ICS 13.030.40  
 
Reference number  
RS 143: 2019  
© RSB 2019  
  
 RWANDA 
STANDARD  RS 
143  
 
Second edition  
2019 -09-13 
 
Wastewater treatment systems — Design 
and construction of septic tanks and 
associated effluent disposal systems — 
Code of practice  
   
RS 143: 2019  
©RSB 2019 - 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 2019  
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 reproduce 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   RS 143: 2019  
iii                ©RSB 2019 - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ....................  v 
1 Scope  ................................ ................................ ................................ ................................ .............  1 
2 Normative references  ................................ ................................ ................................ ...................  1 
3 Terms and definitions  ................................ ................................ ................................ ..................  2 
4 Information required  ................................ ................................ ................................ ....................  7 
4.1 Intended use of building  ................................ ................................ ................................ ..............  7 
4.2 Number and type of users  ................................ ................................ ................................ ...........  7 
4.3 7 
4.4 Natural drainage and ground water table  ................................ ................................ ..................  7 
4.5 Soil conditions  ................................ ................................ ................................ ..............................  7 
4.6 Elevations  ................................ ................................ ................................ ................................ ...... 7 
4.7 Existing soakage pits  ................................ ................................ ................................ ...................  7 
4.8 Maintenance capability  ................................ ................................ ................................ ................  8 
4.9 Options for effluent disposal or reuse  ................................ ................................ .......................  8 
4.10  Neighbourhood land use and environment  ................................ ................................ ...............  8 
4.11  Building layout plan  ................................ ................................ ................................ .....................  8 
5 Flow estimation  ................................ ................................ ................................ ............................  8 
5.1 General  ................................ ................................ ................................ ................................ ..........  8 
5.2 Residential wastewater flows  ................................ ................................ ................................ ...... 9 
5.2.1  Average daily flow  ................................ ................................ ................................ ........................  9 
5.3 Non-residential wastewater flows  ................................ ................................ ...............................  9 
6 Design of waste water control systems  ................................ ................................ ...................  10 
6.1 Process selection  ................................ ................................ ................................ .......................  10 
6.2 Septic tanks  ................................ ................................ ................................ ................................  10 
6.2.1  General provisions  ................................ ................................ ................................ .....................  10 
6.2.2  Location  ................................ ................................ ................................ ................................ ....... 11 
6.2.3  Tank geometry  ................................ ................................ ................................ ............................  12 
6.2.4  Dimensions  ................................ ................................ ................................ ................................ . 12 
6.2.5  Inlet and outlet arrangements  ................................ ................................ ................................ ... 12 
6.2.6  Access openings  ................................ ................................ ................................ ........................  13 
6.2.7  Freeboard  ................................ ................................ ................................ ................................ .... 13 
6.2.8  Chamber partitions  ................................ ................................ ................................ .....................  13 
6.2.9  Vent pipe  ................................ ................................ ................................ ................................ ..... 13 
6.2.10  Principles of operation ................................ ................................ ................................ ...............  13 
6.2.11  Prohibited discharges  ................................ ................................ ................................ ................  14 
6.3 Soakage Pits  ................................ ................................ ................................ ...............................  14 
6.3.1  General  ................................ ................................ ................................ ................................ ........  14 
6.3.2  Applicability  ................................ ................................ ................................ ................................  15 
6.3.3  Location  ................................ ................................ ................................ ................................ ....... 15 
6.3.4  Geometry  ................................ ................................ ................................ ................................ ..... 15 
6.3.5  Dimensions  ................................ ................................ ................................ ................................ . 15 
6.3.6  Cover ................................ ................................ ................................ ................................ ............  16 
6.3.7  Seepage beds  ................................ ................................ ................................ .............................  17 
6.4 Seepage trenches  ................................ ................................ ................................ .......................  19 
6.4.1  General  ................................ ................................ ................................ ................................ ........  19 
6.4.2  Applicability  ................................ ................................ ................................ ................................  19 
6.4.3  Location  ................................ ................................ ................................ ................................ ....... 19 
6.4.4  Geometry  ................................ ................................ ................................ ................................ ..... 19 RS 143: 2019  
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 6.4.5  Dimensions  ................................ ................................ ................................ ................................ . 19 
6.4.6  Aggregate  ................................ ................................ ................................ ................................ ... 20 
6.4.7  Soil cover  ................................ ................................ ................................ ................................ .... 20 
6.4.8  Distribution pipes  ................................ ................................ ................................ ......................  20 
7 Materials and construction  ................................ ................................ ................................ ....... 20 
7.1 Tanks  ................................ ................................ ................................ ................................ ...........  20 
7.2 Soakage pits  ................................ ................................ ................................ ...............................  21 
7.3 Aggregate media  ................................ ................................ ................................ ........................  21 
7.4 Excavation  ................................ ................................ ................................ ................................ .. 21 
7.5 Pipework  ................................ ................................ ................................ ................................ ..... 22 
8 Inspection and testing  ................................ ................................ ................................ ...............  22 
8.1 Water tightness  ................................ ................................ ................................ ..........................  22 
8.2 Partition test  ................................ ................................ ................................ ...............................  23 
9 Plumbing & drainage  ................................ ................................ ................................ .................  23 
10 Maintenance  ................................ ................................ ................................ ...............................  23 
10.1  Septic Tanks  ................................ ................................ ................................ ...............................  23 
10.2  Desludging  ................................ ................................ ................................ ................................ . 23 
10.2.1  Access covers  ................................ ................................ ................................ ............................  24 
10.2.2  Mosquitoes  ................................ ................................ ................................ ................................ . 24 
10.2.3  Blockages  ................................ ................................ ................................ ................................ ... 24 
10.3  Anaerobic filters ................................ ................................ ................................ .........................  24 
10.3.1  Filter cleaning  ................................ ................................ ................................ .............................  24 
Annex  A (normative)  Percolation test ................................ ................................ ................................ .. 25 
Annex  B (informative)  Example computations of average daily flow based on Table 1  ................  26 
Annex  C (informative)  Determination of Working Capacity of Septic Tanks  ................................ .. 28 
Annex  D (informative)  Typical septic tank arrangement  ................................ ................................ ... 30 
Annex  E (informative)  Typical soakage pit arrangement  ................................ ................................ .. 31 
Annex  F (informative)  Typical arrangement of a seepage bed  ................................ .........................  33 
Annex  G (informative)  Typical arrangement of a seepage trench system  ................................ ...... 34 
 RS 143: 2019  
v                ©RSB 2019 - All rights reserved  
 Foreword  
Rwanda Standard s are prepared  by Technical Committees and approved by Rwanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agree ment 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 143 was prepared by Technical Committee RSB/TC 09, Civil engineering and building materials . 
In the preparation of this standard, reference was made to the following standard:  
1) BS EN 1085 :2007  Wastewat er treatment – Vocabulary  
 
