INTRODUCTION:
The twentieth century saw the biggest increase in the population worldwide [1]. Recently, India has crossed the figure of 121 billion in terms of population. This will naturally increase the consumption of energy as it is the paramount importance for an industrialized economy. Currently, to fulfill the needs of energy, people depend mainly on the fossil fuels i.e. on coal and natural gas. A robust growth in the consumption of the fossil fuel has been seen and this was reported in the annual report i.e. Statistical Review of World Energy, published on June 2011. The consumption of coal, oil and natural gas is found to be increased by 7.6%, 3.1% and 7.4% respectively in the year 2010 as compared to the previous year [2]. It is also predicted that the global average temperature will rise from 1.4 to 5.8 °C by year 2100 and continue to rise long after that [3]. This rise is mainly due to the rapid increase in the emission of greenhouse gases
(CO 2 and others) in the atmosphere, which are emitted by the burning of the fossil fuels [4].
To fulfill the energy demand is not the only problem linked with the increase in the population. Beside this, waste generation is also a problem. In India, daily tonnes of waste are generated. Daily 6000 tonnes of waste is generated alone in Delhi and only 62% of the total waste is recycled as against 5,800 tonnes in Mumbai, 2,800 in Bangalore, 2,675 tonnes in Chennai and 4,000 tonnes in Kolkata [5]. Wastes may be of two types i.e. solid waste and liquid waste and both are creating problems as the disposal of the waste is causing nuisance to the society. Water which has been adversely affected by the influence of anthropogenic sources is considered as wastewater [6]. Waste management [7] is becoming one of the key problems of the modern world, an international issue that is intensified by the volume and complexity of domestic and industrial waste discarded by society.
Agra, the city of Taj is also not devoid of this waste problem. Taj city is not only famous for Taj Mahal but also for the petha sweet prepared in the narrow streets of Noori Darwaja. This sweet is prepared from the fruit 'Ash Gourd', the botanical name of which is Benincasa hispida and is commonly known in India as Petha. This has also been named as Kushmand in ancient ayurveda, and is believed to have remarkable curative properties [8]. There are around 500 petha industries units running in Agra [8 10]. These petha industries generate about 30 35 tonnes of solid waste daily [11 12] and uses coal for processing the sweet.
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Supreme Court has banned the use of coal in Agra but most of these units are using it. To avoid the effect of burning of coal (used by petha industry) Development Authority wants to relocate the petha industries at the outskirts of the city [9 & 13]. In addition to this, petha industries use lime water to wash the petha fruit and then drain it as such untreated. This wastewater discarded is highly alkaline. Clean Technology Initiative (CTI) project started a “Waste to energy” [12 & 14] program to generate methane gas from the petha waste which can replace the coal for petha making. But it seems that now days this process is not in action as tons of waste is found nearby these units and create problem to the local people.
Biofuel production [15 20] from the biodegradable waste [21] plays the dual role as this can be used for both i.e. waste disposal and for energy production. Biological processes are considered as the clean and sustainable method to produce hydrogen. There are mainly three methods by which hydrogen can be produced biologically i.e. biophotolysis of water, photofermentation and dark fermentation [22]. In direct biophotolysis, cyanobacteria decompose water to generate hydrogen and oxygen in the presence of light. In photofermentation [23 24], anoxygenic photoheterotrophic bacteria utilize organic feedstock to produce hydrogen in the presence of light. In dark fermentation (DF) also known as anaerobic degradation [25 26], anaerobic heterotrophic bacteria utilize organic feedstock without any light to produce hydrogen [22, 27
29]. To stabilize the waste and convert it into energy anaerobic degradation [30 33] is the process which is mainly used and it acts as the most promising method for hydrogen production [34]. This process is mainly classified into four steps i.e.
Hydrolysis [35] : A chemical reaction where particulates are solubilized and large polymers converted into simpler monomers;
Acidogenesis [36] : A biological reaction where simple monomers are converted into volatile fatty acids;
Acetogenesis [37] : A biological reaction where volatile fatty acids are converted into acetic acid, carbon dioxide, and hydrogen; and
Methanogenesis [38 & 39] : A biological reaction where acetate and hydrogen are converted into methane and carbon dioxide, while hydrogen is consumed.
