ICS 13.030.20  
 
Reference number  
DRS  126-2: 2021  
© RSB 2021  
  
 RWANDA 
STANDARD  DRS  
126-2  
 
Second  edition  
2021-mm-dd 
 
Waste water treatment plants — 
Part 2 : General c onstruction principles  
   
DRS  126-2: 20 21 
©RSB 2021  - 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 2021  
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   DRS  126-2: 2021  
iii                ©RSB 2021  - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ...................  iv 
1 Scope  ................................ ................................ ................................ ................................ .............  1 
2 Normative references  ................................ ................................ ................................ ...................  1 
3 Terms and definitions  ................................ ................................ ................................ ..................  1 
4 Requirements  ................................ ................................ ................................ ................................  4 
4.1 General requirements  ................................ ................................ ................................ ..................  4 
4.2 Design requirements  ................................ ................................ ................................ ....................  5 
4.3 Struct ural requirements  ................................ ................................ ................................ ...............  5 
4.3.1  General  ................................ ................................ ................................ ................................ ..........  5 
4.3.2  Dimensional tolerances  ................................ ................................ ................................ ...............  6 
4.3.3  Concrete tracks  ................................ ................................ ................................ ............................  6 
4.3.4  Fixings and connections between equipment and structures  ................................ ................  6 
4.3.5  Access  ................................ ................................ ................................ ................................ ...........  7 
4.3.6  Building ventilation  ................................ ................................ ................................ ......................  7 
4.3.7  Water supply and drainage  ................................ ................................ ................................ .........  7 
4.3.8  Lifting equipment  ................................ ................................ ................................ .........................  7 
4.3.9  Storage for hazardous chemicals and fuels  ................................ ................................ ..............  7 
4.4 Requirements for equipment  ................................ ................................ ................................ ...... 8 
4.4.1  Principles for mechanical design  ................................ ................................ ...............................  8 
4.4.2  General design requirements  ................................ ................................ ................................ ...... 8 
4.4.3  Environmental impact  ................................ ................................ ................................ ................  11 
4.4.4  Safety requirements  ................................ ................................ ................................ ...................  11 
4.4.5  Documentation  ................................ ................................ ................................ ...........................  11 
4.4.6  Spare parts, special tools  ................................ ................................ ................................ ..........  11 
Annex  A (informative)  Structural tolerances  ................................ ................................ .......................  12 
A.1 Circular tank  ................................ ................................ ................................ ................................  12 
A.1.1  Circular tank with scraper  ................................ ................................ ................................ .........  12 
A.1.2  Circular tank with a scraper bridge travelling on the side wall (track)  ................................ . 12 
A.2 Rectangular tank  ................................ ................................ ................................ ........................  12 
 DRS  126-2: 20 21 
©RSB 2021  - All rights reserved                  iv 
 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 
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 gazett ement as 
Rwanda Standards.  
DRS  126-2 was prepared by Tec hnical Committee RSB/TC 009 on Civil engineering and building materials . 
In the preparation of this standard, reference was made to the following standard s: 
1) BS EN 12255 -1, Wastewater treatment plants  — Part 1: General construction principles   
2) BS EN 12255 -10:2001, Waste Water Treatment Plant  — Safety Principles   
3) EN 752 -6, Drain and sewer systems outside buildings — Part 6: Pumping installations   
4) EN 809, Pumps and pump units for liquids — Common safety requirements   
5) EN 1085, Wastewater treatment — Voca bulary   
6) EN 12255 -9, Wastewater treatment plants — Part 9: Odour control and ventilation   
7) EN 12255 -12, Wastewater treatment plants — Part 12: Control and automation  
The assistance de rived from the above source is hereby acknowledged with thanks.  
This second  edition cancels and replaces the first edition (RS 126 -2: 2012), clause  4 of which  has been 
technically revised.  
DRS  126 consists of the following parts, under the general title Waste water treatment plants : 
⎯ Part 1: Vocabulary  
⎯ Part 2: General construction principles  
⎯ Part 3: Safety requirements   
⎯ Part 4: General data required   
Committee membership  DRS  126-2: 2021  
v                ©RSB 2021  - All rights reserved  
 The following organizations were represented on the Technical Committee on Civil engineering and building 
materials nd Safety  (RSB/TC 13)  in the prepara tion of this standard.  
