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close this bookBiogas Plants in Animal Husbandry (GTZ, 1989, 153 p.)
View the document(introduction...)
View the documentForeword
View the document1. An introduction to biogas technology
Open this folder and view contents2. A planning guide
Open this folder and view contents3. The agricultural setting
Open this folder and view contents4. Balancing the energy demand with the biogas production
Open this folder and view contents5. Biogas technique
View the document6. Large-scale biogas plants
Open this folder and view contents7. Plant operation, maintenance and repair
Open this folder and view contents8. Economic analysis and socioeconomic evaluation
Open this folder and view contents9. Social acceptance and dissemination
Open this folder and view contents10. Appendix

6. Large-scale biogas plants

Biogas technology, or better: anaerobic-process engineering, Is becoming increasingly important as a means of treating and cleaning industrial organic waste materials and highly loaded organic wastewater.

This applies in particular to the following ranges of production:

- large-scale stock farming

- industrial processing of agricultural produce (refining of sugar, production of starch, winning of fibers, processing of coffee, generation of alcohol, slaughterhouses, etc.)

- industrial and urban refuse and sewage (manufacturing of paper, organic household waste, sewage sludge, biotechnological industries).

Most biogas plants used in those areas are large-scale plant systems with volumes ranging from several hundred to several thousand cubic meters.

Compared to aerobic treatment, anaerobic processes offer comparable performance with regard to purification capacity and conversion rates, but also stand apart from the former in that they:

- require less energy to keep the process going and to generate useful energy in the form of biogas, and

- produce less organic sludge, because the growth rate of anaerobic microorganisms is slower than that of aerobic microorganisms.

Consequently, anaerobic treatment of waste materials and wastewater offer some major advantages for a comparable initial invest" meet. Nonetheless, much of the technology has not yet passed the testing stage.

Due to the size of plant, different objectives and special requirements concerning operation and substrates, the anaerobic treatment of waste materials and wastewater involves a different set of planning mechanisms, plant types and implementational factors. To go into detail on this subject would surpass the intended scope of this manual; besides, extension officers hardly need expect to be confronted with the job of planning such plants. Nevertheless, some basic information is offered here to give the reader a general grasp of what large-scale biogas technology involves.

In discussing the various waste-treatment options, differentiation is made between wastewater (organic - highly loaded) and waste materials/residues (organic solids).


Fig. 6.1: Basic principle of organic wastewater treatment (Source: OEKOTOP)

Table 6.1: Some examples of biogas production from agro-industrial residues and wastewater
(Source: OEKOTOP, compiled from various sources)

Area of production

Retention time

Digester loading

Gas production


Degradation rate


[d]

[kg/m³ X d]

[m³/kg]

[m³/m³ X d]

[%]

Slaughterhouse

0.5- 8.5

1.2-3.5 COD

0.3-0.5 COD

0.1 - 2.4

80 COD

Fruit and vegetables

32.0

0.8-1.6 VS

0.3-0.6 VS

-

-

Olive-oil extraction

20.0-25.0

1.2-1.5 TS

0.7 BOD

-

80-85 BOD

Whey

2.0-5.0

6.4 BOD

0.9 BOD

5.5

92 BOD

Potato starch

-

7.5 COD

0.3-0.4 COD

-

90-95 BOD

Yeast factory

0.5-0.7

1.0-8.0 COD

-

0.5-4

60-70 COD

Sugar mill

0.2-1.0

12.0-16.5 COD

-

-

87-97 COD

Milk processing

3.4-7.4

0.7-2.0 VS

0.1-0.4 VS

-

86-99 BOD

Molasses slop

10.0

3.9 VS

0.9 VS

3.5

97 BOD

Molasses distillery

1.2-3.5

18.3 COD

0.6 COD

6.6

45-65 COD

Brewery

2.3-10.0

1.8-5.5 TS

0.3 - 0.4 TS

-

-

Tannery

0.5

2.7-31.9 COD

-

-

80-91 COD

Pharmaceut.ind.

