Master of Science Thesis in Technical Biology
Department of Thematic studies - Environmental Change, Linköping
University, 2020
Investigations of the Effects
of Lowering the
Temperature in Full Scale
Mesophilic Biogas
Digesters at a Wastewater
Treatment Plant
Mesophilic Biogas Digesters at a Wastewater Treatment Plant:
Ella Wilhelmsson Supervisor: Emma Fälth
Investigative engineer, Nodra
Bodil Widell
Manager Development and Laboratory, Nodra
Eva-Maria Ekstrand
Postdoc,TEMA, Linköpings University
Examiner: Annika Björn
Senior Lecturer,TEMALinköpings University
Division of Environmental Change
Department of Thematic studies - Environmental Change Linköping University
SE-581 83 Linköping, Sweden Copyright © 2020 Ella Wilhelmsson
Abstract
This thesis has investigated the effects of running the two full scale biogas digesters at Slottshagen wastewater treatment plant at 34 °C compared to 37 °C, in terms of pro-cess stability, biogas production and energy savings with the aim of saving energy and money by not heating the digesters as much. The main objective was to investigate whether it is at all possible to operate the biogas process at 34 °C or if the process be-comes inhibited or otherwise unstable. If the process could be operated at 34 °C it might mean savings of both energy and money, provided that there is still a sufficient production of biogas.
The experiment lasted for three months and investigated the short-term effects of the reduction of temperature. The process was monitored closely, and samples from the reactors were collected and analysed twice a week to ensure the stability of the biogas process. Several parameters were monitored online, the biogas production and methane content amongst others. Other parameters were calculated, such as the de-gree of degradation and specific methane production. This was done to ensure process stability and a sufficient production of biogas. The energy balance was calculated to evaluate if energy was saved by lowering the temperature in the digesters.
The results show that the biogas process does remain stable at 34 °C while still pro-ducing a satisfactory amount of biogas during the short time of the experiment. Calcu-lations show that both energy and money has been saved during the experiment. How-ever, the system is largely dependent on seasonal variations, therefore further studies over a longer time period would be desirable. During the course of the thesis it has also become evident that the biogas process at Slottshagen is irregular in several as-pects, and that it would be beneficial to even the process out, especially with regards to the hydraulic retention time. Making the process more even would enable further improvements to be made and simplify interpretations and comparisons of process stability data.
Acknowledgments
I would like to extend my warmest thank you to everyone at Nodra, you have all been so kind and welcoming. A special thank you to the people in the lab, Katarina, Niklas and Rihab. I want to express my deep gratitude to my wonderful supervisors Emma and Bodil who has been incredibly supportive during this time.
A big thank you to my marvellous supervisor Eva-Maria, for always guiding me in the right direction. I would also like to acknowledge and thank Annika, my examiner, for taking an interest in this thesis.
Further, I would like to express my appreciation for my opponent Agnes, for broad-ening my perspective with valuable insights.
Lastly, I want to thank Rasmus, for all the love and support.
Linköping, May 2020 Ella Wilhelmsson
Contents
Notation ix
1 Introduction 1
1.1 Biogas production at different temperatures . . . 2
1.2 Aim and research questions . . . 3
1.2.1 Limitations . . . 3
2 Background 5 2.1 Wastewater treatment . . . 5
2.2 The biogas process . . . 7
2.2.1 Sludge digested at Slottshagen . . . 7
2.2.2 The steps of anaerobic digestion . . . 9
2.3 Temperature importance during biogas production . . . 12
2.3.1 Common temperatures in biogas digesters . . . 12
2.4 Important parameters for operation and process stability . . . 12
3 Material and methods 17 3.1 Experimental setup and approach . . . 17
3.2 Parameters monitored . . . 18
3.2.1 Analysis performed . . . 19
3.2.2 Temperature . . . 21
3.2.3 Energy consumption for heating the digesters . . . 22
3.2.4 Hydraulic retention time . . . 22
3.2.5 Specific methane production . . . 22
3.2.6 Energy calculations . . . 22
4 Results 25 4.1 Comparison of experimental temperature 34 °C and historical tempera-ture 37-38 °C . . . 25
4.2 Lab results . . . 30
4.2.1 Sources of error . . . 32
4.3 Use of energy . . . 32
4.3.1 Effect of outside temperature . . . 33
4.3.2 Effect of the hydraulic retention time . . . 34
4.4 Specific methane production . . . 35
4.5 Economy of the biogas process . . . 35
5 Discussion 39 5.1 Outcome of the study . . . 39
5.1.1 Gas production . . . 40
5.1.2 Seasonal variations . . . 41
5.1.3 Effect of hydraulic retention time . . . 42
5.2 Discussion of lab results . . . 42
5.2.1 Experiment in full scale . . . 43
5.3 Economical aspects . . . 44
5.3.1 Foaming . . . 45
5.4 The process at Slottshagen . . . 46
5.5 The future of biogas . . . 47
6 Conclusions 49 6.1 Future work . . . 50
A Appendix A 53 A.1 Data . . . 53
A.2 Events that occurred during the thesis that might have affected the results 59 A.3 Calculating the hydraulic retention time . . . 60
Notation
Abbreviation Meaning
AD Anaerobic digestion
BOD Biochemical oxygen demand
CH4 Methane
CO2 Carbon dioxide
HRT Hydraulic retention time
NH3 Ammonia
NH+4 Ammonium
Nm3 Normal cubic meter, during standard pressure (1 013.25 hPa) and temperature 0 °C
ORC Organic Rankine cycle pe Personal equivalent TS Total solids
VFA Volatile fatty acids VS Volatile solids
WWTP Wastewater treatment plant
1
Introduction
Can a full scale biogas plant operate effectively at a working temperature of 34 °C? Biogas, mainly consisting of methane (CH4) and carbon dioxide (CO2), is the final product of the degradation process of organic compounds performed in several steps by different microorganisms, in the absence of oxygen [20]. Methane is rich in energy and can be utilised as a fuel in vehicles, among other applications. [20]
Commercial biogas production is regarded as an environmentally friendly way of producing fuel since no fossil material is needed, and it does not contribute to global warming, as it comes from atmospheric sources [20]. It can also be used to produce electricity. It is increasingly common with biogas plants, both in Sweden and in other countries, for waste handling and energy transformations. Many different substrates are utilised for biogas production; food waste, manure and rest products from wastew-ater treatment to mention a few. To produce biogas from such substrates is an efficient solution in terms of waste treatment. The digestate that remains after the biogas pro-cess can be spread as fertiliser on fields. [20]
The temperature is one of the key aspects in a biogas process, as it influences the amount of biogas produced and the process stability [16]. Mesophilic temperatures (35-42 °C) are commonly chosen for biogas production at wastewater treatment plants, WWTP, [3], [1].
This thesis has been performed at the company Nodra in Norrköping, at theirWWTP Slottshagen. The thesis investigates how a decrease of a few degrees in temperature af-fects Nodras full scale biogas production. Nodra usually have a temperature of 36-38 °C in their two digesters and during this thesis the temperature has been lowered to 34 °C. The benefit of lowering the temperature is that energy could be saved if the digesters are not heated to such a high temperature, even a few degrees could make a big
ference. The challenge of a reduced temperature is the stability of the process which could be endangered. There is also a risk of a lower biogas yield at a lower tempera-ture. There are very few studies made on the effects of a lower temperature in biogas production, resulting in a need for further research within this aspect of biogas produc-tion. Few studies are performed in full scale, which further increases the importance of this study.
