New York City is one of the few United States cities that does not filter its drinking water. Between source and destination, nothing is removed. The reservoirs that supply NYC water are located in protected watersheds and are subject to a Filtration Avoidance Determination from the EPA.
The Croton Reservoir
These clay-bottom reservoirs contain high levels of extremely fine particles and significant levels of dissolved iron and manganese. While not harmful, 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.
Which is not what buyers and renters in NYC want to see in their new homes.
To remove these particles, there are three filtration options: cartridge, media, and screen.
Cartridge filters must be replaced when they become saturated with particles. For flow rates higher than 30 gallons per minute (gpm), cartridge technology becomes inefficient.
Media vessels are a good choice for flow rates up to roughly 75-100 gpm. To handle higher flow rates, media vessels must be manifolded, increasing footprint, cost, and installation complexity.
For flow rates higher than 100 gpm, screen technology is the most efficient option. It can handle hundreds, even thousands, of gpm within a limited footprint. Note: A large multi-residential building in New York City typically runs at a 400 gpm flow rate.
Compared to media technology, 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 overriding principles of particle filtration essential to keep in mind.
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.
These are universal principles of water filtration, and immutable facts of hydrodynamics. Any claims to the contrary should invite skepticism.
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).
|Unfiltered NTU||Filtered NTU|
|Sample 1||1.3||Sample 2||0.75|
|Sample 3||0.97||Sample 4||0.6|
|Sample 5||0.95||Sample 6||0.65|
|Sample 7||1.2||Sample 8||0.55|
Average Turbidity Reduction: 42%
Filtration under 10 micron
Filtration solutions below 10µ demand more space, a more elaborate and expensive installation, more complicated maintenance protocols, and more water consumed in each self-cleaning cycle. On a practical basis, we see substantially diminished returns from these systems.
That said, if you are interested in exploring finer solutions, from 7µ all the way down to 0.02µ, please contact us for options and details.
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 suction is required to pull particles from the screen during the backwash cycle. The finer the particles, the more power 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.
About the Omicron 10 series
Manufactured to our specifications by STF Filtros (Monzon, Spain), a worldwide company with decades of experience in the screen filtration industry, Omicron technology provides both the sufficient screen surface area for the filtration degree offered and the essential mechanisms to restore screens to a fully clean state after every backwash cycle.
The cleaning nozzles make direct contact with the screen, assuring that backwash water velocity is directed entirely into the scanner.
Impact of the Omicron 10 series
After more than a decade and over 100 major installations in operation, Better Waters has developed a profound understanding of the demands of building-wide water filtration in NYC water conditions, including the overriding constraints of reduced footprint and cost. Less than two years after we first introduced Omicron to New York’s real estate development community, a majority of the most active developers have ordered Omicron 10 for their projects.
The construction and operation of Omicron has been customized to meet the specific demands of NYC water conditions, as well as new water management designs adopted by more and more projects. From extending the duration of flush cycles when 10µ screens are used, to inhibiting flush cycles while house pumps are running, these customizations are the product of years of field experience, and ongoing engagement with the community of engineers, contractors and owners.
To discuss your particular filtration requirements, or for a detailed set of engineering and specification files applicable to your project, please select from our Specifications Library or contact us.