2) ASTM D5879 – 95-18 Standard Practice for Surface Site Characterization for On -Site Septic Systems  
The assistance de rived from the above source is hereby acknowledged with thanks.  
This second edition cancels and replaces the first ed ition RS  143: 2012, [ Clause 2, Normative reference was 
revised and clauses; 7.1 was revised.  )] of which have] been technically revised.  
Committee membership  
The following organizations were represented on the Technical Committee on Civil engineering and building 
materials  (RSB/TC 09) in the preparation of this standard.  
General Reliance Ltd  
Rwanda Polytechnic (RP) - Musanze  
JV CSC&EC(Property) and Fair Constru ction Ltd  
Rwanda Public Procurement Authority (RPPA)  
Rwanda Transport Development Authority (RTDA)  
Standards for Sustainability (SfS)  
University of Rwanda -College of Science and technology (UR -CST)  
Rwanda Standard Board (RSB) - Secretariat  RS 143: 2019  
©RSB 2019 - All rights  reserved                  vi 
  
 RS 143: 2019  
vii                ©RSB 2019 - All rights reserved  
 Introduction  
Septic tanks are widely used in Rwanda for the disposal of domestic wastewater.  
At present, the urban and suburban population of Rwanda use septic tanks. However, unless a septic tank and 
associated effluent disposal system are properly designed and impleme nted, failures occur with consequent 
aesthetic nuisance, public health hazard and environmental pollution.  
It is important that the requirements of the septic tank system be considered during the planning stage of any 
development.  
This code of practice is therefore intended to serve as a guide for the design and construction of septic tanks 
and associated effluent disposal systems in Rwanda. It is intended for use by consult ants, designers, 
manufacturers, certifying bodies, installers and regulators.RS 143: 2019  
1                ©RSB 2019 - All rights reserved  
 Wastewater treatment systems — Design and construction of 
septic tanks and associated effluent disposal systems  
1 Scope  
This Draft Rwanda standard gives technical aspects to be considered in the planning stages of wastewater 
control systems using septic tank systems and sets out the requirements for installation and operation of the 
systems.  
This draft standard covers the design, construction, testing and maintenance of septic tanks for the disposal of 
domestic wastewater  including all waste, black water and grey water systems. It also recommends guidelines 
for the selection, design, construction and m aintenance of systems for the on -site disposal of effluents from 
septic tanks. The systems recommended are soakage systems for the disposal of septic tank effluent below 
ground (soakage pits, seepage trenches, and seepage beds) for the disposal of septic t ank effluents  
2 Normative references  
The following documents are referred to in the text in such a way that some or all of  their content constitutes 
requirements 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 18 -1, Cements — Part 1: Composition, specification and conformity criteria for common cements  
RS EAS 18 -2, Cement — Part 2: Conformity evaluation  
RS EAS 148 (All parts), Cements — Test methods  
RS EAS 94 , Burnt clay building blocks — Specification   
RS 107, Building sand from natural sources — Specification  
RS EAS 54, Burnt building bricks — Specification  
RS 144 , Concrete building blocks and bricks — Specification  
RS ISO 22965 -2, Concrete — Part 2: Specification of constituent materials, production of concrete and 
compliance of concrete  
RS ISO 1920  (All parts) , Methods of testing concrete  
RS EAS 412 -2, Steel for the reinforcement of concrete — Part 2: Ribbed bars   
RS ISO 6935 -1, Steel for the reinforcement of concrete — Part 1: Plain bars   
©RSB 2019 - All rights  reserved                  2 
 RS 373, Aggregates for Concrete — Specification  
RS ISO 22475, Geotechnical investigation and testing — Sampling methods and groundwater measurements 
— Part 1: technical principles for execution  
 
