growing concerns over 
 the large energy requirements needed for 
 effective wastewater treatment has stimulated interest in the use 
 of wastewater as a source of renewable energy .
microbial fuel cells ( mfcs ) are being developed as a sustainable 
 energy technology , as they can directly produce electricity from wastewater 
 allowing for energy recovery to offset the costs of wastewater treatment . in an air - cathode mfc ,
organic matter in wastewater is oxidized 
 by microorganisms , and electrons discharged to the anode travel through 
 an external circuit to the cathode where they combine with oxygen , 
 forming water .
passive transfer of oxygen to 
 the air - cathode avoids the need for energy intensive aeration of the 
 wastewater that is currently required for typical activated sludge 
 or aerobic membrane bioreactor processes .
in addition , mfcs have lower 
 sludge production than conventional aerobic treatment processes , which 
 could reduce treatment costs and the challenges associated with sludge 
 treatment and disposal .
mfcs fed 
 with domestic wastewaters have shown promising performance 
 in terms of achieving electricity generation with simultaneous organics 
 removal , and there continue to be improvements in mfc designs 
 that have produced configurations more suitable for scaling up to 
 larger systems .
capital costs of the materials used in mfcs are also being reduced , 
 for example , by using cathode catalysts such as inexpensive activated 
 carbon .
one operational aspect of using mfcs for 
 wastewater treatment that has not been sufficiently addressed is the 
 need to meet stringent effluent quality requirements .
effluent chemical 
 oxygen demand ( cod ) concentrations with domestic wastewater in mfcs 
 have ranged from 23 to 164 mg / l in fed - batch tests , and 60 to 220 
 mg / l in continuous flow tests , depending on influent cod concentrations , 
 reactor configurations , and cycle time or hydraulic retention time 
 ( hrt ) .
one of the 
 reasons for these high effluent cods is likely inefficient removal 
 of particulate organics , as biofilm reactors 
 such as mfcs and trickling filters are more effective for soluble 
 than particulate cod removal .
thus , post - treatment or integrated processes 
 are needed to further improve the quality of the treated wastewater 
 to meet discharge limits .
one approach to improve the overall 
 extent of wastewater treatment 
 has been to integrate the mfc with a membrane - based process in a single 
 reactor .
this approach has been referred to either as a membrane bioelectrochemical 
 reactor ( mber ) or an electrochemical 
 membrane bioreactor ( embr ) .
although 
 higher treatment efficiencies have been obtained for both acetate 
 solutions and domestic wastewater in tests with this approach , energy 
 consumption has only been balanced with electrical energy production 
 when acetate was used as the substrate .
the main challenges 
 with using both mfcs and membrane processes for domestic wastewater 
 treatment are obtaining high power production from the mfcs , while 
 minimizing membrane fouling . using a 
 shorter hydraulic retention time ( hrt ) in an mfc treating domestic 
 wastewater will usually improve power production , but a shorter hrt could mean a higher organic loading rate 
 on the membrane process , which could result in increased membrane 
 fouling .
membrane fouling control remains 
 the biggest challenge in the use of membranes in both aerobic and anaerobic systems . in previous membrane - based mfc studies , membranes inside the mfcs 
 fouled in 15 days , and
the high maintenance costs 
 due to cleaning processes could limit applications of integrated mfc 
 and membrane bioreactor processes . a new approach to obtain high quality effluent with low energy 
 requirements is proposed here based on using a second stage anaerobic 
 fluidized bed membrane bioreactor ( afmbr ) following wastewater treatment 
 in the mfc . the afmbr has recently been shown to be an effective approach 
 for achieving high quality effluent when used as a post - treatment 
 method for an anaerobic fluidized bioreactor ( afbr ) . in the afmbr , membrane fouling is controlled by using granular activated 
 carbon ( gac ) as the fluidized particles , as these particles can scour 
 the membrane and minimize fouling .
the properties 
 of particles used in the fluidized bed are important , as spherical 
 plastic particles have been shown to not be as effective as gac .
the use of an mfc as the primary treatment process , 
 as opposed to an afbr , may be useful for several reasons .
