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Is there an increased risk of bacterial growth in storage tank type hot
water heaters?

In Sweden (and also Germany and Denmark) people are concerned about the risk
of bacterial growth in hot water heaters. There has been some cases of the
Legionnaires' disease caused by the Legionella bacterium. It seems as the
recommendations of using a rather low hot water temperature (45-50°C) would
promote bacterial growth. Today I read in the newspaper that the Swedish 
authorities seem to recommend 60°C instead, because of a recent investigation
where the Legionella bacterium was found in several water heaters.

Will the use of storage type heaters also increase the risk of bacterial 
growth? As I work with district heating subscriber stations, there is of 
course also the opportunity to use instant water heaters (once-through). 
As the water doesn't rest as long in these types of water heaters they are 
perhaps better. Storage tanks keep the water for a longer time and perhaps
also the temperatures are lower in some places in the tank. This could then
be critical for the bacterial growth.

Below is an extract telling a little more about bacterial growth. What do
you think about this? Which type of water heater is the best one?

Bacterial growth
================
During the last few years, increased concern about microbiological growth in 
domestic water systems has developed. Legionnaires’ disease was discovered in
1976, and since then a considerable literature on the subject has been 
published. It is a typical interdisciplinary field, and much of the literature
does not cover the practical aspects of the subject. An article by Abrahamson 
[18] can serve as an example of the divergence of knowledge, in this case among
tap manufacturers. However, some attention to this has recently been given by
Backer and Frederiksen [19]. Legionnaires’ disease is caused by the Legionella
bacterium, when inhaled. This can occur when bathing because the bacterium 
grows best at around 36°C and can, when attached to aerosols, enter the lungs. 
Legionnaires’ disease is a type of pneumonia with a fatality rate of about 
30% without treatment. There is also a milder form called Pontiac fever, 
which is a type of influenza. It is known that Legionnaires’ disease is 
caused by live bacteria, and it is possible that Pontiac fever is caused by 
dead bacteria.
 
Recent Danish investigations have found the Thermus bacterium in calorifiers, 
causing a deteriorated heat transfer due to biofilms. It is also suspected that 
endotoxins from Thermus can cause allergic reactions. It is possible to kill 
the mesophilic bacterium of Legionella, by raising the temperature. However, 
this is in contrary to today’s energy-saving principles. The calorifier can 
also reach the temperatures where scaling from hard water becomes a serious 
problem, and also there will be an increased risk of Thermus bacterium growth. 
Backer and Frederiksen [19] advise on how to design and operate
district heating subscriber stations. They recommend avoiding stagnation and 
temperatures below 40°C, sedimentation, corrosion and deposition and recommend
using an efficient control method. This advice applies to subscriber stations 
both with and without hot water storage.

_________________________________________________________________
Johan Winberg              		Dept. of Heat & Power Engineering
johan.winberg@vok.lth.se       	Lund Institute of Technology
vox: +46-(0)46 104302          	P.O. Box 118
fax: +46-(0)46 104717          	S-221 00 LUND, Sweden