TRITA-LWR Degree Project ISSN 1651-064X
LWR-EX-11-29
A
MMONIUM
R
EMOVAL
A
ND
E
LECTRICITY
G
ENERATION
B
Y
U
SING
M
ICROBIAL
D
ESALINATION
C
ELLS
Han Wang
ii © Han Wang 2011
Master of Science Degree Project Water System Technology
Department of Land and Water Resources Engineering Royal Institute of Technology (KTH)
SE-100 44 STOCKHOLM, Sweden
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AMMANFATTNINGBiokemiska bränsleceller har blivit ett intressant alternativ för uthållig energiteknik för avloppsvattenbehandling under de senaste åren. Tekniken kombinerar avloppsvattenrening med produktion av elektricitet för att uppnå en positiv elektricitetsbalans. Genom att tillföra mikroorganismer till anoddelen och användning av en katoddel och mellan dessa delar ett membran för transport av vätejoner kan energi alstras i form av elektricitet och värme. Biokemiska bränsleceller för avsaltning kan genomföras genom att tillföra ytterligare en mellandel försett med ett membran för utbyte av anjoner.
Denna studie fokuserar på avlägsnande av ammonium kombinerat med system med biokemiska bränsleceller. Främst två vatten studerades det ena med tillsats av hjorthornssalt (ammoniumvätekarbonat) och det andra med filtrerat rejektvatten från avvattning av rötslam.
Experimenten genomfördes vid försöksanläggningen vid Hammarby Sjöstadsverk och vattenkemiska laboratoriet vid inst. för Mark- och vattenresurser, KTH, Stockholm. Genomfört arbete består av ett förberedelsesteg följt av experiment med avsaltning och med elektricitetskälla från biokemisk bränslecell. Som första del i den experimentella delen utvecklades en biofilm på anoden i anoddelen. Därefter studerades effekten av olika koncentrationer av hjorthornssalt (1,5, 2,5, 5 och 15 g/L) och sedan även rejektvatten från avvattning av rötslam. Hastigheten för avlägsnande av ammonium kan erhållas utifrån initialhalt och sluthalt för en viss tidsperiod och alstring av elektricitet utifrån mätning av volt som kontinuerligt mättes med en elektrisk multimeter.
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CKNOWLEDGEMENTSFirst of all, I would like to thank my supervisor Bengt Hultman, who changed my mind and guided me into this interesting field. His wisdom and strong faith about MFC and MDC always inspired me to rethink what already exists and explore un-known things.
I also want to thank professor Elzbieta Plaza and associate professor Erik Levlin for sharing their theoretical knowledge and encouragement to my thesis work. Thanks Dr. Ann Fylkner for the kindly guidance when I had my first step experiment in LWR laboratory.
Moreover, I would like to express my sincere thanks to Mila Harding, Lars Bengtsson and Christian Baresel in Hammarby Sjöstad research station. They spent a lot of time to help me familiarize with the equipment and for their extreme patience in clearing my doubts.
My appreciation to be conveyed to Junli Jiang and Pegah Piri for they sparing no effort to help me build up the structure. Without their selfless support, I could not have moved smoothly with my thesis work.
A lot of help came from Michelle Maria Jose, who is always encouraging me and gave useful suggestions in the thesis.
My specials thank goes to Jingjing Yang, who gave me the most of supports and encouragement at the experimental stage. From the start point till the end of experiment, she guided me on the operation procedures and also gave many critical comments and suggestions about MDC.
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ABLE OF CONTENTSammanfattning ... iii
Acknowledgements ... v
Table of content ... vii
Acronyms and abbreviations... ix
Abstract ... 1
1. Introduction ... 1
1.1. General background ... 1
1.1.1. Mechanism of water eutrophication ... 2
1.1.2. Sources of nitrogen ... 3
1.2. Risks caused by excessive ammonia nitrogen supply and eutrophication ... 3
1.3. Ammonium removal methods ... 3
1.3.1. Break-point chlorination method... 3
1.3.2. Selective ion exchange method ... 4
1.3.3. Air stripper... 4
1.3.4. Biological method ... 4
1.3.5. Chemical precipitation ... 5
2. Microbial fuel cells and microbial desalination cells ... 5
2.1. General description of fuel cells ... 5
2.1.1. Enzymatic fuel cells ... 5
2.1.2. Microbial fuel cells ... 6
2.2. Working principle of microbial fuel cells ... 6
2.2.1. pH effects in the MFC ... 6
2.2.2. Mediator MFCs and MFCs without mediators ... 7
2.2.3. Thermodynamic theory of MFCs ... 8
2.3. Literature review of microbial desalination cells ... 9
2.3.1. Working principle of MDCs ... 9
2.3.2. Microorganisms involved in MDC experiments ... 10
2.3.3. Commonly used media/substrates and operational conditions ... 11
2.3.4. Hjorthorn salt in MDC experiment ... 11
3. Purpose of present study ... 11
4. Materials and methods ... 13
4.1. Framework of whole experiment ... 13
4.2. Laboratory equipment involved and parameter measurements ... 13
4.2.1. Electronic balance ... 13
4.2.2. Multimeter ... 14
4.2.3. Conductivity meter ... 14
4.2.4. Spectrophotometer ... 15
4.2.5. Incubator ... 17
4.2.6. Resistance decade box ... 17
4.2.7. Air pump ... 18
4.3. Process description of MFC and MDC experiments ... 18
4.3.1. Preparation stage ... 18
4.3.2. MFC stage ... 20
4.3.3. MDC stage ... 21
5. Results and discussion from MFC to MDC stage ... 24
viii 5.2.5. Scenario 5 ... 30 5.2.6. Scenario 6 ... 31 5.2.7. Scenario 7 ... 32 5.2.8. Scenario 8 ... 33 5.2.9. Scenario 9 ... 34 5.2.10. Scenario 10 ... 36
5.2.11. Comparison and discussion ... 38
6. Conclusions... 41
7. Limitations of the study ... 41
8. Suggestions for further research ... 42
8.1. Specific suggestions for the used MDC system ... 42
8.2. Use of MDC system as a part of a treatment system ... 43
8.3. Development of theoretical concepts ... 43
9. References ... 45
10. Other references ... 47
Appendix I ... 48
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CRONYMS AND ABBREVIA TIONSAEM Anion Exchange Membrane
CEM Cation Exchange Membrane
COD Chemical Oxygen Demand
DI water Distilled water
DO Dissolved Oxygen
H_Salt Hjorthorn Salt
MDC Microbial Desalination Cells
MFC Microbial Fuel Cells
N Nitrogen
OCV Open Circuit Voltage
OM Organic Matter
ORR Oxidize-Reduction Reaction
P Phosphorus
PEM Proton Exchange Membrane
S Scenario
SS Suspended Solids
TN Total Nitrogen
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BSTRACTMicrobial fuel cell (MFC) has become one of the energy-sustainable technologies for wastewater treatment purpose in the recent years. It combines wastewater treatment and electricity generation together so as to achieve energy balance. By inoculating microorganism in the anode chamber and filling catholyte in the cathode chamber, and also with the help of a proton exchange membrane (PEM) between them, the MFC can transfer protons and produce power. Microbial desalination cells (MDC) are based on MFC’s structure and can fulfill desalination function by the addition of a middle chamber and anion exchange membrane (AEM).
