1
International Joint Master’s degree
Programme – Second Cycle (D.M.
270/2004) in Environmental Sciences
(Sustainable Development)
Final thesis
Performance testing of a combined solar
and thermal-‐drying system for biological
sludge
Supervisor
Ch. Prof. Francesco Gonella
Assistant supervisors
Ch. Prof. Ernst Worrell (Utrecht University)
Dr. Paolo Franceschetti (Solwa srl)
Graduand
Stefano Grosso
Matriculation Number 830497
Academic Year
2013 / 2014
2
3
TABLE OF CONTENTS
ABSTRACT 5
CHAPTER 1 – INTRODUCTION 6
1.1 WASTEWATER TREATMENT 7
1.1.1 A WASTEWATER TREATMENT PLANT AND SEWAGE SLUDGE 7
1.1.2 SLUDGE TREATMENT PHASES 9
1.1.3 EXISTING SLUDGE–DEWATERING METHODS 11
1.2 WASTEWATER TREATMENT PLANT AT TREVISO (ITALY) 13
1.3 SEWAGE SLUDGE MANAGEMENT AND REGULATION IN EUROPE 14
1.3.1 SEWAGE SLUDGE MANAGEMENT AND REGULATION IN ITALY 16
1.3.2 DIFFERENT SITUATIONS IN EUROPEAN COUNTRIES 17
1.4 BIOLOGICAL SLUDGE–DRYING 19
1.5 BIOLOGICAL SLUDGE INCINERATION 21
1.5.1 ADVANTAGES OF SLUDGE-‐DRYING AND INCINERATION 24
1.5.2 EMISSIONS FROM SLUDGE COMBUSTION 25
CHAPTER 2 – DESCRIPTION OF THE SYSTEMS 28
2.1 SOLAR STILL FOR WATER DEPURATION 28
2.2 FOOD–DRYING SYSTEM 30
2.3 SLUDGE–DRYING SYSTEM 31
2.3.1 COMBUSTION OF THE DRIED SLUDGE 32
2.3.2 TECHNICAL DESCRIPTION 33
2.3.3 COMPETITORS OF THE SLUDGE-‐DRYING SYSTEM 35
CHAPTER 3 – MATERIALS AND METHODS 38
3.1 ANALYSIS OF THE SOLAR-‐DRYING PROCESS 38
3.1.1 CALCULATION OF THE TOTAL SOLAR INCOMING RADIATION 38
3.1.2 DRYING VELOCITY AND DRYING RATE 39
3.1.3 ENTHALPY OF INCOMING AND OUTGOING AIR IN THE SOLAR COLLECTORS 40
3.2 ANALYSIS OF THE THERMAL-‐DRYING PROCESS 43
3.3 DETERMINATION OF SOLID AND ASHES CONTENT IN THE SEWAGE SLUDGE 44
3.4 COD DETERMINATION WITH TITRATION 45
3.4.1. COD DETERMINATION WITH DIGESTION IN MICROWAVES OVEN (MILESTONE) 46
3.5 MATERIALS 47
3.5.1 COLLECTION OF THE SAMPLES 48
CHAPTER 4 – RESULTS 49
4.1 SOLAR DRYING 49
4.1.1 FIRST WEEK OF TESTING 50
4.1.2 SECOND WEEK OF TESTING 52
4.1.3 THIRD WEEK OF TESTING 57
4.1.4 COMMENTS ON THE RESULTS OF THE SOLAR-‐DRYING EXPERIMENTATION 61
4.2 ENTHALPY AND EXCHANGED THERMAL POWER IN THE SOLAR COLLECTORS 62
4.3 THERMAL DRYING 65
4.3.1 FIRST TEST 65
4.3.2 SECOND TEST 67
4.3.3 VOLUME LOSS 72
4.4 TOTAL SOLIDS (TS) AND TOTAL VOLATILE SOLIDS (TVS) DETERMINATION 72
4
4.6 COD ANALYSIS (ORGANIC CONTENT) 74
4.7 INPUT AND OUTPUT DATA OF THE SLUDGE DRYING SYSTEM 75
CHAPTER 5 – CONCLUSIONS 82
BIBLIOGRAPHY 87
TABLE OF FIGURES 90
5
Abstract
The present work has the principal objective of testing and improving the per-‐ formance of an innovative system for biological sludge drying, which combines an inner high-‐performance sludge burner with solar-‐air heaters. The research question is: how to implement the design and structure of the drying system, de-‐ fining its best working conditions (best temperature, materials and dimensions of its components, etc.).
