The aim of study was to enhance the biogas production from cattle manure in a combined microbial electrolysis cell and anaerobic digestion system (MEC + AD). The MEC + AD reactor were operated on semi continuous mode under different voltage applications and different organic loading rates ranging from 6 g VS/L.d to 30 g VS/L.d. The study was carried out in two parts. In the first part, MEC + AD and conventional anaerobic reactors were compared in terms of biogas production. In the second part, MEC + AD reactor was operated under different voltage applications (0.3, 0.6, and 1.0 V) and organic loading rates. MEC + AD system exhibited 28–52% better biogas production performances (2.03 L/L/d; CH4:75.5% vs 1.43 L/L/d; CH4: 70.4%) compared to control reactor in all sets due to voltage application. In the second part, biogas productions of MEC + AD reactor changed between 3.08 L/L/d and 5.13 L/L/d (75–77.8% CH4) depending on the organic loading rates and applied voltages. Higher voltages (0.6 and 1.0 V) showed better methane production performances especially at high OLRs with respect to lower voltages. Consequently, MEC + AD reactor could be operated efficiently at extreme operational conditions (HRT of 2 days and OLR of 30 g VS/L/d) that conventional anaerobic reactors can not be operated.
Research Article
Enhancement of Biogas Production from Cattle Manure in a Combined Microbial Electrolysis Cell and Anaerobic Digestion System At Short Hydraulic Retention Time
https://doi.org/10.21203/rs.3.rs-1501290/v1
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Combined MEC+AD
biogas production
cattle manure
methane
organic loading rate
Microbial electrolysis cell (MEC) technology which is a promosing and sustainable energy production method, can be used for harvesting energy (hydrogen, methane) and value added products (ethanol, hydrogen peroxide, etc.) from organic matters by a small energy input of 0.2–0.8 V [1–3]. MEC process require an anaerobic environment therefore methane production is commonly observed along with hydrogen production [4, 5]. In some cases, methane production can exceed hydrogen production after a period of operation [4, 5]. As a result, many researchers have focused on the pathways and mechanisms of methane production in MECs. It is stated that beside the known pathway of acetoclastic methanogenesis, methane can also be produced directly through conversion of carbon dioxide. Carbon dioxide conversion occur by the help of mediators that transfer electrons from cathode to methanogens [6, 7]. Further studies revealed that in hydrogen producing bioelectrochemical systems (BES), hydrogenotrophic methanogens consume hydrogen that was produced by microbial electrolysis and convert it into methane [8–10]. Cheng et al. [11] proposed an alternative pathway called “electromethanogenesis” in which electrical current or electrons supplied to the system were used by microorganisms directly for the reduction of CO2 into methane. Finally Villano et al. [12] suggested that methane generation in MECs can be in both ways. Following the discovery of spontaneously methane generation pathways in MECs, studies has focused on integrating anaerobic digestion (AD) and MEC technology to enhance methane production, process kinetics and stability [7, 13–15].
The studies conducted with combined MEC + AD systems reported higher methane yield, biogas production and organic substance removal rates than anaerobic digestion alone. MEC + AD systems have better stability and can be operated at shorter hydraulic retention times (HRT) compared to AD [14–16]. It is also stated that unlike anaerobic digestion process, MEC + AD systems can be applied at both low and high organic loading rates (OLR) and they are compatible to lower temperatures [13]. Another prominent advantage of combined MEC + AD reactors pointed out is that methane content in biogas can be upgraded to as high as 80–95% [3, 10, 15]. Methane upgrading is carried out by either direct reduction of CO2 by methanogens or by combination of H2 and CO2 by hydrogentrophic methanogens to form methane [7, 13, 16]. Combined systems are also claimed to be more robust against high levels of toxicants and more durable to VFA accumulation and recalcitrant organics in feedstocks due to the syntrophic relationship between several bacteria type such as exoelectrogens, methanoges and fermentative bacteria [15–17]. Electrogens prevents VFA accumulation by oxidizing simple organic acids more effectively than methanogens [15].
Cattle manure is one of the most generated livestock manure in the world and has a complex content composed of carbohydrates, proteins and fats [18, 19]. It is reported that approximately 1.2 billion tons of manure is generated in Europe [19] and 3.8 billion tons of manure is generated in China [20] annually. In both regions, cattle manure accounts for more than 50% of the total amount [19]. Due to its content such as antibiotics, heavy metals, excess mineral salts and pathogens, it can be detrimental to the plants and can pollute water sources through run-off rains, overflows and infiltration to the soil [19, 20]. Therefore cattle manure must be well treated and turned into a more suitable product such as methane and non-pathogenic fertilizer.
Conventional anaerobic digestion (AD) process is one of the best technological approach that is currently in use for methane and non-pathological fertilizer production from various waste/waste streams [18, 19]. Compared to other wastes, methane yield and methane production available from the cattle manure alone is relatively low due its biodegradable-resistant content (lignocellulosic components) and low C/N ratio [18–20]. Therefore co-digestion of cattlle manure with carbon-rich wastes is recommended by several researchers [18–20]. Even though AD is very usefull to treat a wide range of waste/waste streams, it has some drawbacks such as long HRTs (20–30 days) that is required for a good quality of effluent and high methane generation. This is the case especially when cattle manure is used as the substrate. As a result, higher HRTs necessitate large reactor volumes and consequently high cost. Another distadvantage of AD process is that the microorganisms take place in the process is very sensitive to sudden changes in temperature, pH, excess loadings and to ammonia and sulfide concentrations [21, 22].
Various synthetic and actual waste/wastewater streams have been used as the organic matter source in MECs so far. However there are only one or two studies cunducted with cattle manure as the substrate in MECs which were operated batch-wise. [2, 17]. In light of these facts, in this study, combined MEC + AD system was offered to shorten the required HRT for a considerable biogas production rate from cattle manure without lowering the OLR. The novelty of this study is the enhancement of methane production in a combined MEC + AD system operating on semi continuous mode at different applied voltages and treating cattle manure at short HRTs (2, 3 days) and at high OLRs (20–30 g VS/L/d). As far as we know, the highest OLR applied to a combined MEC + AD reactor on semi continuos mode is presented in this study. At the end of the study, an energy evaluation was also conducted to clarify the energy needed for the voltage application and energy harvested from the extra methane generation.
1.1 Inoculum and cattle manure preparation
Cattle manure used as substrate in this study was delivered from a biogas plant which is located near to an animal husbandry zone in Sincan District of Ankara. The manure was placed into a refrigerator at the temperature of -18oC to prevent biological degredation until it was used. The anaerobic sludge inoculum was obtained from the second stage CSTR of a two stage anaerobic digestion plant operated at mesophilic temperature (35oC) in the same animal husbandry facility. Analysis of the manure and anaerobic sludge inoculum were conducted as soon as they were tranported to the laboratory. The characteristics of the manure and anaerobic sludge inoculum were depicted in Table 1. Prior to feeding the manure to the reactors, it was first blended with a hand blender and then screened through a 1 mm of mesh to remove undesirable solids and particles. Following, total solid (TS) and volatile solid (VS) analyses of the manure was carried out to determine the dilution rate. Then it was diluted to the desired VS concentration by adding tap water. The cattle manure prepared for feeding the reactors was placed in bottles and stored in the refrigerator at 4oC during the feeding period.
