The Biogas Production Plant at Umea Dairy Evaluation of Design and Start Up

7/30/2019 The Biogas Production Plant at Umea Dairy Evaluation of Design and Start Up

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The Biogas Production Plant at Ume Dairy

Evaluation of Design and Start-up

Stina Asplund

2005-10-05

LiU-Tema V-Ex-12

Linkpings universitet, Inst. fr Tema, Avd. fr Vatten i natur och samhlle581 83 Linkping

Tema vatten i natur och samhlle


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Copyright

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publication and for assurance of document integrity, please refer to its www home page:

http://www.ep.liu.se/.

Stina Asplund, 2005


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Defence date

2005-06-21

Department and Division

Institutionen fr Tema

Avdelningen fr Vatten i natur och samhlle

URL, Elektronic version

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-5509

ISBN

ISRN LIU-TEMAV/TBM-EX--06/001--SE

_________________________________________________________________

Title of series, numbering Liu Tema V-Ex 12

LanguageSwedish

English

________________

Report category

Licentiate thesisDegree thesisThesis, C-levelThesis, D-level

_____________

Title The Biogas Production Plant at Ume Dairy Evaluation of Design and Start-upTitel Biogasanlggningen vid Ume mejeri Utvrdering av design och uppstart

Author Stina Asplund

Keywords Biogas, anaerobic digestion, start-up, dairy, whey

Abstract

As a part of a large project at Norrmejerier, a biogas production plant has been constructed at Ume Dairy. In thisplant wastewater, residual milk and whey are decomposed and biogas is produced. The biogas is burned in a steam

boiler. The biogas plant is designed as an anaerobic contact process, with sludge separation and recirculation by a

clarifier. The fat in the substrate is treated in a separate reactor.

The purpose of this study is to evaluate the design and start-up of this biogas production plant. Further, the interaction

with the contractor responsible for construction and start-up is evaluated.

The plant is generally well designed, the process conditions are suitable and the objectives are realistic. However, theseed sludge is unsuitable and the time plan is too optimistic.

At the end of the period of this study, the plant was running and all central components are performing as intended.

Still, the objectives have not been reached. This is mainly attributed to the poor quality of the seed sludge.

The management of the plant and the interaction with the contractor has generally been good. Most problems that

arose were of typical start-up nature. Others were due to insufficient planning or lack of communication. Further,several design flaws were identified during start-up.

Washout of sludge has been one of the most significant drawbacks during start-up. This inconvenience seems to be the

result of improper seed sludge and a too hasty increase of the organic loading rate.


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The Biogas Production Plant at Ume Dairy

Evaluation of Design and Start-up

Stina Asplund

2005-10-05

Supervisor: Tony BckstrmNorrmejerierMejerivgen 2906 22 Ume

Examiner: Prof. Bo H. SvenssonInstitutionen fr TemaAvdelningen fr Vatten i natur och samhlleLinkpings universitet581 83 Linkping


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Sammanfattning

Norrmejerier har som en del av ett strre projekt ltit uppfra en anlggning fr biogasproduktion vid Umemejeri. I anlggningen, som r utformad som en anaerob kontaktprocess, behandlas avloppsvattnen och andraorganiska restprodukter frn mejeriet tillsammans med vassle frn bde Ume och Burtrsk mejeri. Fettet isubstratet avskiljs och behandlas separat. Den biogas som produceras vid nedbrytningen av det organiskamaterialet brnns i en brnnare och nga produceras.

Syftet med den hr studien r att utvrdera anlggningens design, valda processfrhllanden och frfarandetunder uppstarten av biogasanlggningen. Dessutom utvrderas interaktionen med den tyska entreprenr som ransvarig fr konstruktion och uppstart.

Anlggningens utformning och valda processbetingelser r passande och de uppsatta mlen r rimliga. Dremotr valet av ymp olmpligt och tidsplanen fr uppstarten r fr optimistisk.

Nr denna studie avslutades var anlggningen i bruk och biogas producerades. Alla de ml fr som formuleratshade dock inte uppntts. Ympens dliga kvalitet r den mest bidragande orsaken till att uppstartsperioden harblivit frlngd.

Arbetet under uppstarten och samarbetet med entreprenren har generellt sett varit lyckat. Man har dock sttt pmnga komplikationer, varav de flesta har varit av typisk uppstartsnatur. Andra har varit resultatet av bristande

planering och kommunikation. En rad konstruktions- och designfel har ocks identifierats under uppstarten.

Slamflykt frn reaktorerna har varit det mest betydande problemet hos den biologiska processen. Denna frlustav slam frmodas bero p olmpligt val av ymp och en alltfr hastig kning av den organiska belastningen ireaktorerna under uppstarten.

Abstract

As a part of a large project at Norrmejerier, a biogas production plant has been constructed at Ume Dairy. Inthis plant wastewater, residual milk and whey are decomposed and biogas is produced. The biogas is burned in asteam boiler. The biogas plant is designed as an anaerobic contact process, with sludge separation andrecirculation by a clarifier. The fat in the substrate is treated in a separate reactor.

The purpose of this study is to evaluate the design and start-up of this biogas production plant. Further, theinteraction with the contractor responsible for construction and start-up is evaluated.

The plant is generally well designed, the process conditions are suitable and the objectives are realistic.However, the seed sludge is unsuitable and the time plan is too optimistic.

At the end of the period of this study, the plant was running and all central components are performing asintended. Still, the objectives have not been reached. This is mainly attributed to the poor quality of the seedsludge.

The management of the plant and the interaction with the contractor has generally been good. Most problemsthat arose were of typical start-up nature. Others were due to insufficient planning or lack of communication.Further, several design flaws were identified during start-up.

Washout of sludge has been one of the most significant drawbacks during start-up. This inconvenience seems tobe the result of improper seed sludge and a too hasty increase of the organic loading rate.


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Acknowledgements

I would like to thank Norrmejerier for giving me the opportunity to study this interestingproject.

Thanks to Bo Svensson at Linkping University for his support, ideas, interest andcommitment.

I would also like to thank Robert Zarzycki, Enviro-Chemie, for welcoming me at the plant,taking interest in my work and helping me out during his time in Ume.


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Contents

1 Introduction _____________________________________________________________ 1

2 Purpose and objectives ____________________________________________________3

2.1 Purpose _____________________________________________________________ 3

2.2 Scope and discussion of purpose__________________________________________ 3

2.3 Methodology _________________________________________________________ 3

2.4 Terminology _________________________________________________________ 4

2.5 Disposition and audience _______________________________________________ 4

3 Background _____________________________________________________________ 7

3.1 Norrmejerier _________________________________________________________ 7

3.2 Biotrans project _______________________________________________________7

3.2.1 Whey____________________________________________________________73.2.2 Wastewater _______________________________________________________8

3.2.3 The project _______________________________________________________8

3.3 Investigations and purchase _____________________________________________8

3.3.1 Investigations _____________________________________________________8

3.3.2 The contractor_____________________________________________________ 9

4 Biogas production _______________________________________________________11

4.1 Anaerobic digestion __________________________________________________11

4.1.1 Hydrolysis_______________________________________________________114.1.2 Acidogenesis_____________________________________________________12

4.1.3 Acetogenesis_____________________________________________________12

4.1.4 Methanogenesis __________________________________________________12

4.2 Dairy waste products__________________________________________________ 13

4.2.1 Dairy waste products as a substrate for biogas production__________________ 134.2.2 Lactose _________________________________________________________ 13

4.2.3 Milk fat _________________________________________________________ 14

4.2.4 Milk protein _____________________________________________________14

4.3 Process parameters ___________________________________________________14

4.3.1 pH-value ________________________________________________________ 14

4.3.2 Alkalinity _______________________________________________________154.3.3 Temperature _____________________________________________________15

4.3.4 Nutrients and trace elements_________________________________________ 15

4.3.5 Organic loading rate _______________________________________________ 16


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4.3.6 Hydraulic retention time____________________________________________16

4.4 Reactor design_______________________________________________________ 16

4.4.1 High-rate anaerobic reactors_________________________________________ 164.4.2 Two-phase layout _________________________________________________ 17

4.5 Investigations made by Norrmejerier _____________________________________ 184.5.1 Anaerobic digestion experiment at JTI_________________________________ 184.5.2 Study tours to full-scale plants _______________________________________18

4.6 Proper start-up strategy ________________________________________________ 19

5 The plant at Ume Dairy __________________________________________________21

5.1 Feedstock characteristics_______________________________________________ 21

5.2 Design and technology ________________________________________________ 22

5.2.1 Plant layout______________________________________________________225.2.2 Objectives _______________________________________________________23