Anaerobic digestion is mainly based on two important steps i.e.
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Biohydrogen generation [40] :
C6H12 O6 + 2H 2O 4H 2 + 2CH 3COOH + 2CO 2
Methane formation [41] :
CO 2 + 4H 2 CH 4 + 2H 2O
CH 3COOH CH 4 + CO 2
Both hydrogen and methane are important fuel but hydrogen has more advantage over methane as it is clean, safe, renewable and high calorific value [42]. So to increase the production of the hydrogen some pretreatment methods are used to block the reaction at the methanogenesis step and hence the production of hydrogen is increased. Acetate can also be converted into hydrogen and carbon dioxide.
There is another method to produce hydrogen, which is coupled with the electricity production using microbial fuel cell (MFC) in which chemical energy changes into the electrical energy under anaerobic conditions [43 45]. Electrons produced by the bacterial oxidation of the organic matter present in the waste are transferred to the anode (negative terminal) and flow to the cathode (positive terminal) linked by a conductive material containing a resistor, or operated under a load (i.e., producing electricity that runs a device) [46].
Chemical mediators, such as neutral red or anthraquinone 2,6 disulfonate (AQDS), can be added to the system to allow electricity production by bacteria unable to otherwise use the electrode [47]. If no exogenous mediators are added to the system, the MFC is classified as a “mediator less” MFC even though the mechanism of electron transfer may not be known [48].
On the basis of the design, MFCs are classified into two types i.e. single chambered microbial fuel cell (SCMFC) in which cathode and anode are present in the same chamber and dual chambered microbial fuel cell, anode and cathode are separated in two chambers which are separated by a permeable membrane used for proton exchange like Nafion, ultrex etc. [47 & 49].
Related to MFC, microbial electrolysis cell (MEC) partially reverses the process to generate hydrogen or methane from organic material by applying an electric current.
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LITERTAURE CITED:
Biofuel production from the waste (solid and liquid) has been studied by different investigators and they have reported the various results. The effect of the various inoculum pretreatment methods on the production of the biofuel was also reported. In addition to this the working of the microbial fuel cell was also studied by several researchers and the effect of the variations in the components of the cell has been studied recently. Some of these studies are discussed below.
Zhu et al. [50] studied the characterization and the biohydrogen production using food waste (FW), primary sludge (PS) and the waste activated sludge (WAS). The different combinations of the waste was studied (FW+PS, FW+WAS and FW+PS+WAS) and in different ratio. The mixing ratio of 1:1 was found to be the best as compared to the other ratios studied. A hydrogen yield of 112 mL/g volatile solid (VS) added was obtained from a combination of FW, PS and WAS. This yield was equivalent to 250 mL/g VS added if only FW contributed to hydrogen production.
Hydrogen production from the corn syrup waste and the effect of the organic loading rate on the hydrogen production has been studied [51]. Methanogenic activity was also inhibited by heating 0 the mixed culture at 70 C for 30 minutes. Hydrogen production rate increased from 10 L H 2/L·d to 34 L H 2/L·d with the increase of organic loading rate (OLR) from 26 to 81 gCOD/L·d, while a maximum hydrogen yield of 430 mL H 2/gCOD was achieved in the system with an overall average of 385 mL H 2/gCOD .
In another study the thermophillic anaerobic hydrogen production using kitchen waste was estimated [52]. The production of the hydrogen in the different seasons was studied as the chemical oxygen demand varies in the different seasonal dietary habits. The hydrogen 1 1 production rate and yield at volumetric loading rate (VLR) of 28 g COD L day were 1.0 L H2 1 1 1 L day and 1.7mmol H2 g COD respectively, which were higher than at VLR of 19 g COD/L day.
