Application of sediment microbial fuel cell for in situ reclamation of aquaculture pond water quality

Aquacultural Engineering 57 (2013) 101–107

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Application of sediment microbial fuel cell for in situ reclamation of aquaculture pond water quality T.K. Sajana a , M.M. Ghangrekar b,∗ , A. Mitra a a b

Department of Agricultural & Food Engineering, Indian Institute of Technology, Kharagpur 721 302, India Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721 302, India

a r t i c l e

i n f o

Article history: Received 28 September 2012 Accepted 2 September 2013 Keywords: Aquaculture water remediation DO External resistance SMFC Temperature

a b s t r a c t Performance of sediment microbial fuel cell (SMFC) with external resistance (SMFC-1) as well as shortcircuited mode (SMFC-2) was evaluated at different operating temperatures (28–30 ◦ C and 21–25 ◦ C) and in presence and absence of aeration at the cathode. The performance was evaluated in terms of chemical oxygen demand (COD) removal and total kjeldahl nitrogen (TKN) removal for offering in situ treatment of aquaculture pond water. SMFC-2 demonstrated maximum COD and TKN removal efficiencies both in the absence and presence of aeration near cathode as compared to SMFC-1. With aeration at cathode, the COD and TKN removal efficiencies were 79.4% and 92.6% in SMFC-1 and 84.4% and 95.3% in SMFC2, respectively. Without aeration and at lower operating temperature, the COD and TKN removals were slightly lower, yet satisfying aquaculture quality norms. SMFCs demonstrated effective in situ remediation of aquaculture water and can drastically save the operating cost of aquaculture. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Over the last two decades, aquaculture industry is growing due to the increase in per capita consumption of fish products. Therefore, to increase aquaculture production, the industry is moving towards more intensive practices and supplementing protein-rich feed. The uneaten protein rich feed, dead phytoplankton, fish excreta and other metabolic wastes can produce high concentration of ammonia in the water (Chen et al., 1994) which lead to sediment deterioration causing toxic effects in the fish. Further, at higher temperature and pH, concentration of ammonia and nitrite increases whereas that of dissolved oxygen (DO) decreases and this could also pose serious threat to the fish health. In the aquaculture pond, total nitrogen (TN) exists in the inorganic forms of ammonium nitrogen (NH4 + -N), nitrite nitrogen (NO2 − -N), nitrate nitrogen (NO3 − -N), and also in many other forms of organic nitrogen. Nitrogen removal is essential for aquaculture water remediation for the potential reuse of this water and to maintain the fish free of diseases. Maintaining water quality enhances the fish yield and the fish will have good taste. Nitrate contaminated water released into the environment can create serious problems, such as eutrophication of water bodies (Sumino et al., 2006), deterioration of water quality and potential hazard to human or animal health.

∗ Corresponding author. Tel.: +91 3222 283440. E-mail addresses: [email protected], m [email protected] (M.M. Ghangrekar). 0144-8609/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aquaeng.2013.09.002

Constructed wetlands have been investigated as efficient solution for removal of nutrients from effluents (Lin et al., 2002). However, treatment by artificial wetlands outside the fish culture unit is not feasible for small farm holders because of high expenses on acquiring land and power required for pumping the water from pond to wetland. Similarly, other biological treatment or physicochemical treatment of this water is also not economical due to the cost involved in pumping of water from pond to the treatment system and operating cost of treatment system itself. Daily replacement of 5–10% volume of the system with new water may prevent accumulation of nitrate in aquaculture ponds (Masser et al., 1999), which will however lead to more water consumption. Increasing regulatory pressure focused on discharges of this used water to natural water bodies is forcing fish producers to adopt methods that are economical and environmentally friendlier. Sediments are important components of aquatic environment. The condition of pond bottom and the exchange of substances between soil and water strongly influence water quality of aquaculture pond (Boyd, 1995). The surface layer of sediments contain significant amount of pollutants such as organic matter, nitrogen, and phosphorus, thus potentially threatening integrity of the ecosystem (Beg et al., 2001). The oxidized layer at the sediment surface prevents diffusion of most toxic metabolites into the pond water. Ammonia and nitrite will be oxidized to nitrate, ferrous iron will be converted to ferric iron, and hydrogen sulfide will be transformed to sulfate by chemical and biological activity while passing through the aerobic surface layer (Boyd et al., 2002). Thus it is extremely important to maintain the oxidized layer at the sediment surface in aquaculture ponds.

