Comparison between chlorine and chlorine dioxide treatment

We report a typical case related to a cooling circuit where chlorine dioxide is applied, instead of chlorine.

A special monitoring system should also be installed to control the performance of the treatment.

The example cooling system consists of a “once through” system that pump about 100,000 cubic meter of seawater per hour. 

At full load this water undergoes a temperature rise of about 9 °C between the seawater inlet and discharge.

The seawater inlet temperature varies from a low of about 25°C in the winter to a high of about 45°C in the summer—the time weighted average temperature is about 35°C. Seawater is drawn in via an intake structure intended to result in low water velocities located about 300-350 meters off the shoreline. The outfall is via a discharge cap located at a similar distance from shore as the intake structure.

Most part of the cooling seawater pipes are cement or cement lined, the condenser water box is rubber lined and the condenser tubes and tube sheets are titanium.

The first designer’s concept to control biological growth could be chlorination electro-chlorination or via liquid chlorine evaporation.

Chlorinator fed from one-ton liquid chlorine cylinders is no more a technical alternative, since considerations must be highlighted to minimize the risks associated with handling as many as 40 one-ton liquid chlorine cylinders on the site. Due to the concerns of the owners with respect to on-site and off-site safety relative to liquid chlorine, this solution is going to be abandoned. 

Evidence at other power plants that biological growth could not be properly prevented or controlled while maintaining the level of residual chlorine in the discharge water below 0.2 ppm or so, more and more owners are oriented to consider technologies other than electro-chlorination for biocide treatment of the water.

Studies of the biological activity in the local waters, balancing the ability of the chlorinated water to be adequately toxic for the organisms present and simultaneously to comply with the environmental limits established for the cooling water discharge should lead to the following conclusion: the effectiveness dosage of chlorine to kill organisms indicates that high concentrations of chlorine would be required and the residual chlorine value in the water discharge would exceed the environmental permit level. 

The results of typical tests are shown in the following graph:

A small dosage of chlorine dioxide is much more effective (from 2.5 to 7 time) than chlorine to reduce the number of living organisms.

Other tests would show that the chlorine demand of the water a short time after shocking with chlorine significantly increased while the demand for chlorine dioxide remained steady for the same period. That is to say much of the chlorine injected into the water simply reacted with the water and thus became unavailable to affect the organisms.

The reduced chlorine dioxide dosage has also an interesting impact on OPEX costs, as briefly pointed out below:

Electro-chlorination:

1) Water to treat: 100,000 m³/h

2) Electro-chlorinator capacity: 150kg/h Cl2 100%

3) Investment costs: 3,500,000$

4) Operating costs/year: 400,000$ (electrode maintenance, electricity cost)

5) Total costs/year: 700,000$ (including Op. costs + yearly capital costs)

Chlorine Dioxide (ISIA process):

1) Water to treat: 100,000m³/h

2) Chlorine dioxide capacity: 50kg/h Cl2 100%

3) Investment costs: 1,000,000$

4) Operating costs/year: 350,000$ (maintenance, chemicals cost)

5) Total costs/year: 420,000$ (including Op. costs + yearly capital costs)

Conclusions:

• The result of the chlorine dioxide treatment is better than the hypochlorite or chlorine one, as demonstrated in many experiences, and briefly here, especially when a pH problem in the treated water further reduces the low effectiveness of hypochlorite treatments.
• Among chlorine dioxide generators, moreover, ISIA chlorine dioxide plants are considered the safest ones, because of the peculiar underwater technology (recognized to be safer than under vacuum processes).
• The excellent reaction yield and its steadiness during the running years of the plant guarantees the minimum consumption of sodium chlorite.
• When chlorine dioxide is compared with hypochlorite treatments, it should be kept in mind that:
• The investment to erect a chlorine dioxide plant is significantly lower than the electro-chlorinator one, making it attractive not only for a Contractor, but also for a final user.
• ISIA Chlorine Dioxide technology is made by simple devices, does not require any special skill to conduct the plant or to maintain it. No expensive substitution of electrodes or so is required: this is another positive feature for the ClO2 plant, especially in case of prolonged contractual warranty period.