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Economic Feasibility Analysis of Rainwater Harvesting System at Typical Public Buildings in Guangzhou

Objectives : Rainwater harvesting (RWH) is one of the most promising alternative water sources, since rainwater can easily be collected and used without significant treatment for non-potable purposes. However, the economical viability of these systems is not always assured. The objective of this study is to assess the potential water saving and financial performance of an RWH systems for a typical multifunctional building (with a rooftop area of 2,725 m) in Guangzhou, China.


Introduction
With the increasing population, continued urbanization, and over-exploitation of water bodies, Most cities in china have face water scarcity to satisfy the demands for human activities. 1) Guangzhou, which is the third largest city in China, with over 15.9 million populations. The city was under severe water restriction since 2010 due to rapid urbanization and population growth, which prompted the search of new strategies for the sustainable use of water. 2) A number of alternative water sources have received attention in recent years in China, which includes rainwater harvesting (RWH), grey water reuse and waste water recycling. 3) Among these, rainwater harvesting has received the greatest attention as rainwater is fresh in nature and can be easily collected and used for non-potable purposes.
However, due to the lack of sufficient researches on financial viability of RWH system 3,4) , many people in China do not readily see the benefit of RWH system over longer term, and still show reluctance in adopting a rainwater harvesting system. Even the policy makers (e.g., the Ministry of Housing and Urban-Rural Development of PRC) have casted doubts on the energy efficiency of the water reuse systems, and this dilemma is delaying further expansion of implementation of water reuse systems in China. 5) For this reason, a campaign promoting the implementation of RWH systems in urban areas for the purpose of showing their economic benefits and the potable water saving potential become extremely necessary, especially in regions with frequent precipitation and the ones with insufficient water supply. 6) The direct economic viability of RWH system depends on the balance between the investment, operating and maintenance costs of the system and the public water supply cost savings.
This balance comes to the potential amount of available rainwater, the effective amount of rainwater that is used for different purposes and the cost of the alternative water sources. 7) Therefore, whether implementing an RWH system is financially viable or not depends on the specific climate conditions of the site and the consumption distribution pattern of non-potable water in the building.
The research proposed in this paper is undertaken in the light of the current knowledge gaps to assess the financial viability of an RWH system in a specific region. This study quantifies the economic benefits and costs of a rainwater harvesting system in a multi-functional building that located in Guangzhou, to explore whether the utilization of rainwater on the scale of single buildings is an effective water-saving and economic feasibility strategy, and consequently to provide guidance to water authorities and public to enhance the acceptance of this green infrastructure.

Methodology
The following sections presents the case study area, and detail the methodology assumed in this work to compute the water savings and costs in this project.

Study area
Guangzhou was selected as the study region. It locates on the North edge of the Pearl River Delta (PRD) Plain with latitude 22°26′-23°56′ north and longitude 112°57′-114°3′ east. It has a typical subtropical monsoon climate and the average annual precipitation is 1,720 mm (statistical average from 1989s~2019s). The average annual temperature is 22.5 ℃.
shows the geographic locations of the study site.

Data
For the benefit analysis of the proposed RWH system, first step is to collect some basic data consist of the historical precipitation data for the city of Guangzhou, information about the non-potable water demand, and the components and prices of the system.

Rainfall data
To analyze the water supply reliability of RWH system, the latest three decades historical rainfall data were considered.

