SIST-TP CEN/TR 16456:2013
(Main)Characterization of sludges - Good practice of sludge dewatering
Characterization of sludges - Good practice of sludge dewatering
This Technical Report describes good practice for sludge dewatering and belongs to a series on sludge management options. It gives guidance on technical and operational aspects of conditioning, thickening and dewatering processes. Drying, which is another water content reduction process, is not dealt with in this document, but in CEN/TR 15473, Characterization of sludges - Good practice for sludges drying. This report is applicable for sludges from: urban wastewater treatment plants; treatment plants for industrial wastewater similar to urban wastewater; water supply treatment plants. This document may be applicable to sludges of other origin.
Charakterisierung von Schlämmen - Gute fachliche Praxis der Schlammentwässerung
Caractérisation des boues - Bonnes pratiques pour la déshydratation des boues
Le présent Rapport technique CEN décrit les bonnes pratiques pour la déshydratation des boues et fait partie d’une série concernant les filières de gestion des boues.
Il fournit des lignes directrices concernant les aspects techniques et opérationnels des procédés suivants :
- conditionnement, épaississement et déshydratation.
Un autre procédé de réduction de la teneur en eau, le séchage, n’est pas traité dans le présent document mais dans le CEN/TR 15473, Caractérisation des boues - Bonne pratique pour le séchage des boues.
Le présent rapport est applicable aux boues provenant :
- des stations d’épuration des eaux usées urbaines ;
- des stations d’épuration des eaux usées industrielles similaires aux eaux usées urbaines ;
- des stations d’épuration des eaux de distribution.
Le présent document peut s’appliquer aux boues d’origines différentes.
Karakterizacija blata - Dobra praksa za postopek odstranjevanja vode
To tehnično poročilo opisuje dobro prakso za postopek odstranjevanja vode iz blata in je del standardov, ki se nanašajo na možnosti ravnanja z blatom. Podaja navodila glede tehničnih in obratovalnih vidikov priprave, zgoščevanja in odvodnjavanja. Sušenje, ki je še en proces za zmanjšanje vsebnosti vode, ni obravnavano v tem dokumentu, temveč v CEN/TR 15473, Karakterizacija blata – Dobra praksa za sušenje blata. To poročilo velja za blato iz: čistilnih naprav za mestno odpadno vodo; čistilnih naprav za industrijsko odpadno vodo, ki je podobna mestni odpadni vodi; čistilnih naprav v sistemih oskrbe z vodo. Ta dokument se lahko uporablja za blato drugega izvora.
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
SIST-TP CEN/TR 16456:2013
01-december-2013
Karakterizacija blata - Dobra praksa za postopek odstranjevanja vode
Characterization of sludges - Good practice of sludge dewatering
Charakterisierung von Schlämmen - Gute fachliche Praxis der Schlammentwässerung
Caractérisation des boues - Bonnes pratiques pour la déshydratation des boues
Ta slovenski standard je istoveten z: CEN/TR 16456:2013
ICS:
13.030.20 7HNRþLRGSDGNL%ODWR Liquid wastes. Sludge
SIST-TP CEN/TR 16456:2013 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST-TP CEN/TR 16456:2013
TECHNICAL REPORT
CEN/TR 16456
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
August 2013
ICS 13.030.20
English Version
Characterization of sludges - Good practice of sludge
dewatering
Caractérisation des boues - Bonnes pratiques pour la Charakterisierung von Schlämmen - Gute fachliche Praxis
déshydratation des boues der Schlammentwässerung
This Technical Report was approved by CEN on 6 November 2012. It has been drawn up by the Technical Committee CEN/TC 308.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2013 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 16456:2013: E
worldwide for CEN national Members.
