Filter Pressure Advice

[Puffin]

I noticed a few posts where it seems the advice on when to clean your filter has changed. For the longest time the advice (not just from here but in general) was to clean when your pressure goes up 8-10psi. Now it seems the suggestion is to clean when the pressure goes up 20%-25% from your clean reading. In many ways this make a lot more sense than the old advice. My clean pressure is 15psi, 10 psi more is 25psi, a 66% increase, which seems like a lot.

However, the question I have has to do with sand filters. As you know sand filters when clean, filter down about 30 microns or so. As they get dirty, this gets smaller and their ability to remove smaller particles improves. It seems running them somewhat dirty is an intended/expected part design so their particle filtering is more inline with other types of filters. Sometimes we even add DE (or cellulose) to intentionally "dirty" the filter to improve its filtering.

Okay, so on to the question, does the same 20-25% guideline still hold true for sand filters? It seems, like back-washing so often would undermine their ability to filter as effectively.

 

[Bama Rambler]

On mine it does. If I try and wait till the pressure rises 8 to 10 psi I won't be getting anything out of the returns! At about 3 to 4 psi rise and I start getting pretty diminished flow out of the returns. I also can't tell any difference between the filtering of it when clean and when partially dirty.

 

[mas985]

There certainly is a trade off. As the filter gets dirty, it filters better but because of that, the dirt adds to the head loss of the pump so the flow rate reduces. These two things are working against each other. On the one hand, the filter may be removing more debris but on the other hand, the flow rate is reduced so there is less water being filtered. So the real questions is, as the filter pressure rises, is there net benefit or not? Unfortunately, there really is no way to know for sure but there is always a law of diminishing returns and at high PSI rise, I am pretty sure that the filter is not working as well because of the flow rate reduction. The forum collectively agreed to 25% but that is not a firm rule. A 25% PSI rise is about a 10% flow rate loss so if the filter is not filtering 10% more debris, then there is a net loss. A 50% PSI rise, is about 20% flow rate loss. If you want a bit more flexibility, I would target 25%-50% but no more than 50% rise. Just remember that your turnover rate is reduced with high filter pressure.

 

[JasonLion]

As long as you aren't backwashing more than once a week, you will be getting the benefits of the "slightly dirty" filter. Even if the filter isn't catching the fine particles every day, it catches them after a few days, which is plenty good enough. If your filter is so small that you are backwashing more than once a week during normal operation, you should get a larger filter. (What happens during an algae bloom is another story, and not important in this context.)

 

[Bama Rambler]

That's a good point Jason made. I only backwash about 4 times a season. On opening, about twice during the season, and a last time on closing.

 

[Puffin]

Great! Thanks for the replies. My concerns revolved around undermining optimal filter operation.

As for how long it takes for it to get dirty. It takes a long time for the psi to go up 10psi. Last year, after taking care of the initial algae issue I had (thank you TFP for helping through that btw), the pressure didn't go up much at all. In fact it took most of the season to go up to by 10. 20-25% is like 3-4ish psi, which would probably take a month or more. My schedule will probably work out something like your bama.

Thanks again folks for the advice, you guys are always helpful.