This study focuses on ammonium removal and electricity generation in MDC system. Mainly two types of liquid were tested, a solution of Hjorthorn Salt and filtrated supernatant.
The experiments were performed at Hammarby Sjöstad research station and laboratory of Land and Water Resources department, Stockholm. It consists of a preparation stage, a MFC stage and a MDC stage. Until the end of MFC stage, biofilm in the anode chamber had been formed and matured. After that, solutions of different initial concentrations (1.5, 2.5, 5, 15 g/L) of Hjorthorn Salt and also filtrated supernatant have been tested. Ammonium removal degree can be obtained by measuring the initial concentration and cycle end concentration, while electricity generation ability can be calculated by voltage data which was continuously recorded by a multimeter.
Results showed that this MDC system is suitable for ammonium removal in both of Hjorthorn Salt solutions and supernatant. The removal degrees in Hjorthorn Salt solution at desalination chamber were 53.1%, 52.7%, 60.34%, and 27.25% corresponding to initial NH4+ concentration of 340.7, 376, 376 and 2220 mg/L. The ammonium removal degrees in the supernatant were up to 53.4% and 43.7% under 21 and 71 hours operation, respectively. In power production aspect, MDC produced maximum voltage when potassium permanganate was used in the cathode chamber (217 mV). The power density in solutions of Hjorthorn Salt was relative low (46.73 - 86.61 mW/m3), but in the supernatant it showed a good performance, up to 227.7 and 190.8 mW/m3.
Key words: Microbial desalination cell, microbial fuel cell, ammonium removal, power production, digested sludge
1. I
NTRODUCTION1.1.
General background
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resource management experience. To some extent, water scarcity is caused by immature water cleaning technology and low-efficiency water consumption rate.
Moreover, ammonia nitrogen problem is especially important and it is always linked to eutrophication. It is well known that high contents of NH4+ and phosphorus could cause eutrophication, which leads to interruption of water body balance. Eutrophication can only happen in the slow-flow water bodies (lakes, estuaries or bays.) when large proportions of nitrogen and phosphorus are injected. Algae and plankton grow rapidly in a rich nutrient environment thereby reducing the dissolved oxygen in the water body and finally causing the death of fishes and other living beings.
1.1.1. Mechanism of water eutrophication
In the surface fresh water system, phosphate is the limiting factor of plant growth. However, in the seawater system, the factors are ammonia nitrogen and nitrate because seawater has enough phosphate content. In fact sea water contains only limited contents of ammonia nitrogen and nitrate and thus, if pollutants containing ammonia nitrogen and nitrate are injected into seawater, the restricted balance will be eliminated and some special plants will grow rapidly (Figure 1).
The most common dominant species in the seawater are diatoms and chlorella. But after domestic waste water, food and chemical fertilizer industrial wastewater and agricultural drainage are injected into natural water body, with these plenty nutrients, the autotrophic organisms start to grow fast. As time goes by, blue algae will finally be the dominating species. They have a short life-cycle and get easily decomposed by micro-organism. The dissolved oxygen in the water will be consumed and hydrogen sulfide is produced; both reactions will deteriorate water quality. Finally, the fishes and other organisms die. When their bodies decay, nitrogen and phosphate elements are released into the water again and the vicious circle begins. Even cut off external nutrient supply, water body is still difficult to recover to normal conditions.
3 1.1.2. Sources of nitrogen
Agriculture runoff is the main source of nitrogen. It brings large proportions of ammonia nitrogen into the water body and change the original nitrogen balance. A recent study on nitrogen circle conducted by USA Agriculture ministry (China environmental impact assessment, 2011) shows that domestic wastewater and excrement which contain lots of carbamide and ammonia nitrogen may significantly interrupt the nitrogen cycle, Flagellates and Gonyaulax calenella species will be replaced by Nannochloris Genus and Stichococcus Genus (Brett et al., 2010).
1.2.
Risks caused by excessive ammonia nitrogen supply and
eutrophication
Harmful to aquatic animals, mainly fishes
Eutrophication will lead to lower transparency of water body, photosynthesis restriction due to the difficulty of sunlight to reach the aquatic plants and gradual reduction in the dissolved oxygen content. At the same time, plankton grows fast and consumes a large proportion of DO. All of these phenomena will be harmful to the aquatic animals, especially the fishes.
Moreover, accumulated organic matters at the bottom layer will produce hydrogen sulfide under anaerobic condition, which also cause damage to fishes.
Harmful to the human body and livestock when use it as drinking purpose in a long-term basis
Water in the eutrophication area contains nitrite and nitrate. If consumed, it might be toxic and harmful to the body.
1.3.
Ammonium removal methods
Ammonia nitrogen wastewater can be defined as 3 classes based on the concentration: NH3-N > 500 mg/L as high concentration, NH3-N between 50 to 500 mg/L as middle concentration, NH3-N < 50 mg/L as low concentration. Removal methods generally include physical, chemical and biological methods (Table 1), and the most common methods are break-point chlorination method, selective ion exchange method, air stripper method, biological methods and chemical precipitation methods.
1.3.1. Break-point chlorination method
This method uses chlorine or sodium hypochlorite which is injected into the wastewater and the NH3-N gets oxidized to N2. When supplied with appropriate volume of chlorine gas, the ammonia concentration will be zero and free chlorine concentration will be the lowest. After the supply volume exceeds this point, the free chlorine concentration will increase and thus it is called break-point (Baidu website, 2011).
Table 1. List of different approaches for NH4+ removal
Physical method and so on. Reverse osmosis; Distillation; Soil irrigation Chemical method chlorination; Ion exchange; Air stripper; Break-point Incineration; Chemical
4 2 2 Cl H OHOClHCl 4 2 2 NHHOClNH ClHH O 2 2 2 2 NHCl H ONOH H Cl 2 2 NHCl NaOHN HOClHCl
The advantage of break-point chlorination method is effective ammonia nitrogen removal rate as well as disinfection. It will be economy efficient if applied to low NH3-N concentration wastewater. However, for higher concentration wastewater, the addition of chlorine will be a large amount. Moreover, chloramine as by-product may cause secondary pollution. 1.3.2. Selective ion exchange method
Ion exchange occurs between solid interface and liquid interface. Zeolite in most cases has been chosen as exchange resin because of higher adsorption effect to un-ionized ammonia as well as exchange effect to ionized ammonia. It is a simple and reliable technology, with high efficiency and low cost. It is suitable for treating middle and low concentrated (< 500 mg/L) wastewater (Baidu website, 2011).
1.3.3. Air stripper
By using gas to supply wastewater, ammonia nitrogen can be transferred from the liquid phase to gas phase. It takes advantage of the difference between real concentration and balance concentration in both phases. When pH is adjusted to alkalinity, ionized ammonia (NH4+) turns to free ammonia (NH3). With an air stripper, NH3 can be removed at the efficiency of 60-95% (Baidu website, 2011). After that, ammonia gas can be collected by hydrochloric acid and produce ammonium chloride which could be used to soda ash production process. The advantage of the process is that it is simple and stable. However, it is of low efficiency if wastewater temperature is low, and thus it might be not be suitable for operation in winter period.
