Full length articleUnderstanding the impact of hybrid water supply systems on wastewater and stormwater flows
Introduction
More than 50% of the global population lives in cities and this proportion is anticipated to rise to 70% by 2050 (Gregory and Hall, 2011). Countries like Australia are already highly urbanised with almost 90% people living in cities. This figure is likely to keep rising in the future leading to an ongoing increase in water demand. As an example, Australia’s largest cities are forecast to require additional 73% water above the current water supply by 2050 (Burn, 2011). On the other hand, rain-dependent supply is expected to become more unreliable as a result of an unpredictable change in climate. The most recent example of this is the Millennium drought, which occurred in Melbourne during 1997–2009. During this period, the average inflow decreased by almost 40% (Fig. 1) which caused a decline in the city’s drinking water reservoirs (Melbourne Water, 2015). In contrast, the average inflow between 2010 and 2013 is only slightly lower than the long-term average inflow (Fig. 1) highlighting the significant variability in annual water supply inflows if compared on yearly basis.
Cities around the world are typically served by centralized water supply and sewage collection and disposal system (Arora et al., 2015, Sharma et al., 2009). However, sustainability of centralized urban water supply system is in question due to increasing concentration and growth of the global population in cities, the uncertainty and variability in the availability of traditional water source due to periodic droughts and the projected climate change impacts (Cai et al., 2016, Cook et al., 2014, Imteaz et al., 2012, Sharma et al., 2013). Therefore, there is a need to augment the traditional water supply with more diverse water sources such as rainwater tanks, stormwater, and recycled wastewater (Garcia-Cuerva et al., 2016, Gurung et al., 2014, Nair et al., 2014, Sharma et al., 2010a). Implementation of these systems will require integration of traditional and non-traditional supply sources. In recent years, many alternative (non-traditional) water supply options like stormwater, rainwater, desalination and recycled water have become more common-place (Domènech and Saurí, 2010). A combination of such decentralized water supply options in conjunction with the centralized system is referred to as hybrid water supply systems (HWSS) by Sapkota et al. (2015). HWSS is expected to better meet the increasing water demand and provide other benefits such as infrastructure savings on the sewerage system, energy self-sufficiency, higher flexibility and reduction in greenhouse gas reduction (Bieker et al., 2010, Chen et al., 2017, Makropoulos and Butler, 2010, Marleni et al., 2015, Radcliffe, 2010). However, it is still largely unknown how these technologies impact on the operational performance of the existing downstream infrastructure and treatment processes (Marleni et al., 2012, Sapkota et al., 2013, Sapkota et al., 2016).
Several studies have explored and quantified the urban water cycle benefits of alternative water systems (Burns et al., 2015, Fletcher et al., 2007, Grant et al., 2012, Hamel and Fletcher, 2014, Mitchell et al., 2007). Furthermore, the implementation of hybrid water supply systems can alter the wastewater and stormwater quantity and quality and may impact infrastructure performance. For example, different wastewater characteristics trigger various problems such as blockages, odour and corrosion in sewage system (Marleni et al., 2012). Therefore, it is essential to understand the possible impacts of alternative water supply options such as wastewater reuse, stormwater harvesting and rainwater tanks combined with centralized systems on overall range of performance indicators (Fletcher et al., 2007, Sapkota et al., 2015, Sapkota et al., 2016).
This paper presents the evaluation of the impacts of a range of possible hybrid water supply scenarios set in consultation with Victorian water utilities and analyses their potential impacts of hybrid system on wastewater and stormwater flow as well as contaminants. Developed scenarios which are combinations of greywater, stormwater, rainwater and recycled along with centralized system are based on the combination of the infrastructure provision, available water supply servicing choices, and selected technologies. It has been ensured that the developed scenarios comply with the current state and local Government policy as well as water legislation. Scenario analysis results in the form of flow and contaminants in this paper will help in assessing the impact of decentralized systems on the existing centralized infrastructure in terms of wastewater and stormwater quality and quantity change. This improved understanding of the interaction between centralized and decentralized system will aid in improved sewerage and drainage design and facilitate operation and maintenance planning (Sapkota et al., 2015).
Section snippets
Case study
The case study area that has been selected for this study includes the suburb of Quarry Hill Epping North East, Aurora, and Wollert (Fig. 2) in Victoria, Australia which is located in the Northern Growth Area (NGA). NGA is around 20 km north of the Melbourne Central Business District with an area of 22.57 square kilometres. This is a Greenfield area still under development with 3455 households as on 27 August 2012, with a plan to build 30,000 by 2030. The area is currently supplied with Class A
Scenario overview
Before developing the scenarios, a range of alternative water supply choices were assessed for the case study area. These include potable water, treated wastewater via third pipe, treated stormwater via third pipe system, rainwater tanks at the household level and treated grey water. Other options such as direct potable reuse and desalination are not considered as direct potable reuse is not permissible in line with current Australian regulation (DSE, 2009, Higgins et al., 2004) and
Selected water quality and quantity parameters and associated sewer and drainage problems
Water flow rates, volumes and contaminants are considered based on the generalized framework developed by Sapkota et al. (2016). Water contaminants considered for the study are Total Nitrogen (TN), Total Phosphorous (TP), Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). TSS, TP, TN, BOD and COD are selected to represent the contaminants as they are important measures of pollution in both stormwater and wastewater (Last, 2010). To determine
Simulation of water servicing scenarios
Urban Volume and Quality (UVQ) (Mitchell and Diaper, 2006) modelling tool was selected for the simulation of water servicing scenarios. UVQ not only describes water and contaminant flows and balances but also represents the flow paths and concentrations of contaminant from the source to the point of discharge (Marleni et al., 2015). It models the total urban water cycle’s daily water balance using nested catchment, cluster and lot, scales (Mitchell and Diaper, 2006). Most importantly, UVQ
Model calibration and validation
UVQ model has been calibrated against sewage outflow data and recycled water use by changing the model variables based on trial and error until there is good match between the observed and simulated flows. The good agreement is judged by visual inspection of graphical plots, flow volume error and peak flow error. Also, Nash Sutcliffe Efficiency (NSE) being reliable statistics for assessing the goodness of fit, is used to assess the model performance (McCuen et al., 2006). The calibration is
Results and discussions
Seven potential scenarios are run over a thirty-four year time period of climate data diurnally to model seasonal and annual variability to estimate the potable water, wastewater and stormwater flows along with wastewater concentration and stormwater contaminants loads. The results from water and contaminant balance analysis are evaluated in terms of the relative benefits of various hybrid water supply systems. This section presents and compares the results for all the scenarios. This section
Conclusion
Water and contaminant balance modelling in this study provides a preliminary analysis of the impacts of various hybrid water supply scenarios on wastewater flow, wastewater contaminant concentration, stormwater flow and stormwater contaminant loads. From the results, it can be concluded that the use of hybrid water supply systems reduces the potable water demand considerably. The hybrid scenario using rainwater tanks and treated wastewater combined with centralized water supply system is found
Acknowledgments
We would like to acknowledge Bureau of Meteorology (Dockland, Australia), Melbourne Water (Dockland, Australia) and Yarra Valley Water (Mitcham, Australia) for kindly assisting in the study, particularly providing the data and sharing their valuable experiences in the field. Also, the authors would like to thank Dr. Biju George and Dr. Bandara Nawarathna from Bureau of Meteorology, Dockland Australia for their valuable suggestions during the study.
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