**2. Rainwater Tank Analysis Model**

A computer model, which is referred to as the Rainwater Tank Analysis Model (RWTAM) is presented here. The RWTAM can be used to examine the water saving potential and financial viability of a rainwater harvesting system in multi-storey residential buildings. The RWTAM can provide a wide range of results for a proposed rainwater harvesting system including the major cost and benefit elements.

The RWTAM is Windows-based and was developed using Visual Basic. It has various input and output interfaces to enter data and obtain results on water savings and costings of a proposed rainwater harvesting system. The program has 5 main menus: File, Water Savings, Cost Analysis, Settings and Help. Each of these menus has a number of sub-menus as shown in Table 1.

A 'continuous simulation type' water balance model based on daily time steps is used in the program, which calculates the inflow to and outflow from the rainwater tank based on water demand and rainfall data on a given day. The water demand is assumed to consist of toilet flushing, laundry, car washing and irrigation demand. The irrigation demand is difficult to estimate. This is due to the fact that on the days of rainfall and possibly on a number of subsequent days after a significant rainfall event, the irrigation demand would be nil or would be smaller than a normal dry day. In order to account for this, the following approximate but simple procedures are adopted: (i) For 1 day of rainfall, there would be no

of suburbs in Melbourne. A paper by Ming-Daw et al. (2009) focused on the development of a relationship between storage and deficit rates for rainwater harvesting systems. Results showed that as the deficit rate increased so too did the storage size of the tanks. Eroksuz and Rahman (2010) conducted research on the use of rainwater harvesting systems for multi-unit blocks in three cities of New South Wales (NSW), Australia. They found that in order to maximise water savings, larger tanks would be more appropriate and these tanks could provide significant water savings, even in dry years. A study in Brazil by Ghisi et al. (2009) aimed to assess the potential for potable water savings for car washing at petrol stations in the City of Brasilia found that an increase in the tank size enhanced the reliability of the rainwater tank notably in meeting demand. Kyoungjun and Chulsang (2009) showed that rainwater collection would only be feasible in South Korea during six months of the year. They also found that increased cost and marginal increase in reliability make larger tanks unsustainable. They also found that a benefit cost ratio higher than 20% could not be gained due to the low cost of water in South Korea. They suggested the cost of water supply would need to be increased by a factor of five for the rainwater harvesting system to become

There is often a lack of 'easy to use' computing tools which examine the viability of rainwater harvesting system in large buildings. The financial viability of a rainwater harvesting system depends on factors such as local rainfall, roof size, water demand, capital cost, interest rate, maintenance cost and mains water price. This chapter presents a computing tool that can be used to examine various scenarios of a rainwater harvesting system to compare water savings and financial benefits based on life cycle cost analysis. The first part of the chapter presents the computing tool followed by a case study illustrating the

A computer model, which is referred to as the Rainwater Tank Analysis Model (RWTAM) is presented here. The RWTAM can be used to examine the water saving potential and financial viability of a rainwater harvesting system in multi-storey residential buildings. The RWTAM can provide a wide range of results for a proposed rainwater harvesting system

The RWTAM is Windows-based and was developed using Visual Basic. It has various input and output interfaces to enter data and obtain results on water savings and costings of a proposed rainwater harvesting system. The program has 5 main menus: File, Water Savings, Cost Analysis, Settings and Help. Each of these menus has a number of sub-menus as shown

A 'continuous simulation type' water balance model based on daily time steps is used in the program, which calculates the inflow to and outflow from the rainwater tank based on water demand and rainfall data on a given day. The water demand is assumed to consist of toilet flushing, laundry, car washing and irrigation demand. The irrigation demand is difficult to estimate. This is due to the fact that on the days of rainfall and possibly on a number of subsequent days after a significant rainfall event, the irrigation demand would be nil or would be smaller than a normal dry day. In order to account for this, the following approximate but simple procedures are adopted: (i) For 1 day of rainfall, there would be no

economically viable in South Korea.

use of the computing tool and the associated results.

**2. Rainwater Tank Analysis Model** 

including the major cost and benefit elements.

in Table 1.


