Bsi bs en 12255 8 2001 (2004)

BRITISH STANDARD

BS EN
12255-8:2001
Incorporating
Corrigendum No. 1

Wastewater treatment
plants —
Part 8: Sludge treatment and storage

The European Standard EN 12255-8:2001 has the status of a
British Standard

ICS 13.060.30

12&23<,1*:,7+287%6,3(50,66,21(;&(37$63(50,77('%<&23<5,*+7/$:


BS EN 12255-8:2001

National foreword
This British Standard is the official English language version of
EN 12255-8:2001.
The UK participation in its preparation was entrusted to Technical Committee
B/505, Wastewater engineering, which has the responsibility to:
—

aid enquirers to understand the text;

—

present to the responsible European committee any enquiries on the
interpretation, or proposals for change, and keep the UK interests
informed;

—

monitor related international and European developments and
promulgate them in the UK.

National annex NA provides further guidance on design and loading data for
use in the UK.
A list of organizations represented on this committee can be obtained on
request to its secretary.
Cross-references
The British Standards which implement international or European
publications referred to in this document may be found in the BSI Catalogue
under the section entitled “International Standards Correspondence Index”, or
by using the “Search” facility of the BSI Electronic Catalogue or of
British Standards Online.
This publication does not purport to include all the necessary provisions of a
contract. Users are responsible for its correct application.
Compliance with a British Standard does not of itself confer immunity
from legal obligations.

This British Standard, having
been prepared under the
direction of the Sector
Committee for Building and
Civil Engineering, was

published under the authority
of the Standards Committee
and comes into effect on
15 July 2001

Summary of pages
This document comprises a front cover, an inside front cover, the EN title page,
pages 2 to 16, an inside back cover and a back cover.
The BSI copyright date displayed in this document indicates when the
document was last issued.
Amendments issued since publication

© BSI 13 December 2004

ISBN 0 580 37451 3

Amd. No.

Date

15294

13 December 2004 Addition of National annex NA

Corrigendum No. 1

Comments


EN 12255-8

EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM

May 2001

ICS 13.060.30

English version

Wastewater treatment plants — Part 8: Sludge treatment and
storage
Stations d'épuration — Partie 8: Stockage et traitement des
boues

Kläranlagen — Teil 8: Schlammbehandlung und -lagerung

This European Standard was approved by CEN on 8 March 2001.
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 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 Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: rue de Stassart, 36

© 2001 CEN

All rights of exploitation in any form and by any means reserved
worldwide for CEN national Members.

B-1050 Brussels

Ref. No. EN 12255-8:2001 E


Page 2

EN 12255−8:2001
Contents
Page
Foreword ................................................................................................................................................................... 3
1 Scope..................................................................................................................................................................... 3
2 Normative references........................................................................................................................................... 4
3 Terms and definitions .......................................................................................................................................... 4
4 Planning ................................................................................................................................................................ 4
5 Process requirements.......................................................................................................................................... 5
5.1 General................................................................................................................................................................ 5
5.2 Thickening ........................................................................................................................................................... 5
5.3 Disinfection.......................................................................................................................................................... 6
5.4 Stabilization and pseudo stabilization ................................................................................................................. 7
5.5 Sludge dewatering............................................................................................................................................. 10

5.6 Composting ....................................................................................................................................................... 11
5.7 Handling and storage ........................................................................................................................................ 12
6 Construction principles ..................................................................................................................................... 12
6.1 Service life......................................................................................................................................................... 12
6.2 Pipelines............................................................................................................................................................ 13
6.3 Sludge pumps ................................................................................................................................................... 13
7 Safety................................................................................................................................................................... 13
Bibliography............................................................................................................................................................ 14


Page 3

EN 12255−8:2001
Foreword
This European Standard has been prepared by Technical Committee CEN/TC 165, Waste water 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 November 2001, and conflicting national standards shall be withdrawn at the
latest by December 2001.
It is the eighth part prepared by the Working Groups CEN/TC 165/WG 42 and WG 43 relating to the general
requirements and processes for treatment plants for over 50 PT. The parts of the series are as follows:
Part 1:
Part 3:
Part 4:
Part 5:
Part 6:
Part 7:
Part 8:
Part 9:
Part 10:

Part 11:
Part 12:
Part 13:
Part 14:
Part 15:
Part 16:

General construction principles;
Preliminary treatment;
Primary settlement;
Lagooning processes;
Activated sludge processes;
Biological fixed-film reactors;
Sludge treatment and storage;
Odour control and ventilation;
Safety principles;
General data required;
Control and automation1);
Chemical treatment;
Disinfection1);
Measurement of the oxygen transfer in clean water in aeration tanks of activated sludge plants;
Physical (mechanical) filtration1).

