Dilution of sewage: Is it, after all, really worth the bother?

•On average, we found that 50% of dry weather flow is caused by I&I (Flanders, B).•Removing I&I is only relevant for treatment plants if the standards are endangered.•Catchments with high I&I fractions show an increase of CSO spill volumes up to 35%.•Economic consequences of extraneous water are modest.•Strategies to tackle dilution input should mainly focus on quick-win projects.