EN 12255-14:2023

SLOVENSKI STANDARD
01-september-2023
Nadomešča:
SIST EN 12255-14:2004
Čistilne naprave za odpadno vodo - 14. del: Dezinfekcija
Wastewater treatment plants - Part 14: Disinfection
Kläranlagen - Teil 14: Desinfektion
Stations d’épuration - Partie 14 : Désinfection
Ta slovenski standard je istoveten z: EN 12255-14:2023
ICS:
13.060.30 Odpadna voda Sewage water
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 12255-14
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2023
EUROPÄISCHE NORM
ICS 13.060.30 Supersedes EN 12255-14:2003
English Version
Wastewater treatment plants - Part 14: Disinfection
Stations d'épuration - Partie 14: Désinfection Kläranlagen - Teil 14: Desinfektion
This European Standard was approved by CEN on 28 May 2023.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 12255-14:2023 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Introduction . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Symbols and abbreviations . 9
5 Design requirements . 10
5.1 General. 10
5.2 Planning . 11
5.2.1 General. 11
5.2.2 Level of disinfection . 11
5.3 Process design . 11
5.3.1 General. 11
5.3.2 UV radiation . 12
5.3.3 Ozonation . 13
5.3.4 Chlorination . 16
5.3.5 Peracids (Peracetic acid) . 18
5.3.6 Membrane filtration . 18
5.3.7 Effluent maturation ponds . 19
5.3.8 Soil filtration . 19
5.3.9 Hydrogen peroxide . 20
5.4 Process control . 20
5.5 Structures . 21
5.6 Health and safety . 21
Annex A (normative) Ozone system classification . 23
Annex B (informative) Measurement of ozone concentration in water. 24
B.1 General. 24
B.2 Titrimetric determination of the ozone concentration according to the KI method . 24
B.3 Photometric determination within the UV range . 28
Bibliography . 31
European foreword
This document (EN 12255-14:2023) has been prepared by Technical Committee CEN/TC 165
“Wastewater engineering”, the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by January 2024, and conflicting national standards shall
be withdrawn at the latest by January 2024.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 12255-14:2003.
th
It is the 14 part prepared by Working Group CEN/TC 165/WG 40 relating to the general requirements
and processes for treatment plants for a total number of inhabitants and population equivalents (PT)
over 50.
The EN 12255 series with the generic title “Wastewater treatment plants” consists of the following parts:
• Part 1: General construction principles
• Part 2: Storm management systems
• Part 3: Preliminary treatment
• Part 4: Primary treatment
• Part 5: Lagooning processes
• Part 6: Activated sludge process
• Part 7: Biological fixed-film reactors
• Part 8: Sludge treatment and storage
• Part 9: Odour control and ventilation
• Part 10: Safety principles
• Part 11: General data required
• Part 12: Control and automation
• Part 13: Chemical treatment — Treatment of wastewater by precipitation/flocculation
• Part 14: Disinfection
• Part 15: Measurement of the oxygen transfer in clean water in aeration tanks of activated sludge plants
• Part 16: Physical (mechanical) filtration
NOTE Part 2 is under preparation.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North
Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the United
Kingdom.
Introduction
Differences in wastewater treatment throughout Europe have led to a variety of systems being developed.
This document gives fundamental information about the systems; this document has not attempted to
specify all available systems. A generic arrangement of wastewater treatment plants is illustrated below
in Figure 1:
Key
1 preliminary treatment
2 treatment
3 secondary treatment
4 tertiary treatment
5 additional treatment (e.g. disinfection or removal of micropollutants)
6 sludge treatment
7 lagoons (as an alternative)
A raw wastewater
B effluent for re-use (e.g. irrigation)
C discharged effluent
D screenings and grit
E primary sludge
F secondary sludge
G tertiary sludge
H stabilized sludge
I digester gas
J returned water from dewatering
Figure 1 — Schematic diagram of wastewater treatment plants
Detailed information additional to that contained in this document can be obtained by referring to the
Bibliography.
