Reference number  
DRS  126-1: 2018  
© RSB 2018  
 
 
 RWANDA 
STANDARD  DRS  
126-1 
 
Second  edition  
2018 -mm-dd 
 
Wastewater treatment plants  
Part 1: 
Vocabulary  
  DRS 126-1: 2018  
©RSB 2018 - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2018  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to 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  126-1: 2018  
iii                ©RSB 2018 - All rights reserved  
 Contents  Page 
1 Scope  ................................ ................................ ................................ ................................ .............  1 
2 Terms and definitions  ................................ ................................ ................................ ..................  1 
2.1 General definitions  ................................ ................................ ................................ .......................  1 
2.2 Types of wastewater and wastewater collection  ................................ ................................ ...... 5 
2.3 Wastewater quantity and quality  ................................ ................................ ................................  9 
2.4 Methods, characteristics and Impact on the environment  ................................ .....................  12 
2.5 Preliminary and primary treatment  ................................ ................................ ...........................  19 
2.6 Fixed film treatment  ................................ ................................ ................................ ...................  21 
2.7 Activated sludge treatment  ................................ ................................ ................................ ....... 23 
2.8 Other wastewater treatment  ................................ ................................ ................................ ...... 28 
2.9 Sludge treatment  ................................ ................................ ................................ ........................  31 
 DRS 126-1: 2018  
©RSB 2018 - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The ma in 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 126-1 was prepared by Technical Committee RSB/TC 013, Water and Sanitation  
In the preparation of this standard, reference was made to the following standard s: 
1) EN 1085, Wastewater treatment – Vocabulary  
2) EN 752 -6, Drain and sewer systems outside buildings – Part 6: Pumping installations.  
3) EN 809, Pumps and pump units for liquids – Common safety requirements.  
4) EN 12255 -9, Wastewater treatment plants — Part 9: Odour control and ventilation.  
5) EN 12255 -10, Wastewater treatment plants – Part 10: Safety principles.  
6) EN 122 55-12, 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 126-1: 2012  of which  has been technically re vised.  
RS 126 consists of the following parts, under the general title Wastewater treatment plants : 
 Part 1: Vocabulary  
 Part 2: General construction principles   
 Part 3: Safety principles   
 Part 4: General data required   
  DRS  126-1: 2018  
v                ©RSB 2018 - All rights reserved  
 Committee membership  
The following organizations were represented on the Technical Committee on Water and Sanitation  (RSB/TC 
013) in the preparation of this standard.  
Ministry of Environment (MoE)  
Ministry of Disaster Management and Refugee Affairs (MIDIMAR)  
Ruliba Clays Limited  
Shine Engineers Multisectoral Company Limited  (SEMC Ltd )  
Sulfo Rwanda Industries (SULFO)  
University of Rwanda/College of Science and Technology (UR/CST)  
Water and Sanitation Corporation Ltd (WASAC Ltd)  
Rwanda  Standards Board  (RSB) – Secretariat  
 DRS  126-1: 2018  
1                ©RSB 2018 - All rights reserved  
 Waste water treatment plant — part 1: vocabulary   
1 Scope   
This Draft Standard defines  key terms used in the field of wastewater treatment plants.  
2 Terms and definitions  
For the purposes of this document, the following terms and definitions apply.  
The terms are arranged in 9 groups, as indicated in the contents. Each definition within one group  has a 
successive number with a difference of ten. Remark on the given units: In the last column are shown the most 
common units.  
2.1 General definitions  
2.1.1  
wastewater  
water composed of any combination of water discharged from domestic, industrial, commerci al premises  or 
agricultural activities , surface run -off and accidentally any sewer infiltration water  
2.1.2  
foul wastewater  
wastewater comprising domestic and industrial wastewater  
2.1.3  
sludge  
Thick, soft wet mud or similar viscous mixture of liquid and solid components separated from suspension in a 
liquid in industrial processes and treatment of sewage and wastewater  
NOTE  the residues of preliminary treatment are not considered as sludge.  
2.1.4  
population P  
number of inhabitants  
Example: of a settlement  area  
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 2.1.5  
population equivalent PE  
conversion value which aims at evaluating non-domestic  pollution in reference to a domestic pollution fixed at 
54 g/d related to BOD 5 
Note  By extension, other parameters to define the population equivalent can be used ( COD, Suspended  Solids, daily 
quantity of wastewater, N, P etc.)  
𝑃𝐸=𝐵𝑂𝐷  𝑙𝑜𝑎𝑑  𝑓𝑟𝑜𝑚  𝑖𝑛𝑑𝑢𝑠𝑡𝑟𝑦  [𝑘𝑔
𝑑𝑎𝑦]
0.054 [𝑘𝑔
𝑖𝑛ℎ𝑎𝑏.𝑑𝑎𝑦] 
2.1.6  
total number of inhabitants and population equivalents PT  
sum of population and population equivalent:  
PT = P + PE  
Note  This sum should be specified with the expression of the equivalence used, e.g. 2 000 PT based on PEB60. The 
parameter and its value should be the same for the population and the population equivalent  
2.1.7  
design capacity  
maxim um flows and loads of the wastewater the facilities are designed for, which conform to the specified 
consent effluent standard  
Note  several loads (e.g. BOD 5 load, KN load) and several flows can be used (e.g. dry weather flow Design peak flow).  
