**3. Start-up and trial operation of biogas plant for maize silage processing**

Despite the 17,000 biogas plants in EU [24], many of which use maize silage as the main substrate, only a little information on their start-up and trial operation can be found in literature. Start-up and trial operation of a biogas plant for processing of maize silage as the main substrate are described.

Technology of a biogas plant is depicted in **Figure 5**. Effective volume of the used anaerobic reactor was 2450 m3 . Two high-speed blade mixers with horizontal rotational axis and with the immersion depth and mixing direction regulation were used. Fresh silage was loaded into the reactor by means of a conveyor belt. Silage pits were located next to the anaerobic reactor; an average TS of the silage was used during the start-up, and pilot plant operation was 35%; and the expected biogas production was 4200 m3 /d. Biogas was incinerated in a cogeneration unit (ELTECO, Slovakia) with the electric power of 276 kW (electric efficiency of 32%) and the heat power of 479 kW. The reactor was operated at the temperature of 37°C and the volume of the gasholder was 80 m3 , which was assumed as sufficient at stabilized production and consumption of biogas. Also a gas boiler enabling biogas as well as a natural gas incineration with the heat power of 470 kW was included in the technology. This boiler plays an important role during the start-up of the anaerobic reactor when biogas is not available and the reactor has to be heated to the operation temperature by natural gas.

The anaerobic reactor was inoculated with aerobically stabilized sewage sludge from a brewery, which is not often used as an inoculation medium for anaerobic reactors. Normally, for the

**Figure 5.** Diagram of biogas plant for anaerobic digestion of the maize silage.

Considering that from 1 ha of arable land, 30 t of TS silage (VS of 95%) per annum are obtained,

a cogeneration unit with the electric power of 1 MW (electric energy production efficiency of 35%), the daily maize silage demand represents the area of 0.77 ha. This means that the annual

Biomethane potential tests provided the measured specific methane production of 0.215

obtained for long-term reactor operation is due to the adaptation of the anaerobic microor-

Long-term operation of the anaerobic reactor for maize silage processing as the only substrate showed significant instability caused by the low alkalinity of maize silage (high C:N ratio). To stabilize the anaerobic processes, other alkaline reagents or a co-substrate with higher content

Daily operation of a biogas plant with biogas incinerated in a cogeneration unit with the electric

**3. Start-up and trial operation of biogas plant for maize silage processing**

Despite the 17,000 biogas plants in EU [24], many of which use maize silage as the main substrate, only a little information on their start-up and trial operation can be found in literature. Start-up and trial operation of a biogas plant for processing of maize silage as the

Technology of a biogas plant is depicted in **Figure 5**. Effective volume of the used anaerobic

immersion depth and mixing direction regulation were used. Fresh silage was loaded into the reactor by means of a conveyor belt. Silage pits were located next to the anaerobic reactor; an average TS of the silage was used during the start-up, and pilot plant operation was 35%; and

(ELTECO, Slovakia) with the electric power of 276 kW (electric efficiency of 32%) and the heat power of 479 kW. The reactor was operated at the temperature of 37°C and the volume of the

tion of biogas. Also a gas boiler enabling biogas as well as a natural gas incineration with the heat power of 470 kW was included in the technology. This boiler plays an important role during the start-up of the anaerobic reactor when biogas is not available and the reactor has

The anaerobic reactor was inoculated with aerobically stabilized sewage sludge from a brewery, which is not often used as an inoculation medium for anaerobic reactors. Normally, for the

. Two high-speed blade mixers with horizontal rotational axis and with the

, which was assumed as sufficient at stabilized production and consump-

/d. Biogas was incinerated in a cogeneration unit

power of 1 MW requires the amount of silage from an area of 0.77 ha of arable land.

/kg of VS. For long-term maize silage processing in a mixed laboratory anaerobic reac-

operation of biogas plants needs 8431.5 t of TS silage, grown on 281 ha of arable land.