NOTE
For requirements on pumping installations at wastewater treatment plants, provided initially as
Part 2: Pumping installations for wastewater treatment plants, see EN 752-6, Drain and sewer systems
outside buildings — Part 6: Pumping installations.
The parts EN 12255-1, EN 12255-3 to EN 12255-8 and EN 12255-10 and EN 12255-11 became implemented
together as a European package (Resolution BT 152/1998). The date of withdrawal (dow) of all conflicting national
standards is 2001-12-31. Until the date of withdrawal is reached, the national and the already published European

standards both coexist.
This standard includes a Bibliography.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland,
France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden,
Switzerland and the United Kingdom.

1 Scope
This European Standard gives design principles and specifies construction requirements for sludge treatment and
storage facilities in wastewater treatment plants for more than 50 PT.
Other sludges and organic wastes may be treated together with the municipal sewage sludge.
Differences in wastewater treatment throughout Europe have led to a variety of systems being developed. This
standard gives fundamental informations about the systems; this standard has not attempted to specify all
available systems.
Detailed information additional to that contained in this standard may be obtained by referring to the bibliography.

1)

In preparation.


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EN 12255−8:2001
2 Normative references
This European Standard incorporates by dated or undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed hereafter. For
dated references, subsequent amendments to or revisions of any of these publications apply to this European
Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the
publication referred to applies (including amendments).

EN 1085, Wastewater treatment — Vocabulary.
EN 12176, Characterization of sludge — Determination of pH-value.

prEN 12255-1:1996, Wastewater treatment plants — Part 1: General construction principles.
prEN 12255-4:1997, Wastewater treatment plants — Part 4: Primary settlement.
EN 12255-5, Wastewater treatment plants — Part 5: Lagooning processes.
prEN 12255-6:1997, Wastewater treatment plants — Part 6: Activated sludge processes.
prEN 12255-9:1999, Wastewater treatment plants — Part 9: Odour control and ventilation.
EN 12255-10, Wastewater treatment plants — Part 10: Safety principles for the construction of wastewater
treatment plants.
EN 12880, Characterization of sludges — Determination of dry residue and water content.
EN ISO 5667-13, Water quality — Sampling — Part 13: Guidance on sampling of sludges from sewage and
water treatment works (ISO 5667-13:1997).

3 Terms and definitions
For the purposes of this European Standard the terms and definitions given in EN 1085 and the following apply.

3.1
psychrophilic
process conditions for organisms which are active below 30 °C

3.2 mesophilic
process conditions for organisms which are active at temperatures between 30 °C and 45 °C, with an optimum of
32 °C to 37 °C

3.3
thermophilic
process conditions for organisms which are active at temperatures between 45 °C and 80 °C, and have an
optimum of 55 °C to 65 °C

3.4
pseudo stabilization
process which prevents organic degradation so long as particular conditions (such as pH value or dryness) are
maintained, but for which degradation recommences when the conditions are no longer met

4 Planning
Sludge treatment and storage influences subsequent utilization. It may be subject to a variety of regulations
dependent upon the site of the treatment plant and the proposed routes for use or disposal. For new works or
major upgrading, an environmental impact assessment should be carried out.
The choice of the sludge treatment process depends on the size of the treatment plant, the type, origin and
characteristics of the sludge to be treated and the final method of utilization or disposal. Processes which allow for
more than one sludge utilization or disposal option are preferable.


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EN 12255−8:2001
Consideration should be given to the possibility of centralized sludge treatment facilities which allow a wider range
of treatment techniques. Special care is needed in respect of extra loads, e.g. of nitrogen generated from sludge
liquors at centralized facilities.
Sufficient storage capacity shall be available on the source site for raw or treated sludge to prevent sludge
overflow under all likely conditions.
The following factors shall be considered in planning sludge treatment:
— the utilization or disposal route and related quality requirements, e.g. nutrients, harmful substances and
calorific value;
— sludge characteristics;
— import of sludges and other organic wastes;
— minimum and maximum daily sludge production (volume and mass);
— future sludge production;

— range of solids concentrations (total and volatile solids);
— physical characteristics (viscosity, temperature);
— biological properties (degradability, inhibitors and toxicants);
— aggressive or corrosive conditions;
— likely emissions including greenhouse gases, and odours (see also prEN 12255-9:1999);
— removal or disintegration of gross solids which may cause blockage or malfunction;
— effect of abrasive or deposit forming solids such as grit;
— effect of additives used in wastewater treatment, such as precipitants, coagulants and flocculants and their
influence on utilization;
— impact of return liquors on the wastewater treatment process, e.g. peak loads of ammonia and phosphorus
resolubilization from sludge processing;
— health and safety of operators and the general public (see also EN 12255-10), e.g. the generation of toxic
and/or explosive atmospheres;
— nuisance, e.g. smell and visual intrusion;
— environment, e.g. effect of leakage.