2.1.8  
treatm ent capacity  
maximum flows and loads of the wastewater that can be treated by an existing plant so that it delivers a related 
effluent which conforms to the specified consent effluent standard  
Note  several loads (e.g. BOD 5 load, KN load) and several flows  can be used (e.g. dry weather flow, design peak flow).  
2.1.9  
degree of utilization  
ratio of actual load to treatment or design capacity of a wastewater system  DRS  126-1: 2018  
3                ©RSB 2018 - All rights reserved  
 2.1.10  
receiving water  
any type of water body where water or wastewater is discharged  
2.1.11  
final effluent  
treated wastewater effluent from the last stage of a wastewater treatment plant  
2.1.12  
final effluent discharge point  
point where the final effluent is discharged into the receiving body  
2.1.13  
discharge point  
point where the final effluent  is discharged into the receiving water  
2.1.14  
discharge consent  
statutory approval to discharge wastewater to receiving body or foul sewer  
2.1.15  
consent effluent standard  
limitation on flow, physical, biological and chemical characteristics of effluents,  included in a discharge consent  
2.1.16  
grab sample  
‹wastewater engineering› discrete sample taken instantaneously at a precise location  
2.1.17  
composite sample  
‹wastewater engineering› two or more samples or subsamples (either discretely or continuously) mixed together 
in appropriate known proportions from which the average result of a desired characteristic may be obtained   
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 2.1.18  
time proportional sample  
Time composite sample made up of equal volumes withdrawn at equal intervals of time  
2.1.19  
flow proportional sample  
flow composite sample made up of either equal volumes taken after equal volumes of liquid have passed the 
sampling point, or made up of flow proportional volumes taken at equal intervals of time  
2.1.20  
groundwater table  
level below whic h the ground is saturated with water  
2.1.21  
bottom water level BWL  
minimum operating water level in any structure  
2.1.22  
top water level TWL  
maximum operating water level in any structure  
2.1.23  
tightness testing  
non-destructive test to measure leakage on an isolated structure, pipe etc.  
2.1.24  
package plant  
prefabricated factory -built sewage treatment installation  
2.1.25  
on site construction  
site construction which can include prefabricated modules or units  DRS  126-1: 2018  
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 2.1.26  
site assembly  
site configuration of prefa bricated modules or units  
2.1.27  
catchment area  
area draining to a drain, sewer or watercourse  
2.1.28  
outfall  
structure or point from which wastewater is discharged to a wastewater treatment plant or receiving water  
2.2 Types of wastewater and wastewater collection  
2.2.1  
raw wastewater  
wastewater in 2.1.1 which has not undergone any treatment  
2.2.2  
septic sewage  
septic anaerobic wastewater which usually contains hydrogen sulphide  
2.2.3  
domestic wastewater  
wastewater discharged from kitchens, laundry rooms,  lavatories, bathrooms, toilets and similar Facilities  
2.2.4  
industrial wastewater  
water discharged after being used in, or produced by, an industrial process, and which is of no further immediate 
value to that process  
2.2.5  
urban wastewater  
municipal wast ewater from municipal areas consisting predominantly of domestic wastewater and additionally 
it may also contain surface water, infiltration water, trade or industrial wastewater   
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 2.2.6  
rain water  
water arising from atmospheric precipitation, which has not yet collected matter from the surface  
2.2.7  
surface water  
water from precipitation, which has not seeped into the ground and which is discharged to the drain or sewer 
system directly from the ground or from exterior building surfaces  
2.2.8  
heat exchange water  
water used to transfer heat  
2.2.9  
leachate  
water which has percolated through contaminated material,  
E.g. tipped refuse  
2.2.10  
combined wastewater  
water conveyed in a combined system  
2.2.11  
combined system  
drain and sewer system designed to carry bot h foul wastewater and surface water in the same pipeline(s)  
2.2.12  
separate system  
drain and sewer system, usually of two pipelines, one carrying foul wastewater and the other surface water  
2.2.13  
combined sewer overflow  
device on a combined system that re lieves the system of excess flow  DRS  126-1: 2018  
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 2.2.14  
storm water overflow  
device within a wastewater treatment plant that relieves the system of excess flow  
2.2.15  
storm water overflows discharge  
excess water from storm water overflow  
2.2.16  
cesspool  
watertight tank, mostly underground, used for collecting sewage from premises not connected to the public 
sewer and which, unlike a septic tank, has no outflow  
2.2.17  
pressure main  
rising main ‹wastewater engineering› pipeline for conveying wastewater under pressure  
2.2.18  
vacuum main  
‹wastewater engineering› pipeline for conveying wastewater under vacuum  
2.2.19  
grey water  
domestic wastewater excluding faecal matter and urine  
2.2.20  
black water  
domestic wastewater with only faecal matter and urine  
2.2.21  
infiltration  
‹into the ground› the movement of surface water or treated effluent into the ground   
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 2.2.22  
infiltration  
‹into the drain or sewer system› unwanted flow resulting from ingress of groundwater into a drain or sewer 
system  
2.2.23  
exfiltration  
escape of wastewater fr om a drain or sewer system into surrounding ground  
2.2.24  
detention tank  
tank or reservoir for the temporary storage of wastewater  
2.2.25  
drain  
pipeline, usually underground, designed to carry wastewater from a source to a sewer  
2.2.26  
gravity system  
drain or sewer system where flow is caused by the force of gravity and where the pipeline is designed to usually 