Economic data
The costs analysis of the system considers the capital and operating costs of the RWH system. The capital costs include the collection facilities, rainwater tank, pipe lines, treatment devices and installation costs, the labour costs are already included in the installation costs.
The RWH system used in this study consist of a flush diverter, a stainless tank, mosquito nets, PVC pipelines, a set of physical filters (GDM system), and a Chlorinator. The capital of which can be divided into two categories: the constant investment (e.g., the treatment device, pipelines, valve and additional facilities) and the variable capital (i.e., the storage tank, the price of which is varies with its capacity).
Costs of the rainwater harvesting system components and price The storage tank represents the most significant initial costs for an RWH system, its volume should maximize the efficiency of the system. In this study, the tank size was defined based on market availability with the goal of estimating the optimal capacity through evaluating the influence of the various commercial tanks in water saving efficiency and benefits cost ratio of the RWH system. The price of storage tank (stainless steel) with different size are listed in .
The annual operating cost here refers to the expenses of purchases, installs, operates and maintains the infrastructure (water tanks, pumps and pipes) needed for the RWH system by the building owners, as well as depreciation of fixed assets.
In this analysis, the periodic replacements of filter materials were taken into accounted. However, the labor costs and losses during suspended period have been neglected. Because the harvested rainwater from roof to storage tank and from the storage tank to the consumption points are delivered by gravity, thus no power consumption occurred in this case. The annual operating cost is estimated at 10% of the total capital cost according to empirical data collected from existing RWH system in the regions and from some historical data in previous studies. The annual depreciation cost of equipment is estimated at 4% of the total capital costs.
The financial benefit comes from the water savings by applying an RWH system, which is estimated based on the volume of tap water savings and water price. The value of possible environmental benefits from an RWH system (e.g. reducing greenhouse gas emissions from water treatment processes) is not considered in the costs benefits analysis due to limited data availability, which would have disfavored the

Data analysis
The rainfall and water consumption data was subsequently used to assess the performance of the RWH system through a water quantity balance model, which focused on: (1) The water saving potential, (2) the economical feasibility.
In order to calculate the water saving of an RWH system, This algorithm starts by setting the initial stored rainwater to zero. The available rainwater is then calculated from the catchment area, the daily rainfall and the runoff coefficient.
According to the YAS model described by Mitchell (2007), the stored rainwater is updated based on the available rainwater and previous stored rainwater. If the calculated stored rainwater is less than the tank capacity, all available rainwater is collected and the stored rainwater remains unchanged. If not, the stored rainwater is limited to the tank capacity and the harvested rainwater is calculated as the difference between the tank capacity and the stored rainwater at the end of the previous time step (shows in ). Afterwards, the used rainwater is determined depending on whether the stored rainwater is enough to satisfy the daily non-potable demanded or not. For example, if the stored rainwater is greater than or equal to the non-potable demanded, the used rainwater are equivalent to the non-potable water consumption and the remaining stored rainwater will be the difference between the stored rainwater and the non-potable consumption. Otherwise, the used rainwater is limited by the stored rainwater and the rainwater tank will be empty. This compute procedure will repeat for each day of the year. 7) For daily scale, the available rainwater in this case is mathematically expressed as: Where V c is the volume of available rainwater (m 3 ), R is the local daily rainfall (m), A c is the catchment area of roof (m 2 ), R c is the surface runoff coefficient, assumed equal to 0.8 to represent losses of 20%.
For water saving efficiency (WSE), the method outlined in Ward et al. (2012) 11) was adopted. It is calculated by dividing the volume of rainwater consumed by the total demand for water, as follows: Where, V is the volume of drinking water substituted by harvested rainwater and D is total water demand (m 3 ).
In this work, the economical feasibility of the proposed

Results and analysis
These sections present the water saving and economic analysis of the proposed RWH system in target building.

Water savings analysis
The performance of an RWH system in reducing potable water consumption is examined by quantifying its water saving defined as the total volume of rainwater supplied by the system.