Gomez et al. studied the hydrogen production through different methods [53]. Dark fermentation has two main advantages: fulfilling requirements for mild operational conditions and gaining benefit from the residual biomass. It generated a hydrogen yield of 33%. Photofermentation and
4 bioelectrochemical systems (BES) are the processes capable of converting fermentation sub products (like acetic acid) into hydrogen.
Hydrogen production in the combined light and dark conditions from the ground wheat powder solution (10g L 1) has also been reported by some researchers [54]. The highest cumulative 1 hydrogen formation (CHF = 3460 ml), hydrogen yield (201 ml H 2 g starch) and formation rate (18.1 ml h 1) were obtained with a starch loading rate of 80.4 mg S h 1.
The biohydrogen production from the dairy wastewater and the effect of the organic loading rate on the production of the hydrogen was examined by Mohan et al. [32]. Hydrogen evolution rate differed significantly with the substrate/OLR of wastewater used as substrate [OLR 2.4 Kg 3 3 COD/m day volumetric hydrogen production rate: 0.3683 mmol H 2/m min; specific hydrogen 3 production rate: 0.0184 mmol H 2/min gCOD L; OLR 3.5 Kg COD/m day volumetric hydrogen 3 production rate: 1.105 mmol H 2/m min; specific hydrogen production rate: 0.0245 mmol 3 H2/min gCOD L and OLR 4.7 Kg COD/m day volumetric hydrogen production rate: 0.7367 3 mmol H 2/m min; specific hydrogen production rate: 0.0107 mmol H 2/min gCOD L].
Guo et al. reviewed the hydrogen production from the agricultural waste in the dark fermentation [29]. The report states that the hydrogen is one of the most promising candidates as a substitute for fossil fuels. Three categories of agricultural residue have been considered: (i) the waste directly generated from agricultural production (ii) animal manure and (iii) food waste. It has been shown that all three possess great potential as a substrate for hydrogen production by dark fermentation, in decreasing order as food waste, crop residues and livestock waste.
Ginkel et al. studied wastewaters obtained from four different food processing industries that had chemical oxygen demands of 9 g/L (apple processing), 21 g/L (potato processing), and 0.6 and 20 g/L (confectioners A and B respectively) [55]. Hydrogen gas production was generally in the range of 5–11%. Overall hydrogen gas conversions were 0.7–0.9 L H2/L wastewater for the apple wastewater, 2.1–2.8 L H2/L for the potato wastewater , 0.1 L H2/L for confectioner A and
0.4–2.0 L H2/L for confectioner B. When nutrients were added to samples, there was a good correlation between hydrogen production and COD removal, with an average of 0 .10±0.01 L
H2/g COD. Gas produced by a domestic wastewater sample (concentrated 25×) contained only 23±8% hydrogen.
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The fermentative hydrogen production in the dark and acidic conditions was investigated and it was found that the bacterial culture is capable of producing hydrogen at low pH (LpH) i.e. 3.3 to 4.3 [56]. The composition of the hydrogen in the headspace ranging from 50% to 60%, conversion efficiency of ~43% achieved in LpH systems is comparable to that of other buffered systems.
Another study shows the effect of temperature and pH to enhance the hydrogen production [57]. High hydrogen gas concentrations (57 72%) were produced in all tests and it was found that the heat treatment (HT) of the inoculum (pH 6.2 or 7.5) produced greater hydrogen yields than low pH (6.2) conditions with a nonheat treated inoculum (NHT). Conversion efficiencies of glucose to hydrogen (based on a theoretical yield of 4 mol H2/mol glucose) were as follows: 24.2% (HT, pH = 6.2), 18.5% (HT, pH =7.5), 14.9% (NHT, pH = 6.2), and 12.1% (NHT, pH = 7.5).
Besides using thermophillic reactors conditions, mesophilic condition also helps in treating the wastewater and lead to the production of the biogas [58]. A lab scale high rate mesophilic anaerobic contact reactor was operated with wastewater originated from a potato processing plant, at six different loading rates of 1.1 to 5 g COD/L per day. COD removal efficiencies were found to be 78–92% and the methane percentage of the biogas produced was 80–89%.