1.3.4. Biological method
The biological method to remove ammonia nitrogen is a process where various bacterial reactions take place and by passing through nitrification and denitrification processes, finally nitrogen gas is formed.
Generally, the nitrification process should be under aerobic condition. The aerobic nitrifying bacterium oxidizes ammonia nitrogen into nitrite or nitrate using carbon source in the wastewater. The reaction contains two parts; the first step is the transformation of ammonia nitrogen to
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nitrite by nitrosomonas bacteria and the second step is the transformation of nitrite to nitrate by the participation of nitrobacteria. Nitrosobacteria and nitrobacter bacteria are autotrophic bacteria and by taking part in the redox reaction they gain energy.
4 2 2 2
2NH3O 2NO2H O4H
2 2 3
2NOO 2NO
After this, denitrification takes place and nitrate is transformed into nitrogen gas by denitrifying bacteria (Figure 2).
The biological treatment method can be applied to remove various nitrogen compounds and the efficiency of 70-95% can be achieved. It also has limited secondary pollution and is economical. Thus, biological method is the most used technology all over the world, despite its large land occupation.
1.3.5. Chemical precipitation
Chemical precipitation method mainly relies on the chemical reaction to remove ammonia nitrogen compounds. Based on appropriate temperature, pH value, pressure, residence time etc., pollutants will form sparingly soluble compounds or insoluble gas, so as to purify wastewater. The principle behind chemical precipitation is that NH4+, Mg2+, PO 43-react with pollutants and form a precipitate. This method can be used to recycle ammonia nitrogen as agriculture fertilizer.
2. M
ICROBIAL FUEL CELLS AND MICROBIAL DESALINATION CELLS2.1.
General description of fuel cells
Fuel cell in general refers to a device that directly can convert chemical energy into electrical energy. It produces electricity in the anode chamber while at the same time transfer electrons from anode to cathode. The anode chamber and cathode chamber is separated by an ion or proton exchange membrane and this membrane plays a critical role in closing the circuit and electricity generation. It is a passive device that only allows protons to pass through the membrane so that an electrical circuit is formed. Fuel cells are different from batteries because it is sustainable if energy rich compounds are continuously supplied, whereas, batteries consume the limited volume of energy stored by chemical compounds in a closed system (Brett et al., 2010).
Biofuel cells are one of the most important type of fuel cells. It is defined mainly by way of electron formation. In a biofuel cell, electrons are generated by the use of biocatalysts. Based on the biocatalyst that has been used, biofuel cells can be characterized as enzymatic fuel cells (EFC) or microbial fuel cells (MFC). The EFC and MFC use enzymes and bacteria respectively (Brett et al., 2010).
2.1.1. Enzymatic fuel cells
6 2.1.2. Microbial fuel cells
Microbial fuel cells are almost the same as enzymatic fuel cells, but the main difference is the use of bacteria as a power source. The oxidation reactions happen inside the bacteria, and then electrons are transferred to the anode via cell membrane. With the biomass as substrate and microorganisms as the catalyst, microbial fuel cells can work continuously under stable nutrient supply conditions.
2.2.
Working principle of microbial fuel cells
A standard MFC should consist of two chambers, anode chamber and cathode chamber respectively (Figure 3). The anode chamber is used to inoculate liquid media. In many cases of wastewater treatment process, activated sludge or digested sludge is inoculated as bacteria source. By feeding with specific nutrients and ensuring anaerobic conditions in anode chamber, advantage bacteria are growing up and metabolism take place. The most important step, which is the electricity generation, occurs in this chamber. In the metabolism process, carbohydrate glucose is oxidized under anaerobic condition and electrons are released by enzymatic reactions. Those electrons are to be reduced after transferring to the cathode chamber (Wikipedia, 2011). The simplified anode half-cell reaction of oxidation of glucose is as follows.
6 12 6 6 2 6 2 24 24
C H O H O CO H e
While in the cathode chamber, oxygen is required so as to reduce electrons and maintain pH neutral.
2 2
6O 24H24e12H O
2.2.1. pH effects in the MFC
In the MFC, it is vital to maintain the pH in the anode chamber and feed substrate to ensure the target bacteria groups grow. The microorganisms prefer neutral pH rather than acidic or basic condition. Thus, monitoring pH value in the MFC is necessary so as to avoid wide variations. If the interval time of substrate replacement stands too long, pH value in the anode chamber may drop. The growth and metabolism of the microorganism can be inhibited or even halted. An explanation of this inhibition phenomenon is the change in the shape of proteins due to the presence of more H+ ions.
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Moreover, ion exchange membrane can be another important factor in pH maintenance. By placing membrane between the anode chamber and cathode chamber, different aquatic environments occur and different pH ranges arise. The membrane inhibits oxygen transfer from cathode chamber to anode chamber, which is vital to ensure anaerobic condition and also to the corresponding pH range. Also, the bacteria that take part in the biological reactions in the anode chamber cannot move freely. With the membrane, the bacteria in the anode chamber can have a relatively stable environment for favorated group growth. More importantly, membrane plays the role as that of a bridge, to transport ions through the membrane and continue with cathode chamber reactions. Movement of protons from the anode chamber can make an ionic equilibrium, and thus the movement efficiency mainly depends on ionic concentration gradient and membrane material. If protons cannot be transported at a sufficient rate, the pH will surely drop at the anode chamber and rise at the cathode chamber while charge balance can still make equilibrium because of other ions complement (Brett et al., 2010). 2.2.2. Mediator MFCs and MFCs without mediators
Mediator MFCs uses some specific mediators to help electrons transfer from the inside of the microbial cell membrane to the outside, so the electrons can reach the electrodes. Since the electrodes are solid entities, they are impossible to reach the inside of the microbial cells. Therefore, mediators play a role of a bridge to work effectively. Thionine, methyl blue, humic acid, methyl viologen, neutral read belong to the group of mediators that can help to transfer electrons and cope with the problem of electrochemically inactive microbial cells (Wikipedia, 2011).
Mediatorless MFCs are basically divided into two different types, one is direct oxidation of secondary fuels at the anode, and the other one is to use biofilm.
Direct oxidation of secondary fuels at the anode is a reliable MFC method based on the catalyst usage at the anode. Those fermentative microorganisms take advantage of molecular hydrogen as nutrients and with the addition of a catalyst (such as platinum) on the anode, the conversion efficiency can be extremely high. Nevertheless, the catalyst may also cause problems like low stability and high costs of platinum.
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Thus, it would be difficult to put this approach into large-scale practice at present research level.
The more commonly used approach is biofilm MFC (Figure 4). It is based on the direct physical and electronic interaction between the anode and bacteria. By continuous feeding and replacing substrates to the anode, the bacteria colony will slowly grow on the electrode in the anode chamber. It may take a few days or a few weeks according to different articles, but after that it becomes stable, the biofilm will adhere to the electrode surface and produce electricity. The internal mechanism of power production is still unclear and requires further research, and three theories have been proposed (Brett et al., 2010): i. Iron molecules exist on the surface of electrochemically active redox enzymes membrane. These iron molecules play a role as a bridge to enable electrons transfer to external materials (anode electrode) and do not require any chemical assistance to complete the transfer process. ii. The pili or protein appendages may be conductive and can transfer electrons when the pili have physical contact with the electrode in the anode chamber. iii. Some of the microorganisms can self-produce micromolar amounts of their own redox mediators and these products can be directly used as mediators to conduct electrons.
