## 4 Discussion
### Toxicity of the PAM and its Possible Degradation under Anaerobic Conditions
The acids were mainly produced in the course of degradation of organic matter of the digested sludge used as the inoculum. A certain amount of organic acids resulted from PAM degradation, as was indicated by increased amounts of acetate produced in the samples with higher PAM concentrations. Partial PAM hydrolysis and conversion of the intermediate products of PAM decomposition to acetate is another possible cause for acetate accumulation in the flocculant-treated samples [25]. Thus, [PERSON] et al. showed polyacrylic acid to be the mainsource for acetate formation during PAM degradation [25]. Acetate was also a major product of mesophilic AD of SS treated with different concentrations of PAM, and its amount increased with increasing PAM concentration [24]. Increases in the concentrations of ammonium nitrogen were observed in the flocculant-containing samples rather than in the control ones, indicating partial PAM degradation (removal of amide side groups). Similar experiments by [PERSON] et al. revealed that the cationic PAM promoted the release of ammonium nitrogen from SS [17]. [PERSON] et al. found that increasing PAM concentration in the medium resulted in the formation of a lower specific amount of ammonium nitrogen (mg N-NH4/g PAM) [18]. A similar tendency was observed in our experiments. Our data indicated that the PAM was partially biodegradable even at its highest tested concentration of 400 mg/g TS. These results do not agree with the observations of [PERSON] et al., who found that while biological PAM hydrolysis occurred at its concentrations of 100-200 mg/L, its efficiency decreased drastically at PAM concentrations above 300 mg/L [25]. In our experiments, PAM concentration was 37 mg/L in B-5 samples and 296-2964 mg/L in other samples. However, PAM mineralization (stepwise degradation up to methane formation) did not occur. The degradation was limited to the formation of various intermediate products, as well as detachment of nitrogen-containing side groups (the latter resulted in the accumulation of ammonium nitrogen). The possibility of partial PAM biodegradation under anaerobic conditions was reported in several works [17; 18; 23]. In the absence of alternative substrates, microorganisms can use PAM as a source of nitrogen and even of carbon [18; 25; 27]. [PERSON] et al. showed that PAM is relatively resistant to anaerobic degradation compared to polysaccharides and proteins and therefore is used at the later stages when other substrates are available [17]. Thermophilic conditions (50-55\({}^{\circ}\)C) promote partial PAM depolymerization, which increases its availability to microorganisms and the degree of its biodegradation [23]. The polymeric structure of PAM and its high molecular mass, as well as resistance of the floccules to hydrolases, are the main factors determining PAM resistance to microbial degradation [22; 28]. A correlation was observed between the size of the floccules formed in the presence of PAM and the flocculant concentration: larger floccules were formed at higher flocculant concentrations. A similar correlation was reported by other authors [16; 17; 18]. The size of floccules can also affect the efficiency of hydrolysis processes and slow down the conversion of the products of partial decomposition of VS into methane, which is reflected in a decrease in the rate of methanogenesis.