Abstract :

Over the last years anaerobic methods treating wastes got an increasing importance. Nowadays approx. 1.5 Mio Mg/a of organic wastes (without sewage sludge and industrial plants) are digested in the EU, partly with liquid manure and sewage sludge (Co-digestion). In this report there will be a short documentation of the state of biowaste digestion in Germany first. On two examples methods with different research and operating results are shown.

Anaerobic digestion systems for waste treatment can be classified as follows:

Process single stage / multi- stage
Water content:        high 
solids digestion:      DS > 10% - 20%
low solids digestion: DS < 20%
Process temperature : mesophilic (35°C – 37°C)
                              thermophilic (50°C – 55°C)
Operational mode :   continuous / non continuous
Mixing system:        stirring, moving, circulation, percolation

It is to distinguish between plants only for biowaste-digestion and co-digestion processes normally with agricultural wastes from animal farming or sewage sludge. There are about 8 Mio Mg/a biowastes treated in Germany. 85% are composted 15% are treated anaerobically. At the moment there are 44 plants for anaerobic treatment in Germany with a capacity of 1,2 Mio. Mg/a. 

Following examples are shown:

The digestion plant in Braunschweig-Watenbüttel (D) (20 000 Mg/a) shows the balance of energy and mass flux. The trend of relevant system parameters as biogas production, organic acids, decomposition, etc. for a defined period will be shown.

For the combined plant for composting and digestion of the Biogenes Zentrum Peine (D) (24 000 Mg/a) the balance of mass flux was made and a report about sand content in bio waste will be added.

The report will be concluded by a carbon dioxide balance and a resume about the relevancy of different biological waste treatment processes for the global climate. 

Table of Contents

1.   Summary
2.   Introduction
3.   State of biowaste digestion in Germany
4.    Digestion plant Braunschweig-Watenbüttel
      4.1. General description
      4.2. Process parameters and balances
5.    Digestion plant Biogenes Zentrum Peine
      5.1. General description
      5.2. Mass balance and sand content
6.    Consideration of climate pertinence
7.    Literature
 

Summary

The digestion of biowastes got an increasing importance over the last years. The examples of two different digestion plants in Germany (Braunschweig-Watenbüttel; Biogenes Zentrum Peine) show the balances of mass and energy flux during the digestion. Relevant process parameters represent the operational sequence and the study of sand contents in biowastes gives a short overview about the problems of these high technology processes. The conclusion with a consideration of the climate pertinence of digestion (carbon dioxide balance) gives an impression of the advantages by integrating digestion into processes of waste management.

1. Introduction

Over the last years anaerobic methods treating wastes got an increasing importance. Nowadays appr. 1.5 Mio Mg/a of organic wastes are digested in the EU, partly with liquid manure and sewage sludge (Co-digestion). In this report there is a short documentation of the state of biowaste digestion in Germany first. On two examples methods with different research and operating results are shown.

The digestion plant in Braunschweig-Watenbüttel (Germany) (20 000 Mg/a) is an example for balancing the energy and mass flux. The trend of relevant system parameters as biogas production, organic acids, decomposition, etc. for a defined period are shown.

For the combined plant for composting and digestion of the Biogenes Zentrum in Peine (Germany) (24 000 Mg/a) the balance of mass flux was made and a report about sand content in bio waste is added.

The report is concluded by a carbon dioxide balance and a resume about the relevancy of different biological waste treatment processes for the global climate.
 

2. State of biowaste digestion in Germany

Anaerobic, biological methods treating wastes are to classify basically into the following systems, due to their different process parameters.

It is to distinguish between plants for biowaste digestion only and co-digestion processes in general with agricultural wastes from animal farming or sewage sludge.

There are about 8 Mio. Mg/a biowastes treated in Germany, 85 % are composted 15 % are treated anaerobically. Presently there are 44 plants for anaerobic treatment of organic wastes in Germany with a capacity of 1.22 Mio. Mg/a.

