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Development Project: Use of Rainwater as Feedwater in Public Swimming
Baths
In connection with the renovation of the roof of the public swimming baths Vestbadet
I/S, it was decided to establish a system for collection of rainwater. On a yearly basis,
it is possible to collect approximately 950 m3 of rainwater from the renovated
roof surface of 2,200 m2. The annual water consumption at Vestbadet is
approximately 20,000 m3, of which 2-3,000 m3 are used as feed-water
for the 50 metre indoor swimming pool.
The purpose of the project is to develop a simple system for treating rainwater, for
the use as feed-water in public swimming baths, which at the same time satisfies the
requirements of operational economy.
The rainwater system consists of two units. One unit for collection, rough cleaning,
short-term storage and distribution to the building, in the following called the
collection unit. And one unit for cleaning and treatment of the rainwater, in the
following called the cleaning unit.
The cleaning unit consists of the following components:
- A 3M solid particulate filter (10 µm)
- An ozone unit (Prominent) with a maximum capacity of 5g O3/h. Water flow of
2.5 - 3 m3 water/h.
- A charcoal filter (Triton TR 60), active charcoal based on peat
- A UV system (Trojan UV 8002). UV dose of more than 30,000 mJ/cm2 10 cm from
the light source. Two lamps, totalling 130 W. Water flow of 3 to 6 m3 water/h.
The project is a development and test plant. Therefore the design of the system allows
a random combination of the two treatment methods. Furthermore, the flow speed is variable
and can be set at 0.5 to 3 m3 water/hour.
To achieve a good result and to gain valuable experience, it was important that the
system could combine different types of water treatment and at the same time measure the
quality of the "same" rainwater (from the same rain occurrence).
A very extensive measuring program was carried out, where the effect of various
treatment combinations of collected rainwater was analysed.
The project management group continuously discussed the analytic program and the chosen
parameters, which were all approved by the Danish Environmental Protection Agency and the
Medical Health Officers. An initial screening was carried out to determine the chemical
and microbiological composition of the rainwater. Based on this screening the final
analytic programme was determined.
In the course of phases 1 and 2 of the analytic programme more than 90 water samples
were analysed in the period from June 1999 to May 2001. Samples were taken from the
rainwater collected immediately after run-off from the roof surface; from the untreated
rainwater in the collection tank and finally samples were taken of the rainwater before
and after treatment by filtration, charcoal filters, ozone treatment and UV-light
treatment. During the period from February 2002 to October 2002, a further 48 water
samples were analysed to determine the effect on bacterial numbers resulting from various
alterations of the plant.
Total viable counts at 21ºC and 37ºC and the number of enterococci were chosen as
indicator parameters to examine the effect of the various rainwater treatments. The
results from a series of analyses showed comparable reductions in the total viable counts
at 21ºC and 37ºC and the number of enterococci.
Enterococci were not found in any samples after UV-light treatment. Untreated rainwater
typically contained approximately 1,000 enterococci per 100 ml. Thus the treatment reduced
the number of enterococci by 99%.
The analysis results show that UV-light treatment in five comparable analyses reduced
the total viable counts at 21ºC and 37ºC to a level very close to the quality
requirements for drinking water. The concentration in the UV-treated rainwater of
thermo-tolerant coliforms, enterococci and pseudomonas was either below or equal to the
quality requirements for swimming pool water.
From the samples taken on August 27th and October 15th tests were
made of the water immediately after UV-light treatment, i.e. from the actual UV unit. One
test was carried out after the first UV unit and one test after the second UV unit, where
the rainwater had been exposed to UV-light twice. The results of all the analyses show
comparable levels of total viable counts at 21ºC and 37ºC below the quality requirements
for drinking water.
Based on the results, it is proposed that UV-treated rainwater is used as feed-water
for the Vestbadet swimming pool for a test period. The preparation of such a test
including a water quality monitoring programme should be determined in consultation with
the Danish Environmental Protection Agency and the Medical Health Officers.
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