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Bioconversion of organic residues

Methane from integrated biological systems

T.K. Ghose
Biochemical Engineering Research Centre, Indian Institute of Technology, New Delhi, India

INTRODUCTION

In principle, production of methane by anaerobic digestion is a two-step process: (i) photosynthetic fixation of carbon dioxide into biomass, and (ii) a partial microbial conversion of this biomass into CH₄ + CO₂. Biogas ;is an important energy source for organized use in rural development. It is a gaseous mixture comprised chiefly of methane and carbon dioxide, of which methane (60 to 70 per cent) is the combustible component. In most developing countries, animal dung, agricultural residues, and firewoods are used as fuel. The air pollutants in the smoke as a result of burning these fuels cause serious health problems and should be avoided. Biogas, a smokeless fuel, offers an excellent substitute. The importance of biogas as a fuel has been discussed in several reports in the literature (1 - 3).

CURRENT STATUS

At present, biogas production is carried out employing mainly cow/buffalo dung and, to some extent, domestic sewage. But biodegradable cellulosics, several agricultural residues, and water weeds, in addition to industrial wastes, can serve as resources. Daily availability of dung and a few of the major industrial wastes in India is shown in table 1. Computed daily availability of fuel gas, its energy equivalent, and NPK nutrients from these wastes are presented in table 2. Evaluation of some other agricultural residue materials not used currently in bioenergy production is given in table 3. It appears that, although biogas production is a universally important project, biodegradable waste resources have not been fully exploited for their large-scale conversion ;into methane anywhere in the world. More ;important ;is the fact that not many are engaged ;in studies directed toward understanding the basic problems of biogas production, namely, the low ratio of methane to carbon dioxide and low conversion of volatile digestibles into energy-rich methane. Theoretical analysis of the system indicates the possibilities of substantial ;improvement on both these scores. The critical factor for maximal generation of CH₄ therefore lies in the engineering of further photosynthetic reduction of CO₂ into CH₄.

INTEGRATED SCHEME FOR METHANE PRODUCTION

For large-scale CH₄ production by bioconversion, an integrated scheme based on one of the concepts discussed above is illustrated in figure 1. In this energy flow sheet, three important raw materials as alternate or simultaneous input resources are indicated. All of them constitute principally photosynthetically-produced residues in rural areas. Basic studies done at the Bioengineering Research Centre (BERC), Indian Institute of Technology (IIT), Delhi indicate that each one of these wastes can serve as a complementary H-donor to the in situ increase of methane production.

TABLE 1. A Few Important Sources of Biodegradable Waste Materials Available Daily in India

Source: Ref. 3.

TABLE 2. Recoverable Energy and Essential Farm Nutrients Available Daily from a Few Wastes

Source: Ref. 3.

TABLE 3. Recoverable Energy from Major Agricultural Waste
Available, Dry and Unused (Basis 1972 - 1973)

(Kerosene equivalent" of these fuels = 107,000 toes)

Gas productivities per unit weight of various waste resources and their mixtures are compiled in table 4. The energy balance of the scheme indicases that the maximum conversion efficiency (ratio of energy content in CH₄ to the energy conserved in the biomass from solar source) can vary between 20 to 70 per cent.

CONVERSION OF BIOGAS INTO SYNTHETIC NATURAL GAS (SNG): SOME IMPORTANT CONSIDERATIONS

The most important" reaction in biogas production is beta oxidation of organic acids produced by acidogenic flora, ultimately yielding 60 to 70 per cent CH₄ and 30 to 40 per cent CO₂ by the action of methanogenic flora. The C0₂ content in biogas limite its quality as a fuel. To improve substantially the fuel value of biogas, induction of a mechanism to convert as much as possible of its CO₂ through regulations of the bioredox pathway seems possible. In this mechanism, participation of a hydrogen carrier and energy-rich ATP are essential in the reduction of CO₂, as it is an endergonic (energy-absorbing) reaction. Kirsch and Sykes (4) proposed that the enhancement of bioreduction is primarily dependent" upon the ATP pool and hydrogen-carrier regeneration in the organism. By increasing their levels by a biochemical engineering technique, CO₂ reduction can be enhanced, thereby increasing the CH₄ content in biogas and raising its fuel value comparable to that in SNG (3, 5).