Moreover, when microorganisms oxidize the organic matter dissolved in the substrate, the protons are released into the water and are transferred to the cathode via proton exchange membrane. In the cathode chamber, oxygen will be supplied and the protons then combine with water. If platinum is coated to cathode surface, the electricity generation efficiency will be higher compared to bare electrode because platinum is able to donate electrons during oxygen reduction to produce water. While using it as the anode electrode, platinum can also work well as electron acceptor in the oxidation reaction.
Based on what has been discussed above, biofilm MFC has been chosen as an experimental subject and will be described later.
2.2.3. Thermodynamic theory of MFCs
In a reversible chemical reaction, the Gibbs free energy equation can be written as (Bard 1985, Newman 1973):
0 ln( ) G G RT Where: G
= Gibbs free energy 0
G
= Gibbs free energy under standard conditions R = universal gas constantT = absolute temperature
=reaction quotient of the product divided by the reactants Logan and his research group indicate that the Gibbs free energy under standard conditions is derived from the tabulated energies linked to the formation for OC (Logan 2006). If give a negative sign to the value ofG
, it then means maximum theoretical work and electromotive force (emf) can also be deduced (Logan, 2006).
max ( ) ( ) r emf emf G W E Q E n F where max
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emf
E = potential difference between the cathode and anode
Q
= chargen
= number of electrons per reactionF
= Faraday’s constantRearrangement of equations above,
(
)
r emfG
E
n F
At the standard conditions,0 0
(
)
r emfG
E
n F
Therefore, the overall electromotive force in all conditions can be described as, 0
ln( )
emf emfRT
E
E
nF
Finally, the MFC second law efficiency is given as the ratio between actual work output and maximum theoretical work. The formula is valid under the assumption that simple reactions evaluated at the anode and cathode are treated as parallel as more complicated reactions in bio-degradable wastewater (Zielke, 2006).
max
(
)
(
)
actual measured measured
MFC emf emf
W
V
n F
V
W
E
n F
E
where MFC
= MFC second law efficiencyactual
W = Actual work output
measured
V = Measured voltage potential
2.3.
Literature review of microbial desalination cells
2.3.1. Working principle of MDCs
Microbial desalination cells are based on the MFC structure and functions, but with 3 chambers and separated by cation exchange
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membrane (CEM) and anion exchange membrane (AEM) (Figure 5). It is a new technology that can be used in wastewater treatment and potable water production from brackish water or seawater by means of transfer of different ions from middle chamber (desalination chamber) to anode and cathode chamber.
As shown in Figure 5, the AEM is closed to the anode chamber while CEM is closed to the cathode chamber. When the MDC starts to work, the microorganisms oxidize substrate in the anode chamber and release the same amount of protons into the water. The protons cannot pass through AEM (AEM only allows anions across the membrane; the primary species are Cl-, HCO3- and HPO4-). Based on the pH neutrality principle, the anions in the middle chamber have to be transferred to the anode chamber. While the similar situation happens in cathode chamber, the reaction in this chamber results in the reduction of protons. CEM prevents anions in the cathode chamber transfer to the middle chamber, the membrane mainly allows Na+, K+ and H+ in the middle chamber to pass through (Cao et al., 2009). Therefore, the result is transportation of protons and anions in the middle chamber (desalination chamber) and the purpose of desalting can be achieved.
Compared with traditional approaches, the MDC method does not require water pressure or electrical energy (oxygen aeration in the cathode chamber is selectable and will be discussed later). Instead, by using MDC approach, electricity can be generated during proper operation process. (Cao et al., 2009)
2.3.2. Microorganisms involved in MDC experiments
In the sediments many kind of metal-reducing bacteria are easily found. They utilize insoluble electron acceptors (Fe3+ and Mn4+) in the surroundings. Shewanella putrefaciens is one type and its specific cytochromes at the outer membrane make the cell active (Kornell et al., 2005). Another bacteria family called Geobacteraceae can form biofilm on the anode electrode after feeding with some days, and then the biofilm transfers electrons from acetate (Bond and Lovley, 2003).
Based on Chaudhuri and Lovley’s research, Rhodoferax species can take glucose as substrate and convert it to CO2. The conversion efficiency is quite high and can reach as much as 90%. The rest groups of bacteria perform similarly at 1-17 mW/m2 graphite surfaces (Table 2).
Even though these bacteria all show high electron transfer efficiency, they still have drawbacks. Most of them require specific substrates such as acetate or lactate. They grow slowly and to obtain a stable output. It requires lots of time, few days or even few weeks, depending on the feeding approach and laboratory conditions etc. Moreover, compared to Table 2. Microorganism in MFC/MDC (Kornell et al., 2005)
Microorganism References
Desulfuromonas acetoxidans Bond et al., 2002 Geobacter metallireducens Bond et al., 2002
Shewanella putrefaciens Bond and Lovley 2003, Kim et al., 1999a, Kim et al. 1999b, Kim et al., 2002, Schroder et al., 2003
Geobacter sulfurreducens Bond and Lovley 2003
Rhodoferax ferrireducens Chaudhuri and Lovley 2003
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the mixed culture, the axenic bacterial culture holds a relatively low position because of low energy transfer efficiency. The mixed bacterial cultures are easily adaptable to different substrates and environments, with very high power output (Rabaey et al., 2004a, b).
Active mixed cultures can be gained from many places such as sediments or wastewater treatment plant (WWTP). Kim et al., (2004) and Rabaey et al., (2004) indicated nitrogen fixing bacteria (such as Azospirillum) and facultative anaerobic bacteria (such as Pseudomonas and Enterococcus), respectively.
2.3.3. Commonly used media/substrates and operational conditions
The substrates most commonly used in MDC are acetate, lactate and glucose. They are easily dissolved in water and play a role as nutrient supplier.
In the MDC, before the set-up of the chambers, electrodes and wire, it is vital to fix which type of chamber or electrode should be used. For the anode part, the electrode material to be selected should meet requirements like adequate surface for the formation of biofilm; conductive surface. If the MDC uses recycle system, it is better to make sure the electrode will not affect the free flow of influent and effluent. Generally, anode electrode has several options such as graphite felt, graphite granules and plate shaped plain graphite. On the other hand, electrodes in the cathode chamber can be basically divided into two situations, with or without oxygen. The cathodes are often either platinum-coated carbon electrodes with extra oxygen supply or plain carbon electrodes immersed in ferricyanide solution. From up-to-date experiment results, it can be concluded that the addition of ferricyanide works better than the others. For instance, Sangeun et al., (2004) and Logan (2004) inoculated wastewater sludge with 20 mM acetate and got a maximum of 0.097 mW within 120h after inoculation.
2.3.4. Hjorthorn salt in MDC experiment
Hjorthorn salt is a Swedish name of deer horn salt. It has its name because it was obtained from deer horns. It mainly consists of ammonium hydrogen carbonate (NH4HCO3). Usually it is used for food purpose: bakery. It is easily decomposed by heating, and the product will be ammonia and carbon dioxide.