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Contents Page
Foreword . 4
Introduction . 5
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Description and features of thickening / dewatering systems . 11
4.1 Thickening devices . 11
4.1.1 General . 11
4.1.2 Devices based on natural forces (gravity) . 11
4.1.3 Devices based on flotation . 13
4.1.4 Devices based on filtration . 14
4.1.5 Devices based on centrifugation . 19
4.2 Dewatering devices . 20
4.2.1 General . 20
4.2.2 Filter press (plate, membrane) . 20
4.2.3 Belt (filter) press . 22
4.2.4 Centrifuge . 23
4.2.5 Screw press . 23
4.2.6 Others . 24
5 Conditioning . 25
5.1 General . 25
5.2 Conditioning processes . 25
5.2.1 General . 25
5.2.2 Coagulation . 25
5.2.3 Flocculation . 25
5.2.4 Physical processes . 27
5.3 Conditioners . 28
5.3.1 General . 28
5.3.2 Polymers . 28
5.3.3 Inorganic chemicals (multivalent salts, lime) . 29
5.3.4 Other products . 29
5.4 Technical aspects . 30
5.4.1 Storage of conditioner . 30
5.4.2 Selection of conditioner . 30
5.4.3 Preparation of conditioners . 31
5.4.4 Injection, dosing and mixing with sludge . 34
5.4.5 Automation . 37
6 Parameters / Methods for the evaluation of sludge thickenability or dewaterability . 37
6.1 General . 37
6.2 Mechanisms description . 37
6.2.1 Settling / Flotation . 37
6.2.2 Centrifugation . 39
6.2.3 Filtration . 39
6.3 Basic theories and parameters . 41
6.3.1 Settling / Flotation . 41
6.3.2 Centrifugation . 42
6.3.3 Filtration . 42
6.4 Methods of evaluation . 44
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6.4.1 General .4 4
6.4.2 Settleability / Thickenability .4 4
6.4.3 Centrifugability .4 5
6.4.4 Filterability .4 6
6.4.5 Basic parameters .4 8
7 Critical parameters for sizing and optimisation of thickening/dewatering systems . 49
7.1 General .4 9
7.2 Gravity thickeners .5 0
7.3 Belt thickeners.5 0
7.4 Centrifuges .5 0
7.5 Filter-presses .5 2
7.6 Belt-presses .5 2
7.7 Screw-presses .5 3
8 Operational and economic aspects of thickening/dewatering systems . 53
8.1 General .5 3
8.2 Performances . 53
8.3 Energy consumption .5 6
8.4 Labour requirements .5 7
8.5 Water consumption .5 7
8.6 Maintenance .5 8
8.7 Safety aspects .5 8
8.8 Automation .5 8
8.9 Cost aspects .5 9
8.10 Final considerations .6 0
9 Conclusions .6 3
Annex A (informative) Environmental checklist .6 5
Bibliography .6 6
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Foreword
This document (CEN/TR 16456:2013) has been prepared by Technical Committee CEN/TC 308
“Characterization of sludges”, the secretariat of which is held by AFNOR.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
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Introduction
Sludge processing train is a major problem in water and wastewater treatment, as it can account for up to
50 % of total operating costs. The effectiveness and cost of sludge treatment and disposal operations are
strongly affected by its volume and, consequently, by its water content or solids concentration. Thickening and
dewatering are therefore important steps in the total sludge processing train and have serious impact on
subsequent operations.
For illustration, Figure 1 shows the existing solutions for sludge water content reduction, and Figure 2 shows
the level of dry matter content required for intended utilisation and disposal routes.
Figure 1 — Principal thickening / dewatering processes
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This guide deals with the dewatering and thickening techniques quoted in Figure 1.
Figure 2 — Percentage Dry Solids (DS) usually required after thickening and dewatering for intended
routes
Sludges management options are developed in a series of CEN Technical Reports to which belong the
present report, see Figure 3 below.
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Figure 3 — A basic scheme for deciding on sewage sludge use/disposal options and the relevant
CEN/TC 308 guidance documents
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1 Scope
This Technical Report describes good practice for sludge dewatering and belongs to a series on sludge
management options.
It gives guidance on technical and operational aspects of conditioning, thickening and dewatering processes.
Drying, which is another water content reduction process, is not dealt with in this document, but in
CEN/TR 15473, Characterization of sludges — Good practice for sludges drying.
This report is applicable for sludges from:
urban wastewater treatment plants;
treatment plants for industrial wastewater similar to urban wastewater;
water supply treatment plants.
This document may be applicable to sludges of other origin.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
EN 12832:1999, Characterization of sludges — Utilization and disposal of sludges — Vocabulary
prEN 16323:2011, Glossary of wastewater engineering terms
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 12832:1999, prEN 16323:2011 and
the following (taken either from the normative references or from a technical dictionary [1]) apply.
