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Bioelectrochemically-assisted anaerobic composting process enhancing compost maturity of dewatered sludge with synchronous electricity generation
Accepted Manuscript Bioelectrochemically-assisted anaerobic composting process enhancing compost maturity of dewatered sludge with synchronous electricity generation Hang Yu, Junqiu Jiang, Qingliang Zhao, Kun Wang, Yunshu Zhang, Zhen Zheng, Xiaodi Hao PII: DOI: Reference:
S0960-8524(15)00848-2 http://dx.doi.org/10.1016/j.biortech.2015.06.057 BITE 15137
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
9 April 2015 10 June 2015 12 June 2015
Please cite this article as: Yu, H., Jiang, J., Zhao, Q., Wang, K., Zhang, Y., Zheng, Z., Hao, X., Bioelectrochemicallyassisted anaerobic composting process enhancing compost maturity of dewatered sludge with synchronous electricity generation, Bioresource Technology (2015), doi: http://dx.doi.org/10.1016/j.biortech.2015.06.057
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Bioelectrochemically-assisted anaerobic composting process
2
enhancing compost maturity of dewatered sludge with
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synchronous electricity generation
4 5
Hang Yua, Junqiu Jianga, Qingliang Zhaoa,b*, Kun Wanga,b, Yunshu Zhanga, Zhen
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Zhengc, Xiaodi Hao d
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a School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090,
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China
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b State Key Laboratory of Urban Water Resources and Environments (SKLURE), Harbin Institute of
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Technology, Harbin 150090, China
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c School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
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d School of Environment and Energy Engineering (The R & D Centre for Sustainable Environmental
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∗
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[email protected]
Biotechnology), Beijing University of Civil Engineering and Architecture, Beijing, China
Corresponding author. Tel: +86 451 8628 3017; Fax: +86 451 8628 3017. E-mail address:
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1
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Abstract: Bioelectrochemically-assisted anaerobic composting process (AnCBE) with
2
dewatered sludge as the anode fuel was constructed to accelerate composting of
3
dewatered sludge, which could increase the quality of the compost and harvest electric
4
energy in comparison with the traditional anaerobic composting (AnC). Results
5
revealed that the AnCBE yielded a voltage of 0.60±0.02 V, and total COD (TCOD)
6
removal reached 19.8±0.2% at the end of 35 d. The maximum power density was
7
5.6W/m3. At the end of composting, organic matter content (OM) reduction rate
8
increased to 19.5±0.2% in AnCBE and to 12.9±0.1% in AnC. The Fuzzy Comprehensive
9
Assessment (FCA) result indicated that the membership degree of class I of AnCBE
10
compost (0.64) was higher than that of AnC compost (0.44). It was demonstrated that
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electrogenesis in the AnCBE could improve the sludge stabilization degree, accelerate
12
anaerobic composting process and enhance composting maturity with bioelectricity
13
generation.
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Keywords: Anaerobic composting (AnC); bioelectrochemical devices; dewatered
15
sludge; anaerobic compost maturity; fuzzy comprehensive assessment (FCA).
16
2
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1. Introduction
2
Sewage sludge, as an inevitable byproduct of wastewater treatment, needs to be
3
dewatered to reduce its volume and thus dewatered sludge is generated. The disposal
4
processes of dewatered sludge include incineration, sanitary landfill and composting, etc.
5
(Blazy et al., 2014). Among those processes, inorganic particulate matter (PM)
6
containing harmful substances is formed during incineration, and a series of
7
geoenviromental problems (viz., leachability, total and differential settlement and slope
8
stability) are triggered during landfill (Kavouras et al., 2012). Moreover, aerobic or
9
anaerobic composting is widely used in most countries all over the world since the
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composting product can be used as soil conditioner to recycle a substantial amount of
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nutrients and organic matters. To a large extent, aerobic composting requires strict
12
control of blast volume, high energy input and skilful operation of mechanical
13
equipment. In comparison with aerobic composting, anaerobic composting (AnC) has
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such merits as maximum retention of sludge nutrients and lower power consumption
15
while stabilizing the organics. Nevertheless, the AnC process faces the problems of low
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stabilization degree, long composting cycle (40~60 days or even longer) and
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dissatisfactory maturity without long period of composting (Himanen and Hanninen,
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2011; Banegas et al., 2007). These problems limit its widespread application to sludge
19
disposal and utilization.
20 21
The bioelectrochemical devices such as microbial fuel cell (MFC) have been proven to 3
1
be capable of using various sludges (e.g. manure sludge, surplus sludge, petroleum
2
sludge and fermented primary sludge) as anodic fuels for electricity generation while
3
accelerating anaerobic degradation (Dentel et al., 2004; Jiang et al., 2009; Mohan and
4
Chandrasekhar, 2011; Yang et al., 2013). Restated, Jiang et al. (2011) has proven the
5
‘‘fuel-cell” reactions degraded additional 7.9% total COD (TCOD) of raw sludge in
6
MFC, and sludge degradation in MFC can be characterized as a combined process of
7
anaerobic digestion and electrochemical oxidation, being complementary in function to
8
each other. In a sludge composting system, the first and limited step was the insoluble
9
macromolecules in extracellular polymeric substances (EPS) of sludge hydrolysed into
10
soluble organic compounds (viz., proteins, carbohydrates, etc.) (Appels et al., 2008).
11
Moreover, the microbes within MFC could only utilize soluble organic matter to
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product electricity power (Vavilin et al., 1996). So, if the bioelectrochemical process is
13
integrated into the conventional anaerobic composting process, the sludge degradation
14
rate of anaerobic composting might be accelerated since these simple organic
15
compounds can be directly consumed by the available electricigens within the
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bioelectrochemically-assisted anaerobic composting system (AnCBE).
17 18
The objective of this study is to develop an efficient bioelectrochemically-assisted
19
anaerobic composting process for dewatered sludge, focusing on the compost maturity
20
while synchronously generating electricity. The scopes of this work are: (1) to examine
21
the degree of organic degradation and sludge stabilization during composting in AnCBE;
4
1
(2) to assess the maturity degree of composts by using the fuzzy comprehensive
2
assessment (FCA) approach; and (3) to investigate into the simultaneous
3
bioelectrogenesis performance of AnCBE. The results obtained were attempted to
4
accelerate composting of dewatered sludge, which could increase the quality of the
5
compost and harvest electric energy in comparison with the traditional anaerobic
6
composting.
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2. Materials and Methods
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2.1 Experimental set-up of AnCBE and operation
10
The AnCBE reactor made of plexiglas comprised two compartments and a proton
11
exchange membrane (PEM, Nafion 117, Dupont Company) located between an anode
12
and cathode. The anodic compartment was a cylinder (Φ 12 cm×8 cm) and the cathodic
13
was a cube (8 cm×7 cm×12cm). The two compartments were positioned and held
14
together by bolts with the membrane in the middle providing a large surface area. Both
15
the anode and cathode electrodes consisted of a graphite fiber brush and titanium wire
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that collected electrons for the external circuit. The effective volume of the anodic
17
compartment was 780 mL and that of the cathodic compartment was 350 mL. The
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catholyte was potassium ferricyanide as the main ingredient, with the advantages of
19
stability, low over potential and cathodic work potential close to the open circle
20
potential (Aelterman et.al., 2006). A fixed resistance (Rex=1000 Ω) was applied as an
21
external load in the circuit. The anodic compartment was purged with nitrogen to keep
5
1
anaerobic condition after sludge addition. For control, an AnC reactor was constructed
2
with the same configuration of the anodic compartment of AnCBE, being filled with the
3
same dewatered sludge and operated at the same room temperature conditions. Both
4
AnCBE and AnC reactors were wrapped by insulation cotton to prevent heat loss and
5
ensure the smooth progress of composting.
6
7
2.2 AnCBE inoculum and start-up
8
The inoculated sludge was collected from the secondary clarifier of Taiping Wastewater
9
Treatment Plant in Harbin, China. The anodic compartment of AnCBE was inoculated
10
progressively with sewage sludge and dewatered sludge after belt press filtration.
11
Specifically, during the start-up period, only the sewage sludge was fed into the anodic
12
compartment of AnCBE. After three days, a mixture of dewatered sludge and sewage
13
sludge with a ratio of 1:3 (v/v) was filled in the anodic compartment instead. The
14
percentage of dewatered sludge in the mixture was increased every three days until
15
complete replacement into dewatered sludge as the anodic fuel. The successful start-up
16
was indicated by the colonization of electricigens on the electrode and constant
17
production of electric power. The pH, moisture content (MC), OM and total COD
18
(TCOD) of the dewatered sludge were 7.62 ± 0.20, 86.04 ± 0.20%, 54.28 ± 0.06% and
19
227800± 100 mg/L, respectively. Fresh dewatered sludge was stored at 4 oC prior to
20
use.
6
1
2.3 Analysis and computation
2
Before analyses of pH, conductivity, TCOD and NH4 +-N, dewatered sludge samples
3
were diluted with distilled water (1:10 w/v) in a horizontal shaker for 24h at room
4
temperature. Distilled water was utilized to test TCOD, and was filtered through
5
0.45µm membrane to extract soluble component for pH, conductivity and NH4+-N. The
6
statistical analysis was conducted using SPSS software version 12.0 for Windows.
7
Pearson’s correlation coefficient (r) was used to evaluate the linear correlation between
8
two parameters. OM and MC were measured by weighting method (APHA, 2002).
