Water Journal : Water Journal December 2011
wastewater systems water DECEMBER 2011 69 Abstract Decentralised water services have been considered a poor or temporary alternative to centralised water servicing. Recently, a combination of governments seeking to diversify water sources, community sustainability drivers and technology improvements have resulted in decentralised systems being considered for urban wastewater treatment and recycled water provision. However, the terms "decentralised wastewater" and "decentralised water" have been used to describe a vastly diverse set of applications. The understanding of the key parameters of a decentralised system varies with the purpose, origin and application of the particular system. This variation makes it difficult to compare, discuss and analyse the performance and acceptance of decentralised systems. This paper reviews definitions from the literature, different Australian and international regulatory environments, Australian and international codes, and water utilities themselves. It specifically identifies key characteristics of a decentralised wastewater system. Recognising the broad range of alternatives covered by the term "decentralised" will help the industry appreciate why it is so difficult to make generalisations and comparisons in this area. It also highlights the importance of authors clearly stating the parameters covered by their work, rather than assuming a constant definition is understood. Introduction Since the mid-19th Century best practice in water and wastewater has been to centralise services whenever it was economically and technically feasible (Gikas and Tchobanoglous, 2009). Centralised servicing collects water, usually far outside the urban area, and transports it through a large network of pipes to where it is used. Wastewater is transported out of the urban area, treated and usually discharged into a receiving water body. While this centralisation has produced well documented and essential public health benefits (Gikas and Tchobanoglous, 2009; Harremoës, 1998) it has also resulted in significant capital investment and large complex systems. In recent times, the sustainability and resilience of single large systems has been questioned. Community sustainability drivers, supply constraints, water restrictions and technology improvements have led to the consideration of other service alternatives. Changes in social attitudes, financing arrangements and hydrological regimes are fuelling the drive for smaller, more flexible, systems. The term "decentralised water systems" can cover the entire range of water services, including: • Smaller water sources such as rainwater tanks and local groundwater extraction; • Local wastewater treatment, including on-site septic tanks and a range of different small wastewater treatment technologies; • Non-potable water supply including greywater diversion or treatment, stormwater recycling, wastewater recycling and groundwater recharge. Crites and Tchobanoglous (1998) describe decentralised systems as ones that collect, treat and use rainwater, stormwater, groundwater or wastewater at different spatial scales, from individual homes, clusters of homes, urban communities, industries or built facilities, and portions of existing communities either independent from, or as part of, a larger system. However, within the literature decentralised servicing can mean many different things. This makes it hard to compare and fairly consider decentralised systems in relation to better-understood conventional water and wastewater solutions. This paper recognises that decentralised water services cover the whole gamut of water services. However, it focuses on the use of the term "decentralised" (and, later, "distributed") in the literature, specifically in relation to decentralised wastewater and decentralised recycled water services. The paper summarises the commonalities and differences in the use of the term decentralised wastewater and discusses the significance of these variations. It also looks at some of the emerging trends in terminology and definitions as decentralised systems are installed as a complement or competitor to existing centralised networks. In 2009, Cook et al. explored the varying definitions of decentralised systems, outlined the key aspects of a decentralised approach and set a definition specific to the South- East Queensland perspective. This paper builds on Cook et al.'s work, but rather than try to develop a single all- encompassing definition of decentralised systems it looks at key parameters that practitioners can use to clearly define the subset of decentralised systems they are discussing. The paper also seeks to clearly identify an emerging subset of decentralised systems that exist within or close to a large centralised network. One thing is clear: the range of definitions for decentralised systems is as wide ranging as the systems themselves. This reflects their great flexibility and adaptability to local needs, including demand, end uses, regulations, reliability of other supplies, costs of discharge, topography and population density. Decentralised Wastewater Services One of the most widely used definitions of decentralised wastewater systems is taken from Crites and Tchobanoglous (1998): 'Decentralised wastewater management may be defined as the collection, treatment, and disposal/reuse of wastewater from individual homes, clusters of homes, isolated communities, industries, or institutional facilities, as well as from portions of existing communities at or near the point of generation' R Watson A review of terms used in the water industry WASTEWATER SYSTEMS: DECENTRALISED OR DISTRIBUTED?
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