Comparative Kinetic Study of Anaerobic Treatment of Thermally Pretreated Source-Sorted Organic Market Refuse

The present study deals with extensive investigations of the effect of thermal pretreatment on whole-sale market rejects for their biogas production potential. Market reject considered as biomass for this study has been treated at two different temperatures 85C and 135C for 8 h each and subjected as feedstock for anaerobic digestion (AD) process. The AD process has been operated in the mesophilic range (35–38C) of bacterial growth. Various kinetic models have been used to simulate the experimental data. Kinetic modeling revealed that biogas production rate exhibited better coefficient of determination (R) in the range of 0.973–0.989 with exponential model for the ascending limb whereas the descending limb resulted in good linear correlation with R2 as 0.911–0.976. Logistic growth model and Gompertz relation simulation of cumulative biogas production resulted in better R2 values in the range of 0.994–0.997 and 0.998–0.999, whereas the R2 values for exponential rise to maximum plots ranged from 0.722 to 0.800.


Introduction
Anaerobic digestion (AD) is the process of waste treatment leading to biogas sanitation.Several biodegradable substances freely occurring in the environment in the form of market refuse, organic kitchen wastes, farm wastes, yard wastes, sewage, manures, and so forth together form municipal solid waste possessing enough potential for energy recovery to be utilized as substitute for nonrenewable sources of energy.AD is well-suited waste treatment technique for wastes containing higher levels of organic matter and much less expensive than aerobic waste treatment processes as there is no requirement of energy for aeration systems and nevertheless the volatile solid removal rate from the wastes is higher due to rapid volatilization of wastes to biogas.Biogas can be upgraded to the quality of natural gas and can be further used for power automobiles.Methane (CH 4 ) is a potential component of biogas that possesses a calorific value of 6 kwh/m 3 that corresponds to half a litre of diesel oil [1].The resulting sludge from the process is biologically more or less stable and rich in nutrients which makes it a valuable soil conditioner and fertilizer.Regular withdrawing of biogas produces the chances of global warming and gives a source of renewable energy.The composition of biogas is mostly methane (50-75%), carbon dioxide (25-50%), hydrogen (5-10%), nitrogen (1-2%), and hydrogen sulphide (traces) [1][2][3].
The process is globally carried out in four steps, namely, hydrolysis, fermentation and acidification, acetogenesis, and methanogenesis (Figure 2).These sequential stages are carried out by separate microbial populations, namely, the hydrolytic, fermenting, acetogenic, and methanogenic bacteria.None of the groups can produce biogas alone and hence they work together unitedly to synthesize biogas [1].The cleavage of a chemical compound through reaction with water is known as hydrolysis.A hydrogen atom gets attached to one side of the split chain of compound and the remaining hydroxyl group of water gets attached to the other side.Hydrolysis is the first step of AD process where carbohydrates are broken down to sugars, proteins to amino acids, fats to fatty acids, and so on.The second step of AD process is fermentation where the fermentative bacteria transform sugars and other organic products of the hydrolysis step to organic acids, alcohols, hydrogen, carbon dioxide, and ammonia.Acetogenesis is the third step of AD process where the acetogenic bacteria utilize the products of the fermentation step and produce hydrogen, carbon dioxide, and acetic acid by utilizing present carbon of biomass and the dissolved or bound oxygen.Hence, the acid producing bacteria create anaerobic environment for the methanogens.Further, methanogenesis is the final step of AD process where the carbon dioxide, hydrogen, and acetic acid are converted to methane and carbon dioxide and variable trace quantities of nitrogen, hydrogen sulphide, and other components.Operating temperature is one of the important process parameters needed for the design of biogas generation [4].Likewise, other essential process parameters to be considered are pH, TS, C/N ratio, COD, mixing, and reactor setup.
Though anaerobic treatment is one of the best methods for efficient solid waste management, the kinetics of the process is very slow.Thermal pretreatment of the wastes carried out in this research work has been observed to improve the kinetics of AD process in terms of higher biogas yield as compared to the process with untreated wastes within the same hydraulic retention time of 45 days under mesophilic conditions (35-38 ∘ C).The increased interest in the process stimulated the idea of mathematical modelling and simulation which is much less expensive than performing extensive laboratory experiments.Application of sophisticated methods of process control is only possible if mathematical models are available for the system to be optimized.Several simulation models of AD process have been proposed [5][6][7][8][9][10][11][12][13].The models described require simultaneous solutions of mass balance equations for each individual substrate and bacterial population, yielding equations with numerous unknown parameters.Hence, a simpler approach has been made in this study to describe the anaerobic biogas fermentation process.
This work primarily deals with study of the biogas production kinetics for the description and evaluation of methanogenesis by fitting the experimental data of biogas production to linear, exponential, and Gaussian equations.In addition the cumulative biogas production has been simulated using logistic growth model, exponential rise to maximum, and modified Gompertz plots.
The main goal of our work is to compare and contrast between low and high temperature treated and untreated wastes.The extensive research work during this course of study has been carried out from January 2015 to March 2015 in the Energy and Combustion Laboratory, Department of Chemical Engineering, Jadavpur University, Kolkata, India.