Conclusions of the anaerobic digestion of maize silage in laboratory conditions:

tor, the measured specific methane production was 0.316 Nm3

/ha. For a biogas plant with produced biogas incineration in

/kg of VSS. The higher value

methane production is 9006 Nm3

180 Advances in Silage Production and Utilization

ganisms to the maize silage substrate.

main substrate are described.

the expected biogas production was 4200 m3

to be heated to the operation temperature by natural gas.

reactor was 2450 m3

gasholder was 80 m3

of nitrogen (sewage sludge or manure) can be used.

Nm3

inoculation of the anaerobic reactor the anaerobically stabilized sludge is used, but the distance to the nearest wastewater plant with anaerobically stabilized sludge was 15 km, and the required amount of sludge could not be provided. Aerobically stabilized sludge from brewery wastewater plant was available for only as far as 2 km from the biogas plant. The amount of aerobically stabilized sludge added to the anaerobic reactor for inoculation before its start-up was 1700 m3 with an average concentration of suspended solids (SS) of 30 g/L. After the inoculation, the reactor was heated to 37°C and gradually loaded with maize silage. During the start-up of the anaerobic reactor, biogas production, pH, VFA, NH4-N, PO4-P, and suspended solids concentration were monitored. The course of these parameters was also monitored during the first 200 days of the pilot plant operation (**Figures 6**–**10**).

**Figure 6.** Course of silage dose and biogas production during the start-up of the anaerobic reactor.

**Figure 7.** Concentration of VFA in filtered sludge water from the anaerobic reactor during the start-up.

amount was divided into six parts and every 6 hours 3.33 t was dosed. Average specific biogas

Increase of these concentrations within the first 20 days of operation is related to the degradation of sludge used as the inoculum (similarly as for the VFA concentration—**Figure 7**). NH4-N concentration gradually decreased to approximately 200 mg/L and that of PO4-P to below 20 mg/ L. Low concentrations of ammonia nitrogen were followed by a pH decrease (**Figure 9**) due to the low alkalinity of the silage. Values of pH below 6.5 led to methanogenesis inhibition which increased the VFA concentration above 7500 mg/L (**Figure 7**) and decreased the biogas production significantly. From day 120, it was started with dosing of aerobically stabilized sludge (the same one that was used for inoculation) to increase the NH4-N concentration and

**Figures 7**–**9**, the NH4-N concentration increased to above 600 mg/L, VFA concentration decreased and pH was stabilized at around 7.2. The silage dose after stabilization of the reactor

the cogeneration unit worked with its 100% capacity. OLR of the anaerobic reactor was in the

During the anaerobic reactor start-up, some interesting phenomena have been observed: after each silage dosing, a temporary increase in biogas production and the resulting increase in the cogeneration unit electrical power output, **Figure 11** shows the response of the electrical power output for a silage dose every 3 h (16 t per day, each dose of 2 t), with the total biogas production

and the cogeneration unit efficiency of 67% (day 140). The period of increased biogas

/d) and the SS concentration in sludge water of the anaerobic reactor after

/kg of silage TS.

Maize Silage as Substrate for Biogas Production

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183

/kg of silage TS, and

/d (SS concentration of 30 g/L). As it is evident from

production between days 50 and 100 of the reactor operation was 0.726 m3

operation was increased to 24 t/d, specific biogas production reached 0.7 m3

**Figure 8** presents the course of the NH4-N and PO4-P concentrations.

**Figure 9.** Course of pH in the anaerobic reactor during the start-up.

stabilize pH. The sludge dose was 7–10 m3

200 days of operation was 60 g/L (**Figure 10**).

range of 2.3–2.7 kg/(m3

of 2800 m3

**Figure 8.** Concentration of NH4-N and PO4-P in filtered sludge water from the anaerobic during the start-up.

The silage load was gradually increased (**Figure 6**), with the starting load of 2 t/d. As it follows from **Figures 6** and **7** (biogas production and VFA concentration), anaerobic reactor operation was stable, and the biogas production was proportional to the increasing load approximately until the end of day 100. Maximum load of silage in this period was 20 t/d. This

