**3. A case study in Sydney Australia**

164 Urban Development

Fig. 4. Whole life cycle (WLC) cost breakdown as an output from the RWTAM program

Fig. 5. Breakdown of capital cost as an output from the RWTAM program

The various steps involved in running the RWTAM are as follows. (a) Install the program in a local directory called 'Rain Water' (b) Open the program by double clicking on the 'Rainwater Tank' icon. (c) Open the rainwater data by going to the 'File' icon and selecting An example is presented here to illustrate the application of the RWTAM. For this example, a hypothetical multi-storey building is considered, located in the Botany Bay Council area in Sydney, Australia. Daily rainfall records over 60 years (Jan 1946 to Dec 2005) from Sydney Airport station are used. A 75 kL tank size is selected for the purpose of this case study. Two different site areas are considered: 2000 m2 and 4000 m2 with roof areas of 800 m2 and 1600 m2 respectively. For each of these two site areas, three different floor arrangements are considered assuming four apartments per floor and 3 persons per apartment: (a) 4 floors consisting of 16 apartments having 48 persons (b) 6 floors with 24 apartments having 72 persons (c) 8 floors with 32 apartments having 96 persons. In the life cycle cost analysis (LCCA), it is assumed that the rainwater harvesting system has a life of 60 years.

According to the Building Sustainability Index (referred to as BASIX) for multi-unit buildings, all new houses in the state of New South Wales (NSW) must save at least 40% of potable water as compared to an average traditional non-BASIX house (NSW Department of Planning, 2005). This involves rainwater harvesting, the use of various water efficient appliances in the apartment such as 4A rated washing machines and dishwashers, 3A rated dual flush toilets (the higher the A-rating, the more water efficient the device is), water efficient shower heads and taps and native, low-water-use plants. Both BASIX and non-BASIX (i.e. traditional) approaches with rainwater harvesting systems are examined in this case study. It is assumed that rainwater is used for toilet flushing and laundry (indoor water use) and irrigation (garden and lawn); the relevant water demand data is obtained from Sydney Water Corporation.

In the water balance model, the effective runoff is generated by calculating the precipitation minus the losses which are the runoff coefficient and first flush losses. A plot of the annual precipitation is shown in Figure 6 which shows a notable variability of total rainfall from year to year and also a drop in total rainfall values from 1991 to 2005. However, annual total rainfall values may not have direct influence on the water yield of rainwater tanks which is mainly governed by distribution of rainfall events in a year and magnitude of rainfall events. For example, if the event rainfall is too high, most of the runoff will leave the rainwater tank as overflow as the tank would overflow very quickly.

The building area or the catchment area is assumed to be 40% of the total site area, which forms the tank footprint. The loss arises from gutter overflow, evaporation and first flush. It is assumed here that one litre of water is diverted to first flush per square metre of roof area. Once the first flush device is full, the remaining rainwater is diverted to the rainwater tank. Therefore the first flush losses are 800 litres for the 800m2 roof and 1,600 litres for the 1,600m2 roof. The total losses as a component of the runoff for each of the roof areas are presented in Figures 7 and 8. It can be seen in Figures 7 and 8 that the total losses increase

Rainwater Harvesting in Large Residential Buildings in Australia 167

0.00 500.00 1000.00 1500.00 2000.00 2500.00 3000.00 3500.00

into the tank.

**Runoff (kL)**

1946

1950

Fig. 8. Loss and runoff (4,000m2 site area)

1954

1958

seen that the runoff is directly proportional to the rainfall.

the water demand once for the eight-floor scenario.

six-floor and eight-floor scenarios require mains top-up every year.

1962

1966

1970

Figure 9 illustrates the difference between the effective runoff generated from the two roof areas, with the 1,600m2 roof generating twice the runoff of the 800m2 roof area. It can also be

In order to assess how much water is available per year in relation to the water demand, four graphs representing different scenarios are presented in Figures 10 to 13. The water available is the effective runoff entering the tank minus the overflows. It can be seen in Figure 10 that the water available (or the net water entering the tank) exceeds the water demand for more than half of the years out of the sixty years analysed. This does not mean that no mains top-up is required as the rainfall can happen in large storm events resulting in greater tank overflow. With an increased water demand relating to the six-floor scenario, the water available only exceeds the water demand for a few of the sixty years and only exceeds

As the water demand keeps on increasing, the water available cannot keep up and mains top-up is required. Ironically, the higher water demand means that more mains top-up needs to be used which results in higher water savings. It can be seen from Figure 11 that the water availability exceeds the water demand only once for the four-floor scenario. The

