Urban Water Systems: Drivers of Climate Change?

Transcription

1 Urban Water Systems: Drivers of Climate Change? D J M Flower 1, V G Mitchell 1, G P Codner 1 1 Institute for Sustainable Water Resources & ewater CRC, Department of Civil Engineering, Monash University, Victoria, 3800, David.Flower@eng.monash.edu.au Abstract Urban water systems contribute to climate change both directly through the fugitive greenhouse gas (GHG) emissions associated with water storage reservoirs and wastewater treatment processes, and indirectly through significant energy and materials consumption. This paper presents the findings of an investigation of the GHG emissions associated with operating a case study urban water system in Melbourne, Australia. It was revealed that the appliances associated with the residential end uses of water were responsible for significantly more GHG emissions than all upstream and downstream operations. These findings led to the conclusions that any project seeking to minimise the energy consumption and GHG emissions associated with urban water systems should: (a) consider the whole system, including the end uses of water, in a holistic application of life cycle thinking, and (b) focus on reducing the energy and water consumption associated with the end uses of water, since this will yield multiple benefits upstream, downstream and at the point of use. 1. Introduction Induced climate change, largely associated with anthropogenic greenhouse gas (GHG) emissions, has the potential to become one of the most significant environmental changes caused by humans. It is widely recognised that urban water systems are highly vulnerable to induced climate change, with many temperature and rainfall related impacts directly impeding their operation (see, for example Howe et al., 2005). However, it is not so well understood that urban water systems contribute to climate change as a result of: (a) consumption of energy derived from carbon based fuels, (b) biochemical processes which directly generate fugitive GHGs, and (c) consumption of goods and services that involved energy consumption or the biochemical generation of fugitive GHGs at some upstream point. This nexus of climate change drivers and impacts associated with urban water systems results in the development of a feedback loop, within which the impacts of various future urban water management strategies are currently poorly understood. This paper presents the findings of an investigation of the GHG emissions associated with operating a case study urban water system located in Melbourne, Australia using a life cycle assessment approach. Importantly, and in contrast to most previous studies, the system boundary used throughout this investigation was expanded to holistically include all aspects of the urban water system. The outcomes of this case study analysis are informing the ongoing research project by ensuring the most critical urban water system configurations and components are targeted. Life cycle assessment (LCA) can be split into three broad categories, process based, economic input-output based and hybrid based (Suh and Huppes, 2005, Suh et al., 2004). The underlying principles of each approach are the same; the inputs, outputs and potential environmental impacts that should be allocated to a particular product or activity include the sum of those associated with pre-operation, operation and post-operation life cycle stages. LCA is formally recognised and defined by the AS/NZS ISO series of standards, and as such, is a widely accepted methodology for determining the environmental impacts associated with a product or service, such as the provision of water, wastewater and drainage services. For the research project outlined in this paper, an LCA approach was determined to be the most appropriate methodology, since while it is appropriate for the present analysis of GHG emissions, it can also be extended later to include a wide range of other environmental