About 80 % of all the plants are run as low solids digestion plants. Especially at plants bigger than 
20 000 Mg/a, located mostly in the east of Germany as co-digestion plants, the part of biowastes is about 25 %. 25 % are food and wastes from production processes, and half of the delivered wastes are from animal farming and sewage sludge. 

The high solids digestion is run with 2/3 of biowastes from households and 1/3 foodstuff, waste from the industrial production and green prunings. 

About 3/4 of the total plant capacity (60% of the plants) have a single stage process. 75% of the single stage, low solids digestion is run with a mesophilic process temperature (35°C – 37°C), single stage, high solids digestion are mostly (80%) run thermophilic

In the east of Germany the treating capacities are about 41 kg/E*a, 50% are needed for the treatment of sewage sludge [1]. About 9 kg/E*a of organic biowaste is digested in the west of Germany.
 

 3. Digestion plant „Braunschweig-Watenbüttel“

3.1  General description

20 000 Mg/a of organic wastes are treated by the KOMPO-GAS-plant of the  Braunschweiger Kompost GmbH in Braunschweig-Watenbüttel, built by Bühler GmbH Germany  in 1997 (fig. 1; fig.2)).

The completion of the windrow composting was necessary because of the increasing initiation of the bio-waste-container in the urban area of the city of Braunschweig and an increasing quantity of organic wastes as a result.

Separate collected organic wastes are treated by a thermophilic high solids anaerobic process. During the first step of the process the sampled biowaste is separated from impurities and reduced to small pieces to get a suitable specific surface for the digestion of the biowaste particles. After this pretreatment the whole biowaste enters the process of digestion. 

Through heat exchanger, which guarantee a steady substrate temperature of 55°C for the process of digestion, the substrate is pumped into the reactor. Digestate lead back over an internal system (cycle) is used as a kind of „inoculation-material“.

The KOMPO-GAS-digester is constructed as a horizontal digester. The consequence is a grafting flow, typical for this semi-continous process, a constant dwell of the digestate, to prevent a short circuit flow, a process water cycle with only dewatering-water, a minimized volume of the fermenting-vat and a totally hygenisation by thermophilic process circumstances. 80 m3 – 140 m3 of biogas with about 60 % Methane-rate per Mg biowaste can be produced. The biogas is used in a block-type thermal powerstation after dewatering and seperating solids. 
 

 3.2 Process parameters and Balances

The balance of the mass flux in the Digestion plant „Braunschweig-Watenbüttel“ is based on data of the period 11/98 to 12/98. 

While the first treatment in the plant impurities are seperated and the waste is shreddered mechanically. By adding presswater from the dewatering unit the higher water content for digestion is adjusted and the mixture is pumped into the reactor. As inoculum, batches of material are taken out of the horizontal digester and added to the mixture after biowaste pre treatment. The retention time of the organic material in the reactor is about 20-22 days. The retention time depends on the volume of capacity and seasonal influences. The produced biogas is utilized in the nearby block-type thermal power station of the sewage treatment plant of the city of Braunschweig. After dewatering and adjusting the water content at about 65 % the digestate is stored another 7 to 10 days for aerobic post treatment in the composting area. The exhaust air is purificated by a biofilter [10], [11]. 

The daily input ranges between 25 Mg and 55 Mg organic wastes (water content 50% - 70%) and 15 Mg to 30 Mg presswater. Biogas up to 8350 m³/d (period 19.11.98 – 22.12.98, leaving the digester, ~ 50°C, 100% H2O-Saturation) is produced (Fig. 3).

The quantities of biowastes (input) and presswater utilized for digestion can be recovered in the volume of produced biogas. As expected there is a time charge between input and output because digestion run over a period of about 20 days (fig. 4).
 
 

Fig 4

Fig 5

 There are 4 sample points placed directly at the digester, one at the mixer and one at the delivery bunker (fig. 5). The pH-value in the digester ranges between 7 und 8,2 with a stabilasation during the fermentation. In the Mixer (mixing biowaste and presswater) pH-values down to 4.5 were measured (fig. 6).
 