 
3 Terms and definitions  
For the purposes of this standard, the follow ing terms and definitions apply . 
3.1 
aerobic bacteria  
bacteria that grow in the presence of oxy gen 
3.2 
all waste septic tank  
tank receiving the discharge of combined  black water and grey water  
3.3 
anaerobic bacteria  
bacteria that grow in the absence of oxygen  
3.4 
daily inflow  
volume in litres of sewage and liquid wastes flowing into  a septic tank during a 24 h  period  
3.5 
desludging  
removal of the accumulated sludge and scum from the septic tank  
3.6 
DN100  
nominal pipe diameter in millimetres  RS 143: 2019  
3                ©RSB 2019 - All rights reserved  
 3.7 
drain  
underground pipe for conveying sewage and liquid wastes to the septic tank  
3.8 
effluent  
liquid discharged from a septic tank  
3.9 
effluent disposal system  
constructed system utilising various methods and materials to effectively dispose of septic tank effluent  
.11 
hydraulic loading  
liquid flow required to be handled by the treatment p rocess  
3.12 
multiple occupancy residential premises   
more than one residential dwelling unit or flat constructed on one or more allotments and discharging into a 
common or multiple septic tanks  
3.14 
percolation  
movement of water into the soil  
3.15 
soakage trench  
subsurface soakage system using perforated pipe to convey the effluent along a trench  
3.16 
primary treatment  
treatment of sewage that occurs in the septic tank compartment/s of a waste control system   
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 3.17 
relevant authority  
local authority w here the proposed septic tank system is to be installed in an area subject to local government 
control  
3.18 
reticulated water  
any water supply obtained from a reticulated system and includes any bore, river or dam water supply  
3.19 
sanitary fixtures  
plumbi ng fixtures connected to the system including a bath, basin, clothes and dishwashing machines, food 
waste disposal unit, kitchen sink, laundry trough, spa bath, toilet and other sanitary  
3.20 
scum  
material floating on the surface of the septic tank which u sually  contains fats, oils and greases  
3.21 
septic tank system  
includes the sanitary plumbing fixtures, traps, waste pipes, vents, inspection openings, drains, septic tank and/or 
other treatment units and methods of effluent disposal  
3.22 
sludge  
solids which have settled to the bottom of the septic tank  
3.24 
soil permeability  
capability of a soil to allow water to percolate  
3.25 
subsurface soakage  
trench, bed, well or pipe system from which effluent percolates into the soil  RS 143: 2019  
5                ©RSB 2019 - All rights reserved  
 3.26 
suspended solids  
solid particles held in suspension including settleable and non settleable matter  
3.27 
waste control system  
includes any system providing for the collection, treatment or disposal of human, commercial or industrial wastes 
in solid or liquid form incorporati ng biological, chemical or artificial means and fixtures, fitting, appliances, plant, 
processes associated with such a system  
3.28 
wastewater  
water which is collected and transported through waste pipes and sewers and normally includes water from 
domestic,  commercial and industrial sources  
3.29 
grey water  
wastewater drained from sinks, tubs, showers, dishwashers, clothes washers, and other non -toilet source s 
3.30 
black water  
liquid and solid human body wastewater with only faecal matter and urine  and the ca rriage waters generated 
through toilet usage  
3.31 
septic tank  
closed sedimentation tank in which settled sludge is in immediate contact with the wastewater flowing through 
the tank, and the organic solids are partially decomposed by anaerobic  bacterial action  
3.32 
baffle  
device used in a tank to reduce eddies and promote a more uniform flow through the tank   
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 3.33 
Biochemical Oxygen Demand (BODt)  
concentration of dissolved oxygen consumed under specified conditions ( t days at 20 °C with or without 
nitrification inhibition) by the biological oxidation of organic and/or inorganic matter in water  
3.34 
Chemical Oxygen Demand (COD)  
concentration of oxygen equivalent to the amount of dichromate consumed when a water sample is  treated with 
that oxidant under defined conditions  
3.35 
discharge point  
point where the final effluent is discharged into the receiving water  
3.36 
inspection port  
opening in the top of the tank which allows inspection of the contents  
3.37 
invert  
lowest po int on the inside of a pipe or drain at a given cross section  
3.38 
competent authority  
organization with appropriate statutory powers of controlling environment protection  
3.39 
wastewater treatment system  
system relying on natural processes and/or mechanic al components that is used to collect, treat, and 
disperse/discharge wastewater from single dwellings or buildings.  RS 143: 2019  
7                ©RSB 2019 - All rights reserved  
 4 Information required  
4.1 Intended use of building  
The intended and/or current use of the building shall be identified (e.g. individual home, shop, office building, 
etc.). Possible future uses, which could differ from the current use, should also be identified, as usually, septic 
tanks are not refitted when buildings undergo change of usage.  
4.2 Number and type of users  
4.2.1  The maximum numbe r of users who would reasonably be expected to use the building and facilities 
on a daily basis shall be estimated as accurately as possible. It should be noted that septic tank systems shall 
be designed for the maximum  and not the average  number of daily users. In the case of separate dwelling units, 
the number of estimated users shall be at least 6.  
4.2.2  In the case of commercial buildings, the number of each type of users such as residents, day -time 
users, visitors, customers, service staff etc. shall b e determined according to the provisions given in Annex B.  
Site plan . 
4.2.3  A site plan and layout of the building and facilities shall be obtained. The plan should show all relevant 
details such as building outlines, location of water sumps, wells, proper ty boundaries, street lines, etc.  
4.3 Natural drainage and ground water table  
The natural drainage features of the site shall be assessed, either by means of a contour plan of the site or by 
visual assessment or both. Storm water drainage paths should be ident ified. The highest seasonal level of the 
local groundwater table shall be determined. This should be done either by observing water levels in wells on 
the site or in the immediate vicinity, or by digging trial pits to determine the location of the groundwa ter table  or 
in accordance with the provisions given in RS ISO 22475 . Information on seasonal variation of the local 
groundwater table can often be obtained by interviewing residents in the locality.   
4.4 Soil conditions  
The soil shall be explored to sufficien t depth to ascertain the soil types, grading structure, stability and 
permeability. Where soakage of effluent is being considered, a percolation test shall be carried out as described 
in Annex A . 
4.5 Elevations  
Invert elevations of all wastewater outlets from the building shall be determined, together with any external 
factors which may affect the invert elevation of the inlet to the septic tank. In cases where treated effluent is to 
be discharged to the surface, the elevations of potential discharge points sha ll be determined.  
4.6 Existing soakage pits  
The locations and size of any existing soakage pits in the vicinity shall be noted.   
©RSB 2019 - All rights  reserved                  8 
 4.7 Maintenance capability  
The maintenance capability of the potential users or owners of the facility should be assessed. Septic system s 
frequently fail due to inadequate or inappropriate maintenance and ignorance of users.  
4.8 Options for effluent disposal or reuse  
Potential options for effluent disposal shall be explored with the consultation of the users and/or developers. 
Potential for ef fluent reuse for uses such as gardening, irrigation, toilet -flushing, and landscaping shall be 
evaluated. Significant savings in fresh water consumption should be achieved through on -site effluent reuse.  
4.9 Neighbourhood land use and environment  
The land use and local environmental conditions in the immediate neighbourhood of the site shall be 
investigated and relevant details noted (e.g. are there drinking water wells in the immediate vicinity? Are any 
streams used for drinking / bathing downstream? What is t he condition of the roadside drains? etc.).  
4.10 Building layout plan  
A detailed building layout plan shall be provided showing:  
a) the position and description of all the sanitary fixtures to be connected to the septic tank e.g. water closet 
pan, basin, bath, sho wer, laundry trough, washing machine, kitchen sink, dishwasher and food waste 
disposal unit;  
b) method of connecting the fixtures to the drainage system  including location of the sewer, drain, inspection 
openings, junctions and bends, size and grade of sewer  drain, position and size of traps (sanitary fixture, 
floor waste and overflow relief gully), vents and waste pipes.  
c) Site plan layout - should include setbacks from septic tank to adjacent buil dings and to the nearest road.  
5 Flow estimation  
5.1 General  
The required hydraulic capacity for an on -site wastewater treatment system is determined initially from the 
estimated wastewater flow. Reliable data on existing and projected flows should be used if onsite systems are 
to be designed properly. In situations whe re onsite wastewater flow data are limited or unavailable, estimates 
should be developed from water consumption records.  
The overall urban water consumption in Rwanda for drinking and basic sanitation services, human hygiene and 
food preparation should be 180 liters  average per capita per day for high standing premises, 80  liters  average 
per capita per day for medium  standing premises and 50 liters  average per capita per day for low standing 
premises of which 60  % - 70 % average is transformed into greywater, while most of the rest is consumed for 
toilet flushing and released as blackwater and this shall be the basis for the system design.  
In rural areas, the system design should base on the average daily per capita consumption of 20 liters.   RS 143: 2019  
9                ©RSB 2019 - All rights reserved  
 When using water meter readings or other water use records, outdoor water use should be subtracted to develop 
wastewater flow estimates.  
Accurate wastewater ch aracterization data and appropriate factors of safety to minimize the  possibility of system 
failure are required elements of a successful design. System design varies  considerably and is based largely 
on the type of dwelling under consideration. For exampl e, daily flows are usually expressed on a per capita 
basis.  
Applying these data to characterize residential or non residential wastewater therefore requires that a second 
parameter of the number of persons living or present in the residence be considered.  
5.2 Residential wastewater flows  
5.2.1  Average daily flow  
The average daily wastewater flow from residential premises shall be estimated fro m indoor water use in the 
home.  
Average daily flow is the average total flow generated on a daily basis from individual waste water generating 
activities in a building. These activities typically include toilet flushing, showering and  bathing, clothes washing, 
dishwashin g and other miscellaneous uses.  
5.3 Non-residential wastewater flows  
For non -residential premises, daily flows are generated from a variety of commercial, institutio nal, and 
recreational premises.  
The average daily flow of wastewater to the system shall be estimated as accurately as possible. In the absence 
of more accurate information, the flow shall be estimated for each category of user by multiplying the maximum 
number of users in each category by the respective per capita daily flows. The average daily flow is the 
summation of the flows of all the contributory user categories.  
The per capita daily wastewater flow f or each category of user based on daily water consumption per capita is 
given in Table 1 below . 
   
