Linköpings universitet SE–581 83 Linköping
Linköping University | Department of Thematic Studies - Environmental Change
Master’s thesis, 30 ECTS | Engineering Biotechnology
2021 | LIU-TEMA/LITH-EX-A--2021/--SE
Hydrolysis of waste activated
sludge from pulp and paper mills
- effect on dewatering properties
and biogas potential by utilizing
existing side streams
A case study of two pulp- and paper mills in Sweden
Louise Hjalmarsson
Supervisor: Eva-Maria Ekstrand (LiU) External supervisor: Anna Karlsson (SBF) Examiner: Alex Enrich Prast
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Abstract
A big challenge within pulp and paper mills is the large quantities of waste activated sludge (WAS) that is produced during the wastewater treatment. The WAS is made up of biological cells and extra polymeric substances (EPS) and can bind a large amount of water causing difficulties to dewater the WAS. This study aimed to determine how to improve the dewatering properties of the WAS by using hydrolysis. Hydrolysis will cause the cells to disrupt and the bound water in the cells and the water trapped by the EPS can be re-leased. Specifically, this study investigated what impact hydrolysis with heat, alkalis, and acids had on the WAS dewatering properties. In addition to the impact on the dewatering properties, the release of organic material and nutrients from the cells has also been impor-tant for biomethane production. In this study, it was specifically NH4– N, PO43 –and COD
that have been studied. WAS from paper mills have in general poor methane potential so it was of interest to see how the WAS was affected by hydrolysis and how hydrolysis could improve the methane production.
To test the hypothesis of whether hydrolysis could affect the WAS and improve the dewa-tering properties, several experiments were performed. The experiments included thermal hydrolysis at temperatures of 70-90˝C, acidic hydrolysis with acids such as spent acid and
acid water, and alkalis such as green liquor sludge and EOP. All acids and alkalis used in the study were chemicals that exist at the paper mills included in this study. To test the dewatering properties, methods such as TS analysis on the accept, CST-analysis, and a belt press were used. Analysis was also performed on the reject to measure the suspended solids and the nutrients NH4– N, PO43 – and COD in the WAS. This study did also
deter-mine what effect hydrolysed WAS had on the biomethane potential. In this study were the paper mills BillerudKorsnäs in Skärblacka and SCA in Östrand included. Hence was sludge from the two mills of interest to analyse.
This study has shown better dewatering properties with an increase in the total solids (in the accept) after the thermal hydrolysis, the acidic hydrolysis with spent acid, and the alkali hydrolysis with green liquor sludge. Specifically did the acidic hydrolysis with spent acid improve the dewatering properties in terms of an increase in TS in %. The biggest increase in TS in % could be seen after using 10% spent acid ratio. The TS for the WAS from SCA Östrand increased in this experiment by 107 %. The thermal hydrolysis also showed promising results both in terms of dewatering properties and in the release of organic material.
The biochemical methane potential test results showed a better and more rapid stabilized production of biomethane after hydrolysis of WAS compared to untreated WAS. The ther-mal hydrolysis both increased the rate of production and the total amount of methane produced. The thermally hydrolysed WAS from SCA Östrand improved the methane pro-duction from 77 Nml methane/g VS to 95 Nml methane/ g VS. The WAS from BillerudKo-rsnäs improved the methane production from 40 Nml methane/ g VS to 55 Nml methane/ g VS.
These results, both from the methane potential tests and the results of the increased dewa-tering properties, show that the concept with hydrolysing should be evaluated further for improving the dewatering of the WAS.
Acknowledgements
Five years of study have soon come to an end and this master´s thesis summarizes the expe-riences and knowledge I have gained during my years at Linköping University. But I would never have been able to complete this essay without all the help and feedback I received while working on this essay.
Therefore I would like to thank my supervisors Anna Karlsson and Eva-Maria Ekstrand for all your valuable insights, your feedback, your help, and for always answering my questions. Also thanks to my examiner Alex Enrich Prast for taking an interest in this report.
I would also like to thank all personnel at the lab of Scandinavian Biogas for your patience when I performed my analyses and for performing the biomethane potential test for me. I would also like to thank the reference group of AFRY, for always answering my questions and given me valuable feedback during this thesis.
Last but not least I would like to thank my friends and family for always supporting me whenever I need your help♥
Louise Hjalmarsson Linköping, 18 May 2021
Sammanfattning
Pappers- och massaindustrierna står idag inför ett stort problem gällande den stora mängd slam som dagligen produceras vid reningen av det interna avloppsvattnet. Avloppsvattnet som bildas i både massatillverkningen och papperstillverkningen renas idag genom åtmin-stone två steg där det första steget, primärsedimenteringen, tar bort större material. Det återstående rejektet går sedare vidare till en luftad biologisk process där mikroorganismer bryter ned det organiska materialet i avloppsvattnet. Slammet som bildas i denna process är av biologisk karaktär och utgör svårigheter för bruken att hantera och avvattna.
Det producerade bioslammet utgör en stor del av pappersindustriernas avfall. På grund av den stora volym som dagligen produceras har pappersindustrierna ständigt ett behov av att hitta nya lösningar för att kunna hantera slammet på ett bättre sätt. Idag förbränns slammet ofta tillsammans med det slam som produceras i det första steget i reningsprocessen, även kallat fiberslam, för att kunna uppnå tillräckligt höga torrhalter.
Tidigare forskning har visat på att slamhydrolys kan förbättra avvattningsegenskaperna. Med slamhydrolys kan komplexa organiska molekyler sönderdelas och en större del av det organiska materialet frigörs då från cellerna. Utöver att organiskt material frigörs från cellerna, frigörs även bundet vatten från cellerna vilket förbättrar avvattningen.
De metoder för hydrolys som undersökts i tidigare studier är baserade på kemiska, termiska, mekaniska och biologiska principer som kan användas var för sig eller i kombination. I den här studien har främst kemiska och termiska hydrolysmetoder använts.
Syftet med denna studie var att förbättra slammets avvattningegenskaper och undersöka om det gick att göra med de sidoströmmar och den restvärme som redan finns tillgängliga på pappersbruken. I den här studien har därför värme, restsyra, grönlutslam, EOP, bakvatten, CTMP-totalavlopp och syravatten använts, vilka alla är befintliga sidoströmmar på pappers-bruken BillerudKorsnäs Skärblacka och SCA Östrand som är med i den här studien. Ström-marna har valts ut baserat på deras pH, temperatur, deras sura samt alkaliska egenskaper i hopp om att dessa skulle kunna verka hydrolyserande på slammet.
Slammet blandades med de utvalda kemikalieströmmarna och analyserades därefter för att ta reda på hur slammets egenskaper förändradrades av hydrolysen. För att undersöka om värme kunde förbättra avvattningsegenskaperna, placerades slam i vattenbad som hade tem-peraturerna 70-90˝C. Hydrolys med de kemiska strömmarna skedde genom att tillsätta dessa till slammet till olika koncentrationer och låta blandningen stå i 1 timme.
Det som har kunnat konstateras i den här studien är att värme har kunnat förbättra avvat-tningsegenskaperna i slammet. Torrsubstansen (TS) för slammet från BillerudKorsnäs ökade med 14.4 % efter 80 graders värmehydrolys i 2h. För slammet från SCA Östrand ökade TS med 11.6 % efter 90 graders värmehydrolys i 1h. Dessutom har de organiska materialet i slammet ökat så pass mycket att den biokemiska metanpotentialen undersöktes efter värme-hydrolysen. Även restsyran visade sig kunna förändra slammets egenskaper så pass mycket att avvattningegenskaperna förbättrades. Resultaten från biometanpotentialtesten visade på att det värmehydrolyserade slammet gav ett bättre totalt utbyte i metanproduktion än obe-handlat slam. Det värmehydrolyserade slammet resulterade i en slutlig produktion på 55
Nml metan/g VS jämfört mot 40 Nml/g VS för ohydrolyserat slam från BillerudKorsnäs. För slammet från SCA Östrand resulterade värmehydrolysen i en slutlig metanproduktion på 95 Nml metan/g VS jämfört mot 77 Nml metan/g VS för ohydrolyserat slam.