5 Process requirements
5.1 General
Provision shall be made to allow the sampling of input and output for each unit process (see EN ISO 5667-13).
Flow measurement for each unit process should be considered.
The design shall take account of any requirements for control of odour, noise, vibration and explosive
atmospheres in accordance with prEN 12255-9:1999 and EN 12255-10.

5.2 Thickening
5.2.1 General
Sludge thickening is carried out in a continuous or batch mode of operation, using gravity thickeners, mechanical
thickening equipment such as filters or centrifuges, or dissolved air flotation.
The selection of the thickening method and its design shall take account of the following factors:
—
—

—
—

the sludge solids concentration required by subsequent processes;
the solids recovery from the process;
resolubilization of phosphorus in gravity thickeners;
retention times, which when exceeding one day can result in anaerobic degradation, causing odour
emission, foaming, bulking and impaired dewaterability;
— control of the sludge feed and liquor removal rates;
— the storage and controlled return of sludge liquor where nitrification or nitrogen removal is required.
Due to enhanced viscosity, positive displacement pumps should be used for transferring the thickened sludge.
A programme of sludge testing and analysis shall be considered where practicable to assist in the design of gravity
thickeners.


Page 6

EN 12255−8:2001
5.2.2 Gravity thickening
Gravity thickeners should have a depth of at least 3 m, have a bottom slope of at least 50° (conical) or
60° (pyramidal) to the horizontal or be equipped with either an agitator or a rake which includes a bottom scraper
(e.g. picket fence). Other features which shall be considered include:
—
—
—
—

retention and removal of scum;
supernatant withdrawal at different levels (e.g. using a vertically moveable device);
observing the quality of the supernatant liquor during removal;

ventilation and exhaust air deodorization if thickeners are covered.

Factors which affect the design of gravity thickeners include:
—
—
—
—

the surface loading rate;
the mass surface loading rate;
the solids detention time;
the total depth of the consolidation zone.

5.2.3 Mechanical thickening
Where thickening equipment is similar to that used for mechanical dewatering, the relevant construction principles
apply. The most common machines for mechanical thickening are:
— drum filters;
— belt filters;
— centrifuges.
Mechanical sludge thickening equipment should:
— normally operate automatically with the facility for manual override;
— include all equipment required for storage, preparation and dosage of any necessary flocculant;
— be enclosed or located in adequately ventilated rooms to reduce corrosion and for the health and safety of
the operator.
The requirements and guidelines for mechanical sludge dewatering equipment in 5.5.2 are also appropriate to
mechanical sludge thickening.
5.2.4 Air flotation
Waste activated sludge or backwash water from biofilters can be thickened by dissolved air flotation with or without
chemical flocculation.
Dimensioning of a dissolved air flotation unit shall take account of the following:

— the surface loading rate;
— the mass surface loading rate;
— the air/solids ratio.

5.3 Disinfection
Sludge disinfection may be achieved chemically (see 5.4.4) or thermally.
Processes which can achieve disinfection include:
—
—
—
—
—
—
—

thermophilic aerobic digestion;
thermic processes, e.g. heat treatment, thermal drying;
thermophilic aerobic digestion as a pre-treatment before mesophilic anaerobic digestion;
thermophilic anaerobic digestion as a pre-treatment before mesophilic anaerobic digestion;
composting;
addition of lime to liquid sludge or sludge cake;
mesophilic anaerobic digestion in combination with long term storage.
NOTE
Pasteurization is time/temperature dependent. It may take place before or simultaneously with
any stabilization process used.


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EN 12255−8:2001

5.4 Stabilization and pseudo stabilization
5.4.1 General
Stabilization is a process for transformation of readily degradable organic substances into mineral or slowly
degradable organic substances. Treatment of sludge with lime or thermal drying is known as
“pseudo stabilization”. It can prevent organic degradation so long as particular conditions (pH value or dryness)
are maintained, but degradation recommences when the conditions are no longer met.
Pseudo stabilization processes may be used to reduce odour emission during storage, to improve sludge handling
as well as to achieve disinfection. They remain an option for treatment before land application, but they do not
reduce the long-term potential for gas production which shall be considered if the sludges are to be landfilled.
Methods which measure degradability may be used to characterize the quality of stabilization.
Methods which measure sulfide evolution may be used to characterize septicity (or the potential for odour
formation and emission).
A degree of sludge stabilization can be achieved by the extended aeration process (see prEN 12255-6:1997).
5.4.2 Anaerobic digestion
5.4.2.1 Design considerations
When designing an anaerobic digestion plant, the following factors shall be considered depending upon whether
the plant is heated:
—
—
—
—
—
—
—
—
—
—
—
—
—