1.1
Biogas production at different temperatures
Anaerobic digestion,AD, is performed by microorganisms and it occurs in several dif-ferent steps, with difdif-ferent microorganisms performing various tasks. Studies around 34 °C have been performed at lab-scale with promising results [3], [10], but every bio-gas process is unique and will respond in its own way.
There is a study by Andersson (2019) performed at a wastewater treatment plant in Uppsala which suggests that a lowering of the temperature to 34.5 °C can be done without any losses in biogas production [3]. The aim of the study was to investigate the effects of a lower temperature when producing biogas. [3] However, the experiments by Andersson were done at lab-scale whereas this thesis has been performed at full-scale.
A resembling experiment was done at Tekniska Verken in Linköping, the temper-ature was lowered to 34 °C from 38 °C in a lab-scaleAD reactor with substrate from the wastewater treatment plant [10]. Rönnberg et al. (2017) concluded that the gas production rate was lowered with a lowered temperature, but there was not a loss in the specific methane production. Another observation was that the levels of foam in-creased greatly, this was the main disadvantages of the temperature change and it nega-tively affected the following process step. The lower temperature also slightly increased the sludge mass after the digestion. Rönnberg et al. (2017) concluded that the lower temperature means an economic benefit. [10] These results are very interesting and motivates further investigations at 34 °C, as well as investigations in full scale, which has been the focus of this thesis. However, since the study in Linköping showed prob-lems with foaming at a lower temperature, the foam levels during this thesis have been closely monitored.
A study investigating process stability when the temperature was changed from 35 °C to 30 °C showed that such a change does not affect the biogas production or the per-formance of the biogas process, once the system had stabilised at the new temperature [14]. This indicates that biogas processes can be subjected to a decreased temperature and not be damaged, which is promising for this thesis.
A greatly resembling experiment was conducted in Sweden where the temperature in anADprocess at aWWTPwas lowered from 37.5 °C to 34.5 °C in lab scale and from 37.5 °C to 35 °C in full scale [13]. The results show that by reducing the temperature with 2.5 °C approximately 13 % energy can be saved. In addition, there were no effects
1.2 Aim and research questions 3
on the biogas production or the process stability. [13] These results are very promising and suggests that the results in this thesis could have a profitable outcome.
Earlier studies suggest that it is possible to lower the temperature without any large losses in biogas production. Since each biogas process is unique, it is necessary to in-vestigate how a change in temperature affects the process at Slottshagen, which has been done in this thesis. However, earlier studies show promising results which mo-tivates performance of this study. The objection with this study for Nodra is to lower the operating costs of the biogas process. Nodra is owned by the municipality of Nor-rköping and the entire company is self-funded, the incomes are from the fees charged for their services and may not exceed the costs [19].
1.2
Aim and research questions
This thesis will investigate if it is possible to maintain a sufficient biogas production in full scale at the lower temperature of 34 °C, instead of the more common 37 °C, and thus lower the costs of heating the digesters. The expectation is that a lower temperature would mean energy savings and then also money savings, when there is a lesser need to heat the digesters. The aim of this thesis is summarised in a few research questions, as follows:
• How will the biogas process stability, in terms of volatile fatty acids (VFA), alkalin-ity, total solids (TS) and pH be affected by the temperature decrease to 34 °C from 37 °C?
• How will the production of biogas respond to a lowering the temperature to 34 °C and how will it affect Nodras ability to deliver biogas?
• How much energy used for heating the digesters could be saved by lowering the temperature to 34 °C?
• What other consequences are there to this change in the process? Especially, how will the economical aspects of the biogas process be affected, will the degree of degradation be affected and how will the foaming be affected?
1.2.1
Limitations
It is of paramount importance that the process remains stable throughout the experi-ment, since it is performed in full scale. For this reason, several parameters were mon-itored to ensure that the process remained stable, and all values remained within the range of what is considered normal at Slottshagen. If any of the monitored parameters had reached unusual levels, it could have been harmful to the process. If the process had shown signs of collapsing, the experiment within this study would have had to be terminated.
The time frame of this thesis is a limiting factor, thus limitations of parameters to be investigated as well as the extent of the experimental time period had to be imple-mented. Many parameters play an important role in the process, but in this study the following parameters were investigated; TS, ash, pH, volatile fatty acids (VFA), ammo-nium, alkalinity, temperature in the digesters, amount of biogas produced, methane content in raw gas, use of Organic Rankine Cycle (ORC), gas flared, suspended solids in reject water, outside temperature, Hydraulic Retention Time (HRT), antifoam needed, energy used for heating the digesters, degree of degradation and specific methane pro-duction. When investigating the energy that could be saved, only the energy needed for heating the digesters and the energy content in the produced gas have been taken into account. The theoretical methane potential of the substrate has not been investi-gated. This thesis has been performed as Nodras activity have carried on as usual, no precautions have been taken to try to stabilise the process more than usual during this period. Events that affect the process, see Appendix A, have happened on a few occa-sions, as it normally happens every once in a while. These events have most likely had an impact on the process, in particular the emptying of the external sludge tank, but this has not been looked into in detail.
2
Background
Nodra has several important objectives in order to keep the city functional and pro-vide its citizens with the necessary facilities for a normal everyday life. Nodra handles and provides drinking water, collects and handles waste, provides internet and purifies wastewater. The majority of all wastewater in Norrköping is purified at Nodras facility, Slottshagen. The sludge that remains after the purification is made into biogas. The sludge is digested in two parallel digesters and the produced gas is upgraded to vehi-cle fuel, which powers the busses in Norrköping. To use biogas as fuel, the carbon dioxide must first be removed in an upgrading process, so that the gas consists only of pure methane [20]. Methane is also the substance of natural gas from fossil sources. However, there is no differences between the two, except for origin, which means that methane from biogas production can be used directly in vehicles and thus replace nat-ural gas.
There are many parameters that affect the biogas process, such as temperature, HRT and the typ of substrate fed to the digesters. The process is sensitive since it consists of microorganisms that needs certain conditions to thrive. In this chapter an overview of the biogas process is given, as well as a brief description of the wastewater treatment at Slottshagen. Parameters of importance for biogas process stability is also presented in this chapter.
2.1
Wastewater treatment
At SlottshagenWWTP, the wastewater from Norrköping is treated and purified before it is released into the sea in Bråviken. Rest products from the wastewater treatment process are made into biogas. An overview of the process can be seen in Figure 2.1. Slottshagen is built for 200 000 pe. The unit pe stands for personal equivalent and is used to describe the capacity of aWWTP[2]. It is calculated with the biochemical
Separation of sand and solid
items
Pre-sedimentation Biologicalstep
Iron chloride
Chemical step, sedimentation
Primary sludge Biological sludge
Sediment, goes back to the beginning of the
process
Pure water, released into Bråviken Iron chloride
Figure 2.1: An overview of the different steps of the wastewater treatment process
at Slottshagen.
gen demand,BOD, as an impurity amount of 70 g BOD7/person and day. Normally in Sweden, 25-30 l raw gas/pe and day is produced at wastewater treatment plants when primary and biological sludge is digested, as it is at Slottshagen. [2] During 2019 the load was 178 000 pe and during 2018 it was 157 800 pe at Slottshagen. This corresponds to 30.5 l raw gas/pe and day for 2019 and 34.8 l raw gas/pe and day for 2018. At Slottsha-gen external sludge is digested together with the primary and biological sludge as well, making the number higher than the theoretical value. The external sludge constitutes about 10 % of the total amount of sludge fed to the digesters.