5.2.3 Pumping station and sieve __________________________________________235.2.4 Mixing and equalisation tank ________________________________________23

5.2.5 Fat flotation______________________________________________________23

5.2.6 Conditioning tank _________________________________________________ 23

5.2.7 Methane reactors__________________________________________________ 23

5.2.8 Clarifier_________________________________________________________ 24

5.2.9 Fat treatment_____________________________________________________255.2.10 Biofilters _______________________________________________________25

5.2.11 Steam boiler and flare _____________________________________________ 265.2.12 Post- aeration____________________________________________________ 26

5.3 Planned start-up procedure _____________________________________________ 26

5.4 Seed sludge _________________________________________________________ 27

5.4.1 Municipal sewage treatment plant at n, Ume__________________________275.4.2 Paper mill in Germany _____________________________________________ 27

6 Discussion of plant and process design_______________________________________29

6.1 Plant layout and reactor design__________________________________________29

6.2 Process parameters ___________________________________________________29

6.3 Feedstock___________________________________________________________30

6.4 Seed sludge _________________________________________________________ 30

6.5 Planned start-up procedure _____________________________________________ 31

7 Conclusions regarding plant and process design________________________________33

8 Hypotheses regarding start-up______________________________________________ 35

8.1 Questions at issue ____________________________________________________35


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9 Course of events ________________________________________________________ 37

9.1 Start-up procedure____________________________________________________ 37

9.1.1 Start of mixing and equalisation tank__________________________________ 379.1.2 Inoculation ______________________________________________________37

9.1.3 Feeding _________________________________________________________ 379.1.4 Start-up of fat treatment ____________________________________________38

9.1.5 Start of steam boiler _______________________________________________ 38

9.2 Variables monitored __________________________________________________38

9.2.1 Analytical procedures______________________________________________ 38

9.3 Problems encountered and measures taken_________________________________39

9.3.1 Inconveniences due to inadequate plant design __________________________ 399.3.2 Problems that arose during start-up ___________________________________ 39

9.3.3 Sludge washout___________________________________________________ 40

10 Results and discussion ___________________________________________________43

10.1 The start-up ________________________________________________________ 43

10.1.1 Inconveniences due to inadequate plant design _________________________ 4310.1.2 Inconveniences due to inadequate project planning ______________________ 44

10.1.3 Precautions and preparations _______________________________________4410.1.4 Seed sludge _____________________________________________________45

10.2 Interaction with contractor ____________________________________________45

10.2.1 Communication difficulties_________________________________________ 45

10.2.2 Chain of decisions and responsibility allotment_________________________ 4610.2.3 Training of the staff_______________________________________________46

10.3 Analysis results _____________________________________________________47

10.3.1 Organic loading rate ______________________________________________ 4810.3.2 Hydraulic retention time ___________________________________________50

10.3.3 COD and VFA __________________________________________________50

10.3.4 pH value _______________________________________________________51

10.3.5 Degradation_____________________________________________________ 52

10.3.6 Biogas production ________________________________________________ 53

10.3.7 MET performance________________________________________________5410.3.8 Fat flotation_____________________________________________________54

10.3.9 Sludge washout__________________________________________________55

11 Conclusions and recommendations _________________________________________57

11.1 Conclusions regarding start-up _________________________________________57

11.2 Recommendations ___________________________________________________58

11.2.1 Recommendations regarding the plant ________________________________58

11.2.2 Recommendations to Norrmejerier___________________________________ 58

11.2.3 Recommendations regarding start-up of biogas plants____________________58


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11.2.4 Improvements ___________________________________________________59

12 References ____________________________________________________________ 61

12.1 Articles ___________________________________________________________ 61

12.2 Reports____________________________________________________________ 6313 Appendices____________________________________________________________65

13.1 Appendix 1 - Calculation of biogas yield from lactose_______________________ 65

13.2 Appendix 2 - Substrate profile _________________________________________65

13.3 Appendix 3 - Plant layout _____________________________________________ 66

13.4 Appendix 4 - Analysis results __________________________________________67

13.4.1 HRT___________________________________________________________ 6713.4.2 pH ____________________________________________________________ 67

13.4.3 Biogas production ________________________________________________ 6813.4.4 Fat flotation_____________________________________________________70

13.4.5 Sludge washout__________________________________________________71


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1 IntroductionThe issue of finding an effective disposal of wastewater and reduction of pollution is a

problem for many food-processing industries. The high concentration of organic material andnutrients in such wastewater makes satisfactory waste treatment a matter of great importance.

Anaerobic (i.e. in the absence of oxygen) degradation of organic material is an establishedtechnology for wastewater treatment. In this process, the organic material is digested bymicroorganisms and the polluting potential of the wastewater is reduced. At the same time, amixture of gases called biogas is formed.

Biogas is an energy rich, combustible gas derived from the anaerobic decomposition oforganic matter. It primarily consists of methane and carbon dioxide. Biogas can be producedin an industrial manner using large-scale anaerobic reactors. These reactors consist of a large

vessel with inflow of substrate, e.g. wastewater, and effluent of produced gas and treatedwater. The gas is burned for utilisation of the energy in form of electricity or heat.

Appropriate reactor design and suitable choice of substrate and process parameters areessential for success when dealing with any kind of biotechnology production. When treatingindustrial wastewaters in an anaerobic manner, the substrate is pre-determined. Inconsequence of this, even more concern must be given to reactor and process design.

As the microorganisms in the form of seed sludge are inoculated to the unfamiliarenvironment of a new biogas reactor they need to adapt to the new conditions. The seedsludge needs time to grow and become acclimatised to the new substrate. This adaptation isfacilitated by a cautious start-up procedure, allowing the culture to develop withoutdisturbances due to over feeding. Inadequate start-up will lead to digestion failure, caused byimproper microbial community structure development. Hence, the start-up of an anaerobicdigester requires caution and knowledge in the art of bioreactor processing.

The study presented in this thesis was performed to evaluate the design and start-up of abiogas production plant. It was carried out to document relevant information, to acquireexperiences from the start-up and to promote awareness of the significance of proper plantlayout and start-up procedures.


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2 Purpose and objectives2.1 PurposeThe purpose of this study is to evaluate design and start-up of a biogas plant at Norrmejerierin Ume. The evaluation of the design consists of a review of plant layout and processconditions. The start-up of the biogas plant is evaluated with regard to planned start-up

procedure, line of action during start-up and managing of the plant. Further, interaction andNorrmejeriers cooperation with the contractor is studied.

2.2 Scope and discussion of purposeThe evaluation of plant layout will be performed with a biotechnology approach, with regardto the plant and reactor design, process conditions and seed sludge of choice.

The start-up procedure will be studied with special consideration given to supervision andmanaging of methane reactors and the active sludge therein. Extra attention will also be givento other elements of the plant processing the organic material, such as hydrolysis, fat flotation,fat treatment and biofilters. Problems due to design inadequacies identified during start-upwill be considered as well. The aim of this study is to evaluate the start-up period of the plant,i.e. the 16-week period between first inoculation (2005-02-16) and the official opening of the

plant (2005-06-08).

Start-up of the fat treatment was postponed until after the period of this study. Therefore, theline of action during the start-up of the reactor for fat treatment will not be evaluated.

The plant is constructed and built by Enviro-Chemie, a German contractor. This company isalso responsible for the start-up. As a part of the evaluation presented in this thesis, theinteraction and cooperation with the contractor will be studied. Norrmejeriers actions withany shortages in the contractors performance will be evaluated. That is, only the acts of

Norrmejerier will be evaluated. The fulfilment of the contract by the contractor will hence notbe considered in this thesis.

The emphasis in this thesis is on biogas production by anaerobic digestion. Thus, it will not be

evaluated if any other treatment method of the waste products, such as ethanol production oraerobic treatment would have been more suitable for Norrmejerier.

2.3 MethodologyTo allow a better understanding of biogas production, recommended plant design and start-up

procedure, an extensive literature study was performed. The evaluation of plant design isbased on published articles as well as reports from Norrmejeriers own studies on the subject.Information given by the contractor Enviro-Chemie has also been considered and criticallyreviewed.


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The evaluation concerning the start-up of the plant at Ume Dairy is mainly based on theauthors observations at site. It is the result from discussions and interaction with the partiesinvolved, as well as on comparisons with literature. Various analysis results from the start-up

phase has been studied and reviewed to evaluate the management of the plant during thisperiod.

2.4 Terminology

Alkalinity The sum of all the titratable bases in a liquid

Anaerobic digestion Decomposition of organic matter by microorganisms in the absence of oxygen.

Biogas A mixture of methane and carbon dioxide, produced by microorganisms.

Bioreactor An apparatus used to carry out any kind of bioprocess.