B. J Construction Ltd  
Bridge to Prosperity  
CIMERWA  
Green Pact Africa  
Mass Design  
Rwanda Housing Authority  
Rwanda Inspectorate, Competition and Consumer Protection Authority (RICA)  
UR-CST 
Rwanda  Standards Board  (RSB) – Secretariat  
  DRS  126-2: 2021  
1                ©RSB 2021  - All rights reserved  
 Waste water treatment plant — Part 2: Gene ral construction principles  
1 Scope  
This Draft Rwanda Standard specifies general requirements for structures and equipment as they relate to 
wastewater treatment plants for a total population of more than 50 PT.  
The primary application is designed for wastewater treatment plants for the treatment of d omestic and municipal 
wastewater.  
Requirements for structures which are not specific for wastewater treatment plants are not within the scope of 
this Draft Rwanda Standard. Other Rwanda Standards can apply.  
Equipment which is not solely used in wastewate r treatment plants is subject to applicable product standards. 
However, specific requirements for such equipment when used in wastewater treatment plants are included in 
this part.  
General principles of building construction, mechanical and electrical eng ineering are not subject of this 
standard. This Rwanda Standard does not cover the design of treatment processes.  
This standard gives fundamental information about the systems; this standard has not attempted to specify all 
available systems.  
2 Normative r eferences  
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) appl ies.  
RS 126 -3, Waste water treatment plants  — Part 3: Safety requirements   
3 Terms and definitions  
For the purposes of this standard, the follow ing terms and definitions . 
3.1 
structure  
any construction and its components built for the accommodation of equipment   
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 3.2  
equipment  
any component which is installed in, mounted on, attached to, or operated on structures, in the performance of 
their intended function  
3.3   
units  
any structure  including any related equipment which is used as a process stage and which can be isolated from 
other parallel, upstream or downstream structures.  
NOTE  Examples for a unit s are a grit chamber, a clarifier, an aeration tank, a thickener, a digester.  
3.4   
assembly  
mechanical equipment that can be removed and replaced as a whole  
NOTE  Examples for an assembly are a pump, a compressor, a gas engine, an aerator.  
3.5  
wastewater treatment plant  
system for the purification of wastewater including structures and equipment  
3.6   
client  
municipality, city or other organization or its representative which intends to build a wastewater treatment plant 
or parts thereof  
3.7   
bidder  
company or other organization which offers to build a plant, or to build or supply parts thereof  
3.8  
contractors  
company or organization which received a contract to build a plant, or to build or supply parts thereof  DRS  126-2: 2021  
3                ©RSB 2021  - All rights reserved  
 3.9  
tracks  
those parts of a structure on which whe els run  
3.10   
design loading Y N  
effective average loading in continuous operation under full load  
NOTE  It is greater than or equal to the value of the operating loading which, for example, fluctuates as a function of 
the given load.  
3.11   
continuous  load bearing capacity YC  
load bearing capacity in continuous operation under full load  
3.12  
maximum loading Ymax  
peak loading which is taken as the switch -off value to which, for example, overload circuit breakers are adjusted  
3.13   
maximum load bearing capacity YB  
highest possible load bearing capacity limited to short -term load peaks, such as occur on switching on and off  
NOTE  In addition, alarm loadings Y S, lying between the design loading Y N and the switch -off loading Y max, can be 
agreed as required, Y N and Y max being stated by the equipment supplier.  
3.14   
utilization factor K A  
parameter for the effects on drive units etc., intrinsic to their operation  
NOTE  Usually KA includes either directly or indirectly information on the loading, running time and temperature and is 
an overall value of the relationship between load bearing ca pacity and loading.  
3.15   
design service life   
operating time until break -down of a machinery element under design loading, which is reached by a certain 
percentage of the elements tested   
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 NOTE As an example, the percentage for rolling bearings is 90 %;  the design service life is different from both the 
warranty time and an average service life of use, as used for cost efficiency calculations.  
