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CLOSE THIS BOOKBiogas plants in Animal Husbandry (GTZ, 1989, 153 p.)
7. Plant operation, maintenance and repair
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT7.1 Commissioning of biogas plants
VIEW THE DOCUMENT7.2 Plant operation
VIEW THE DOCUMENT7.3 Plant maintenance
VIEW THE DOCUMENT7.4 Plant repair
VIEW THE DOCUMENT7.5 Safety measures

Biogas plants in Animal Husbandry (GTZ, 1989, 153 p.)

7. Plant operation, maintenance and repair

The main objective of any plant owner/user is to have a well-functioning biogas plant that involves a modest amount of work for operating it and requires very little effort and expense for maintenance and repair in the long run. Smooth running of a biogas plant is dependent on good information and careful planning and construction. Operating errors and false expectations are the most frequent causes of plant outage.

7.1 Commissioning of biogas plants

The commissioning procedure for a biogas plant includes:
- inspection and final acceptance of all components
- initial filling
- starting the plant
- user familiarization

Inspection and final acceptance

Prior to filing the plant, all components must be carefully inspected for proper function and suitability for acceptance. Of particular importance at the time of final acceptance is seal testing of the digester, gasholder and gas pipes.

It must be kept in mind that the seal tests described below are very laborious without pumps (e.g. hauling of more than 10 m³ water) and may not even be feasible if water is scarce, in which case such testing must be dispensed with. The time and effort involved must be weighed against the risk of having to empty the plant after completely filling it with slurry. In either case, it is very advisable to use a motor pump.

Water-seal testing
Fill the entire digester with water and check the fill level in all components.

Once all components have become saturated with water (after about 1 day), refill to the zero line, wait one day, and then remeasure. If the water loss amounts to less than 2% of the digester volume, the plant may be regarded as leaktight.

Seal testing (water and gas) of a fixed-dome plant
Fixed-dome plants are regarded as leaktight if the water-seal test shows less than 2% water loss, and the gas-seal test shows less than 5% gas loss.

Gas-seal testing of a floating-drum plant
In the case of floating-drum plant, only the metal gasholder must be subjected to gas seal testing; any leaks are detected with the aid of soap water.

Fig. 7.1: Water-seal testing of a digester. 1 Fill the plant with water, Check the fill levels: 21 Inlet no water in the mixing pit, 22 Digester - at least 10 cm neck height above water level. 3 Refill to compensate for moisture absorbed by the masonry. 31 Mark the water level. 4 Measure the drop in water level as basis for calculating the water loss (W1 = p r² X h). 5 Repeat measurements as necessary. (Source: OEKOTOP)

Fig. 7.2: Seal testing (water and gas) of a fixed-dome plant. 1 Fill the plant up to the zero line; Check the fill levels: 21 Gas extraction points at least 10 cm above discharge level, 22 Water level in the displacement pit. 3 Perform water-seal test and level-drop check (cf. fig. 7.1). 4 Close the entry hatch. 5 Fill with gas up to maximum a1lowable plant pressure a) with air (pump), b) with exhaust gas (vehicle exhaust), 51 . . . until the displacement pit overflows, or 52 . . . until gas bubbles out of the inlet pipe. 6 Refill the plant to compensate for saturation losses. 7 Measure the level drop (h) after one day, and calculate the gas losses. (Source: OEKOTOP)

Fig. 7.3: Gas-seal testing of a metal gasholder. 1 Place the gasholder in position with the gas valve closed. 2 Mark the top edge of the digester neck on the gasholder. 3 Check the location of the mark one day later. 4 If the mark is found to have dropped by 1-3 cm, use soapy water to check for leaks in the gasholder. (Source: OEKOTOP)

Fig. 7.4: Pressure testing a gas pipe. 1 Close all gas valves and fill the water trap. Find the maximum pipe pressure, i.e. how high the pressure in the pipe can go until the water trap blows off (not more than 50 cmWG). 2 Adjust the test pressure with the aid of a manometer-equipped test pump or the gasholder (10% below max. pressure). Check the pressure loss after one day. 3 Use soapy water to detect leaks. (Source: OEKOTOP)

Pressure testing of the gas pipe
The test must be performed while all gas pipe connections are still accessible. Pressurize the gas pipe with the aid of a test pump or by placing weights on the gasholder. If there is no noticeable loss of gas after one day, the pipe may be regarded as gaslight.

