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water quality monitoring
Purified water quality is monitored continously with a conductivity sensor (E25)
positioned in the water flow just before the purified water outlet.
Conductivity is the measure of the ability for a material to conduct electricity, in
this case in water. With a high concentration of ions in the water will the conductivity be high, and vice versa.
Ions are salts or minerals, for example Sodium, Iron, Lead, Chloride, Sulphate, dissolved in water. Conductivity is measured in mS/m (milliSiemens/meter) or µS/cm.
The alternative is to indicate concentration of salts, measured in mg/l or ppm (parts
per million). The term TDS (Total Dissolved Solids) is often used and it is the same
as salt content.
During production of purified water is the measured value compared with a preset
value determined at installation. The current purification level is displayed on the
control panel ().
Ions are much smaller than most other possible contaminants in the inlet water. The
conclusion is if ions are separated, are also heavy metals, organics etc also reduced
with the same or better efficiency.
There are exceptions to this rule. The rejection efficiency through the membrane
depends on the substance size (molecular weight), shape (on molecular/ion level)
and electrical charge. The larger and bulkier the substance is, and with electrical
charge, the better is the rejection rate. Examples of exceptions are gasses dissolved
in water, and small organical molecules (THM’s e.g.). These have a rejection of
>60% compared with >95% for ions in general.
When the membrane is in the beginning of break-through, will the smallest ions
pass first: Sodium and Chloride. In most waters are these the highest concentrated.
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