Sample Collection and Preparation
. Source-sorted OFMSW has been collected locally from places in and around Jadavpur University campus, cleaned, and sun dried for 2 days with exposure to bright sunlight for 8 hours daily in accordance with the working hours of the University in order to remove the superficial moisture.After sufficient drying, bone dry biomass has been graded and sorted to prevent inclusion of unwanted and possible contaminant materials (detergents, sand, dirt, dust, bones, etc.).
Further the dried materials have been charged for particle size reduction using a Wiley mill (make: DLF Motors India Pvt.Ltd.) and passed through IS sieves of size 7 mm.The free-flowing finely ground and screened sample of OFMSW has been stored in air tight vessel to prevent absorption of atmospheric moisture, till use as feed material for AD.Fresh cattle manure (CM) has been obtained from nearby cow shed around the University campus where cows are fed with special feed consisting of silage, hay forage, agricultural residues, flour mill residues, and so forth.Finally, CM has been collected from randomly selected 5 cows and sun dried on plastic trays for 2 days.Similarly, the dried CM has been finely ground using ball mill (make: SC Dey & Co.) and strained using IS sieves of 1 mm size, filled in containers (Scott-Duran), and kept in refrigerator at 4 ∘ C till further use.1).Fresh CM slurry has been used as inoculum in this study, as it contains all the required groups of microbial consortium essential for AD process.S/I ratio has been maintained as 2 for this study and accordingly requisite amount of CM has been soaked in distilled water and stirred at 500 rpm using a magnetic stirrer for 30 min to form slurry and kept in incubator at 37 ∘ C for 7 days.Later the conical flask and its contents have been kept in refrigerator at 4 ∘ C until use.Prior to use the inoculum has been incubated for one week at 37 ∘ C with constant mixing to reactivate microbiological activity.The pH, total solid, and volatile solid of the inoculum have been determined, discussed in characterization subsection, and tabulated in Table 2.