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Microbial fuel cell (MFC) is the most promising approach to treat domestic as well as industrial wastewater along with generation of electricity (Liu et al., 2004; Ghangrekar and Shinde, 2008). Sediment microbial fuel cells (SMFCs) or benthic microbial fuel cells (BMFCs) are a special application of MFC to generate electricity from the electro-potential difference between oxic water and anoxic sediments present in the water body. An anode of such SMFC is embedded in pond sediment and the cathode is placed in overlying water. Microorganisms degrade organic compounds present in sediment and wastewater, generating electrons and protons. Electrons are transferred from anode to cathode through an external circuit and protons flow from sediment to the cathode side and combine with oxygen on cathode to produce water. Reimers et al. (2001) employed platinum mesh electrodes to produce current from both salt-marsh and estuarine sediments. Microbial electrochemical snorkel (MES), which is a simplified design of a short-circuited MFC, was reported to be successful to optimize wastewater treatment without producing any current (Erable et al., 2011). The intensive fish culture system often needs the introduction of artificial aeration systems (Boyd and Ahmad, 1987). However, for extensive aquaculture there is no need of aeration system. Hence, depending on the type of fish culture, oxygen may or may not be available through active aeration for cathodic reaction. However, even in the absence of active aeration, some amount of oxygen is always available near the cathode, due to passive aeration when cathodes are placed close to the water surface. In the competitive world today, it has become absolutely essential for the aquaculture projects to reduce production cost by minimizing the cost involved in external treatment cum recirculation of water and increase fish yield by maintaining water quality in the prescribed optimum limit. A good solution for in situ remediation of aquaculture water is the use of SMFC, which offers simultaneous treatment to the water while generating some electricity. Hence, this study was aimed to investigate the performance of SMFC for in situ aquaculture water remediation in terms of chemical oxygen demand (COD) and nitrogenous compounds removal. The performance of SMFCs was investigated with an external load resistance and short-circuited connection, with and without aeration at the cathode. To take into account variation in the performance due to seasonal changes in temperature, the performance was evaluated in the operating temperature range of 28–30 ◦ C and 21–25 ◦ C, respectively.

2. Materials and methods 2.1. SMFC configuration and operation Four SMFC experimental columns were constructed from a PVC cylinder, with three rectangular graphite plates working as anode and three plates making cathode. These electrodes had a total projected surface area of 1418 cm2 for anode as well as cathode, and each graphite plate had dimension of 21 cm × 10.5 cm × 0.5 cm. The graphite plates were attached together with a stainless steel nut and bolt and the gap between each plate was kept at 1 cm. The cylinder had internal diameter of 11 cm and height of 1.5 m (Fig. 1). The anodes were installed vertically in the sediment zone. The sediment was collected from existing aquaculture pond and filled in the experimental column up to a height of 50 cm from bottom. The cathode was positioned vertically in the oxic water at a nearest distance of 77 cm from anode top edge. The remaining volume of the cylinder was filled with used water from the practicing aquaculture pond. Three ports were provided to the cylinder at a distance of 25 cm centre to centre between them from top of column for collection of water samples along the height. The anode and the cathode were connected with concealed copper wire through external load

Air pump

Air stone

25 cm

25 cm

Water collecting ports

Cathode

R

25 cm

25 cm

50 cm

Pond sediment

Anode Fig. 1. Laboratory scale sediment microbial fuel cell used in the study.

of 100  in SMFC-1 and they were short-circuited in SMFC-2. Two experimental columns were operated as SMFC-1 and the remaining two were operated as SMFC-2. The results presented are average performance of the two SMFCs. For the experiments with aeration in cathode chamber, air was supplied through commercially available aquarium aerator (Zhongshan RISHENG Electrical Product Co. Ltd.) at a depth of 23 cm from top. Experiments were performed in a batch mode. Fresh feeding was given after achieving the water quality suitable for aquaculture. The performance of these SMFCs was evaluated under active aeration for the first 35 days. Later, they were operated under passive aeration at cathode. The aeration was stopped and the same anode and cathode were used after removing the biofilm developed on them during the earlier phase of cathodic aeration. Performance of these SMFCs was monitored for 22 days covering total four feed cycles under passive aeration at the cathode. Initially, the performance of these SMFCs was evaluated at ambient temperature in the range of 28–31 ◦ C with aeration and without aeration in cathode. Later, performance of these SMFCs was evaluated at a lower operating temperature, due to winter season, in the range of 21–25 ◦ C with aeration in cathode and without disturbing biofilm developed on the electrodes. A control experiment was also operated with and without aeration to evaluate performance without the presence of bio-electrode system. Control set up had no electrode system in it and was filled with the same quantity of sediments and aquaculture water as used in SMFCs. 2.2. SMFC inoculation The sediment was collected from the fish pond bottom having oxidizable organic matter (OM) of 2.1% (w/w). The SMFCs were filled with this sediment up to a height of 50 cm from bottom. No other culture was added in these SMFCs as inoculum and entire experiments relied on the presence of naturally occurring bacteria in the aquaculture sediments and water. Water was collected from 15-year-old operating aquaculture pond in IIT Kharagpur. The pond had a dimension of 14 m × 10 m × 1.5 m. Stocking density of 3 fish/m2 is being cultured in the pond. Fishes cultured were Rohu, Catla and Mrigal in the ratio of 1:1:1. Fish feeding rate of 5 kg per day is being adopted. The total kjeldahl nitrogen (TKN) concentration in the feed aquaculture used water collected from this pond was

