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The influence of household rainwater harvesting system design on
water supply and stormwater management efficiency
Sangaralingam Ahilan, Peter Melville-Shreeve, Zoran Kapelan and David Butler
University of Exeter, Centre for Water Systems, Exeter, UK
Abstract: Rainwater harvesting is increasingly being recognised as a sustainable option for
both urban water and stormwater management. This study explores the potential impact of
household rainwater harvesting on water supply augmentation and stormwater management
in a typical three-bedroom house in Newcastle-upon Tyne, NE England. The continuous
simulation of historical rainfall events at 15-minutes resolution over a 30-year period (1984-
2013) is carried out to evaluate the system’s water saving and stormwater control
efficiencies. Current and future rainfall projections are also incorporated in the analysis. The
British Code of practice (BS 8515) is adopted to design the rainwater harvesting system.
Results indicate that a rainwater harvesting system which is primarily designed for water
supply augmentation with the size of 2.4 m3 contributes 64% of non-potable water demand
(toilet flushing) and an 86% reduction of stormwater runoff volume into the sewer system. A
larger system (6.5 m3) which is sized for both water supply augmentation and flood
management provides 70% non-potable water supply and 96% reduction of stormwater
runoff volume, indicating that a system which is designed for water supply only may be
sufficient to achieve dual benefits. The relationship between storage and system efficiencies
are explored for commercially available tanks for historical and future rainfall events. The
influence of storage volume on flood peak attenuation is also explored for the historical flood
events.
Keywords: Rainwater harvesting; Water supply; Stormwater management, Continuous simulation
1. INTRODUCTION
Growing urban populations and changes in rainfall patterns lead to ever-increasing pressure
on potable water resources and urban flood risk in many countries including the UK. In recent
years, Rainwater Harvesting (RWH) is increasingly being recognised as a potential
decentralised option for both water supply augmentation and stormwater management. Earlier
UK RWH studies typically focused on the potential impact of RWH on water supply
augmentation only (Fewkes & Warm, 2000; Fewkes & Butler, 2000). Recent studies are
beginning to explore their associated stormwater management benefits (Melville-Shreeve et
al., 2017). However, it is still unclear is what benefits can be obtained from a simple, single
volume tank, what efficiencies are achievable with respect to standard design methods and
what is the relationship between water supply augmentation and stormwater management.
This paper presents a preliminary modelling exercise to systematically evaluate both water
supply and source control benefits of a household RWH system in a typical three-bed room
house in Newcastle-upon-Tyne, NE England. In the paper, the study site is described, together
with-it rainfall regime and water demand. Evaluation is carried out using three indices to
compare the performance of systems designed using British Standard Code of Practice for
RWH design (BSI, 2013) under current and future climatic scenarios.
2. METHODOLOGY
2.1 Study site
This study focuses on a household rainwater harvesting system in a typical three-bedroom
house in Newcastle-upon Tyne, NE England with a roof area of 80 m2 (A) and an occupancy
of 4 (Figure 1). Newcastle is vulnerable to surface flooding as over 90% of the city centre
surface is impermeable. Recent widespread, localised flooding incidents have had a
significant impact on society and the economy which indicates the need for more sustainable
stormwater management solutions the city. Rainfall data sets over a 30-year period (1984-
2013) at 15 minutes resolution at the Jesmond Dene rainfall gauging station were obtained
from the Environment Agency and utilised in the study. UK Climate Projection 2009 (UKCP09)
rainfall data sets at the daily interval were also generated and incorporated to evaluate the
relative impact of future rainfall patterns (2050 - high emission scenario). The water
consumption for toilet flushing is assumed as 50 l/person/day and evenly distributed
throughout the day.
Figure 1. Schematic of the household rainwater harvesting system
2.2 System’s performance evaluation
The performance of the rainwater harvesting system was modelled using a yield-after –storage
(YAS) reservoir operating regime. Three non-dimensional indices are considered to evaluate
system performance. The first index measuring water-saving efficiency, ET, as defined by
Dixon et al. (1999), is the ratio between the volume of rainwater supplied and the non-potable
water demand (e.g. toilet flushing) during the simulation period is defined as:
(1)
where Yt [L3] represents the rainwater supply (yield) at each time step t, Dt [L3] is the water
demand at each time step, and T is the total number of time steps.
The second index, rainwater overflow ratio, OT, is defined as the volume of rainwater stored in
the system divided by the inflow to the rainwater harvesting system during the specified time
interval as modified from Campisano & Modica (2012):
(2)
where QD t [L3] represents the rainwater exceeding the system capacity at each time step t, A
is the roof area, Rt [L] is the rainfall amount at each time step, and T is the entire period under
consideration.