Current practices of wastewater treatment, in particular the treatment of biolog-‐ ical sludge from urban sewage, are introduced first, with insight into current management issues in Europe and Italy. Solar-‐drying technology is then de-‐ scribed, along with a presentation of the system developed for this project. In addition, its technical advantages and the possible environmental benefits that the technology could bring, especially in comparison to other solar-‐drying plants available on the market, are demonstrated.
Testing was done separately on two solar-‐drying systems and in a laboratory ov-‐ en, in order to assess the response of the biological sludge to solar and thermal heating.
Moreover, the measure of the heating value during the combustion phase is pre-‐ sented, followed by a brief literature review regarding possible emissions into the atmosphere: this data is necessary for the development of the project, be-‐ cause the heat produced by combustion will be put back into the system to dry the new wet sludge, obtaining a final product with about 85% dry matter.
This system is expected to have a strong economic appeal, since it can produce large cost savings: this product is completely new on the market in terms of ex-‐ pected performance, design, technology and dimensions.
6
Chapter 1 – Introduction
The present work is embedded in a field of research started at Ca’ Foscari Uni-‐ versity of Venice and further developed by Solwa srl, a start-‐up company found-‐ ed by Dr. Paolo Franceschetti. He was a former Ph.D. student at Ca’ Foscari Uni-‐ versity studying “Renewable energy and distributed microgeneration”. With the creation of an innovative solar still for water depuration called Solwa, he was awarded with many national and international prizes1 (the system will be de-‐
scribed in Paragraph 2.1). Starting from the knowledge acquired in the develop-‐ ment of the Solwa prototype, other innovative systems were proposed, such as the one for food drying (see Paragraph 2.2).
The prototypes developed are innovative in terms of size, design and technologi-‐ cal features and aim to enter the market as sustainable and environmentally friendly systems, with, at the same time, better performances in terms of costs and efficiency, compared to their competitors. It is from this line of research that the system for sludge drying and burning has been designed.
The structure of the thesis will be organized as follows: there will be a descrip-‐ tion of wastewater and sewage sludge treatment and management, and the exist-‐ ing technologies for its disposal; the regulatory framework will be described and the current emissions from sludge incineration, as described in literature, will be analysed. Then, the two prototypes (solar still and food drying) will be intro-‐ duced, and the sludge drying and burning project will be fully described. Finally, the experimental part will be presented, including materials and method applied, presentation and discussion of the results, and conclusions derived from the whole project.
1 2010: Solwa solar still has been included in the IDEASS UN Program for the “Innovation for Development
of Humanity”.
2011: MIT – Massachusetts Institute Of Technology (Boston – USA): Solwa project is awarded as “Italian innovation of the year” by the Journal “Technology review”.
2012: Solwa srl. is awarded first place by Huffington Post (World) in an evaluation of the 10 technological successes of the year worldwide.
2012: Gaetano Marzotto award (Vicenza – Italy): winner of the award “Impresa del futuro” (Firm of the fu-‐ ture). [http://www.solwa.it]
7 1.1 Wastewater treatment
Wastewater Treatment (WWT) is a process that makes water suitable for partic-‐ ular uses, such as drinking, industry or medicine. Depending on the end use, the process is very different. In general, wastewater treatment can be divided into three categories:
1) Purification for domestic use
2) Treatment for industrial application
3) Treatment of the wastewater before discharge or reuse
The different types of treatment also depend on the quality of the original water.
Urban wastewater generally contains a mix of various substances: oxygen con-‐
suming material, sediments, fats, oils, foam, salts, nutrients, pathogens and a lot of other objects that ends up in the discharge.
In WWT, the removed substances are: -‐ the sifted material;
-‐ the material after coarse screening; -‐ foam and sludge.
Sludge is generally the substance produced most abundantly and can be in liquid or semi-‐solid form, with a typical solid content between 0.25 and 12% (Stoddard, et al., 2003). Sludge, then, is the side-‐product of wastewater treat-‐ ment that interests more the purpose of this thesis project because, as it will be presented in the following sections, its disposal is a great issue of modern socie-‐ ty.
1.1.1 A Wastewater Treatment Plant and Sewage Sludge
The wastewater treatment takes normally place in one assigned area: a conven-‐ tional wastewater treatment plant (WWTP), which comprises different pro-‐ cesses. The result is generally purified water on one side and biological sludge on the other; this is a general scheme:
8
Figure 1 – Basic flow diagram for conventional wastewater treatment plant
It is then important to distinguish the different types of sludge and from which phase of the process they are taken, in order to better understand which might be their composition and physic-‐chemical characteristics. A WWTP in general produces three types of sludge (Reverdy, et al., 2013):
-‐ Primary biological sludge comes from the settling of the effluents, pre-‐ viously suspended in water.