3.1
cake
solid fraction of sludge as resulting from a solid-liquid separation process
3.2
centrate
sludge liquor separated by centrifugation
3.3
centrifugation
partial separation of solid from liquid under centrifugal forces
3.4
charge density
percentage of positive or negative charge
3.5
compressibility
ability of a sludge to be compressed under pressure
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3.6
compression point
sludge solids concentration at which compression begins in a sedimentation process
3.7
desaturation
removal of water due to displacement of water by air
3.8
draining / drainage of sludge
separation of water from sludge liquor by gravity filtration
3.9
dryness
ratio of dry solids to sludge mass
3.10
electroosmosis
movement of liquid relative to a stationary charged surface as induced by an electrical field
3.11
expression
removal of sludge water due to deformation of solids under pressure
3.12
filter
device for the removal of sludge water whereby solids are retained on a water-permeable filter medium
3.13
filter medium
material where through a fluid flows and which retains matter contained in the fluid
3.14
filterability
characteristic describing the ability of sludge to be filtered
3.15
filtrate
sludge liquor separated by filtration
3.16
filtration
process of retention of the suspended matter by passing through a medium
3.17
floc
aggregate of particles that results from a flocculation process
3.18
flotation
raising of suspended matter in liquid to the surface by the entrainment of a gas
3.19
“g“
2
gravitational acceleration (9,81 m/s )
3.20
isolelectric point
condition in which a substance has a neutral charge
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3.21
mesh
interlacing of crossed wires that determines the openings which can be square, triangular or rectangular
3.22
molecular weight
chain length of a polymer
3.23
particle size distribution
relative amount of particles classified per size ranges
3.24
polymer
class of natural and synthetic materials which are formed by association of structural units (monomers) by
covalent bonds
3.25
porosity
ratio of the void volume to the total volume of material
3.26
pre-treatment
improvement of sludge characteristics by physical or chemical means
3.27
rheology
study of flow and deformation properties under the influence of an applied stress
3.28
saturation
ratio of the volumes of water and pores in a solid matrix
3.29
sieve (sludge treatment)
device for removing solids from fluids whereby the fluid flows through slots, perforations or a mesh
3.30
settling
ability for sludge solids to separate from water by sedimentation under gravity
3.31
sludge liquor
liquor separated from sludge. Sludge liquor can be called supernatant, filtrate and centrate
3.32
specific cake resistance
property representing the resistance to filtration of a layer of particles, having a unit mass of dry solids
deposited on a unit filtering area
3.33
supernatant
sludge liquor separated by gravity thickening
3.34
water distribution
different physical states of water associated with sludge solid particles
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3.35
zeta potential
electrical potential present at the plane of slip when a particle moves relative to its suspending liquid (or vice
versa)
4 Description and features of thickening / dewatering systems
4.1 Thickening devices
4.1.1 General
Thickening devices enable the removal of free water from sludge. They are based on:
natural (static) forces;
artificial forces.
Thickening presents the following advantages:
reduction of sludge volume with low energy consumption;
reduction of storage capacities and volumes for subsequent treatment;
reduction of transport costs;
improvement of performance of dewatering machines;
decrease in quantity of chemicals for dewatering in some cases.
This section discusses the most commonly used devices for thickening.
4.1.2 Devices based on natural forces (gravity)
4.1.2.1 General
The principle of gravity thickening relies on sludge settling under the effect of gravitational forces. It enables
the raising of the concentration of a suspension through sedimentation to produce a thickened sludge with a
relatively clear liquid as overflow. Thickeners can be designed to operate in either the batch or continuous
mode.
Sludge thickening can be achieved in clarifiers or separate thickeners which provide for a greater sludge
storage capacity.
4.1.2.2 Gravity thickener
The traditional gravity thickener (Figure 4) comprises a relatively shallow, open top cylindrical/rectangular tank
with either a flat bottom or a bottom shaped in the form of an inverted cone. The feed mixture is gently and
continuously introduced to the feedwell. The supernatant is removed via an annular weir at the top of the unit
and sludge solids are removed from a well at the bottom. Slowly rotating rakes mounted on a central shaft aid
the thickening process by directing thickened solids towards the well for subsequent discharge and by
creating channels to release further liquid from the sludge.
Tanks with a diameter smaller than 25 m are usually formed from steel and have bottoms with an angle
usually less than 10° equipped with rake arms. Larger tanks between 25 m and 200 m diameter are normally
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made from a combination of concrete and steel and employ rakes designed to match the angle of the conical
bottom.
