Flow rate for backwash

[Benj]

Hello all,
I want to put in a 2.0 - 2.5 cu ft carbon tank filter. I am uncertain if I have enough flow for the 10+ gallons/minute for backwash. I have two water meters to the property. I get 20 gallon/minute from my sprinker system side. When I measure flow on a house hose bib, I get 5 gallon/minute with the PRV at 60 psi and 7.5 gallon with it opened to 80 PSI. Currently there's a small PRV I plan to replace. My question is; Is the small PRV I have now combined with reduced frow through 1/2 pex and a hose bib enough to account for the difference in flow rate between my house and sprinker system flow rates. IE...if I put 1" PVC/PEX-A from the street to the carbon tank with a larger/higher flow rate PRV, will this likely give me 10+ gallons per minute, or should I go with a 2.0 cu ft tank. My assumption is that my reduced flow rate will resolve with a bigger PRV and 1" pipe, but would hate to get a 2.5 cu ft tank and not be able to backwash it.

 

[Bannerman]

A 2.0 ft3 carbon filter will typically utilize a 12" diameter media tank.

The recommended Backwash flow rate for carbon media is 8-12 GPM per square foot (ft2), so the appropriate backwash rate for a 12" tank will therefore calculate to 6.2-9.4 GPM, which equals an average of 7.8 GPM.

For a 13" diameter tank appropriate for 2.5 ft3 media, the appropriate backwash rate will be 7.4-11 GPM, thereby giving an average of 9.2 GPM.

Using the average flow rate to size the backwashing filter's DLFC button (Drain Line Flow Control = drain flow restrictor), will allow a safety margin incase your incoming water supply temperature is slightly below 60°F, or slightly warmer than 60°F. A colder supply temperature will result in greater media lift, thereby increasing the potential for the media to become lifted too high within the tank, and therefore increasing the potential for some media to be back washed out to drain. When the incoming temperature is warmer than 60°F, the media will not be lifted as high, thereby often requiring a higher flow rate DLFC depending on the water temperature.

I assume the supply line from the street is already 3/4" inch or larger diameter, and probably not PEX. The larger diameter seems to be supported as you are measuring 20 GPM through the sprinkler system supply (irrigation sprinklers or fire suppression?), which likely is supplied using the same supply line from the street.

In any event, if your incoming supply from the street is 3/4" or larger, the the supply piping in/out for the point-of-entry carbon system, should be of equal or larger diameter.

1/2" PEX is far too restrictive, and garden hose spigots will commonly further increase the amount of restriction, thereby substantially further limiting the flow rate.

 

[Sponsor]

 

[Benj]

A 2.0 ft3 carbon filter will typically utilize a 12" diameter media tank.

The recommended Backwash flow rate for carbon media is 8-12 GPM per square foot (ft2), so the appropriate backwash rate for a 12" tank will therefore calculate to 6.2-9.4 GPM, which equals an average of 7.8 GPM.

For a 13" diameter tank appropriate for 2.5 ft3 media, the appropriate backwash rate will be 7.4-11 GPM, thereby giving an average of 9.2 GPM.

Using the average flow rate to size the backwashing filter's DLFC button (Drain Line Flow Control = drain flow restrictor), will allow a safety margin incase your incoming water supply temperature is slightly below 60°F, or slightly warmer than 60°F. A colder supply temperature will result in greater media lift, thereby increasing the potential for the media to become lifted too high within the tank, and therefore increasing the potential for some media to be back washed out to drain. When the incoming temperature is warmer than 60°F, the media will not be lifted as high, thereby often requiring a higher flow rate DLFC depending on the water temperature.

I assume the supply line from the street is already 3/4" inch or larger diameter, and probably not PEX. The larger diameter seems to be supported as you are measuring 20 GPM through the sprinkler system supply (irrigation sprinklers or fire suppression?), which likely is supplied using the same supply line from the street.

In any event, if your incoming supply from the street is 3/4" or larger, the the supply piping in/out for the point-of-entry carbon system, should be of equal or larger diameter.

1/2" PEX is far too restrictive, and garden hose spigots will commonly increase the amount of restriction, thereby substantially limiting the flow rate.

