**3. Results and evaluation**

We assumed that the balance of the multi-stage anaerobic digestion process can be influenced by setting the ratio of the seeding material which results in greater degradation of the organic content of the treated waste, as well as in greater methane production.

We evaluated the experimental results based on two aspects:


Co-Digestion of Organic Waste

of the 1:1.5 and 1:2 mixing ratios.

Fig. 4. The degradation rate of organic wastes against time

rate of organic material can be achieved for the whole treatment period.

According to our measurement results, with a 60-days treatment with 1:3 biowaste to seed ratio, 54% organic material degradation can be achieved. In the case of biowaste to seed mixtures of 1:1.5 and 1:2 ratios, only 41-43% of the organic material became decomposed during the same period of time. Thus, when increasing the amount of seeding material with improving the initial conditions of the treatment, a considerable impact in the degradation

0 10 20 30 40 50 60

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

**Time (d)**

application.

0

10

20

30

**Rate of degradation (D%)**

40

50

60

and Sewage Sludge by Dry Batch Anaerobic Treatment 105

containing VFG as well, the easily degradable organic material content was higher than in the case of the reactor containing only sewage sludge. The fatty acid accumulated the in reactor containing VFG which led to the acidification of the reactor, in the end. Against the acidification of the biowaste, in the case of the sludge, the values of pH and hydrogen concentration were better than the critical level even in the initial critical phase of the treatment. This calls the attention to that, because of the varying quality of VFG waste, the determination of the seeding material ratio has to be estimated case by case in each practical

We can calculate the degradation of organic material of the waste from the quotient of the methane production totalled in the time and of the chemical oxygen demand of the waste mixture. Figure 4 shows the rate of degradation against time for different seeding rates and substrates (the methane production of the seeding material is deducted). Onto the measurement results we fitted the logistic function describing biological processes (Dt% = Dmax / (1 + e -k(t-t0)). The reaction kinetic parameters are shown in Table 2. The value of *k*  reaction rate constant rose with the increase of the seeding material ratio which resulted in decrease of the value of *t0*. Significant differences cannot be detected in the values of *k* and *t0*


#### **3.1 The results of the organic matter degradation**

The actual methane production of different mixtures of organic wastes and seed, referred to one unit of treated organic material, is shown in Figure 3. The methane production of the seeding material present in the reactor was deducted from the methane production of the mixture of the waste and seeding material. As a result, because of the relatively high degradability of the seeding material, in the case of unbalanced reactors caused by low seeding material ratios, we had even negative methane production in the first 20 days which was indicated as zero value.

Fig. 3. Actual methane production referred to one unit of treated organic material

We reached the highest methane yield with the 1:3 biowaste to seed ratio. With the increase of the seed ratio, the methane production grew, too. The methane yield was very low in the case of 1:0.5 and 1:1 biowaste to seed ratios. Due to the low seed ratio, the waste became acidified (pH 5.5-5.8), so thus the process of methane production was also inhibited. Since our goal was to determine the optimal seeding ratio, we carried out the test in these reactors only for 15 days. The maximal methane production of the seeding material (digested sludge) occurred on day 10, however its extent was one eighth of that of the balanced reactors and the methane production decreased to zero after the 30 days.

Having compared the treatability of the sewage sludge and of the biowaste, with the 1:1 seed ratio applied in practice, we can state that in the case of the sewage sludge, a more balanced reactor performance can be observed. The results suggest that in the case of reactor


The actual methane production of different mixtures of organic wastes and seed, referred to one unit of treated organic material, is shown in Figure 3. The methane production of the seeding material present in the reactor was deducted from the methane production of the mixture of the waste and seeding material. As a result, because of the relatively high degradability of the seeding material, in the case of unbalanced reactors caused by low seeding material ratios, we had even negative methane production in the first 20 days which

Fig. 3. Actual methane production referred to one unit of treated organic material

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

the methane production decreased to zero after the 30 days.

We reached the highest methane yield with the 1:3 biowaste to seed ratio. With the increase of the seed ratio, the methane production grew, too. The methane yield was very low in the case of 1:0.5 and 1:1 biowaste to seed ratios. Due to the low seed ratio, the waste became acidified (pH 5.5-5.8), so thus the process of methane production was also inhibited. Since our goal was to determine the optimal seeding ratio, we carried out the test in these reactors only for 15 days. The maximal methane production of the seeding material (digested sludge) occurred on day 10, however its extent was one eighth of that of the balanced reactors and

0 10 20 30 40 50 60 70 80 90

seed 1:0.5 biowaste:seed 1:1 biowaste:seed 1:1.5 biowaste:seed

**Time (d)**

Having compared the treatability of the sewage sludge and of the biowaste, with the 1:1 seed ratio applied in practice, we can state that in the case of the sewage sludge, a more balanced reactor performance can be observed. The results suggest that in the case of reactor

reactor volume.

was indicated as zero value.