2 impacts. In this case, the product under investigation was identified as a unit volume of water as it travels through an urban water system. Pre- and post-operational life cycle stages were defined as the periods during which the unit volume of water was found in water supply and wastewater systems, while the operational life cycle stage was defined as the intermediate period during which the unit volume of water was used for various residential end uses. 2. Literature Review There have been many studies in recent times that have investigated the energy consumption and/or GHG emissions associated with a single component of the urban water system, while a smaller number of studies have taken a wider view and investigated more than one urban water system component. It is important to note that the majority of the reviewed studies neglected the environmental impacts intrinsic to the end uses of water (see, for example Lundie et al., 2004, Grant et al., 2006a, Grant et al., 2006b, Grant et al., 2006c, Saliba and Gan, 2006, Gardner et al., 2006, Crettaz et al., 1999). Those that included the environmental impacts associated with the residential end uses of water within their system boundary consistently found them to be the most significant system component in terms of operational energy (Wolff et al., 2004, Arpke and Hutzler, 2006, Arpke and Hutzler, 2005). No previous Australian study has holistically investigated the operational energy consumption and/or GHG emissions associated with all urban water system components. While some international studies have attempted such a task, none have succeeded in determining the disaggregated system-wide energy consumption and/or GHG emissions associated with each residential end use of water. 3. Methodology 3.1 Case Study Description To determine the relative importance of the GHG emissions associated with operating various urban water system components, a case study for a single residential house was developed for the eastern suburbs of Melbourne, Australia. The water consumption patterns discussed later in this paper were applied to a typical household of three adults sharing the same average pattern, rounded up from the Australian national average of 2.6 individuals per household (Australian Bureau of Statistics, 2007). This case study was selected to establish a baseline water consumption and GHG emissions profile for a typical residence in this area, such that impacts of future changes to water consumption patterns and/or infrastructure configurations could be quantified, and extrapolated to a large number of similar situations. 3.2 System Boundary The investigated urban water system components and associated responsible agencies are outlined in Figure 3.1. Note the comprehensive system boundary selected for this investigation relative to most of the reviewed previous investigations. Note also that stormwater systems were excluded from this analysis because: (a) the authors were informed that there is neglibible energy input to the stormwater system in Melbourne (Baxter, 2006) and, (b) there was no reliable data available regarding any other potential sources of GHG emissions, such as any anaerobic processes that may occur in some wetlands leading to the biochemical generation of fugitive GHG emissions. These issues will be explored in further detail in future research.

3 System Boundary (current investigation) System Boundary (most previous investigations) System Boundary (most previous investigations) Unexploited water resources Energy Supply (electrical, thermal, transport, etc) Water resource exploitation Bulk distribution Water treatment Local distribution Residential end uses of water Wastewater removal Wastewater treatment Effluent disposal Greenhouse gas emissions (including both energy related and fugitive emissions) Melbourne Water Yarra Valley Water Consumers Yarra Valley Water and/or Melbourne Water Figure 3.1 System boundary containing the urban water system components and responsible agencies considered in this study 3.3 Input Data The data for the centralised water supply and wastewater services used throughout this investigation were sourced from two water utilities, Melbourne Water and Yarra Valley Water. Both of these organizations have implemented programs to minimize GHG emissions. It is important to note that the GHG emissions reported in this paper do not account for any GHG abatement credits purchased by either Melbourne Water or Yarra Valley Water from other organizations, but do include the impacts of any internal changes to the operations of these water utilities which have physically decreased total GHG emissions. This was considered an important issue, since a key goal of this study was to identify the most critical urban water system components that should be targeted for improvement, without relying on abatement credits to allow business to continue as usual, although this can be a positive step under some circumstances. The total GHG emissions associated with centralised water supply and wastewater systems were determined by combining data collected and published by Melbourne Water and Yarra Valley Water, originally developed using the factors and methods developed by the Australian Greenhouse Office (Melbourne Water, 2006, Melbourne Water, 2005, Yarra Valley Water, 2006, Commonwealth of Australia, 2005a). Spatially and temporally averaged figures for were used, which were believed to be more widely representative than location specific or instantaneous data. The final GHG emissions used were compared to others quoted in the literature and found to lie within the typical range, confirming the validity of this approach. Note that no fugitive GHG emissions were included for water storage reservoirs, due to a lack of reliable data. Significant amounts of energy are generated at the two large wastewater treatment plants in Melbourne by using the captured fugitive GHG emissions associated with anaerobic biochemical wastewater treatment processes (also known as biogas, composed mainly of methane). The impact of biogas capture at these plants is reflected in the final GHG emissions. Table 3.1 below shows the residential end uses of water considered in this investigation, and the associated baseline consumption patterns, originally developed by Yarra Valley Water (Roberts, 2005, Roberts, 2004). Note that evaporative air conditioning can be a significant non-seasonal end use of water, but because only 20% of households in the end use measurement survey had evaporative air conditioners, a figure also shared by the rest of Australia, it was considered more representative of the whole population to exclude it from this investigation (Roberts, 2005, Australian Bureau of Statistics, 2005).

