## 4 Conclusions  
In the present study, the anaerobic co-digestion (ACD) of wet hydrolyzed DAF sludge (WHDS) and stock yard (SY) waste was investigated. It was demonstrated that the anaerobic co-digestion of WHDS and SY waste always resulted in enhanced biomethane potentials compared to biomethane potentials obtainable from the anaerobic digestion of the individual WHDS and SY substrates. This overall improved biomethane yield associated with the co-digestion systems was attributed to the presence of substrate synergizing effects. It was also shown that the WHDS to SY waste mix ratios of 1:4 on a volatile solids mass basis enhanced biomethane production with a CPI of 1.78. An increase in the solubilization rate of the macromolecules present in the substrate was insufficient to explain the synergistic effects, since the high solubilization rate constant did not necessarily translate to an equally high biomethane yield. Other factors, such as the nutrients balance for enhanced biodiversity and microbial activity and enhanced dilution of inhibitory compounds, e.g., the residual toxic hexane in the substrates, were therefore suggested as worthy factors that positively influenced biomethane yields. Finally, the possibility resource recovery from digestate was also proposed via the coupling of the one-step HTL process with the ACD system. This study demonstrated that, based on the chemical composition of the digestate, satisfactory HTL useful product yields were feasible. The preliminary energetic assessments highlighted the dependence of environmental performance of the integrated process of HTL and ACD technologies on the moisture content of the digestate. It is therefore inferred that there might be a trade-off between the mass of high moisture buffer solution introduced for pH maintenance and the need for a favorable environmental performance. Improved heat recovery was also identified as an approach that would encourage favorable energetic performance of the proposed integrated system with opportunities of recovery of useful products of biochar and biocrude clearly elucidated.  
The following are available online at [[http://www.mdpi.com/2076-3417/8/11/2290/s1](http://www.mdpi.com/2076-3417/8/11/2290/s1)]([http://www.mdpi.com/2076-3417/8/11/2290/s1](http://www.mdpi.com/2076-3417/8/11/2290/s1)), Section S1, Table S1: Gas volumes obtained during the anaerobic co-digestion of the substrate mixtures, Section S1, Table S2: Variation in the pH of each substrate mixtures during the anaerobic co-digestion process, Section S1, Table S3: Variation in the pH of each substrate mixtures during the anaerobic co-digestion process, Section S1, Table S4: Variation in the pH of each substrate mixtures during the anaerobic co-digestion process, Section S1, Table S5: Variation in the pH of each substrate mixtures during the anaerobic co-digestion process, Section S1, Table S6: Variation in the pH of each substrate mixtures during the anaerobic co-digestion process, Section S1, Table S7: Variation in the pH of each  
S2.1: Calculations for carbon balance, Section S2.2: Explanation for the estimated losses in the mass of carbon in product streams.  
[PERSON] and [PERSON] conceptualized the study; [PERSON] designed the approaches used in this study, conducted experiments and drafted the manuscript, together with [PERSON] and [PERSON].  
This research received no external funding.  
[PERSON] gratefully acknowledges the financial support of the University of Otago via the Otago Doctoral scholarship. He is grateful to [PERSON] of the Green Island waste treatment plant, Otago for his help in providing access to a steady supply of inoculum. He also acknowledges the input of [PERSON] of the Department of Human Nutrition, University of Otago for providing excellent suggestions during preliminary experimental runs.  
The authors declare no conflict of interest.