Akron Water Pollution Control Station No Feasible Alternative

to 135% of this value. The final settling tank capacity is not constrained by the peak solids
loading rate of 35 lbs./day/ft. 2 due to the typical mixed liquor suspended solids concentration of
1,800-2,000 mg/l, and the 16% RAS pumping rate. Based on a maximum 1,000 gpd/sf surface
overflow rate, the FSTs should be limited to a process flow of 141 MGD.
“However, based on Water Environment Federation Manual of Practice No. 8
(MOP 8), it would be appropriate to limit the overflow rate to 800 gpd per square
foot for a 100 foot diameter tank with a ten foot side water depth (SWD). Ten of
the 18 final settling tanks at the Akron WPCS have ten foot SWD…If the twelve
foot deep final settling tanks were operated to the designed maximum SOR of
1,000 gpd/sf and the ten foot deep final settling tanks were operated to a
maximum SOR of 800 gpd/sf, a peak flow of 119 MGD may be processed (one
ten foot deep unit out-of-service). Although linear regression analysis of available
data suggests acceptable performance would occur near this rate, the number of
data points is inadequate to provide a high confidence level for this scenario. The
relatively shallow depth (10 foot sidewater depth) of ten of these tanks makes it
questionable to predict performance at higher SORs. Density current baffles
were recently installed to reduce upflow currents along the sidewalls. Historically,
a fairly persistent pin floc problem had caused concern for the effluent TSS
concentration at high SORs. Recent operational and physical improvements
have diminished the pin floc problem.” (Facilities Plan 98 6 )
3.3.2. Hydraulic Limitations
The 1980 Facilities Plan 1 recommended operating the Akron WPCS to provide complete
treatment of all flows up to a rate of 150 MGD. It is the historical understanding of the City that
the 150 MGD rate included RAS flow (110 MGD wastewater plus 40 MGD RAS). All flows to
210 MGD were to receive preliminary treatment, including screening and grit removal, and
disinfection. The Plan provided for using the storm retention basins to store the first 10 MG of
flow in excess of 150 MGD.
In 1996 the plant treated an average day flow of approximately 89 MGD with 80.8 MGD average
flow through the secondary treatment facility. Past experience and studies of the various
treatment facilities have resulted in the Akron WPCS being operated to limit flows through
primary and secondary treatment to approximately 150 MGD and 110 MGD, respectively. The
purpose of this hydraulic evaluation (originally presented in the Facilities Plan 98 6 ), and in
conjunction with the 1993 and 1997 stress tests and the process performance evaluation, was
to study the hydraulic feasibility of processing higher flow rates through various treatment
facilities.
The following three general operating requirements specific to plant hydraulics are considered
respective of Ohio EPA and the Recommended Standards for Wastewater Facilities 2 (Ten
States Standards).
• all treatment facilities shall be protected from flooding caused by the 100-year flood
recurrence level in the receiving stream (Cuyahoga River);
• treatment facilities shall function uninterrupted and provide adequate treatment for permit
compliance when faced with the 25-year flood level in the receiving stream (Cuyahoga
River); and
• treatment facilities shall function adequately and provide treatment to a level meeting permit
requirements with the largest unit of any treatment process out-of-service.
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