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 decentralised.

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.

Big savings
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 pay back.

"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 Laden.

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 solution.

"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

Long-term planning
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 sewers.

"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.