II
SUSTAINABILITY
URBAN DOMESTIC WATER TANKS: LIFE CYCLE ASSESSMENT T Grant, M Hallmann Abstract
Sinc e potable s u ppl y wa s not considered, first flush rejection was not necessa ry.
R egion, over 7 years to determine a m edian year. Data were not averaged across years, as the aim of the model was to maintain the sporadic nature of rainfall events, rather than smooth this ou t with yea rly averaging. Ideally a single weather station would have been better, however, un-avernged data was not readily available. Th e savi ngs for each tank were p lotted against the total roof co llection area using rainfall data from 1994 to year 2000. The results from this are shown in Figure I for the 600-litre tank and Figure 2 for the 2,250 L water tank. The graphs show substantial yearly variation in water savings from high in 1995 and very low water savings in 1997. From th ese grap hs, the year of 1998 was selected as a median year in terms of potential savi ngs for a given sec of rainfall data. A nominal roof area of 220m 2 was selected for th e study as no data was ava il abl e o n ave ra ge roof ar eas of M elbourne households. While n o fi rst flush system was assumed 0 .5mm of rai nwater was assu m ed to be lost to roof adsorption and wetting. This was purely an estimate as no data for this was found.
Water Supply
Water Demand
T he rainwater supply m odel looked at daily rain events averaged across fiv e weather stati ons in the Yarra Valley
T he water demand model from the households also aimed to represe nt the typical variation in water use by h o use-
Life cycle assessment and costing of a 600 L and a 2250 L domestic water tank in the M elbourne north-eastern suburbs has shown that sign ificant water savings are possible, that although the energy and materials impact are higher than for reticu lated supp ly they are insign ifica nt compared to other domestic impacts, but with current Melbourne water prices and electricity costs the payback period is longer than the 30 year life assumed. The discounted costs fo r th e 600 L tank are 13% higher and the 2250 L tank some 20% higher than reti culated water costs for garden and toilet usage .
Introduction Yan-a Valley Water, w hich supplies the north - eastern subu rbs of Melbourne, commissioned th e Centre fo r D esign at the Roya l M e lb o urn e Institute of Tech nology to research the fo llowing three questions: • H ow mu ch water is likely to b e saved by usi ng a water tank, given th e seasonality of Melbo urne rainfall and seasonality of water usage patterns? • What are the broader en vironmental implications of the water tank production, installation , use and eventual disposa l? • What are the lon g-term fina ncial implications for residents purchasing water tanks? This paper summarises the results of that study.
600-litre plastic tank used for garden watering only using gravity feed.
Savings for last 7 years with different roof areas (112) 6001 tank supplying garden only
35,c,
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1994 1995 1996 1997 - 1 998 1999
Methods The two water tank scenarios tested in the study were: • A 600-litre plastic tank used for garden watering only using gravity feed (i.e., no pump installed); and • A 2,250-litre plastic tan k used for garden wa tering and toilet flushin g, with a pump installed with an automatic pressure switch. Mains water (via a backflow preventer) is connected to the tank via a float switch to maintain a minimum level of water in the tank for toilet flushing in times of low rainfall. 36
WATER AUGUST 2003
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0,C, +-- - - - ~ - - - ~ - - - - ~ -- - - ~ - - -- ~ - - -~ 100
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Rool Sliz•
Figure 1 . Percentage of garden water demand supplied by 600-litre water tank over 7 years weather data showing selected "average" year for use in thi s study.