COD Chemical oxygen demand. The mass of oxygen needed to oxidise a volume of

organic material in liquid solution.

Fermentation Energy-yielding anaerobic metabolic digestion.

HRT Hydraulic retention time. The average time a volume element of a liquidmedium resides inside a vessel.

OLR Organic loading rate. The amount of organic material that is fed to each cubicmeter of reactor volume during one day.

Residual milk A waste product formed at dairies: an intermediate phase between a dairyproduct and water.

Seed sludge Biological sludge that is used for inoculation of a biogas reactor.

Sludge washout Sludge leaves the plant with the effluent water stream.

VFA Volatile fatty acids.Whey Protein and sugar rich by-product from cheese production.

Whey permeate The liquid rich in sugar formed when whey is ultrafiltered and proteins areseparated.

2.5 Disposition and audienceThis research consists of a theoretical background followed by an evaluation. To start with,the reader is introduced to the background of the Biotrans project at Norrmejerier. Anaerobicdigestion, reactor design and biogas production from dairy waste products is reviewed. This isfollowed by a description of the biogas plant at Ume Dairy.

The evaluation presented in this masters thesis consists of two main sections. The firstsection is an evaluation of plant design, process conditions, seed sludge of choice and plannedstart-up strategy. The conclusions made from this evaluation are used to formulate hypothesesregarding the second part of evaluation, considering the actual start-up and managing of the

plant. The thesis is finalised with a concluding discussion, concerning both parts ofevaluation.


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The reader is assumed to possess basic knowledge of the principles in microbiology andchemistry. Familiarity with the area of biotechnology is helpful, but not required.

When reading this thesis, it is important to keep in mind that the words methane reactors,anaerobic reactors and biogas reactors are used synonymously.


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3 Background3.1 Norrmejerier

Norrmejerier is an economic association owned by almost 700 milk farmers in the Norrbottenand Vsterbotten provinces in the north of Sweden. Norrmejerier consists of five dairies,Burtrsk Dairy, Lycksele Dairy, Ume Dairy, Hedenset Dairy and Lule Dairy, of whichUme Dairy is the largest. Every year, almost 200 000 metric tonnes of raw milk is refined toconsumption milk, cheese, yoghurt, soured milk, cream etc. by these dairies. The associationhas around 490 employees and 219 of these are active at Ume Dairy. [Norrmejerier, 2005]

3.2 Biotrans project3.2.1 WheyDisposal and utilisation of whey from cheese production has long been a problem for

Norrmejerier. The whey formed during cheese production has been concentrated byevaporation into whey powder or sold as pig food to local farmers. The transportation of thewet un-concentrated whey to pig farmers has lead to large handling and transportation costs.Furthermore, pig farming is not very common in Vsterbotten and Norrbotten. There aresimply not enough pig farmers in Norrmejeriers region of activities to find an outlet for the

whey produced. This is not a problem for other dairies in Sweden and has therefore been acompetitive disadvantage for Norrmejerier. A good utilisation of whey would hence enhancethe profitability of cheese production. [Norrmejerier, 2003a]

The consumption of low-refined dairy products such as milk, cream, soured milk etc. isdecreasing in Sweden. To make use of the surplus of raw milk produced by milk farmers inVsterbotten and Norrbotten the cheese production at Norrmejerier must increase. This will inturn lead to production of even more whey. [Norrmejerier, 2003a]

Whey powder can be used as an additive at dairies and other food industries. Unfortunately,the evaporation into powder is a highly energy demanding process. The energy needed toconcentrate the whey into powder has been as high as 40% of the total energy consumption at

Ume Dairy. Further investments in evaporation equipment have therefore not been an option.[Norrmejerier, 2003a]

Whey can also be ultra filtered into permeate and whey protein concentrate (WPC). Permeatecan be used in sweets industries and as carbon source in fermentation processes. [Atra et al,2005] The high value proteins in WPC can be used to increase protein content in or to changetexture and flavour of a range of different foods [El-Garawany and Abd El Salam, 2005].


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3.2.2 WastewaterThe wastewater from Ume Dairy has previously been treated at the municipal sewagetreatment plant at n in Ume. The content of chemical oxygen demand (COD) in thiswastewater has been as high as 15% of the total COD load at the municipal plant. This haslead to high fees for Norrmejerier and a burden on the municipal wastewater treatment plant.

3.2.3 The projectNorrmejerier aims to enhance the environmental-friendly profile of the association. A goodutilisation of the whey and reduced emissions of organic material would facilitate thisambition. [Norrmejerier, 2003a]

To reduce the problems with whey and waste disposal an anaerobic wastewater treatmentplant has been planned. In this plant wastewater and residual milk from Ume Dairy shall betreated together with whey permeate from both Ume and Burtrsk Dairy. The biogas

produced in this plant will be burned and steam produced. This hot steam will be utilised in

the production at Ume Dairy. As a part of this project, a plant for ultra filtration of wheyproducing whey permeate and WPC will be built. In addition, heat pumps and heatexchangers utilising the heat energy of the wastewater will be installed. [Norrmejerier, 2003a]

The project as a whole is called Biotrans and represented Norrmejeriers largest investmentthe financial year of 2004 [Norrmejerier, 2005]. It is partially financed by the CountyAdministration of Vsterbotten and by the European Unions LIFE program [Norrmejerier,2003a].

Due to reduced electricity needs, production of steam and heat and lowered waste fees,Norrmejerier has calculated to save up to 10 million SEK annually by the Biotrans project.

3.3 Investigations and purchase3.3.1 InvestigationsExtensive investigations were made during the planning of the plant. Different kinds oftreatment methods were considered and finally an anaerobic process for biogas productionwas chosen.

Study tours to full-scale biogas plants at dairies in Belgium, Germany and Switzerland weremade during the year of 2003. The experiences from these study-tours laid the foundations for

the plant design at Ume Dairy. The conclusions made after these studies are discussed below(see 4.5.2).

Two anaerobic digestion experiments commissioned by Norrmejerier were performed atSwedish Institute of Agricultural and Environmental Engineering (JTI), Ultuna, Sweden.These experiments confirmed that biogas could be produced from the waste products at UmeDairy. The results of the experiments at JTI are discussed further below (see 4.5.1).


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3.3.2 The contractorTwo sets of purchase were made. The offers from the first purchase were so diverse that theycould not be compared. Therefore, pre-planning was performed with help from SWECO AB(Sweden), to investigate what plant layout and process method to use. At the second round of

purchase, the same basis data could be supplied to everyone concerned.

Eventually, the tender submitted by Enviro-Chemie was chosen. Enviro-Chemie is a Germancompany, founded in 1979. The main activity is construction of custom-made plants for waterand wastewater treatment, and the company offers complete solutions including planning,construction and service. Enviro-Chemie has 165 employees.

The contractor is responsible for design, engineering, construction and start-up of the plant.They are also obliged to provide the staff of Norrmejerier with proper training. Since theresponsibility of the start-up is on the contractor, Norrmejerier shall not carry out any actionsduring start-up without first consulting Enviro-Chemie.


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4 Biogas production4.1 Anaerobic digestionAnaerobic digestion of organic compounds is a complex process, involving several differenttypes of microorganisms. The four main steps of this decomposition are hydrolysis,acidogenesis, acetogenesis and methanogenesis [de Mes et al, 2003]. The anaerobic digestion

process is illustrated in Figure 1.

Figure 1 An illustration of the anaerobic digestion process, where complex molecules aredecomposed to methane and carbon dioxide. The four main steps of this processare hydrolysis, acidogenesis, acetogenesis and methanogenesis.

4.1.1 HydrolysisEnzymes are excreted by a group of anaerobic bacteria called acidogenes. The extracellularenzymes de-polymerise proteins, carbohydrates and lipids into oligomers and monomers byhydrolysis [Lastella et al, 2000]. Smaller, water-soluble compounds such as amino acids,

sugars, alcohols and long chain fatty acids are formed [Ghaly et al, 2000].


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4.1.2 AcidogenesisAfter hydrolysis, the acidogenes convert the small water-soluble molecules by fermentation[Schink, 1997]. The most important products of this process are acetate, carbon dioxide andmolecular hydrogen [Bjrnsson, 2000]. Other digestions products from fermentation arealcohols, succinate, lactate and aromates [Schink, 1997]. Volatile fatty acids (VFA) such as

propionate, butyrate, isobutyrate, valerate and isovalerate are also formed [Griffin et al,1998].

Acetate, carbon dioxide and molecular hydrogen can be directly utilised as a substrate byanother group of anaerobic microorganisms called methanogens [Bjrnsson, 2000].