Additionally, the methane yield coefficient was found to be 0.394 L CH 4/gTCOD rem .
Sugar beet processing wastewater and beet pulp batch reactors were operated under mesophilic conditions. Food to microorganisms (F/M) range studies (0.51–2.56 g COD/g VSS) observed treatment efficiencies i.e. 63.7–87.3% COD removal and 69.6–89.3% VS reduction which indicates high biodegradability for both wastewater and beet pulp, which decreased with increasing F/M [59]. The co digestion of the wastes was evaluated, and it was concluded that, major outcome of wastewater addition was to increase methane production rate of beet pulp, rather than increasing its ultimate biodegradability.
In a study the ratio of the kitchen waste (KW) to mixed culture (MC) was varied and the production of hydrogen was monitored [60]. In 80:20 (KW:MC) ratio, the biohydrogen significantly increased with time as compared to the 60:40 (KW:MC) and 50:50 ratio (KW:MC). The maximum biohydrogen production in 80:20 batch reactors at 8th day was 13.3 % and the reduction rates in physical parameters were: TS 49.25 %, TDS 68.75 %, TSS 55.83 % and COD 35.75 % also studied.
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Sivaramakrishna et al. evaluated the hydrogen production in the thermo acidophilic conditions from the rice bran de oiled wastewater [61]. The batch reactor was operated at 57 0C and at pH 6 and an increase in the production of hydrogen (1861±14ml/L WW/d) as compared to the reactor operated at 37 0C (651±30ml/L WW/d) was observed.
In contrast to the results reported above Cai et al. investigated the production of hydrogen from the sewage sludge using alkaline treatment and studied the increase in hydrogen yield from 9.1 mL of H 2/g of dry solids (DS) of the raw sludge to 16.6 mL of H 2/g of DS of the alkaline pretreated sludge [62]. Methane was absent and low amount of carbon dioxide (0.8% of control) was present in the biohydrogen production from the alkaline pretreated sludge.
Benyi et al. studied the possibility of hydrogen production from alkali pre treated sludge without seed [63]. The maximum hydrogen yield was obtained at initial pH value of 11.0 (14.4 1 mL·g VS ). This study concludes that the hydrogen production is much more at alkaline pH as compared to the production in acidic or in neutral conditions.
Recent research has demonstrated that orange peel waste represents a potentially valuable resource that can be developed into high value products i.e. biogas, acids etc [64]. The applications described together with those that will no doubt be developed in the future, represent great opportunities to harness the economical benefit of this agro industrial waste and to develop even more efficient and sustainable systems.
There is a study to examine the effects of organic loading rate and hydraulic retention time on volatile fatty acid composition and treatment efficiency of high rate thermophilic anaerobic contact reactor (TACR) treating potato chips wastewaters [65]. The COD removal efficiencies were found to be 86–97% and the methane percentage of the biogas produced was 68–89% during the OLRs studied. Acetate, propionate, iso butyrate, butyrate, iso valerate, valerate, iso caproate and caproate was found as the intermediates in the anaerobic digestion of the wastewater.
The hydrolysis and acidification of sugar beet processing wastewater and beet pulp for volatile fatty acid (VFA) production through acidogenic anaerobic metabolism has been studied [66]. Reactors were operated at 35 ± 1 °C with different combinations of HRT (2–4 days), wastewater
7 pulp mixing ratios (1:0 to 1:1, in terms of COD) and pH ranges (5.7 to 7.5). Due to increase in the acidification process, production of VFA in the reactors takes place and hence pH is lowered (5.7 to 6.8). In this pH range, methanogenic activity was successfully inhibited and the lowest methane percentages (5.6 to 16.3%) were observed in the produced biogas.
Production of the organic acids from simulated kitchen waste and original kitchen waste was compared [67]. The maximum organic acids produced in the both kitchen waste and model kitchen waste were 48.64g/L and 37.49g/L respectively at pH 5 and 37°C. Lactic acid was found dominant in the both condition followed by acetic and butyric and their percentage production were 76.2%, 17.7% and 6.1% respectively.