1.2 Configuration of combined MEC+AD reactors
In this study, control and MEC+AD reactors were made up of cylindrical borosilicate glass reactors with operational capacity of 0.8 L (inner diameter: 9 cm, height: 17 cm). Control reactor was a conventional anaerobic reactor without electrodes and voltage application. Combined MEC+AD reactor was formed by installment of anode and cathode electrodes into an identical anaerobic reactor. The inlet and outlet faucet of the reactors was at the bottom of the lateral surface with a diameter of 3 to 4 mm. Decanting and feeding of the reactors during the experiments were achieved through these faucets using scaled plastic syringes. Anode and cathode electrodes were made up of carbon fiber cloth and activated carbon pellets respectively. Carbon based materials were selected as electrodes due to their conductivity, chemical and biological stability, availability and economic cost [3, 17]. The electrode materials were purchased from online suppliers. All electrode materials were pretreated in an oven at 450 °C for 30 min to carbonize combustible organics and to remove the impurities from the surface. Then the carbon materials were washed with deionized water and finally dried in a stove at 105 °C. For the anode electrode preparation, a piece of carbon fiber cloth (length:20 cm; width:10 cm; thickness:1 mm) were folded in two from the long side and covered by a plastic net. Plastic net is the same with the ones placed on the windows of the households to avoid flies and bugs. The edges of the plastic net and carbon fiber cloth was siliconed and attached together to prevent the carbon cloth folding down inside the plastic net. A piece of titanium wire (length:15-20 cm, diameter: 1.5 mm) was attached to the carbon cloth and flat plastic net together and stretched out from the top lateral surface of the reactor for electrical connection. For the cathode electrode fabrication, 50 gr of activated carbon pellets (length:3-7 mm; diameter: 3 mm) were placed into a cylindrical chamber (length: 12-13 cm, diameter: 2.5-3 cm) made up of with the similar plastic net. The pellets inside the chamber were squezed and a piece of titanium wire was passed firmly through the pellets/chamber from bottom to the top. Then the excess part of the titanium wire of the cathode electrode was bonded to a rubber stopper at the top of the reactor and thus the cathode electrode was fixed hanging steady in the reactor. The distance between the anode and cathode electrodes inside the reactors was about 2 cm. The construction of electrodes and schematic presentation of MEC+AD reactors are shown in Figure 1. Power application to the MEC+AD reactors was supplied by connecting the titanium wires stretched out of the reactors to the power supply unit (PSU). Positive and negative ends of the PSU were connected to the anode and cathode electrodes respectively. A 10 ohm (Ω) resistance was connected in series between the cathode and the negative end of the PSU in order to measure the voltage accross resistance and calculate the current according to the Ohm`s Law [5, 8].
1.3 Experimental setup and operational methods
The experiments were conducted in two parts. In the first part, control and MEC+AD reactors were compared in terms of biogas production and organic removal performances at different HRTs (3, 4, 5 days) and OLRs (6-10 g VS/L/d). Also, MEC+AD reactor supplied with voltages of 0.6 and 0.3 V in the first part. In the second part of the study, MEC+AD reactor was operated at different OLRs varying from 14-15 g VS/L/d to 29-30 g VS/L/d at HRT of 2 days. The reason of choosing HRT between 5 days to 2 days is to show the MEC+AD system can achieve similar treatment performance with those AD reactors treating cattle manure at relatively high HRTs (10-30 days) [18-20]. Three different voltages (0.3, 0.6, 1 V) were applied to the reactor to demonstrate the voltage effects. When one set of study was completed, the voltage applied to the reactor were increased respectively. The operational conditions used in MEC+AD reactors are summarized in Table 2.
Every set of the experiments was conducted twice for at least 15 days of period to ensure the reproducibility and the steady state process for all operational conditions. All the reactors were operated at mesophilic condition (37±2oC) on semi continuous mode by feeding and decanting once a day. Heating and stirring (100-120 rpm) of the reactors were achieved by magnetic stirrers. No external chemicals were used to control the pH level of the reactors. Prior to beginning of the experiments, reactors were filled with anaerobic digested sludge and cattle manure (3 % VS, 4.15 TS: w/w) at the ratio of 1:1 and operated for a month to enable biofilm formation on the electrodes and acclimation of the microorganisms at mesophilic temperature and applied voltage of 0.6 V [15].
1.4 Analytical methods
The performance of the MEC+AD reactors and control reactor were evaluated by measuring biogas productions and methane contents and COD, VS and TS removal rates in the reactors. pH and ORP in the reactors were also measured. Current productions in MEC+AD reactors were measured to observe voltage effects. Biogas production of the reactors were determined daily using water displacement method. Methane content (CH4) of the biogas was measured according to Orsat Method. KOH solution of 40 % by mass was used in the Orsat apparatus to absorp CO2, NH3 and H2S [23]. TS and VS analysis were conducted according to standard methods [24]. COD was analyzed by Hach Lange Cadas 200 model spectrophotometer using commercially available testing kits (Hach-Lange LCK 514). pH and ORP values of the effluent waste measured by a benchtop pH meter. Finally, current production of the reactors was calculated according to Ohm`s Law, by measuring the voltage across the 10 Ω resistance connected in series between negative end of the PSU and cathode tip of the MEC+AD reactors [5, 8, 10]. Current density was calculated by dividing current production with the liquid volume in the MEC+AD reactors [5, 10].
It is claimed that especially hydrolysis and acetoclastic methanogenesis are often considered as the rate limiting satges for anaerobic digestion process [18, 21, 25]. It is also suggested that deficiency of essential elements and low C/N ratio in organic wastes can be an obstacle for high performance in anaerobic digestion [18]. MEC technology as a new alternative bioenergy production method can be combined with anaerobic digestion to overcome the rate limiting barriers. VFA accumulation, slow degredation of the organics, poor stability, long HRTs and etc. are the obstacles needed to be addressed in methane production processes [3, 26]. Therefore, the aim of this study was to enhance the biogas production from cattle manure in a combined MEC+AD reactor operated at high OLRs and short HRTs with the voltage addition to the system and compare MEC+AD reactor with a conventional anaerobic reactor.
2.1 Comparison of MEC+AD and control reactors
In the first part of the study, anaerobic control reactor and MEC+AD reactor were compared in terms of biogas production, methane content of biogas, COD, TS and VS removal. Four set of experiments were conducted in the 1st part where OLR, HRT and applied voltage were the main variables.
Biogas production performances
Following the start-up period, MEC+AD and control reactors were operated at HRT of 5 days with the OLR of 6.0 g VS/L/d (TS:4.15 %; VS: 3.01 %; w/w) for 17 days (1st set). MEC+AD reactor was supplied with a voltage of 0.6 V in this set. In the 2nd set, OLR was increased to 7.5 g VS/L/d by means of shortening the HRT to 4 days for both of the reactors. Applied voltage was kept at 0.6 V for the MEC+AD reactor. In the 3rd set, reactors were operated at the same HRT and OLR as in 2nd set. However, voltage application to MEC+AD reactor was decreased to 0.3 V in the 3rd set to observe the differences in biogas production performances at different voltage applications. Finally, at the 4th set, OLR was increased to 10 g VS/L/d by shortening the HRT of the reactors from 4 days to 3 days. Applied voltage for MEC+AD reactor was kept at 0.3 V in the last set of the first part. The aim of shortening the HRT from 5 days to 3 days in the first part of the study was to show whether the MEC+AD and control reactors could operate efficiently at challenging conditions which are not common for anaerobic process only [18, 19]. Also, effects of applied voltages of 0.3 V and 0.6 V on the biogas production was compared at the same feeding conditions. Daily average biogas production and methane content of the biogas produced in MEC+AD reactor were between 1.75 - 2.39 L/L/d and 72-78 % respectively in the first part. On the other hand, excluding the 4th set, biogas production and methane content of the biogas in control reactor were between 1.14 - 1.43 L/L/d and 70-78 % respectively. Biogas production and methane content of control reactor decreased drastically in the 4th set due to the high OLR as it can be seen in Figure 2. Biogas production rate of MEC+AD reactor increased when OLR was increased (or HRT was decreased) where as it was the opposite situation for control reactor. Methane yields of the reactors were calculated according to feed VS concentration in this study. Highest methane yields of MEC+AD and control reactors were 0.217 L CH4/g VS and 0.167 L CH4/g VS respectively and observed at HRT of 5 days (OLR= 6 g VS/L/d). All the experimental results observed in the first part isshown in Table 2 and Figure 2. MEC+AD system exhibited 28 % to 52 % better biogas production performances compared to control reactor in all sets due to voltage application. It was reported by several researchers that combined MEC+AD reactors were superior to anaerobic digester only in terms of biogas production and organic removal rates [27-29]. In this study applied HRT was shorter compared to other studies conducted with cattle manure or co-digestion of cattle manure [19, 20]. On the other hand, applied OLRs (6-10 g VS/L/d) were similar with other studies that aimed biogas production from cattle manure [18-20]. For example, Bi et al. [20] treated mixture of cattle manure and food waste at various HRTs and reported that the highest methane production and methane yield of 1.48 L CH4/L/d and 0.236 L/g VS were observed at HRT of 5 days and over 15 days respectively. It is known that methane yield of cattle manure is around 0.21-0.25 L CH4/g VS [18] and HRT can vary between 10-30 days depending on the temperature, substate type and OLR of the process in anaerobic continuously stirred tank reactors (CSTR) [30]. In the present study, HRTs of 3 to 5 days were applied successfully to a combined MEC+AD reactor even when cattle manure was the sole substrate. Therefore it can be claimed that the findings (1.75 Lbiog./L/d and 0.217 L CH4/g VS) were convenient with the literature even at shorter applied HRTs.