operate partially full  
2.2.27  
sewer  
pipeline or other construction, usually underground, designed to carry wastewater from more than one source   
2.2.28  
sewer system  
network of pipelines and ancillary works which conveys wastewater from drains to a treatment plant or other 
place of disposal  DRS  126-1: 2018  
9                ©RSB 2018 - All rights reserved  
 2.3 Wastewater quantity and quality  
2.3.1  
flow 
‹wastewater engineering› volume of fluid passing a certain cross section per unit of time  
2.3.2  
peak flow  
maximum volume of fluid passing a certain cross -section per unit of time  
2.3.3  
design peak flow  
maximum flow of wastewater a plant is designed to treat  
2.3.4  
dry weather conditions  
period of time during which the  influence of rainfall precipitation or snow melt is negligible in terms of flow  
Example:  consecutive 5 days without rainfall over neither 1 mm/d nor snow melt  
2.3.5  
dry weather flow  
flow not affected by rainfall or snow melt  
2.3.6  
dry weather peak hourly flow  
maximum hourly flow of wastewater during a 24 h -period measured under dry weather conditions  
2.3.7  
volume of water discharge  
Integral of flow over a given time interval   
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 2.3.8  
load 
‹wastewater engineering› ratio of mass to time  
Example  mass of  BOD 5 per unit of time  
2.3.9  
concentration  
‹wastewater engineering› ratio of mass to volume  
Example: mass of BOD 5 per unit of volume  
2.3.10  
biochemical oxygen demand BOD t 
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  
2.3.11  
chemical oxygen demand COD  
concentration of oxygen equivalent to the amount of dichromate consumed when a water sample is treated w ith 
that oxidant under defined conditions  
2.3.12  
total organic carbon TOC  
concentration of carbon present in the organic matter which is dissolved or suspended in water  
2.3.13  
dissolved organic carbon DOC  
concentration of organic carbon remaining in solution after filtration under defined conditions  
2.3.14  
total solids TS  
concentrations of the sum of dissolved, suspended and floating solids  DRS  126-1: 2018  
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 2.3.15  
suspended solids SS  
concentration of solids in a liquid usually determined by filtration or centrifuging and then drying all under 
specified conditions  
2.3.16  
dissolved solids  
concentration of substances remaining after filtration and evaporation to dryness determined under specified 
conditions  
2.3.17  
settle able solids  
concentration of the suspended solids h aving settled under specified conditions  
2.3.18  
floating solids  
undisclosed matter floating on water  
2.3.19  
total nitrogen N tot 
sum of the concentrations of Kjeldahl, nitrite and nitrate nitrogen  
2.3.20  
kjeldahl Nitrogen KN  
concentration of the sum of orga nic and ammoniacal nitrogen  
2.3.21  
total phosphorus  
concentration of the sum of organic and inorganic phosphorus  
2.3.22  
dissolved phosphorus  
concentration of the sum of organic and inorganic phosphorus measured after filtration under defined conditions   
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 2.3.23  
toxic substance  
substance which in a low concentration is liable to inhibit biological processes  
2.4 Methods, characteristics and Impact on the environment  
2.4.1  
pretreatment  
‹wastewater engineering› modification of the characteristic of a wastewater be fore discharge to the sewer  
2.4.2  
preliminary treatment  
stage of treatment involving gross solids, sand, grit or floating matter removal from wastewater  
2.4.3  
primary treatment  
stage of treatment involving the removal of suspended solids from raw wastewate r or after preliminary treatment  
2.4.4  
secondary treatment  
stage of treatment by biological processes, such as activated sludge or other (even no biological) processes 
giving equivalent results  
2.4.5  
biological treatment  
stage of treatment by biological pr ocesses such as activated sludge  
2.4.6  
tertiary treatment  
advanced treatment additional treatment processes which result in further purification than that obtained by 
applying primary and secondary treatment  
Note  it is recommended that the expression for the precise treatment, e.g. nitrogen removal, phosphorus removal, 
polishing lagoons, disinfection, filtration, is used since in some cases the tertiary treatment can also be integrated in the  
secondary treatment.  DRS  126-1: 2018  
13                ©RSB 2018 - All rights reserved  
 2.4.7  
aerobic wastewater treatment  
purifica tion of wastewater with the help of aerobic organisms under aerobic or anoxic conditions  
2.4.8  
anaerobic wastewater treatment  
purification of wastewater with the help of microorganisms under anaerobic conditions  
2.4.9  
multistage biological treatment  
series  arrangement of identical or different biological process stages with separate biological sledges  
2.4.10  
aerobic  
dissolved oxygen is present  
2.4.11  
anoxic  
dissolved oxygen is absent and nitrite or nitrate is present  
2.4.12  
anaerobic  
dissolved oxygen, nitr ate, nitrite and sulphate are absent  
2.4.13  
degradation  
‹wastewater engineering› physical, chemical or bio -chemical process by which wastewater or sludge 
components are broken down  
2.4.14  
biodegradation  
‹wastewater engineering› degradation of wastewater o r sludge components resulting from the activity of living 
organisms   
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 2.4.15  
aerobic degradation  
‹wastewater engineering› biodegradation by microorganisms under aerobic or anoxic conditions  
2.4.16  
anaerobic degradation  
‹wastewater engineering› biodegradation  by microorganisms under anaerobic conditions  
2.4.17  
total respiration  
exchange of gases between an organism and its environment resulting from oxidation with the release of energy 
under aerobic or anoxic conditions  
Note  Sum of substrate respiration and en dogenous respiration  
2.4.18  
substrate  
nutrition for microorganisms in the wastewater treatment  
