ok-biogas training material - vinavac

8/3/2019 Ok-Biogas Training Material - VINAVAC

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Enabling Access to Sustainable Energy Program

http://ease-vn.org.vn

Training document for

building biogas - vacvina digester


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The center for rural communities

research and development

Chapter I

Biogas technology Instructions for the construction

of improved VACVINA biogas plants

I.INTRODUCTION:

Organic substances exist in a wide variety of forms from living beings to dead organisms and even animal

droppings. Organic matter is composed mostly of carbon (C), combined with such other elements ashydrogen (H), oxygen (O), nitrogen (N), sulphur (S), etc. to form organic compounds such as

carbohydrates, proteins and lipids. In nature, dead and inert organic matter is quickly disposed of by

micro organisms, mainly bacteria1, through a digestion process that breaks the complex carbon chains

into smaller substances.

The digestion process occurring in presence of oxygen is called aerobic digestion and produces a mixture

of gases having for component carbon dioxide (CO2). The chemical equation can be expressed as:

1

n(C6H10 O5) + nH2O + n6O2Aerobic bacteria

n (6CO2+6H2O) + Q Kcalories.

The digestion process occurring without oxygen is called anaerobic digestion and generates a mixture of

gases having for main component methane (CH ) The chemical equation can be expressed as:4

Anaerobic bacterian (C6 H10O5)+ nH2O n (3CH + 3CO )4 2

Biogas is the name given to the mixture of gases generated by the bio-degradation of organic substances

under anaerobic conditions. Composed mostly of methane, biogas produces 5200-5800 Kj/m3 burned at

normal temperature, and thus represents a viable environmentally friendly energy source to replace fossil

fuel.

II.ADVANTAGES OF BIOGAS TECHNOLOGY

Sewage and agricultural waste contain organic substances with high molecular compounds. Given the

proper temperature and humidity conditions, these substances are broken into lower molecular compound

material, inorganic matter and gases. In high concentration, this process creates pollution and adverse

hygienic conditions for humans and animals. On the other hand, by properly treating the waste, usefulrenewable energy can be obtained as well as organic fertilizer, thus effectively changing waste into

money. This technique becomes an important addition to the Vietnamese integrated agricultural model

VAC2. Biogas technology can bring many benefits to farmers, among which:

1. The production of clean and inexpensive renewable cooking fuel.2. Improved hygiene and health conditions in the household compounds (elimination of raw and

untreated animal manure and night soil, elimination of the bad smell caused by pigs, reduction in the

1 These include fat-decomposing bacteria, cellulose-decomposing bacteria, protein-decomposing bacteria, acid-producing

bacteria and methane-producing bacteria.2VAC is an acronym formed by three Vietnamese words (Vuon, Ao, Chuong) meaning Garden, Pond and Stable and

refers to a model of small-scale bio intensive farming where gardening, fish rearing and animal husbandry are closely

integrated. VAC makes optimal use of land, water and solar energy to achieve high economic efficiency with low capitalinvestment.


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number of flies and parasites around the house, cleaner animal pens, elimination of toxic wood and

coal smoke in the kitchen area). The introduction of biogas plants is also a mean to raise overall

awareness on the importance of environmental issues and sustainable agricultural practices.

3. Reduction in CO2 emission and reduced deforestation pressure in the midlands and mountainous areasby substituting wood with biogas as cooking fuel.

4. Long-term improvement in the financial situation of households by reducing or eliminating cookingfuel expenses.

5. Reduction in soil degradation, improvement of soil fertility and agricultural production by the use ofdigested effluent as organic fertilizer in replacement of chemical fertilizers.

6. Facilitates animal husbandry activities especially in crowded and peri-urban areas.7. Reduces the workload of women for fuel gathering and cleaning of pots and pans.III.STATUS OF BIOGAS TECHNOLOGY IN VIETNAM

Family-sized affordable biogas technology was introduced in Vietnam in the early nineties. Since then,

various models have been implemented, evaluated and modified, including some indigenous biodigester

designs.Flow-through bio-digesters in usage in Vietnam can be broadly divided into two main types: fixed dome

and plastic bag. Recently, with the introduction of VACVINAs HTASC model, a third category has

emerged.