There are contradictory claims on the optimal voltage that should to be applied to the MEC systems. Yu et al. [31] reported that 0.7 V was the optimum voltage for the MECs for the highest methane yield. On the other hand, other studies suggested that 1.0 V of power application enhanced methane generation the most [32-34]. In another study, Xiao et al. [35] stated that methane production was higher at applied voltage of 1.8 V. Moreover, Feng et al. [36] found out that optimal voltage was between 0.3-0.5 V when treating sewage sludge. Different optimal voltage results can be attributed to the operating conditions, substrate type and reactor design. Therefore, applied voltage was decreased from 0.6 V to 0.3 V in the 3rd set to observe the effects on the MEC+AD reactor. It is thought that, as a result of lower voltage supplementation, biogas production and methane yield of MEC+AD reactor decreased slightly (1.75 L/L/d, 0.177 L CH4/g VS) respect to previous set [32, 34]. On the other hand, control reactor exhibited an unsteady state in the 3rd set, similar to 2nd set but more unstable and inefficient. It can be attributed to the shorter HRT of 4 days which are not favorable for methanogenic process [18, 20, 22]. Finally, when the OLR was increased to 10 g VS/L/d by means of shortening HRT to 3 days, MEC+AD reactor with applied voltage of 0.3 V presented an increase in biogas broduction (2.39 L/L/d, 76.25 %) due to the available organic in the medium. Methane yield calculated for MEC+AD reactor in the 4th set was nearly the same with the previous set, indicating an efficient operation at those conditions. On the other hand, it is assumed that control reactor failed due to the high OLR which might be resulted due to the VFA accumulation and/or change in structure of microbiome community at HRT of 3 days [20, 22]. Although there is not a direct evidence for this assumption, it can be seen in Figure 3 that organic removal, pH and oxidation and reduction potential (ORP) of the control reactor diverges drastically from previous sets.
Biogas productions obtained in MEC+AD reactor in the first part of the study complies with the other studies conducted in continuously operated MEC or MEC+AD reactors. For instance, Park et al. [37] reported a methane yield of 0.3-0.36 L CH4/g CODrem. when treating food waste at the OLR of 2-10 kg/m3/d and HRT of 15-20 days in a combined MEC+AD reactor operated at temperature of 35oC and the applied voltage of 0.3 V. The electrodes used in the study were coated with Ni, Cu and Fe which helps efficient electron transfer. In another study, a single chamber MEC was operated on sequencing batch mode at 35oC by feeding sewage sludge at HRTs of 5 to 20 days and OLRs of 1.44 to 5.76 kg VS/m3/d. Methane yields were 0.368 L CH4/g CODrem.(20 days) and 0.479 L CH4/g CODrem.(10 days) at a voltage of 0.3 V. Maximum methane production was reported as 1.34 L/L/d at 5 days of HRT [38]. Li et al. [9] used a high rate anaerobic sludge blanket (UASB) reactor for methane production from synthetic wastewater (glucose) at the concentration of 3 g COD/L. It was reported that at HRT of 6 h and OLR of 12 g COD/L/d methane production was around 3 L CH4/d at applied voltage of 1.0 V. Even though cattle manure was the sole substrate fed to the MEC+AD reactor, the results obtained in the first part of the present study (2.39 L/L/d, 76.25 %) was comparable with those studies.
It is known that about 30 % of methane production in anaerobic processes originates from hydrogentrophic methanognesis [30]. However, power application to the MEC reactors can enhance the growth of hydrogenotrophic methanogens faster than acetotrophic methanogens at short HRTs [9, 11, 35]. Thereby, contribution of hydrogen to methane formation can exceed 30 % owing to the voltage application. Electrons given to the system lead the way to the hydrogen formation and afterwards utilization of hydrogen and CO2 by hydrogenotrophic methanogens ends up with CH4 formation [8, 11-13]. Also, by voltage application in MECs, direct conversion of CO2 into methane can be carried out by methanogens that capture electrons. In this study, methane production of MEC+AD reactor surpassed methane production of control reactor by 28 to 52 % respectively in the first part. Higher methane production in MEC+AD reactor can be explained by the voltage application which stimulated reduction of CO2 with electrons and reduction of CO2 with H2 through methanogens [11, 12]. CH4 content of biogas usually vary between 55-65 % in anaerobic processes [1, 39]. In the first part of the study, CH4 content of the biogas produced from MEC+AD reactor was mostly between 75-80 %. This can be attributed to the methane formation pathways such as electromethanogenesis and hydrogenotrophic methanogenesis. Methane content of the biogas produced from control reactor was similar with the MEC+AD reactor until the HRT was decreased to 3 days. However, methane content declined to around 40 % at the HRT of 3 days. The decrease in biogas production and methane content in control reactor can be attributed to the insufficient time for acetogens and methanogens in order to fully operate at high OLRs and short HRTs [22, 39].
Organic removal rates
Organic removal in anaerobic processes is related with the stability and robustness of the reactor process. In a well functioning reactor, biogas production and organic removal should be at desired rates. COD, TS and VS removal rates of both MEC+AD and control reactors are presented in Figure 3(A). It can be seen that substrate removal rates were higher in the MEC+AD reactor compared to the control. COD removal rates of MEC+AD reactor changed from 40 % to 33 % in the descending order from 1st set to 4th set. On the other hand, COD removal rates of the control reactor decreased from 35.5 % to 13.8 % during the first part of the study. The lowest removal rate of 13.8% at the last set was due to the deterioration of the control reactor caused by high OLR. Similarly, TS and VS removal rates of MEC+AD reactor were superior to the control reactor. TS and VS removal rates of MEC+AD were between 28.3 – 20.1% and 34.1-26.2 % respectively. On the other hand, TS and VS removal rates of the control reactor were between 25.5 – 7.7 % and 32 – 9.2 % respectively. In the 4th set TS and VS removal rates of the control reactor decreased drastically due to the high OLR and short HRT. The decrease in organic removal rates in control reactor was parallel with the decrease in biogas productions and methane content in the last set. Higher organic removal rates of MEC+AD reactor can be explained by the voltage application which could have facilitated the growth of hydrogenotrophic methanogens faster than acetotrophic methanogens [8, 11, 13]. As a result of this process, MEC+AD was able to comply with shorter HRTs compared to the control reactor [9, 11, 35].
pH and oxyidation reduction potential of the reactors
pH and Oxidation Reduction Potential (ORP) values in anaerobic digestion can be good indicators for favourable process. pH and ORP values of both MEC+AD and control reactors were presented in Figure 3(B). It is known that methanogens are optimally active at a pH around 7 or between 6.5-7.5 [22, 30, 40]. However optimal pH range for methanogens can vary according to the substrate type, temperature of the process, buffering capacity, alkalinity, VFA concentration, CO2 concentration in liquid phase and etc. [40]. pH of the MEC+AD reactor varied between 7.4 and 7.6 during the first part of the study. pH values were convenient for methanogenic activity at high OLR [22, 30]. In the control reactor, pH varied between 7.65 and 7.40 in the first three sets of the first part. But in the last set (Set 4), pH declined instantly from 7.4 to 6.6 in a few days due to the short HRT and high OLR. pH decline in the control reactor was parallel with the reduction of biogas production and methane yield. It was obvious that methanogenic activity was severely inhibited in the control reactor. This inhibition can be attributed to VFA accumulation and pH decrease due to overloading [21, 22, 40].
