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Fe compost clear air

Fig. 13.14 Flow sheet of a composting plant with rotting drum in closed shed.

sition. Maturity describes the suitability of the compost for plant growth and has been associated with the degree of humification. Several methods are used in practice to determine maturity and stability: e.g., simple field tests in Dewar flasks, tests on plants, respiration activity, chemical analyses, nuclear magnetic resonance (NMR) [1 to 26]. The various procedures for determining the maturity and stability of compost are discussed in the literature; general, final tests have not been agreed upon.

Concerning the compost quality factors that relate to human health, such as the contents of heavy metals, hazardous organic compounds, parasites, and other disease organisms, several countries have passed laws, ordinances, regulations, and norms.

References

Section 13.1: Introduction

1 DeBertoldi M (ed.), Compost: Production, Quality and Use. London 1987: Elsevier.

2 Haug RT, The Practical Handbook of Composting. Boca Raton, FL 1993: Lewis.

3 Thomé-Kozmiensky KJ (ed.), Biologische Abfallbehandlung. Berlin 1995: Erich Freitag.

4 White P, Franke M, Hindle P, Integrated Solid Waste Management: A Lifecycle Inventory. London 1995: Blackie.

5 DeBertoldi M, Sequi P, Lemmes B, Papi T (eds.), The Science of Composting. London 1996: Blackie.

6 Epstein E, The Science of Composting. Lancaster, Basel 1997: Technomic.

7 Bidlingmaier W (ed.), Biologische Abfallverwertung. Stuttgart 2000: Ulmer.

8 Insam H, Riddech N, Klammer S (eds.), Microbiology of Composting. Berlin/Heidelberg 2002: Springer.

Section 13.2: Waste Materials for Composting

1 Fricke K, Stellenwert der biologischen Abfallbehandlung in integrierten Entsorgungskonzepten, in: Biologische Verfahren der Abfallbehandlung (Dott W, Fricke K, Oetjen R, eds.), pp. 1-58. Berlin 1990: Erich Freitag Verlag.

2 Fricke K, Turk T, Stand und Stellenwert der Kompostierung in der Abfallwirtschaft, in: Bioabfallkompostierung -Flächendeckende Einführung (Wiemer K, Kern M, eds.), pp. 13-98. Witzenhausen 1991: M.I.C. Baeza.

3 Bidlingmaier W, Anlageninput und erzeugte Kompostqualität. in: Biologische Abfallbehandlung II (Wiemer K, Kern M., eds.), pp. 109-120. Witzenhausen 1995: M.I.C. Baeza

4 Thomé-Kozmiensky KJ (ed.), Biologische Abfallbehandlung. Berlin 1995: Erich Freitag

5 Schuchardt F, Composting of plant residues and waste plant materials. in: Biotechnology (Rehm HJ, Reed G, eds.) Vol. 11c: Environmental Processes III, pp. 101-125, Weinheim 2000: Wiley-VCH

Section 13.3: Fundamentals of Composting Process

1 Edwards CA, Neuhauser EF (eds.), Earthworms in Waste and Environmental Management. The Hague 1988: SPB Academic

2 Michel FC, Reddy CA, Effect of oxygenation level on yard trimmings composting rate, odor production, and compost quality in bench-scale reactors. Compost Sci Util 1998, 6(4), 6-14

3 Schaub-Szabo SM, Leonard JJ, Compost Sci Util 1999, 7(4), 15-24

4 Shi W, Norton JM, Miller BE, et al., Appl Soil Ecol 1999, 11(1), 17-28

5 Sommer SG, Dahl P, J Agric Eng Res 1999, 74(2), 145-153

6 Van Ginkel JT, Raats PAC, Van Haneghem IA, Neth J Agric Sci 1999, 47(2), 105-121

7 Chamuris GP, Koziol-Kotch S, Brouse TM, Compost Sci Util 2000, 8(1), 6-11

8 Dominguez J, Edwards CA, Webster M, Vermicomposting of sewage sludge: effect of bulking materials on the growth and reproduction of the earthworm Eisenia andrei. Pedobiologia 2000, 44(1), 24-32

9 El-Din SMSB, Attia M, Abo-Sedera SA, Field assessment of composts produced by highly effective cellulolytic microorganisms. Biol Fert Soils 2000, 32(1), 35-40