It can be seen in Figure 12 that with the larger site area (i.e. 4,000m2), despite the increased irrigation demand, the water availability far exceeds the water demand for the majority of the years for the four and six floor scenarios. In fact, the water availability exceeds the water demand for all but two years for the four floor scenario. Only twelve years miss out for the six floor scenario and about half the years for the eight floor scenario. Figure 12 shows the advantage of having a larger roof area to capture a greater amount of rainfall

1974

**Year**

Losses Net Runoff

1978

1982

1986

1990

1994

1998

2002

with the runoff generated. For this scenario, the loss generated from the 800m2 roof is exactly half of that generated by the 1,600m2 roof. The runoff coefficient is assumed to remain constant throughout the life cycle analysis period, which is assumed to be 60 years.

Fig. 6. Variability in annual precipitation values from 1946 to 2005 at Botany Bay (Sydney)

Fig. 7. Loss and runoff (2,000m2 site area)

with the runoff generated. For this scenario, the loss generated from the 800m2 roof is exactly half of that generated by the 1,600m2 roof. The runoff coefficient is assumed to remain constant throughout the life cycle analysis period, which is assumed to be 60 years.

0.00

0.00 200.00 400.00 600.00 800.00 1000.00 1200.00 1400.00 1600.00 1800.00

**Runoff (kL)**

1946

1950

Fig. 7. Loss and runoff (2,000m2 site area)

1954

1958

1962

1966

1970

1974

**Year**

Losses Net Runoff

1978

1982

1986

1990

1994

1998

2002

1946

1950

1954

1958

1962

1966

1970

Fig. 6. Variability in annual precipitation values from 1946 to 2005 at Botany Bay (Sydney)

1974

**Year**

1978

1982

1986

1990

1994

1998

2002

500.00

1000.00

**Precipitation (mm)**

1500.00

2000.00

2500.00

Fig. 8. Loss and runoff (4,000m2 site area)

Figure 9 illustrates the difference between the effective runoff generated from the two roof areas, with the 1,600m2 roof generating twice the runoff of the 800m2 roof area. It can also be seen that the runoff is directly proportional to the rainfall.

In order to assess how much water is available per year in relation to the water demand, four graphs representing different scenarios are presented in Figures 10 to 13. The water available is the effective runoff entering the tank minus the overflows. It can be seen in Figure 10 that the water available (or the net water entering the tank) exceeds the water demand for more than half of the years out of the sixty years analysed. This does not mean that no mains top-up is required as the rainfall can happen in large storm events resulting in greater tank overflow. With an increased water demand relating to the six-floor scenario, the water available only exceeds the water demand for a few of the sixty years and only exceeds the water demand once for the eight-floor scenario.

As the water demand keeps on increasing, the water available cannot keep up and mains top-up is required. Ironically, the higher water demand means that more mains top-up needs to be used which results in higher water savings. It can be seen from Figure 11 that the water availability exceeds the water demand only once for the four-floor scenario. The six-floor and eight-floor scenarios require mains top-up every year.

It can be seen in Figure 12 that with the larger site area (i.e. 4,000m2), despite the increased irrigation demand, the water availability far exceeds the water demand for the majority of the years for the four and six floor scenarios. In fact, the water availability exceeds the water demand for all but two years for the four floor scenario. Only twelve years miss out for the six floor scenario and about half the years for the eight floor scenario. Figure 12 shows the advantage of having a larger roof area to capture a greater amount of rainfall into the tank.

Rainwater Harvesting in Large Residential Buildings in Australia 169

0.00

0.00

1946

1950

1954

1958

1962

1966

Fig. 12. Water entering tank vs. water demand (BASIX and 4,000m2 site area)

demand only twice for the six floor scenario and none for the eight floor scenario.

