The values of volumetric methane production rates for each OLR studied in natural and previously fermented molasses are also given in Tables 3 and 4, respectively. In the case of untreated molasses, it can be seen that the volume of methane produced per day increased progressively with increased OLR up to OLR values of 5.5 g COD per l day, after which a marked decrease was observed over the range tested. Apparently, the activity of methanogenic bacteria was not impaired up to OLR values of 5.5 g COD per l day because of the adequate buffering capacities provided in the experimental system. Nevertheless, the methane production rate decreased from 0.925 to 0.755 l/l day when the OLR was increased from 5.5 to 7.5 g COD per l day. This decrease in the methane production at the highest OLR values might be attributed to an inhibition
of the methanogenic bacteria at high OLR values, which caused an increase in effluent VFA contents and VFA/ alkalinity ratios, as can be seen in Table 4. Specifically, VFA content increased from 4.7 to 9.3 g/l (as acetic acid) when the OLR was increased from 5.5 to 7.5 g COD per l day. In the case of molasses previously treated with P. decumbens, the methane production rate increased linearly from 0.337 to 1.331 l/l day with increased OLR in all the range of OLR tested (1.5/7.5 g COD per l day). In addition, the values found for this parameter were always higher than those observed for anaerobic digestion of untreated molasses for the same values of OLR, which clearly shows the advantage of previously fermenting the molasses to be treated by anaerobic digestion. On the other hand, the methane yield coefficient (volume of methane per COD added to the reactor) can be calculated from Tables 3 and 4. As can be observed in Table 5, a gradual decrease in the fraction of organic matter transformed into methane was found after an OLR value of 5.5 g COD per l day, decreasing the values of methane yield coefficient 49% (untreated molasses) and 20% (pre-treated molasses) for the experiments corresponding to an OLR of 7.5 g COD per l day in relation to the values found at an OLR of 1.5 g COD per l day. Finally, the values of the substrate removal rate can be calculated from data summarized in Tables 3 and 4. Fig. 5 shows the variation of the HRT with the organic loading removal rate in the cases of the anaerobic digestion of untreated and pre-treated molasses. This plot also clearly shows the advantage of the pretreatment of molasses with P. decumbens previous to its anaerobic digestion. For a given value of the organic loading removal rate, the HRT decreased considerably when the molasses are previously fermented. In order to establish a more rigorous comparison, the time used in the pre-treatment (4 days) has been added to the HRT of the pre-treated molasses, obtaining a curve located in an intermediate position (Fig. 5). It is apparent that, even under these circumstances, the previous statement continues being valid. Therefore, the time necessary for digesting a given organic load, in the case of using a combined aerobic/anaerobic treatment, is always lower than that necessary to anaerobically treat the waste. On the other hand, the performance of the reactor processing untreated molasses starts to become destabilized for OLR values of 5.5 g COD per l day, which does not happen when the wastewater has been pre-treated, since the reactor admits loading rates of up to 7.5 g COD per l day without destabilization symptoms appreciated.