data analysis and results

The average rainwater consumption per household (which equates to mains water savings) was 36.1 kL/

hh for a total period of the 11 months during which a complete data set was available over the 12-month monitoring period (Umapathi et al. 2012). The total water use per household (combination of rainwater and mains water) was 136 kL/hh. Scaling up the data to a 12-month period gave an average rainwater use of 40 kL/hh/yr and a total water demand of 151 kL/hh/yr (Figure 4.12). As shown in Figure 4.12, the rainwater consumption in the households varied between 0.6 to 69 kL/hh/yr and the total water consumption from 50 to 398 kL/hh/yr. The validity of the monitored potable water use data was verified

through cross-correlation with the water billing records obtained from the local water utility. The correlation coefficient was 0.98.

Figure 4.12 Partitioning of the 12 month consumption for 20 monitored homes in SEQ. Data includes total potable, total rainwater, and mains top-up to the tanks. Source: (Umapathi et al. 2012).

The results obtained are in contrast with findings from desktop studies (Chong et  al. 2011) of 691 households using rainwater tanks under very similar circumstances in South East Queensland, which identified an average annual mains water saving of 58 kL/hh/yr and 59 kL/hh/yr for 2009 and 2010 respectively. It may be due to the small sample size of 20 homes. Although the study represented a smaller sample, the results obtained gave a better indication of the various factors that can be involved in influencing the water consumption patterns in everyday households. These factors were: differences in per capita water usage; effects of locally imposed water use restrictions; differences in external water use (irrigation) habits; and variations in local rainfall which varied substantially between households that were located only a few kilometres apart.

The rainwater supply as a percentage of total household water demand for 20 homes is shown in Figure 4.13. The average overall rainwater supply based on the 20 households was 31% and the contribution of rainwater top-up (backup supply) was 14% of total household water usage (Umapathi et al. 2012). The results suggests that physical factors such as rooftop collection areas and rainwater tank sizes have a direct impact on the mains water offset, and hence the rainwater reliability of the rainwater in the system. The rainwater tank system could be better designed to reduce backup supply (from 14% of total demand) by either increasing the tank sizes, or the rainwater catchment areas, or a combination of both.

Figure 4.14 depicts the household rainwater demand on the tank for intended use and the corresponding supply of rainwater from the tank. Household rainwater tank systems on, or close to, the optimum demand-supply line demonstrate high volumetric reliability. The rainwater demand was almost completely satisfied by rainwater supply in 6 households as shown by their proximity to the water demand-supply

line in Figure 4.14 also corresponding to very low mains water top-up for these households, as shown in Figure 4.12. The homes farthest from the optimum demand-supply line were the least rainwater sufficient.

Figure 4.13 Rainwater reliability of rainwater tank systems at 20 monitored homes.

Figure 4.14 Rainwater demand and rainwater supply of 20 households.

Figure 4.15 shows the hourly diurnal water demand pattern for one of the 20 studied households using data recorded over a 4-month period. The graph shows the average hourly water demand of 29.1 L/day, with average peak water usage of 61.4 L/day. The peaking water use factor (ratio of average peak demand divided by average hourly demand) is 2.1, which is low compared to the peak factor of 5 suggested for the design of water supply network systems for developments with a population below 2000 (Swamee &

Sharma, 2008). This may be due to a very low external water demand for garden irrigation recorded during the study period. Figure 4.15 also shows the diurnal pattern water supplied from the mains for top-up to the rain tank (MIRW), and the water supply from rainwater tank (TRW).

Figure 4.15 Diurnal water use pattern averaged over 4 months for one of the 20 households in the South East Queensland monitoring study.

On the energy front, the average specific energy (SE) for the pumping systems at 19 homes was 1.52 kWh/kL (one home was omitted due to a faulty pump). The average SE for homes with trickle top-up system was slightly higher than those with automatic switching devices, at 1.59 kWh/kL and 1.46 kWh/

kL respectively. In the case of the trickle top-up systems, the total water (rainwater + mains water top-up) supplied from the tank would require pumping. In comparison, other alternative potable water supplies such as indirect potable reuse require more than 2.8 kWh/kL and energy required to make freshwater from seawater by desalination is around 3.5 kWh/kL. Thus, rainwater is the least energy intensive alternative water source compared to desalination and indirect potable reuse. Traditional potable water supply from catchment reservoirs, which is often gravity assisted, is less energy intensive than rainwater supply, requiring less than 0.9 kWh/kL (Tjandraatmadja, 2012).

