In a recirculation system chlorine residual should be monitored
in the cooling water return line or the tower basin, as well
as in the plant effluent (effluent residual level requirements
by EPA: less than average 0.2 ppm for any 24 hours period
and not exceed 0.5 ppm maximum level in 24 hours). The time
to detect the residual in the return line of a recirculating
system will depend upon many factors among these are:
- Water Volume
- Number of Cycles of Circulation
- Chemical Contamination Present
- Degree of Biofouling
The total residual chlorine (TRC)
in the cooling water return line consists of two parts: the
free residual chlorine (FRC), and the combined residual chlorine
(CRC). The two together make up the total residual chlorine
(TRC).
The free residual consists of available
unreacted chlorine that can react immediately with any new
biological or chemical demand. The combined residual includes
chlorine that is combined with ammonia, certain amines, and
other compounds. Combined chlorine residual (monochloramine,
dichloramine & trichloramine) exhibits biological kill,
but sometimes less effectively and at a slower rate (free
chlorine is higher than to mono-chloramine by 25 times in
the strength). Alternative chlorine bearing compounds, such
as chlorine dioxide (ClO2), may be preferred where
ammonia concentration is so high that chlorine alone will
not control the biofouling.
When chlorine addition is started,
the combined residual chlorine will be the first to appear
at the sampling point. The time elapsed until the appearance
of free residual will range from a minute in a once through
system to as long as an hour in a large recirculating system
having a long holding time and substantial chlorine demand.
The appearance of a free residual,
in the discharge, indicates that all demand has been satisfied
and chlorination can be stopped. It is not necessary to have
a persistent free residual to get effective biofouling control.
With experience in a given system, it is possible to develop
a relation ship between the amount and timing of the total
residual, and the biofouling control obtained. This relationship
then becomes the basis for designing the chlorination program.
Many large industrial systems, especially those operating
in the alkaline pH range, operate very well with a continuous
low level addition program, never showing any appearance of
free residual chlorine.
In electric utility plants, the
main contamination source is the make-up water. This is a
predictable source unless the water supply quality varies
widely. For this reason, the relationship between the free
and combined residuals should remain fairly constant except
for seasonal variations. An unusual loss of free residual
should be an immediate warning of possible biofouling in the
system.
In an industrial plant such as
a still mill or petrochemical plant, many factors contribute
to the chlorine demand. Make-up water contamination, leakage
from process streams, groundwater runoff, airborne process
gases (such as ammonia and hydrogen sulfide) and other elements
as all provides either nutrients that encourage biological
growth, or compounds that react directly with chlorine. Techniques
for measuring the free and combined residual chlorine in cooling
water vary from simple chemical test to sophisticated on-line
electro chemical methods. All methods require careful maintenance
and frequent calibration to ensure accurate readings.
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