©RSB 2019 - All rights  reserved                  10 
 Example computations of average daily flow based on per capita daily flow  are given in Table B.1 . 
Table 1 — Per capita daily flows for different user categori es 
User Category  Per capita wastewater flow (litres/person/day)* 
All waste  Blackwater  Greywater  
Houses, housing 
estates and apartment 
complexes  Residents  130 39 91 
Shops, Offices, etc.  Daytime employees  50 15 35 
Overnight employees  120 36 84 
Customers / Visitors  10 3 7 
Schools, Universities, 
etc Residents  120 36 84 
Daytime  50 15 35 
Restaurants (dine -in) Overnight employees  120 36 84 
Day-time employees  50 15 35 
Meals served  25 l/meal  10 l/meal  15 l/meal  
Restaurants (take -
away)  Overnight employees  200 50 150 
Day-time employees  50 30 20 
Meals served  15 l/meal  N/A 15 l/meal  
Hotels  Guest  240 60 180 
Staff (residential)  200 50 150 
Staff (non -residential)  100 50 50 
Kitchen  l/meal  N/A l/meal  
Swimming pool  l/user/d  N/A l/user/d  
* Except where otherwise shown.  
6 Design of waste water control systems  
6.1 Process selection  
The proper selection of an appropriate effluent disposal process is essential to the satisfactory functioning of a 
septic tank. This code of practice recommends the use of soakage pit, seepage trench and seepage beds for 
the disposal of septic tank effluen t or other approved means of subsurface disposal of effluent, or by 
conservancy tank, which shall be emptied regularly.  
6.2 Septic tanks  
6.2.1  General provisions  
6.2.1.1  The main function of a septic tank is to separate, retain and partially digest settleable and f loatable 
solids in wastewater. The working capacity of a septic tank shall be sufficient for all these functions to occur.   RS 143: 2019  
11                ©RSB 2019 - All rights reserved  
 6.2.1. 2 Septic tanks shall be watertight, with sufficient structural strength and integrity to withstand external 
soil pressures, in ternal and external water pressures and any likely imposed loading.  
6.2.1. 3 Septic tanks situated under driveways and parking areas shall be designed to carry the appropriate 
vehicle loads.  
6.2.1. 4 All septic tanks either precast concrete septic tank or c ast in -situ septic tank, unless otherwise specified 
by the competent authority, shall be designed and constructed as follows:  
a) septic tanks over 2500 litres shall be divided into two chambers so that the effective capacity of the first 
chamber is twice that  of the second chamber,  
b) the length to width ratio shall be not less than 2:1 and a length to depth ratio of approximately 2:1,  
c) the effective liquid depth shall be not less than 900  mm, 
d) connections for the inlet, outlet and inspection openings shall be inte grally cast for concrete constructed  
septic tanks , 
e) the septic tank shall be installed on a compacted, level base and the top  cover for inspection chamber or 
manhole of the tank shall terminate at least 50mm above the finished ground surface level, with the  
surrounding surface graded away from the septic tank and be provided with access covers  constructed of 
a material as approved and be of sufficient strength to withstand all imposed loadings including vehicle 
loads where situated in vehicle access areas, b e constructed so as to be child proof and effectively sealed 
to prevent the ingress and/or egress of water or gas and be removable for maintenance purposes, be 
positioned centrally over the dividing compartment wall and have an access opening of at least 9 00mm 
long and 500mm wide, or be positioned over the inlet and outlet fittings and have dimensions to ensure an 
access opening of at least DN500 or 500  mm x 450  mm, and 
f) for septic tanks over 5000 litres  capacity, they shall be provided with access opening o f at least 900  mm 
long and 500  mm wide and be positioned centrally over the dividing wall or have access openings of at 
least DN600 and be positioned to permit inspection of the inlet and outlet fitting and enable access to each 
compartment for maintenance.  
6.2.1. 5 Cast in situ concrete septic tanks should be generally used  where larger capacity septic tanks are 
required e.g., greater than 10 000 litres.  The tank shall be structurally sound, smooth internally, watertight and 
the concrete used sha ll have a compressive strength of not less than 25 MPa at 28 days.  
6.2.2  Location  
Septic tanks shall be located in an open area wherever possible. However,  they may be located under car parks, 
driveways, terraces etc., in order to save space, provided due consid eration is given to the structural integrity 
of the tank and adjacent features.  
Sufficient access to the tank shall be available for inspection and desludging activities.   
©RSB 2019 - All rights  reserved                  12 
 6.2.3  Tank geometry  
The geometry of a septic tank should be rectangular, with length betwee n 2 to 4 times the width. Tanks of other 
shapes such as circular section may be used provided the area of the water surface in the tank during normal 
operation is sufficient to ensure proper solids separation.  
In the absence of detailed analysis, the minim um surface area requirement may be estimated empirically as 
follows:  
Minimum surface area (m2) = working capacity of tank (m3) / 3. 
In the case of multi -compartment tanks the compartments shall be of unequal size to avoid mass oscillations of 
liquid in the  tank.  
6.2.4  Dimensions  
The working capacity of the septic tank shall be estimated as described in Annex C.  
In any event the working capacity of a single tank shall always be greater than 1m3 and less than 12 m3. 
Where the required working capacity exceeds 12 m3, parallel sets of tanks shall be used such that the working 
capacity of each is less than 12 m3.  
Septic tank for all  wastes for residential dwellings shall have the capacity of not less than 3  m3 for all water 
closet and household liquid was tes for up to 6 persons. For each additional 2 persons 1  m3 shall be added.  
Septic tanks for sewage only shall have the capacity of not less than 1 .620 m3 for sewage from pan for up to 6 
persons. For each additional 2 persons, 0.540  m3 shall be added.  
For multiple occupancy residential premises such as flats, units and town houses the capacity of the septic tank 
shall be calculated on the basis of total number of bedrooms plus one bedroom and multiply by 2 persons per 
bedroom.  
The minimum internal width of a tank shall be 750 mm . 
The minimum depth below liquid level shall be 1  m. 
Annex D  shows the arrangement of a typical septic tank.  
6.2.5  Inlet and outlet arrangements  
Tee fittings, elbows or baffles shall be provided at the inlet and outlet.  
The inlet fitting shall extend a minimum of 20 % of the liquid depth below the liquid level.  
The outlet fitting shall extend a minimum of 300 mm below the liquid level of the tank.  RS 143: 2019  
13                ©RSB 2019 - All rights reserved  
 The invert of the outlet pipe shall be at least 50 mm below the invert of t he inlet pipe.  
6.2.6  Access openings  
One or more access openings shall be provided for inspection and desludging.  
Openings may be circular, square or rectangular. Circular access openings shall be at least 500 mm in diameter. 
Square or rectangular openings shall  have a minimum minor dimension of 500 mm.  
Openings shall be provided with a suitable cover to prevent the ingress of surface and ground water into the 
tank.  
6.2.7  Freeboard  
A minimum of 200 mm freeboard shall be provided between the liquid level and the highest  point on the ceiling 
of the tank.  
The air space thus provided shall have a volume equivalent to at least 10 % of the total tank volume.  
6.2.8  Chamber partitions  
In the case of multiple compartment tanks, chamber partitions shall have one or more openings, of to tal area 
greater than the area of the inlet to the tank, at a height between 30 % – 70 % of liquid depth from the bottom 
of the tank. The minimum dimension of an opening shall be 100 mm.  
6.2.9  Vent pipe  
Each chamber in a septic tank shall be vented through an op ening of minimum 25 mm diameter.  
A vent pipe of minimum 25 mm diameter shall be provided extending outside the tank to a height sufficient to 
avoid odour nuisance.  
The pipe shall be covered with a suitable mosquito proof mesh at the top.  
A single vent pipe is sufficient, provided the air space in each chamber of the tank is interconnected with another 
through an opening of minimum 25 mm diameter. If not, multiple vent pipes shall be provided to ensure that 
each chamber of the tank is vented.  
6.2.10  Principles of operation  
Wastewater enters the septic tank where settlement of solid matter subsequently occurs. Anaerobic bacteria 
then partly breakdown this solid matter within the tank. The heavier solid matter falls to the bottom and forms 
sludge, whilst the fats and other lighter matter float to the surface and form a scum.  
The effective settling of solids is directly dependent upon the detention time within the tank.   
©RSB 2019 - All rights  reserved                  14 
 Excessive build  up of sludge and scum reduces the capacity of the detention zone, resulting in di scharge of 
suspended solids to the effluent disposal system.  
The minimum period of time in the detention zone should be at least 24 h  to ensure 60  % to 70  % of the 
suspended solids are removed and that the Biochemical Oxygen Demand  (BOD5) is reduced by 30  %. 
Therefore, the septic tank should be of sufficient capacity to provide for a -24 h retention of the daily inflow into 
the tank.  
Small capacity septic tanks are adversely affected by flow surge, hence it is important that the types of discharge 
be taken i nto account when sizing the septic tank.  
The inlet and outlet shall be baffled to avoid undue disturbance to the contents, especially the scum.  
Where the daily inflow for the septic tank is excessive, or the tank is not regularly desludged, the inflow rete ntion 
time may be reduced, thus increasing the carryover of suspended solids to the effluent disposal system. This 
may decrease the effective life of the disposal system, failure of which may result in backflow into the house 
system, or overflow to the gro und surface.  
6.2.11  Prohibited discharges  
Unless otherwise approved by the relevant authority,  no person shall permit or cause any of the following 
discharges into a septic tank system:  
a) any storm water, including roof and rainwater tank overflow, and surface drai nage waters,  
b) any back  flush waters from a swimming pool or water softener;  
c) any sanitary napkin, clothing or plastic material or liner;  
d) any trade waste;  
e) any petrol or other flammable or explosive substance whether solid, liquid or gaseous;  
f) any disinfectant or deodorant, antiseptic or germicide powder or fluid, unless specifically stated to be 
suitable for use in a septic tank;  
g) any other matter or substance which would impair the effective working of a septic tank.  
6.3 Soakage pits 
6.3.1  General  
Soakage pi ts are used to soak septic tank effluent into the surrounding soil. They do not provide any direct 
treatment and are based on the principle that the effluent gets treated as it moves through the surrounding soil 
before entering the groundwater table or oth er water body.  RS 143: 2019  
15                ©RSB 2019 - All rights reserved  
 6.3.2  Applicability  
Soakage pits shall be applicable only in areas where the seasonal high groundwater table is greater than 2.5 m 
below the ground surface and the soil percolation rate is between 25 mm and 125 mm/hr.  
6.3.3  Location  
6.3.3.1  Soakage pits shall be located in an open area and satisfy the following requirements:  
a) at least 18  m away from the nearest well or other drinking water source;  
b) at least 5 m away  from the nearest building;  
c) a minimum distance from other soakage pits, either e xisting or proposed, within or outside the property 
shall be maintained as specified in Table 2  below.  
Table 2 — Minimum distance between soakage pits  
Average daily flow  
(m3/d) Minimum distance between soakage pits 
(m) 
< 2 10 
2 - 10 20 
10 - 30 36 
6.3.3.2  In the case of soakage pits with flows differing from each other, the spacing requirement shall be 
that required for the pit receiving the highest flow.  
6.3.4  Geometry  
Soakage pits are usually circular or square in plan, although other regular shapes may  be used.  
6.3.5  Dimensions  
Soakage pits shall be sized to provide sufficient effective area for the absorption of the average daily flow.  
The effective area of a soakage pit shall be the area of the side walls lying between a level 150 mm below the 
invert of the  inlet pipe and the bottom of the pit.  
The required effective area shall be determined by multiplying the appropriate specific effective area given in 
Table 3  by the average daily flow.  
   