Further information on the performance of pumping systems and the energy use associated with rainwater tank systems is detailed in Chapter 6.

4.7 cAse study 2: sydney wAter, sydney, new south wAles, AustrAlIA

The New South Wales Government introduced BASIX (the Building Sustainability Index) policy in 2004, which required all single and multi-unit residential buildings to be designed to use less potable water and emit fewer greenhouse gases (NSW Department of Planning, 2008). BASIX set the reduction targets at

40% less for potable water use and 40% fewer greenhouse gas emissions than the average NSW dwelling (Ferguson, 2011). The main objective of the 12-month study conducted by Sydney Water in 2011 was to confirm the water savings being achieved for 52 newly built BASIX compliant households spread broadly across the Sydney basin to capture the variation in climatic range (Ferguson, 2011). The study also aimed at identifying opportunities for improving the water saving capacity and reducing the pumping energy use at the rainwater tank.

The metering arrangement was setup to monitor: 1) the water supply from the rainwater system for connected non-potable end uses; 2) mains water demand for the top-up system; and 3) total mains water use in the household (including top-up), and energy demand from the rainwater tank pumping system. The instrumentation setup was similar to that shown in Figure 4.11. The monitoring set-up used in this study was adopted in the South East Queensland study by Umapathi et al. (2013), which additionally monitored the external water use for garden irrigation. All water flow data were logged in one-minute intervals using meters that generated 0.5 L/pulse.

4.7.1 data analysis and results

Using a sample size of 40 detached households out of 52 monitored homes, Ferguson (2011) found that the total household water demand ranged from 84 to 556 kL/yr, with a mean demand of 197 kL/yr. These results were shown to coincide closely with the potable demands for new homes built under BASIX regulations. The water demand from the rain tanks ranged from 5 to 161 kL/year with a mean demand of 59 kL/yr.

The water savings achieved from rainwater use in 46 households with complete data ranged from 0 to 96 kL/yr (Figure 4.16), with a mean savings of 38 kL/yr (median of 39 kL/yr). Hence, although the demand for water from the rainwater tank was on average of 30% of the total household water demand, only 19% was met by rainwater from the tanks. The difference was supplied by mains water top-up.

Figure 4.16 Water savings from rainwater tanks in 46 monitored households in the Sydney study.

The study found that the connected roof area (rain catchment area) has a major impact on the volumes of rainwater collected in the tanks. A case study on a single home with a 5 kL rainwater tank and 140 m² of connected roof area found that the demand for rainwater was 161 kL/yr out of a total household demand of 556 kL/yr (Ferguson, 2011). Modelling using household specific data with local rainfall showed that

the water savings (i.e., rainwater yield) could be increased by 12 kL to 81 kL/year by increasing the roof collection area to 210 m².

Modelling assessment of another single household indicated the underperformance of the rainwater tank system that achieved a savings of 28 kL per year, compared with a potential of 45 kL per year. This could be attributed to an unusually high cut-in level in the tank for activation of the backup system, thus leaving less capacity in tank for captured rainwater. Hence, notwithstanding the system having adequate roof catchment area and storage capacity, there is a need to have a clear understanding of settings for the other system components which are determined at the time of tank installation.

The 12-month study period also allowed researchers to assess the seasonality of water use in the households. Water demand for indoor end uses such as toilets and washing machines was found to be constant throughout the year in contrast to outdoor end uses that showed seasonal fluctuations (water demand was higher in summer season than the winter season). The study also found that the water demand between November 2009 and January 2010 (summer months in Australia) appeared to be met by both potable (mains water) and non-potable (rainwater) water sources.

A major part of the study focussed on linking the water use from the rainwater tank systems with the energy consumed in supplying the assigned end uses. Energy use by rainwater pumps was found to be an average of 78 kWh/hh/yr per household per year, with the median energy intensity of 1.48 kWh/kL.

The median active pumping energy (energy used for pumping water only) intensity was 1.42 kWh/kL.