electrical 
 energy is directly produced in the mfc , whereas in the afbr electricity 
 would have to be produced in a separate process from biogas that might 
 need to be cleaned and purified to remove hydrogen sulfide and water 
 to improve utilization efficiencies .
any 
 hydrogen sulfide generated in situ in an mfc would be expected to 
 be rapidly oxidized in the mfc as it is a good electron donor to the 
 anode .
there should be very little methane 
 in the mfc effluent compared to that produced by the afbr , as organic 
 matter is mainly converted into current or lost to aerobic degradation 
 due to oxygen transfer across the cathode .
it is important to remove 
 dissolved methane , which can be supersaturated in these systems , to 
 minimize its release into the atmosphere as it is a potent greenhouse 
 gas . in this study we examined domestic 
 wastewater treatment using a 
 two - stage mfc - afmbr system , containing four mfcs and one afmbr , at 
 ambient temperature .
there were two separate flow lines into the afmbr , 
 with two mfcs connected hydraulically in series ( with separate electrical 
 circuits ) in each flow line ( figure 1 ) .
the 
 use of two mfcs in series avoided large changes in cod concentrations 
 in each mfc , as such large cod changes have previously been shown 
 to adversely affect current generation .
each pair 
 of mfcs had a different electrode configuration in order to compare 
 two design approaches : using a separator electrode assembly ( sea ) , 
 where the electrodes are sandwiched together and a separator was placed 
 between them to prevent short circuiting and reduce oxygen crossover 
 from the cathode ; and using a spaced electrode assembly ( spa ) , where 
 the electrodes are kept close to each other , but with sufficient space 
 to avoid direct contact ( no separator was used ) ( figure s1 , supporting information ( si ) ) .
it has recently 
 been shown that the spa design can reduce treatment time compared 
 to the sea , although less energy may be recovered in the spa configuration 
 due to the loss of organic matter to aerobic processes rather than 
 current generation .
treatment efficiency 
 was evaluated in terms of cod and total suspended solid ( tss ) removals , 
 and energy efficiency was quantified for both processes in terms of 
 production and demands , under continuous flow conditions .
schematic diagram 
 ( a ) and photo ( b ) of the two - stage mfc - afmbr 
 system .
( u = the first upstream mfc , and d = the second downstream mfc prior to the afmbr ) .
the two - stage mfc - afmbr system 
 consisted of four mfc reactors and one afmbr reactor .
the four mfc 
 reactors were arranged in two groups , each group having two mfc reactors 
 with the same electrode configuration that were hydraulically connected 
 in series ( figure 1 ) .
single - chamber , air - cathode 
 mfcs ( 130 ml ) were constructed as previously described .
each reactor contained three brush anodes connected 
 together externally with copper wire , and a single air - cathode . the 
 anodes were graphite fiber brushes with a titanium wire core ( 25 mm 
 diameter by 35 mm length ) ( mill - rose , mentor , oh ) that were heat treated 
 at 450 c for 30 min before use .
the cathode ( 35 cm projected surface area ) was made of wet - proofed carbon cloth ( 30 
 wt.% , # cc640wp30 , fuel cell earth , stoneham , ma ) , with a platinum 
 catalyst ( 0.5 mg / cm ) on the water side and four ptfe diffusion 
 layers on the air side .
for the 
 sea mfcs , the brush anodes were trimmed in half along the direction 
 parallel to the core to prevent contact by the bristles with the cathode 
 through the separator , as both electrodes were pressed against the 
 separator ( si figure s1 ) .
two layers of 
 textile separator ( 46% cellulose and 54% polyester , 0.3 mm thickness , 
 amplitude prozorb , contec inc . ,
spartanburg , sc ) were used in the 
 sea reactors to prevent short - circuiting and to minimize oxygen crossover . 

the spa mfcs did not contain separators , so the edges of the brush 
 anodes were set 0.8 cm from the surface of the cathodes .
the 
 afmbr ( 65 ml ) consisted of a 300 mm long by 16 mm diameter 
 clear polyvinyl chloride ( pvc ) tube ( u.s . plastic corp . ) containing 
 10 g ( wet weight ) of gac ( darco mrx , 10  30 mesh , norit activated 
 carbon ) as the fluidized bed medium and support for bacterial growth . 