The primary application is for wastewater treatment plants designed for the treatment of domestic and
municipal wastewater.
NOTE For requirements on pumping installations at wastewater treatment plants see EN 752 and the
EN 16932 series:
—  Part 1: General requirements;
—  Part 2: Positive pressure systems;
—  Part 3: Vacuum systems.
1 Scope
This document specifies design principles and performance requirements for disinfection of effluents
(excluding sludge) at wastewater treatment plants serving more than 50 PT.
NOTE Sludge disinfection is described in EN 12255-8.
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.
EN 12255-1, Wastewater treatment plants - Part 1: General construction principles
EN 12255-5, Wastewater treatment plants - Part 5: Lagooning processes
EN 12255-10, Wastewater treatment plants - Part 10: Safety principles
EN 12255-12, Wastewater treatment plants - Part 12: Control and automation
EN 12255-15, Wastewater treatment plants - Part 15: Measurement of the oxygen transfer in clean water
in aeration tanks of activated sludge plants
EN 16323, Glossary of wastewater engineering terms
ISO 15727, UV-C devices — Measurement of the output of a UV-C lamp
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 12255-1, EN 16323, and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
• ISO Online browsing platform: available at https://www.iso.org/obp/
• IEC Electropedia: available at https://www.electropedia.org/
3.1
residual concentration
concentration of a substance in the final effluent of a treatment stage
3.2
UV dose
product of UV irradiance and specific exposure time along the pathway of an infinitesimal small water
volume
Note 1 to entry: UV dose is expressed in millijoules per square centimetre (mJ/cm ).
3.3
UV intensity
quotient of the energy of the UV radiation received on the surface of an infinitesimal small area divided
by the size of the area
Note 1 to entry: The unit of UV intensity is W/m , measured in accordance with ISO 15727.
3.4
UV-reactor
closed vessel or an open channel section with an assembly of UV-lamps irradiating the water passing
through
3.5
bioassay
measurement of the concentration or potency of a substance by its effect on living cells or tissues
[SOURCE: EN 16323:2014, 2.3.5.4, modified to remove limitations]
3.6
specific ozone demand
required dissolved ozone concentration in the wastewater to achieve a level of disinfection
Note 1 to entry: The unit of specific ozone demand is typically g O /m or g O /l.
3 3
3.7
ozone destructor
device for destruction of residual ozone that has not been consumed in the ozonation process and is
accumulated in the gaseous form in an off-gas stream
Note 1 to entry: The destruction takes place in gas-phase by converting ozone (O3) into oxygen (O2).
3.8
chlorinator
equipment for dosing chlorine into water
Note 1 to entry: Includes in situ generation.
3.9
contact tank
tank for providing the required retention time for certain reactions to take place
3.10
contact time
required retention time at a certain concentration for a specific reaction to occur
3.11
membrane
semipermeable material used as filter media in membrane filtration processes
Note 1 to entry: Membranes normally are flat sheets, tubes or hollow fibres composed of a thin semipermeable
layer on a structural material.
3.12
permeate (noun)
liquid or gas that diffuses through a permeable membrane
[SOURCE: ISO 3857-4:2012, 2.54]
3.13
concentrate (noun)
fluids enriched with substances not passing the membranes in membrane filtration processes
3.14
membrane flux
amount of permeate produced per unit area of membrane surface per unit time
3.15
transmembrane pressure
mean pressure exerted across the semipermeable membrane
[SOURCE: ISO 8637-3:2018, 3.7]
3.16
cross flow filtration
filtration with a significant flow parallel to the membrane surface
Note 1 to entry: This is intended to prevent substances from accumulating on the surface of the membrane.
[SOURCE: EN 16323:2014, 2.3.3.5]
3.17
perpendicular mixing
mixing perpendicular to flow direction
3.18
feed-gas
gas or gas mixture which is supplied to the ozone generation system
3.19
normal cubic metre
cubic metre of gas, usually dry, referenced to 1 atmosphere (101,325 kPa) and 0 °C
[SOURCE: ISO 20675:2018, 3.41]
3 3
Note 1 to entry: The unit is expressed m n. In other documents the unit Nm is sometimes used.
