SIST-TP CEN/TR 15473:2008

SLOVENSKI STANDARD
SIST-TP CEN/TR 15473:2008
01-julij-2008
Karakterizacija blata - Dobra praksa pri sušenju blata
Characterization of sludges - Good practice for sludges drying
Charakterisierung von Schlämmen - Gute fachliche Praxis zur Schlammtrocknung
Caractérisation des boues - Bonne pratique pour le séchage des boues
Ta slovenski standard je istoveten z: CEN/TR 15473:2007
ICS:
13.030.20 7HNRþLRGSDGNL%ODWR Liquid wastes. Sludge
SIST-TP CEN/TR 15473:2008 en,fr
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------

TECHNICAL REPORT
CEN/TR 15473
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
March 2007
ICS 13.030.20

English Version
Characterization of sludges - Good practice for sludges drying
Caractérisation des boues - Bonne pratique pour le Charakterisierung von Schlämmen - Gute fachliche Praxis
séchage des boues zur Schlammtrocknung
This Technical Report was approved by CEN on 27 August 2006. It has been drawn up by the Technical Committee CEN/TC 308.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2007 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 15473:2007: E
worldwide for CEN national Members.

---------------------- Page: 2 ----------------------

CEN/TR 15473:2007 (E)
Contents Page
Foreword.3
Introduction .4
1 Scope .5
2 Normative references .5
3 Terms , definitions and abbreviated terms .5
4 General.7
5 Treatment process description .9
6 Drying plant ancillaries .35
7 Operation .38
8 Safety considerations .42
9 Characteristics of dried sludge products .47
10 Outlets available .51
Bibliography .56

2

---------------------- Page: 3 ----------------------

CEN/TR 15473:2007 (E)
Foreword
This document (CEN/TR 15473:2007) has been prepared by Technical Committee CEN/TC 308
“Characterization of sludges”, the secretariat of which is held by AFNOR.
The status of this document as CEN/TR has been chosen because much of its content is not completely in
line with practice and regulations in each member state. This document gives recommendations for good
practice concerning the drying of sludges, but existing national regulations remain in force.
3

---------------------- Page: 4 ----------------------

CEN/TR 15473:2007 (E)
Introduction
All the information of this CEN Technical report constitutes a framework for the process of drying sludges.
Various Directives will apply to thermally dried sludge products depending on the use to which they are to be
put. These Directives include Directive 86/278/EEC(see [1]) for recycling to land, Directive1999/31/EC (see
[2]) for disposal to landfill and Directive 2000/76/EC (see [3]) for incineration and energy recovery and
Directive 94/9 for equipment intended for use in potentially explosive atmospheres (see [4]).
This document should be read in the context of the requirements of these Directives and any other relevant
regulations, standards and codes of practice, which may prevail locally within Member States.
4

---------------------- Page: 5 ----------------------

CEN/TR 15473:2007 (E)
1 Scope
This CEN Technical report describes good practices for sludge drying and it is one of a series on sludge
management options. It gives guidance on
- drying processes;
- characteristics of dried sludge products;
- recycling or disposal of dried sludge products.
from urban wastewater treatment plants.
Sludges of other origin, like sludge from water supply or industrial treatment plants are not in the exact scope
of this CEN Technical Report, however the handling of most of these sludges will comply to a large extent with
the leads given in this CEN Technical Report.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
EN 1085, Wastewater treatment – Vocabulary
EN 12832, Characterisation of sludges – Utilisation and disposal of sludges –Vocabulary
CR 13714, Characterisation of sludges – Sludge management in relation to use or disposal
CEN/TR 13767, Characterisation of sludges – Good practice for sludges incineration with and without grease
and screenings
CEN/TR 13768, Characterisation of sludges – Good practice for combined incineration of sludges and
household wastes
CEN/TR 15126, Characterisation of sludges – Good practice for landfilling of sludges and sludge treatment
residues
3 Terms , definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document the terms and definitions given in EN 1085, EN 12832 and CR 13714
apply and also those given in:
Directive 91/271/EC (see [5]) concerning urban wastewater treatment;
Directive 75/442/EC (see [6]) the waste framework directive as amended by Directive 91/156/EC (see [7]);
Directive 99/31/EC (see [2]) on the landfill of waste;
Directive 86/278/EEC (see [1]) on the protection of the environment, and in particular the soil, when
sewage sludge is used in agriculture;
Directive 2000/76/EC (see [3]) on incineration;
Directive 94/9 (see [4]) for equipment intended for use in potentially explosive atmospheres;
5