Initial filling of the plant

The initial filling for a new biogas plant should, if possible, consist of either digested slurry from another plant or cattle dung. It is advisable to start collecting cattle dung during the construction phase in order to have enough by the time the plant is finished. When the plant is being filled for the first time, the substrate can be diluted with more water than usual.

Starting the plant

Depending on the type of substrate in use, the plant may need from several days to several weeks to achieve a stable digesting process. Cattle dung can usually be expected to yield good gas production within one or two days. The breaking-in period is characterized by:

- low-quality biogas containing more than 60% CO2
- very odorous biogas
- sinking pH end
- erratic gas production.

The digesting process will stabilize more quickly if the slurry is agitated frequently and intensively. Only if the process shows extreme resistance to stabilization should lime or more cattle dung be added in order to balance with pH. No additional biomass should be put into the biogas plant during the remainder of the starting phase. Once the process has stabilized, the large volume of unfermented biomass will give rise to a high rate of gas production. Regular loading can commence after gas production has dropped off to the intended level.

As soon as the biogas becomes reliably combustible, it can be used for the intended purposes. Less-than-optimum performance of the appliances due to inferior gas quality should be regarded as acceptable at first. However, the first two gasholder fillings should be vented unused for reasons of safety, since residual oxygen poses an explosion hazard.

User familiarization

The plant owner should be familiarized with the details of plant operation and maintenance at the time of commissioning. It is important that he be not only familiarized with the theory of function but given ample opportunity to practice using all parts of the plant. The user-familiarization procedure should be built up around an operational/maintenance checklist (cf. table 7.2).

Table 7.1: Checklist for the inspection and acceptance of biogas plants (Source: OEKOTOP)

Check item




I Interim inspection prior to backfilling


· unsuitable

· dismantle/demolish the plant

Digester masonry

· cracked/broken bricks/ stones

· demolish the masonry

Fixed-dome plants in particular call for high-quality work. Tearing down the plant at this point is often . the least-painful solution

· seriously false dimen- signs

· improperly jointed masonry

· fix with mortar, incl. foundation

inlet/outlet pipes

· false configuration

· demolish

· poor backfilling

· refill

II Final inspection and acceptance

Rendering in digester/ gasholder

· cracks, sandy rendering, low strength, cavities, poorly worked edges and coving

a) for the gas space of a fixed-dome plant: replace rendering

Ensure high-quality work man-ship for fixed-dome plants

b) for the slurry space: repair or add a new layer of rendering (usually sufficient)

Masonry seal coats

· cracks, flaws, thin spots, voids

· renew the coating

(applies only to fixed-dome plants)

Mixing pit/slurry sotre/displacement pit

· cracks, poor workman-ship

· demolish or repair

Inlet/outlet pipes

· plugged, poorly worked transitions

· remove mortar plug, repair with mortar/ concrete

Earth fill

· insufficient fill height

· recompact, add

· poorly compacted earth

Water-fill check of all component heights(relative): inlet, outlet, digester, gas- holder cf. chapter 7.1

for floating-drum plants:

· water in mixing pit, too low; water spills over the digester, i.e. digester too low; outlet too low for fixed dome plants:

· shorten the outlet

Differentiation must be made between nonconfor mances that impair the plant's function and nonconformances that merely prevent optimal utilization

· water above gas zero line -

· add wall height to digester

· lengthen the outlet

· alter the elevation of the

a) gas outlet

b) displacement pit

· water in mixing pit

· alteration too much trouble

· water in displacement pit

· alteration too much trouble

Seal testing (see chapter 7.1 for details)

for floating-drum plants:

· water loss < 2%

· acceptable as is

In case of doubt, locate and repair all defects, even if it seems like a lot of trouble at the moment; repairs at a later date would be more troublesome!

