Reference number  
RS 187: 2018  
© RSB 2018  
 
 
 RWANDA 
STANDARD  DRS  
187 
 
Second  edition  
2018 -mm-dd 
(Reaffirmed yyyy)  
Rainwater harvesting systems — Code of 
practice  
 
 
  DRS  187: 2018  
©RSB 2018 - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2018  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reprodu ce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw   
ePortal: www.portal.rsb.gov.rw   DRS  187: 2018  
iii                ©RSB 2018 - All rights reserved  
 Contents  Page 
1 Scope  ................................ ................................ ................................ ................................ .............  1 
2 Normative references  ................................ ................................ ................................ ...................  1 
3 Term s and definitions  ................................ ................................ ................................ ..................  1 
4 Design  ................................ ................................ ................................ ................................ ............  4 
4.1 Sizing  ................................ ................................ ................................ ................................ .............  4 
4.2 Collection  ................................ ................................ ................................ ................................ ...... 8 
4.3 Filtration and treatment  ................................ ................................ ................................ ...............  9 
4.4 Storage  ................................ ................................ ................................ ................................ ..........  9 
4.5 Materials and fittings ................................ ................................ ................................ ..................  10 
4.6 Power supply  ................................ ................................ ................................ ..............................  11 
4.7 Back -up water supply and backflow prevention  ................................ ................................ ..... 11 
4.8 Overflow and drainage  ................................ ................................ ................................ ...............  13 
4.9 Controls and metering  ................................ ................................ ................................ ...............  14 
4.10  Distribution  ................................ ................................ ................................ ................................ . 15 
5 Installa tion ................................ ................................ ................................ ................................ ... 16 
5.1 General  ................................ ................................ ................................ ................................ ........  16 
5.2 Tank installation  ................................ ................................ ................................ .........................  16 
5.3 Cistern installation  ................................ ................................ ................................ .....................  17 
5.4 Pipework installation ................................ ................................ ................................ ..................  17 
5.5 Testing and comm issioning  ................................ ................................ ................................ ...... 18 
6 Water quality  ................................ ................................ ................................ ...............................  18 
7 Maintenanc e ................................ ................................ ................................ ................................  19 
8 Risk management  ................................ ................................ ................................ .......................  20 
8.1 General  ................................ ................................ ................................ ................................ ........  20 
8.2 Water quality  ................................ ................................ ................................ ...............................  21 
 DRS  187: 2018  
©RSB 2018 - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The main task of technical committees is to prepare national standards.  Final Draft Rwanda S tandards adopted 
by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and gazettment as 
Rwanda Standards.  
RS 187 was prepared by Technical Co mmittee RSB/TC 013, Water and Sanitation . 
In the preparation of this standard, reference was made to the following standard:  
BS 8515:2009: Rain water harvesting  
The assistance de rived from the above source is hereby acknowledged with thanks.  
This second edition cancels and replaces the first. edition (RS  187: 2013), which has been technically revised  
  DRS  187: 2018  
v                ©RSB 2018 - All rights reserved  
 Committee membership  
The following organizations were represented on the Technical Committee on Water and Sanitation  (RSB/TC 
013) in the preparation of thi s standard.  
AQUASAN Ltd  
ELECTROMAX  
Entreprise U RWIBUTSO  
Inyange Industries Ltd  
JIBU Corporate  
Ministry in charge of Emergency management  
ROTO Tank Ltd  
RULIBA Clays  
Rwanda Mines, Petroleum and gas Board  (RMB)  
Rwand a Utility Regulatory Authority ( RURA ) 
SONAT UBE 
SULFO Industries  
University of Rwanda ( UR-CST) 
Water and Sanitation Corporation (WASAC)  
Rwanda  Standards Board  (RSB) – Secretariat  DRS  187: 2018  
©RSB 2018 - All rights reserved                  vi 
 Introduction  
0.1. General  
On-site collection and use of rainwater is an alternative to public mains water supply for a variety of non and 
potable  water uses in the home, workplace and garden. It can also provide benefits for the attenuation of  surface 
water run -off. 
As the rainwate r harvesting sector expands, there is a need for standardization to protect the public and to  
ensure that reliable systems are designed, installed and maintained.  
0.2. Types of rainwater harvesting  
 
0.2.1  There are three basic types of rainwater harvesting systems (s ee Figure 1):  
a) water collected in storage  tank(s) and pumped  directly to the points of use ;  
b) water collected in storage tank(s) and fed by gravity to the points of use; and   
c) water collected in storage tank(s), pumped to an elevated cistern and fed by gravi ty to the points of use.  
0.2.2  Within these basic types, there are variations such as:  
d) internal or external locations for tanks;  
e) single or multiple linked tanks;  
f) freestanding or fully or partially buried tanks;  
g) communal tanks supplying multiple properties; and  
h) packaged systems or components.  