1 Fixed dome biogas plant

Developed mostly in China and India, the fixed dome bio-digester is composed of several interconnected

elements:

Figure 1b: Construction

of a fixed dome digesterFigure 1: Scheme of

Chinese fixed dome digester

- An underground digestion chamber (in yellow) topped by a dome structure where the gasaccumulates. The top can be made of brick, Ferro-concrete or composite.

-

The inlet system (red arrow) where input material is mixed and fed into the digester.- The outlet system (blue arrow) to retain the digested slurry.As organic material is fed into the digester, gas is produced and accumulates in the dome. As gas

formation increases, the liquid in the digestion chamber is forced down and overflows in the retention

tank compressing the gas in the upper part of the chamber. Normally a fixed dome plant will produce

constant pressures equivalent up to 80 cm of water column. As the gas is used, the liquid in outlet tank

will flow back to the digester. This simple hydraulic system ensures a regular flow of gas at good pressure

levels.

However, the fixed-dome bio-digester model has a number of disadvantages when applied in Vietnams

rural areas. The model is rather complicated to build given the current construction skills and work

practice of rural masons. Overall construction, especially of the dome, must be of high quality, otherwise,cracks can develop leading to gas leakage and reduction of pressure. Quality control becomes of utmost

importance but is unfortunately still difficult to implement. In addition, investment cost is still rather high

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in comparison with income of households. The cost of the system, already rather high, is increased by the

fact that the foundation of the digestion chamber must be meticulously compacted before casting. Over

time, in the digestion chamber, a layer of scum can accumulate which will prevent gas from escaping to

the dome. Yearly cleaning and maintenance is required.

These conditions might explain the fact that many farmers who decided to build such biogas plants by

themselves have run into problems with their system breaking down after a short time of operation.

4.2 Plastic bag bio-digesters

This model was first introduced from Columbia to Vietnam in 1994 by doctor Reg Preston, rector of the

University of Tropical Agriculture.

Figure 2: Plastic bag biodigester

Digestion bag

InletOutlet

Gas reservoir

The system consists of a long digestion bag or fermentation bag (Fig.2) and a gas reservoir bag. Both are

made of two or three layers of polyethylene tubing, 21c thick and 1m in diameter. The length of themain bag determines the total digestion chamber volume (a 10 m bag results in a 7 cubic meter digestion

chamber). The digestion bag is half buried in a trench and is connected at one end to a small mixing and

collection tank and at the other end to a slurry holding pit. The gas reservoir has a capacity of 1,8 m3 and

is usually suspendedeither in the kitchen or the stable (Fig. 3).

Figure 3: Plastic bag biodigester

As organic material is fed into the digestion bag, it slowly travels down the length of the tube while the

fermentation process occurs and is then expelled at the other end as digested slurry. The biogas produced

accumulates in the main bag and, through gravity as well as pressure, finds its way to the gas reservoirs.Thus biogas is stored partly in the digestion bag and partly in the gas reservoir. Users can easily judge the

quantity of gas available by the level of inflation of the bags.

Plastic bag digesters are inexpensive, easy to install and operate and scum formation can be controlled

easily by gently slapping the scum to make it sink to the bottom. On the other hand, the polyethylene bags

are somewhat fragile they can be perforated or torn and their life expectancy is but 3-5 years. They must

be protected from wandering animals and also from sunlight as UV rays will degrade the plastic. In

addition, the model requires a large area of 10-15m2 near the animal stable. This is a problem for many

households in the Red River Delta where household compound is not large enough to allow its

installation.

4.3 The VACVINA biogas model

OBJECTIVE: The VACVINA biogas digester was designed with the following objectives:

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Simple design and construction with high tolerance to construction flaws and defects.Making the best use possible of the restricted compound space.Long life expectancy.As low cost as possible without sacrificing life expectancyConstant gas production over time by controlling the formation of scum.Low maintenance and adapted to the habits and perceptions of the intended users.

Taking the best elements of the fixed dome digester, with those of the plastic bag digester and using the

common and widespread construction technology of septic tanks, the first VACVINA biogas units werebuilt in May 1998.

Septic tank

Plastic bag biodigester

CH4

Fixed domeBiodigester

Inlet

Figure 4:

Improved VACVINA Biodigester

Toilet

Digestion

tankOutlet

Gas reservoir

Although no one model is perfect, the improved VACVINA biodigester has proved to be a good

compromise between simplicity, low cost and effectiveness. Designed with the Vietnamese mentality and

context in mind, it has been well received by farmers and has maintained a very good success rate.

