Raw influent currently enter the plant on the north of the site, from the intersection of Mastick and Shepherds Hill Road. A 36-inch sanitary sewer is the main conveyance pipe into the plant. A second 18-inch sanitary sewer runs parallel to the 36-inch at a much steeper slope.
Flow is directed through both manholes and east to the Parshall Flume Structure. Flow combines downstream of the flumes, and the EQ pump station discharge is also discharged at this structure.
Influent continues to the Preliminary Treatment Facility (PTF) through a 54-inch pipe and combines with Return Activated Sludge (RAS) from the RAS Conditioning Tank. RAS is added to the raw influent (for odor control) prior to screening.
RAS Conditioning Tank
RAS removed at the Final Clarifiers is pumped from the RAS/WAS pump station to the RAS Conditioning Tank (RCT). The RAS conditioning tank has a total volume of 0.5 million gallons (MG) and is used to: 1. de-gas and remove nitrates from the return sludge, 2 lower the ORP (Oxidation Reduction Potential of the RAS, 3 de-oxygenate and “jump start” for biological phosphorus removal, 4. condition the RAS to operate as a selector once combined with the influent. 5 and provides the North Olmsted biological process with an OSA (oxic, settled, anoxic) process modification that reduces sludge yield. The tank has a vertical turbine mixer to keep solids suspended, and a series of weirs and gates to divert RAS to Aeration.
During normal flow conditions, two (2) 16-inch pipes from the RAS/WAS pump station return RAS at to a chamber adjacent to the RCT. Normally RAS overflows the weirs in the tank and is blended with plant influent. Blending the RAS with the raw influent prior to screening and grit removal provides another “level” of odor control at the headworks as the conditioned RAS absorbs volatile organic compounds. All the headworks air exchange is provided with odor control prior to exhaust to the atmosphere. Piping also allows RAS to be blended with flow at the Aeration Distribution Chamber.
During high flow conditions, the gates to the Influent channels are closed and RAS is discharged directly to the VLRs. Isolation gates are provided at the Aeration tanks to allow all or a selected tank to receive flow. If the conditioning tank needs to be taken out of service, the gate at the RCT chamber can be closed. A set of weirs allow flow to pass directly into the influent channel without filling and overflowing the RCT. Service can then commence on the conditioning tank.