**Figure 9.** Course of pH in the anaerobic reactor during the start-up.

**Figure 7.** Concentration of VFA in filtered sludge water from the anaerobic reactor during the start-up.

182 Advances in Silage Production and Utilization

**Figure 8.** Concentration of NH4-N and PO4-P in filtered sludge water from the anaerobic during the start-up.

The silage load was gradually increased (**Figure 6**), with the starting load of 2 t/d. As it follows from **Figures 6** and **7** (biogas production and VFA concentration), anaerobic reactor operation was stable, and the biogas production was proportional to the increasing load approximately until the end of day 100. Maximum load of silage in this period was 20 t/d. This amount was divided into six parts and every 6 hours 3.33 t was dosed. Average specific biogas production between days 50 and 100 of the reactor operation was 0.726 m3 /kg of silage TS. **Figure 8** presents the course of the NH4-N and PO4-P concentrations.

Increase of these concentrations within the first 20 days of operation is related to the degradation of sludge used as the inoculum (similarly as for the VFA concentration—**Figure 7**). NH4-N concentration gradually decreased to approximately 200 mg/L and that of PO4-P to below 20 mg/ L. Low concentrations of ammonia nitrogen were followed by a pH decrease (**Figure 9**) due to the low alkalinity of the silage. Values of pH below 6.5 led to methanogenesis inhibition which increased the VFA concentration above 7500 mg/L (**Figure 7**) and decreased the biogas production significantly. From day 120, it was started with dosing of aerobically stabilized sludge (the same one that was used for inoculation) to increase the NH4-N concentration and stabilize pH. The sludge dose was 7–10 m3 /d (SS concentration of 30 g/L). As it is evident from **Figures 7**–**9**, the NH4-N concentration increased to above 600 mg/L, VFA concentration decreased and pH was stabilized at around 7.2. The silage dose after stabilization of the reactor operation was increased to 24 t/d, specific biogas production reached 0.7 m3 /kg of silage TS, and the cogeneration unit worked with its 100% capacity. OLR of the anaerobic reactor was in the range of 2.3–2.7 kg/(m3 /d) and the SS concentration in sludge water of the anaerobic reactor after 200 days of operation was 60 g/L (**Figure 10**).

During the anaerobic reactor start-up, some interesting phenomena have been observed: after each silage dosing, a temporary increase in biogas production and the resulting increase in the cogeneration unit electrical power output, **Figure 11** shows the response of the electrical power output for a silage dose every 3 h (16 t per day, each dose of 2 t), with the total biogas production of 2800 m3 and the cogeneration unit efficiency of 67% (day 140). The period of increased biogas production was ca. 1 h; the increase in biogas production showed in **Figure 11** represents 5.13% of the total biogas production per silage dose. Such an increase is related to the content of readily biodegradable organic matter in maize silage (VFA, alcohols, lower saccharides, etc.), which can vary in the range of 2.1–11.1% (**Table 4**).

**TS [%] Acid Ethanol Glucose Fructose Reference**

Between two doses, also the quality of the biogas produced changed; average methane concentration in biogas was 54.5% and that of hydrogen sulfide was 160 ppm. In the first two hours after the loading, the methane concentration decreased by ca. 2% (from 55 to 53%), which can be explained by higher CO2 production due to the degradation of readily biodegradable matter. More significant changes in the biogas composition were observed for a five day silage loading interruption, when the methane concentration in biogas increased from 52.8 to 65%.

In the steady state, when the full capacity of the cogeneration unit was achieved, the biogas

of ca. 11,500 kWh/d were provided at the daily maize silage dose of 6–7 t of TS. Electric energy was sold to the electric grid, produced heat was employed for the anaerobic reactor heating (12–13% of the produced heat), for greenhouses heating and also for drying maize grains between September and December produced on the premises as well as that produced by neighboring farmers. The digestate was stored and used if necessary as a fertilizer on the arable

Conclusions from the start-up and trial operation of a biogas plant for anaerobic digestion of

Results of the start-up of the anaerobic reactor have proved the suitability of aerobically stabilized sludge for the anaerobic reactor inoculation despite the substrate not being used for

Results of the laboratory experiments were confirmed – the low alkalinity of maize silage and the need for additional substrates with higher nitrogen content to stabilize the reactor operation. In the present case, aerobically stabilized sludge from a brewery wastewater plant was

After ca. 150 days of the biogas plant operation, the designed parameters were stabilized. At

production of ca. 6600 kWh/d and heat production of ca. 11,500 kWh/d were achieved. Daily

/d, electric energy

the full capacity of the cogeneration unit, the biogas production of 4200 m3

dose of silage was 24 t/d, divided into six portions every 4 hours.