9
Elemental composition (C and N) was determined with an element analyser (vario EL
10
III, Elementar German). The pH, electrical conductivity (EC), NH4+-N, and TCOD were
11
analysed according to Standard Methods (APHA, 2002). The voltage difference
12
between two electrodes was recorded across a fixed load (1000 Ω) by a voltage
13
collection instrument (12 bit A/D conversion chips, US) connected to a personal
14
computer. The anode and cathode potentials were measured against an Ag/AgCl
15
reference electrode (+ 0.195 V vs. standard hydrogen electrode, SHE). The maximum
16
power density and polarization curve were determined by adjusting the external
17
resistance to 10~9999 Ω for recording the corresponding voltage drop, where power
18
density (W/m3) was calculated through the effective volume of the anode compartment.
19
Electrochemical impedance spectroscopy (EIS) were measured by using CHI 660
20
electrochemical working station (CH Instrument, USA) . EIS measurements were
21
carried out for the anode in a frequency range of 100 kHz to 1 MHz with an AC signal 7
1
of 10mV amplitude. The real (Z′), imaginary (Z″) and frequency (ƒ) components of EIS
2
in Nyquist plot and Bode plot were analysed using ZsimDemo 12.0 software to simulate
3
the equivalent resistances and capacitances.
4 5
TCOD removal rate was calculated by the equation of TCOD reduction =
6
(TCOD0-TCODi)/TCOD0 × 100%, where TCOD0 is the initial TCOD of the dewatered
7
sludge, and TCODi stands for the actual monitoring data of TCOD at ith day of
8
operation. Similarly, OM reduction rate could be calculated by the equation of OM
9
reduction = (OM0-OMi)/OM0×100%, where OM0 is the initial organic matter content of
10
the dewatered sludge, and OMi stands for the actual monitoring data of organic matter
11
content at ith day of operation. Germination index (GI) was calculated using the
12
comparative seed germination test by the equation GI(%) =(Seed germination × Root
13
length of the treatment)/(Seed germination × Root length of the control)×100% (Tiquia
14
and Tam, 1998). The compost maturity was evaluated by fuzzy comprehensive
15
assessment (FCA). The FCA was conducted based on the fuzzy set theory used in
16
various environmental areas divided into the following 5 steps: (1) determination of an
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evaluation factor set U based on the actual local situation, (2) establishment of
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membership functions of fuzzy composting maturity, (3) calculation of the membership
19
function matrix, (4) calculation of the weights matrix, and (5) determination of the
20
assessment results (Mi et al., 2011). The maximum power density and polarization
21
curve were determined by adjusting the external resistance to 10~9999 Ω for recording
8
1
the corresponding voltage drop after MFC reached a constant power. The internal
2
resistance (Rint) were determined by the peak of the power density curve, and it was also
3
determined by the slope of polarization curves generated by the equation U = V0-IRint
4
under a steady-state condition, where V0 is the electromotive force (V) and Rint is the
5
internal resistance of the cell (Ω) calculated by Rint = ∆V0/∆I, which is deduced from the
6
polarization curve (Logan, 2008).
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3. Results and Discussion
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3.1 Sludge degradation
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3.1.1 NH4+-N variations
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On the whole, the NH4 +-N contents in both AnCBE and AnC reactors increased on the
12
initial 10 days and then gradually decreased (Fig. 1). The elevated NH4 +-N content
13
during the initial 10 days of composting was as a consequence of an intense
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mineralization of sludge organic matter with ammonia being released. At the end of
15
composting, the NH4 +-N contents of both composts (33.28 ± 1.6 for AnCBE and 50.04 ±
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2.5 mg/L for AnC) were lower than the initial contents, which was attribute to
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nitrification, ammonia volatilization and microbial immobilization during
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decomposition of organic matter. The lower NH4+-N content in AnCBE than in AnC
19
might result from ammonium removal by the anodes in MFC (Cheng and Logan, 2007).
20
However, the NH4+-N content showed increase trend during 20th to 25th day of AnCBE
9
1
operation, which was mainly owing to the soluble protein hydrolysed at the later stage
2
of degradation (Zhang et al, 2012). This phenomenon was in accordance with the
3
fluctuation of NH4+-N content observed by other researchers (Meng et al., 2014; Zhang
4
et al., 2012). Moreover, the absence or decreases in NH4+-N content was an indication
5
of both good compost quality and completion of maturation process (Tiquia et al.,
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1997).
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Fig. 1
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10
3.1.2 OM reduction
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OM reduction rates increased remarkably during the initial 25 days because of the high
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microbial activity with abundant organic substrate (Nakhshiniev et al., 2014). The
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surplus substrate in both systems was refractory organics in sludge (Zhang et al., 2012),
14
so the increase tend of OM reduction rates was not obvious with the operation time
15
extending. Restated, the available easily-biodegraded substances to microbes in sludge
16
were gradually converted to CO2, H2O and energy, while the remainder was eventually
17
the stable compounds (Sanabria-Leon et al., 2007). This coincided well with the results
18
of the NH4+-N variation (Fig. 1), suggesting almost the completion of composting.
19
Furthermore, during the course of composting, the OM reduction rate achieved
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19.50±0.20% for AnCBE and 12.90±0.10% for AnC, respectively. The OM reduction 10
1
rates in AnCBE were higher than those in AnC all throughout the composting process,
2
indicating that the OM had been consumed by other microorganisms such as
3
eletricigens besides the common fermentation bacterial colonies in AnC. In the
4
later-stage of composting (the 25th -35th day), OM reduction rates increased tardily
5
with the growth of 1.80±0.02% for AnCBE vesus 0.60±0.01% for AnC, which showed
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the eletricigens activity was superior to the common anaerobic microbe in AnC for
7
refractory organics in sludge. The OM reduction rates in AnCBE were higher than those
8
in AnC throughout the composting process, due to the OM consumed by other
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microorganisms (viz., eletricigens, the common fermentation bacterial colonies). From
10
this point of view, anaerobic composting assisted by bioelectrochemical process could
11
be accelerated, thus reducing composting time.
12
13
3.2 Assessment of composts maturity
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3.2.1 Moisture content (MC)
15
Fig. 2 showed the MC profiles of substrates both in the anodic compartment of AnCBE
16
and in AnC. During the initial 20 days, the MC in both devices varied within small
17
ranges, indicating the water demanding for hydrolysis and water consumption for
18
degradation were close in volume. From the 20th to 35th day, MC in AnCBE declined
19
dramatically, with the water losing as water vapor and participating in the biochemical
20
reaction. However, the decline of MC for AnC appeared 5 days later than for AnC BE, 11
1
which demonstrated that the metabolism process of AnC was slower than that of AnC BE.
2
As reported by other researchers, the MC reduction during anaerobic composting was
3
also ascribed to water evaporation resulting from heat generation, which was in return
4
viewed as an index of decomposition rate (Hassan et al., 2009). The MC was decreased
5
by 2.41±0.20% in AnCBE, which was more than that of 0.62±0.20% in AnC during the
6
whole composting period, because water had an additional reaction by participating in
7
electrogenesis within AnCBE as shown in equation of C12H22O11+13H2O →
8
12CO2+48H++48e- (Rebaey and Verstraete 2005). Water in AnCBE transported through
9
the membrane was primarily in the way of electro-osmotic drag by protons from the
10
anode to the cathode (Springer et al., 1991). Results indicated that the AnCBE process
11
had the functions of further dehydration for sludge and decrease of sludge volume.
12 13
Fig. 2
14
15
3.2.2 pH and electricity conductivity (EC)
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As shown in Fig. 3, the pH of the sludge first increased to 8.04 ± 0.12 for AnCBE and to
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8.40±0.12 for AnC on the 25th day and then declined until the end of the 35th day,
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overall. pH value was affected by the VFA and NH4+-N contents simultaneously. This
19
phenomenon was consistent with the relationship of NH4 +-N between the two devices
20
mentioned above, where more VFA content was produced during hydrolysis in AnCBE
21
than in AnC (Jiang et al., 2009). That pH value of AnCBE decreased by 0.10 ± 0.01 12
1
during the initial 5 days was opposite to the pH value of AnC. VFA produced in MFC
2
seeded with sludge at significant level during the first stage may be the main cause of
3
this variation (Zhang et al, 2012). Similarly, that pH value increased by 0.12 ± 0.01
4
from the 30th to the 35th day could be explained by the low content of VFA at the later
5
stage (Zhang et al, 2012). As revealed by other researchers, the pH value of compost
6
ranging from 5.5 to 8.5 was acceptable, and the pH ranging from 6 to 8 was suitable for
7
composting (Silva et al., 2013). The close proximity of pH values in AnCBE and AnC
8
within the optimal composting pH range (Fig. 3) indicated that the pH adjustment was
9
not needed after the incorporation of electrogenesis into the conventional anaerobic
10
composting system.
11 12
Fig. 3
13 14
The parameter of EC is usually used to monitor the composting effects. As shown in Fig.