 DRS  187: 2018  
vii                ©RSB 2018 - All rights reserved  
  
Figure 1 — Outline examples of rainwater harvesting systems
DRS  187: 2018  
1                ©RSB 2018 - All rights reserved  
 Rainwater harvesting systems — Code of practice  
1 Scope   
1.1 This Rwanda Standard gives recommendations on the design, installation, testing and maintenance of 
rainwate r harvesting systems in Rwanda.  
1.2 It covers systems supplying water for domestic uses (i n residential, commercial, industrial or public 
premises) that do not necessarily require potable water quality such as laundry, WC flushing and garden 
watering. It may however cover the potable purpose after meeting required standards RS EAS 12 Potable  
Water-Specification . 
1.3 It covers individual and communal systems and those providing stormwater control. It does not cover water 
butts. It also does not cover product design for specific system components. It applies to retrofitting and new 
build.  
NOTE Alt hough this Standard does not give specific recommendations relating to the use of rainwater for fire suppression 
or commercial irrigation, these applications are not excluded.  
2 Normative references  
The following referenced documents are indispensable for the application of this document. For dated 
references, only the edition cited applies. For undated references, the latest edition of the referenced document 
(including any amendments) applies.  
RS EAS 12 Potable Water -Specification   
3 Terms and definitions  
For the purposes of this standard, the following terms and definitions apply.  
3.1 air gap  
physical break between the lowest level of the water inlet and the maximum fault level or critical level of an 
appliance or installation, a feed pipe, or an air inlet orifice incorporated into a hydraulic circuit   
3.2 anti-surcharge  valve  
valved device, installed directly in the pipework of a drainage system intended to protect buildings from 
backflows and flooding from drains or sewers  
3.3 backflow  
movement of the fluid from downstream to upstream within an installation    
©RSB 2018 - All rights reserved                  2 
 3.4 backflow prevention device   
device which is intended to prevent contamination of potable water by backflow in a water supply system   
3.5 back -up supply  
supply of potable water, e.g. from the public mains water supply or private borehole, that can supplement the 
non-potable supply  in times of drought and/or heavy demand  
3.6 break cistern  
cistern used to separate two plumbing systems of different pressures, water qualities or flow rates, where the 
water from one system flows through an air gap and into the storage cistern feeding the second system  
3.7 calmed inlet  
fitting on the end of the drainage pipe feeding the storage tank that minimizes turbulence and slows the water 
flow into the tank  
NOTE  The calmed inlet is used to prevent disturbance of any sediments near the base of the tank.  
3.8 cistern  
fixed container for holding water at atmospheric pressure for subsequent reuse as part of a plumbing system  
3.9 control unit  
unit which automatically controls and monitors the function of the rainwater harvesting system to facilit ate 
effective operation  
3.10 cross -connection  
physical hydraulic link or a removable link between two separate systems, which can lead to cross  contamination  
3.11 dead leg  
section of pipework through which no water flows, usually created by closing a p ipe after the removal of a 
terminal fitting  
3.12 depression storage  
volume of water lost from surfaces, e.g. by evaporation or absorption, before run -off commences  
3.13 dip sample  
sample of water collected by immersing a container into a body of water and withdrawing it  DRS  187: 2018  
3                ©RSB 2018 - All rights reserved  
 3.14 domestic use  
use related to residential or similar dwellings  
NOTE 1 Potable domestic use includes water for the kitchen sink, wash and hand basins, bath, sho wer and dishwasher. 
Non-potable domestic use includes water for WC flushing, domestic washing machines and garden watering.  
NOTE 2 In commercial, industrial or public premises, “domestic use” is limited to water used for those 
applications/appliances desc ribed above and excludes, for example, water used for firefighting , central heating or irrigation 
systems.  
3.15 infiltration  
<into the ground> the movement of surface water or treated effluent into the ground  
3.16 green roof  
roof covered with vegetation  
3.17 nominal capacity  
dimensional volume of the maximum capacity of water that can be retained within the tank, e.g. up to the 
overflow  
3.18  non-potable water  
any water other than potable water  
NOTE  Non-potable water can  also be referred to as “unwholesome” water.  
3.19  overflow  
device that relieves the system of excess volume  
3.20  point of use  
point where water is drawn by the user either directly or by connecting an apparatus  
3.21  potable water  
water suitable fo r human consumption   
NOTE  Potable water can also be referred to as “wholesome” water.  
