## 3 Process Optimisation
### Pre-Treatment of High-Fat RMP Wastewater Substrates to Enhance Anaerobic Digestion  
There has been extensive investigation into various substrate pre-treatments to enhance substrate performance in anaerobic digestion. Pre-treatments can be categorised as physical, thermal, thermobaric, chemical, thermochemical and biochemical and have been explored in depth [6; 40; 41]. Within the literature, pre-treatment of RMP waste has been a relatively minor focus and warrants further investigation. Furthermore, enhancing digestion of FOG has been an interesting prospect academically due to the high specific methane potential (SMP) of fat of around 1000 mL CH\({}_{4}\cdot\)g VS\({}^{-1}\). [PERSON], [PERSON] and [PERSON] [42] investigated the impact of thermal, chemical and thermochemical pre-treatments and bile as a novel biochemical pre-treatment respectively. Each pre-treatment demonstrated only a minor improvement to biomethane potential. However, the greatest impact of treatment was on the degradation rate of the substrates. Each of the thermal (Figure 5a), chemical (Figure 5b) and thermochemical (Figure 5c) pre-treatments impacted the rate of degradation and methane yields. The greatest improvement was observed following thermal pre-treatment in which hydrolysis was enhanced to the point of entirely eliminating the lag phase observed in the control and other trials (Figure 5a). Evidence of enhanced hydrolysis was also observed in the thermochemical pre-treatment, but the degree of impact was reduced in comparison to the thermal pre-treatment and appeared to be due to the added chemical component. Indeed, this trend was not evident in the chemical treatment, which itself had minimal impact but did slightly improve the rate of substrate degradation. [PERSON], [PERSON] and [PERSON] [43] investigated the effect of a novel bio-surfactant, bovine bile, on treating FOG-rich DAF sludge using anaerobic digestion. Two outcomes were anticipated based on the in vivo function of bile: (1) bile would act as a bio-surfactant and promote dissolution of fat particles  
Figure 5: Specific methane potentials and production rates of: (**a**) thermally treated dissolved air flotation (DAF) sludge from a red meat processor [42]; (**b**) chemically treated DAF sludge [42]; (**c**) thermos-chemically treated DAF sludge [42]. Error bars represent one standard deviation from the mean. and thereby enhance the rate of degradation and; (2) bile would act to inhibit bacteria. To answer these questions and better assess the BMP data, curve fitting was conducted using two equations in order to extract information (Equations (1) and (2)). While a low dose of 200-600 mg bile-L\({}^{-1}\) induced a small increase in methane yield, there was neither evidence for enhanced FOG dissolution nor improved FOG degradation rate (Table 2). However, as dose increased beyond 2 g bile-L\({}^{-1}\) the inhibitory effects were observed and increased in magnitude with respect to increasing dose (Figure 6; Table 2). Equation (1): Growth curve logistic equation. \[B=\frac{B_{0}}{1+e^{-k(t-t_{0})}} \tag{1}\]  
From Equation (1): \(B\) is the cumulative specific methane potential (SMP; mL CH\({}_{4}\cdot\)g VS\({}^{-1}\)) at time \(t\) (days); \(B_{0}\) is the maximum SMP achieved by end of digestion; \(k\) is the rate constant; \(t_{0}\) is the time