T.K. Sajana et al. / Aquacultural Engineering 57 (2013) 101–107 Table 1 Composition of pond water. Pond water composition NH4 + -N concentration (mg/L) TKN concentration (mg/L) COD concentration (mg/L) DO concentration (mg/L) Conductivity (mS/cm) pH

1.2–3.2 3.8–5.2 97–103 5.3–5.8 0.29–0.32 7.6–7.7

in the range of 3.8–5.2 mg/L. The DO concentration and pH of the aquaculture water was in the range of 5.31–5.78 mg/L and 7.6–7.7, respectively. Chemical oxygen demand (COD) and the conductivity were in the range of 97–103 mg/L and 0.29–0.32 mS/cm, respectively. The composition of pond water is represented in Table 1. 2.3. Analyses and calculation Analysis of the parameters such as COD, TKN, ammonium nitrogen, nitrite nitrogen and nitrate nitrogen was done regularly according to APHA standard methods (APHA, 1998). The voltage and current were measured using a digital multimeter (RISH Multi 15S, India) and converted to power according to Ohm’s law, P = IV, where, P = power (W), I = current (A), and V = voltage (V). Power density (mW/m2 ) and current density (mA/m2 ) were calculated by dividing the power and current by the total anode surface area (m2 ). Polarization studies were carried out by varying the external resistance in the range from 100  to 30 k after allowing the circuit to stabilize for five to ten minutes at each resistance. Internal resistance of the MFC was measured from the slope of the plot of voltage versus current (Picioreanu et al., 2007). Organic matter of the aquaculture pond sediment was determined by Walkely and Black rapid titration method (Walkley and Black, 1934). 3. Results and discussion 3.1. Wastewater treatment 3.1.1. COD removal The control set up was operated during the experiment with and without aeration. The initial COD concentration in the influent aquaculture water was 103 mg/L. When the COD concentration in the water is less than 30 mg/L is considered optimal for fish culture and up to 45 mg/L is acceptable (Santhosh and Singh, 2007). Due to natural degradation of organic matter, in case of control experiment with aeration, the COD concentration was reduced to 26 mg/L after 17 days, and in the control without aeration, it was reduced to 29 mg/L after 22 days of operation. This indicates that

75 60 45 30 15 0

5 SMFC-1

10

15 20 Time (Days) SMFC-2

25 Control

30

b

105 COD concentration (mg/L)

COD concentration (mg/L)

