Optimal sizing of storage tanks for domestic rainwater harvesting in Sicily
Highlights
► A dimensionless methodology for the optimal design of domestic rainwater harvesting systems is developed. ► A novel parameter to describe the intra-annual rainfall patterns is proposed. ► Easy-to-use regional regressive models to evaluate water saving and overflow performances are derived. ► Economic analysis based on minimum cost criteria is carried out to evaluate the optimal DRWH tank size.
Introduction
In many parts of the world, domestic water saving practices have awakened increasing attention due to the growth of water demands in urban areas. Some of these practices involve the installation of systems able to reduce the household water consumption (i.e. dual flush toilet systems, tap flow aerators, etc.) and the adoption of educational programs to diffuse water sustainability concepts within the population.
In such a context also, domestic rainwater harvesting (DRWH) has recently become an important option as a water source, especially for urban and peri-urban areas affected by restricted availability of freshwater.
DRWH defines the small-scale concentration, collection, storage and use of rainwater runoff coming from rooftops, courtyards and other impervious surfaces for domestic use. It is largely recognized (USEPA, 2004) that rainwater can replace potable water for several less quality-demanding water uses such as house toilet flushing, terrace cleaning or private garden watering. Besides, despite studies from different parts of the world reveal the presence of contaminants in rooftop rainwaters (Meera and Mansoor Ahammed, 2006), in specific geographical contexts these waters have been identified as a major source also for drinking, cooking and sanitary purposes (Duncker, 2000) since they did not present increased risk of gastrointestinal illness when compared with water from public mains (Heyworth, 2001, Abdulla and Al-Shareef, 2009).
Results of investigations concerning the analysis of water uses in urban households for water saving objectives (Mukhopadhyay et al., 2001, Lazarova et al., 2003, Campisano and Modica, 2010) have shown that a significant amount (up to 30%) of water in houses is typically used for toilet flushing. This value suggests potentially high water saving benefits deriving from the primary use of harvested rainwater to toilet flushing (Glist, 2005, Jones et al., 2009). For such use, only basic water treatment (i.e. filtration and chlorination) should be accomplished and storage tanks with limited size would be required since the daily toilet water demand is relatively constant during the year.
Many DRWH schemes were proposed by researchers in the past few decades (Donati, 1995, Aylward et al., 2006). In most of the cases, the most important design decision concerns the evaluation of the storage capacity according to the desired level of system performance.
There is a large literature concerning methodologies to estimate performance and sizing of rainwater harvesting systems. Approaches include analysis based on water balance simulation (Fewkes and Butler, 2000, Villarreal and Dixon, 2005, Ghisi and Ferreira, 2007) and probabilistic methods (Lee et al., 2000, Guo and Baetz, 2007). In general, results indicate that storage capacity cannot be standardized, being markedly influenced by site-specific variables such as local rainfall, roof area, potable water demand and number of people in the household (Mwenge Kahinda et al., 2007, Aladenola and Adeboye, 2010, Eroksuz and Rahman, 2010).
In order to generalise results, some authors have recently started exploring the variability of water saving at different spatial and temporal scales. In this context, Fewkes (2000) investigated how spatial and temporal fluctuations in rainfall could be incorporated into behavioural models of DRWH systems. In particular, water balance simulations were conducted according to different reservoir operating algorithms and various temporal scales for a few rain series in the UK.
Cheng and Liao (2009) explored regional zoning for rainwater harvesting systems in northern Taiwan using cluster analysis. Using the precipitation data from 72 stations, they derived a dimensional indicator to score rainwater harvesting system potential as function of regional rainfall characteristics and system storage size.
Hanson et al. (2009) provided a log-linear regressive relationship to calculate required storage capacity for a rainwater harvesting system which is generally applicable in the U.S. The equation is based on results of a behavioral model of a DRWH system and applied to daily time step records at 232 U.S. precipitation gauging stations. Although, the equation demonstrates good predictive performances at national scale, its application requires data elaboration to calculate climatic variables (daily rainfall statistics) to predict storage capacity.
An assessment of economical feasibility of DRWH systems is also needed in the design phase. A recent Australian study (ABS, 2009) shows that rainwater harvesting is perceived by the population as a costly home facility. Other authors (Mikkelsen et al., 1999, Liaw and Tsai, 2004) investigated the economical issues of DRWH systems with particular reference to rooftop rainwater harvesting techniques. Rahman et al. (2010) found that it could be possible to achieve pay-back for the DRWH system in case of multi-storey buildings, great number of users and small discount rates. In a successive study Rahman et al. (2012) showed that, for detached houses, the benefit for the rainwater tanks result less than costs without government rebate.
Domenech and Sauri (2010) investigated the financial variability of DRWH systems in single and multi-family buildings in the metropolitan area of Barcelona, Spain. Pay-back periods were found to vary between 30 and 60 years.
Tam et al. (2010) investigated the cost effectiveness of rainwater harvesting in residential houses in Australia finding high financial benefits in high-rainfall areas.
Zhang et al. (2010) examined the financial variability of DRWH systems in high rise buildings in four capital cities in Australia and found Sidney to have the shortest pay-back period (about 10 years).
In 2009, a research program has been launched at the University of Catania (Italy) to investigate water saving obtainable by rooftop rainwater harvesting for domestic use.
In this paper, a dimensionless methodology to determine the optimal size of DRWH tanks is presented, based on the results of daily water balance simulations for 17 rainfall gauging stations in Sicily (Italy). Compared to literature, a novel dimensionless parameter is defined to allow a better description of the intra-annual character of rainfall patterns. Regional regressive models were derived enabling the evaluation of water savings and overflow discharges from DRWH systems. A relationship to evaluate the optimal size of the tank was also derived based on the minimum cost criterion. An explicative case was finally introduced to show the procedure in detail.
Section snippets
Hydraulic scheme and water balance equations
The typical scheme of a domestic rainwater harvesting system is based on the collection of rainwaters coming from the building roof (and/or other surfaces) and on their temporary storage within a rainwater tank. Under this scheme, demand for water uses in the house which are compatible with rainwater quality is satisfied primarily by water accumulated in the storage tank, and only then by water from the mains supply. In the present research, the demand was limited to the toilet flushing use and
Used data
To evaluate potential of rooftop rainwater harvesting, the presented methodology was applied to the rain data series of rain gauging stations in Sicily. The island is the largest Italian region with a total surface area of 25,711 km2 and a population over 5 million people, mostly localised in coastal areas. The climate is typically Mediterranean with an average yearly rainfall of about 720 mm prevalently concentrated within the semester October–March. Peaks of precipitation (above 1000 mm/year)
Conclusions
A dimensionless methodology to determine the optimal size of DRWH tanks was presented in the paper, based on the results of daily water balance simulations applied to precipitation data series recorded in Sicilian rainfall stations.
Compared to previous literature research, a novel dimensionless parameter, namely modified storage fraction, was introduced to allow an improved description of the rainfall patterns. Results of simulations showed increased ability of the chosen parameters to model
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