The first step in the purification process when the wastewater comes to Slottsha-gen is the sand separation [2]. Since the water always contains sand it is important to separate it as soon as possible because otherwise it can damage the equipment in the plant. Then comes the grid that separates solid items and litter. [2]
The next step is the pre-sedimentation [2]. Iron chloride is added and the water goes into giant pools where fat and other floating materials are separated from the wa-ter. Particles with higher density than water sinks to the bottom and particles with lower density floats on the surface. Scrapers slowly move along the surface and the bot-tom of the pools and scrape away the sludge. This sludge is called primary sludge or chemical sludge, and it later goes into the biogas digesters, but firstly it is collected in a storage tank. To have an efficient pre-sedimentation step decreases the load on the following steps and reduces the risk of disturbances in the following process. [2]
The water then moves on to the biological step, where microorganisms are used to remove nitrogen from the wastewater [2]. This takes place in giant pools with anoxic and aerobic zones to benefit bacterial nitrifiers and denitrifiers. This is followed by a sedimentation pool where the sludge is scraped from the bottom of the pool. The bi-ological material is recirculated into the pools. However, some of the sludge goes to a
2.2 The biogas process 7
storage and later into the biogas digesters and is called biological sludge. [2]
As the water moves on, iron chloride is added [2]. It reacts with phosphorus and forms a precipitate. This sediments in giant pools. [2] The sediment produced in this step goes back to the incoming water, the beginning of the process, for purification. This final step is called the chemical step and it concludes the water purification pro-cess and the water is then released into Bråviken.
2.2
The biogas process
During the wastewater purification process sewage sludge, in form of primary and bi-ological sludge, are produced. There are three different substrates that are fed to the digesters at Slottshagen. It is primary sludge, biological sludge and external sludge. Once in the digester theAD begins. Different microorganisms cooperates to break down the substrate and if it is completely degraded mainly methane and carbon diox-ide are formed in the process. The whole process consists of several complex steps performed by different groups of microorganisms.
2.2.1
Sludge digested at Slottshagen
The substrate that enters the biogas digesters at Slottshagen comes from three sources, as can be seen in Figure 2.2. There is primary and biological sludge that comes from purification steps in the wastewater treatment. There is also external sludge that is delivered in trucks, which comes from smallerWWTPs on the countryside, restaurants that separate their fat, sludge from cleaning of pumping stations, households that have waste wells of their own, etc. The inflow of the external sludge is not constant.
Digester 1 Digester 2 External sludge storage tank Biological sludge storage tank Primary sludge storage tank Belt filter Polymer Heat exchanger Antifoam
Figure 2.2: An overview of the flow of the different types of sludge before it enters
The primary and biological sludge is separately stored in tanks before it is pumped into the digesters. In these tanks the sludge sediments before it is withdrawn from the bottom. This is done in order to not get more water than necessary into the digesters. Before entering the biogas digesters the primary and biological sludge is mixed and then dewatered further, through adding of a polymer and passing a belt filter. There-after it is mixed with digester sludge that is circulated from the digesters. Sludge goes from the digester, through the heat exchanger and is then mixed with the sludge com-ing from the belt filter and together it goes back into the digesters, as is illustrated in Figure 2.2. In that way, the cold primary and biological sludge is heated up before en-tering the digesters. There are online measurements of how much sludge (m3) that goes into each digester. The external sludge is added as well, but it is not dewatered, it goes directly from its tank into the digesters. However, the external sludge is mixed with recirculated digester sludge that has passed the heat exchanger to warm it up on the way into the digesters. The aim is to divide it equal between the digesters, but there are no online measurements of how much sludge that goes into each digester, only of how much is added in total. Once in the digesters, the sludge is digested and biogas is formed. The sludge is heated by being pumped from the digesters, to a heat exchanger located in the building next to the digesters. There it is warmed and then pumped back into the digesters. The pipes that run between the digesters and the heat exchanger are insulated to prevent loss of heat. The flow of the sludge after the digesters is illustrated in Figure 2.3. Digester 1 Digester 2 Storage tank Centrifuges Digestate stored for six months
Digestate spread on fields Unqualified digestate to landfill Reject water to anammox purification Polymer
Figure 2.3: An overview of the flow of sludge after it leaves the digesters at
Slottshagen.
The process after the digestion in the digesters can be seen in Figure 2.3. The di-gested sludge is taken out of the digesters at the bottom and pumped into a storage tank. Thereafter, a polymer is added and the digested sludge goes to a decanter cen-trifuge to get dewatered. The reject water that is removed from the sludge goes to a process step with anammox purification. The sludge that remains, the digestate, is arranged in large piles by a wheel loader and stored for six months to ensure that no pathogens remains. The digestate is analysed for heavy metals and that which passed
2.2 The biogas process 9
the test is spread on farm land as a fertiliser. Otherwise the digestate must be placed in a landfill.
2.2.2
The steps of anaerobic digestion
The biogas process is a degradation of substrates under anaerobic conditions. It con-sists of several steps, performed by different microorganisms. An overview of these steps can be seen in Figure 2.4 [20]. The main steps in the degradation are hydrolysis, fermentation, acetogenesis and methanogenesis [9]. The final product of the process is biogas and it generally consists of 60-70 % methane [15]. The biogas, also called raw gas, can be upgraded by removing the carbon dioxide resulting in a gas consisting of almost pure methane and then be used as vehicle fuel. The energy content of pure methane gas is 9.97 kWh/m3[27].
HYDROLYSIS Carbohydrates, lipids, proteins
FERMENTATION Monosaccharides, amino acids
ACETOGENESIS Organic acids, alcohols
METHANOGENESIS CH4+ CO2
H2+ CO2
Acetic acid
Figure 2.4: An overview of the steps of anaerobic degradation and the compounds
involved in the different steps.