90

0

without the presence of bio-electrode system, the water quality was remediated naturally and it took 17 and 22 days, respectively, for operation with and without aeration, to meet the water quality requirement for residual organic matter measured in terms of COD. SMFCs were operated under batch mode with aeration in the cathodic side for consecutive five feed cycles and without aeration for next four feed cycles. When operated with aeration, SMFC-1 had initial COD concentration of 103 mg/L which became 23 mg/L within 9 days during first feeding and for the next feed cycles, optimum level of COD was obtained within 6, 4, 4 and 4 days, respectively. SMFC-2 also showed similar results (Fig. 2a). When SMFC-1 was operated without aeration, the COD concentration reduced from 103 to 40 mg/L within 9 days, and for the next feed cycles, the optimal COD concentration was achieved within 5, 4, and 4 days, respectively. In SMFC-2, when operated without aeration, COD concentration came down from 104 to 30 mg/L within 9 days in first feeding, and for the next feed cycles, the days were reduced to 5, 4, and 4 (Fig. 2b) while meeting the required COD concentration for aquaculture. Comparing the performance of SMFCs with the control experiment, it is evident that less number of days (4 days instead of 17 or 22 days) of operation are required to meet the optimum limit for COD concentration when SMFCs are present. Hence, it is clear that SMFCs effectively offered in situ treatment to the aquaculture water for reduction of organic matter concentration and making it suitable for rearing fish. With aeration at cathode, in SMFC-1 and SMFC-2, the maximum COD removal efficiency of 80.84% and 85.8%, respectively, were obtained after 23 days of operation (at 4th feed cycle). COD concentration was reduced from 102 to 19 mg/L in SMFC-1, and 101 to 14 mg/L in SMFC-2. In case of aeration in cathode, the average COD removal efficiency was 79.4 ± 1.4% in SMFC-1 and 84.4 ± 1.3% in SMFC-2 (Table 2). When SMFCs were operated without aeration at cathode, the maximum COD removal efficiency of 60.7% (1st feed cycle) was obtained in SMFC-1 and 73.3% (2nd feed cycle) was achieved in SMFC-2. In SMFC-1, COD concentration was reduced from 103 to 40 mg/L, and in SMFC-2 it was reduced from 102 to 27 mg/L. Without aeration, the average COD removal efficiencies were 58.7 ± 2% in SMFC-1 and 72.5 ± 0.8% in SMFC-2. Microbial electrochemical snorkel (MES) technology as well as a short-circuited MFC is reported to enhance COD removal by 57% than a MFC connected with 1000  external resistance, confirming its promising potential for wastewater treatment. Higher substrate removal efficiency has been reported with short-circuited operation of MFC compared to resistance-connected MFC (Erable et al., 2011). In the present experiment, due to short circuited mode of operation, SMFC-2 demonstrated higher COD removal efficiency than SMFC-1 under both the operating conditions, i.e. with and without aeration at the cathode. As the external resistance was

a

105

103

90 75 60 45 30 15

0

5 SMFC-1

10 15 20 Time (Days) SMFC-2

25

30

Control

Fig. 2. Variation of COD with time in SMFC-1 and SMFC-2 (a) with aeration and (b) without aeration.

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negligible in SMFC-2, the electrons produced from the oxidation of organic matter at the anode could easily reach the cathode, enhancing the process kinetics. 3.1.2. Nitrogen removal The influent aquaculture water was monitored for nitrogen content in the form of TKN, ammonium nitrogen, nitrite nitrogen and nitrate nitrogen. The initial concentration of TKN in the aquaculture water was 3.89 mg/L. When SMFCs were operated with aeration in the cathodic side, average TKN removal efficiency of 92.6 ± 3.4% and 95.3 ± 2.9% were obtained in SMFC-1 and SMFC-2, respectively (Fig. 3). Without aeration, the average TKN removal efficiencies were 71.7 ± 2.9% in SMFC-1 and 79.9 ± 1.5% in case of SMFC-2. When both SMFCs were operated with aeration at cathode, the initial concentrations of ammonium-nitrogen (NH4 + -N) and nitrate nitrogen (NO3 − -N) in SMFC-1 and SMFC-2 were 1.22, 0.42, and 1.14, 0.42 mg/L, respectively (Table 3). The DO near the cathode was 5.7 mg/L for SMFC-1 and 5.8 mg/L in SMFC-2. During the first feeding after 9 days, NH4 + -N concentration reduced to 0.36 mg/L in SMFC-1 and 0.007 mg/L in SMFC-2. However, NO3 − -N concentration increased to 1.58 mg/L in SMFC-1 and 1.55 mg/L in SMFC-2. For the next 4 cycles, the required concentration of NH4 + -N was obtained within 6, 4, 4 and 4 days. This indicates that the days of operation were reduced due to better acclimation and biofilm formation on the electrodes in subsequent feedings. Total ammonium nitrogen concentration of less than 1 mg/L is tolerable for most of the fish species (Brune and Gunther, 1981). Further, the SMFCs were operated without aeration at cathode. The average DO concentration near cathode in SMFC-1 and SMFC-2 was 4 ± 0.1 mg/L due to natural diffusion of oxygen from air–water interface. The initial NH4 + -N concentration of 3.2 mg/L was present in both the SMFCs, whereas NO3 − -N concentration was 0.04 and 0.07 mg/L, respectively. Within 9 days of operation, the NH4 + -N concentration was reduced to 0.56 and 0.38 mg/L in SMFC1 and NO3 − -N concentration increased to 2.3 mg/L and 2.65 mg/L in SMFC-2. For the next three feed cycles, desired concentration of NH4 + -N was achieved within 5, 4 and 4 days. Above results show that the concentration of ammonium nitrogen was decreasing and that of nitrate nitrogen was increasing in case of both these SMFCs operated with and without aeration. This states that the nitrifying bacteria like Nitrosomonas and Nitrobacter were developed in both the SMFCs. Virdis et al. (2010) reported operation of MFC at different DO levels at the cathode. During the operation, at a DO value of 4.35 ± 0.08 mg/L, ammonium nitrogen level as low as 2.13 ± 0.05 mg NH4 + -N/L and NO3 − -N level of 1.0 ± 0.5 mg NO3 − N/L in the effluent were reported. At the same time, a higher DO of 5.02 ± 0.02 mg/L yielded lower values of ammonium nitrogen level at 1.71 ± 0.02 mg NH4 + -N/L. However, NO3 − -N level of 8.7 ± 0.7 mg NO3 − -N/L was reported. When SMFCs were operated with aeration, in SMFC-1 pH near the cathode and anode was varying from 8.4 to 8.6. Similarly, in SMFC-2 pH near the cathode was varying from 7.9 to 8.7, while the value was in the range of 7.8 to 8.6 near the anode. However,

