**1. Introduction**

96 Management of Organic Waste

Nooren, C. A. M., Van Breeman, N., Stoorvogel, J. J. and Jongmans, A. G. 1995. The role of

Piearce, T. G., Oates, K. and Carruthers, W. J. 1990. A fossil earthworm embryo

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(Oligochaeta) from beneath a late bronze age midden at Potterna, Wiltshire, UK. *J.* 

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dynamics of the earthworms *Hormogaster elisae* (Oligochaeta, Hormogastridae) in

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Organic waste and waste water sludges can be stabilized both in anaerobic and in aerobic processes. The advantage of the anaerobic treatment of the waste is that biogas develops during the degradation process. Instead of energy consumption which is a usual characteristic of aerobic-processes it is accompanied with energy production, which can be utilized as energy source (Kayhanian & Tchobanoglous, 1992; Cout et al., 1994).

The anaerobic processes can be further classified according to the dry mater content and the feeding of the fermenting reactor. According to Tchobanoglous (1993) and his work team, we can talk about semidry procedures in the range of 15-20% dry matter contents. If the dry matter content is high, only batch reactors operated with the principle of filling and emptying can be applied, in order to avoid difficulties due to the continuous feeding.

On waste water treatment plants of small and medium capacity, the waste water sludge can not be economically stabilized by the conventional anaerobic treatment of low dry matter content and continuous feeding. Thus, the sludge is usually stabilized by composting in that cases. At waste water treatment plants of great capacity, the sludge is stabilized by anaerobic treatment of liquid, continuous technology which is often followed by composting, in order to achieve better material characteristics of the end product.

The municipal waste management directives require that the organic content of the wastes to be dumped should be reduced. The realization of the waste management goals requires the stabilization of municipal organic wastes, where generally composting is applied. The sewage sludge and the organic fraction of municipal solid waste, called vegetable, fruit and garden (VFG) waste are different from each other regarding their materials and quality, yet for their stabilization, combined treatment is more and more often applied. The quantity and quality of the VFG varies with time and space, depending on the season, the structure of the settlement and the standard of living.

The novel dry batch BIOCEL technology was introduced for the treatment of municipal solid organic waste in the Netherlands. It has the advantage that it is simple to operate, and

Co-Digestion of Organic Waste

and Sewage Sludge by Dry Batch Anaerobic Treatment 99

Fig. 1. Metabolic stages and products in anaerobic digestion of complex organic material

production (Gujer & Zehnder, 1983), because the biomass production is negligible.

Among them, the usual rate-limiting factor of the whole procedure is the methagonesis (Gosh & Klass, 1978). The rate of the acid production is great compared to that of the methane production. At a balanced anaerobic degradation process, the elimination rate of the biologically degradable organic dry material is almost equal to that of the methane

The research and the thermodynamic calculations show that 70% of the methane is generated during the decarboxylation of the acetic acid, and the remaining 30% comes from the reduction of the carbon dioxide (Jeris & McCarthy, 1965; Kaspar & Wuhrmann, 1978).

CH3COOH → CH4 + CO2 (∆G° = - 39,5 kJ)

CO2 +4 H2 → CH4 + 2 H2O (∆G° = - 145 kJ) So, the efficiency of the methane production is characterized not only by measuring the methane content, but also by determining the hydrogen content. In case the hydrogen is accumulated and is not converted to methane, then the accumulated hydrogen will immediately inhibit the oxidation of the propionic acid and the accumulated acetic acid. This will result in decrease in pH and, thus, in acidification of the reactors. This again will

its specific reactor volume projected to the treated material flow is low (Brummeler et al., 1991; Brummeler, 2000; Simon, 2000). The investment costs of the dry batch BIOCEL technology are lower by 40% than those of the continuous anaerobic systems (Brummeler et al., 1992). Its advantage over composting by state-of-the-art technology comes from a simpler technical solution and a more economical operation.