4 Symbols and abbreviations
AOX halogenated organic compounds
BOD5 biochemical oxygen demand in 5 days (expressed as milligrams of oxygen needed to
break down the organic matter contained in a litre of water over five days (mg/l))
COD chemical oxygen demand
CT product of concentration and contact time
LOX liquid oxygen
NOx nitrogen oxides
PAA peracetic acid
PE polyethylene
PTFE polytetrafluoroethylene
PVC polyvinyl chloride
Q wastewater flow rate
RH relative humidity, (expressed as a percentage of present state of absolute humidity
relative to a maximum humidity given the same temperature)
SS suspended solids, (expressed as milligrams of small solids contained in a litre of water
(mg/l))
P concentrations of total phosphorus compounds expressed in milligrams of
tot
phosphorous in a litre of water (mg/l)
T contact time
THM trihalomethanes
tR retention time
UV ultraviolet, electromagnetic radiation with wavelength 100 nm to 400 nm
V active contact volume
5 Design requirements
5.1 General
Disinfection processes are used to improve the microbiological quality of effluents, if required, e.g.
because of sensitive uses of the receiving waters downstream. A disinfection of effluents from wastewater
treatment plants can reduce public health risks by preventing contamination by human pathogens in:
• waters used for bathing and other recreational activities involving immersion;
• shellfisheries;
• treated wastewater to be used for irrigation or as process water or other compatible uses;
• sources used for potable water supply.
Disinfection of effluents from wastewater treatment can be attained by two possible mechanisms:
• inactivation of microorganisms rendering microorganisms incapable of reproduction;
• removing the microorganisms from an effluent (e.g. by filtration) but not necessarily inactivating
them.
Although other methods exist, the processes most commonly used for disinfecting wastewater by
inactivating microorganisms are:
• ultraviolet (UV)-radiation;
• chlorination;
• ozonation;
• peracetic acid.
The processes most commonly used for disinfecting wastewater by removing respectively reducing
microorganisms are:
• membrane filtration;
• effluent maturation ponds;
• soil filtration.
Combinations of different processes are possible e.g. flocculation filtration with ozonisation.
5.2 Planning
5.2.1 General
Disinfection, if required, should be the last stage in the wastewater treatment process. Poor performance
by upstream processes will affect the performance of the disinfection process. If an effluent has to be
stored prior to discharge – e.g. in case of discharge to tidal water or irrigation – it should preferably be
disinfected after storage directly prior to discharge in order to limit regrowth hazards.
When planning disinfection systems consideration shall be given to the:
a) level of disinfection required;
b) stability and efficiency of disinfection process;
c) technological level of disinfection process;
d) operational requirements;
e) monitoring of water quality;
f) safety hazards;
g) environmental impacts, e.g.:
1) effects on the quality of the effluents (reduction of BOD ,COD, SS, Ptot);
2) deleterious effects of residual disinfectants;
3) production of toxic or bioaccumulating by-products;
h) power requirements.
5.2.2 Level of disinfection
Disinfection processes shall reduce or inactivate human pathogens to a level that the risk of the
disinfected wastewater being a source of infections is minimized. Disinfection processes are not intended
to remove all micro- organisms, or even remove all human pathogens.
National or local regulations or the relevant authority can specify the required level of disinfection to be
achieved.
The specification of the planned level of disinfection shall include procedures for sampling, analysis and
evaluation. Statistical criteria for complying with the level of disinfection required shall be named
explicitly e.g. for dry weather and storm water conditions. These procedures shall be defined by the
customer.
5.3 Process design
5.3.1 General
With respect to the high required reduction rate no short circuiting, by-passing, or incomplete treatment
is permitted. The required treatment shall be applied to all wastewater because the microbiological
quality of disinfected wastewater reacts very sensitively to any wastewater not being disinfected
properly.