when SMFCs were operated without aeration, pH near the anode and cathode were varying from 7.5 to 7.6, and 7.6 to 7.9, respectively, in SMFC-1. In SMFC-2, pH near cathode varied from 7.7 to 7.8, while that near anode it was in the range of 7.6–8.0. The optimum pH for Nitrosomonas and Nitrobacter is between 7.5 and 8.5 which supports that nitrification was occurring in the cathode. Nitrification also produces acid. This acid formation lowers the pH and it consumes alkalinity in the water. When SMFC-1 and SMFC-2 were operated with aeration, pH near the cathode was higher than that without aeration. Aeration provided oxidant to consume proton, leading to higher pH near the cathode. The pH difference between anodic and cathodic solutions in SMFC-2 was less compared to that in SMFC-1. It can be inferred that due to short circuit mode of operation in SMFC-2, better charge transfer might have supported minimizing pH imbalance in the system. The average water temperature was 30.4 ± 0.1 ◦ C in both SMFCs when operated with aeration, and 30.6 ± 0.6 ◦ C, when operated without aeration. Water temperature also affects the rate of nitrification. Nitrification reaches a maximum rate at temperatures of 30–35 ◦ C. When the SMFCs were operated with aeration, the average TN removal rate was 40.2 ± 0.2 mg/m2 d (removal efficiency of 44 ± 5.4%) in SMFC-1 and 43.5 ± 3.4 mg/m2 d (49 ± 8.3%) in SMFC-2. When operated without aeration, the average TN removal rate of 20.9 ± 3.4 mg/m2 d (27 ± 6.0%) was observed in SMFC-1, while that in SMFC-2 it was 31.4 ± 4.9 mg/m2 d (36 ± 7.8%). Puig et al. (2011) found that MFC with biofilm in the cathode removed nitrogen at the rate of 75.7 ± 12.4 g N/m3 d (removal efficiency of 30 ± 4%). The nitrogen removal efficiency observed in present experiment is in agreement with the values reported in literature under both the operating conditions, i.e. with and without aeration at the cathode. 3.2. Organic matter removal from sediment The sediment was collected from the bottom of fish pond having oxidizable organic matter of 2.1%. At the end of the experiment, oxidizable organic matter in the sediment was reduced to 0.64% and 0.58% in SMFC-1 and SMFC-2, respectively, when operated with aeration. These results indicate that the organic matter in the sediment was oxidized by the bacteria on the anode. Oxidation of organic matter present in the sediment was slightly more in the short circuited SMFC. The oxidized layer at the sediment surface layer prevents diffusion of toxic compounds into pond water, favouring better water quality suitable for fish growth. This oxidized layer at the bottom of pond can be maintained by employing SMFC. 3.3. Power generation As the used aquaculture water containing very low organic matter concentration, which means low energy content, was used in these experiments, electricity generation is not expected to be as high as that reported in the microbial fuel cell using

Table 2 Performance of SMFCs operated with and without aeration near cathode. With aeration SMFC-1 Average COD removal efficiency (%) Average TN removal efficiency (%) Average OCV (mV) Average sustainable power density (␮W/m2 ) Average maximum power density (␮W/m2 ) a

Without resistance (short circuit mode).