Oxidation reduction potential (ORP) is a measure of the net values of oxidation-reduction reactions within an aqueous environment. ORP can be used as an indicator for monitoring the anaerobic digesters because CH4 production mostly takes place at ORP values between -175 and -400 mV [41, 42]. Acidogenesis take place at ORPs between -250 and-300 mV whereas methanogenesis take place at ORPs -300 and -360 mV [42]. In the first part of the study, ORP values varied between -300 and -350 mV in MEC+AD reactor and between -245 and -325 mV in control reactor. ORP values of MEC+AD reactor showed that methanogenic activity was quite stable during the study. On the other hand, ORP values of the control reactor ranged between -295 and -330 mV in the first three sets. However at the last set, when the HRT was reduced to 3 days, ORP values increased to -245 mV which was not in the range of methanogenic activity. ORP increase can be explained by the deterioration of the methanogenic activity which may have resulted due to the change in anaerobic conditions at short HRT [42].
2.2 Biogas production of the MEC+AD reactors at different voltages
In the second part of the study, combined MEC+AD reactors were operated under different voltage applications (0.3, 0.6, and 1.0 V) and different OLRs at constant HRT of 2 days to observe the effects of voltage applications on high OLRs and short HRT. There is not a study so far that was conducted with a combined MEC+AD reactor operated at HRT as short as 2 days. Also, to our knowledge OLR of as high as 29-30 g VS/L/d was not applied to a combined MEC+AD reactor so far.
Biogas production and methane contents obtained in the second part are presented in Figure 4. Daily average biogas productions of MEC+AD reactors fed with three different OLRs changed between 3.08 and 4 L/L/d for (MEC+AD)0.3V, 3.14 and 4.86 L/L/d for (MEC+AD)0.6V, 3.43 and 5.13 L/L/d for (MEC+AD)1.0V. Average methane content of the biogas produced in the reactors were between 75-76 %, 76.1-76.6 % and 75.1-77.8 % for (MEC+AD)0.3V, (MEC+AD)0.6V and (MEC+AD)1V respectively. Many studies have been conducted in continuously or semi continuously operated MECs so far. Therefore, Table 3 was organized to present the results of those studies conducted with different kind of substrates and operational conditions (HRT, OLR, voltage, temperature) with additional informations. In some of these studies lower operating temperatures [8, 27, 36, 43, 44] were applied and in others, synthetic wastewater or easily degradable waste streams were used [45-47, 50, 51]. Expensive and specially designed electrodes [36, 37, 38, 45-47] or pH adjustment and substrate pretreatment [27, 43] were applied in several of those studies as well. Sangeetha et al. [43] operated an upflow-MEC by feeding artificial beer wastewater at COD concentration of 1.5-2.0 g/L at the temperature of 30oC. Methane production and yield were 0.367 L CH4/L/d and 0.143 L/g COD respectively at HRT of 1 day and applied voltage of 0.8 V in the study. The results obtained in the present study were higher than the results obtained by Sangeetha et al. [43]. In another study, a combined MEC+AD reactor was operated for methane production at mesophilic temperature from acetate (OLR: 2 g COD/L/d; HRT: 20 days) at an applied voltage of 0.3 V. The methane production and yield were 0.27 L CH4/L/d and 0.34 L CH4/g CODrem. repectively [45]. Although operational conditions were different than the present study, methane production was quite low compared to the methane production of the present study. Park et al. [47] treated food waste in a combined MEC+AD reactor operated on sequencing batch mode at HRT of 20 days and OLR of 8 kg TCOD/m3/d with the voltage application of 0.3 V. The electrodes of the reactor were carbon graphite mesh electrodes coated with metal catalyzer that enables faster electron transfer. Methane production and yield were 3 L/L/d (calculated) and 0.36 L CH4/g CODrem. respectively which were similar to the results of the present study [47]. Nevertheless, it is shown in Table 3 that biogas production rates (3.08 – 5.13 L/L/d) obtained in the present study were compatible with many of the studies. In some cases, the results were higher than the other studies even though very high OLR (29-30 g VS/L/d) and short HRT (2 days) were applied in the present study. It was obvious that when the OLR was increased from 15 g VS/L/d to 22.5 g VS/L/d in the 2nd set, biogas productions of all reactors increased independently from the voltage application. The increase rates in biogas production from 1st set to 2nd set were 28, 46, and 39 % in the MEC+AD reactors operated at 0.3, 0.6 and 1.0 V respectively. In the 3rd set when the OLR was increased from 22.5 g VS/L/d to 30 g VS/L/d biogas production rate of (MEC+AD)0.3V did not change and the increase in biogas productions of (MEC+AD)0.6V and (MEC+AD)1.0V were in the range of 6-8 %.
The effects of different voltages on MECs and MEC+AD reactors were studied by several researchers and different results were reported. A single chamber MEC operated on batch mode was fed with acetate at concentration of 2 g/L. The MEC was supplied with voltages of 0.5, 1.0, and 1.5 V [34]. Electrogenic activity and methane production of 0.35 L CH4/g COD were found highest at applied voltage of 1.0 V which was reported as the optimum voltage required for the growth of exoelectrogens on the electrodes. It was stated in the study that lower voltage application was insufficient to drive the favorable reactions whereas higher voltage application than 1.0 V caused high-voltage inhibition. Similar results and conclusions were revealed by Choi et al. [33]. However, Feng et al. [36] claimed applied voltages between 0.3 and 0.5 V was optimum for the highest methane generation/yield (0.35-0.37 L/L/d and 0.33-0.35 L/g CODrem.). The results in that study were obtained with a MEC equipped with expensive and special electrode materials that was operated on sequencing batch mode at 25oC. The study was conducted by feeding sewage sludge at concentration of 43.3–51 mg VS/L at HRT of 20 days [36]. Although there are contradictory statements on optimum voltage range, it is obvious that the applied voltage is very influential on the current production in MECs [34, 48, 49]. In the present study, highest biogas production and methane yield and current production were achieved at applied voltage of 1.0 V at all three OLRs as it is shown in Figure 4. However, the methane production at applied voltage of 0.6 V were nearly as high as the results obtained at 1.0 V. Higher biogas production at high applied voltages can be attributed to the electrons given to the system. The more electrons captured by the methanogens, the more methane generation take place at the electrodes [48, 49].