We assume that the dry batch anaerobic treatment could be used for combined anaerobic treatment of VFG wastes and municipal sewage sludge. When treating the waste water sludge and other municipal organic wastes together by anaerobic method, the possible too high easily degradable organic content of the VFG might be a problem, because in lack of sufficient seeding material, it can lead to acidification of the system. A number of literature reports about the anaerobic treatment of different organic wastes separately (Brummeler, 1993), but there are no results available regarding co-digestion by dry anaerobic treatment.

The effective anaerobic conversion of organic substances into methane depends on the activity of miscellaneous microbial populations. A diagram of the consecutive metabolic stages, which can be distinguished in anaerobic digestion, is shown in figure 1. In well balanced digestion, all products of a previous metabolic stage are converted into the next one. The overall result is a nearly complete conversion of the biodegradable organic material in the waste into end products such as methane, carbon dioxide, hydrogen sulphide, ammonia, etc. without significant build-up of intermediate products.

The products of the fermentation vary depending on quality of raw material and environmental conditions applied. Low pH values decreases the relative amount of acetic acid and increases the relative amount of propionic acid (Breure & van Andel, 1984). Partial pressure of hydrogen in the gas phase can significantly influence the kind of products formed by fermentative bacteria (Wolin & Miller, 1982). Hanaki et al. (1981) stated that βoxidation of long-chain fatty acids is thermodynamically unfavourable unless the hydrogen partial pressure is maintained at a very low level. The dependence of a low hydrogen partial pressure makes, that long chain fatty acids degradation can be inhibited indirectly by inhibition of hydrogen consuming organisms (Koster, 1989).

The high substrate affinity of the hydrogen-consuming micro organisms makes it possible to maintain low hydrogen concentrations. According to Robinson & Tiedje (1982), the Michaelis-Menten half-saturation constant (Km) for hydrogen is in the range of 5.8-7.1 µM. Zehnder et al. (1982) stated, that in a well balanced methane fermentation, the hydrogen partial pressure does not exceed 10-4 atm and in most cases approximately 10-6 atm.

Macro and micro nutrients contents of the treated raw material effects the yield of micro organisms. Lettinga et al. (1981) advised for macro nutrients a COD to nitrogen to phosphorus ratio in a range of 350 : 5 : 1 – 1000 : 5 : 1.

The precondition of the efficient application of the anaerobic batch reactors is the establishment of the balance between the acid production and the methane production, in the absence of the reactor getting acidified (Benedek, 1990). During the anaerobic degradation of the organic material, four consecutive metabolic steps can be distinguished: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Batstone et al., 2002).

its specific reactor volume projected to the treated material flow is low (Brummeler et al., 1991; Brummeler, 2000; Simon, 2000). The investment costs of the dry batch BIOCEL technology are lower by 40% than those of the continuous anaerobic systems (Brummeler et al., 1992). Its advantage over composting by state-of-the-art technology comes from a

We assume that the dry batch anaerobic treatment could be used for combined anaerobic treatment of VFG wastes and municipal sewage sludge. When treating the waste water sludge and other municipal organic wastes together by anaerobic method, the possible too high easily degradable organic content of the VFG might be a problem, because in lack of sufficient seeding material, it can lead to acidification of the system. A number of literature reports about the anaerobic treatment of different organic wastes separately (Brummeler, 1993), but there are no results available regarding co-digestion by dry

The effective anaerobic conversion of organic substances into methane depends on the activity of miscellaneous microbial populations. A diagram of the consecutive metabolic stages, which can be distinguished in anaerobic digestion, is shown in figure 1. In well balanced digestion, all products of a previous metabolic stage are converted into the next one. The overall result is a nearly complete conversion of the biodegradable organic material in the waste into end products such as methane, carbon dioxide, hydrogen sulphide,