79.4 44 499 18.8 106.7

± ± ± ± ±

Without aeration SMFC-2

1.4 5.4 0.8 1.3 1.1

84.4 ± 49.4 ± 725 ± 0a 241 ±

1.3 8.3 1.3 1.8

SMFC-1 58.7 27 359 17.3 51.8

± ± ± ± ±

2 6 7.4 1.3 0.8

SMFC-2 72.5 ± 36 ± 711 ± 0a 98.4 ±

0.8 7.8 3.9 1.5

T.K. Sajana et al. / Aquacultural Engineering 57 (2013) 101–107

105

Table 3 Performance of SMFCs at different nitrogenous compound concentrations. Days of operation (days)

With aeration (all concentrations are in mg/L)

Days of operation (days)

Without aeration (all concentrations are in mg/L)

NO3 − -N

NH4 + -N

NO3 − -N

NH4 + -N

Initial

Final

Initial

Final

Initial

Final

Initial

Final

9 6 4 4 4

SMFC-1

1.22 1.16 0.75 1.16 0.76

0.36 0.41 0.30 0.35 0.07

0.42 0.35 1.90 0.35 0.80

1.58 1.15 3.76 2.80 3.87

9 5 4 4

3.2 2.8 2.6 2.5

0.56 0.98 0.97 0.98

0.04 0.82 0.94 0.93

2.30 1.98 2.62 3.23

9 6 4 4 4

SMFC-2

1.14 1.11 0.70 1.11 0.70

0.007 0.009 0.19 0.30 0.006

0.42 1.15 2.70 1.15 0.90

1.55 1.95 4.56 3.59 4.02

9 5 4 4

3.20 2.80 2.60 2.50

0.38 0.73 0.68 0.63

0.07 0.82 0.92 0.93

2.65 3.00 3.20 3.23

industrial wastewater containing higher concentration of organic matter. Also, the electricity recovery should not be compared with BMFCs due to far lower conductivity of aquaculture water as compared to sea water present in case of BMFC. However, the small amount of energy recovered in the form of direct electricity as byproduct during the in situ remediation of aquaculture water can be used for powering water quality sensors or water level indicators by integrating few SMFCs together. SMFC-1 and SMFC-2 started generating current from first day of operation. On the first day of operation, in SMFC-1 short circuit current of 0.049 mA and open circuit voltage (OCV) of 0.130 V were generated. Similarly, in SMFC-2, the short circuit current of 0.115 mA and OCV of 0.543 V were obtained on the starting day of operation. The current and voltage gradually increased with time in both these SMFCs. SMFC-1 generated a maximum OCV of 0.499 V after 7 days of operation. Similarly, SMFC-2 generated a maximum OCV of 0.726 V after 22 days of operation (Fig. 4a). Maximum short circuit current of 0.162 mA and 0.282 mA were observed in SMFC-1 and SMFC-2, respectively. In SMFC-1, the maximum sustainable power of 2.86 ␮W (power density 20.38 ␮W/m2 ) was observed at an external resistance of 100 . Due to short circuit operation, SMFC-2 showed better performance and had higher OCV than SMFC-1. Variation of OCV with time in case of SMFC-1 and SMFC-2 when SMFCs were operated with and without aeration is shown in Fig. 4. When SMFCs were operated without aeration on the first day of operation, in SMFC-1 and in SMFC-2, short circuit current of 0.048 and 0.064 mA and OCV of 0.167 V and 0.182 V were observed. Without aeration at cathode, SMFC-1 generated a maximum OCV of 0.365 V after 11 days of operation, and for SMFC-2, a maximum OCV of 0.715 V was observed after 20 days of operation (Fig. 4b). Even without aeration, SMFCs generated high OCV possibly due to the

a

60 40 20

0

Polarization study was carried out by varying external circuit load resistance from 30 k to 100 . Current generation showed decreasing trend with increase in the resistance, and voltage stabilization was relatively rapid at higher resistances. Similar power generation trend was reported in the literature, which indicated typical fuel cell behaviour (Jang et al., 2004; Oh and Logan, 2005). Voltage drop was very rapid at lower external resistance and it got stabilized comparatively faster at higher resistances. For SMFCs operated with aeration, in SMFC-1 when the resistance was reduced from 30 to 10 k, the voltage was reduced from 0.126 V (0.004 mA) to 0.107 V (0.011 mA); whereas, when the external resistance was reduced from 1000 to 100  the voltage rapidly dropped from 37 mV (0.037 mA) to 4.6 mV (0.046 mA). Similarly, in SMFC-2, the voltage was reduced from 0.521 V (0.017 mA) to 0.449 V (0.045 mA), when the resistance was reduced from 30 to 10 k and a rapid voltage drop was observed when the external resistance was reduced from 1 k (0.165 V, 0.165 mA) to 100  (0.022 V, 0.22 mA). Maximum power densities of 107.3 ␮W/m2 at external resistance of 900  in SMFC-1 and 242.1 ␮W/m2 at external resistance 100

80

0

3.4. Polarization and internal resistance

TKN removal efficiency (%)

TKN removal efficiency (%)

100

absence of mixing, i.e. minimum oxygen diffusion near the anode. The maximum sustainable power in SMFC-1 was 2.56 ␮W (power density 18 ␮W/m2 ) at 100 . SMFC-1 when operated with aeration gave better power production than without aeration, i.e. 10.5% more. This was due to the oxygen availability and mixing of water at the cathode. Mass transfer limitation for the electron donors to reach the anode due to lower concentration was a major limitation for sustainable power production from SMFC.

