## 1 Introduction
They demonstrated the presence of synergizing effects, manifested as higher biomethane potentials recorded when the substrates were co-digested, relative to the predicted biomethane potential when the substrates were digested independently of the synergizing effects. They also showed that the ACD of aloe peel waste and dairy manure in a substrate mix ratio by mass of 3:1 resulted in the highest biomethane potential. [PERSON] et al. [11] investigated the ACD of substrate mixtures composed of vinasse and press mud substrates in the mix ratios of 0:1, 1:0, 1:3, 3:1 and 1:1 on a chemical oxygen demand (COD) mass basis. In the work of [PERSON] et al., it was demonstrated that the ACD of substrate mixtures composed of vinasse and press mud substrates resulted in the introduction of synergizing effects which were also expressed as enhanced biomethane potentials. They showed that the ACD of vinasse and press mud substrates in the substrate mass mix ratio of 3:1 resulted in the highest biomethane potential. The present study, therefore, seeks to experimentally investigate the preferred substrate mix ratio for the co-digestion of WHDS residue and stockyard (SY) waste. The influence of the carbon to nitrogen (C/N) ratio of each substrate mixture on the biomethane potential is also investigated, as previous studies have demonstrated that the preferred C/N ratio for an improved biomethane potential are largely substrate specific and may range from 9 [12] to 30 [13] for different AD processes. The stockyard waste employed as a co-digestion substrate is obtained from animal holding pens and is composed of mainly livestock fecal matter, with over 15 megatons of fecal matter estimated to be generated per year in New Zealand [14]. The SY waste also contains residual animal food due to spills from feeding troughs, water due to spills from watering troughs, water from periodic 'wash-down' operations and liquid animal secretions (urine). Therefore, the SY waste stream was selected as a viable co-digestion substrate, as it provides a sustainable supply of useful biomass in New Zealand due to its obvious abundance ([PERSON], Group Environmental Manager, Alliance Group Limited, New Zealand personal communication, 15 September 2016). This investigation also recognizes that while the ACD of WHDS and SY waste may lead to improvements in specific biomethane productivity, the associated increase in substrate concentrations will lead to an upsurge in the volume of wet residual digestate generated. This upsurge in the generation of the wet residual digestate will exacerbate existing challenges associated with the storage, handling and management of digestate [15]. The handling of the wet residual digestate is usually achieved via complex processing steps that emphasize digestate treatment prior to its disposal to the environment rather than value extraction [16]. The digestate processing steps applied serve to minimize associated health risks due to the exposure of livestock and humans to zoonotic agents that may be retained in the digestate [17]. These zoonotic agents are responsible for infections in animals such as foot-and-mouth disease and infections in humans such as influenza and salmonellosis [17]. The need for extensive digestate processing is also reinforced by [PERSON] et al. [18]. They stated that, in addition to the presence of residual pathogens, digestate could also contain heavy metals and antibiotic residues. This explains the varying effects (positive or negative) of the application of organic fertilizers on the soil microbial community. Given that there is a possibility of escalating digestate handling costs due to the increased mass of digestate residue obtainable from the application of the ACD approach, an alternative one-step hydrothermal liquefaction (HTL) digestate processing operation is proposed as a viable intensification technology that will simplify existing digestate processing methods.