0

5

10

**Methane production (CH**

**4-COD g . kg VS-1 .d-1)**

15

20

25

30

35

40

**3.1 The results of the organic matter degradation** 

containing VFG as well, the easily degradable organic material content was higher than in the case of the reactor containing only sewage sludge. The fatty acid accumulated the in reactor containing VFG which led to the acidification of the reactor, in the end. Against the acidification of the biowaste, in the case of the sludge, the values of pH and hydrogen concentration were better than the critical level even in the initial critical phase of the treatment. This calls the attention to that, because of the varying quality of VFG waste, the determination of the seeding material ratio has to be estimated case by case in each practical application.

We can calculate the degradation of organic material of the waste from the quotient of the methane production totalled in the time and of the chemical oxygen demand of the waste mixture. Figure 4 shows the rate of degradation against time for different seeding rates and substrates (the methane production of the seeding material is deducted). Onto the measurement results we fitted the logistic function describing biological processes (Dt% = Dmax / (1 + e -k(t-t0)). The reaction kinetic parameters are shown in Table 2. The value of *k*  reaction rate constant rose with the increase of the seeding material ratio which resulted in decrease of the value of *t0*. Significant differences cannot be detected in the values of *k* and *t0* of the 1:1.5 and 1:2 mixing ratios.

Fig. 4. The degradation rate of organic wastes against time

According to our measurement results, with a 60-days treatment with 1:3 biowaste to seed ratio, 54% organic material degradation can be achieved. In the case of biowaste to seed mixtures of 1:1.5 and 1:2 ratios, only 41-43% of the organic material became decomposed during the same period of time. Thus, when increasing the amount of seeding material with improving the initial conditions of the treatment, a considerable impact in the degradation rate of organic material can be achieved for the whole treatment period.

Co-Digestion of Organic Waste

kg VS-1 unit used by us.

plant treating VFG

it, too) was lower.

sludge digestion alone.

Full scale BIOCEL plant1

Time (d)

more favourable values measured in the case of sludge.

Cumulative methane production

1:3 seed to biowaste2

kg VS-1)

1Value calculated according to Brummeler (1993), 450 m3 reactor, waste TS 36%, VS 65% 2Methane production together with the methane production of the seeding material

(CH4-COD g .

and Sewage Sludge by Dry Batch Anaerobic Treatment 107

accumulation of hydrogen and volatile fatty acids. At sewage sludge digestion, often the hydrolysis appears as the process limiting step (Koster, 1989) which could contribute to the

We compared our measurement results with the operation data of a full scale BIOCEL plant (Brummeler, 1993) (Table 4.). The literature refers the biogas quantities to wet waste mass, to standard condition. For comparability we recalculated the literature data to the CH4-COD g .

Laboratory scale Full scale

5 110.0 22.9 18.4 7.7 1.2 1.6 10 297.7 157.2 98.1 20.9 13.4 8.4 20 660.0 278.7 173.9 46.3 23.7 14.9 40 - 453.5 286.5 - 38.6 24.6 60 - 551.6 325.9 - 47.0 28.0

Table 4. Comparison of the laboratory results with the operation data of a full scale BIOCEL

The results of Table 4 show that the results of the methane production referring to VFG waste reviewed in the literature, at the same moment, significantly exceed the results of the co-digestion of the biowaste with sewage sludge. In our experiment, the difference resulting from the lag phase as well as the lower degradability of the biowaste containing sludge and VFG can be definitely pointed. In our experiment half of the waste mixture was sewage sludge. The sewage sludge applied by us was less degradable than the biowaste, thus, the degradability of one unit of waste mixture (and so the amount of methane production from

The results of Table 4 show that higher gas yield referred to one unit of organic matter can be reached in the case of co-digestion of VFG and sewage sludge than in the case of sewage

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

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

1:1 seed to sludge

BIOCEL plant2

Degradation of the organic material (D%)

> 1:3 seed to biowaste2

Laboratory scale

1:1 seed to sludge


Table 2. Kinetic parameters of the degradation process

Considering the rates of actual methane production, we can see that the actual rates measured on day 10 significantly increase with the growth of the amount of seeding material. At the values related to day 30, the effect of seed ratio onto the methane production can be still well detected. The actual rate of the methane production further increased from day 30 also in each cases, which suggests that we can count on a considerable degree of degradation even after day 30. This is confirmed by the *t0* value, as well.

To characterize the process of the anaerobic degradation, we checked the hydrogen content of the biogas, as well as the temporal evolution of the pH of the reactors in the most critical initial phase of the treatment (Table 3). The hydrogen content of the biogas was above the value of the detection limit only in the first 9 days.


