Feasibility Analysis of Rainwater Harvesting Systems for
Non-potable Water in a Public Building
Weilun Chen1, Weijun Gao2,3, Xindong Wei4, Jinming Jiang5

1 Doctor student, Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu
2 Professor, Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu
3 Academy Professor, iSMART, Qingdao University of Technology
4 Professor, School of Environmental and Municipal Engineering, Jilin Jianzhu University
5 Doctor, Innovation Institute for Sustainable Maritime Architecture Research and Technology, Qingdao University of Technology

水資源には限りがあり、雨水を貯めて非飲料水として活用することは大切だ。ではどのようなシステムが有効なのか。その答えを北九州大学における実証試験を通じて検証した。

Abstract

Rainwater harvesting is one of the important measures to alleviate water scarcity. However, the performance ofrainwater harvesting systems (RWH) in different regions and buildings is different. Public buildings have more
non-potable water demands, such as air conditioning cooling and building cleaning. Some demands are stablethroughout the year, but others are seasonally changed, especially for cooling. The changing demands may or may
not increase the feasibility of RWHs. In order to explore the feasibility of RWHs in public buildings, a Japanese
campus was selected as a case study. A water balance model and a life cycle cost model with actual monitoringdata were carried out to simulate the water-saving performance and the economic benefits of the RWH under different non-potable water demand scenarios. The results show that the installation of RWH in public buildings has
prominent water-saving performance and economic benefits. The optimal rainwater tank size of RWH should be
expanded when the non-potable water demand changes seasonally. The results obtained can not only serve as acomparison tool for other studies, but also provide data support for large-scale RWH research to promote the RWH.
Keywords

Rainwater harvesting system, public building, water conservation, economic analysis

1. Introduction

With the trend of increasing water demand caused by
global warming and population growth, water scarcity
worldwide is getting worse and worse. According to theprediction of the World Water Assessment Program(UNESCO WWAP), there will be 47% of the world population living in water scarcity regions in 2030[1]. Thus,
alternative water sources such as rainwater have been
widely used in many developed and developing countries to alleviate the pressure on water supply from centralized main water plants. Rainwater harvesting
systems (RWHs) are derived for supplying rainwater tobuildings as decentralized water reuse systems to substitute non-potable water because some water uses do
not rely on high-quality water such as laundry, toilets,
and irrigation taps[2].

Rainwater is one of the most common alternative
water sources. Rainwater harvested by RWH can beeasily reused without complicated purification facilities
after the first washing, and can even be reused directlyin some countries[3]. However, the water-saving efficiency of RWH is limited by many factors, such as theamount of precipitation and the rainwater capture areasof buildings[4]. This has led to the fact that the water-saving efficiency of RWH in single-family houses is higher
than that in multi-store houses under the same captureareas, and the water-saving efficiency of RWHs is negative in cold semi-arid and warm desert areas[5]. As aresult, the economic benefits brought by the RWHs insome building types and some regions cannot offset theinvestment cost. Therefore, although most countries
encourage the installation of RWHs on new buildings,

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Weilun Chen, et al.

there are still several regions that still lack confidence inthe economic benefits of spreading RWH to cities[6].
Compared with residential and commercial buildings, public buildings have a larger scale of non-potablewater consumption and more non-potable water uses,
such as cleaning and sanitation[7]. Previous studies haveshown that installing water reuse systems in campus
buildings is more feasible than other building types.
However, the evaluation of installing RWH in this typeof building is still in its infancy[8]. Evaluating thewater-saving performance and economic benefits of
RWH in campus buildings, which is a largely under-explored domain in public buildings, has far-reaching significance for the promotion of RWH.
Therefore, the motivation of this work is to encourage the use of RWHs in the context of public buildings,
as an integral part of sustainable water management.
This study aims to evaluate the feasibility of installing alarge-scale RWH on campus in Japan. A water balance
model and a life cycle cost model are used to determine
the water-saving efficiency and economic benefits of theRWH. The results found in this research can serve as acomparison tool for other studies and provide data support for stakeholders to popularize HRG.

