It was found that both diesel fuel and spent engine oil visibly
influenced the number of methanogenic bacteria—a microbial
group directly responsible for the biogas production, after 7 days
of the process, while after 14 days the effect was not observed. The
differences in results between 7th and 14th day of the experiment
concerning the numbers of bacteria can be probably explained by
microbial adaptation to the process condition. Comparison of the
results of numbers of methanogenic bacteria and sulphate reducing
bacteria in the presence of diesel fuel (which was probably not so
toxic to microorganisms) can reflect the competitive mechanism
for protons—the increase in number of methanogenic bacteria is
accompanied by the decrease in sulphate-reducing microorganisms.
(Tables 3 and 4).
Barret et al. [13] stated, that the hydrocarbons anaerobic digestion
was not strongly related to the overall metabolism of the
biomass. In this research the total dehydrogenase activity assessment
was applied for the evaluation of microbial enzymatic
activity. The results obtained in the presence of the diesel fuel were
similar to the results for the control bioreactor, both in endogenous
and substrate-supplied samples In case of engine oil, the
visible decrease in the microbial dehydogenase activity occurred.
The similar tendencies in enzymatic activity in the control and
experimental reactors were observed for both petroleum products.
The chemical analyses, excluding the COD assessment, did not
reflect the diffrences in biogas production efficiency between the
petroleum product contaminated sludge and the control samples(Tables 5 and 6), although there are some evidence that nitrogen
and phosphorus may be limiting factors for the hydrocarbons
biodegradation [36].
The results for COD values confirmed the previous observation
concerning the sligtly stimulating effect of the addition of diesel
fuel on the anaerobic sludge digestion and the inhibition of the
process in the presence of spent engine oil.
The observed negative impact of spent engine oil ofthe methane
fermentation process and the toxicity may be related to the presence
of metal particles and the products of mechanical and thermal
decomposition of oil components which are formed during the
oil exploitation. It was noticed that the presence of heavy metals
can diminish hydrocarbons biodegradation [31] The increase
of the density and the viscosity of the oil inside the engine results
in a decrease of its biodegradability [37]. For the fresh commercial
diesel fuel the effect was not observed.