THE DESIGN & WORKING PRINCIPLES: The design and working principles of the VACVINA

biodigester is illustrated in Fig. 4 and Fig.5.

Note:

1. Inlet,2. Digestion tank,3. Outlet,

4. Gas reservoirFigure 5: General Scheme

of VACVINA Biodigester

It consists of a flat-top rectangular underground digestion chamber with external plastic gas reservoirs.

An ingenious system of inlet siphon allows the new material to be dropped unto the surface of the

fermenting liquid thus breaking any accumulated scum.

1.The inlet

The inlet is a simple inexpensive off-the-shelf siphon pipe (also called rabbit joint) made of glazed-terra

cotta (Fig.6).

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Figure 6:

The inlet siphon (by glazed terra-cotta ) allows animal dung to be fedinto the di

Animal dung

estion chamber


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The system normally comprises two or three siphon inlets and can also be fed by the lavatory pan. These

inlets play an important role in preventing the formation of a hardened scum layer by allowing the new

material to drop on top of the liquid surface, thus wetting and breaking the top layer. As new material isfed daily the surface is continually stirred and broken. In addition, the siphons also act as pressure safety

valves. With 15 cm of water column, they effectively ensure gas tightness of the main chamber but,

should the internal pressure increase over this limit, excess gas will bubble out. (Fig. 5, on the right).

2. The digester

The main digestion chamber is an underground rectangular tank made of bricks and mortar (Fig.7).

However, the shape of the tank can be adjusted to the specific configuration and constraints of the family

compound while retaining a sufficient volume. One of the great advantages of this model is that the

concrete flat top of the digester can provide a clean and dry floor for the pigsty or stable.

.

5

Figure 7:The costructing

digester

3.The outlet

The outlet system consists of a straight pipe, normally made of PVC, 110mm diameter and 1m long,

inserted at a 45

o

angle in the digestion chamber. Its function is to drain the effluent (liquid form) from thedigester to the slurry pit and to set the level of liquid in the digester.

The outlet pipe must therefore be positioned lower than the inlet pipe, with its end opening about 35 cm

lower than the digester cover. To ensure gas tightness of the digester, the other end of the pipe should

extend deep under the static surface of liquid in the digestion chamber (Fig.34).

4. Slurry pit

The slurry retention pit is constructed near the outlet of the digestion chamber and is used to store the

systems effluent for later use (Fig.28). The amount of effluent flowing into the slurry pit is equivalent to

that of the input fed into the digester. However, the volume of slurry retention pit must be calculated

according to the intended usage of the slurry. It is important to remember that, in order to ensure the

normal functioning of the system, the slurry level must always be kept lower than the digesters outlet.

5. Gas reservoir

The external gas reservoirs are used to collect and store the biogas before being used as cooking fuel (Fig

8). One gas reservoir should be enough for a normal family. However, if the household has a large

number of animals, more gas will be generated and more storage bags will be required. The bags are

made of two layers of 20 mc thick polyethylene tubing, and have a storage capacity of 1.5 2 cubic

meters. In order to increase the gas pressure during cooking, a rubber band is tightened around the

reservoir and then loosened again after finished using the burner to allow the bag to re-inflate itself.


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Chapter 2

Instructions for the construction

of improved VACVINA biogas plants

Objective

Participants will gain practical knowledge and skills on the techniques required for the construction of

improved VACVINA biogas plants, namely:

The design of biogas plants adapted to different topographies and animal husbandry situation. The construction steps The installation procedures The operation and utilization

I.PREPARATION

1.1 Calculating the digestion chamber volume

The size of any biodigester plant must be based on the number and species of animals owned by the

family. The calculations take into consideration the following basic parameters: daily amount of

collectable animal dung, minimum required retention time of the slurry in the digester, and dilution ratio

of manure to water (TS content). For the improved VACVINA biogas plants, actual volumes can be

derived through the following formulas:

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SV = Vgas+ Vdig (1) (Figure 9)

VWhere gas

VckhVgas:Gas volume in the digestion V1 digchamber

hV : Slurry volume in the digestion 2dig

chamberFigure 9: Volum

of Biodigester

And, Vdig

= T x Vdm

(2)

Where

T: Retention time of slurry in the digester (e.g. 40 50 days)

Vdm: Daily amount of water and dung (liter/day)

