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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 the 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%.

When the thickening and digestion phase of the MBC startup took place, only one plant was sending its solids to MBC through a 4.25 mile long force main. The long pipeline detention time, as well as the variable nature of the primary sludge, presented several challenges that had to be addressed in the startup and operation of the centrifuge thickening system. With the assistance of Alfa Laval's engineering and technical staff as well as diligent efforts of the construction manager startup staff and MBC operation's staff, the centrifuge thickening process is now producing approximately 55,000 lbs/day of 4.5 to 5% thickened solids.

The results of the past two year's startup and operation experience are contained in this paper. This paper is a follow up to one we presented resented at the 14th Annual Residuals and Biosolids Management Conference entitled “Startup Experience of the City of San Diego Metro Biosolids Center Dewatering Process”, Fonda et al, March 2000.
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Document Type: Research Article

Publication date: 01 January 2002

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