**3.2 The results of methane production referred to reactor volume**

We assume that the seed ratio, as a result of two opposing effects, influences the methane production per reactor volume unit. The increase of the seed ratio makes the anaerobic process balanced but at the same time decreases the amount of degradable organic matter per reactor volume unit. That is a question, to what extent the already digested material should be recycled for seeding. Another question is how the co-digestion of the easily degradable VFG and sewage sludge affects the gas production of the reactors. To answer the question, we checked the values of the totalled methane production referred to reactor

Co-Digestion of Organic Waste

of the reactor

Type of the reactor CH4 produced

(CH4-COD g .

production gradually increased until day 40.

0.00

0.05

**Reactor-specific CH4**

 **prod. (Ndm3. dm-3 . d-1)**

0.10

0.15

0.20

0.25

dm-3)

and Sewage Sludge by Dry Batch Anaerobic Treatment 109

(CH4-COD g .

v10d

1:1.5 biowaste:seed 23.56 0.529 0.503 0.143 20.85 1:2 biowaste:seed 21.68 0.505 0.485 0.123 20.59 1:3 biowaste:seed 22.26 0.586 0.442 0.127 18.75 1:1 sludge:seed 19.97 0.578 0.305 0.144 17.49 Table 5. Kinetic parameters of summarized methane production appertaining to the volume

In the case of the values of actual methane production relating to day 10, we did not gain in each case higher v10 value when increasing the seeding ratio. The v30 value relating to day 30 is in all cases less than the v10 value which indicates the decrease of methane production. In the case of v30 values, we experienced that, when increasing the seeding, the value of actual methane production referred to reactor volume unit and relating to day 30 decreased.

We assume that the treatment period (retention time) affects the gas production of the reactor. The question is, taking into account the enhancement of gas production in the reactor and at the same time the degradation rate indicating the efficiency of the treatment, what retention time the reactors ought to be designed to. Figure 6 shows the average methane production determined for the treatment period (specific methane production

Figure 6 clearly shows the differences between the sewage sludge and the biowaste containing VFG, too. At the biowaste, as a result of the higher proportion of the easily degradable organic material due to the VFG, with the reduction of the retention time from 30 to 10 days, the gas yield grew in the case of 1:2 and 1:3 seeding ratios. At the 1:1.5 seeding ratio, because of the initial unfavourable conditions (pH, hydrogen), this effect occurred later between day 20 and 40. Because of the sludge being less degradable, the methane

referred to time and volume unit) depending on the duration of the treatment.

Fig. 6. Average methane production in the case of different biowaste : seed ratios

0 10 20 30 40 50 60

1:1.5 biowaste:seed 1:2 biowaste:seed 1:3 biowaste:seed 1:1 sludge:seed

 dm-3 . d-1) v30d

 dm-3 . d-1)

k (1 . d-1)

**Time (d)**

t0 (d)

(CH4-COD g .

volume unit for the sewage sludge and biowaste (consisting of 50% sludge and 50% VFG) at seed ratios shown in Figure 5.

Fig. 5. Summarized, specific methane production on the basis of reactor volume against time

Figure 5 shows that in the case of methane production referred to reactor volume, the methane production of the sewage sludge above 30-day retention time is lower than that of the combined treatments, so thus, we can achieve higher gas yield from one unit of reactor volume when the sewage sludge and the biowastes are treated together than in case the sewage sludge is applied alone. We achieved maximal methane production with codigestion at the 1:1.5 waste to seed ratio, this is followed by the 1:3 and 1:2 waste to seed ratios, however, significant difference between the measurement results cannot be detected. The increase of the seed ratio, in spite of the more inert material filling up the reactor volume, did not considerably reduce the methane production projected to reactor volume unit until day 30 of the treatment. A great increase of the amount of the seeding material, however, results in increase of the reactor volume necessary to the actual treatment capacity which, at the same time, is associated with the same rate of increase in gas production. Taking into consideration also the goal of stabilization, based on the comparison of Figures 4 and 5, we can state that is may be worth to count on the reduction of the retention time while increasing the seed ratio, for the purpose of optimization of the gas yield, degradation and volume demand.

