BCWWA – Modified Slow Sand Filtration

Rossland has a six-cell slow sand filtration treatment plant, which
operates efficiently year round. This system is simple and effectively
protects the community from most threats to public health.
Slow Sand Filtration is undergoing a resurgence in North America, mainly due to a few simple factors:

  • Low O&M
  • Lower skill requirements than other technology
  • Ease of Operation
  • Ease of Maintenance, Minimal Service Calls
  • Simple Chemical Requirements
  • Meets Regulartory Requirements
  • Passive Process

There is a need to entice new blood into the industry…

  • 35% of qualified operators are eligible to retire within the next 10 years.
  • Low Compensation for High Level of Responsibility.

Life Cycle Costs for treatment plants…

Engineering Cost 1-2%
Construction Costs 16%
O&M 80-93%

Modified Slow Sand Filtration can include…

  • Ozonation (oxidation) – Disinfection and Color removal
  • Roughing Filtration (solids removal) – removes the residual ozone and extends the range of raw water turbidity removal.
  • Post Treatment Biologically Activated Carbon (can remove some pharmaceuticals)
  • Post treatment limestone contactor (corrosion)
  • Post treatment disinfection (required)

Seems like an effective combination is Ozonation and Biological Treatment for most systems.

A disadvantage for slow sand technologyis the slow ripening period, (up to 12 weeks of operation with new sand). To shorten this timeframe in new construction – often the sand is loaded in prior to completion of the building, and water is run through as the building is built around it.

Overall, Slow Sand Filtration is a simple, effective technology worth looking at for smaller water networks.

Published by Mike Thomas

Mike Thomas P.Eng. ENV SP, is the author of UrbanWorkbench.com and Director of Engineering at the City of Revelstoke in the Interior of British Columbia, Canada.