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CHAPTER 1
Performance and economics of internally plumbed rainwater tanks: An Australian perspective
Rodney Anthony Stewart, Oz Sahin, Raymond Siems, Mohammad Reza Talebpour and Damien Giurco
1.1 INTRODUCTION
Water security is becoming a global issue of concern. In developed nations like Australia, high population growth and strong economic development are increasing demand, while supply is under threat from environmental degradation and climate change. Centralised reservoir and distribution networks have long served major metropolitan centres with potable water supply. However, the capture capacity of traditional supply sources is approaching a limit in many areas, leading to a host of new supply options coming into consideration (WWAP, 2012). Correspondingly, water security is considered as one of the six key risks in Australia under a changing climate. The Intergovernmental Panel on Climate Change asserts that climate change will lead to a reduction in water supply for irrigation, cities, industry and riverine environments in those areas where stream flow is expected to decline (for example in the Murray-Darling Basin, Australia) and annual mean flow may drop 10 to 25% by 2050 and 16 to 48% by 2100 (Hennessy et al. 2007).
Rainwater tank systems, collecting and distributing water at a decentralised level, are one potential solution to assist in bridging supply-demand gaps. The basic principle of these decentralised systems is the capture of precipitation collected from the available roof area, which flows by gravity into a storage tank, where it can serve demand for water end-uses. Historically, internally plumbed rainwater tanks (IPRWTs), serving water end-uses inside the house, have only been prevalent in rural areas in the absence of centralised supply infrastructure. In the last 10 to 20 years, amid new concerns over water security, a variety of water businesses, governments and other stakeholders have been advocating the use of IPRWTs in urban areas. However, almost universally these systems have been recommended and implemented without a proper understanding of their underlying viability and performance. In an urban setting, there are a multitude of alternative water supply options and any chosen supply system must be both competitive and sustainable.
This chapter details an investigation into the economics and performance of IPRWTs conducted in Australia's South-east Queensland (SEQ) region and examines these findings in an international context. The study utilises a combination of modelling and empirical data to generate a range of unit life cycle costs (LCC) under different scenarios and conducts a sensitivity analysis on pertinent variables.
1.2 BACKGROUND
The practice of rainwater harvesting (RWH) can be traced back at least 4000 years BC (Gould and Nissen-Peterson, 1999; Mays et al. 2007), with systems employing cisterns fed with rainwater attached to single households in ancient civilisations such as Jordan, Rome, Greece and Asia. In more modern times, they have primarily been used in the rural domain where the construction of centralised infrastructure was not feasible. In the new age of sustainability, RWH has enjoyed something of a renaissance; systems have again penetrated into cities where the bulk of the world's population resides. In excess of 100,000,000 people worldwide are estimated to be using a RWH system of some form (Heggen, 2000).
RWH systems can be separated into a number of subcategories based on how they are configured. They may be communal, whereby a number of residences are connected to a tank that is fed from a large roof area, or installed on an individual basis to stand-alone households. Many systems only supply outdoor uses such as garden irrigation and pools, while the popular trend recently has been to internally plumb systems to supply a range of in-home end-uses to maximise savings (via substitution) from centralised sources. The advent of modern appliances requires that the water supply to the house be pressurised. Therefore, the vast majority of IPRWTs contain a pump that can extract water from tanks and deliver it under pressure to the house. These pumps may operate at different levels based on a flow rate or be single speed. More complex pressure vessel setups may also be employed. Switch systems that allow end-uses to be supplied by either the tank or central mains supply are commonplace, so that when a tank is empty essential supply is maintained. This chapter focuses on typical IPRWTs installed on single detached residential households configured with single speed pump and switch systems supplying water for toilets, clothes washers and external use.
There are many purported benefits of RWH and the herein focused upon contemporary IPRWT systems; the predominant benefit being a reduction in urban water demand. For residents, this can offer reduced water bills and decreased reliance on mains supplies. For communities and governments, this can delay the need for centralised infrastructure upgrades and reduce peak stormwater volumes (Coombes et al. 2003). By decreasing the amount of water required from central supplies, RWH can also assist in raising groundwater levels; an urgent task in many urban locations. The major negatives associated with RWH arise from a lack of reliable supply and potentially poor water quality; both of which can be circumvented with the right system setup in the presence of a backup or mains supply.