10 Kutzner HJ, Microbiology of composting. in: Biotechnology (Rehm HJ, Reed G. eds.) Vol. 11c: Environmental Processes III, pp 35-100, Weinheim 2000: Wiley-VCH

11 Larney FJ, Olson AF, Carcamo AA et al., Physical changes during active and passive composting of beef feedlot manure in winter and summer. Bioresource Technol 2000, 75(2), 139-148

12 Moller HB, Sommer SG, Andersen BH, Nitrogen mass balance in deep litter during the pig fattening cycle and during composting. J Agric Sci 2000, 135, 287-296

13 Schuchardt F, Composting of plant residues and waste plant materials. in: Biotechnology (Rehm HJ, Reed G eds.) Vol. 11c: Environmental Processes III, pp 101-125, Weinheim 2000: Wiley-VCH

14 Amon B, Amon T, Boxberger J, et al., Emissions of NH3, N2O and CH4from dairy cows housed in a farmyard manure tying stall (housing, manure storage, ma nure spreading). Nutr Cycl Agroecosys

2001, 60, 103-113

15 Beck-Friis B, Smars S, Jonsson H, et al., Gaseous emissions of carbon dioxide, ammonia and nitrous oxide from organic household waste in a compost reactor under different temperature regimes J Agric Eng Res 2001, 78, 423-430

16 Elwell DL, Keener HM, Wiles MC, et al., T ASAE 2001, 44, 1307-1316

17 Hao XY, Chang C, Larney FJ, et al., J Environ Qual 2001, 30, 376-386

18 Hassen A, Belguith K, Jedidi N, et al., Microbial characterization during composting of municipal solid waste. Biore-source Technol 2001, 80, 217-225

19 Huang GF, Wu QT, Li FB, et al., Nitrogen transformations during pig manure composting. J Environ Sci China 2001, 13, 401-405

20 Ndegwa PM, Thompson SA, Integrating composting and vermicomposting in the treatment and bioconversion of biosol-ids. Bioresource Technol 2001, 76, 107-112

21 Barrington S, Choiniere D, Trigu, M, et al., Effect of carbon source on compost nitrogen and carbon losses. Bioresource Technol 2002, 83(3), 189-194

22 (Barrington S, Choiniere D, Trigui M, et al., Compost airflow resistance. Biosyst Eng 2002, 81, 433-441

23 Hart TD, De Leij FAAM, Kinsey G, et al., Strategies for the isolation of cellulolytic fungi for composting of wheat straw. World J Microb Biot 2002, 18, 471-480

24 Jensen HEK, Leth M, Iversen JJL, Effect of compost age and concentration of pig slurry on plant growth. Compost Sci Util

25 Noble, R, Hobbs, PJ, Mead, A, et al., Influence of straw types and nitrogen sources on mushroom composting emissions and compost productivity. J Ind Mi-crobiol Biot 2002, 29(3), 99-110

26 Richard TL, Hamelers HVM, Veeken A, et al., Moisture relationships in composting processes. Compost Sci Util 2002, 10(4), 286-302

27 Singh A, Sharma S, Composting of a crop residue through treatment with microorganisms and subsequent vermi-composting. Bioresource Technol 2002, 85(2), 107-111

28 Tiquia SM, Tam NFY, Characterization and composting of poultry litter in forced-aeration piles. Process Biochem 2002, 37, 869-880

29 Veeken A, de Wilde V, Hamelers B, Passively aerated composting of straw-rich pig manure: effect of compost bed porosity. Compost Sci Util 2002, 10(2), 114-128

30 Wolter M, Prayitno S, Schuchardt F, Comparison of greenhouse gas emissions from solid pig manure during storage versus during composting with respect to different dry matter contents. Landbauforsch Volk 2002, 52(3), 167-174

31 Agnew JM, Leonard JJ, The physical properties of compost. Compost Sci Util 2003, 11, 238-264

32 Barrington S, Choiniere D, Trigui M, Knight W, Compost convective airflow under passive aeration Bioresource Technol 2003, 86, 259-266

33 Bolta SV, Mihelic R, Lobnik F, et al., Microbial community structure during composting with and without mass inocula. Compost Sci Util 2003, 11, 6-15