1970

1974

Figure 13 shows that as the water demand increases, the water availability is unable to meet the demand. Despite the larger roof area, the water availability exceeds demand for about eighteen of the total sixty years for the four-floor scenario. The water availability exceeds

1978

4 Floors 6 Floors 8 Floors Net Water Entering Tank

1982

1986

1990

1994

1998

2002

500.00

1000.00

1500.00

2000.00

2500.00

1946

1950

1954

1958

1962

1966

1970

Fig. 11. Water entering tank vs. water demand (Non-BASIX and 2,000m2 site area)

1974

1978

4 Floors 6 Floors 8 Floors Net Water Entering Tank

1982

1986

1990

1994

1998

2002

500.00

1000.00

1500.00

2000.00

2500.00

Fig. 9. Comparison of effective runoff for two roof areas

Fig. 10. Water entering tank vs. water demand (BASIX and 2,000m2 site area)

0.00

0.00

1946

1950

1954

1958

1962

1966

Fig. 10. Water entering tank vs. water demand (BASIX and 2,000m2 site area)

1970

1974

1978

4 Floors 6 Floors 8 Floors Net Water Entering Tank

1982

1986

1990

1994

1998

2002

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1946

1950

1954

1958

Fig. 9. Comparison of effective runoff for two roof areas

1962

1966

1970

1974

**Year**

1978

Rainfall 2000 sqm 4000 sqm

1982

1986

1990

1994

1998

2002

0.00

500.00

1000.00

1500.00

**Runoff (kL)**

2000.00

2500.00

3000.00

500.00

1000.00

**Precipitation (mm)**

1500.00

2000.00

2500.00

Fig. 11. Water entering tank vs. water demand (Non-BASIX and 2,000m2 site area)

Fig. 12. Water entering tank vs. water demand (BASIX and 4,000m2 site area)

Figure 13 shows that as the water demand increases, the water availability is unable to meet the demand. Despite the larger roof area, the water availability exceeds demand for about eighteen of the total sixty years for the four-floor scenario. The water availability exceeds demand only twice for the six floor scenario and none for the eight floor scenario.

Rainwater Harvesting in Large Residential Buildings in Australia 171

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00

0.00

1946

1950

1954

1958

1962

1966

Fig. 16. Mains top-up required vs. water demand (Non-BASIX and 2,000m2 site area)

1970

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

500.00

1000.00

**Mains Required (kL)**

1500.00

2000.00

2500.00

**Mains Required (kL)**

1946

1950

1954

1958

1962

1966

Fig. 15. Mains top-up required vs. water demand (BASIX and 4,000m2 site area)

1970

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

The average mains top-up required per year over the sixty year analysis period is shown in Figures 14 to 17. It can be seen that the mains top-up required increases with the water demand, with the eight floor scenario requiring significantly more mains top-up than the four and six floor scenarios. There is also a significant difference in the mains top-up required between the BASIX and non-BASIX approaches. It is also noted that the mains topup required decreases when the roof area is increased as a result of the increased runoff entering into the rainwater tank.

Fig. 13. Water entering tank vs. water demand (Non-BASIX and 4,000m2 site area)

Fig. 14. Mains top-up required vs. water demand (BASIX and 2,000m2 site area)

The average mains top-up required per year over the sixty year analysis period is shown in Figures 14 to 17. It can be seen that the mains top-up required increases with the water demand, with the eight floor scenario requiring significantly more mains top-up than the four and six floor scenarios. There is also a significant difference in the mains top-up required between the BASIX and non-BASIX approaches. It is also noted that the mains topup required decreases when the roof area is increased as a result of the increased runoff

entering into the rainwater tank.

0.00

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00

**Mains Required (kL)**

1946

1950

1954

1958

1962

1966

Fig. 14. Mains top-up required vs. water demand (BASIX and 2,000m2 site area)

1970

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

1946

1950

1954

1958

1962

1966

Fig. 13. Water entering tank vs. water demand (Non-BASIX and 4,000m2 site area)

1970

1974

**Year**

1978

4 Floors 6 Floors 8 Floors Net Water Entering Tank

1982

1986

1990

1994

1998

2002

500.00

1000.00

1500.00

**Net Water Entering Tank (kL)**

2000.00

2500.00

3000.00

Fig. 15. Mains top-up required vs. water demand (BASIX and 4,000m2 site area)

Fig. 16. Mains top-up required vs. water demand (Non-BASIX and 2,000m2 site area)

Rainwater Harvesting in Large Residential Buildings in Australia 173

0.00

0.00

1946

1950

1954

Fig. 20. Water savings (Non-BASIX and 2,000m2 site area)

1958

1962

1966

1970

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

200.00

400.00

600.00

**Water Savings (kL)**

800.00

1000.00

1200.00

1946

1950

1954

Fig. 19. Water savings (BASIX and 4,000m2 site area)

1958

1962

1966

1970

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

200.00

400.00

600.00

**Water Savings (kL)**

800.00

1000.00

1200.00

Fig. 17. Mains top-up required vs. water demand (Non-BASIX and 4,000m2 site area)

The water saving is the most significant component of a rainwater harvesting system as this eventually determines the viability of the system. A rainwater harvesting system that produces little water savings is unlikely to be financially viable. Figures 18 to 21 compare the average water savings over the sixty year analysis period for a number of scenarios.