the gac was washed with deionized water for three times prior to use 
 to remove any residuals .
the afmbr contained a submerged membrane 
 module with eight 200 mm long polyvinylidenefluoride ( pvdf ) hollow 
 fiber membranes ( 2.0 mm outside diameter , 0.8 mm inside diameter , 
 0.1 m pore size , kolon inc .
, south korea ) , having a total membrane 
 surface area of 0.004 m. a hungate tube ( 10 ml , bellco 
 glass inc . , vineland , nj ) with the bottom cut off was glued onto the 
 top of the pvc tube and sealed with a thick butyl rubber stopper ( 20 
 mm diameter , chemglass inc . , vineland , nj ) , for biogas collection 
 and measurement .
the mfcs were inoculated and fed with domestic 
 wastewater collected 
 from the primary clarifier of the pennsylvania state university wastewater 
 treatment plant , and operated in continuous flow mode at an hrt of 
 4 h. the primary clarifier effluent was collected weekly and stored 
 in a refrigerator ( 4 c ) to minimize cod changes over time . during 
 tests ,
a container of wastewater was placed in an ice bucket to keep 
 it cool in order to minimize degradation prior to being fed into the 
 mfcs .
the wastewater warmed in the tube when it was transferred into 
 the bottom of the reactors using a peristaltic pump ( model no . 7523 - 90 , 
 masterflex , vernon hills , il ) at an overall flow rate of 1560 ml / d , 
 with 780 ml / d for each flow line .
the effluent from the top of the 
 upstream mfc reactor flowed into the bottom of the downstream mfc 
 due to the hydraulic pressure .
the effluent from the two mfcs series 
 was delivered to the afmbr using another peristaltic pump ( as above ) 
 at a flow rate of 1560 ml / d , producing an hrt of 1 h. the top of the 
 membrane module was connected to the same peristaltic pump , to maintain 
 a constant permeate flux of 16 l / m / h ( lmh ) from the afmbr . 

the pump was operated with a 10 min on and 1 min off cycle time , as 
 it was previously shown that periodic relaxation of the membrane reduced 
 trans - membrane pressure ( tmp ) .
tmp was 
 monitored continuously using a vacuum pressure gauge ( type1490 , ashcroft , 
 stratford , ct ) .
fluidization of gac was maintained with a peristaltic 
 pump ( model no . 7523 - 80 masterflex , vernon hills , il ) at the desired 
 flow rate of 170 ml / min , resulting in bed expansion of 7080% 
 to a height of 210240 mm .
the two - stage mfc - afmbr system was 
 operated at room temperature ( 25 c ) .
the voltage across 
 an external resistor for the mfc circuit was measured every 20 min 
 using a multimeter ( model 2700 ; keithley instruments , inc . ) .
current 
 was calculated using ohm s law ( i = u / r ) , with power calculated as p = iu , where u is the 
 measured voltage ( v ) , and r the external resistance 
 (  ) .
a reference electrode [ ag / agcl ; 
 + 200 mv vs standard hydrogen electrode ( she ) ; basi ] was inserted into 
 the upper middle of the mfc reactor to determine the anode and cathode 
 potentials .
polarization and power curves were obtained by changing 
 the external resistances from open circuit to 1600 , 800 , 400 , 200 , 
 and 100  , with one day at each resistance ( six hrts ) .
the averaged 
 voltage at each external resistance was used to obtain the polarization 
 curve .
columbic efficiency ( ce ) was calculated as ce = ct / cth  100% , where ct was the total coulombs calculated by integrating 
 the current over time ( ct =  i t , where t is the time interval of one hrt ) , and cth was the theoretical amount of coulombs available based 
 on the cod removed in the mfc over the same amount of time , calculated 
 as cth = [ fb ( codin  codout ) q t]/m , where f is faraday s 
 constant , b = 4 is the number of electrons exchanged 
 per mole of oxygen , codin and codout are the 
 influent and effluent cod , q is the flow rate , t is the time interval ( hrt ) , and m = 32 
 is the molecular weight of oxygen .
total suspended solids ( tss ) were measured using standard methods 
 ( apha , 1998 ) .
total cod ( tcod ) and soluble cod ( scod ) were measured 
 using standard methods ( method 5220 , hach company , loveland , co ) . 