Thermal Pretreatment.
Thermal treatment has been widely studied earlier and most successfully applied at industrial scale operations [14][15][16] but no systematic research works on various temperatures and treatment times have been done to enhance the biodegradability of OFMSW.Thermal treatment causes disintegration of cell membrane leading to solubilization of organic compounds [17][18][19][20].Heating has been achieved by various methods, namely, steam, electric, and microwave.No significant difference has been observed between steam and electric heating though microwave heating solubilized more biopolymers [21].In present study, thermal pretreatment at both low (<110 ∘ C) and high (>110 ∘ C) temperature has been studied.Heating of biomass has been achieved by means of electric heating in a hot air oven (make: SICCO Instruments).For instance preheating at 85 ∘ C and 135 ∘ C has been conducted for 8 h, respectively, and resulted in enhancement of biogas production as compared to that of untreated biomass.A trial experiment has also been conducted at preheating conditions of 165 ∘ C and did not result in any enhancement of biogas production; rather hardening and darkening of biomass have been observed and hence are not reported in present study.The dark brownish color development of the substrate indicated occurrence of Maillard reactions.In this regard, for lignocellulosic biomass, temperatures above 160 ∘ C results in solubilization of hemicellulose and lignin, which are phenolic compounds inhibitory to anaerobic microbial consortium [22].Exposure of substrate to severe conditions can result in loss of volatile organic matter and decrease the potential biogas production [23].Hence, higher solubilization of substrate components can be achieved with lower temperature but longer retention times [23].

Feedstock Preparation.
Thermally pretreated and untreated samples of OFMSW have been mixed with inoculum prior to use as feed to bioreactor.Table 2 represents the results of characterization of biomass on dry weight basis.A study reported that total solid content in the range of 7.4-9.2%with CM exhibits best performance for digestibility [24].Similarly another study [4] reported that for CM at 35 ∘ C temperature maximum gas production has been obtained with 8% total solid.In present study, a higher TS content of 15% and initial substrate concentration of 100 g L −1 have been maintained for all the reactors.Hence, biomass and inoculum have been mixed with soft water in such a manner that the total solid content of test reactor is maintained at 15% of the effective volume of reactor.S/I ratio has been maintained as 2 on the basis of total solid content of the test reactor.The characterization results revealed that biomass possesses sufficient volatile matter indicating good potential for generation of biogas.Rubber cork has been fitted tightly to the neck of each bottle to help maintain the anaerobic environment.Three glass tubes made of borosilicate glass have been fitted to rubber cork tightly.Each glass tube consisted of a glass knob or valve at the opening end.Among three tubes one of them has been used as gas delivery tube and the other two have been kept dipped in slurry, one for sampling out from time to time for determination of pH, total solid, and volatile solid.A cylindrical jar essentially fitted with glass knobs or valves at both ends, made of borosilicate glass and with 500 mL capacity, has been used for measuring biogas by water displacement method.The gas delivery tube has been connected to one end of the jar by means of rubber tube.
Similarly the other end of the tube has been connected to a glass jar of 500 mL capacity to facilitate the volume of displaced water upon commencement of biogas into the jar.
A weighing balance has been utilized for measuring the required mass of cattle manure and biomass.The constant temperature of the processes has been maintained by putting the bench top reactors into an incubator (make: SC Dey & Co.).A digital pH meter (make: Eutech Instruments, Singapore) has been used to determine pH of the fermentation slurry at an interval of 48 hours.Certain drop in pH of fermentation slurry has been adjusted by the addition of 4 to 5 drops of 6N NaOH solution.The entire experiment has been conducted for a period of 45 days till significant reduction in the production of biogas.Apart from test reactors, CM has been digested alone at a loading rate of 100 g L −1 as a control reactor against all the test reactors so as to monitor the performance of AD process of untreated and treated biomass.All the reactors have been subjected to gentle shaking and swirling prior to monitoring of gas production, once in every 24 hours to maintain intimate contact of microorganisms with the substrate.The reactors with individual processes have been designated as group A (biomass treated at 135 ∘ C), group B (biomass treated at 85 ∘ C), group C (untreated biomass), and group D (control reactor consisting of inoculum).Biogas production has been observed to be slow at the beginning and end of AD process because in batch reactors biogas production directly corresponds to the specific growth rate of methanogens [24,25].
During this process a small amount of atmospheric air enters the headspace but in insufficient quantities to affect the redox conditions in the reactor: any nitrogen detected in the gas composition is corrected for, as this is not normally produced as a result of the digestion process [26].