4.1.3 AcetogenesisFermentation products with more than one carbon atom can not be directly digested bymethanogens and must first be converted into acetic acid, carbon dioxide and hydrogen byacetogenic bacteria [Bjrnsson, 2000, Schink, 1997].

Acetogens are slow-growing microorganisms that are sensitive to environmental changes suchas changes in the organic loading rate (see 4.3.5) and flow rate. The acetogenic process isdependent on low partial pressure of hydrogen to remain thermodynamically favourable. Co-cultivation with hydrogen-consuming methanogens is one way to secure that the partial

pressure remains at a low level. [Bjrnsson, 2000].

4.1.4 MethanogenesisMethanogens are anaerobes and belong to the phylogenetic domainArchae. Methane can besynthesised via two different pathways, of which one involves acetate and the other molecularhydrogen. About 70% of the methane is produced from acetate. The acetate-consumingorganisms are slow-growing and highly sensitive to changes in the environment.Hydrogenotrophic methanogens grow faster and are more robust to environmental changes.[Bjrnsson, 2000]

Accumulation of VFA results in a lowered pH, which in turn leads to a decrease of themethanogenic activity. If the rate of methanogenesis is decreased, further accumulation ofVFA will lower the pH-value even more and resulting in a further decrease of methanogenicactivity. [de Mes et al, 2003]

When biogas is produced in an industrial manner, the aim is to attain as much biogas with ashigh methane content as possible.


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4.2 Dairy waste products4.2.1 Dairy waste products as a substrate for biogas productionWhey is a by-product from cheese production with high concentrations of proteins and lactose

[Ergrderet al, 2000] Cheese whey is highly biodegradable (up to 99%) and the organiccontent is high. However, the alkalinity (see 4.3.2) is low. Undiluted, raw whey thereforetends to rapidly acidify and is therefore difficult to treat by anaerobic digestion. To achievestability in the process, the whey must first be diluted or deproteinated. [Malaspina et al,1996, Ergrderet al, 2000] Mixing whey with dairy wastewater and by that means reducingthe organic concentration is one method to circumvent the issue of instability [Gavala et al,1999].

4.2.2 LactoseDuring anaerobic digestion, the disaccharide lactose is hydrolysed into D-glucose and D-

galactose. These monosaccharides are degraded to pyruvate, which in turn is converted toacetic acid. This acid is then converted to methane and carbon dioxide. [Gavala andLyberatos, 2001]

The theoretical yield of methane and carbon dioxide, i.e. biogas, produced by anaerobicdigestion can be calculated from Buswells formula [Ghaly and Ramkumar, 1999]:

242ban CO)4

b

8

a-

2

n(CH)

4

b-

8

a

2

n(OH)

2

b-

4

a-(nOHC +++!+ (Eq. 1)

For lactose (C12H22O11), the fractions are calculated as follows [Ghaly and Ramkumar, 1999]:

CO6CH6OHOHC242112212

+!+ (Eq. 2)

At room temperature (25C), this means that 0.849 l biogas is produced for each gram oflactose that is completely decomposed. For calculations, see Appendix 1. The theoretical

biogas yield is, in other words, 0.849 m3/kg lactose. [Ghaly and Ramkumar, 1999] 1.1 mg/l oflactose equals a COD of 1 mg O2/l [Balannec et al, 2005]. This implies that one kilogram ofCOD should result in the formation of 0.772 m3 (i.e. 0.849/1.1) of biogas, if the CODoriginates from lactose only. The fractions of methane and carbon dioxide in this biogas will,according to the reaction formula above, both be 50%.


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4.2.3 Milk fatMilk fat is not easily bio-degraded. To achieve a satisfactory COD-removal by anaerobicdigestion of milk fat, a relatively long hydraulic retention time (see 4.3.6) is necessary. [Omilet al, 2003] Hydrolysis has been found to be the rate-limiting step during degradation of lipids[Petruy and Lettinga, 1997]. The break-down of fat generates long chain fatty acids, whichcauses inhibition of the methanogenesis. To avoid these problems, fat can be removed fromthe rest of the feedstock before entering the anaerobic reactors. Usually this removal is made

by flotation (See 5.2.5). [Omil et al, 2003] In anaerobic treatment of dairy waste products, thefat content of the substrate should be reduced to a concentration below 100 mg/l [Perle et al,1994].

Sludge flotation and development of sludge with low activity may be attributed to thepresence of lipids in the culture [Perle et al, 1994]. Milk fat has been shown to rapidly adsorbto anaerobic sludge. If lipids cover the sludge, the gas will have difficulties to escape, thus

bringing the sludge up to the surface and causing sludge washout. [Petruy and Lettinga, 1997]

4.2.4 Milk proteinCasein is the most important milk protein, comprising 80% of the total protein content in rawmilk. The rate of casein hydrolysis is lower than the hydrolysis of lactose. [Vidal et al, 2000]

Degradation of casein has been shown to be highly dependent of biomass acclimatisation. Aculture that is pre-acclimated for casein will utilise it efficiently, but without acclimatisationthe compound will not be degraded to any significant extent. [Perle et al, 1994, Vidal et al,2000]

The presence of proteins can also influence the qualities of the sludge. It may for examplecause sludge with a slimy appearance and bad settling characteristics [Perle et al, 1994].

4.3 Process parameters4.3.1 pH-valueHydrolysis, acidogenesis and acetogenesis can proceed at a wide range of pH-values, butmethanogenesis is dependent on a neutral pH. Production rate of methane is lower for pHvalues outside the range of 6.5-7.5. [de Mes et al, 2003] If pH is maintained within theoptimum range of 6.8-7.2, the percentage of methane in the produced biogas will be at itsmaximum. [Ghaly, 1996].

As stated above, lactose is easily degraded by acidogenic microorganisms. If not correctlypH-controlled, this will lead to acid inhibition of the methanogenic stage and digester failure.Proper pH-control will increase the COD-reduction, the biogas production rate and themethane yield of the process. [Ghaly, 1996]

High concentrations of free ammonia in the anaerobic culture may cause a partial inhibition ofthe biogas formation. On the other hand, presence of ammonia facilitates the regulation of pHand may by that means prevent VFA inhibition. [Vidal et al, 2000]


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4.3.2 AlkalinityAlkalinity of a liquid is the sum of all the titratable bases and is a measure of its acid-neutralising capacity. The neutralising qualities are primarily due to the presence ofcarbonate, bicarbonate and hydroxide ions.

Due to the low alkalinity of cheese whey, extra addition of chemicals to increase this valuemay be needed for successful biogas production. For this purpose sodium bicarbonate(NaHCO3) is found to be most suitable [Rajeshwari et al, 2000].

If both VFA concentration and alkalinity are monitored, the ratio of these can be calculated.An increase of this ratio precedes a pH decrease and implies that the digester is about to

become unstable. [Griffin et al, 1998]

4.3.3 TemperatureAnaerobic digestion and biogas production can operate at three different temperature ranges:

psychrophilic (0-20C), mesophilic (20-42C) and thermophilic (42-75C). The higher thetemperature, the shorter digestion time needed to attain a specific rate of biogas production.The hydrolytic and acidogenic phases of anaerobic digestion are not very sensitive towardstemperature changes. On the other hand, the effect of temperature on acetogenic andmethanogenic microorganisms is significant. [Rajeshwari et al, 2000] Propionate oxidizershave been found to be most sensitive to temperature changes of all microorganisms in theanaerobic culture [Leito et al, 2005]. Thus, raising the temperature over the range ofoperation will lead to inhibition of methane formation [Rajeshwari et al, 2000]. A shockchange in temperature may also lead to pH drop in the reactor. This drop is due to temperatureinhibition of methanogenesis, leading to accumulation of fermentation products. [Leito et al,2005].

Anaerobic digestion of cheese whey and dairy wastewater at temperatures between 20-42C(i.e. within the mesophilic range) has been recommended by several authors [Ghaly, 1996,Kalyuzhnyi et al, 1997, Rajeshwari et al, 2000]

4.3.4 Nutrients and trace elementsNutrients and trace elements such as phosphorous, nitrogen, sulphur, calcium, potassium,iron, nickel, cobalt, zinc and copper are essential for optimum activity of the microorganismsinvolved in anaerobic digestion [Rajeshwari et al, 2000].

Iron, nickel and cobalt are vital parts of important enzymes in anaerobic organisms. Nickelconstitutes, for example, the active centre of acetate formation enzymes and H2-consuminghydrogenases. As a part of different hydrogenases, iron is essential for H2-uptake. Cobalt

plays an important role in the transfer of methyl-groups by the enzyme colbamin. [Aresta etal, 2003]

It has been shown that addition of trace elements and nutrition is essential for anaerobicdigestion of cheese whey [Ergrderet al, 2000]. Further, an anaerobic digestion experiment atJTI showed that addition of cobalt enhanced the gas production when dairy waste productswere treated [Nordberg et al, 2002].