3.22  public mains water  
wholesome water supplied by a water undertaker, licensed water supplier   
©RSB 2018 - All rights reserved                  4 
 3.23  rainwater  
water arising from atmospheric precipitation  
3.24  rainwater butt  
cask set on end to store rainwater for garden watering  
3.25  return period  
average period of time within which the depth or intensity of rainfall for a given duration, e.g. 5 min, 24 hr, will 
be equalled or exceeded once  
3.26  soakaway  
pit or other drainage arrangement prepared in permeable ground to which surplus surface water is fed and from 
which it soaks into the ground  
3.27  spillover level  
level at which water will start to flow over the receiving vessel with all outlets closed  
3.28  stormwater control  
measures to control the rate and quantity of surface water run -off  
3.29  surface water  
water from precipitation, which has not seeped into the ground and which is discharged to the drain or sewer 
system directly from the ground  or from exterior building surfaces  
3.30  tank  
closed, watertight, vented container for rainwater, which forms part of a drainage system  
3.31  working capacity  
maximum capacity of water that can be extracted from a tank in normal use, e.g. from the ove rflow to the lowest 
extraction point  
4 Design  
4.1 Sizing  
4.1.1  General  
As the optimum storage capacity for a rainwater harvesting system is a function of the rainwater availability and 
the non -potable water demand, the following factors should be identified in ord er to calculate the size of the 
system:  DRS  187: 2018  
5                ©RSB 2018 - All rights reserved  
 i) the amount and intensity of rainfall;  
j) the size and type of the collection surface; and  
k) the number and type of intended applications, both present and future.  
4.1.2  Calculation methods  
4.1.2.1  General  
4.1.2.1.1  The storage capacity of the rainwater harvesting system should be determined using one of the 
following methods (a simplified approach for residential properties):  
a) where there is consistent daily demand, for which no calculations have to be carried out ( see 4.1.2.2);  
b) an intermediate approach which uses simple formulae to calculate a more accurate estimation of storage 
capacity than the simplified approach (see 4.1.2.3); and  
c) a detailed approach for non -standard systems, where there is variable demand thr ough the year (see 
4.1.2.4).  
NOTE 1 The simplified approach is not suitable for commercial premises as the assumptions relating to demand are not 
applicable. The intermediate approach may be used for certain commercial and industrial premises, such as sch ools and 
offices.  
NOTE 2 For larger rainwater harvesting systems, the size of the system needs to be analysed using a detailed approach to 
ensure a cost -effective solution is developed, as seasonal variations in rainfall can affect sizing requirements eve n where 
demand is relatively predictable and consistent.  
4.1.2.1.2  Once the storage capacity has been determined, storage tanks should be selected on the basis of 
working capacity, rather than the total capacity of the container.  
4.1.2.1.3   The size of the tank should allow for rainfall variation; however, it should be noted that construction 
above a certain size based on rainfall for that area provides very limited additional benefit unless stormwater 
attenuation is intended.  
NOTE The size of the tank will also affect how often the stored water overflows. Occasional overflowing can be useful for 
maintenance and might have benefits for water quality.  
4.1.2.2  The simplified approach  
4.1.2.2.1  To apply the simplified approach to sizing the rainwater harvesting s ystem for non -potable domestic 
use, storage capacity should be estimated using the following method.  
4.1.2.2.2  First, the roof plan area draining to the storage tank should be established and the annual average 
rainfall depth for the location of the site should be determined from Hydrological regions of Rwanda for sizing 
rainwater harvesting systems.   
©RSB 2018 - All rights reserved                  6 
 4.1.2.2.3  In most cases, the storage capacity should then be read from the Y axis of Figure 2, using the 
appropriate diagonal line for the rainfall depth. Ho wever, where the site has a large roof plan area and/or is in a 
region with high annual rainfall, the storage capacity should be determined in relation to the population in the 
house.  
NOTE  The simplified approach is based on the following assumptions:   
d) relatively constant daily domestic use through the year of 50 litres per day per person for WC flushing and clothes 
washing;  
e) annual average rainfall depth for the site location; and  
f) the use of standard tiled pitched roofs for the collection surface.  
  
Figure 2 — Storage capacities for non -potable domestic water based on maximum average annual 
rainfall and roof size for small populations (simplified approach)  
DRS  187: 2018  
7                ©RSB 2018 - All rights reserved  
 4.1.2.3  The intermediate approach  
4.1.2.3.1  To apply the intermediate approach to sizing the rainwater harvesting system for non -potable 
domestic use, storage capacity should be calculated from the following equations and should be the littlest of 
5 % of the annual rainwater yield or 5 % of the annu al non -potable water demand. 5 % of the annual rainwater 
yield should be calculated using the equation:  
YR = A × e × h × h × 0.05  (1)  
where:  
YR is the annual rainwater yield (L);  
A is the collecting area (m2);  
e is the yield coefficient (%);  
h is the  depth of rainfall (mm);  
h is the hydraulic filter efficiency.  
5 % of the annual non -potable water demand should be calculated using the equation:  
DN = P d × n × 365 × 0.05                  (2)  
where:  
DN is the annual non -potable water demand (L);  
Pd is the daily requirement per person (L);  
n is the number of persons.  
4.1.2.3.1   Where the system is to provide both non -potable water for domestic use and stormwater control, 
the integrated sizing approach given in A.2 should be used to estimate the addit ional storage capacity needed.  
NOTE  The intermediate approach is similar to the method described in 4.1.2.2 and thus the results obtained are likely 
to be similar. Equations are provided to allow a more flexible and accurate facility for calculating the s torage needed. The 
advantage of this is the variables can be modified to reflect the situation being considered.  
