NYCDEPCostAnalys isReport-0607 .pdf - New York-New Jersey ...

extracted through the membranes as a permeate. Compared to filtration, the removals are greater
due to typical pore sizes between 0.1 to 0.2 m.
The mechanism of membrane filtration is to use hydrostatic pressure to force liquid against a
semipermeable membrane. Only the liquid and particles smaller than the membrane's pore size
can pass through. Pore size refers to the diameter of the spherical particle that the filter would
retain. A filter is rated with two pore sizes corresponding to two tiers of retention efficiency:
• Absolute pore size - the particle size retained with 100% efficiency
• Nominal pore size - the particle size retained with a selected majority efficiency
Membrane filters are designated by pore sizes as shown in Table 1-3.
Filtration
Type
Membrane
filtration
Ultrafiltration
Nanofiltration
Pore
Size
(µm)
0.1-10
0.01
0.001
Reverse
Osmosis 0.0001
Table 1-3: Membrane Filter Designations
Pressure
low-pressure
cross-flow
up to 145 psi
(10 bar)
mid-range
pressure
30–250 psi
(2–17 bar)
high-pressure
Removal
Turbidity. Colloidal and suspended particles including
fats
Microorganisms/pathogens, viruses, high molecularweight
substances, organic and inorganic polymeric
molecules including proteins, silicates
Color. Demineralization, desalination (divalent salts),
softening (polyvalent cation removal), sugars, pesticides,
herbicides
All suspended and dissolved solids. Low-molecularweight
substances such as monovalent salts
Membrane filtration is not commonly used on the scale of full tertiary treatment. It achieves
removals far in excess of current regulation, and does so at significantly greater cost than
conventional filtration.
A schematic of the process is shown in Figure 1-2.