---------------------- Page: 6 ----------------------

CEN/TR 15473:2007 (E)
Directive 99/92/EC (see [8]) on minimum requirements for improving the safety and health protection of
workers potentially at risk from explosive atmospheres;
and the following terms and definitions apply:
3.1.1
adhesion or shearing phase
phase, which exists in a range of some 40 % to 60% dry residue content, where the sewage sludge changes
its rheological behaviour. Within this phase there is a "sticky mass" whose treatment and transportation is to
be given special attention. Above the adhesion phase the sewage sludge has, in many cases, depending on
the drying equipment, a more crumbly/lumpy structure that makes it easier to handle
3.1.2
fully dried / partly dried sludge
sludge dried above 85% dry residue content is defined as “fully dried” and sludge of dry residue content
below 85% as “partly dried”
3.1.3
convection dryer
drying system where the heat is transferred to the product by a gaseous medium which is in intimate and
direct contact with the product
NOTE The evaporated water is thus mixed with the drying medium and the exhaust gases from the dryer consist of
the drying gas including leakage air plus the evaporated water. Convection dryer can operate with direct or indirect heating.
3.1.4
conduction dryer
drying system where the heat is transferred through an intermediate heat transfer surface to the product
NOTE The medium, which supplies the heat to the product, is never in direct contact with it. The total exhaust gas
amount leaving the dryer is the evaporated water plus some leakage air. Therefore a conduction dryer is always operated
with indirect heating.
3.1.5
solar dryer
drying system where the heat is transferred to the product by solar radiation
3.1.6
combined drying system
system, which uses both principles, convection and conduction in the same dryer
3.1.7
hybrid drying system
system that consists of a combination of a conduction dryer and a convection dryer
3.1.8
direct heating
off gas from the burner is in contact with the drying product
3.1.9
indirect heating
heating loop is crossing a heat exchanger
3.2 Abbreviated terms
BOD Biological Oxygen Demand
COD Chemical Oxygen Demand
6

---------------------- Page: 7 ----------------------

CEN/TR 15473:2007 (E)
HAZOP Hazard and Operability Studies
LCV Lower Calorific Value
LIT Layer Ignition Temperature
LOC Limiting Oxygen Concentration
MEC Minimum Explosion Concentration
MIE Minimum Ignition Energy
MIT Minimum Ignition Temperature
MPOC Maximum Permissible Oxygen Concentration
PLC Programmable Logic Controller
RTO Regenerative Thermal Oxydizer
SCADA System Control, Alarm and Data Acquisition
VOC Volatile Organic Carbon
WWTP Waste Water Treatment Plant
4 General
The drying of sewage sludge is a complex process but it can contribute to the need for increased disposal
security for sewage sludge. With dried sewage sludge, a wider potential customer market can be approached
than for liquid or dewatered products. It can be recorded that the range of those willing to accept sewage
sludge can be expanded if one offers dried sewage sludge. In any case, the opportunity for marketing sewage
sludge can be extended considerably which, in turn, represents an additional security for disposal and/or
utilisation. The overview of thermal dryer types is given in Figure 1.
7

---------------------- Page: 8 ----------------------

CEN/TR 15473:2007 (E)
Dryers
Convection Solar dryers Conduction
dryers dryers
with direct with indirect with indirect
heating heating heating
Hybrid drying systems
+
Convection dryer Conduction dryer
Combined drying systems:

The principles of convection and conduction are applied in one dryer


Figure 1: Overview on thermal dryer types
Thermal drying of sludge can result in the following advantages for almost all outlet routes:
• Substantial minimisation of the bulk of sludge for disposal. Thus 1 t dry residue of sludge at 90%
represents 1,1 t actual or wet mass of sludge for disposal, but 1 t dry residue of sludge at 5% represents
20 t wet mass of sludge for disposal. Minimisation of sludge mass and volume by thermal drying results
in savings in transport costs, which can be a major component of disposal costs.
• The removal of water leaves a thermally dried sludge product with a lower calorific value (LCV) (about 10
MJ/kg to 15 MJ/kg, depending e.g. upon the extent of pre-treatment), which can be used for thermal
recycling.
• The thermally dried sludge product has favourable properties, as it is usually handable and storable,
which lends flexibility to operations.
• For recycling to agriculture, in particular, there is the advantage that thermal drying is an ‘advanced
treatment process whereby the dried sludge product gets sanitised and effectively free of pathogens at
the end of the process.
Whilst the thermally dried product has definite advantages over other types of sewage sludge, there are some
reservations (Brown and Jacobs, 2001 (see [9])) about the thermal drying process as follows:
• High capital cost (see 7.2.1);
• High operating cost, mainly energy consumption (see 7.2.1);
• Safety issues, particularly risk of fire and explosion (see clause 8);
• Technology can be complex and needs some well trained operators (see 7.3);
• The thermally dried product can be re-infected by micro-organisms and as a result of rewetting odour
can be released after storage dependent upon the conditions of storage.
8

---------------------- Page: 9 ----------------------

CEN/TR 15473:2007 (E)
The thermally dried sludge product can reach a dry residue content up to about 95% mass fraction. In most
instances thermal drying of sludge will aim to achieve a dry residue content of more than 50% in order to
avoid the adhesion or shearing phase.
5 Treatment process description
5.1 General
There are numerous designs of thermal drying equipment available on the market and many of these have
been adapted from other industries and used for sewage sludge drying. Few have been designed specifically
for sewage sludge.
The equipment can be classified into four main groups defined by the drying process:
• Conduction dryers;
• Convection dryers;
• Combined or hybrid drying system;
• Solar dryers (radiation dryers).
5.2 Conduction dryers
5.2.1 General
A conduction dryer is a drying system where the heat is transferred through an intermediate heat transfer
surface to the product.
The medium, which supplies the heat to the product, is never in direct contact with it. The total exhaust gas
volume leaving the dryer is the evaporated water plus some leakage air. Therefore conduction dryers are
always operated with indirect heating.
5.2.2 Disc dryers
Disc drying plants are - dependent on their shape - in a position to dry sewage sludge, both partially and
completely. With this, complete drying is made possible using a mixing machine placed before the dryer. Here,
a part of the already dried product is mixed with the dewatered sludge and thus overcoming of the adhesion
phase is achieved outside the dryer. Plants for full drying, as special structures, are also used by which the
return admixture takes place in the input area of the dryer.
Wear problems and dried sludge behaviour (fines and dust) have contributed to the decline in use of disc
dryers for full drying. In the majority of cases, disc dryers are used to dry sludge up to a dry residue content
below the shearing phase (for an auto thermal incineration).
9

---------------------- Page: 10 ----------------------

CEN/TR 15473:2007 (E)