15
3, the EC of the compost in AnCBE increased remarkably as compared to that in AnC
16
within the initial 15 days (1755 ± 84 µS/cm versus 1306 ± 76 µS/cm), always higher
17
than those in AnC. This meant that most of the organics were mineralized with more
18
soluble organic matters and inorganic ions being released. With the continuous
19
consumption of these soluble OM by the available microorganisms, the EC stabilized
20
gradually after composting for 35 days with the final values of 975 ± 58 µS/cm for
21
AnCBE and 867 ± 52 µS/cm for AnC (close to the initial 798 ± 45 µS/cm). EC of the
13
1
compost in AnCBE was higher than that in AnC all throughout the 35 days, since the
2
higher NH4 +-N content in the anodic compartment of MFC could increase the ionic
3
strength of the medium, and thus enhance the anodic conductivity (Mohan and Das,
4
2009). The much lower EC values than the upper limit of 4000 µS/cm implied the
5
favorable tolerance of the compost for seeds (Mohan and Das, 2009).
6
7
3.2.3 C/N ratio and germination index (GI)
8
The C/N ratio is regarded as an important indicator to evaluate compost maturity, and
9
the variations of compost C/N ratio in AnCBE and AnC were shown in Fig. 4. Overall,
10
the C/N ratio experienced a process of first increase (0-10 days) and subsequent
11
decrease (10-30 days). The composting of materials with C/N ratio lower than 20:1
12
could lead to the production of excess ammonia (Sreesai et al., 2013), which coincided
13
with the results of NH4 +-N content. On the 35th day, the C/N ratios of the composts
14
were 13.84 ± 0.20 for AnCBE and 15.42 ± 0.20 for AnC, much lower than the initial C/N
15
ratio of 17.42 ± 0.30, indicating that the OM had been decomposed and lost in form of
16
small-molecule gases such as CO2. The C/N ratio of 14 or less for the compost is
17
usually considered as its maturity (Barrington et al., 2002). The lower compost C/N
18
ratio in AnCBE than that in AnC suggested that the biodegradation of dewatered sludge
19
was enhanced by electrogenesis, with the compost being more mature than that in AnC.
20 21
Fig. 4 14
1 2
During anaerobic composting of sludge, a variety of metabolic compounds are released,
3
which might be toxic to plants. In this case, the parameter of GI can be used to evaluate
4
the phytotoxicity of compost (Tiquia et al., 1997). The variation of GI (Fig. 4) indicated
5
that the composts from AnCBE and AnC almost had the same trend during composting
6
of dewatered sludge for 35 days. Previous research reported that the compost with GI
7
≥50% represents the level which plants could withstand, that with GI ≥80% is
8
considered phytotoxic-free and complete maturity, and that with GI ≥100% is
9
considered as mature compost where the substrate has a promotive reaction to the plants
10
(Sellami et al., 2008). In this sense, the compost with better quality was obtained from
11
AnCBE than from AnC (Fig. 4). On the 35th day, the GI of compost in AnCBE and AnC
12
reached 117.6±4.2% and 94.1±3.2% (initial GI 63.2±2.6%), respectively, demonstrating
13
that the compost from AnCBE could boost plant growth. The lowest GI on the 10th day
14
was probably associated with NH4+-N release during the early stage of composting
15
(Briton, 2000). The gradual increase of GI after composting for 10 days (>80%)
16
indicated the disappearance of phytotoxic compounds (Tiquia et al., 1997). The higher
17
GI for AnCBE compost than that for AnC was ascribed to the higher EC (ref. to Fig. 3)
18
and lower NH4+-N (ref. to Fig. 1), since high ion concentration may be helpful for the
19
growth of the seeds, and GI could be to some extent controlled by high EC (Briton,
20
2000; Nakhshiniev et al., 2014). These results demonstrated that the compost would not
21
have any phytotoxic effects to plants, and the compost from AnCBE exerted more
15
1
positive influence on GI.
2
3
3.2.4 FCA of compost maturity
4
To select an appropriate assessment methodology, the Pearson correlation coefficient (r)
5
is usually adopted to find out the relationship among different parameters. The very
6
weak statistical correlation among various compost maturity parameters (Table S1 and
7
S2) indicated that the principal component analysis (PCA) was not suitable for
8
assessing the compost maturity, which required very strong correlation among
9
parameters (r≈1). Thus, the FCA methodology (when r<0.50) was chosen to assess the
10
compost maturity by using parameters of NH4+-N, C/N, GI and EC, with the classes of
11
compost maturity degree shown in Table 1.
12 13
Table 1
14 15
Following the establishment of membership functions (Table 1), the membership
16
function matrix (R) for raw sludge (RS) and composts from AnCBE and AnC was
17
obtained respectively. Based on the assessment parameters of RS (Table S3), AnCBE
18
(Table S4) and AnC (Table S5), the fuzzy algorithm W⋅R gave the following FCA
19
results:
20 21
WRS⋅RRS= (0.19, 0.06, 0.56,0.19)T; 16
1
WAnCBE⋅RAnCBE= (0.64, 0.24, 0.12, 0)T;
2
WAnC⋅RAnC = (0.44, 0.35, 0.21, 0)T.
3 4
The above assessment results indicated that the membership degree of class I for AnCBE
5
compost (0.64) was much higher than that of AnC compost (0.44), while that for RS
6
belonged to Class III (0.56). Thus, the conclusion might be drawn that the incorporation
7
of bioelectrochemical process enhanced anaerobic composting of dewatered sludge with
8
much more mature compost obtained.
9 10
To verify whether the bioelectrochemical process can accelerate anaerobic composting
11
process, the fuzzy algorithm W⋅R for AnCBE and AnC composts (on the 15th and the
12
25th day) are calculated following the same procedures as mentioned above with the
13
results shown as follows (also in Table S6-S9):
14
WAnCBE,15d ⋅RAnCBE,15d = (0.19, 0.46, 0.23, 0.13)T
15
WAnC,15d ⋅RAnC,15d = (0.15, 0.31, 0.44, 0.10)T
16
WAnCBE,25d ⋅RAnCBE,25d = (0.48, 0.29, 0.23, 0)T
17
WAnC,25d ⋅RAnC,25d = (0.14, 0.45, 0.33, 0.08)T
18 19
The above results showed that the maturity degree for both AnCBE,15d (II, 0.46) and
20
AnCBE,25d (I, 0.48) composts were higher than those for AnC,15d (III, 0.44) and AnC,25d
21
(II, 0.45), respectively. The results revealed the compost of AnCBE was much more
22
mature than that of AnC under the same composting time. Furthermore, the membership 17
1
degree of Class I for AnCBE,25d compost (0.48) was close to that for AnC,35d compost
2
(0.44), and the membership degree of Class II for AnCBE,15d compost (0.46)
3
approximated to that for AnC,25d compost (0.45), demonstrating the bioelectrochemical
4
process could accelerate anaerobic composting of dewatered sludge.
5
6
3.3 Bioelectrogenesis performance of AnCBE
7
3.3.1 Voltage and power density
8
Following a successful acclimation period, the AnCBE generated a voltage of 0.60±0.02
9
V followed by slight decrease after operation for about 20 days. Because the active
10
electricigenic microbial biofilm formed on the anode electrodes and utilized organic
11
matters in the sludge, the TCOD of dewatered sludge was removed linearly with time
12
and reached 19.8±0.2% at the end of the 35th day (Fig 5a). In the meantime, the
13
particulate COD of dewatered sludge was hydrolyzed and transformed into soluble
14
organics, which were eventually utilized by electricigenic microorganisms to generate
15
electricity (Xiao et al., 2013). The maximum power density reached 5.6W/m3 with an
16
OCV of 0.87V on the 15th day (Fig 5b). With the aid of least squares linear regression,
17
the calculated Rint of AnCBE was 97 Ω, which was consistent with 100Ω according to the
18
peak power density curve method. After composting for 25 days, the surplus substrate in
19
both systems was refractory organics in sludge, and the MC in the anodic compartment
20
of AnCBE was not in favour of electrogenesis any more, corresponding to the voltage
18
1
drop of AnCBE.
2
3.3.2 EIS measurement and analysis
3
To further investigate the distribution of internal resistance in the AnCBE, the EIS of the
4
anode was measured (Fig. 5c, d, e) at an open circuit potential (OCP) on the 3rd, 13th
5
and 20th day. The EIS data are presented in the forms of the Nyquist and Bode plots, in
6
which the impedance modulus |Z| and the phase angle (Φ) are plotted vs. the logarithm
7
of the frequency ƒ of the applied AC signal.
8 9
Fig. 5
10 11 12
As shown in Fig. 5c, the low-frequency region of the Bode plots exhibited that Rct+Rs
13
(3d) > Rct+Rs (20d) ≈ Rct+Rs (13d) while the ohmic resistance Rs were close in the
14
high-frequency region. The lowest Rct on the 13th day revealed that the bioactivity of
15
electricigens on the anode was higher than those on the 13th day and the 3th day,
16
corresponding to the high TCOD removal from the 10th day to 20th day. The phase
17
angle at the low-frequency region of the Bode plots (Fig. 5d) showed that the closeness
18
to -90° followed the order of Φ (3d) > Φ (20d) > Φ (13d), indicating that there was a
19
higher double layer capacitance on the 3th day than on the other days.
