The biological treatment of a synthetic slaughterhouse wastewater (SSWW) was studied using an anaerobic baffled reactor (ABR) and an aerobic activated sludge (AS) at a laboratory scale, with total organic carbon (TOC) loading rates of 0.03–1.01 g/(L.day), total nitrogen (TN) loading rates of 0.01–0.19 g/(L.day), and a flow rate of 2.93 to 11.70 mL/min in continuous mode. Results revealed that combined anaerobic-aerobic processes had higher efficiency to treat SSWW than a single process. Up to 96.36% TOC, 80.53% TN, and 99.38% 5-day carbonaceous biochemical oxygen demand (CBOD5) removal from an influent concentration of 1,008.85 mgTOC/L, 419.77 mgTN/L, and 640 mgCBOD5/L at the hydraulic retention time (HRT) of 6.24 days and a flow rate of 3.75 mL/min was achieved. The UV/H2O2 process was studied to treat a secondary effluent of SSWW with TOC loadings of 64.88–349.84 mg/L. Up to 75.22% TOC and 84.38% CBOD5 removal were obtained for an influent concentration of 64.88 mgTOC/L at the HRT of 3 h with H2O2 concentration of 900 mg/L. An optimum molar ratio dosage of 13.87 mgH2O2/mgTOCin was also obtained. Combined anaerobic-aerobic and UV/H2O2 processes enhanced the biodegradability of the TOC, TN, and CBOD5 present in the SSWW. Up to 99.98% TOC, 82.84% TN, and 99.69% CBOD5 overall removals were obtained for an influent concentration of 1,004.88 mgTOC/L, 200.03 mgTN/L, and 640 mgCBOD5/L at the HRT of 4 days and a flow rate of 5.90 mL/min. A cost-effectiveness analysis (CEA) was performed for the optimum conditions for the SSWW treatment by optimizing total electricity cost and HRT, in which the combined anaerobic-aerobic and UV/H2O2 processes had an optimal TOC removal of 92.46% at an HRT of 41 h, a cost of $1.25/kg of TOC removed, and $11.60/m3 of treated SSWW. This process reaches a maximum TOC removal of 99% in 76.5 h with an estimated cost of $2.19/kg TOC removed and $21.65/m3 treated SSWW.