4.1.2.4  The detailed approach  
4.1.2.4.1  The detailed approach should be used to calculate the storage size more accurately for all situations 
and par ticularly where:  
a) the demand is irregular (e.g. external use, non -residential use, tourism);  
b) the yield is uncertain (e.g. due to the use of green roofs, permeable pavements); and   
©RSB 2018 - All rights reserved                  8 
 c) costly, larger or complex rainwater harvesting systems are proposed.  
NOTE  Computer models may be used to simulate the performance of the rainwater harvesting system as accurately 
as possible. The uncertainty associated with both the demand and the supply needs to be considered using appropriate 
uncertainty methods.  
4.1.2.4.2  To apply the detailed approach to sizing the rainwater harvesting system for non -potable domestic 
use, storage capacity should be estimated by building a model of yield and demand, which is based on a 
continuous rainfall time series for a minimum of 3 years and preferably 5 years. This time series should use 
daily rainfall data for assessing non -potable system storage.  
NOTE 1  From an analysis of the results of using a time series for design, the frequency of overflowing can be found. 
Occasional overflowing a llows any floating material to be discharged and offers potential benefits for water quality.  
NOTE 2  The analysis also enables an assessment of how much water might be saved annually and the number of days 
that no rainwater is available, which is particul arly useful if there is no back -up supply.   
4.2 Collection  
4.2.1  Surface  
4.2.1.1  When selecting a collection surface, the following factors should be taken into account, as these 
can affect the quality and quantity of the collected water: the surface’s materials and their drainage shall meet 
the following characteristics:  
a) run‑off from  a green roof will be significantly less than that from a hard roof and is also likely to be affected 
by a colouration caused by the soil or fertilizers; and  
b) the levels of pollution and the risk of contaminants entering the system.  
4.2.1.2  Surfaces subje ct to as little pollution as possible should be used. Ground level or trafficked surfaces 
can provide large areas for collection and may be used in areas where there is a high demand for non -potable 
water (e.g. commercial, industrial or public premises). A s these surfaces carry a greater risk of pollutants 
entering the system, they should only be used once a specific risk assessment has been completed (see Clause 
8).  
4.2.2  Guttering and collection pipework  
4.2.2.1  Roof outlets, guttering and pipework should fun ction as an integral part of the whole system, with 
access for routine maintenance and cleaning.  
4.2.2.2  Collection pipework should allow the rainwater to flow from the collection surface to the storage 
tank by gravity or syphonic action.  
4.2.2.3  Pipewor k should be free draining to avoid stagnation and should prevent contaminated water 
entering the system from other sources.  DRS  187: 2018  
9                ©RSB 2018 - All rights reserved  
 4.2.2.4  In addition, sealed gullies should be used at ground level to minimize the risk of pollutants entering 
the system.  
NOTE  Conventional rainwater goods and drainage pipes may be used.  
4.3 Filtration and treatment  
4.3.1  Filtration should be incorporated in the system before the collected rainwater enters the main body of 
stored water, to prevent debris accumulating in the tank e.g. a filter may be placed in the collection pipework 
upstream of the tank.  
4.3.2  The filter s ystem should include a filter which:  
a) is water and weather resistant;  
b) is removable and readily accessible for maintenance purposes;  
c) has an efficiency of at least 90%; and  
d) passes a maximum particle size of <1.25 mm.  
4.3.3  Additionally, to prevent any ot her floating debris from entering the distribution system, the storage tank 
should be fitted with a calmed inlet.  
4.3.4  Where feasible, a floating extraction point from the tank should be used, which is approximately 100 
mm to 150 mm below the surface of the water. If floating extraction is not practicable, a fixed extraction  
Note 1 Hard roof surfaces are considered the most suitable for rainwater collection, although many common roofing 
materials may also be used. It is important to note that most collec tion surfaces are likely to be affected by some form of 
contamination, e.g. animal and bird faeces, soil, grit, hydrocarbons and various chemicals. These contaminants can have 
negative effects on the quality of the water collected.   
Note 2 A rainwater har vesting system with filtration conforming to 4.3 provides water of a suitable quality for WC flushing, 
laundry and garden watering in most residential, commercial and industrial situations. However, readers might wish to note 
that some situations, e.g. whe re greater human exposure to the water is anticipated or where the water is to be used in 
public premises, could require higher water quality. In such cases, the system may incorporate treatment processes such 
as ultraviolet (UV) or chemical disinfection.   
Note 3  An additional filter may also be fitted at the extraction point.  
4.4 Storage  
4.4.1  General  
4.4.1.1  The rainwater harvesting system should, as a minimum, include a tank for primary storage, which 
may be positioned either above or below ground. All tanks should be appropriate to the site (see 5.2.1).  
NOTE 1  Tanks are normally prefabricated off site.   
©RSB 2018 - All rights reserved                  10 
 4.4.1.2  The tank(s) used in the system should be constructed from materials that create watertight 
structures without encouraging microbial growth.  