1.2.1 IPRWT systems in Australia
IPRWT systems have been utilised for generations in rural Australia (EHAA, 1999; Marsden Jacob Associates, 2007). Deployment in urban areas was widely discouraged for many years with a number of local governments banning rainwater tanks in the 1960s, citing water quality as a prohibitive hazard (White, 2009). A severe drought that ran from 2000 until 2009 affected large portions of south-eastern and south-western Australia (CSIRO, 2011), leading to critical depletion of freshwater reservoirs. This triggered the introduction of legislation and Government-backed incentives to install IPRWTs in urban households. They were championed as 'green' and 'sustainable' solutions to the water security crisis, with limited research available to verify such notions at the time.
As of 2007, about 20% of Australian households had some form of RWH system (ABS, 2007). Retamal et al. (2009) provide a comprehensive description of a range of IPRWT configurations in Australia and their advantages and disadvantages. The majority of residential dwellings being constructed use fixed speed pumps with potable switch systems or tank top-up systems. The more elaborate and efficient designs, incorporating pressure vessels and variable speed pumps, are rarely considered by house builders as they are predominantly concerned with satisfying mandated building code requirements at least capital cost (in locations where IPRWTs are mandated). The IPRWTs examined in this study were mandated by the Queensland Government to be installed in new houses built or those substantially renovated.
1.2.2 RWH and IPRWTs around the globe
RWH in one form or another is practiced very widely around the globe. Two purpose driven groups can be considered: those that are using rainwater as a supplement to already existing water supply systems and those using rainwater as basic supply (König & Sperfeld, 2006). IPRWTs similar to those examined in Australia's SEQ require a certain socioeconomic level to be present in homes. Some of the nations with widespread IPRWTs are listed below. It should be noted that in Australian literature the distinction between RWH and IPRWTs is explicit, while in much of the international literature this is not the case.
• Germany: Regarded as a leader in IPRWT technology, some 35% of new buildings are installed with a RWH system (EA, 2010). Germany has groundwater over abstraction problems in many regions and RWH systems have been promoted through legislation and incentives as a means to reduce this issue (Herrmann & Schmida, 2000). 1.5 million systems are estimated to be supplying toilet flushing, clothes washers (washing machines) and garden irrigation (Galbraith, 2012).
• United Kingdom: RWH was a traditional water source before central mains supply became widespread. Modern RWH systems have only been introduced recently. Adoption is supported and encouraged by the Code for Sustainable Homes under which all new houses must have a rating of 3, with IPRWT installation a means of raising this score. The UK Rainwater Harvesting Association (2006) reports that approximately 4000 RWH systems are installed in the UK each year with approximately 100,000 already in existence. These systems are commonly internally plumbed to supply toilet flushing as well as garden irrigation (EA, 2010).
• Malaysia: Introduced after the 1998 drought, rainwater use is encouraged for domestic purposes under Water Services Industry legislation (Shaari et al. 2009).
• Sri Lanka: RWH was initially popular rurally and is now also promoted in cities through the country's Urban Development Authority (2007).
• China: Gansu province began research and implementation, with 17 provinces now adopting RWH. Over 5.6 million tanks supply potable water to 15 million people (UNEP, 2001).
• Bermuda: Mandated by law for all buildings, rainwater is the primary source of domestic water (Rowe, 2011).
Trends around the world appear similar, with urban penetration increasing with advocacy from governments. König and Sperfeld (2006) noted that amortisation (pay back) of IPRWTs increases with the cost of mains water, therefore those nations with the highest cost of mains water are typically the highest adopters of IPRWTs technology. In terms of the cost and effectiveness of IPRWTs, regardless of location or configuration, there are a number of factors that determine the cost and effectiveness of IPRWTs, which are summarised in Table 1.1. Any location will have its own make-up of these variables. However, the relationships that govern many of these variables will be very similar between locations. A well-documented investigation conducted in one area can provide insight into the performance and economics of IPRWTs on a wider scale. This is undertaken in the following sections in an Australian context.
1.3 AUSTRALIAN CASE STUDY
The study presented in this chapter identified that Australian water businesses have been implementing a range of alternative water supply schemes, in an attempt to conserve centralised supplies of potable water. However, they undertook such schemes with only best guess potable savings figures and alternative source demand values to serve as justification. Seeking a more rigorous assessment process, the present study followed an evidence-based approach whereby the water consumption of IPRWTs was monitored through end-use studies and costs were evaluated using actual cost and performance data. The end goal of this assessment process was to arrive at an accurate total resource perspective unit cost ($/m3) for IPRWTs in order to better inform decision-making regarding their use. The IPRWT performance and economic analysis was completed alongside evaluations of three other alternative supply schemes, including desalination and recycled water. Readers are referred to Stewart (2011) if they seek information on the latter two schemes.