And, Vck

= h1S

Where

S: Area of the flat bottom slab of the digester (m2)

h1: Distance between the bottom of the cover slab and

the static liquid surface in the digester (m)

When using mainly pig manure, Vdm can be calculated as follows:

Vdm

= (w + nL)T (3)

Where

w: The amount water to dilute the dung of n pigs (l)L: The daily average amount of dung per pig (l/day)

The optimal dilution ratio of the input material is 1:5 (one part of manure for five parts of water)


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Put w = 5nL into (3), and from (1), (2) and (3) we have:

V= Vck + (5nL + nL)T = Vck + 6nLT

As a result, the formula for calculating the digester volume for pig farming is

V= Vck

+ 6nLT (4)

Where

n: Regular number of pigs (pig)

L: Daily average amount of dung per animal (2 liters/head/day)T: Retention time (day)

For a retention time of 40 days (T = 40),

V = Vck

+ 240nL

1.2 Construction material

The construction material required to build a 7 cubic meter VACVINA biogas digester is detailed in the

following table:

Table 9: The construction material required to build a 7 m

3

VACVINA biogas digester

Item Unit of measure Quantity

1 A- standard solid brick piece 1.400

2 Cement kg 600

3 Coarse yellow sand m3

1,5

4 Gravel m3

0.5

5 Iron bars (8) kg 30

6 Zinc joint pipe piece 1

7 PVC pipe (21) m 4

8 Valve, joints piece 15

9 Plastic pipe (21) m 15

10 PVC pipe (110) m 1

11 Polyethylene bag piece 1

12 Siphon pipe piece 2

13 Single burner stove piece 2

14 Glue, rubber strips

15 Hoe, shovel, knife, trowel and other masonry tools...

II.TECHNICALCONSTRUCTIONOFDIGESTIONTANK

The construction steps are as the followings:

2.1 Use canvas to cover the construction site

If the digester is erected outside the animal stable, the first thing to is to install a large canvas covering the

construction site (Fig.1). This will reduce the overall construction time by sheltering the site from bad

weather and will also ensure a better quality of work.

2.2 Excavating the pit


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Selecting the construction site:

The location of the digester is determined by the current installations in the household compound. With

its flat concrete top, the most suitable place for the system is under (or near) the pigsty or animal stable.

The shape of the digester does not necessarily have to be rectangular and can be designed to

accommodate the disposition and topography of the family plot.

Excavating the pit:

After careful selection of the construction site, excavation is started. The dimensions of the pit must be

larger than those of digester so as to facilitate the execution of the work.

The dimensions of the digester can be tailored to specific configurations. However, to minimize

investment and maximize efficiency and ease of operation, it is suggested to keep the depth to 2m, the

width to 1.5 2 m and to adjust the length to obtain the total volume required.

Note: When excavation is

carried out in an area of high

water level, it is necessary to

dig a ditch. Water accumulated

in the ditch can be pumped out

during the construction of the

digester base. (Fig. 11) Figure 11: Drainage ditch

2.3 Building the base (foundation) of the digester

After completing the excavation of the pit, the following steps are required to build the base:

- Prepare a first 15 cm layer of tightly compacted broken bricks or crushed stone (4 x 6cm)- Pour the foundation of 5 cm concrete mix (1 part cement, 2 parts yellow sand and 3 parts crushed

stone) (Fig.12).Note:

8

-Where the soil is of weak bearing

capacity, metal rods (rebar) 10 should be

used for the concrete layer. Any

accumulated water must be pumped

regularly. It is possible to use inexpensive

plastic sheeting to protect the base from

water.

-Use inexpensive plastic sheets to protect

the base from high water levels (Fig.13,

left). Add metal rods (rebars) in the

concrete slab when the soil has weak

bearing capacity.

Concrete 5cm

Crushed stone

Fi ure 12: The concrete baseg

2.4 Building the walls of the digesterWalls are constructed after the base is completed. The thickness of the walls is the standard 10 (i.e. the

thickness of a brick plus plaster Fig.15). The bricks used must be solid grade A. The mortar is made of 1

part cement to 4 parts coarse sand. While laying bricks, ensure that the space (joints) between them is

well compacted with mortar.

Picture 15: Building the

Outletinlet

Figure 14: Positioning

the inlet and outletwalls of the digester


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Note: During the construction of the walls, remember to leave the technical openings for the installation

of inlet/outlet pipes as indicated in figure 14.