1)
Figure 2: Disc dryer
The disc dryers (see Figures 2 and 3) are constituted of:
• A stator or body
• A rotor composed of a hollow shaft with hollow discs shaped as plates and welded on it.
A heating fluid crosses the rotor, either saturated steam up to approximately 1 MPa (10 bar) or thermal oil,
which transfer thermal energy to the dewatered sludge, through the disc’s surface. A process variant can
additionally have the stator heated. The thermal oil (or steam) is heated in a boiler, usually fired with fossil fuel
or biogas.
Vapours exhaust from the superior dome of the dryer, which should be correctly designed for effective transfer
to a cyclone or a scrubber (partial drying at 45 %). As the residence time of the sludge in this type of dryer is
rather large (around 1h), the vapours are usually highly polluted. However, one advantage of an indirect
conduction dryer is that the volume of polluted vapours is small and kept separated from the flue gases of the
energy source.
Due to the slow rotation of the rotor (circumferential velocity approximately 1 m/s) the sewage sludge is well
mixed and a new interface for drying is continuously created. The requirements of the start-up condition are,
met with the design of the drive.
Transport paddles, by which the sewage sludge is transported axially in the dryer, are additionally mounted on
the rotor discs. However, in order to dry past the sticky phase, it is necessary to recycle some dry product
upstream.

1) This dryer is an example of a suitable design of thermal drying 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 SIL documentation.
10

---------------------- Page: 11 ----------------------

CEN/TR 15473:2007 (E)
The dryer discs are subjected to high wear and corrosion loads, which should be taken into account through
the selection of suitable materials or appropriate wear reserves, and in terms of maintenance and life time.
Particularly stressed areas can additionally be armoured. The rotor has a considerable mass so that reverse-
bending stresses also need to be accounted for.
Through the small separation of the discs it is possible to create a large surface area for heat transfer- related
to the dryer volume. This means that disc dryers can be made very compact. The following specific
evaporation performances have been documented (ATV-DVWK 2001 see [10]):
2
• With full drying plants of approximately 7 kg to 10 kg H O evaporated / (m .h) and
2
2
• With partial drying plants of greater than 11 kg H O evaporated / (m .h)
2
The degree of drying of the product is, as a rule, set via the input sludge quantity, which is proportional to the
filling level of the sludge in the dryer and to the ratio of return, mixed dried material. To control the filling level
and maximise the contact surface with the sludge, either pressure pick-up cells or gamma radiation level
detectors are used.
In practice, for a daily start up and shut down, the operational flexibility of disc dryers is difficult to manage
because of the high energy heat stored in the heating medium. As there are permanently large sewage sludge
quantities with varying degrees of dryness in disc dryers, a quick shutdown of the plant is not possible.
Consequently, there is a danger that sewage sludge can bake on to the discs (particularly with a sudden
removal from service and extended standstill periods). Therefore several hours are to be allowed for start-up
and closedown procedures so that disc-drying plants are best operated continuously.
11

---------------------- Page: 12 ----------------------

CEN/TR 15473:2007 (E)


Key
1 Dewatered sludge
2 Mixer
3 Disc dryer
4 Solids separator
5 Exhaust vapour treatment
6 Dry material
7 Cooler
8 Grinder
9 Sieving plant
10 Feed screw
11 Oil/gas
12 Burner
13 Steam
14 Condensate
15 Air feed
16 Exhaust gas
2)
Figure 3: Disc drying plant for full drying
5.2.3 Paddle dryers
These dryers are composed of a horizontal body, in which two shafts are rotating in opposite directions. Each
shaft bears special paddles, of wedge-shaped type. The paddles of the first shaft are staggered, in relation to
the second one.
The main difference with a disc dryer is in the design of the paddles and the high torque, which allow complete
drying without dry sludge feedback.

2) This is an example of a suitable design of thermal drying 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 ATV-DVWK has given the authorisation to reproduce this scheme included in document ATV-DVWK 2004,
see [11].
12