The dormant (stand-by) energy use was a negligible 4 kWh/year. Due to the detailed nature of the data obtained, significant uses of dormant energy (up to 8 Wh/minute) were identified in some households which warranted further investigation as they suggest faulty pumps or leaks in the rainwater system.

Results also showed that most household water uses had low flow rates, with over 75% of the household uses being 10 L/minute or less and 50% were 6 L/minute or less. The maximum water flow rates measured ranged from 10 to 22 L/minute. However, some of the installed pumps (more specifically, submersible pumps) were designed for much higher flow rates, which suggest they were oversized for the task and hence used more energy than needed to provide the acceptable level of service.

The majority of water use events were between 4.5 to 9 L per event, which were most likely full-flush toilet events. Flow events between 50 and 90 L accounted for 15% of the demand and were most likely washing machine end uses, whereas large water use events (>99 L) that contributed about 20% of the total demand may have been associated with some (top loading) washing machines and garden uses. The study highlighted the significance of choosing the correct pump capacity (kW) matched to the flow rates of the connected end uses. The study also suggested the positive benefits of connecting large volume, high flow rate end uses, such as garden taps, to rainwater tanks to ensure effective use of tank storage (adequate storage space preceding rainfall events) and efficient pumping operation. These aspects in details are also covered in more detail in Chapter 6.

4.8 other cost consIderAtIons AssocIAted wIth monItorIng

Apart from the obvious costs involved in the procurement of monitoring equipment such as loggers, meters, cables and so on, there are some other factors that affect the overall cost of the monitoring, and these should also be considered in the contingencies for planning the study.

Participant recruitment – This can be done by a contractor or the water utility with a mail out to selected properties known to have rainwater tanks. The records of rainwater tanks are varied, depending on the location. In the Australian state of New South Wales (NSW), the Department of Planning holds records on BASIX certification and details on size and end use for rainwater systems. Councils and utilities may also have records of tank installations as a result of rebates.

Installation scheduling – The installation process requires careful coordination with participant homeowners. Maps are used to determine the location of households, the time required to install equipment, and then the travel time to the next location.

Other factors – Other issues can arise during the installation process which adds to the time and cost of the project, these include:

• Rescheduling: Occasional delays in installation of equipment may be expected due to unforeseen circumstances which may prolong installation time periods.

• Damage or failure of equipment: Equipment, particularly for mains meter monitoring, can be damaged by vandals, lawn mowers and even cars throughout the project. These faulty units need to be replaced as soon as practicable to maintain dataflow.

• Mobile phone reception: Some areas have insufficient mobile phone reception to regularly upload the data, requiring site visits for manual downloads.

• Inaccessible or inappropriate systems: There have been cases where the plumbing required to install the meters on the inlets and outlet lines cannot be accessed because the pipes are located within walls or covered by other materials, or the pipe fittings were of a particular material unavailable to the installer. There are about 15 different pipe types (in Australia) with different fittings, which need different crimping tools. In many cases, it was easier to move on to another house than purchase a new set of tools and fittings for just the one installation.

4.9 conclusIon

The increasing integration of rainwater tanks and any alternative water supply technologies is expected to have an impact on the future demands of centralised systems. Therefore, the need to validate and assess the influence of these alternative systems on existing centralised infrastructure is important. Monitoring the rainwater tanks will help determine the reliability of these systems and enable better planning by policy makers. Studying the rainwater use patterns within households enables governments and utility planners to develop suitable guidelines for installing rainwater tanks to reduce the reliance on potable water.

Nonetheless, monitoring is a comprehensive process. Monitoring programmes may face a range of setbacks, from exorbitant costs to the inability to obtain statistical significance in analytical outcomes due to variations in the samples, or simply the failure of all or part of the monitoring equipment. The chapter has also briefly discussed an emerging trend in instrumentation available for water consumption assessments.

Experimental and analytical methodologies, and also issues associated with monitoring based assessments have been discussed using studies conducted in a few major cities across Australia as examples. The monitoring studies have identified a gap between modelled and monitored data on household rainwater usage, providing valuable information for water planners for developing regional strategic water plans. The main conclusions are summarised below:

• Technology allows sub-hourly monitoring of water use, tank water supply and energy consumption for investigating the rainwater usage, diurnal patters of water supplied by various sources and specific energy consumption in rainwater supply.

