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Published byCarmen McManus Modified over 2 years ago

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Dwell Time By Steve Childers, VP of Education Posting on the NI (Nishikigoi International) Board New article to be published on NI Magazine What is dwell time?

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Filtration Issues Time for water to pass through a given volume Used to be a measure of debris settlement performance in mechanical filtration Is it a valid measure? Is longer dwell time better for performance in biological filtration?

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Passive Settlement Vortex chamber for solids separation

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Volume calculations Discounting cone area, 4 feet by 4 feet volume Area is 3.14 x R squared Volume = Area x Depth = cubic feet 1 cubic feet is US gallons Volume is US gallons

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Dwell Time calculations Flow rate of 3000 US gph, with 60 minutes in an hour, equivalent to 50 gpm Dwell time = volume / flow rate / 50 gpm = 7.5 minutes

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Taller or Wider Better? Same volume, different dimensions Taller: 3 ft diameter, 7 ft tall Almost same dwell time

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Inlets and Outlets Original chamber, gives reduced volume of 235 gallons Dwell time of 4.7 minutes Taller chamber, with depth of 4.75 ft gives just over 250 gallons Dwell time of 5 minutes

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Critical factor for settlement Forces that keep solids in suspension Velocity of water current Boundary layer is slower Wider surface layer is better

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The Answer Settlement is not passive Micro-screen stops velocity to zero Removal of solids is active washing

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Matala in Bio Chambers 94% pass through, for 1 inch thickness

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Volume calculations Surface area 171 sq ft/ cubic ft Total volume of matting just over 7 cu ft Void space at 94% is 6.58 cu ft Matting space is 6% or.42 cu ft Media surface area is 1197 sq ft Empty chamber volume 209 gallons Chamber with matting is 206 gallons

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Dwell time calculations At 3000 gph, as before Dwell time on empty is 209/50 = 4.18 minutes Dwell time with matting is 206/50 = 4.12 minutes

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Velocity calculations 3000 gph moving through a 4 line 3000 gallons is cubic inches Velocity = flow rate/area = inches per minute Hitting the biochamber, it slows to inches per minute

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Calculations continued Takes 50 gallons just under one minute Each gallon passes through 24 sq ft of media surface area. How much of this hits the bio film to be converted?

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What if it was all a 4 pipe? Jam it with Matala Dwell time is reduced to 1 minute Every gallon passes through 1187 sq ft of surface area (compared to 24 sq ft) Do we really want to do this?

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How do you improve contact? Reduce void space – lava rock

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Best solution Kaldness moving Increase possibility of contact No increase in volume No blockage

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