In the MDC system, Hjorthorn salt can be used to simulate ammonium removal. As AEM and CEM applied and biofilm formed, protons will be released into water in the anode chamber while cation will be required in the cathode reaction. Thus, NH4+ and HCO3- in the Hjorthorn salt solution will be attracted by cathode and anode chamber respectively. By this way, the NH4+ can be removed from the desalination chamber.
3. P
URPOSE OF PRESENT ST UDY12 1. Preparation stage/MFC stage:
Setting up of two parallel MFCs, inoculating different kinds of sludge and then feeding with different nutrients (sugar sold in the supermarket and acetate solution) so as to find out biofilm formation times, maximum voltages and open circuit voltages and efficient nutrients.
2. Ammonium removal stage/MDC stage:
Combining the two MFC’s into one MDC and keeping the previous anode electrodes (with biofilm on the electrode), AEM and CEM are inserted adjacent to the anode chamber and cathode chamber respectively. By changing test solutions in desalination chamber and catholyte, it is possible to collect these data:
Ammonium removal degree from the addition of artificial solution (Hjorthorn salt) under different replacement times; Internal resistance at different running stages in MDC (start-up
phase, rise phase, peak phase, decline phase, low stable phase); Energy aspect: Maximum voltage, maximum power and power
density;
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4. M
ATERIALS AND METHODS4.1.
Framework of whole experiment
The experiment process (Figure 6) is divided into 3 stages: preparation stage, MFC stage and MDC stage. It was started from 29th, June and ended on 17th, August. Details will be explained in later 4.3.
4.2.
Laboratory equipment involved and parameter measurements
During the 2 month of experiment process, the equipment been used are listed as Table 3 shown.4.2.1. Electronic balance
The electronic balance used during the whole experiment is LA-110, ACCULAB Corporation’s product, made in Germany.
Figure 7. Electronic balance used in this experiment Table 3. Equipment list in the MFC and MDC experiment
Name No. Country of origin Description
Air pump 302S Sweden Christian Berner AB
Mixer WM/220/F England Fisons Scientific Apparatus Multimeter BS1901W Sweden Caltek Industrial (H.K.) Ltd. Conductivity meter Cond 330i Germany WTW 82362 Wellheim
Spectrophotometer XION 500 Germany DR LANGE
Electronic balance LA-110 Germany ACCULAB
Incubator Germany MEMMERT
Resistance decade
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LA-110 is an analytical balance with high performance. The display update only requires 0.1 to 0.4 second and also with selectable weighing units (g, kg, GN, mg, ct, lb, oz, thl, tlt, dwt etc.). The maximum capacity is 110 g, which means it is only for small scale experiment usage. The readability and reproducibility specifications are 0.0001 g and ±0.0001 g, respectively.
LA-110 is frequently used in the experiment (Figure 7). It was mainly responsible for weighing chemical compounds in addition to be added to MFC/MDC as nutrient solutions. In this whole experiment, chemical compounds includes sugar, potassium chloride, sodium chloride, disodium hydrogen phosphate, potassium hydrogen phosphate, sodium acetate, ammonium chloride, magnesium chloride hexahydrate and calcium chloride hexahydrate etc.
4.2.2. Multimeter
Multimeter is a hand-held electronic measuring device that can be applied in many circumstances. It has lots of basic functions such as voltage, current and resistance measurement. Usually it provides measurement function of both alternating current/voltage and direct current/voltage. In this experiment, all of the measurements are set as direct current/voltage.
BS1901W series has been chosen as the voltage measurement device (Figure 8). It is a 3 1/2 Digits all-ranges protection meter with battery test. The measuring range is between 200 mV to 600 V and DC current from 20 mA to 200 mA. Due to limitation of present lab conditions (no data logger is available), it is possible to record voltage data by connecting multimeter to the circuit 24 hours together with video monitoring (for instance, open video monitoring software before the lab is closed, and on return the next morning, voltage data at every moment can be seen from the software “YouCam”).
4.2.3. Conductivity meter
15
strength of conductivity, the higher salt concentration, the higher conductivity the solution has.
Cond 330i meter (Figure 9) is a robust and waterproof handheld device with parallel temperature display. It also provides manual temperature compensation with linear temperature function and non-linear function for ultrapure water and natural waters. The working temperature is between -5 ℃ to 105 ℃ and resolution is 0.00 S cm/ . In this experiment, due to the small volume of MFC and MDC (8.4 mL volume in each chamber), it is difficult to measure conductivity by using Cond 330i meter (The normal procedure of measurement is to use probe immersed in the solutions, but in this situation there is not enough solution). Therefore, as a parameter, conductivity value will only appear occasionally in the text.
4.2.4. Spectrophotometer
Spectrophotometer is one kind of photometer that makes it possible to test the intensity based on the light source wavelength. Spectral bandwidth and linear range of absorption are two vital characteristics of
16
spectrophotometer. Due to the fast speed and high precision, it is widely used for measurement of transmittance/reflectance of solutions.
Spectrophotometer used in the laboratory is XION 500, DR LANGE Corporation, made in Germany (Figure 10). Simply attach ProID clips to the cuvette containing the sample, read the identification with the scanner and read out the right measurement procedure. It ranges from 340 to 900 nm, wavelength accuracy is ±2 nm with automatic zeroing. In this MFC/MDC experiment, it might be the most crucial device because all of the ion concentrations are measured by spectrophotometer. The concentration tested in this device included ammonium and COD. The process of ammonium measurement is: Firstly, the rough value of solution was estimated; secondly, the solution was diluted in a 50 mL flask and mixed; thirdly, 0.2 mL solution was extracted from flask and Figure 12. Cuvettes used in the measurement
17
been added to the cuvette (LCK 302 cuvette ranging from 47 -130 mg/L of NH4-N, Figure 12) and been shacked; fourthly, 15 minutes was for cooling and then tested in the spectrophotometer. The value will be finally shown in the screen and NH4+ concentration can be calculated. 4.2.5. Incubator
Incubator is a container that provides a constant temperature (adjustable) so as to ensure a stable growing environment for microorganism. Usually the incubator used in the laboratory is electric heating, but the old method of using hot water may also achieve the heating function. At the Hammarby Sjöstadsverk Laboratory (as shown in Figure 11), electrical heating made by MEMMERT, Germany is used. During the experiment, incubator was used when membrane required to be soaked in DI water before 24 hours of usage.
4.2.6. Resistance decade box
Resistance decade box is a device that can change external resistance and at the same time the resistance value can be displayed. By connecting it to the MFC/MDC circuit, external resistance can be adjusted to test internal resistance. In this MDC experiment, internal resistance is required to be measured not only because it is a key factor that affects electricity generation, but also reflects the advantages/drawbacks of MFC/MDC structures and membrane block effect.
Generally, when the external resistance equals to the internal resistance, the value of maximum power density Pm will reach the peak.
2
4
m iE
P
R
where mP = Maximum power density
E
= Electromotive forcei
18
Thus, it can be seen that MFC/MDC power output is restricted by E and Ri. E is the driving force of the cell, due to the reactions that happens in the MFC/MDC, it is difficult to enhance E. However, the internal resistance is closely linked to MFC/MDC structure and there is a great space for improvement. Thus, it is vital to reduce internal resistance in the MFC/MDC. The first part is to measure true internal resistance by adjusting external resistance to draw a polarization curve VS power density-current curve.