all samples for scod measurement were filtered through 0.45 m 
 pore diameter syringe filters ( polyvinylidenedifluoride , pvdf , 25 
 mm size , restek corporation ) . conductivity and ph were measured immediately 
 after sampling using a probe ( sevenmulti , mettler - toledo international 
 inc . ) .
the sampling points on the flow line for the chemical analyses 
 were indicated in the si ( figure s2 ) .
biogas 
 ( 200 l samples ) of the afmbr headspace was sampled using gastight 
 syringes ( 250 l ; hamilton samplelock syringe ) and analyzed 
 using two gas chromatographs ( sri instruments ) for h2 , 
 n2 , ch4 and co2 , as described previously .
gas was collected and measured directly using 
 a 10 ml glass syringe ( air - tite products co. , inc . , va ) inserted into 
 the top of the afmbr .
dissolved methane was also measured as described 
 previously by transferring a liquid sample 
 from the afmbr reactor into a serum bottle ( 6.5 ml , wheaton , millville , 
 nj ) without any air contact or headspace , and sealed with a thick 
 butyl rubber stopper ( 20 mm diameter , chemglass inc . ) .
the serum bottle 
 full of the liquid sample was then autoclaved to prevent biological 
 activity . some liquid sample ( 1.5 ml )
was then replaced by n2 gas from this serum bottle with a syringe . after establishing gas 
liquid 
 equilibrium by shaking the serum bottle for six hours at room temperature , 
 the amount of dissolved methane was back - calculated from the measured 
 methane amount in the headspace ( detailed information in the si ) .
the start - up time needed for the 
 sea mfcs was shorter than that required for the spas .
the sea mfcs 
 produced a stable voltage of 0.59  0.03 v ( 1000  ) after 
 3 days , while the spas produced 0.51  0.04 v after 3 days , and 
 required 10 days to achieve a stable voltage of 0.58  0.01 v. 
 stable voltage production was indicated by a deviation between the 
 daily averaged voltage values that was < 0.006 v ( 1% of 
 the daily averaged voltage ) over three consecutive days .
there was 
 no appreciable difference in start - up time between the upstream or 
 downstream mfc within the individual flow paths ( data not shown ) .
the power produced by the seas and spas changed over time . based 
 on the polarization data obtained after 1 month
, the sea - u mfc produced 
 a maximum power of 0.31 mw ( 89 mw / m , normalized to the 
 cathode projected surface area of 35 cm ) , which was comparable 
 to that of spa - u ( 0.33 mw ) ( figure 2 ) .
although 
 the same current was produced with these two configurations , the sea - u 
 had better cathode performance but showed poorer anode performance 
 than the spa - u ( figure 3b and c ) .
the downstream 
 mfcs produced slightly lower maximum power than the upstream ones , 
 with 0.28 mw for sea - d and 0.27 mw for spa - d ( figure 2 ) .
the downstream mfcs generally had more positive anode potentials 
 than the first mfcs ( figure 3b ) , likely due 
 to the lower substrate concentrations in the downstream mfcs ( si table s1 ) , as it was shown that the anode 
 potentials became more positive at lower substrate concentrations 
 in a previous study .
power production of the 
 sea and spa mfcs at different time after 
 start - up , after ( a ) 1 month and ( b ) 5 months .
( u = 
 the first upstream mfc , and d = the second downstream 
 mfc prior to the afmbr ) .
voltage , anode potential and cathode potential of the sea and spa 
 mfcs at different time after start - up : ( a ) voltage , ( b ) anode potential 
 and ( c ) cathode potential at 1 month , and ( d ) voltage , ( e ) anode potential 
 and ( f ) cathode potential at 5 month . the letters  a  
 in
all electrode potentials were reported versus the ag / agcl reference 
 electrode [ + 200 mv vs a standard hydrogen electrode ( she ) ; basi ] .
after 5 months , the maximum power 
 densities of the sea mfcs were 
 relatively unchanged ( 0.33 mw for sea - u and 0.32 mw for sea - d ) , and 
 the wastewater composition fed into the reactor was relatively unchanged 
 based on the influent tcod concentrations ( 210  11 mg
/ l at 5 
 months , compared to 224  17 mg / l at 1 month ) . however , the maximum 
 power produced by spa mfcs substantially decreased to 0.16 mw ( spa - u ) 
 and 0.18 mw ( spa - d ) .
