Water Journal : Water Journal December 2011
wastewater systems water DECEMBER 2011 71 Whether decentralised systems are assumed to include or exclude on-site systems, they all have an implied smaller scale than centralised systems. Occasionally the scale is specifically defined. This is more frequently done in standards or regulations. The US EPA (2005) definition suggests decentralised systems typically provide treatment on the property of individual homes or businesses. However, when scale is discussed explicitly in a definition it can vary considerably from only including on-site (US EPA, 2005), to including large recycled water schemes such as Rouse Hill (18,000 homes) in Sydney (Gordon, 2008). The Interim Final Queensland Guidelines for Decentralised Wastewater Systems (2007) (unpublished) were developed for systems with capacity from 21 to 1,000 EP (Cook, 2009). In Switzerland, treatment plants for less than 500 residents are considered as decentralised systems (Adler, 2006). NSW Department of Local Government, in its guidelines for on-site reuse for single dwellings, defines centralised and on-site systems (1998). It suggests centralised systems can be built to service from less than 10 to many thousands of households. This is the exception rather than the rule, as usually 10 houses would be considered decentralised. These examples show the wide range of scales the term decentralised systems cover. Perceptions of Improved Sustainability Nelson (2008) suggests that as integrated water solutions become the preferred way of servicing, decentralised technologies will become more dominant. Certainly within major Australian water utilities the process of integrated water planning has spurred the consideration, if not the implementation, of decentralised water and wastewater alternatives. Decentralised definitions often included perceptions of greater sustainability and opportunities for recycling (see Venhuizen, 1986, for example). Results from Cook et al.'s (2009) interviews with industry practitioners in Australia reiterated that the concept of decentralised treatment included being fit for purpose and sustainability objectives, including better integration of services and water-sensitive urban design. This is reflected in current trends in Australia. Voluntary green building codes such as the Green Building Council of Australia's 'Green Star Building Rating Tool' have encouraged the use of decentralised recycled water systems. Alternative Management and Ownership Models Occasionally the management and ownership model is referred to. When it is, generally the more decentralised the system the more private the ownership and management structures. The larger and more centralised systems tended to have an assumption of public ownership and management. The Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT) suggests cluster treatment systems are typically privately owned, community systems are publically owned and on-site systems are owned by individuals (2010). Cook et al. (2009) also suggested on-site systems were predominantly individually owned, cluster systems suggested some form of common ownership and distributed systems were operated by specialised utilities. Perception of Inferior Performance and Increased Risk Decentralised systems, particularly on- site systems, have a poor performance perception. They have generally only been used at the fringes of traditional centralised servicing (Nelson, 2008) and have often been viewed as a temporary or second-class measure. Governments throughout Australia (see, for example, Victorian Auditor- General, 2006) have identified and responded to the high levels of failing on- site systems (mainly septic tanks) through long and expensive centralised sewering programs.2 This response has reinforced the perception that centralising rather than improved management techniques is the best way to overcome the social (odour, amenity), health and environmental issues associated with these failing tanks. Advocates for decentralised services dispute decentralised systems present higher risks. Pinkham et al.'s (2004) review of risks concludes generally, the risk and cost of failure are less for decentralised as the consequences are small and widely distributed. Greater Variety in Source, Treatment and Transport Technologies and Discharge Location The final area where decentralised systems were differentiated from centralised systems was the source and discharge methods. These concepts were not often referred to, but when they were, decentralised systems were generally seen as more flexible and capable of meeting the local needs and opportunities. A wide variety of sources for decentralised systems was evident PHOTO: © PERMEATE PARTNERS 2011 The distributed recycled water system at Pennant Hills Golf Club in NSW.
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