The products of the fermentation vary depending on quality of raw material and environmental conditions applied. Low pH values decreases the relative amount of acetic acid and increases the relative amount of propionic acid (Breure & van Andel, 1984). Partial pressure of hydrogen in the gas phase can significantly influence the kind of products formed by fermentative bacteria (Wolin & Miller, 1982). Hanaki et al. (1981) stated that βoxidation of long-chain fatty acids is thermodynamically unfavourable unless the hydrogen partial pressure is maintained at a very low level. The dependence of a low hydrogen partial pressure makes, that long chain fatty acids degradation can be inhibited indirectly by

The high substrate affinity of the hydrogen-consuming micro organisms makes it possible to maintain low hydrogen concentrations. According to Robinson & Tiedje (1982), the Michaelis-Menten half-saturation constant (Km) for hydrogen is in the range of 5.8-7.1 µM. Zehnder et al. (1982) stated, that in a well balanced methane fermentation, the hydrogen

Macro and micro nutrients contents of the treated raw material effects the yield of micro organisms. Lettinga et al. (1981) advised for macro nutrients a COD to nitrogen to

The precondition of the efficient application of the anaerobic batch reactors is the establishment of the balance between the acid production and the methane production, in the absence of the reactor getting acidified (Benedek, 1990). During the anaerobic degradation of the organic material, four consecutive metabolic steps can be distinguished:

partial pressure does not exceed 10-4 atm and in most cases approximately 10-6 atm.

hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Batstone et al., 2002).

simpler technical solution and a more economical operation.

ammonia, etc. without significant build-up of intermediate products.

inhibition of hydrogen consuming organisms (Koster, 1989).

phosphorus ratio in a range of 350 : 5 : 1 – 1000 : 5 : 1.

anaerobic treatment.

Fig. 1. Metabolic stages and products in anaerobic digestion of complex organic material

Among them, the usual rate-limiting factor of the whole procedure is the methagonesis (Gosh & Klass, 1978). The rate of the acid production is great compared to that of the methane production. At a balanced anaerobic degradation process, the elimination rate of the biologically degradable organic dry material is almost equal to that of the methane production (Gujer & Zehnder, 1983), because the biomass production is negligible.

The research and the thermodynamic calculations show that 70% of the methane is generated during the decarboxylation of the acetic acid, and the remaining 30% comes from the reduction of the carbon dioxide (Jeris & McCarthy, 1965; Kaspar & Wuhrmann, 1978).


So, the efficiency of the methane production is characterized not only by measuring the methane content, but also by determining the hydrogen content. In case the hydrogen is accumulated and is not converted to methane, then the accumulated hydrogen will immediately inhibit the oxidation of the propionic acid and the accumulated acetic acid. This will result in decrease in pH and, thus, in acidification of the reactors. This again will

Co-Digestion of Organic Waste

presented in Table 1.

methanogenic seed

0.026 CH4-COD .

**2.2 Methods** 

MSZ 21976-10:1982.

time.

g VS-1 .

cabbage, 10% paper and 10% coffee grounds.

(TS %)

(digested sludge) 24.54 56.86 667 excess sludge 28.87 48.22 556 fresh grass 31.30 92.20 985 potato peel 18.51 94.33 1 074 lettuce 7.69 85.67 1 193 bread 65.35 97.36 1 094 cucumber peel 4.82 84.44 1 486 cabbage 8.79 90.98 1 086 paper 92.49 98.98 1 288 coffee grounds 34.30 99.28 1 145

Table 1. Characteristics of waste and sludge used for the experiment

Materials Total solids

and Sewage Sludge by Dry Batch Anaerobic Treatment 101

follows: 50% municipal excess sludge, 50% VFG consisting 25% fresh grass and 25% kitchen waste. The excess sludge came from the activated sludge technology of a municipal waste water plant, which can be characterized with a 20-day sludge retention time. The kitchen waste consisted of 25% potato peel, 15% lettuce, 15% bread, 15% cucumber peel, 10%

