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Best Practices for Wet Weather Concerns at Combined Sewer Wastewater Treatment Facilities

  • • Develop a comprehensive wet weather operating plan for combined sewers to prevent or minimize the discharge of combined sewer overflows into bodies of water without proper treatment and disinfection. Include in the plan, the nature and condition of the collection system, typical flows and pollutant loadings at the wastewater treatment plant, what occurs during exceptional wet weather flows, and how it affects the operation of the plant. Monitor the typical amount of grit, screenings, BOD, and total suspended solids in primary sedimentation, operation of the aeration tanks, final settling of the solids, and the disinfection system, and predict the increase due to extreme weather conditions.
  • • Determine the amount of space available for storage of access sewage flow in unused equipment and tanks for later treatment as well as maximum hydraulic capacity, maximum treatment capacity, and how to improve these to avoid sewage overflows.
  • • Determine how to bypass a portion of the treatment process when hydraulic overload will occur and yet not release the flow into surface bodies of water.
  • • Reduce plant recycling such as the supernatant fluid from the sludge digesters and store until the flow rate through the plant is reduced and then treat it.
  • • Equally distribute the flow into multiple units in any given process by using special flow splitting devices.
  • • Determine if the secondary clarifiers in the treatment plant are working effectively during wet weather and if not alter the process.
  • • Determine prior to wet weather if the pumping stations and the collection system are working properly and any unused storage of fluid will be able to be used.
  • • Determine before wet weather if the bar screens and aeration tanks are working appropriately and if cleaning and maintenance needs to be carried out immediately.
  • • Place all units of the sewage treatment system into active service and set the controls for continuous operation during wet weather. Cleanout all channels and screening containers, as well as grit and other substances frequently.
  • • Constantly evaluate the velocity of the sewage flow as it moves into the plant and through the process to prevent hydraulic overloading and make necessary corrections rapidly and efficiently.
  • • Avoid high solids loading in the primary treatment process by removing them more frequently during wet weather. Check all baffles and weirs and make sure they are operating properly.
  • • Discontinue sending secondary sludge to the primary clarifiers until the extreme wet weather is over.
  • • Use chemical coagulants to help remove the total suspended solids in the primary clarifiers during primary treatment.
  • • Activated sludge processes may be severely affected by large flows which will cause a loss of the biomass in the aeration tanks and in the secondary clarifiers. Utilize the step feed and context stabilization mode which handles higher hydraulic loadings, but will not be as efficient as the normal mode in BOD removal, in order to prevent this from occurring.

Do not allow excess solid storage in this process.

• Determine what amount of chlorine and contact time is necessary to effectively reduce any solids and the amount of E. coli present before discharge of the effluent.

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