NOTE 1 Suitable materials include concrete, glass reinforced plastic (GRP), polyethylene or polypropylene, and steel coated 
with non -corrodible materials,   
NOTE 2  Storage may be accommodated in permeable pavement constructions or other structures, such as geo -cells.  
4.4.1.3  All tanks and cisterns, whether used separately or connected to each other in order to create greater 
capacity, should avoid stagnation, e.g. by ensuring that pipework connections allow the through -flow of water.  
4.4.1.4  All tanks and ciste rns should have screened ventilation and fitted lids to prevent contamination of 
the water.  
4.4.1.5  All tanks and cisterns should be sited so that the stored water does not attain temperatures that 
could encourage multiplication of Legionella.  
4.4.1.6  Where tanks are positioned above habitable or vulnerable areas, the risk of water leakage should 
be considered, e.g. bunding, additional drainage, sump pump.  
4.4.1.7  The loading of the structure should be taken into account when locating tanks.  
4.4.2  Above ground tanks and cisterns  
4.4.2.1  Where they are used, above ground tanks and cisterns should be insulated and opaque to minimize 
the potential problems of warming and algal blooms.  
4.4.2.2  The use of disinfection equipment is site and material spe cific depending on the user’s 
requirements and therefore expert advice is to be taken if such treatment is deemed appropriate.  
4.4.2.3  Consideration also needs to be given to the environmental impact of disinfection treatments.   
NOTE  Above ground tanks are particularly cost effective for retrofit applications.  
4.4.3  Below ground tanks  
Below ground tanks (and their covers) should be sufficiently rigid to resist likely ground and traffic loadings. 
Tanks should be installed to resist flotation.  
NOTE  This migh t require the use of concrete for backfilling.  
4.5 Materials and fittings  
4.5.1  The materials selected for the tank and other components should be suitable for the location and 
temperature ranges anticipated.  DRS  187: 2018  
11                ©RSB 2018 - All rights reserved  
 4.5.2  All components of the system should be cap able of withstanding pH levels as low as 5 for the lifetime 
of the products.  
4.5.3  Consideration should be given to the environmental impact of materials used. Existing resources on 
site should be utilized, where appropriate, and materials re -used where p ossible to limit the environmental 
impacts of the system.  
4.6 Power supply  
The power supply of the rainwater harvesting system should be readily accessible but also guarded to ensure 
against the inadvertent isolation or disconnection of electricity.  
4.7 Back -up water supply and backflow prevention  
NOTE  Attention is drawn to the Water Fittings standards which require adequate backflow prevention to be provided 
so that water supplied from the public mains water supply for domestic uses does not become contaminate d.  
4.7.1  Back -up water supply  
4.7.1.1 The rainwater harvesting system should incorporate a back -up water supply, which may be introduced 
into a purpose -designed module, incorporating a:  
a) break cistern prior to its pump, for delivery to the distribution pipework;  
b) an intermediate storage cistern, usually located at high level; or  
c) the main collection tank, via a direct connection or discharging into the collection pipework, but not before 
filtration.  
4.7.1.2  The back -up supply should be fitted with a control mechanism which ensures that the amount of 
water supplied is minimized to that needed for immediate use. It is recommended that this is provided from a 
make -up module or an intermediate  storage cistern.  
4.7.1.3  The back -up supply should be sized to allow it to meet the full demand requirements in dry periods.  
NOTE 1 It is important to consider the implications of using a back -up supply derived from public mains water during 
extended dr y periods. As the rainwater harvesting system is likely to rely on the back -up supply to satisfy much of the 
demand, certain applications will not be appropriate.  
NOTE 2  For instance, the use of the system for garden watering might be prohibited when wat er use restrictions (e.g. a 
hose pipe ban) are in place.  
NOTE 3 In order to bring this to the user’s attention, advice ought to be included in user instructions, wherever possible.  
4.7.1.4  If the back -up supply is to be fed into the main collection tank,  careful consideration should be given 
to tank selection in order to minimize the amount of water needed for continued normal operation before the 
next rainfall event, e.g. it might be beneficial to use a tank with a small sump area.   
©RSB 2018 - All rights reserved                  12 
 4.7.2  Backflow prevention  
4.7.2.1  To prevent non -potable water entering the potable or public mains water supply, the back -up supply 
should be fitted with a backflow prevention device   
4.7.2.2  Flow rates, head loss and installation requirements should be taken into account when se lecting 
the backflow prevention device.  
4.7.2.3  The backflow prevention device should be located upstream of, or at, the point of delivery where 
the two supplies come into contact with each other.  
4.7.2.4  The impact that a sudden demand from the back -up mechanism might create in operation on the 
water supply, particularly in large communal systems, should be considered and it is important that supply 
infrastructure is capable of meeting this increase.  
4.7.2.5  The design of the system should ensure that t here are no dead legs and suitable turnover of water 
is achieved, reducing the opportunity for water to become stagnant when not required. Where this is 
unavoidable, additional backflow prevention in the form of a single check valve should be provided at t he branch 
of the pipework supplying the back -up mechanism to protect the potable water supply.  