Förhoppningen är att resultaten från denna studie kan leda till förbättrad avvattnningen och att man sedan kan avvattna bioslammet och fiberslammet var för sig. Andra förhoppningar är även att man kan föra tillbaka en del av de närsalter som löses ut från cellerna till den biologiska reningsprocessen. Därmed skulle man kunna minska behovet av externt tillförda näringsämnen. Vidare är också förhoppningen att genom att utnyttja den restvärme och de kemiska sidoströmmar som finns på bruken, att detta skulle kunna förbättra slamhanteringen samt förbättra industrins cirkularitet och resursutnyttjande.
Definitions
Acid water Acid from the tallow separation in the process at SCA Östrand Biochemical Methane Potential
(BMP) Determines the methane potential of sludge
Capillary Suction Time (CST) Method and device used to evaluate the water holding capacity of sludge
Crown Press/Belt press Belt press simulator used to dewater the sludge a form the sludge to a solid cake
CTMP effluent (total) Effluent from the chemical thermo-mechanical pulping (CTMP) process at SCA Östrand (Sv: Totalavlopp)
EOP An alkaline bleaching step in the pulp and paper process.
E=extraction, O=oxygen and P=Peroxide
Green Liquor Sludge Alkaline solid by-product from the chemical recovery process at pulp and paper mills
Hydrolysis The breaking of chemical bonds in a reaction involving water. In a biological context, it will cause the cells to disrupt
Primary treatment
The first step in the wastewater treatment process where larger material are removed with a combination of filtration and sedi-mentation
Primary Sludge Sludge composed of fibers that sediments in the first step in the wastewater treatment
sCOD
Soluble chemical oxygen demand. A method to determine the concentration of organic material in for example wastewater after the supernatant have been filtered
Secondary Treatment The second step in the wastewater treatment process where or-ganic material is removed through a biological process
Suspended solids Small particles that are kept liquid/suspended in a solution Spent acid Acidic sidestream in the paper process
Total solids The material that is left in a sample after being heated
Waste activated Sludge (WAS) Sludge produced in the secondary treatment which mainly con-sist of biomass
per-Contents
Abstract iii Acknowledgments iv Sammanfattning vi Definitions viii Contents ix 1 Introduction 11.1 Aim and Research questions . . . 2
1.2 Limitations . . . 2
1.3 Goals connected to the research questions . . . 2
2 Background 4 2.1 Pulp- and paper process and the generation of WAS . . . 4
2.2 The wastewater treatment process at pulp- and paper mills . . . 5
2.2.1 Primary treatment . . . 6
2.2.2 Secondary treatment - aerated process . . . 6
2.3 Characteristics and water holding properties in WAS . . . 7
2.4 Hydrolyse methods to improve dewatering . . . 8
2.4.1 Thermal hydrolysis . . . 8 2.4.2 Alkali hydrolysis . . . 8 2.4.2.1 pH and alkalinity . . . 8 2.4.3 Acidic hydrolysis . . . 9 2.5 Assessment of dewaterability . . . 9 2.5.1 Belt press . . . 9
2.5.2 Capillary suction time test . . . 10
2.6 Effect of hydrolysis . . . 10
2.6.1 Total solids and volatile solids . . . 10
2.6.2 Suspended solids . . . 11
2.6.3 COD, NH4– N and phosphorus measurements on the reject . . . 11
2.7 Anaerobic digestion . . . 11
2.7.1 Biochemical methane potential test . . . 12
2.8 Statistical determination . . . 12
3 Paper mills involved in the thesis 13 3.1 BillerudKorsnäs - Skärblacka . . . 13
3.2 SCA Östrand . . . 13
4 Materials and method 15 4.1 Overview of analyses . . . 15
4.2 Sludge and sidestreams used . . . 16
4.3 Protocols for hydrolysis . . . 17
4.3.1 Thermal hydrolysis . . . 17
4.3.2 Acidic hydrolysis . . . 17
4.3.2.1 Hydrolysis with residual peroxide and fibers from CTMP ef-fluent . . . 18
4.3.2.2 Hydrolysis with heat and acids . . . 18
4.3.3 Alkaline hydrolysis . . . 19
4.3.3.1 Hydrolysis with NaOH as control on WAS from SCA Östrand and BillerudKorsnäs Skärblacka . . . 20
4.4 Analytical methods . . . 20
4.4.1 Centrifugation . . . 20
4.4.2 Total Solids (TS) and volatile Solids (VS) . . . 20
4.4.3 Suspended solids . . . 21
4.4.4 Alkalinity . . . 21
4.4.5 pH . . . 22
4.4.6 Acidity . . . 22
4.4.7 Measurement of NH4– N, PO43 –, and sCOD . . . 22
4.4.8 Biochemical methane potential test . . . 22
4.5 Assessment of dewatering properties . . . 23
4.5.1 Dewatering with belt press . . . 23
4.5.2 Capillary suction time . . . 24
4.5.3 Statistical analysis . . . 24
4.5.4 Sludge volume index - SVI . . . 24
5 Results 25 5.1 Results on untreated WAS - i.e Controls . . . 25
5.1.1 Sludge Volume Index . . . 26
5.2 Overview of results - BillerudKorsnäs Skärblacka . . . 27
5.3 Overview of results - SCA Östrand . . . 28
5.4 Thermal hydrolysis . . . 29
5.4.1 Analysis of TS . . . 29
5.4.2 Analysis of suspended solids . . . 29
5.4.3 Analysis of sCOD . . . 30
5.4.4 NH4– N analysis . . . 30
5.4.5 Phosphate analysis . . . 31
5.4.6 CST analysis . . . 31
5.5 Acidic hydrolysis with spent acid . . . 32
5.5.1 TS analysis . . . 32
5.5.2 Analysis of suspended solids . . . 33
5.5.3 Analysis of sCOD . . . 33
5.5.4 Analysis of phosphate . . . 34
5.5.5 CST analysis . . . 34
5.6 Alkaline hydrolysis using green liquor sludge (GLS) and NaOH - BillerudKo-rsnäs . . . 35
5.6.1 Hydrolysis with NaOH as reference . . . 35
5.6.2 TS analysis . . . 35
5.6.3 Analysis of sCOD . . . 36
5.6.4 Analysis of phosphate . . . 36
5.7 Results on the dewatering properties using belt press and polymers . . . 37
5.8 Results of the BMP test . . . 37
6.1 The impact of thermal hydrolysis on WAS . . . 39
6.1.1 The impact of acidic hydrolysis with spent acid on the WAS . . . 41
6.1.2 Hydrolysis with GLS . . . 43
6.1.3 Result from the dewatering using belt press . . . 44
6.2 Biochemical methane potential test . . . 44
6.3 General discussion regarding the research questions and the goals . . . 45
7 Conclusion and future work 47 7.1 Conclusion . . . 47 7.2 Future work . . . 48 Bibliography 49 A Appendix 55 A.1 Experiments . . . 55 B.2 Results - BillerudKorsnäs . . . 57
B.2.1 Alkaline hydrolysis using EOP . . . 58
B.2.1.1 Analysis of total solids and suspended solids . . . 58
B.2.1.2 Analysis of sCOD and phosphate . . . 58
B.2.1.3 CST analysis . . . 58
B.2.2 Alkaline hydrolysis using green liquor sludge (GLS) . . . 59
B.2.2.1 Analysis of total solids and suspended solids . . . 59
B.2.2.2 Analysis of sCOD and phosphate . . . 59
C.3 Results - SCA Östrand . . . 60
C.3.1 Acidic hydrolysis with acid water . . . 61
C.3.1.1 Analysis of total solids . . . 61
C.3.1.2 Analysis of sCOD . . . 61
C.3.1.3 Analysis of phosphate . . . 61
C.3.2 Acidic/peroxide hydrolysis with CTMP . . . 62
C.3.2.1 Analysis of total solids and suspended solids . . . 62
C.3.2.2 Analysis of sCOD and phosphate . . . 62
C.3.3 Alkaline hydrolysis with green liquor sludge (GLS) . . . 63
C.3.3.1 Analysis of total solids and suspended solids . . . 63
C.3.3.2 Analysis of CST . . . 63
C.3.3.3 Analysis of sCOD and phosphate . . . 63
C.3.4 Hydroxide hydrolysis with white water effluent (Bleaching filtrate) . . . 64
C.3.4.1 TS analysis . . . 64
C.3.4.2 Analysis of sCOD and phosphate . . . 64
D.4 Titration curves . . . 64
D.4.1 Titration curve of EOP . . . 65
D.4.2 Titration curve of waste activated sludge . . . 65
D.4.3 Titration curve of green liquor sludge . . . 65
D.4.4 Titration curves of spent acid . . . 66
E.5 Statistical determination of BMP test . . . 67
1
Introduction
A major challenge within the pulp and paper industry is the large amount of excess sludge that is produced daily [1]. A mapping of the sludge at the pulp and paper mills in Sweden shows that the quantities of sludge are around 200 000 – 300 000 ton annually [2]. To manage those large quantities incineration of the sludge is often used. But the high water content in the sludge makes the management of the sludge costly and can stand for up to 65 % of the total operating costs at paper mills [3].
