Co-thickening of primary and waste activated biosolids (WAS) using dissolved air flotation (DAF) thickeners was chosen for the biosolids processing facilities (BPF) for the City of San Diego Clean Water Program. Selection of the process was based on an evaluation that compared separate to combined thickening of primary biosolids and WAS. The northern and southern BPF will receive an estimated 239 dry metric tons/day (266 dry tons/day) of raw biosolids. Data from other plants were collected to confirm operating performance as well as design criteria. It appears that primary solids assist the WAS in floating probably because of their larger particle size and material strength. A parametric analysis identified several key factors that influenced selection including biosolids characteristics, influent peaking factors, solids loading rate, energy and polymer usage, land and labor costs. The results of this evaluation showed that co-thickening with DAF thickeners had the lowest life cycle costs (LCC) when solids loading rates were relatively high. Co-thickening also had the lowest LCC for all polymer dosage rates evaluated. Separate thickening had the lowest LCC when additional costs were added for larger primary sedimentation tanks and digesters. This evaluation will be beneficial to engineers evaluation thickening methods by identifying the key parameters that determine the cost of constructing and operating thickening facilities.