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4.3.5 Organic loading rateThe organic loading rate (OLR) of a process is a measure of how much organic material thatis fed to each cubic meter of reactor volume during on day (i.e. kg COD/m3day).

How high OLR an anaerobic reactor can withstand is highly dependent of the reactor design.

Attached biomass reactors, such as fluidized bed and fixed film reactors (see 4.4.1), canendure higher OLR:s than non-attached biomass reactor types. Overloading of the reactorwith organic matter can lead to sludge washout. [Rajeshwari et al, 2000]

Large fluctuations of flow and concentration of substrate can affect the biogas production.The magnitude of these effects depends on reactor design and process conditions.Accumulation of VFA in the culture and drastic changes in the biogas production rate aretypical responses to such variations. [Leito et al, 2005]

4.3.6 Hydraulic retention timeHydraulic retention time (HRT) of an anaerobic digester is the average time a volume elementof the liquid medium resides inside the reactor. When all other parameters are remainingconstant, an increase HRT leads to better productivity [Ghaly, 1996].

Anaerobic digestion can be performed with a relative short HRT, i.e. high rate systems, orwith long HRT, i.e. low rate systems. High rate systems are usually used for treatment ofwastewater, while low rate systems are applied on slurries and solid waste. To achievestability and avoid sludge washout, high rate systems may require some kind of biomassrecirculation or attachment, resulting in a sludge retention time longer than HRT. [de Mes etal. 2003]

4.4 Reactor design4.4.1 High-rate anaerobic reactorsHigh-rate anaerobic digesters are designed with emphasis on retaining a high viable biomassconcentration inside the rector. This can be achieved with several different methods involvingeither biomass aggregation or biomass adhesion. [Rajeshwari et al, 2000] A few of thesemethods are discussed below.

Biomass can be attached to high density particulate carrier materials such as sand or activatedcarbon. This method is used in anaerobic fluidized bed reactors (AFB). The attachment of

microorganisms to carrier materials enables higher biomass concentrations inside the reactorand an opportunity for higher OLR:s. [Rajeshwari et al, 2000]

Biomass support media such as PVC (polyvinyl chloride) supports, hard rock particles orceramic rings can be introduced into the reactor. One example of such a reactor is the fixedfilm reactor (FFR) It is simple in construction and do not require any kind of mechanicalmixing. It can withstand high organic loading rates, but since a large volume is occupied bythe media, the reactor volume must be relatively big. [Rajeshwari et al, 2000]


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The formation of sludge granules facilitates biomass settling and prevents biomass washout.This is the case in the upflow anaerobic sludge blanket reactor (UASB), in which the substrateis introduced from the bottom and passes upward through a granular sludge bed. The UASBreactor requires low investments, but has a long and sensitive start-up period. When in steady-state, it can withstand high organic loading rates. [Rajeshwari et al, 2000]

When utilising whey as a substrate for anaerobic digestion, granulation of the biomass is notlikely to occur. Using UASB reactors for whey degradation is therefore difficult. [Malaspinaet al, 1996, Ergrderet al, 2000] Despite this, a few successful attempts to treat whey in aUASB digester have been made [Kalyuzhnyi et al, 1997, Ergrderet al, 2000].

Another method of retaining high biomass concentration is separating and recirculatingbiomass in effluent streams. This method is mainly applied to reactors without biomasssupport media, such as the continuously stirred tank reactor (CSTR). In the CSTR,mechanical agitation ensures that wastewater and sludge always is in contact. A CSTRsupplemented with biomass separation and recirculation with a clarifier is called an anaerobiccontact (AC) process. [de Mes etal, 2003]

The AC process is suited for soluble organic waste streams with high strength andbiodegradability. Due to the need of an external clarifier the space requirement of an ACprocess is relatively high. [Given et al, 2005]

4.4.2 Two-phase layoutTwo reactors of the same or different types can be combined to achieve a so called two-phaselayout. Spatially separating the hydrolytic/acidogenic and methanogenic steps into individualchambers will according to several authors decrease the risk of acid inhibition of themethanogenesis [Malaspina et al, 1996, Rajeshwari et al, 2000, Yang et al, 2003]. Comparedto a single-phase digester, a two-phase arrangement may increase the acidogenic rate andallow a reduction of the total reactor volume [Liu et al, 2002].

The products from the hydrolytic/acidogenic stage should be volatile acids and some carbondioxide. The volatile acids act as substrate for the second methanogenic stage, where methaneand carbon dioxide is formed. [Ghaly and Ramkumar, 1999]


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4.5 Investigations made by NorrmejerierThe two most important investigations made by Norrmejerier are the anaerobic experiments atJTI and the study tours to full-scale plants. The results from these investigations have

significantly influenced the choice of plant layout and process conditions of the biogas plantat Ume Dairy.

4.5.1 Anaerobic digestion experiment at JTIDuring planning of the project, two anaerobic digestion experiments were performed atSwedish Institute of Agricultural and Environmental Engineering (JTI), Ultuna, Sweden.Wastewater and residual milk from Norrmejerier and whey permeate from Bollns wastreated in an anaerobic filter (i.e. anaerobic reactor with carrier material). The experimentsindicated that anaerobic digestion of wastes from Norrmejerier is feasible. Some of theconclusions drawn by JTI were [Nordberg et al, 2002]:

- At a temperature of 37C and with a HRT of 3 days, COD can be reduced with 84%.The methane yield was 0.28 g/l COD. A methane percentage of 55% was achieved.

- Addition of cobalt improved the COD reduction and methane yield.- pH - regulation of the process is vital. During the experiments sodium bicarbonate

(NaHCO3) was used for this purpose.

4.5.2 Study tours to full-scale plantsStudy tours to full-scale biogas plants at dairies on the continent were very informative for

Norrmejerier. Some of the most important conclusions drawn from these study tours byNorrmejerier were as follows [Norrmejerier, 2003b]:

- Fat in the wastewater must be removed by flotation and treated separately.- Due to problems with clogging, filling material for biomass support in the reactor is

not suitable.

- Continuously stirred tank reactors with sedimentation in a clarifier (i.e. contactprocess) are recommended.

- Regulation of pH with NaOH is needed.- The need of alkalinity enhancing substances such as NaHCO3 is small.


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4.6 Proper start-up strategyWithout proper start-up of the anaerobic reactor, the system is not likely to reach itsmaximum degree of efficient removal of soluble nutrients and organic matter [Liu et al,2002]. It is, especially in mesophilic conditions, important to use a gradual start-up approach.During the first few days of feeding, caution is extra crucial. [Griffin et al, 1998]

The source and size of seed sludge is together with the initial mode of operation the mostcritical factors during start-up of anaerobic digesters. The inoculated volume should be atleast 10% of the total reactor volume. [Griffin et al, 1998] It is beneficial to use sludge from areactor that is treating similar waste products. If that it not possible, acclimatisation of thesludge to the substrate in question is needed. [Wolmarans and de Villiers, 2002] Griffin andcolleagues (1998) suggested that it may not always be advantageous to choose inoculum froma stable, well-balanced digester. For an effective start-up, it is more important that there arehigh levels of methanogens present in the seed sludge.

If instability in the process arises during start-up, different approaches are recommended. Oneschool of art is to add NaHCO3 to enhance of the alkalinity and decrease the rate of feeding[Griffin et al, 1998, Bjrnsson, 2000]. Another claims that decrease of feeding rate issufficient and that no chemicals should be added [Griffin et al, 1998].


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5 The plant at Ume DairyAs discussed above, a range of variables may contribute to the overall performance of a

biogas plant. Some of these are plant layout, reactor design, feedstock characteristics, processparameters, seed sludge and start-up procedure.

5.1 Feedstock characteristicsIn the biogas plant, wastewater from Ume Dairy is treated together with whey permeate fromUme and Burtrsk Dairies. Residual milk (i.e. a mixture of different dairy waste productsand water) is also fed to the plant, together with the content of a vessel called the pig tank.

The content of this tank is quite similar to residual milk in characteristics and was formerlysold as pig food, therefore the name.

During the anaerobic digestion experiment at JTI [Nordberg et al, 2002], wastewater andresidual milk from Ume Dairy and whey permeate from a dairy in Bollns were mixed. Thefractions of the mixture were proportioned to simulate the incoming substrate to the full-scale

plant. After the mixture had been prepared, it was found that the whey permeate from Bollnswas a concentrate. To achieve a suitable COD load, the mixture had to be diluted with water.The mixture was sent to an external laboratory for analysis. The results from this analysis are

presented in Table I, Appendix 2. Due to the dilution, the absolute values obtained in thisanalysis do not correspond to the real values at Ume Dairy.

