Rainwater tanks have been in use for many centuries in non-arid regions. Traditionally, it is the prime source of water for many remote communities where there is no other suitable water source. Such remote communities even use rainwater for potable purposes after having some basic treatments. Recently, given the tremendous increase in population and the limited sources of potable water, urban water authorities are adopting several measures including demand management and identifying alternative water sources such as stormwater harvesting, grey water, and wastewater reuse. Among all the alternative water sources, stormwater harvesting is the most suitable and easily achievable. Despite having centralised water supply systems, modern urban communities have started adopting rainwater tanks, mainly for non-potable purposes. As such, the original focus of rainwater tanks as being only a means of water supply has shifted to being an alternate source of water augmentation. 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 the implementation of such water saving methodologies.
There have been numerous studies on rainwater tanks, with many treating rainwater harvesting potentials or quantifications, among which some recent ones are Imteaz et al. [
1], Khan et al. [
2], Santos et al. [
3], and Imteaz and Moniruzzaman [
4]. Continuing from water harvesting potentials, some other studies have investigated the reliability and financial benefits related to rainwater tanks [
5,
6,
7]. There are several studies that have investigated the quality of harvested/captured rainwater, as in many cases, it is used for potable purposes. Boulomytis [
8] assessed the quality of harvested rainwater over clay tiles in Brazil, analysing the turbidity, colour, and pH parameters. The author found that due to the antecedent drought prior to the first intense rainfall, the quality of the first flush was poor due to the contaminants initially spread over the roof. However, the samples collected after 30 min had sufficient quality to be used for the purpose of primary food irrigation (e.g., lettuce), in keeping with the potability standards of the Brazilian Federal Law. Farreny et al. [
9] investigated the effect of different types of roofs, taking into consideration factors such as the roof slope and material. In addition to investigating the effects on runoff quantity, they have also investigated the physicochemical characteristics of the collected rainwater from different types of roofs. They have reported that sloping roofs provide better quality runoff compared to flat roofs in regard to some of the tested parameters (conductivity, total organic carbon, and total carbonates except for ammonium). Lee et al. [
10] investigated the effect of different types of roof material on runoff water quality in South Korea. Among the tested roofing materials (i.e., wooden shingle tiles, concrete tiles, clay tiles, and galvanized steel), they have found that galvanized steel provides the most suitable runoff quality and that even the collected rainwater meets the Korean drinking water quality standard regarding the pH, TSS, NO
3, SO
4, Al, Cu, Fe, Pb, Zn, and
E. coli. Rahman et al. [
11] investigated the quality of harvested rainwater on several occasions within a year in Bangladesh. They reported that the quality of harvested water is satisfactory and fulfils the potable water standard for Bangladesh in regard to faecal coliform, total coliform, total suspended solids (TSS), turbidity, NH3–N, lead, and BOD
5. Charters et al. [
12] investigated stormwater runoff quality from different impervious surfaces including roads and roofs of different materials. They have found that the road surface produces the highest concentrations of TSS, while the highest copper and zinc concentrations are drained from copper and galvanized roofs, respectively. Obviously, these copper and zinc concentrations are derived from the dissolution of copper and galvanized roofing materials. They have also found that the concentrations of pollutants are much higher in the first samples (i.e., first flush). Leong et al. [
13] have monitored harvested rainwater quality in six different sites in Malaysia for a period of 8 months. They have reported that the harvested rainwater is not suitable for drinking. However, it was suitable for recreational purposes, although on some occasions it did not even meet the criteria of recreational water in regard to pH, ammonia, phosphates, and total coliforms.
Many of the above-mentioned studies considered the quality of stormwater at the first flush stored in a first-flush diverter. Quality of stormwater at first flush is expected to be much inferior compared to the subsequent collections, as a first-flush diverter is provided to capture higher concentrations of pollutants accumulated from antecedent dry days. However, none of the studies considered treatment and improvement of stormwater quality within the tank. As the harvested water stays within the tank, the sediments tend to settle at the bottom of the tank. Because some nutrients are attached to the sediment particles, settlements of sediments also result in settlements of some nutrients. Moreover, several bacteria are likely to die within the tank after a few days due to a lack of food (i.e., organic matter), if the stored rainwater was not highly enriched with organic matters. With the aim of investigating this phenomenon, earlier rainwater samples collected directly from the roof and from the tank were tested, and similar results were reported by Imteaz et al. [
14]. To ascertain such conclusions, this study aimed to thoroughly review the existing literature on the qualities of harvested rainwater from two different sources, i.e., roof and tank, and then to analyse such segregated rainwater quality data to establish the water quality benefit of rainwater tanks. Additionally, the experimental measurements were compared through a mathematical modelling study on the same phenomena applying the widely used simulation software titled Modelling for Urban Stormwater Improvement Conceptualisation, MUSIC [
15]. In the past, there were several studies on the experimental measurements of rainwater quality from the roof and/or tank. However, no study has verified such measurements through the simulation model.