2.2.3. Methane Production Kinetics

The results of the BMP test were subjected to kinetics determination. The first-order equation (Equation (6)) was used to provide information about the rate of methane production and the estimated value of maximum methane production potential with the application of Statistica 13.0 software (TIBCO Software Inc., Palo Alto, CA, USA). Afterward, the methane production rate was calculated (Equation (7)) [18]:

$$BMP = EBMP\_{\varepsilon} \times \left(1 - \varepsilon^{(-k \times t)}\right) \tag{6}$$

$$r = k \times EBMP\_c \tag{7}$$

where:

*BMP*—the cumulative methane production obtained from a substrate after time t, mlCH4 × gVS<sup>−</sup>1;

*EBMPe*—the estimated value of experimental maximum methane production obtains from a substrate, mlCH4 × gVS<sup>−</sup>1;

*k*—constant reaction rate, d<sup>−</sup>1;

*t*—process time, d;

*<sup>r</sup>*—methane production rate, mlCH4 × (gVS × d)<sup>−</sup>1.

## 2.2.4. Statistical Analysis of Biochar Effect

To check if biochar addition had a statistically significant effect (positive or negative) on the methane fermentation, the one-way analysis of variance with post-hoc Tukey tests was performed at the level of α = 0.05, with the application of Statistica 13.0 software (TIBCO Software Inc., Palo Alto, CA, USA).

#### **3. Results and Discussion**