3.16  
mode of operation  
characteristic value related to the effects on motors and other electrical components intrinsic to their operation 
(e.g. frequency of starts, temperatures)  
3.17  
degree of protection  
characteristic value related to the effects on motors and other electrical components intrinsic to their 
environmental conditions (e.g. effects of water or d ust) 
4 Requirements  
4.1 General requirements  
Wastewater treatment plants shall meet the following requirements:  
a) National regulations shall be observed ;  
b) The discharge limits shall be met;  
c) be capable of satisfactory treatment of the full range of flows and loa ds;  
d) personal safety;  
e) nuisance, odour, noise and toxicity, aerosols and foam shall be considered and shall meet the relevant 
requirements  of BS EN 12255 -9 and RS 126 -3;  
f) danger to operating personnel shall be minimized;  
g) the required service life and long term structural integrity shall be achieved, including water and gas;  
h)  tightness;  
i) provisions shall be made for case of operation and maintenance;  
j) provision for future e xtensions or modifications of the plant shall be considered;  
k) the reliability of operation shall be high and risk of danger and the impact of malfunctions shall be        limited;  
l) be cost effective in respect of total costs (capital and operating costs);  DRS  126-2: 2021  
5                ©RSB 2021  - All rights reserved  
 m) the energy consumption during construction and operation shall be considered; and 
n) the waste products shall be reduced in quantity and improved in quality as far as reasonably achievable to 
allow for reuse or safe disposal.  
4.2 Design requirements  
4.2.1  All assemblies that are subject to occasional failure (e.g. pumps and compressors) shall be installed 
with sufficient stand -by capacity so as to achieve full treatment capacity and efficiency with one assembly out 
of service. In the case where stand -by assembl ies cannot be practically installed, provisions shall be made to 
replace rapidly by another one kept in stock.  
4.2.2 Where practicable and necessary for maintenance work it shall be possible to bypass every unit or 
assembly, either by a parallel unit or a ssembly, channel or pipe.  
4.2.3 Where necessary the inlet to the treatment plant shall include a facility which limits the flow. Such 
facilities may be balancing tanks and/or storm water overflows as required by the authorities.  
4.2.4 Where power supply is subject to prolonged interruption, wastewater treatment plants shall have 
emergency power generation or an equivalent facility to provide a sufficient power supply during power failure 
of the network, e.g. a terminal for easy connection with a readily a vailable mobile power generator. Connected 
to the emergency power supply shall as a minimum include the measuring and control system, the pumps for 
waste water and return sludge and any aeration equipment (at a designed minimum capacity).  
4.2.5 When the p ower supply is restored after an interruption, the treatment plant shall be designed so that 
normal operating status is resumed automatically.  
4.2.6 Provision shall be made for taking representative samples upstream and downstream of each unit and 
of any flow whose characteristics are important for operation and supervision.  
4.2.7 The design shall ensure that all information (quantities and qualities) that is important for effective 
operation of the plant is readily obtainable (e.g. flows, levels, pressur es, temperatures, dissolved oxygen 
concentrations, pH -values, other concentrations).  
4.2.8 The design shall enable cleaning, maintenance and repairs to be carried out easily and safely (e.g. 
access, flushing connections to pipes, isolation means).  
4.2.9 Appropriate provision shall be made for the case of malfunction or emergency.  
4.3 Structural requirements  
4.3.1  General  
Structures shall be :  
a) stable to bear all loads during construction, operation and maintenance periods   
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 (e.g. water pressures, static and dynamic forces being induced by the equipment );  
b) resistant against chemical and biological attack from wastewater, sludge, air and gas components and 
against temperatures and temperature changes as appropriate ; and   
c) protected a gainst flotation.  
4.3.2  Dimensional tolerances  
The permissible dimensional tolerances for structures which are required for the function of the equipment are 
specified  in Annex A . Other dimensional tolerances shall be agreed with the supplier of the equipment .  
4.3.3  Concrete tracks  
4.3.3.1  Tracks shall be identified in the drawings.  
4.3.3. 2 Tracks shall be level and free of ridges.  
4.3.3. 3 Particular requirements in regard of the quality and placing of the concrete shall be met in order to 
reinforce the tracks a gainst the effects of compression and shear forces.  
4.3.3. 4 The strength of concrete shall not be less than 35 N/mm².  
4.3.3. 5 The maximum pressure on the wheels shall be limited to:  
a) rubber wheels 2 .5 MN/m2;  
b) polyurethane wheels 5 .0 MN/m2  
In the la tter case, protection of the track with steel plates or other suitable material may be necessary.  
4.3.4  Fixings and connections between equipment and structures  
4.3.4.1  The possibility of differential settlement between structures, and between structures and e quipment 
(such as pipelines) shall be taken into account. Sufficient flexible joints and flexibility in the equipment itself or 
in its connections to the structures shall be provided.  
4.3.4. 2 Reinforcement in the structure shall not be used for securing e quipment.  
