Quality distribution of artificially induced ground water at the drinking water plant of Luleå

Abstract: The purpose of this report is to present a study on the water quality of
the Drinking water-treatment plant of Luleå (Sweden): to give an
explanation to the observed difference in temperature, using both sampling
period data and historical data found: finally some alternatives will be
suggested to try to improve this quality.

With respect to the water quality, three aspects have been differentiated,
studying them separately with the purpose of locating the points to
improve:
• Turbidity and conductivity: Generally the quality of the water sent
to the distribution system is within Swedish standards (0.5NTU for the
turbidity and conductivity < μs/cm), but in some points these
values are surpassed.
• Particle size distribution: it has been drawn up a map of the
particle distribution, classified by large vs. number, done to the three
wells with more turbidity of the system. Approximately the 91% of particles
are less than 5μm of equivalent diameter.
• Organic matter: a study was made for three wells with a high
turbidity, where the organic matter relatively the inorganic one –as
volatile carbon (loss)- was under (approx.) to 30% in the considered cases.

In order to explain the observed temperature differences throughout the
years, three hypotheses have been studied:

• Different retention times: from the time the water enters the
infiltration ponds until it leaves by the wells, an average of two months
passes.
• Soil as a heat exchanger: by means of an energy balance it has been
seen that the Earth absorbs energy from the water when it’s warms, cooling
it and vice versa.
• Inner heat of the Earth: it has been obtained an approximate
temperature gradient of the deepest point of the wells using the thermal
gradient.

Last, some tests were carried out to see the viability of introducing a
chemical precipitation in the process to decrease the turbidity, but there
was no perceivable coagulation in any studied cases. With respect to the
possibility of introducing membrane filtration technology, the most viable
filtration for the type of particles we have is the microfiltration.

Therefore, the conclusion is that there are problems with the high ionic
content in some of the wells which provoke salinity tastes on the water,
and also turbidity exceedes the thershold values according to the Swedish
Standards for drinking water in some cases. The organic matter is not high,
hence there is no need of introducing a biological treatment in the plant
and finally as the particle size distribution test showed, the best
alternative would be introducing microfiltration, since it would retain the
90% of the particles.