 
 

Fig 6

Fig 7

The static view on acetic acid concentration inside the reactor shows at the beginning of the process (after 2 – 5 days) concentrations from 163 mg/kg to 2 250 mg/kg depending on the different samples. During digestion the concentration of acetic acids get reduced continuosly. After 18 –22 days the concentration of acetic acids decreased below 150 mg/kg during the normal process (fig. 7).

While digestion propionic acid concentration get reduced on values about 1.7 mg/kg to 6.5 mg/kg (sample point ¼). The values before  digestion are about 75 mg/kg to 495 mg/kg (fig. 8).

The view on the concentration of the „low organic acids“ during the digestion shows a clear degradation rate. The dynamic view on the contrations of „low organic acids“ on the way of the organic material starting at the mixer over the sample points trough the digester shows their reduction during the whole digestion (fig. 9 + fig. 10).
 
 
 

Fig 9

Fig 10

The samples of organic material at the delivery bunker and mixer are very inhomogenious regarding on „low organic acids“. The material gets more homogen because of mixing even with the inoculum in the fermentation vat during digestion. 

As a rule the input has a water content about 50 % to 65 %. At the outlet of digestion the water content has raised to 80 %. The raised water content is adjusted activly by adding presswater and inoculum (fig.10).
 

Fig 11

Fig 12

The organic substance of the input material differs because of the input composition. Volutile solids between 58 % and 92 % where measured during the study period. After digestion  the digestate has a relatively constant Ignition loss of about 47 % to 52 %. The biodegradation of organic material varies between 34 % and 91 % depending on the input material. 
 

The energy balance of the digestion plant „Braunschweig-Watenbüttel“ for the period 19.11.1998 to 22.12.1998 is based on a total input of 1 407 Mg biowaste. During this period about 132 500 m³ biogas (normed m³) were produced which is equal to a energy value of about 715 000 kWh. About 60 % of the energy (430 000 kWh) was thermic. 30% of the energy (215 000kWh) was changed by a block-type thermal power station into electricity and 10% (70 000 kWh) got lost. At the digestion plant „Braunschweig-Watenbüttel“ 75 000 kWh of electricity were needed for the plant itself. 140 000 kWh were lead into the local electricity net. 120 000 kWh thermal energy was needed for the digestion process and the heating of the building. 310 000 kWh of the thermal-energy left the power station as waste heat.
 

4 Digestion plant  „Biogenes Zentrum Peine“

4.1 General description

The combined plant for composting and digestion of the „Biogenes Zentrum Peine“ (Germany) was built in 1997/1998 for a biowaste capacity of about 24 000 Mg/a (fig. 14). The main intention of a combination plant was to have a optimal process engineering correlation between composting (ligno/cellulose containing, high structured wastes and biowastes from the biobin) and digestion (structured, wet biowastes and organic wastes from industry (food-industry)). The produced energy from digestion has to be used for the the total energy demand of the whole plant. Target situation is a CO2 neutral operation, a injection of energy into the local electricity net and the substitution of fossil energy sources under the view of climat relevancy. The digestion plant was built by D.U.T. and is run by Rethmann. The capacity of the plant is about 10 000 Mg/a with biowaste input pieces smaller 40 mm. The operational sequence is shown in fig. 15. Fig. 16 shows the mass balance of the digestion plant. In table 3 relevant process parameters can be seen which are the results of the proof operation period and the regular operation period.

4.2 Mass balance and sand content

The mass balance shows that about 60 % of the input material leaves the plant as digestate. Remarkable is a very high quota of separated impurities. Especially during the vegetation period because of sticked soil in the fraction < 40 mm gravel, sand and smaller mineral fractions are carried with the biowaste into the plant. This is responsible for a high falt rate of pumps and pipelines. It is necessary to reduce the sedimentation in parts of the plant which are not walkable (reactor) by separating the sandy fraction in a pretreatment.