20
19
1
The Nyquist plots of EIS (Fig. 5e) displayed depressed semicircles in the high
2
frequency range followed with another depressed semicircles in the medium frequency
3
range. The first loops on the 3th day and the 20th day had the similar larger diameters
4
than those on the 13th day. The appearance of depressed semicircles could be attributed
5
to the ionic migration in the passivating film, the charge transfer (in the medium
6
frequency range) and the resistance of the electrolyte in the pores of the passivating film
7
formed on the AC signal (Takami et al., 1995). The diameter of the second semicircle
8
was expected for the adsorption-desorption of reactants (Wang et al., 1999). In addition,
9
a short line close to straight with an angle of approximately 45° to the X-axis appeared
10
in the plots, which illustrated the characteristics of the semi-finite diffusion (Warburg
11
impedance) on the flat electrode. The Warburg slope was interpreted as an empirical
12
qualitative parameter related to the diffusion resistance, which was caused by the rolling
13
catalyst layer and gas diffusion layer (Wang et al., 1999).
14 15
The impedance spectra of the anode was analysed by fitting to the circuit of EC, i.e. the
16
R(QR)W, and the fitted data of parameters in the equivalent circuit (Fig. 5e) were listed
17
in Table 2. The Rs, as the key performance parameter of a fuel cell, had almost no
18
change (Barbir. F., 2013). The W on the 3th day was higher than that on the 13th and the
19
20th day which was possibly due to the smooth electron transfer state with high and
20
constant voltage output (Fig. 5). The lowest Rs value on the 13th day indicated the best
21
performance of AnCBE. Similar to the Rs, the Cdl value of anode reached the minimum
20
1
on the 13th day, implying that the electric double layer of the anode surface was
2
thickened by the multiplication of adsorbed organic.
3 4
Table 2
5
6
4 Conclusion
7
Based on the study of bioelectrochemically-assisted anaerobic composting process, the
8
conclusions were drawn as follows. The AnCBE improved the sludge stabilization with
9
a higher TCOD removal efficiency (19.8±0.2%) and OM reduction rate (19.5±0.2%)
10
after 35 days of composting. FCA results demonstrated that the membership degree of
11
class I for AnCBE compost (0.64) was higher than that of AnC compost (0.44), and
12
bioelectrogenesis accelerated the anaerobic composting progress and improved the
13
composting maturity. The AnCBE yielded a voltage of 0.60±0.02 V and maximum
14
power density of 5.6W/m3 on the 15th day with the corresponding OCV of 0.87V.
15 16
Acknowledgement
17
The authors greatly acknowledge funding from Project 51378144 and 51206036
18
supported by National Nature Science Foundation of China, and the supports by State
19
Key Laboratory of Urban Water Resource and Environment (2013DX04), Harbin
20
Institute of Technology.
21
21
1 2
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3
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12. Himanen, M., Hanninen, K., 2011. Composting of bio-waste, aerobic and anaerobic sludges Effect of feedstock on the process and quality of compost. Bioresour Technol, 102, 2842-2852. 13. Jiang, J., Zhao, Q., Zhang, J., Zhang, G., Lee, D., 2009. Electricity generation from bio-treatment of sewage sludge with microbial fuel cell. Bioresour Technol, 100, 5808-5812. 14. Jiang, J., Zhao, Q., Wei, L., Wang, K., Lee, D., 2011. Degradation and characteristic changes of
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organic matter in sewage sludge using microbial fuel cell with ultrasound pretreatment.
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Bioresour Technol, 102, 272-277.
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15. Kavouras, P., Pantazopoulou, 6E., Varitis, S., Vourlias, G., Chrissafis, K., Dimitrakopulos, G.P.,
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16. Logan B.E., 2008. Microbial fuel cells. John Wiley & Sons, USA.
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Bioelectrochemical desalination and electricity generation in microbial desalination cell with
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19. Mohan, S.V., Chandrasekhar, K., 2011. Self-induced bio-potential and graphite electron
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with simultaneous power generation. Bioresour Technol, 102, 9532-9541.
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20. Mohan, Y., Das, D., 2009. Effect of ionic strength, cation exchanger and inoculum age on the
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performance of Microbial Fuel Cells. Int J Hydrogen Energ, 34, 7542-7546.
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21. Nges, I.A., Liu, J., 2009. Effects of anaerobic pre-treatment on the degradation of
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dewatered-sewage sludge. Renew Energ, 34, 1795-1800.
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22. Sanabria-Leon, R., Cruz-Arroyo, L.A., Rodriguez, A.A., Alameda, M., 2007. Chemical and
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1800-1807.
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24. Silva, M.E.F., Lemos, L.T., Bastos, M.M.S.M., Nunes, O.C., Cunha-Queda, A.C., 2013.
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Recovery of humic-like susbtances from low quality composts. Int J Hydrogen Energ, 128,
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624-632.
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25. Springer, T.E., Zawodzinski, T.A., Gottesfeld, S., 1991. Polymer Electrolyte Fuel-Cell Model. J Electrochem Soc, 138, 2334-2342.
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1
26. Sreesai, S., Peapueng, P., Tippayamongkonkun, T., Sthiannopkao, S., 2013. Assessment of a
2
potential agricultural application of Bangkok-digested sewage sludge and finished compost
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products. Waste Manage Res, 31, 925-936.
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27. Takami, N., Satoh, A., Hara, M., Ohsaki, T., 1995. Rechargeable Lithium-Ion Cells Using
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Graphitized Mesophase-Pitch-Based Carbon-Fiber Anodes. J Electrochem Soc, 142,
6
2564-2571.
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28. Tiquia, S.M., Tam, N., Hodgkiss, I.J., 1997. Effects of turning frequency on composting of spent pig-manure sawdust litter. Bioresour Technol, 62, 37-42. 29. Tiquia, S.M., Tam, N., 1998. Elimination of phytotoxicity during co-composting of spent pig-manure sawdust litter and pig sludge. Bioresour Technol, 65, 43-49. 30. Vavilin, V.A., Rytov, S.V., Lokshina, L.Y., 1996. A description of hydrolysis kinetics in anaerobic degradation of particulate organic matter. Bioresour Technol, 56, 229-237. 31. Wang, X.Y., Yan, J., Yuan, H.T., Zhang, Y.S., Song, D.Y., 1999. Impedance studies of nickel hydroxide microencapsulated by cobalt. Int J Hydrogen Energ, 24, 973-980. 32. Xiao, B., Yang, F., Liu, J., 2013. Evaluation of electricity production from alkaline pretreated sludge using two-chamber microbial fuel cell. J Hazard Mater, 254, 57-63.
33. Yang, F., Ren, L., Pu, Y., Logan, B.E., 2013. Electricity generation from fermented primary
18
sludge using single-chamber air-cathode microbial fuel cells. Bioresour Technol, 128, 784-787.
19
34. Zhang, G., Zhao, Q., Jiao, Y., Wang, K., Lee, D., Ren, N., 2012. Efficient electricity generation
20 21
from sewage sludge using biocathode microbial fuel cell. Water Res, 46, 43-52. 35. Zouboulis, A.I., Karakostas, T., Xenidis, A., 2015. Incineration of tannery sludge under oxic
25
1
and anoxic conditions: Study of chromium speciation. J Hazard Mater, 283, 672-679.
2
26
Table and Figure captions
1 2
Table 1 Classes of compost maturity degree for assessment parameters
3
Table 2 Comparison of the anode performances at different operational time
4
Fig. 1 Variation of NH4 +-N during composting
5
Fig. 2 Variation of MC during composting
6
Fig. 3 Variations of pH and EC during composting
7
Fig. 4 Variations of C/N ratio and GI percentage during composting and seed
8
germination
9
Fig. 5 Variations of (a) TCOD removal efficiency and (b) power density curve of AnCBE
10
on the 15th day; electrochemical impedance spectra at open circuit: (c) Bode plot for
11
modulus impedance; (d) Bode plot for phase angle; (e) Nyquist plot and equivalent
12
circuit [i.e. the inserted R(QR)W]: Rs- ohmic resistance; Rct- polarization resistance (or
13
charge transfer resistance); W- Warburg impedance, the diffusion resistance; Cdl-double
14
layer capacitances; CPE-associated with the Cdl
15
27
Table 1 Classes of compost maturity degree for assessment parameters
1
NH4 +-N Class
C/N
GI
EC(µS/cm)
NH4 +-N＜1.0
C/N＜12
GI>1
EC<500
1.0≤NH4+-N＜4.0
12≤C/N＜14
0.8＜GI≤1
500≤EC＜1500
4.0≤NH4+-N＜8.0
14≤C/N＜16
0.6＜GI≤0.8
1500≤EC＜2500
NH4 +-N≥10
C/N≥20
GI≤0.4
EC≥3000
(mg/g dry sludge) I
(Good)
II
(Preferable)
III (Common) IV (Bad)
28
1 2
Table 2 Comparison of the anode performances at different operational time Time (d)
Rs(Ω)
Cdl1(10 -3 F)
Rct(Ω)
W(Ω)
3
4.612
0.018
25.65
0.435
13
4.505
0.022
21.55
0.423
20
4.599
0.020
22.21
0.430
29
1 2
Fig. 1 Variation of NH4 +-N during composting
30
1 2
Fig. 2 Variation of MC during composting
31
1 2
Fig. 3 Variations of pH and EC during composting
32
1 2 3
Fig. 4 Variations of C/N ratio and GI percentage during composting and seed germination
33
6
0.75
5
20
0.6
4
0.45
3
0.3
2
0.15
1
0
15
30
2
4 6 8 Current (mA)
20
10
15
10
10
5
5
0
0
0
1
5
10
15 20 Time(d)
25
30
10
1000
100000
lgƒ(Hz)
(e) 20 16 14
-Zim(Ω)
25
3d 13d 20d
18
3d 13d 20d
30
Phase angle (Ф)
0
35
(d) 35
20 15
12 10 8 6
10
4
5
2
0
0
0
2
3d 13d 20d
25
0 0
35
|Z|
Potential (V)
TCOD removal (%)
25
(c) Power density (W/m3)
(a) 30 (b) 0.9
100 lgƒ(Hz)
100000
0
5
10
15
20 25 Zre(Ω)
30
35
40
3 4
Fig. 5 Variations of (a) TCOD removal efficiency and (b) power density curve of AnCBE on the 15th
5
day; electrochemical impedance spectra at open circuit: (c) Bode plot for modulus impedance; (d)
6
Bode plot for phase angle; (e) Nyquist plot and equivalent circuit [i.e. the inserted R(QR)W]: Rs-
7
ohmic resistance; Rct- polarization resistance (or charge transfer resistance); W- Warburg impedance,
8
the diffusion resistance; Cdl-double layer capacitances; CPE-associated with the Cdl
9 10
34
1
Highlights
2 3
► Bioelectrochemically-assisted anaerobic composting for dewatered sludge was achieved.
