Performance evaluation of various bioreactors for methane fermentation of pretreated wheat straw with cattle manure

2.1 Pretreatment of wheat straw

Fresh wheat straw samples were obtained from a local farm of New Delhi, India. The samples were ground to particle size <1–2 mm by using a blender (Hummer 900, Delhi, India) and stored until used in experiments. Fresh cattle manure was also obtained from a dairy farm situated in New Delhi. The pretreated sample of wheat straw was blended with cattle manure in a ratio of 40:60 for the anaerobic digestion process. Details of the pretreatment carried out on wheat straw are given below.

The feedstock material with a combination of calcium hydroxide (Ca[OH]₂) and sodium carbonate (Na₂CO₃) was incubated for 48 h at ambient temperature which varied from 25°C to 30°C [4]. The concentration of both catalysts, i.e. Ca(OH)₂ and Na₂CO₃, was 3% on the basis of weight- to-volume ratio. The ratio of substrate-to-dissolved catalyst liquid was 10% on the basis of weight-to-volume ratio.

2.2 Reactors experimental setup

A pilot scale study on effective methane fermentation of wheat straw was conducted at the Micro Model Complex of the Indian Institute of Technology Delhi. Experimental reactors with a capacity of 2.2 m³ were fabricated with some of the necessary modifications required for agricultural residues. Three reactors were installed for individual experimental evaluation. One was a CSTR, a second reactor was an FFR, and a third reactor was a conventional floating drum reactor (CR) which was used as the control reactor. These three reactors were modified for use of wheat straw biomass as feed material into them. The inlet and outlet diameter of all three reactors was 152.4 mm. The guide frame was improved in all reactors. The CSTR reactor was equipped with a steel made stirrer for mixing of the substrate, which enabled ease of accessibility of substrate to the anaerobic microorganisms. The stirrer was operated by using a 0.746 kW electric motor connected through a gear box, which maintained the speed of rotation of stirrer as 10 rpm. The fixed film in the FFR was made of bamboo and fitted vertically into the reactor. The CR was used as a control to compare the performance of the other two reactors. Details of these three reactors are presented below.

2.2.1 CSTR experiment:

The CSTR with a capacity of 2.2 m³ was operated in semi-continuous mode for a period of 90 days. Figure 1 shows the schematic and pictorial views, which represent various components attached with the CSTR. The main reactor had a provision to connect with a separate floating drum, with a capacity of 1.0 m³, which enabled measurement of the volume of biogas production, produced from the reactor. The reactor was fed with 25 kg of substrate once in a day, with the optimized ratio of pretreated wheat straw to cattle manure. Total solids concentration in the fed substrate was maintained at 10%, with an organic loading rate of 2.2 kg volatile solids (VS)/day to the reactor.

Figure 1:

Schematic and pictorial view of various components attached with the continuous stirred tank reactor (CSTR).

2.2.2 FFR experiment:

The FFR with a capacity of 2.2 m³ was also operated under mesophilic temperature for a period of 90 days. Fixed films were prepared of bamboo in a mesh shape. Figure 2 shows the schematic and pictorial views of the FFR with its various attachments. Film material was selected by considering factors such as long life, strength, cost, local availability, and noncorrosive nature. The shape of the fixed film was prepared by keeping the following points in view: i) it should provide a large surface area for immobilizing the microbes and at the same time it should occupy less space; ii) it should not clog the substrate; and iii) height of the film should remain immersed in the slurry. The produced biogas quantity was measured by using the floating drum. The reactor feeding rate, total solids concentration, and organic loading rate were maintained the same as in the case of the CSTR experiment (as mentioned under section 2.2.1).

Figure 2:

Schematic and photographic view of the fixed film reactor (FFR).

2.2.3 CR experiment:

The CR with a capacity of 2.2 m³ was also operated in similar conditions as those of the CSTR and FFR. This reactor was used as a control for the CSTR and FFR. Figure 3 shows the schematic and pictorial views of the CR with its various attachments. The reactor feeding rate, total solids concentration, and organic loading rate were maintained the same as in the case of the CSTR experiment (as mentioned under section 2.2.1).

Figure 3:

Schematic and photographic view of the conventional reactor (CR).

All three types of semi-continuous reactors were fed with the optimized pretreated substrates of wheat straw: 3% Ca(OH)₂+3% Na₂CO₃ (temperature 33.66^oC, incubation time 27.87 h) blended with cattle manure in the ratio of 40:60 (pretreated wheat straw: cattle manure), maintaining total solids contents of 10% in the substrate. The methane fermentation experiments were carried out from March 2012 to August 2012. The inoculum used for start-up of the reactors was developed individually for each reactor by charging the full volume of reactors with fresh cattle manure at once and keeping for 60 days for full-flash developed inoculum in each reactor.