Key
1 feed 6 rake
2 drive head 7 feedwell
3 walkway 8 thickened suspension (underflow)
4 supernantant (overflow) 9 well
5 flocculant
Figure 4 — Gravity thickener [1]
When space is limited, the lamellar separator is used. It is a rectangular tank containing a series of closely
spaced rectangular plates inclined at an angle of higher than 50° to the horizontal.
Commercial designs provide three flow patterns, cross-flow, parallel flow and the most common counter–flow
where the feed and supernatant flows can be most simply arranged.
The choice of a lamella separator is mainly related to the concentration of the input sludge.
4.1.2.3 Deep cone thickener
A deep cone thickener (Figure 5) has the same operation principle as a conventional circular gravity thickener
but the slopes of the bottom are far steeper and have an angle in the region of 37°. Units are available with
diameters of up 15 m. A rake rotating at speeds between 0,25 rpm and 2 rpm is usually provided in order to
aid the thickening process and increase the underflow concentrations.
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Key
1 fast acting flocculant 5 supernatant (overflow)
2 feed 6 rake and scraping arms
3 mixing device 7 thickened suspension (underlow)
4 motor drive
Figure 5 — Deep cone thickener [1]
4.1.3 Devices based on flotation
Flotation thickeners are process devices wherein solid particles are separated from the liquid phase by
becoming attached to air bubbles. The particles float to the water surface and are removed with skimmers.
The most common device is dissolved air flotation (Figure 6) which uses pressurised air 300 kPa to 600 kPa
and dissolves it in pressurised water. The pressure is then suddenly released to form small bubbles with a
diameter of 40 µm to 80 µm. Bubbles are mixed with sludge (direct flotation) or with sludge diluted by
underflow water (indirect flotation).
Other systems are also used:
• vacuum flotation thickeners employ air that is dissolved at atmospheric pressure followed by a pressure
drop to allow the formation of bubbles with a few millimeters diameter;
• induced air flotation thickeners generate bubbles of 0,2 mm to 1 mm diameter by injecting air into water,
e.g by means of a Venturi nozzles.
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Figure 6 — Dissolved air flotation
4.1.4 Devices based on filtration
4.1.4.1 General
Many kinds of devices are commercially available and the most common ones are described below. They are
usually fed with flocculated sludges.
4.1.4.2 Belt thickener
The sludge is uniformly distributed on a travelling filter belt (width: 800 mm to 2 700 mm, length: 2 m to 5 m)
that moves slowly (7 m/min to 30 m/min). The filtrate drains through the continuously travelling filter in the
horizontal filter zone. Solids are retained on the belt. Specially designed “baffles” divert the sludge in order to
1)
facilitate water drainage. Spray nozzles are used to wash the belt while it returns to the front end (Figure 7 ).
1) This belt thickener is an example of a suitable design thickening and dewatering equipment available commercially.
This information is given for the convenience of users of this CEN Technical Report and does not constitute an
endorsement by CEN of this equipment. The manufacturer has given the authorisation to reproduce the scheme included
in Huber documentation (www.huber.de).
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Figure 7 — Belt thickener
4.1.4.3 Disc thickener
Flocculated sludge overflows into the disc thickener consisting of an inclined and slowly rotating disc
2)
(diameter: 1 500 mm to 1 800 mm) that is lined with a filter cloth (Figure 8 ). Sludge water drains by gravity
2) This disc thickener is an example of a suitable design of thickening and dewatering equipment available commercially.
This information is given for the convenience of users of this CEN Technical Report and does not constitute an
endorsement by CEN of this equipment. The manufacturer has given the authorisation to reproduce the scheme included
in Huber documentation (www.huber.de).
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through the filter. While the sludge moves upwards, it is turned over by ploughs to open up free filter surface in
their wake. A scraper removes thickened sludge from the disk at its upper side. Before sludge is fed again, the
filter cloth is backwashed by means of a spray bar.
Figure 8 — Disc thickener
4.1.4.4 Drum thickener
Sludge is fed from the bottom and is thickened in a drum (diameter: 600 mm to 1 200 mm) rotating at low
speed and equipped with a metallic mesh (500 µm to 600 µm) or belt. Sludge water drains through the mesh
and is collected in a trough. Thickened sludge is driven by rotating baffles through the drum (which might be
slightly inclined) and drops at the drum’s end into a chute. The exit of the sludge is allowed by the inclination
3)
of the drum (Figure 9 ).
