## 1 Introduction  
Previous investigations have proposed the feasibility of utilizing readily available meat processing waste streams as sustainable biorefinery feedstocks for biochemical and biofuel production [1]. Recent studies have demonstrated that meat processing dissolved air flotation (DAF) sludge can serve as a cheap and sustainable biodiesel feedstock via an integrated two-step in-situ hydrolysis and esterification pathway [2]. Crucially however, the utilization of meat processing DAF sludge as a biodiesel feedstock will result in the generation of significant masses of wet hydrolyzed DAF sludge (WHDS) after the biodiesel production process [3,4]. This WHDS residue was identified as constituting a secondary waste stream that must be efficiently utilized to prevent possible secondary pollution issues. Recognizing that the WHDS residue contains masses of useful degradable organics, the anaerobic digestion of the WHDS residue has been proposed as a viable pathway for the efficient low-cost utilization of this residue. This is because anaerobic digestion (AD) conversion processes are recognized as highly effective pathways for cheap biomethane generation via an oxygen-free microbial degradation of organics [5]. The high moisture content of WHDS, however, suggests that the technical feasibility of utilizing WHDS as a sustainable AD feedstock may be severely inhibited, since reduced concentrations of degradable substrates will lead to uncompetitive biomethane potentials [6,7]. A review of literature has suggested that the application of the anaerobic co-digestion (ACD) technological pathway may constitute a viable approach that will efficiently circumvent concerns associated with poor biomethane generation arising from low substrate concentrations [6, 7, 8]. This is because apart from enabling an increase in the mass of biodegradable material available for degradation, ACD may provide an opportunity for the introduction of favorable substrate synergizing effects via the dilution of any unwanted residual compounds that may influence nutrient availability to anaerobic microbes [6,7]. One possible unwanted chemical that may be retained in trace quantities within the WHDS substrate is non-polar hexane that is employed during the recovery of the fatty acid products from the lab-scale in-situ hydrolysis process [3]. There is, however, also a risk of introducing unfavorable antagonizing effects in a co-digestion mixture. In the case of such antagonizing effects, the potential benefits of increased substrate concentration available for biodegradation are outweighed by the negative effect of possible toxicity of one of the substrates [9]. Evidently, the abundance of several ACD studies in the literatures reinforces the current research interest in the co-digestion approach. Examples of such studies include the researches by [PERSON] et al. [9], [PERSON] et al. [10] and [PERSON] et al. [11]. [PERSON] et al. [9] investigated the ACD of chicken processing waste and Miscanthus and ACD of chicken manure and sea grass for different mix ratios by mass of 1:1, 1:3 and 3:1. They showed that the ACD of chicken processing waste and Miscanthus presented antagonizing effects, manifested as reduced biomethane potentials relative to the predicted biomethane potential when the substrates were degraded in the absence of antagonizing effects. Additionally, they showed that Synergizing effects were introduced when the ACD of chicken manure and sea grass was undertaken. These synergizing effects were manifested by enhanced biomethane potentials in all co-digestion substrate mixtures investigated relative to the predicted biomethane potentials when the substrates were degraded in the absence of synergizing effects. The study undertaken by [PERSON] et al. [10] assessed biomethane potentials from the ACD of aloe peel waste and dairy manure for different substrate mix ratios by mass of 1:0, 3:1, 1:1, 1:3 and 0:1.