The Croton Reservoir
New York City is one of the few United States cities that does not filter its drinking water.
NYC water is subject to a Filtration Avoidance Determination from the EPA.
The clay-bottom reservoirs contain high levels of extremely fine particles and significant levels of dissolved iron and manganese.
During the roughly 100 mile transit from source to NYC these dissolved metals oxidize and transform into particles.
By the time water reaches a NYC building, excess suspended solids create an unusually high degree of water turbidity (an optical measure of light refraction) and cause brown water conditions.
For flow rates higher than about 100 gpm, automatic screen technology is the most efficient option. It can handle hundreds, even thousands, of gpm within a limited footprint.
Compared to traditional media filters, screen filtration requires only a fraction of the amount of water used for self cleaning.
Particle Size and Impact
Below is a typical stratified particle analysis of Manhattan water, showing particle size distribution (in the columns) and then Total Suspended Solids (TSS) volume.
|Size in Microns||Count per cc tested||Percent of total count|
The analysis shows more than 99% of particles are smaller than 15 microns (µ).
Note that the TSS volume is 60% higher than the EPA's maximum contaminant level (mcl) for drinking water.
As the filtration degree becomes finer, a higher percentage of particles will be captured.
Note that screen perforation (10µ, 25µ, etc.) is not pass / fail.
Screen pore size should be seen as delivering statistical probabilities.
A smaller pore size traps exponentially more dirt than a coarser one. Thus, while both a 25µ and 10µ screen will capture particles in the 1 - 5µ range, the 10µ screen will capture many times more.
The question of what filtration degree is optimal for NYC water conditions is subject to practical considerations, including cost, physical space, and basic principles of mechanical physics. For reasons we explain below, in most cases 10µ filtration is the optimal value.
Considerations regarding 10 micron filtration in NYC
There are two essential principles of particle filtration:
At a given flow rate, the finer the filtration, the more filtration area is required. (i.e: 10µ filtration requires at least twice the surface area of 25µ.)
The finer the filtration, the more force is required to clean off the collected particulate matter during a cleaning cycle.
A comparison of a 25µ screen vs. a 10µ screen, viewed at the same magnification
With reliable 10µ filtration, you can expect Turbidity levels to be reduced to below the 1 NTU (Nephelometric Turbidity Units) threshold required for compliance with the WELL Building Standard for domestic water from IWBI (International WELL Building Institute).
Below are the results of before / after 10 filtration in Brooklyn NY. They represent the highest and lowest values recorded for incoming and product water samples obtained on October 18, 2018.
|Unfiltered NTU||Filtered NTU|
|Sample 1||0.75 (highest)||Sample 2||0.15 (highest)|
|Sample 3||0.66 (lowest)||Sample 4||0.12 (lowest)|
Average Turbidity Reduction: >80%
Importance of the Cleaning Cycle
Reliably achieving 10µ screen filtration at a high flow rate of hundreds of gallons per minute depends on one essential factor: the cleaning cycle.
Discharge from a cleaning cycle
Powerful velocity of water entering the cleaning nozzles is required to pull particles from the screen during the backwash cycle. The finer the filtration degree, the more velocity is required. This power cannot be realized unless the suction scanner nozzles come into direct contact with the screen. Contact must be firm enough to capture all accumulated particles, yet gentle enough not to damage the screen over the course of tens of thousands of cleaning cycles.
Without contact, the turbulence in a water environment will decimate the suction power. With even a fraction of an inch distance from the screen, cross currents will assure that part of the dirt load, intended to be removed, will instead re-enter the filtration chamber. This will cause a backwash loop. The system will continually try to clean the screen, but be unable to do so.
Note in this example how the cleaning nozzles make direct contact with the screen, assuring that backwash water velocity is directed entirely into the scanner.
Today most owners and engineers want 10 micron.
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
Thus 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.
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 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 -- the 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, specify high pressure filters to be installed downstream of house pumps, designed to leverage high pressure as a value to support reliable 10 micron.
Just as important as adequate pressure is adequate screen area.
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.
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.
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.