• High cost of installation (approximately $5000 per house) limits the number of monitoring sites.

Hence, these studies are generally conducted at pilot scale and the data collected may not be representative of suburb-scale tanks behaviour.

• Technology supports rich data collection of rainwater supply, which can be done remotely.

However, this can be labour intensive and more time consuming if the study sites are wide

spread. Daily or sub-daily data collection will also be impossible, thus limiting the scope of the study.

• Rainwater tank monitoring allows checking on compliance with government policies and building codes, for example, the Queensland Development Code and BASIX. The South East Queensland and the Sydney Water studies both showed significant rainwater supply undershoot (40 kL/hh/yr and 38 kL/hh/yr respectively).

• Water flow data collected at hourly, monthly or even daily time-steps (intervals) are sufficient if the objective of the research is to quantify the gross water consumption in households. However, smaller sub-hourly intervals, such as one-minute intervals (or lower), help in understanding the finer details associated with end use consumption characteristics and can provide insights into leakage occurrence, malfunctioning of monitoring equipment or of the end-use appliances, as well in the design of reticulation pipe sizing in new suburbs with mandated tanks. Nevertheless, analysis of fine resolution data can be cumbersome and thus the selection of the time interval should be based on the study objectives.

• One minute monitoring intervals were chosen in this case study to avoid problems attributed to larger time intervals, however study indicated that 6-minute monitoring intervals will also be suitable for understanding diurnal flow patterns.

• Tank water level monitoring provides rain catch figures on an event basis, which can be compared with modelled rain capture information data. Very few studies of this type are reported in the literature.

4.10 reFerences

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Beal C., Sharma A., Gardner T. and Chong M. (2012). A desktop analysis of potable water savings from internally plumbed rainwater tanks in South-East Queensland, Australia. Water Resources Management, 26(6), 1577–1590.

Beal C., Stewart R A., Huang T. and Rey E. (2011). SEQ residential end use study. Smart Water Systems and Metering, Journal of the Australian Water Association, 38(1), 80–84.

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Chong M. N., Umapathi S., Mankad A., Gardner E. A., Sharma A. and Biermann S. (2011). Estimating water savings from mandated rainwater tanks in South East Queensland. In: Begbie D. K. and Wakem S. L. (eds) (2011) Science Forum and Stakeholder Engagement Building linkages, Collaboration and Science Quality, Urban Water Security Research Alliance 14–15 September, Brisbane, Queensland, pp. 31–34, http://www.urbanwateralliance.

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Chong M. N., Sharma A., Umapathi S. and Cook S. (2012). Understanding the Mains Water Saving from Mandated Rainwater Tanks using Water Balance Modelling and Analysis with Inputs from On-Site Audited Parameters.

Urban Water Security Research Alliance Technical Report No. 65, Urban Water Security Research Alliance, Available: http://www.urbanwateralliance.org.au/publications/technicalreports/ (accessed August 2014).

Chong M. N., Sharma A. K., Umapathi S. and Cook S. (2014). Performance of Plumbed Rainwater Tanks in Urban Australia: Understanding the potential mains water saving using integrated water balance modelling, regression and response surface analysis. Manuscript submitted.

Chowdhury R. (2013). A stochastic model of domestic water consumption and greywater generation in the Al Ain city. In: 20th International Congress on Modelling and Simulation, Adelaide, Australia, 1–6 December 2013.

Cook S., Sharma A. and Chong M. (2013). Performance analysis of a communal residential rainwater system for potable supply: A case study in Brisbane, Australia. Water Resources Management, 27(14), 4865–4876.

Coombes P. J. and Kuczera G. (2003). Analysis of the Performance of Rainwater Tanks in Australian Capital Cities.

In: Paper Presented to the 28th International Hydrology and Water Resources Symposium, 10–14 November 2003, Wollongong, New South Wales, Engineers Australia.

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Davison G. (2008). Down the gurgler: historical influences on Australian domestic water consumption. In: Troubled Waters: Confronting the Water Crisis in Australia’s Cities, P. Troy (ed.). Australian National University Press, Canberra, Australia, pp. 37–65.

DSITIA (2013). SILO Climate Data. The State of Queensland Department of Science, Information Technology, Innovation and the Arts.

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Canberra, University of Technology Sydney.

Canberra, University of Technology Sydney.

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