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 difference between 25 and 10 micron in a domestic water application.
A good way to understand the value of 10 micron 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
From the above perspective, the true difference can be more easily appreciated. It's not as much about comparing the micron values of 25 vs. 10 as it is about 19% open vs. 6% open. It illuminates 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 open, generally delivers a profound reduction of Turbidity levels.
Now let's look at it from the technical side. The challenge posed by filtering through a screen only 6% open is not the screen itself. The hard part is assuring what it takes to clean a 10 micron screen during the automatic backwash cycle. Above all, sufficient incoming water pressure is required to ensure enough backwash water velocity that a screen, having collected particulate matter (the "filter cake"), can successfully be cleaned and restored to be ready for the next filtration cycle.
The essential pressure values:
For 25 micron the filter requires not less than 45 psi at the filter inlet. For 10 micron, not less than 58 psi.
The difference between 45 and 58 is not trivial. 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 appears to be 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, designed to leverage high pressure as a value to support reliable 10 micron. Today more than half of all the point-of-entry filtration systems we supply are high pressure units located after house pumps, or in constant pressure applications.
Just as important as adequate pressure is adequate screen area.
To put it simply: More is better. Take two 10 micron filters running at a given pressure and max flow rate, filtering the same loads of suspended particles (equal TSS levels). The only difference in this example is that one has more screen area than the other. The system with more screen will operate with greater reliability than the one with less.
While both filters in the above scenario will capture the same TSS volume over a given period of time, the system with more screen area will distribute those captured solids over greater surface volume. Since backwash is triggered by the system reaching its pressure differential threshold (a settable value), the amount of time between backwash cycles will be longer for the system with more screen area.
At the moment either system reaches PD threshold, the relative amount of screen occlusion is therefore about the same. But what is critically different between the two systems is that at all times -- whether during 100% forward flow or backwash -- the larger system retains a significantly higher ratio of open area to flow rate, which allows more water to pass through.
This simple fact is a decisive factor in assuring trouble-free operation on an ongoing basis. In cases where incoming pressure is at the cusp of the minimum required to supply the system, it can mean the night-and-day difference between ongoing operational stability and frequent system fault.
Here we draw more on our field experience than on defined threshold values such as minimum pressure required. In cases where the specification is "generous" -- say a Twin 16500-10 (33,000 cm2 screen area) rather than a single 21300-10 (21,000 cm2 screen area) for the same flow rate / inlet pressure / TSS level -- the operational difference is profound. The larger system has an exponentially higher probability of working without incident (not requiring field maintenance) than the smaller system does.
For the above reasons, we are seeing more and more projects engineer-specified for 10 micron to be (1) high-pressure rated equipment located downstream of house pumps and (2) sized to provide greater than the minimum screen area stated for a given flow rate.
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.