Methane yields were in the range of 0.103 L/g VS [(MEC+AD)0.3V] and 0.173 L/g VS [(MEC+AD)1.0V] under different OLRs during the present study. Highest methane yield was observed at applied voltage of 1.0 V at the lowest OLR (14-15 g VS/L/d) in the second part. Methane yields of the MEC+AD reactors decreased from 1st set to 3rd set while the OLR was increased gradually as it is shown in Table 2. The decrease in methane yields can be attributed to the instability of the process due to the insufficient time for acetogens and methanogens in order to fully operate at high OLRs and short HRTs [22, 39]. And as a result, accumulation of intermediate products (VFA, ammonia, etc.) may have increased due to insufficient time [22, 38]. These results were also in consistency with the previous studies [36, 37, 43-45]. In those studies, MEC reactors were operated at either higher HRTs (> 10 days) or lower OLRs (< 2-3 g COD/L/d) compared to this study. In this study, biogas productions ranging from 1.75 to 5.13 L/L/d (71.6-77.8 % CH4) were promising despite the fact that MEC+AD reactors were operated at challenging conditions compared to the studies mentioned in Table 3.
2.3 Organic removal in MEC+AD reactors
COD, TS, and VS removal rates of MEC+AD reactor operated at voltages of 0.3, 0.6 and 1.0 V were presented in Figure 5(A). It was clear that organic removal rates decreased in all MEC+AD reactors when the OLR was increased step by step. The effect of applied voltages was slightly significant on the organic removal efficiency. VS removal rates at OLR of 15 g VS/L/d were between 23-26 %, however removal rates decreased to around 14-16 % when the OLR was increased to 29-30 g VS/L/d. The decrease in organic removal rate was in parallel with the decrease in methane yield in the reactors. VS removal rates and methane yield reductions can be attributed to the high OLR [22, 39]. COD and TS removal rates of the MEC+AD reactors at all applied voltages followed the same pattern with the VS removal rates; the COD and TS removal rates decreased when the OLR was increased. The results were presented in Table 2 and Figure 5(A). Even though OLR was very high with 2-day HRT in the 2nd part of this study, organic removal rates were compatible with the previous studies that used real livestock wastes as the substrate. In a two-chamber MEC, pig slurry was treated at HRT of 32.4 hours and at set potential of 0.8 V vs. SHE (Standart Hydrogen Electrode), [52]. In that study, COD removal rate was 24±8 % in the anode chamber. In another study, a single cell MEC operated continuously at 35oC and 0.3 V voltage application. Mixed sewage sludge was treated at different HRTs and 39 % of TCOD removal rate was obtained at 5 days of HRT and 5.76 kg VS/m3/d of OLR [38]. In their study a special electrode assembly were used to minimize the distance between electrodes which included carbon nanotubes in their structure. In another study, synthetic brewery wastewater was fed to an upflow combined MEC+AD reactor at HRT and OLR of 5.6 d and 5.8 g COD L/d respectively. Total organic carbon removal rate was higher than 90 % at applied voltage of 0.5 V. However maximum methane production was 1.16 L/L/d even with usage of powdered activated carbon for microorganism adhesion [53] Asztalos and Kim, [54] used waste activated sludge (7.9 g COD/L) as the substrate in a single chamber MEC which had 3 carbon fiber anodes and stainless steel mesh cathode. At different solids retention time of 7, 10 and 14 days, COD removal efficiencies obtained were between 30-34 % at applied voltage of 1.2 V.
2.4 Current production of the MEC+AD reactors at different applied voltages
Application of different voltages naturally ended up with different current productions in MEC+AD reactor. Volumetric current densities of MEC+AD reactor were calculated by dividing the average current produced in the reactor by the liquid volume of the reactor. Current densities of MEC+AD reactor operated at different voltage supplementations (0.3, 0.6 and 1.0 V) were presented in Figure 5(B). Current density of (MEC+AD)0.3V varied from 0.5 to 2.0 A/m3 and was mostly around 1.0 A/m3 during all three sets. Current density of (MEC+AD)0.6V was around 2-3 A/m3 during the study which was higher than the current density of (MEC+AD)0.3V due to the higher voltage input. Higher voltage input enhanced the biogas production due to the electrogen activity and electron transfer [31, 34, 36]. When the voltage application was increased to 1.0 V, current densities of (MEC+AD)1.0V also increased to around 4-6 A/m3. It can be concluded that an increase in applied voltage resulted in an increase at biogas production as a consequence of an increase in electron transfer and electrogen activity in MEC+AD reactors. Highest biogas productions were observed in (MEC+AD)1.0V at all OLRs. However, similar biogas productions were observed in (MEC+AD)0.6V at the 2nd and 3rd sets as well. Biogas production rates and current densities at different OLRs and applied voltages are presented in Table 2. The current density values obtained in this study were consistent with the other studies conducted in MEC reactors operated in continuous mode [50-53]. Cusick et al. [50] obtained max. current density of 7.4 A/m3 (after 100 days of operation) in a 1100 L of MEC which were fed with winery wastewater (0.7−2.0 g SCOD/L) and operated at 31oC with 1 day HRT. In that study, pH adjustment and acetate supplementation were performed as well. Also specially designed 144 pairs of electrodes were used to enhance the electrogenic activity and methane production at a voltage of 0.9 V. Xu et al. reported [53] current productions of 6 to 8 mA with an upflow MEC that was operated at 35oC and applied voltages of 0.5 and 1.0 V. The current productions obtained in that study was similar to the currents of the the present study (4-6 mA).
pH and ORP values of MEC+AD reactors supplied with different voltages were presented in Figure 5(C). pH and ORP values of the reactors varied between 7.18 and 7.53 and -355 mV and -320 mV respectively during the second part of the study. These results were in the convenient range for the methanogenic activity [22, 30]. It was noticed that different voltage applications did not affect the pH and ORP values in the reactors. According to the ORP values of MEC+AD reactors, it can be concluded that higher methanogenic activity dominated the reactors in the second part of the study [42].
In the present study it was shown that high biogas productions of 3.08-5.13 L/L/d and methane content of 75-77.8 % in the biogas can be obtained from MEC+AD reactors treating cattle manure at HRT of 2 days. Unlike anaerobic control reactor, MEC+AD reactors could be operated efficiently at high OLRs (15-30 g VS/L/d) and shorter HRTs (2 and 3 days) without any sign of inhibiton. MEC+AD reactor was found superior to anaerobic reactor at HRT of 4 and 5 days in terms of methane productions (1.33-1.54 vs. 0.93-1.12 L/L/d) and organic removal rates (31.5-34.1 % vs. 27.2-31.9 %) owing to the voltage application. The effects of different applied voltages (0.3, 0.6 and 1.0 V) on the performance of MEC+AD reactors were also observed. Higher biogas productions were obtained at voltages of 0.6 V and 1.0 V compared to 0.3 V. Nevertheless, voltage addition to MEC+AD reactors enhanced biogas production and organic removal at operating conditions which can not be applied for conventional anaerobic reactors.
Energy efficiency evaluation showed that MEC+AD reactor can harvest several times more energy from substrate than the energy needed for voltage application. These results indicate that reactor volumes can be lowered in the case of a combined system which reduces the investment cost. In addition to this, treatment capacity can be increased by shortening the operational HRTs. Finally, combined MEC+AD reactors can be used as a pretreatment method prior to a full scale anaerobic/aerobic process as another alternative application.
2.5 Energy evaluation of the MEC+AD reactor
In MEC process, energy is needed for voltage supplementation, heating, mixing and other operational activities. However, energy optimization must be applied for sustainability and energy efficiency. It is important to clarify that in some studies, energy efficiencies were calculated based on the additional methane production in MECs compared to control anaerobic digesters [9, 44]. In this case energy efficiency relative to the electrical input was calculated according to the equation below for MEC+AD reactors [44, 54].
(WCH4) is the energy content of the methane produced by voltage application. ΔHCH4 is the energy content of CH4 based on the heat of combustion value (890.8 kJ/mol) [9, 51]. VMEC (1.45 L CH4/L/d) and VC (1.12 L CH4/L/d) are the daily average CH4 productions at MEC+AD and control reactors respectively at HRT of 5 days.
Electrical energy provided by external power supply was calculated with the equation below [9, 38].