34 Fukumoto Y, Osada T, Hanajima D, Haga K, Patterns and quantities of NH3, N2O and CH4 emissions during swine manure composting without forced aeration: effect of compost pile scale. Bioresource Technol 2003, 89(2), 109-114

35 Liang C, Das KC, McClendon RW, The influence of temperature and moisture contents regimes on the aerobic microbial activity of a biosolids composting blend. Bio-resource Technol 2003, 86(2), 131-137

36 Principi P, Ranalli G, da Borso F, et al., Microbiological aspects of humid husk composting. J Environ Sci Heal B 2003, 38, 645-661

Section 13.4: Composting Technologies

1 Schuchardt F, Composting of plant residues and waste plant materials. in: Biotechnology (Rehm HJ, Reed G, eds.) Vol. 11c: Environmental Processes III, 101-125, Weinheim 2000: Wiley-VCH

Section 13.5: Composting Systems 10

1 Haug RT, The Practical Handbook of Composting. Boca Raton, FL 1993: Lewis.

2 Thomé-Kozmiensky KJ (ed.), Biologische Abfallbehandlung. Berlin 1995: Erich Frei- 11 tag Verlag.

3 Gronauer A, Claassen N, Ebertseder T et al., Bioabfallkompostierung. BayLfU 139 12 (1997)

4 Bidlingmaier W (Ed.), Biologische Abfallverwertung. Stuttgart 2000: Ulmer Verlag

5 Krogmann U, Körner I, Technology and strategies of composting. in: Biotechnology (Rehm HJ, Reed G, eds.) Vol. 11c: En- 13 vironmental Processes III, pp 127-150, Weinheim 2000: Wiley-VCH

6 Schuchardt F, Composting of plant residues and waste plant materials. in: Bio- 14 technology (Rehm HJ, Reed G, eds.) Vol.

11c: Environmental Processes III, 101-125, Weinheim 2000: Wiley-VCH

Section 13.6: Compost Quality

1 Adani F, Genevini PL, Gasperi F, Zorzi G, Organic matter evolution index

(OMEI) as a measure of composting effi- 17

ciency. Compost Sci Util 1997, 5(2),

53-62

2 Popp L, Fischer P. Claassen N, Biologisch-biochemische Methoden zur Reife- 18 bestimmung von Komposten. Agrobiol

3 Saharinen MH, Evaluation of changes in CEC during composting. Compost Sci

4 Fauci MF, Bezdicek DF, Caldwell D, et al., Compost Sci Util 1999, 7(2), 17-29

5 Namkoong W, Hwang EY, Cheong JG, et al., Compost Sci Util 1999, 7(2), 55-62 20

6 Warman PR, Compost Sci Util 1999, 7(3), 33-37

7 Koenig A, Bari QH, Application of self-heating test for indirect estimation of 21 respirometric activity of compost: theory and practice. Compost Sci Util 2000, 8(2), 99-107

8 Ouatmane A, Provenzano MR, Hafidi M, et al., Compost Sci Util 2000, 8(2), 22

124-134

9 Wu L, Ma LQ, Martinez GA, J Environ Qual 2000,. 29, 424-429

Butler TA, Sikora LJ, Steinhilber PM, et al., Compost age and sample storage effects on maturity indicators of biosolids compost. J Environ Qual 2001, 30, 2141-2148

Eggen T, Vethe O, Stability indices for different composts. Compost Sci Util 2001, 9(1), 19-26

Provenzano MR, de Oliveira SC, Silva MRS, et al., Assessment of maturity degree of composts from domestic solid wastes by fluorescence and Fourier transform infrared spectroscopies. J Agric Food Chem 2001, 49, 5874-5879 Smith DC, Hughes JC, A simple test to determine cellulolytic activity as indicator of compost maturity. Commun Soil Sci Plan 2001,32,1735-1749 Levanon D, Pluda D, Chemical, physical and biological criteria for maturity in composts for organic farming. Compost Sci Util