Setup of Biogas Measurement.
In order to prevent the dissolution of biogas into water, brine solution has been prepared and used to measure yield of biogas by displacement of water (Table 1).Acidified brine solution has been prepared according to the process mentioned by [27].Laboratory grade sodium chloride has been dissolved in distilled water until formation of a supersaturated solution and further amount of 4 to 5 drops of concentrated sulfuric acid has been added to lower the pH.As soon as the glass cork of delivery tube was opened, the positive pressure of biogas within the reactor caused the biogas to flow through the delivery tube and a pressure gradient provided the driving force for displacement of the solution to adjacent bottle connected to the gas measuring jar by means of rubber pipe.Hence, the displaced volume of brine solution has been measured and considered as the amount of biogas produced.Biogas yield has been monitored daily and measured after every 72-hour interval.All gas volumes have been reported as correct to STP of 0 ∘ C and 101.325 kPa.[28].Bulk density has been determined using suitable method involving a graduated measuring cylinder estimating the weight of a known volume.pH has been determined using bench meter (Eutech Instruments, Singapore) with a combination of glass electrode calibrated in buffers 4, 7 and 10.Calorific value of biomass has been determined using Karas-Simek bomb calorimeter following the method of [28].
The temperature-time plot for calorific value determination has been represented in Figure 4. Elemental composition (C, H, and N) has been analyzed using Elemental Analyzer of Indian Association for the Cultivation of Science and presented in Table 3.The values of VS destructions have been calculated for each reactor before and after the digestion test by subtracting the VS contents of the control reactor from that of the test reactor.

Simulation of Biogas Production
Kinetic study of biogas production has been conducted for description and evaluation of methanogenesis by fitting the experimental data of biogas production to various kinetic equations.Biogas production rates of biomass digested with CM have been simulated using linear, exponential, Gaussian, and Gompertz plots.It has been assumed that biogas production rate increases linearly with increase in time and after reaching a peak value it decreases linearly to a low significant value.The linear equation for biogas production rate can be expressed by the equation below [4,11,12]: where  is biogas production rate (mL g −1 VS d −1 ),  is hydraulic retention time in days, and  and  are constants obtained from the intercept and slope of the graph plotted  versus .Slope  is positive for the ascending limb and negative for the descending limb.
The exponential plot can be represented by (2), where it has been assumed that the biogas production rate increases exponentially with increase in time and reaches a peak value and thereafter decreases to a low significant value exponentially with increase in time.Consider where  is biogas production rate (mL g −1 VS d −1 ),  is hydraulic retention time in days,  (mL g −1 VS d −1 ),  (mL g −1 d −1 ), and  (day −1 ) are constants, and  is positive for the ascending limb and negative for descending limb.The Gaussian plot including both ascending and descending limbs can be represented by (3), where it has been assumed that the biogas production rate follows normal distribution over the hydraulic retention time.Consider where  is biogas production rate (mL g −1 VS d −1 ),  is hydraulic retention time in days,  0 is time at which the maximum biogas production occurred, and  (mL g −1 VS d −1 ) and  (day) are constants.Logistic growth equation, Gompertz equation, and exponential rise to maximum have been used to simulate the cumulative biogas production in each case.Equations ( 4), ( 5), and (6) represent linear, Gompertz, and exponential rise to maximum models, respectively.Consider where  is cumulative biogas production (mL g −1 VS),  is kinetic constant (day −1 ),  is hydraulic retention time (days), and  and  are constants.Consider where  is cumulative biogas production (mL g −1 VS),  is hydraulic retention time (days), and  and  are positive numbers. is biogas production potential (mL g −1 ),  is minimum time required to produce biogas (day), and  sets the growth rate (-scaling), which are constants.Consider where  is cumulative biogas production (mL g −1 VS),  is biogas production potential (mL g −1 ),  is kinetic rate constant (day −1 ), and  is hydraulic retention time (days).