Start-up of the anaerobic reactor took approximately 100 days.

/d, electric energy production of ca. 6600 kWh/d and the heat production

Maize Silage as Substrate for Biogas Production

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185

39.2 6.2 2.6 0.2 <1 1.1 – – [25] 36.0 4.2 1.5 0.32 1.04 1.5 – – [26] 31.0 5.21 1.28 – – – 0.29 0.47 [27] 41.0 2.12 0.86 0.05 0.24 0.17 – – [28] 28.5 0.56 0.47 0.01 0.01 0.27 – – [29]

**Table 4.** Concentration of volatile and readily biodegradable matter in maize silage (% of TS).

**Lactic Acetic Propionic Butyric**

production of 4200 m3

lands of the farm.

this purpose usually.

maize silage:

used.

**Figure 10.** Concentration of suspended solids in the anaerobic reactor during the start-up.

**Figure 11.** Increase of power output of the cogeneration unit (biogas production) after silage dosing.


**Table 4.** Concentration of volatile and readily biodegradable matter in maize silage (% of TS).

production was ca. 1 h; the increase in biogas production showed in **Figure 11** represents 5.13% of the total biogas production per silage dose. Such an increase is related to the content of readily biodegradable organic matter in maize silage (VFA, alcohols, lower saccharides, etc.),

which can vary in the range of 2.1–11.1% (**Table 4**).

184 Advances in Silage Production and Utilization

**Figure 10.** Concentration of suspended solids in the anaerobic reactor during the start-up.

**Figure 11.** Increase of power output of the cogeneration unit (biogas production) after silage dosing.

Between two doses, also the quality of the biogas produced changed; average methane concentration in biogas was 54.5% and that of hydrogen sulfide was 160 ppm. In the first two hours after the loading, the methane concentration decreased by ca. 2% (from 55 to 53%), which can be explained by higher CO2 production due to the degradation of readily biodegradable matter. More significant changes in the biogas composition were observed for a five day silage loading interruption, when the methane concentration in biogas increased from 52.8 to 65%.

In the steady state, when the full capacity of the cogeneration unit was achieved, the biogas production of 4200 m3 /d, electric energy production of ca. 6600 kWh/d and the heat production of ca. 11,500 kWh/d were provided at the daily maize silage dose of 6–7 t of TS. Electric energy was sold to the electric grid, produced heat was employed for the anaerobic reactor heating (12–13% of the produced heat), for greenhouses heating and also for drying maize grains between September and December produced on the premises as well as that produced by neighboring farmers. The digestate was stored and used if necessary as a fertilizer on the arable lands of the farm.

Conclusions from the start-up and trial operation of a biogas plant for anaerobic digestion of maize silage:

Results of the start-up of the anaerobic reactor have proved the suitability of aerobically stabilized sludge for the anaerobic reactor inoculation despite the substrate not being used for this purpose usually.

Start-up of the anaerobic reactor took approximately 100 days.

Results of the laboratory experiments were confirmed – the low alkalinity of maize silage and the need for additional substrates with higher nitrogen content to stabilize the reactor operation. In the present case, aerobically stabilized sludge from a brewery wastewater plant was used.

After ca. 150 days of the biogas plant operation, the designed parameters were stabilized. At the full capacity of the cogeneration unit, the biogas production of 4200 m3 /d, electric energy production of ca. 6600 kWh/d and heat production of ca. 11,500 kWh/d were achieved. Daily dose of silage was 24 t/d, divided into six portions every 4 hours.