5

10

15 20 25 Time (Days)

SMFC-1

30

SMFC-2

35

b

80 60 40 20

0

5

10

15 20 25 Time (Days)

SMFC-1

30

SMFC-2

Fig. 3. TKN removal efficiency of SMFC-1 and SMFC-2 (a) with aeration (b) without aeration.

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a

Open circuit voltage (V)

0.7 0.6 0.5 0.4 0.3 0.2 0.1

0.6 0.5 0.4 0.3 0.2 0.1

0

5

10

15 20 25 Time (Days)

SMFC-1

30

35

SMFC-2

b

0.7

Open circuit voltage (V)

106

0

5

10

15 20 25 Time (Days)

SMFC-1

30

35

SMFC-2

Fig. 4. Open circuit voltage of SMFC-1 and SMFC-2 (a) with aeration (b) without aeration.

of 700  in SMFC-2 were observed. In SMFC-1, internal resistance was 840  and in SMFC-2 it was 616 . Due to larger distance between anode and cathode, the internal resistance of both the SMFCs was higher. However, SMFC-2 showed slightly lower internal resistance due to better charge transfer ability because of short circuit operation from the beginning. Polarization was also carried out in case of SMFC-1 and SMFC2, when they were operated without aeration. Maximum power densities of 52.23 ␮W/m2 (700 ) and 99.48 ␮W/m2 (600 ) were observed in SMFC-1 and SMFC-2, respectively. In SMFC-1, internal resistance of 675  was observed and it was 546  in SMFC-2. Surprisingly, SMFCs operated with aeration showed more internal resistance compared to that without aeration. It has been observed that rotating cathode in a river SMFC increased the oxygen availability to the cathode, and therefore improved the cathode reaction rate, resulting in a higher power production (49 mW/m2 ) compared to that in a non-rotating cathode system (29 mW/m2 ); which is 69% improvement over nonrotating cathode (He et al., 2007). The low rate of oxygen reduction on cathode was directly related to low output power densities of SMFCs (Song et al., 2011). During polarization the maximum power density of SMFC-1 operated with aeration was 105% higher (107.3 ␮W/m2 ) than that without aeration (52.23 ␮W/m2 ). This was due to the enhanced cathodic reaction rate when operated with aeration. In case of SMFC-2, operation with cathodic aeration gave 143% higher power density than an operation without aeration. With aeration, the pH difference between cathode and anode was negligible in SMFC-1 and SMFC-2. However, without aeration the pH difference was apparent. The pH difference between anodic and cathodic solutions was observed to change the internal resistance of MFC. Internal resistance decreases with increasing pH difference between anode and cathode solutions. When the pH difference was 2 units, the internal resistance of 523  and for zero units, internal resistance of 547  was reported earlier (Jadhav and Ghangrekar, 2009). Hence, more the difference in pH between anolyte and catholyte, more is the power output of MFC.

3.5. Effect of external resistance SMFCs were operated with an external resistance of 100  (SMFC-1) and short circuited electrode mode (SMFC-2). It was observed that during the experiments with and without aeration at cathode, the short-circuited SMFC (SMFC-2) showed higher COD removal efficiency when compared to SMFC with 100  external resistance (SMFC-1). TKN removal efficiency was also higher in short-circuited SMFC (SMFC-2) than the SMFC-1. At lower external resistance, the COD removal efficiency was more, and with increase in external resistance it decreased, as reported earlier by Jadhav and Ghangrekar (2009).