In this case, RBOX-408 (LUTRON Electronic Enterprise CO., LTD) has been chosen as external resistance. It is able to provide a wide range of resistance from 1 ohm to 11111110 ohm (Figure 13). The working temperature is between 0 to 50 degrees and accuracy is ±1%. The operation is simple: by switching the button with different combinations, the preferred resistance value can be obtained.
4.2.7. Air pump
The only purpose of using air pump is to provide air in the cathode chamber. The cathode chamber with mixed catholyte continuously requires oxygen for water formation purpose. The soft pipe is across the pump and on one side (No.1 in the Figure 14) the pipe is exposed to air while the other side (No. 2 in the Figure 14) is connected to a well-sealed probe which is inserted to catholyte. Due to the rotation of the pump, air is continuously transferred to the catholyte and catholyte is almost saturated.
4.3.
Process description of MFC and MDC experiments
As mentioned in Figure 6, the whole experiment process is divided into 3 parts: preparation stage, MFC stage and MDC stage.
4.3.1. Preparation stage
At the first stage, it is interesting to see which sludge (activated sludge or digested sludge) is able to produce more electricity. Thus, in the preparation stage, both activated sludge and digested sludge were fed by sugar or mixed nutrients in order to screen out suitable microorganism. The process was carried out in the following way:
Activated sludge and digested sludge were obtained from Hammarby Sjöstadsverk by using the container (No. 1) shown in Figure 15. Figure 14. Air pump
19
Preparation stage: container. Sludge was supplied to a 50 mL volume bottle (No. 2) for feeding purpose. Sugar (No. 3) was fed every 24 hours and continuously for 5 days as mentioned in the Table 4. The process of adding sugar should be as quick as possible to ensure anaerobic conditions. The dosage operation was carried out by electronic balance. During 7th July to 12th July, digested sludge was extracted and stored in two different bottles. As parallel samples, these two bottles were fed by mixed nutrients and sugar, respectively. The sludge volume and dosage can be seen in Table 4.
Table 4. Nutrient solution contents
Activated sludge Digested sludge Food 1 Digested sludge Food 2
Period 06.29 - 07.04 07.07 - 07.12 07.07 - 07.12
Nutrient Sugar Mixed solution Sugar
Conc. (g/L) 1 3 3 2 1.6 CH COONa H O 2 4 4.4 KH PO 2 4 3 2 3.4 K HPO H O 4 1.5 NH Cl 2 6 2 0.1 MgCl H O 2 2 2 0.1 CaCl H O 0.1 KCl 1
Sludge volume 30 mL 22 mL (17 mL sludge+5 mL DI water) 24 mL (19+5)
Dosage 0.03 g 0.0352g 0.0968g 0.0748g 0.033g 0.0022g 0.0022g 0.0022g 0.024 g Figure 15. Preparation stage: container
1
20 4.3.2. MFC stage
Two sets of MFC package were ordered from Reading University, UK. They required to be assembled before usage. When building MFC together with cation exchange membrane inside, the whole container should be soaked in the DI water for 24 hours. The purpose of this is to moist the carbon paper electrodes as well as CEM (Figure 16).
This MFC (Reading University, made in UK) is small scale and sealed by rubber. Electrodes were cut into suitable size and placed into the chambers, and most importantly, the electrodes were crossed through the hole and can be linked to the outside. While at the same time, measuring the dimension of each chamber, it was found that (Figure 17): Chamber volume = Length * Width * Height
= 4 cm * 3 cm * 0.7 cm = 8.4 mL Electrode area = Length * Width
= 4 cm* 2.7 cm = 10.8 cm2
After being soaked in DI water for 24 hours, the container would be empty and then have to wait a few minutes to dry naturally. A 2.5 g of activated sludge (fed with sugar in the previous 5 days) and injected into anode chamber, and the rest space was occupied by supernatant in the fed bottle. The holes were sealed properly by PTFE and adhesive tape as soon as possible.
After the anode chamber has been done, the next step was injecting catholyte in the cathode chamber. The catholyte was made by 4 chemical compounds as shown in Table 5. As a matter of convenience, 500 mL of catholyte was made and stored in the fridge.
The final step was to connect the circuit by simple electric wires and clamps. The external resistance was set to 220 ohm with a multimeter to measure its voltage, the measuring range set to 200 mV. After the circuit was connected, voltage change was recorded. The room temperature was checked was checked and 23±1℃.
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of carbon paper electrode. Thus, for bacteria safety and membrane performance, it is necessary to keep liquid level and supply catholyte every 24 hours (anode chamber does not have these problems since it was blocked and has no air contact). The anode chamber, however, requires substrate replacement. As microorganism in the anode chamber lives on nutrients and uses them to form biofilm, the nutrient solution (Table 4) should be replaced every day. The ideal condition is to extract all substance from the anode chamber and put it into centrifuge, and then replace the liquid supernatant. However, since there was no centrifuge available at the lab, liquid was kept stable and liquid from the upper layer was extracted as an alternative way for replacement purpose. Most importantly, after 7 days of running, MFC with activated sludge could not produce detectable electricity (resolution: 1 mV) and two bottled digested sludge fed by different nutrients were taken over by it. They followed the same operation procedures but slight difference in the substrate constitute.
The NH4+ concentration and COD measurement were not proceeding in this stage and the value will be absent in the result tables. This stage was focused on biofilm’s formation and electricity generation.
4.3.3. MDC stage
MDC stage is the core component in the whole thesis work. It has two main tasks, ammonium removal and electricity generation.
MDC build-up:
When two MFCs are stabilized to produce electricity, then it could be possible to consider MDC construction. The small size of container may cause difficulties to replace substrate, therefore two anode chambers instead of one (Figure 19) maybe considered wisely. Before constructing Figure 17. Structure of MFC in use
Table 5. Catholyte composition in MFC process Chemical component Concentration (g/L)
KCl 0.1
NaCl 1.0
Na2HPO4 2.75
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the MDC, another membrane called anion exchange membrane (Membrane international, US), requires to be soaked in NaCl solution for 24 hours. Here I weighed 12.5 g of NaCl powder and dissolved it into 250 mL DI water (Figure 18).
The biofilm already formed in both of the anode electrodes was moved to the new structured MDC. AEM was closed to anode chamber and CEM was closed to cathode chamber (Figure 19). The gaps were sealed by rubber and electrodes were connected to resistance box by wire lines. The external resistance first chosen was 440 ohm and which was replaced by resistance box purchased on 27th, July. Moreover, after the digested sludge is injected in the anode chamber, use nitrogen gas to expel oxygen for 3 minutes.
After the MDC construction finished, it was shown as Figure 21.
MDC operation:
With a probe supplying oxygen continuously, the MDC started to work. It tested 10 scenarios during 22 days (from 26th, July to 17th, August) and basically followed the same procedure (Figure 22).
Figure 19. MDC construction plan
NH4+
HCO
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The normal procedure is firstly to measure the input NH4+ concentration and COD value, and then after several hours’ reaction measure the output NH4+ and COD value and finally calculate the ammonium removal degree and COD change. Measurement method has been described in 4.2.4 Spectrophotometer. During the cycle time, it is required to record voltage data continuously. As mentioned before, as there was no data logger of voltage parameter in the lab, laptop camera with the software called “YouCam” was applied as substitution (Figure 20). Set the photo interval time as 10 seconds and put the multimeter in front of camera, it can be totally the same effect as data logger.