3) This drum thickener is an example of a suitable design of thickening and dewatering equipment available
commercially. This information is given for the convenience of users of this CEN Technical Report and does not constitute
an endorsement by CEN of this equipment. The manufacturer has given the authorisation to reproduce the scheme
included in Huber documentation (www.huber.de).
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Figure 9 — Drum thickener
4.1.4.5 Screw thickener
Flocculated sludge overflows into the screw thickener consisting of an inclined screen drum (diameter:
300 mm to 1 200 mm) and a flighted screw slowly turning therein (Figure 10). The screen drum is completely
filled with sludge. The screw transports the sludge slowly upwards, whereby sludge water drains by gravity
through the screen. Thickened sludge is discharged at the upper end of the screen drum. The screen drum is
backwashed at regular intervals by means of rotating spray bars.
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Figure 10 — Screw thickener
4.1.4.6 Draining bag/tubes
Specific synthetic filter cloths of high permeability and mechanical resistance are assembled to form draining
bags/tubes into which sludge is pumped while sludge water drains through the cloth. During subsequent
storage, consolidation of sludge continues as water evaporates through the pores of the filter cloth
(Figure 11).
Figure 11 — Draining tubes
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4.1.4.7 Horizontal grids / deck thickeners
In these filter thickeners, the separati
...
SLOVENSKI STANDARD
kSIST-TP FprCEN/TR 16456:2012
01-september-2012
Karakterizacija blata - Dobra praksa za postopek/ke odstranjevanja vode
Characterization of sludges - Good practice of sludge dewatering
Charakterisierung von Schlämmen - Gute fachliche Praxis der Schlammentwässerung
Caractérisation des boues - Bonnes pratiques pour la déshydratation des boues
Ta slovenski standard je istoveten z: FprCEN/TR 16456
ICS:
13.030.20 7HNRþLRGSDGNL%ODWR Liquid wastes. Sludge
kSIST-TP FprCEN/TR 16456:2012 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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kSIST-TP FprCEN/TR 16456:2012
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kSIST-TP FprCEN/TR 16456:2012
TECHNICAL REPORT
FINAL DRAFT
FprCEN/TR 16456
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
July 2012
ICS 13.030.20
English Version
Characterization of sludges - Good practice of sludge
dewatering
Caractérisation des boues - Bonnes pratiques pour la Charakterisierung von Schlämmen - Gute fachliche Praxis
déshydratation des boues der Schlammentwässerung
This draft Technical Report is submitted to CEN members for Technical Committee Approval. It has been drawn up by the Technical
Committee CEN/TC 308.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.
Warning : This document is not a Technical Report. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a Technical Report.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2012 CEN All rights of exploitation in any form and by any means reserved Ref. No. FprCEN/TR 16456:2012: E
worldwide for CEN national Members.
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FprCEN/TR 16456:2012 (E)
Contents Page
Foreword .4
Introduction .5
1 Scope .8
2 Normative references .8
3 Terms and definitions .9
4 Description and features of thickening / dewatering systems . 11
4.1 Thickening devices . 11
4.1.1 General . 11
4.1.2 Devices based on natural forces (gravity) . 12
4.1.3 Devices based on flotation . 13
4.1.4 Devices based on filtration . 14
4.1.5 Devices based on centrifugation . 19
4.2 Dewatering devices . 20
4.2.1 General . 20
4.2.2 Filter press (plate, membrane) . 20
4.2.3 Belt (filter) press . 23
4.2.4 Centrifuge . 23
4.2.5 Screw press . 24
4.2.6 Others . 25
5 Conditioning . 26
5.1 General . 26
5.2 Conditioning processes . 26
5.2.1 General . 26
5.2.2 Coagulation . 26
5.2.3 Flocculation . 26
5.2.4 Physical processes . 29
5.3 Conditioners . 29
5.3.1 General . 29
5.3.2 Polymers . 29
5.3.3 Inorganic chemicals (multivalent salts, lime) . 30
5.3.4 Other products . 30
5.4 Technical aspects . 31