NOTE This is because the required reduction of indicator organisms is usually in the magnitude of 99,9 % to
99,99 %. A leakage or short circuiting of 0,01 % to 0,1 % of the wastewater or a reduced reduction rate of only 99 %
in 1 % to 10 % of the wastewater due to incomplete treatment can cause germ counts that already exceed the
effluent standards.
5.3.2 UV radiation
UV disinfection is the application of UV radiation artificially generated in UV lamps in UV reactors to the
wastewater to be disinfected. An appropriate dose of UV radiation will cause an irreversible inactivation
of microorganisms with no other significant effects on the wastewater.
NOTE The disinfection by UV radiation is due to a photochemical effect. UV radiation of germicidal wavelength
causes the formation of dimers of neighbouring thymine bases in nucleic acids. These dimers disturb the replication
of the nucleic acids and cause an irreversible inactivation of the microorganisms if the formation of dimers is too
numerous to be repaired by the cells repair mechanisms.
UV radiation systems for wastewater disinfection can be classified as follows:
• type of UV reactor (open channel gravity flow systems, closed vessel systems);
• type of UV-lamps (low pressure or medium pressure mercury discharge lamps);
• configuration of UV-lamps (in wastewater immersed lamps housed in quartz glass sleeves, non-
contact systems).
Radiation systems can consist of one or more UV reactors. UV reactors can be in series or parallel.
Designing and sizing a UV radiation system for wastewater disinfection, the following site specific
parameters shall be taken into consideration:
• minimum UV dose;
• peak flow;
• minimal UV transmittance of effluent.
The minimum UV-dose is the UV irradiation required to reduce the concentration of microorganisms in
an effluent to the requested level of disinfection. The minimum UV dose is independent of the UV
radiation system used for the disinfection. The minimum UV dose is only determined by:
a) the level of disinfection required specified in terms of
• relevant indicator and/or pathogen organisms concentrations;
• sampling and analysis procedures (photo-reactivation);
• statistical criteria for approval.
b) characteristics of the wastewater
• suspended solids concentration;
• concentrations of microorganisms before disinfection.
The required minimum UV-dose can be estimated on the basis of experimental data determined by
collimated beam tests, pilot plant studies, or experience from other installations.
On the basis of the minimum UV dose, peak flow, and minimum UV transmittance a UV radiation system
can be designed and sized appropriate to deliver the required minimum UV dose to all wastewater to be
disinfected. Design and sizing of UV radiation systems are system specific. The system supplier shall
provide a verifiable UV dose calculation based on a bioassay study or on an UV intensity distribution
calculation combined with a detention time distribution study (tracer study).
For a safe disinfection and a good efficiency of a UV radiation system the hydraulic design and the
efficiency of the UV-lamps are most important. The UV-lamp-ballast-systems provider should provide an
expertise on efficiency and out-put drop over time from an independent source. The system shall be such
that, if the lamps efficiency drops, leading to a UV-dose lower than the minimum UV-dose, maintenance
shall occur for cleaning the sleeves and/or replacing the lamps. As an alternative, the system can be such,
that the UV input can be increased-so that the UV-dose is higher than the minimum UV-dose.
The hydraulic design of UV radiation system shall ensure that:
• no wastewater to be disinfected can by-pass the UV radiation system at any time;
• all the cross sections of the UV reactors are irradiated (no shadowed areas);
• hydraulic flow is as close to plug flow as practicable;
• hydraulic flow is as close to perfect perpendicular mixing as practicable.
Efficiencies of UV radiation systems claimed by contractors can be verified by:
• bioassay studies (see [15], [24]);
• pilot plant studies;
• full scale experience.
For systems with submerged UV-lamps a cleaning routine for the quartz glass sleeves of the UV-lamps
shall be established. For systems with low pressure mercury discharge UV-lamps cleaning frequencies of
less than once every two weeks can be expected. For systems using medium pressure mercury discharge
UV-lamps cleaning might be required much more frequently. In addition to the existence of automatic
cleaning systems it may be necessary to implement manual cleaning routines for the quartz glass sleeves
of the UV-lamps. Contractors shall provide information to the operational/maintenance team and a
maintenance plan.