Table 3. The hydrogen content of biogas and the pH of wastes in the case of different wastes and seeding ratios

It is seen in the case of biowaste that, by the increase of seeding ratio, the hydrogen content of the biogas decreases and the pH of the waste in reactors increased. During the test period, the hydrogen content of the biogas also decreases and then, following day 9, it is under the value of detection limit. The critical hydrogen concentration, above the 0.01 % as calculated based on the literature (Zehnder et al., 1982), measured in the first 5 days had a negative effect on the methane production in the case of each seeding ratio (Figure 3). By the increase of the seeding, above the ratio of 1:1.5, the hydrogen concentration decreased below the critical value from day 9 and the methane production started to increase. The unfavourable values of hydrogen and pH measured in the case of 1:0.5 and 1:1 ratios led to the acidification of the reactors. The pH of the reactors increased during the test which resulted in the rise of biogas production. During the anaerobic treatment of the sewage sludge, we did not measure significant hydrogen quantity in the biogas even in the case of 1:1 sludge to seed mixing ratio. This can be explained by that there is less easily degradable organic material in the sewage sludge than in the tested biowaste which is responsible for the

1:1.5 biowaste:seed 0.055 44.33 0.936 0.424 0.788 1:2 biowaste:seed 0.054 42.58 0.918 0.453 0.782 1:3 biowaste:seed 0.078 28.29 0.939 0.820 1.256 1:1 sludge:seed 0.060 31.38 0.876 0.526 0.749

Considering the rates of actual methane production, we can see that the actual rates measured on day 10 significantly increase with the growth of the amount of seeding material. At the values related to day 30, the effect of seed ratio onto the methane production can be still well detected. The actual rate of the methane production further increased from day 30 also in each cases, which suggests that we can count on a considerable degree of degradation even after day 30. This is confirmed by the *t0* value, as

To characterize the process of the anaerobic degradation, we checked the hydrogen content of the biogas, as well as the temporal evolution of the pH of the reactors in the most critical initial phase of the treatment (Table 3). The hydrogen content of the biogas was above the

(%) pH H2

1:0.5 biowaste:seed 9.66 5.47 0.38 5.51 1.08 5.55 0.08 5.65 0.06 5.70 1:1 biowaste:seed 4.27 5.70 0.19 5.80 0.13 5.75 0.02 5.78 0.02 5.83 1:1.5 biowaste:seed 3.58 5.90 0.14 5.84 0.10 6.13 0.02 6.06 0.01 6.34 1:2 biowaste:seed 1.40 6.27 0.04 6.32 0.02 6.34 0.01 6.43 0.01 6.59 1:3 biowaste:seed 0.37 6.10 0.05 6.23 <dl 6.45 0.01 6.65 0.01 6.72 1:1 sludge:seed 0.62 6.68 <dl 6.94 <dl 7.09 <dl 7.28 <dl 7.37 Table 3. The hydrogen content of biogas and the pH of wastes in the case of different wastes

It is seen in the case of biowaste that, by the increase of seeding ratio, the hydrogen content of the biogas decreases and the pH of the waste in reactors increased. During the test period, the hydrogen content of the biogas also decreases and then, following day 9, it is under the value of detection limit. The critical hydrogen concentration, above the 0.01 % as calculated based on the literature (Zehnder et al., 1982), measured in the first 5 days had a negative effect on the methane production in the case of each seeding ratio (Figure 3). By the increase of the seeding, above the ratio of 1:1.5, the hydrogen concentration decreased below the critical value from day 9 and the methane production started to increase. The unfavourable values of hydrogen and pH measured in the case of 1:0.5 and 1:1 ratios led to the acidification of the reactors. The pH of the reactors increased during the test which resulted in the rise of biogas production. During the anaerobic treatment of the sewage sludge, we did not measure significant hydrogen quantity in the biogas even in the case of 1:1 sludge to seed mixing ratio. This can be explained by that there is less easily degradable organic material in the sewage sludge than in the tested biowaste which is responsible for the

2nd day 3rd day 5th day 7th day 9th day

(%) pH H2

t0

(d) R2 v10d

(D% .

d-1)

(%) pH H2

(%) pH

v30d (D% .

d-1)

(1 . d-1)

The description of the sample <sup>k</sup>

Table 2. Kinetic parameters of the degradation process

value of the detection limit only in the first 9 days.

H2

(%) pH H2

Type of the reactor

and seeding ratios

well.

accumulation of hydrogen and volatile fatty acids. At sewage sludge digestion, often the hydrolysis appears as the process limiting step (Koster, 1989) which could contribute to the more favourable values measured in the case of sludge.

We compared our measurement results with the operation data of a full scale BIOCEL plant (Brummeler, 1993) (Table 4.). The literature refers the biogas quantities to wet waste mass, to standard condition. For comparability we recalculated the literature data to the CH4-COD g . kg VS-1 unit used by us.


1Value calculated according to Brummeler (1993), 450 m3 reactor, waste TS 36%, VS 65%

2Methane production together with the methane production of the seeding material

Table 4. Comparison of the laboratory results with the operation data of a full scale BIOCEL plant treating VFG

The results of Table 4 show that the results of the methane production referring to VFG waste reviewed in the literature, at the same moment, significantly exceed the results of the co-digestion of the biowaste with sewage sludge. In our experiment, the difference resulting from the lag phase as well as the lower degradability of the biowaste containing sludge and VFG can be definitely pointed. In our experiment half of the waste mixture was sewage sludge. The sewage sludge applied by us was less degradable than the biowaste, thus, the degradability of one unit of waste mixture (and so the amount of methane production from it, too) was lower.

The results of Table 4 show that higher gas yield referred to one unit of organic matter can be reached in the case of co-digestion of VFG and sewage sludge than in the case of sewage sludge digestion alone.