The reaction kinetic parameters of results referred to the reactor volume are shown in Table 5. It is apparent from the results of the table that the values of the maximal methane production are nearly the same, no significant differences can be detected. The value of *k*  reaction rate constant is the highest in the case of 1:1.5 biowaste to seed ratio and its value equals to the *k* value relating to the sewage sludge.

volume unit for the sewage sludge and biowaste (consisting of 50% sludge and 50% VFG) at

Fig. 5. Summarized, specific methane production on the basis of reactor volume against time Figure 5 shows that in the case of methane production referred to reactor volume, the methane production of the sewage sludge above 30-day retention time is lower than that of the combined treatments, so thus, we can achieve higher gas yield from one unit of reactor volume when the sewage sludge and the biowastes are treated together than in case the sewage sludge is applied alone. We achieved maximal methane production with codigestion at the 1:1.5 waste to seed ratio, this is followed by the 1:3 and 1:2 waste to seed ratios, however, significant difference between the measurement results cannot be detected. The increase of the seed ratio, in spite of the more inert material filling up the reactor volume, did not considerably reduce the methane production projected to reactor volume unit until day 30 of the treatment. A great increase of the amount of the seeding material, however, results in increase of the reactor volume necessary to the actual treatment capacity which, at the same time, is associated with the same rate of increase in gas production. Taking into consideration also the goal of stabilization, based on the comparison of Figures 4 and 5, we can state that is may be worth to count on the reduction of the retention time while increasing the seed ratio, for the purpose of optimization of the gas yield, degradation

0 10 20 30 40 50 60

1:1.5 biowaste:seed 1:2 biowaste:seed 1:3 biowaste:seed 1:1sludge:seed

**Time (d)**

The reaction kinetic parameters of results referred to the reactor volume are shown in Table 5. It is apparent from the results of the table that the values of the maximal methane production are nearly the same, no significant differences can be detected. The value of *k*  reaction rate constant is the highest in the case of 1:1.5 biowaste to seed ratio and its value

seed ratios shown in Figure 5.

and volume demand.

0

5

10

**Methane produced (CH4-COD g .**

 **dm-3)**

15

20

25

30

equals to the *k* value relating to the sewage sludge.


Table 5. Kinetic parameters of summarized methane production appertaining to the volume of the reactor

In the case of the values of actual methane production relating to day 10, we did not gain in each case higher v10 value when increasing the seeding ratio. The v30 value relating to day 30 is in all cases less than the v10 value which indicates the decrease of methane production. In the case of v30 values, we experienced that, when increasing the seeding, the value of actual methane production referred to reactor volume unit and relating to day 30 decreased.

We assume that the treatment period (retention time) affects the gas production of the reactor. The question is, taking into account the enhancement of gas production in the reactor and at the same time the degradation rate indicating the efficiency of the treatment, what retention time the reactors ought to be designed to. Figure 6 shows the average methane production determined for the treatment period (specific methane production referred to time and volume unit) depending on the duration of the treatment.

Figure 6 clearly shows the differences between the sewage sludge and the biowaste containing VFG, too. At the biowaste, as a result of the higher proportion of the easily degradable organic material due to the VFG, with the reduction of the retention time from 30 to 10 days, the gas yield grew in the case of 1:2 and 1:3 seeding ratios. At the 1:1.5 seeding ratio, because of the initial unfavourable conditions (pH, hydrogen), this effect occurred later between day 20 and 40. Because of the sludge being less degradable, the methane production gradually increased until day 40.

Fig. 6. Average methane production in the case of different biowaste : seed ratios

Co-Digestion of Organic Waste

The Netherlands

pp. 40-45

Netherlands.