UV-lamps should be replaced at intervals recommended by the manufacturers. It is recommended that
the minimum lamp lifetime should be greater than one year (8760 h) for continuous disinfection systems.
5.3.3 Ozonation
Ozone is typically produced by transforming oxygen (O ) to ozone (O ) from oxygen sources such as:
2 3
a) ambient air, typically 21 % oxygen, 78 % nitrogen and 20 % to 40 % RH;
b) compressed air, from e.g. a compressor, typically 21 % oxygen, 78 % nitrogen. RH depending on the
installation/dryer;
c) oxygen generated from air at the installation site using an oxygen generator, typically below −45°C
dew point, ≥ 85 % oxygen, ≤ 15 % nitrogen;
d) gaseous oxygen from e.g. pressurized cylinders;
e) liquid oxygen (LOX) from e.g. pressure tanks.
A feed-gas is considered dry at dew point (humidity) below of −45°C.
The main objective of an ozone generator is to produce ozone. However, by-product formations of
nitrogen oxides (NOx) and nitric acid/nitric salts are directly correlated to the quality of the feed-gas.
The use of on-site generated oxygen feed-gas also removes potential hazards and manual handling
commonly associated with storing pressurized oxygen bottles or liquid oxygen.
The purity requirements for the feed-gas, including requirements of dew-point (humidity in the feed-
gas), shall be fulfilled and stated by the manufacturer of the ozone generator manufacturer.
Ozone generation from supply air inevitably produces by-products in the form of nitrogen oxides (NOx),
mainly N2O and N2O5 [47]-[50], which are classified significant greenhouse gases. In addition, depending
on the bromide concentration and ozone dose, bromate can be formed in a concentration that is not
suitable for drinking water production.
If the feed-gas is insufficiently dried (>-45°C dewpoint), N O will react with the water (humidity) present
2 5
in the feed-gas (e.g. air) and form nitric acid (HNO ). The involved transient nitrogen oxides NO, NO and
3 2
NO react in catalytic processes with the ozone and consequently reduce the ozone yield of the ozone
generator [54].
The nitric acid itself can accumulate inside the ozone generator and lead to corrosion or affect
components inside the ozone generator as well as downstream components in the overall plant. The
formation of nitric acid will be effectively inhibited using dried feed-gas (dew point below −45 ° at normal
temperature and pressure).
However, diluting of the feed-gas with a small amount of nitrogen when generating ozone will improve
the ozone yield [51].
Furthermore, by drying the feed-gas, the energy demand for the generation of ozone will be decreased
(due to lack of above-mentioned reduction of ozone yield) [52]-[53].
Always follow the ozone generator manufacturer’s recommendations for feed-gas regarding purity, dew
point, pressure, volumetric flow, etc. An ozonation system shall at least include the following
components:
• ozone generator(s);
• oxygen concentrator or oxygen cylinder;
• air compressor with dryer if an oxygen concentrator is used;
• contact tank for mass transfer of ozone from gas to liquid phase;
• contact tank;
• if necessary; process water pump;
• automation components (control cabinet, sensors/gauges, control valves, etc.);
• other process and mechanical equipment used to achieve the required ozone residual concentration
in the water;
• ozone destructor for off-gas.
Ozonation systems shall be classified in accordance with Annex A.
Ozone is an oxidizing gas that is produced on site. Exposure to ozone can be hazardous to health so
exposure levels may need to be monitored e.g. humans shall not be exposed above 0,1 ppm (0,2 mg/m )
in gas phase as mean value for a period longer than eight (8) hours. Ozonation can lead to formation of
other by-products. In designing an ozonation plant all relevant safety considerations for generating and
handling ozone, in accordance with EN 12255-10, shall be respected.
The efficiency of the ozonation process is highly dependent of an effective ozone mass transfer, through
the contact tank, into the effluent in order to achieve the required CT-value, as defined in Formula (1)
below.
