# Efficiency of Chemical Toilet Sewage (CTS) Co-Fermentation with Typical Energy Crops
## 3 Results
### FOS/TAC and pH  
The study also measured FOS/TAC, which can often be used to verify whether the organic load rates (OLRs) in the digesters are within optimal levels. The FOS quotient is the sum of volatile fatty acids (VFAs) in the medium during anaerobic digestion, measured in mgCH\({}_{3}\)COOH/dm\({}^{3}\), whereas TAC is used to determine alkaline buffer capacity, expressed  
Figure 3: Correlations between **(a)** CTS in the feedstock and the contribution of basic taxonomic groups of methanogens, **(b)** the contribution of individual taxonomic groups of methanogens and CH\({}_{4}\) in the biogas. in mgCaCO\({}_{3}\)/dm\({}^{3}\). FOS/TAC also has other uses, apart from measuring OLR and HRT. It can be used to indirectly evaluate methanogenic bacteria activity, whose populations can be inhibited by factors that hamper their growth or metabolism, as well as by toxic substances in the medium. This can cause unmetabolized VFAs to accumulate during methanogenesis, which in turn leads to increased FOS/TAC. In fact, this was the case in the present experiment. While the OLR was maintained at a constant and relatively low 2.2 kgVS/m\({}^{3}\)-day across all variants, a steady increase in FOS/TAC values was still observed, clearly indicating VFA accumulation in the anaerobic medium. The VFA levels were found to consistently increase with the CTS input. V1 (% CTS) had 1680 \(\pm\) 121 mgCH\({}_{3}\)COOH/dm\({}^{3}\), whereas V7 (60% CTS) contained 2840 \(\pm\) 180 mgCH\({}_{3}\)COOH/dm\({}^{3}\). In variants V1-V3 (CTS in the dilution fluid = 0-20%), the FOS/TAC ranged from 0.30 \(\pm\) 0.01 to 0.33 \(\pm\) 0.04. These values show that the AD process was running at optimal performance, as further evidenced by the biogas yield and quality. Increased CTS doses led directly to higher FOS/TAC, with the latter reaching 0.39 \(\pm\) 0.02 in V4 and 0.43 \(\pm\) 0.04 in V5, indicating impaired VFA removal. The highest FOS/TAC values (over 0.50) were noted in V6 and V7. Total alkalinity values were similar across variants, from 4923 \(\pm\) 538 mgCaCO\({}_{3}\)/dm\({}^{3}\) (V4) to 5717 \(\pm\) 533 mgCaCO\({}_{3}\)/dm\({}^{3}\) (V3). This translates to a range of 14%, compared to the much higher 42% for VFAs. The values of the primary anaerobic digestion indicators are presented in Table 6.  
The present study also showed that the reduction in the total activity of methanogenic bacteria--and the resultant increase in VFAs in the system--was reflected in the observed pH values. The pH was at fairly stable, near-optimal levels across variants V1 to V4: between 7.34 \(\pm\) 0.02 in V2 and 6.89 \(\pm\) 0.11 in V4. Subsequent variants showed a sharp decrease in pH, as well as dramatic fluctuations thereof. V5 had a pH of 6.42 \(\pm\) 0.19, which dropped to just 6.11 \(\pm\) 0.14 in V7. For their study on CTS digestion, [PERSON] et al. (2021) adjusted the pH to a target range of 9.4 to 7.0 [29]. These are the optimal levels for anaerobic processes [20; 43]. [PERSON] et al. (2017) also found that stable pH improves performance [30]. Efficient mesophilic anaerobic digestion of the CTS was achieved at pH 7.3 [30]. [PERSON] (2008) [31] obtained good results when anaerobically digesting toilet sewage with kitchen waste in a CSTR. Again, stable AD was predicated on maintaining near-neutral pH [31]. In our study, rising CTS inputs were found to strongly and positively correlate with the VFA levels (R\({}^{2}\) = 0.8889) (Figure 4a). On the other hand, no such correlation was found between the experimental variant and total alkalinity in the digesters (R\({}^{2}\) = 0.0351) (Figure 4a). Even stronger relationships were noted between the FOS/TAC and the output of gas metabolites. A particularly pronounced negative correlation was detected between FOS/TAC and CH\({}_{4}\) production (R\({}^{2}\) = 0.9646), while a slightly weaker one was found between FOS/TAC and biogas yield (R\({}^{2}\) = 0.8998) (Figure 4b).