NOTE  Where the backflow prevention device is to be provided by a tundish arrangement and there is a risk of odours 
venting back into the premises, the use of a  waterless trap may be considered downstream of the tundish.  
4.7.2.6  When designing for backflow prevention, the appliances to be connected to the system should also 
be taken into account. Where an appliance with a single fill connection, e.g. washing mac hine, is to be supplied 
solely with rainwater, additional backflow prevention might not be required. However, if the appliance needs a 
supplementary potable source of water, e.g. public mains water.  
4.7.3  General  
4.7.3.1  For most systems, other than those whic h distribute the collected rainwater by gravity, a pump(s) 
should be used to ensure its continual availability.  
NOTE The operational safety and hydraulic demand will dictate whether a single pump or multiple -pump systems are 
needed.  
4.7.2.2  The pump sho uld be selected and arranged such that:  
a) energy use and noise are minimized;  
b) cavitation is prevented; and  
c) air is not introduced into the system.  
4.7.2.3  The pump should be equipped with dry -run protection, which may be either integral to the pump or 
provided by an external control device.  DRS  187: 2018  
13                ©RSB 2018 - All rights reserved  
 4.7.2.4  Surges, water hammer and hunting from the pump should be absorbed and prevented from causing 
undue high pressures, e.g. by the incorporation of expansion vessels or pressure controls, in order to prevent 
bursting and excessive draw off.  
4.7.4  Pumps outside the tank  
4.7.4.1  If installed outside the tank, the pump should have its own self ‑priming mechanism or a control 
system which ensures a constant fully primed condition. The suction line to the pump should be laid with a 
steady gradient upwards towards the pum p. The pump should be placed in a well -ventilated location and 
protected from extremes of temperature, with sound and vibration ‑free mountings.  
4.7.4.2  A non -return valve should be provided in the suction line to the pump in order to prevent the water 
column from draining down. The pressure line of the pump should be supplied with an isolating valve.  
4.7.5  Pumps inside the tank  
4.7.5.1  The immersion depth should be in accordance with the pump manufacturer’s instructions. The 
pump should be removable for mainte nance purposes.  
4.7.5.2  A non -return valve should be provided, with an isolating valve to enable the non -return valve to be 
maintained.  
NOTE A minimum level of water needs to be maintained above the pump inlet in order to prevent damage by sucking in air , 
sediment or debris.  
4.7.6  Multiple pump systems  
Multiple pump systems should conform to applicable standards, with a standby pump as necessary.  
4.7.7  Pump control unit  
4.7.7.1  The pump control unit should:  
a) operate the pump(s) to match demand;  
b) protect the pumps  from running dry; and  
c) protect the motor from over -heating and electric overload.  
4.7.7.2  The pump control unit should permit manual override.  
4.8 Overflow and drainage  
4.8.1  An overflow should be fitted to all tanks/cisterns to allow excess water to be di scharged during extreme 
rainfall events. The overflow should be such that any backflow is prevented and vermin are unable to enter the 
tank/cistern. Overflows fitted to above ground tanks/cisterns should be screened.   
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 4.8.2  The capacity of outlet pipe on t he overflow should be equal to or greater than the capacity of the inlet 
pipe.  
4.8.3  For the majority of systems, it is recommended that the overflow is connected to a soakaway.  
NOTE 1 For some systems, it might be more appropriate to either pass the flo w into the surface water drainage system or 
allow it to flood. The choice of drainage is dependent on factors such as ground conditions and the consequences of the 
performance being exceeded. For example, in locations where soils have very low permeability , a soakaway might overfill 
as a result of large rainfall events although this will result in minimal consequences like infrequent temporary local wet ar eas 
and run‑off in most instances.  
NOTE 2 Where the tank is likely to be full on a regular basis, due to high regional rainfall, the overflow may be located at a 
lower level and designed to throttle and attenuate the flow. However, if the throttle takes the form of a small orifice, it w ill be 
liable to blockage from floating material.  
4.8.4  The overflow from the primary storage tank or cistern is likely to contain a small amount of floating 
material such as leaves washed from filters so, if the water is to be passed to a soakaway, appropria te trapping 
of the material should be provided.  
4.9 Controls and metering  
4.9.1 A control unit should be incorporated in the rainwater harvesting system to ensure, as a minimum, that 
users are aware of whether the system is operating effectively. The control  unit should:  
a) control pumps and minimize operational wear and energy use;  
b) activate the back -up water supply automatically when the minimum water volume in the tank is reached; 
and  
c) provide a volt -free output to enable the system to be linked to a buildin g management system (BMS), where 
appropriate.  
NOTE 1  In order to prevent waste, storage tanks/cisterns with valve -controlled water inputs should have a warning 
system so any failure is readily noticeable.  
NOTE 2  In addition to the control unit, system st atus monitoring may be incorporated that informs the user of:  
a) whether rainwater or back -up supply water is being used;  
b) the volume of rainwater used and the volume of water used from the back -up supply. This can be logged and displayed;  
c) how full the tank  is; and  
d) any malfunctions. These should relate to the specific fault, e.g. pump failure, back -up supply failure.   
