WEFTEC 2004 USE OF CENTRIFUGES FOR THICKENING PRIMARY AND SECONDARY SOLIDS: A SURVEY OF NORTH AMERICAN AND EUROPEAN WASTEWATER FACILITIES

Due to improvements in technology over the past decade, centrifuges are now commonly used for thickening primary and secondary solids. In comparison to other thickening technologies, such as gravity thickening, dissolved air flotation, and gravity belt filtration, centrifuges are often preferable because they require less space, are effective at containing odors, and can produce thickened sludge with a relatively high solids content (6 to 12% for centrifuges vs. 5 to 7% for other technologies). Increased solids content reduces the volume of thickened sludge by about 20 to 70%, which results in corresponding reductions in costs of downstream stabilization processes such as anaerobic digesters. This paper will present the results of a survey of facilities throughout North America and Europe that are using centrifuges to thicken primary and secondary solids. Detailed operational data will be provided, including flow and loading, type of solids, usage of flocculant aids (i.e., polymer), energy consumption and maintenance history.Most of the facilities surveyed thicken secondary solids or a blend of primary and secondary solids. Use of centrifuges for thickening primary solids alone is a relatively new application, so the number of operating facilities is significantly fewer. Special considerations for thickening primary solids include using inline grinders or screens upstream of the centrifuges to prevent large solids and rags from damaging the machines and removing grit from influent raw sewage or dilute primary solids that would otherwise cause premature wear in the machines. Some facilities have tanks upstream of the centrifuges that are used to equalize flow, blend primary and secondary solids, and maintain a consistent solids concentration in the feed.