Figure 21. Real product of MDC
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Figure 22. Working conditions of MDC system
There are totally 10 scenarios that has been tested (Table 6).
No. 1 to No.5 scenarios were designed to test pure and simple chemical solution performance in MDC system, since the Hjorthorn salt mainly contains NH4HCO3. No.6 to No. 8 scenarios were more valuable to provide data that was more closed to reality (filtrated supernatant and even wastewater with low NH4+ concentration). No. 9 and No. 10 were designed to see the influence of different catholyte, based on the previous catholyte composition, 20 mg/L KMnO4 was added to the original catholyte and tested the NH4+ removal rate and power generation ability.
It is necessary to mention that the resistance box purchased on 27th, July. Therefore, experiments performed before that day could not measure the internal resistance.
5. R
ESULTS AND DISCUSSIO N FROMMFC
TOMDC
STAGEThe MFC stage, as mentioned in Figure 6, is mainly responsible for biofilm’s formation while in the MDC stage, the attention is more focused on ammonium removal and electricity generation. Therefore, voltage data in both of the MFC stage and MDC stage will be presented in the Appendix and core data and voltage-time figures will be shown in the text.
5.1.
MFC stage
25 Table 6. Scenarios tested in MDC stage
No Desalination chamber Process description
1 H_Salt 1.5 g/L Test the NHgeneration under 48 hours. 4+ removal degree as well as electricity 2 H_Salt 2.5 g/L Test the NH4+ removal degree as well as electricity
generation under 22 hours.
3 H_Salt 2.5 g/L Test the NHgeneration under 45 hours. 4+ removal degree as well as electricity 4 H_Salt 5 g/L Test the NHgeneration under 67 hours. 4+ removal degree as well as electricity 5 H_Salt 15 g/L Under the high NHremoval degree as well as electricity generation under 24 4+ concentration condition, test NH4+
hours.
6 Filtrated supernatant Test the filtrated supernatant sample in 21 hours. Sample was taken from Hammarby Sjöstadsverk. 7 Filtrated supernatant Test the filtrated supernatant sample as comparison in 71 hours. Sample was taken from Hammarby Sjöstadsverk. 8 Filtrated influent wastewater Test the filtrated influent wastewater sample under 24 hours. Sample was taken from Hammarby Sjöstadsverk. 9 H_Salt 15 g/L Under the high NHKMnO4 to the former catholyte, test the electricity 4+ concentration condition, add the
generation performance under 24 hours.
10 H_Salt 15 g/L Under the high NHKMnO4 to the former catholyte, test the electricity 4+ concentration condition, add the
generation performance under 92 hours.
The MFC of FOOD 2 produced maximum voltage of 13.4 mV and OCV of 233.5 mV under 400 ohm resistance condition.
In the MFC stage, the MFC of FOOD 1 performed generally better than the MFC of FOOD 2 (Table 11 in the appendix). While it is admitted that due to lack of operation experience and monitoring video, the biofilm formation time can only be roughly estimated, ~50 hours. Compared to many other researches, this result does not even reach the average power production level (ranging from 200 mV to 600 mV). However it is still reasonable due to the container size, lack of stirrer and centrifuge, oxygen diffusion from cathode chamber to anode chamber, which will be discussed in 5.2.11.
5.2.
MDC stage
5.2.1. Scenario 1During the 48 hours, the MDC produced maximum voltage of 26.3 mV, maximum power of 0.00157 mW, maximum power density of 1.45 mW/m2 (based on 10.8 cm2 electrode area) and 46.73 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 13 and Table 12). The time for MDC to reach maximum voltage is 27 hours (Figure 23). However, as the curve rises slowly after 2.03 h, it is possible to use 2.03 as the “arrival time”.
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Figure 23. Voltage generated by using 1.5 g/L H_Salt
crash (Figure 23). Since the voltage - time curve is still rising before 14:15, the real maximum voltage value may not be known. However, based on the predictions and Scenario 2-10, the value should be in the range of 26.3 to 30 mV.
The ammonium removal degree is 53.1% based on the original NH4+ concentration of 340.7 mg/L. The ammonium concentration (430.5 mg/L) in cathode chamber was even higher than for fresh H_Salt solution. This phenomenon could be explained either by evaporation effect or by ammonium ion residual, or both.
The internal resistance can be roughly measured by resistance box. When the peak voltage appears, test a series of resistance value (from 1 ohm to 100000 ohm) and record the data of voltage. Based on the voltage and resistance value, parameters such as current, current density, power and power density could be figured out (Appendix II). In this experiment, the MDC showed a maximum power density of 2.32 mW/m2 at ~3000 ohm and 0.100 mW/m2 at ~5500 ohm, in high and low efficiency respectively (Table 20 and Table 21).
The maximum power production was 0.00250 mW at current of 0.029 mA, where the voltage was 86.6 mV correspondingly (Figure 24).
Figure 24. Voltage and power generated as a function of current in Scenario 1 0 5 10 15 20 25 30 35 0 5 10 15 20 25 Vo ltage (m V) Time (h)
Voltage - time curve
27
Figure 25. Voltage generated by using 2.5 g/L H_Salt 5.2.2. Scenario 2
During the 22 hours, the MDC produced maximum voltage of 29.6 mV, maximum power of 0.00199 mW, maximum power density of 1.84 mW/m2 and 59.23 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 14). The time for MDC to reach maximum voltage is 3 hours (Figure 25).
The ammonium removal degree is 52.7% based on the original NH4+ concentration of 376 mg/L. The ammonium concentration in cathode chamber (452.4 mg/L) was even higher than fresh H_Salt solution. This phenomenon could be explained either by evaporation effect or by ammonium ion residual, or both.
The internal resistance can be roughly measured by resistance box as described in Scenario 1. In this experiment, the MDC showed a maximum power density of 0.514 mW/m2 at ~6000 ohm in low efficiency condition (Table 22). The peak performance in high efficiency condition was missed.
Due to miss of high efficiency data, the maximum power production in low efficiency was 0.00056 mW at current of 0.0096 mA, where the voltage was 57.7 mV correspondingly (Figure 26).
Figure 26. Voltage and power generated as a function of current in Scenario 2 0 5 10 15 20 25 30 35 0 4 8 12 16 20 Vo ltage (m V) Time (h)
Voltage - time curve
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Table 7. Basic information and critical data of scenario 3
Time 2011/7/26 14:14 – 2011/7/28 11:26
Duration 45 hours
Anode chamber Mixed acetate solutions (Same as FOOD 1 in Table 4) Desalination chamber Hjorthorn salt 2.5 g/L
Cathode chamber Catholyte shown in Table 5
NH4+ concentration comparison(mg/L)
Before After
Middle chamber 376 149.1
Cathode chamber 0 409.4
Removal degree 60.3%
Internal resistance (Ohm)
Peak performance /
Stable low performance /
Power production under 440 ohm
Maximum voltage 28.8 mV
Maximum power 0.00178 mW
Maximum power density 1.65 mW/m2
52.98 mW/m3 5.2.3. Scenario 3
During the 45 hours, the MDC produced maximum voltage of 28.8 mV, maximum power of 0.00179 mW, maximum power density of 1.65 mW/m2 (based on 10.8 cm2 electrode area) and 52.98 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 7). The time for MDC to reach maximum voltage is 34.3 hours (Figure 27). The ammonium removal degree is 60.3% based on the original NH4+ concentration of 376 mg/L. The ammonium concentration in cathode chamber (409.4 mg/L) was even higher than fresh H_Salt solution. This phenomenon could be explained either by evaporation effect or by ammonium ion residual, or both.