2.4.19  
respiration rate  
rate of oxygen consumption due to respiration  
2.4.20  
substrate respiration  
consumption of oxygen by organisms due to the biodegradation o f added substrate  
2.4.21  
endogenous respiration  
consumption of oxygen by organisms without addition of a substrate  
2.4.22  
eutrophication  
enrichment of water, both fresh and saline, by nutrient salts, especially compounds of phosphorus and nitrogen, 
that wi ll accelerate the growth of algae and higher forms of plant life  DRS  126-1: 2018  
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 2.4.23  
nutrient salts  
inorganic matter necessary for the nutrition of organisms  
Example  N, S, P and trace elements  
2.4.24  
nutrient salts removal  
biological, physical or chemical processes used in wastewater treatment specifically for the removal of 
phosphorus and nitrogen compounds  
2.4.25  
ammonification  
conversion of compounds containing organic nitrogen to ammonium ions  
2.4.26  
nitrification  
oxidation of ammonium ions by bacteria, usually  to the end product nitrate  
2.4.27  
denitrification  
reduction of nitrate or nitrite to liberate mainly nitrogen gas by the action of bacteria  
2.4.28  
plug -flow system  
theoretical system with complete mixing in the cross -section of the flow and with neither d iffusion nor dispersion 
in the direction of flow  
2.4.29  
completely mixed system  
theoretical system with a uniform concentration of components  
2.4.30  
power per unit volume of reactor  
measured or installed electrical power of driving motor of a mixing and/or  aeration device, relative to volume of 
a reactor   
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 2.4.31  
retention period  
detention time theoretical period during which a fluid is retained in a particular unit or system as calculated by 
the ratio of its volume divided by the flow of fluid excluding rec ycled flows  
2.4.32  
degree of separation  
‹wastewater engineering› ratio of mass separated in a separation process to the introduced mass of a substance  
2.4.33  
contact time  
‹wastewater engineering› time available to wastewater for contact with other media or  substances  
2.4.34  
volumetric loading  
ratio of load to volume of treatment module  
2.4.35  
surface loading rate  
ratio of flow to surface area  
Example  The volume of wastewater or sludge treated per unit time per unit horizontal cross -sectional area of that part of 
the treatment plant under consideration.  
2.4.36  
mass surface loading rate  
mass of components introduced per unit time and surface area unit  
Example  Solids loading of sedimentation tanks or BOD 5 loading of the filter medium of biological filters.  
2.4.37  
settling velocity  
‹wastewater engineering› rate of settling of solids determined under specified conditions  
Example  by settling curve  DRS  126-1: 2018  
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 2.4.38  
weir-overflow rate  
flow passing over the unit length of weir  
2.4.39  
settlement tank  
sedimentation tank stru cture for separation of particles from wastewater under the influence of gravity  
Example  primary settlement tank, intermediate settlement tank, secondary settlement tank or clarifier  
2.4.40  
scraper  
‹wastewater engineering› mechanical device for removal of settled or floating material  
2.4.41  
flotation process  
raising of suspended matter in liquid to the surface by the entrainment of a gas  
2.4.42  
floating sludge  
scum floating solid material separated from sludge or wastewater  
2.4.42  
scum board  
vertical board partly immersed in a tank or pond to retain floating material  
2.4.43  
stripping  
removal of volatile components from liquids by gas exchange  
2.4.44  
biological seeding  
introduction of appropriate microorganisms into a biological system in order to initiate or  enhance a biological 
process   
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 2.4.45  
disinfection  
‹wastewater engineering› treatment of wastewater or sludge to reduce pathogenic activity below a specified 
level 
2.4.46  
flow splitter  
chamber or channel arrangement designed to divide a flow into required p roportions  
2.4.47  
baffle  
device used in a tank to reduce eddies and promote a more uniform flow through the tank  
2.4.48  
dosing point  
‹wastewater engineering› position at which chemicals are added to wastewater or sludge in a treatment plant  
2.4.49  
wastewater treatment plant  
facility for the physical, biological and/or chemical treatment of wastewater  
2.4.50  
physico -chemical treatment of wastewater  
treatment of wastewater by addition of chemical(s)  
2.4.51  
semi permeable membrane  
material used as filt er material in membrane filtration processes  
2.4.52  
cross flow filtration  
filtration with a significant flow parallel to the membrane surface, which is intended to prevent substances from 
accumulating on the surface of the membrane  DRS  126-1: 2018  
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 2.4.53  
dead end filtrati on 
filtration without a significant flow parallel to the membrane surface  
2.4.54  
granular media filter  
bed of filter material which is submerged in either an up flow or down flow of effluent to remove solids within the 
bed 
2.4.55  
drum filter  
cylindrical cl oth filter which rotates about a horizontal axis and is immersed in a horizontal flow of effluent to 
remove solids  
2.4.56  
micros trainer  
cylindrical sieve which rotates about a horizontal axis and is partially immersed in a horizontal flow of effluent to 
remove solids  
2.5 Preliminary and primary treatment  
2.5.1  
screen  
device for removing particles and objects from a flow of wastewater  
Example  by retention on manually or mechanically raked bars, on moving bands, rotating discs or drums of perforated 
metal sheet or of wire mesh  
2.5.2  
sieve  
device for removing fine solids from a flow of wastewater  
EXAMPLE  by retention on moving bands, rotating discs or drums of perforated sheet, wire mesh or filter bags  
Note  Sieves usually have smaller openings than screens . 
