CEN/TR 16456:2013
(Main)Characterization of sludges - Good practice of sludge dewatering
Characterization of sludges - Good practice of sludge dewatering
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, 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
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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.
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
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
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.
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
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.
Figure 3 — A basic scheme for deciding on sewage sludge use/disposal options and the relevant
CEN/TC 308 guidance documents
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
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“
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
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.
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
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.
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.
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).
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).
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).
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.
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
4.1.4.7 Horizontal grids / deck thickeners
In these filter thickeners, the separation of the solid and liquid phase is mainly achieved by means of a two
dimensional mesh of 100 µm to 500 µm. Flocculated sludge is fed onto the gravity drainage grid/deck
(Figure 12), which retains the thickened sludge. Thickened sludge is removed by gravity or by a mechanical
scraper or by vibrators. Grids are washed by spray water.
Figure 12 — Horizontal grid with scraper
4.1.5 Devices based on centrifugation
Centrifuges are commonly used both for thickening and dewatering.
Centrifuges permit accelerated sedimentation of particles under the force of centrifugation. This centrifugal
force is up to 3 000 "g" depending on the machine size and sludge characteristics (Figure 13).
Key
A feed sludge
B cake
C centrate
Figure 13 — Schematic diagram of a centrifuge
The suspension to be treated is introduced via a fixed tube to a rotating distributor. Under centrifugal force,
heavy particles settle on the interior wall of the bowl. They are scraped by a conveyor scroll and conveyed
towards a cone. The scroll rotates slightly faster or slower than the bowl thanks to a gear box and this
difference is called differential speed. Compacted sediment in the cone is driven through nozzles. Centrate
flows over a circular and usually adjustable weir.
The weir is adjusted while the machine is stopped.
4.2 Dewatering devices
4.2.1 General
Dewatering devices can also remove interstitial water (water that is attached to solids by surface tension and
can be partially removed by sludge compression) and vicinal water (water that is physically bound to solid
surfaces and can only be partially removed even by extreme mechanical force) from sludge. Filter presses,
belt presses, screw presses and centrifuges are the most common techniques.
4.2.2 Filter press (plate, membrane)
A filter press includes a plurality of plates arranged in a horizontal stack, together with a head piece and a
pressure piece, the latter being hydraulically pushed towards the head piece. The plates and head pieces
have a slurry feed port and a number of filtrate outlet ports usually located at the corner of the plates. Each
plate has a cloth on both sides with appropriate holes for the feed and filtrate ports, thus creating a series of
chambers when the plates are held together (Figure 14).
The sludge is pumped into the chambers allowing solids to build up in the filter and filtrate to flow through the
filter cloth and along the ribbed plate surface to their filtrate outlets. The press is fed under pressure until the
set pressure is reached and/or until the filtrate flow drops below a minimum value.
In the conventional plate filter press, after a press cycle, the feed ports are blown out with air and the plates
are removed from each other, allowing cake to drop out of the bottom of the press. After discharge of all
cakes, the press is closed again for the next cycle.
Some filter presses (membrane filter presses) are additionally provided with flexible diaphragms between the
filter plates and cloth, whereby a fluid medium (water or air) is pressed into the space between the plates and
cloth. This further compresses the filter cake (until 1 500 kPa) in the chamber by inflating the membrane. The
compression process also tends to produce more uniform cake. Although membrane presses are more
expensive than conventional filter presses, the additional capital and operating costs are often justified by
better performance (Figure 15).
Key
A feed sludge
B cake
C filtrate
Figure 14 Schematic diagram of a plate filterpress
a) Filtration phase b) Compression phase
Key
A feed sludge
B cake
C filtrate
Figure 15 — Schematic diagram of a membrane filter press
The chamber volume of filter-presses is in the range: 200 l to 1 500 l with plate dimensions from 0,5 m x 0,5 m
up to 3,0 m x 3,0 m. Filter presses can have 200 plates and a filtration area of up to 1 000 m .
