Wastewater—Sewage and Stormwater Management in Ashfall

Wastewater (sewage and stormwater) collection systems are vulnerable to damage and disruption from volcanic ash. Ash can enter sewer lines where there is inflow or infiltration through illegal connections, cross connections, gully traps, manhole covers and cracks in sewer pipework. Cities that have combined sewer and stormwater collection systems are particularly vulnerable as ash will readily enter stormwater drains and can then cross over into sewer lines.

  • Ash can form unpumpable masses, block catchpits, sumps and drains and cause surface flooding. It can be very difficult to remove from underground drainages.
  • Ash can cause severe wear and tear to pump impellers (pitting and thinning of metal).
  • Ashfalls can cause power outages which will affect pumping stations that do not have backup power generation. Interruptions in pumping can lead to overflows if storage capacity is exceeded.

In general, few effects can be expected for very thin ashfalls (<2 mm). At ash depths of >20 mm, effects may include severe intractable blockages and surface flooding which may require extensive repairs and/or maintenance. Factors affecting the severity of impacts include:

  • the extent and speed of ash cleanup, and the steps taken to keep ash out of sewers and storm drains;
  • rainfall (heavy rains may wash ash into drainage systems before cleanup has begun)
  • ash characteristics, particularly particle size and density.