The produced sludge is unavoidable since it is a result of the required wastewater treatment process. The wastewater treatment process removes large material and toxic particles in order to release the water to nearby recipient or reuse the water again in the process [4].
The wastewater treatment process contains a primary treatment step and secondary treat-ment step [5]. In the primary step, large particles are removed, and the sludge produced in this step, also referred to primary or fiber sludge, will sink to the bottom of the basin due to gravitation [5]. The second step in the wastewater treatment process is usually a biological step and usually consist of an aerobic process. In this step, a lot of the remaining organic material (dissolved and suspended) is removed and the sludge produced in this step is called waste activated sludge (WAS) [6]. The WAS mainly consist of biomass produced in the degra-dation of the organic material in the aerobic process [7].
The primary or fiber sludge is easy to dewater and do not usually cause the mills any prob-lem. However, the WAS from the secondary treatment has a higher water-holding capacity which makes it difficult to manage and dewater [1]. The high water-holding capacity in the WAS is due to the extracellular polymeric substances (EPS) in the sludge and due to the bounded water in the cells [1].
To overcome the obstacle with the high water holding properties in WAS there are several ways to improve the dewatering. One way is to mix it with primary sludge and costly poly-mers to achieve dryness suitable enough for incineration [1]. But there are other methods that can be used. One method is hydrolysis. Hydrolysis breaks the chemical bonds in a reaction involving water. In terms of biosludge, thermal or chemical hydrolysis with heat, alkalis and acids can break the cell membrane and cause the cells to disrupt and water and organic ma-terial will be released [8][9]. Another possibility with a successful hydrolysis is to utilize the WAS for the production of biogas [10].
1.1. Aim and Research questions
1.1
Aim and Research questions
AFRY, the Department of Thematic Studies at LiU, and Scandinavian Biogas have now started a project financed and requested by Energiforsk. This master´s thesis is a part of this project and the aim of the thesis was to investigate what effect hydrolysis has on the dewatering properties of the WAS. In the project, an investigation was also conducted to determine if chemical/thermal hydrolysis of the sludge had any effect on the biomethane potential. The hydrolysis was performed with existing side streams obtained from two pulp and paper mills. By using sidestreams that already exist at the mills this could result in reduced costs for the mills due to less use of chemicals such as polymers, imply a reduced carbon footprint, and a more flexible management of the WAS.
The sidestreams that were included in this project were residual heat sidestreams and sidestreams/filtrates such as green liquor sludge, white water, acid water from tallow sepa-ration, CTMP effluent, spent acid, and a bleaching effluent (EOP) from the pulping process. To determine what effect the hydrolysis had on the dewatering properties, the key questions for the project were therefore:
1. How is the dewaterability of the waste activated sludge affected by hydrolysis with chemical/thermal side streams?
2. What temperature in the thermal hydrolysis is suitable for improving the dewatering properties? How much acids/alkalis should be added to the WAS in the chemical hy-drolysis for improving the dewatering properties?
3. Can a combination of a specific temperature, alkalinity/acidity of the side streams fur-ther improve the dewatering properties and/or the biogas potential?
4. What effect does hydrolysis have on the suspended solids after dewatering the WAS? 5. How is the biochemical methane potential affected by hydrolysis?
1.2
Limitations
To narrow this master´s thesis project, only WAS from two paper mills have been in-cluded in the experimental part. The mills that were involved in the project were one with kraft pulping, BillerudKorsnäs in Skärblacka, and one with both kraft pulping and chemi-thermomechanical pulp (CTMP), SCA Östrand.
The time frame of this thesis was a limiting factor regarding how many parameters and lab-oratory experiments that could be executed. To optimize the time frame, the lablab-oratory ex-periments were primary a screening with less time-consuming tests. The most promising treatments were then combined and further tested.
Many parameters and analysis are important to determine and measure but in this thesis, the following parameters were investigated: total solids (TS) after centrifuging the WAS, volatile solids (VS), measurements on the reject water including sCOD, NH3– N, PO43 – after centrifuging the WAS, CST, pH, acidity, and alkalinity of some substrates.
1.3
Goals connected to the research questions
To determine the effect of the hydrolysis, goals connected to the research questions were formulated. The measurable goals are listed in Table 1.1.
1.3. Goals connected to the research questions
Table 1.1: Measurable goals connected to the research questions
Measurable goals
Increase the dewaterability and thus the TS on the WAS with 100 % compared to the control (after centrifugation)
Have a total solid (TS) content of at least 5-6 % on the WAS for biomethane potential test
Increase the bio methane from 100 L methane/kg VS to 150-200 L methane/kg VS
Increase the amount of soluble COD with 30% for an acceptable hydrolysis
2
Background
2.1
Pulp- and paper process and the generation of WAS
The produced sludge is a result of the pulp- and paper process which involved a number of steps where water is needed and thus creates huge amounts of waste water. The pulping process is the first step in the process that generates wastewater. Wastewater in this step is caused by the incoming logs that need to be debarked and chipped into wood chips. Water is needed to wash the logs sufficiently in order to remove all pollutants that can interfere with the coming steps in the process[11].
The next step in the pulping process is the pulping. In this step are the wood chips cooked in chemicals or mechanically ground to a pulping mass. The pulp is then washed to remove excess material also causing wastewater. Next is the bleaching step. In the bleaching step are some of the pulp bleached with chemicals such as chlorine dioxide ClO2, peroxides or sodium dithionite [12][13]. The ClO2is used in two steps with an EOP step in between. The E (=Extraction) step has oxygen and/or hydrogen peroxide added to increase the removal of lignin in the wood chips [14]. The result from the bleaching process is pulp that will be used in the paper-making process [13]. Like the other steps, the bleaching process also generates wastewater in order to remove excess chemicals and to clean the pulp before it is used in the paper-making process [11]. Some of the wastewater will then be transported into the wastewater treatment system.