©RSB 2019 - All rights  reserved                  16 
 Table 3 — Specific effective areas for soakage pits  
Percolation rate  
(mm/hr)  Specific effective area  
(m2/m3.d) 
25 34 
50 17 
75 11 
100 8.4 
125 6.6 
NOTE  Intermediate values may be determined by linear interpolation.  
No single soakage pit shall be designed for an average daily flow greater than 30 m3/d. 
Multiple soakage pits may be used in parallel, provided the spacing requirements specified in Table 2  are 
satisfied.  
The minor dimension of a soakage pit shall be greater than 900 mm and the major dimension less than 3 m.  
The depth of the soakage pit s hall be such that a minimum distance is maintained between the bottom of the 
pit and the highest seasonal groundwater table (GWT) according to the values given in Table 4 below.  
Table 4 — Minimum depth to Ground Water Table (GWT) from bottom of soakage pit  
Percolation rate  
(mm/hr)  Minimum depth to GW  
(m) 
25 – 50 1.2 
50 – 75 1.8 
75 – 100 2.4 
100 – 125 3.0 
If a rock layer exists below the soakage pit, a similar minimum distance to the rock layer from the bottom of the 
pit shall be maintained.  
Annex E  shows a typical arrangement of a soakage pit.  
6.3.6  Cover  
Soakage pits shall be provided with a suitable permanent cover that prevents the ingress of surface water, 
insects and rodents into the pit.  
The cover shall be fixed in place and capable of withstanding reasonable imposed loads.  RS 143: 2019  
17                ©RSB 2019 - All rights reserved  
 6.3.7  Seepage beds  
6.3.7.1  General  
 Seepage beds are used to soak septic tank effluent into the surrounding soil in situations where soakage pits 
are not applicable. They provide some partial treatment of the septic tank effluent p rior to soil absorption.  
Seepage beds are a bed of prepared aggregate, usually sand, gravel or other inert media, through which the 
effluent percolates prior to soaking into the soil. Septic tank effluent is applied to the bed through perforated 
distributo r pipes laid at intervals along the bed.  
Seepage beds may be either gravity fed or pressure dosed at intervals using an automated pump and sump 
arrangement.  
A typical arrangement of a seepage bed is shown in Annex F.  
6.3.7.2  Applicability  
Seepage beds shall be app licable only in areas where the seasonal high groundwater table is greater than 1.5 
m below the surface, the soil percolation r ate is between 25 mm and 250 mm/hr , and the ground slope is less 
than 5 %. 
6.3.7.3  Location  
Seepage beds shall be located in an open area  and satisfy the following requirements:  
a) at least 18 m away from the nearest well or other drinking water source;  
b) at least 5 m away from the nearest building;  
c) at least 5 m away from the nearest soakage pit;  and 
d) at least 1 m away from the nearest seepage bed or trench.  
6.3.7.4  Geometry  
Seepage beds are usually rectangular but may be of any convenient shape in plan.  
6.3.7.5  Dimensions  
Seepage beds shall be sized to provide sufficient effective area for the absorption of the average daily flow.  
The effective area of  a seepage bed shall be the area of the bottom of the bed.  
The required effective area shall be determined by multiplying the appropriate specific effective area given in 
Table 5  by the average daily flow.   
©RSB 2019 - All rights  reserved                  18 
 Table 5 — Specific effective areas for seepage bed s and seepage trenches  
Percolation rate  
(mm/hr)  Specific effective area  
(m2/m3.d) 
25 50 
50 25 
75 17 
100 12.5 
125 10 
150 8.3 
175 7.1 
200 6.25 
225 5.6 
250 5.0 
Minimum, maximum and typical values of bed dimensions are given in Table 6  below.  
Table 6 — Minimum, maximum and typical dimensions of seepage beds  
Bed dimension  Typical  
(mm)  Maximum  
(mm)  Minimum  
(mm)  
Width  1000  - 6000  6 000 1 000 
Depth of aggregate  300 - 600 600 300 
Depth of topsoil  100 - 150 N/A 100 
Spacing between  beds 
(sidewall to sidewall)  - N/A 1000  
The maximum bed length shall be 20m.  
6.3.7.6  Aggregate  
Sand, gravel, stone chips or any other suitable inert material, which is insoluble in water and resistant to the 
corrosive nature of septic tank effluent, may be used as aggregate.  
The depth of aggregate in the bed shall be greater than 300 mm and less than 600  mm. 
The nominal aggregate size shall be greater than 20 mm and less than 40 mm.  
6.3.7.7  Soil cover  
A minimum depth of 100 mm of topsoil shall be provided above the bed as a soil cover.  
An effective soil barrier such as a reverse filter arrangement, filter cloth or polyethylene liner shall be provided 
between the soil cover la yer and the aggregate layer in the bed to prevent infiltration of soil into the bed.  RS 143: 2019  
19                ©RSB 2019 - All rights reserved  
 The soil cover may be turfed or vegetated with shallow rooted plants.  
6.3.7.8  Distribution pipes  
The minimum internal diameter of distribution pipes shall be 100 mm.  
Perforations in pipes shall be between 10 mm – 15 mm diameter with a total area of 10000 mm2 / m length of 
pipe.  
The minimum spacing between adjacent pipes in a bed  shall be between 300 mm min. and 2000 mm max . 
Pipes shall be laid horizontally.  
A minimum of 2 distribut ors shall be provided per bed.  
6.4 Seepage trenches  
6.4.1  General  
Seepage trenches are similar to seepage beds except that they are suitable for sloping  ground where the ground 
slope is less than 25 %. 
A typical arrangement of a seepage trench is shown in Annex G .The maximum length of a trench shall be 20 
m. 
6.4.2  Applicability  
Seepage trenches shall be applicable only in areas where the seasonal high groundwater table is greater than 
1.5 m below the surface, the soil percolation rate is between 25 mm and 250 mm/hr, and th e ground slope is 
less than 25 % across trenches and less than 5 % along trenches.  
6.4.3  Location  
Same as in sub -clause 6.3.7.3  
6.4.4  Geometry  
Seepage trenches shall be rectangular in plan.  
6.4.5  Dimensions  
Seepage trenches shall be sized to provide sufficient effective are a for the absorption of the average daily flow.  
The effective area of a seepage trench shall be the area of the trench bottom.   
©RSB 2019 - All rights  reserved                  20 
 The required effective area shall be determined by multiplying the appropriate specific effective area given in 
Table 5  by the average daily flow.  
Minimum, maximum and typical values of trench dimensions are given in Table 7  below.  
Table 7 — Typical dimens ions of seepage trenches  
Bed dimension  Typical  
(mm)  Maximum  
(mm)  Minimum  
(mm)  
Width  300 - 600 600 300 
Depth of aggregate  300 - 600 600 300 
Depth of topsoil  100 - 150 N/A 100 
Spacing between adjacent 
trenches  (sidewall to side wall)  - N/A 1000  
The maximum length of a trench shall be 20 m.  
6.4.6  Aggregate  
Same as in sub -clause 6.7.7.6  
6.4.7  Soil cover  
Same as in sub -clause 6.3.7.7  
6.4.8  Distribution pipes  
The minimum internal diameter of distribution pipes shall be 100 mm.  
Perforations in pipes shall be between 10 mm – 15 mm diameter with a total area of 10000 mm2 per metre 
length. Pipes shall be laid horizontal.  
7 Materials and construction  
7.1 Tanks  
Septic tanks may be constructed in -situ in reinforced concrete, lined brick maso nry or lined cement block 
masonry. Alternatively, they may be pre -cast in reinforced concrete, steel or Polyethylene.  
The material and construction details of pre -cast septic tanks are outside the scope of this standard. However, 
in all cases, tanks shall be water -tight, structurally sound, and resistant to the corrosive nature of a septic, 
anaerobic environment.  
Brick masonry shall not be used in situations where the tank would be either wholly or partly below the seasonal 
high groundwater table. However, solid cement block masonry may be used in such cases.  RS 143: 2019  
21                ©RSB 2019 - All rights reserved  
 Materials used in construction of septic tanks shall conform to the requirement specified in relevant existing 
standards , some of them are mentioned below .  
The cement to be used for construction of tan ks shall conform to RS EAS 18 -1 and RS EAS 18 -2. The cement 
shall be testes in accordance with relevant part of RS EAS 148 . The building sand for masonry works shall 
comply with RS107 . 