· water loss > 10% for fixed-dome plants:

· repair masonry

· water loss < 2%

· acceptable as is

(pressureless) > 10%

· repair masonry

· water loss < 5 %

· acceptable as is

(pressurized) > 10%

· repair masonry

Gasholder (of floating- drum plants only)

·false dimensions,wrong

· reject gasholder,

· grade of steel,

build new one

· faulty welds

· repair welds

· faulty painting

· repaint

Gas valves

· stuck valves

· repair or replace

Check carefully; defects most likely in the gas system

· missing valves

· false water trap

Gas pipe

· false gradient

· re-install

· leak found upon pressure testing

· repair leaks

· poor workmanship

· re-install

Gas appliances

· functional defects

· repair or replace

7.2 Plant operation

The operation of a simple biogas plant is relatively uncomplicated. The user must be given all the information and practical assistance he needs before and during the early phases of plant operation.

Collecting substrate

The collection of substrate is a simple matter when combined with work that has to be done anyway, e.g. cleaning the stables. It can be made even easier by arranging for the manure to flow directly into the mixing pit. Experience shows that it is not a good idea to gather dung from fields, roads, etc. or to go to the trouble of elaborately chopping up or otherwise preprocessing plant material for use as substrate. The work involved is usually underestimated, while the motivation is overestimated.

Filling the plant

Filling means: mixing the substrate with water, removing bouyant materials, allowing the fill material to warm up, flushing it into the digester, and removing sand and stones. The simple mixing pit shown in figure 5.16 can handle a daily fill quantity of up to 500 1.

Digested-slurry storage/utilization

The further processing of digested slurry is a critical point in that it can be quite toilsome (cf. chapter 3.4).

In designing the plant, care must be taken to ensure that the slurry store will be large enough. Fixed-dome plants in particular should be equipped with an overflow, so that the digested slurry does not have to be hauled away every day.

Table 7.2: Checklist for the daily operation and regular maintenance of biogas plants (Source: OEKOTOP)

Daily activities:
- fill the plant
- clean the mixing pit
- agitate the digester contents
- check the gas pressure
- check the gasholder contents
- check the appearance and odor of the digested slurry

Weekly/monthly activities:
- remove/use the digested slurry
- clean and inspect the gas appliances
- check the gas valves, fittings and appliances for leaks
- inspect the water trap

Annual activities:
- inspect the digester for scum formation and remove as necessary by opening the plant
- inspect the plant for water tightness and gas tightness
- pressure-test the gas valves, fittings and pipes
- check the gasholder for rust and repaint as necessary

Monitoring the process

If the plant is properly started before being handed over to the user, it may be assumed to be in proper working order. The user will have become familiar with what optimum plant operation involves. This is very important, because from then on he himself will have to watch for any appreciable changes in how the plant functions; the main indication of a beginning malfunction is a change in the daily gas output.

7.3 Plant maintenance

The maintenance scope for a biogas plant includes all work and inspections needed to ensure smooth functioning and long service life. To the extent possible, all maintenance work should be done by the user.

Biogas plants can develop a number of operational malfunctions. The most frequent problem, "insufficient gas production", has various causes. Often enough, it takes the work of a "detective" to locate and remedy the trouble. It may be necessary to experiment with and monitor the plant for months on end in cooperation with the user.

7.4 Plant repair

Repair measures for biogas plants (cf. table 7.5) are necessary in case of acute malfunctions and as indicated by routine monitoring. Repair measures exceeding simple maintenance work usually require outside assistance, since the user himself may not have the necessary tools or know-how.

It is advisable to have the annual maintenance work mentioned in chapter 7.3 performed by external artisans With prior experience in biogas technology. Such maintenance and repair work should be ordered on a contract basis. Past project experience shows that professional biogas repair and maintenance services can be very important for ensuring long-term plant performance. Such services should include general advice, functional testing, troubleshooting, spare-parts delivery and the performance of repair work.