-‐ Secondary biological sludge is produced from the settling of the organic matter (including bacteria). Part of the sludge is regularly removed from the tanks in order to avoid an excess of biomass.
-‐ Tertiary biological sludge or physical-‐chemical sludge (derived from primary sludge): with the addition of a coagulant, the organic matter com-‐ ing from wastewaters is agglomerated; 90% of the suspended matter can be captured and settled, forming tertiary sludge, that contains a major part of water mineral salts and coagulant agent.
The characteristics of sewage sludge are also very different depending on the
origin of the wastewater. In particular, different hazardous compounds can
be present in the sewage from industrial process, depending on the production chain. For example, tannery sludge generally has a high Cr6+ content or sludge
from paper industry contains various bleaching compounds. Another type of sludge is the so-‐called “Red mud” which is a waste product of the production of
aluminium in the mining industry through the Bayer process (refining bauxite
en route to alumina).
One interesting possibility that has to be tested in future research is the incin-‐
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11
It can be observed that every phase has a different result in the sludge composi-‐ tion: the thickener increases the solid content while the digestion reduces the organic content and the amount of bacteria.
1.1.3 Existing sludge–dewatering methods
Large amounts of sludge are produced from a WWTP every day. For this rea-‐
son, various technologies for sludge dewatering have been developed. In this section, the most common existing methods are presented; from an overall per-‐ spective, they can be divided into natural or artificial methods:
-‐ Natural methods: the sludge is arranged on an open-‐air bed where it is dehydrated through evaporation. Reinforced concrete tanks are normally used, with the bottom covered by a draining layer, made of coarse gravel, over fine gravel and sand. The main problem with this method is rainfall, which causes most of the water lost by evaporation to be reintroduced in-‐ to the system. Other disadvantages are the large ground space required, the noxious fumes produced and the possibility of retaining viruses and bacteria in the sludge (with the hazard of spreading pathogens by air). -‐ Artificial methods: they are all the techniques that include the applica-‐
tion of machinery. They can be distinguished into: • Mechanical treatments:
o Filtration: the sludge passes through a filtering medium such as a vacuum pump or pressure plate partially immersed in the sludge itself (belt or filter press, Figure 5).
! "P!
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13 1.2 Wastewater treatment plant at Treviso (Italy)
The WWTP of Treviso is here described as it is where part of the experimenta-‐ tion took place and the sludge samples were collected. It can be also taken as an example of how a common WWTP is organised and which treatment processes operate in it.
The plant of the City of Treviso is located in the area of “Sant’Antonino” (via
Cesare Pavese n. 18) and discharges the treated water into a final dead branch of
the Sile River. The plant has been in operation since 1975 with an original capac-‐ ity of 30000 Population Equivalent (PE) and a conventional line of activated sludge (Regione del Veneto, 2011).
Due to the increase in the hydraulic load and with the aim of ensuring the strict-‐ er standards of new directives, the plant was enlarged and restored until achiev-‐ ing a total capacity of 70000 PE and the presence of machinery both for water and sludge treatment (see Table 1).
Table 1 – Process chain of the WWTP of Treviso (Regione del Veneto, 2011)
Water treatment Sludge treatment
Coarse screen Thickener F and H
Lifting station Anaerobic digester
Grit removal Belt press
Biological process – Anaerobic-‐Anoxic-‐Aerobic
Gasometer
Secondary sedimentation Cogenerator
Disinfection Torch
Figure 6 presents a general scheme of all the treatments applied in the WWTP of Treviso.
14
Figure 6 – Block scheme of the line system of WWTP in Treviso (Regione del Veneto, 2011)
1.3 Sewage sludge management and regulation in Europe
Sewage sludge management is a complex issue worldwide since its production
is continuously increasing, particularly in industrialized countries (Gálvez, et
al., 2007). Due to the high quantitative produced every day, in fact, the treatment of wastewater shows huge expenses for the purification process and for the sludge disposal. It is from this market request of new sludge disposal method that the research described in this thesis take its origin.
In Europe, sewage sludge production was nearly 11 million tons of dry matter (DM) in 2012 and is expected to increase by at least 10% by 2020 (Oikonomidis & Marinos, 2014). The progressive implementation of the Urban
Waste Water Treatment Directive 91/271/CEE on urban WWT in all the EU
Member States has been increasing the amount of sewage sludge requiring dis-‐ posal: this Directive stated that waste waters from agglomeration with more than 2000 Population Equivalent (PE) had to undergo to secondary treatment by
15 the end of 2005: this increased the number of houses connected to the sewage system (EC, 2014a).