2.5.3   
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 screenings  
matter retained by a screen or a sieve  
2.5.4  
screening press  
mechanical device for reducing volume and water content of screenings  
2.5.5  
macerator  
comminutor shredding device for reducing the size of coarse particles  
2.5.6  
grit separat or 
structure or mechanical device for separating and removing grit, sand or similar mineral material from 
wastewater  
2.5.7  
grit chamber  
structure for separating grit, sand or similar mineral matter from wastewater  
2.5.8  
aerated grit chamber  
structure for separating sand and other mineral matter from wastewater, using air to induce circulation and may 
be combined with the removal of floating matter  
2.5.9  
grit classifier  
mechanical device to remove solid material from a grit separator while simultaneously re ducing its water and 
organic content  
2.5.10  
grease separator  
oil separator structure or device for separation of grease, oil or other floating material from wastewater  DRS  126-1: 2018  
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 2.5.11  
balancing  
equalization reduction in variations in flow, concentration, temperatu re etc.  
2.5.12  
balancing tank  
structure which permits reduction in variations in flow, concentration, temperature etc.  
2.5.13  
primary settlement tank  
settlement tank in which the majority of settle able solids are removed by sedimentation from raw or prel iminarily 
treated wastewater  
2.5.14  
lamella separator  
system of regularly spaced, inclined plates or tubes, designed to increase the effective settling area in a 
settlement tank  
2.6 Fixed film treatment  
2.6.1  
biological film  
layer consisting of microorganisms which forms on the surface of support media  
2.6.2  
fixed film reactor  
reactor in which most of the biological treatment is achieved by a biological film attached to support media  
2.6.3  
biological filter  
trickling filter percolating filter fixed film reactor with a bed of support media through which wastewater percolates  
Note  Aeration may occur by natural or artificial ventilation.   
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 2.6.4  
biological contactor  
fixed film reactor the biological film of which is intermittently or continuously immersed in wastewater, sometimes 
combining active aeration  
2.6.5  
submerged bed reactor  
type of fixed film reactor in which the support media is submerged in the wastewater flow  
2.6.6  
granular fixed bed reactor  
biofilterfixed film reactor using granular material as a  medium which combines filtration and biochemical 
degradation  
2.6.7  
fluidized bed reactor  
‹wastewater engineering› fixed film reactor in which a bed of particles is freely suspended by an upward flow of 
liquid, gas or combined liquid and gas  
2.6.8  
sand fil ter 
constructed filter using sand as filter material  
2.6.9  
constructed wetland  
construction near to nature using helophyte plants supporting bacterial action in the gravel or sand for treatment 
of wastewater  
2.6.10  
support media  
inert material of various specific surfaces on which an attached film grows  
2.6.11  
support media specific surface  
property of support media expressed as surface area per unit volume measured under specified conditions,  DRS  126-1: 2018  
23                ©RSB 2018 - All rights reserved  
 e.g. material without biofilm  
2.6.12  
filter dosing chamber  
small tank which receives settled wastewater until the desired volume has accumulated, when it is discharged 
automatically to the distributor of a biological filter  
2.6.13  
filter distributor  
device for uniform distribution of wastewater on biological filte rs 
2.6.14  
humus sludge  
biological film which has sloughed off from a biological filter and is usually separated from the treated wastewater 
in an intermediate or secondary settlement tank  
2.6.15  
humus tank  
intermediate or secondary settlement tank downstre am of a biological filter  
2.6.16  
recirculation  
Return of a proportion of fixed film reactor effluent to mix with the influent  
2.7 Activated sludge treatment  
2.7.1  
activated sludge process  
process for the biological treatment of wastewater in which a mixture of  wastewater and activated sludge is 
agitated and aerated, subsequently separated from the treated wastewater and is returned to the process  
Note  some activated sludge is removed from the process as excess sludge.  