At lower resistance, the electrons move more easily through the circuit than at higher resistance, oxidizing electron carriers of the microbes in the anode. Higher fuel oxidation by the microbes is expected with higher ratio of oxidized electron carriers in the culture at a lower resistance. Therefore, the MFCs can be operated at lower resistances to remove the organic matter at a higher rate (Jang et al., 2004). Enhanced rate of electron discharge at lower resistances might be responsible for rapid voltage drop and slow stabilization of the voltage at lower resistances. 3.6. Effect of temperature on the performance of SMFC During winter season, the ambient temperature drops from 28–30 ◦ C to 21–25 ◦ C. When SMFCs were operated with aeration at this lower temperature, the average COD concentration in SMFC-1 was reduced from 90 ± 3 mg/L to 40 ± 1.56 mg/L and in SMFC-2; it was reduced from 90 ± 3 mg/L to 30 ± 0.5 mg/L. The average TKN removal efficiencies of 68 ± 2.7% and 87 ± 1.58% were obtained in case of SMFC-1 and SMFC-2, respectively. The TKN removal efficiencies at lower temperatures were 24.6 ± 0.7% and 8.3 ± 1.32% less as compared to operation at higher temperature for SMFC-1 and SMFC-2, respectively. A maximum OCV of 0.751 V and short circuit current of 0.3 mA were observed in SMFC-1 at lower operating temperature. Similarly, in SMFC-2, the maximum OCV of 0.875 V and short circuit current of 0.365 mA were observed. SMFC-1 and SMFC-2 operated at lower temperatures showed higher OCV compared to the values at higher temperature. However, this lower operating temperature proved that, the electrochemically active bacteria could remain active even at lower temperature (Jadhav and Ghangrekar, 2009). Capability of MFC converting substrate at lower temperature below 20 ◦ C has also been reported earlier (Pham et al., 2006). 3.7. SMFC in aquaculture Excellent COD removal efficiency and the capability to remove ammonium nitrogen suggest that SMFC can be used effectively for in situ remediation of aquaculture water. The decomposition of nitrogenous compounds is particularly important in aquaculture water. As the concentration of ammonium nitrogen is reduced with the use of SMFC, the current practice of water exchange can be avoided, thus saving the pumping cost involved in external treatment of this water as well as recirculation. Also, oxidized layer at the pond bottom sediments is maintained by SMFC which in turn helps in preventing diffusion of toxic gases into the pond water. SMFC can also be used to effectively reduce the oxygen demand of bottom sediments before starting the next cycle of fish farming. Colt (1987) determined that the oxygen requirement for 1 kg of feed as 0.25 kg of oxygen. Paddle wheel aerators have a standard

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aeration efficiency of 2.7 kg O2 /kW h (Ahmad and Boyd, 1988). In case of 1 ha pond having 15,750 kg of Catla fish, 473 kg of feed is required approximately. The power required per day for the oxidation of organic matter and nitrification using external treatment system for this aquaculture used water is 3.65 kW. This power can be saved if SMFC is used. Water exchange is the method that is presently used for maintaining ammonium nitrogen concentration in aquaculture ponds. The power required for pumping water for 6 months is approximately 820 kW/ha pond area. By employing SMFC, water exchange can be avoided, thereby resulting in considerable savings in pumping cost as well as consumption of water. Although sediment MFCs are reported earlier to be successful in marine water (Reimers et al., 2001), the present experiments demonstrated successful implementation of SMFCs in water with low conductivity (0.3 mS/cm) containing relatively very low concentration of pollutants. But the power output of such SMFCs is expected to be very low due to low concentration of pollutant and low conductivity of the medium and may not be sufficient to power any device, particularly in case of small ponds. However, the results of these experiments are encouraging for in situ remediation of aquaculture pond water. This will drastically reduce the cost of treatment apart from saving the pumping cost. There is still a lot to be learnt in the scale up of SMFC for large-scale field applications. Further, the performance of SMFC under different field conditions such as with variation in pH of water, depth of pond, and presence of cellulose in sediment needs further investigation for full scale application. 4. Conclusions The effects of aeration at cathode, external resistance, and operating temperature were evaluated on the performance of SMFCs. SMFCs operated with aeration showed higher COD removal efficiency, TKN removal efficiency, removal of NH4 + -N, and power density than SMFCs operated without aeration. However, SMFC in short circuit operation showed better results than SMFC operated with external resistance. SMFCs operated with aeration at lower operating temperature also gave better COD and TKN removal efficiencies. The COD, TKN and nitrogenous compounds removal demonstrated by SMFC is encouraging for in situ remediation of aquaculture pond water to make overall operation of aquaculture economical. However, for the full scale application of SMFC in the aquaculture pond, more experiments must be conducted on the factors affecting the performance of SMFC such as temperature, pH of the aquaculture water, algal growth in the pond, distance between electrodes, anode to cathode surface area ratio, electrode materials, and stocking density of fish in the pond. References Ahmad, T., Boyd, C.E., 1988. Design and performance of paddle wheel aerators. Aquacultural Engineering 7, 39–62. APHA, AWWA, WPCF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC. Beg, M.U., Al-Muzaini, S., Saeed, T., Jacob, P.G., Beg, K.R., Al-Bahloul, M., Al-Matrouk, K., Al-Obaid, T., Kurian, A., 2001. Chemical contamination and toxicity of

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