Hu et al. investigated the effect of chloroform pretreated methanogenic granules on the production of hydrogen [68]. They stated that the possibility of integrating immobilized hydrogen fermentation using chloroform treated granules with immobilized methane production using untreated granular sludge. The results showed that the integrated batch cultures produced 1.01 mol hydrogen and 2 mol methane per mol glucose.
The effect of Ni 2+ (0–10 mmol/l), Fe 2+ (0–200 mmol/l) and Mg 2+ (0–15 mmol/l) concentration on photo hydrogen production from acetate was investigated by batch culture [69]. When Ni 2+ and Fe 2+ concentrations were 4 mmol/l and 80 mmol/l respectively, maximal hydrogen yield of 2.87 0 and 2.78 mol H 2/mol acetate was obtained in a batch culture at 35 C with initial pH 7.0. There was no remarkable effect of Mg 2+ on the biohydrogen production.
The impact of the chemical components of the organic waste on the production of the lactic acid was investigated by Ohkouchi et al. [70]. It was found that the bioconversion of sugars to lactic acid was affected by the ratio of total sugars to total nitrogen content (the TS/N ratio), and was improved by nitrogen supplementation to adjust the TS/N ratio upto 10. Lactic acid yield was also affected by the fermentable sugars contents, i.e. various oligosaccharides constituted of mainly C6 sugars.
Lata et al. extracted the organic content from vegetable market waste and tea waste in a packed digester for 24 and 300 h respectively [71]. The sequence was (Acetic, Propionic) > (Isobutyric, Butyric) > Valeric for digestion of vegetable market waste while it was Isovaleric > (Isobutyric, Acetic) > Propionic during digestion of tea waste.
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Primary sludge (PS) was given alkaline pretreatment and the mechanism of the accumulation of short chain fatty acids (SCFAs) was investigated [72]. The maximum ratio of acetic, propionic, iso butyric, n butytic, iso valetic and n valetic acids was determined as 49.4%, 34.4%, 14.6%, 12.2%, 17.9% and 6.3% respectively. The maximum SCFAs yields from PS fermentation was 312.9mg COD/g VSS at pH 10.0–11.0 after 5 days reaction time, which was 1.8 times of that at neutral and acid pHs.
Guo et al. studied the effect of the sterilization, sonication and microwave on the hydrogen production and found the maximum hydrogen production i.e. 15.02 ml/gTCOD in the sterilization pretreatment given at 121 0C for 20 minutes [73]. The sludge was sonicated at the power density of 2 W/ml using ultrasonication probe for 5 minutes which leads to the 4.68 ml/gTCOD of hydrogen production. For the microwave pretreatment, sludge was heated for 2 min at the power of 560 W and the hydrogen produced was 11.44 ml/gTCOD.
In another study, the sludge was sonicated for 15 min. using the probe sonicater supplying the power density of 0.51 W/mL and the sonication intensity of 4.8 W/cm 2 [74]. This enhanced the production of the methane due to the increase in the rate of the anaerobic digestion of the waste.
The effect of sonication on the biodegradability and flowability was also investigated by a group of researchers [75]. In this study the wastewater sludge was sonicated by using the ultrasound of frequency 20 KHz and the power intensity of 0.75 W/cm 2 for 60 min. An increase in the soluble chemical oxygen demand and biodegradability by 45.5% and 56% respectively, in terms of total solids consumption has been observed.
Yan et al. reported the enhancement of the waste activated sludge (WAS) hydrolysis using ultrasonication and its effect on the production of the volatile fatty acids (VFA) [76]. The ultrasonic pretreatment of WAS was performed with an ultrasonicator (US Sonics, VCX 105) at a frequency of 20 kHz and time of 10 min. The energy density was controlled at 0.25, 0.5, 1.0, 2.0 and 4.0 kW/L in reactors 1 to 5, pH of these reactors were adjusted to 10.0 by adding 2 M sodium hydroxide (NaOH) or 2 M hydrochloric acid (HCl). The maximal VFA accumulation (3109.8 mg COD/L) occurred at ultrasonic energy density of 1.0 kW/L and fermentation time of 72 h, which was more than two times that without ultrasonic treatment (1275.0 mgCOD/L).