The plant is dimensioned for an organic load of 15000 kg COD/day and an average flow of1400 m3/day, but with variations from 800 m3/day to 1700 m3/day. This load consists of awastewater flow of 1100 m3/day, 250 m3/day of whey permeate and 25 m3/day of residualmilk (including the pig tank). The temperature of incoming substrates is 22-25C, and theaverage total fat load is 200 mg/l. Flow rates, pH and COD content of the waste products are

presented in Table 1.

Table 1 The characteristics of the waste products at Ume Dairy.

Waste product Flow rate (m3/day) COD concentration (mg/l) pH

Wastewater 1100 1200 9 - 11

Whey permeate 250 44000 6

Residual milk 25 70000 7

The variations of pH in the wastewater are quite large. This variability is attributed to acid(HNO3) and base (NaOH) washing of the process tanks at the dairy.


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5.2 Design and technology5.2.1 Plant layoutThe wastewater treatment plant at Ume Dairy uses anaerobic technology to produce biogas

and reduce the polluting potential of the organic waste products. The energy rich biogas isused to produce steam utilising a steam boiler.

The plant consists of, but is not limited to: pumping station and sieve, mixing and equalisationtank, fat flotation, conditioning tank, two methane reactors, clarifier, post-aeration, fattreatment reactor, biofilters, steam boiler and flare. These units are described below. Inaddition to these components the plant also includes dosing stations for chemicals, a sludge

buffer tank, heat exchangers, heat pumps and a condensation trap.

The plant is controlled by a programmable logic controller (PLC) from Siemens (Sweden).The visualisation system InTouch (Wonderware Corporation, Sweden) assists and facilitatesthe management of the plant. The choice to use InTouch for visualisation at the biogas plant

was made by Norrmejerier. The same system is used for visualisation of production at UmeDairy.

Figure 2 shows the outlines of the plant and the flows of water and gas. A more schematicfigure of the plant layout is presented in Figure I, Appendix 3.

Figure 2 The plant, including reactors and other vessels, main building and the flare.(Courtesy of Norrmejerier.)


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5.2.2 ObjectivesThe organic loading rate to the methane reactors will be around 2.6 kg COD/m3 and the HRTis calculated to 3.6 days. The contractor has guaranteed a COD reduction of at least 80%. It isclaimed that each kilogram of COD gives rise to 0.35 m3 of biogas with a methane content of70%. The total rate of biogas production, including gas produced in the fat treatment, iscalculated to 177 m3/h. There shall be no odour emissions, not within the plant, nor in theoutgoing air.

5.2.3 Pumping station and sieveA drum sieve removes any solids larger than 0.8 mm from the wastewater flowing from thedairy. The water flows by gravity to a pumping station, from where it is pumped to the mixingand equalisation tank.

5.2.4 Mixing and equalisation tankThe mixing and equalization tank (MET) has a volume of 800 m3. Here discontinuouswastewater discharge from the dairy, residual milk and whey permeate is equalised.Temperature and pH-value is monitored and regulated. In the MET the organic material ishydrolysed and pre-acidified; organic acids are produced.

5.2.5 Fat flotationFrom the MET the wastewater flows continuously to the flotation unit. In the fat flotationunit, the wastewater flows through an open basin while bubbles of air well up from the bottomof the vessel. The bubbles lift up particles to the surface of the liquid, where sludge is formed.The flotation sludge, which consists mainly of fat and proteins, is removed from the surfacewith a scraper. The capacity of the fat flotation unit is removal of 1000 1500 kg fat/day. Thatrepresents about 20% of the total COD load.

The sludge is collected and is then pumped to the fat treatment tank for further processing.The excess sludge is pumped to the fat buffer tank for disposal.

5.2.6 Conditioning tankAfter flotation the water flows continuously to the conditioning tank. The pH-value ismeasured and regulated by addition of sodium hydroxide (NaOH) or hydrochloric acid (HCl).If necessary, the conditioning tank can be used for dosing of nutrients. The conditioned

wastewater is distributed into the feeding system of both methane reactors.

5.2.7 Methane reactorsThe two methane reactors operate in parallel. They are constructed as CSTR:s withrecirculation, that is, they are designed as AC processes. Mixing of the reactors is performed

by one main agitator situated at the top, complemented with two smaller agitators at thebottom of each reactor. A baffle in each reactor facilitates the mixing. Each methane reactoris 15 m high and has a volume of 2500 m3. The HRT is calculated to an average of 3.6 days.


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The temperature of the wastewater fed to these reactors shall be between 35-38C. pH of theculture is monitored and shall be regulated to 6.8-7.2. It is stated by the contractor that theoptimal ratio of COD: N: P is 800: 5: 1.

After it has been heated with two heat exchangers, the wastewater is fed to the top of thereactors. The top agitator pumps the substrate down through a central pipe. At the end of the

pipe, the wastewater flows upwards through the reactor. During this circulation, organicmaterial in the wastewater is digested by anaerobic microorganisms and biogas is formed. Atthe top of the reactor, the treated water is collected and enters a degassing device, where

biogas is removed from the liquid. The effluent water is then transferred to the clarifier.

Sample ports are positioned at five different heights on each reactor. The lowest is situatedfour meters from the bottom of the reactor.

5.2.8 ClarifierA clarifier separates and recirculates biomass in the effluent from the methane reactors. The

effluent water enters a funnel with lamellae, where the sludge is separated. It is thenrecirculated to the methane reactors. The volume of the clarifier is 400 m3.

The methane reactors and the clarifier are illustrated in Figure 3 and Figure 4.

Figure 3 Schematics of the clarifier and the methane reactors. The clarifier has a funnelwith lamellae, where the sludge is separated. The methane reactors have topagitators and baffles. (Courtesy of Norrmejerier.)


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Figure 4 Influent and effluent wastewater streams of the clarifier and methane reactors.The uppermost arrow symbolises the flow of biogas from the reactors. (Courtesyof Norrmejerier.)

5.2.9 Fat treatmentAs fat is collected in the fat flotation unit, it is transferred to the fat treatment process. In avessel with a volume of 125 m3 anaerobic microorganisms digest the fat and biogas is formed.Addition of enzymes can make the process more efficient, splitting tri-glycerol into fatty acidsand glycerol. The HRT of this process is 22 days.

The plant also includes a vessel for storage of excess sludge. During start-up, this tank wasused as a fat buffer. The two vessels were constructed by modification of two wastewaterneutralising tanks formerly used by the dairy.

5.2.10BiofiltersIn the two biofilters, odours in the exhaust air from the post aeration, clarifier, sieve and fattreatment is removed. The air is pressed through a mulch bed before it leaves the filtersthrough a ventilation system to the atmosphere. Microorganisms in the mulch break downodorous compounds, and the gas becomes odourless. The biofilters are illustrated in Figure 5.

Figure 5 A biofilter and the mulch bed therein. Gas flows into the biofilter and odorouscompounds are decomposed. (Courtesy of Norrmejerier.)


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5.2.11Steam boiler and flareThe steam boiler is made by Weishaupt (Sweden) and has en effect of 1.5 MW. The flare isoriginally dimensioned for a flow of 250m3/h, but will later be upgraded to process a flow of500 m3/h.

5.2.12Post- aerationThe effluent from the clarifier is aerated using compressed air. Since the microorganisms areanaerobes, this aeration stops the fermentative and methanogenic activity before the water isdischarged to the municipal sewer.

5.3 Planned start-up procedureStart-up of the anaerobic reactors is planned by the contractor. The key elements of the planare filling the reactors with water, initial warm-up, adjustment of pH-value, filling of seed

sludge, additional warm up, start feeding wastewater and start feeding whey.

The entire start-up procedure is planned to proceed for 18 weeks, but no actual time limit forthe start-up procedure is given by the contractor. However, it has been stated that reaching fullcapacity can take up to six months.

The flow of wastewater is planned to start from day one, with a flow of 500 m3/day. It shouldbe gradually increased during 18 weeks, reaching 2 780 m3/day the last week of start-up.

Addition of whey is planned to start week number three. The flow of whey or whey permeateis planned to increase with 14 m3/day each week (i.e. 212 m3/day week 17), with a CODconcentration of 52 000mg/l. In the start-up plan, neither the material in the pig tank nor the

residual milk is taken into account. An outline of the increase of COD load during start-up isdemonstrated in Figure 6. During the start-up period parameters such as HRT, temperatureand pH are to be maintained constant and at the values planned for steady-state operation.