—
—
—
—
—
—
—
—
—
—
—
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—
—
—
—

the required volatile solids reduction;
degradability;
operating temperature;
temperature control;
hydraulic retention time;
average and peak loads;
dimensions of the digester;
one- or two-stage processes;
gas production (average and peak);
gas storage and use;
limitation on gas emissions;
limits and controls on odour emissions;
frequency of feeding;

scum and foam control and removal;
seeding;
mixing;
short-circuiting and dead-space;
mixing energy (Wh/m3 d) and mixing intensity (W/m3);
thermal insulation;
generation of aggressive components in sludge or gas;
corrosion protection to the interior surface in contact with biogas;
corrosion protection for gas holders, or means of adding inhibitors to water seals;
the sum of the maximum hydrostatic pressure plus the maximum gas pressure;
effect of static and dynamic forces (for example, due to mixers, recirculation, pumps, or temperature
variations);
equipment repair or replacement without emptying the digester;
overflow routes shall not be obstructed by any valve arrangement;
provision of a view port with external and internal wiper on top of the digester;
pressure relief mechanisms;
equipment for dosing, e.g. alkali or anti-foam agents.

The average influent solids concentration should be greater than 4 % total dry solids by mass (see EN 12880).
Pipelines connected to the digester below the minimum sludge level should have a section between the isolation
valve and the digester which can be isolated by freezing.
Gas filters, desulfurizers and gas measurement equipment arranged between digesters and gas holders shall be
provided with by-passes. Where digester gas is collected, it should be used or burnt and not vented.
NOTE

Processes for gas treatment storage and utilization are not included in this standard.


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EN 12255−8:2001
For heated digestion as a minimum, equipment should be provided to monitor or record the following as a
minimum:
—
—
—
—
—

temperature;
sludge level;
sludge input and gas production;
the volume of gas in gas holders;
pressure loss in the gas system.

All sensors shall be removable without emptying the digester. Means for sampling raw sludge, sludge in the
digester, digested sludge and biogas shall be considered.
5.4.2.2 Cold digestion
This may be carried out in open digesters, such as lagoons, open tanks, in enclosed vessels and Imhoff tanks.
Open digestion of raw sludge should be used only at wastewater treatment plants of less than 1 000 PT, and only
where odour and other volatile emissions including methane, are environmentally acceptable.
The following factors shall be considered:
—
—
—
—

minimum of two anaerobic lagoons or vessels;
operation in parallel;
sludge removal;

the need for scum boards at the overflow.

For details about Imhoff tanks and lagoons see prEN 12255-4:1997 and EN 12255-5 respectively.
5.4.2.3 Heated digestion
Heated digestion is preferable to cold digestion for sewage sludge, as stabilization and process control are more
reliable. Imported wastes should be screened or disintegrated as appropriate, and either fed directly or in
admixture into digesters.
Factors which shall be considered in design of heating systems and heat exchangers include:
—
—
—
—
—
—

installation of equipment external to digesters;
condensate hammer (in the case of steam injection);
removal of precipitates and deposits;
minimum flow velocity in pipework (if less than 1 m/s, then a regular flushing procedure is required);
head loss;
thermal insulation and heat balance.

Factors which shall be considered in design of mixing systems include:
—
—
—
—

external or internal sludge recirculation;
digester size and shape;

completely mixed digesters or digesters with simultaneous thickening and supernatant liquor removal;
prevention of dead spaces and short-circuits.

Efficient mixing by recirculation requires at least five times the digester volume/day.
Scum, foam and bottom deposits lead to severe disturbances during the process of digestion. The following
factors shall be considered:
—
—
—
—
—

means to prevent formation of scum layers and bottom deposits;
scum removal without emptying tanks;
health and safety measures during maintenance;
prevention of foam entry to gas lines, e.g. by foam traps;
grit removal during normal digester operation.

5.4.3 Aerobic digestion
Aerobic digestion is usually thermophilic and carried out in closed tanks.


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EN 12255−8:2001
The following factors shall be considered for aerobic thermophilic digestion:
—
—
—
—

—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—

sludge type;
solids concentration;
viscosity;
degree of stabilization;
need for disinfection;
temperature and temperature control;
retention time;
average and peak loads;
frequency of feeding;
dimensions of reactors;
thermal insulation;
one- or two-stage processes;
mixing intensity (W/m3);

prevention of anaerobic conditions;
prevention of solids deposition;
oxygen transfer capacity (kg/h) and oxygen transfer efficiency (kg/kWh);
foam control and maintenance of aeration during foaming;
odour control;
heat recovery and heat balances;
accessibility of heat exchanger surfaces for mechanical cleaning.