the reason for these decreases was a large reduction 
 in cathode potentials ( figure 3f ) , which was 
 likely due to biofouling .
while the cathodes 
 used for the sea configuration contained a separator that covered 
 the cathode , the spa cathodes were directly exposed to the wastewater , 
 and thus they were more prone to fouling ( figure 3f ) .
the maximum power density of 89  6 mw / m produced 
 by the sea mfc in these continuous flow tests was lower than the maximum 
 power densities obtained in two other studies with domestic wastewater 
 when the mfc was operated in fed - batch mode ( 120 mw / m or 328  11 mw / m )
. the lower power density here was likely due 
 to a lower influent cod ( 217  18 mg
/ l , compared to 275  
 71 mg / l and 303  69 mg / l ) , and operation under continuous flow conditions , 
 where the average substrate concentration was lower than that in the 
 fed - batch reactors at the beginning of the cycle .
the different electrode configurations ( sea or spa ) 
 did not appreciably 
 affect the extent of cod removal .
tcod removals were 28  7% 
 for sea - u , and 34  3% for spa - u , which are comparable removals 
 within the calculated standard deviations .
the downstream mfcs had 
 slightly lower tcod removals than the upstream mfcs , with 17  
 5% for sea - d and 19 
fed - batch tests with domestic wastewater have shown 
 that cod removal in mfcs is first order with respect to concentration 
 ( unpublished data ) .
thus , the reduction in cod concentration would 
 have reduced removal rates in the downstream reactors .
scod removals 
 showed the same trends as tcod , with greater removals in the upstream 
 reactors ( 27  10% for sea - u , 32  5% for spa - u ) than the 
 downstream ones ( 19  5% for sea - d , and 26  7% for spa - d ) . 

note that these cod removals were based on the combination of all 
 data in tests at the different external resistances used in the polarization 
 tests , as the effluent cod concentrations in these tests did not change 
 substantially with the different external resistances ( cod concentrations 
 in the si , table s1 ) .
these cod removals 
 were lower than those obtained in previous studies operated in fed - batch 
 mode using the same domestic wastewater source ( 6293% ) , due 
 to the short hrt ( 4 h ) in this study compared to much longer fed - batch 
 cycle times ( 1236 h ) . in additional tests , 
 increasing the hrt to 24 h increased cod removals to 67  2% , 
 which was about the same as that obtained in fed - batch mode with a 
 cycle length of 24 h ( 65  1% ; 500  resistance , data not 
 shown ) .
however , a long hrt is not desirable for efficient wastewater 
 treatment , and thus the shorter hrt was used here .
the ces increased 
 in proportion to the current ( lower resistance ) , 
 even though the cod removals remained relatively constant with different 
 resistances . at the maximum power density ( 0.31  0.02 mw , 0.86 
  0.02 ma ) , the overall ce of the sea mfcs was 18% ( 13% for sea - u 
 and 28% for sea - d ) . over time
the sea and spa mfcs had comparable overall 
 ces at 1 month , ( range of 629% ) , but after 5 months the sea 
 mfcs remained relatively unchanged while those of the spa mfcs decreased 
 ( range of 420% ) with the decreased currents .
ce values obtained 
 here under continuous flow conditions were comparable to those previously 
 reported for fed - batch conditions ( 231% ) .
overall , these results suggest that the sea configuration 
 was superior to the spa design on the basis of fixed hrts as it maintained 
 higher power densities and ces over time with the same level of wastewater 
 treatment .
the 
 two - stage mfc - afmbr system achieved excellent treatment levels in 
 terms of cod and tss removals .
the afmbr was first inoculated with 
 anaerobic sludge and fed the effluent from mfcs for two weeks .
after 
 that , the membrane module was installed , and preliminary tests were 
 conducted to optimize the design and operation of the afmbr over a 
 period of approximately two months , with occasional system shutdown 
 to address problems related to consistent flow and treatment . following 
 this system optimization , a new membrane module was installed , and 
 the mfc - afmbr system was operated continuously for 50 days in concert 
 with the mfcs operation over the last two of the five months .