WEis the electrical energy input, I is the daily average current (V=IxR, R=1 Ω and V= 40 mV, I = 40 mA, not given in the study) obtained in the system, Vap is the applied voltage (1.0 V) and is the time (1 day=24x3600 s).
Thereby energy efficiency (ƞe) can be determined by the equation:
.
Energy efficiency relative to additional methane production and electrical input in MEC+AD was found approximately 380 %. This result is appropriate with those studies conducted in MECs at different constructional and operational conditions [9, 50, 51, 55]. An UASB-MEC reactor succeeded energy efficiency of more than 1200% with synthetic wastewater (7 g COD/L) at HRT of 6 hours and applied voltage of 1.0 V [9]. Guo et al. [55] designed a specially stacked cathode in a MEC which treated synthetic beer brewery wastewater at 35 oC. They reported energy efficiency between 378 and 1584 % at various applied voltages.
It is known that acetoclastic methanogenesis can compete electrogens over acetate utilization at high organic concentrations. Therefore, acetoclastic methanogenesis can gradually increase and even replace anodic oxidation (electrogens) and become the dominant pathway [29, 56]. In this study, low current production can be attributed to the suppressed electrogen growth on anode which may have arouse from acetoclastic methanogens domination on anode at high VS concentrations [57]. It is thought that methanogens and electrogens in the biofilm of electrodes competed over the electrons and substrate. This competition caused lower current productions in the MEC+AD reactors.
Funding: This work was supported by the Scientific Research Projects Coordination Unit (Project No: FHD-2020-18581) of Hacettepe University.
Competing Interest: The authors have no relevant financial or non-financial interests to disclose. All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Kenan Dalkilic. The first draft of the manuscript was written by Kenan Dalkilic and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Data availability: The datasets generated during and/or analysed during the current study are not publicly available due to personal reasons such as the study is a part of Ph.D thesis and have the chance to be prepared as another manuscript. But the datasets are available from the corresponding author on reasonable request.
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Table 1. Characteristics of inoculum, raw and fed manure.
|
Parameter |
Inoculum |
Raw manure |
Fed Manure |
|
TS, g/L |
77.5±2.0 |
113±7 |
41.1±3 – 87.5±5 |
|
VS, g/L |
51±2 |
89.5±2.5 |
29.8±1.5 – 60.1±1.5 |
|
TCOD, g/L |
65±2 |
98±3 |
35±2 – 75±3 |
|
SCOD, g/L |
- |
6±0.5 |
4±0.3-5±0.3 |
|
TN |
- |
3.8±0.4 |
1±0.2-2.5±0.3 |
|
pH |
7.30±0.1 |
7.0±0.2 |
7.25±0.15 |
|
EC (mS) |
12.3 |
>20 |
>13 |
|
ORP (mV) |
-345 |
-175 |
-170 |
|
VS/TS % |
66±2% |
77±3 % |
71±3 % |
TS, total solids; VS, volatile solids; TCOD, total chemical oxygen demand; SCOD, soluble COD; TN, total nitrogen
Table 2. Performances of MEC+AD and control reactor.
|
Part 1 |
Set 1 |
Set 2 |
Set 3 |
Set 4 |
|||||||||
|
Operational conditions |
Control |
MEC+ AD |
Control |
MEC+ AD |
Control |
MEC+AD |
Control |
MEC+ AD |
|||||
|
HRT (day) |
5 |
4 |
4 |
3 |
|||||||||
|
OLR (gVS/L/d) |
6.0 |
7.5 |
7.5 |
10.0 |
|||||||||
|
Voltage (V) |
|
0.6 |
|
0.6 |
|
0.3 |
|
0.3 |
|||||
|
Biogas pro. (L/L/d) |
1.31 |
1.82 |
1.43 |
2.03 |
1.14 |
1.75 |
0.52 |
2.39 |
|||||
|
CH4 content (%) |
78 |
71.6 |
70.4 |
75.5 |
77.5 |
75.9 |
40.9 |
76.3 |
|||||
|
CH4 yield (L/g VS) |
0.167 |
0.217 |
0.137 |
0.207 |
0.125 |
0.177 |
- |
0.182 |
|||||
|
VS Rem. (%) |
31.9 |
34.1 |
30 |
32.8 |
27.2 |
31.5 |
9.2 |
26.2 |
|||||
|
Part 2 |
Set 1 |
Set 2 |
Set 3 |
||||||||||
|
OLR(g VS/L/d) |
14.5-15.5 |
22-23 |
29-30 |
||||||||||
|
HRT (days) |
2 |
2 |
2 |
||||||||||
|
Voltage |
0.3 |
0.6 |
1 |
0.3 |
0.6 |
1 |
0.3 |
0.6 |
1 |
||||
|
Biogas production (L/L/d) |
3.08 |
3.14 |
3.43 |
3.97 |
4.59 |
4.76 |
4.0 |
4.86 |
5.13 |
||||
|
CH4 content (%) |
76 |
76.1 |
75.5 |
75 |
76.6 |
75.1 |
76 |
76.1 |
77.8 |
||||
|
CH4 yield (L/g VS) |
0.156 |
0.159 |
0.173 |
0.132 |
0.156 |
0.159 |
0.103 |
0.125 |
0.135 |
||||
|
VS Rem. (%) |
23.35 |
24.43 |
25.64 |
16.93 |
18.67 |
19.10 |
13.95 |
14.81 |
15.89 |
||||
|
Current dens.(mA/L) |
1.2-1.9 |
1.7-3.1 |
3.9-6.3 |
0.7-1.2 |
2.3-3.7 |
3.3-6 |
0.5-1.2 |
2.3-4.4 |
4.4-6.5 |
||||
Table 3. Comparative assesments of continuously/semi continuously operated MECs.