2002, 10, 339-346

Weppen P, Determining compost maturity: evaluation of analytical properties. Compost Sci Util 2002, 10(1), 6-15 Wu L, Ma LQ, Relationship between compost stability and extractable organic carbon. J Environ Qual 2002, 31, 1323-1328 Adani F, Gigliotti G, Valentini F, Laraia R, Respiration index determination: a comparative study of different methods. Compost Sci Util 2003, 11, 144-151 Benito M, Masaguer A, Moliner A, Arrigo N, Palma RM, Chemical and microbiological parameters for the characterisation of the stability and maturity of pruning waste compost. Biol Fert Soils 2003, 37, 184-189 Brewer LJ, Sullivan DM, Maturity and stability evaluation of composted yard trimmings. Compost Sci Util 2003, 11, 96-112

Changa CM, Wang P, Watson ME, et al., Assessment of the reliability of a commercial maturity test kit for composted manures. Compost Sci Util 2003, 11, 125-143 Chen YN, Nuclear magnetic resonance, infra-red and pyrolysis: application of spectroscopic methodologies to maturity determination of composts. Compost Sci Util

2003, 11, 152-168

Chica A, Mohedo JJ, Martin MA, Martin A., Determination of the stability of MSW compost using a respirometric technique. Compost Sci Util 2003, 11, 169-175

23 Cooperband LR, Stone AG, Fryda MR, 25 Ravet JL., Relating compost measures of stability and maturity to plant growth. 26 Compost Sci Util 2003, 11, 113-124

24 Korner, I, Braukmeier, J, Herrenklage, J, et al., Investigation and optimization of composting processes: test systems and practical examples. Waste Manage 2003, 23, 17-26

Rynk R, The art in the science of compost maturity. Compost Sci Util 2003, 11, 94-95 Zubillaga MS, Lavado RS, Stability indexes of sewage sludge compost obtained with different proportions of a bulking agent. Commun Soil Sci Plan 2003, 34, 581-591

Anaerobic Fermentation of Wet and Semidry Garbage Waste Fractions

Norbert Rilling

14.1

Introduction

During the last 30 years the amount of solid wastes has rapidly increased. Now one of the primary aims in waste management is to reduce the amount of waste to be disposed by prevention, reduction, and utilization (KrW-/AbfG, 1994).

By means of separate collection and biological treatment of biowaste, the amount of municipal solid waste (MSW) to be incinerated or landfilled will be significantly reduced.

Biological treatment of garbage waste fractions can be carried out aerobically (composting) or anaerobically (anaerobic digestion). Each technique is appropriate for a certain spectrum of wastes. Today, most biological waste is composted because this technology is already well developed, with quite a lot of experience at hand, but anaerobic processes advance in their importance for the utilization of solid organic waste.

Anaerobic digestion, which is typically conducted inside a closed vessel in which the temperature and moisture are controlled, is particularly suited to wastes with a high moisture content and a high amount of readily biodegradable components. In Sections 14.2 and 14.3 the characteristics of the anaerobic process are discussed with respect to their ecological and economical aspects.

14.2

Basic Aspects of Biological Waste Treatment

The leading aim of separate collection and treatment of biological wastes is stabilization of the waste by microbial degradation. The product is a compost that, if its pollutant content is low, can be used as a fertilizer or soil conditioner and thus be fed back into the natural cycle.

In contrast to the commonly established composting processes, the technique of anaerobic fermentation of waste is relatively young and dynamic. With great scien tific expenditure, process developments and optimizations are being pursued, so it may be assumed that the technological potential of biowaste fermentation has not yet been fully exhausted.

Biochemical Fundamentals of Anaerobic Fermentation

Biogas is produced whenever organic matter is microbially degraded in the absence of oxygen. In nature this process can be observed in marshlands, in marine sediments, in flooded rice fields, in the rumen of ruminants, and in landfill sites (Maurer and Winkler, 1982).

Anaerobic degradation is effected by various specialized groups of bacteria in several successive steps, each step depending on the preceding one. For industrial-scale application of anaerobic fermentation processes it is necessary to have a thorough knowledge of these interactions, to avoid substrate limitation and product inhibition.

The entire anaerobic fermentation process can be divided into three steps (Fig. 14.1):

1. hydrolysis

2. acidification

3. methane formation

At least three groups of bacteria are involved in the anaerobic fermentation process.

