Drinking Water Filter Backwashing

2. Drinking Water Filter Backwashing • Backwashing – reversing and increasing the water flow to flush particles out of the filter media. • Backwashing is vital to the life of the filter media. • Backwashing is also fundamental to the quality of the water coming out of the filter. • This presentation examines the most common filters, pressure or rapid-rate gravity.

3. What is Filter Backwashing? • One of the best ways to clean the filter is to backwash it. • Meaning to reverse the flow while increasing the velocity. • This reversed flow passing back through the filter blasts the clogged particles off and out of the filter.

4. Video Showing a Rapid Sand Filter in Backwash Seymour Capilano Water Plant Backwash http://www.youtube.com/watch?v=whwEBxqa3yU

5. Why Drinking Water Filters Need Backwashing • Drinking water filters collect, catch, or gather particles from incoming flow. • When the filter’s pores become clogged the flow is restricted and contaminants may bleed through to the clear well. • Turbidity or cloudiness rises on the out flow of the filters.

7. Why Drinking Water Filters Need Backwashing • Although every filter is unique, the principles of backwashing are similar for all of them. • One key ingredient to a good filter backwash is clean water (usually out of the clear well or first storage tank).

9. When to Backwash • Cloudiness (turbidity) of the water coming out of the filter is one of the best ways to determine when to backwash. • Good rule of thumb is 0.1 nephelometric turbidity units (NTU). • Head loss on the filter is another indicator. When the filter gets clogged, more negative pressure is created. –2.5 to –4 psi or about 10 feet of head loss is a common point to start the backwash.

10. When to Backwash • Some small plants may have a clear tube when water rises due to head loss. Every 2.31 feet is 1 psi (Usually pre-marked to indicate when to backwash). • Another indicator is gallons filtered or the pump’s run time, this varies from filter to filter.

12. How Long to Backwash • Backwash until the water runs clear (provided there is enough clean water). • If there is not enough clean water to backwash until clear, the filter should be backwashed more often to eliminate overextended filter run times.

13. Techniques to Enhance and Speed Up Backwashing • Add surface wash system (a series of water jets can be fixed or revolving apparatus) that starts at the beginning of the backwash and ends at the middle of the backwash. • Subsurface wash—another series of water jets in the media bed itself, usually when the bed is fully fluidized (full backwash).

14. Techniques to Enhance and Speed Up Backwashing • For surface or subsurface wash to work adequately, 45 psi minimum is needed. • The flow for fixed nozzles should be 2 gallons per minute per square foot. • The flow for rotating nozzles should be 0.5 gallons per minute per square foot.

15. Techniques to Enhance and Speed Up Backwashing • Add air scour system (a series of small air pipes with diffusers in the under-drain or just above the under–drain that blow uncontaminated air) This helps break-up the mud balls. Air flow should be 3 to 5 cu ft. per minute per sq. ft. of filter area for air pipe in the under–drain with lower air rates for pipes above–under drain.

16. Techniques to Enhance and Speed Up Backwashing • Sometimes with air scour systems, the backwash rate must be reduced or varied. (usually a back wash rate of 8 gallons per minute per sq. ft. of filter area). • Air scour can be used the entire backwash cycle but usually it is used in the first few minutes.

17. Techniques to Enhance and Speed Up Backwashing • Another technique to help the backwash is to use an ordinary, disinfected gravel rake on an open rapid–rate gravity filter, this helps break-up mud balls. • Spraying the sides of the filter walls down with clean potable water when the filter is in full backwash cycle for open rapid–rate gravity filters.

18. Techniques to Enhance and Speed Up Backwashing • Increasing or varying the backwash rate helps the the backwash process but not too much as the support gravel could be displaced.

19. What Should the Backwash Rate Be? • Most design manuals for a rapid–rate gravity filter – min. 8 to 15 gal. per minute per sq. ft. of filter area. For those filters with air scour the rate should be at 8 gals. per minute per sq. ft. • For pressure filters the backwash rate is 15 gals. per minute per sq. ft.

20. How Much Expansion Should the Filter Bed Have in the Full Backwash Cycle? • During backwash the filter bed expands. The filter bed should have as much expansion as possible without losing media or displacing the support gravel. • Most engineering manuals – bed expansion should be 30 to 50 percent • Realistically, a 15 to 20 percent expansion will work with proper backwash duration.

21. How Much Expansion Should the Filter Bed Have in the Full Backwash Cycle? • A 30 inch thick filter media should expand the bed from 4.5 to 6 inches when in full backwash. • To much expansion and filter media can be lost or the support gravel displaced.

22. Backwash Water • The easiest way is to have the backwash line connected to the nearest sanitary sewer line (with proper backflow prevention). Take care not to overload the sanitary sewer system. • If a sanitary sewer is not present then a basin or basins can be used.

23. Backwash Water • With backwash basins the solids are allowed to settle and the top lay can be drained off to discharge point to a river or a stream with (NPDES) permit.

24. Backwash Water • Backwash water recycling can be an option. In some places it is required and in some places it is needed for drought conditions. • This recycled water goes to the head of the plant. • Filter Backwash Recycling Rule – states no more than 10% of the incoming flow can be settled/decanted backwash water.

25. After Backwash • Turbidity spike — the first slug of water that comes from the filter after the backwash and the filter is back to normal operation. • This turbidity spike can occur anywhere from a few minutes to 40 minutes or more after the filter is in operational mode.

26. After Backwash • “Ripening” the filter can take 30 minutes to 24 hours. • Some small plants backwash at the end of the day and allow ripening to occur overnight. • The longer the filter can sit the better for ripening and does not cost anything or require special piping.

27. How Long Should Filter-to-Waste be? • Many plants do not have the luxury to “ripen” the filter to avoid the turbidity spike. This is where filter-to-waste comes in. • Filter-to-waste is the most common technique of eliminating the turbidity spike, meaning that the first slug of filtered water is directed to the sanitary sewer or backwash basin.

28. How Long Should Filter-to-Waste be? • Filter-to-waste should continue until the turbidity spike subsides – the less turbidity in the clear well the better. • If no plumbing is in place for filter-to-waste this would be a good investment • Best way to monitor the turbidity is with the continuous in-line turbidity monitors.

29. How Long Should Filter-to-Waste be? • If in-line monitors are not available then a tap off the individual filters can be used to take samples. • Use a Stop watch and a bench top turbidity meter. • Samples should be taken every minute for the first 5 minutes and then at 2 minute intervals for the next 20 minutes or until the turbidity goes below +/- 0.1 NTU. Be sure to use clean containers for sampling.

30. P.O. Box 6898 Morgantown, WV 26506-6898 Phone (toll free) (800) 624-8301 Web www.nesc.wvu.edu E-mail info@mail.nesc.wvu.edu