4
► Bioelectrogenesis accelerated the anaerobic composting progress.
5
► TCOD removal in AnCBE(19.8±0.2%) was higher than that in AnC composting after 35d.
6
► Membership degree of class I for AnCBEcompost (0.64) was higher than for AnC(0.44).
7
► Maximum power density of 5.6W/m with a corresponding OCV of 0.87V.
3
8 9 10
35
1 2
Graphical Abstract
3
4 5
36
S0960-8524(15)00848-2 http://dx.doi.org/10.1016/j.biortech.2015.06.057 BITE 15137
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
9 April 2015 10 June 2015 12 June 2015
Please cite this article as: Yu, H., Jiang, J., Zhao, Q., Wang, K., Zhang, Y., Zheng, Z., Hao, X., Bioelectrochemicallyassisted anaerobic composting process enhancing compost maturity of dewatered sludge with synchronous electricity generation, Bioresource Technology (2015), doi: http://dx.doi.org/10.1016/j.biortech.2015.06.057
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1
Bioelectrochemically-assisted anaerobic composting process
2
enhancing compost maturity of dewatered sludge with
3
synchronous electricity generation
4 5
Hang Yua, Junqiu Jianga, Qingliang Zhaoa,b*, Kun Wanga,b, Yunshu Zhanga, Zhen
6
Zhengc, Xiaodi Hao d
7 8
a School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090,
9
China
10
b State Key Laboratory of Urban Water Resources and Environments (SKLURE), Harbin Institute of
11
Technology, Harbin 150090, China
12
c School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
13 14 15
d School of Environment and Energy Engineering (The R & D Centre for Sustainable Environmental
16
∗
17
[email protected]
Biotechnology), Beijing University of Civil Engineering and Architecture, Beijing, China
Corresponding author. Tel: +86 451 8628 3017; Fax: +86 451 8628 3017. E-mail address:
18
1
1
Abstract: Bioelectrochemically-assisted anaerobic composting process (AnCBE) with
2
dewatered sludge as the anode fuel was constructed to accelerate composting of
3
dewatered sludge, which could increase the quality of the compost and harvest electric
4
energy in comparison with the traditional anaerobic composting (AnC). Results
5
revealed that the AnCBE yielded a voltage of 0.60±0.02 V, and total COD (TCOD)
6
removal reached 19.8±0.2% at the end of 35 d. The maximum power density was
7
5.6W/m3. At the end of composting, organic matter content (OM) reduction rate
8
increased to 19.5±0.2% in AnCBE and to 12.9±0.1% in AnC. The Fuzzy Comprehensive
9
Assessment (FCA) result indicated that the membership degree of class I of AnCBE
10
compost (0.64) was higher than that of AnC compost (0.44). It was demonstrated that
11
electrogenesis in the AnCBE could improve the sludge stabilization degree, accelerate
12
anaerobic composting process and enhance composting maturity with bioelectricity
13
generation.
14
Keywords: Anaerobic composting (AnC); bioelectrochemical devices; dewatered
15
sludge; anaerobic compost maturity; fuzzy comprehensive assessment (FCA).
16
2
1
1. Introduction
2
Sewage sludge, as an inevitable byproduct of wastewater treatment, needs to be
3
dewatered to reduce its volume and thus dewatered sludge is generated. The disposal
4
processes of dewatered sludge include incineration, sanitary landfill and composting, etc.
5
(Blazy et al., 2014). Among those processes, inorganic particulate matter (PM)
6
containing harmful substances is formed during incineration, and a series of
7
geoenviromental problems (viz., leachability, total and differential settlement and slope
8
stability) are triggered during landfill (Kavouras et al., 2012). Moreover, aerobic or
9
anaerobic composting is widely used in most countries all over the world since the
10
composting product can be used as soil conditioner to recycle a substantial amount of
11
nutrients and organic matters. To a large extent, aerobic composting requires strict
12
control of blast volume, high energy input and skilful operation of mechanical
13
equipment. In comparison with aerobic composting, anaerobic composting (AnC) has
14
such merits as maximum retention of sludge nutrients and lower power consumption
15
while stabilizing the organics. Nevertheless, the AnC process faces the problems of low
16
stabilization degree, long composting cycle (40~60 days or even longer) and
17
dissatisfactory maturity without long period of composting (Himanen and Hanninen,
18
2011; Banegas et al., 2007). These problems limit its widespread application to sludge
19
disposal and utilization.
20 21
The bioelectrochemical devices such as microbial fuel cell (MFC) have been proven to 3
1
be capable of using various sludges (e.g. manure sludge, surplus sludge, petroleum
2
sludge and fermented primary sludge) as anodic fuels for electricity generation while
3
accelerating anaerobic degradation (Dentel et al., 2004; Jiang et al., 2009; Mohan and
4
Chandrasekhar, 2011; Yang et al., 2013). Restated, Jiang et al. (2011) has proven the
5
‘‘fuel-cell” reactions degraded additional 7.9% total COD (TCOD) of raw sludge in
6
MFC, and sludge degradation in MFC can be characterized as a combined process of
7
anaerobic digestion and electrochemical oxidation, being complementary in function to
8
each other. In a sludge composting system, the first and limited step was the insoluble
9
macromolecules in extracellular polymeric substances (EPS) of sludge hydrolysed into
10
soluble organic compounds (viz., proteins, carbohydrates, etc.) (Appels et al., 2008).
11
Moreover, the microbes within MFC could only utilize soluble organic matter to
12
product electricity power (Vavilin et al., 1996). So, if the bioelectrochemical process is
13
integrated into the conventional anaerobic composting process, the sludge degradation
14
rate of anaerobic composting might be accelerated since these simple organic
15
compounds can be directly consumed by the available electricigens within the
16
bioelectrochemically-assisted anaerobic composting system (AnCBE).
17 18
The objective of this study is to develop an efficient bioelectrochemically-assisted
19
anaerobic composting process for dewatered sludge, focusing on the compost maturity
20
while synchronously generating electricity. The scopes of this work are: (1) to examine
21
the degree of organic degradation and sludge stabilization during composting in AnCBE;
4
1
(2) to assess the maturity degree of composts by using the fuzzy comprehensive
2
assessment (FCA) approach; and (3) to investigate into the simultaneous
3
bioelectrogenesis performance of AnCBE. The results obtained were attempted to
4
accelerate composting of dewatered sludge, which could increase the quality of the
5
compost and harvest electric energy in comparison with the traditional anaerobic
6
composting.
7 8
2. Materials and Methods
9
2.1 Experimental set-up of AnCBE and operation
10
The AnCBE reactor made of plexiglas comprised two compartments and a proton
11
exchange membrane (PEM, Nafion 117, Dupont Company) located between an anode
12
and cathode. The anodic compartment was a cylinder (Φ 12 cm×8 cm) and the cathodic
13
was a cube (8 cm×7 cm×12cm). The two compartments were positioned and held
14
together by bolts with the membrane in the middle providing a large surface area. Both
15
the anode and cathode electrodes consisted of a graphite fiber brush and titanium wire
16
that collected electrons for the external circuit. The effective volume of the anodic
17
compartment was 780 mL and that of the cathodic compartment was 350 mL. The
18
catholyte was potassium ferricyanide as the main ingredient, with the advantages of
19
stability, low over potential and cathodic work potential close to the open circle
20
potential (Aelterman et.al., 2006). A fixed resistance (Rex=1000 Ω) was applied as an
21
external load in the circuit. The anodic compartment was purged with nitrogen to keep
5
1
anaerobic condition after sludge addition. For control, an AnC reactor was constructed
2
with the same configuration of the anodic compartment of AnCBE, being filled with the
3
same dewatered sludge and operated at the same room temperature conditions. Both
4
AnCBE and AnC reactors were wrapped by insulation cotton to prevent heat loss and
5
ensure the smooth progress of composting.
6
7
2.2 AnCBE inoculum and start-up
8
The inoculated sludge was collected from the secondary clarifier of Taiping Wastewater
9
Treatment Plant in Harbin, China. The anodic compartment of AnCBE was inoculated
10
progressively with sewage sludge and dewatered sludge after belt press filtration.