For thermophilic operation and disinfection, reactors shall be covered and thermally insulated. The sludge feed
should have a volatile solids concentration higher than 25 kg/m3 to avoid the need for subsidiary heating.
5.4.4 Chemical treatment, conditioning and flocculation
Pseudo stabilization can be achieved by adding an appropriate chemical substance (for example, lime to a pH
value greater than pH 12), in order to stop the microbiological activity. Disinfection can occur as a secondary
benefit of addition of lime.
Chemical treatment of raw sludge may be sufficient for agricultural utilization, depending on national regulations,
or it can be an alternative solution in case of failure of a main treatment such as anaerobic digestion.
The following factors shall be considered:
— emission of ammonia due to the high pH values;
— ventilation and air scrubbing.
If disinfection of liquid sludge is required, the following shall be considered:
—
—
—
—
—
—

the pH value should be raised to greater than 12;
required storage time for treated sludge;
use of multiple storage vessels;

pH value monitoring during storage (see EN 12176);
disintegration or screening sludge before lime conditioning;
agitation to prevent formation of deposits.

Dewatered sludge can be treated by the addition of quicklime. The exothermic reaction can achieve temperatures
in the sludge of more than 55 °C. Quicklime treatment of dewatered sludge may be sufficient to meet the
rheological properties required for landfilling.
The following factors shall be considered when using quicklime to disinfect dewatered sludge:
— treatment conditions of a minimum of 24 h at 55 °C and pH value greater than 12 (e.g. by storage of limed
sludge in thermally insulated containers);
— monitoring of temperature of sludge near the surfaces;
— sludge particle size should not exceed 10 mm;
— efficiency of mixing (small particles ≤ 10 mm generally only need dusting with lime);
— solidification and increase in solids concentration and mass due to addition of material.


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EN 12255−8:2001
Silos or tanks for holding chemicals should have more than one tanker capacity, to store the load of a tanker and
the remaining stock. They shall have the following:
—
—
—
—
—
—
—
—

suitable access for maintenance;
feed pipe with isolator;
pressure release valve;
filter for exhaust air;
anti-bridging devices;
means to control dosing;
interlocked security and safety devices;
means to measure contents.

All tanks and equipment shall be resistant to corrosion and chemical attack. Abrasion shall be taken into account.
It shall be possible to remove mechanically chemical deposits from any part of the equipment.
The following shall be considered in the design of a quicklime and dewatered sludge mixer installation:
—
—
—
—
—

dust-tight mixers;
external bearings and shaft seals;
inspection opening with interlocks;
ventilation of buildings;
odour, dust and other air emission controls.

Where organic polymers are used, the following shall be considered:
—
—
—
—
—

—
—
—
—

suitability for liquid and/or solid polymer;
batch preparation in separate containers for make-up and maturation;
sufficient capacity of maturation containers;
source, quantity and quality of the clean water supply for dissolving polymers;
means to avoid lump formation;
dilution of matured polymer with water before contact with sludge;
dilution ratio of polymer to be independent of pressure supply of water;
high mixing intensity during mixing of sludge with polymer;
controls for polymer dose.

5.5 Sludge dewatering
5.5.1 Sludge drying beds
Sludge drying beds, often installed in countries with a drier climate, may consist of a minimum of two cells with
porous filter media and drain pipes. The filter beds are usually built of multiple sand and gravel layers where the
particle size increases from the top to the bottom. The finest upper layer is gradually removed together with the
dried sludge and shall be renewed after several removal cycles.
The upper sand layer shall have a depth of 50 mm to 100 mm and the lower gravel layer shall have a depth of
300 mm to 400 mm. The drainage pipes in the gravel layer shall have a minimum DN 80.
Anaerobically digested sludge is applied to the drying beds to a maximum depth of 300 mm and other sludge to a
depth of 100 mm. It is recommended to apply digested sludge from the bottom of the digester. Due to the pressure
drop, dissolved gas is released and floats the solids to the sludge surface thus enabling the sludge water below to
drain rapidly.
The removal of the sludge is carried out manually or with mechanical scrapers. Adequate access shall be provided
for vehicles to carry out sludge removal.
5.5.2 Mechanical dewatering

Mechanical dewatering is carried out after chemical conditioning (with lime and iron or polymer flocculation),
thermal conditioning or freezing, using, e.g.:
—
—
—
—

belt filter press;
centrifuge;
chamber filter press;
membrane filter press.