2.2. The wastewater treatment process at pulp- and paper mills
Figure 2.1: A schematic sketch of the originating of waste water adapted from[15]
2.2
The wastewater treatment process at pulp- and paper mills
The objective of the wastewater treatment process is to remove solubilized pollutants, sus-pended particles, and toxic matters in the incoming water from the process before it is re-leased to an effluent stream or used in the process again [5]. The wastewater treatment sys-tem therefore consists of at least two steps, the primary treatment step and the secondary treatment step [10].
A schematic presentation of the most common wastewater treatment process at the pulp- and paper mill can be seen in Figure 2.2.
2.2. The wastewater treatment process at pulp- and paper mills
Figure 2.2: A schematic sketch of the wastewater treatment process at pulp- and paper mills adapted from [16].
2.2.1
Primary treatment
The primary treatment step is initiated by the water effluent from the pulp- and paper pro-cess that comes into the wastewater treatment system. The primary clarifier can be both a chemical and a mechanic process that utilizes sedimentation and flotation to separate and re-move larger solid particles from the wastewater [6]. Sedimentation and flotation require that the solid particles are large enough to settle at enough rate. The settling can be achieved in a pond or stabilization basin [17]. While the settled material, mainly referred to primary/fiber sludge mostly consists of fibers and other material from the pulp and paper process sink to the bottom of the basin, the separated water will move further to the next step in the process [18][19]. The primary treatment is therefore a way to reduce the load before the water enters the secondary treatment [20].
2.2.2
Secondary treatment - aerated process
The secondary treatment is often an aerobic treatment step in which the organic material is degraded [5] [6]. Common methods to remove the organic material is to use aerated basins or an activated sludge process with bacteria [17]. To maintain the process, the microorganisms often need additions of nutrients such as phosphorous and nitrogen in order to sufficiently degrade the organic substances in the wastewater. The bacteria involved in the secondary treatment use the organic compounds in the wastewater as a source of energy and removes the remaining organic material, phosphor and nitrogen. Hence, while the amount of organic material is removed, the numbers of microbes present increase and flocs of activated sludge is produced [12]. Part of the sludge produced in the activated sludge treatment process is re-circulated back from the secondary clarifier defined as return activated sludge (RAS) to keep a constant concentration of microorganisms in the secondary treatment step [21]. The residue from the aerated lagoons or the activated sludge process is the WAS which mainly consists of biomass. To measure the reduction of the organic material and how efficient the
wastew-2.3. Characteristics and water holding properties in WAS
ater treatment is, chemical oxygen demand - COD is measured. COD is used to quantify the organic material in the sample [22].
Since the WAS from the secondary treatment is composed of biological cells it is often more difficult to dewater it compared to the primary sludge. The difficulty of dewatering the WAS is due to the microorganisms which can hold the water tightly within the cells [5].
2.3
Characteristics and water holding properties in WAS
In WAS, most of the microorganisms are present in form of microbial aggregates, such as flocs, biofilms, and granules [23]. The flocs are in turn often influenced by extracellular polymeric substances (EPS). EPS are complex highly charged polymers that interact with the cells in the WAS. The EPS are therefore mainly responsible for the physicochemical properties of the flocs including flocculation, sludge settling and difficulties with sludge dewatering [24]. The mechanism of EPS is also that they can absorb a large amount of water and bind the water molecules tightly or capsule the water inside the cells or flocs. The binding of EPS with cells forms a vast net-like structure protecting the cells from external stress[25].
The most commonly analysed EPS in sludge are polysaccharides, proteins, nucleic acids, and lipids[25]. The extracellular polymeric substances are also highly negatively charged. The EPS can therefore be affected by other charged molecules in the WAS or that is added to the WAS. One example is calcium ions. The calcium ions can improve the floc strength in the WAS and improve the dewaterability by binding in to the EPS and avoid the EPS to bind to cells. Larger flocs, caused by the binding with positive charged ions are also often easier to dewater and to separate from the water.
However, if no positively charged molecules such as calcium ions are added, the negative charge of the EPS can bind to the bipolar water molecules in the sludge. Consequently, the negative charge of the EPS can therefore convert the loosely bonded water in the cells to be tightly bound to the cell surface [26].
Due to the different water-binding characteristics in WAS, the water in the sludge is often categorized into the subtypes free water, interstitial water, vicinal water and water of hydration [27].
According to Erdincler and Vesilind (2007) the sub types of water in WAS are explained the following way [27]:
• Free water in the sludge is water that is not associated or influenced by the suspended solid particles in the sludge. Free water in the sludge is easy to separate from the WAS. • Interstitial water is water that is trapped in the cell structure and in the interstitial space of the flocs and microorganisms. The water inside the cells can only be released when the cells disrupts. If the cells are disrupts, the interstitial water becomes free water which improves the sludge dewaterability [27].
• Vicinal water is water that is associated with multiple layers of water molecules and are very tight bound to the particle surface.
• Water of hydration is water that is chemically bound to the microorganisms and can only be removed with thermal energy.
To overcome the biological properties and the water-holding characteristics, hydrolysis of the WAS have been seen to work efficiently [23].
2.4. Hydrolyse methods to improve dewatering
2.4
Hydrolyse methods to improve dewatering
2.4.1
Thermal hydrolysis
Thermal hydrolysis will cause the cells to disintegrate and disrupt at high temperatures. It can also cause the EPS in the WAS to degrade. The breakdown of the EPS in the WAS can therefore release the interstitial water trapped inside the cells. The interstitial water is rich in dissolved organic compounds and disruption of cells will therefore cause leakage of cell material which will cause an increase in COD and nutrients [28] [29]. The mechanism of the thermal hydrolysis on WAS is also that it alters the structure of the sludge. In addition to the alter in structure, results on thermal hydrolysis does also show that a destruction of colloidal properties of proteins and carbohydrates can occur. The destruction will cause a release of water that was originally bound to the particles. Lastly, research have also concluded that thermal hydrolysis reduces or denatures substances in WAS with high affinity towards water such as proteins and carbohydrates, leading to better dewatering of the WAS [30].
The methods often used when talking about thermal hydrolysis include hot air oven, auto-clave, microwave, and hot water bath [31]. Temperatures in the hot water bath which have seen to have a hydrolysing effect vary between 60-100˝C [31].
Thermal hydrolysis have also been combined with chemical hydrolysis in order to make the process more rapid and efficient [32].
2.4.2
Alkali hydrolysis
Alkaline hydrolysis is a common subject of investigation due to its high efficiency and sim-plicity on WAS [33]. Using alkalis for hydrolysis enhance the destruction of cell walls causing the cells to disrupt and release its cell material [27][30].
Erdincler & Vesilind (2007) suggest that by using NaOH as an alkali treatment, the cells in the WAS will disrupt [27]. Why the cells disrupt with alkali treatment is because strong alkali can solubilize gels (EPS). The solubilizaton of EPS cause the cell to loose its viability and the cell cannot maintain an appropriate turgor pressure and disrupts. Another possible explanation why the cells disrupt is due to the chemical reaction with the cell wall. Saponification of lipids in the cell wall will also cause solubilization of the membrane, causing the cells to disrupt. Disruption of WAS leads to leakage of intracellular material which can increase both the amount of COD, ammonium, and phosphate as well as the dewaterability due to leakage of water [27][30].
The pH of the alkali used in the alkali hydrolysis have been discussed in many articles. Grü-bel & Suschka suggest that a pH of 9 is suitable for hydrolysing the WAS [34]. However, research have also concluded that a pH of 12 and using NaOH or KOH had the largest effect on the released COD and disruption of cells. The share of dissolved COD (sCOD) seem to rapidly increase as the concentration of NaOH increased up till 7g/L [32].