11
Specifically, during the start-up period, only the sewage sludge was fed into the anodic
12
compartment of AnCBE. After three days, a mixture of dewatered sludge and sewage
13
sludge with a ratio of 1:3 (v/v) was filled in the anodic compartment instead. The
14
percentage of dewatered sludge in the mixture was increased every three days until
15
complete replacement into dewatered sludge as the anodic fuel. The successful start-up
16
was indicated by the colonization of electricigens on the electrode and constant
17
production of electric power. The pH, moisture content (MC), OM and total COD
18
(TCOD) of the dewatered sludge were 7.62 ± 0.20, 86.04 ± 0.20%, 54.28 ± 0.06% and
19
227800± 100 mg/L, respectively. Fresh dewatered sludge was stored at 4 oC prior to
20
use.
6
1
2.3 Analysis and computation
2
Before analyses of pH, conductivity, TCOD and NH4 +-N, dewatered sludge samples
3
were diluted with distilled water (1:10 w/v) in a horizontal shaker for 24h at room
4
temperature. Distilled water was utilized to test TCOD, and was filtered through
5
0.45µm membrane to extract soluble component for pH, conductivity and NH4+-N. The
6
statistical analysis was conducted using SPSS software version 12.0 for Windows.
7
Pearson’s correlation coefficient (r) was used to evaluate the linear correlation between
8
two parameters. OM and MC were measured by weighting method (APHA, 2002).
9
Elemental composition (C and N) was determined with an element analyser (vario EL
10
III, Elementar German). The pH, electrical conductivity (EC), NH4+-N, and TCOD were
11
analysed according to Standard Methods (APHA, 2002). The voltage difference
12
between two electrodes was recorded across a fixed load (1000 Ω) by a voltage
13
collection instrument (12 bit A/D conversion chips, US) connected to a personal
14
computer. The anode and cathode potentials were measured against an Ag/AgCl
15
reference electrode (+ 0.195 V vs. standard hydrogen electrode, SHE). The maximum
16
power density and polarization curve were determined by adjusting the external
17
resistance to 10~9999 Ω for recording the corresponding voltage drop, where power
18
density (W/m3) was calculated through the effective volume of the anode compartment.
19
Electrochemical impedance spectroscopy (EIS) were measured by using CHI 660
20
electrochemical working station (CH Instrument, USA) . EIS measurements were
21
carried out for the anode in a frequency range of 100 kHz to 1 MHz with an AC signal 7
1
of 10mV amplitude. The real (Z′), imaginary (Z″) and frequency (ƒ) components of EIS
2
in Nyquist plot and Bode plot were analysed using ZsimDemo 12.0 software to simulate
3
the equivalent resistances and capacitances.
4 5
TCOD removal rate was calculated by the equation of TCOD reduction =
6
(TCOD0-TCODi)/TCOD0 × 100%, where TCOD0 is the initial TCOD of the dewatered
7
sludge, and TCODi stands for the actual monitoring data of TCOD at ith day of
8
operation. Similarly, OM reduction rate could be calculated by the equation of OM
9
reduction = (OM0-OMi)/OM0×100%, where OM0 is the initial organic matter content of
10
the dewatered sludge, and OMi stands for the actual monitoring data of organic matter
11
content at ith day of operation. Germination index (GI) was calculated using the
12
comparative seed germination test by the equation GI(%) =(Seed germination × Root
13
length of the treatment)/(Seed germination × Root length of the control)×100% (Tiquia
14
and Tam, 1998). The compost maturity was evaluated by fuzzy comprehensive
15
assessment (FCA). The FCA was conducted based on the fuzzy set theory used in
16
various environmental areas divided into the following 5 steps: (1) determination of an
17
evaluation factor set U based on the actual local situation, (2) establishment of
18
membership functions of fuzzy composting maturity, (3) calculation of the membership
19
function matrix, (4) calculation of the weights matrix, and (5) determination of the
20
assessment results (Mi et al., 2011). The maximum power density and polarization
21
curve were determined by adjusting the external resistance to 10~9999 Ω for recording
8
1
the corresponding voltage drop after MFC reached a constant power. The internal
2
resistance (Rint) were determined by the peak of the power density curve, and it was also
3
determined by the slope of polarization curves generated by the equation U = V0-IRint
4
under a steady-state condition, where V0 is the electromotive force (V) and Rint is the
5
internal resistance of the cell (Ω) calculated by Rint = ∆V0/∆I, which is deduced from the
6
polarization curve (Logan, 2008).
7 8
3. Results and Discussion
9
3.1 Sludge degradation
10
3.1.1 NH4+-N variations
11
On the whole, the NH4 +-N contents in both AnCBE and AnC reactors increased on the
12
initial 10 days and then gradually decreased (Fig. 1). The elevated NH4 +-N content
13
during the initial 10 days of composting was as a consequence of an intense
14
mineralization of sludge organic matter with ammonia being released. At the end of
15
composting, the NH4 +-N contents of both composts (33.28 ± 1.6 for AnCBE and 50.04 ±
16
2.5 mg/L for AnC) were lower than the initial contents, which was attribute to
17
nitrification, ammonia volatilization and microbial immobilization during
18
decomposition of organic matter. The lower NH4+-N content in AnCBE than in AnC
19
might result from ammonium removal by the anodes in MFC (Cheng and Logan, 2007).
20
However, the NH4+-N content showed increase trend during 20th to 25th day of AnCBE
9
1
operation, which was mainly owing to the soluble protein hydrolysed at the later stage
2
of degradation (Zhang et al, 2012). This phenomenon was in accordance with the
3
fluctuation of NH4+-N content observed by other researchers (Meng et al., 2014; Zhang
4
et al., 2012). Moreover, the absence or decreases in NH4+-N content was an indication
5
of both good compost quality and completion of maturation process (Tiquia et al.,
6
1997).
7 8
Fig. 1
9
10
3.1.2 OM reduction
11
OM reduction rates increased remarkably during the initial 25 days because of the high
12
microbial activity with abundant organic substrate (Nakhshiniev et al., 2014). The
13
surplus substrate in both systems was refractory organics in sludge (Zhang et al., 2012),
14
so the increase tend of OM reduction rates was not obvious with the operation time
15
extending. Restated, the available easily-biodegraded substances to microbes in sludge
16
were gradually converted to CO2, H2O and energy, while the remainder was eventually
17
the stable compounds (Sanabria-Leon et al., 2007). This coincided well with the results
18
of the NH4+-N variation (Fig. 1), suggesting almost the completion of composting.
19
Furthermore, during the course of composting, the OM reduction rate achieved
20
19.50±0.20% for AnCBE and 12.90±0.10% for AnC, respectively. The OM reduction 10
1
rates in AnCBE were higher than those in AnC all throughout the composting process,
2
indicating that the OM had been consumed by other microorganisms such as
3
eletricigens besides the common fermentation bacterial colonies in AnC. In the
4
later-stage of composting (the 25th -35th day), OM reduction rates increased tardily
5
with the growth of 1.80±0.02% for AnCBE vesus 0.60±0.01% for AnC, which showed
6
the eletricigens activity was superior to the common anaerobic microbe in AnC for
7
refractory organics in sludge. The OM reduction rates in AnCBE were higher than those
8
in AnC throughout the composting process, due to the OM consumed by other
9
microorganisms (viz., eletricigens, the common fermentation bacterial colonies). From
10
this point of view, anaerobic composting assisted by bioelectrochemical process could
11
be accelerated, thus reducing composting time.
12
13
3.2 Assessment of composts maturity
14
3.2.1 Moisture content (MC)
15
Fig. 2 showed the MC profiles of substrates both in the anodic compartment of AnCBE
16
and in AnC. During the initial 20 days, the MC in both devices varied within small
17
ranges, indicating the water demanding for hydrolysis and water consumption for
18
degradation were close in volume. From the 20th to 35th day, MC in AnCBE declined
19
dramatically, with the water losing as water vapor and participating in the biochemical
20
reaction. However, the decline of MC for AnC appeared 5 days later than for AnC BE, 11
1
which demonstrated that the metabolism process of AnC was slower than that of AnC BE.
2
As reported by other researchers, the MC reduction during anaerobic composting was
3
also ascribed to water evaporation resulting from heat generation, which was in return
4
viewed as an index of decomposition rate (Hassan et al., 2009). The MC was decreased
5
by 2.41±0.20% in AnCBE, which was more than that of 0.62±0.20% in AnC during the
6
whole composting period, because water had an additional reaction by participating in
7
electrogenesis within AnCBE as shown in equation of C12H22O11+13H2O →
8
12CO2+48H++48e- (Rebaey and Verstraete 2005). Water in AnCBE transported through
9
the membrane was primarily in the way of electro-osmotic drag by protons from the
10
anode to the cathode (Springer et al., 1991). Results indicated that the AnCBE process
11
had the functions of further dehydration for sludge and decrease of sludge volume.