4.9.2  Additional monitoring of the overflow, water quality, tank temperature and other parameters may also 
be included.  DRS  187: 2018  
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 4.10 Distribution  
4.10.1  Genera l  
4.10.1.1  The system should distribute the collected rainwater by:  
a) pumping it from the storage tank directly to the point of use;  
b) pumping it from the storage tank to intermediate cisterns near the point of use;   
c) using a gravity distribution tank, wher e practicable; or  
d) using a full gravity system, without pumps.  
4.10.1.2  Consideration should be given to minimizing the energy used to distribute rainwater.  
4.10.2  Distribution pipework and fittings  
4.10.2.1  Where practicable, to differentiate rainwater pipework from potable water pipework, a contrasting 
type or colour of pipe material should be used. The pipework of the rainwater harvesting system, including any 
below ground back -up supply pipes, should not b e blue, as this is the recognized standard used for the potable 
water supply. It is recommended that pipes are either green, or black and green., In addition, all pipework and 
fittings should be marked and/or labelled in accordance with applicable regulati ons.  
4.10.2.2  Pipework should be sized to provide adequate flow and pressure, e.g. oversized pipes can cause 
water quality issues from low flows and excessive pressures can cause undue consumption or leakage.  
4.10.2.3  Pipework and fittings should be arra nged in such a way as to:  
e) be sufficiently strong to resist bursting from the pressure they are to be subjected to in operation;  
f) prevent cross -connections with any public mains water or potable water supply; and  
g) prevent the trapping of air during filling , and the formation of air locks during operation, that would cause 
water to be unduly drawn off to clear the system.  
NOTE  A variety of materials may be used for the distribution pipework, including:  
a) polybutylene;  
b) cross -linked polyethylene (PE -X);  
c) copper;   
d) stainless steel containing molybdenum (Mo); and   
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 e) multi -layer barrier pipes. It is important to note that, where polybutylene or cross -linked polyethylene pipes are to 
be installed below ground, ducting is required.    
5 Installation  
5.1 General  
Instal lation should be carried out in accordance with instructions given by the manufacturer or supplier. 
Installation should ensure that all components, including tanks, are accessible for future maintenance and/or 
replacement of consumable parts. In particular , consideration should be given to the following points:  
a)  access to below ground tanks;  
b) access for personnel to above ground tanks and cisterns, e.g. those located in lofts and roofs;  
c) the location of access covers and filters (avoiding the need for acces s equipment wherever possible); and  
d) vehicular access to the site.  
NOTE  Attention is drawn to local planning and national building regulations, including the Water Fittings Regulations.   
5.2 Tank installation  
5.2.1  General  
5.2.1.1  Prior to installation, any site specific factors that might affect the installation process should be 
taken into account. Such factors include:  
a) groundwater levels;  
b) ground strength and stability;  
c) land contamination;  
d) proximity to trees;  
e) proximity to uti lities and foundations;  
f) shading and temperature; and  
g) access routes.  
5.2.1.2  All tanks should be fitted with lids that protect the water from contamination and prevent inadvertent 
human entry. All tanks should be installed so that the stored water does n ot attain temperatures that should 
encourage multiplication of Legionella.  DRS  187: 2018  
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 5.2.1.3  Any holes that have been cut in a tank, other than those provided by the manufacturer, should be 
round, so as to not cause any additional stress on the tank that might resu lt in a split. Where non -circular 
apertures are unavoidable, stress relief should be applied to the aperture to minimize any risk of splitting.  
5.2.2  Above ground tanks  
5.2.2.1  Above ground tanks should be securely mounted and supported on a firm level base ca pable of 
withstanding the weight of the tank when filled with water to the rim.  
5.2.2.2  Tanks that are to be installed within a building should be able to withstand any temporary 
deformation that is required during installation, e.g. when being squeezed t hrough a doorway. Tanks, when 
installed and correctly supported, should not deform as the water level in the tank changes. Tanks should not 
be supported by pipework.  
5.2.3  Below ground tanks  
5.2.3.1  Below ground or partially buried tanks should be installed so  that they are not deformed or 
damaged. Measures, such as concrete surrounds or backfilling and/or controlled filling with water, should be 
taken to ensure the structural stability of these tanks.  
NOTE Issues relating to structural stability include: avoi ding flotation, resisting ground pressures and water table 
fluctuations (structural deformation), resisting vehicle loadings and accommodating differential movement.  
5.2.3.2  The area around the access covers of any below ground tank should be impervious a nd free 
draining away from the covers to avoid contamination during maintenance and inspection.  
5.3 Cistern installation  
5.3.1  Where storage cisterns are needed within buildings, these should be installed as for any cold water 
cistern with appropriate suppor t, insulation and means to prevent contamination. The cistern should be 
supported on a firm level base capable of withstanding the weight of the cistern when filled with water to the 
rim. Flexible cisterns should be supported on a flat rigid platform fully  supporting the bottom of the cistern over 
the whole of its area, e.g. close boarding.  
