Development of dual water supply using rooftop rainwater harvesting and groundwater systems
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The aim of this study was to gain a better understanding on dual water supply system consisting of rainwater and groundwater. The pilot study of the designed system was carried out at the Universiti Tun Hussein Onn Malaysia, Batu Pahat, Johor. The first part of the analysis was carried out to assess the rainwater harvesting system efficiency. The monthly results on total quantities of the collected rainfall in storage tank was 48.97 m3 while the actual rainfall calculated was 56.04 m3 with 75% of collection efficiency. The rainfall-storage rating curve was then plotted using 150 rain events data versus the volume of harvested rainwater collected during the study period. The second part of the analysis was to determine the total average pumping rate by conducting a step drawdown test, which gave Q = 39.5 m3 / h. For the daily discharge rate, the well was capable to supply water approximately 1.69 m3. In the current study, the daily water demand calculated was 0.59 m3, which gave the total volume per month approximately from 16.5 to 18.3 m3. Based on the performance of the system, most of the days rainwater could not meet the water demand, thus have to be supported by the groundwater.
KeywordsConservation Groundwater Rainwater harvesting system Water supply
Malaysia is known to have abundant surface water due high average annual rainfall between 2420 and 3830 mm. In the extreme cases, Malaysia receives heavy rainfall between November and February as much as 600 mm in 24 h. This statistics shows that the annual and monthly rainfall in Malaysia are quite large . However, in some cities water shortage or scarcity continues to persist due to the increasing in the annual population growth of 1.5% and industrial activity. In addition, the water tariffs in Malaysia is extremely cheap compared to Indonesia and Singapore, therefore, the average daily water consumption of Malaysians is between 220 and 250 L, which is much more than that recommended by World Health Organization of 165 L/day. An effective method of water supply and storage system should be taken seriously to avoid the demand problem, especially in drought season. Public and private water user agencies are encouraged to shift to integrated management of water. The ability to be close to being self-sufficient on water resources would render the building to be sustainable in terms of water resources. In order to be self-sufficient, alternative water resources should be utilized instead of relying on public water supply. RWH and groundwater abstraction are among the solutions to the problem in the areas that have inadequate water resources or face long-term water supply disruptions as well as to complement conventional systems for non-potable use, such as toilet flushing, clothes washing, watering plants, irrigation and for potable uses [2, 3, 4, 5, 6].
RWH is said to have very good potential as an alternative water supply in Malaysia. RWH was introduced after the 1998 drought by Ministry of Housing and Local Government. Few guidelines concerning RWH in Malaysia have been developed . According to the recent studies which have been carried out on the rainwater quality in Malaysia, RWH has the potential to reduce cost and the usage of treated water as well as can be considered as one of the alternative for the water resources [8, 9]. Although RWH is gaining much interest, the implementation has been limited due to several disadvantages such as seasonal variation and related cost [7, 10, 11, 12, 13]. Recently, Hafizi Md Lani et al.  concluded that Malaysia is well positioned to harvest rainwater and to be a saviour for the water shortage, but in some places or event, it cannot completely dependable source of water supply due to uneven rainfall distributions. As to support the RWH system, other natural resources can be considered. For instance, by making a dual water supply system consisting of rainwater and groundwater.
Groundwater is an alternative water source in the areas with a limited surface water resources. In Malaysia, the utilization of groundwater is very low compared to other countries such as Denmark, Austria, Thailand, China, USA, due to failure to recognize the vast potential of the invisible groundwater resource . Hence, to test the level of usability of groundwater a comprehensive study needs to be conducted. In addition, rainwater and groundwater are two interdependent water sources. However, people who harvest rainwater generate an additional negative impact on groundwater users. In other words, although the technology of RWH system may allow us to collect the runoffs, it also causes decreasing amount of water available to replenish the aquifer. Therefore, the motivation of this study was to develop a water supply system that integrates both rooftop RWH system and groundwater abstraction as water supply alternatives to fulfil the water demand while adopting the concept of sustainability and conservation on water resources.
2.1 Study area
2.2 Rainwater harvesting system
2.3 Water demand
The rainwater demand depends on the number of people using the water, average consumption per person, and the range of uses by the consumer. The water demand for the rainwater influences the effectiveness and availability of designed rainwater storage tank. Rainwater demand for domestic application recommended by DID  was used as a guideline in designing process. In the current study, for indoor usage with average of four households, a single flush toilet was chosen with average total water used was 0.12 m3 per day. Meanwhile, for a twin tub washing machine, the average total water used was 0.4 m3 per day. Estimated average total for dishwasher and general cleaning were 0.3 and 0.2 m3 per day, respectively. For outdoor uses, total average for drip system, sprinkler, and hosing paths were estimated to be 0.2 m3 per day. Therefore, the total daily water demand was 0.59 m3.