I'm not sure about the size pipe at the meter, but I have 1" PVC coming into the house. I have two different meters (seperate one for sprinkler) as the city charges less for water that won't become sewage but the outgoing pipes are the same size from the meter. I plan to pipe it all the way into the filter tank with 1" Pex A. I'll have a spin down filter (tired of cleaning trash out of my PRV), a pressure reducing valve/Watts LF25AUB-z3 1", a 20" big blue 5 micron sediment filter then either a 2.0 or 2.5 cu ft carbon tank.

Without. hooking up the spin down filter, PRV and sediment filter, I'm not 100% confident I'd have 9 gpm available but I haven't committed to installing it all yet to check.

I hadn't heard of people using a flow restrictor on the drain line. I assume one measures the flow rate then puts a shut off valve partially closed to restrict flow or something more mechanized that regulates it.

I'm not a plumber or a water filter installer, just a home owner, but this is the 4th house I've put a filter system in and I've done a lot of reading on all kinds of different options, so I have some knowledge but no where near being expert.

On another note, I have thought about bone char for flouride removal but have read that it doesn't work well except for at low pH around 5. I can't find any documentation on the web or this site in regards to removal efficiency at different pH levels.

thanks.

 

[Bannerman]

hadn't heard of people using a flow restrictor on the drain line.

Every backwashing filter/softener device, utilizes a DLFC flow restriction button, to limit the backwash flow rate.

The appropriate backwash flow rate needed is fully conditional on the specific media, tank diameter and water temperature. The drain flow rates specified above, are applicable to 12X40 screen size GAC (granular activated carbon) media (most common), and 60°F water temp (also most common). Without the appropriate flow restrictor, the media is likely to become lifted too high during backwash, thereby increasing the potential for loss of media to drain. If the flow rate DLFC is not high enough, then the media will not be back washed sufficiently each cycle.

Bone char requires long contact time to reduce the fluoride level, but even with extending the contact time by using an extremely large amount of media, some fluoride will continue to remain. A much better option will be to utilize an undersink Reverse Osmosis system, to supply purified waterfor cooking and consumption, delivered through a separate low flow rate (</= 1 GPM) faucet.

 

[Benj]

Every backwashing filter/softener device, utilizes a DLFC flow restriction button, to limit the backwash flow rate.

The appropriate backwash flow rate needed is fully conditional on the specific media, tank diameter and water temperature. The drain flow rates specified above, are applicable to 12X40 screen size GAC (granular activated carbon) media (most common), and 60°F water temp (also most common). Without the appropriate flow restrictor, the media is likely to become lifted too high during backwash, thereby increasing the potential for loss of media to drain. If the flow rate DLFC is not high enough, then the media will not be back washed sufficiently each cycle.

Bone char requires long contact time to reduce the fluoride level, but even with extending the contact time by using an extremely large amount of media, some fluoride will continue to remain. A much better option will be to utilize an undersink Reverse Osmosis system, to supply purified waterfor cooking and consumption, delivered through a separate low flow rate (</= 1 GPM) faucet

 

[Benj]

Thanks for the information. The Fleck 5600 doesn't have a flow restriction setting or documentation on it in the manual, so I'm uncertain how people restrict flow if the GPM is too high.

In reading more about flouride removal, it it seems that it's not efficient enough to be worth the hassle/expense of installing a 2nd tank, so I may just not bother with trying to remove it...other than drinking water. I've used RO under the sink before but I plan to get a large inline distiller and run water to a faucet beside the sink. https://mypurewater.com/shop/mega-classic-distiller/

 

[Reach4]

Thanks for the information. The Fleck 5600 doesn't have a flow restriction setting or documentation on it in the manual, so I'm uncertain how people restrict flow if the GPM is too high.

It does. It is listed as a "Washer Flow".

The diagram does not label it. It is a piece of item 2, and is the third thing to the left of the elbow labeled as item 14.

So from right to left, the elbow, the DLFC retainer, o-ring, "Washer Flow" (which is actually the DLFC which gets retained).

 

[Bannerman]

The Fleck 5600 has limited capability of only a~7.0 gpm maximum drain flow rate even while no DLFC restrictor is installed.

Because of the low maximum drain flow rate, the usual recommendation for a 5600 control valve is maximum 12" diameter (2ft3) softener, or a 10" diameter (1.5 ft3) media filter.