|
Reactor type/ volume |
Operation; Substrate; OLR/HRT/Feed cycle; Voltage |
Methane production |
COD (VS) Removal |
Additional info |
Ref. |
|
TC-0.225 L total |
Continuous, 22oC. Synthetic medium in anode and cathode. OLR of fed acetate is 1.6 kg COD/m3 total in anodic compartment. HRT=2.2 h. V:0.6 V. |
0.41 mole CH4/ mole acetate (%57 CH4 and %42 H2). 1.85 mol H2/ mol acetate |
46±8% (Acetate conversion to H2) |
Anode and cathode medium was refreshed by continuous recirculation. Acetate was dosed to the anode batch wise when it was depleted in anode medium. The anodic oxidation of acetate was not hampered by NH4-N conc. up to 5 g N/L. |
[8] |
|
SC-1 L UASB (d:7 cm, h:26 cm) |
Continuous, 35°C. Synthetic WW with glucose, NH4Cl and KHPO4 was fed to the MEC. HRT of 6 h was used for all feeding period. For 7 g COD/L, OLR was 28 g COD/L/d. Applied voltage was 1 V. |
44.6 mL/h for 1g COD/L; 248.5 mL/h for 7 g COD/L |
89.2% for 1 g COD/L; 71.0% for 7 g COD/L |
The distance between anodes and cathode was 2.5 cm. The influent COD was changed from 1 g/L to 7 g/L for two reactors during the 100 days operation period. COD:N:P ratio of 200:5:1 was adjusted. |
[9] |
|
SC-0.256 L total, cubic |
Continuous, 22oC. Substrate was acetic acid solution (8.5 mL/d) added to medium in recirculation loop. HRT=5.3 h, OLR= 1.38 g COD/L/d. A nonwoven cloth was used to prevent contact btw. anodic and cathodic graphite granules. Voltage was 0.8 V. |
0.33 and 0.28 L CH4/L/d with and without recirculation loop respectively |
98% and 93% with and without recirculation loop respct. |
MECs were operated at different OLRs and with/without recirculation loop of fresh medium. Recircul. loop to both anode and cathode enhanced the CH4 production. Max. production was 0.75 L CH4/L/d with OLR of 4.13 g COD/L/d. pH buffer was used in all experiments. |
[27] |
|
SC-12 L, cylindrical (d:24 cm) |
SBR, 25oC. Substrate was sewage sludge (TCOD= 31.7-47 g/L and VS=43,3–51 mg/L) HRT=20 days. Applied voltages were 0.3, 0.5 and 0.7 V. |
370 mL/L/d for 0.3 V. 346 mL/L/d for 0.5 V. 350 and 330 mL/g CODrem. for 0.5 V and 0.3 V respectively |
~55% COD and 64-66% VS removal for both 0.3 and 0.5 V |
The separator and electrode assembly(SEA: 6×24 cm2) was prepared by stacking the anode, a polypropylene nonwoven sheet as a separator, and the cathode to eachother respectively. SEAs were helically installed inside of the digester. |
[36] |
|
Combined AD+MEC; 20 L, cylindrical |
Sequencing batch (SBR), 35oC. Food waste; OLR=2-10 kg/m3/d ; HRT=20 days, 15 days for OLR 10 kg/m3/d; 0.3 V. |
19-76 L/day due to the OLR 2-10 kg/m3/d; 0.3-0.36 L CH4/g CODrem. |
68-76% |
At 10 kg/m3/d, CH4 production was achieved stably. The distance between electrodes were <3mm. Non-woven fabric was placed between electrodes to prevent short circuit. |
[37] |
|
SC-12 L, cylindrical (d:24 cm) |
SBR, 35oC. Substrate was mixed sewage sludge (TCOD:36.6g/L, VS:28.8 g/L). HRT was 20, 15, 10, and 5 days (by increasing of the sewage sludge feeding rate). OLR were in the range of 1.44 to 5.76 kg VS/m3/d. App. voltage was 0.3 V. |
Between 369 and 479 mL CH4/g CODrem. for 20 and 10 days respct. Max. production:1.34 L/L/d at 5 days HRT |
64% to 39%, decreasing by decreasing HRT from 20 to 5 days. |
SEA (6×24 cm2) was prepared by stacking the anode, a polypropylene nonwoven sheet as a separator, and the cathode to eachother respectively. At the start up, 40% of the working volume was inoculant (15.6 g TS/L; 10.6 g VS/L), the rest was sewage sludge. |
[38] |
|
Upflow-MEC, 0.6 L, l=35 cm, d=5 cm |
Continuous, 30oC. Phosphate buffer added to artificial beer wastewater. COD conc. was 1.5-2.0 g/L. HRT=24 h., Applied voltage was 0.8 V. |
143 mL/g COD (Ni cat.), 367 mL CH4/L/d. |
85%-Ni cat., 79% SS cat., 69% Cu cat. |
Distance btw. electrodes was 3 cm. Stainless steel(SS), nickel and copper meshes were compared in three dif. reactors. Ni cathode was superior compared to others. 10 mL of domestic WW was used as inoculum. |
[43] |
|
Upflow-1L (empty bed volume) |
Continuous, 24oC. Acetate based synthetic WW and NH4-N rich synthetic WW, flowrate: 1.6 L/d. HRT=14 h. Influent COD of 1-1.2 g COD/L. App. voltage was 1.3 V for acetate based WW. and App. voltage was 0.6-0.75 V for NH4-N rich WW (0.5-0.65 g COD/L). |
0.27 L CH4/g COD for acetate WW, 0.22 L CH4/g COD for ammonium rich WW |
81-87% |
Inoculum for MECs was the effluent of an MFC. At the condition of non-voltage application (only AD), methane production decrased. COD removals were not effected from the voltage application. The lowest methane production occured when real domestic WW was used. |
[44] |
|
SC-0.625 L, d:10 cm, h:10 cm, cylindrical |
SBR, 35oC. Substrate was acetate. OLR and HRT were 2.0 kg COD/m3/d and 20 days respct. Electrodes were placed at distances of 1, 3 and 5 cm from each other. Mixing velocity was also changed btw. 30 and 60 rpm. V: 0.3 V. |
0.33-0.34 L CH4/g CODrem.(0.26-0.27 L/L/d) for 1 cm distance. |
71-78% at all MECs, but higher in small distances |
Reactors were inoculated with anaerobic sludge. Higher mixing velocity enhanced the CH4 production at higher electrode distances (3-5 cm). Small distance of electrodes had the best CH4 production and performance. |
[45] |
|
Combined AD+MEC; 20 L, cylindrical |
Sequencing batch (SBR), 35oC. Food waste; 60 g TCOD/L; OLR=3 kg/m3/d, HRT=20days; V:0.3 V |
17 L CH4/d; 0.34 L CH4/g CODrem. |
76% COD; 73 % VS |
AD and combined AD+MEC reactors were compared in terms of methane production performances for start-up, intermediate and final stages. |
[46] |
|
Combined AD+MEC; 20 L, cylindrical |
SBR, 35oC. Food waste; 171± 38 g TCOD/L; OLR=8 kg TCOD/m3/d, HRT=20days; Electrodes had biofilm on them already when the reactors were started. Applied voltage was 0.3 V. |
62±2 L/d and 0.36 L CH4/g CODrem. (at the stabilized period) |
Cal. ~88 % of TCOD removal at the stabilized period |
AD and AD+MEC reactors were compared in terms of CH4 yield and CH4 production for dif. periods. AD+MEC stabilized 40 days faster than AD only. |
[47] |
|
Reactor type/ volume |
Operation; Substrate; OLR/HRT/Feed cycle; Voltage |
Methane production |
COD (VS) Removal |
Additional info |
Ref. |
|
SC-1100 L rectangular, h: 0.7 m, w: 0.64 m, l: 2.3 m |
Continuous, 31oC. Substrate was winery WW, HRT=1 day. Feed conc. was within a desired range of 0.7−2.0 g SCOD/L. pH was controlled (pH>6). Acetate amendment was used to enhance the electrogenic microorgorganism conc. V: 0.9 V. |
Total gas (CH4 and H2) production of 0.15-0.28 L/L/day (86±6 % CH4) |
SCOD removal 62±20% (btw. the days of 60-100) |
144 electrode pairs in 24 parallel modules were used in the MEC. Enrichment of the MEC accomplished about 60 days after. Mixing, chemical addition and substrate distribution were accomplished through a recirculation line. |
[50] |
|
TC-Integrated (0.7 L catho.+0.5 L anode) |
Continuous, 25oC at HRT of 1 day. Acetate (1.5 g/L) and glucose (2.5 g/L in 50 mMPBS) were used as the carbon sources in the anode and cathode respectively. OLR=2.5 kg/m3/d of glucose. Cathode was the AD unit for CH4 production. V: 0.8 V. |
0.07 L CH4/L/d for glucose; 0.247 L CH4/L/d for SFL |
40-60% for both glucose and SFL |
