3.2.4. pH and Conductivity

Monitoring pH value is an indicator of the degree of biological and biochemical decomposition [33]. Electrical conductivity is a commonly used parameter for monitoring the amounts of nutrients, salts and impurities in the solution [55]. Since the composition of substrates could significantly change during pretreatment, pH and conductivity values could also significantly fluctuate. pH and conductivity measurements in the analyzed samples are shown in Figure 5.

pH values were generally higher in the untreated samples, and decreased during pretreatment, especially for the grass samples. This can be explained by the fact that, because of the degradation of organic compounds during thermal pretreatment, amino acids, ammonia and fatty acids are formed, which cause a drop in pH [29]. At higher temperatures, amino acids could also be degraded, and the pH increases again [56]. This can be clearly noticed in the sample with the combination of sewage sludge and grass (G + S), where an increase in pH is noticed at 80 ◦C, as compared with the pH value at 36.8 ◦C. Another interesting observation is that, in the case of samples containing only sewage sludge (S), the pH value after thermal pretreatment at 38.6 ◦C increased slightly and decreased after pretreatment at a higher temperature (80 ◦C). This could be related to the release of NH4 <sup>+</sup>, which could increase the pH of the solution when present in higher concentrations.

When biological pretreatment was performed, pH increased slightly with the addition of rumen fluid, most likely because of the slightly alkaline environment of rumen fluid (the pH of untreated sample was 7.5). For high degradation efficiency, maintaining a pH value in the optimal range is important, and the natural buffering ability of rumen fluid plays a major role in that process [57]. However, the optimal pH for most lignocellulose-degrading enzymes should be between 4.5 and 6.0 [58], and for methanogenic bacteria, the optimal range is between 6.6 and 7.6 [59], although a wider

range (between 6.5 and 8.2) has also been reported [60]. Hu et al. [52] reported that acidogenesis of cattail by rumen cultures is possible at higher pH (pH of 6.9). Similar values for treated samples were found in the present study (pH values between 5.1 and 7.2).

**Figure 5.** pH and conductivity (μS/cm) measurements.

Conductivity of the samples increased with pretreatment at lower temperatures (38.6 ◦C) and decreased with pretreatment at higher temperatures (80 ◦C). This can be confirmed by the findings of some authors who stated that electrical conductivity increases at temperatures up to 50 ◦C and becomes linear over time with any further increase in temperature [61].

The highest conductivity was measured in the grass sample (G) after pretreatment at 38.6 ◦C, because ions were dissolved from the grass, which thus increased conductivity. In one of the previous studies [61], it was likewise found that with the temperature increase, the conductivity in vegetable materials increases up to four times. Since the composition of these materials is comparable to grass, similar findings could be expected.

High conductivity values were also noted in the case of a mixture of grass and sludge (G + S), and a combination of grass, sludge and rumen fluid (G + S + R). This is due to the presence of grass, since sludge exhibits smaller conductivity values. In all the samples with rumen fluid (G + R, S +R and G + S + R), conductivity decreases as compared to the samples without rumen fluid (G, S and G + S). On the other hand, the conductivity of untreated rumen fluid is relatively high, owing to dissolved ions. The decrease in conductivity in mixtures with rumen fluid could be connected to the buffering capacity of rumen fluid [62].
