The City of San Diego recently completed the first three years of operation of its new Metro Biosolids Center (MBC) raw solids thickening, anaerobic digestion and biosolids dewatering process. The MBC is designed to process all wastewater solids removed from the City of San Diego and surrounding participating agencies. The thickening process is comprised of raw solids receiving tanks, raw solids degritting, centrifuge thickening, thickened solids screening, and thickened sludge blending prior to anaerobic digestion. At the time these facilities were designed there was a limited amount of experience in centrifuge thickening of combined primary and secondary solids. Typical centrifuge thickening operations only thicken waste activated sludge. Primary sludge is thickened by gravity either in the primary settling basins themselves or gravity thickeners. Due to the centralized nature of the MBC, this typical mode of processing raw solids was not considered reasonable. Three different water reclamation plants were planned to send raw solids by force mains up to 16 miles long. To facilitate transport of the combined sludge, the average concentration was designed to be approximately 0.5% to 1%.