CT C× T (1)
where:
C = ozone residual concentration (mg/l)
T = contact time (expressed in minutes)
NOTE Annex B describes a method used for measuring ozone concentration in water.
The following types of contact tank can be used:
• diffused bubble;
• positive pressure injection;
• negative pressure injection (venturi);
• mechanical agitation;
• packed tower.
A higher ozone concentration (g O /m ) improves the mass transfer efficiency of the contact tank; an
3 n
ozone concentration of at least 50 g/ m is recommended. The contact tank shall provide sufficient
n
retention time for the disinfection reactions of the ozone to be completed. Short circuiting shall be
avoided. The total footprint of the contact tank should be minimized.
With respect to its toxicity any residual ozone in the waste gas shall be destroyed. The volumetric flow
rate of waste gas from the contact tank should be minimized. Where possible, the contact tank and waste
gas destruction should be integrated. All ozone bearing parts of an ozonation plant shall be a closed vessel
system only vented through an ozone destructor. In the case of an elevated ozone concentration
(> 0,2 mg/m ) being detected the ozone generators shall shut down automatically. Systems used for
ozone destruction in the waste gas include:
• thermal destruction (T > 350 °C, t > 2 s);
R
• catalytic destruction (i.e. Palladium/CuO-MnO in accordance with manufacturer’s instructions);
• activated carbon (activated carbon is oxidized and consumed by the ozone destruction).
The ozone systems shall be equipped with ambient detectors and safety mechanisms delivered by the
ozonation system manufacturer. In confined spaces, ambient ozone sensors shall be placed in areas
according to the end-user risk analysis.
3 3
Typical residual concentration of ozone in the wastewater is in range of 0,1 g/m to 1 g/m . For ecological
reasons at particularly sensitive sites lower concentrations can be necessary. Important parameters for
the design of the ozonation system for water treatment are the following:
• If possible, project specific pilot tests should be carried out to determine the actual specific ozone
demand;
=
• If specific pilot tests cannot be performed a CT value of 3 (minutes × mg O /l), can be used;
• An “Active” volume = Tank volume minus the volume occupied by other equipment and materials in
the tank = Net water volume;
• Dissolved ozone residual concentration shall be measured on the wastewater outlet from the contact
tank using ozone or ozone equivalent measuring equipment;
• The ozone system should automatically be able to control the CT-determined dissolved ozone
residual concentration in the water;
• Contact time, T, (minutes) is calculated using Formula (2).
V
(2)
T=
Q
where:
Q = wastewater flow rate (litres/ minute)
V = active contact volume (litres)
The efficacy of the ozonation system shall be expressed as grams of ozone residual concentration per
kWh under nominal test conditions defined in EN 12255-15. The efficacy assessment shall include the
total power consumption of all components of the ozonation system (see Annex A).
5.3.4 Chlorination
Chlorination is a well-established disinfection technique, known to present the potential to form
disinfection by-products, such as THM’s, AOX or PCB’s, among others. The toxicity potential associated
with chlorination should be assessed through a risk analysis, whenever justified by the characteristics of
the receiving water body (e.g. a sensitive water body) and intended use for the treated wastewater [46].
The required dosage of the disinfectant solution depends on the type of disinfectant used and is site
specific. The dosage of the disinfectant chemical shall be adjusted to the flow rate and the disinfectant
consumption rate of the wastewater with the objective to attain a stable residual concentration in the
contact basin effluent and also to prevent by-product formation. The site specific required dosage should
be determined by experiments, if possible, before design is undertaken. Residual concentration for
chlorination in the contact basin effluent should be approximately 0,2 mg/l of free chlorine. With a lower
residual concentration disinfection might not be complete, with a higher residual concentration a severe
damage of the bacterial population in the receiving water and excessive concentrations of toxic by-
products in the effluent might be the consequence. In order to reduce the negative effects of chlorinated
effluents in the receiving water, chlorinated effluents may have to be dechlorinated prior to being
discharged.