In the EU waste disposal (as sludge is considered a waste) is regulated by the
Waste Framework Directive 2008/98/CE (EC, 2014b) that indicates various
measures to protect the environment and human health, based on prevention and reduction of the negative impacts caused by waste production and manage-‐ ment. In particular, risk to water, air, soil, plants or animals should be avoided. The main criteria established by the Directive are summed up in Figure 7:
Figure 7 – Waste management hierarchy established by the Waste Framework Directive (EC, 2014a; EC, 2014b)
In addition, after the promulgation of the Water Framework Directive (Di-‐ rective 2000/60/EC), a number of new treatment plants have been built. This because the Directive requires that all inland and coastal waters within defined river basin districts must reach at least good status by 2015 and defines how this should be achieved through the establishment of environmental objectives and ecological targets for surface waters (EC, 2014a).
A biological sludge is defined as “treated” when it has been subjected to “biolog-‐ ical, chemical or heat treatment, long-‐term storage or any other appropriate pro-‐ cess so as significantly to reduce its fermentability and the health hazards result-‐ ing from its use" (EC, 2014a). In the last decades, the most common ways to treat sludge were landfilling, incineration, agricultural use and alternative fuel in in-‐ dustrial processes. In addition to this, sludge was recycled, including all the pro-‐ cesses that resulted in the reuse of the sludge (Suh & Rousseaux, 2002).
16 The most attractive option for sludge disposal so far has been use in agricul-‐
ture, since the sewage sludge, due to the physical-‐chemical processes involved in
its treatment, is rich in nutrients (mainly nitrogen and phosphorous) and valu-‐
able organic matter, useful when soils are depleted or subject to erosion. This
solution has been, however, recently criticised because the sludge also tends to concentrate heavy metals and poorly-‐biodegradable trace organic compounds and pathogens (viruses, bacteria etc.) present in wastewaters (EC, 2014a). These substances (in particular heavy metals) can cause soil contamination, even though European regulations limit the amount of metallic materials in the sludge.
More recently, another issue has emerged, concerning the organic pollutants sludge can contain, such as pharmaceuticals, pesticides and personal care prod-‐ ucts, which might have an impact on the food chain (Lederer & Rechberger, 2010). European legislation also restricts the final disposal of sludge in landfills, requiring a decrease to 35% biodegradable content in it by 2020 (Stehlik, 2009).
Sludge disposal is then an issue that is going to worsen in the next future: Euro-‐ pean regulations, posing stricter limits regarding landfilling and agriculture, is trying to direct the producer to find new solutions for a safe disposal even though it seems that insufficient effort is brought for a regulation that should fa-‐ vour this alternatives.
1.3.1 Sewage sludge management and regulation in Italy
In Italy the issue of sludge disposal is urgent as well: the amount of sludge pro-‐ duced was around 1.7 million tons of DM in 2005. For sludge from urban wastewater, it was nearly 1 million ton DM/year (ISTAT, 2005).
17
Table 2 – ISTAT (2005) data for sludge amount in Italy
Primary
[Mtons/year] [Mtons/year] Secondary [Mtons/year] Tertiary [Mtons/year] Total
Italy 9002 6049 1850 16901 Northwest 3773 1744 429 5946 Northeast 2752 1109 514 4375 Centre 1452 1482 330 3264 South 907 1165 359 2431 Islands 118 549 218 885
In the Italian regulation, sludge is defined in the Testo Unico Ambientale (D. Lgs. 152/2006) as “treated or untreated residue coming from urban wastewater treatment plant” (art. 74). It is then considered special waste and its manage-‐ ment is governed by part IV of D. Lgs. 152/2006; article 127, however, states that the sludge must be reused whenever it is appropriate. Sludge disposal should then be done by:
-‐ Incineration, aimed at energy recovery of the sludge itself or together with the organic fraction of the urban wastes.
-‐ Disposal in controlled landfills for special waste (resumed from D. Lgs. 36/03 and D.M. 03/08/2005).
In addition, D. Lgs. 99/1992 (embodied in the D. Lgs. 152/2006), which was the implementation of the European Directive 86/278/CEE, regulates the use of sewage sludge in agriculture. This Directive enhances the use of sludge for agri-‐ cultural purposes by spreading onto the soil, trying at the same time to prevent eventual harmful effects on agricultural land, vegetation, animals and man. Com-‐ posting of the sludge is suggested for a subsequent use in agriculture, together with reuse in the production of bricks, asphalt and concrete. The spread onto the soil of untreated sludge, however, is forbidden, unless it is injected or incorpo-‐ rated into the soil, and whenever its heavy metals content does not meet the re-‐ quirements of the legislation.