4.3.4. 3 Where different metals are in contact, measures shall be incorporated to prevent corrosion by 
galvanic action.  
4.3.4. 4 Where metallic fixings might be in electrical contact with the reinforcement of the structure 
appropriate electrical insulation shall be provided, e.g. insulating, chemical anchor with threaded rod.  DRS  126-2: 2021  
7                ©RSB 2021  - All rights reserved  
 4.3.5  Access  
4.3.5.1 Safe and sufficient access in the form of paths, gangways, bridges, stagesand  the like shall be 
provided to allow supervision, operating, servicing, cleaning and maintenance. Openings shall be provided 
which allow easy replacement of equipment.  
4.3.5.2 The location of operating and maintenance points shall allow for adverse weathe r conditions and 
other hazards (e.g. handling of gases, vapours, sludge, oil and grease) and possibility of collapse, squeeze and 
sheer points.  
4.3.5.3 The buildings and access shall be sufficiently large to allow all erecting and dismantling, 
maintenance  and repair operations and replacement of assemblies in an easy manner.  
4.3.5.4 Appropriate means shall be provided to deter access by unauthorized persons.  
4.3.6  Building ventilation  
In enclosed rooms, the possible existence of damp atmospheres, foul air and the risk of explosions shall be 
considered  according to RS 126 -3. Adequate ventilation shall be provided  according to BS EN 12255 -9.  
4.3.7  Water supply and drainage  
4.3.7.1 Where occasional flushing is required, a water supply shall be installed. Process w ater shall preferably 
be used for this purpose. Appropriate means shall be provided to prevent process water from contaminating the 
drinking water network. Any national regulations for the quality of process water used for flushing shall be 
observed. This may be particularly important when water is pressurized.  
4.3.7.2 Appropriate drainage shall be installed where water accumulation may occur due to overflow, leakage 
or flushing. In such locations, all floors shall be sealed and sloped towards a pit from w here the water drains by 
gravity or is automatically pumped away. All tanks should be constructed to allow emptying.   
4.3.8  Lifting equipment  
Lifting equipment or adequate provisions for removal shall be provided where necessary in order to allow all 
maintenanc e work and replacement of all assemblies.  
4.3.9  Storage for hazardous chemicals and fuels  
4.3.9.1 Where hazardous liquid chemicals or fuels are stored or conveyed, provisions must be made to prevent 
environmental  impact in case of leakage. National regulations  and any requirements in RS 126 -3 shall be 
observed. The required safety provisions (e.g. double tank walls, tanks in bunds, leak sensors) will depend on 
the volumes stored and potential risks. .  
4.3.9.2 Tanks containing chemicals that would interact to fo rm a hazardous mixture or could attack the material 
of other tanks shall not share a single bund.   
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 4.4 Requirements for equipment  
4.4.1  Principles for mechanical design  
4.4.1.1 The use and requirements of the equipment shall be specified.  
4.4.1.2 A general descri ption and the following information shall be provided:  
a) loads (e.g. traffic loads, wind loads, operating loads and travelling single loads);  
b) loadings (e.g. design loading, maximum loading, alarm loadings);  
c) load bearing capacities (e.g. continuous load be aring capacity, maximum load bearing capacity);  
d) utilization factor K A;  
e) mode of operation;  
f) degree of protection provided by enclosures. All gears and drives which are located above water, but near 
a place where water jet flushing may take place, shall be  protected against spray water; gears and drives 
which may be directly cleaned with water jets shall be protected.   
g) safety measures and precautions . 
4.4.2  General design requirements  
4.4.2.1  Walkways, stairs, platforms and gratings  
Irrespective of the design traffic load, the load -bearing capacity of walkways shall not be less than 3 .5 kN/ m2. 
In addition, the maximum deflection of a walkway shall not exceed 10 mm or the span divided by 200.  
4.4.2.2  Covers, assembly openings, cleaning openings  
The design and arrangement of covers, assembly openings and cleaning openings shall be consistent with the 
operational requirements of the plant. Openings shall be fitted with secure covers which cannot shut 
accidentally. Where frequent access is required, the covers shall be easily opened and closed  and shall be tight 
to control gas and odo ur.  
4.4.2.3  Cable drums with spring motors  
Spring motors used for cable drums may be employed only where the number of cycles doe s not exceed 1 000 
per year and the path length does not exceed 30 m.  
