Anaerobic digestion (AD) is a biological process which allows the removal of high organic-loading and potentially polluting substances by their transformation into biogas, a mixture of methane and carbon dioxide, prevalently. AD presents many other advantages: it has a low energy consumption and low construction costs with a relatively simple plant technology. Actually, since anaerobic bacteria work more efficiently at room temperature or higher, AD can be profitably applied in developing countries. Biogas production is a foundamental parameter of AD because it is the main index to be considered in a process economic evaluation and also because it gives a measure of its efficiency as well.
Moreover, biogas production, and more frequently methane production, is often used as an index set to control the process.
With the increase of energy price, the specific biogas production (SGP) of primary and residuals crops has become a goal for economic energy supply and, as a consequence, a rapid and effective method for measuring the gas produced has to be put forward, because there is not an accepted international standard yet. The effective knowledge of biogas production rate allows study of the biological process through macroscopic indicators, easily usable in industrial field, as well. The present study concerns the development and the validation of a technique for biogas production measurement and kinetic determination which adopts bench-mark laboratory-scale experiments with complex solid substrates, i.e. primary and residual energy crops.
A laboratory-scale plant was designed and put up to perform this task. The equipment permits to carry out 4 contemporary tests because it is composed of 4 independent gas-lines, each of which connecting an anaerobic reactor to a gas-meter.
Data from the experiments were continuously recorded by a data logger. The equipment was tested with synthetic substrate feeds of ethanol and sodium acetate. By a comparison
between experimental gas production data and the theoretical ones, stoichiometrically calculated, the range of the error on methane productions resulted within ± 5%.
In addition, the presence of oxygen amounts in the mixture, revealed the inconsistency of a test. These positive results allowed the implementation of different experiments to measure biogas produced from natural substrates. Apple, onion, corn straw, potato and winery wastes mixture were therefore tested in various experiments in order to calculate the SGP and SMA of the different crops. A new mathematical model for the description of complex substrate degradation was developed as well. The model was calibrated on the different biological systems and then applyed on real substrates to carry out their COD fractionation, to analyse the biological variable trends and to test the reliability of results. Finally, the reliability of a procedure for the evaluation of a two-step AD as compared to the one-step AD was tested by using apple and potato substrates.