4 Table 3.1 The residential end uses of water and associated total water consumption patterns (modified after Roberts, 2005, Roberts, 2004) Mean Event Mean Event Mean Event Flow End Use Mean Event Volume Frequency (events Duration (minutes Rate (litres per (by appliance) (litres) per person per day) per event) minute) Shower Toilet Dishwasher Clotheswasher Tap Irrigation 0.094* *this figure has been adjusted to reflect the 28 week duration of the irrigation season (Roberts, 2004) The analysis presented in this paper used mean end use volumes. End use volumes can vary from these mean values due to a range of factors, including the water star rating of the associated fixture or appliance. Future papers will explore the impact of this variation in more detail but such analysis is beyond the scope of this paper. Several end uses of water, such as showers, require hot water. This study considered the two most common water heating systems in Australia, continuously energized gas storage and offpeak energized electric storage hot water services, with household penetration rates of 47.5% and 25.2% respectively (Australian Bureau of Statistics, 2005). The energy consumption associated with operating gas storage hot water services was calculated based on the energy star rating system described in AS The energy consumption associated with operating electric storage hot water services was calculated based on the minimum energy performance standards (MEPS) and daily energy consumption calculations described in AS/NZS :2005 and AS/NZS :1997. Thus, the energy consumption associated with each hot water service could be varied based on the prescribed energy efficiency rating. The hot water consumption associated with each appliance was determined based on several assumptions, most importantly a preferred shower and tap water temperature of 40 c, and a top loading warm (40 c) clotheswasher cycle. Note that the dishwasher modeled for this investigation used electricity to heat water internally. The results generated by the hot and cold water consumption model are shown in Figure litres per household per year Shower Toilet Dishwasher Clotheswasher Tap Irrigation Hot water Cold water Figure 3.2 Annual household hot and cold water consumption by end use Additionally, it was found that the appliances associated with some end uses of water, such as dishwashers and clotheswashers, required energy to operate. The energy consumption associated with operating both of these appliances was modeled based on the energy star

5 rating system, described for dishwashers in AS/NZS /.2:2005 and clotheswashers in AS/NZS /.2:2005. Note that the calculations described in the aforementioned standards for clotheswashers had to be modified to reflect the presence of the process inefficiencies encountered when heating water, and to remove an energy consumption penalty associated with the subsequent use of a clothes dryer. The final energy consumption results could be manipulated based on the energy star rating of the installed appliance, although this feature of the model was not exploited to generate the results presented in this paper. The operational energy consumption associated with each appliance, in terms of natural gas and electricity consumption, was converted into GHG emissions using the factors and methods developed by the Australian Greenhouse Office (Commonwealth of Australia, 2005a). Note the use of the carbon dioxide equivalent (CO 2 -e) as the unit of measurement, an index which incorporates the various global warming potentials of a wide range of GHGs, including carbon dioxide, methane and nitrous oxide. 3.4 Integration of Results The urban water system flow pathways approach was developed to integrate the individual component specific GHG emissions into an index for the entire system. Given that there are a wide range of alternative system component configurations through which a hypothetical unit volume of water could pass to supply the various needs of the consumer, it was found that it was necessary to develop a methodology for reporting the GHG emissions associated with an urban water system based on system configuration and intended end use. An example of a flow pathway is illustrated in Figure 3.3 for hot water input to a clotheswasher, where wastewater is used directly for landscape irrigation. Water storage reservoir Centralised treatment Bulk distribution Local distribution Hot water service Clotheswasher Greywater recycling Figure 3.3 Example urban water system flow pathway A GHG emission factor was attached to each system component located on the flow pathway, in terms of GHG emissions per unit volume of throughput water. The model developed during this investigation enabled the analysis of separate flow pathways for each residential end use of water, which made it possible to determine the total operational GHG emissions associated with a particular end use of water, based on the origin of the water, end use related appliance characteristics, and final destination of the subsequently generated wastewater. After the GHG emissions profile for each end use was developed, these profiles could be integrated to determine total system-wide GHG emissions for a particular household, individual, or end use event. 4. Results The GHG emissions associated with operating the end uses of water were determined over a one year period. The results are shown in Figure 4.1 for a household using a gas storage hot water service with an energy star rating index of 3.