- The hole for the inlet pipe is 30cm diameter and starts from the top of the wall. It can be located on any

wall.

- The hole for the outlet pipe is 30cm high by 25 cm wide and is located 30cm below the upper edge of the

wall.

2.5 Plastering the walls

The plaster applied on the inner side of the digester plays a very important role in ensuring permanent gas

and water tightness.

Plastering mortar is made of clean fine yellow sand mixed thoroughly with cement (1 part cement to 3

parts sand).

Special care must be taken to ensure even thickness of the plastering layer. The surface must be smooth

and regular and all edges must be rounded.

The following steps must be followed:

a) Clean and scrub the entire surface to be plasteredb) Plaster a 1 cm thick layer. Wait for this layer to dry slightly and use the hand trowel to press the

entire surface evenly

c) Wait 1-2 hours for the layer to dry then plaster a second layer in the same manner. Polish this finallayer with pure cement solution.

5cm

Plastering

layer

Figure 16:Plastering the walls

Important: The top 5 cm of each wall must not be plastered. This wall surface will be plastered once the

digester cover is installed. This final plastering must be applied properly at the connection area between

the wall and the cover (with rounded edges) to ensure proper gas tightness (Fig.16).

2.6 Casting the ferro-concrete

Figure 17: formwork for

casting the concrete cover

digester cover

After plastering and polishing the walls, it is time to construct the digester

cover.

Because the flat-top cover is used to support the pigsty, animal stable and/or

latrine, it must be 10cm thick, flat shaped and casted in place (DO NOT use

a precast concrete slab). The procedure is the same as that used to build a

house foundation and comprises the following steps:

a) Prepare a solid formwork for casting the concrete cover. Use scaffoldings

to support the wooden framework (Fig.17, Fig.18).

b) Define the location of the manhole to provide easy access to the digestion chamber while not

interrupting animal production activities (Fig.19). Prepare a wooden mould shaped as a truncated V-

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shaped rectangular column with upper dimensions of 50 cm x 50 cm, lower dimensions of 45 cm x 45

cm, and a height of 10cm (Fig.20).

Figure 20: Wooden mould for

the manhole (technical opening)Figure 19: Positioning

the Manhole

The manhole performs the following functions:

- Provides access to the digester inner chamber to remove the

formwork after the concrete has settled and to install the accessories (inlet/outlet pipes, gas pipe...).

- Provides access to the digester to remove accumulated waste and grit (normally after 7-10 years of use).

c) Prepare a 15cm x 15cm mesh of8 steel rods, with hooks at both ends of the rods (Fig.21). Positionthe shorter steel rods under the longer ones for better solidity. If the width of the cover exceeds 2.5 m, a

construction engineer must be consulted to design the proper reinforced mesh configuration.

Attention: Additional reinforcing rods must be installed around the manhole to increase the bearing

capacity.

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Figure 21: Binding steel rods

d) Cast the concrete cover with 200-marked concrete :

The basically proportions of construction materials 1cement /2sand /3 gravel(Fig.22) could be used for

casting the concrete cover with 200-marked concrete . To ensure even thickness of the cover, several

10cm wooden pegs are used. Once the thickness of the concrete layer is evenly compacted, remove the

pegs.

e) The main gas pipe must be fitted during casting. Made up of iron 21 diameter, it is fitted right

beside the wall of the animal stable so as not to disturb the animal production activities in the future.

f) Compact and sprinkle the concrete.

g) After 2 to 3 hours, once the surface of concrete has taken shape, the wooden mould for the manhole

cover can be removed. Smooth and press thoroughly the edges of the opening.

h) The first manhole cover is then casted in place (pay particular attention to this technique during

practice). Coat the surface and lateral side of the manhole opening with cement paper, lay the steel rod

mesh and cast the concrete (thickness of 5 cm). The steel mesh uses masonry 8 rods spaced at 10cmintervals with end-hooks. To provide a convenient way to remove the cover, two carrying handles made

up of10 rods are inserted.

i) A second manhole cover 55cm x 55cm is prepared. After the first cover is placed and sealed with clay

and water, the second cap will be placed on top to provide a flat and dry final surface.

j) To save construction time, a stove platform should be casted at the same time as the concrete digester

cover. This concrete platform will support the stove burner. It is made of 6-8cm of concrete reinforced

by a steel mesh of 20cm x 20cm 8 rods with end-hooks. The breadth and length of platform are 60cm


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and 120cm respectively. For maximum convenience, it is recommend fixing the platform 75 cm above

the kitchen floor (Fig.29).