When the tank is made of lined brick masonry or lined cement block masonry, the burnt bricks and burnt blocks 
shall conform to  RS EAS 94 and RS EAS 54 respectively. Cement block shall conform to RS 144.  
When the tank is constructed in -situ in reinforced co ncrete, or pre -cast in reinforced concrete, the concrete shall 
conform to the requirements specified in RS  ISO 22965 -2. Testing of concrete shall be conducted in accordance 
with the relevant part of RS ISO 1920.  
The steel reinforcement for reinforced conc rete shall comply with RS EAS 412 -2: Steel reinforcement for 
concrete shall conform to RS ISO 6935 -1 for Plain bars and RS ISO 6935 -3 for welded fabric.  Aggregates to be 
used for concrete production, shall comply with RS 373.  
7.2 Soakage pits  
The walls of soa kage pits shall be constructed with open jointed brick or open jointed cement block masonry 
and left unlined. However, walls shall be lined and made impervious up to an appropriate distance from the top 
of the soakage pit in order to prevent ingress of sur face water into the pit, where necessary.  
7.3 Aggregate media  
Aggregate media shall be immersed in water and washed free of fines and debris prior to placing in the beds.  
Where a minimum size of aggregate has been specified, all aggregate shall be sieved throu gh a sieve with the 
appropriate mesh size and only aggregate retained in the sieve shall be used.  
Due care shall be taken when placing aggregate to avoid damage to sidewalls, trench / bed bottoms, liners and 
pipework.  
7.4 Excavation  
Excavation for seepage beds  and trenches shall be done with due care to avoid damaging and / or compacting 
the surrounding soil.  
Damage can be done by:  
a) Smearing where the soil surface is smo othed, filling cracks and pores,  
b) Compacting whe re the soil porosity is reduced, and  
c) Puddling where washed clay settles on the base of the trench /bed.  
©RSB 2019 - All rights  reserved                  22 
 The following guidelines shall be followed for excavation of seepage beds and trenches to reduce the risk of 
damage to the soil.  
Plan to excavate only when the weather is fine.  
During wet se asons or when construction cannot be delayed until weather becomes fine, smeared soil surfaces 
shall be raked carefully with a fine -tined rake to restore a more natural soil surface. Care shall be taken to rake 
only at the surface.  
 Avoid excavation when s oil has moisture content above the plastic limit. If the soil forms a “wire” when rolled 
between the palms, it is generally above the plastic limit.  
When excavating by machine, use a bucket with “raker teeth” and excavate in small “bites” to minimize 
compa ction.  
Avoid compaction by keeping people off the finished trench or bed floor.  
If rain is likely, cover any open trenches to protect them from rain damage.  
Excavate parallel to the contours of sloping ground, wherever possible.  
Ensure that trench or bed b ottoms are horizontal.  
Divert all surface water well away from excavations.  
7.5 Pipework  
All pipework from the building to septic tank and from the septic tank to the soakage system shall be a minimum 
of DN 100.  
8 Inspection and testing  
8.1 General  
Inspection of the system may be made either during installation or upon completion.  
Where regular inspections are made, the work shall be inspected before covering/backfilling.  
All systems shall be inspected for structural defects, defects in construction and conformity with the design 
specifications. All such defects detected shall be repaired and rectified such that the original requirements have 
been satisfied.  
8.2 Water tightness  
All septic tanks shall be tested for water tightness and leaks as follows:  RS 143: 2019  
23                ©RSB 2019 - All rights reserved  
 a) Once construction and installation has been completed, empty any water in the tank and clean the tank, 
making sure to clear the tank bottom of any construction debris, mud, clay, sand etc. and brush the tank 
walls and bottom thoroughly with a stiff bristled b rush. Close the ta nk and let it stand for 24 h . 
b) Inspect the tank walls and bottom for any signs of water leaking in. If evidence of leaks exist, repair as 
necessary and repeat steps a) and b) prior to proceeding to step c).  
c) Fill all chambers of the tank up  to the liquid lev el and let it stand for 24 h . 
d) Check for any drop in water level. If there is a drop in water level, top up to the liquid level and r e-check 
after a further 24 h . If the drop in level persists, empty the tank and repair or replace as neces sary and 
repeat steps c) and d).  
8.3 Partition test  
All multiple chambered septic tanks shall be subject to the following test after satisfying the water tightness test 
in 8.1 above.  
Fill one side only of each partition wall with water up to the level of the l owest opening  in the partition wall. Keep 
the other side emp ty. And let stand for 24 h.  
Inspect the partition wall for cracks or other structural damage. If water has leaked to  the other side, the depth 
of such water shall be less than 250 mm.  
9 Plumbing & d rainage  
All sanitary plumbing and drainage work including the installation of fixtures and connection to the septic tank 
system via traps, waste pipes and drains shall be carried out in accordance with  relevant  plumbing and drainage 
standards.  
10 Maintenance  
10.1 Septic Tanks  
Appropriate and timely maintenance is the key to the successful continued function of any system and due care 
and attention shall be given to inform and educate users and owners of septic tanks and associated effluent 
disposal systems of all the routine maintenance requirements of their system.  
10.2 Desludging  
Septic tanks shall be desludged at the appropriate intervals according to the design and use of the tank. 
Typically, this is when the tank is between one third and half full of sludge and / o r scum.  
Tanks should not be completely emptied during desludging. Between 100  mm – 150 mm of sludge should be 
left in the bottom of the tank as ‘seed’ for the next cycle of operation.   
©RSB 2019 - All rights  reserved                  24 
 Septage from desludged septic tanks shall be either disposed in a facility intended for that purpose, such as a 
sewage treatment plant, or where no such facility is available, shall be buried in a pit with due care being taken 
not to pollute the local groundwater and the neighbouring environment.  
10.2.1  Access covers  
All access covers shall be properly replaced and sealed after each opening.  
Broken and damaged access covers shall be promptly repaired or replaced.  
10.2.2  Mosquitoes  
Septic tanks are a prime breeding location for mosquitoes. Due care shall be taken to ensure that ta nks are 
properly sealed, with particular attention being given to prompt repair of structural cracks which allow 
mosquitoes to enter the tank. The mosquito proof mesh cover over vent pipes shall be inspected regularly and 
replaced as required.  
10.2.3  Blockages  
The commonest form of blockage is due to solids blocking the inlet device in a septic tank. This may be cleared 
by rodding the inlet device from above (through an access opening or inspection port ) with a suitably flexible 
rod. 
10.3 Anaerobic filters  
Anaerobic fi lters do not require regular maintenance as such. However an annual inspection is recommended. 
Bubbles forming at the filter surface are an indicator of a filter which is functioning well.  
10.3.1  Filter cleaning  
The commonest cause of filter backup is due to gas blockages caused by expanding  gas bubbles as they move 
up through the filter bed. These may be cleared by rodding  the filter bed from above with an appropriate rod to 
release the trapped gases.  
If rodding is not sufficient to clear the filter, the filter s hall be emptied by pumping out the contents from the inlet 
chamber. If a layer of scum is present on the surface of the filter this shall be removed prior to emptying. The 
empty filter bed shall then be sprayed from above with water until a continuous flow  of water appears from the 
bottom of the bed. A simple garden hose may be used for the purpose.  
Anaerobic filters should not  be backwashed under pressure in the manner of rapid sand filters as this would 
dislodge the biofilm growth on the filter media.  
 RS 143: 2019  
25                ©RSB 2019 - All rights reserved  
 Annex  A 
(normative)  
 
