## 3 Process Optimisation
### Effect of Temperature and Mixing  
High-fat substrates are typically problematic in anaerobic digestion systems. However, there are a number of opportunities available to enhance FOG digestion with each adaptation representing an incremental step toward becoming a high-rate system. Importantly, not all high-rate systems are suitable for digestion of FOG-rich substrates. In the treatment of high-fat substrates, mixing has two functions. Firstly, it prevents the formation of a fatty crust, which, once formed is difficult to disperse. Secondly, mixing improves contact between the microbial consortium and exo-enzymes with the soluble and insoluble organics to promote degradation. [PERSON], [PERSON] and [PERSON] [11] investigated the effect of stirring in lab-scale reactors treating RMP wastewater. The reactors were operated at 26 \({}^{\circ}\)C to mimic the minimum effluent temperature recorded at the biogas facility during winter. The effects of mixing can be summarised as follows:  
1. Formation of crust/foaming layer development: Crust formation was only observed in reactors with minimal stirring (1 h-day\({}^{-1}\) before and after feeding to prevent washout of active biomass). 2. Process stability: Volatile fatty acids (VFA) accumulated faster in the continuously stirred reactors, indicating a greater rate of hydrolytic degradation of organics, albeit imbalanced with methanogen performance. However, due to the low reactor temperature, this caused the system to fail sooner than the unstirred reactor. The unstirred reactor failed 10 days after the stirred reactor. 3. Biogas quantity and quality: Biogas production was greater in the stirred reactor prior to reactor failure. Methane content was higher in the stirred reactor, and hydrogen sulphide concentration was much lower. CALs are typically operated under ambient conditions--absent of active heating and stirring. In Australia these systems rely on the insulating properties of the lined earth to resist external temperature changes, with heating supplied by incoming wastewater. While this is cost effective, the average temperature of the CAL can be far from optimal. [PERSON], [PERSON] and [PERSON] [11] investigated the effects of digestion temperature in stirred reactors from 26 \({}^{\circ}\)C to the mesophilic optimum of 40 \({}^{\circ}\)C. The effects of temperature can be summarised as follows:  
1. Formation of crust/foaming layer development: As beef grease and tallow has a melting point of 36-42 \({}^{\circ}\)C, an operational temperature of 40 \({}^{\circ}\)C is effective at melting fat to promote hydrolytic degradation. Crust formation was not observed in stirred, heated reactors. 2. Process stability: Heated, stirred reactors maintained greater buffering capacity than the unstirred and unheated reactors and did not experience any process instability as indicated by a consistent pH and VFA:total alkalinity ratio. 3. Biogas quantity and quality: Biogas production continually increased throughout the experiment beyond 70 days in the reactors that received heating and stirring, while the unheated and unstirred reactors failed after 50 and 60 days, respectively. [PERSON] et al. [27] further investigated the effects of site-specific seasonal temperature variation on digester performance. Six reactors were operated at 38 \({}^{\circ}\)C and half were reduced to 25 \({}^{\circ}\)C over a period of 90 days. During this time, the organic loading rate was maintained at 0.31 g COD-L\({}^{-1}\).d\({}^{-1}\). Following temperature reduction, the organic loading rate was progressively increased in a step-wise manner to determine process limitations at the minimum and maximum temperatures (Figure 3a). The effect of temperature reduction on degradation efficiency of FOG was apparent after 25 days, by which time the temperature had been decreased to approximately 35 \({}^{\circ}\)C and coincides closely with the minimum melting temperature of beef tallow.