Chlorinators for the disinfection of wastewater are technologically similar to the systems used for the
chlorination of potable water and include appropriate systems for:
• storage of disinfectant chemicals;
• preparation and dosing of disinfectant solutions;
• mixing of wastewater and disinfectant solution;
• disinfection reactions being completed in reaction tanks commonly referred to as contact tanks;
• the dechlorination before discharge.
Disinfecting chemicals are toxic and hazardous. The more commonly used in chlorinators are:
• sodium hypochlorite solution;
• chlorine gas;
• chlorine dioxide.
The systems for the storage, preparation, and dosing of the disinfecting chemicals depend on the type of
disinfecting chemicals being used.
Sodium hypochlorite solution can be purchased in concentrations of 5 % to 15 % NaOCl. It can be stored
in tanks and dosed with positive displacement pumps. Stored sodium hypochlorite solutions decompose
over time. The decomposition rate increases with rising temperature and/or solar exposure.
Chlorine gas can be stored in pressurized gas tanks. Any rooms possibly affected by chlorine gas in case
of leakage, rupture or malfunctioning shall be controlled by chlorine gas detectors. Chlorine gas can be
dosed with negative pressure injection systems (venturi) into a side stream of the effluent, producing a
solution of hypochlorous acid, which then is mixed with the effluent. Such chlorinators shall have the
following components:
• a pressure/vacuum regulator;
• a feed rate controller;
• a mixing device (e.g. venturi injector);
• a flow indicator for the dosing substance.
Chlorine dioxide is an unstable gas that can explode if not well managed. It should not be stored prior to
use and should be generated on-site as required for disinfection. Storage and use of chlorine dioxide in a
solution of approximately 5 % is possible. The manufacturer's instructions shall be considered. There are
a number of methods which can be used to generate chlorine dioxide solution on site. These include the
following reactions:
• sodium chlorite and chlorine gas;
• sodium chlorite and hydrochloric acid;
• sodium chlorite, hydrochloric acid and sodium hypochlorite.
Chlorine dioxide reactors shall be designed to ensure that there is:
• an efficient generation of chlorine dioxide from the feed chemicals;
• a low concentration of chlorine in the chlorine dioxide solution.
Chlorine dioxide is an effective bactericide over a wide range of pH values and in many circumstances
more effective than chlorine. Unlike chlorine, it does not react with ammonia to form chloramines and
there seems to be considerably less formation of AOX compounds with chlorine dioxide than with
chlorine. It can lead to the formation of other by-products (chlorates, bromates, etc.).
The mixing of the effluent and the disinfectant solution should be very intense and should be completed
in a very short time (within a period of seconds). In line mixing systems or vigorously stirred tank
reactors with a short detention time are appropriate solutions.
Disinfection reactions are completed in contact tanks. The objective of the contact tanks is to maintain
the microorganisms in the effluent stream in intimate contact with the disinfecting chemical for the
required period. A disinfection contact tank shall be designed to avoid short circuiting and should be as
near to a plug flow system as is practicable. It will normally be a pipeline or a serpentine chamber.
The required dosage of the disinfectant solution depends on the type of disinfectant used and is site
specific. The dosage of the disinfectant chemical shall be adjusted to the flow rate and the disinfectant
consumption rate of the wastewater with the objective to attain a stable residual concentration in the
contact tank effluent. The site-specific required dosage should be determined by experiments, if possible,
before design is undertaken. Residual concentration for chlorination in the contact tank effluent should
be approximately 0,2 mg/l of free chlorine. With a lower residual concentration disinfection might not be
complete, with a higher residual concentration a severe damage of the bacterial population in the
receiving water and excessive concentrations of toxic by-products in the effluent might be the
consequence. Negative effects on the receiving water have been reported for chlorine concentrations as
low as 0,05 mg/l to 0,1 mg/l. In order to reduce the negative effects of chlorinated effluents in the
receiving water chlorinated effluents shall be dechlorinated prior to being discharged.
5.3.5 Peracids (Peracetic acid)
Peracetic acid (PAA) is a strong oxidant and disinfectant. The disinfecting action of PAA is through the
release of reactive hydroxyl radicals that react with organisms.
