During the first 16 days of August 2006, Denmark was hit by heavy
downpours, in many places almost as much rainfall as is normal for
an entire summer season.
According to the Danish Meteorological Institute, the town of
Klarup just south-east of Aalborg received as much as 245mm of
rain. "Remember: one millimetre corresponds to one litre or one
kilo of water per square meter. An average 1,000-square-meter
property would therefore have received 250 tonnes of water over the
past three and half weeks," said the Institute's website.
The heavy rain meant that the combined sewer systems were
overloaded in several places in the Municipality's low-lying
residential areas. The rain peaked on 15 March when Aalborg was hit
by a once-every-140-years rain event. On this day, many home owners
were forced to stand passively by while their local area
transformed into a regular wetland, and a great number of basements
and several ground floors of homes were flooded with a combination
stormwater and sewage.
A shower of benefits from separate sewer systems
To minimise the risk of similar situations in future, the
Municipality of Aalborg's Public Utilities Company has
prepared a 'Vision 2100', which is based on the principle that
sewage in the future is to be treated at the Municipality's two
central treatment plants, while stormwater management is to be
This will be made possible by replacing the combined sewer systems
with separate sewer systems. By thus managing sewage and stormwater
runoff in two separate systems, residents in low-lying areas in
particular will avoid having their basements and ground floors
flooded during extreme rain events. Should a rain event lead to
flooding nonetheless, rainwater and not unsanitary sewage from
kitchens and bathrooms will rise up into their basements.
Separation also eliminates the risk of sewage getting into the
environment. Instead, it will be carried on to the treatment plants
via a closed system, while stormwater can be led to detention
basins and watercourses.
"By separating the systems and establishing very small stormwater
runoff systems, we are more likely to be in a position where we
only have to deal with small amounts of water, rather than large,
uncontrollable accumulations. In the final analysis it will mean
better quality bathing and drinking water for the public, and a
cleaner environment for both fauna and flora," says Bo Laden from
the Municipality of Aalborg's Public Utilities Company.
Illustration from the Municipality of Aalborg's information
pamphlet to the public on separate sewer systems.
Separate sewer systems are not entirely new in the Municipality of
Aalborg. Since the 1960s, the municipality has been establishing
separate sewer systems rather than combined sewer systems in new
developments. Thus around 62% of the Municipality's sewer system is
now separate sewer systems.
What is new is that the Public Utilities Company is now addressing
the existing, run down systems in built-up areas. Although laying
two rather than just one pipeline is a somewhat more expensive
solution, Bo Laden has no doubt that the investment will eventually
"Our calculations show that it is around 10% more expensive and
that it corresponds to an extra cost of about DKK 4 million
annually. On the other hand, we are saving a lot on costs of energy
and man-hours when less sewage has to be treated at the treatment
plants. We will be saving DKK 35 million annually in operating
costs once the vision has been implemented in 2100. By investing
DKK 4 million, we will be saving DKK 17-19 million annually on
average over the next 100 years. So yes, it is a good investment.
You don't have to be a banker to figure that out, " says Bo
The public must also take responsibility
An important element for the success of the Public Utilities
Company's vision is to make the public realise that replacing the
old systems with separate sewer systems is the sensible thing to
do. It will cost the individual home owner around DKK 30,000 to dig
a trench on their land and lay the new sewer pipes to where they
can be connected to the main municipal sewer systems.
Bo Laden sometimes hears people complaining that the municipality
is charging them extra and forcing them to pay unnecessary money
for something which the utilities can manage in a combined system.
However, he still believes that people are generally positive
towards separating sewage and stormwater, because it means an
investment in the environment, climate and in better sanitary
conditions, as well as being a future-proof and holistic
"We also want to make the public aware of their obligation to deal
with the rain that falls on their land. We design the public sewer
systems to cope with a given amount of rain; it is not designed to
cope with just any amount of rain. In heavy downpours therefore,
there will always be some water left which the public sewer systems
won't be able to cope with," says Bo Laden.
"I think that this is new to many people, and some might be
frightened by the thought that they cannot merely lead the water
away as quickly as possible. But the point is, if we work together,
we will be able to avoid the extensive flooding in the lower parts
of the system, because if everyone does their bit, then the rain
will be more evenly distributed," he says.
The water is coming! On 15 August 2006, the residents of the
town of Klarup had to take alternative methods into use in order to
keep water from invading their homes. If separate sewer systems for
sewage and stormwater were established in the area, the risk of
similar situations occurring again in the future would be
significantly reduced. Photo: Peter V. Nielsen
To begin with, the Municipality of Aalborg is establishing
separate sewer systems in the town of Klarup, the area most
affected by the rain event in August 2006. This project will end in
2012, while, over the next 90 years, separate sewers will be
established in the remaining areas that currently have combined
"This may seem like a very long time, however I believe that it is
important to set long-term goals and visions which are based on
current issues and systems. In the Municipality of Aalborg, we have
decided that separate sewers are the way forward, but this may not
be the case in other parts of Denmark. It depends solely on the
local conditions in the specific area," says Bo Laden.