In a preliminary test the optimum ratio of substrate to inoculum was investigated. Five digesters were filled with different quantities of P. stratiotes homogenate and inoculum as shown in Table 3. Inoculum 1 (Table 2) was used for this test, carried outat 35°C. The results from the preliminary experiments revealed that there was no effect on the specific gas production (17"5 + 1'1 litres kg- 1 fresh P. stratiotes STP) by varying the substrate:inoculum ratio. Furthermore, the methane content of the biogas was the same in all cases (59-7+ 1.1%). However, there were slight differences in the concentrations of acetic and propionic acids in the slurry samples. As shown in Fig. 1(a), the acetic acid was rapidly decomposed in digesters 1, 2 and 5 (Table 3), while, in digesters 3 and 4, conversion was slower. This indicates a slight inhibition in the more highly loaded digesters which can also be seen from the accumulation of propionic acid in these digesters in the initial stage (Fig. l(b)).
In a preliminary test the optimum ratio of substrate to inoculum was investigated. Five digesters were filled with different quantities of P. stratiotes homogenate and inoculum as shown in Table 3. Inoculum 1 (Table 2) was used for this test, carried outat 35°C. The results from the preliminary experiments revealed that there was no effect on the specific gas production (17"5 + 1'1 litres kg- 1 fresh P. stratiotes STP) by varying the substrate:inoculum ratio. Furthermore, the methane content of the biogas was the same in all cases (59-7+ 1.1%). However, there were slight differences in the concentrations of acetic and propionic acids in the slurry samples. As shown in Fig. 1(a), the acetic acid was rapidly decomposed in digesters 1, 2 and 5 (Table 3), while, in digesters 3 and 4, conversion was slower. This indicates a slight inhibition in the more highly loaded digesters which can also be seen from the accumulation of propionic acid in these digesters in the initial stage (Fig. l(b)).
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