The third index RPO represents the reduction in peak discharge because of rainwater storage
over the simulation period as modified from Melville-Shreeve et al. (2017):
(3)
where RPO, QP [L3/T] and QRWH [L3/T] represent the ‘Reduction in Peak Overflow’, peak
discharge and peak discharge with rainwater harvesting system over the simulation period
respectively.
3. RESULTS AND DISCUSSION
Household RWH system performance evaluation
The modelled water-saving efficiency (ET) and the rainwater overflow ratio (OT) of RWH
systems over a 30-year (1984 - 2013) period for a range commercially available RWH
system sizes (1.5 – 15 m3) is shown in Figure 2 (ET (1984 - 2013 & OT (1984 – 2013). In
addition, 100 equiprobable 30-year, daily rainfall time series were also used in the simulation
to evaluate the potential impact of future rainfall patterns under a high emissions scenario.
The mean of these results is shown in Figure 2 (ET UKCP09 2050 & OT UKCP09 2050).
Figure 2. Water-saving and stormwater control efficiencies of the RWH systems
As shown in Figure 2, both ET and OT increase with tank size to a certain point whilst further
increases in storage doesn’t improve the efficiencies greater extent since inflow is fixed. The
(BSI, 2013) is adopted to dimension the RWH tank. As shown in Figure 2, the system which
is designed for water supply augmentation only has a volume of 2.4 m3 contributes 64% of the
non-potable water demand (WC) and a 86% reduction in stormwater runoff into the sewer
system. A larger system (6.5 m3), which is dimensioned for both water supply augmentation
and stormwater management provides 70% of non-potable water supply and 96% stormwater
control. Changes in future rainfall patterns with more dry spells lower the RWH system’s water-
saving efficiency (up to 6%) and increases the stormwater control efficiency (up to 3%). A 25%
larger size tank (3 m3) is required to meet the same (64%) non-potable water demand in 2050.
Table 1 summarises the impact of the RWH system on flood peak attenuation for the 20 largest
roof runoff events over a 30-year period with 2.4 m3 and 6.5 m3 tank sizes. Estimated Minimum
(Min), first quartile (Q1), Median, third quartile (Q3) and Maximum RPO of these 20 runoff
events are shown in Table 1.
Table 1. RPO for the 20 largest historical events
2400 l
6500 l
Min
0%
9%
Q1
32%
100%
Median
77%
100%
Q3
100%
100%
Max
100%
100%
The 2.4 m3 tank exhibits relatively larger variations in RPO than 6.5m3 tank as expected. As
indicted these are substantial reductions in the peak of major storms even for the smaller tank.
However, both tanks would have been unable to cope with most extreme historical rainfall
event (circa. 100-year event) which occurred on 28th June 2012 when 50 mm rain fell in around
2 hours of which 22.6 mm fell in 15 minutes. For Newcastle, this is equivalent to the expected
rainfall for the whole month of June. The smaller and larger tank provides 0% and 9% RPO
for the 2012 event respectively. This is because the volume of the 2012 rainfall event is so
high and the smaller tank has reached its full storage capacity.
CONCLUSIONS
This study explores potential benefits of RWH system on water supply augmentation and
stormwater control in a three-bed rooms house in Newcastle-upon Tyne, NE England. The
simulation results indicate that around 2/3 of WC demand could be supplemented by the
domestic rainwater system. Simulation results also indicate that future rainfall patterns (2050
– high emission scenario) would require a 25% increase in the tank storage to meet the
current water supply capacity. Furthermore, continuous simulation over the 30-year period
(1984-2013) shows that the RWH system could provide over 75% flood peak attenuation and
over 85% reduction in stormwater runoff volume.
References
BSI (2013) BS 8515:2009 + A1:2013 Rainwater harvesting systems – Code of practice. BSI, London.
Campisano, A., Modica, C. (2012) Optimal sizing of storage tanks for domestic rainwater harvesting in Sicily.
Resources, Conservation and Recycling, 63, 9-16.
Dixon, A., Butler, D. and Fewkes, A. (1999) Computer simulation of domestic water reuse systems: investigating
greywater and rainwater in combination. Wat. Sci. Tech. 39(5), 25-32.
Fewkes, A. (2000) Method of modelling the performance of rainwater collection systems in the United Kingdom.
Proc. CIBSE A: Building Serv. Eng. Res. Tech. 21(4), 257-265.
Fewkes, A. and Butler, D. (2000) Simulating the performance of rainwater collection and reuse systems using
behavior models. Proc. CIBSE A: Building Serv. Eng. Res. Tech. 21(2), 99-106.
Melville-Shreeve, P.J., Ward, S. and Butler, D. (2017) Dual-Purpose Rainwater Harvesting System Design.
Sustainable Surface Water Management: A Handbook for SuDS, Edited by Susanne M. Charlesworth & Colin
Booth.
