## 2 Wastewater Treatment and Process Limitations  
Anaerobic digestion is the technology of choice to treat and recover biogas from the large quantities of Australian RMP wastewater that remain following solids separation. Although efforts are made to remove solids, the macromolecular composition of RMP wastewater remains largely proteins and fats [15; 16]. While proteins have a modest biochemical methane potential of 480 m\({}^{3}\) CH\({}_{4}\) per tonne of volatile solids (VS), they are readily degradable and provide a source of nitrogen to the system. By comparison, fats have the largest biochemical methane potential of 1001 m\({}^{3}\) CH\({}_{4}\)-tVS\({}^{-1}\)[17] and provide a large amount of carbon. Consequently, the methane potential of RMP wastewater is around 700 m\({}^{3}\) CH\({}_{4}\)-tVS\({}^{-1}\) or 4-5 m\({}^{3}\) CH\({}_{4}\)-m\({}^{-3}\) of wastewater [16; 18]. This, among other reasons, has prompted red meat processors to retrofit covers to existing lagoons and build new covered anaerobic lagoons (CALs) to tap into this potential [19; 20]. Covered anaerobic lagoons are the preferred technology to treat wastewater in rural Australian RMP facilities, where land is available, prior to discharge to adjacent agricultural land [9; 20; 21; 22; 23; 24; 25]. CALs are a cost effective option and relatively simple to operate [13] and carry further benefits with relatively small sludge generation in comparison with aerobic systems [26]. However, there are process limitations due to the absence of mixing and heating in many CAL designs. Consequently, suboptimal temperatures restrict the metabolic rates of digester microbes; passive mixing and resulting sedimentation limits contact between microbes and waste organics and; trace element (TE) limitation can down-regulate or eliminate essential biochemical pathways [27; 28]. High-fat substrates in particular introduce a host of additional problems including:  
1. Crust formation and loss of effective volume: Fatty and buoyant substrates are less dense than water and separate out to form a fatty crust on the surface of lagoon systems. As the crust mass accumulates it occupies greater volume within the lagoon, limiting the space for the digester liquor to accommodate and effectively reducing the effective volume of the lagoon (Figure 2) [6; 10; 20]. This simplified conceptual diagram (i.e., Figure 2) assumes a coupled hydraulic and solids retention time and a static flow rate. As dead space increases it is expected that a reduction in treatment time and consequently a reduction in treatment efficiency will result. This loss of treatment efficiency resulting from insufficient residence time is known as short-circuiting. As the lagoons are typically designed to accommodate a particular waste volume, there are two options: (1) divert wastewater flow to reduce the amount of wastewater going to the digester [29] or (2) overload the digester [29]. 2. Reduced treatment efficiency: If digester inflow is not reduced to accommodate for the reduction in effective volume resulting from crust accumulation, the digester will experience both organic and hydraulic overloading [11; 30]. The effect of hydraulic overload is short-circuiting in which wastewater passes from inlet to outlet without spending sufficient time in the digester to achieve sufficient organics degradation [31] and may result in the loss of active biomass (i.e., sludge washout) due to the higher flow rate resulting from forcing the same volume of wastewater through an ever-narrowing corridor (Figure 2). The effect of organic overloading is reduced treatment efficiency and potential toxicity to the microbial consortium [11; 32]. The impact of crust accumulation under the cover of a CAL can be less apparent and effluent characteristics can give a false impression of digester performance prior to signs of failure. As covers are typically opaque it is difficult to gauge crust formation.