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EN 12255−8:2001
Chemical handling and storage shall be carried out in accordance with EN 12255-10.
The following factors shall be considered in design and arrangement of sludge dewatering processes:
— frost protection of pipework and operating rooms;
— local exhaust ventilation for machinery;
— ventilation of operating rooms sufficient to meet the requirements for minimum working place air pollution
and corrosion prevention;
— wash-down of rooms;
— anti-slip floors;
— sludge storage capacity prior to dewatering, sufficient for reasonably foreseeable interruptions to
dewatering operations;
— need for homogenization of sludge in storage tanks;
— disintegration of gross solids in the sludge upstream of sludge feed pump;
— sludge liquor treatment;
— sludge liquor storage and flow control, particularly for ammonia rich sludges.

The following features shall be considered in the selection and design of mechanical dewatering equipment:
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—

type and total solids concentration of the sludge to be dewatered;
dewaterability characteristics of the sludge;
sludge properties required for the disposal route or destination of the sludge;
availability of process water (quality, flow and pressure);
compressed air required (flow and pressure);
daily net dewatering capacity (the output mass of dewatered dried solids), and maximum daily dewatering
capacity, which includes any recirculation of solids (for example, solids return from a centrifuge);
maximum sludge feed rate, hydraulic (m3/h) and mass (kg DS/h);
consumption of polymers or conditioning agents per mass of dried solids fed to the equipment;

dry mass concentration in the dewatered sludge at the specified net capacity;
dry mass capture rate attainable at the specified net capacity;
consumption of polymers or conditioning agents at the specified net capacity;
type of the mechanical equipment, relevant dimensions and specification of all relevant materials and
means of corrosion protection;
nominal power consumption of all electrical drives, maximum and average power consumption;
automatic operation when possible;
failure indication at the control panel;
automatic shut-down on failure;
automatic start-up and shut-down;
consumption of process and clean water (including water for preparation and dilution of polymers or
conditioning agents);
labour time required during regular operations.

5.6 Composting
Composting can achieve the following:
— aerobic stabilization;
— disinfection;
— drying.
Factors which shall be considered in the design of a composting plant include:
—
—
—
—
—
—
—
—
—
—

—
—

windrowing, aerated piles or in-vessel system;
porosity and aeration of the material;
the required nutrient content;
mixing/blending;
odour, dust, volatile emissions and bio-hazards;
land area required;
water content;
temperature control;
drainage of compost storage and traffic areas;
storage of product;
weather protection during and after composting;
source, availability and sustainability of bulking agent.


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EN 12255−8:2001
A bulking agent may be used to achieve the following:
— maintenance of aerobic conditions;
— an increase in solids concentration after maturation;
— an increase of carbon concentration.

5.7 Handling and storage
5.7.1 General
Liquid sludge is stored in sludge holding tanks or sludge lagoons. Dewatered sludge is stored on hard standing
areas or in sludge silos. Factors which shall be considered during the design of sludge handling and storage
facilities include:

—
—
—
—
—
—
—

the extent of sludge production;
the frequency of sludge discharge;
the interruptions of transfers to final destination, e.g. agricultural land or landfill;
the impact of sludge liquors on wastewater treatment;
the rheological characteristics of the sludges;
the odour and gas emissions;
the explosion risk.

5.7.2 Liquid sludge holding tanks
For the storage of liquid sludges in holding tanks the following shall be considered:
— means to remove supernatant;
— mixing or scraping devices for shallow sloping bases;
— means to remove scum.
5.7.3 Sludge lagoons
For sludge lagoons see EN 12255-5.
5.7.4 Dewatered sludge storage areas
For the storage of dewatered sludge the following shall be considered:
— water-tight base;
— roofing;
— collection, drainage and balancing of rain-water and run-off to wastewater treatment.
5.7.5 Dewatered or dry sludge silos
For the storage of dewatered or dry sludge in silos the following shall be considered:

—
—
—
—
—

water-tight base;
control of bridging;
control of output;
fire hazard and control of storage temperature;
explosion from dust or biogas.

6 Construction principles
6.1 Service life
The service life of all motors shall be a minimum of class 3 (see prEN 12255-1:1996). The service life of all
bearings and gears for pumps, compressors, rakes, mixers, sludge dewatering machines and similar machinery
and components shall be a minimum of class 4 (see prEN 12255-1:1996), unless otherwise specified.


Page 13

EN 12255−8:2001
6.2 Pipelines
The flow velocity in liquid sludge and sludge liquor pipelines shall not be continuously less than 1 m/s unless
measures for the prevention of sedimentation/encrustation are taken. If the rate of gravity flow is too low then
pumping shall be considered.
Systems shall be designed so that regular operations do not require pressure tight isolation of any section. This is
to avoid the risk that high or damaging gas pressures could develop in a sealed section.
In sludge pipelines which are connected to permanently filled sludge tanks below the minimum sludge level and
which include a frequently operated isolation valve, a second manual valve shall be installed between the reactor

and the valve which is frequently operated.
Pipelines and other equipment which are installed in sludge tanks should be corrosion resistant.

