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Operation of biogas plants

The day-to day operation of a biogas unit requires a high level of discipline and routine to maintain a high gas production and to ensure a long life-span of the biogas unit. Many problems in the performance of biogas plants occur due to user mistakes or operational neglect.

Often, these problems can be reduced
• by less complicated designs that are adapted to the substrate, the climatic conditions and the technical competence of the user,
• by high-quality and user-friendly appliances,
• by design and lay-out of the biogas for convenient work routine,
• by proper training and easy access to advice on operation problems.

Preparation to starting the plant
This preparation stage includes carrying out of impermeability check of digester and gas system. To do this it is necessary to attach a water manometer to the gas system and to close all the vents so that it becomes possible to measure excess air pressure in the digester.

Digester is filled with water up to operational level. Excess air will be let out through the safety vent. After this manometer reading is taken and filled digester is left for a day. If after a day manometer reading is the same or is changed only insignificantly then the system is considered impermeable.

If the pressure in digester is significantly lower, it is necessary to find and repair the leak. Biogas plant shall be started only when plant as a whole and all its elements can be considered operational and conform to safety regulations.

Starting operations
The initial filling of a new biogas plant should, if possible, consist of either digested slurry from another plant (around 10%) or fresh cattle dung.

The age and quantity of the inoculant (starter sludge) have a decisive effect on the course of fermentation. 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 to allow filling of the digester p to 2/3 of its volume.

Type of substrate
Depending on the type of substrate in use, the plant may need from several days to several weeks to achieve a stable digesting process. After diluting substrate with water until homogenous liquid substrate results it is loaded into digester that is filled up to no more than 2/3 of inside volume. The rest of the digester volume is used for biogas collection. The substrate loaded into digester should not be cold – it’s temperature has to be close to chosen optimal digestion temperature.

Optimization of startup process
For optimization of the initial digestion process several well-known startup methods can be used:
• feeding active ferment from other well-functioning digester;
• adding reagents, such as lime, carbonic acid, alkali and others;
• initial filling of dieter with warm water and gradual addition of manure;
• filling digester with fresh manure;
• filling of digester with hot gases and gradual feeding of manure.

For proving stable growth of microorganisms during the startup period the heating of the substrate should be gradually increased for no more than 2ºС daily until it reaches 35-37ºС. During the heat up process substrate has to be intensively agitated. After 7-8 days bacteria become very active and biogas production starts.

Characteristics of breaking-in period
The breaking-in period is characterized by:
• low quality biogas containing more than 60% CO2
• very odorous biogas
• sinking pH and
• erratic gas production

Process stabilization
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 the pH value. 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 result in a high rate of gas production. Regular loading can commence after gas production has dropped off to the expected level.

Gas holder preparation
Gas holders can only be prepared for use after acceptance and checkout according to specifications and after examination by State engineering supervision service.

To avoid formation of explosive gas mixture before gas holder is filled with biogas it is necessary to displace air from the whole gas system. Air displacement is normally carried out by filling gas holder with water followed by displacement of water with biogas or non-flammable gases under pressure. Air displacement can be considered as successful if oxygen content of the gas probe from the gas holder does not exceed 5 %.

Outside check has to confirm proper functioning of control equipment of the gas holder (check and safety valves, manometer, pressure reducer). Reliability of the grounding and lightning protection has to be checked with help of grounding measure. Earth-connection resistance has to be less than 4 Ohm.

Gas quality
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.