The variation of temperature during the anaerobic digestion process of two studied substrates is presented in Figure . One can observe that the temperature average value is around 31 – 32°C, with peaks at 36 – 37°C. The general behavior is connected with a combined regime: mesophilic regime for the first 40
days, for the wheat bran batch, and 50
days for the mix batch, including cryophilic regime for the last part of the process.
Temperature variation during anaerobic digestion process.
In addition to temperature, the pH is an important process parameter for the management of the biogas processes. The pH variation during anaerobic digestion process is presented in Figure . For wheat bran, the initial pH values are lower than for the mix batch and a general tendency of increasing from values of 6.5 up to 7 – 7.1 after 40
days is confirmed. In comparison, the behavior for the mix batch is much more linear, starting from pH values of 6, with small peaks inside the first 12
days, and stabilizing near the domain of 6.8 – 7.1, for the rest of the process. This indicates a better behavior of the mix during anaerobic fermentation, and has as advantage to be much easier controlled than for the first batch of material.
pH variation during anaerobic digestion process.
From Figure it can be observed that the daily biogas production underlines the fact that the mix batch is able to produce larger quantities of biogas, with average value of 0.405
/day, while the wheat bran batch had smaller average value of 0.323
/day. The obtained volumes are considered at normal pressure and the temperature existent inside the anaerobic tanks.
Daily biogas production during anaerobic digestion process.
By using the MATLAB software, the experimental data were processed and analyzed. The clusters of the temperature and pH values for mix and wheat bran batches and the histograms for the produced consequent biogas amounts are presented in Figures and . The existing data can be used to determine the biogas quantities produced by each substrate for a known anaerobic digestion temperature and pH [13
Cluster of the temperature and pH values for the mix and histogram for the produced consequent biogas amounts, in m3/day.
Cluster of the temperature and pH values for the wheat bran and histogram for the produced consequent biogas amounts, in m3/day.
From Figure , it can be noticed that the higher value of biogas yield in case of an anaerobic digestion of mix is 0.955
and correspond to a pH of 7 and a temperature of 33°C. The mix experiments were based on values measured during 65
The correlation coefficient between T and the biogas volume generated during the anaerobic digestion (T-temperature in degree C and Q - amount of biogas in m3
) (Figure ) is 0.4642 that represents not a very large value, but a significant one, according to [14
], where three domains −1 to −0.33; -0.33 to 0.33; and 0.33 to 1; are given. The correlation coefficient between pH and Q is 0.2737. This result is insignificant but still a positive value. The two quantities are still positively correlated meaning that the growth of one trains the increase of the other. These values are well corresponding to the location of the maxima in the cluster plan for the biogas production (Figure ).
In correspondence, Figure indicates that the highest yields of biogas for anaerobic digestion of wheat bran is 0.963
/day and corresponds to a pH of 6.9 and a temperature of 29°C .
The anaerobic digestion of this batch was monitored during 65
The correlation coefficient between T and Q (temperature and amount of biogas) is - 0.508, and is considered as a significant one. The coefficient between the pH and Q value is 0.6892, and is also considered significant. The two quantities are positively correlated, the correlation coefficients corresponding to the location of the maxima in the cluster plan of the biogas production. The two parameters are positively correlated, indicating clearly that the growth of one trains the increase of the other.