tcod further decreased from the influent concentration to the afmbr 
 of 107  10 mg
/ l to 16  3 mg / l in the effluent , providing 
 an overall tcod removal for the two stages of 92.5% ( 49.1% for the 
 mfcs , and 43.4% for the afmbr ) ( figure 4 and si table s2 ) . the effluent scod and tcod concentrations 
 from the afmbr were identical , and therefore there was a lower overall 
 scod removal of 86.2% ( influent scod to mfcs of 114  8 mg / l , 
 compared to tcod of 210  11
mg / l ) ( figure 4 and si table s2 ) . a larger percent of 
 scod was removed by the mfcs ( 50.3% ) than by the afmbr ( 35.9% ) , while 
 particulate cod removal was 47.9% for the mfcs compared to 100% for 
 the afmbr .
changes in the forms of cod might occur through hydrolysis 
 from particulate to soluble cod , and through biomass growth from soluble 
 to particulate
. the particulate cod removal in mfcs might be partially 
 due to settling in the reactor chambers , while that in the afmbr was 
 primarily due to membrane filtration .
l of tss due to filtration of the wastewater through the membrane , 
 resulting in > 99.6% tss removal ( figure 4 and si table s2 ) .
these cod and tss removals 
 are 
 comparable to those obtained using a staged anaerobic fluidized membrane 
 bioreactor ( saf - mbr ) treating domestic wastewater .
there was little overall change in ph , as the influent 
 ph to the mfcs of 7.6  0.1 decreased to 7.1  0.1 in the 
 mfcs effluent , but it increased to 7.5  0.2 following treatment 
 in the afmbr ( si table s2 ) .
these ph changes 
 might result from losses of co2 and volatile fatty acids , 
 for example from methanogenesis processes occurring in the mfc - afmbr 
 system .
also there were no large changes in conductivity , with 1473 
  33 ms / cm for the mfcs influent , 1457  15
ms / cm for the 
 mfcs effluent , and 1420  19 ms / cm for the afmbr effluent ( si table s2 ) .
influent and effluent concentrations , 
 and removals of tcod , scod 
 and tss for the combined mfc - afmbr system .
the values inside the figures 
 were the percent of the influent concentration that was removed by 
 the mfcs , afmbr , and the whole system .
the afmbr was operated continuously for 50 days at a high 
 membrane 
 flux of 16 lmh , even without cleaning by backwashing or using chemicals . 

most of the increase in the tmp , from 0.015 to 0.035 bar , occurred 
 during the first 8 days of operation ( figure 5 ) .
thereafter , it slowly increased to 0.050 bar during the rest of 
 the test ( figure 5 ) .
liquid ( 9 ml ) was withdrawn 
 from the afmbr every 3.5 days ( 0.16% of the total influent flow ) to 
 removal finer material and excess suspended solids from days 8 to 
 50 , as suggested in a previous afmbr study .
the membrane flux of 16 lmh here is higher than that previously 
 reported for the afmbr following an afbr ( 11 lmh ) , with a pvdf hollow - fiber 
 membrane with the same pore size as the one here ( 0.1 m ) . in that study ,
the tmp reached 0.25 bar in 3 
 days when the membrane flux was increased to 14 lmh , which is much higher than the maximum tmp observed here . 

the stable operation of the flux through the afmbr without appreciable 
 membrane fouling was likely due to a combination of factors here that 
 included the scouring effect of the gac particles on the membrane 
 surface , intermittent filtration , and periodic removal of suspended 
 solids .
the use of mfcs as the primary treatment process likely contributed 
 to the high flux and stable performance of the afmbr , due to the removals 
 of cod and tss in the mfcs .
the improved flux with the first - stage 
 mfc treatment , compared to that previously obtained with a first - stage 
 afbr treatment , suggests that mfcs might be a better first stage treatment 
 than afbr , but this can not be concluded without direct side - by - side 
 tests of the two different systems .
the operation of the afmbr without 
 wastewater pretreatment was not examined here as that would represent 
 a different treatment process , and one that would not allow for electrical 
 power generation and recovery from cod removal .