---------------------- Page: 13 ----------------------

CEN/TR 15473:2007 (E)
The dewatered sludge to be dried is fed at one extremity of the dryer and flows out to the other extremity,
carried in part by the rotation of shafts and inclination of the dryer.
The heating medium (steam or thermal oil from the boiler) is fed simultaneously to the dryer shell, the shafts
and the paddles.
The vapours escape to a lateral condenser. There is a little control air swept in.
The body, shafts and paddles are manufactured with special and resistant steel. The surface of the paddles
and body are quite smooth.
Typical features of a paddle dryer are:
• Low shaft rotation speed (about 10 tr/min), in order to limit the risks of wear;
• Good homogeneity of temperature in the sludge bulk, which allows a good control and avoids a local
overheating;
• The paddle construction and the smooth surface enables self cleaning and an excellent thermal
2
transfer (up to 20 kg H O evaporated /(m .h);
2
• For full drying, there is no need for any back mixing of dried sludge; the shearing phase is overcome
by kneading action;
• It can accommodate possible variations of sludge dry residue content;
• The high residence time (up to 4 h) is effective for pathogen reduction.
The operation of paddle dryers is similar to disc dryers, but special attention is required to control the plastic
phase, which can move along the dryer when different sludges are processed.
5.2.4 Thin film dryers
Thin film dryers consist of a horizontal stator with double walled cylinder and an internal rotor. The heat
energy in the form of saturated steam or thermal oil is fed to the dryer via the double jacket of the cylinder.
Two kinds of thin film dryers can be differentiated:
• Total indirect dryer;
• Mixed dryer with an internal fluidised effect for sludge transportation.
a) Thin film dryer with mechanical transportation of sludge (see Figures 4 and 5)
The function of the internal rotor, with its welded-on distributor and transport elements, is to build up and
spread the dewatered sewage sludge in a 5 mm to 15 mm thick layer on the inner circumference of the stator.
In this manner a continuously renewed contact interface is ensured and sludge is exchanged very intensively
at the heated wall.
The design of the rotor provides a spiral shaped carrier for the dried material along the heating surfaces as far
as the discharge side. A continuous blending and breakdown of agglomerates, which could possibly form in
the adhesion phase, is achieved through the freely moving pivoted flaps of the rotor. The rotor can be
matched to various sludges by changes to the paddles (rotor design) during stoppages.
By using a special blade configuration fitted on the rotor, sludge is exchanged very intensively at the heated
wall. These blades have different functions as:
• Feed transfer elements. These pieces transport the sludge from the feed point into the heated area,
they are located at the dryer inlet;
• Twisted transportation elements. They have the function of conveying the sludge through the dryer at
slow or high speed;
13

---------------------- Page: 14 ----------------------

CEN/TR 15473:2007 (E)
• Cleaning elements which remove the sludge from one area of the heated wall and push it to an
another area to continuous mixing;
• Back return blades to improve the residence time in the dryer.
The sludge is continuously fed into the dryer at one end, where the feed blades of the rotor distribute the pasty
slurry onto the heated wall. The subsequent mixing and transport blades carry the sludge in a highly mixed
manner in a spiral path on the heated surface, thus accomplishing the required evaporation of water. Sludge
retention time is in the range of 4 min to 10 min and can be adjusted by using various combinations of blades.
NOTE Thin film dryers do not tolerate fluctuations in the quality of the dewatered cake that is fed into them, which
makes them inappropriate for sludge treatment centres.
The rotor can be driven at different and adjustable rotation speeds.
The sludge, after drying, is released from the dryer in a form of granules or coarser particles, at a temperature
between 80 °C to 95°C. This is then loaded, via a spiral screw, for example into containers.

Key
1 Dewatered sludge
2 Steam
3 Thin film dryer
4 Exhaust vapour treatment
5 Dried sludge
6 Cooler
7 Exhaust gas
8 Air feed
9 Condensate
10 Burner
11 Oil/gas
3)
Figure 4: Thin film dryer plant

3) This is an example of a suitable design of thermal drying 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 ATV-DVWK has given the authorisation to reproduce this scheme included in document ATV-DVWK 2004,
see [11].
14