The amount of total solids (furthermore as TS), volatile solids (furthermore as VS) and the value of chemical oxygen demand (furthermore as COD) of the waste and the sludge are

> Volatile solids (VS %)

To characterize the seed, we defined its stability and methanogenic activity. The seed was not stable, it could be degraded by a further 13%. The organic degradation occurred mostly within the first 30 days. The digested sludge came from a completely stirred tank reactor operated with 20 days hydraulic retention time. The methanogenic activity of the seed was

chemical oxygen demand (COD) of digested sludge for a unit of volatile solid in a unit of

The TS content and the volatile solids (VS) content of the samples were determined by drying and burning to constant weight at a temperature of 105°C and 650°C, respectively. The chemical oxygen demand (COD) of the sludges was measured by the standard method

The amount of biogas generated, was measured by an "A1" type, Schlumberger wet gas meter. The methane and hydrogen content of biogas was measured by a Shimadzu 2014 gas chromatograph. The temperature of the column was 60°C, the temperature of the injector was 170°C, and the temperature of the detector was 250°C. As carrier gas we used nitrogen with 20 mL/min gas flow. In the 3.0-m long, 3.00-mm internal diameter glass column, Supelco Molecular sieve filling was put. The detection was done with TCD detector. We measured the quantity and the methane content of the biogas every day at the beginning, and then, when the amount of the biogas decreased, every other and then every fifth day.

(g O2 .

d –1, which shows the maximum methane production measured in

Chemical oxygen demand (COD )

kg TS-1)

affect the oxidation of the hydrogen, decreasing the reaction efficiency and increasing the partial pressure of the hydrogen (Gujer & Zehnder, 1983).

To keep the degradation process balanced, seeding material is needed. A significant effect in balancing the process can be achieved by properly setting the ratio of the methanogen seeding material. However, the determination of the optimal amount of the seeding material is a complex issue which is of great importance to the operation of batch reactors being economical. Low seeding material ratio, in extreme cases, can lead to acidification of the reactor or, in better cases, moderates the process rate. This can be compensated by extending the retention time associated with increased reactor volume.

The increase of the biogas production and the decrease of the treatment time can be achieved also by increasing the quantity of the seeding material since this way a more effective degradation can be counted on. However, increasing the quantity of the seeding material can result in the increase of the reactor volume, too.

The optimal waste to seeding material ratio in the case of municipal solid organic wastes is 1:2.3 in laboratory, while less than 1 : 1 in full scale conditions (Brummeler, 1993). According to related literature data, the duration of the treatment in cases of municipal organic waste is around 30-36 days (Brummeler et al., 1991, 1992); however in cases of a low seeding material ratio, the duration of the treatment can be 50 days or more (Brummeler et al., 1992). There is no published data about seeding material demand for the dry, batch anaerobic co-treatment of the biowaste and waste water sludge.

We assume that the combined dry batch treatment of VFG waste generated on settlements and of sewage sludge has many advantages. As a result of the co-digestion, because of the different easily degradable organic contents of the sewage sludge and VFG, we can count on the increase of the gas yield projected to reactor volume, compared to a separate treatment of the VFG and sludge. We can assume as an advantage that a more balanced quality of the sewage sludge can have a positive effect on the co-digestion with organic wastes having quality varying with time and space. A further advantage can be, from the aspect as a potential of anaerobic treatment of the sewage sludges generated on smaller settlements, that the increased waste flow with VFG can make it economical. Nevertheless, it is necessary to investigate the appropriate seeding material ratio, the determination of which does not depend only on the achievable methane yield but on the required duration of the treatment and on the targeted stabilization goal of organic material, too.

Our aim is to study the combined dry batch treatment of VFG and sewage sludge. Our goal is to evaluate the aspects of determination of the optimal seeding material ratio, besides the study of the avoidance of acidification of the reactors, the achievable greater degradation rate of organic material and the maximal gas yield.