4.4.2.4  Pumps and pipelines  
4.4.2.4.1  All pumps shall be appropriate to the conveyed medium and its condition.   DRS  126-2: 2021  
9                ©RSB 2021  - All rights reserved  
 4.4.2.4. 2 The minimum nominal diameter of pipes and pumps shall be specified ap propriately to the medium 
conveyed. Generally, the minimum nominal diameter shall be DN 80 if mixtures of water and grit or sludges are 
conveyed. Smaller minimum nominal diameters may be agreed if upstream comminution or sieving is 
incorporated or where th ere is no risk of blockage.  
4.4.2.4. 3 Pumps shall be provided with individual isolating valves and check valves, if not otherwise 
specified. Positive displacement pumps shall be equipped with a sensor and a pressure switch to detect lack of 
liquid inflow and prevent damage.  
4.4.2.4. 4 Isolating and check valves shall be tight when closed and be appropriate for the medium and its 
condition (e.g. pressure, temperature, composition). If not otherwise specified, there shall be no internal 
obstruction to the fl ow when opened.  
4.4.2.4. 5 The forces and vibrations arising in pipe systems shall be taken into account in the design.  
4.4.2.4. 6 For pipelines conveying wastewater, sludge or digester gas, the pipe arrangement, the profile and 
the velocity shall be such that sedimentation (and condensation accumulation in gas or air pipes) and gas 
accumulation are avoided. Where this is not possible, means for the removal of sediment, condensate and gas 
accumulation shall be provided. Branches shall not be made in such a way that obstructions are likely to be 
formed. If not otherwise specified, the radius of bends shall be minimum three times the nominal diameter.  
4.4.2.4. 7 Pipes shall be readily identifiable or marked to ease of identity.  
4.4.2.4. 8 Pipe systems shall be  water and gas tight as necessary.  
4.4.2.5  Blowers, compressors  
4.4.2. 5.1 Blowers and compressors shall be appropriate for the intended use and blowers for aeration shall 
deliver an air sufficiently free from oil.  
4.4.2. 5.2 Blowers and compressors shall be equipped with appropriate isolating and check valves and, where 
necessary, with temperature and pressure switches.  
4.4.2.6  Measuring and control equipment  
4.4.2. 6.1 Measuring and control equipment serves to acquire process inform ation which is necessary for the 
safe reliable and efficient operation of the wastewater treatment plant and its equipment.  
4.4.2. 6.2 The necessary measuring and control equipment shall be specified at an early planning stage, 
taking into account the inst allation conditions. This applies both to its location within the plant and to the layout 
and size of the structures as a function of the type of equipment.  
4.4.2.7  Electrical equipment  
4.4.2. 7.1 The necessary electrical equipment shall be specified at an early planning stage, taking into 
account the installation conditions. This applies both to its location within the plant and to the layout and size of 
the structures depending on the type of equipment.   
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 4.4.2. 7.2 Additional information on specific structures an d assemblies is given in the relevant specific 
standards.   
4.4.2.8  Materials and corrosion protection  
4.4.2. 8.1 Materials used for the equipment shall be resistant to attack by the constituents of municipal 
wastewater and sludge, aerosols, sewage gases and atmos pheric influences (e.g. micro -atmosphere) as 
appropriate and consistent with the relevant requirements. The client shall inform the equipment supplier of any 
special factors, such as the presence of septic sewage. If different materials are connected, detr imental galvanic 
corrosion shall be prevented. If load -bearing components are made of plastic material, detrimental effects of the 
environment (e.g. UV -radiation, temperature) shall be considered.  
4.4.2. 8.2 Unless otherwise specified, the supplier may assume that the wastewater concerned is municipal 
containing industrial effluents only in such portions that the characteristics lie within the limits given by the 
relevant consent standards for the discharge of  wastewater into municipal sewers. On this basis, the equipment 
supplier shall select the materials.  
4.4.2. 8.3 Local conditions may require the use of particularly durable materials; this being subject to special 
agreement between contractor and client. D urability can be achieved by the use of materials inherently resistant 
to corrosion or by the application of a suitable coating. Where possible, anti -corrosion protection shall form part 
of the manufacturing process.  
4.4.2. 8.4 Jointing elements (e.g. nuts , bolts, washers and screws) which are in contact with water or corrosive 
atmosphere should be made of stainless steel, with the exception that use of high -strength material is necessary 
to bear strong forces.  