5.3.2  Overflows fitted to storage cisterns should be capable of discharging all inflows into the cistern.  
NOTE  In addition, an automatic supply cut -off device activate d by an overflow may be installed to minimize the waste 
of water.  
5.4 Pipework installation  
5.4.1  General  
5.4.1.1  The pipework connecting the collection surface to the tank should be installed so that water losses 
are minimized. Pipes should not discharge into open gullies where splashing or additional contamination could 
occur.   
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 5.4.1.2  Where specified in the design,  it should be ensured that an anti ‑surcharge valve is fitted to the 
overflow to prevent wastewater backflow.  
5.4.2  Labelling and identification  
Where two or more water systems, i.e. potable and non -potable, supply one property, all pipework, fittings and 
point s of use for the rainwater harvesting system should be marked and/or labelled in  order to facilitate 
identification, to prevent inadvertent consumption or cross -connection between the systems, and to avoid 
operating errors.  
5.5 Testing and commissioning  
5.5.1  The system should be flushed and tested prior to handover to ensure that pipework and containers are 
watertight and that there are no cross -connections and the manufacturer’s recommendations.  
NOTE  Running coloured dye through the system and carryin g out a visual inspection is regarded as a suitable test.  
5.5.2  All pipework and fittings should be tested and meet the requirements of applicable standards. The 
system should also be tested to ensure that wiring is electrically safe and that there is no interference to or from 
other electrical or electronic equipment, or wiring in the vicinity. Where the installation conforms to this code of 
practice, a certificate(s) of installation/commissioning should be provided.  
6 Water quality  
6.1 Frequent water sam ple testing is not necessary; however, observations for water quality should be made 
during maintenance visits to check the performance of the system. Tests should then be undertaken to 
investigate the cause of any system that is not operating satisfactori ly and any complaints of illness associated 
with water use from the system. Sampling for tests should be carried out.   
NOTE It is essential that rainwater harvesting systems are designed in a way that ensures the water produced is fit for 
purpose and pres ents no undue risk to health.  
6.2 Water quality should be measured in relation to the guideline values given in Table 1 for parameters 
relating to health risk, and Table 2 for parameters relating to system operation, which provide an indication of 
the wat er quality that a well-designed  and maintained system is expected to achieve for the majority of operating 
conditions.  DRS  187: 2018  
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 Table 1 — Guideline values (G) for bacteriological monitoring  
  
Table 2 — Guideline values for general system monitoring  
  
NOTE  Water quality will fluctuate particularly following rainfall events when there might be a short -term change.  
7 Maintenance  
7.1 Human entry into tanks should be avoided, wherever possible. Where entry is essential, it should only be 
undertaken by trained pe rsonnel with personal protection equipment suitable for confined spaces. Maintenance 
procedures should be in accordance with manufacturer’s maintenance recommendations.  
7.2 In the absence of any manufacturer’s recommendations, the maintenance schedule giv en in Table 3 
should be followed. The maintenance intervals listed here are for initial guidance but the frequency should be 
modified in the light of operational experience. A log should be kept of inspections and maintenance.  
 
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 Table 3 — Maintenance schedu le  
  
8 Risk management  
8.1 General  
8.1.1  A risk assessment should be carried out to determine whether the system is safe and fit for purpose. 
This should take place when the system is being designed.   
8.1.2  The risk assessment should consider the design, in stallation, testing and commissioning, operation and 
maintenance of the system, including water quality, structural stability, electrical safety and access provision.  
DRS  187: 2018  
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 8.1.3  The risk assessment should consider the effects of exposure to and the potential i mpacts of, the system 
on: people, including operators, installers, maintainers, a) and water users, particularly those who might be more 
susceptible to poor water quality (e.g. children or the elderly);  
a) the environment, including domestic and feral animal s, birds and fish, plants, water courses and 
groundwater; and  
b) physical assets, including buildings, foundations, drains, paved areas and gardens.  
8.1.4  The risk assessment should be used to identify additional actions, process improvements or enhanced 
controls that can reduce risks in a cost -effective manner.  
NOTE The use of rainwater for WC flushing and general garden watering is considered to be a low -risk 
application due to the low level of human exposure. However, there are some factors, such as the use of 
pressure washers and garden sprinklers that increase the extent of exposure through aerosols, thus making 
risk assessment necessary.  
8.2 Water quality  
The risk assessment should consider potential sources of contamination of water entering or already in the 
system. The risk assessment should be used to identify the need for any further water quality control measures, 
including additional monitoring, for systems where a ground -level and/or highly trafficked collection surface is 
to be used.  
NOTE  The Wo rld Health Organization endorses the “water safety plan” approach to protect the safety of water supplies. 
This involves a system of risk assessment and risk management.   
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 Bibliography  
[There is no bibliography]  
 
 DRS  187: 2018  
ICS 13.060.99  
 
 
©RSB 2018 - All right s reserved