2.7.2  
activated sludge  
biological mass (fl ocs) produced in the treatment of wastewater by the growth of suspended bacteria and other 
microorganisms under aerobic or anoxic conditions   
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 2.7.3  
return ed activated sludge  
activated sludge which has been separated from mixed liquor in the secondary settlement tank for further use 
in the activated sludge process  
2.7.4  
return sludge ratio  
ratio of return activated sludge flow to the wastewater inflow  
2.7.5  
denitrification mixed liquor Recirculation ratio  
ratio of recirculated nitrate rich mixed liquor from an aeration tank to the gentrification zone divided by the 
incoming flow  
2.7.6  
mixed liquor  
mixture of wastewater and activated sludge undergoing treatment of an activated sludge plant  
2.7.7  
mixed liquor suspended solids MLSS  
concentration of suspend ed solids in the mixed liquor  
2.7.8  
mixed liquor volatile suspended Solids MLVSS  
concentration of organic suspended solids in the mixed liquor  
2.7.9  
surplus activated sludge  
waste activated sludge excess sludge which is removed from an activated sludge pro cess 
2.7.10  
specific surplus sludge production  
ratio of mass of suspended solids of surplus sludge to unit mass of BOD5 removed  DRS  126-1: 2018  
25                ©RSB 2018 - All rights reserved  
 2.7.11  
sludge age  
calculated time required to waste the total inventory of sludge being in the process tanks (excluding the clar ifiers 
and anaerobic zones and including the aerobic and anoxic zones) at a constant wastage rate and taking the 
treated Wastewater solids into account  
2.7.12  
mean cell residence time MCRT  
calculated time required to waste the total inventory of sludge in an activated sludge plant (including clarifiers, 
aerobic, anoxic and anaerobic zones etc.) at a constant wastage rate and taking the treated wastewater solids 
into account  
Note  Sludge age taking into account all the treatment works.  
2.7.13  
sludge loading F /M 
load of pollutants entering the biological treatment per unit mass of mixed liquor suspended solids or mixed 
liquor volatile suspended solids  
Note  the basis may be total or volatile suspended solids.  
2.7.14  
SBR reactor  
activated sludge wastewater treatm ent with discontinuous operation in one tank  
Note  Operations are filling, mixing, aerating, settling and decanting.  
2.7.15  
aeration  
‹wastewater engineering› introduction of air or Oxygen  
2.7.16  
tapered aeration  /Stepped aeration)  
step aeration type of act ivated sludge plant aeration whereby a greater quantity of air is admitted to the upstream 
end of the plug flow aeration tank where the highest biological activity exists, and a lesser mount of air is 
admitted to the downstream end of the tank   
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 2.7.17  
aerat ion tank  
structure in which wastewater and activated sludge are mixed and aerated  
2.7.18  
oxidation ditch  
type of aeration tank taking the form of usually parallel channels joined at the ends to form a closed circuit  
2.7.19  
contact  stabilization  
type of activated sludge process whereby the return sludge is aerated separately  
2.7.20  
extended aeration  
activated sludge process where a long aeration phase enables reduction of organic material in the sludge  
2.7.21  
oxygen concentration  
mass of oxygen dissolved  per unit volume of water or wastewater  
2.7.22  
oxygen saturation value  
concentration of dissolved oxygen in water or wastewater in equilibrium, either with air (natural systems) or with 
pure oxygen (oxygen wastewater treatment systems)  
Note  it varies with  temperature, partial pressure of oxygen and salinity  
2.7.23  
oxygen uptake rate  
mass of oxygen consumed per unit time and per unit volume of mixed liquor  
2.7.24  
oxygen transfer capacity under Process conditions 
 OC 
mass of oxygen that under process conditi ons can be transferred into mixed liquor per unit time  DRS  126-1: 2018  
27                ©RSB 2018 - All rights reserved  
 2.7.25  
oxygen transfer capacity in clean water OC  
mass of oxygen that under standard conditions an aeration device or system can transfer into clean water per 
unit time  
2.7.26  
OC/load in clean water  
ratio of oxygen transfer capacity in clean water to BOD -load 
2.7.27  
alpha factor  
ratio of the oxygen transfer coefficient in mixed liquor to the oxygen transfer coefficient in clean water  
2.7.28  
oxygen saturation factor  
beta factor ratio of the oxygen saturation value in mixed liquor to the oxygen saturation value in clean Water at 
the same temperature and atmospheric pressure  
2.7.29  
gross oxygen transfer efficiency under process conditions  
ratio of oxygen transfer capacity in mixed liquor suspended sol ids to power consumption measured at the motor 
terminals  
2.7.30  
clarifier  
secondary settlement tank settlement tank in which activated or humus sludge is separated from the effluent of 
an activated sludge plant or biological filter  
2.7.31  
static, up flow  settlement tank  
dortmund tank funnel -shaped sedimentation tank with mainly vertical flow and mostly central wastewater Inlet  
2.7.32  
sludge blanket  
layer of freely suspended sludge between the overflow level and the inflow level in an up -flow clarifier or in an 
up-flow anaerobic sludge blanket reactor   
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 2.7.33  
sludge blanket filtration  
effect of the sludge blanket as filter when being passed by mixed liquor  
2.7.34  
sludge blanket level  
depth of sludge/supernatant interface below the surface of the clarifier  
2.7.35 
sludge volume surface loading  
volume of sludge being passed through the horizontal cross -sectional area of a settlement tank per unit of time  
Note  it is calculated as the product of surface loading rate and settled sludge volume.  