1.3.2 Different Situations in European countries
The situation of sludge production and disposal varies widely by country: this section will briefly present the actual situation in European Union Member
States (EU-‐27) and the future predictions by 2020 provided by Kelessidis &
18
In the European Union countries, specific sludge production ranges from 0.1
kg per PE and year (Malta) to 30.8 kg per PE and year (Austria) (Kelessidis &
Stasinakis, 2012). As said before, the legislation on this sector is outlined by the European Directive 86/278/EC but many countries have adopted more strin-‐
gent regulations, with lower limit values for heavy metals, organic micropollu-‐
tants and pathogens (Kelessidis & Stasinakis, 2012).
The type of treatments adopted, as well, varies between countries: the most popular stabilisation methods are aerobic (in 24 countries) and anaerobic di-‐ gestion (in 20 countries); mechanical sludge dewatering is generally preferred comparing to the use of drying beds in all Member states (EU-‐27), while thermal
drying is the most common practice in old Member States (EU-‐15 countries), in
particular in Germany, Italy, France and United Kingdom.
Regarding the final disposal of sludge, reuse is the most applied in EU-‐15 coun-‐ tries (53% of produced sludge), followed by incineration (21% of produced sludge), while in the new Member States that joined EU after 2004 (EU-‐12), the most common disposal method is still landfilling (Kelessidis & Stasinakis, 2012).
Based on current trends, Kelessidis & Stasinakis (2012) have also produced fore-‐ casts for sludge management by 2020, dividing the countries into five groups:
• Group 1 – Increasing agriculture use only: France, Malta. • Group 2 – Status quo: Germany, Estonia, Netherlands, Cyprus.
• Group 3 – Increasing incineration only: Austria Portugal Slovakia, Hungary, Belgium (mainly) and Latvia, Denmark, Ireland, Luxembourg (less).
• Group 4 – Increasing (mainly) agriculture and incineration: Sweden (major shift to composting and co-‐incineration), Czech Republic (com-‐ posting), Lithuania, Poland (composting), Romania, Slovenia, United Kingdom.
19 • Group 5 – Increasing agriculture and (mainly) incineration: Spain, It-‐
aly (composting), Bulgaria, Finland, Greece (Kelessidis & Stasinakis, 2012).
It is then interesting to observe that the situation is very faceted: EU-‐12 coun-‐ tries will temporary enhance landfilling since they will have to face at first a ur-‐ gent situation but then up to 2020 an increase in agricultural reuse can be ex-‐ pected. Regarding EU-‐15 countries, agricultural recycling and incineration seems to be the practices most favourable to be adopted by 2020.
1.4 Biological sludge–drying
Figure 8 – Distribution of sludge drying plants in European countries (Kelessidis & Stasinakis, 2012)
Sludge drying is here analysed more deeply, since the projected system will ex-‐ ploit both thermal drying and incineration (see Paragraph 1.5).
Drying the sludge, in fact, is a possibility to reduce the expenses for its disposal; this is an important step showing many positive results (Oikonomidis & Marinos, 2014):
1. Drying means a reduction in mass and volume, thus saving in transpor-‐ tation costs because the sludge is often moved by truck from the produc-‐ tion site to the place of final use; a smaller amount of sludge thus means
less costs for transport, handling and storage. This is also an envi-‐ As far as concerning the new EU countries, Czech Republic is the
region’s leader in sludge management innovation (Le Blanc et al., 2008). This can be illustrated by the full-scale use of mechanical sludge disintegration and the use of sludge lysate being produced during the disintegration or by rich experience on thermophilic anaerobic digestion (Zabranska et al., 2009). Such innovative tech-niques of disintegration by mechanical (ultrasound, mills, homog-enizers), thermal, chemical (acids, lyes) and biological (enzymes) means have also been studied and applied mainly in Germany and less in Sweden and Italy with encouraging results (WPCF, 1989; Kunz et al., 1996; Lee and Welander, 1996; Sakai et al.,
1997; Krogmann et al., 1997; Muller, 2000; Le Blanc et al., 2008).
On the other hand, sludge dewatering seems to be an important step in sludge management of most EU-27 countries. According to
Table 3, the majority of European WWTSs use mechanical
dewatering instead of drying beds that are preferred mainly in small WWTSs and are reported in 6 out of 27 European countries. From financial point of view, the prevailing sludge dewatering technologies in descending order are centrifuges (41%), belt filter presses (28%) and filter presses (23%) (www.frost.com).