Percolation test  
Whenever soil absorption of septic tank effluent is considered, a percolation test shall be performed prior to the 
design of a soil absorption system in the following steps:  
A.1 The percolation test shall be carried out at the location where the soil absorption system is to be installed. 
In cases where this is uncertain, several percolation tests at likely locations shall be performed.  
A.2 A square or circular hole of side (or diameter) between 100 mm – 300 mm shall be dug or bo red up to 
the depth where soakage is to take place.  
A.3 The bottom and sides of the hole shall be carefully scraped with a fine tined fork to remove any smeared 
soil and restore a natural soil surface.  
A.4 All loose materials shall be removed from the bott om of the hole and fine gravel or coarse sand of nominal 
size approximately 6 mm placed at the bottom to a depth of 50 mm.  
A.5 The hole shall then be filled with water up to a depth of 300 mm above the gravel layer and let stand for 
24 hours.  
A.6 After 24 h, the water level in the hole shall be adjusted to 150 mm above the gravel layer, and the drop 
in water level over a 30 minutes period shall be measured.  
A.7 The percolation rate shall be calculated as follows:  
A.8 Percolation rate (mm/hr) = drop in water level (mm) / (0.5 hrs.)  
A.9 Where there is no water remaining in the hole after step A.6, water shall be added to the hole up to a 
level 150 mm above the gravel layer and the drop in water level measured at 30 minute s intervals over a period 
of 4 h , topping up the level to 150 mm after each measurement. The drop in water level over the final 30 minute 
period shall be taken to calculate the percolation rate.  
A.10 Where the water in the hole drains out completely, within the 30 minutes measurement perio d, the soil 
percolation rate exceeds 300 mm/hr.  and is unsuitable for soakage of septic tank effluent.  
 