Results and Discussions
The research work has been carried out in order to study the influence of thermal pretreatment on the kinetics of biogas synthesis.Exhaustive investigations with thermal treatment of wastes have been carried out at both low (85 ∘ C) and high (135 ∘ C) temperatures for 8 h, respectively.Thereafter the pretreated materials have been charged for AD.Biogas production rate and cumulative biogas production have been plotted against hydraulic retention time as shown in Figures 5(a) and 5(b).Peak biogas production occurred on days 15, 21, 12, and 12 for groups A, B, C, and D. Maximum biogas production rate has been observed in the order of group A (12.77 mL g −1 VS d −1 ) > group C (11.87 mL g −1 VS d −1 ) > group B (11.62 mL g −1 VS d −1 ) > group D (6.58 mL g −1 VS d −1 ).Similarly, cumulative biogas production was also found to be highest for group A (137.73 mL g −1 VS), followed by groups B (101.76 mL g −1 VS), C (89.35 mL g −1 VS), and D (44.81 mL g −1 VS).Hence, thermal pretreatment could enhance the biogas production potential resulting in enhanced ultimate biogas yield as shown in Figures 5(a

Modelling
Analysis of experimental data has been performed in MS-Excel and Curve Expert 1.4 using the solver feature by nonlinear regression.Figures 6(a) and 6(b) represent linear model fitting of biogas production rates for all the groups according to (1) mentioned earlier.The coefficient of determination and standard error values have been determined for all the mathematical models and tabulated in Tables 4 and 5.The coefficient ( 2 ) including both the ascending and descending limbs for all the groups ranged from 0.911 to 0.976.Similarly, Figures 7(a) and 7(b) show the exponential plot of biogas production rates including the ascending and descending limbs for each case.The coefficient of determination  2 for the ascending and descending limb has been observed to be in the range of 0.441-0.989,which signifies that linear regression was better than that of exponential plot.The coefficient  2 for ascending limb shown in Figure 7   The coefficient  2 has been observed to be in the range of 0.994-0.997for logistic plot with maximum  2 value for group B, which signifies that logistic plot for cumulative biogas production is ideal for group B at specified conditions.For the Gompertz relation, the  2 value ranged from 0.998 to 0.999 indicating better coefficient of determination compared to that of logistic model plot.In logistic growth equation, the kinetic constant "" has been found to be in the order of group D (1.758) > group B (1.598) > group A (1.527) > group C (1.494), but cumulative biogas production was in the order of group A (137.73 mL/g VS) > group B (101.76 mL/g VS) > group C (89.35 mL/g VS) > group D (44.81 mL/g VS).
Figure 10(e) shows that experimental cumulative yield of biogas is graphically almost close to simulated results of Gompertz relation curve fitting of cumulative yield of biogas.It signifies that cumulative yield of biogas from biomass follows nonlinear relationship with retention time.
In Figure 11 cumulative biogas production has been simulated using (6) and exponential rise to maximum plots has been presented of which the  2 values ranged from 0.722 to 0.800, indicating poor correlation as compared with Gompertz relation and logistic growth model plots.

Approximate Analysis of Cost-Benefit Ratio
Significant amount of energy has been required for preconditioning of biomass prior to anaerobic digestion.The approximate cost-benefit ratio has been evaluated on the basis of total input energy required for anaerobic digestion divided by the output energy equivalent of biogas yield.Total solid (TS) of biomass charged into each reactor = 75.0g.Total solid (TS) of biomass content of 2 reactors (groups A and B) = (75 × 2) = 150 g.Energy required to remove moisture from total solids of biomass of 2 reactors = (150 × 37.8) = 5670 cal = 5.67 kcal ( in 1 ).