Regarding other sludge treatment methods applied in European countries, thermal drying has prevailing position in sludge man-agement of EU-15 (Table 3). It should be mentioned that 110 ther-mal drying plants were operated in EU in 1995 (Hall, 1995), the drying lines were increased to about 370 in 1999 (EC, 1999), while today they exceed 450. Most of these plants constitute the first stage of incineration units.Fig. 1represents distribution of sludge drying plants in European countries. Except of Luxembourg and Finland, all EU-15 countries apply this technology (Drace
medio-ambiente, 2010; Milieu Ltd., WRc and RPA, 2010). AsFig. 1reveals,
the big majority of thermal drying plants (almost half of them) are operated in Germany, following by Italy, UK and France. Rotary Drum Dryers (RDD) is the most commonly used system, following by other types as Fluidized Bed Dryers (FBD) or Belt Dryers (BD)
(http://www.web4water.com/library/print.asp?id=3539;
Arlabos-se et al., 2012). An innovative method called Direct Microwave
Drying has also been used in Ireland (Turovskiy and Mathai, 2006). On the other hand, excepting Slovenia, there are no thermal drying units in the new EU-12 countries (Fig. 1).
Long term storage is also applied in several old or new Member States (9 out of 27) as it is an easy and cheap method for sludge
management but it requires proper climates and great areas. Other methods such as cold fermentation, solar drying or pasteurization are scarcely referred in a limited number of countries (Table 3). 5. Sewage sludge disposal in EU-27
The change of sewage sludge disposal methods in EU-15 after implementation of 91/271 Directive (CEC, 1991) can be seen in
Fig. 2. It should be mentioned that the most recent available data
for all European countries are these of 2005. According to the re-sults, landfilling presents a significant and continuing decrease be-tween 1992 and 2005, from 33% to 15%. On the other hand, sludge incineration has been almost doubled (from 11% to 21%), following the estimate-target (EC, 1999). Biosolids reuse, which mainly in-cludes agricultural utilization and composting, has been slightly increased, while an important part of total sludge production (9% in 2005) has been managed using several practices. This part of sludge is reported in several reports as ‘‘others’’ and include meth-ods such as pyrolysis, temporary storage (e.g. Greece, Italy), long storage (e.g. Poland, Estonia, Lithuania), reuse in green areas and forestry (e.g. Ireland, Latvia, Slovakia), landfill cover (e.g. Sweden, Flanders), exportation of sludge amounts to other countries (e.g. granulated sludge from Netherlands to Germany for incineration, sludge for composting or incineration from Luxembourg to Ger-many) as well as possible differences between total sludge produc-tion and disposal amounts. Besides the banning of sludge dumping to the sea after 1998, it is possible that high values of ‘‘others’’ ob-served in 2000 could also be due to the continued apply of this practice in some European countries (EC, 2004).
Sludge disposal methods for year 2005 in EU-15 and EU-12 are presented inFig. 3a and b, respectively. As it can be seen, the phi-losophy of sludge management is quite different between old and new Member States. The prevailing technology in EU-15 is recy-cling in agriculture (44%). In contrary, the status in new countries is quite unclear, as for 35% of sludge no specific disposal manner is declared. This uncertainty mainly originates from Poland which, as it was mentioned in Section 2, is the greatest sludge producer in EU-12 countries. Almost half of produced sludge in Poland (48%) has no specific outlet, while according to data reported in BIOPROS project (2006), it seems that this percentage include stockpiling and lagooning. Taking into account this notice, landfilling (28%)
0% 10% 20% 30% 40% 50% 60% Germany Italy UK France Netherlands Spain Austria Denmark Belgium Ireland Sweden Portugal Greece Slovenia
Sludge drying plants in European countries (%)
Fig. 1. Distribution of sludge drying plants in European countries (EC, 1999; Le Blanc et al., 2008; HMEPPPW, 2009; Drace medioambiente, 2010; http://www.wat-erworld.com; http://andritz.com; http://www.environ.ie/en/; http://www.web4-water.com;http://www.waterworld.com;http://www.hse.gov.uk).
48 47 57 50 54 11 13 15 18 21 33 29 18 18 15 6 5 4 2 6 6 14 9 0 10 20 30 40 50 60 70 80 90 100 1992 1995 1998 2000 2005 Year
Sludge disposal methods used in EU-15 (%)
Reuse Incineration Landfill Surface waters Others
Fig. 2. Sludge disposal methods applied in EU-15 between 1992 and 2005 (year 1992 does not include Italy, Sweden, while year 1998 does not take into account Italy due to lack of data) (http://epp.eurostat.ec.europa.eu;Hall, 1995; EC, 1999, 2004, 2006; EEA, 2002; BIOPROS, 2006; HMEPPPW, 2007; Milieu Ltd., WRc and RPA, 2010).
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21
fans discharge the saturated air stored in the plant and provide air re-‐
newal.
3) Robots or drums with rotating scrapers (or similar devices) renew the sludge by mixing it.
A SSDP can be pure when the only source of energy is the solar one, or hybrid when there is a second source, e.g. waste heat from combined heat and power (CHP) unit (Oikonomidis & Marinos, 2014).