2.7.36  
settled sludge  volume  
volume of sludge per litre of wastewater or mixed liquor settled after 30 min  
2.7.37  
sludge volume index SVI  
volume in milliliters occupied by 1 g of activated sludge after settlement under specified conditions  
Note  Specific conditions can include dilution, stirring and specified time, usually 30 min.  
2.7.38  
sludge bulking  
activated sludge that occupies an excessive volume and is difficult to be settled and thickened  
Note  usually associated with the excessive presence of filamentous organisms.  
2.8 Othe r wastewater treatment  
2.8.1  
wastewater lagoon  
wastewater pond basin of simple construction, mostly earth bank structure for wastewater treatment  DRS  126-1: 2018  
29                ©RSB 2018 - All rights reserved  
 Example: oxidation pond, aerated lagoon or maturation pond  
2.8.2  
settlement lagoon  
settlement pond wastewater  lagoon used for the separation of solids from wastewater  
2.8.4  
oxidation pond  
stabilization pond wastewater lagoon without artificial aeration in which mainly aerobic degradation occurs  
2.8.5  
anaerobic lagoon  
wastewater lagoon for wastewater settlement an d anaerobic degradation and digestion of sludge  
2.8.6  
maturation pond  
wastewater lagoon used as tertiary treatment, typically for the removal of pathogenic microorganisms by 
exposure to solar radiation by competition and predation mechanisms  
Note  See also 8080 “effluent polishing”.  
2.8.7  
natural lagooning  
biological wastewater treatment consisting of a series of wastewater lagoons without artificial aeration  
2.8.8  
effluent polishing  
further stage of treatment improving the quality of secondary effluent by removing suspended solids  
Example: polishing lagoon or filtration  
Note  Consequential removal of residual BOD 5 may occur.  
2.8.9  
land treatment  
irrigation treatment (and usually disposal) of wastewater, by spreading it on the land for subsoil infiltration  
2.8.10 
percolation   
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 disposal of suitably treated wastewater into subsoil without agricultural objective  
Example: through a soak away, an infiltration gallery or a network of subsoil drains  
2.8.11  
subsoil drain  
pipe that disposes water into subsoil  
2.8.12  
soak  away  
pit or other drainage arrangement prepared in permeable ground to which treated wastewater is fed and from 
which it soaks into the ground  
2.8.13  
chemical precipitation  
conversion of components dissolved in water into undissolved form by chemical reac tion with a precipitant  
2.8.14  
precipitant  
chemical used to bring about precipitation  
2.8.15  
coagulation  
‹wastewater engineering› destabilization of undissolved and colloidally dispersed matter to allow aggregation, 
usually by addition of coagulants  
2.8.16  
coagulant  
‹wastewater engineering› chemical added to destabilize suspensions or emulsions  
2.8.17  
flocculation  
‹wastewater engineering› formation of separable flocs by aggregation of small particles  
Note  the process is usually assisted by mechanical,  physical, chemical or biological means.  
2.8.18  
flocculent  DRS  126-1: 2018  
31                ©RSB 2018 - All rights reserved  
 ‹wastewater engineering› chemical which is added to produce flock aggregates or to increase or strengthen 
flocs 
2.8.19  
filter material  
‹wastewater engineering› inert material with various particle  or pore sizes used for filtration  
2.9 Sludge treatment  
2.9.1  
sludge treatment  
all stages of transformation of sludge for its utilization or disposal  
Note  this can include sludge thickening, sludge stabilization, sludge conditioning, thermal hydrolysis, dewat ering, 
drying, disinfection, incineration.  