Hydrolysis
In the hydrolysis, which is the first step of the biogas process, carbohydrates, lipids and proteins are broken down into smaller compounds; monosaccharides, fatty acids, amino acids and alcohols [20]. This is done for the microorganisms to be able to take up the nutrients and use them as substrate, otherwise they are to big and cannot be utilised by the organisms. [20] It is a complex process to break down the organic com-pounds into soluble monomers, it occurs in several steps, generally by extracellular enzymes [9]. The enzymes are secreted into the solution and there breaks down the compounds. For some substrates, hydrolysis is the rate-limiting step of the biogas pro-cess and therefor the rate of the entire propro-cess could be dependent on this step. [9]
Fermentation
The next step is fermentation, which also consist of a number of reactions [20]. The reactions that occurs depends on which microorganisms that are present, as well as on the substrate. The occurring reactions also influences which compounds that are formed in this step. There are several different microorganisms that performs fermen-tation, some of them are the same ones that performs the hydrolysis. The products from the hydrolysis step is used as substrate in the fermentation. However, the fatty acids produced in the hydrolysis is not used as a substrate in the fermentation, they are instead broken down in the next step in the process, the acetogenesis. The inter-mediary products of the different fermentation processes are several different organic acids, alcohols, ammonia, hydrogen sulphide, carbon dioxide and hydrogen. The acids formed are usually in their charged form, but there is an equilibrium with the un-charged form, as can be seen in equation 2.1. [20]
CH3COOHCH3COO−+ H+ (2.1)
The pH in the digesters regulates which form is the most common one. When the pH is above 7, as it is in biogas digesters, the acids are mostly in their charged form and can then form salts with various metals, sodium and potassium for example. The products of the fermentation step are merely a waste product that is of no use for the organisms that have formed them, but it is the substrate for the organisms in the fol-lowing steps of the biogas process. [20]
Acetogenesis
The following step is the acetogenesis. Here, acetate is produced from the products of fermentation and hydrolysis; fatty acids and alcohols [20]. This is performed by acetogens, organisms whose main product is acetate. [20] The products from the fer-mentation step are oxidised by acetogens and hydrogen and acetate is formed [9]. In the process electrons are transferred to protons, H+, and hydrogen, H2, is formed. This is a complex process and the final product, acetate and hydrogen, is the substrate of the next step in the biogas process, the methane forming step. [9] During the aceto-genesis a close cooperation between different microorganisms, the ones that produce hydrogen and the ones that uses hydrogen, is absolutely essential [20]. The complex process of acetogenesis is inhibited at high concentrations of hydrogen gas. Therefore, the hydrogen that is produced must constantly be removed for the process to continue. [20] This is taken care of by the methanogenic archaea that produce methane, they use up the hydrogen and thus the acetogenesis can continue [9]. This cooperation between microorganisms is called interspecies hydrogen transfer. Together, the aceto-genic bacteria and methanoaceto-genic archaea can break down fatty acids, which neither of them would be able to do on their own. [9] This requires that the two microorganisms are in close contact with each other [20]. However, in the absence of a hydrogen con-sumer, many hydrogen producers can use an alternative pathway and break down the material in a way that does not produce hydrogen. If hydrogen cannot be produced, generally more fatty acids of different kinds and alcohols are produced instead. [20]
2.2 The biogas process 11
Methanogenesis
The final step in the production of biogas is the methanogenesis, this is where the final product, methane, is produced [20]. Carbon dioxide is also formed as a by-product. The organisms responsible for producing methane are the methanogens, which uses the products formed in the acetogenesis as substrate. There are several different types of the methanogens, which grow at different rates, generally quite slowly, requiring be-tween 1 and 12 days to divide into two. Therefore, the hydraulic retention time,HRT, which is the average time the substrate is inside the digesters, cannot be shorter than that, otherwise the organisms are at risk of being flushed out of the digesters. [20] Acetate, carbon dioxide and hydrogen are the main substrates in the production of methane, but other compounds could also serve as substrates [9]. There are three known ways for the methanogens to produce methane, and which way being used de-pends on which strains of microorganisms that are present. The possible reactions can be seen in equation 2.2, 2.3 and 2.4. [9]
CH3COO−+ H+−−→ CH4+ CO2 (2.2)
4 H2+ CO2−−→ CH4+ 2 H2O (2.3)
4 CH3OH −−→ 3CH4+ CO2+ 2 H2O (2.4)
Acetate can be cleaved into methane and carbon dioxide as in equation 2.2. Car-bon dioxide can be reduced to methane as described in equation 2.3. Methylated com-pounds can be converted to methane as in equation 2.4. Acetate is the most important substrate in the production of methane [9]. The formation of methane that is depen-dent of a hydrogen source occurs in flocks or biofilms, where the organisms are close to those that produces hydrogen. There are different methanogens with different prop-erties. Since the methanogens are archaea and not bacteria, they are more sensitive to disturbances in the process than the microorganisms in the other steps. [9] If the temperature or the pH is changed, the methanogens are usually the first ones to be af-fected [20]. Some are sensitive to changes in pH and high ammonia levels, and these are most common at low acetate concentrations [9]. Most methanogens prefer a neu-tral pH and mesophilic temperatures. With the exception of one methanogen (who has its optimum at 84-110 °C) all known methanogens prosper at the temperature in ques-tion for this thesis (as well as higher temperatures), the temperature has to go below 25 °C before the organisms are harmed. For some substrates can the methanogenesis be the rate limiting step. [9]
The formation of biogas is a complex process that require certain conditions to function. Several parameters have to be monitored in order to ensure that the process remains stable.
2.3
Temperature importance during biogas
production
Due to different microorganisms having their optimum growth rate and metabolism at different temperatures [15], the temperature in the digesters has a large effect on the performance of the biogas process [16]. The temperature should not be varying more than 0.6 °C/day [15].
2.3.1
Common temperatures in biogas digesters
There are different temperature intervals at which different microorganisms thrive. Psy-chrophilic (15-25 °C), mesophilic (35-42 °C) and thermophilic (46-60 °C) [1] are the three main intervals mentioned in the literature, though there are other temperatures that microorganisms can live at. However, the intervals differ a bit in the literature, mesophilic temperatures are sometimes defined as 25-40 °C [3] or 10-48 °C [20]. Mesophilic temperature is commonly chosen for biogas production at wastewater treatment plants [3]. For biogas processes in general a temperature of 37-39 °C (mesophilic) or 52-55 °C (thermophilic) is chosen, but 40-44 °C (hyper-mesophilic) is becoming more common [10].
2.4
Important parameters for operation and process
stability
The aim of this study is to investigate the effects of lowering the temperature in two full scale anaerobic digesters at Slottshagens wastewater treatment plant. To be able to analyse the results several parameters need to be monitored. It is both parameters that has an effect on the temperature as well as parameters that are affected by the temperature. The parameters that could affect the temperature in the digesters are; the outside temperature and how often the organic rankine cycle,ORC, runs. The pa-rameters that could be affected by the temperature are; the amount of gas produced and the composition of the gas, the amount antifoam needed, the level of suspended solids in outgoing water from the digesters and the energy consumption for heating the digesters.
Hydraulic retention time
Hydraulic retention time, is the time that the substrate is inside the digester before it is flushed out [20]. With the current inflow it is the time it will take to replace all the material in the digester. The degree of degradation affects the amount of sludge that re-mains. If the degree of degradation is high, it means that more of the substrate has been converted into biogas, leaving less sludge after the digestion process. The HRT also af-fects the degree of degradation. It takes time for the microorganisms to break down the material and produce biogas, if they are not given enough time they will not be able to convert as much material. A shorter retention time therefore generally leads to a lower
2.4 Important parameters for operation and process stability 13
degree of degradation. However, it is not that simple as other factors also matters. The temperature is important, at higher temperatures the microorganisms can work faster, and the need for a longer retention time lessen, a high degree of degradation can still be achieved. How difficult the substrate is to break down for the microorganisms also matters how long time the microorganisms need to convert it into biogas. [20] TheHRT for the past two years at Nodra can be seen in Appendix A, Figure A.1.
Organic Rankine Cycle
Normally, the upgraded gas is pumped directly into the gas grid. However, sometimes the gas grid is full or the gas cannot be upgraded at the moment, since the upgrading equipment is at a momentary standstill. Gas is still being produced, but there is very little possibility of storing raw gas at Slottshagen. Some raw gas can be stored in a gas clock, however it is very small compared to the daily gas production. There are then two remaining options on what to do with the gas. Firstly, the gas is used to power the Organic Rankine cycle,ORC, which generates heat utilised to warm the digesters. When theORCruns there is no need to use the district heating, which saves money for Nodra. The second option is to simply burn the gas in a torch, which is called flaring. Flaring occurs when the gas grid is full and theORCcannot run, because the temperature in the digesters is high enough and no heating is needed at the moment. To burn the gas is a complete waste of the gas, the energy cannot be harvested. Flaring is done as seldom as possible, but sometimes there is no choice. The reason for burning the gas is to lower the impact on the environment instead of releasing methane into the atmosphere. The usage of theORCduring the past two years can be seen in Appendix A, Figure A.2.