   
©RSB 2019 - All rights  reserved                  26 
 Annex  B 
(informative)  
 
Example computations of average daily flow based on Table 1  
Example 1:  
For an office building with 50 day -time employees, 6 overnight employees and 200 customers / day, the average 
daily flow would be computed as follows:  
Average  daily  flow  (all waste )
=(50 day time  employees  ×50l/p/d )+( 6 overnight  employees  × 200  l/p/d)
+ (200  customers   × 10 l/p/d
=5700 /l/d                                                                                                                                
Average  daily  flow  (blackwater )
=(50 day time  employees  ×30l/p/d )+( 6 overnight  employees  × 50 l/p/d)
+ (200  custo mers   × 10 l/p/d
=2800 /l/d                                                                                                                                
Average  daily  flow  (greywater )
=(50 day time  employees  ×20l/p/d )+( 6 overnight  employees  × 150  l/p/d)
+ (200 customers   × 5 l/p/d
=2900 /l/d                                                                                                                                
Example 2:  
For a hotel building with 20 rooms, 12 residential staff, 8 non -residential staff, se rving a maximum of 400 
meals/day, the average daily flow would be computed as follows:  
Average  daily  flow  (all waste )
=(20 rooms  ×2 guests /room  ×240 /l/p/d )+( 8 non  residential  staff  × 100  l/p/d)
+ (12 residential  staff  × 200  l/p/d+400  meals  ×15 l/meal  
=18 800 /l/d                                                                                                                                
 
Average  daily  flow  (blackwater )
=(20 rooms  ×2 guests /room  ×60/l/p/d )+( 8 non  residentia l staff  × 50 l/p/d)
+ (12 residential  staff  × 50 l/p/d 
=3 400 /l/d                                                                                                                                
 RS 143: 2019  
27                ©RSB 2019 - All rights reserved  
 Average  daily  flow  (greywater )
=(20 rooms  ×2 guests /room  ×180 /l/p/d )+( 8 non  residential  staff  × 50 l/p/d)
+ (12 residential  staff  × 150  l/p/d+400  meals  ×15 l/meal  
=15 400 /l/d                                                                                                                     
 
Example 3:  
For a restaurant with 20 non -residential staff, 5 residential staff, serving a maximu m of 150 dine -in meals / day 
and 100 take -away meals / day, the average daily flow would be computed as follows:  
Average  daily  flow  (all waste )
=(20 non  residential  staff  ×50 l/p/d )+( 5 residential  staff  × 200  l/p/d)
+ (150  dine  in meals /d × 25 l/meal +100  take away  meals ×15 l/meal                
=7250 /l/d         
 
Average  daily  flow  (black  water )
=(20 non  residential  staff  ×30 l/p/d )+( 5 residential  staff  × 50 l/p/d)
+ (150  dine  in meals /d× 10 l/meal                                                                                                     
=2350  /l/d       
   
Average  daily  flow  (grey  water )
=(20 non  residential  staff  ×20 l/p/d )+( 5 residential  staff  × 150  l/p/d)
+ (150  dine  in meals /d× 10 l/meal  
+100  take  away  meals /d ×   15 l
/meal                                                                                                                                                                                     
=4900  /l/d       
 
  
©RSB 2019 - All rights  reserved                  28 
 Annex  C 
(informative)  
 
Determination of Working Capacity of Septic Tanks  
The working capacity of a septic tank shall be the sum of the volumes required for settling, sludge digestion, 
sludge and scum storage and shal l be estimated as follows.  
The volume required for settling, shall be calculated as follows:  
𝑽𝒔=𝒕𝒔.𝑸 
where,  
𝑉𝑠 volume required for settling (m3) 
𝑄 average daily flow of wastewater (m3/d) 
𝑡𝑠 time required for settling (days)  
= (1.5 – 0.3.logQ), (> 0.2) (days.)  
The volume required for sludge digestion shall be calculated as follows:  
𝑽𝒅=𝒒𝒔 .𝒕𝒅.𝑷 
where , 
𝑉𝑑 Volume required for sludge digestion (m3) 
𝑞𝑠  volume of fresh sludge per person per day (m3/person/day)  
= 0.001 m3/p/d for all waste or grey water  
= 0.00055 m3/p/d for black water only  
𝑡𝑑 time required for sludge digestion (days)  
= 33 days (for an ambient temperature of 200C) 
𝑃 population equivalent.  
= Q (m3/d)/ 0.2 (m3/p/d) for all waste  RS 143: 2019  
29                ©RSB 2019 - All rights reserved  
 = Q (m3/d)/ 0.05 (m3/p/d) for black water  
= Q (m3/d)/ 0.15 (m3/p/d) for grey water  
The volume required for sludge storage shall be calculated as follows:  
𝑽𝒔𝒕=𝒓.𝒑.𝒏 
where , 
𝑽𝒔𝒕 volume required for sludge storage (m3) 
𝒏 desludging interval (> 1) (years)  
𝒓 Volume of digested sludge per person per year (m3/p/y) 
= 0.04 m3/p/y for all waste or grey water  
= 0.022 m3/p/y for black water only  
The volume required for scum storage shall be taken as 0.5  Vst. 
Then the design working capacity of the septic tank shall be the requ ired volume , 𝑽=𝑽𝒔 + 𝑽𝒅+ 𝟏.𝟓 𝑽𝒔𝒕 (𝒎𝟑)  
©RSB 2019 - All rights  reserved                  30 
 Annex  D 
(informative)  
 
Typical septic tank arrangement  
 
Plan  
 
Section  
RS 143: 2019  
31                ©RSB 2019 - All rights reserved  
 Annex  E 
(informative)  
 
Typical soakage pit arrangement  
 
                    Plan (Circular type)                                                                Plan (Square type)  
 
Plan (Rectangular type)  
 
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Section  
RS 143: 2019  
33                ©RSB 2019 - All rights reserved  
 Annex  F 
(informative)  
 
Typical arrangement of a seepage bed  
 
Plan  
 
Section  
 
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 Annex  G 
(informative)  
 
Typical arrangement of a seepage trench system  
 
Plan  
RS 143: 2019  
35                ©RSB 2019 - All rights reserved  
  
Section  
 
 
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 Bibliography  
[1] IS SLS 745:2003 Wastewater treatment systems  − Design and construction of septic tanks and 
associated effluent disposal systems − Code of practice  
[2] Waste Control Systems - Standard for the construction, installation and operation of septic tank in South 
Australia , March, 1995  
[3] BS 6297:2007+A1:2008 Code of practice for the design and installation of drainage fields for use in 
wastewater treatment  
[4] BS EN 12566 -1:2000 Small waste water treatment systems for up to 50 PT - Part 1: Prefabricated septic 
tanks  
[5] BS EN 12566 -4: 2007 Small wastewater treatment systems for up to 50 PT -Part 4: Septic tanks 
assembled in situ from prefabricated kits  
[6] Sewerage system  - Standard practice manual , September, 2006  
 RS 143: 2019  
 
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