As illustrates in
, the annual water saving of the RWH system increased significantly with increasing tank sizes before reaching a plateau (first stage). It is calculated that no more than 400 m 3 potable water can be saved by a 1 m 3 storage tank, nevertheless, this is boosted to over 3,300 m 3 if a 15 m 3 storage tank is used ( ). These patterns are in agreement with previous studies. [13][14][15][16] For the first stage, it is evident that increasing the tank volume affords an opportunity to store more rainwater. However, the amount of water saving reaches a plateau since the rainwater tank is able to store the total rainwater captured from the rooftop.
Therefore, it is useless to increase the tank size once it exceeding 20 m 3 .
Another indicator is the water savings efficiency (WSE), defined as ratio between the used rainwater and the non-potable water demand. Generally, high water saving is well in line with high water saving efficiency of RWH system under the same water demand pattern. 12) also shows the effect of increasing storage capacity on the water saving efficiency of an RWH system. As obvious, the WSE, follow the same pattern as the water saving curves. The results demonstrate that with successive increases in the tank capacity up to 20 m 3 , the WSE increase up to 57%. Afterwards, this indicator reaches a plateau for the range of 20 m 3 to 30 m 3 .
Significantly, an RWH system can achieves an annual water saving of 3,923.56 m 3 and a WSE of 56.86%, respectively, with a storage capacity equal to or greater than 20 m 3 ( ).

Payback period analysis
One method to estimate the financial viability of installing an RWH system is to evaluate the time period required to recover the project investment, namely payback period. In order to be competitive, a return on investment is expected within a reasonable period of time, and the lower the payback period is, the more attractive the project becomes. It is obvious that the payback period increase again when tank capacity over 10 m 3 , this is due to the fact that increases tank size raises initial investment and operating costs of the RWH system, while the amount of captured rainwater does not increase with the increase of tank capacity since the available rainwater is limited. Thus a successive increase in tank size will eventually leading to a point that the annual increase in expenditures exceeds the increase in revenues.
According to , it is not difficult to find that the growth rates of payback period decreased as the capacity beyond 5 m 3 , for instance, a nearly 80% decrease in payback period was found when rainwater capacity increased from 3 m 3 to 5 m 3 .
Nevertheless, the payback period decreased by only 26.9% when rainwater capacity increased from 6 m 3 to 8 m 3 , and only a 5.6% increase in PBP was witnessed with tank size increasing from 8 m 3 to 20 m 3 . This is because, with the continuous increasing of storage capacity, the tank size became a less limited factor, instead, the rainfall will becomes a more relevant variable for benefit effectiveness of an RWH system. The current work found that the maximum BCR (1.50) of the RWH system is achieved for a 10 m 3 tank scheme. The present study also found that the benefit cost ratio were below the desired value of 1.00 for small tank scenarios (between 1 m 3 and 4 m 3 ).
Indeed, the correlation between benefits costs ratio with the capacity of storage tank can be expressed by the following equations: Where y is benefits-costs ratio of the RWH system and x is the capacity of rainwater tank.

Sensitivity analysis
A sensitivity analysis was carried out for the water price,

Discussion
In this study, the water saving potential and costs effectiveness of an RWH system at a typical building were examined using a yield after supply model. According to the simulation result. The proposed RWH system achieved an annual potable water saving of 11,496.04 m 3 . The economic indicators in term of benefit cost ratio is 1.50 and payback periods are within 6.26 years, respectively, implying that this green infrastructures is economically viable for the a public building in Guangzhou.
However, it is found that the annual water savings and economic benefits from RWH system are strongly correlated

