Secondary treatment is a complex biological process that uses bacteria to convert dissolved organic matter into suspended matter that can be settled out as sludge. The remaining wastewater is then disinfected and discharged, while the solids are stabilized, dewatered and hauled away.
The first stage of the secondary treatment process is the aeration basins, where oxygen is introduced to the wastewater. This serves two purposes. First, it creates an oxygen-rich environment to sustain the microorganisms that feed on the suspended and dissolved organic matter in the wastewater. These microorganisms and the biodegradable matter they consume are together called activated sludge. Secondly, it supplies oxygen that the organic material uses as it decays. When treated wastewater is ultimately released into San Francisco Bay, it won’t demand oxygen from the Bay that fish and wildlife need to thrive.
From the aeration basins, the treated wastewater flows to the secondary clarifiers. The activated sludge settles out, where it is either sent back to the aeration basins to maintain the microorganism population, or thickened and sent to the digesters. The remaining clarified water, called secondary effluent, is sent to the final stage in the treatment process, which is disinfection.
Treated effluent from the secondary clarifiers flows into disinfection tanks, where sodium hypochlorite, commonly known as bleach, is added to kill pathogenic organisms. The disinfected water is then sent to the San Francisco Bay via the East Bay Dischargers Authority (EBDA).
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