Backwashing problems on Look at backwash rates and other an individual filter features. Visible mudballs or Backwash unable to Investigate increasing backwash and air cracking remove mud and silt scour rates. The mudballs may have to be physically removed or broken up. Chemical cleaning and lancing may help. Media in localised Possible problem with the It may be necessary to inspect the ‘boils’ areas; possible under-drains nozzles, which will require digging out the media depth sand. Uneven flow into troughs Troughs not Check and resolve as appropriate. Troughs in the wrong places Encrustation of media of scale or other Fouling can usually be cleaned off (which changes media size fouling on media chemicals or, where it is surface build-up, and shape) scraped off the top. Filter in troughs Loss of with the This problem has a range of possible backwash water causes. Resolve as appropriate if losses are significant. Spikes in filtered water Sudden changes in flow Look for and control situations where flow turbidity during a filter run rate to a filter or filters rate changes suddenly. For example, when one filter is taken off-line for backwash, the other may step up in flow and generate a spike. Poor coagulant dosing Investigate optimising the coagulant dose or type. Growths in the Investigate chlorinating backwash water or unfiltered water (only if unfiltered water is very clean and if the disinfection by products will not cause problems). High turbidity at the end of Initiating backwashes too Are methods available to identify when to a filter run late start a backwash, other than waiting for turbidity to increase? Methods could refer to the time or volume filtered since the last wash, the water level increase above the filter, or the pressure drop across the filter. High turbidity at the start of Residual turbidity being Only restart a dirty filter after backwashing a filter run washed out of media it first. filters into service by ramping up flow very slowly. Add a to waste’ facility where water ‘filter – is diverted away to waste while the water is turbid after a backwash. Dose polyelectrolyte as a filter aid to mature the filter
Backwashing problems on an individual filter Look at backwash rates and other features. Visible mudballs or cracking Backwash unable to remove mud and silt Investigate increasing backwash and air scour rates. The mudballs may have to be physically removed or broken up. Chemical cleaning and lancing may help. Media ‘boils’ in localised areas; possible media depth Possible problem with the under-drains It may be necessary to inspect the nozzles, which will require digging out the sand. Uneven flow into troughs Troughs not Troughs in the wrong places Check and resolve as appropriate. Encrustation of media (which changes media size and shape) of scale or other fouling on media Fouling can usually be cleaned off chemicals or, where it is surface build-up, scraped off the top. Filter in troughs Loss of with the backwash water This problem has a range of possible causes. Resolve as appropriate if losses are significant. Spikes in filtered water turbidity during a filter run Sudden changes in flow rate to a filter or filters Look for and control situations where flow rate changes suddenly. For example, when one filter is taken off-line for backwash, the other may step up in flow and generate a spike. Poor coagulant dosing Investigate optimising the coagulant dose or type. Growths in the Investigate chlorinating backwash water or unfiltered water (only if unfiltered water is very clean and if the disinfection by products will not cause problems). High turbidity at the end of a filter run Initiating backwashes too late Are methods available to identify when to start a backwash, other than waiting for turbidity to increase? Methods could refer to the time or volume filtered since the last wash, the water level increase above the filter, or the pressure drop across the filter. High turbidity at the start of a filter run Residual turbidity being washed out of media Only restart a dirty filter after backwashing it first. filters into service by ramping up flow very slowly. Add a ‘filter to waste’ facility – where water is diverted away to waste while the water is turbid after a backwash. Dose polyelectrolyte as a filter aid to mature the filter
optimisation-small-drinking-water-treatment-systems-jan14.docx