0.5-2.0

0.2-3.5 COD

0.6 COD

0.1-2.5

94-98 COD

Refuse + sewage sludge

11.0-22.5

1.2-3.1 VS

1.0 VS

-

-

Refuse

25.0-30.0

0.7-3.2 VS

0.1-0.4 VS

-

-

Cattle farming

15.0-35.0

0.5-2.5 VS

0.2-0.4 VS

0.6-1.4

-

Pig farming

10.0-25.0

0.8-4.1 VS

0.1-0.5 VS

0.8-2.1

-

Poultry farming

15.0-35.0

0.6-3.6 VS

0.2-0.5 VS

0.7-1.8

-

Sewage sludge

20.0-30.0

1.2-4.5 VS

0.1-0.6VS

0.8-1.5

-

Wastewater treatment

Organically contaminated wastewater contains mostly dissolved substances that are measured in terms of COD (chemical oxygen demand) and BOD (biochemical oxygen demand, i.e. oxygen required for mineralizing the organic contents).

The main purpose of wastewater treatment is to remove or mineralize the organic substances, i.e. to prepare them for release into a receiving body of water or the agricultural environment.

Anaerobic fermentation serves as the biological purifying process. Purification performance rates of up to 95% BOD are achievable. The choice of process and the achievable purification performance rates are determined by the type and composition of the substrate/wastewater. In general, dissolved organic substances are readily biodegradable. Retention times ranging from a few hours to a few days are not uncommon. On the other hand, some organic substances are hard to break down (paints, aromates, etc.), while others are toxic and/o,r capable of causing a shortage of nutrients and adverse medium characteristics (e.g. pH-shifts). A number of special-purpose processes have been developed for use in anaerobic wastewater treatment in order to compensate for the high hydraulic loads and lack of bacterial colonization areas:

Contact fermenter

Digested slurry is recycled through a continuously stirred reactor in order to maintain a high level of bacterial concentration and, hence high performance. The contact process is a suitable approach for both mobile substrates and substrates with a high concentration of solids.

Upflow fermenter
An upflow-type fermenter with a special hydraulic configuration serves simultaneously as a suspended-solids filter with a high bacterial density and correspondingly high biodegradation performance.

Fluidized-bed fermenter
A vehicle (balls of plastic or clay) is kept "floating" in the fermenter to serve as a colonizing area for the bacteria.

Fixed-bed fermenter
A vehicle (plastic pellets or lumps of clay, rock or glass) provides a large, stationary colonization area within the fermenter. Fixed-bed fermenters are suitable for wastewater containing only dissolved solids. If the wastewater also contains suspended solids, the fermenter is liable to plug up.

Two-phase fermentation
The acidic and methanogenic phases of fermentation are conducted separately, each under its own optimum conditions, in order to maximize the fermentation rates and achieve good gas quality.

The treatment of wastewater marked by heavy organic pollution must always be looked upon as an individual problem that may require different processes from one case to the next, even though the initial products are identical. Consequently, trials must always be conducted for the entire chain: production process - purification - wastewater utilization - and energy supply/ use.

Thanks to their uncomplicated, robust equipment, the contact process and fixed-bed fermentation stand the best chance of success in developing countries.

Waste materials/residues
The fact that practically identical production processes often yield residues that hardly resemble one another also applies to industrial waste materials. Here, too, pretrials and individual, problem-specific testing are called for in any case.

The potential range of organic waste materials is practically unlimited. Of particular interest for the purposes of this manual, however, are waste materials from factory farms and slaughterhouses.

Large-scale stock farming
The characteristics of dung from cattle, pigs and chickens were described in chapter 3.2. In factory farming, the dung yield is heavily dependent on the given type of fodder and how the stables are cleaned. Thus, pinpoint inquiries are always necessary.

The large quantities of substrate, often exceeding 50 m³/d, lead to qualitative differences in the planning and implementation of large-scale plants, as opposed to small-scale plants. This has consequences with regard to substrate handling and size of plant:

- Daily substrate-input volumes of more than 1 m³ cannot be managed by hand. Pumps for filing the plant and machines for chopping up the substrate are expensive to buy and run, in addition to being susceptible to wear & tear. In many cases, careful planning can make it possible to use gravity-flow channels for filling the plant.

- Plants of a size exceeding 100 m³ usually cannot be made of masonry, i.e. the types of plant discussed in chapter 5 cannot be used.

The choice of plant is limited to either the mechanized types used in industrial countries or simple, large-scale plants. Experience shows that most simple, large-scale plants are
- of modular design,
- usually equipped with channel digesters,
- and require the use of substrate from which the scum-forming material has been removed in order to get by with either low-power mechanical mixers or none at all.

Since large-scale biogas plants produce accordingly large volumes of biogas, the generation of electricity with the aid of a motor-generator set is of main interest.