First, during hydrolysis, the mostly water-insoluble biopolymers such as carbohydrates, proteins, and fats are decomposed by extracellular enzymes to water-soluble monomers (e.g., monosaccharides, amino acids, glycerin, fatty acids) and thus made accessible to further degradation.

In the second step (acidification) the intermediates of hydrolysis are converted into acetic acid (CH3COOH), hydrogen (H2), carbon dioxide (CO2), organic acids,

Fig. 14.1 The three stages of anaerobic processing of organic matter (Sahm, 1981).

(1) hydrolysis (2) acidification

Fig. 14.1 The three stages of anaerobic processing of organic matter (Sahm, 1981).

amino acids, and alcohols by different groups of bacteria. Some of these intermediate products (acetic acid, hydrogen, and carbon dioxide) can be directly used by me-thanogenic bacteria, but most of the organic acids and alcohol are decomposed into acetic acid, hydrogen, and carbon dioxide during acidogenesis. Only these products, as well as methanol, methylamine, and formate, can be transformed into carbon dioxide and methane (CH4) by methanogenic bacteria during the third and last step, methane formation.

14.2.1.1 Hydrolytic and Acid-forming (Fermentative) Bacteria

The first group of bacteria is very heterogeneous: besides obligate anaerobic bacteria, also facultative anaerobic bacterial strains occur. The high molecular weight compounds of the waste biomass (proteins, polysaccharides, fats) are decomposed into low molecular weight components by enzymes that are excreted by fermentative bacteria. This first step is inhibited by lignocellulose-containing materials, which are degraded only very slowly or incompletely. Subsequently, acid-forming bacteria transform the hydrolysis products into hydrogen, carbon dioxide, alcohols, and organic acids such as acetic, propionic, butyric, lactic, and valeric acids. The formation of the acids decreases the pH.

14.2.1.2 Acetic Acid- and Hydrogen-forming (Acetogenic) Bacteria

The group of acetogenic microorganisms represents the link between fermentative and methanogenic bacteria. They decompose alcohols and long-chain fatty acids into acetic acid, hydrogen, and carbon dioxide. It is a characteristic of acetogenic bacteria that they can grow only at a very low hydrogen partial pressure. For this reason they live in close symbiosis with methanogenic and sulfidogenic bacteria, which use hydrogen as an energy source (Sahm, 1981).

14.2.1.3 Methane-forming (Methanogenic) Bacteria

The group of methanogenic bacteria is formed of extreme obligate anaerobic microorganisms which are very sensitive to environmental changes. They transform the final products of acidic and acetogenic fermentation into methane and carbon dioxide. About 70% of the methane is produced by the degradation of acetic acid and about 30% by a redox reaction from hydrogen and carbon dioxide (Roediger et al., 1990).

The slowest step is rate-determining for the whole process of anaerobic fermentation. Although methanogenesis of acetic acids is the rate-limiting step in the anaerobic fermentation of easily degradable substances, hydrolysis can be rate-limiting when sparingly degradable substances occur. Because of the complexity of the anaerobic degradation mechanisms and the stringent requirements of the microorganisms, process operation is very important for fermentation processes. To achieve optimized, undisturbed anaerobic degradation, the speed of decomposition in the consecutive steps should be equal.

358 | 14 Anaerobic Fermentation of Wet and Semidry Garbage Waste Fractions 14.2.2

Influence of Processing Conditions on Fermentation

The activity of microorganisms in anaerobic fermentation processes depends mainly on water content, temperature, pH, redox potential, and the presence of inhibitory factors.

14.2.2.1 Water Content

Bacteria take up the available substrates in dissolved form. Therefore, biogas production and the water content of the initial material are interdependent. When the water content is below 20% by weight, hardly any biogas is produced. With increasing water content biogas production is enhanced, reaching its optimum at 91%-98% water by weight (Kaltwasser, 1980).

14.2.2.2 Temperature

The process of biomethanation is very sensitive to changes in temperature, and the degree of sensitivity depends on the temperature range. For most methane bacteria, the optimum temperature range is between 30 and 37 °C (Maurer and Winkler, 1982). Here, temperature variations of ±3 °C have minor effect on the fermentation (Winter, 1985). In the thermophilic range, however, i.e., at temperatures between 55 and 65 °C, a fairly constant temperature has to be maintained, since deviations by only a few degrees cause a drastic reduction of the degradation rates and thus of biogas production.

