TITULO Performance of air-cathode stacked microbial fuel cells systems for wastewater • SUBTÍTULO treatment and electricity production Edson Baltazar Estrada-Arriaga a *, Yvonne Guillen-Alonso b , Cornelio Morales- Morales b , Luis A. Pliego-Sánchez b , Liliana García-Sánchez b , Erick O. Bahena- Bahena b and Oscar Pérez Guadarrama b a Mexican Institute of Water Technology b Polytechnic University of State Morelos
Microbial Fuel Cells (MFCs) The MFCs are bio-electrochemical systems, capable of generating electricity from the oxidation of organic matter from wastewater treatment and simultaneously the contaminant removal. External resistor MFC effluent - + Cathode Ánod o Biofilm Membranes Anode -Nafion Sustrate -Cation-exchange (wastewater) Advantages of MFC over current energy generating technologies from organics * Ambient temperature * High conversion efficiency * No gas treatment * Application for wide locations and diverse fuels * No energy input for aeration
Principies of MFC • Biochemical degradation – microorganism growth : Oxidation- reduction Electrochemistry, Catalyst reaction Mass transport, Mixing of substrate • @ anode, acetate ( substrate ) is oxidized by bacteria (catalyst), mixed culture. CH 3 COO - + 4 H 2 O 2 HCO 3 - + 9H + + 8e - (E an = - 0.296 V) Electrons produced are transferred to the anode by mediator/shuttles, direct membrane, or nano-wire. 2.2 kW/h.m 3 considering • Electrons flow to cathode through a conductive material containing a that the energy of glucose resistor or operated under a load to produce electricity . molecule containing • Protons diffuse from anode to cathode through cation exchange 4.4 kW/h.kg COD membrane. (chemical oxygen demand • @ cathode, the electrons are combined with proton and oxygen or (COD) of 500 mg/L) chemical oxidizer. Diverse bacterial community is working as a catalyst. O 2 +4H + +4e - 2H 2 O (E cat = 0.805 V)
Voltage productions by MFCs • Standard electrode potential, at SC (25 o C, 1 atm, 1 M) = zero, relative Single chamber air-cathode MFC to normal hydrogen electrode (NHE) • Maximum attainable cell voltage can be calculated by, E emf = E cat – E an According to Nernst equation . �� �� ��� � � ��� � � � �� � � ��� � � ��� ��� � � ��� � � Acetate oxidized at anode, oxygen used as e-acceptor at cathode - + 9H + + 8e - CH3COO - + 4 H 2 O Anode 2 HCO 3 Cathode O 2 +4H + +4e - 2H 2 O E an = E 0 an – RT/8F ln ([CH 3 COO-]/[HCO3-] 2 [H+] 9 ) = - 0.296 V E cat = E 0 cat – RT/4F ln (1/pO 2 [H+] 4 ) = 0.805 V Oxidation-reduction potentials E emf = E cat – E an = 0.805 – (- 0.296) = 1.101 V Maximum MFC voltage Theoretical = 1.1 V Open circuit mode (OCV)) = 0.6-0.8 V(without current) Real voltage during current generation < 0.62 V
Stacked MFC (Multi-electrode MFCs) A single cell delivering about 0.2-0.8 V (too low for most applications). Just like batteries, individual cells are stacked to achieve a higher voltage and power. This assembly of cells is called a cell stack, multi-electrode or just a stack. Series connections Increasing voltage - + Stacked MFC Vt = MFC1 V1 +MFC2 V2 +MFC3 V3 ...MFC n+1 Vn+1 - MFC1 - + MFC2 The power density and voltages can be + MFC3 increased when MFCs are stacked in series or in parallel. V = voltage (volts) j = electric current (amperes) Parallel connections Increasing electric current (Stacked MFC j = MFC1 j 1 + MFC2 j 2 +MFC3 j 3 …MFC n+1 j n+1 ) + + + - - - MFC1 MFC3 MFC2
Decentralized wastewater treatment system MFCs as alternative for wastewater treatment. New Process- Microbial Electrochemical Technologies * Removal organic matter * Electricity direct Stacked MFC system
Objective The main objective of this study was to evaluate the performance of two air-cathode stacked MFC systems at different HRT (3, 1 and 0.5 d) during wastewater treatment and electricity production.
Materials and Methods Architecture of stacked MFCs Stacked MFC 1 (un-shared reactor)-shared anolyte MFC 1 MFC 6 MFC 18 MFC 2 Anode and cathode MFC 5 MFC 19 superficial area of 0.0036 m 2 MFC 3 MFC 4 MFC 20 • 20 individual MFC unit (800 mL MFC); Total volume 16 L. • Single chamber air-cathode MFC • Stacked MFC 1 was fed in continuous flow cascade mode (flow was transported through of the each MFC compartment
Materials and Methods Stacked MFC 2 (shared reactor)-shared anolyte • 40 MFC unit into shared reactor. • Total volume 16 L. • Single chamber air-cathode MFC. • Stacked MFC 2 was fed in continuous flow (not separator between in each cell was used).