Two holes are placedwhen casting stove

form for installing the

gas pipes coming frombeneath the platform

Position of 2 stoveburners installed on

stove platform

Figure 28: Thestove platform

Note: While constructing the stove platform, two holes 25 cm in diameter must be made. The holes are

placed 10 cm from the edge of platform and the distance between the holes is 50 cm. These will be used

to install the gas pipes coming from beneath the platform (Fig. 28).

III.INSTALLATION:

INLET,OUTLET,EQUIPMENTS

After constructing the digester cover, manhole cap and stove platform, the entire concrete work must be

protected and maintained in proper conditions to ensure drying and curing. The formwork is removed after7 days, and the inlet and outlet pipes are then installed.

Important: as the concrete has not yet reached its load capacity, weight must not be applied yet to the

surface of the cover.

3.1 The inlet system

The inlet system of the digester consists of a mixing tank connected to the main reservoir by

one or more siphon pipe (also called U-band, rabbit-shaped joint, cattail-shaped joint) The

mixing tank is usually located close to the digester and at the corner of the animal stable.

The standard dimensions of the mixing tank are 0.2 m x 0.4 m with a depth of 0.3 m (Fig.6).

This system prevents the formation of a scum layer by regularly wetting and mixing the surface with

fresh input. The number of pipes is based upon the size of digester, commonly 2-3 pipes per digester.

The latrine can also be linked to the input system (Fig. 31 and Fig.32).

Animal dung

Figure 31: The correct position

of siphon pipe in inlet of

biodigester

Figure 32: Inlet

including siphon pipes

5 cm

As the siphon also acts as a water valve to prevent gas leakage, proper installation is very important.

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Once positioned in its proper place, the siphon pipe is slightly sloped at a 30o angle. This condition could

be reached when the highest point of the pipe should be 5cm higher than the top of the opening (Fig.31).

In this position, when water is poured to fill the siphon, it will start running at the other end as the level

reaches the lip of the opening. Fix the pipe firmly in this position and seal the joint with mortar.

The mixing tank is then constructed around the inlet. Its function is to collect and mix the fresh material

before feeding it to the digester (Fig.32). It can be covered by a net to prevent larger debris and stones

from entering the digester.

Important: Do not place the siphon pipe before or duringthe construction of the digester cover.

3.2 The outlet system

The outlet system consists of an outlet pipe and a slurry holding pit.

The holding pit is commonly built alongside the outlet and its volume depends on the usage by the family

of the digested slurry. However, irrespective of its dimensions, the top of the slurry pit will determine the

level of slurry in the main chamber as the outlet pipe will rest upon its upper ledge. Hence, the level of

slurry in the slurry pit should always be kept equal or below the outlet mouth.

The outlet pipe allows the digested slurry to flow from the digester to the slurry pit. Its length and

position are important as they determine the level of slurry in the digester. The PVC pipe is 110

150 mm in diameter with a length equal to one third of the slurry height (80 100 cm).

It must be positioned at a 45o angle, making sure it extends under the surface of the slurry the equivalent

of one third of the height of the slurry (Fig.34). For example, if the slurry in the main chamber is 1.50 m

deep, the end of the pipe must extend 50cm below the surface.

Figure 34:

Design ofoutlet

systemand Static

slurry level

3.3 The gas pipeline

The gas pipeline connects the gas outlet pipe with the gas reservoirs, the safety valve and the burner.

Attention: Before the pipeline is fitted on the gas outlet pipe, the latter should be cleaned by poking a

fixing the platform 75 cm above the kitchen floor6 steel rod.

3.4 The safety valve

Purpose: The safety valve is important to ensure that the gas pressure in the system does not exceed 15cm

of water column. It consists of a 1 1.5 litre , 1 T-joint and 21 Tienphong plastic tubing (Fig.35).

Installation: To install the safety valve, make a hole 1.5-2 cm in diameter at the neck of the bottle. Insert

a short length of pipe into the bottle and connect the other end to the T-joint. Insert the T-joint as part of

the main pipeline (Fig. 35).