12 13
Fig. 2
14
15
3.2.2 pH and electricity conductivity (EC)
16
As shown in Fig. 3, the pH of the sludge first increased to 8.04 ± 0.12 for AnCBE and to
17
8.40±0.12 for AnC on the 25th day and then declined until the end of the 35th day,
18
overall. pH value was affected by the VFA and NH4+-N contents simultaneously. This
19
phenomenon was consistent with the relationship of NH4 +-N between the two devices
20
mentioned above, where more VFA content was produced during hydrolysis in AnCBE
21
than in AnC (Jiang et al., 2009). That pH value of AnCBE decreased by 0.10 ± 0.01 12
1
during the initial 5 days was opposite to the pH value of AnC. VFA produced in MFC
2
seeded with sludge at significant level during the first stage may be the main cause of
3
this variation (Zhang et al, 2012). Similarly, that pH value increased by 0.12 ± 0.01
4
from the 30th to the 35th day could be explained by the low content of VFA at the later
5
stage (Zhang et al, 2012). As revealed by other researchers, the pH value of compost
6
ranging from 5.5 to 8.5 was acceptable, and the pH ranging from 6 to 8 was suitable for
7
composting (Silva et al., 2013). The close proximity of pH values in AnCBE and AnC
8
within the optimal composting pH range (Fig. 3) indicated that the pH adjustment was
9
not needed after the incorporation of electrogenesis into the conventional anaerobic
10
composting system.
11 12
Fig. 3
13 14
The parameter of EC is usually used to monitor the composting effects. As shown in Fig.
15
3, the EC of the compost in AnCBE increased remarkably as compared to that in AnC
16
within the initial 15 days (1755 ± 84 µS/cm versus 1306 ± 76 µS/cm), always higher
17
than those in AnC. This meant that most of the organics were mineralized with more
18
soluble organic matters and inorganic ions being released. With the continuous
19
consumption of these soluble OM by the available microorganisms, the EC stabilized
20
gradually after composting for 35 days with the final values of 975 ± 58 µS/cm for
21
AnCBE and 867 ± 52 µS/cm for AnC (close to the initial 798 ± 45 µS/cm). EC of the
13
1
compost in AnCBE was higher than that in AnC all throughout the 35 days, since the
2
higher NH4 +-N content in the anodic compartment of MFC could increase the ionic
3
strength of the medium, and thus enhance the anodic conductivity (Mohan and Das,
4
2009). The much lower EC values than the upper limit of 4000 µS/cm implied the
5
favorable tolerance of the compost for seeds (Mohan and Das, 2009).
6
7
3.2.3 C/N ratio and germination index (GI)
8
The C/N ratio is regarded as an important indicator to evaluate compost maturity, and
9
the variations of compost C/N ratio in AnCBE and AnC were shown in Fig. 4. Overall,
10
the C/N ratio experienced a process of first increase (0-10 days) and subsequent
11
decrease (10-30 days). The composting of materials with C/N ratio lower than 20:1
12
could lead to the production of excess ammonia (Sreesai et al., 2013), which coincided
13
with the results of NH4 +-N content. On the 35th day, the C/N ratios of the composts
14
were 13.84 ± 0.20 for AnCBE and 15.42 ± 0.20 for AnC, much lower than the initial C/N
15
ratio of 17.42 ± 0.30, indicating that the OM had been decomposed and lost in form of
16
small-molecule gases such as CO2. The C/N ratio of 14 or less for the compost is
17
usually considered as its maturity (Barrington et al., 2002). The lower compost C/N
18
ratio in AnCBE than that in AnC suggested that the biodegradation of dewatered sludge
19
was enhanced by electrogenesis, with the compost being more mature than that in AnC.
20 21
Fig. 4 14
1 2
During anaerobic composting of sludge, a variety of metabolic compounds are released,
3
which might be toxic to plants. In this case, the parameter of GI can be used to evaluate
4
the phytotoxicity of compost (Tiquia et al., 1997). The variation of GI (Fig. 4) indicated
5
that the composts from AnCBE and AnC almost had the same trend during composting
6
of dewatered sludge for 35 days. Previous research reported that the compost with GI
7
≥50% represents the level which plants could withstand, that with GI ≥80% is
8
considered phytotoxic-free and complete maturity, and that with GI ≥100% is
9
considered as mature compost where the substrate has a promotive reaction to the plants
10
(Sellami et al., 2008). In this sense, the compost with better quality was obtained from
11
AnCBE than from AnC (Fig. 4). On the 35th day, the GI of compost in AnCBE and AnC
12
reached 117.6±4.2% and 94.1±3.2% (initial GI 63.2±2.6%), respectively, demonstrating
13
that the compost from AnCBE could boost plant growth. The lowest GI on the 10th day
14
was probably associated with NH4+-N release during the early stage of composting
15
(Briton, 2000). The gradual increase of GI after composting for 10 days (>80%)
16
indicated the disappearance of phytotoxic compounds (Tiquia et al., 1997). The higher
17
GI for AnCBE compost than that for AnC was ascribed to the higher EC (ref. to Fig. 3)
18
and lower NH4+-N (ref. to Fig. 1), since high ion concentration may be helpful for the
19
growth of the seeds, and GI could be to some extent controlled by high EC (Briton,
20
2000; Nakhshiniev et al., 2014). These results demonstrated that the compost would not
21
have any phytotoxic effects to plants, and the compost from AnCBE exerted more
15
1
positive influence on GI.
2
3
3.2.4 FCA of compost maturity
4
To select an appropriate assessment methodology, the Pearson correlation coefficient (r)
5
is usually adopted to find out the relationship among different parameters. The very
6
weak statistical correlation among various compost maturity parameters (Table S1 and
7
S2) indicated that the principal component analysis (PCA) was not suitable for
8
assessing the compost maturity, which required very strong correlation among
9
parameters (r≈1). Thus, the FCA methodology (when r<0.50) was chosen to assess the
10
compost maturity by using parameters of NH4+-N, C/N, GI and EC, with the classes of
11
compost maturity degree shown in Table 1.
12 13
Table 1
14 15
Following the establishment of membership functions (Table 1), the membership
16
function matrix (R) for raw sludge (RS) and composts from AnCBE and AnC was
17
obtained respectively. Based on the assessment parameters of RS (Table S3), AnCBE
18
(Table S4) and AnC (Table S5), the fuzzy algorithm W⋅R gave the following FCA
19
results:
20 21
WRS⋅RRS= (0.19, 0.06, 0.56,0.19)T; 16
1
WAnCBE⋅RAnCBE= (0.64, 0.24, 0.12, 0)T;
2
WAnC⋅RAnC = (0.44, 0.35, 0.21, 0)T.
3 4
The above assessment results indicated that the membership degree of class I for AnCBE
5
compost (0.64) was much higher than that of AnC compost (0.44), while that for RS
6
belonged to Class III (0.56). Thus, the conclusion might be drawn that the incorporation
7
of bioelectrochemical process enhanced anaerobic composting of dewatered sludge with
8
much more mature compost obtained.
9 10
To verify whether the bioelectrochemical process can accelerate anaerobic composting
11
process, the fuzzy algorithm W⋅R for AnCBE and AnC composts (on the 15th and the
12
25th day) are calculated following the same procedures as mentioned above with the
13
results shown as follows (also in Table S6-S9):
14
WAnCBE,15d ⋅RAnCBE,15d = (0.19, 0.46, 0.23, 0.13)T
15
WAnC,15d ⋅RAnC,15d = (0.15, 0.31, 0.44, 0.10)T
16
WAnCBE,25d ⋅RAnCBE,25d = (0.48, 0.29, 0.23, 0)T
17
WAnC,25d ⋅RAnC,25d = (0.14, 0.45, 0.33, 0.08)T
18 19
The above results showed that the maturity degree for both AnCBE,15d (II, 0.46) and
20
AnCBE,25d (I, 0.48) composts were higher than those for AnC,15d (III, 0.44) and AnC,25d
21
(II, 0.45), respectively. The results revealed the compost of AnCBE was much more
22
mature than that of AnC under the same composting time. Furthermore, the membership 17
1
degree of Class I for AnCBE,25d compost (0.48) was close to that for AnC,35d compost
2
(0.44), and the membership degree of Class II for AnCBE,15d compost (0.46)
3
approximated to that for AnC,25d compost (0.45), demonstrating the bioelectrochemical
4
process could accelerate anaerobic composting of dewatered sludge.
5
6
3.3 Bioelectrogenesis performance of AnCBE
7
3.3.1 Voltage and power density
8
Following a successful acclimation period, the AnCBE generated a voltage of 0.60±0.02
9
V followed by slight decrease after operation for about 20 days. Because the active
10
electricigenic microbial biofilm formed on the anode electrodes and utilized organic
11
matters in the sludge, the TCOD of dewatered sludge was removed linearly with time
12
and reached 19.8±0.2% at the end of the 35th day (Fig 5a). In the meantime, the
13
particulate COD of dewatered sludge was hydrolyzed and transformed into soluble
14
organics, which were eventually utilized by electricigenic microorganisms to generate
15
electricity (Xiao et al., 2013). The maximum power density reached 5.6W/m3 with an
16
OCV of 0.87V on the 15th day (Fig 5b). With the aid of least squares linear regression,
17
the calculated Rint of AnCBE was 97 Ω, which was consistent with 100Ω according to the
18
peak power density curve method. After composting for 25 days, the surplus substrate in
19
both systems was refractory organics in sludge, and the MC in the anodic compartment
20
of AnCBE was not in favour of electrogenesis any more, corresponding to the voltage
18
1
drop of AnCBE.
2
3.3.2 EIS measurement and analysis
3
To further investigate the distribution of internal resistance in the AnCBE, the EIS of the
4
anode was measured (Fig. 5c, d, e) at an open circuit potential (OCP) on the 3rd, 13th
5
and 20th day. The EIS data are presented in the forms of the Nyquist and Bode plots, in
6
which the impedance modulus |Z| and the phase angle (Φ) are plotted vs. the logarithm
7
of the frequency ƒ of the applied AC signal.