Table 1. Main menus and submenus of RWTAM model

irrigation during the day but irrigation would resume on the next day. (ii) For 1 to 7 days of consecutive rainfall, there would be no irrigation during the rainfall days plus none for the equal number of previous days of consecutive rainfall. (iii) For 8 to 21 days of consecutive

Rainwater Harvesting in Large Residential Buildings in Australia 163

Fig. 2. Input interface of RWTAM program for life cycle cost analysis

Fig. 3. Average annual water savings graph as an output from the RWTAM program

rainfall, there would be no irrigation during the rainfall days plus no irrigation for the equal number of previous days of consecutive rainfall up to 7 days. The water demand on a particular day is then calculated by adding the indoor demand, car washing demand and the required irrigation (garden and lawn) demand for the day.

From the water balance model, the following output values are estimated on a daily basis: (i) net rainfall entering into the tank (ii) water in the tank (ii) water demand (iii) mains topup and (iv) water savings. The mains top-up is the amount of mains water needed to top-up the rainwater tank to the specified minimum level (e.g. 10% of the tank volume). For the cost analysis, RWTAM undertakes life cycle cost analysis (LCCA), which is the procedure of assessing the cost of a product over its life cycle or portion thereof (AS/NZS, 1999). The life cycle cost is the sum of acquisition and ownership of a product over its life cycle (AS/NZS, 1999). All past, present and future cash flows identified in the LCCA are converted to present day dollar value and are a function of discount rates. All costs considered here are in Australian dollars. This study uses the concept of nominal cost (the expected price that will be paid when a cost is due to be paid, including estimated changes in price due to changes in efficiency, inflation/deflation, technology and the like) and nominal discount rate (the rate to use when converting nominal costs to discounted costs). To convert a nominal cost (*CN*) to discounted cost (*CD*), following equation is used (AS/NZS, 1999):

$$\mathbf{C}\_{D} = \mathbf{C}\_{N} \times \left(\frac{1}{\left(1 + d\_{n}\right)^{y}}\right) \tag{1}$$

where *dn* is the nominal discount rate per annum and *y* is the appropriate number of years.


Fig. 1. Input interface of the RWTAM program for water savings

rainfall, there would be no irrigation during the rainfall days plus no irrigation for the equal number of previous days of consecutive rainfall up to 7 days. The water demand on a particular day is then calculated by adding the indoor demand, car washing demand and

From the water balance model, the following output values are estimated on a daily basis: (i) net rainfall entering into the tank (ii) water in the tank (ii) water demand (iii) mains topup and (iv) water savings. The mains top-up is the amount of mains water needed to top-up the rainwater tank to the specified minimum level (e.g. 10% of the tank volume). For the cost analysis, RWTAM undertakes life cycle cost analysis (LCCA), which is the procedure of assessing the cost of a product over its life cycle or portion thereof (AS/NZS, 1999). The life cycle cost is the sum of acquisition and ownership of a product over its life cycle (AS/NZS, 1999). All past, present and future cash flows identified in the LCCA are converted to present day dollar value and are a function of discount rates. All costs considered here are in Australian dollars. This study uses the concept of nominal cost (the expected price that will be paid when a cost is due to be paid, including estimated changes in price due to changes in efficiency, inflation/deflation, technology and the like) and nominal discount rate (the rate to use when converting nominal costs to discounted costs). To convert a nominal cost

> 1 1 *D N y*

where *dn* is the nominal discount rate per annum and *y* is the appropriate number of years.

*d*

 

*n*

(1)

the required irrigation (garden and lawn) demand for the day.

(*CN*) to discounted cost (*CD*), following equation is used (AS/NZS, 1999):

Fig. 1. Input interface of the RWTAM program for water savings

*C C*


Fig. 2. Input interface of RWTAM program for life cycle cost analysis

Fig. 3. Average annual water savings graph as an output from the RWTAM program

Rainwater Harvesting in Large Residential Buildings in Australia 165

the daily rainfall data file relevant to the multi-storey building site in question. (d) Select 'Setting menu' and enter the data for Rainwater Tank Water Balance Model and LCCA. (e) Obtain output/results on water savings and cost analysis selecting the 'Water Savings' and 'Cost Analysis' icon, respectively. The various menus and submenus are self-explanatory

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

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

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

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 tank as overflow as the tank would overflow very quickly.

(LCCA), it is assumed that the rainwater harvesting system has a life of 60 years.

and easy to work with.

Sydney Water Corporation.

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


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 the daily rainfall data file relevant to the multi-storey building site in question. (d) Select 'Setting menu' and enter the data for Rainwater Tank Water Balance Model and LCCA. (e) Obtain output/results on water savings and cost analysis selecting the 'Water Savings' and 'Cost Analysis' icon, respectively. The various menus and submenus are self-explanatory and easy to work with.