Operation: Pour water into the bottle to cover the end of the pipe by 10 cm. Mark the water level on the

bottle as a guide to add water when necessary. If the pressure in the system reaches more than 10 cm of

water column, gas will automatically bubble out of the pipe and of the water bottle.Location: position the safety valve in a well ventilated area and where it is easy to see the water level

and add water when required.

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Important: Check the water level regularly. If le level falls below the 15 cm mark, water must be added

immediately to prevent gas leakage and reduction in gas pressure.

Transparent

astic bottlePl

T-joint fromPVC material

Permanent water levelFigure 35:The Safety valve

3.5 The gas reservoirs

Gas is stored in the gas reservoir until used for cooking. The plastic reservoirs are suspended under the

stable roof or in an area where they are not subject to be punctured or affected by sunlight. In order to

store enough gas for the 4-5 hours of daily cooking required by a typical family (of 5-6 persons and

raising 6-7 pigs), 1 bag is needed. More gas reservoirs are needed in case if quantity of pigs is higher.

The reservoir is made of two layers of 20-24c polyethylene tubing, 100cm in diameter and 2.5 m inlength capable of storing 1.8m3 of gas. One end of bag is tightly tied while the other is connected to the

pipeline with 21 plastic tubing.

Figure 36: Gas reservoir,Conected to gas pipe

using T-joint

T-joint

Figure 37: Gas reservoirSuspended under the roof

of the pigsty or kitchen

Installation:

- The double layer is achieved by inserting two polyethylenes tubing one in the other making sure not to

trap significant amount of air between the layers.

- Binding technique: For the bag to inflate evenly when storing gas, the two ends of the bag are folded

from the outer edge to the center of the bag (this technique will be demonstrated in detail during thepractice sessions). Elastic rubber strips are used to tie the ends.

- Insert a 30-cm-long T-shaped pipe made of PVC 21 (Fig.38) in the two layer bag to convey gas. Thebuilt-in pressure will ensure that gas flows automatically to the burner.

- The gas reservoir should be hung tidily, out of harms way and in a convenient place for use. They are

normally hung below the stable roof, not too far from the kitchen area where the gas is used.

Operation:

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An elastic rope can be tied around the bags to create a stable output of gas during cooking (Fig.38).

Loosen the rope after usage to allow the reservoirs to inflate back to their original size.

Figure 38: An elastic ropecan be tied around the bagsto create a stable output of

gas during cooking

Figure 39: The small gas-fan installedto help suck out gas from the digester

and push it towards the burner

3.6 The burner

VACVINA manufactures affordable and easy to use biogas burners (Fig.29). The burners are made of

cast iron with copper regulation valve. Designed like the standard LPG burners, they are nonethelessmuch less costly.

The burners are installed on the stove platform (see section II/6-j and Fig.41) once the installation of the

gas pipeline and all the fittings has been completed.

Figure 41: Installation

of the burners

The biogas burners are connected to the pipeline through a valve (Taiwan valve) fitted under the platform(Fig.30 and Fig.39 and Fig.41). Before fitting the valve, apply lubricant on the surface of the ball inside

the valve to ensure smooth and easy operation.

Important: For safety reasons, especially to prevent children from opening the burners, a main valve

must be installed at the suitable elevation and near the platform. The main valve as well as the burner

valve are closed when not using gas. Before using the burner, the main valve is opened first before the

gas valve..

3.7 Site filling

After completing the construction and installation of the system, all remaining excavated areas must be

carefully filled back. The best material to use is sand, especially around the digester outer walls. The sand

layers should be sprinkled with water and tightly compacted.

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Chapter 3

Operation and maintenance

of the VACVINA biogas plant

I.FEEDING THE BIODIGESTER

Before putting the biogas system into operation, the digester should be cleaned thoroughly to remove any

spilled mortar or construction debris. In addition all components of the system should be tested to ensure

gas-tightness.

Feeding material

Although all organic matter, from animal or vegetal origin can be biologically digested, the VACVINA

biodigester was specially designed to recycle animal and human waste products, mainly fresh manure and

night soil.

1.1 Initial feeding of the digester

Once the construction of biodigester and the installation of appliances and accessories are completed, the

biodigester can and should be operated immediately. To kick-start the anaerobic process and ensure a

proper colonization of methanogenic bacteria, the most appropriate inputs are cattle, poultry or pig

manure. In order to ensure a rapid production of gas for cooking, it is advisable to prepare 700-800 kg of

fresh dung (to be collected and stored in a 7-day period) as initial input. The dung must be properly

hydrated and flushed into the digester with sufficient amount of water to prevent clogging and scum

formation in the digestion chamber. The chamber must be filled with at least enough material to cover the

bottom of the outlet pipe.