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Microwave assisted alkali pretreatment method was used to improve the saccharification in enzymatic hydrolysis and hydrogen yield in combined dark and photo fermentation [77]. Microwave heating was conducted in a microwave digestion system at a frequency of 2.45 GHz had a maximum power of 1000W with 10 discrete settings, and at a highest working temperature of 235 0C and working pressure of 3.5 MPa. Microwave heating was designed at 140 0C (working pressure: 0.2 0.6 MPa) for 15 min and the maximum hydrogen yield was greatly enhanced to 463 ml/g TVS, which is 43.2% of the theoretical hydrogen yield.
Microwave heating was found to be a rapid and efficient process for releasing organic substrates from sludge [78]. Sludge microwave heating was performed in batch tests using 30 mL of sludge in a 70 mL PTFE vessel. All test samples were subject to microwave heating at temperatures of 80, 120, 150 and 170°C. The microwave heating holding times were 1, 5, 10, 20 and 30 min. The total biogas increased by 12.9% to 20.2% over control after 30 days of digestion using microwave treated mixture sludge. Biogas production was 11.1% to 25.9% higher for excess sludge than for untreated sludge.
MFCs (microbial fuel cells) are used for the production of hydrogen from the organic waste which is coupled with the electricity production. Initially, the concept of the generation of electricity was given by M.C. Potter [79]. He stated that “the disintegration of organic compounds by microorganisms is accompanied by the liberation of electrical energy. These fuel cells are based on the metabolic activities of the microorganisms on the organic substrates particularly the natural sources which contain the sugars as the main component.
Wei et al. investigated the production of hydrogen and electricity liberation using starch wastewater with a proton exchange microbial fuel cell (PEMFC) [80]. The maximum hydrogen production and the power output reported by these researchers was 186mL/g starch and 0.428 W respectively.
The power generation in a single chambered microbial fuel cell was also explored [81] and it was found that the maximum coulombic efficiency (CE) and power density taking place at initial concentration of 300 mg COD/L were 8.16% and 68.5 mW/m 2 respectively.
Kim et al. compared the production of electricity in microbial fuel cell using ethanol and methanol and run the cyclic voltammetry (CV) to demonstrate the electricity production is
10 coupled with the oxidation of the ethanol [82]. Electricity production was estimated in the two chambered MFC using ethanol with the power density of 40+2mW/m 2 and a coulombic efficiency (CE) ranging from 42% to 61%. The culture was then transferred to the single chambered MFC and an increase in the power output was observed i.e. 488+12mW/m 2 (CE=10%) with ethanol.
Production of electricity from the different organic substrate (acetate, ethanol and glucose) with concentration range of 0.5 35mM of was investigated in single chambered microbial fuel cell (SCMFC) [83]. The power density generated from the glucose, acetate and ethanol was 401 mW/m 2, 368 mW/m 2 and 302 mW/m 2 respectively. It was also observed that the increase in the voltage output was in the range of 0.5 20mM substrate concentration but it significantly decreases at higher concentration level.
Microbial fuel cell was examined to treat the biodiesel waste (BW) and electricity generation was studied simultaneously [84]. The production of electricity was compared on the basis of the medium used i.e. 50mM phosphate buffer solution (PBS) and glycerin. In the PBS medium, the power density output was 487 ± 28 mW/m 2, cathode (1.5 A/m 2 cathode) and in the glycerin medium it was 533 ± 14 mW/m 2. In this study, two types of electrodes was used i.e. carbon cloth and carbon brush. The power density increased from 778 ± 67 mW/m 2 cathode using carbon cloth to 1310 ± 15 mW/m 2 cathode using carbon brush as anode which was further increased to 2110 ± 68 mW/m 2 cathode using the heat treated carbon brush anode in 200 mM PBS electrolyte.