The sand content (0,06 mm - 2 mm) of the input material out of the bunker was at about 6 % to 24 % (by weight). The average value was about 16 % (by weight). The separation rate by a heavy solids sluice was about 80 % at the suspensor. Fig. 17 shows a typical grading curve of the separated material out of the sand separator.
 
 

About 33 % of the potential energy of the biogas can be changed via transducer and used as electric power. One third is needed for the plant itself. The surplus is lead into the local power supply system. The thermal power needed for the heating of the biowastes can be totally covered by the block-type thermal power station; the surplus of thermal power could be given to third, but it`s not used yet. 

Waste water produced in the composting (codensate) and digestion (dewatering) (about 17 % of the biowaste) is mostly utilized for agricultural needs.
 

5 Considerations of climate pertinence

Even waste management activities connected with the carbon dioxide discussion have to be viewed under the increasing climate relevancy aspect. In the case of biowaste treatment there is the choice between the technologies of composting and digestion. Basically both technologies have their right to exist depending on waste quantity, waste quality, waste structure, local and economical circumstances.

Under the aspect of carbon dioxide emissions it is to determine that for the composting of 1 Mg biowaste, 30 kg of carbon dioxide will be produced [7]. The cause is the production of electric power (0,61 kg CO2/kWh) for preparation (15 kWh/Mg) and decomposition and air management (35 kWh/Mg). 240 kg to 300 kg climate neutral carbon dioxide out of the aerobic biodegradation of renewable biological resources are to add into the balance.

Digestion technology processes produce about 80 kg/Mg carbon dioxide and 150 kg/Mg out of the thermal oxidation of methane. This thermal oxidation of methane produces electric and thermal power, which can be used for the process and given to third. As an alternative biogas can be lead into the local power supply system. This calculation (electric power production without thermal power use of third) causes a carbon dioxide credit of 100 kg/Mg.

The combination of aerobic treatment of ligno-cellulose containing wastes and anaerobic treatment of  insignificant structured biowastes is the presently best solution under the aspect of carbon dioxide emissions because of a potential substitution of fossil energy resources combined with energy self-sufficient operating.

80 kg/Mg carbon dioxide emissions could be avoided compared with the composting of biowastes. Calculated with 10 Mio. Mg biowastes in Germany each year it is only a very small part of the total carbon dioxide emissions but these examples show that there is the possibility to optimize the treatment of wastes in detail and to reduce climate pertinence emissions.
 

Literature

[1] Kern, M. et al. : Stand der biologischen Abfallbehandlung in Deutschland, Teil II Vergärung, Müll und Abfall 2/99, S. 78 bis 81 (1999)

[2] Pelz, A. : Optimierung des Suspensers einer Vergärungsanlage für Bioabfälle Diplomarbeit an der Fachhochschule Braunschweig/Wolfenbüttel (1998)

[3] Grüner, S. : personal information (Biogenes Zentrum Peine) (1999)

[4] Anonym : brochure of „Biogenes Zentrum Peine“  (1998)

[5] Anonym: Company documents Steinmüller-Rompf (1998, 1999)

[6] Polster, D. : personal information  (Steinmüller-Rompf) (1999) 

[7] Anonym : Schlußbericht der Enquete-Komission  ”Schutz der Erdatmosphäre”. Bundestagsdrucksache 12/8600 vom 31.10.1994 (1994)

[8] Stadt Braunschweig – Stadtreinigungsamt: Bioabfallvergärungsanlage Braunschweig Watenbüttel, Stadtreinigungsamt/Fa. Bühler (1997)

[9] Anonym: KOMPOGAS – Verfahren zur Vergärung biogener Abfälle

[10] personal information: Herr Harmsen (1999)

[11] personal information: Herr Hüttner, Fa. KOGAS (1999)

[12] Operation datas: KOGAS AG und KOGAS GmbH (1999)