## 3 Results  
### DNA Quantity, Concentration, and Quality Assessment  
The molecular weight of the DNA varied dramatically between kits (Figure 1). The PUREL kit generated the highest weight of DNA. Mostly degraded DNA was observed when DNA was extracted using the FAVOR method, especially when samples of activated sludge and biofilm were processed. In the case of activated sludge, when the DNA concentration was investigated using the fluorescent approach, the highest mean value of DNA yield was noticed when the FASTD method of DNA extraction was used (0.84 \(\pm\) 0.17 ng of DNA/1 mg of biomass) (Tables S1-S3). For comparison, the respective values were 0.41 (\(\pm\)0.02) and 0.11 (\(\pm\)0.02) when the PUREL and FAVOR methods were used. When the spectrophotometric approach was used, the mean values were 1.42 (\(\pm\)0.17), 0.54 (\(\pm\)0.03), and 2.34 (\(\pm\)0.66) for the FASTD, PUREL, and FAVOR methods, respectively. All differences between all methods of DNA extraction were statistically significant when using both the fluorescent and spectrophotometric approaches. The quality of purified DNA was the worst when FAVOR was used (260/280 = 2.09 \(\pm\) 0.04); when FASTD and PUREL were used, this value was 1.91(\(\pm\)0.03, and \(\pm\)0.04, respectively). This difference was statistically significant.  
In the case of biofilm, when the fluorescent approach was used for DNA concentration analysis, the highest mean value of DNA yield was noticed when the FASTD method of DNA extraction was used (0.38 \(\pm\) 0.15 ng of DNA/1 mg of biomass). With the PUREL and FAVOR methods, these values were 0.14 (\(\pm\)0.06) and 0.13 (\(\pm\)0.05), respectively. When the spectrophotometric approach was used, the mean values were 0.57 (\(\pm\)0.16), 0.16 (\(\pm\)0.06), and 5.37 (\(\pm\)4.49) for the FASTD, PUREL, and FAVOR methods, respectively. With the fluorescence measurements, significant differences were found between FASTD and PUREL, as well as between FASTD and FAVOR. With the spectrophotometric approach, all methods of DNA extraction differed significantly. The quality of DNA was best when the PUREL method was used for DNA purification (260/280 = 1.83 \(\pm\) 0.07). With FASTD and FAVOR, these values were 1.98 (\(\pm\)0.05) and 2.12 (\(\pm\)0.08), respectively. All these differences were statistically significant.  
When the fluorescent approach was used to measure the concentration of DNA extracted from digestate, the highest yield was noticed with FASTD (0.20 \(\pm\) 0.07 rg of DNA/1 mg of biomass). With PUREL and FAVOR, these values were 0.07 (\(\pm\)0.01) and 0.05 (\(\pm\)0.01), respectively. The calculated DNA yield values when the spectrophotometric approach was used were as follows: 0.82 (\(\pm\)0.28), 0,16 (\(\pm\)0.04), and 0.43 (\(\pm\)0.17) for FASTD, PUREL, and FAVOR, respectively. All differences between all methods of DNA extraction were statistically significant with both the fluorescent and spectrophotometric approaches. The quality of DNA was the best when the PUREL method was used for DNA purification (260/280 = 1.81(\(\pm\)0.11). These values were 1.93 (\(\pm\)0.07) and 2.07 (\(\pm\)0.07) when FASTD and FAVOR were used, respectively. The differences between FASTD and FAVOR as well as between PUREL and FAVOR were statistically significant.