Calculations for Input Energy Needed for Anaerobic
Digestion.Amount of heat required to raise the slurry to reactor operating temperature (35 ∘ C) has been calculated from where  is digester temperature ( ∘ C),   is influent slurry temperature ( ∘ C),   is total weight of slurry added to reactor,   is specific heat capacity of added slurry, and   is given by the equation

2. 5 . Experimental Procedure 2 . 5 . 1 .
Setup of Test Reactors.The schematic diagram of the experimental setup is shown in Figure 3.It consists of a completely randomized arrangement of bench top batch reactors in a 3 × 3 replicated laboratory experiment and has been conducted in a series of 3 laboratory aspirator bottles made of borosilicate glass of 1000 mL capacity each.

Figure 4 :
Figure 4: Temperature-time plot for determination of calorific value.

Figure 5 :Figure 6 :
Figure 5: (a) Biogas production rate of thermally treated (A and B), untreated (C) biomass and inoculum (D) for a retention time of 45 days.(b) Cumulative biogas production of thermally treated (groups A and B), untreated (group C) biomass and inoculum (group D) for a retention time of 45 days.
(a) ranged from 0.973 to 0.989, which is far better than the  2 values of the descending limb as shown in Figure 7(b).Comparing Figures 6(a), 7(a), 6(b), and 7(b) we can say that the  2 values are better in case of exponential curve fitting for ascending limb and linear model fitting is better for descending limb.Hence, biogas production increases exponentially with time and reaches a peak value at a particular time period and then decreases linearly with time to a very low significant value.

Figure 7 :
Figure 7: (a) Exponential plot of the ascending limb of biogas production rate for thermally treated (groups A and B), untreated (group C) biomass and inoculum (group D).(b) Exponential plot of the descending limb of biogas production rate for thermally treated (groups A and B), untreated (group C) biomass and inoculum (group D).
Needed for Thermal Pretreatment.It has been assumed that at pretreatment temperatures of 85 ∘ C and 135 ∘ C only superficial moisture gets vaporized.Initially 1.0 g of sample biomass has been taken for analysis separately.All necessary calculations for 1.0 g sample have been further made for the actual amount (dry weight basis) of pretreated biomass charged into reactors (groups A and B) for anaerobic digestion: Weight of sample biomass taken for analysis = 1.0 g.Latent heat of vaporization of water = 540 cal g −1 .Initial moisture content of sample biomass = 12%.Final (after pretreatment) moisture content of sample biomass = 5%.Reduction in moisture content of sample biomass = 7%.Energy required to remove moisture from 1.0 g sample biomass = (540 × 0.07) = 37.8 cal g −1 .Number of test reactors considered for study of pretreated sample biomass = 2.

Figure 8 :
Figure 8: (a) Gaussian plot of cumulative biogas production for thermally treated biomass at 135 ∘ C (group A).(b) Gaussian plot of cumulative biogas production for treated biomass at 85 ∘ C (group B).(c) Gaussian plot of cumulative biogas production for untreated biomass (group C).(d) Gaussian plot of cumulative biogas production for the inoculum (group D).

Figure 9 :
Figure 9: (a) Logistic growth model plot of cumulative biogas production for treated biomass at 135 ∘ C (group A).(b) Logistic growth model plot of cumulative biogas production for treated biomass at 85 ∘ C (group B).(c) Logistic growth model plot of cumulative biogas production for untreated biomass (group C).(d) Logistic growth model plot of cumulative biogas production for inoculum (group D).

Figure 10 :
Figure 10: (a) Gompertz relation plot of cumulative biogas production for treated biomass at 135 ∘ C (group A).(b) Gompertz relation plot of cumulative biogas production for treated biomass at 85 ∘ C (group B).(c) Gompertz relation plot of cumulative biogas production for untreated biomass (group C).(d) Gompertz relation plot of cumulative biogas production for inoculum (group D).(e) Comparative plot for experimental and Gompertz relation simulated cumulative biogas yield.

Table 2 :
Properties of feedstock and inoculum.
Data shown are the average and standard deviation based on triplicate runs (mean value ± SD); ND = not determined.

Table 3 :
Ultimate properties of OFMSW and CM cake.

Table 4 :
Coefficient of determination.