Cooling Water TreatmentBack
Complex fouling deposits, which may include micro and macro-biological matter, inorganic particles and products of corrosion, are a significant concern for the efficiency of cooling water systems, heat exchangers and pipes in general. The mechanism of biofilm (film of biological origin) formation can be summarized as follows:
1. Surfaces are covered by primary colonizing bacteria and other organisms;
2. There is a transition stage in which multi-layers of cell develop, embedding themselves in their polymer material;
3. Finally, the population density quickly increases resulting in a mature biofilm.
Biofilm is the substrate conducting to settlement and adhesion of other biological and inorganic materials.
In general, “macro-fouling” refers to the growth of crustaceans (Barnacles), molluscs (Mussels and Clams) and Coelenterates (Hydroids), whereas “micro-fouling” is referred to algae and bacteria.
The Biofouling issues in cooling circuits
The presence of fouling causes:
1. Higher operation costs, because lower heat transfer causes production losses and increase in flow resistance demands more pumping energy;
2. Higher maintenance costs, for cleaning operations or replacement of pipes broken by under deposit corrosion or by over-heating;
3. Less operational time because more shutdowns are necessary to clean or repair the equipment.
The following parameters concerning water quality also play a role in the development of fouling:
a. Temperature - the rate of microbial growth depends on seasonal temperature fluctuation;
b. Dissolved gas - the content of oxygen affects the growth of several aquatic species;
c. The availability of nutrients (Phosphorus and Nitrogen) - these are the basic elements required for biosynthesis;
d. pH and suspended solids.
Chlorine vs. Chlorine Dioxide
Chlorine (as hypochlorite or chlorine gas) and chlorine dioxide are two of the most common biocides, used in many applications.
Learn more: Chlorine vs. Chlorine Dioxide different features against biofilm growth
Here is a comparison based on their chemical characteristics:
1. Chlorine dioxide has a biocide activity that is constant over a wide range of pH (6-9), this fact makes it suitable for sea water systems (pH>8).When pH is higher than 8, the active species, i.e. the efficacy, for the equilibrium NaClO/ClO- decrease (-80%).
2. Using chlorine dioxide, there is no formation of halogenated compound (as TTHMs) and bromate, which are hazardous for human health (and in outlet water to environment for animal species).
3. Chlorine dioxide is much more soluble in water than chlorine (10gr/l), so the diluted solution used in ISIA plants (1-2gr/l) are widely below the solubility and this fact increases the intrinsic safety.
4. Chlorine dioxide is not affected by hydrolysis.
5. Chlorine dioxide is immediately effective, as shown in the graph below:
The immediateness of the biocide effect also allows for different dosing strategies, i.e. shot modes, when normally with hypochlorite a continuous dosing is required. This implies a greater consumption of the latter to have the same – or less- efficacy.
The use of chlorine dioxide, thus, allows the plant to very effectively prevent growth and the plant can be compliant with all environmental permit conditions.
Learn more on environmental laws and regulatory aspects
EasyaReadox: a proper monitoring system can be the “Brain” of the treatment program. It manages to inject chlorine dioxide in a proper quantity and at the right moment, saving material and protecting the environment.
Additional benefits are accomplished in annual cost and safety.
Learn more on heat exchangers performances: biofouling effect