There has been an increase in research efforts aimed at improving the energy efficiency of wastewater treatment processes at large centralized wastewater treatment plants (WWTPs). Concerns over global warming impacts, energy sustainability and biosolids generation are among several key drivers toward the establishment of energy-efficient WWTPs. A two-stage process, the so-called AB process, has been suggested for the recovery of caloric energy content from sewage organic. The first stage is primary treatment in an extremely high loaded biosorption stage (A-stage) also called the high-rate conventional activated sludge process, which is subsequently followed by secondary treatment in a low loaded biological stage (B-stage) to ensure the removal of dissolved organics and ammonia. High-Rate Contact Stabilization processes have gained attention as pre-concentration technologies because of their capability to remove particulate, colloidal and dissolved organic matter, and produce high amounts of sludge.