Overcoming Obstacles with a Difficult to Handle Sludge - Centrifuge Piloting at Miami’s Central District WWTP

A five-month centrifuge thickening, digestion and centrifuge dewatering pilot study was conducted at Miami-Dade’s Central District Wastewater Treatment Plant (CDWWTP). The pilot study was set up to simulate future thickening, digester and dewatering operating conditions to establish thickening and dewatering performance criteria. CDWWTP currently processes the sludge produced at both CDWWTP and the North District Wastewater Treatment Plant (NDWWTP) which is transferred via a 16 mile pipeline. During pilot thickening operation, which started in May 2016, three different types of sludges were tested in the centrifuge thickener; CDWWTP’s sludge alone, NDWWTP sludge alone and a blend of both CDWWTP and NDWWTP sludges. Although the CDWWTP WAS only sludge was easily able to thicken in the pilot centrifuge, using 1.5 to 2 gram per dry kilogram (g/kg) or 3 to 4 pound per dry ton (lb/DT) of dry polymer, the NDWWTP Primary Sludge and WAS, which was more dilute, was difficult to handle. The testing showed that stable operation for thickening the NDWWTP sludge could only be achieved when blended with the CDWWTP WAS in a blend tank which allowed stable operation of thickening using 0.75 to 1.5 g/kg (1.5 to 3 lb/DT) of dry polymer. The centrifuge thickened waste activated sludge (WAS) was fed to Plant 2, Cluster 1, Digester 3 to simulate high rate anaerobic digestion. For a period from mid-June to mid-August, the centrifuge was operated 24 hours per day, 7 days a week. The purpose of the continuous operation was to increase the solid content inside Plant 2, Cluster 1, Digester 3 to best simulate future digestion conditions. The dewatering piloting was conducted using the sludge from Plant 2, Cluster 1, Digester 3 that was isolated and fed mechanically thickened sludge. The testing showed that >24% TS cake could be achieved with 12.5 g/kg (25 lb/DT) active dry polymer and a ferric sulfate dose equal to 1.9 gal ferric sulfate per 1,000 gal of sludge. Testing conducted without the use of ferric sulfate conditioning showed that dewatering performance was reduced by 2 to 4% point in cake solids and that solids recovery percentages were lower. Dryer cake at 26-28% TS could be produced using emulsion polymer but required almost double the active polymer dosages than used for the dry polymer.