2.3 Analytical tools and methods

Measurements of daily gas production, substrate temperature, pH of slurry, volatile fatty acids (VFA), and gas composition were performed. Slurry samples were collected from each reactor after every 5 day period, and were analyzed for proximate parameters, total carbon, total nitrogen, phosphorous, and potassium contents. Total nitrogen was analyzed by the CHN analyzer (vario EL Perkin Elmer, USA). Total solids, VS, phosphorous (P) and potassium (K) were analyzed according to standard analysis methods of American Public Health Association [15]. Total VFA was determined using gas liquid chromatography (Nucon 5700), fitted with an Flame ionization detector (FID), a thermal conductivity detector and a 6 m×3 m Chromasorb 101 column. The concentrations of methane and carbon dioxide in produced biogas were analyzed by using a gas chromatograph (Model 7890A, Agilent, Germany) fitted with a Porapak Q column of stainless steel with diameter of 3.175 mm, length of 2.74 m and with a thermal conductivity detector.

3.1 Reactor operating parameters

3.1.1 Operating substrate temperature

Figure 4 shows the temperature of the substrate observed during the anaerobic digestion process. The maximum to minimum variation of substrate temperature for the CSTR was found to be 34.0–38.0°C. However, the variations of substrate temperature in the cases of the FFR and CR were observed to be 35–39°C, and 36.0–39.5°C, respectively. The observed results clearly showed that all three digesters were operated under mesophilic temperature conditions. The optimum temperature for mesophilic operation of anaerobic digesters is reported to be within 30–35°C with minimum to maximum marginal limits of 20–40°C [16]. It has been reported that an increase in operating temperature also increases ammonia toxicity [5, 17]. Furthermore, mesophilic temperature digesters have been found to improve more volatile degradation rates as compared to thermophilic digesters. However, these digesters require a higher hydraulic retention time at a mesophilic temperature [18]. The operation of anaerobic digesters under mesophilic temperature is suitable in Indian climate conditions, which avoids extra energy input cost required in temperature maintenance.

Figure 4:

Variation of substrate temperature during operation of various reactors.

3.1.2 Substrate pH

The variations in pH values of substrates in different reactors during the methane fermentation are shown in Figure 5. The initial pH values of substrate just after feeding into the CSTR, FFR, and CR were found to be 7.5, 7.6, and 7.2, respectively. The minimum and maximum values of variation of pH while operating the CSTR digester were observed to be 7.5 and 8.4, and similar values of 7.6 and 8.5 were found in the case of the FFR. Further, the minimum and maximum values of pH for the conventional digester were 7.2 and 8.2, respectively.

Figure 5:

Variation of substrate pH in different reactors during methane fermentation process.

It is clearly evident from the figure that the observed pH values do not show a clear and stable relationship among the three different reactors. A rapid drop in pH for all reactors was observed during the third week of digestion, and started to increase followed by stabilization after the fourth week. The optimum pH for operation of anaerobic digesters has been reported to be 6.8–7.2, with marginal values of 6.6–7.6 [16]. Thus, the observed results in all three cases of reactor operation pH values were within the normal limit. The plotting line of substrate’s pH for the FFR was found in between the CSTR and CR plots. The pH value was found to increase on 49th day in the case of the FFR, which might be due to the accumulation of some ammonia, while a decrease in pH was observed in the case of the CSTR on the 21st day, which resulted due to the accumulation of VFA in the reactor (as clearly evident in Figure 6) caused by the higher digestion of organic matter to acids.

Figure 6:

Observed production of volatile fatty acids in various reactors during methane fermentation.

3.2 Performance analysis of CSTR

Daily biogas production yield of various reactors (CSTR, FFR, and CR) is shown in Figure 7. Further, the volumetric methane concentration in biogas produced from various reactors, and daily methane production yields are shown in Figures 8 and 9, respectively. The methane content in biogas produced from the conventional reactor was observed to be in the range of 55–56% during the course of investigation. Further, the observed variation in methane content for the CSTR was found to vary from 55% to 57%, and was found almost similar to that of the conventional reactor.

Figure 7:

Variation of daily biogas production yield in different types of reactors.

Figure 8:

Volumetric concentration of methane in biogas produced in different reactors.

Figure 9:

Variation of daily methane production yield in different types of reactors.

The performance of the CSTR was found to have only marginal increase in methane production yield compared with that of the conventional reactor (CR). Methane production yield of the CSTR reactor was found to be 0.289 m³/kg VS, with a biogas production yield of 0.538 m³/kg VS, as shown in Table 1. The observed average VS degradation efficiency was found to be 32.0%, as shown in Figure 10. The methane production yield was found to be 10.6% higher compared to the conventional reactor, although the substrate inside the reactor was continuously mixed thoroughly using mechanical stirrer.