6.3 Sludge pumps
Sludge pump selection shall take the following into consideration:
—
—
—
—
—
—
—
—

thickness and viscosity of sludge;
grit, rag and other gross solids and fibres in sludge;
incorporation of either, or both, sludge screening and disintegration;
risk of blockage, abrasion and cavitation;
pump wear;
energy efficiency;
pump duty including suction and delivery heads and flow rate;
local and operational conditions, e.g. submersed or dry well, available space.

Pump casings with ventilation and dewatering bores should be provided. The leaking water from lubricated glands
shall be drained. The circumferential velocity of the rotors of eccentric screw pumps should not exceed 2 m/s
during regular operation in order to avoid excessive wear.

7 Safety
The risk of accumulation of toxic or explosive gases shall be taken into consideration. Appropriate measures for
natural or mechanical ventilation shall be taken and/or explosion proof equipment shall be selected.

Closed anaerobic digester systems shall be operable in overflow mode.
If the level of sludge in a digester can be adjusted, then appropriated measures, e.g. the use of interlocks, shall be
taken to prevent:
— vacuum formation;
— digester gas overpressure;
— digester gas escape.
Gas conveying systems shall be protected by lightning conductors.
Reference shall be made to EN 12255-10 for safety requirements.


Page 14

EN 12255−8:2001
Bibliography
The following documents provide guidance on design, construction and operation of sludge treatment processes.
This list of documents, which are published and used by the members of CEN, was correct at the time of
publication of this European Standard but should not be considered to be exhaustive.

European Standard
prEN 12832, Characterisation of sludges — Utilisation and disposal of sludges — Vocabulary.

Austria
OEWAV Regelblatt Nr. 30: Sicherheitsrichtlinien für den Bau und Betrieb von Faulgasbehältern auf
Abwasserreinigungsanlagen.
OEWAV Regelblatt Nr. 17: Landwirtschaftliche Verwertung von Klärschlämmen — Empfehlungen für Betreiber von
Abwasserreinigungsanlagen.

France
Ministère de l’équipement, du logement et des transports (96-7 TO)
Conception et exécution d’installations d’épuration d’eaux usées Fascicule no 81 titre II.

Germany
ATV-M 366 2)
Maschinelle Schlammentwässerung, Entwurf 1999:08.
Arbeitsbericht der ATV (1986): Entseuchung von Klärschlamm, Teil 1, Korrespondenz Abwasser (1986), 11, 1141.
Arbeitsbericht der ATV (1988): Entseuchung von Klärschlamm, Teil 2, Korrespondenz Abwasser (1988), 1, 71.
Arbeitsbericht der ATV (1988): Entseuchung von Klärschlamm, Teil 3, Korrespondenz Abwasser (1988), 12, 1325.
Arbeitsbericht der ATV (1992): Auswahl und Einsatz von organischen Flockungshilfsmitteln — Polyelektrolyten —
bei der Klärschlammentwässerung. Korrespondenz Abwasser (1992), 4, 569.
Arbeitsbericht der ATV (1994): Stabilisierungskennwerte für biologische Stabilisierungsverfahren, Korrespondenz
Abwasser (1994), 3, 455.
Arbeitsbericht der ATV (1995): Maschinelle Schlammentwässerung, Korrespondenz Abwasser (1995), 2, 271.
Arbeitsbericht der ATV (1998): Eindickung von Klärschlamm, Korrespondenz Abwasser (1998), 1, 122-134.
Arbeitsbericht der ATV (1999): Einstufung von organischen Flockungsmitteln — Polyelektrolyten — in
Wassergefährdungsklassen, Korrespondenz Abwasser (1999), 2, 267.
Abwassertechnische Vereinigung (1996): Klärschlamm, ATV-Handbuch, 4. Auflage, Verlag Ernst & Sohn, Berlin.

United Kingdom
DEE, A., DAY, M., and B. CHAMBERS, (1994). Guidelines for the design and operation of sewage sludge
3)
consolidation tanks. ISBN 0 902 156 93 4.
— CIWEM Services Ltd (1996). Sewage Sludge: Introducing Treatment and Management.
— CIWEM Services Ltd (1997). Sewage Sludge: Stabilization and Disinfection.
— CIWEM Services Ltd. Sewage Sludge: Dewatering, Drying and Incineration.