14.2.2.3 pH Level

The pH optimum for methane fermentation is between pH 6.7 and 7.4. If the pH of the medium drops below 6, because the balance is disturbed and the acid-producing dominate the acid-consuming bacteria, the medium becomes inhibitory or toxic to the methanogenic bacteria. In addition, strong ammonia production during the degradation of proteins may inhibit methane formation if the pH exceeds 8. Normally, acid and ammonia production vary only slightly due to the buffering effect of carbon dioxide/bicarbonate (CO2/HCO3-) and ammonia/ammonium (NH3/NH+), which are formed during fermentation, and the pH normally stays constant between 7 and 8.

14.2.2.4 Redox Potential and Oxygen

Methane bacteria are very sensitive to oxygen and have lower activity in the presence of oxygen. The anaerobic process, however, shows a certain tolerance to small quantities of oxygen. Even continuous, but limited, oxygen introduction is normally tolerated (Mudrak and Kunst, 1991). The redox potential can be used as an indicator of the process of methane fermentation. Methanogenic bacterial growth requires a relatively low redox potential. Hungate (1966; cited by Braun, 1982) found -300 mV to be the minimum value. Changes in redox potential during the fermentation process are caused by a decrease in oxygen content as well as by the formation of metabolites like formate or acetate.

14.2.2.5 Inhibitory Factors

The presence of heavy metals, antibiotics, and detergents can inhibit the process of biomethanation. With reference to investigations of Konzeli-Katsiri and Kartsonas (1986), Table 14.1 lists the limit concentrations (mg L-1) for inhibition and toxicity of heavy metals in anaerobic digestion.

Gas Quantity and Composition

Biogas is a mixture of various gases. Independent of the fermentation temperature, a biogas is produced which consists of 60%-70% methane and 30%-40% carbon dioxide. Trace components of ammonia (NH3) and hydrogen sulfide (H2S) can be detected. The caloric value of the biogas is about 5.5-6.0 kWh m-3. This corresponds to about 0.5 L of diesel oil.

If the chemical composition of the substrate is known, the yield and composition of the biogas can be estimated from Eq. (1) (with reference to Symons and Buswell, 1933):

C"H"Ob + (n - 4- 2) H2O - 8 - 4 ) CH4+ (7-i + 4 ) CO2 (1)

Table 14.2 shows the mean composition and specific quantity of biogas as dependent on the kind of degraded substances.

For anaerobic digestion of the organic fraction of municipal solid waste, an average biogas yield of 100 m31-1 moist biowaste and having a methane content of about 60% by volume may be assumed.

Table 14.1 Inhibition of anaerobic digestion by heavy metals (Konzell-Katsiri and Kartsonas, 1986).

Heavy Metal Inhibition Toxicity

Copper (Cu) 40-250 170-300

Zinc (Zn) 150-400 250-600

Nickel (Ni) 10-300 30-1000

Lead (Pb) 300-340 340

Chromium III (Cr) 120-300 200-500

Chromium VI(Cr) 100-110 200-420

Table 14.2 Mean composition and specific yields of biogas in relation to the kind of substances degraded.

ch4

CO2

Substance

Gas Yield

Methane Content

Carbon Dioxide Content

(m3 kg-1 TS)

(Vol. %)

(Vol. %)

Carbohydrates

0.79

50

50

Fats

1.27

68

32

Proteins

0.70

71

29

Municipal solid waste (MSW)

0.1-0.2

55-65

35-45

Biowaste

0.2-0.3

55-65

35-45

Sewage sludge

0.2-0.4

60-70

30-40

Manure

0.1-0.3

60-65

Comparison of Aerobic and Anaerobic Waste Treatment

Professional biological treatment must include the separation of interfering matter, sanitation, and microbial degradation of the readily and moderately degradable substances so that the final product is biologically stable, compatible with plant roots, and, as far as possible, free of pollutants and can be applied as a soil improver in horticulture and agriculture. Aerobic composting and anaerobic fermentation are, in principle, available as processes for the biological treatment of organic residues.

Composting is suitable for the stabilization of rather dry solid waste, and anaerobic processes are used for very moist waste (e.g., kitchen garbage) which is easy to degrade. Both systems have advantages and disadvantages. Their general properties are listed and compared in Table 14.3.