Total solids, ash and degree of degradation
The sludge consists of water, volatile solids,VS, and total solids,TS. The total solids con-sists ofVSand other non-organic compounds, such as minerals, rocks and dirt. [20]VS is a measurement of the organic matter content in the sludge [9]. The content of a sam-ple of sludge is illustrated in Table 2.1.
Table 2.1: The content of sludge sample from the digesters.
Sample TS
Water Ash VS
Ash is the residual ash, the residue after ignition, mainly non organic compounds [20]. When the analysis ofTSand ash is done there is a risk that the result is underesti-mated since the volatile substances could evaporate as the analysis is being performed [24].
Using theVSconcentration the degree of degradation can be calculated. The degree of degradation is also calledVSreduction (VS red) [1]. It is a measurement of how much of the available organic substrate that is being degraded and turned into biogas. A high
degree of degradation suggests an efficient process, since a lot of the organic substrate is converted to biogas. [1] Nodra measuresTSand ash, as well as calculates the degree of degradation, once a week and values for these parameters during the last two years can be found in Appendix A, for TS in Figure A.3 and Figure A.4, for ash in Figure A.5 and Figure A.6, and for degree of degradation in Figure A.7 and Figure A.8.
pH
The microorganisms in the biogas process are sensitive to changes in pH and therefore it is important to monitor pH [1], [15]. The pH levels of a biogas process is generally 7-8.5 [20]. A drop in pH would mean that the buffering capacity is used up in the sys-tem. To avoid this from happening the alkalinity is also measured. Nodra measures pH once a week, the data for the last two years for the pH in the digesters can be found in Appendix A, Figure A.9 and Figure A.10.
Alkalinity
Measuring alkalinity is a way of tracking the buffering capacity of a system [20]. It is a measurement of the amount of alkaline substances, which can neutralise strong acids. VFAs are naturally formed in the biogas process, however, as long as the system have a good buffering capacity, meaning a high alkalinity, the acids will be neutralised and the pH will remain stable. If the alkaline substances are used up and the system loses its buffering capacity, disturbances can have large effects quickly. If acids accumulate when there is no buffering capacity it quickly lowers the pH causing the system to be-come unstable. [2] [21] [20] It is good to have a high alkalinity, but if it bebe-comes too high it could cause problems. It could cause release of ammonia which inhibits the mi-croorganisms producing methane. [20] Nodra does not measure alkalinity as a part of their regular analyses and therefore there is no available historical data regarding the alkalinity levels.
Volatile fatty acids
The amount of volatile fatty acids,VFAs, in the digesters is a good measurement of how well the process is performing and if there is an imbalance in the system the levels of VFAs usually rise [1]. VFAis the collection name for several fatty acids, the most com-mon ones being acetic acid, propionic acid, valeric acid, butyric acid, isovaleric acid, isobutyric acid and caproic acid [6]. For the process to be considered stable the levels ofVFAs should not exceed 300 mg CH3COOH/l [2]. If the levels ofVFAs are high theVS andTScontent could be underestimated, showing lower results than the actual values [1]. The risk of a lowered pH is also increased ifVFAlevels are high, especially if the system has a low buffering capacity. It can also lead to a lower production of biogas. High levels ofVFAs could also cause foaming, and eventually at high concentrations it becomes toxic to the microorganisms. [1]. The accumulation ofVFAs is caused by an imbalance in the process, when the different microorganisms work at different rates [1]. This could be induced by overloading the system or changes in temperature [1], which is why it is important to monitor theVFA levels during this thesis. Processes
2.4 Important parameters for operation and process stability 15
loaded with materials containing a high degree of fat are generally at higher risk of ac-cumulatingVFA[21]. Nodra measures the levels ofVFAapproximately once a month. Historical values for theVFAlevels for the past two years can be seen in Table A.1 in Appendix A.
Ammonium
Ammonium (NH+4) and ammonia (NH3) are important for process stability [15]. Ele-vated levels of ammonium suggests an imbalance in the system [1], [15] and is there-fore monitored. With the level of ammonium the ammonia level can be calculated using equation 2.5 [1]
NH3= NH4( 1 +
10−pH 10−(0.09018+2729.92T )
) (2.5)
where T is the temperature in Kelvin. High ammonium gives the process a good buffer capacity, however, it can also inhibit the microorganisms producing methane [1]. The levels of ammonia can be around 1000 mg/l without any disturbances in the process, however there are large differences between different processes. Some processes gets inhibited at lower concentrations whereas some do not experience any problems even at higher concentrations. There is an equilibrium between ammonium and ammonia and it is mainly ammonia that inhibits the biogas process and can cause the process to become unstable. Parameters that affects the equilibrium are temperature and pH. [1] Problems with inhibition caused by ammonia is more common at higher temper-atures [11]. As to the levels of ammonium acceptable for the process, generally 2-3 g NH+
4/l does not cause any problems [20]. Although, the same principle as for ammo-nia applies to ammonium, there are large variations between different processes when it comes to at which level inhibition starts. [20] Nodra does not measure ammonium as a part of their regular analyses and therefore there is no available historical data regarding the ammonium or ammonia levels.
Suspended solids
Suspended solids,SS, is a measurement of the amount of solid particles in water [2]. The levels ofSSare measured in the reject water before it enters the anammox step. High levels ofSScould contain organic material and a carbon source, which could in-hibit the anammox specific bacteria. There is a risk that increased levels of foam in the digesters affect the dewatering of the sludge which could cause an increase inSS, which is whySSis an important parameter to monitor. Historical data of theSSlevels at Nodra can be found in Appendix A, Figure A.11, Nodra measuresSSapproximately once a week.
Antifoam
Antifoam is a chemical agent that is added to the digesters to decrease the levels of foam that is created by the microorganisms. Nodra uses a type called Antispumin. How much foam that is produced depends on several factors, temperature being one of
them [8]. Foam creates a layer on the top of the sludge in the digesters and if it becomes too thick the produced gas could have trouble getting through. [8] At really high levels of foam it can even flood the digesters, causing the foam to stream down the sides of the digester. This has happened at Nodra on a few occasions in the past. Antifoam is added automatically. The antifoam is added to the sludge as it goes into the digesters. The inflow of antifoam is set and regulated manually. It is increased when higher foam levels are noticed, which is checked manually.
About ten years ago the bacteria stems growing in the biological purification step at Slottshagen were analysed. A large portion of these bacteria goes into the biogas digesters with the biological sludge. One of the bacteria stems is Microthrix parvicella which is known to cause foaming in digesters [10]. In the biological step it thrives dur-ing the cold months, from November to March, due to the lower temperatures. It has been observed to cause foaming in December and January in other full scale digesters at wastewater treatment plants [8]. When the temperature is lowered in the digesters it benefits this bacteria and it produces more foam. High amounts of fat have previously been shown to increase foaming at wastewater treatment plants [8] and the external sludge is usually rich in fat.
3
Material and methods
In this chapter, the experimental set up, approach and the methods used for analysing samples from the digesters are described. Analysis of some parameters have been made in the laboratory at Slottshagen and some parameters have been measured on-line and the data obtained from Nodras systems. An overview of the parameters that have been measured and monitored can be seen in Table 3.1.
3.1
Experimental setup and approach
The experiment was performed on the two full scale biogas digesters at Nodras facility Slottshagen. One of the digesters have is filled with about 2300 m3sludge, this digester is referred to as Digester 1 in this study, and the other one holds about 1800 m3of sludge and is referred to as Digester 2 in this study. They run parallel to each other and the process is continuous, meaning that substrate i pumped in and taken out regularly, without any clear start or stop in the process. The digesters are stirred by gas bubbles being released from four rods (in each digester) that are inserted from the top of the digesters and they go down into the digesters where gas bubbles are released. The gas that bubbles is recirculated biogas that is produced in the process. The oldest digester was built when theWWTPwas built, in 1956. The second one was built in the beginning of the seventies.