Figure 6 The outlines of the planned start-up procedure between 18/2 (day 1) and 8/6(day 110). The figure illustrates an approximation of the planned increase in

COD load to the plant.


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5.4 Seed sludgeSeed sludge for inoculation of the anaerobic reactors was collected from two differentsources. The largest volume was taken from the biogas reactor at the municipal sewagetreatment plant at n in Ume. This reactor was primarily chosen because of the shorttransportation distance, and not because of good sludge quality. A smaller volume of sludgewas transported from Germany, where it had been collected from an UASB reactor at a papermill. At the initial inoculation, a volume of about 200 m3 was added to each of the twomethane reactors.

The decision of which seed sludge to use was not made until quite a short time before start-up. Choice of inocula was thus not made at the time of plant and reactor design, nor when thestart-up plan was prepared.

5.4.1 Municipal sewage treatment plant at n, UmeThe digester at the municipal sewage treatment plant has a HRT of 20 days. Primary

municipal sewage sludge is used as a substrate, and approximately 50% of the organic contentin it is reduced in this process. The methane content of the biogas produced at n ranges 60-65%. The performance of the digester has been stable since start-up.1

The seed sludge from n was black and smooth without granules. It had an oily appearanceand a scent of diesel. The total solids (TS) content of this sludge was around 2%. About 150m3 of this municipal sewage sludge was pumped into each reactor.

The seed sludge from the municipal sewage treatment works was tested by Enviro-Chemie.Conclusions drawn from these tests were [Engelhart, 2004]:

- The sludge settling characteristics is poor. Only 1/4 of the sludge volume is settling onthe bottom, while 3/4 forms a floating layer on top of the vessel.

- The suspended solids (SS) content is 20.6 g/l, which is low for municipal digestersludge.

- A methanogenic activity test with acetic acid showed a lag phase (i.e. no gasproduction) of several hours, followed by steady but poor methane production.

- The sludge quality is poor and the low settleability will cause sludge washout duringstart-up.

5.4.2 Paper mill in GermanySeed sludge was also collected from an UASB reactor at a paper mill in Germany. Thesubstrate treated in this reactor was paper mill wastewater. The sludge was dark grey, wellgranulated and had a TS of 7%. During the primary inoculation in February, 50 m3 granularsludge was inoculated in each reactor. At re-seeding in June, an additional volume of 25 m3sludge from Germany was inoculated in each reactor. Altogether, 150 m3 seed sludge wastransported from the paper mill in Germany. One tanker with seed sludge from Germany witha volume of 25 m3,costs about 30000 SEK, including transportation.

1 Personal communication with Per Rendahl, UMEVA, Ume


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6 Discussion of plant and process design6.1 Plant layout and reactor designThe MET offers an opportunity to equalise the incoming substrate. Since the variationsregarding pH and COD load in the wastewater from the dairy are so significant, such a vesselis necessary to achieve a stable operation. For example, a batch of whey from Burtrskrepresents more than 30 m3 of high-strength organic material that needs to be properly diluted

before entering the methane reactors. The MET is, however, quite small and has a HRT ofonly about 14 h. A larger tank would equalise the COD flow and pH fluctuations even more.

The conclusion that fat has to be removed and treated separately is in accordance with what is

recommended in the literature [Omil et al, 2003, Perle et al, 1994]. To prevent instability andsludge washout, it is obviously important that as little fat as possible enters the methanereactors. The use of a flotation unit to remove fat from wastewater is a standard method, andseems to be suitable.

Since granulation of sludge is not likely to occur when dealing with dairy wastewater andwhey, an UASB reactor would have been inappropriate. However, successful attempts todigest dairy waste products in an UASB reactor have been made [Kalyuzhnyi et al, 1997,Ergrderet al, 2000]. A reactor with carry material such as the fixed film reactor would also

be unsuitable, due to problems with clogging attributed to the presence of fats and calcium inthe influent substrate [Norrmejerier, 2003b].

The contact process is, however, a high-rate anaerobic process that is suitable for treatment ofwastewater with high organic content. It seems to be an appropriate choice for Norrmejerier.Because of the short HRT, it is essential to include a device for sludge separation andrecirculation. The use of a clarifier for this purpose is a common method.

The two methane reactors are to be operated in a parallel manner. Many authors haverecommended a two-phase process, where hydrolysis/acidogenesis and methanogenesis areseparated [Malaspina et al, 1996, Rajeshwari et al, 2000, Yang et al, 2003]. A two-phase

process is less sensitive and easier to pH-regulate, and is therefore suitable for treatment ofthe easily-degraded substances of dairy waste products. It is possible that a two-phase designcould have been applied at Norrmejerier.

6.2 Process parametersOperating a mesophilic anaerobic digester at a temperature of 35-38C is suitable and inaccordance with what has been recommended in the literature. The ambition to retain the pHvalue in the methane reactors between 6.8 and 7.2 is in line with what has been concluded inother studies.

Organic loading rates as high as 20 kg COD/m3day from dairy wastewater and whey havebeen reported by several authors [Ghaly, 1996, Kalyuzhnyi et al, 1997]. However, none ofthese studies were performed in an anaerobic contact process.


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Since cheese whey and dairy wastewater is easily degradable it can be digested at short HRT.It can be as short at 2-3 days in an UASB reactor treating un-diluted whey [Ergrderet al,2000]. This despite the common view that UASB:s are unsuitable for digestion of dairy waste

products.

The HRT and OLR of the methane reactors at Ume Dairy are not extremely high, nor lowcompared to other studies. These values are therefore presumed to be suitable.

The HRT for the fat treatment (e.g. 22 days) seems to be appropriate, since anaerobicdigestion of fat requires a relatively long hydraulic retention time. It can, however, bediscussed whether thermophilic processing of the fat would have been an option. It wouldenhance the productivity, hence allowing a shorter HRT to be implemented. On the otherhand, finding a suitable thermophilic seed sludge would probably be difficult.

6.3 FeedstockAs mentioned above, several trace elements and nutrients are essential for optimal activity ofthe microorganisms involved in anaerobic digestion. Examples of such substances are

phosphorus, nitrogen, sulphur, calcium, potassium, iron, nickel, cobalt, zinc and copper.

The waste products from Ume Dairy were analysed during the anaerobic digestionexperiments at JTI. Since the sample contained concentrated whey permeate, it was dilutedwith water to obtain a suitable COD concentration before analysis. Therefore, the exact valuesof this test result do not correspond to the concentrations in the feedstock. It is, however,assumed that the result from this test can be used to get an indication of the feedstockcharacteristics.

Nitrogen, sulphur and phosphorus are most likely present in required amounts. That is also the

case for calcium, since the concentration of calcium in milk is quite high. The substrateprofile from the experiments at JTI shows that also potassium is present in relatively highconcentrations.

The substrate profile gives no information regarding the levels of iron, zinc or copper. It istherefore impossible to judge whether the concentrations of these elements are sufficient.

Nickel and cobalt may be present in the substrate, but in low concentrations. The experimentat JTI showed that addition of cobalt enhanced the biogas production. Other studies made ondairy wastes show that addition of trace elements to the anaerobic digester is vital.

Further investigations on this issue should be made. It is possible that addition of cobalt,copper, iron, zinc or nickel would enhance the overall performance of the plant.

6.4 Seed sludgeThe settleability and activity tests of the sludge from the municipal treatment works at n inUme showed that the sludge quality is poor. Despite this, it was decided to use the sludge asinoculum. That seems to be an inappropriate choice; the activity and settleability of the sludgeis too poor. Studies have shown that the seed sludge needs to be carefully acclimated in orderto digest the milk protein casein. Without proper acclimatisation, casein will inhibit thedigestion process.


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Since no sludge of good quality was found within a close geographical range, purchase andtransportation costs must be considered in relation to lack of profit due to ineffective start-up.In Norrmejeriers case, every day of full production at the biogas plant and the filtration ofwhey, more than 27000 SEK will be saved (e.g. 10 millions per year.) When comparing this

profit with the cost of good quality sludge from Germany (e.g. 30000 SEK per 25 m3), it is

obvious that it would have been beneficial to transport more sludge from the continent.The decision of which seed sludge to use was made only a short time before start-up. That is,there was no possibility to take the quality of the seed sludge into consideration at plantdesign or in planning of start-up. The quality of the sludge is an important factor, and thestart-up plan must be adjusted to suit the seed sludge of choice. Had it been clear that such

poor quality sludge as that from the municipal sewage treatment works at n was to be usedfor inoculation, maybe a more delicate start-up procedure would have been planned.