2.4.2.1 pH and alkalinity
While pH is a measurement of the hydrogen concentration in the solution, the alkalinity is a measurement of the buffering capacity or its ability to neutralize acids. The higher alkalinity, the higher buffering capacity against pH changes does the solution has. Alkaline compounds in a solution can be bicarbonates (HCO3–), carbonates (CO32) or hydroxides (OH–) that neu-tralises the H+ ions by binding to them. In turn, this formation will decrease the acidity of the solution. The total alkalinity, also called titration alkalinity, measures all carbonate, bicar-bonate, and hydroxide alkalinity at a pH of 4.5. A pH at 4.5 represents the number of bases in a sample that will accept a proton when being titrated with a strong acid [35].
2.5. Assessment of dewaterability
The importance of measuring not only the pH but also the alkalinity is due to the importance for the bacteria and other biological entities in WAS. pH changes can also lead to an increase or a decrease in flocculation whereupon measuring the alkalinity, therefore, is of importance [36].
The quantitative definition of the total alkalinity can be seen in (1) [37]. Total alkalinity(eq L)´´HCO3 ´+2 CO 32´+OH´´H+ {1} H++CO32´ÝÝÑHCO3´ {2} H++HCO3´ÝÝÑH2CO3 {3} H++OH´ÝÝÑH2O {4}
Measuring the alkalinity can both be done by calculating the total amount of CaCO3or the amount of HCO3–. To convert CaCO3to HCO3–, CaCO3in mg/L can be multiplied with 1.22[38].
2.4.3
Acidic hydrolysis
Using acids to disrupt the cell walls and increase the dewaterability have also been shown to work efficiently on WAS [39]. Like the alkali treatment, extremes of pH will cause the EPS proteins to loose their natural shape. This happens since the EPS are unstable in strong acids which will lead to acidic hydrolysis of the glycosidic linkages in the EPS. The EPS will therefore loose its viability and consequently disrupt. Disruption of the cells in the WAS leads to leakage of intracellular material and water comes out of the cells which improves the dewaterability sludge[30]. The pH of the acid that should be used in acid hydrolysis have been discussed in many articles. Devlin (2011) suggests that a pH of 2-3 is the most beneficial to destroy the EPS and improve to the dewaterability of the WAS [39]. Another type of acidic hydrolysis that have been discussed is to utilize hydrogen peroxide H2O2. Hydrogen peroxide have been reported to successfully disintegrate cells in biomass. Using hydrogen peroxide as hydrolysis method, the H2O2will produce free radicals that disrupt the cell walls, the proteins, the membrane phospholipids of the cells, and also disrupt the EPS. This means that the hydrogen peroxide itself can cause the cells to disrupt without having extremes of pH [40].
2.5
Assessment of dewaterability
Dewatering is of paramount importance in sludge management since the WAS before dewa-tering contains a lot of water. Reducing the WAS volume for transportation and disposal requires that the WAS is properly dewatered. Incineration of the sludge, which is the most common method to manage the sludge, requires for example sludge with low water content, preferably lower than 50% [29].
I a full scale production, the sludge is often dewatered with a centrifuge and a large-scale belt press.
2.5.1
Belt press
To measure the dewatering properties of the sludge in lab scale, a bench top belt filter press, also called Crown Press can be used. The belt press is an instrument designed to dewater the WAS and simulates the industrial belt press at pulp- and paper mills [41].
2.6. Effect of hydrolysis
The belt press consists of a gravity filtration unit and a pair of filter belts that are pulled against a static curved surface. To perform an analysis with the belt press, research suggests that adding 200 ml- 400 ml of WAS on the belt Press is adequate to get qualitative results. Sometimes are polymers added to the WAS when performing tests with the belt press to fully simulate the large scale belt press. Polymers are used at the paper mills to promote floccula-tion of the WAS. The flocs are then separated from the water making it easier to dewater the sludge. Polymers exist in a lot of variations. They can have a negative charge, positive charge, non ionic (neutral), and amphoteric (positive and negative) depending on the WAS[41].
2.5.2
Capillary suction time test
Capillary suction time (CST) is a simple and precise measurement of the rate water is released from the WAS. The CST device is made up of two plastic blocks, a filter paper, a steel funnel, electrodes that are fixed in the upper block, and an electrical timer that measure the capillary suction time. The filter paper is placed between the two plastic blocks and the stainless-steel collar is placed on the upper plastic block. WAS is then added to the steel funnel and the test is initiated. The time starts when the water in the sludge has reached the first electrode. After the water has moved another 0,7 cm the timer will stop [42].
When sludge is added to the cylindrical collar, Vesilind (1988) describe the mathematical formula of the velocity of the water as following:
ν= Q
πDd (2.1)
where d=filter paper depth and Q=Flow rates from the collar into the filter. This can be expressed as a differential where
dD dt =
Q
πDd (2.2)
and t=the time it takes for the water to travel from D22to D12
The calculated time between the two sensors will then have the time: t= (D22´D12)
πd
Q (2.3)
A CST-value of approximately 20 s is said to represent WAS with good dewatering properties [43]. An improved dewaterability is also said to be indicated by a decrease in CST. However, the CST does only measure the rate of dewatering by capillary suction and to actually be able to say something about the dewaterability it is valuable to measure the total solids, nitrogen and the EPS content since these can correlate with high CST values [42] [44].
2.6
Effect of hydrolysis
To determine the effect of the hydrolysis and the effects of the dewatering properties, mea-surements on the total solids, the suspended solids, meamea-surements on COD, phosphate and ammonium-nitrogen can indicate if the cells have disrupted or not [45].
2.6.1
Total solids and volatile solids
The WAS mostly contains water and biological material. When analyzing the sludge, a com-mon methodology is to determine the TS and VS. TS stands for total solids and is a ratio of weight obtained before and after the drying process in an oven at a temperature of 105˝C. Volatile solids, VS, is measure as % of TS. The VS content indicates the organic part of the
2.7. Anaerobic digestion
TS and is a way to represent the part of the material that is biodegradable in the WAS [46]. A successful hydrolysis will preferably lead to increased TS values after centrifugation since the bound water in the EPS will be released. It is also possible that the suspended solids have bounded to the floc also leading to an increase in TS [47].
2.6.2
Suspended solids
Suspended solids refers to small particles that remain in suspension when the bigger fraction of the WAS settle. The suspended solids can describe the water quality, since less suspended solids is favourable. The suspended solids are negatively charged. this causes them to repel each other and also repel the bigger flocs which also are negatively charged. During hydroly-sis, it is not uncommon that the suspended solids become positively charged which can make them bind to the negatively charged floc [48]. Hydrolysis of sludge can hence be a way to decrease the suspended solids and yield a better quality on the water phase in sludge [49].
2.6.3
COD, NH
4– N and phosphorus measurements on the reject
Chemical oxygen demand or COD is a way to measure the organic material in a sample. COD is defined as how much oxygen that is consumed in the oxidation of organic material in the sample. The more oxygen consumed the more oxidizable organic material is in the sample. To oxidize the organic matter in a sample, a strong oxidant is used. The concentrations of organic matter in terms of oxygen equivalents are then determined from the difference of initial oxidant concentrations and the remaining oxidant concentration in the sample [50]. If the COD-value increases compared to a control without any hydrolyse, it can give a hint on the effect of the hydrolysis and how much of the cellular material that have leaked from the cells [51].
The leakage of cellular material and trapped water inside the cells will not only lead to an increase in COD but in ammonium and phosphorus as well [52]. Measuring the ammonium and phosphorus after hydrolysis, is therefore a way to quantify the release of organic com-pounds after cell disruption. Comparing the values with non-hydrolysed WAS can therefore be a way to distinguish between a successful disruption of cells [53].
However, ammonium is very volatile and can easily change forms between ammonium (NH4+) and ammonia (NH3) which is a gas. This is called ammonia volatilization and means that when measuring ammonia it is possible that some part of the ammonium might have been converted to ammonia which will not be detected when measuring the amount of NH4– M in a spectrophotometer[54]. NH4+ is also very sensitive to pH changes. Alkaline or acid hydrolysis could therefore affect ammonium in the sample [55].
