Optimisation of rainwater tank design from large roofs: A case study in Melbourne, Australia

https://doi.org/10.1016/j.resconrec.2011.05.013Get rights and content

Abstract

Rainwater tanks for larger roof areas need optimisation of tank size, which is often not carried out before installation of these tanks. This paper presents a case study of rainwater tank evaluation and design for large roof areas, located in Melbourne, Australia, based on observed daily rainfall data representing three different climatic regimes (i.e. dry average, and wet years). With the aim of developing a comprehensive Decision Support Tool for the performance analysis and design of rainwater tanks, a simple spreadsheet based daily water balance model is developed using daily rainfall data, contributing roof area, rainfall loss factor, available storage volume, tank overflow and irrigation water demand. In this case study, two (185 m3 and 110 m3) underground rainwater tanks are considered. Using the developed model, effectiveness of each tank under different climatic scenarios are assessed. The analysis shows that both the tanks are quite effective in wet and average years, however less effective in dry years. A payback period analysis of the tanks is preformed which reveals that the total construction cost of the tanks can be recovered within 15–21 years time depending on tank size, climatic conditions and future water price increase rates. For the tanks, a relationship between water price increase rates and payback periods is developed. The study highlights the need for detailed optimisation and financial analysis for large rainwater tanks to maximise the benefits.

Highlights

► Investigates the effectiveness of stormwater harvesting and payback periods for large rainwater tanks using recorded daily rainfall data. ► Analysis revealed that two tanks (considered in the case study) have payback periods of 21 years and 19 years. ► Relationships between different percentage increase in yearly water prices and payback periods are presented. ► Several hypothetical scenarios to optimise the size and connected roof area of the tanks were presented. ► Difference in optimisation results for rainfalls in a real average year and a theoratical annual average assumption are presented.

Introduction

With increasing population and changing climate regime, water supply systems in many cities of the world are under stress. To tackle this problem, water authorities are adopting several measures including demand management and identifying alternative water sources such as stormwater harvesting, greywater and wastewater reuse and desalination. Among all the alternative water sources, stormwater harvesting perhaps has received the most attention. In Australia, federal, state and local government authorities have been promoting stormwater harvesting through campaigns, as well as offering financial incentives and grants to promote water saving ideas and innovations.

Among all the stormwater harvesting options, rainwater tanks have received the greatest attention. Fewkes (1999) conducted studies on residential rainwater tanks in the United Kingdom, producing a series of dimensionless design curves which allows estimation of the rainwater tank size required to obtain a desired performance measure given the roof area and water demand patterns. Vaes and Berlamont (2001) developed a model to determine the effectiveness of rainwater tanks and stormwater runoff using long term historical rainfall data. Coombes and Kuczera (2003) found that for an individual building with a 150 m2 roof area and 1–5 kL tank in Sydney can yield 10–58% mains water savings (depending on the number of people using the building). According to Coombes and Kuczera (2003), depending on roof area and number of occupants, rainwater tank use can result in mains water annual savings of 18–55 kL for 1 kL sized tanks and 25–144 kL for 10 kL sized tanks. In Sweden, Villarreal and Dixon (2005) investigated water savings potential of stormwater harvesting systems from roof areas. Villarreal and Dixon discovered that a mains water saving of 30% can be achieved using a 40 m3 sized tank (toilet and washing machine use only).

Coombes (2007) conducted studies on the modelling of the rainwater tanks and the opportunities for effective retention storage using the PURRS (Probabilistic Urban Rainwater and Wastewater Reuse Simulator) water balance model. Following over a decade of research into the quality of rainwater collected from roofs, Coombes (2007) has identified the potential for rainwater to be utilised far more extensively than many government regulators are recommending. Australian Capital Territory (ACT) Planning & Land Authority (2008) developed charts of available rainwater that can be harvested for various activities on different size tanks and for different size roof catchments. Such charts can be used as a quick reference for choosing an initial tank size. The predicted change in rainfall patterns in Australia as a result of global warming adds further complexity to planning adequate rainwater harvesting schemes (CSIRO, 2007). Ghisi et al. (2007) investigated the water savings potential from rainwater harvesting systems in Brazil (South America) and found that average potential for potable water savings of 12–79% per year for the cities analysed. Ghisi et al. (2009) evaluated that savings of potable water by using stormwater harvesting for washing vehicles in petrol stations located in Brasilia (Brazil, South America) and found that the average potential for potable water savings by using rainwater is 32%.

Tam et al. (2010) investigated cost effectiveness of rainwater tank application for residential buildings in Australia. Based on the data from seven cities, Tam et al. (2010) concluded that using rainwater would be a logical choice for households in Brisbane, Gold Coast and Sydney. Khastagir and Jayasuriya (2010) presented a theory for optimal sizing of rainwater tanks in Melbourne.

Despite positive outcome from many studies, there remains a general community reluctance to adopt stormwater harvesting on a wider scale. Part of the reason for this reluctance can be attributed to lack of information about the effectiveness of a stormwater harvesting system and the optimum storage size required to satisfy the performance requirements under the specific site conditions (Imteaz and Shanableh, 2009). A proper in-depth understanding of the effectiveness of any proposed on-site stormwater harvesting system including a life cycle cost analysis is often lacking. Also, there have been limited researches on rainwater harvesting from commercial and large roof areas in Australia. Furthermore, many studies have used mean annual rainfall data or generated rainfall data in modelling rainwater harvesting system. This paper presents development of a daily water balance model for the optimisation of rain water tank size. As a case study, two rainwater tanks constructed within the campus of Swinburne University of Technology (Melbourne, Australia) were considered. Analysis shows detailed assessment of effectiveness and payback periods for the rainwater tanks. Also, differences in analyses and results using average annual rainfall and daily rainfall data have been discussed.

Section snippets

Methodology

A spreadsheet based daily water balance model was developed considering daily rainfall, contributing catchment (roof) area, spillage/leakage losses, storage volume and water uses. In the model, the prime input value was the daily rainfall amount for three differeent rainfall regimes/years (dry year, average year and wet year). Through statistical analysis using historic daily rainfall data three separate years (dry year, average year and wet year) were selected from 1st decile, 5th decile and

Case study

The study uses two large underground rainwater tanks (185 m3 and 110 m3), located in Swinburne University of Technology campus, located in Melbourne, Australia (Fig. 1). Melbourne is the 2nd largest city in Australia with population over 4 million. Melbourne City has the culture of water conservation. The main purpose of these two tanks in the university campus is to capture stormwater from the roof of selected buildings and use the water for landscape irrigation. However, no optimisation study

Tank size optimisation

For the above-mentioned case study it is obvious that both the tanks are not having an optimised design; north tank is larger in size and connected with a smaller roof, on the other hand south tank is smaller in size and connected with a bigger roof. Developed daily water balance model has been used for several different roof size and tank volume scenarios, to achieve an optimised design outcome. For the North tank, optimisation was performed through increasing connected roof area and for the

Summary and conclusion

This paper investigates the effectiveness of stormwater harvesting and payback periods for large rainwater tanks. This uses data from two underground rainwater tanks constructed in Swinburne University campus in Melbourne, Australia. The study considered recorded daily rainfall data instead of average annual rainfall data or generated rainfall data. Analysis using recorded daily rainfall data reveals possible realistic scenrios instead of a complete hypothetical average scenario. The study

Cited by (152)

View all citing articles on Scopus
View full text