energy usage for the two - stage mfc - afmbr 
 system was calculated as previously described .
all 
 the volumetric energy densities were reported on the basis of normalizing 
 to 1 m of wastewater treated .
the energy requirements 
 were calculated as 0.0107 kwh / m for fluidizing the gac 
 particles , and 0.0014 kwh / m for pumping permeate through 
 the membranes , resulting in a total electrical energy requirement 
 for the afmbr of 0.0186 kwh / m ( table 1 ) .
the electrical energy requirement for pumping liquid through 
 the mfcs was negligible compared to that needed for the afmbr ( table 1 ) .
the higher energy requirement for the afmbr was 
 primarily due to pumping needed for liquid recirculation to maintain 
 the gac fluidization .
the energy needed for this is proportional to 
 the total reactor flow rate and the hydraulic head loss of the system . in an afmbr reactor with a given configuration , 

the minimum recirculation flow rate and the hydraulic head loss are 
 fixed , and thus the energy requirement for recirculation is inversely 
 proportional to the permeate flow rate or the hrt .
therefore , the high permeate flux and low hrt achieved 
 for the afmbr in this study were favorable for achieving a low energy 
 requirement of 0.0186 kwh / m .
this energy requirement is 
 lower than previous reports using the afmbr ( 0.0270.040 kwh / m ) , but there 
 are many differences in these studies that preclude a direct comparison 
 of these values .
energy requirement = 9.8qe , where q ( m / s ) is flow rate and e ( m 
 h2o ) is head loss .
assume energy efficiency 
 of 65% 
 in conversion of electrical energy to pump energy .
this maximum power output was quite similar 
 to that obtained during steady operation , and therefore it represents 
 power production that could be obtained during continuous treatment 
 tests ( si figure s3 ) .
the ratio of the electrical energy 
 produced to that required by the mfc - afmbr system .
based 
 on the maximum power that could be produced by the sea configuration 
 after 5 months of operation ( 0.33 mw for sea - u and 0.32 mw for sea - d , 
 figure 2 ) , the total power that could be produced 
 by the four mfcs coupled to the afmbr was 1.28 mw ( four times 0.32 
 mw ) ( table 1 ) .
if all of this electrical energy 
 was recovered , the net electrical energy available for the system 
 operation would be 0.0197 kwh / m .
this would be enough 
 to supply the 0.0186 kwh / m required to operate the system 
 if these values could be maintained for larger - scale systems .
however , 
 this energy balance would likely change as the size of the system 
 increases .
also , in practice there might be other energy losses that 
 would affect overall energy recovery , that have not been included 
 here .
direct electricity production by mfcs is advantageous , compared 
 to methanogenic reactors that require combustion of the methane to 
 produce power , as the conversion efficiency of methane to electricity 
 is typically only 33% .
however , there 
 will also be energy losses in converting the low voltage dc power 
 into higher voltage dc or ac power .
the 
 additional energy that could be recovered from the methane 
 production in the afmbr was not included in this energy balance as 
 it would have been difficult to recover .
total methane production 
 in the afmbr was 1.67 ml / l liquid treated at ambient temperature and 
 pressure , with most of this present as dissolved methane ( 1.5
this 
 concentration was estimated to be 125% oversaturation relative to 
 the concentration of methane in the afmbr headspace .
the energy value 
 of this amount of methane is 0.016 kwh / m ( methane combustion , 
 assuming 800 kj / mol ) , equivalent to electrical energy of 0.005 kwh / m ( 33% energy recovery ) which could theoretically add 27% more 
 energy production into the system .
however , as most of this methane 
 is dissolved , and at a low concentration , it would have been difficult 
 to recover .
the methane yield from the afmbr was 0.75 mmol / g cod removed , 
 indicating only 5% of the cod removal in the afmbr could be attributed 
 to methane generation .
the amount of methane that theoretically could 
 be produced in the afmbr was estimated as 17 ml / l , based on the cod 
 removal and assuming a conversion of 0.017 mol ch4/g cod 
 ( detailed information and calculations in the si ) .
it is not clear what other processes occurred relative 
 to cod removal as the methane production was only about /10 of that possible by methanogenesis alone .