15-24.

and Sewage Sludge by Dry Batch Anaerobic Treatment 111

*Modelling of Anaerobic Wastewater* IWA Publishing, London, England, pp. 88. Benedek, P., Major, V., Réczey, G., & Takács I. (1990). *Biotechnológia a környezetvédelemben,*

Biotechnion (1996): *Laboratory methods and procedures for anaerobic wastewater treatment,*

Breure A. M., & van Andel J. G. (1984). *Hydrolysi´s and acidogenic fermentation of a protein,* 

Brummeler ten, E., Horbach H. C. J. M., & Koster I. W. (1991). *Dry anaerobic batch digestion of* 

Brummelet ten, E., Aarnink M. M. J., & Koster I. W. (1992). *Dry anaerobic-digestion of solid* 

Brummeler ten, E. (1993). *Dry Anaerobic Digestion of the Organic Fraction of Municipal Solid* 

Brummeler ten, E. (2000). *Full scale experience with the BIOCEL process.* Water Science and

Cout, D., Gennon, G., Ranzini, M., & Romano, P. (1994). Anaerobic co-digestion of

*International Symposium on Anaerobic Digestion.* Johannesburg, South Africa. Ghosh, S., & Klass, D. L. (1978). *Two-phase anaerobic digestion.* Process Biochemistry 13(4) pp.

Gujer, W., & Zehnder A. J. B. (1983). *Conversion processes in anaerobic digestion.* Water Science

Hanaki, K., Matsuo T., & Nagase M. (1981). *Mechanism of inhibition caused by long-chain fatty acids in anaerobic digestion process*, Biotechnology Bioengineering, 23. pp. 1591-1610 Haug, R. T. (1980). *The Compost Engineering. Principles and Practice,* Ann Arbor Science

Jeris, J. S., & McCarty, P. L. (1965). *The Biochemistry of Methane Fermentation Using 14C tracers,*

Kaspar, H. F., & Wuhrmann, K. (1978). *Kinetic parameters and relative turnover of some important catabolic reactions in degesting sludge*. Appl. Environ. Microbiol. Kayhanian, M., & Tchobanoglous, G. (1992). Pilot investigation of an innovative two-stage

Koster, I. W. (1989). *Toxicity in anaerobic digestion with emphasis on the effect of ammonia, sulfide* 

Lettinga, G. & Hulshoff Pol, L. W. (1990). Basic aspects of anaerobic wastewater treatment

anaerobic digestion and aerobic composting process for the recovery of energy from the organic fraction of MSW. In: *Proceedings of the 5th International Symposium* 

*and long-chain fatty acids on methanogenesis,* Doctoral dissertation, Wageningen

technology. In: *Anaerobic reactor technology, International Course on Anaerobic Waste* 

Műszaki Könyvkiadó, ISBN 963-10-8224-5, Budapest, Hungary

Biotechnology, Volume: 50 Issue: 2, pp. 191-209.

Volume: 25 Issue: 7, pp. 301-310.

Technology: Vol. 41 (3) pp. 299-304.

and Technology 15. pp. 127-167

Journal WPCF 37, No. 2 . pp. 178-192.

*on Anaerobic Digestion.* Venice, Italy.

Publishers, ISBN 0-250-40347-1, Michigan, The USA

Agricultural University, Wageningen, The Netherlands

*Water Treatment*, Wageningen Agricultural University

*1 (ADM1) Scientific and Technical Report No. 13 IWA Task Group for Mathematical* 

Wageningen Agricultural University, Department of Environmental Biotechnology,

*gelatine, in an anaerobic continuous culture,* Applied Microbiology Biotechnology 20.

*the organic fraction of municipal solid-waste,* Journal of Chemical Technology and

*organic waste in a BIOCEL reaktor at pilot-plant scale,* Water Science and Technology,

*Waste*, Doctoral Thesis, Wageningen Agricultural University, Wageningen, The

municipal sludges and industrial organic wastes. In: *Proceedings of the 7th* 

Figure 6 shows that in each case of waste to seed mixture, the average methane production reaches its maximum after day 10 and then after day 30 it starts to decline. This means that the retention time has to be minimum 30 days in the case of a combined dry batch treatment of VFG waste and sewage sludge. In the case of higher seeding ratios, following 30-40 days, the average methane production is almost the same in the case of each seeding, so thus, the effect of the seeding prevails less. Figure 6 confirms, that optimizing the anaerobic treatment, it is worth to check, together with the increase of the seed ratio, the option of reducing the retention time. It can be stated that the application of the seeding in 1:3 ratio has no negative impact on the gas production of the reactors even above a 40-day retention time assuring high grade stabilization.