6 kg CO2-e per household per year Shower Toilet Dishwasher Clotheswasher Tap Irrigation Wastewater removal and treatment Cold water supply Hot water supply (to hot water service) Heating water Appliance operation Figure 4.1 Annual household GHG emissions associated with each end use of water (continuous energization gas storage hot water service) The total mass of GHG emissions associated with the end uses of water, including those allocated from upstream and downstream activities, was found to be 2320 kg CO 2 -e per household per year. The operation of water supply and wastewater systems was responsible for less than 10% of this total. Note the consistent significance of heating water and/or appliance operation in the emissions profile of each end use that uses hot water and/or requires energy input to operate. To further investigate the patterns observed in the previous example, a MEPS compliant electric storage hot water service using off-peak boost was substituted into the model, without changing the water consumption patterns. The results of this analysis are shown in Figure 4.2. kg CO2-e per household per year Shower Toilet Dishwasher Clotheswasher Tap Irrigation Wastewater removal and treatment Cold water supply Hot water supply (to hot water service) Heating water Appliance operation Figure 4.2 Annual household GHG emissions associated with each end use of water (off-peak energization electric storage hot water service) In this example, the total mass of GHG emissions associated with the end uses of water, including those allocated from upstream and downstream activities, was found to be 7146 kg CO 2 -e per household per year. In this case, the operation of the water supply and wastewater systems was responsible for less than 4% of the total. Note the increased significance of heating water in the emissions profile of each end use that uses hot water. While the electric hot water service used in this example was significantly more energy efficient than the gas powered unit used in the previous example, the GHG emissions associated with electricity consumption in Victoria, and in most of the other states of Australia, are substantially higher than those for natural gas for the same quantity of energy. This is largely a local issue, caused

7 by the heavy dependence on fossil fuels for electricity in Australia, which fuel over 90% of total electricity generation (Commonwealth of Australia, 2005b). To place these results in context, consider that, according to the factors and methods developed by the Australian Greenhouse Office, a petrol powered passenger car, traveling km each year generates approximately 4500 kg CO 2 -e each year (Commonwealth of Australia, 2005a). Given that cars are often presented as one of the most critical targets for the reduction of household GHG emissions, it is interesting to see that, under some circumstances, the water related household GHG emissions can be greater than those associated with the use of a car. 5. Discussion The results presented here have established that the GHG emissions associated with heating water and operating the appliances linked with the residential end uses of water are far greater than those associated with the upstream and downstream activities of urban water utilities. This key finding was identified by extending the system boundary of this analysis beyond the infrastructure managed by water utilities, which has previously been extensively investigated in isolation, and by allocating the upstream and downstream GHG emissions to each individual residential end use of water. This approach to allocating responsibility for GHG emissions was considered to be valid since without consumption, there would be no requirement for production. Hence, from a certain point of view, the end user of a product, water in this case, is at least partially responsible for the upstream and downstream repercussions of their consumption, in addition to any immediate impacts. However, given that water is essential to life, and that the end user does not necessarily have any control over how water is managed upstream and downstream, the water utilities also have some responsibility for the GHG emissions associated with their operations. Similarly to the approach taken to water demand management, the water utilities should provide leadership in managing this joint responsibility for total GHG emissions associated with urban water systems, assisting end users in minimising not only their water consumption, but also the GHG emissions associated with that consumption, which, as shown in this paper, are not necessarily linked activities. Further research, beyond the scope of this paper, will include detailed analysis of the systemwide consequences of both structural and non-structural water demand management strategies, and changing energy supply mixes. The sensitivity of the final results obtained from the model described in this paper to the input data will also be extensively tested. 6. Conclusions In this paper, it has been shown that the GHG emissions associated with operating urban water systems need to be analysed in a holistic manner, including all system components. By extending the system boundary to include the residential end uses of water, and developing a procedure to allocate upstream and downstream GHG emissions to each such end use, it was demonstrated that the GHG emissions associated with the components of urban water systems operated by water utilities are relatively insignificant. It was found that the operation of hot water services, dishwashers and clotheswashers was responsible for 90% to 96% of the total GHG emissions associated with the residential end uses of water. For Victorian households using a natural gas powered storage hot water service, the total mass of GHG emissions associated with the residential end uses of water, including those

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