## 2 Wastewater Treatment and Process Limitations
As fatty and buoyant organics separate to the surface of the lagoon they become isolated from the liquid phase and are not removed in the digester effluent stream [10]. As samples are typically collected from the digester outlet, subsequent analyses on the effluent wastewater can give false indications of degradation efficiency [29]. However, in comparison with theoretical or laboratory-determined BMP results, an early indicator may be a low in situ gas yield due to the physical limitation of microbes and exoenzymes to access the organic mass that is effectively isolated in the crust. Essentially, the surface-area-to-volume ratio of the crust decreases as the crust accumulates, reducing the available area for enzyme-substrate interaction and resulting degradation [6]. 3. Damage to infrastructure: The accumulation of a fatty crust has three main implications for infrastructure and maintenance including pipe blockages and clogging of gas collection and handling systems [33; 34], reduced cover integrity [10; 19; 35; 36] and the difficulty and cost of crust removal [25]. (1) Pipe blockages are commonly reported and can result in accumulation of hazardous waste upstream at the RMP facility, or accumulation within the digester. (2) Covers are typically made from high-density polyethylene which over time reacts when in contact with fat to weaken the cover integrity, allowing for gas permeation, risk of tears and risk to workers who may walk on the cover [19]. Raft style covers rely on a skeleton of pipework which can easily become bent, warped and generally compromised by accumulated fatty crust [10]. Removal of covers to remove fatty crusts often results in the destruction of the cover. Furthermore, removal of crust has been described as difficult, time consuming and expensive [25]. 4. Reduced gas yield: Fatty crusts also limit the potential to recover biogas produced from a waste stream. While gas emerges through thin crust layers [26] as preferential pathways for gas release, thick crusts restrict the pathways by which gas can escape the liquid medium into the cover for capture [10]. In these cases, gas becomes compressed in the liquid phase and escapes at the next possible point which may evade capture and contribute to greenhouse gas emissions. Furthermore, fat that is broken down to form long-chain fatty acids may contribute to microbial inhibition and thereby limit the amount of methane gas production [37]. Concerted efforts should be made to reduce FOG entry into wastewater and to recover FOG from wastewater prior to entering a secondary waste treatment system. [PERSON], [PERSON] and [PERSON] [11] used data from sites reported by [PERSON] and [PERSON] [38] and [PERSON] et al. [18] to determine that the reported sites could recover 50k-1.12M AUD-year\({}^{-1}\) through increased tallow sales at 100% FOG recovery at a cost of $605-tonne\({}^{-1}\). At 2019 prices this would be worth 68k-1.53M AUD-year\({}^{-1}\). However, it is unlikely that such high degree of FOG recovery will be practical in the near future. Consequently, secondary treatment systems (i.e., anaerobic digesters) need to be capable of robust operation under suboptimal conditions.