Environmental Engineering Reference
In-Depth Information
application of nanomembranes in removing hardness, NOM and trace contaminants such
as disinfection by-products (DBPs) and estrogenic compounds, as well as improving
biostability of the drinking water.
Table 12.2
Properties of various drinking water plants.
Parameter
Raw Water Type
Seawater
Groundwater
High
Colloids
Medium
Colloids
Process
Desalination
Softening
Color Removal
2-5 bar
2-5 nm
NF, UF
Plant:
−
50-60 bar
< 0.5 nm
RO
10-20 bar
0.5-1 nm
NF
4-8 bar
1-2 nm
NF
Operating Pressure
−
Pore Diameter
−
Membrane methods
Removal Efficiency:
−
> 96%
100%
100%
100%
100%
> 96%
100%
100%
100%
100%
90-95%
100%
100%
100%
100%
80-90%
100%
100%
100%
100%
Color (in NOM)
−
Suspended Matter
−
Salts
−
Bacteria
−
Virus
12.2.1 Hardness Removal
Water hardness is caused by soluble ions of the alkaline earth metals, calcium,
magnesium, strontium and barium. Hardness in natural water is caused mainly by
calcium and magnesium ions, since strontium and barium rarely occur in substantial
concentrations. Water hardness is not a health risk, but it is unwanted because it reduces
the effect of soap and detergents used in laundry and dish washing; and it may cause
visible deposits on surfaces such as forming a scale of calcium and magnesium minerals
in boilers, or reducing the water flow by deposits in pipes.
Nanofiltration processes are capable of removing hardness, heavy metals, NOM,
particles and a number of other organic and inorganic substances in one single treatment
step. NF membranes have a reasonable high rejection of bivalent ions whereas the
rejection of monovalent ions is moderate to low, as shown in Table 12.3 (Mulder, 1996;
GE Osmonics, Inc., 1997).
Gorenflo et al. (2002) studied nanofiltration of a hard groundwater with high
content of NOM. The membrane filtration was carried out at a water treatment plant of
the public works at the City of Mainz, Germany. The membrane used in this case was a
NF200B membrane from FilmTec. The molecular weight cut-off (MWCO) given by the
manufacturer is 200 Dalton and the membrane surface is reported to be slightly
negatively charged. Calcium rejection (74.9-78.9%) was higher than expected from the
producer's specifications. Magnesium was rejected significantly better (86.7-90.3%)
Search WWH ::
Custom Search