2. Material and methods

2.1 Study area and system description
In order to explore the feasibility of RWHs in publicbuildings, a campus located in Kitakyushu, Fukuoka,
Japan was selected as a case study. The layout of thecampus is shown in Fig.1. The campus includes 7 buildings with 53214.35 m2 of service areas and 10632 m2 ofrainwater capture areas. An integrated RWH is stalled
in one of the buildings, which is mark with a red dot in
Fig.1. All rainwater from the campus is transferred to
the RWH and distributed to water end-uses.
The RWH on the campus includes rainwater storage
tanks, a piece of filtration equipment, rear rainwatertanks to store the treated rainwater, and the corresponding pumps and plumbing systems (Fig.2). Thus, there isno first flushing device in the RWH. Main water supplements are directly supplied into the rear rainwater tanks
to avoid insufficient rainwater supply because of the096

courtesy Google Inc.

Fig. 1 Location of the study area

RainfallRainwater
storage tanks

Spillage

Filtration
equimpent

Main
water

Rear
rainwatertanks

Cooling towers
Toilets
Irrigation

Fig. 2 Lay out of the RWH

scarcity of rainwater.
Non-potable water end-uses of the campus include
cooling towers for air conditioning and electricity systems, irrigation, and toilets of all the buildings. The
water consumption data of the campus is obtained byfield investigation of the water consumption inventory
in the campus. Fig.3 shows the annual trend of thosewater consumption. According to Fig.3, there are twokinds of water consumption modes on the campus, one
is the water consumption of toilets and irrigation, theother is the water consumption of cooling towers. The
former water consumption has a small change in a year.
The later water consumption mode, however, has obvious seasonal characteristics, which is topped in summer
and lower in other seasons. Thus, in this paper, three6th AFC Best Papers

the end of time step t (m3); SPIt is the rainwater spillageof rainwater tanks (m3); and Yt is the rainwater yield of
RWHs.
“Yield after spillage” algorithm (YAS) assumes thatthe yield of rainwater in rainwater tanks occurs after
excess rainwater spillage. This algorithm can simulate
the operation state of RWHs conservatively to obtain
more appropriate results than the “yield before spillage”
algorithm (YBS)[9]. Thus, in this paper, YAS is used tocarry out the simulation of the RWH:

600

Toilets
Irrigation
Cooling towers

Water consumption (m3)

500

400

300

200

100

0
0

45

90

135

180

Day

225

270

315

360

365

Fig. 3 Non-potable water consumption of the campus

kinds of non-potable water demands are assumed toevaluate the feasibility of the RWH in the public building: only for cooling towers, only for toilets and irrigation, and for all the non-potable water end-uses. The
annual water demands of those scenarios are listed inTable 1.

Annualwater
demand14879 m3

Scenario 2:
Toilets andirrigation

Scenario 3:
Cooling towers,
toilets, andirrigation

9847 m3

24726 m3

2.2 Methodology
2.2.1 Water balance model
Water balance model is used widely in the simulationof RWHs because it can accurately describe the operation state of RWHs according to daily step size. The
mathematical expression is:
+

−1 +

=

+

+

= {

= {

−1 +

−1 +

−

= {
0

Table 1 Annual water demand of the public building
Scenario 1:
Cooling towers(2)= 1000(1)

Where It is the rainwater harvested by rainwater storage tanks (m3); Rt-1 is the rainwater remaining in rainwater tanks at the beginning of time step t (m3); SUPt is
the main water supplements from municipal pipelines(m3); Rt is the rainwater remaining in rainwater tanks at

0
= {

−1 +

−
−

−1 +

−

+
+

−

−1 −

+

−

< 0;
0;

(3)

−
−

< 0;
0;

−1 +

−
−

< 0;
0;

(5)

−1 +

−
−

< 0;
0;

(6)

−1 +

−1 +

(4)

Where φ is the runoff coefficient (dimensionless,0-1), and 0.8 is considered in this paper; F is the rainwater capture areas (m2); H is the precipitation (mm); D isthe non-potable water demand of rainwater (m3); and V
is the size of rainwater tanks (m3).
Water-saving efficiency (W, %) is the proportion of
non-potable water that can be replaced by reusing
rainwater:∑= 100 × ∑(7)

Non-potable water supply capacity (NSC, %) refers
to the proportion of days in a year that the main water is
not necessary to be supplied to the RWH:
= 100 ×

−

(8)

097

Weilun Chen, et al.

