Secondary treatment at the 80-mgd Fresno-Clovis Regional Wastewater Reclamation Facilities (RWRF) has always been a challenge because of difficulties with achieving good sludge settleability. Sludge volume index (SVI) measurements between 300 mL/g and 600 mL/g are often encountered,
mainly because of a combination of viscous and filamentous bulking. A number of factors are thought to be responsible, including the high industrial content of the wastewater, long sewer residence times, and even possible nutrient deficiency. While long-term solutions were being sought for
the plant, an innovative operational approach was implemented to help limit solids washout in the clarifiers and avoid discharge violations. The aeration basins were operated in a semi-settling mode, with the air being turned off in the last three zones of each basin two hours at a time. This
was done on a timed cycle so that at any given time, half of the aeration basins were operating in the normal mode, while the other half were operating in the semi-settling mode. This approach allowed for a substantial reduction in the solids loading to the clarifiers without adversely impacting
the nitrification process. In addition, some denitrification was also accomplished, thereby alleviating problems with denitrification and rising sludge in the secondary clarifiers. This operational approach offers treatment plants with poorly settling sludge operational relief while long-term
solutions are being sought.
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