In this thesis, the anaerobic co-digestion of thickened waste activated sludge (TWAS) and, fat, oil and grease (FOG) was investigated as a method for TWAS:FOG treatment, stabilization, reduction and conversion to bio-methane gas as a valuable source of renewable energy. In the first phase, thermophilic and hyper-thermophilic anaerobic co-digestion of TWAS and FOG were investigated and compared. 20 – 80%FOG (based on total volatile solids) were tested using two sets of biochemical methane potential assays (BMP). Hyper-thermophilic co-digestion of TWAS with up to 60%FOG was shown to significantly increase the methane production and VS reduction as compared to the thermophilic co-digestion of the same TWAS:FOG mixture and as compared to the control (TWAS thermophilic mono-digestion). Both linear and non-linear regression models were used to represent the co-digestion results. In the second phase, the feasibility of the thermophilic and hyper-thermophilic co-digestion of TWAS and FOG were more investigated using lab scale semi-continuous reactors. The dual stage hyper-thermophilic reactor was introduced for the first time in this work for co-digesting TWAS and FOG. The dual stage co-digestion reactor was shown to significantly outperform the single-stage thermophilic mono-digestion reactor (the control) and the single-stage thermophilic co-digestion reactor at all three hydraulic retention times (HRTs) considered in the study namely, 15, 12 and 9 days. The dual-stage hyper-thermophilic co-digester digested up to 70%FOG at 15 days HRT without any stressing signs and produced a methane yield that was 148.2% higher compared to the control methane yield at the same HRT. It also produced a class A effluent at all three tested HRTs and positive net energy for 15 and 12 days HRT. The effects of microwave (MW) pretreatment, and combined alkaline-MW pretreatment on the co-digestion of TWAS:FOG mixtures with 20, 40 and 60% FOG were investigated in the third phase of this study. MW pretreatment at a high temperature of 175ᵒC was shown to be the most effective MW pretreatment option in solubilizing TWAS:FOG mixtures and in boosting the methane yield. It resulted in maximum solubilization for the 20%FOG samples and maximum methane yield for samples with 60%FOG. The combined alkaline-MW (NaOH-MW) pretreatment at a pH 10 showed to be an ineffective option for TWAS:FOG pretreatment before the anaerobic co-digestion process. In the fourth phase, the effects of the three selected pretreatments on the solubilization of TWAS and 20%FOG mixture on the molecular scale were investigated. The pretreatments used included: (i) MW pretreatment at 175ᵒC (since this was the best MW pretreatment condition according to the results of phase 3), (ii) hyper-thermophilic stage @ 70ᵒC and 2days HRT (effectively used in phases 1 and 2), and (iii) conventional heat at 70ᵒC. The analysis involved separation of the solubilized substrates after pretreatment using ultrafiltration (UF) at four different sizes (1, 10, 100 and 300 kDa). The results showed that each pretreatment method uniquely changed the particle size distribution. These changes showed to affect the biodegradability of substrates with different class size. Finally, two brief studies were performed using BMP tests to investigate the feasibility of FOG addition as a biogas booster in TWAS anaerobic digestion. First, the effect of FOG addition on TWAS and organic fraction of municipal solid waste (OFMSW) co-digestion was tested using hyper-thermophilic BMP tests. The addition of 30% FOG (based on total volatile solids) was shown very effective in improving the methane yield. The 30% FOG addition to TWAS:OFMSW mixture resulted in 59.9 and 84.4% higher methane yield compared to the methane yields of TWAS:OFMSW and TWAS samples, respectively. Second, the feasibility of using the soluble part of FOG (L-FOG) as a co-digestion substrate to increase the biogas production from the thermophilic digestion of TWAS was investigated. The results showed that co-digestion of TWAS and 20 to 80% (based on total VS) of L-FOG using a substrate to inoculum ratio (S/I) of 1 improved the biogas yield by 13.5 to 83.0%, respectively. No inhibition was reported at high L-FOG %.