Materials and Methods Monitoring in series connection Stack MFC 2 Resistor portable box/load bank 10Ω-40kΩ. Monitoring in individual MFC unit Data acquisition PC Resistor change switch Stack MFC 1
Materials and Methods Table 1. Operational conditions Open Circuit Mode Closed Circuit Mode (1,000 Ω) Polarization curves Biomass HRT 10 d - aclimatation Power density (mW/m 2 ) HRT (d) 3, 1 and 0.5 3, 1 and 0.5 Voltage (mV) INDIVIDUAL MFC UNIT SERIES CONNECTION Current density (mA/m 2 ) � � � ∗ � � � � � � � ∗ � ��� � ��� � � � � � ∗ � COD, TN, and TP were measured using �� ��� � � � � �� � 1 � � � standard methods (APHA, 2005). � � �� � � � � �� � � ���
Materials and Methods Characteristic of municipal Residential housing wastewater COD = 209 ±41 mg/L. Total Nitrogen (TN) = 38 ±11 mg/L. Total Phosphorus (TP) = 15 ±3 mg/L. pH = 6.9-7.5 Total Suspended Solids (TSS) = 200 mg/L
Results Stacked MFC 1 Stacked MFC 2 (HRT 3, 1 and 0.5 d) (HRT 3, 1 and 0.5 d) Individual 2-440 mV 263-600 mV MFC unit Acclimated period OCV 1,000 Ω (OCV) HRT of stacked MFC 800 0.5 d Series HRT 3 HRT HRT HRT 1 HRT 0.5 20 cells 40 cells 700 1 d 3 d d d connection 580 ±65 mV 540 ±35 mV 600 (OCV) Voltage (mV) 500 400 300 Individual 6-50 mV 1-30 mV 200 MFC unit 100 (1,000 ohms) 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Series Time (d) connection 46 ±28 mV 30 ±8 mV MFC 1 MFC 2 MFC 3 MFC 4 MFC 5 MFC 6 MFC 7 MFC 8 MFC 9 MFC 10 (1,000 ohms) MFC 11 MFC 12 MFC 13 MFC 14 MFC 15 MFC 16 MFC 17 MFC 18 MFC 19 MFC 20 Series voltage OCV 1,000 Ω HRT 0.5 d V total = V MFC 1 + V MFC 2 + V MFCn+1 ) 700 HRT 3 HRT HRT HRT HRT 600 1 d 3 d 1 d 0.5 d 500 Acclimated period of stacked MFC Voltage (mV) 400 voltage dropped phenomenon 300 200 100 Same anolyte 0 Shared reactor 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Stacked MFC 1 and 2 (Stacked MFC 2) Time (d) MFC 1 MFC 2 MFC 3 MFC 4 MFC 5 MFC 6 MFC 7 MFC 8 MFC 9 MFC 10 MFC 11 MFC 12 MFC 13 MFC 14 MFC 15 MFC 16 MFC 17 MFC 18 MFC 19 MFC 20 MFC 21 MFC 22 MFC 23 MFC 24 MFC 25 MFC 26 MFC 27 MFC 28 MFC 29 MFC 30 ionic cross-conduction between units MFC 31 MFC 32 MFC 33 MFC 34 MFC 35 MFC 36 MFC 37 MFC 38 MFC 39 MFC 40 Series voltage
Stacked MFC 1 High Power at HRT 3 d 100 Power density in series connection HRT 3 d 80 -Individual cell 20 Power = 1,107 mW/m 2 60 (mW/m 2 ) -Series connection Power = 79 mW/m 2 40 20 0 0 2 4 6 8 10 12 14 16 18 20 Current density (mA/m2) 45 HRT 1 d 40 Power density in series connection (mW/m 2 ) -Individual cell 9 Power = 870 mW/m 2 35 30 -Series connection Power = 40 mW/m 2 25 20 15 10 5 0 0 2 4 6 8 10 12 14 16 18 20 Current density (mA/m 2 ) 40 HRT 0.5 35 -Individual cell 1 Power = 430 mW/m 2 ensity in series tion (mW/m 2 ) 30 -Series connection Power = 33 mW/m 2 25 20 15
ked MFC 2 High Power at HRT 3 d 500 5 MFC 1 MFC 2 MFC 3 MFC 4 ual cell 11 Power = 472 mW/m 2 HRT 3 MFC 5 MFC 6 Power density in series MFC 7 MFC 8 Power density (mW/m 2 ) 400 4 connection (mW/m 2 ) MFC 9 MFC 10 MFC 11 MFC 12 MFC 13 MFC 14 MFC 15 MFC 16 300 3 MFC 17 MFC 18 MFC 19 MFC 20 ual cell 34 Power = 292 mW/m 2 MFC 21 MFC 22 200 2 MFC 23 MFC 24 MFC 25 MFC 26 MFC 27 MFC 28 MFC 29 MFC 30 100 1 MFC 31 MFC 32 MFC 33 MFC 34 ual cell 34 Power = 275 mW/m 2 MFC 35 MFC 36 MFC 37 MFC 38 0 0 MFC 39 MFC 40 Series connection 0 1 2 3 4 5 6 7 8 Current density (mA/m 2 ) 350 5 Power density in series HRT 1 d connection (mW/m 2 ) 300 4 Power density connection 250 (mW/m 2 ) 3 200 r 3.8-4.2 mW/m 2 150 2 100 nt density < 0.1 mA/m 2 1 50 0 0 0 2 4 6 8 10 Current density (mA/m 2 ) 300 5 HRT 0.5 (mW/m^2 250 y in series 4 mW/m 2 ) 200 3 ohmic and activation losses 150
Removal COD from Stacked MFC 1 and 2 at different HRT (3, 1 and 0.5 d At closed circuit mode, COD removal were decreased OCV 1,000 Ω 100 Acclimated period of stacked MFC systems 80 Removal (%) 60 HRT 3 d HRT 3 d HRT 1 d HRT 1 d 40 HRT 0.5 d HRT 0.5 d 20 TN and TP removal 0 were < 47 % 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Time (d) nfluent stacked MFC 1 effluent stacked MFC 2 effluent
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