2.7
Anaerobic digestion
WAS can also be utilized for biogas production where hydrolysis can improve the degrad-ability of the organic material in the WAS [56]. In anaerobic digestion (AD), anaerobic mi-croorganisms convert organic material to biogas [57]. The AD process consists of the steps hydrolysis, acidogenesis, acetogenesis, and methanogenesis [58].
The first step in AD is the hydrolysis step. In this step are large complex organic molecules such as proteins, carbohydrates and lipids degraded to simple soluble monomers such as fatty acids, sugars and amino acids [59]. The hydrolysis step is often considered to be the slowest step and the step that limits the whole AD [60]. To carry out this process, different groups of hydrolytic bacteria are involved. To catalyze the hydrolysis step, enzymes are often involved in the substrate degradation. Utilising chemical and thermal hydrolysis beforehand, is therefore a way to improve the degradation of biological material and a possibility to move directly to step two is possible [61].
2.8. Statistical determination
The second step in AD is the acidogenesis. In the acidogenesis, the monomers from the hydrolysis step are converted to short-chain carboxylic acids and alcohols [62].
The third step in the AD process, is acetogenesis. In this step, bacteria called acetogens use the product from the previous steps to generate H2+ CO2and acetate as main products [63][46]. The last step in the AD process is methanogenesis. In this step, methane and carbon dioxide are produced. The organisms that are involved in the process are methanogenes which use the acetate, H2and CO2formed in the previous step as a direct substrate to form methane [58]. The possible chemical reactions to form methane and carbon dioxide in the methanogenesis can be seen in 5, 6, and 7 [57].
CH3COO´+H+ÝÝÑCH4+CO2 {5}
4 H2+CO2ÝÝÑCH4+2 H2O {6}
4 CH3OH ÝÝÑ 3 CH4+CO2+2 H2O {7}
2.7.1
Biochemical methane potential test
The AD process can be evaluated with a biochemical methane potential test (BMP) during anaerobic conditions [64].
During the BMP test, substrate is mixed with an anaerobic bacteria culture, often inoculum from wastewater treatment plants. The bottles are then incubated at a stable temperature, normally 38 °C or 55 °C and are intermittently stirred. Methane and carbon dioxide will be produced during the test due to the anaerobic degradation of organic contents in the substrate as stated in 5 to 7. The measurement continues until the production of gas is stabilized or starts to decrease [64]. The amount methane produced is calculated under normal conditions with 1 atm, and 0°C and is specified in normal ml (Nml) per g VS [65].
2.8
Statistical determination
To be able to draw any conclusions on the generated data it is of importance to evaluate the data statistically. A statistical test that often is performed is a normality test which tests if the data is normally distributed. Normally distributed data is a prerequisite for further tests and to being able to draw conclusions when comparing means for example. The normality test compares the scores of the data to a normally distributed set of scores with the same mean and standard deviation. The null hypothesis for a normality test is that the sample distribution is normal. To have normally distributed data, the p-value should be above the significance level - α [66].
Another test valuable to perform is the F-test. The F-test, test for equality of variances from two populations provided that the data is normally distributed. The result from the F-test will determine if the two populations have equal variances or not [67].
In case the variances are significantly equal, a t-test assuming equal variances can be per-formed. The t-test compare the means of two normal populations and with the t-test it is possible to determine if there is a significant difference in means from the different treatments [68].
3
Paper mills involved in the thesis
Included in the thesis are two mills, one with a kraft process, and one with both a CTMP and kraft process. In this chapter, a summary of the two mills, their difficulties, and their aspirations about the results from this thesis is presented.
3.1
BillerudKorsnäs - Skärblacka
At BillerudKorsnäs in Skärblacka, a total of 20 ton primary sludge and 10 ton of WAS is produced each day. Some of the sludge (primary and WAS) is incinerated in the bark fire boiler resulting in an increase in NOX emissions. But all of the produced sludge (primary sludge and WAS) cannot be incinerated. The WAS that is not incinerated is managed by external companies that produces soil of it in a mixture with other raw materials. This is very expensive and it is also difficult to find external companies that can handle the large amounts of WAS.
The WAS is therefore one of the biggest environmental and sustainability issues that exist at Skärblacka. Even though centrifuging and use of polymers result in a TS at about 16-17 % this is still not a desirable TS for incineration. BillerudKorsnäs Skärblacka is therefore involved in the project with AFRY, Scandinavian Biogas, and LiU to determine if hydrolysis potentially could yield easier management, increased TS, and better dewatering properties of the WAS. A desirable result of the project would be to increase TS (with the use of polymers) to at least 30 %.
3.2
SCA Östrand
SCA in Östrand is both a CTMP mill and a kraft mill. Today is the dewatered WAS, after centrifugation, mixed with white liquor, a chemical involved in the pulping process and then evaporated and incinerated in the recovery boiler. The CTMP-production at SCA Östrand will later be moved to SCA Ortviken where no recovery boiler is available. Hydrolysis with the existing side streams at the mills could therefore be one way to improve the dewatering properties. Just like BillerudKorsnäs involvement in the project with AFRY, Scandinavian Biogas and LiU is SCA Östrand interested in an improved management of the WAS and
3.2. SCA Östrand
better dewatering properties. A goal set for the project is to have an improved TS (with the use of polymers) to at least 30 % after centrifugation.
4
Materials and method
In this chapter, the material and methods used throughout all experiments are presented.
4.1
Overview of analyses
In order to test the hypothesis that hydrolysis could improve the dewatering properties and investigate the research question formulated in chapter 1 the two WAS selected (Skärblacka and Östrand) were subjected to a number of different treatments. An overview of the treat-ments is given in Table A.1 in Appendix A. Below in this chapter are the protocols for the different hydrolysis methods and the associated analyses given.
4.2. Sludge and sidestreams used
Figure 4.1: The analysis and methods performed on the WAS
4.2
Sludge and sidestreams used
The side streams of interest at BillerudKorsnäs - Skärblacka and SCA - Östrand that poten-tially could have a hydrolysing effect on the WAS can be seen in Table 4.1 and Table 4.3. These sidestreams have been selected based on interviews made by AFRY, TEMA, and Scan-dinavian Biogas with the selected mills before this master´s thesis project was initiated. The potential with these side streams is due to its pH (acid or alkaline), its content of peroxide, and/or the temperature of the stream that could act hydrolysing on the WAS.
Table 4.1: Side streams of interest at BillerudKorsnäs - Skärblacka
Side stream Volume/day pH of side stream TS (%) Temperature
Waste activated sludge 900 m3 7 1%
-EOP 10 000 m3 9 65-70˝C
Green Liquor Dregs 11 m3 13 50%
-Sesquisulphate/spent acid 1 m3 <1 65˝C
Residual cooling water 1800 m3 7 80˝C
The ratios in Table 4.2 and Table 4.3 below have been considered when choosing the propor-tions between WAS and the side stream in the experiments.
Table 4.2: Ratio between the WAS and the side streams at BillerudKorsnäs - Skärblacka.