Where Dtot is the total number of days in a year of
365 d; Dsup is the number of days that the main watersupplements to the RWH when the non-potable water
yield from the RWH cannot meet the demand (d).
Rainwater spillage rate (R, %) is the proportion of
the excess rainwater spilling to the harvested rainwater:∑= 100 × ∑(9)In the simulation process, the selection principle ofrainfall time series should be long enough to reflect thestability of rainfall and the time resolution of rainfall ispreferable into hour interval than daily interval thanmonth interval. However, previous studies have shownthat 20-year daily rainfall data can achieve similar
results to 50-year hourly rainfall data in continuous simulation[10]. Thus, a 20-year daily rainfall data from Japan
Meteorological Agency is used in this paper[11].
2.2.2 Economic analysis
Life cycle cost model is used to determine the economic benefits of the RWH in the campus. A life cycle
cost model includes the investment cost, operation andmaintenance cost, and dismantling cost of the RWH. A
20-year life cycle is assumed of the RWH because thelife cycle of almost all pumps and valves is 20 years, andthere is no replacement process in the 20-year life cycle.
On the other hand, because the dismantling cost has lessproportion during the whole life cycle cost[12], the dismantling cost is ignored in this paper. The rainwatertank is the most expensive component of RWHs in theinvestment cost compared to plumbing systems, thus theinvestment cost of the RWH includes rainwater tankcost, pumps cost, and filtration equipment cost. The
ideal life cycle cost model is:
=

+

(10)

Where COSTRWH is the life cycle cost of RWH (JPY);COSTV is the cost of rainwater tank (JPY); COSTC is theother components cost (JPY). The cost of RWH’s components is listed in Table 2.

098

Table 2 The cost of RWH’s components
Items
Rainwater tanks
Filtration equipment
Pumps

Price (JPY)9730 per m3
700,000
900,000

The direct economic benefits of RWH is to reducewater tariffs:
= ∑

×

(11)

(1+ )

Where BENEFITRWH is the economic benefit of
RWH (JPY); COSTW is the water tariff of the Kitakyushu(JPY); r is the discount rate of Japan (%), which is 3.5%
according to the Global Economic Data[13]; y is the year
in the life cycle of the RWH. The water tariff ofKitakyushu includes basic fee and usage fee, and thespecific price is shown in Table 3.
Benefit-Cost rate (BCR) was used to evaluate the investment feasibility of RWHs:=(12)

If the BCR is more than 1 shows that the RWH has economic benefits and it’s feasible for investment, otherwise it is negatively feasible to install the RWH in thecampus building. In addition, the sale tax rate is 8% inthis paper.
Table 3 The water tariffs of the Kitakyushu, Japan(monthly)
Price (JPY)
4500
122
156
208
288
310

Base
1 m3 – 25 m3
26 m3 – 50 m3
51 m3 – 200 m3
201 m3 – 1000 m3
1001 m3

Net present value (NPV) is used to determine the
cash flow of the RWH on the campus over a 20-year life
cycle:=−(13)

6th AFC Best Papers

3.1 water-saving efficiency
Water-saving efficiency of RWH on the campus
under three non-potable water demand scenarios isshown in Fig.4.
100

Water-saving efficiency (%)