The approach of the study has been to monitor the biogas process closely during the experiment, partly to ensure that it remains stable and partly to have data to com-pare with historical data in order to draw conclusions regarding the effects of the new temperature. Different time periods have been investigated to provide answers to dif-ferent questions. The historical values for the same parameters that have been moni-tored during the experiment have been analysed, to get a sense of what is normal for SlottshagenWWTP, meaning how much, and between what values does each
ter usually vary. Data from two years back, 2018-2019, have been studied to see how the process at Nodra behaves under a longer period of time, and if there has been any deviation from what can be considered to be normal. This was done to investigate how the process behaves under a longer time period and to analyse how the process varies with season. It also provided information on how much different parameters vary over time and how large variations that can be considered to be normal for Slottshagen.
Data from October 2019 until January 2020 have also been investigated, for the same parameters that were investigated during the experiment. This provides infor-mation on how the system behaved before the change of temperature was made, and how it has behaved for the last threeHRTs before this thesis. The system can be consid-ered stable during this time period, since no larger changes were made for threeHRTs [11]. This provides the starting conditions for the experiment, and if parameters are different during the experiment compared to the time before the experiment (October 2019-January 2020) it can be assumed that the lower temperature may be a contribut-ing factor to the change. The comparisons between the experiment and the time before the experiment were thus made to ensure process stability.
Some comparisons were also made between the months of the experiment and the corresponding months of 2019. This was done to account for the seasonal changes that the process is subjected to. In particular, this type of comparisons was used when pos-sible energy savings were calculated, since the seasonal effects are especially palpable when it comes to the use of energy for heating the digesters.
The initial temperature in the digesters was 36-37 °C at the start of this study. On February 4th 2020 was the temperature lowered to 34 °C and kept there until the end of April, generating an experimental time of three months. The data for the three experi-mental months could be compared with data from the same months of previous years. When the temperature is altered in a biogas digester it has to be kept steady for 2-3 HRTs in order for the system to become stable [11]. TheHRTfor the digesters is usually between 15-25 days, but occasionally it can be both longer and shorter. The reason for lowering the temperature in both digesters instead of keeping one at a higher tempera-ture as a reference has to do with how the gas production is measured. The amount of produced biogas is measured online at Nodra, but only a value for the total gas produc-tion from both digesters is measured. It cannot be determined from which digester the gas comes, and therefore the temperature of both digesters was altered and the results compared with historical data instead of having one of the digesters as a reference. The HRTis also calculated for both digesters together, it cannot be calculated for just one di-gester, and since theHRTis an important parameter it motivates the alteration of both digesters instead of one. More on theHRTin subsection 3.2.4.
3.2
Parameters monitored
During the course of this thesis several parameters were closely monitored, an overview can be seen in Table 3.1. The parameters that were analysed in the lab during the
ex-3.2 Parameters monitored 19
perimental period are pH,VFA,TS, ash, ammonium and alkalinity. The parameters that were monitored by other means are temperature in the digesters, amount gas pro-duced, methane content in raw gas, use ofORC, amount gas flared, suspended solids in reject water, outside temperature,HRT, amount antifoam used, energy usage for heat-ing the digesters and degree of degradation. The results can be seen in chapter 4.
Table 3.1: An overview of measured, monitored and calculated parameters during
the experiment.
Parameter monitored/
measured Frequency
TS, ash Twice a week, analysed in the lab VFA Twice a week, analysed in the lab pH Twice a week, analysed in the lab Ammonium Twice a week, analysed in the lab Alkalinity Twice a week, analysed in the lab Temperature in digesters Online
Amount biogas produced Online Methane content in raw gas Online
Use ofORC Online
Flaring Online
Suspended solids
in reject water Measured by Nodra once a week Outside temperature Daily average
temperature obtained fromSMHI HRT Calculated for every day using
online data of inflow and fill of digesters Antifoam needed Changed manually when necessary Energy usage for heating digesters Monthly value summarised by Nodra Degree of degradation Calculated weekly by Nodra
Specific methane production Calculated for each month during the experiment
3.2.1
Analysis performed
The analyses were performed at the laboratory at SlottshagenWWTP. Samples was col-lected from each digester on the day of the analyses, which were performed twice a week, throughout the whole experiment which lasted three months. This was done as a part of this thesis, in addition to Nodras regular analyses. Each analysis was per-formed in duplicate for each digester. However, during the first weeks the analyses were only performed in singles.
pH
The pH-instrument was calibrated before each measurement, using two buffer solu-tions, one at pH 7 and one at pH 4 and then controlled with a control solution at pH 6 [25]. Calibrating solutions, control solution and the digester sludge were tempered in a water bath for 30 minutes to 25 °C before the measurements. The temperature
was verified with a thermometer. Samples with a temperature of 25 °C ± 0.5 °C were accepted. The pH value for the control buffer was accepted if it was within ± 0.03 units from the value stated on the bottle by the manufacturer. The control sample was noted with two decimals and the sludge sample was noted with one decimal. [25] Nodra has an uncertainty of measurements of ±0.2 pH units for the pH-meter.
Volatile fatty acids
Approximately 10 ml sludge was centrifuged at 2800 rpm for 10 minutes [12]. The su-pernatant was then filtered using a membrane filtration set with a pore width of 1.2µm from Hach LangeGMBH. The analysis was performed with the kit for organic acids anal-ysisLKC365 from Hach LangeGMBH. Two cuvettes for each sample was needed, one for the sludge sample and one for the blank sample. The cuvettes were prepared according to the instructions of the kit, and were then measured in the spectrophotometer Xion 500. The result was calculated by subtracting the blank sample from the sludge sample and the concentration is given in mg CH3COOH/l. It cannot be determined the con-centration of different fatty acids from this method, only the total concon-centration. [12] There is no determined measurement uncertainty of this method at Nodra.
TS, VS, ash and degree of degradation
To begin the analysis of total solids, one crucible for each sample was weighed (Cempty) and the weight was noted [24]. The crucibles were filled with sample and the weight noted. One crucible was filled with a control substance consisting of 50 % kaloin and 50 % cellulose. The crucibles were then placed in an oven with a temperature of 108 °C over night, for approximately 20 hours. After cooling for one hour in a desiccator the crucibles (Cdried) were weighed and thereafter they were placed in a furnace with a tem-perature of 540 °C for two hours. After cooling for one hour the weight of the crucibles (Cburned) was noted andTScould be calculated. [24] The measurement uncertainty of theTSanalysis has been determined to be 9 % and for the the ash it is 10 % at Nodra. TSis calculated according to equation 3.1 [24].
TS=Cdried−Cempty
V × 100 (3.1)
whereVis the weight of the sample without the crucible. The result is a value of how many percent of the sample that consists of solids.
Ash is calculated as described by equation 3.2 [24], as follows: Ash =Cburned−Cempty
Cdried−Cempty × 100
(3.2) The result is given in ash of percentTS.
Volatile solids is calculated as described by equation 3.3 [24].
3.2 Parameters monitored 21
Nodra calculates the degree of degradation for each digester once a week and it has been monitored during this thesis. The degree of degradation is calculated according to equation 3.4 [1]
VSred=Qs×VSin−Qr ×VSr
Qs×VSin (3.4)
whereQsis the inflow to the digester each day,VSinis theVScontent in the substrate going into the digester,Qris the flow out of the digester each day,VSris theVScontent in the sludge going out of the digester.
