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Manganese:
Manganese is a commonly occurring mineral found in ground water. Mn
will leave black, scaly deposits and will eventually foul membranes in concentrations
greater than .05 ppm. Manganese must be treated with a manganese greensand filter,
regenerated with potassium permanganate.
Hydrogen Sulphide:
H
2
S is
known for its “rotten egg” odour. In addition to having a
objectionable odour, it is highly corrosive and will damage plumbing and equipment. At
lower concentrations (< 1 ppm) it can be removed with a Paterson Iron Remover (air
injection). At levels exceeding 1 ppm it will need to be treated with a manganese
greensand filter.
Nitrates:
The presence of nitrates in the water is usually an indicator for other problems
with the water source. Nitrates are a by-product of the decomposition of organic
material that may include sewage or run-off from septic systems. Surface runoff
containing fertilizers will also be high in nitrates. The source of the nitrates should be
found and eliminated – feed water should be treated with nitrate-selective ion exchange
resins to reduce nitrates to < 5 mg/l.
Tannins and Lignin:
These are organic materials that are often contained in surface
water and shallow well water. Their origin is the natural decomposition of plant matter
and tree barks, leaving a brown colour ranging from a pale yellow to dark amber. Feed
water tannins must be pre-treated to levels < .5 mg/l to avoid membrane fouling. This is
usually achieved through the use of organic scavenging ion exchange resin
(polystyrene or polyacrylic), regenerated with salt.
Chlorine:
Feed water should be free of chlorine
(< .1 ppm), as TFC membranes are
easily damaged by chronic chlorine exposure. This is an important consideration when
municipal water is used as the feed water source. Pre-treatment by carbon filtration is
very effective for chlorine reduction – carbon eventually becomes exhausted and must
be replaced to ensure continuous filtration. If stabilized chlorine is present, size your
carbon filter 2 to 3 times larger than for unstabilized chlorine to ensure removal.
Consider installing a post-carbon filter test port for ease of sampling.
Organic and Microbiological Activity:
Raw water sources, particularly surface water,
often contain algae and naturally occurring bacteria. Ground water may contain sulphur
or iron reducing bacteria or may be contaminated with other more dangerous strains of
coliforms or
E. coli.
These must be treated to avoid contamination or fouling of the pre-
treatment systems or membrane element(s).
pH:
Most membrane manufacturers
recommend a pH range between 3.0 and 11.0 for
the membrane element. We recommend that the system operate at between 5.5 and
9.5, to minimize the effects of extremely acidic or scale-forming water at either ends of
the range. Extremes in pH will be harder on your system, either causing premature
corrosion on metallic parts or scaling internal components upstream from the
membrane, and likely the membrane itself. Keep in mind that reverse osmosis lowers
the pH of the feed water, by up to 1.0, depending on the water chemistry. This will be a
consideration for the end use of product water and for the vessels and plumbing system
that will be used to store and control it.
Turbidity:
This is the term to describe suspended particulate in the raw water.
Turbidity is treated by the sediment cartridges that your RO system has been equipped
with. Reduction of turbidity is necessary to avoid fouling your membrane and damaging
your drive pump. Make sure that your pre-filters are changed regularly and that quality
Excelpure 5-micron or 1-micron cartridges are used as replacements. The SDI (silt
density index) on feed water reaching the membrane must be kept < 5 to assure long
membrane life
