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OSMOSIS
Osmosis is the phenomenon of lower dissolved
solids in water passing through a semi-permeable membrane into higher dissolved
solids water until a near equilibrium is reached. Reverse Osmosis (RO) is a
membrane process of purification which removes most of the the total dissolved
solids (TDS) in water by reversing the natural process of osmosis. Pressure is
applied to a TDS-concentrated solution against a semi-permeable membrane,
causing pure water to diffuse through the membrane. RO has become an important
process for a wide variety of applications including: medical, laboratory,
desalination, industrial wastewater, Deionized (Dl) pretreatment, and drinking
water.
TESTING RO WATER QUALITY
Electrical conductivity is the most
convenient method for testing RO water quality and membrane performance. Pure
water is actually a poor electrical conductor. The amount of ionized substances
(salts, acids, or bases) dissolved in water determines its conductivity.
Normally, the vast majority of the dissolved minerals in tap, surface or ground
water are conductive impurities. Myron L Company has conducted extensive
research relating conductivity to TDS, resulting in instrumentation and
calibration solutions which have become the standard of the RO industry.
When calibrating your conductivity instrument
for testing fresh water, the "442 Natural Water Standard™" solutions are the
best choice. These solutions are available in various concentrations. 442
solutions contain the following salts diluted in pure water: 40% sodium
bicarbonate, 40% sodium sulfate and 20% sodium chloride. These are the most
common salt compounds in surface and ground water. A sodium chloride solution
provides better results in brackish or sea water because the predominant salt in
these waters is sodium chloride.
ORP
ORP (Oxidation Reduction Potential/Redox) and
pH are important parameters in measuring the success and useful life of an RO
membrane. The ORP may be used to determine the activity of an oxidizer. RO
membranes are susceptible to attack by oxidizers such as chlorine, bromine,
ozone and hydrogen peroxide. The activity of the oxidizer is more informative
than the chemical residual because it determines the ability and speed of
oxidation. A high ORP reading would indicate a need for pretreatment. A low ORP
may indicate biological activity which may cause fouling of the membranes.
ORP can also be used to determine an overfeed
of sodium bisulfite, which is used to reduce chlorine. If the ORP reading is
under 200 mV, you have a reducing condition. This overfeed costs extra money and
can lead to environmental discharge problems. It is best to check the reject
water, where the concentration is highest. This will show even minute quantities
of oxidizers or reducers.
pH
pH is very useful in predicting membrane life
and the scaling potential of feedwater. The higher the pH and calcium, the more
likely it is that scale will form on the membranes. However, with silicon based
compounds, a low pH will increase the tendency for scaling. Membranes also have
a pH range where operation is optimal. It is often useful to check the pH of the
reject water to help determine scaling potential.
RECOMMENDED INSTRUMENTATION
HOME SYSTEM SALES
Since 1963, Myron L has manufactured
single and multiple range handheld instruments. Model
RO-1 and RO-1NC are
reliable, single range instruments used to demonstrate the RO process to a
prospective buyer. The color coding of the model RO-1 dial dramatizes the
difference between high TDS (red-above EPA recommended limits for drinking
water), medium TDS (orange - within EPA recommended standards for drinking
water), and low TDS RO water (blue-high purity water). Installers prefer the
three range 532 models or
TECHPRO
II™
TP1 or TPH1 because they are ideal for accurately testing both feed and product
water.
COMMERCIAL / INDUSTRIAL
Larger RO systems such as those found in
bottled water plants, hospitals, industrial process, or seawater desalination
require continuous monitoring to verify water quality and membrane condition.
For continuous measurement of water quality, Myron L manufactures the
720 and
750 Series II
Monitor/controllers. Monitor only, and monitor/controller models are available.
Monitor/controller models contain an adjustable set point and a heavy-duty 10
amp relay which can be used to activate alarms, valves, auto dialers, etc. A
variety of options and outputs are available to cost-effectively tailor the
monitor to the particular RO application.
The
ULTRAMETER
II™ 6P and 4P are
preferred by water treatment professionals for calibrating and checking
Commercial/industrial RO systems. They appreciate the waterproof case, ability
to store and record 100 memory data records, and 3 preprogrammed solution
curves.
ULTRAMETER IIs
are compact, but their multiple parameters give them the versatility of several
instruments.
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