5.4.1 Storage of conditioner . 31
5.4.2 Selection of conditioner . 31
5.4.3 Preparation of conditioners . 32
5.4.4 Injection, dosing and mixing with sludge . 35
5.4.5 Automation . 38
6 Parameters / Methods for the evaluation of sludge thickenability or dewaterability . 39
6.1 General . 39
6.2 Mechanisms description . 39
6.2.1 Settling / Flotation. 39
6.2.2 Centrifugation . 41
6.2.3 Filtration . 42
6.3 Basic theories and parameters . 44
6.3.1 Settling / Flotation. 44
6.3.2 Centrifugation . 44
6.3.3 Filtration . 45
6.4 Methods of evaluation . 46
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6.4.1 General . 46
6.4.2 Settleability / Thickenability . 46
6.4.3 Centrifugability . 47
6.4.4 Filterability . 48
6.4.5 Basic parameters . 50
7 Critical parameters for sizing and optimization of thickening/dewatering systems . 51
7.1 General . 51
7.2 Gravity thickeners . 52
7.3 Belt thickeners . 52
7.4 Centrifuges . 52
7.5 Filter-presses . 54
7.6 Belt-presses . 54
7.7 Screw-presses . 55
8 Operational and economic aspects of thickening/dewatering systems . 55
8.1 General . 55
8.2 Performances . 56
8.3 Energy consumption . 59
8.4 Labour requirements . 59
8.5 Water consumption . 59
8.6 Maintenance . 60
8.7 Safety aspects . 60
8.8 Automation . 60
8.9 Cost aspects . 61
8.10 Final considerations . 62
9 Conclusions . 65
Annex A (informative) Environmental checklist . 67
Bibliography . 68
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Foreword
This document (FprCEN/TR 16456:2012) has been prepared by Technical Committee CEN/TC 308
“Characterization of sludges”, the secretariat of which is held by AFNOR.
This document is currently submitted to the Technical Committee Approval.
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Introduction
Sludge processing train is a major problem in water and wastewater treatment, as it can account for up to
50 % of total operating costs. The effectiveness and cost of sludge treatment and disposal operations are
strongly affected by its volume and, consequently, by its water content or solids concentration. Thickening and
dewatering are therefore important steps in the total sludge processing train and have serious impact on
subsequent operations.
For illustration, Figure 1 shows the existing solutions for sludge water content reduction, and Figure 2 shows
the level of dry matter content required for intended utilization and disposal routes.
Figure 1 — Principal thickening / dewatering processes
This guide deals with the dewatering and thickening techniques quoted in Figure 1.
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Figure 2 — Percentage Dry Solids (DS) usually required after thickening and dewatering for intended
routes
Sludges management options are developed in a series of CEN Technical Reports to which belong the
present report, see Figure 3 below.
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Figure 3 — A basic scheme for deciding on sewage sludge use/disposal options and the relevant
CEN/TC 308 guidance documents
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1 Scope
This CEN Technical Report describes good practice for sludge dewatering and belongs to a series on sludge
management options.
It gives guidance on technical and operational aspects of:
Conditioning, thickening and dewatering processes.
Drying, which is another water content reduction process, is not dealt with in this document but in
CEN/TR 15473, Characterisation of sludges — Good practice for sludges drying.
This report is applicable for sludges from:
urban wastewater treatment plants;
treatment plants for industrial wastewater similar to urban wastewater;
water supply treatment plants.
This document may be applicable to sludges of other origin.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
EN 12832:1999, Characterization of sludges — Utilization and disposal of sludges — Vocabulary
prEN 16323:2011, Glossary of wastewater engineering terms
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3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 12832:1999, in prEN 16323:2011
and the following, taken either from the normative references or from technical dictionary [1], apply.