Min et al. studied the effect of the membrane and salt bridge on the electricity production and found that the power output was high while using proton exchange membrane i.e. 40+1mW/m 2 for G. metallireducens and 38+1mW/m 2 for the wastewater inoculum [85]. In case of salt bridge, 2.2mW/m 2 power output was reported and it is mainly due to the high internal resistance in the salt bridge.
MFC has the capability to convert the glucose into electricity at high rate and efficiency [86]. Microbial culture was enriched to investigate the power output. Electron recovery in terms of electricity up to 89% occurred for glucose feeding rates in the range 0.5–3 g l −1 d−1, at powers up to 3.6 W m −2 of electrode surface. In addition to this, performance of the MFC was enhanced by using two different cathode operation modes i.e. aerated and open air cathode [87]. The
11 relatively stable average output voltage and the maximum power density in the aerated cathode operation was obtained as 45mV and 0.45 mW/m 2 respectively and the corresponding values obtained in much shorter time during the open air cathode operation were 80 mV and 0.77mW/m 2.
Performance of the air cathode MFC using wood charcoal was studied by Chai et al. [88]. The air cathode was build with finely milled charcoal powder and cement plaster as binder; while anode was made up of a packed bed of charcoal granules. The MFC generated a stable power density at 33mW/m 2 (0.5V) under a load of 200 after 72 hours of operation. An open circuit voltage (OCV) of 0.7mV was obtained after 15 hours operating under open circuit.
The capability of the experimental systems used in two chambered microbial fuel cell was tested in terms of repeatability and reproducibility [43]. For N = 4 replicates, these differences were set to 9.0% COD R units, 261 mV and 63 mg/L in VFAs for open circuit experiments and 3.6%,
30.2mV and 45 mg/L in closed circuit experiments. There is an intrinsic lack of repeatability in the experiments derived from the large variability in the microbial communities while using mixed culture.
Using nafion solution, cathode binding agent was studied in MFC and compared with the MFC working without using nafion solution [89]. The current output of both types of MFCs was monitored as a function of time using an external resistor. The current did not change much with time and was higher for the water cell (WC) than for the Nafion cell (NC). The maximum power concentration produced by the WC was about 1.8 W/cm 3 compared to 1.2 W/cm 3 for the NC.
Cusick et al. investigated microbial fuel (MFCs) and electrolysis cells (MECs) to recover energy directly as electricity or hydrogen from fed winery or domestic wastewater [90]. Winery wastewater fed MFCs produced a maximal voltage of 441+17 mV (1 k ) and electrical energy at 31.7+2.1 Wh/m 3, compared to MFCs fed domestic wastewater that produced 381+10 mV and 22.5+1.9 Wh/m 3. At an applied voltage of ~0.9 V, winery wastewater fed MECs produced ~0.9 3 3 mA of electrical current and hydrogen at a maximum flow rate of 0.17+0.01 m H2/m d, with a coulombic efficiency of 50+8% and TCOD removal of 47+3%. The gas composition for the winery waste MECs was 70+8% hydrogen, 26+7% carbon dioxide, and 4.3+ 3% methane. MECs fed domestic wastewater briefly produced more than 1 mA of electrical current and the coulombic efficiency was 64+9% with a TCOD removal of 58+3%. The gas produced by the
12 domestic wastewater fed MECs (70+1% hydrogen, 28+2% carbon dioxide, and 2.5+0.6% methane) was similar to that produced using winery wastewater.
MFCs at room or higher temperatures (20–35 0C) are relatively well studied compared to those at lower temperatures [91]. MFC performance was examined over a temperature range of 4–30 0C MFCs initially operated at 15 0C or higher all attained reproducible cycles of power generation, but the startup time to reach stable operation increased from 50 h at 30 0C to 210 h at 15 0C. However, reproducible cycles of power generation could then be achieved at even the two lowest temperatures of 4 0C and 10 0C. Power production increased linearly with temperature at a rate of 33±4mW/0C, from 425±2mWm −2 at 40C to 1260±10mWm −2 at 30 0C.