90
80
70
60
50
40
30

Cooling
Toilets+irrigation
Colling+toilets+irrigation

20
10
0

0

500

1000 1500 2000 2500 3000 3500 4000 4500 5000

Rainwater tank size (m 3)Fig. 4 Water-saving efficiency of RWH on the campus

According to Fig.4, the water-saving efficiency of
installing RWH on campus can be divided into threestages: rapid rise stage, slow rise stage, and thresholdstage. The rainwater tank size before the water-saving
efficiency of RWH reaches the threshold is assumed tobe the optimal rainwater tank of RWH. First, when the
non-potable water demand scenario only has toilets andirrigation, the water-saving efficiency threshold of RWH
is 87.23%; when the non-potable water demand is only
for cooling towers, the water-saving efficiency thresholdof RWH drops to 68.46%. Finally, when all non-potablewater demands are provided by rainwater, the water-saving efficiency drops to 54.76%. This indicates that nomatter how to increase the rainwater tank size, RWH
will not be able to meet the demand for non-potablewater on campus. In addition, since all the roofs of thecase campus are set as rainwater capture areas, it is
impossible to increase the non-potable water-saving
efficiency by increasing the rainwater capture areas.
This indicates that installing a large-scale RWH on
campus will not be able to meet all the non-potablewater demands.

3.2 non-potable water supply capacity
The non-potable water supply capacity of RWH on
the campus is shown in Fig.5. Interestingly, although thetrends of the curves under the three non-potable water
demand scenarios are similar to those in Fig.4, the orderof the curve thresholds under the three scenarios is different from Fig.4. The highest value of NSC occurs
when rainwater is only used for cooling tower supplemental water (the water demand varies greatly with theseasons), reaching 92.88%. The second is to use rainwater only for toilets and irrigation, which is 83.38%.
When all non-potable water demands are provided by
rainwater, the NSC is only 66.30%.
Non-potable water supply capacity (%)

3. Results and discussion

100
90
80
70
60
50
40
30

Cooling
Toilets+irrigation
Colling+toilets+irrigation

20
10
0

0

500

1000 1500 2000 2500 3000 3500 4000 4500 5000

Rainwater tank size (m3)

Fig. 5 non-potable water supply capacity of RWH on thecampus

The water-saving efficiency of scenario 1 is lower
than that of scenario 2, but the number of stable watersupply days (NSC) of scenario 1 is higher than that of
scenario 2 is that the rainy season in the study area isconcentrated in the peak water demand period of scenario 1, thus more rainwater is used instead of spillingout of the rainwater tank (Fig.6). Compared to Scenario
1 with Scenario 3, in which the demand for non-potablewater varies with the seasons, Scenario 3 has a much
lower NSC value than Scenario 1 due to the greater
water demand. RWH under scenario 3 cannot meet
non-potable water demand nearly half of the year.
According to Fig.6, in Scenario 1 and Scenario 3,
almost no rainwater spills from the rainwater tank when099

Weilun Chen, et al.

100
90

Cooling
Toilets+irrigation
Colling+toilets+irrigation

Rainwater spillage rate (%)

80
70
60
50
40
30
20
10
0

0

500

1000 1500 2000 2500 3000 3500 4000 4500 5000

Rainwater tank size (m3)

Fig. 6 Rainwater spillage rate of RWH on the campus

This is still caused by the short peak period of
non-potable water in the rainwater demand model ofscenario 1, and the smaller water tank cannot store alarge amount of rainwater for use during the peak
non-potable water demand. A seasonal demand patternplaces a higher demand on the storage capacity of RWH.
3.3 Economic benefits
The economic benefits of RWH on campus are shownin Fig.7. It can be seen from Fig.7 that regardless ofwhether the non-potable water demand changes with theseasons, it is economically feasible to install RWH in
public buildings. In scenario 1 and scenario 2, when the
rainwater tank of RWH exceeds 4000 m3 and 3500 m3,
the BCR is less than 1. However, the water-saving efficiency thresholds of Scenario 1 and Scenario 2 arereached when the rainwater tank size is 2500 m3 and1300 m3, respectively. This indicates that when rainwater is only used for air conditioning cooling towers andonly for toilets and irrigation, there is no need to expand
the rainwater tank size of RWH to a higher level.
From the value of BCR, when the water-saving