2.4 Rainfall analysis
Rainfall data was measured and digitally recorded using HOBO tipping bucket rain gauge. The rain gauge was installed on a levelled platform on a tower to make sure there was no surrounding obstacle. A rating curve was established by making a number of concurrent observations of rainfall and storage over a period of time, covering the expected range of stages at the rainfall gauging section. The physical change to the rainfall data alters this relation and these changes were accounted for in the derivation of a storage from a time series of storage data. The idea of rainfall-storage rating curve was adopted from the conventional stage–discharge rating curve where the data are plotted versus the concurrent stage to define the rating curve for the stream .
2.5 Groundwater pumping test
2.6 Dual water supply system
In the current study, rainwater was used in preference to groundwater and the system was equipped with automatic devices. The switching device used floats to automatically detect when sufficient rainwater was available for use in the building. When rainwater levels were low, the device automatically changed back to groundwater so there was no an uninterrupted supply of water to the building.
Main components of the installed systems
Length = 14 m and Width = 9 m
Ø = 0.3 m
Ø = 0.3 m
Capacity = 1000 L
Length = 14 m
Length = 2 m, Ø = 0.3 m
Capacity = 2000 L
Capacity = 1000 L
11 and 12
Length = 20 m
3 Results and discussion
3.1 Rainfall-storage rating curve
3.2 Constant rate test
3.3 Water demand and water usage
Meanwhile, March recorded the least amount of monthly groundwater usage of 6.2 m3 (Fig. 5d). The recorded rainfall depth was 197 mm with the total amount of rainwater collected in the storage tank of 11.5 m3. Increase in rainwater storage allows to reduce groundwater pumping by a more or less equivalent amount. Almost similar results found in April 2019. The amount of harvested rainwater and groundwater used were 11 m3 and 6.7 m3, respectively. In May 2019, the total volume of harvested rainwater was 8.6 m3 and complemented with 9.7 m3 of groundwater. Overall, it can be seen that most of the days rainwater could not meet the water demand thus have to be supported by the groundwater. However, in doing so, not only the rainwater could be captured and reused but there is no over-exploitation of groundwater issues occur in the future.
From the results discussed earlier, January recorded the highest monthly groundwater usage with 12.3 m3. Meanwhile, March recorded the least amount of monthly groundwater usage of 6.2 m3. From the constant rate test conducted, the daily discharge rate of the groundwater was 1.69 m3. Therefore, the pump was capable to extract approximately 50.7 m3 of groundwater per month. Overall, it can be concluded that the groundwater was able to be used as supplement water to the system without any over exploitation issues since the required groundwater volume to meet the monthly water demand ranged from 6.2 to 12.3 m3.
3.4 Observed and actual amount of rainwater collection
During the study period from December 2018 to May 2019, the monthly rainfall depth recorded at the study area were as follows: 100.4 mm in December 2018, 204.4 mm in January 2019, 93.8 mm in February 2019, 197 mm in March 2019, 279 mm in April 2019 and 318.6 mm in May 2019. In order to determine the effectiveness of the installed RWH system, actual volume of rainwater collected was identified. Equation 4 was used to identify the actual amount of rainfall that can be collected. The harvested rainwater were monitored daily at the study site. In the present study, the runoff coefficient was 0.75 since it gave the least differences volume for both observed and actual rainwater collected. This result was comparable to that suggested by NAHRIM (2014) which for corrugated metal sheet roofs, the runoff coefficients ranged between 0.7 and 0.9. In Seoul, South Korea, Mun and Han  estimated the collection efficiency of rainwater from a building rooftop of 0.8. A runoff coefficient of 0.85 was employed in a study carried out by Adugna et al.  to account for evaporation loss and possible first flush diversion.
Differences between observed and actual amount of harvested rainwater
Volume of harvested rainwater (m3)
In Malaysia, exploring alternative water sources and conserving natural resources have always been important issues in the context of water resources management. Therefore, the main aim of this study was to adopting the RWH system and groundwater abstraction as part of the conservation of natural water resources. This has also been investigated in some countries such as in France [24, 25] and Vietnam  and successfully implemented. To determine the collection efficiencies of RWH system, a comparison between the actual rainwater that can be harvested and the observed harvested rainwater was made. It can be concluded that 0.75 was the most accurate runoff coefficient since it gave the least differences volume for both observed and actual rainwater collected. To assess the yield of the tube well, a constant rate pumping test was carried out and the safest discharge rate was determined. Overall, by implementing this system, it can be observed that most of the months, rainwater could not meet the water demand thus have to be supported by the groundwater.
Overall, this study provides a significant reference for sustainable water management and this approach can be efficiently applied especially for rural areas, where there is no piped water supply. Water quality for both rainwater and groundwater warrant further investigations as the current study only focuses on the water quantity.
Siti Nazahiyah Rahmat was supported by a Fundamental Research Grant Scheme (FRGS) granted by the Ministry of Higher Education, Malaysia, Grant Number: 1523.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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