The two Ferkessedougou biogas plants situated in the northern part of Cote d' Ivoire stand as examples of a successful large-scale biogas-plant concept based on a simple design. They have been in operation at the local cattle-fattening station and slaughterhouse since 1982 and 1986, respectively, where they serve in the disposal of some of the excrements produced by an average number of 2500 head of cattle. The plant consists of a simple, unlined earth-pit digester with a plastic-sheet cover serving as gasholder. The gas is used for generating electricity, heating water and producing steam.


Fig. 6.2: Biogas plant in Ferkessedougou - system OEKOTOP. 1 Cattle feedlot, 2 Manure gutter, 3 Feedpipe, 4 Sluice, 5 Rubber-sheet gasholder, 6 Earth-pit digester, 7 Discharge pipe, 8 Impounding weir, 9 Slurry storage (Source: OEKOTOP)

At present, some 20% of the slaughterhouse's electricity requirement is covered by the biogas plants, and the biogas-driven steam sterilizer saves 50 000 I diesel fuel each year. The total initial investment amounting to 60 million F.CFA yields annual savings of approximately 12 million F.CFA after deduction of the operating costs (1 DM = 150 F.CFA).
The Ferkessedougou biogas plants demonstrate how even large-scale installations can keep biogas technology cost-efficient by relying on simple designs, e.g. large digester volume despite low cost of construction.

Table 6.2: Technical data of the Ferkessedougou biogas plant (Source: OEKOTOP)


Biogas plant I

Biogas plant II

No. of animals

700 head of cattle in 12 feedlots


Digester volume

400 m³

810 m³

Gasholder volume

80 m³

>600 m³

Slurry storage volume

300 m³

3500 m³

Retention time

40-2s days

40 -22 days

Daily substrate input1

10-18 m3/d

20-38 m³/d

TS-content


4-8%

Daily gas production

250 m³/d

450 m³/d

Specific gas production

0.6 m³/m³ Vd

0.55 m³/m³ Vd

Gas utilization

MWM gas-powered

Deutz gas-powered


motor-generator set

motor generator set


15 kWel

32 kWel, with exhaust heat recovery for heating water

Operating time

22 h/d

10 h/d

Power generation

270 kWh/d

245 kWh/d



Combination gas-oil burner for steam sterilizer, 130-355 kW

1 Fluctuation due to seasonal factors (rainy/dry season)

Slaughterhouses

The proper disposal of paunch and intestinal contents (fecal matter), dung and urine and, in some cases, blood and offal is not always ensured in slaughterhouses. Such residues can be put to good use in a biogas plant, since:

- the energy demand and the substrate incidence are extensively parallel and usually involve short distances for transportation;

- the biogas technique is more cost-efffcient and yields more energy than aerobic processes, so that most slaughterhouses could cover their own energy demand with such a plant.

Slaughterhouses in developing countries span a wide size range. Consequently, various techniques are needed for treating and/or disposing of waste materials and wastewater. While little experience has been gained to date in connection with the disposal of slaughterhouse wastes via biogas technology, the following assessment can nonetheless be arrived at:

- Small, village-scale slaughterhouses

in which 50 - 100 animals are slaughtered each week can make use of simple agricultural biogas plants like those discussed in chapter 5 for disposing of all offal and other residues, and the digested slurry can be used as agricultural fertilizer.

The main problem in such plants is the formation of a thick layer of scum made up of the contents of paunches and fecal matter. For that reason, and in order to achieve good hygiene, retention times of 100 days or more are considered practical.

- Medium-sized slaughterhouses (200-500 slaughterings per week)

Here, too, biogas plants are able to provide complete disposal, although large-scale types like those used in Ferkessedougou are required. Sometimes, it is a good idea to separate the solid wastes from the wastewater and possibly compost the solids.

- Large-scale slaughterhouses

Most such slaughterhouses are quite similar to those found in European cities and are usually located in urban areas. Consequently, proper waste disposal and wastewater purification call for integrated concepts in line with European standards.

Table 6.3: Slaughterhouse waste quantities (Source: OEKOTOP)

Type of waste

Cattle

Sheep

Pigs

Stomach contents

11.6%¹

4.3%¹

2.8%¹

Intestinal contents

3.3%¹



Blood

~14 kg

~2 kg

~4 kg

Offal

2-5 kg

0.5-1 kg

1-1.5 kg

Dung (without fodder)

5 kg

0.8 kg

1.5 kg

¹ Expressed as percentages of live weight