The internal resistance value could not be measured due to lack of resistance box (still in delivery). Without these data, V-P curve cannot be produced.
Figure 27. Voltage generated by using 2.5 g/L H_Salt 0 5 10 15 20 25 30 35 0 10 20 30 40 Vo ltage (m V) Time (h)
Voltage - time curve
29
Figure 28. Voltage generated by using 5 g/L H_Salt 5.2.4. Scenario 4
During the 67 hours, the MDC produced maximum voltage of 27.8 mV, maximum power of 0.00176 mW, maximum power density of 1.630 mW/m2 (based on 10.8 cm2 electrode area) and 52.4 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 15). The time for MDC to reach maximum voltage is 1.43 hours (Figure 28). The ammonium removal degree is 20.2% based on the original NH4+ concentration of 536 mg/L. The ammonium concentration in cathode chamber (558 mg/L) was even higher than fresh H_Salt solution. This phenomenon could be explained either by evaporation effect or by ammonium ion residual, or both.
The internal resistance can be roughly measured by resistance box as described in Scenario 1. In this experiment, the MDC showed a maximum power density of 2.5 mW/m2 at ~2000 ohm in high efficiency condition (Table 23). The peak performance in low efficiency condition was missed.
The maximum power production in high efficiency was 0.00269 mW at current of 0.0367 mA, where the voltage was 73.4 mV correspondingly (Figure 29).
Figure 29. Voltage and power generated as a function of current in Scenario 4 20 22 24 26 28 30 0 20 40 60 80 100 Vo ltage (m V) Time (minute)
Voltage - time curve
30
Figure 30. Voltage generated by using 15 g/L H_Salt 5.2.5. Scenario 5
During the 24 hours, the MDC produced maximum voltage of 35.8 mV, maximum power of 0.00291 mW, maximum power density of 2.7 mW/m2 (based on 10.8 cm2 electrode area) and 86.60 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 16). The time for MDC to reach maximum voltage is 2.4 hours (Figure 30). The ammonium removal degree is 27.3% based on the original NH4+ concentration of 2220 mg/L. The ammonium concentration in cathode chamber was 965 mg/L.
The internal resistance can be roughly measured by resistance box as described in Scenario 1. In this experiment, the MDC showed a maximum power density of 4.15 mW/m2 at ~2100 ohm under high efficiency condition and 0.15 mW/m2 at 6000 ohm under low efficiency condition (Table 24 and Table 25).
The maximum power production was 0.00448 mW at current of 0.0462 mA, where the voltage was 97 mV correspondingly (Figure 31).
Figure 31. Voltage and power generated as a function of current in Scenario 5 0 10 20 30 40 0 4 8 12 16 20 Vo ltage (m V) Time (h)
Voltage - time curve
31
Figure 32. Voltage generated by using filtrated supernatant 5.2.6. Scenario 6
During the 21 hours, the MDC produced maximum voltage of 58 mV, maximum power of 0.00765 mW, maximum power density of 7.08 mW/m2 (based on 10.8 cm2 electrode area) and 227.68 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 17). The time for MDC to reach maximum voltage is 8.08 hours (Figure 32). The ammonium removal degree is 53.4% based on the original NH4+ concentration of 993 mg/L. The ammonium concentration in cathode chamber (712 mg/L) was even higher than fresh H_Salt solution. This phenomenon could be explained either by evaporation effect or by ammonium ion residual, or both.
The internal resistance can be roughly measured by resistance box as described in Scenario 1. In this experiment, the MDC showed a maximum power density of 7.86 mW/m2 at ~1250 ohm under the high efficiency condition and 0.700 mW/m2 at ~5500 ohm under low efficiency (Table 26 and Table 27).
The maximum power production was 0.00849 mW at current of 0.0824 mA, where the voltage was 103 mV correspondingly (Figure 33).
Figure 33. Voltage and power generated as a function of current in Scenario 6 0 10 20 30 40 50 60 70 0 4 8 12 16 20 Vo ltage (m V) Time (h)
Voltage - time curve
32
Figure 34. Voltage generated by using filtrated supernatant 5.2.7. Scenario 7
During the 71 hours, the MDC produced maximum voltage of 53.1 mV, maximum power of 0.00641 mW, maximum power density of 5.94 mW/m2 (based on 10.8 cm2 electrode area) and 190.8 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 18). The time for MDC to reach maximum voltage is 5.42 hours (Figure 34). The ammonium removal degree is 42.7% based on the original NH4+ concentration of 1221 mg/L. The ammonium concentration in cathode chamber was 801 mg/L after the cycle.
The internal resistance can be roughly measured by resistance box as described in Scenario 1. In this experiment, the MDC showed a maximum power density of 6.93 mW/m2 at ~1600 ohm under high efficiency condition and 0.0677 mW/m2 at ~6500 ohm under low efficiency (Table 28 and Table 29).
The maximum power production was 0.00748 mW at current of 0.0684 mA, where the voltage was 109.4 mV correspondingly (Figure 35).
Figure 35. Voltage and power generated as a function of current in Scenario 7 0 10 20 30 40 50 60 0 10 20 30 40 50 60 70 Vo ltage (m V) Time (h)
Voltage - time curve
33
Figure 36. Voltage generated by using filtrated wastewater 5.2.8. Scenario 8
During the 24 hours, the MDC produced maximum voltage of 19.7 mV, maximum power of 0.00088 mW, maximum power density of 0.817 mW/m2 (based on 10.8 cm2 electrode area) and 26.25 mW/m3 (based on 33.6 mL volume) under 440 ohm condition (Table 19). The time for MDC to reach maximum voltage is 12.8 hours (Figure 36). The ammonium removal degree is insignificant based on the original NH4+ concentration of 40.7 mg/L. The ammonium concentration in cathode chamber was 226 mg/L after the cycle. This phenomenon reflected the truth that ammonium residual existed in both of desalination chamber and cathode chamber. It means chambers didn’t washed well by DI water before this scenario started. But if combine the power generation data, it might be not satisfied using MDC to treat influent wastewater due to low ammonium concentration.
The internal resistance can be roughly measured by resistance box as described in Scenario 1. In this experiment, the MDC showed a maximum power density of 0.398 mW/m2 at ~3000 ohm under high efficiency condition and 0.0226 mW/m2 at ~6000 ohm under low efficiency (Table 30 and Table 31).
The maximum power production was 0.00043 mW at current of 0.0120 mA, where the voltage was 35.9 mV correspondingly (Figure 37).
Figure 37. Voltage and power generated as a function of current in Scenario 8 -10 -5 0 5 10 15 20 25 0 4 8 12 16 20 Vo ltage (m V) Time (h)