A great advantage of anaerobic fermentation is the production of biogas that can be used as a source of energy. Either local users can be found for the gas recovered from the process, or it can be cleaned and upgraded for inclusion in a gas supply network. By way of comparison, during composting all the energy is released as heat and cannot be used. In addition, intensive composting requires a lot of energy for artificial aeration of the waste material.

The technical expenditures for anaerobic fermentation are higher than for composting, but if standards, especially concerning the reduction of emissions (odor, germs, noise, dust), are raised, the technical expenditures for composting can be expected to grow as well.

The duration of anaerobic and aerobic treatment depends very much on the substrate and the process used, so that the times required cannot be compared in general. The same is true for the floor space required.

A major advantage of anaerobic digestion in comparison with aerobic composting is the ability of engineers to have total control over gaseous and liquid emissions, as well as having the potential to recover and use methane gas generated as the wastes degrade. For composting, the problem of odor control has not yet been sufficiently

Table 14.3 Comparison of aerobic and anaerobic waste treatment (according to Rilling, 1994a).

Characteristics

Anaerobic Digestion

Aerobic Composting

Phases

solids, liquid

solids, liquid, gas

Degradation rate

up to 80% volatile solids

up to 50% volatile solids

Energy consumption

excess produced

demands input

Technical expenditure

in the same range

Duration of the process

1-4 weeks

4-16 weeks

(only anaerobic stage)

(depending on the process)

Post-treatment

generally necessary

generally none

(post-composting about 2-8 weeks)

Floor space required

comparatively low

comparatively low or high

(depending on the process)

Odor emission

comparatively low

comparatively high

Stage of development

little experience but increasing

much experience

Costs

in the same range

Sanitation

external process step

integrated

Suitability of wastes

wide (wet and dry wastes)

narrow (dry wastes)

solved. Large volumes of odiferous waste air have to be treated by costly means to avoid complaints from surround communities. In contrast, hardly any odorous emissions occur in anaerobic fermentation, because this biological treatment takes place in closed reactors. Malodorous waste air is produced only during loading and unloading the reactor.

No general statement can be made as to whether anaerobic or aerobic fermentation is the more favorable process, as in each individual case many factors must be considered. This means that in the field of waste treatment, as well as in the field of wastewater treatment, several different processes with similar goals can have certain advantages - depending on the operational area.

14.3

Processes of Anaerobic Waste Treatment

Anaerobic fermentation has been used successfully for many years as a treatment for wastewater, sewage sludge, and manure. However, anaerobic digestion of municipal solid waste is a relatively new technique which has been developed in the past 10-15 years.

Only biodegradable household wastes - i.e., those of organic or vegetable origin -can be processed in anaerobic digestion plants. Garden waste, or green wastes as they are often called, can also be included, but woodier materials (like branches) are less suitable because of their relatively longer decomposition time under anaerobic conditions.

As a result of anaerobic fermentation combined with an additional post-composting step, a material is produced that is usually similar to the compost produced by aerobic processes. It can be used as fertilizer, soil conditioner, or peat substitute.

Although composting is widely used for wastes containing high amounts of dry matter, anaerobic digestion has turned out to be a good alternative for treating wet organic wastes (Fig. 14.2). At present, the anaerobic fermentation technique is mainly used in Western Europe, where more than 30 companies offer anaerobic treatment plants commercially for the digestion of putrefiable solid waste.

Procedures of Anaerobic Waste Fermentation

Generally, the following steps are required for the anaerobic treatment of organic waste (Rilling, 1994a):

1. delivery and storage of the biological waste

2. preprocessing of the incoming biological waste

3. anaerobic fermentation

4. storage and treatment of the digester gas

5. treatment of the process water

6. post-processing of the digested material

Figure 14.3 shows the possible treatment steps used in biowaste fermentation. In principle, all fermentation processes can be described as a combination of a selection of these treatment steps. The process technology demanded for the implementation of the different steps of the treatment differs very much, depending on the anaerobic process chosen. In general, the gas production increases and the detention time decreases with increasing energy input for preparation of the material and the fermentation itself (mesophilic/thermophilic).

aerobic composting

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