Biofouling growth may be partly prevented during the
design phase, by using suitable materials
(i.e. copper, AISI 316 stainless
steel or treatment of the surfaces with special polymers)
and by dimensioning
the pipes in such a way as to obtain a flow rate (> 1 m/s) which
will hinder
adhesion of the organisms and to avoid stagnant points as much as possible.
There are physical and chemical preventive methods for
controlling fouling in cooling
water systems. Physical treatments are mainly
used during shutdowns.
Chemical treatments are based on non-oxidizing
biocides or, more commonly, on
oxidizing biocides, like chlorine gas. Compliance
with regulations on water
discharges and the necessity of safe biocides to
handle often leads to the
choice of chlorine dioxide as biocide for cooling
systems of large plants.
In this kind of plants, experiences have proved that
Chlorine Dioxide is much
more effective than Hypochlorite. The reason for that
better effectiveness of
Chlorine Dioxide is briefly here explained.
If we think at the cell walls (of bacteria or marine
organisms), they can be imagined
as a long chain of phospholipids. Each phospholipid
has in its head a phosphate
group which has a negative charge. When
Hypochlorite is injected, it rapidly
dissociates in Hypochlorose Acid (HOCl)
and Hypochlorite Ion (ClO-) and,
at the PH of the seawater (7.5 – 8.5), the
dissociation is around 20% of acid and 80%
of ion.
Figure 1 –Form in which
available chlorine is present
Since the Hypochlorite Ion has the same negative
charge as the phospholipids of the cell wall,
there is an electro-magnetic
repulsion between the two molecules. This means that if 1 PPM
of Hypochlorite
is injected, only 0.2 PPM is actually effective against the fouling, the
remaining
80% is simply wasted in the water. Chlorine Dioxide is a gas diluted
in the water and it follows
the same pattern of Oxygen. Bacteria and marine
organisms breathe ClO2 along with Oxygen,
herefore it is very easy for it to
enter the cell wall and start its oxidation action. Besides, when
Chlorine
Dioxide oxidises the organic material inside the cells it is reduced to
Chlorite (ClO-2),
which is not a biocide, but has the very interesting feature
of being a biostatic agent, it stops
the organisms’ reproduction cycle.
Organisms recover their reproduction capability in 6-12 hours
(depending on
some environmental parameters). This behaviour allows shot treatment instead of
the continuous one. As soon as they recover the reproduction capability in the
interval between two
shots, another one is performed and the bacteria colony
has no chance to grow.
Figure 2 –Different behaviour
of Chlorine and Chlorine Dioxide
So it is normally suggested a shot strategy for dosage
(instead of continuous one): since the goal
to be achieved is to limit the
bio-fouling growth on the surfaces (pipes etc…), treating the intake
basins the
bio-fouling has no sufficient time to grow in the hours between two shots.
By the way, it is important to know that a stronger
strategy can be managed by the chlorine
dioxide treatment, when needed, thanks
to the better tolerability shown (for instance) by RO plants
and distillers.
Moreover, it’s well known that Chlorine Dioxide is a
bromate-free water treatment.
Bromate is one of the greatest concerns for potable
water and/or for the outfall of cooling circuits,
when seawater is used
(bromate can be formed also in the intake treatment and, therefore, its
releases in the environment can create a toxic issue in the outfall of the
plant or can cause cross
contamination of bromate-free waters).
That is the reason why Dubai first and Qatar later
have switched all their potable water
treatment from hypochlorite/chlorine to
chlorine dioxide.
