Climate change could put more pressure on the sewage systems in the future. Climate scenarios indicate critical factors such as increased volumes of rain and rising water levels.
“There is of course a lot of uncertainty surrounding both climate change and the population increase. But with what we know today, we can discern a development which indicates problems for Stockholm as early as the middle of this century,” says Jonas Olsson, a hydrology scientist at SMHI.
Risk of more overflow
Stockholm’s sewage system deals with and treats used wastewater for over a million people. A large volume of water from rain and melted snow, for example, also enters the system. The water is flushed into the drains and transported through the pipe network to the treatment plants in Henriksdal and Bromma. The treated wastewater then flows with the outward streams into the archipelago, and out into the Baltic Sea.
The future rising volumes of water pose a greater risk of the pipe network being unable to receive all of it in connection with rain. Untreated water will then be spilled – or overflow – into Lake Mälaren and the Baltic Sea.
The results of the new study indicate that the inflow to Henriksdal treatment plant could increase by between 15% and 20% by the end of the century. The projections also show that the volume of untreated water overflowed could increase by 5-10% over the next 30 years, and by 20-40% by the end of the century. This would create problems in meeting the current goals for the amount of overflow.
Altered precipitation patterns
Another challenge is that the precipitation is likely to have a different pattern in future, with more rain during the winter half of the year and less in the summer compared to at present. The increased flows during the winter affect the treatment processes, for example, to a great extent, as cold water is more complicated to treat than warm water.
Higher water levels in Lake Mälaren and the Baltic Sea, more frequent and more intensive rain also increase the risk of various flooding problems.
“The rain analyses are very important. The sewage systems are strongly affected by how precipitation shifts in terms of intensity, duration and time of year, such as changes in the summer and winter,” says Bo Westergren from water company Stockholm Vatten.
Continued action programmes
The results of the analysis will be followed up by further studies. Stockholm Vatten will use ongoing sensitivity analyses and developments regarding overflowing and flooding in light of altered water levels, as a basis for programmes of action.
“To secure deliveries to our customers also in the future, we must take a long-term approach in our planning. We need to consider climate change now,” says Westergren.
Analyses and method development is taking place at SMHI in various research projects, in order to enable similar studies in other cities. This primarily involves evaluating and improving climate scenarios on a local scale, and making the scenarios accessible and of practical use for urban hydrological effect studies.
The report entitled En studie av framtida flödesbelastning på Stockholms huvudavloppssystem (‘A study of the future discharge load on Stockholm’s sewer system’) was carried out within the framework of the Mistra SWECIA research programme by SMHI and Stockholm Vatten. The results are based on extensive projections with mathematical models using climate scenarios and historical data for precipitation, temperature, evaporation and water levels in Lake Mälaren.