The economic and environmental advantages (since no fossil fuel are burned) of solar systems are clearly described in Figure 10, an analysis provided by the multinational Parkson Company (the worldwide leader in the field) (Parkson & KET, 2010), which compares the traditional technology with the solar one (gas
fired dryer costs vs solar dryer costs):
Figure 10 – Thermal energy consumption comparison between gas-‐fired and solar dryer (Parkson & KET, 2010)
It is starting from an analysis of the current situation on sludge disposal by dry-‐ ing that the idea of an innovative system came out, since the market seems to be very favourable to this new disposal solution.
1.5 Biological sludge incineration
The innovative idea behind the projected system is that solar/thermal drying should be combined with incineration of the dry sludge (for a fully description
22 see Paragraph 2.3). The incineration is here described for a better comprehen-‐ sion of the mechanism involved in the system.
Incineration can be defined as the complete combustion with a rapid exo-‐
thermic oxidation of the fuel elements contained in the sludge: it requires
temperatures of 420°-‐500° C and the presence of oxygen. Complete combustion of all organic solids requires temperatures over 760°-‐820°C. (Turovskiy & Mathai, 2006). During the incineration process, the organic components of sludge are converted into oxidised end products, such as carbon dioxide (CO2), water vapour and ash; particulates and other gases could also be present in the
final product, which is why these gases are brought into a post-‐combustion chamber before being released into the atmosphere.
Figure 11 – Heating Values of Sludge and Other Residuals (Turovskiy & Mathai, 2006)
One of the principal parameter that has to be taken into account in the sludge incineration is moisture content. Sludge cake with 30 to 50% of solids content (50 to 70% of moisture) is autogenous, which means that it can be burned without auxiliary fuel. Percentages lower than those ones (20-‐30% solids) could indicate that an auxiliary fuel for combustion is required. For this reason, there is the necessity of reducing the moisture content of the sludge by mechanical dewatering or thermal drying before incineration (Turovskiy & Mathai, 2006).
Another important parameter of sludge incineration is the heating value of sludge. It represents the quantity of heat released per unit mass of solids. The amount of heat released from sludge is a function of the types and com-bustible elements present in sludge. The primary comcom-bustible elements in sludge (and in most available auxiliary fuels) are carbon, hydrogen, and sulfur. Carbon burned to carbon dioxide has a heating value of 34 MJ/kg
(14.6 × 103 Btu/lb), hydrogen has a heating value of 144 MJ/kg (62 × 103 Btu/
lb), and sulfur has a heating value of 10 MJ/kg (4.5 × 103 Btu/lb).
Conse-quently, any changes in the carbon, hydrogen, or sulfur content of sludge will raise or lower its heating value. Table 8.4 shows the heating values of various types of sludge, grease and scum, and screenings.
8.3.1 Methods of Incineration
The process of sludge incineration in furnaces can be divided into the follow-ing stages: heatfollow-ing, dryfollow-ing, distillation of volatile matter, combustion of the organic fuel matter, and calcination to burn the residual carbon. Heating the sludge to 100°C (212°F) and then drying it at about 200°C (392°F) consume the principal quantity of heat and are generally required for the incineration process. These parameters also affect the selection of the size of the main and auxiliary equipment and consequently, determine the cost in general. In the course of moisture evaporation in the drying zone, volatile substances are liberated together with the moisture, which sometimes results in objection-able odors.
The combustion of the sludge takes place at temperatures between 200 and 500°C (392 and 932°F), due to the thermal radiation of the fl ame and the incandescent walls of the combustion chamber, as well as the convection heat transfer from the exhaust gases. The calcination of the ash fraction of the sludge is completed by its cooling to a temperature at which it can be removed from the site.
The design temperature in the furnace should not exceed the melting point of ash [usually, about 1050°C (1922°F)] and should not be below 700°C (1292°F), thus providing reliable deodorizing of the gases. Systems for sludge incineration should provide complete combustion of the organic fraction of the sludge and utilization of the heat of the exhaust gases.
INCINERATION 291
TABLE 8.4 Heating Values of Sludge and Other Residuals Dry Solids
Type of Sludge/Residual MJ/kg Btu/lb
Primary sludge 20–28 8600–12,000
Activated sludge 16–22 6,900–9,500
Digested sludge 10–15 4,300–6,500
Grease and scum 39 16,800
23
Figure 12 – Advantages and Disadvantages of Incineration (Turovskiy & Mathai, 2006)
As said in Paragraph 1.3.2, incineration (together with agricultural recycling) seems to be the practice most applied by 2020 in the Old Member States (EU-‐ 15) of the European Union (which are also the more industrialised). In these countries thermal treatment with energy recovery is expected to have a share
till 37% (double compared to EU-‐12 countries) (Kelessidis & Stasinakis, 2012).