Side stream Ratio in %
Green Liquor Dregs : Waste activated sludge 1.2 % Sesquisulphate/Spent Acid : Waste activated sludge 0.1 %
4.3. Protocols for hydrolysis
Table 4.3: Side streams of interest at SCA Östrand
Side stream/filtrate Volume/day pH of side stream TS (%)/susp (mg/L) Temperature
Waste activated sludge 2400 m3 7 TS: 1 %
-White water after bleaching 840 m3 6 Susp: 500-1000 mg/L 80-90˝C
Green Liquor Dregs 216 m3 13 50%
-Sesquisulphate/spent acid 7 m3 <1 60-70˝C
CTMP effluent 10 800 m3 6 Susp: 3000-3500 mg/L 85˝C
Acid water from tallow separation 312 m3 3 80˝C
Table 4.4: Ratio between the WAS and side streams at SCA Östrand
Side stream Ratio in %
White water: Waste activated sludge 35 %
Green Liqour Dregs : Waste activated sludge 9 % Sesquisulphate/Spent Acid : Waste activated sludge 0.3 %
CTMP Effluent : Waste activated sludge 450 %
Acid water from tallow separation : Waste activated sludge 13 %
4.3
Protocols for hydrolysis
4.3.1
Thermal hydrolysis
The thermal hydrolysis was performed by incubating 400 ml WAS in 1L bottles at three dif-ferent temperatures. The chosen temperatures were 70, 80, and 90˝C and incubation was performed at 1-2 hours according to literature [31]. The WAS was not stirred during the ex-periment and the screw caps were not fully tightened to avoid pressure in the bottles. After a complete incubation the bottles were left for 20 minutes to cool at room temperature. The protocol for the thermal hydrolysis performed on WAS from both BillerudKorsnäs and Östrand can be seen in Table 4.5.
Table 4.5: Thermal hydrolysis performed on WAS from both BillerudKorsnäs and Östrand
Amount of added WAS Temperature˝C Time incubated in water bath
400 ml 70 1 hour
400 ml 80 1 hour
400 ml 80 2 hours
400 ml 90 1 hour
4.3.2
Acidic hydrolysis
The acidic hydrolysis was performed using the chemical side streams from the mills with low pH i.e sesquisulphate/spent acid from BillerudKorsnäs AB Skärblacka with pH <1 and acid water from the tallow separation at SCA Östrand with pH at 3. Spent acid is a chemical side stream that exists at both Skärblacka and Östrand but in this experiment was only spent acid from Skärblacka used. Acid water exists only at SCA Östrand and therefore was only WAS from Östrand treated with acid water.
The amount of chemicals added to the sludge in the experiments was based on the ratio between the acidic chemicals and the amount WAS that existed at the mills, see Table 4.4 and Table 4.3.
To perform the acidic hydrolysis, the acids were added to the WAS. The mix was then stirred every 10 min. The experiment lasted for 1 hour. The protocol for the acidic hydrolysis with the including controls can be seen in Table 4.6 and Table 4.7.
4.3. Protocols for hydrolysis
Table 4.6: Acidic Hydrolysis with spent acid on WAS obtained at BillerudKorsnäs Skärblacka and SCA Östrand
Amount WAS Amount spent acid Dilution % Time incubated
199 ml 1ml 0,5 % 1 h
198 ml 2 ml 1% 1h
360 ml 40ml 10 % 1h
Control Amount water Dilution % Time incubated
200 ml WAS - -
-360 ml WAS 40 ml 10 %
-Table 4.7: Acidic hydrolysis with acid water on WAS obtained from SCA Östrand
Amount WAS Amount acid water Dilution % Time incubated
199 ml 1ml 0,5 % 1h
198 ml 2 ml 1% 1h
Control Amount water Dilution % Time incubated
200 ml WAS - -
-4.3.2.1 Hydrolysis with residual peroxide and fibers from CTMP effluent
To perform the experiments with residual peroxide, the same method used for the acidic hydrolysis was used in this experiment. See Table 4.8 for protocol.
Table 4.8: Acidic hydrolysis with CTMP effluent on WAS from SCA Östrand
Amount of added WAS Amount CTMP Dilution % Time incubated
200 ml 200 ml 50% 1 h
Control Amount water Dilution % Time incubated
200 ml WAS 200 ml 50 %
-Another side stream with the potential to hydrolyse the WAS with its content of residual peroxide, was the bleaching filtrate from the process at SCA Östrand called white water. Of the two mills included in this study does white water only exists at SCA Östrand. Therefore was only WAS from SCA Östrand involved in the experiment. The method to perform this experiment was the same as above. The protocol for the experiment can be seen in Table 4.9.
Table 4.9: Acid hydrolysis with white water from SCA Östrand on WAS from Östrand
Amount WAS Amount white water Dilution % Time incubated
300 ml 100 ml 25% 1h
Control Amount water Dilution % Time incubated
300 ml WAS 100 ml 50 %
-4.3.2.2 Hydrolysis with heat and acids
In order to test if a combination of heat and acid could increase the effects of the hydrolysis, an experiment that combined those two was performed. WAS from both BillerudKorsnäs and SCA Östrand (separately) was mixed with spent acid to a dilution of 1 % in the WAS. The mix of WAS and spent acid was then incubated at 70 degrees for 1 hour. The bottles were not stirred during the experiment. After the incubation was completed, the bottles were left at room temperature to cool for 20 minutes. The protocol for the experiment can be seen in Table 4.10.
4.3. Protocols for hydrolysis
Table 4.10: Thermal and acid hydrolysis at 70˝C and with spent acid performed on WAS from both BillerudKorsnäs and SCA Östrand
Amount WAS Amount spent acid Dilution Temperature Time incubated
396 ml 4ml 1 % 70˝C 1 h
4.3.3
Alkaline hydrolysis
To perform the experiments with the alkaline side streams, WAS was mixed with green liquor sludge (GLS) and EOP in two different experiments. EOP has a pH at 9 and GLS a pH at 13 (after being diluted in water, see below). Both side streams were obtained from BillerudKo-rsnäs in Skärblacka.
The experiments with GLS were performed on WAS both from SCA Östrand and Billerud-Korsnäs Skärblacka.
The GLS used in this experiment was dewatered GLS with a TS of 50 %. To perform the experiments with GLS, the GLS was therefore mixed with water to get a more soluble form of the GLS. A 10 % dilution was obtained by adding 20.161 g of GLS in a measuring flask and then fill the flask with MilliQ water up to 200 ml to get a TS of 5 %.
To perform the experiment the 5% TS GLS, WAS was mixed with GLS at three different ratios. The amount of GLS added to the WAS was based on the ratio between GLS and WAS at the mills, see Table 4.4, Table 4.2. The hydrolysis test was performed as described for the acid hydrolysis above. Controls with water were included to compensate for the dilution effect. An experiment with GLS was also performed without dissolving the GLS in water. Instead, 50 % TS GLS was added straight to the WAS. 35.1 g of GLS was added in a measuring flask and then was WAS added up to 350 ml. The protocol can be seen in Table 4.11.
Table 4.11: Alkaline Hydrolysis with GLS on WAS from SCA Östrand and BillerudKorsnäs Skärblacka
Amount WAS Amount GLS Dilution % Time incubated
198 ml 2 ml 1 % 1 h
190 ml 10 ml 5 % 1 h
180 ml 20 ml 10 % 1 h
350 ml 35.1 g 1 h
Control Amount water Dilution % Time incubated
198 ml WAS 2 ml 1 %
-195 ml WAS 5 ml 5 %
-190 ml WAS 10 ml 10 %
-An experiment combining GLS and heat was also performed to determine if the combination had an increased hydrolysing effect on the WAS. The mix of WAS and GLS was incubated in a water bath at 70˝C for 1 hour. The bottles were not stirred due to the heat. When the incubation was completed, the bottles were left in room temperature for 20 minutes to cool. The protocol for the combination of the thermal and alkaline experiment can be seen in Ta-ble 4.12. Control to this experiment was the non-dissolved GLS added to the WAS described in subsection 4.3.3.
Table 4.12: Thermal and alkaline hydrolysis at 70˝C and GLS on WAS from SCA Östrand and BillerudKorsnäs Skärblacka
Amount WAS Amount green liquor sludge Temperature Time incubated
4.4. Analytical methods
To perform the experiments with EOP, WAS from BillerudKorsnäs was mixed with EOP at three different ratios. The method for the experiments with EOP was the same as acidic hydrolysis. The protocol for the experiment and the controls performed can be seen in Ta-ble 4.13.
