Following an eruption, it is very common for the public to hold fears about chemical contamination of water supplies. However, in general, the physical impacts of an ashfall will tend to overwhelm problems caused by the release of chemical contaminants from fresh volcanic ash. Impacts vary according to the type of treatment system.
It is very difficult to exclude ash from the sewerage and stormwater collection networks. Systems with combined storm water/sewer lines are most at risk. Ash will enter sewer lines where there is inflow/infiltration by stormwater (e.g., cross connections, around manhole covers, or through holes and cracks in sewer pipes).
Ash-laden sewage entering a treatment plant is likely to cause failure of mechanical prescreening equipment such as step screens or rotating screens. Ash that penetrates further into the system will settle and reduce the capacity of biological reactors as well as increasing the volume of sludge and changing its composition.