The cathode was the inner cylinder and the anode was the outer cylinder surrounding cathode. Sludge fermentation liquid (SFL) containing acetate, polysaccharide, protein and VFAs was also fed to the cathode subsequently. OLR of SFL was 3.8 kg/m3/d. |
[51] |
|
TC- 0.5 L Anode + 0.265 L Cathode |
Continuous, 23oC. Pig slurry, 7.87 kg COD/m3/d and 0.44 kg N/m3/d. HRTanode=32.4 h, HRTcathode=14.1 h. 0.8 V set potential vs. SHE. |
79 L CH4/m3/d for MEC; 450 L CH4/m3/d for CSTR |
24±8 % for MEC; 54±8% for CSTR; 30±6% N-NH4 for MEC |
A thermophilic anaerobic 4 L CSTR was connected in series with a TC-MEC (1 L). Cathode was fed with synthetic solution. And inoculated with 30 mL of resuspension liquid of an UASB reactor effluent. |
[52] |
|
Upflow MEC- 5.6 L, cylindrical |
SBR, 35oC. Synthetic brewery wastewater (65.3 g COD/L). OLR and HRT were 5.8 g COD L/d and 5.6 d respectively. Applied voltages were 0.5 and 1 V. |
0.91 and 1.16 L/L/d for reactors with GAC and PAC respct. at 0.5 V. |
> 90 % for TOC at 0.5 V |
Two different particle sizes of coal-based activated carbon (5 g/L), [GAC (0.84–2.00 mm) and PAC (75–177 μm)] were added in two different reactors to compare their performances. |
[53] |
|
SC-0.18 L |
Fed-batch, 22-23oC. Raw sludge conc. was 7.9±2 g COD/L. SRT of 7, 10 and 14 days were applied. Feeding was done by the replacement of treated waste with the raw waste in every 7 days. V: 1.2 V. |
CH4 rate was 95 % in the biogas. Cal. production was btw. 14 to 25.6 mL CH4/d for all SRTs |
30-34 % at all SRTs |
Carbon fiber brushes were pretreated. No catalyts were used on cathode.The reactors were started with an influent containing 50% digested sludge and 50% waste activated sludge (WAS). |
[54] |
|
SC-0.8 L, cylindrical |
Fed batch mode (Fed 3 times in a week), 34oC. Anaerobic sludge from WW plant was the inoculum (10 g VSS/L). SRT was 20 days. Molasses as a biorefinery sidestream was the substrate (OLR=2 g COD/L/D). Applied voltages were 0.5, 1 V and open circuit. |
0.57-0.59 mL CH4/L/d for MECs including open circuit MEC also. |
The distance btw. electrodes was 1 cm. 60 days of biomass retention on the electrodes was effective on the biogas production. Similar results of voltage applied MECs and open circuit MEC showed the effect of biomass retention on the electrodes. |
[56] |
|
|
SC- 3 L total volume, cylindrical |
Continuous fllow, 21oC. Synthetic WW with acetate (450 mg COD/L) and domestic WW (78 mg COD/L)were fed to the MEC at HRTs of 4, 8, 12 and 24 h at different stages. Applied voltage was 1 V. |
61 mL CH4/L/d for synthetic WW and 12 mL CH4/L/d for domestic WW at 8 h and 24 h HRT respct. |
~50% for synt. WW and real WW at 8 h and 24 h HRTs respct. |
Synthetic WW and real domestic WW were used as influents for the MEC. A piece of polyester cloth with 0.6 mm thickness was sandwiched between the anode and the cathode to avoid electrical contact. |
[57] |
|
SC- 2.4 L (26.7cm x 11.5cm x 8.8cm) |
Continuous flow, 30oC. The medium contained 1 g/L acetic acid with buffer. HRT: 1 d. Anodes and cathodes were grouped into four each with two pairs of anode and cathode. 0.9 V was applied. |
0.118 L CH4/L/d |
Between 31-47% |
At the first 10 days of operation there was significant H2 production from MEC. On the 3rd day max. H2 production was 0.53 L/L/d. However H2 production decreased afterwards. |
[58] |
|
TC-2x0.86 L, rectangular: 17×17×3 cm3 |
Continuous flow in anode, 25oC. Anolyte: synthetic WW with acetate flushed with N2/CO2 (70/30) mixture, OLR=1.08 g COD/L/d, HRT=0.6 d. Catholyte: synthetic WW without acetate flushed with N2/CO2 (70/30) mixture and recirculated. Anode was controlled at potential of +0.2 V (vs. SHE). |
0.28 L CH4/ L/d (91.2 meq/L/d) |
Acetate removal of 94 ± 1% |
103 mL of catholyte was replaced by the effluent of anode that was coming through PEM resulting in an HRT of 8.35 d. Electrodes and PEM were pretreated. pH was controlled in catholyte by gas mixture of N2/CO2. |
[59] |
|
TC-2x0.86 L, rectangular: 17×17×3 cm3 |
Continuous flow in anode, 25oC. Anolyte: synthetic WW with acetate flushed with N2/CO2 (70/30) mixture, OLR=1.08 g COD/L/d, HRT=0.6 d. Catholyte: synthetic WW without acetate flushed with N2/CO2 (70/30) mixture and recirculated. Set anode potentials of btw. +0.2 V and −0.2 V (vs. SHE) |
Increased from 4.2 mmol/L day 9.7 mmol/L day when set potential changed from -0.2 V to -0.1 V |
Acetate removal rate increased from 35% to 88% when set potential changed from -0.2 V to -0.1 V |
103 mL of catholyte was replaced by the effluent of anode that was infiltrating through PEM resulting in an HRT of 8.35 d. Electrodes and PEM were pretreated. pH was controlled in catholyte by gas mixture of N2/CO2. |
[60] |
|
Reactor type/ volume |
Operation; Substrate; OLR/HRT/Feed cycle; Applied voltage |
Methane production |
COD (VS) Removal |
Additional info |
Ref. |
|
SC-15 L (d:28 cm, h: 41 cm), cylindrical |
Fed-batch (SBR), 35oC. Food waste, TCOD : 63 g/L, VS :37 g/L. OLR= 2 kg VS/m3/d, HRT=20 days. Applied voltage was 0.3 V. |
0.56 L CH4/g VSrem. or 0.34 L CH4/g CODrem. |
Anodes and cathodes were combined together with a non-woven fabric (1mm thick) placed btw. electrodes. In MEC, CH4 production enhanced due to the increased bacterial populations compared to AD. |
[61] |
|
|
SC (0.35 L), TC(0.5 L anode+0.35L cathode) |
SBR, 55oC. 3.5 mL glucose (1 M) was fed as substrate to MECs daily. In anode of TC-MEC, 350 mL of 150 mM NaHCO solution was used as the electrolyte. Set cathode potential of -0.8, -1.0 and -1.2 V (vs.Ag/AgCl) were applied. |
TC: 75-93% CH4, ~0.7 L/L/d; SC: 54% CH4, ~0.5 L/L/d at -1.2 V. TC: 60-77% CH4, ~1.0 L/L/d; SC: 56% CH4, ~1.0 L/L/d at -1.0 V |
SC and TC reactors were compared in terms of their performances. Thermophilic inoculum was used at both MECs. The distance between TC electrodes was 10 cm. CH4 content was higher in TC and SC MECs compared to control. |
[62] |
|
|
Combined MEC+AD (0.8 L) |
Semi continuous, 37oC. Cattle manure was fed once a day at HRTs of 5, 4, 3 and 2 days. OLR increased from 6.0 to 30 g VS/L/d. Applied voltages were 0.3, 0.6 and 1.0 V. |
Btw. 1.33 (at 0.3 V) -3.99 L (at 1.0 V) CH4/L/d and 0.103 - 0.217 L/g VS |
Btw. 16.47 - 40 % COD removal (13.95 % - 34.1 % VS) |
CH4 production and organic removal rates presented here are the max. and min.values obtained in the study. pH adjustment and manure pretreatment were not applied. |
This study |
|
Abbreviations: MEC: microbial electrolysis cell; AD: anaerobic digestion; COD: biochemical oxygen demand; TC: two chamber; OLR: organic loading rate; HRT: hydraulic retention time; conc.:concentration; PEM: proton exchange membrane; SHE: standart hydrogen electrode; btw: between; SC: single chamber; SCOD: soluble COD; AD-MEC (MEC+AD): combined MEC and anerobic digestion; VSS: volatile suspended solid; rem.: removed; SBR: sequencing batch reactor; TCOD: total COD; respct.: respectively; WW: wastewater; UASB: upflow anaerobic sludge blanket reactor; app.:applied; dif.:different; synt.:synthetic: MFC: microbial fuel cell; cal.: calculated; WAS: waste activated sludge; PBS: phosphate buffer solution; GAC: granular activated carbon; PAC: powered activated carbon; TOC: total organic carbon |
|||||