3.1
cake
solid fraction of sludge as resulting from a solid-liquid separation process
3.2
centrate
sludge liquor separated by centrifugation
3.3
centrifugation
partial separation of solid from liquid under centrifugal forces
3.4
charge density
percentage of positive or negative charge
3.5
compressibility
ability of a sludge to be compressed under pressure
3.6
compression point
sludge solids concentration at which compression begins in a sedimentation process
3.7
desaturation
removal of water due to displacement of water by air
3.8
draining / drainage of sludge
separation of water from sludge liquor by gravity filtration
3.9
dryness
ratio of dry solids to sludge mass
3.10
electroosmosis
movement of liquid relative to a stationary charged surface as induced by an electrical field
3.11
expression
removal of sludge water due to deformation of solids under pressure
3.12
filter
device for the removal of sludge water whereby solids are retained on a water-permeable filter medium
3.13
filter medium
material where through a fluid flows and which retains matter contained in the fluid
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3.14
filterability
characteristic describing the ability of sludge to be filtered
3.15
filtrate
sludge liquor separated by filtration
3.16
filtration
process of retention of the suspended matter by passing through a medium
3.17
floc
aggregate of particles that results from a flocculation process
3.18
flotation
raising of suspended matter in liquid to the surface by the entrainment of a gas
3.19
“g“
2
gravitational acceleration (9,81 m/s )
3.20
isolelectric point
condition in which a substance has a neutral charge
3.21
mesh
interlacing of crossed wires that determines the openings which can be square, triangular or rectangular
3.22
molecular weight
chain length of a polymer
3.23
particle size distribution
relative amount of particles classified per size ranges
3.24
polymer
class of natural and synthetic materials which are formed by association of structural units (monomers) by
covalent bonds
3.25
porosity
ratio of the void volume to the total volume of material
3.26
pre-treatment
improvement of sludge characteristics by physical or chemical means
3.27
rheology
study of flow and deformation properties under the influence of an applied stress
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3.28
saturation
ratio of the volumes of water and pores in a solid matrix
3.29
sieve (sludge treatment)
device for removing solids from fluids whereby the fluid flows through slots, perforations or a mesh
3.30
settling
ability for sludge solids to separate from water by sedimentation under gravity
3.31
sludge liquor
liquor separated from sludge. Sludge liquor can be called supernatant, filtrate and centrate
3.32
specific cake resistance
property representing the resistance to filtration of a layer of particles, having a unit mass of dry solids
deposited on a unit filtering area
3.33
supernatant
sludge liquor separated by gravity thickening
3.34
water distribution
different physical states of water associated with sludge solid particles
3.35
zeta potential
electrical potential present at the plane of slip when a particle moves relative to its suspending liquid (or vice
versa)
4 Description and features of thickening / dewatering systems
4.1 Thickening devices
4.1.1 General
Thickening devices enable the removal of free water from sludge. They are based on:
natural (static) forces;
artificial forces.
Thickening presents the following advantages:
reduction of sludge volume with low energy consumption;
reduction of storage capacities and volumes for subsequent treatment;
reduction of transport costs;
improvement of performance of dewatering machines;
decrease in quantity of chemicals for dewatering in some cases.
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This section discusses the most commonly used devices for thickening.
4.1.2 Devices based on natural forces (gravity)
4.1.2.1 General
The principle of gravity thickening relies on sludge settling under the effect of gravitational forces. It enables
the raising of the concentration of a suspension through sedimentation to produce a thickened sludge with a
relatively clear liquid as overflow. Thickeners can be designed to operate in either the batch or continuous
mode.
Sludge thickening can be achieved in clarifiers or separate thickeners which provide for a greater sludge
storage capacity.
4.1.2.2 Gravity thickener
The traditional gravity thickener (Figure 4) comprises a relatively shallow, open top cylindrical/rectangular tank
with either a flat bottom or a bottom shaped in the form of an inverted cone. The feed mixture is gently and
continuously introduced to the feedwell. The supernatant is removed via an annular weir at the top of the unit
and sludge solids are removed from a well at the bottom. Slowly rotating rakes mounted on a central shaft aid
the thickening process by directing thickened solids towards the well for subsequent discharge and by
creating channels to release further liquid from the sludge.
Tanks with a diameter smaller than 25 m are usually formed from steel and have bottoms with an angle
usually less than 10° equipped with rake arms. Larger tanks between 25 and 200 m diameter are normally
made from a combination of concrete and steel and employ rakes designed to match the angle of the conical
bottom.
Key
1 feed 6 rake
2 drive head 7 feedwell
3 walkway 8 thickened suspension (underflow)
4 supernantant (overflow) 9 well
5 flocculant
Figure 4 — Gravity thickener [1]
When space is limited, the lamellar separator is used. It is a rectangular tank containing a series of closely
spaced rectangular plates inclined at an angle of higher than 50° to the horizontal.
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Commercial designs provide three flow patterns, cross-flow, parallel flow and the most common counter–flow
where the feed and supernatant flows can be most simply arranged.
The choice of a lamella separator is mainly related to the concentration of the input sludge.
