sion or galvanic corrosion. Research over
the past several years has identified many
species of bacteria and complex metabolic
pathways leading to the corrosion of metals. Every common metal and metal alloy
with the exception of titanium is susceptible to MIC.
If a district cooling system is ‘
microbiologically active,’ it is likely there will be
bacteria in the deposits. Biofilm acts as the
glue that binds a deposit to the metal
surface due to its gelatinous matrix structure. Consequently, aerobic bacteria (like
Pseudomonas and Thiobacillus), as well
as anaerobic bacteria such as sulfate-reducing bacteria (Desulfovibro, which
reduce sulfates to sulfide) and iron-reduc-
ing bacteria (Gallionella, which secretes
iron deposits as a byproduct for metabolism), contribute to MIC formation.
Preventing the Problem
The first step, both for corrective
action and to prevent additional future
problems, is to determine the presence of
microbiological activity within the district
cooling system. Unfortunately, planktonic
dipstick measurements only measure the
presence of free-floating bacteria, and it
is the ‘sessile’ (permanently attached)
biofilm growth colonies that are responsible for MIC. Planktonic tests, however,
can be used as an indicator of the presence of sessile bacteria: Ideally, a total
Biofilm Basics
Biofilm is composed of ‘sessile’ (permanently attached) growth microorganisms embedded in a
gelatinous matrix, anchored to a surface substrate – like chilled-water piping and storage tanks
– by polymeric sugars (polysaccharides). Several factors play a role in biofilm growth within
both open- and closed-loop heat exchange systems. These include:
Temperature
Bacteria populations can be psychrophilic (cold-temperature thriving), mesophilic (
moderate-temperature thriving) and thermophilic (high-temperature thriving) in nature. Although they
primarily exist in mesophilic and thermophilic environments, biofilms can coexist in an array
of environmental settings regardless of temperature – including both district heating and
cooling systems.
Flow Hydrodynamics
Metal surfaces exposed to lower velocities of water may tend to accumulate more biofilm,
although there may be some minimum flow velocity at which mass transport of nutrient
loading will limit biofilm accumulation.
Nutrient Loading
A lack of nutrients will limit the growth and the amount of biofilm; however, in both open- and
closed-loop recirculation systems this is hardly the case. In open cooling water recirculation
systems with significant airborne and process contamination, the risk is greater for developing
problematic biofilms. This is especially true in systems that are operated at higher concentration
ratios where nutrient loading is allowed to accumulate. In closed-loop systems, bacterial biofilms
have the potential to coexist due to the presence of aerobic and anaerobic bacteria.
Microbial Composition
The composition of the microbial growth is complex, due to the nature of different bacterial
biofilms to coexist. A case in point is the highly adaptive nature of the Legionella species,
specifically Legionella pneumophila, which can coexist in slime layers and become active when
conditions are right, spreading through piping.
Sunlight
The presence or absence of sunlight also factors into microbe and biofilm growth. Algae, for
example, require sunlight even more than nutrients to grow (certainly additional phosphates
will exacerbate growth). In open recirculation systems, algae is of prime concern not only as a
potential foulant but also as a nutrient source for other microorganisms. In closed-loop systems,
the primary bacterial biofilm incurred is pseudomonas (slimers).
colony count of 103 CFM/ml is a good,
safe level, but realistically, a 105 CFM/ml
total colony count is acceptable. Any count
higher than 105 CFM/ml is a sure sign
that sessile biofilm is active and present.
Given the synergistic properties of
biofilm and MIC, good bio-control measures are a necessity. One such measure,
which district energy systems have only
recently begun to take advantage of, is
the application of chlorine dioxide (ClO2).
For years, ClO2 has proved its worth in the
remediation control of sanitation and disinfection in other industries.
For example, municipalities have
successfully utilized ClO2 to treat wastewater and drinking water. Hospitals use it
to eradicate biofilms harboring Legionella
colonies in their domestic water supply.
Various food industry segments – from
wineries and dairies to vegetable growers
and poultry and beef processors – rely on
ClO2 to ensure sanitary water in their operations. Chlorine dioxide is also used in
ethanol production and the pulp and
paper industry. During the past five years,
numerous district energy systems in the
U.S. have also employed ClO2 to combat
biocorrosion. These include the district
cooling operations of commercial buildings, hospitals and university campuses
(fig. 2).
Chlorine Dioxide Advantages
How does ClO2 do its job? It disinfects
and inactivates pathogens by ‘searching’
for di-sulfide amino acids (e.g., cystine,
thymine and tyrosine) within the cell
membranes of the bacteria. These amino
acids are the building blocks for protein
synthesis. Chlorine dioxide ultimately kills
the microorganism by disrupting nutrient
and waste transfer across the cell.
Chlorine dioxide has several advantages over other biocidal agents:
● Chlorine dioxide is a broad-spectrum
biocidal agent that is non-pH-depend-ent.
● Chlorine ‘chlorinates’ (disassociates
in water to form hypochlorous acid
and the hypochlorite ion). Chlorine is
pH-dependent, and therefore in alka-line-laden reticulating waters exceeding pH 7. 3 the predominant form of
chlorine will be the hypochlorite ion
rather than hypochlorous acid.
● Since ClO2 is a soluble gas in solution