100

efficiency reaches the threshold, the BCR of RWH is
1.62 (rainwater is used only for cooling towers), 2.52
(rainwater is used only for toilets and irrigation), and 2.2(rainwater is used for cooling towers, toilets, and irrigation), respectively. This shows that when rainwater isonly used for toilets and irrigation, RWH has stronger
investment potential. However, BCR is different fromthe NPV, the BCR emphasizes the investment feasibility
that it does not present the overall monetary value of thebenefits and costs[14], while the NPV emphasizes theabsolute of monetary value.
The NPV of RWH on campus for 20 years is shownin Fig.8. It can be seen from Fig.8 that when rainwater isused for all non-potable water supplies, although thewater-saving efficiency is low, the NPV value is thehighest due to the high water-saving amount. This showsthat in this scenario, although the reliability of watersupply cannot be guaranteed, the economic benefits or
RWH are the highest. Interestingly, the highest value of
NPV of RWH in the three scenarios occurs when the
rainwater tank volume is 900 m3 (25,044,591 JPY), 500
m3 (26,876,332 JPY) and 700 m3 (41,900,728 JPY),
respectively, instead of the threshold of water-saving
efficiency, which 2500 m3 (16,667,382 JPY), 1300
m3(22,033,471 JPY), and 2500 m3 (32,612,799 JPY).
This value appears in the first stage of the RWH
water-saving efficiency curve, that is, the rapid rise stage.
14
13

Cooling
Toilets+irrigation
Colling+toilets+irrigation

12
11
10
9

BCR

the water-saving efficiency of RWH reaches the threshold (2500 m3, rainwater spillage rate is 7.3% and 0%,
respectively). In addition, it can be seen that in the
non-potable water demand mode of scenario 2, when the
rainwater tank of the RWH is less than 600 m3, the rainwater spillage rate is smaller than that of scenario 1.

8
7
6
5
4
3
2
1
0

0

500

1000 1500 2000 2500 3000 3500 4000 4500 5000

Rainwater tank size (m3)

Fig. 7 BCR of RWH on the campus

6th AFC Best Papers

the economics of RWH, especially the investment in theoperation phase of RWH, more accurate modelingshould be carried out to more accurately analyze theeconomic benefits of RWH in different life cycles.

50000000
45000000
40000000
35000000
30000000

NPV

25000000
20000000

References

15000000
10000000
50000000- 5000000

Cooling
Toilets+irrigation
Colling+toilets+irrigation- 10000000- 15000000- 20000000

05001000 1500 2000 2500 3000 3500 4000 4500 5000

Rainwater tank size (m3)

Fig. 8 NPV of RWH on the campus

4. Conclusion

In order to evaluate the feasibility of rainwater harvesting systems in public buildings, an on-site decentralized rainwater harvesting system on a Japanese
campus is selected in this study. The water-saving performance and economic benefits of RWH are analyzed.
The conclusions are as follows:
Installing RWH in public buildings has a prominent
water-saving efficiency, but it cannot meet all non-potable water demands. When all non-potable water isreplaced by rainwater, RWH needs to rely on main water
for almost half of the year. A seasonally changing
demand for non-potable water places higher requirementson the rainwater tank size of RWH. Regardless of thewater demand model, it is economically feasible toinstall RWH on campus. However, the maximum economic benefits of RWH do not appear at the threshold ofwater-saving efficiency, but at the later stage of the rapid
rise stage of water-saving efficiency. As the water-saving efficiency of RWH enters a period of slow increase,
the economic benefit of RWH has a downward trend.
Therefore, a reasonable selection of RWH rainwatertank size between the best water-saving efficiency andthe best economic benefits has become the focus of
RWH design in public buildings.
Future research should increase the case studies of
RWH’s feasibility analysis in different building types,
especially with actual data as the background to improve
the accuracy of the results. In addition, in response to

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