2.9.2  
sludge disposal  
‹wastewater engineering› disposal of sludge without environmental benefit  
EXAMPLE  sludge or incineration ash to landfill  
2.9.3  
sludge utilization  
‹wastewater engineering› utilization of sludge with environmental benefit  
2.9.4  
sludge characterization  
description of the physical, mechanical, chemical and biological properties of sludge  
2.9.5  
raw sludge  
 
sludge removed from primary sedimentation tanks  
Note 1 to entry: It may include primary sludge, co -settled with recycled secondary sludge   
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 2.9.6  
primary sludge  
sludge removed from primary treatment unmixed with other recycled sludges  
2.9.7  
mixed primary sludge  
sludge removed from primary treatment which contains other sludges, e.g. waste activ ated sludge  
2.9.8  
secondary sludge  
sludge separated after secondary treatment  
2.9.9  
biological sludge  
sludge separated after biological treatment  
2.9.10  
physico -chemical sludge  
sludge separated after chemical precipitation  
2.9.11  
stabilization  
‹wastewater engineering› process whereby organic substances (dissolved or particulate) are converted to 
materials which are either inorganic or very slowly degradable  
2.9.12  
stabilized sludge  
sludge which has been subjected to a stabilization process, thereby reducing  its tendency for biological 
degradation  
2.9.13  
degree of stabilization  
degree of degradation achievable by sludge stabilization  
Note  Measured e.g. by the reduction of organic material.  DRS  126-1: 2018  
33                ©RSB 2018 - All rights reserved  
 2.9.14  
anaerobic sludge digestion  
anaerobic process which reduces the  organic content of sludge  
2.9.15  
aerobic sludge digestion  
aerobic process which reduces the organic content of sludge  
2.9.16  
anaerobicly digested sludge  
sludge stabilized by anaerobic digestion  
2.9.17  
aerobically digested sludge  
sludge stabilized by aerob ic digestion  
2.9.18  
digester  
digestion tank reactor for digestion  
2.9.19  
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  
2.9.20  
Imhoff tank  
two-storey structure the upper part of which serves as a settlement tank whilst its lower part serves as an 
anaerobic digester  
2.9.21  
digester gas  
biogas gas mixture generated during anaerobic digestion compri sing mainly methane (CH 4) and carbon dioxide 
(CO 2)  
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 2.9.22  
digestion time  
ratio of effective volume of digester to mean daily sludge volume input  
Note  if supernatant is withdrawn from the digester, the digestion time is shorter than the mean solids retentio n time.  
2.9.23  
digester solids retention time  
ratio of total inventory of solids in a completely mixed digester to the mean daily output of suspended solids, 
taking the suspended solids of removed supernatant into account  
2.9.24  
pasteurization  
process, inv olving the elevation of temperature for an appropriate period of time, for the purpose of either 
inactivating microorganisms, particularly pathogens, or decreasing their number for a limited period of time 
below a specified level or a value lower than the infectious threshold  
2.9.25  
sludge thickening  
sludge concentration process, with or without chemical treatment, such as gravity decantation or flotation and 
keeping it in liquid state  
2.9.26  
thickener  
device for sludge thickening usually proceeding dewatering  
2.9.27  
picket fence  
slow-speed rotary device in a thickener comprising vertical bars mostly provided with a scraper  
2.9.28  
sludge lagoon  
lagoon for storage of sludge  DRS  126-1: 2018  
35                ©RSB 2018 - All rights reserved  
 2.9.29  
sludge conditioning  
physical, chemical, thermal or other treatment of sludge to facilitate dewatering  
2.9.30  
chemical conditioning  
conditioning by addition of chemicals  
2.9.31  
thermal conditioning  
conditioning by altering sludge temperature  
2.9.32  
sludge dewatering  
reduction of the water content of sludge by the use of one or several technologies, usually by natural or 
mechanical means  
2.9.33  
sludge drying bed  
structure for sludge dewatering and eventually drying of sludge by drainage and evaporation  
2.9.34  
filter capacity  
mass of dry solids retained or sludge volume passed per unit time and per unit filter area or other suitable 
dimension  
2.9.35  
filter cake  
‹wastewater engineering› solid or semi -solid residue produced during sludge dewatering by a filtering -process  
2.9.36  
sludge liquor  
Liquor separated from sludge  
NOTE  Sludge liquor can be called:   
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 a) supernatant (thickener, digestes),  
b) filtrate (filter),  
c) centrate (centrifuge) etc.  
2.9.37  
supernatant liquor  
liquor in a tank lying above the deposited solids  
2.9.38  
post conditioning of sludge  
physical, chemical, thermal or othe r treatment of sludge to facilitate sludge disposal or utilization after 
dewatering  
2.9.39  
thermal sludge drying  
process whereby water is removed from sludge by evaporation  
2.9.40  
sludge incineration  
high-temperature oxidation of sludge organic material  
2.9.41 
dried solid content  
ratio of the mass of total solids to total mass of sludge  
2.9.42  
psychrophilic  
temperature range for organisms active typically below 30 °C  
2.9.43  
mesophilic  
temperature range for organisms active at temperatures typically between 30 °C and 45 °C  DRS  126-1: 2018  
37                ©RSB 2018 - All rights reserved  
 2.9.44  
thermophilic  
temperature range for organisms active at temperatures typically over 45 °C  
2.9.45  
conditioned sludge  
sludge treated physically or chemically to improve dewater ability  
2.9.46  
dewatered sludge  
sludge, in which the water content has been reduced by natural or mechanical means  
2.9.47  
dried sludge  
sludge in which the water content has been reduced to a low level by evaporation  
2.9.48  
industrial sludge  
sludge from the treatment of industrial wastewater  
2.9.49  
sludge cake  
sludge generated from dewatering devices  
Example  filter press, centrifuge .. 
   
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 Bibliography  
 DRS  126-1: 2018  
ICS 13.030.20  
 
 
©RSB 2018 - All right s reserved