Water Journal : Water Journal December 2011
wastewater systems technical features 70 DECEMBER 2011 water This definition is representative of the wide range of terminology used in the literature when discussing decentralised systems. Terms used include: • Local treatment (Cook, 2009; NSW Department of Health, 2010); • On-site treatment (many, including Sydney Water, 2009; Geisinger and Chartier, 2005; US EPA, 2005; WERF, 2010); • Allotment (Mitchell and White, 2003); • Cluster (many, including Geisinger and Chartier, 2005; Mitchell, Abeysuriya and Willetts, 2008; US EPA, 2005; WERF, 2010; Willets, Fane and Mitchell, 2007); • Distributed systems (Cook, 2009; Mitchell, Abeysuriya and Willetts, 2008); • Community (Auckland Council; NSW Department of Health, 2010); • Development scale (Tchobanoglous & Leverenz, 2008); • Hybrid on-site (Tchobanoglous and Leverenz, 2008); • Satellite treatment plants (Gikas and Tchobanoglous, 2009; Tchobanoglous and Leverenz, 2008). In addition to using a wide range of terms, the definition of decentralised systems can vary depending on the specific immediate context. Yarra Valley Water's (2009) discussion paper recognised that the terms "distributed", "decentralised", "on-site", and "cluster" are used interchangeably to refer to wastewater treatment systems that operate outside the reticulated sewage network. Sydney Water's sewer mining policy (2008) refers to decentralised wastewater treatment as a type of sewer mining that involves extracting domestic wastewater from a private sewer and treating it on site for reuse as recycled water. Sydney Water's EIS for West Camden Sewage Treatment Plant (2001) upgrade and amplification considered three 'decentralised' options including: • On-site systems; • Community systems that were not connected to the centralised network; • A combination of on-site and community systems. Key Themes in Definitions Although the definition of decentralised wastewater can vary, there are some key similarities in the majority of definitions. These issues are similar to those found by Ackermann et al. (2001) and Pepermans et al. (2005) when they attempted to define distributed electricity generation. They both found it beneficial to look at the concepts used to describe the systems to better understand their components. Specifically, Pepermans et al. (2005) found that the lack of agreement on a precise definition was due to the concept including many technologies and applications in a range of environments (page 797), which is also the case for decentralised wastewater systems. The two most consistent concepts in decentralised wastewater systems were the proximity of the generation and/or treatment to use and smaller size. However, what represented small size and close proximity varied between authors. Other concepts varied in consistency, as will be discussed below. What emerged was not a single concept of decentralised systems, but more a moving continuum, where most agreed on what centralised and on-site systems were, with decentralised being somewhere in between (or including on site). The concept of a continuum somewhere between centralised and on-site was also discussed in Pinkham et al. (2004). The themes identified in decentralised wastewater definitions include: • Close proximity of wastewater source, treatment and use/disposal; • Smaller size and relationship to on-site systems; • Perception of improved sustainability; • Alternative management and ownership models; • Perception of inferior performance; • Greater variety in source, treatment and transport technologies and discharge locations; • Relationship of decentralised systems to centralised systems. These themes are not dissimilar to Pinkham et al. (2004), varying mainly in the inclusion of sustainability and performance perceptions and grouping together of treatment/discharge and sewer type. The final grouping reflects a subtle difference between the American and Australian experiences, particularly in recent years where technological, demand and cost advances have changed the technologies used both in decentralised treatment and centralised transport.1 Close Proximity of Wastewater Source, Treatment and Use/Disposal According to the Oxford Dictionary, "decentralise" is "to undo the centralisation of". The two definitions in Crites and Tchobanoglous (1998) are representative of this concept of decentralised being the opposite of centralised. They use networks to separate the two, with centralised systems having large and long transport networks and decentralised systems having little to no transport network. The idea of wastewater treatment being close to the waste generation, and the disposal or end use being close to the treatment, is well accepted in decentralised literature. Crites and Tchobanoglous (1998) use "at or near the point of generation" in their definition of decentralised. In New Zealand, (Auckland Council) defines decentralised systems as: 'The collection, treatment and disposal/ reuse of limited volumes of wastewater, generally from cluster(s) of dwellings and/or accommodation facilities that are usually located relatively close together, with the wastewater system relatively close to the source (also referred to as "community", "neighbourhood" or "cluster" systems)' [the author's emphasis) Generally the overarching concept of proximity for generators to each other, generators to treatment and generators to end use is to keep systems small and local. It is very hard to specifically define and is referred to only in general terms (eg, relatively close, at or near). Smaller Size and Relationship to On-site Systems While all definitions agree decentralised schemes are not centralised, many have variations within the "not centralised" component. Many of these explicitly exclude on site as a separate category (for example, see NSW Department of Health, 2010 and Tchobanoglous and Leverenz, 2008). Others, like Sydney Water (2009) and Geisinger and Chartier (2005), specifically include on-site systems under the banner of decentralised. This is perhaps another example of the advances in technology and new applications changing the understanding of terms. In the past, 'on-site' would generally refer to a single household; now on-site systems can refer to basement plants in large commercial or residential towers.
Water Journal April 2012
Water Journal November 2011