LEGIONELLA IN COOLING WATER
WHAT IS THE ISSUE AND WHAT ARE THE POSSIBILITIES
TO CONTROL THE SITUATION
Introduction
Raw surface water that is used for cooling processes in industry contains a wide variety of microorganisms species. In a continuous biological dynamical process the different species of bacteria form
biofilms on all available, submerged substrates. Within the biofilm the different species will occupy
their own space conform their physiological demands. The bacteria will create their optimal microclimate through which a substrate bound micro-community develops. Legionella is a common species
in surface water, so also in cooling water they are common and they will occupy a specific niche in the
developing bacterial micro-community. This has to be taken into account literally because they are
‘eaten’ by the present protozoa in the biofilm. The latter form the ‘grazers’ in the biofilm, specifically
amoebae. Once absorbed by the host cell, they force their host to dispose nutrients necessary for
growth and reproduction. For information: in surface water an average CFU of 1500/L is common.
Biofilms
All submerged substrates and surfaces in
contact with cooling water will be conditioned
immediately by absorption of organic material
and bacteria which are mobile in the (water)
phase. Micro-organisms attach within a few
hours to surfaces of the cooling water system
and start producing extra cellular substances
or EPS (mostly polysaccharides). This forms a
slime-layer matrix, the EPS layer in which
bacteria are trapped and subsequently start
their life cycle as sessile species. This whole
complex is known as biofilm. The architecture
of the biofilm is influenced by physical and
chemical parameters like temperature, flow,
pH, available nutrients, roughness and material of the substrate surface. In systems with
low water velocities the biofilm thickness can
obtain several centimetres. High velocities do
not hinder biofilm development but results in
thin (millimetres, or microns) and vast biofilms.
In the continuous process bacteria and organic
material stick onto the biofilm, and the bacteria
search actively their best position in the developing biofilm. Within the biofilm the bacteria
are sitting literally on their food and at an optimal temperature so the develop excellently. It
is estimated that more then 99% of the living
bacteria on earth are living in a biofilm and
there is hardly no substrate not colonised or
not can be colonised.
Amoebe
Biofilm and Legionella
The detected high numbers of Legionella bacteria in re-circulating cooling water compared
to surface water is caused by the following
factors:
- The existence of substrate bound biofilms and presence of protozoan species,
more specifically amoebae in which Legionella reproduce.
Compared to the natural situation, cooling
water systems have explicit large surfaces
(substrates), formed by the heat exchangers
and cooling tower fillings for heat dissipation,
on which biofilms can develop. . Legionella
reproduces in the protozoa and have their
highest virulence on the moment of release
from the host cells.
- A surplus of nutrients
Beside an increase of the salt concentration
and suspended matter due to the phenomenon known as ‘cycle of concentration’
(evaporation divided by make-up water), also
particles are effectively scrubbed from the air
as it is pulled through cooling water cascading over the fill, and are concentrated in the
sump water. This provides all nutrition ingredients for biofilm development, which derives
in the tower basins.
Also the presence of sand filters operating
according the biological active modus may
play a stimulating role in the development of
biofilms by the production of metabolic waste
products
- Cooling water temperatures
The water temperatures are logically higher,
resulting in promoted biofilm development
and growth of protozoa, and Legionella. Legionella has an optimum temperature of 36
degrees Celsius
-
Application of shock treatments with
biocides for disinfection
These treatments kill or inactivate the biofilms and protozoa for a short period, but the
conditions remain for fast re-growth, because the architecture of the biofilm is not
eliminated. In other words there is a biocidal
effect, but not a structural cleaning effect. It
is even plausible that during dosage of biocides the probability for infection increases.
The protozoa with Legionellae inside are released from the biofilm, potentially acting as
‘Trojan horses’. Also,the Legionella bacteria
in fagosomes (membrane bound vesicles inside the protozoa) can be suddenly liberated
in huge quantities. It is well known that Legionella bacteria in fagosomes are resistant
to high chlorine concentrations for instance.
There are references about adaptation of
amoebae to biocides and even stimulation of
growth.
Legionella measurements
Measuring the number of bacteria is as a rule
done by sampling water for bacterial counts by
accredited laboratories. The samples are
treated in such way that optimal conditions are
created for growth. The number of colony
forming units (cfu/L) is determined by cultivation on special growth media and species recognition is done by DNA techniques. However,
these control activities are all based on the
existence of Legionella in the planktonic phase
in the water. As mentioned above, the presence of biofilms and amoeba cause the existence of Legionella. Strictly spoken the measured numbers are a strong underestimation of
the real number because more then 95% of
the real number of the Legionella are associated directly with biofilms and protozoa’s. So,
cooling water systems with low numbers of
micro organisms in the water phase can have
a severe active biofilm with high numbers of
Legionella bacteria.
What is going wrong now:
- The application of biocides does not lower
the influx of nutrients
- Disinfection with biocides does not guarantee that surfaces are free from EPS, allowing
rapid re-growth.
- The effect of the biocide dosing programmes
on amoebae and Legionella is unknown as
the programmes are designed on minimal
concentration and regime which has to be
maintained for cost reduction
- The objective is not the free-living Legionella
in the water phase, but the Legionella-host
relation inside the biofilm. The present
knowledge is however fragmentary
- Legionella measuring by conventional plate
counts takes more then a week, so results
are walking continuously behind present-day
situations. Besides the measurements being
hardly accurate due to so-called by-growth of
other micro-organisms, there is also a wide
dispersion in results by different laboratory
analyses in round robin tests by a factor up
to 10 to 20.
Summarised, biofilm formation is hardly to
avoid or to mitigate without excessive use of
biocides, which is opposite to the aim of the
water authorities, water administrations and
legislation authorities.
Recommendations
A more holistic approach should be regarded,
emphasising aspects like other types of substrates, control of existing biofilms, influencing
of biofilms, better analysing of biofilms and
monitoring.
With respect to the latter, monitoring offers
good prospects, providing an ‘early warning’
signal, not for Legionella but focused directly
on the activity of biofilms and bacteria and indirectly the activity of protozoa. Well-controlled
biofilms offers the first shield against Legionella infection.
At present, there is monitoring equipment
available, specifically the BIoGEORGE® system. This monitor enables ‘on-line’ applications
and produces a continuous (10 min interval)
output of the bacterial activity in a biofilm. In
contrast to the Legionella issue in drinking
water, where the legal standard is set on 50
CFU/L, a continuous monitoring performs a
more practical (procedural) approach for industrial cooling water systems. In these systems biofilm formation can not be avoided and
the standards as stated for tap-water would be
absolutely far from reality. Monitoring enables
the possibilities for:
- Early warning for bacterial activity increase
- Biofilms mitigation and detection of re-growth
- Indirect control on Legionella infection
- Optimisation of biocide dosing
- Development of environmental friendly mitigation procedures focused on biofilm suppression by changing the parameters
needed for biofilm growth.
Information is available at KEMA Power Generation
& Sustainables; web site www.kema-kps.nl or by
email: [email protected]
Tel: 026 3 56 30 08 GSM: 06 51 61 06 06
BIoGEORGE™ probe