Benefits Costs Ratio
with tank sizes. As mentioned in sections 3.1, both the annual water saving and the water saving efficiency of the RWH system for this multifunctional building increased significantly with increasing tank volumes before reaching a plateau. These findings are in agreement with previous studies. [13][14][15][16] In fact, the potential water saving depending on the precipitation amount and its temporal distribution, catchment area, as well as tank capacity. For small tank schemes (< 20 m 3 ), the tank capacity is a limiting factor. It is evident that increasing the tank capacity within these ranges offered an opportunity to store more rainwater. However, with a further increase in tank sizes, the annual water saving and WSE reached a plateau of 3,923.56 m 3 and 56.86%, respectively. In this case, tank capacity is no longer a limiting factor. Instead, the catchment area of target building becomes a new constraint to the water saving potential of the RWH system, Hence, it will be uneconomical to increase the tank capacity considering the total amount of rainwater available is limited.
In terms of the PBP and BCR, their increasing patterns for the RWH system suggest increasing benefit by increasing the tank capacity. Conversely, the decreasing PBP and BCR patterns are due to the potential economic benefit that can't offsets the higher operating cost. As obvious, a small tank scenario would leading to extremely long payback periods. With regard to economic feasibility of RWH system, several studies have reported that a rainwater harvesting system can not achieve financial viability in the case of without government rebate, E.g., Domenech and Sauri (2010) 18) investigated the financial viability of the RWH system in single and multi-family buildings in the metropolitan area of Barcelona (Spain), they found that in single-family households an expected payback period was found to be between 33 and 43 years depending on the tank size, while in a multi-family building a payback period was 61 years for a 20 m 3 tank.
Whereas in another study, Tam et al. (2010) 19) investigated the cost effectiveness of RWH system in residential houses around Australia and found that although this system can offer a considerable portion of non-drinking water, but is hard to achieve desirable financial benefit for Brisbane. The same study found that it could not be possible to achieve "pay back" for the RWH system without some favourable scenarios and conditions for multi-storey buildings in Sydney. 9) Literature also reported that rainwater collection would only be feasible in South Korea for during 6 months of the year. 20) As has been noted, the financial viability of an RWH system depending on the tank size, climatic conditions, non-potable water consumption pattern, public water price as well as a discount rate considered. Our study found that for a multifunctional building in Guangzhou, the total investment of an RWH system can be recovered within 6.26-16.56 years time depending on the tank size. The humid climates in South China (with average annual precipitation higher than 1,700 mm) is the main reason account for these high costs effectiveness of an RWH system in Guangzhou. This finding is also consistent with the results found by Imteaz 21) and Jing 22) , as they stated that the local rainfall is the major variables of interest in the design and evaluation of RWH systems, as it determines the amount of collectible rainwater generated from a given contributing catchment.
It should be mentioned here that in this study, the results of the economical feasibility analysis performed should be considered conservative for the economic benefit only considers the reduction of potable water consumption resulting from rainwater reuses. Firstly, in this economic evaluation, the value of possible environmental and social benefits from an RWH system were not considered due to limited data availability.
For example, RWH system has the potential for reducing resources and energy consumption requiring to assure drinking water quality, convey it to end-user and removal pollutants from waste water before discharge 23) , these may contribute to the reduction of greenhouse gas emissions from water pumping and water treatment processes which causing climate change. Secondly, the widespread implementation of RWH system in densely urbanized areas may contribute to the reduction of the peak discharges in stormwater drainage systems 8) , reduce the size of downstream drainage infrastructure 24) , and with potential benefits on reduce urban floods frequency. 25) If all these benefits have been incorporated into the economic evaluation of the RWH system, the actual benefits costs ratio would increase and the payback period would be shorter than predicted.
Information from this study will be useful for local government authorities, who can forming a policy that will contributing to addressing urban water issues and provides guidance for building-owners to promote the campaign of green architecture construction. Moreover, government should take initiatives to educate the urban dwellers on water conservation benefits associated with RWH implementation in urban areas to achieve a sustainable development. 26) Although the current study is specific to Guangzhou, China, However, this paper presents an insight on potential economic returns of RWH system and such study will motivate others to conduct similar investigations in elsewhere.

Conclusions
In this study, a yield after supply model was used to examines the water savings potential and assesses the economic performance of an RWH system at a multifunctional building in Guangzhou, China. The simulation results show that, the RWH system is able to provide excellent financial returns to building owners. The costs effectiveness of the proposed RWH system in terms of payback period and benefit cost ratio is 6.26 years and 1.50, respectively. These results demonstrating that the concept of RWH is financial viability in Guangzhou.
Nevertheless, a widespread implementation of a rainwater harvesting system in the public buildings will not only lead to economic savings, but also go further to relive pressure on urban drainage systems and natural water body. These benefits were not considered in the economic analysis due to inaccessible of sufficient data. For practical purposes, it will be wise for other cities to quantify these benefits that will affect the financial returns from installation of RWH systems.
Future research should focus on refining the data and expanding the study regions.