For building-wide filtration, today most owners and engineers want 10 micron. After all, if you are going to the trouble providing filtration, why not get the finest?
Let's look in detail at the choice between 25 and 10 micron in a domestic water application. First, just how significant is the difference?
A good way to understand the real difference is first by comparing the open area of 25 and 10 micron screens:
25 micron: 81% steel barrier, 19% open
10 micron: 94% steel barrier, 6% open
Seen from the above perspective, the true difference can be more easily appreciated. It's not as much about comparing the values of 25 and 10 as it is about 19% vs. 6%. It explains why as good as 25 micron is, there's little impact on actual Turbidity (a metric for measuring water clarity by quantifying refracted light). Whereas 10 micron, with only 6% of the filtration area actually open, generally delivers a profound reduction of Turbidity levels.
Now let's look at the technical side. The challenge posed by filtering through a screen only 6% open is not the screen itself -- that's mechanically simple. The hard part is cleaning a 10 micron screen during the automatic backwash cycle. Above all, sufficient incoming water pressure is required to ensure enough suction power so that a screen, having collected particulate matter (the "filter cake"), can successfully be cleaned and restored to be ready for the next filtration cycle.
Here are the essential values:
For 25 micron the filter requires not less than 45 psi at the filter inlet. For 10 micron, it's not less than 58 psi.
The difference between 45 and 58 is significant. The net pressure from the street after RPZs and before the house pumps at New York City buildings is nowadays rarely as high as 58 psi. If it's in the vicinity of about 50 psi, that's still OK -- Omicron filters routinely include a flush line suction pump to boost pressure during the backwash cycles. But where net pressure is under say 40 psi -- and this is more and more common in New York -- no flush line pump can generate enough added pressure to compensate for that low value sufficient to assure reliable 10 micron filtration.
Accordingly we offer high pressure filters to be installed downstream of house pumps. They are designed to leverage high pressure as a value to support reliable 10 micron. Today about half of all the point-of-entry filtration systems we supply are high pressure units located after house pumps, or in constant pressure applications, precisely to enable reliable 10 micron.
In cases where downstream of the pumps is not an option, and incoming pressure is simply too low to support 10 micron, we advise a 25 micron solution. Yes 10 micron delivers more transparent water than 25 micron, but a working 25 micron filter is more effective than a non-working 10 micron unit on bypass -- i.e. doing nothing at all.
Note that for any given flow rate the amount of screen area required for 10 micron filtration is about twice the area required for 25 micron.
For more about Omicron sizing and selection:
Or call us to discuss. We will be delighted to assist in your selection and specification process.