Ammonium
For the analysis of ammonium levels a kit from Hach LangeGMBHcalledLCK305 was used, according to the manufacturer’s instructions. The sample was diluted 100 times, 5 ml sample was mixed with 495 ml water. 0.5 ml diluted sample was added to a cuvette and left to rest for 15 minutes. Thereafter the result was measured in the spectropho-tometer Xion 500 and the result is given in mg NH+4/l.
Alkalinity
The alkalinity of the sludge was measured using the instrument Dosimat 665 [23]. A control was performed before each measurement using 5 ml 5.00 mmol/l sodium car-bonate stock solution mixed with 45 ml water, creating a 50 ml control solution. The solution was poured into the container and a titration to pH 5.4 was performed. When the sample was stable at pH 5.4 for at least 30 s the amount HCO3needed was noted. The sludge was diluted 50 times, 5 ml sludge was mixed with 245 ml analysis water. 50 ml of the diluted sample was used for the alkalinity analysis, which was performed in the same way as the control sample. The result was given in mg HCO3/l. [23]
3.2.2
Temperature
The temperature in the digesters is measured online with the accuracy of two decimals and automatically regulated to remain stable at a chosen level. A control of the ther-mometers in the digesters were performed at the start of the thesis, before the start of the experiment. It revealed that the thermometer in digester 1 showed +0.1 °C and the one in digester 2 showed -0.4 °C in comparison with a reference thermometer. After a few weeks the thermometers in both digesters were exchanged for new ones showing the actual temperature in a correct way. All data presented in this report that contains temperature has been modified to show the actual temperature, the errors of the ther-mometers have been corrected, until the day the therther-mometers were exchanged for new ones. The outside temperature was also monitored during the experiment, by us-ing the values from Sveriges Meteorologiska och Hydrologiska Institut,SMHI[22].
3.2.3
Energy consumption for heating the digesters
The digesters are heated with district heating and a value for the amount needed for only the digesters at Nodra can be obtained. The amount is summarised to a monthly value of how much energy (kWh) that has been used to heat the digesters.
3.2.4
Hydraulic retention time
TheHRTis calculated according to equation 3.5 [1]
HRT=VR
QS (3.5)
whereQSis the flow into the digester each day andVRis the current volume inside the digester. Generally, in processes like the one at Nodra, theHRTis between 15-40 days. [1] Since there are no measurements of which digester the external sludge is added to, only how much that is added, the two digesters are seen as one unit when theHRTis calculated. The aim is to divide the external sludge equally between the two digesters, but since there are no measurements it cannot be assumed to be exactly equal. Calcu-lations made to determine theHRTcan be found in Appendix A.
3.2.5
Specific methane production
Specific methane production is a measurement of how much methane that is obtained from the organic substrate that is fed to the digesters. It is calculated according to Equation 3.6
m3CH4/ m3VS=
Vg × % CH4
Qs×VSin×TSin (3.6)
whereVgis the amount of gas produced andQsis the inflow of substrate to the digesters. It is calculated for both digesters.
3.2.6
Energy calculations
The energy content in the produced methane has been calculated and compared with the previous year, this is notated∆kW hmet hane= ∆kW hmet hane2019−∆kW hmet hane2020. The same comparison has been made for the amount of energy used for heating the di-gesters,∆kW hheat i ng= ∆kW hheat i ng 2019− ∆kW hheat i ng 2020. Then a comparison be-tween the energy content in the biogas and the used up energy for heating has been made, ∆kW hmet hane− ∆kW hheat i ng. These calculations have been performed for each month of the experiment, meaning that February 2020 was compared to February 2019, and likewise for March and April. The months are compared to the correspond-ing months of last year and nothcorrespond-ing else since the biogas process, and in particular the amount of energy used for heating the digesters, varies with season. If that balance is
3.2 Parameters monitored 23
negative it means that the methane produced contains more energy that has been used to heat the digesters, which is the desirable scenario. This means that the reduction of used energy is greater than the reduction of produced methane.
4
Results
The results of the experiment are presented in this chapter. The prerequisites for the experiment are described, important parameters that largely effect the results are con-veyed and the economic effects are estimated.
4.1
Comparison of experimental temperature 34 °C
and historical temperature 37-38 °C
In Table 4.1 the average values for the digesters can be seen, this is during the time pe-riod from October 2019 until January 2020, which is the time pepe-riod before the exper-iment begun. The values that can only be measured for the two digesters combined are seen in Table 4.3, for the same time period. This is how the system was before the thesis and before the change of temperature, which took place on the 4th of February 2020. The values in these two tables can be compared with the ones in Table 4.2 and Table 4.4, which displays the same values, but during the time of the experiment, from February 2020 until April 2020.
Table 4.1: Average values for the digesters from October 2019 until January 2020.
The standard deviation for the period is also stated.
Parameter Average Digester 1 Average Digester 2
pH 7.2 ± 0.09 7.2 ± 0.08 VFA (mg CH3COOH/l) 90 ± 11.0 86 ± 13.3 Temperature (°C) 37.1 ± 0.41 36.8 ± 0.58 Degree of degradation (%) 54 ± 5.7 53 ± 6.3 TS (%) 3.6 ± 0.25 3.8 ± 0.32 Ash (ash in % of TS) 36 ± 1.5 36 ± 1.3 25
Table 4.2: Average values for the digesters during the experiment from February
2020 until April 2020. The standard deviation for the period is also stated.
Parameter Average Digester 1 Average Digester 2
pH 7.2 ± 0.04 7.2 ± 0.05 VFA (mg CH3COOH/l) 123 ± 13.3 128 ± 17.1 Temperature (°C) 33.8 ± 0.80 34.0 ± 0.67 Degree of degradation (%) 55 ± 5.5 56 ± 5.5 TS (%) 3.7 ± 0.19 3.7 ± 0.22 Ash (ash in % of TS) 35 ± 1.7 36 ± 1.1
The pH remains unchanged at 7.2, graphs over the pH during the experiment is found in Figure A.12 and Figure A.12, and for the time before the experiment in Fig-ure A.14, as well as for the past two years in FigFig-ure A.9 and FigFig-ure A.10, all in Appendix A. There are no considerable changes from the period before the experiment compared to the time of the experiment in theTS, ash or degree of degradation either, theTSis about 3.6-3.8 %, the ash is between 35-35 % ofTSand the degree of degradation varies between 53-56 % over the two time periods. TheVFAlevels are a bit higher during the experiment, 123-128 during the experiment compared to 86-90 during the time period before the experiment from October 2019 until January 2020. The temperature is of course different, as that was the objective of the experiment. For digester 1 the average temperature has been 33.8 °C and for digester 2 34.0 °C during the experiment. During the period before the experiment the temperature in digester 1 was on average 37.1 °C and 36.8 °C for digester 2. The same values for the past two years can be found in Appendix A, Table A.1 and Table A.2, this gives a sense of how the system has behaved over a longer period of time.