6.5 Planned start-up procedureThe start-up plan was settled before the choice of inoculum was made. Thus, it is not adaptedto the poor quality and settleability of the sludge. Further, the plan leaves no room fordisturbances or practical complications. It is therefore obvious that the time plan is toooptimistic. To what extent the start-up will be prolonged is, however, hard to tell. Theliterature has little information to offer regarding recommended start-up procedures ofanaerobic contact processes. In fact, the start-up plan provided by the contractor is also quitevague. For example, it offers no information regarding within which ranges parameters suchas concentrations of COD, VFA and nutrients should be retained. The only information givenregarding the OLR is that the feed of whey should be increased with 14 m3 per week, and thatthe flow of wastewater should be gradually increased from 500 m3/day to 2780 m3/day.


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7 Conclusions regarding plant and process designThe plant design and process conditions agree with what is recommended in the literature.The objectives are in level with results from other anaerobic treatments of dairy waste

products, including the anaerobic digestion experiment performed at JTI. This indicates thatdesign as well as process conditions are suitable for anaerobic digestion of wastewater andwhey at Ume Dairy.

The poor quality of the seed sludge from the municipal wastewater treatment works maycause problems such as ineffective acclimatisation and sludge washout during start-up.Further efforts to find more appropriate seed sludge should have been made and the decisionof which sludge to use should have been made at an earlier stage of the project planning.

The start-up plan is too optimistic and offers no room for disturbances. It will therefore takelonger time than planned before the start-up is completed and steady-state has been achieved.


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8 Hypotheses regarding start-upSince the design and process conditions agree with what is recommended in the literature andthe objectives are in level with results from other anaerobic treatments of dairy wastes, it ishypothesized that the performance of the anaerobic methane reactors will be satisfactory andthat the objectives of the project will be achieved.

Since whey and other dairy wastes are easily bio-degraded, pH regulation is essential. It ishypothesized that relatively large amounts of pH-increasing additive will be needed tomaintain a stable operation.

The start-up plan leaves no room for disturbances and is therefore assumed to be toooptimistic. Even though the aims and goals eventually will be achieved, it will not be done

within the allotted time.The seed sludge is mainly from a municipal sewage treatment works and has poor qualities. Itwill acquire time to acclimatize and adapt to the new substrate. This will complicate the start-up and make the process particularly sensitive.

8.1 Questions at issueTo help investigate the validity of the hypotheses, a few questions have been formulated.

- Will the start-up of the biogas plant at Ume Dairy be successful, despite thesensitivity of the process due to easily bio-degraded substrate and unsuitable seedsludge?

- Will the objectives of the project be achieved and will this be done within the 18weeks suggested in the start-up plan?

- Will the interaction and cooperation with the contractor be successful?


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9 Course of events9.1 Start-up procedureThe start-up of the biogas plant at Ume Dairy began in February 2005. Before the plant wastaken into operation, it was inspected by local authorities.

9.1.1 Start of mixing and equalisation tankThe MET tank was started about four weeks before the rest of the plant was set into operation.During this period, it was found that the temperature of the wastewater from the dairy was

higher than initially believed. It was also confirmed that the variations of pH were significant.

9.1.2 InoculationThe reactors were completely filled with water and warmed to 35 C. The pH-value was thenadjusted. Both methane reactors were inoculated with seed sludge from n and fromGermany. Each reactor was filled with 200 m3 of sludge, and about one quarter of thisoriginated from Germany. The inoculation was performed in stages during a period of sixdays, from 16/2 to 21/2.

Additional inoculation was performed 2/6. This time, additional 25 m3 of sludge fromGermany was inoculated in each reactor.

9.1.3 FeedingFeeding of the reactors with wastewater started on the third day of inoculation (i.e. 18/2), butonly a fraction of the wastewater was fed to the reactors. The initial loading rate was about0.12 kg COD/m3 day.Addition of whey started after about two weeks, on 4/3. How much whey or whey permeateto be fed was decided the same day and the decision was made depending on the VFA andCOD concentrations in the reactors (see 9.2 and 10.3) and to meet the wishes of the dairy.Please notice that it is not always quite clear when whey and when whey permeate is fed to

the plant.Addition of residual milk and material from the pig tank started after yet another week (i.e.after about three weeks of operation, 9/3). Residual milk was thereafter fed every day, andmaterial from the pig tank every weekday.

After one month of operation (i.e. 18/3), whey from Burtrsk was received for the first time.Such deliveries were made from time to time during start-up, often a few days in a row.


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9.1.4 Start-up of fat treatmentThe fat flotation unit was started as soon as the flow of wastewater to the plant began, so thatas little fat as possible should pass into the anaerobic reactors. Due to problems with theagitator installations, the start-up of fat treatment was postponed. As the reactor was ready forinoculation, it was, for several different reasons, decided to postpone the start-up further.

During this period, fat separated in the flotation was either bypassed to the municipal sewer orstored in the sludge buffer tank.

9.1.5 Start of steam boilerThe steam boiler was set into operation 11/5. At this date biogas was produced in sufficientamounts and the methane content was high enough.

9.2 Variables monitoredDuring start-up and operation of the plant, several parameters were monitored. To follow theOLR and degradation rate of the reactors, COD concentration was measured daily in theMET, after flotation, in methane reactors and in the final effluent. Under the period of start-upin-line COD sensors were installed at three positions: before and after MET and in the finaleffluent. To ensure that the OLR was not too high, VFA concentration in the methane reactorswas measured on a daily basis. During the first period of start-up, VFA in the MET was alsomeasured.

Influent, effluent and recycled flow of the methane reactors were monitored as well as flow offinal effluent to the municipal sewer. Total flow of waste products into the plant, that is, to theMET, was also recorded. pH and temperature of influent and effluent was monitored andregulated to maintain the process conditions of choice. Together with the different flowvalues, the value of these parameters were read and recorded daily at 10:00.

The biogas flow and the methane content of the produced biogas was measured and recordedonce a day. Since no measure device for gas flow or methane content was on hand until 4/5,no such values are available before that date.

During the first period of the start-up, additional analyses were made approximately once aweek. Ammonium (NH4-N), nitrate (NO3-N), nitrite (NO2-N), total nitrogen and phosphate(PO4-P), were monitored to ensure that nitrogen and phosphorus was present in properamounts in the culture. Later on, these values were not analysed.

9.2.1 Analytical proceduresCOD, VFA, ammonium, nitrate, nitrite, total nitrogen and phosphate were analysed with DrLange (Hach Lange, Germany) cuvette tests, using the standard methods recommended forsuch test kits. Samples from the methane reactors were filtrated before analysis, thus notincluding the biomass. Samples from the MET, after flotation, final effluent etc. were notfiltrated before analysis.


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9.3 Problems encountered and measures takenDuring start-up, several practical problems were encountered. Most difficulties were of atechnical nature due to design or installation flaws. Some of the more critical and significant

problems that arose are discussed below.

9.3.1 Inconveniences due to inadequate plant designMany design flaws were identified during start-up. Even though they were of a technicalnature attributed to inadequate plant design, these flaws are reviewed and discussed as start-up problems in this study.

The capacity of the steam boiler was found to be too low. This inconvenience seems to be theresult of incorrect calculations. It has been speculated that the steam boiler may only have onethird of the needed capacity. The biogas yield and methane percentage in the biogas is higherthan initially expected, and that may be some of the reasons for this inconvenience.

The consumption of NaOH for pH-regulation was larger than calculated. With a NaOHconsumption of around 10 m3/week, there is a need for weekly deliveries of NaOH to the

plant.

Though the methane reactors are designed as CSTR:s and the content should be completelymixed, it has been obvious during start-up that this is not the case. The biomass concentrationis higher at the lower parts of the reactors. Sampling ports should therefore have beeninstalled at the very bottom of each methane reactor. With the first port situated four metersabove bottom level, there is no possibility to access the region with the highest biomassconcentration. A valve at the exterior of each reactor can be opened if the culture at the

bottom needs to be accessed. This is, however, probably a quite complicated procedure,especially during wintertime.

The lack of proper insulation of measuring equipment and valves located outdoors causedseveral problems during the first period of start-up, which took place in wintertime. Theoutdoor temperature during this period ranged between -5 and -20C. As the equipment froze,it gave incorrect signals. For example, the level indicators in one methane reactor incorrectlyshowed that the tank was about to overflow on one occasion.

The basin of the fat flotation unit was not equipped with any kind of cover in the originaldesign. This caused extensive emissions of odours in the main building and the problemremained for several weeks. Later on and at the initiative of Norrmejerier, the basin wascovered. The cover removed the odours almost completely.

During start-up, it was found that gas was produced in the clarifier. This had not beenconsidered at plant desi

Documents

The Biogas Production Plant at Umea Dairy Evaluation of Design and Start Up