## 2 Materials and Methods
### Experimental Design  
The batch-wise anaerobic digestion of the substrate mixtures was undertaken in a 500 mL bioreactor (conical flask, Schott Duran, Germany) such that a working volume of 400 mL was maintained. Biomethane potential was measured using a eudiometer with each bioreactor subjected to a constant mesophilic temperature of 37.0 \(\pm\) 0.1 \({}^{\circ}\)C using a general purpose stirred thermostatic water bath (GD100, Cambridgeshire, UK). Substrate mixtures characterized by mass ratios of the volatile solids of WHDS (VS\({}_{\rm{WHDS}}\)) to the volatile solids of SY (VS\({}_{\rm{SY}}\)) of 1:0, 4:1, 3:2, 2:3, 1:4 and 0:1 were degraded anaerobically to determine which VS mass ratio will best promote enhanced \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Inoculum** & **WHDS** & **SY** \\ \hline Total solids (\%w/w, wet feedstock) & 2.33 (0.07) & 7.35 (0.65) & 16.73 (0.01) \\ Volatile solids (\%w/w TS 1) & 41.89 (1.64) & 24.86 (0.42) & 42.45 (0.94) \\ Fixed carbon (\%w/w TS) & 23.21 (1.94) & 49.57 (0.64) & 23.68 (1.16) \\ VS 2 to TS mass ratio & 0.42 (0.02) & 0.25 (0.03) & 0.42 (0.02) \\ Ash (\%w/w TS) & 34.90 (0.07) & 25.57 (0.48) & 33.87 (0.68) \\ Carbon content (\%w/w TS) & 29.66 (0.14) & 42.88 (0.13) & 33.37 (0.29) \\ Nitrogen content (\%w/w TS) & 3.61 (0.05) & 2.49 (0.07) & 2.40 (0.02) \\ Sulphur content (\%w/w TS) & 0.795 (0.03) & 2.40 (0.06) & 0.30 (0.00) \\ Hydrogen content (\%w/w TS) & 4.87 (0.07) & 5.91 (0.07) & 4.73 (0.02) \\ Oxygen content (\%w/w TS) & 26.18 (0.16) & 20.76 (0.51) & 25.33 (0.68) \\ Carbon to nitrogen mass ratio & 8.23 (0.11) & 17.22 (0.48) & 13.90 (0.167) \\ pH value & 7.46 (0.01) & 8.09 (0.00) & 7.56 (0.00) \\ Specific gravity at 20 \({}^{\circ}\)C & 0.97 (0.01) & 0.96 (0.01) & 1.07 (0.05) \\ Empirical formula (ash free db 3) & C\({}_{100}\)H\({}_{196}\)N\({}_{10}\)C\({}_{66}\)S & C\({}_{48}\)H\({}_{79}\)N\({}_{2}\)O\({}_{17}\)S & C\({}_{297}\)H\({}_{504}\)N\({}_{18}\)O\({}_{169}\)S \\ \hline \hline \end{tabular}  
* TS represents total solids, 2 VS represents volatile solids, standard errors are shown in parentheses based on duplicate measurements and 3 db represents dry weight basis.  
\end{table} 
Table 1: Characterization of the WHDS (wet hydrolyzed DAF sludge), SY (stockyard) waste and inoculum. biomethane generation. The substrate-inoculum mixture was prepared such that a volatile solid mass ratio of inoculum to a substrate (ISR) of 2 was maintained. This ISR ratio will eliminate possible biomass limiting kinetics in the system via the prevention of excessive acidification of the substrate media [27; 28]. The mass characteristics of the co-digestion mixtures are presented in Table 2.  
The buffer system provided in each bioreactor was a solution composed of 350 mg/L of anhydrous di-potassium hydrogen orthophosphate (K\({}_{2}\)HPO\({}_{4}\)), 250 mg/L of potassium dihydrogen orthophosphate (KH\({}_{2}\)PO\({}_{4}\)) and 5000 mg/L of sodium hydrogen carbonate (NaHCO\({}_{3}\)) [29]. No external nutrient sources (trace metals) were introduced to the bioreactor mix, since nutrient requirements for microbes can be sufficiently provided internally in substrates mixtures composed of organic fractions of animal excreta, municipal or sewage solid wastes [30; 31]. Prior to initializing the experiment, each bioreactor was purged with nitrogen gas to displace atmospheric oxygen so that anaerobic digestion was encouraged. An illustration of the experimental setup discussed is presented in Figure 2. During the AD process, the volume of biomethane produced was measured using a eudiometer unit as shown in Figure 2.  
The AD biogas generated was purified for biomethane volume measurement by stripping it of (mainly CO\({}_{2}\)) impurities via bubbling of the biogas using a 3 N barrier solution of sodium hydroxide (NaOH) [32]. The volumetric displacement of the barrier solution of NaOH in a connecting reservoir was subsequently utilized in measuring the daily biomethane potential, for 20 days, according to ISO/DIS 14853 standard methods [33].