1. Introduction
Biogas plants are environmental friendly due to their capability of
methane emission reduction. The fermentation process in such biogas
plants is a very complex process which can be compared to the digestion
process in mammals (e.g. cows and pigs) stomach because during chemical
digestion, which occurs in the gastrointestinal tract, food is digest
into small molecules by digestive enzymes. Consequently, the biogas
plants often use piggery manure1 or cattle-manure2 as an inoculum. Such
material, containing ready culture of bacteria and partially digested,
allows the fermentation process in the tank to begin3. In addition to the
inoculum, temperature (psychrophilic, mesophilic, thermophilic)4,total solid5, carbon in glucose6 and pH7, which influence
archaea 8 survival
capability also influence the methane production9.
It is known that magnesium (Mg) takes part in some important processes
in our life such as carbohydrate and glucose metabolism10. The glucose
metabolism for the formation of thiamine diphosphate (TDP) from thiamine
requires Mg 11.
Therefore, the microorganisms that are involved in methane production
may also need the Mg. For instance, Li et al. reported that magnesium
oxide in the co-fermentation process was an effective additive12. In another similar
study by Wang et al., it was discovered that hydrogen production was
linearly increased by using Mg element13. Not only Mg could
have influence on methane production, but other elements like sodium
(Na) and calcium (Ca) may also influence this process. This assumption
was backed from the previous reports in literature indicating that
methane production could be increased by using potassium hydroxide,
calcium hydroxide and sodium hydroxide 14-17.
Besides, the information about influence these elements on methane
production can be used for improving efficiency of mathematical model
which could be a good tool for biogas plants. Such way could be used for
prediction methane production before real production would be started.
Unfortunately, many presented models took into account not sufficient
amount of factors or not crucial factors for methane production. For
instance model presented by Masse and Droste contained a large amount of
factors like for example: requirement of ammonia-N, total concentration
of acetate or rate of hydrolysis. Besides for this model was conducted
only few tests and average error for calculation methane production was
28% 9. However, most of
the studies focused on all four stages18-21 (hydrolysis,
acidogenesis, acetogenesis, methanogenesis22). This generate a
large amount of variables but it does not guaranty a high model
efficiency. For instance model created by Minot included a large amount
properties and based on bacterial growth curve23. Moreover in this
work was found only one sample for testing. Therefore this model cannot
be reliable. Similar situation can be found in model presented by P.
Axaopoulos and P. Panagakis24. Situation looks
better for next work, where relative error was smaller than 20%25.
Obviously, the information presented above does not fully describe the
factors and models of methane production, but it can be claimed that
most of them need very precise information about the process. Besides,
because information about the influence of crucial elements during
fermentation process is limited, it was decided to extend this knowledge
and improve mathematical model. Therefore this study aimed to present
mathematical model for continuous technology in biogas plant, which take
into account influence of the crucial elements: initial carbon content
in the dry mass of the substrate, influence Mg on methane production,
volumetric load, temperature, pH, etc.