It is important to ensure that the following materials are not fed in the digester:

-

Soil, sand and stones.- Branches, twigs and straw.- Soap and detergent solution, cleaning chemical, antiseptic products, rain water.

And all components must be checked to ensure gas-tightness:

- Sufficient amount of water in safety valve.- All siphons filled with water.- Lavatory pan (if any) siphon filled with water.- All gas valves (main gas valve, stoves tap) closed.

The process of bio-degradation and biogas generation will take 5-10 days (slightly more in cold

temperatures). Once the gas reservoirs start to inflate, the biogas can be used for cooking. Initially, the gaswill contain some amount of air that will be flushed out as the system is used.

1.2 Daily feeding of material

The initial 700-800 kg of input material will kick-start the digestion process but is not sufficient to

produce a large quantity of biogas for daily gas using. Fresh material must be fed daily in the digester to

satisfy the cooking fuel needs of the family. To produce enough gas for a family of 7 persons, an average

of 15-20 kg of pig or cattle dung must be fed daily. In addition, night soil will be added if a latrine is

connected to the biogas system.

Beside dung, water must be added daily in the ratio of 5 parts of water for each 1 part of dung. The

proportion of water is an important factor for optimum biogas production. Adding too much water willflush partly-digested slurry out of the digestion chamber. Less gas will be produced and the slurry will not

be sufficiently treated to remove all odour and parasites.


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When large quantities of water are used to clean the pigsty or bathe the pigs during the hot summer days,

this excess water must be drained and prevented to enter the biodigester (Fig.42). A drainage ditch can be

built to drain the excess water while feeding only the animal waste to the digester. The ditch and the inlet

opening must be carefully designed to prevent rainwater from entering the digester.

Taking full advantage of the VACVINA flat-top design with multiple siphon inlets, a latrine is often

constructed at the same time as the biodigester. Water used for cleaning the latrine must be taken into

account when calculating overall water-dung ratio. The siphon in the latrine must also be checked

regularly and kept full at all times to prevent gas leakage.

II.MAINTENNCE AND TROUBLESHOOTING

2.1 Using the biogas for cooking

1. When the gas reservoir is inflated, gas can be used for cooking. At he beginning, the biogas mightcontain too much air to be readily flammable. Discharge this first bagful of gas and wait one day

for the production of new biogas.

2. When gas is not used, all gas valves should be closed.3. When using the stove, remember to first light a flame then slowly open the gas valve and adjust

the flame height and intensity according to the cooking need.

4. To ensure stable and clean combustion, the copper regulator must be cleaned at least once a week.5. If the gas pressure is too low, use a rubber band to constrict the gas reservoir (Fig.38). The band

must be loosened when finish cooking to allow the bag to re-inflate. A small gas-fan can also be

used to extract more gas from the reservoir (Fig. 39, Fig.41).

6. A main gas valve should be installed out of reach of children this main valve should be keptlocked when fuel is not needed.

2.2 Common Problems and Solutions

Problem Cause Potential Solution

Bio-degradation

process has not

startedWait a few days for the process to start

Feed the digester with the digested slurry of another digesterInsufficient bacteria

contentFeed the digester with pre-treated material

Gas leakage

Check the gas pipeline

Check the water level in the safety valve

Check the water level on digestion chamber cover, in all siphons

and in the lavatory panCarefully close all gas valves after each use and make sure the are

out of reach of children

1. No gas or

not enough

gas

Not enough feeding

material

Feed as per recommendation

Holes or tears in the

bag

Repair it with a nylon bandage or replace the bag, if possible.2. Gas

reservoir do

not inflate

as they

should

Gas is leaking from

the gas pipeline

Repair the gas pipeline by welding

3. Gas does

not burn

Too much air or CO2 Flush the gas reservoir and wait for its replenishment with new

biogas


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properlyAdd a small amount of lime solution into the digester (be very

careful)

Not enough gas

pressure

Constrict the gas reservoir with a rubber band or install a gas

extracting device

4. The

flame often

extinguishes

Water has

accumulated in gas

pipeline

Drain water from the gas pipeline

ok-biogas training material - vinavac

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