1.3.1 Context of investigation
In 2007, the Queensland state government introduced new legislation, namely the Queensland Development Code Mandatory Part 4.2 (QDC). This stipulated that all detached residential households needed to achieve potable water savings (DIP, 2009). Under this legislation, water savings targets are mandated for new detached houses in Queensland, ranging from 16 to 70 m3 per household per year (m3/hh/y), depending on the local government area. The widely accepted solution to reduce potable water use was through the installation of a 5 m3 polymer rain tank plumbed to the toilet, laundry and external taps of detached, single residential households. A minimum of 100 m2 of roof area must divert rainwater into the tank. Internal fixtures supplied from a rain tank are required to have a backup supply of potable water using a trickle top-up or automatic switching system. Gardiner (2009) notes that, of more than 300,000 tanks in SEQ, about 30,000 were installed under the QDC. Inspections revealed that in most cases house builders chose the least cost IPRWTs with a single speed pump and switch system. Three successive wet years in SEQ saw reservoirs return to capacity and pressure on water supply decrease. Consequently, the Queensland State Government removed the requirement for new houses to have IPRWT from late 2012 due to a number of reasons. These included the need to recoup the construction cost of bulk water infrastructure (such as desalination) constructed during the drought, reduced water consumption due to behaviour change and housing affordability.
1.3.2 Data gathering and end-use study experimental procedure
Data gathering was conducted to inform modelling and the LCC analysis. Eighty-seven (n = 87) Gold Coast City (GCC) detached households (a single dwelling on a single lot) without IPRWTs were sampled during two cross-sectional periods during 2010. This case serves as the business-as-usual water supply scheme for the purposes of this study and is used for baseline potable water savings comparisons. The sample provides a reasonable representation of household types with a strong mix of family types, income categories and household occupancies.
High-resolution smart metering equipment was employed to enable the collection of water consumption data and subsequent end-use analysis. The relationship between smart metering equipment, household stock inventory surveys and flow trace analysis is shown in Figure 1.1. Essentially, a mixed-method approach was used to obtain and analyse water-use data. Two aligned main processes were adopted: (1) physical measurement of water use via smart meters with subsequent remote transfer of high-resolution data; and (2) documentation of water-use behaviours and compilation of water appliance stock via individual household audits and self-reported water-use diaries.
1.3.2.1 Instrumentation
Standard local government residential water meters were replaced with high resolution water meters. These meters measured flow to a resolution of 72 pulses per litre, or one pulse every 0.014 litres. The smart meters were connected to data loggers programmed to record pulse counts at 5-second intervals. Each logger was wired to a meter, labelled and activated prior to installation to reduce reliance on plumbing contractors to prepare and activate the equipment; all equipment was installed by approved plumbing contractors.
1.3.2.2 Data transfer and storage
As the loggers were wireless, data was transferred remotely to a server at Griffith University through a General Packet Radio Service (GPRS) network (such as a 2G or 3G phone network) via email. Removable SIM cards were inserted in each logger and tested prior to installation. The data was transferred weekly, creating approximately 120,000 data records, sent to email addresses before being downloaded and processed. Raw data files in the ASCII format were modified to .txt files for flow trace analysis.
1.3.2.3 End-use analysis process
End-use data in the .txt file format were analysed using Trace Wizard version 4.1 (Aquacraft, 1997). Water diaries and stock appliance audits were used to help identify flow trace patterns for each household. A template was created for each household and data for a sampled 2-week period were analysed. Trace Wizard was employed in conjunction with water audits and diaries to analyse and disaggregate consumption into a number of end-uses, including toilets, irrigation, showers, clothes washers and taps. A Microsoft Excel spread sheet was utilised as a final output for more detailed statistical trend analysis and chart production.
1.3.2.4 End-use results summary
There was a notable difference in irrigation between the two seasonal periods monitored. Winter 2010 irrigation end-use was 9.4 litres per person per day (lpd), representing only 7% of total consumption. This was less than half of the 21.9 lpd recorded in summer 2010, supporting historical bulk reading data that irrigation in GCC is greater during summer. The average sampled total per capita residential consumption value of 156.5 lpd was very close to the Queensland Water
Commission (2009) reported SEQ monthly per capita residential consumption average for the 2010 period (140–160 lpd). This indicated that the end-use results were representative and useful for comparisons. A summary of the summer and winter 2010 end-use breakdown for the single detached, potable-only reticulated scheme end-use values is presented in Table 1.3. Readers are referred to Stewart (2011) and Beal et al. (2011) for a full description of the end-use data used in this current study. This sample of potable-only homes situated on the Gold Coast is used for comparison with the potable plus IPRWT supplied households discussed below.
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