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Impact of backwash water turning off method on grains stratification and filtrocycles

Congress: 2008
Author(s): Korczak P., Dąbrowski W.
Cracow University of Technology 24 Warszawska Street, 31-155 Krakow, Poland tel. +48-12-6282551, fax. +48-12- 6282042 email: wdabrow@usk.pk.edu.pl

Keyword(s): filter backwash, extended terminal subfluidisation wash
AbstractImpact of backwash water turning off method on grains stratification and filtrocycles Korczak P., Dąbrowski W. Cracow University of Technology 24 Warszawska Street, 31-155 Krakow, Poland tel. +48-12-6282551, fax. +48-12-6282042 email: wdabrow@usk.pk.edu.pl Oral Congress Sub-themes 1. Development of water resources and infrastructure - Data, monitoring and information technology 2. Water availability, use and management - Water quality management: surface and ground water Abstract The purpose of the research was to develop a backwash method which ensures both fast ripening of filtration quality and long filtrocycles. Based on recent strategies the filter backwash should be completed with subfluidisation velocity, to remove some impurities realized from filter grains while they are settling. This method is called extended terminal subfluidisation wash (ETSW). However, we have been proved that slow decrease in backwash velocity from above to below the value of minimum fluidisation results in more distinguish grain stratification and shortened filtration runs. The effect of the backwash water turning off method on filtration runs and filtrate quality has been investigated experimentally. Three methods have been considered : 1 fast closing of backwash water, 2 slow decrease in backwash intensity , 3 first fast decrease in backwash intensity much below the minimum fluidisation, then increase to a value slightly below the minimum fluidisation velocity with extending of the backwash period, and finally shutting down the backwash water entirely. Fast decrease in backwash water intensities at the end of a filter run for the first method produced longer and much more repeatable filter runs while long decrease resulted in faster and much less repeatable head loss build up in media. Based on these results a new method of backwash water turning off has been suggested and verified in laboratory experiments. This method is based on a fast decrease in backwash intensity much below the minimum fluidisation velocity, then in increase slightly below this value, extending backwash and finally in shutting down. The experiments were carried out on filter media fulfilling British standards and using coagulated and flocculated suspensions. In the first series of experiments a filtration column has been not disturbed for 24 hours between a backwash and the start of the next filter run, to ensure the same self compaction of filter grains due to gravity. This unpractical circumstances allowed to receive quite repeatable head loss build up in all filter runs, especially for the method of fast closing down the inflow of backwash water. In the second series of experiments the filter operation have been started each time just immediately after a backwash as it is the case in all filtration plants of a technical scale. The same conclusions resulted from this series of experiments but the results were less repeatable and the differences in lengths of filtration runs less meaningful, but still of a practical importance.
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