FIG. 1. Methane Production and Energy Flow in Integrated Biogas Plant

TABLE 4. Conversion Efficiencies of Carbon and Energy in Single-Stage Anaerobic Digestion

Computed from data in Ref. 6.
*VS = volatile solids

Another important aspect to consider is the effect of digestion temperature on the conversion efficiency of both carbon and energy. Based on a number of experiments conducted to evaluate the conversion efficiencies of biomass carbon into methane in single-stage anaerobic digesters, a very important observation has been reported (6) namely, mesophilic (37 °C) digestion is superior to thermophilic (48 °C), expressed as carbon and energy conversion efficiencies. The computations have been made using the value of 38.1 per cent carbon in the dry feed material, with the input energy of the biomass being equal to 5,248 kca./kg dry matter in biomass. The energy input value was calculated on the basis of a feed material composition of 40.5 per cent protein (5.7 per cent kcal/g dry volatile solids), 8 per cent fat (9.5 kcal/g), and 51.5 per cent carbobydrate (4.2 kcal/g dry volatile solids). The average efficiencies are given in table 4.

TABLE 5. Daily Availability of CH4 Per Unit Weight of Various Waste Resources

Adapted from Ref 7.
Caloric value of biogas = 4.81 kcal/litre; caloric value of CH4 = 8.0 kcal/litre.

TABLE 6. Removal of Volatile Matter and Production of Methane from Various Agricultural Residues and Gobar

CH₄ PRODUCTION FROM GOBAR-ALGAE-WATER HYACINTH MIXTURE

Results of studies conducted at BERC, IIT, Delhi (7) on CH₄ production at 35 °C, using gobar, algal biomass, water hyacinth, or a mixture of all three, are shown in table 5. It is clear that supplementation of gotear with water hyacinth algal biomass would produce a gas rich in CH₄ content.". It seems that the mixture favours the regeneration of hydrogen carrier in the reactor flora and enhances reduction of CO₂ to CH₄.

Some of our more recent experimental studies (8) on a two phase system (acid-producing and methane-generating phases) employing a mixed substrate, namely: gobar,, water hyacinth, and algae (volatile solids 1:1:1 on dry basis) are presented in table 6. It can therefore be concluded that by increasing compatibility of digestion of several agricultural! residues used as substrates in biogas production, it is possible to increase not only the quantum of gas by about 100 per cent, but also the methane content by nearly 20 per cent in excess of what is generally available from gobar in most digesters operating in India.

CONCLUSION

This brief review of the work done at BERC, IIT, Delhi indicases the possibility of increasing the quantum of gas and its methane content substantially by admixing gobar with water hyacinth and algae at the same volatile content levels as that of gobar.. Such a scheme will also reduce the problem of availability of dung from a restricted number of cattle owned by small families, and will increase the possibility of using the digested sludge as an improved manure compared to digested dung, which has a higher residual content of Iignin.

REFERENCES

1. C.R. Prasad, K.K. Prasad, and A.K.N. Reddy, in Econ. Polit. Weekly, 9: 132 - 34), 1347, (1974).

2. M.A. Sathianathan, "Biogas- Achievements and Challenges," AVARD Pub. 5th ed. 11975).

3. T.K. Ghose and S.N. Mukhopadhyaya, in Indian Chem. Engr. 18 (4): 12(1976).

4. E.S. Kirsch and R.M. Sykes, in Prog. Indian Microbiol., 9: 155 (1971).

5. M.D. Akbar, P.A.D. Rickard, and F.J. Moss, in Biotechnol. Bioeng., 16: 455 (1974).

6. D.l. de Renzo, in Energy from Bioconversion of Waste Materials" (Noyes Data Corp., Park Ridge, N.J., 1977), p.96.

7. A. Singh, M. Tech. thesis, IIT, Delhi (1978).

8. N.K. Nigam, M.Tech. thesis, IIT, Delhi (1979).

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