As for the combined values for the two digesters, which can be seen in Table 4.3 and Table 4.4, it can be observed that the amount of gas produced is very similar dur-ing the two time periods, only 30 m3more gas was produced during the time before the experiment. This is however an average value for the whole time period, and the gas production varies over the time of the experiment, as can be seen in Figure 4.7. A bit more (127 m3, corresponding to 42 %) gas has been flared, and theORChas been run-ning a bit more (21 m3, corresponding to 11%) during the experiment compared to the time period before. On average, the methane content in the raw gas has not changed, it has remained on 61 %. However, it has varied during the experiment, see Figure A.16, compared to the time before, see Figure A.15, in Appendix A. During the past three years the methane content has on average varied between 60-65 %. Note that theHRT is different for the two time periods, it has been 5 days shorter during the experiment. There is a difference of 150 mg/l in the amount of suspended substances in the reject water, however this does vary overtime, as can be seen in Table 4.5.
4.1 Comparison of experimental temperature 34 °C and historical temperature 37-38 °C 27
Table 4.3: Combined values for
the digesters from October 2019 until January 2020.The standard deviation for the period is also stated.
Parameter Average
Produced gas (m3/day) 5 433 ± 312.6 Flared (m3/day) 173 ± 250.7 ORC (m3/day) 169 ± 225.6 Amount methane in raw gas (%) 61 ± 2.0
HRT (days) 20 ± 5.9
Suspended solids (mg/l) 611 ± 928.1
Table 4.4: Combined values for
the digesters during the experi-ment from February 2020 until April 2020. The standard devia-tion for the period is also stated.
Parameter Average
Produced gas (m3/day) 5 403 ± 704.7 Flared (m3/day) 300 ± 528.9 ORC (m3/day) 190 ± 335.0 Amount methane in raw gas (%) 61 ± 3.0
HRT (days) 15 ± 3.7
Suspended solids (mg/l) 761 ± 1481.3
The temperature in the digesters at Nodra has varied a lot during the past years, between 35-38 °C, as can be seen in Figure 4.1 and Figure 4.2, these graphs show a monthly average temperature for each digester, during the past two years. The temper-ature for the period before the experiment can be seen in Figure 4.3. Here, the average daily temperature for each digester is shown. There are variations during this period as well, the temperature varies between approximately 35-38 °C. The temperature during the time of the experiment is seen in Figure 4.4. In the beginning the drop from about 37 °C to 34 °C can be seen, the change was made on February 4th. However, there was some difficulty getting the temperature to remain steady at 34.0 °C and additional ad-justments were made to stabilise it.
34 35 36 37 38 39 Temp er atu re (° C) Month 2019 2018
Figure 4.1: Average monthly
temper-ature for digester 1 during 2018 and 2019. 34 35 36 37 38 39 Tem p er atu re (° C) Month 2019 2018
Figure 4.2: Average monthly
temper-ature for digester 2 during 2018 and 2019.
32 33 34 35 36 37 38 Temp er at ur e (° C) Date Digester 1 Digester 2
Figure 4.3: Average daily temperature
in the digesters from October 2019 un-til January 2020. Standard deviation for digester 1 was ± 0.41 and ± 0.58 for digester 2 during this period.
32 33 34 35 36 37 38 Tempe ra tur e (° C) Date Digester 1 Digester 2
Figure 4.4: Average daily
tempera-ture in the digesters during the exper-iment, from February 2020 until April 2020. Standard deviation for digester 1 was ± 0.80 and ± 0.67 for digester 2 during this period.
The amount of produced raw gas for the last two years is presented in Figure 4.5. As can be seen, the production varies largely seen over longer time periods, during the past two years it has varied between approximately 4 500-6 500 m3. In Figure 4.6 the gas production for the time period before the experiment can be seen. It varies slightly, but keeps around 5 500 m3per day. Next to it, the gas production during the experiment can be seen, in Figure 4.7. There are larger variations in gas production during this time period, but on average it is also around 5 500 m3per day, as was also seen in Table 4.4. 4000 5000 6000 7000 Pr od uc ed r aw gas ( m 3/da y) Month 2018 2019
Figure 4.5: Average daily amount of raw gas produced each month during 2018
and 2019.
Gas has been flared during the experiment, as can be seen in Figure A.18, which can be compared with how much that was flared the time before the experiment, in Figure A.17 and the past two years, in Figure A.19, all in Appendix A. TheORChas been running at times during the experiment, as shown in Figure A.20, which can be com-pared with the time before in Figure A.21, these in Appendix A as well.
4.1 Comparison of experimental temperature 34 °C and historical temperature 37-38 °C 29 3500 4000 4500 5000 5500 6000 6500 7000 7500 Pr od uc ed r aw gas ( m 3/da y) Date
Figure 4.6: Total amount of raw gas
produced each day from October 2019 until January 2020. The standard devi-ation for the period was ± 312.6.
3500 4000 4500 5000 5500 6000 6500 7000 7500 P roduc ed ra w gas ( m3 /da y) Date
Figure 4.7: Amount of raw gas
pro-duced each day during the experi-ment, from February 2020 until April 2020. The standard deviation for this period was ± 704.7.
The levels ofSSin the reject water after the digesters can be seen in Table 4.5 for the time before as well as during the experiment.SSis usually around 200 mg/l, however it can on occasion reach levels of around 4 500 mg/l. It is measured approximately one a week. Values under 1 000 mg/l are considered acceptable.
Three weeks after the start of the experiment, on the 26th of February, the amount of antifoam was increased, from about 0.6 l/h to 0.8 l/h. On the 31st of March the antifoam was increased even further, to 0.9 l/h. Towards the end of the experiment the antifoam was reduced to the original amount of 0.6 l/h as it was when the experiment started, since the need had lessened because the foam levels had decreased. During 2019 Nodra used about 3 100 l of antifoam in total. During 2018 that number was 2 400 l, although there are no records of how much was added during different months.
Table 4.5: Average monthly levels of suspended substances in the reject water
af-ter the digesaf-ters from October 2019 until April 2020. The standard deviation for the period is also stated.
Month Suspended substances (mg/l) October 2019 1 649 ± 1572.7 November 2019 234 ± 32.0 December 2019 251 ± 81.8 January 2020 311 ± 93.7 February 2020 325 ± 104.2 March 2020 358 ± 146.2 April 2020 1902 ± 2493.0
4.2
Lab results
Here, the results from the analyses performed at the laboratory at SlottshagenWWTP during this thesis are presented. The experiments have been made in duplicates (ex-cept for the first few weeks) and the standard deviation for the two measurement points has been calculated and is presented in the graphs. The values in the graphs are the av-erage value from the two measurement points.
The pH varied between 7.1 and 7.3 during the experiment. Graphs over the pH variation can be seen in Figure A.12 and Figure A.13 in Appendix A.
Volatile fatty acids
In Figure 4.8 and Figure 4.9 the levels of volatile fatty acids can be seen for the two digesters. It has varied between about 100-180 mg CH3COOH/l during the experiment. No accumulation over time was observed.
80,0 110,0 140,0 170,0 200,0 VF A ( mg CH 3 C OOH/ l) Date
Figure 4.8: VFA levels for digester 1
during the experiment.
80,0 110,0 140,0 170,0 200,0 VF A ( mg C H3 C OOH /l) Date
Figure 4.9: VFA levels for digester 2
during the experiment.
Ammonium
In Figure 4.10 and Figure 4.11 the ammonium levels for the digesters are shown, it has been about 800-1 200 mg/l. This has not previously been measured at Nodra and there-fore there is nothing to compare it to. The levels dropped slightly in the middle of the experiment but increased again, but not to any extreme levels, and no accumulation over time was observed. With the concentration of ammonium have the concentra-tion of ammonia been calculated, the levels of ammonia can be seen in graphs in Ap-pendix A, Figure A.24 and Figure A.25. The ammonia levels have been about 40 000-70 000 mg/l.
