Infiltration and inflow of non-sewer water to the wastewater network (I/I-water) may have a number of both financial and environmental consequences. In Norway, there are two commonly used methods for calculating the volume of I/I-water, The Dilution method (DM) and the Water Balance Method (WBM). When comparing the methods, the WBM gives a lower value of I/I-water than the DM. Analysis shows that the volume of I/I-water for some large Norwegian wastewater plants is decreasing. From 2009 to 2016, the average value has decreased from 70% to 66% of the total annual flow. For investigated Danish districts the average amount of I/I-water is stable, on about 30%. Calculations performed by the Finnish Water Utilities Association shows a stable percentage of I/I-water on about 40% in Finland from 2010 to 2016. Calculations on Swedish wastewater plants show a reduction in I/I-water from 58% to 46% from 2010 to 2016. For the districts Asker, Bærum, and Drammen in Norway, the amount of I/I-water is increasing with increasing percentage of combined sewer systems. This is also the case for investigated plants in Norway, Sweden, and Finland. The exception is Denmark, with a high percentage of combined systems, but a low percentage of I/I-water. Investigations done for Asker, Bærum, Drammen, and the two Danish districts Randers and Esbjerg vest, show a correlation between rainfall and I/I-water only for Asker and Esbjerg vest.

Urban flooding caused by heavy rainfall is expected to increase in the future. The main purpose of this study was to investigate the variables characterizing the placement of a house, which seem to have an impact when it comes to the exposure to floods. From the same region in Norway, data from 347 addresses were derived. All addresses were either associated with insurance claims caused by flooding or were randomly selected. A multivariate statistical model, Partial Least Square Regression (PLS), was used. Among others, the analysis has shown that the upstream, sealed area is the most significant variable for characterizing properties’ exposure to urban flooding. The model confirms that flooding tends to occur near old combined sewer mains and in concave curvature, and houses located in steep slopes seem to be less exposed. Using this method, it is possible to rank and quantify significant exposure variables contributing to urban floods within a region. Results from the PLS-analysis might provide important input to professionals, when planning and prioritizing measures. It can also predict flood-prone areas and make residents aware of the risks, which may induce them to implement preventive measures.

Infiltration and inflow water (I/I-water) is a big challenge in sewage systems in many countries. I/I-water above an acceptable level indicates that the sewage system is not functioning properly. I/I-water leads to increased pumping costs and increased sewage overflow, leading to increased pollution of the receiving waters. Many rehabilitation projects are driven by the need to reduce the share of I/I-water and common measures are to replace pipes and manholes. The share of I/I-water is predominantly driven by rainfall. This makes it difficult to document the efficiency of mitigating measures. One way to address this issue is to compare data from rehabilitation areas to areas where no measures have been implemented. Three rehabilitation areas in Asker Municipality, Norway, were successfully assessed by applying this approach. Asker has a 100% separate system. The strategy to reduce I/I-water in Asker Municipality was to rehabilitate sewage mains, either by full replacement or lining the old pipes, and replacement of manholes. The assessment shows that rehabilitation of selected municipal pipes, pipes proven to be in bad condition through closed circuit TV inspection, reduced the share of I/I-water only to a limited extent. Since the rehabilitation done was not a complete replacement of all pipes and manholes, the limited effects are assumed to be caused by the water finding other ways into the system. In separate systems other measures than renovations of pipes should be considered when aiming to reduce I/I-water.