4.2.3 Belt (filter) press
Flocculated sludge is spread on a filter belt (width from 0,5 m to 3 m). In a pre-dewatering or draining zone the
belt and the sludge thereon travel more or less horizontally. Baffles, e.g. in the form of ploughs, are usually
provided to turn over the sludge and open up a free filter area in their wake. Sludge water drains by gravity
through the belt. In a wedge zone the sludge is squeezed between a pair of belts whereby gradually rising
pressure is applied on the sludge. In a press-shear zone the two belts travel around rollers with decreasing
diameters, whereby the sludge pressure increases. Alternating shear forces are generated by slightly different
belt velocities supporting sludge compression. At the end of the press-shear zone sludge cake is released and
scraped off the belts with blades. Filtrate from the various zones is collected in one or several troughs. Sludge
cake can drop directly into a container or can be transported with a belt or a screw conveyor. The belts are
continuously cleaned by high pressure spray water (Figure 16).
Key
A feed sludge
B cake
C filtrate
Figure 16 — Schematic diagram of a belt press
2 2 2
Filtration surfaces are in the following range: 3 m to 15 m for a low pressure machine (400 kPa), 8 m to
2 2 2
25 m for a medium pressure machine (500 kPa) and 14 m to 46 m for a high pressure machine (700 kPa).
4.2.4 Centrifuge
The principle of a centrifuge has already been described (4.1.5). The counterflow principle has become
dominant for sludge dewatering. The differential speed of modern dewatering centrifuges is very low (1 rpm to
10 rpm) in order to maximise the solid mass in the rotating bowl and thus its retention time. Under such
conditions, the torque and energy consumption is increased but this leads to higher solids concentration.
Centrates are somewhat more concentrated in suspended solids than filtrates obtained by filtration processes.
Centrifuges operate up to 4 000 g to achieve maximal performance (solids concentration, centrate quality)
depending on sludge characteristics.
The main differences between dewatering and thickening centrifuges are higher “g” force, lower differential
speed, torque based control and higher overflow weir.
4.2.5 Screw press
A screw turns (0,2 rpm to 1,5 rpm) within a cylindrical screen. Its flight generates pressure and sludge water is
driven out through the screen. Pressure is controlled by screw speed and an adjustable cone at the discharge
end. The cone is either adjusted manually or moved by pneumatic cylinders, depending on type and quality of
the sludge. Sludge cake is squeezed through a ring gap between the cone and an orifice. Brushes on the
screw’s flight keep the screen open from the inside; the screen is also periodically washed with spray water
4)
from the outside. Filtrate is collected in an enclosure (Figure 17 ).
Key
1 motor drive 6 pressure cone
2 pressure probe 7 compressed air
3 service water 8 solids discharge
4 screen basket 9 filtrate outlet
5 screw shaft 10 sludge feeding
Figure 17 — Schematic diagram of a screw press
4.2.6 Others
Devices listed below are less common but are still used for some applications.
The following are examples:
vacuum rotating drum filter;
lamellar centrifuge system;
rotary press;
drying beds and lagoons;
4) This screw press 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).
centrifuge dryer;
vacuum filter dryer;
electrosmosis systems;
dewatering containers.
Their performances are compared to classical systems in Clause 8.
5 Conditioning
5.1 General
Sludge conditioning is a pre-treatment to improve the removal of water during the thickening/dewatering
process and a wide range of products/processes are commercially available to do that.
5.2 Conditioning processes
5.2.1 General
Conditioning can be carried out by chemical and/or physical means. The chemical conditioning includes
coagulation (charge neutralisation) and flocculation (bridging to form larger flocs). The most appropriate
conditioning system has to be chosen depending on the sludge properties, dewatering equipment and the
desired degree of thickening or dewatering.
5.2.2 Coagulation
A suspension of dispersed particles is stabilised by electrical charges on the particle surface, causing it to
repel neighboring particles [2]. This prevents charged particles from aggregating to form larger flocs, so solids-
liquid separation is difficult. Coagulation is the destabilisation of these suspensions by neutralising the charges
that keep them apart.
Coagulants can be synthetic or natural products. They are essentially 100 % charged and have low molecular
weights. Since sludges possess a surface charge, the chemical that is used to neutralise that charge should
be the opposite.