4.1.2.3 Deep cone thickener
A deep cone thickener (Figure 5) has the same operation principle as a conventional circular gravity thickener
but the slopes of the bottom are far steeper and have an angle in the region of 37°. Units are available with
diameters of up 15 m. A rake rotating at speeds between 0,25 and 2 rpm is usually provided in order to aid the
thickening process and increase the underflow concentrations.
Key
1 fast acting flocculant 5 supernatant (overflow)
2 feed 6 rake and scraping arms
3 mixing device 7 thickened suspension (underlow)
4 motor drive
Figure 5 — Deep cone thickener [1]
4.1.3 Devices based on flotation
Flotation thickeners are process devices wherein solid particles are separated from the liquid phase by
becoming attached to air bubbles. The particles float to the water surface and are removed with skimmers.
The most common device is dissolved air flotation (Figure 6) which uses pressurised air (300 to 600 kPa) and
dissolves it in pressurized water. The pressure is then suddenly released to form small bubbles with a
diameter of 40 µm to 80 µm. Bubbles are mixed with sludge (direct flotation) or with sludge diluted by
underflow water (indirect flotation).
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Other systems are also used:
• vacuum flotation thickeners employ air that is dissolved at atmospheric pressure followed by a pressure
drop to allow the formation of bubbles with a few millimeters diameter;
• induced air flotation thickeners generate bubbles of 0,2 mm to 1 mm diameter by injecting air into water,
e.g by means of a Venturi nozzles.
Figure 6 — Dissolved air flotation
4.1.4 Devices based on filtration
4.1.4.1 General
Many kinds of devices are commercially available and the most common ones are described below. They are
usually fed with flocculated sludges.
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4.1.4.2 Belt thickener
The sludge is uniformly distributed on a travelling filter belt (width: 800 mm to 2700 mm, length: 2 m to 5 m)
that moves slowly (7 m/min to 30 m/min). The filtrate drains through the continuously travelling filter in the
horizontal filter zone. Solids are retained on the belt. Specially designed “baffles” divert the sludge in order to
1)
facilitate water drainage. Spray nozzles are used to wash the belt while it returns to the front end (Figure 7 ).
Figure 7 — Belt thickener
1) This belt thickener is an example of a suitable design thickening and dewatering equipment available commercially.
This information is given for the convenience of users of this CEN Technical Report and does not constitute an
endorsement by CEN of this equipment. The manufacturer has given the authorisation to reproduce the scheme included
in Huber documentation (www.huber.de).
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4.1.4.3 Disc thickener
Flocculated sludge overflows into the disc thickener consisting of an inclined and slowly rotating disc
2)
(diameter: 1 500 mm to 1 800 mm) that is lined with a filter cloth (Figure 8 ). Sludge water drains by gravity
through the filter. While the sludge moves upwards, it is turned over by ploughs to open up free filter surface in
their wake. A scraper removes thickened sludge from the disk at its upper side. Before sludge is fed again, the
filter cloth is backwashed by means of a spray bar.
Figure 8 — Disc thickener
2) This disc thickener is an example of a suitable design of thickening and dewatering equipment available commercially.
This information is given for the convenience of users of this CEN Technical Report and does not constitute an
endorsement by CEN of this equipment. The manufacturer has given the authorisation to reproduce the scheme included
in Huber documentation (www.huber.de).
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4.1.4.4 Drum thickener
Sludge is fed from the bottom and is thickened in a drum (diameter: 600 mm to 1 200 mm) rotating at low
speed and equipped with a metallic mesh (500 µm to 600 µm) or belt. Sludge water drains through the mesh
and is collected in a trough. Thickened sludge is driven by rotating baffles through the drum (which might be
slightly inclined) and drops at the drum’s end into a chute. The exit of the sludge is allowed by the inclination
3)
of the drum (Figure 9 ).
Figure 9 — Drum thickener
3) This drum thickener is an example of a suitable design of thickening and dewatering equipment available
commercially. This information is given for the convenience of users of this CEN Technical Report and does not constitute
an endorsement by CEN of this equipment. The manufacturer has given the authorisation to reproduce the scheme
included in Huber documentation (www.huber.de).
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4.1.4.5 Screw thickener
Flocculated sludge overflows into the screw thickener consisting of an inclined screen drum (diameter:
300 mm to 1 200 mm) and a flighted screw slowly turning therein (Figure 10). The screen drum is completely
filled with sludge. The screw transports the sludge slowly upwards, whereby sludge water drains by gravity
through the screen. Thickened sludge is discharged at the upp
...
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