Table 4.13: Alkaline Hydrolysis with EOP on WAS from BillerudKorsnäs Skärblacka
Amount WAS Amount EOP Dilution % Time incubated
300 ml 100 ml 25 % 1h
200 ml 200 ml 50 % 1h
100 ml 300 ml 75 % 1h
Control Amount water Dilution % Time incubated
300 ml WAS 100 ml 25 %
-200 ml WAS 200 ml 50 %
-100 ml WAS 300 ml 75 %
-4.3.3.1 Hydrolysis with NaOH as control on WAS from SCA Östrand and BillerudKorsnäs Skärblacka
To determine what alkaline effect EOP and green liquor sludge had on the WAS, experiments with NaOH, was performed as a reference since NaOH is known to have an alkaline effect on WAS. The NaOH experiment was performed by mixing 0.32 g NaOH in 200 ml WAS (from both BillerudKorsnäs and SCA Östrand). The WAS was treated for 1 hour and was stirred every 10 min. The choice of 0.32 g NaOH was due to literature suggesting that 16 g NaOH/100g TS would have an hydrolysing effect on the WAS [69] .
The protocol for the experiment with NaOH can be seen in Table 4.14.
Table 4.14: Alkaline Hydrolysis with NaOH on WAS from both mills
Amount WAS Amount NaOH Dilution % Time incubated
200 ml 0.32 g 1h
4.4
Analytical methods
Most of the analyses performed after the WAS was centrifuged. The analyses that was per-formed without centrifuging the WAS beforehand was when analysing TS as total, when performing the CST-analysis and measuring the TS after using the Crown Press.
4.4.1
Centrifugation
The centrifuged used in this project was a Thermofisher Hereaus Megafuge 8 and a Beckman Coulter centrifuge with rotor JA-10. The WAS was centrifuged at 4700 G for 3 minutes in both centrigues. Two different centrifuges were used since the first one broke in the middle of the project.
4.4.2
Total Solids (TS) and volatile Solids (VS)
Analyses of the total solids and volatile solids were performed on the pellet both on untreated WAS and after centrifugation of the WAS. To measure the TS and VS, crucibles and aluminum forms were used. The crucibles were used when measuring both TS and VS and aluminum forms were used when only measuring the TS. The crucibles and aluminum forms were first weighed without sample. The weight was noted and denoted as Cno sample. The sample was then added and the weight was noted once again. The crucibles/aluminum forms were then
4.4. Analytical methods
placed in an oven at 105˝C for at least 20h. To cool and at the same time preventing the samples from being damp, they were then put in a desiccator for 20 minutes. The samples were then weighed again denoted as Csample. To calculate the TS, Equation 4.1 was used.
TS(%) = C105´Cno sample
msample 105
˚100 (4.1)
where m is the weight of the sample added to the aluminum form or to the crucible.
The analysis of the volatile solids, VS, was conducted by putting the samples from the TS measurement into a muffle furnace at 550˝C for 2 hours + 4 hours cooling. The samples were then put into a desiccator. To calculate the volatile solids (VS), Equation 4.2 was used.
VS(%) = C550 degrees´Cno sample
C105 degrees´Cno sample
˚100 (4.2)
4.4.3
Suspended solids
The suspended solids were measured on the supernatant after centrifuging the WAS. The supernatant was then filtered using a suction filtration set up and a glass filter with a diameter of 47 mm and pore size of 1.6 µm. The suspended solids was measured by keep track of the volume added to the filter and then put the filter in an oven at 105˝C over for at least 20 hours. The filters were then weighed again and the results represented the suspended solids (Standard SS-EN 872:2005). The analysis on the suspended solids were performed in triplicates.
4.4.4
Alkalinity
The alkalinity was determined by titrating EOP, GLS, and WAS with 0.1 M HCl (i.e 0.1 Nor-mal), and the alkalinity was expressed in mg CaCO3/L
The following formula was used:
millieqvivalents Alkalinity =Vadded acid˚
normals ml ˚
meq Alk
meqH+ (4.3)
Using the equations (4.3), (4.4) (4.5), and (4.6) with numbers the alkalinity is calculated to:
15ml ˚0,1 meq H +
ml ˚
1 meq Alk
1meqH+ =1,5 meq Alk (4.4)
1,5 meq Alk 100ml ˚
1000 ml
L =15 meq/L Alk (4.5)
with the knowledge that the molar mass of CaCO3=100 mg/mol and that there are 2 meq/mmol we get: 15 meq Alk L ˚ 1 mmol CaCO3 2 meq CaCO3 ˚100 mg CaCO3 1mmol =750 mg L as CaCO3 (4.6) 15 ml was the volume that was added to reach a pH of 4,5, see titration curve in the Appendix Figure A.16
Using the same equations the alkalinity of the WAS was determined to 325 mg/L as CaCO3 and the alkalinity of the green liquor sludge (20.191 g up to 200 ml) in WAS was determined to 5000 mg/L as CaCO3.
4.4. Analytical methods
4.4.5
pH
pH was measured on all sidestreams and on all WAS-mixes. The pH meter used was an inoLab 7320 pH meter with a HAM polilyte Bridge Lab pH electrode. The equipment was calibrated by using commercial solution at pH 7 and pH 4 at 20˝C once a week. A control with a pH of 8 at 20˝C was measured before and after measurement of the samples.
4.4.6
Acidity
The acidity of the acids was also calculated in order to determine the strength of the acids. To determine the acid’s ability to ionize, i.e ability to release its H+, the Ka- value was calculated. To calculate Ka, the following formula can be used:
Ka =
[H3O+][A´]
[HA] (4.7)
The pKa of a solution can also be determine by the Henderson-Hasselbalch equation which utilize a titration curve. The Henderson-Hasselbalch equation is
pH=pKa+log[base]
[acid] (4.8)
By utilizing titration curves after titrating the acids with a known base (NaOH), it was possi-ble to determine the Kavalue of the acid with Henderson-Hasselbalch equation. The Ka-value can be determined by finding the equivalence point in the titrations curves. Then, the pH can be found and also how much NaOH that was added in total. By dividing the added NaOH by half, the log[base][acid] will be equal to 0 since the added base will be equal to the consumed acid in the reaction, thus pH=pKa. Looking at the titration curves in Figure A.19, the pKa of the spent acid was « 2 in all curves. Hence, the Ka of spent acid was calculated to 0.01. Titration of acid water was also performed. But after adding 1 ml NaOH to the mix with WAS and acid water, the pH changed from 7 to 9. Hence, the titration was ended with the assumption that the acid water was a weak acid.
4.4.7
Measurement of NH
4– N, PO
43 –, and sCOD
Analysis on sCOD, NH4– N and PO43 –was performed by filtrating the supernatant through a glass filter with 1.6 µm in pore size. The filtrate was then added to cuvettes from Hach Lange. To measure the sCOD were the kits LCK514 and LCK014 used. For the measurements of NH4– N were the kits LCK302 and LCK 303 used, and for the PO43 –were the kits LCK349, and LCK350 used. All analyses were conducted according to the manufacturer’s instructions.
4.4.8
Biochemical methane potential test
Biochemical methane potential tests were performed to determine the methane potential of the hydrolysed WAS. The chosen samples were the hydrolysed WAS with both an increase in TS of the pellet and an increase in sCOD in the supernatant. The chosen samples can be seen in Table 4.15 and Table 4.16.
The biochemical methane potential tests were performed using two different systems. The first system was provided by Scandinavian Biogas and was a CJC Labs biomethane poten-tial test (BMP) system (CJC Labs Ltd., UK). The method used in this test was conducted by Ometto et al. (2017) [70]. This system was used for the WAS obtained at SCA Östrand. The other system was provided by TEMA and was a AMPTS II system. The method for this system was the same but with some modifications.