Fig. 18. Water savings (BASIX and 2,000m2 site area)

0.00

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00

**Water Savings (kL)**

1946

1950

1954

Fig. 18. Water savings (BASIX and 2,000m2 site area)

1958

1962

1966

1970

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

1946

1950

1954

1958

1962

1966

Fig. 17. Mains top-up required vs. water demand (Non-BASIX and 4,000m2 site area)

1970

The water saving is the most significant component of a rainwater harvesting system as this eventually determines the viability of the system. A rainwater harvesting system that produces little water savings is unlikely to be financially viable. Figures 18 to 21 compare the average water savings over the sixty year analysis period for a number of scenarios.

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

500.00

1000.00

**Mains Required (kL)**

1500.00

2000.00

2500.00

Fig. 19. Water savings (BASIX and 4,000m2 site area)

Fig. 20. Water savings (Non-BASIX and 2,000m2 site area)

Rainwater Harvesting in Large Residential Buildings in Australia 175

The maximum water savings are achieved when water demand is the highest. This occurs for Scenario 12 where the annual water savings achieved is 934kL. The minimum water savings occurs for Scenario 1 which produces an average of 446kL water saving per year. The minimum mains water requirement, however, occurs for Scenario 4 which on average requires 95kL annually and produces yearly water savings of 555kL. Furthermore, the model shows that for some years, mains top-up would not be required at all. It is also found that the performance of the rainwater tank improves significantly with the increasing size of the roof catchment. The larger roof area results in a larger inflow to the rainwater tank

The capital and operating costs are estimated using the Sydney market price for each of the scenarios mentioned above. The highest capital and operating costs are produced for Scenario 12 as a result of the increased plumbing reticulation costs involved with plumbing the extra floors and additional lengths of down piping required for the larger building area. An increased water demand also results in higher pump operating costs than the other

A LCCA is performed on each of the above scenarios to determine the most viable option i.e. the highest benefit/cost ratio. The price of water, the inflation rate of water and the interest rate/discount rate are also considered as variables. The best case benefit/cost ratio is found to occur for Scenario 10 and the worst benefit/cost ratio for Scenario 3. It is found that the financial viability improves at lower interest rates and higher water prices. The best case scenario is therefore found to occur at a water price of \$1.634/kL at 4.5% inflation rate for water price and an interest rate of 5%. The benefit/cost ratio produced is 1.39 which results in a payback period of 38 years. It is noted that the rainwater harvesting system is not able to payback at an interest rate of 7.5% and other higher rates for the scenarios considered here. At the current water price, it is only possible to payback if the inflation rate of water is at 4.5% which is likely to happen considering dwindling water supplies in Sydney and recent water price increases. At the higher water rate of \$1.634/kL and 4.5% inflation, the BASIX compliant unit is able to payback with the eight-floor scenario being the

Figure 22 shows the yearly cumulative costs and benefits for the best possible scenario. In the first year, the difference between cost and benefit is \$33,904 which indicates that there is a loss of -\$33,904. As the years go on, the cumulative benefits increase and the cumulative costs decrease. At year 38, the benefit is equal to the cost when the savings crosses the x-axis. The water price, rate of inflation, and operating cost determine how fast the benefit becomes equal to the cost. It can be seen that the total benefit in 60 years is \$20,539 indicating that not only has the rainwater harvesting system is paid back, it has saved the owner \$20,539.

It can therefore be concluded, from a financially viable perspective, that it is possible to achieve a payback for a rainwater harvesting system under some favourable conditions. The largest single factor affecting the viability of a rainwater harvesting system is the cost of mains water. The higher the cost of mains water, the more viable the rainwater harvesting system becomes. From an environmental perspective, rainwater harvesting systems have the ability to reduce reliance on mains water leading to lower infrastructure cost and

most viable at a benefit/cost ratio of 1.15 and a payback period of 50 years.

possible deferment of new infrastructure such as dams.

providing greater savings, if the harvested water can be utilised.

scenarios.

Fig. 21. Water savings (Non-BASIX and 2,000m2 site area)

These figures show an increase in water savings in relation to an increasing water demand. The water savings also increase with an increased roof area despite the mains top-up required decreasing for larger roof areas. It can also be seen from these figures that the maximum water savings occur with the non-BASIX approach for an eight-floor scenario with a 4,000m2 site area. It is this scenario that is likely to be the most financially viable option although the increased installation costs of the additional floors might offset the additional savings gained.

