WWTP Capacity Evaluation Completed

Recently the City of Ashland contracted with Carollo Engineers to perform a capacity evaluation of the wastewater treatment Plant (WWTP) as a guide to future capital planning.  The work included a review of three years of recent operating data and an evaluation of each major unit process in the plant to determine process capacity. Both the secondary process and the tertiary process were evaluated. The secondary system consists of the biological portion of the WWTP with the membrane filtration process as the tertiary system. My intent here is to give you a brief summery of the completed capacity evaluation study.



The current population of 20,868 adds a dry weather average flow of 2.1 million gallons per day (MGD) to the treatment plant.  This flow represents a load of 3, 275 pounds carbonaceous biochemical oxygen demand (CBOD) and 4,200 pounds total suspended solids (TSS).   The plant has been fully operational since 2002.


Secondary System


The following major unit processes in the secondary system were evaluated:


1)      Influent Pumps

2)      Influent Bar Screens

3)      Grit Removal

4)      Oxidation Ditches and Secondary Clarifiers

5)      RAS/WAS Pumps

6)      UV Disinfection

7)      Dewatering System

8)      Lime Stabilization System


All of the major processes that were evaluated show that they have capacity for flow and load through the design year of 2015.  Assuming the population growth trend remains the same, the oxidation ditches will be the first to reach capacity. This will occur in about 2025.




Tertiary System


The membrane filtration system was installed when the city opted to abandon the offsite portion of the original project. The filtration system was designed for phosphorus removal and to operate during the summer months only, with a maximum flow rate of 2.5 MGD.  The current system is operating very near capacity, especially during the shoulder months of each season.  The shoulder months include the four to six weeks at the beginning and end of each season, when weather can add additional flow to the treatment plant. This additional flow takes up capacity of the system.  The city’s ongoing efforts to reduce these inflow and infiltration (I&I) areas, will help extend the capacity of the system.  With this in mind, along with some operational concerns, it was recommended that the current system be upgraded in the near future.  The existing membrane cassettes were designed with the capability to be increased in size by 10 percent.  This upgrade would add additional capacity of 0.4 MGD per day.  Anticipating the need, city staff has already added this to the capital projects budget and scheduled for budget year 2006-2007.  With this upgrade, and our ongoing I&I reduction efforts, the membrane filtration system will have capacity through the year 2012.

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