Dwell Time

1
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?

2
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?

3
Passive Settlement

• Vortex chamber for solids separation

4
Volume calculations

• Discounting cone area, 4 feet by 4 feet volume
• Area is 3.14 x R squared
• Volume Area x Depth 50.24 cubic feet
• 1 cubic feet is 7.481 US gallons
• Volume is 375.8 US gallons

5
Dwell Time calculations

• Flow rate of 3000 US gph, with 60 minutes in an
  hour, equivalent to 50 gpm
• Dwell time volume / flow rate
• 375.8 / 50 gpm 7.5 minutes

6
Taller or Wider Better?

• Same volume, different dimensions
• Taller 3 ft diameter, 7 ft tall
• Almost same dwell time

7
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

8
Critical factor for settlement

• Forces that keep solids in suspension
• Velocity of water current
• Boundary layer is slower
• Wider surface layer is better

9
The Answer

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

10
Matala in Bio Chambers

• 94 pass through, for 1 inch thickness

11
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

12
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

13
Velocity calculations

• 3000 gph moving through a 4 line
• 3000 gallons is 11577.6 cubic inches
• Velocity flow rate/area 921.78 inches per
  minute
• Hitting the biochamber, it slows to 11.38 inches
  per minute

14
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?

15
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?

16
How do you improve contact?

• Reduce void space lava rock

17
Best solution

• Kaldness moving
• Increase possibility of contact
• No increase in volume
• No blockage