The following scenarios are considered in the life cycle cost analysis (LCCA). Four different interest rates/discount rates are considered 5%, 7.5%, 10% and 15% per annum. It is also assumed that water price would increase at three different inflation rates: 2.6%, 3.5% and 4.5% per annum. Two different water prices are considered: \$1.264/kL and \$1.634 per kL. All costs considered here are in Australian dollars.

Scenario 1: BASIX compliant four-floor case built on a site area of 2,000m2 Scenario 2: BASIX compliant six-floor case built on a site area of 2,000m2 Scenario 3: BASIX compliant eight-floor case built on a site area of 2,000m2 Scenario 4: BASIX compliant four-floor case built on a site area of 4,000m2 Scenario 5: BASIX compliant six-floor case built on a site area of 4,000m2 Scenario 6: BASIX compliant eight-floor case built on a site area of 4,000m2 Scenario 7: Non-BASIX compliant four-floor case built on a site area of 2,000m2 Scenario 8: Non-BASIX compliant six-floor case built on a site area of 2,000m2 Scenario 9: Non-BASIX compliant eight-floor case built on a site area of 2,000m2 Scenario 10: Non-BASIX compliant four-floor case built on a site area of 4,000m2 Scenario 11: Non-BASIX compliant six-floor case built on a site area of 4,000m2 Scenario 12: Non-BASIX compliant eight-floor case built on a site area of 4,000m2.

0.00 200.00 400.00 600.00 800.00 1000.00 1200.00 1400.00 1600.00

additional savings gained.

**Water Savings (kL)**

1946

1950

1954

Fig. 21. Water savings (Non-BASIX and 2,000m2 site area)

All costs considered here are in Australian dollars.

Scenario 1: BASIX compliant four-floor case built on a site area of 2,000m2 Scenario 2: BASIX compliant six-floor case built on a site area of 2,000m2 Scenario 3: BASIX compliant eight-floor case built on a site area of 2,000m2 Scenario 4: BASIX compliant four-floor case built on a site area of 4,000m2 Scenario 5: BASIX compliant six-floor case built on a site area of 4,000m2 Scenario 6: BASIX compliant eight-floor case built on a site area of 4,000m2 Scenario 7: Non-BASIX compliant four-floor case built on a site area of 2,000m2 Scenario 8: Non-BASIX compliant six-floor case built on a site area of 2,000m2 Scenario 9: Non-BASIX compliant eight-floor case built on a site area of 2,000m2 Scenario 10: Non-BASIX compliant four-floor case built on a site area of 4,000m2 Scenario 11: Non-BASIX compliant six-floor case built on a site area of 4,000m2 Scenario 12: Non-BASIX compliant eight-floor case built on a site area of 4,000m2.

1958

1962

1966

1970

These figures show an increase in water savings in relation to an increasing water demand. The water savings also increase with an increased roof area despite the mains top-up required decreasing for larger roof areas. It can also be seen from these figures that the maximum water savings occur with the non-BASIX approach for an eight-floor scenario with a 4,000m2 site area. It is this scenario that is likely to be the most financially viable option although the increased installation costs of the additional floors might offset the

The following scenarios are considered in the life cycle cost analysis (LCCA). Four different interest rates/discount rates are considered 5%, 7.5%, 10% and 15% per annum. It is also assumed that water price would increase at three different inflation rates: 2.6%, 3.5% and 4.5% per annum. Two different water prices are considered: \$1.264/kL and \$1.634 per kL.

1974

**Year**

1978

4 Floors Six Floors Eight Floors

1982

1986

1990

1994

1998

2002

The maximum water savings are achieved when water demand is the highest. This occurs for Scenario 12 where the annual water savings achieved is 934kL. The minimum water savings occurs for Scenario 1 which produces an average of 446kL water saving per year. The minimum mains water requirement, however, occurs for Scenario 4 which on average requires 95kL annually and produces yearly water savings of 555kL. Furthermore, the model shows that for some years, mains top-up would not be required at all. It is also found that the performance of the rainwater tank improves significantly with the increasing size of the roof catchment. The larger roof area results in a larger inflow to the rainwater tank providing greater savings, if the harvested water can be utilised.