There has been already important improvements in the incineration tech-‐ niques during the last years, in terms of technological level, cost reduction and
environmental protection. Innovative technologies, such as pyrolysis or phos-‐
phorous recovery from sewage sludge have been already developed in large-‐ scale project worldwide but it is possible that current technology, like co-‐ incineration in coal-‐fires, use of cement kilns plants of incineration of Municipal Solid Waste (WSW) will still be preferred in the following years (Kelessidis & Stasinakis, 2012). One key point is that the adoption of sludge incineration tech-‐ nologies is strictly related to the adoption of drying technology, being the ne-‐ cessity of increase sludge heating value and transfer cost reduction. A favoura-‐
ble alternative to current drying technologies, as described in the previous sec-‐
tion, is solar drying.
290 THERMAL DRYING AND INCINERATION
or supplement plant heating requirements. The dried sludge itself has a fuel value and may be used as a heat source for the drying medium.
8.3 INCINERATION
Incineration is complete combustion, which is the rapid exothermic oxidiza-tion of combustible elements in sludge. Dewatered sludge will ignite at temperatures of 420 to 500°C (788 to 932°F) in the presence of oxygen. Tem-peratures of 760 to 820°C (1400 to 1508°F) are required for complete combus-tion of organic solids. In the incineracombus-tion of sludge, the organic solids are converted to the oxidized end products, primarily carbon dioxide, water vapor, and ash. Particulates and other gases will also be present in the exhaust, which determines the selection of the treatment scheme for the exhaust gases before venting them to the atmosphere.
The principal advantages and disadvantages of incineration over other methods of sludge stabilization are listed in Table 8.3. Sludge is incinerated if its utilization is impossible or economically infeasible, if storage area is limited or unavailable, and in cases where it is required for hygienic reasons.
One of the principal parameters of sludge incineration is the sludge mois-ture. Sludge cake with 30 to 50% solids (50 to 70% moisture) is autogenous; that is, it can be burned without auxiliary fuel. Sludge cake with 20 to 30% solids (70 to 80% moisture) may require an auxiliary fuel for combustion. Therefore, before incineration, the moisture content of the sludge should be reduced by mechanical dewatering or thermal drying.
TABLE 8.3 Advantages and Disadvantages of Incineration
Advantages Disadvantages
1. Reduces the volume and weight of wet 1. High capital and operating costs. sludge cake by approximately 95%, 2. Reduces the potential benefi cial thereby reducing disposal requirements. use of biosolids.
2. Complete destruction of pathogens. 3. Highly skilled and experienced 3. Destroys or reduces toxins. operating and maintenance staffs 4. Potentially recovers energy through the are required.
combustion of waste products, thereby 4. If residuals (ash) exceeds the reducing the overall expenditure prescribed maximum pollutant
of energy. concentrations, they may be
classifi ed as hazardous waste, which requires special disposal. 5. Discharges to atmosphere (particulates and other toxic or noxious emissions) require extensive treatment to assure protection of the environment.
24 Despite the many improvements of the last years, incineration presents some
important issues: in the emissions, dioxins and furans are often detected,
which are very hazardous compounds, and heavy metals are generally released;
flue gases and ashes (side-‐products of the process) lead to high cost of treat-‐
ment, while solid residues present the problem of their handling (Fytili & Zabaniotou, 2008).
1.5.1 Advantages of sludge-‐drying and incineration
Drying combined with incineration is a sludge disposal method that clearly opens to wide economic chances: many sludge-‐producing plants, in fact, have to transport the sludge by truck from the site of production to a treatment plant, which could be many kilometres far from it. The cost of transport, together with the price required for the treatment, are generally huge expenses for the plant’s owner. Suffice to say that 50-‐60% of the management costs of a depura-‐ tion plant are made by sludge treatment and disposal.
In Italy, the cost of sludge (20% DM) disposal can be indicatively estimated as
100-‐300 euro/ton, depending on the distance between the place of disposal and
the depuration plant, on the type of disposal (landfilling, composting, spreading etc.) and on the region in which the plant is located. This expense regards a product that is 80% water: it clearly emerges that, removing the water content of the sludge, dramatically reduces the cost of disposal. Incinerating the sludge gives, in fact, an amount of ashes, which is approximately 10% in volume com-‐ pared to the starting product (20% DM sludge). This would mean that the pro-‐ ducer should have to dispose of an amount of special waste significantly low-‐
er. Future research, however, will have to focus on the emissions that the pro-‐
cess produces into the atmosphere, in order to see if there could be a hazard for the human heath (see following section).
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