2)

3)

Available at: Gesellschaft zur Fưrderung der Abwassertechnik e. V. (GFA), Theodor-He-Allee 17,

53773 Hennef.
Published by WRc, Frankland Road, Blagrove, Swindon SN5 8YR.


Page 15

BS EN 12255−8:2001
National annex NA (informative)
Design and loading data
NA.1 Introduction
Clause 5 of the standard gives guidance on the design of plants to highlight the main design
considerations for thickening, disinfection (pasteurization), stabilization, dewatering, composting,
thermal drying and incineration of sludge so as to highlight the main design considerations for each
type of process. This annex gives more detailed information on typical retention times/operating
temperatures used for design of biological stabilisation processes, typical target solids,
concentrations for sludge thickening, dewatering and drying, and typical combustion intensity for
incineration.
Further information can be obtained from the Manuals of British Practice on Sewage Sludge
Treatment [1, 2 and 3]. The design of sludge treatment plants to serve population equivalents of
1000 or less is considered in more detail in BS 6297:1983 [4].
The retention times and target solids concentrations are based on typical sludges arising from
treatment of mainly domestic wastewater. Significant deviations can occur from the presented
values, if sludge:
— contains significant levels of solids arising from trade waste or waterworks sludge;
— has components which are inhibitory to digestion;
— has a high ratio of secondary to primary solids.
NA.2 Biological stabilization
Table NA.1 — Retention times and temperatures for biological stabilization
Treatment required

Type of process

Descriptions

Pasteurization

Thermophilic aerobic
digestion

Thermophilic digestion at a temperature of 70 °C for a
minimum period of 30 min or at 55 °C for minimum period
of 4 h (or appropriate intermediate conditions). Followed in
all cases by primary mesophilic anaerobic digestion.

Stabilization

Mesophilic anaerobic
digestion

Primary digestion in the temperature range 350 °C ± 3 °C
for a mean retention period of at least 12 days. Followed in
each case by a secondary digestion stage which provides a
mean retention period of at least 14 days.

Thermophilic
anaerobic digestion

Mean retention period of at least 7 days for digestion. All
sludge must be subject to a minimum temperature of 55 °C
for a period of at least 4 h.

Windrows or aerated
piles

Minimum temperature of 40 °C for at least 5 days and,
during this period within the body of the pile, a minimum
temperature of 55 °C for 4 h. A subsequent period of
maturation is required to ensure that the compost reaction
process is substantially complete.

Composting


Page 16

BS EN 12255−8:2001
NA.3 Thickening and dewatering
Table NA.2 — Typical solids concentrations for thickening and dewatering plants
Treatment
required

Type of
process

Thickening

Gravity

Primary

a
b

Digested

6-9

2.5-3.5

3-4

5-8

4-6

a,b

-

6-9

-

-

-

a,b

30-40

15-25

25-35

30-40

20-35

Mechanical
Dewatering

Typical solids concentration (% DS)
Activated
Pri/act
Pri/humus

Mechanical

Chemical flocculant addition required.
Solids recovery (proportion of the feed solids entering the product sludge) should be greater than 90 %.

NA.4 Thermal drying
Table NA.3 — Target solids concentrations for drying plantsa
Use

Required dried sludge solids concentration
(% DS)

Landfill disposal

Recycling on to agricultural land as a soil improver
or co-incineration with municipal waste
a
b

> 90
b

90-95

In direct driers the temperature of air at the inlet is between 300 °C and 500 °C and about 100 °C at the outlet. Indirect
driers use intermediate pressure steam at a pressure (3-20 bar) or thermal oil (200 °C - 250 °C).
Disposal of sludge is restricted to short periods of the year. Hence it should be dried to a high solids concentration and
pelletized to make it suitable for long-term storage and easier to handle.

NA.5 Incineration
Fluidized bed incinerators operate at a temperature of about 800 °C to 900 °C to ensure
combustion of odorous compounds. The typical combustion intensity of such an incinerator is
about 10 MW/m³.
List of references
[1] Manuals of British Practice in Water Pollution Control
Unit Processes Sewage Sludge I: Production, Preliminary Treatment and Digestion
The Institution of Water Pollution Control (1979)
[2] Manuals of British Practice in Water Pollution Control
Unit Processes Sewage Sludge II: Conditioning, Dewatering and Drying
The Institution of Water Pollution Control (1981)
[3] Manuals of British Practice in Water Pollution Control
Unit Processes Sewage Sludge III: Utilization and Disposal
The Institution of Water Pollution Control (1978)
[4] BS 6297:1983 Code of practice for design and installation of small sewage and treatment

works and cesspools
[5] DoE (1989) “Code of Practice for the Agricultural Use of Sewage Sludge” HMSO
[6] CEN/TC 308 EN #CFCR “Guidelines of Good Practice for Sludge Production”
[7] CEN/TC 308 EN #CKHP “Characterization of Sludges — Guidelines for Combustion”


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BS EN
12255-8:2001

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