The capital and operating costs are estimated using the Sydney market price for each of the scenarios mentioned above. The highest capital and operating costs are produced for Scenario 12 as a result of the increased plumbing reticulation costs involved with plumbing the extra floors and additional lengths of down piping required for the larger building area. An increased water demand also results in higher pump operating costs than the other scenarios.

A LCCA is performed on each of the above scenarios to determine the most viable option i.e. the highest benefit/cost ratio. The price of water, the inflation rate of water and the interest rate/discount rate are also considered as variables. The best case benefit/cost ratio is found to occur for Scenario 10 and the worst benefit/cost ratio for Scenario 3. It is found that the financial viability improves at lower interest rates and higher water prices. The best case scenario is therefore found to occur at a water price of \$1.634/kL at 4.5% inflation rate for water price and an interest rate of 5%. The benefit/cost ratio produced is 1.39 which results in a payback period of 38 years. It is noted that the rainwater harvesting system is not able to payback at an interest rate of 7.5% and other higher rates for the scenarios considered here. At the current water price, it is only possible to payback if the inflation rate of water is at 4.5% which is likely to happen considering dwindling water supplies in Sydney and recent water price increases. At the higher water rate of \$1.634/kL and 4.5% inflation, the BASIX compliant unit is able to payback with the eight-floor scenario being the most viable at a benefit/cost ratio of 1.15 and a payback period of 50 years.

Figure 22 shows the yearly cumulative costs and benefits for the best possible scenario. In the first year, the difference between cost and benefit is \$33,904 which indicates that there is a loss of -\$33,904. As the years go on, the cumulative benefits increase and the cumulative costs decrease. At year 38, the benefit is equal to the cost when the savings crosses the x-axis. The water price, rate of inflation, and operating cost determine how fast the benefit becomes equal to the cost. It can be seen that the total benefit in 60 years is \$20,539 indicating that not only has the rainwater harvesting system is paid back, it has saved the owner \$20,539.

It can therefore be concluded, from a financially viable perspective, that it is possible to achieve a payback for a rainwater harvesting system under some favourable conditions. The largest single factor affecting the viability of a rainwater harvesting system is the cost of mains water. The higher the cost of mains water, the more viable the rainwater harvesting system becomes. From an environmental perspective, rainwater harvesting systems have the ability to reduce reliance on mains water leading to lower infrastructure cost and possible deferment of new infrastructure such as dams.

Rainwater Harvesting in Large Residential Buildings in Australia 177

A case study is presented for a 75kL rainwater tank, located in Sydney, Australia. It is found that the performance of a rain water harvesting system in terms of water savings improves significantly with the increasing roof size and water demand. It is also found that for most of the typical scenarios the rain water harvesting system is not financially viable at the current water prices in Australia, which is highly subsidized and in the current high interest regime (greater than 7%). In a few cases however, the rain water harvesting system is likely to be financially viable, in particular at smaller interest rates and higher water prices. It is also found that the capital cost represents the highest component in the whole life cycle cost of a rain water harvesting system followed by the maintenance cost. The outcomes of this study suggests that government authorities should consider increasing the subsidy for a rain water harvesting system to offset the financial burden of the home owners to encourage the installation of rain water harvesting systems. It should be noted that there are significant environmental benefits of a rain water harvesting system such as water conservation and on-site retention of pollutants. Rainwater harvesting system also increases the resilience of the urban water supply system, which is important during drought years, which is common in Australia. Rainwater harvesting system is also likely to defer construction of major water

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harvesting facilities: Case study on several rainwater harvesting facilities in Korea.

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http://www.basix.nsw.gov.au/information/common/pdf/sustainable\_feat\_multi

supply dam and desalination plant.

AS/NZS 4536.

*Production,* Vol. 11, pp. 1-11.

**5. References** 

Fig. 22. Annual benefits and costs of the best possible scenario for the rainwater harvesting system

A breakdown of the different cost components of the whole life cycle cost is presented in Table 2. It can be seen that the capital cost comprises the highest component (66%) whereas the maintenance cost is the second highest contributing 18%. The pump operating cost only contributes 6% of the total cost although when added to the pump capital, replacement and maintenance costs the total expenditure of the pump jumps to \$9,872 or 19% of the total life cycle cost. This is quite significant and whether or not a rooftop rainwater tank is justified may be a subject to further research as with a rooftop tank there would be no pump cost. Although, the weight of a 75kL rainwater tank is likely to add significant structural cost to the building which may not justify a rooftop rainwater tank.


Table 2. Breakdown of whole life cycle cost for the best possible scenario of the rainwater harvesting system