Table 7.3: Checklist for troubleshooting in case of insufficient gas production (Source: OEKOTOP)

Quantity and quality of substrate
- low/less daily input
- excessive dilution with water
Ascertain by control measurements

Gas system leaks
- gasholder
- gas pipe
- valves and fittings
Ascertain by checking all components and connections for leaks with the aid of soapy water

Disturbance of the biological process
- heavy odor
- change of color of digested material
- drop in pH

Possible remedial measures:
- inspect the quality of the substrate
- stop biomass until the process returns to normal
- stabilize the pH, e.g. with lime
- add cattle dung or healthy slurry
- investigate the user's filling methods to determine if pollutants or noxious substances (detergents, pesticides, etc.) are getting into the plant

Table 7.4: Simple-plant malfunctions and remedial measures (Source: OEKOTOP)


Possible cause


Plugged-up inlet pipe

fibrous substrate

use rod to unplug the pipe

Stuck gasholder

floating scum

1. turn the gasholder

2. take off the gasholder

and remove the scum

Tilted gasholder

broken guideframe


Low gas production, poor gas quality

cf. table 7.3

cf. table 7.3

Receding slurry level

leak in plant


Inadequate gas storage in fixed-dome

leak in gasholder



Stuck gas cocks


apply oil, ,operate repeatedly

Leaky gas pipe

corrosion, inferior workman-



Sudden loss of gas

- broken gas line


- blown-off water trap

refill with water

- open gas cock


Pulsating gas pressure

water in the gas pipe

pump out the pipe, relocate that

section of pipe

plugged-up gas pipe

push rod through pipe

Malfunctioning gas appliances

cf. chapter 5.5.3

cf. chapter 5.5.3

Structural damage

cf. table 7.1

cf. table 7.1

Table 7.5: Potential repair situations for simple biogas plants (Source: OEKOTOP)



Remedial measures

Damaged masonry and rendering

Inspection of open plant, subsiding slurry level,

Inspect the plant and repair any seal test damage; tear down the plant if large cracks are found

Damaged or broken inlet/outlet pipe

(see above)

Replace pipe, repair connection to masonry

Damaged/leaky fixed-dome plants

Gas seal test, visual inspection of open plant

Repair rendering, renew seal coatings

Damaged/corroded gasholder

High gas losses, seal test

Replace gasholder, renew weldings

Broken guide frame

Tilted/immobile gasholder

Remove gasholder, repair guide frame

Leaky/broken/corroded gas pipe

High gas losses, gas seal test of pipe

Locate damage, repair or replace as necessary

Damaged valves/fittings

Functional inspection

as a rule: replace

Damaged gas appliances

Functional inspection

cf. "Gas appliances"

7.5 Safety measures

Biogas is a combustible, explosive gas. Its safe handling and use can be ensured, or at least promoted, by:
- educational measures and operating instructions
- good, careful planning and execution
- timely detection of damage and gas leaks
- installation of safety equipment

Safety aspects of planning and implementation

The following basic rules should be adhered to:
- plant located outside of buildings used for other purposes, e.g. stablings
- underground installation of pipes
- no use of hoses
- careful installation and regular inspection of gas pipes
- regular inspection of gas appliances
- good ventilation of rooms containing gas appliances without safety pilots
- installation of safety stop valves
- one directly on the plant, and another on each appliance.

Educational measures and operating instructions

The user must be made fully aware of the explosive nature of biogas, possibly by way of demonstration (e.g. by producing a flash flame). He must learn by heart the following basic rules:
- Never leave an open flame unattended!
- Always close the gas and safety valves of each appliance properly and immediately after each use!
- Close the plant's safety valves each night and whenever the plant is left unattended!

Experience shows that leaks and open gas cocks can be detected very quickly, i.e. before an explosive mixture forms, by watching for the conspicuous odor of unburned biogas.

Safety devices
As long as the above safety aspects are adhered to, small biogas plants in rural areas require few or no special safety devices, the one major exception being appliances that operate on their own, i.e. refrigerators, radiant heaters, etc., in which case the use of safety pilots is obligatory.