5.2.3 Flocculation
5.2.3.1 Floculation by polymers
While coagulation neutralises electro-static forces, the flocculation process bridges particles. This leads to
bigger flocs that are easily separable from the liquid phase with different mechanisms. This is illustrated in
Figure 18.
a) Initial adsorption b) Floc formation
Key
I) initial adsorption at the optimum polymer dosage IV) initial adsorption excess polymer dosage
II) floc formation V) rupture of floc
III) secondary adsorption of polymer VI) secondary adsorption of polymer
1 polymer 8 restabilised particle
2 particle 9 excess polymer
3 destabilised particle 10 stable particle (no vacant sites)
4 destabilised particles 11 intense or prolonged agitation
5 flocculation 12 floc fragments
6 floc particle 13 restabilised floc fragment
7 no contact with vacant sites on another particle
Figure 18 — Representation of the bridging model for the destabilisation of colloids by polymers (in
six steps) [3]
Factors affecting flocculation are:
energy input or mixing intensity: It enables the number of collisions to be improved because particles
need to collide before forming aggregates. However, too much energy could have a detrimental effect on
the collision efficiency by breaking up already formed floc. When two colloidal particles having the same
charge approach each other, the possibility of their coagulation will depend on the difference in their
electrostatic charge which is reduced by coagulant addition (zeta potential) and their kinetic energies
which are supplied by turbulent mixing;
energy input time: Coagulation normally requires an initial short time of high mixing energy followed by a
longer time of lower energy mixing;
amount of solids: The lower the amount of suspended particles, the lower the flocculation rate because of
collision infrequency. Thus, a longer mixing time can be required with more dilute solution and smaller
particles;
flocculant addition: flocculants are chemicals that increase particle size by bridging particles together.
Underdosage leads to small flocs difficult to dewater and overdosage increases the viscosity of water and
leads to big and sticky flocs;
temperature: it will affect flocculation efficiency. Decrease of temperature increases viscosity of interstitial
water causing an increase in chemical demand or a decrease of liquid-solid separation efficiency.
Temperatures higher than 60 °C can cause the degradation of the polymer.
5.2.3.2 Flocculation by inorganic chemicals
Inorganic conditioning consists of 2 consecutive steps (Figure 19).
First, in case there is negatively charged sludge, bivalent or trivalent salt is added to the sludge in order to
enhance the neutralisation (e.g. at a pH lower than the isoelectrical point for the metal hydroxides)
Secondly, lime can be added as calcium hydroxide, to increase pH for rapid precipitation of amorphous gels
(Fe(OH) , Al(OH) ) which act as inorganic polymers bridging particles. Lime is often used together with iron
3 3
chloride. Lime also reacts with bicarbonate to form calcium carbonate, which provides the sludge with
additional structural integrity and the porosity needed to increase its dewaterability.
In addition to inorganic conditioning alone, a combination with organic polymers could be useful.
Key
1 FeCl addition
2 Zeta poential decrease
3 first step: Coagulation
4 lime addition
5 Fe (OH) precipitation
6 second step: Floculation
Figure 19 — Representation of conditioning by inorganic chemicals
5.2.4 Physical processes
Physical conditioning processes include mechanical, thermal and freeze conditioning.
Mechanical conditioning alters the structure of the sludge and enhances its dewaterability by the addition of
inert inorganic or organic additives (ashes, fine coal, sawdust, sand or gravel products).
In thermal conditioning, the sludge is heated to temperatures between 180 °C to 230 °C under pressure
(10 bar to 25 bar) for 30 min to 60 min, whereby the cell structures of organic substances will be broken, so
water removal is improved. Another positive effect is the hygienisation of sludges [4].
This conditioning method can be used with both raw sludges as well as stabilised sludge. An additional
chemical stabilisation is not necessary.
With this process a large part of the organic substances go into solution which leads to an increase in the
organic load of filtrate/centrate.
In addition attention is drawn to the problem of odour formation, which requires a relevant gas treatment.
Freezing is frequently used for natural sludge conditioning in cold climate zones.
5.3 Conditioners
5.3.1 General
There exist various types of conditioning chemicals. Organic polymers, trivalent metal salts (aluminum or iron),
or polym
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