The different factors such as electrode surface area and reactor geometry relative to solution conditions such as conductivity and substrate concentration was scaled up to enhanced the working of the MFC [92]. The substrate concentration has significant effect on anode but not on cathode performance, while the solution conductivity has a significant effect on the cathode but not on the anode. At 0.15 g/L, the power density was 27 W/m 3. When the substrate concentration increased, power increased by 33% to 36 W/m 3 at 0.5 g/L and by 56% to 42 W/m 3 at 1 g/L. At the solution conductivity of 1.7 mS/cm (10 mM phosphate buffer solution) which is typical for domestic wastewater, the power density was only 16 W/m 3 with a current density of 2.8 A/m 2. Power density increased by 107% to 33 W/m 3 when the solution conductivity increased to 7.8 mS/cm (50 mM). Doubling the cathode size was predicted to increase power by 62%, while only 12% for the anode with domestic wastewater.
In addition to this, Ghangrekar et al. studied performance of mediator less and membrane less microbial fuel cell (ML–MFC) [93]. The ML–MFC gave COD and BOD removal efficiencies of 88% and 87% respectively and TKN removal was around 45– 50%. The current productions were 0.71 and 0.67mA when lactose and dextrose were used in the feed respectively. Maximum power density of 10.9 and 10.13mW/m 2 was observed at lower spacing between the electrodes (20 cm) and for lesser surface area of the anode respectively.
Electricity generation using single chambered microbial fuel cell was examined using acetate, without giving any external voltage to the reactor [94]. A maximum of 0.15 volts was obtained as output. This corresponds to a current of 2.8 µA at pH 6.8. A maximum of 92.3 mV (current
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1.3 µA) was obtained without the assistance of any external device. This output can further be enhanced after repeated inoculations.
MOTIVATION:
Based on the literature reviewed, it has been observed that:
Biodegradable waste (both solid and liquid) like fruit waste, food waste, kitchen waste, wastewater has been used to produce hydrogen using mixed culture but no work has been done on the petha waste which is of concern in the city, Agra. It has been seen that there is a contradiction in the results generated from the researchers using alkaline and acidic pretreatment methods for the production of hydrogen. Not much study has been done on the optimization of the pretreatment using ultrasonication, microwave and laser techniques. Most of the work done is in the batch reactors but not much work has been carried out in the continuous.
OBJECTIVES:
Efficient hydrogen production would be studied by blocking the reaction which leads to the production of methane and carbon dioxide using pretreatment methods. Pretreatment methods like microwave heating, ultrasonication, laser will be studied to enhanced the hydrogen production. Degradation of the organic matter will be studied by comparing the initial and final physical and the chemical parameters like sugar, lipid, TS, TSS, COD etc. studies would also be carried out in the reactor operated in the continuous reactors. The main objectives of the study may be outlined as:
1. The effect of the different types of inoculum pretreatment methods (such as heat shock, pH, chemical, sonication, laser, microwave etc.) on the production of the hydrogen would be investigated. 2. The optimization of the different pretreatment methods. 3. Degradation of the organic matter will be studied by the periodic estimation of physical and the chemical parameters like sugar, lipid, TS, TSS, COD etc. 4. Comparative production of hydrogen production methods in the batch and continuous reactor.
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5. The feasibility of the hydrogen production using the microbial fuel cell would also be studied using different types of electrodes.
METHODOLOGY:
Different pretreatment methods will be carried out on the active anaerobic mixed culture taken from cow dung. The results obtained will be compared with the reactors operated in the normal conditions i.e. when no pretreatment will be given to the inoculum. Monitoring of the hydrogen and methane will be done using gas chromatography (GC) equipped with the thermal conductivity detector (TCD) using nitrogen as the carrier gas.
The various physical and chemical experimental parameters would be studied initially and finally using APHA AWWA standard methods. COD will be done as the main parameter to study the biodegradation of the waste. Power generation would also be studied using microbial fuel cell (MFC) and the comparison would also be studied using various types of electrodes in them. Hydrogen produced in the MFC would also be monitored using TCD in GC.
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