more methane might be recovered in the 
 future with improvements in the configuration and operation of the 
 afmbr .
the two - stage mfc - afmbr system was shown to 
 effectively treat domestic wastewater ( primary clarifier effluent ) 
 at ambient temperatures in terms of cod and tss removals , producing 
 a high effluent quality with a near neutral ( or net positive ) energy 
 requirement , without the need for membrane cleaning even after 50 
 days of operation .
tcod was reduced from 210  11 mg / l to 16 
  3 mg / l , resulting in 92.5% overall cod removal with > 99% 
 removal 
 of tss to a final effluent concentration of < 1 mg / l .
the energy 
 requirement of the afmbr is much less than that needed for aerobic 
 mbrs with internal membranes that require air sparging to control 
 membrane fouling .
the high permeate flux 
 ( 16 lmh ) and low hrt ( 1 h ) here resulted in an overall low energy 
 requirement for the afmbr of only 0.0186 kwh / m .
thus , 
 the energy produced by only the mfcs ( 0.0197 kwh / m ) was 
 theoretically sufficient here to meet the energy demands for the system 
 operation , although the energy balance for a larger system would likely 
 change .
an additional benefit of the mfc - afmbr system should be a 
 low sludge production rate .
although sludge production was not measured 
 here ( for an estimate , see the si ) , previous 
 studies have shown that the sludge production by anaerobic mfc and 
 afmbr processes are less than those of conventional aerobic processes 
 such as activated sludge .
while feasibility 
 of the combined process was shown here , additional work will be needed 
 to optimize the performance of the individual and combined mfc and 
 afmbr reactors .
more optimal treatment could likely be obtained by 
 adjusting the hrts of the two systems , and examining benefits of treatment 
 rates compared to electrical power production .
the current findings 
 were sufficient to show that the two - stage mfc - afmbr system is useful 
 for treatment of low strength wastewater even at ambient temperatures . 

the robustness of the system at other temperatures , particularly lower 
 ones , would need to be investigated .
however , mfcs have been shown 
 to function over a wide range of temperatures .
the 
 assessment of nitrogen , phosphorus , and pathogen removals will be 
 examined in future studies to see to what extent the mfc - afmbr system 
 can be used for these wastewater components .
other issues to address 
 are capture and use(or destruction ) of the methane to avoid its release 
 into the air , and efficient use of the electricity produced by mfcs . 
 following these optimization studies
, it should be possible to better 
 evaluate the economics of the system compared to traditional treatment 
 systems .
 microbial fuel cells ( mfcs ) are a 
 promising technology for energy - efficient 
 domestic wastewater treatment , but the effluent quality has typically 
 not been sufficient for discharge without further treatment . 
 a two - stage 
 laboratory - scale combined treatment process , consisting of microbial 
 fuel cells and an anaerobic fluidized bed membrane bioreactor ( mfc - afmbr ) , 
 was examined here to produce high quality effluent with minimal energy 
 demands . 
 the combined system was operated continuously for 50 days 
 at room temperature ( 25 c ) with domestic wastewater 
 having a total chemical oxygen demand ( tcod ) of 210  11 
 mg / l . 
 at a combined hydraulic retention time ( hrt ) for both processes of 
 9 h , 
 the effluent tcod was reduced to 16  3 mg / l ( 92.5% removal ) , 
 and there was nearly complete removal of total suspended solids ( tss ; 
 from 45  10 mg 
 / l to < 1 
 mg / l ) . 
 the afmbr was operated at a 
 constant high permeate flux of 16 l / m2/h over 50 days , 
 without the need or use of any membrane cleaning or backwashing . 
 total 
 electrical energy required for the operation of the mfc - afmbr system 
 was 0.0186 kwh / m3 , which was slightly less than the electrical 
 energy produced by the mfcs ( 0.0197 kwh / m3 ) . 
 the energy 
 in the methane produced in the afmbr was comparatively negligible 
 ( 0.005 kwh / m3 ) . 
 these results show that a combined mfc - afmbr 
 system could be used to effectively treat domestic primary effluent 
 at ambient temperatures , producing high effluent quality with low 
 energy requirements . 
Introduction
Materials
and Methods
Results and Discussion