# Efficiency of Chemical Toilet Sewage (CTS) Co-Fermentation with Typical Energy Crops
## 3 Results  
### Biogas and CH\({}_{4}\) Production  
V1 (maize silage only) yielded 390 \(\pm\) 44 cm\({}^{3}\)/gVS biogas containing 59 \(\pm\) 3.1% CH\({}_{4}\). Higher average biogas yields were noted for V2 and V3, where the dilution fluid contained 10% and 20% CTS, respectively, the former producing 411 \(\pm\) 37 cm\({}^{3}\)/gVS (57 \(\pm\) 2.6% CH\({}_{4}\)). Biogas production peaked in V3 at 425 \(\pm\) 39 cm\({}^{3}\)/g vs. (58 \(\pm\) 1.4% CH\({}_{4}\)). Differences across V1-V3 were not statistically significant (\(p\) = 0.05). V4 (30% CTS) and V5 (40% CTS) fared significantly worse in terms of gas output--384 \(\pm\) 37 cm\({}^{3}\)/gVS (55 \(\pm\) 2.7% CH\({}_{4}\)) and 361 \(\pm\) 48 cm\({}^{3}\)/gVS (47 \(\pm\) 3.9% CH\({}_{4}\)), respectively. The lower CH\({}_{4}\) fractions translated to significantly reduced nominal methane yield in V5 (170 \(\pm\) 23 cm\({}^{3}\)CH\({}_{4}\)/gVS) comparedwith V4 (211 \(\pm\) 20 cm\({}^{3}\)CH\({}_{4}\)/gVS). Marked incremental decreases in AD performance were observed across V6 (50% CTS) and V7 (60% CTS) at 211 \(\pm\) 46 cm\({}^{3}\)/gVS (40 \(\pm\) 4.1% CH\({}_{4}\)) and 190 \(\pm\) 39 cm\({}^{3}\)/gVS (39 \(\pm\) 3.4% CH\({}_{4}\)), respectively. Methane production varied from 1085 \(\pm\) 100 cm\({}^{3}\)CH\({}_{4}\)/day (V3) to just 326 \(\pm\) 66 cm\({}^{3}\)CH\({}_{4}\)/day (V7) across the variants. Full biogas and methane production data are given in Table 4. The worldwide literature has seen few investigations into the impact of chemical toilet waste on AD. [PERSON] et al. (2021) [29] tested whether biogas can be extracted from fecal sludge (FS) sourced from toilet complexes (ESTC) of passenger railcars. The FS was chemically treated (either through acidification or alkalization) and digested under thermophilic conditions. Acidification produced better results, with biogas yields of 3050 dm\({}^{3}\)/t containing 80% methane on average [29]. Other researchers have also found that CTS amendment improved anaerobic digestion. [PERSON] et al. (2017) [30] treated toilet sewage in a pilot-scale CSTR reactor. As the COD loads in the feedstock were raised from 0% to 35%, methane production similarly rose from 222 to 332 dm\({}^{3}\)CH\({}_{4}\) kg/COD\({}_{\rm{rem}}\). The methane fraction in this biogas was approx. 60% [30]. [PERSON] (2008) [31] anaerobically digested toilet sewage with kitchen waste in a laboratory-scale CSTR at a ratio of 40 g kitchen refuse to 1 L of toilet wastewater. The methane fraction was similar across all variants at around 65 \(\pm\) 2.0%, which translated to nominal yields of 172 to 255 dm\({}^{3}\)CH\({}_{4}\)/kg COD\({}_{\rm{input}}\). Similarly, [PERSON] (2021) [32] posited that cistern-flush and pour-flush toilets, non-sewer toilets, and chemical toilets can be used for biogas production. The author believes that, although the quantity and/or seasonality of such waste preclude it from being used as the sole feedstock, it can feasibly be used in mixtures with other feedstocks [32]. The results indicate a strong correlation between biogas/CH\({}_{4}\) production trends and the CTS dose in the feedstock. This linear correlation progresses in a down-curve pattern (Figure 1). The increase in CTS dose (0% to 20%) in variants V1-V2 was found to very strongly and positively correlate with biogas production (R\({}^{2}\) = 0.9868), as well as CH\({}_{4}\) production (R\({}^{2}\) = 0.9255) (Figure 1). This trend was reversed in variants V3 (20% CTS) to V7 (60% CTS), where the increasing of CTS dose was found to very strongly and negatively correlate with biogas production (R\({}^{2}\) = 0.8910) and specific CH\({}_{4}\) production (R\({}^{2}\) = 0.9286) (Figure 1).