Slaughterhouses release a significant amount of wastewater containing varying concentration of organic matter, nutrients, and other pollutants that necessitate robust treatment solutions. In this paper, a multi-stage lab-scale plant that consists of a pre-treatment stage, two trains with either a Static Granular Bed Reactor (SGBR) and Expanded Granular Sludge-bed Bioreactor (EGSB) coupled individually to membrane bioreactors (MBRs), were used. This configuration was designed to systematically evaluate and compare the efficacy of each anaerobic bioreactor type within an integrated treatment train. The lab-scale plant was operated for 77 days, a duration selected to ensure process stability and collect sufficient data for statistical analysis, and quality performance parameters were investigated using capability indices (Cp, Cpk, Pp, and Ppk). The secondary experimental data was analysed through QI Macros (SPC software for Microsoft Excel) to provide a rigorous, statistical evaluation of process control and performance. The lab-scale plant designed with the three stages, i.e. bio-physical pre-treatment stage – SGBR or EGSB units – MBRs, indicated a significant potential and capability of the overall process to treat PSW, whereby treatment stages had Pp = Ppk = 1.00 with 99.73% of each treatment stage outputs being within specifications. This high degree of statistical capability demonstrates exceptional process control and a low probability of producing non-conforming effluent. Furthermore, all stages performed with Cpk equal to 0.99, 1.06, 0.83, 0.86, and 0.79, respectively. The key findings of the paper reveal the treatment efficiency, Operational Stability, and Statistical Performance; which is a focused interpretation of the Capability Indices (Cp, Cpk) for each reactor, explicitly stating which system showed better performance.