I cant comment on filter size or type or venturi/air-pump because I dont know, sorry.
Re the BD 3" is sometimes used especially in the US. Which size depends on flowrate, pipe length and configuration.
Have a look at http://www.mdminc.com/Friction_Loss_Chart.htm, if you dont understand that, say so. Note it is US so for a guide only and divide the flowrates by 1.2 to get UK gph
With 3" you probably get less settling of sediment in the actual pipe because, for a given flowrate, the actual speed of the flow is higher than in 4", however the losses are greater. Losses reduce the flowrate for a given pump and 'waste' money.
A BD is generally used to feed filtration, that is because the driving force for the water movement, gravity, doesnt liquify the sediment that you are trying to remove, the sediment reaches the filter system in particles of a larger size which are more easily filtered out. Pump to filtration and you reduce the particle size.
Posted on Apr 20, 2005, 8:55 AM from IP address 81.77.115.21