4.4.2.9  Fabrication of welded equipment  
4.4.2. 9.1 Proof of qualification is required for the personnel fabricating welded structures and equipment (e.g. 
isolating gates, working platforms, conveyor systems, screening assemblies and sludge scrapers).  
4.4.2. 9.2 Special skills of the personnel in charge of wel ding work for systems which are to contain 
inflammable or explosive fluids, such as fuel or gas, are required.  
4.4.2. 9.3 These requirements do not invalidate other specifications for individual equipment, such as 
pressure vessels.  
4.4.2.10  Scrapers  
Since scraper s are used in several units, their design principles are specified in this general part.  
Loads and dimensioning:  
The traffic load on bridges shall be assumed to be 1 .5 kN/m2. Higher traffic loads may be agreed. The maximum 
deflection under combined actio n of its own weight and the main loads, with the exception of the traffic load, 
shall be the span divided by 500. The supporting structure shall be such that the main loads including operational 
loads do not cause any torsion liable to impair the scraper p erformance or resulting in its permanent deformation.  DRS  126-2: 2021  
11                ©RSB 2021  - All rights reserved  
 4.4.3  Environmental impact  
4.4.3.1  All relevant requirements concerning emission control shall be observed. All wastewater treatment 
plants shall be located and designed with due regard for their effects on the environment.  
4.4.3. 2 Where significant emission to the environment is likely as  a result of the operation of a wastewater 
treatment plant, such emission shall be abated by special measures relating to structures, equipment and 
methods of operation, due consideration being given to the distance of the area to be protected from the 
treatment plant. Emission of odour, noise and pollutants (e.g. oil and grease) shall be prevented by means of 
suitable structures, equipment and mode of operation.   
4.4.3. 3 Where additional measures to prevent or abate such emissions are required, these shal l not impair 
the function, reliability, safety and maintenance of the plant and its components.  
4.4.3. 4 Soil and groundwater shall be protected from systems leakage.  
4.4.4  Safety requirements  
See RS 126 -3.  
  
4.4.5  Documentation  
4.4.5.1 The basic documentation for wastewater treatment plants shall include "as completed" drawings 
for structures, parts of structures and equipment, means of corrosion protection, pipe work plans, wiring 
diagrams, operating instructions, lubrication plans and lists of spare parts and wea ring parts, all these requiring 
regular updating. The documentation shall enable the client to undertake all service, maintenance and repair 
work and shall contain the essential information for future modification or extension works.  
4.4.5.2 The client sh all specify the language in which the basic documentation is to be supplied.  
4.4.5.3 Operating instructions shall cover the general process and any specific local modifications or 
peculiarities. Frequency and scope of routine tasks as well as the necessar y servicing measures for all parts of 
the plant, including checking its quality of operation, shall be described.  
4.4.6  Spare parts, special tools  
Spare parts shall be recommended by the supplier and be listed separately and satisfactory provision must be 
made for their availability.   
 
 
  
©RSB 2021  - All rights reserved                  12 
 Annex  A 
(informative)  
 
Structural tolerances  
A.1 Circular tank  
A.1.1  Circular tank with scraper  
a) Inner diameter of the tank: ± 0,  03 m;  
b) Bottom contour: ± 0, 03 m.  
A.1.2  Circular tank with a scraper bridge travelling on the side wall (track)  
a) Inner diameter of the tank: ± 0, 03 m;  
b) Bottom contour: ± 0, 03 m;  
c) Inner and outer diameter of the track: ± 0, 03 m.  
A.2 Rectangular tank  
a) Distance o f the side walls and tracks from the middle axis: ± 0, 02 m;  
b) Distance of the tracks from each other: ± 0, 02 m;  
c) Distance of the side walls from each other: ± 0, 02 m;  
d) bottom surface laterally: ± 0, 01 m;  
e) Level of tracks a distance of 4 m: ± 0, 02 m.  
f) Only for suction type scraper and other non -swiveling scrapers:  
g) Depth of tank (bottom to track): ± 0, 02 m.  
  DRS  126-2: 2021  
 
Price based on 1 2 pages  
©RSB 2021 - All rights reserved   
 Bibliography  
[1] Regulation No 004/R/SAN/EWS/RURA/2016 of 10/11/2016, Governing decentralized waste water 
treatment systems  
[2] RS 126 -2:2012 Waste water treatment plants – Part 2: Construction principles