PAA produces by-products with little toxic potential for the aquatic environment [38], [42], [43].
However, its mode of action could dictate that PAA may potentially provoke oxidative stress to beneficial
organisms in waterbodies receiving PAA-treated wastewater [39][44].
PAA will decompose into hydrogen peroxide and acetic acid. PAA disinfection has the potential to
increase the organic content in the treated effluent due to the presence of acetic acid.
Peracetic acid can be used in aqueous solution of 10 % to 18 %. Solutions with more than 18 % of PPA
exhibit some degree of explosiveness, instability, and reactivity. Attention shall be paid to the loss of
disinfection activity over time. Peracetic acid should be stored at cool temperatures in containers or other
recipients of glass, plastics, or stainless steel.
The required dosage and the contact time depend strongly on:
• the quality of the wastewater;
• the target organism; and
• the level of inactivation required.
To achieve best performance the pH of the water to be disinfected, should be below 8,2. Therefore, the
site-specific required dosage should be determined by experiments.
5.3.6 Membrane filtration
The membrane filtration processes used for wastewater disinfection are ultra- and microfiltration. Both
membrane filtration processes use porous membranes as filter media and behave as sieving filters. In
membrane filtration the effluent is forced through the membrane pores under pressure. The
transmembrane pressure is normally generated by a pressure pump on the effluent side, static height
difference or a vacuum pump on the permeate side. Membrane filtration systems include the following
elements:
• modules which contain membranes in the form of hollow fibres, tubes, discs or pleated cartridges,
flat or spiral wound sheets, and provide adequate systems for distributing the inflowing effluent and
for collecting the concentrate and the permeate;
• pressure or vacuum pumps that provide an appropriate transmembrane pressure;
• systems for backwashing and/or chemical cleaning of the membranes.
Membrane filtration processes are characterized e.g. by:
• size of the pores in the membranes (microfiltration or ultrafiltration);
• material of the membranes (organic or inorganic);
• type of the modules (hollow fibres, tubes, discs or pleated cartridges, flat or spiral wound sheets);
• mode of operation (dead end or cross flow filtration);
• type of influent (settled effluent or mixed liquor).
Designing and sizing a membrane filtration system the following additional factors shall be considered:
• membrane flux achievable in operation just before backwashing or cleaning the membrane;
• backwashing and cleaning procedures;
• energy consumption.
Consideration shall also be given to the safe disposal of the concentrate. Design and operation of the
secondary treatment process shall then consider any such additional inputs. Care should be exercised to
avoid the build-up of solids within such a system which are removed by the membrane filtration process
but are not removed by the secondary treatment process. The addition of small amounts of coagulant to
the concentrate is one method of avoiding this problem.
A routine for cleaning the membrane shall be established. Cleaning can be accomplished using back-
washing, air scouring or chemical cleaning. The interval between cleaning will be dependent on the
reduction in membrane flux or alternatively can be based on a fixed time interval. An appropriate cleaning
regime shall be established during commissioning. The cleaning regime shall be reviewed periodically.
A method shall be provided to identify and isolate and replace membranes that have failed.
5.3.7 Effluent maturation ponds
The basic design requirements for effluent maturation ponds are set out in EN 12255-5. Retention time
should be 5 d to 20 d. The design of the ponds shall aim to avoid short circuiting. Flow patterns in effluent
maturation ponds can be improved by a high length to width ratio, a meandering design of the ponds or
by dividing the volume into several ponds in series.
NOTE The efficiency and reliability of maturation ponds is generally far less than for other disinfection
processes due to climate influences such as solar radiation and temperature and their variability.
5.3.8 Soil filtration
In general soil filtration is not suitable for wastewater treatment plants for over 50 PT.
National or local regulations or the requirements of the relevant authority can apply to the minimum
horizontal distance between any soil filter and manmade features and surface waters. Such features will
include water supply wells, property boundaries and the foundations of buildings.
NOTE National or local regul
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