Figure 10:

Volatile solids degradation efficiency during methane fermentation in different reactors.

The observed results showed that continuous mixing of substrate is not a desirable option for enhancing bacterial activities to get higher methane production yield. Further again, continuous stirring requires more energy input in the process. In an experimental study conducted on the effect of mixing on methane production yield, it was found that in comparison to continuous mixing, in intermittent and minimal mixing strategies methane production improves by 1.3% and 12.5%, respectively [3]. Moreover, the stirring is responsible for up to 54% of the power consumption of current biogas plants, and it has been recommended as a possibility to operate biogas plants more efficiently to reduce the energy consumption by avoiding constant stirring [20].

3.3 Performance analysis of FFR

Daily biogas production yield of THE FFR is again shown in Figure 7, and the volumetric methane concentration in biogas produced, and daily methane production yield are shown in Figures 8 and, respectively. The observed methane content for the FFR was found to vary from 56% to 60%, and was found to be the highest compared to the other two reactors. A minimum, but considerable increase in methane content was observed in the case of the FFR compared to the other two reactors (CSTR and CR).

It has been reported that the methane content in the produced biogas is dependent on the breakdown of smaller organic matter (such as acetate, carbon dioxide, and hydrogen) into methane through both acetoclastic and hydrogenotrophic pathways [16]. The increased methane content in the FFR was due to enhanced performance of microbial consortium of methanogenic archaea, which depends on many parameters, i.e. pH, temperature, and inocula. It has been reported that the FFR provides a large surface area for microbes to stay for a long time, therefore, increasing the conversion efficiency of volatile matters into methane with the increase in population of methanogenic bacteria [8].

It is evidenced from Table 1 that the observed mean methane and biogas production yield for the CR was 0.264 m³/kg and 0.484 m³/kg of VS, respectively. The mean methane production yield during anaerobic digestion of wheat straw co-digested with cattle manure in the FFR was found to be 0.342 m³/kg VS, with a specific biogas production yield of 0.601 m³/kg VS. The observed VS degradation efficiency was found to be 34.75%. Low methane production was observed during the initial phase of the methane fermentation process, which is obviously due to the hydrolysis process, and transformation of intermediates took place very slowly. The observed results showed that process performance, biogas production, and methane production yields were better than with other reactors at the same organic loading rate, feed ratio, temperature, and volume of rectors. A maximum methane production yield of 0.414 m³/kg VS was noticed on the 14th day of reactors operating with co-digestion of wheat straw with cattle manure, which resulted from increase in operating temperature causing a higher degradation rate of VS. The FFR also showed a slight increase in biogas as well as methane productions in the 2nd month, again due to increase in operating temperature.

In overall analysis, the results from the pilot scale study conducted on co-digestion of pretreated wheat straw with cattle manure revealed that the reactor configuration has a profound influence on the anaerobic digestion of the substrates. The process was found to be more efficient in the case of the FFR than in the case of the CSTR and CR. The obtained methane yield increased by 41.1% (included startup period) compared to the control reactor (CR). Enhanced performance of the FFR is due to enhancement in the bacterial activities, as because of the fixed film conditions, the anaerobic microbes are retained for a longer duration in the substrate, and the fixed film does not allow them to wash out through the outlet of the reactor. The layering of microorganisms for non-stirred digesters was investigated by Alexandra et al. [20]. In the first case, the digester was not stirred for a period of 24 h, and in the second case, the digester was not stirred for a period of 2 months to investigate the effect of layering of microorganisms in the digesting substrate. The results revealed that the methanogene microorganisms prefer the lower layers of a non-stirred digester. Further, the performance of the CSTR and FFR confirm that the FFR reactor configuration is simpler to operate and comparatively more efficient than the other reactors.

3.4 Manurial analysis of digested slurry

The manurial contents (nitrogen, phosphorus, and potassium) in the digested slurry obtained from all three reactors are presented in Table 2. Maximum nitrogen content was found in the CSTR as 1.16% and an almost similar amount of nitrogen content was analyzed in both the FFR and CR as 1.13%. Phosphorus content in the digested slurry was found as highest 0.56% in the FFR digestate. However, it was observed to be 0.51% and 0.54% for the CR and CSTR, respectively. Further, the potassium content in the digested slurry from the CR, CSTR, and FFR was observed as 0.86%, 0.86%, and 0.87%, respectively. The digested slurry is considered as good quality organic manure which can replace chemical fertilizers in agricultural crop production systems, along with the soil health maintenance, and promote organic farming.