8 9
Fig. 5
10 11 12
As shown in Fig. 5c, the low-frequency region of the Bode plots exhibited that Rct+Rs
13
(3d) > Rct+Rs (20d) ≈ Rct+Rs (13d) while the ohmic resistance Rs were close in the
14
high-frequency region. The lowest Rct on the 13th day revealed that the bioactivity of
15
electricigens on the anode was higher than those on the 13th day and the 3th day,
16
corresponding to the high TCOD removal from the 10th day to 20th day. The phase
17
angle at the low-frequency region of the Bode plots (Fig. 5d) showed that the closeness
18
to -90° followed the order of Φ (3d) > Φ (20d) > Φ (13d), indicating that there was a
19
higher double layer capacitance on the 3th day than on the other days.
20
19
1
The Nyquist plots of EIS (Fig. 5e) displayed depressed semicircles in the high
2
frequency range followed with another depressed semicircles in the medium frequency
3
range. The first loops on the 3th day and the 20th day had the similar larger diameters
4
than those on the 13th day. The appearance of depressed semicircles could be attributed
5
to the ionic migration in the passivating film, the charge transfer (in the medium
6
frequency range) and the resistance of the electrolyte in the pores of the passivating film
7
formed on the AC signal (Takami et al., 1995). The diameter of the second semicircle
8
was expected for the adsorption-desorption of reactants (Wang et al., 1999). In addition,
9
a short line close to straight with an angle of approximately 45° to the X-axis appeared
10
in the plots, which illustrated the characteristics of the semi-finite diffusion (Warburg
11
impedance) on the flat electrode. The Warburg slope was interpreted as an empirical
12
qualitative parameter related to the diffusion resistance, which was caused by the rolling
13
catalyst layer and gas diffusion layer (Wang et al., 1999).
14 15
The impedance spectra of the anode was analysed by fitting to the circuit of EC, i.e. the
16
R(QR)W, and the fitted data of parameters in the equivalent circuit (Fig. 5e) were listed
17
in Table 2. The Rs, as the key performance parameter of a fuel cell, had almost no
18
change (Barbir. F., 2013). The W on the 3th day was higher than that on the 13th and the
19
20th day which was possibly due to the smooth electron transfer state with high and
20
constant voltage output (Fig. 5). The lowest Rs value on the 13th day indicated the best
21
performance of AnCBE. Similar to the Rs, the Cdl value of anode reached the minimum
20
1
on the 13th day, implying that the electric double layer of the anode surface was
2
thickened by the multiplication of adsorbed organic.
3 4
Table 2
5
6
4 Conclusion
7
Based on the study of bioelectrochemically-assisted anaerobic composting process, the
8
conclusions were drawn as follows. The AnCBE improved the sludge stabilization with
9
a higher TCOD removal efficiency (19.8±0.2%) and OM reduction rate (19.5±0.2%)
10
after 35 days of composting. FCA results demonstrated that the membership degree of
11
class I for AnCBE compost (0.64) was higher than that of AnC compost (0.44), and
12
bioelectrogenesis accelerated the anaerobic composting progress and improved the
13
composting maturity. The AnCBE yielded a voltage of 0.60±0.02 V and maximum
14
power density of 5.6W/m3 on the 15th day with the corresponding OCV of 0.87V.
15 16
Acknowledgement
17
The authors greatly acknowledge funding from Project 51378144 and 51206036
18
supported by National Nature Science Foundation of China, and the supports by State
19
Key Laboratory of Urban Water Resource and Environment (2013DX04), Harbin
20
Institute of Technology.
21
21
1 2
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28. Tiquia, S.M., Tam, N., Hodgkiss, I.J., 1997. Effects of turning frequency on composting of spent pig-manure sawdust litter. Bioresour Technol, 62, 37-42. 29. Tiquia, S.M., Tam, N., 1998. Elimination of phytotoxicity during co-composting of spent pig-manure sawdust litter and pig sludge. Bioresour Technol, 65, 43-49. 30. Vavilin, V.A., Rytov, S.V., Lokshina, L.Y., 1996. A description of hydrolysis kinetics in anaerobic degradation of particulate organic matter. Bioresour Technol, 56, 229-237. 31. Wang, X.Y., Yan, J., Yuan, H.T., Zhang, Y.S., Song, D.Y., 1999. Impedance studies of nickel hydroxide microencapsulated by cobalt. Int J Hydrogen Energ, 24, 973-980. 32. Xiao, B., Yang, F., Liu, J., 2013. Evaluation of electricity production from alkaline pretreated sludge using two-chamber microbial fuel cell. J Hazard Mater, 254, 57-63.
33. Yang, F., Ren, L., Pu, Y., Logan, B.E., 2013. Electricity generation from fermented primary
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sludge using single-chamber air-cathode microbial fuel cells. Bioresour Technol, 128, 784-787.
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34. Zhang, G., Zhao, Q., Jiao, Y., Wang, K., Lee, D., Ren, N., 2012. Efficient electricity generation
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from sewage sludge using biocathode microbial fuel cell. Water Res, 46, 43-52. 35. Zouboulis, A.I., Karakostas, T., Xenidis, A., 2015. Incineration of tannery sludge under oxic
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and anoxic conditions: Study of chromium speciation. J Hazard Mater, 283, 672-679.
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Table and Figure captions
1 2
Table 1 Classes of compost maturity degree for assessment parameters
3
Table 2 Comparison of the anode performances at different operational time
4
Fig. 1 Variation of NH4 +-N during composting
5
Fig. 2 Variation of MC during composting
6
Fig. 3 Variations of pH and EC during composting
7
Fig. 4 Variations of C/N ratio and GI percentage during composting and seed
8
germination
9
Fig. 5 Variations of (a) TCOD removal efficiency and (b) power density curve of AnCBE
10
on the 15th day; electrochemical impedance spectra at open circuit: (c) Bode plot for
11
modulus impedance; (d) Bode plot for phase angle; (e) Nyquist plot and equivalent
12
circuit [i.e. the inserted R(QR)W]: Rs- ohmic resistance; Rct- polarization resistance (or
13
charge transfer resistance); W- Warburg impedance, the diffusion resistance; Cdl-double
14
layer capacitances; CPE-associated with the Cdl
15
27
Table 1 Classes of compost maturity degree for assessment parameters
1
NH4 +-N Class
C/N
GI
EC(µS/cm)
NH4 +-N＜1.0
C/N＜12
GI>1
EC<500
1.0≤NH4+-N＜4.0
12≤C/N＜14
0.8＜GI≤1
500≤EC＜1500
4.0≤NH4+-N＜8.0
14≤C/N＜16
0.6＜GI≤0.8
1500≤EC＜2500
NH4 +-N≥10
C/N≥20
GI≤0.4
EC≥3000
(mg/g dry sludge) I
(Good)
II
(Preferable)
III (Common) IV (Bad)
28
1 2
Table 2 Comparison of the anode performances at different operational time Time (d)
Rs(Ω)
Cdl1(10 -3 F)
Rct(Ω)
W(Ω)
3
4.612
0.018
25.65
0.435
13
4.505
0.022
21.55
0.423
20
4.599
0.020
22.21
0.430
29
1 2
Fig. 1 Variation of NH4 +-N during composting
30
1 2
Fig. 2 Variation of MC during composting
31
1 2
Fig. 3 Variations of pH and EC during composting
32
1 2 3
Fig. 4 Variations of C/N ratio and GI percentage during composting and seed germination
33
6
0.75
5
20
0.6
4
0.45
3
0.3
2
0.15
1
0
15
30
2
4 6 8 Current (mA)
20
10
15
10
10
5
5
0
0
0
1
5
10
15 20 Time(d)
25
30
10
1000
100000
lgƒ(Hz)
(e) 20 16 14
-Zim(Ω)
25
3d 13d 20d
18
3d 13d 20d
30
Phase angle (Ф)
0
35
(d) 35
20 15
12 10 8 6
10
4
5
2
0
0
0
2
3d 13d 20d
25
0 0
35
|Z|
Potential (V)
TCOD removal (%)
25
(c) Power density (W/m3)
(a) 30 (b) 0.9
100 lgƒ(Hz)
100000
0
5
10
15
20 25 Zre(Ω)
30
35
40
3 4
Fig. 5 Variations of (a) TCOD removal efficiency and (b) power density curve of AnCBE on the 15th
5
day; electrochemical impedance spectra at open circuit: (c) Bode plot for modulus impedance; (d)
6
Bode plot for phase angle; (e) Nyquist plot and equivalent circuit [i.e. the inserted R(QR)W]: Rs-
7
ohmic resistance; Rct- polarization resistance (or charge transfer resistance); W- Warburg impedance,
8
the diffusion resistance; Cdl-double layer capacitances; CPE-associated with the Cdl
9 10
34
1
Highlights
2 3
► Bioelectrochemically-assisted anaerobic composting for dewatered sludge was achieved.
4
► Bioelectrogenesis accelerated the anaerobic composting progress.
5
► TCOD removal in AnCBE(19.8±0.2%) was higher than that in AnC composting after 35d.
6
► Membership degree of class I for AnCBEcompost (0.64) was higher than for AnC(0.44).
7
► Maximum power density of 5.6W/m with a corresponding OCV of 0.87V.
3
8 9 10
35
1 2
Graphical Abstract
3
4 5
36