environmental approvals: - Onus on the operator/owner to demonstrate adequate controls to manage any impacts due to insufficient buffers for both existing and proposed works; - Heritage and ecological surveys may be required. • Planning: - Surrounding zoning may include sensitive land uses (residential) within recommended buffer distances. On the other hand, some of these implementation issues can result in positive outcomes, including finding reuse opportunities for biosolids. The need for solids reduction is becoming increasingly evident as the volume of waste to landfill grows, while the available space for landfills shrinks. Biosolids produced as a by-product of the digestion process contain many nutrients required by plants for strong growth, including nitrogen, phosphorous, potassium and micronutrients. Therefore, biosolids can be an excellent fertiliser for use in agriculture and forestry, and as a soil improver in composting. Adding biosolids to soil can improve water retention, help retain nutrients, accelerate plant growth, and potentially reduce stormwater runoff and erosion. There are also opportunities for funding and revenue generation at co- digestion facilities to improve payback. The most likely opportunities for revenue generation are: • Large Scale Generation Certificates (LGCs); • Revenue from excess electricity fed back into the grid; • Tipping fees for the acceptance of high- strength waste streams that can be equal to those charged at landfills. Recent experience with these types of projects has found that the tipping fee is a very important parameter for reducing pay-back periods and increasing lifecycle savings. The availability of LGCs coupled with on-site savings from electricity and heat production also improve the cost- effectiveness of these facilities significantly. Existing Facilities -- Lessons Learned There are a number of facilities in operation globally that co-digest sewage and non-sewage wastes, including some that operate only on non-sewage waste streams. Through extensive desktop study and case study review of local and global facilities, commonalities have been identified. Some of the key findings include: • Facilities that have supplemented sewage sludge with non-sewage waste streams have observed significant increases in biogas yield. • The most effective waste streams for biogas production have high energy content. Examples include fats, oils, grease, cheese whey, abattoir waste, piggery waste, sewage sludge (from the earlier stages of the process), some forms of sugar wastes such as concentrated syrup or molasses, and food waste. • The biogas produced from various wastes will vary in quality and sometimes requires treatment prior to combustion -- this will reduce the risk of wear and tear on engines and equipment downstream, as well as increase co-generation system efficiency. • The types of wastes used, the ratios at which each is fed into the digestion process, and the approach to collection, treatment and combustion of biogas, are all highly variable and site specific. • The overall beneficial effectiveness of a biogas facility is multifaceted -- the reduction of industrial waste, reduction of environmental impacts of the wastes, increase in renewable energy capability in a community, reduction of dependence on fossil fuels, reduction in fugitive emissions and general increase in process efficiency, are all positive aspects common to all case studies. Summary The utilisation of anaerobic digestion of various high-strength wastes for the production of energy, heat and other end products is a highly beneficial process that is increasingly becoming adopted around the world. There are an increasing number of small-scale wastewater treatment facilities worldwide that take advantage of the availability of high- strength wastes for increased biogas production. The additional benefit of heat and power generation, and subsequent reduction in environmental footprint and lifecycle operating cost, is aiding many facilities in meeting sustainability objectives. The co-digestion facilities also allow improved management of a second waste source, diverting waste away from landfills or separate treatment facilities. The processes used and the waste streams fed into the system are well- proven at full scale implementation, and there are many examples of successful schemes from many perspectives including financial, social and environmental. Work continues at laboratory, pilot and full-scale developments to further optimise these practices and technologies; however, there is general consensus that this is a highly beneficial technology that is likely to significantly increase over the coming years throughout the world. Acknowledgements The authors would like to acknowledge Zeynep Erdal, Wastewater Technology Leader, and Timothy Shea, Technology Fellow, both at CH2M HILL (US) for their contribution and review of this article. The Authors Kate Simmonds (email: Kate.Simmonds@ ch2m.com.au) has nine years' experience in the water and wastewater industry, working as an environmental engineer and project manager in New Zealand, the United Arab Emirates and Australia. Kate is CH2M HILL's ANZ Region Sustainability Coordinator, and in this role communicates the value of sustainable water, assisting with developing, integrating, delivering and coordinating the resources, tools, training and strategies for sustainability services and projects in the ANZ region. Dr John Kabouris (email: John.Kabouris@ch2m.com) is a Senior Technologist with CH2M HILL. He has expertise in process optimisation and advanced biosolids and biogas solutions. He has worked in projects involving high-strength waste codigestion and conducted laboratory research on the codigestion of municipal sludge and fat, oil and grease under conventional and advanced digestion. He also co-authored the 2010 Water Environment Federation (WEF) Technical Practice Update document on codigestion of Fat Oil and Grease (FOG) and High Strength Wastes (HSW).
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