---------------------- Page: 15 ----------------------

CEN/TR 15473:2007 (E)
Basically both a partial and a full drying are possible with a thin film dryer. Its thermal efficiency is excellent
2
(very high coefficient of thermal conductivity W/m ), thanks to the good transfer of energy through the surface,
that ensures high specific drying rate in the range of 25 kg H O evaporated / (m².h) to more than 100 kg H O
2 2
evaporated / (m².h). However, the mechanical power necessary to overcome the shearing phase of sludge is
higher, in comparison to systems utilizing a back mixing of dried sludge.
Operating with higher degrees of drying far above the adhesion phase, vaporisation no longer takes place in a
thin layer but in a form of bulk material drying in which the contact surfaces reduce significantly. From this
results a smaller heat transfer so that larger heating surfaces are necessary. With degrees of drying of up to
approximately 65 % dry residue content one shall reckon with a specific evaporation performance of 25 kg
2 2
H O evaporated / (m .h) to 35 kg H O evaporated / (m .h). With higher degrees of drying the necessary
2 2
evaporation performances are to be considered as economically critical.
Thus, thin film dryers are mostly used for partial drying of sludge up to 65 % dry residue content and
2 2
sometimes for small units (500 kg H 0 evaporated (m /h) to 1000 kg H 0 evaporated (m /h) for full drying
2 2
(85 % dry residue content to 90 % dry residue content).
Start-up and shut-down can be carried out without problem in approximately 1 h, as only relatively small
quantities of sewage sludge are present in the dryer (ATV-DVWK 2001 see [10]).
15

---------------------- Page: 16 ----------------------

CEN/TR 15473:2007 (E)

Key:
1 Steam
2 Vapour
3 Wet sludge
4 Condensate
5 Dried sludge
4)
Figure 5: Thin film dryer plant
Relevant to the degree of drying with thin film dryers is the dry solid content of the input sludge and the
quantity of sludge supplied. The rate of rotation of the paddles influences the product structure and, as a rule,
is not changed during operation. As thin film dryers are used mainly for partial drying, small variations of the
final dryness can be accepted so that, following a single setting of the operational parameters, a further
control is not absolutely necessary (Franke, Günther 1993 (see [12])).
To sum up, a thin film dryer operates at higher tr/min than either disc or paddle dryers, and with a much
shorter residence time. With no recycling of dried sludge, most of the thin film driers in industrial operation
reach a final dry residue content of 55 % to 65 %, versus 90 % for paddle dryers. Disc dryers without recycling
are limited to 40 % to 45 % final dry residue content.
b) Thin film dryer with an internal fluidised effect
The principle of the thin film dryer has been associated with the fluidisation one. This allowed the development
5)
of a “turbo-dryer”
16

---------------------- Page: 17 ----------------------

CEN/TR 15473:2007 (E)
The turbo-dryer is a combined dryer, which associates the twin effects of conduction and convection drying
with a carrier gas. It is one of the few dryers, which use this method of heating. In the dryer, the sludge film, in
contact with the heating body, is constantly renewed; so the sludge area, exposed to the airflow and to the
wall area, is increased.
The drying process deals with a closed loop circuit. Dried sludge is removed from the vapour stream, in an
isolated cyclone locked at the inferior part, by a rotary valve.
The thermal conductance ratio is high. Low air volumes are introduced into the loop and the absence of back
mixing of dried sludge leads to an optimisation of the energy transfer. The construction materials (protected
paddles) shall be very resistant to wear, because the peripheral tangential velocity is quite high (≅ 30 m/s).
The high rotation speed of the rotor requires a perfect equilibrium; a breakdown or the wear of paddles are
drawbacks that can cause damage. Maintenance and control require specialized staff. Carrier gas fluidisation
and mechanical action of the rotor contribute to the fragmentation and division of granules of dried sludge,
especially if the mechanical resistance of the granule is too weak, leading to the formation of fines and dust,
which necessitate a downstream re-agglomeration with a pelletizing machine.
5.2.5 Others
a) Multi-tray dryers
6)
The vertical multi-trays dryers are indirect dryers .
It consists of hollow trays, constructed one above the other inside a cylindrical shell. The trays are heated with
thermal oil running in a closed loop path.
Dewatered sludge is pumped to a coater. In the coater,
...