Wastewater—Sewage and Stormwater Management in Ashfall

Ash-laden sewage can cause extensive and costly damage if it enters a wastewater treatment plant. In particular, mechanical pre-screening equipment is highly vulnerable to damage. It may be preferable to bypass the treatment plant and discharge raw or partially-treated sewage to the environment to protect the plant from damage. Very thin ashfalls of <2 mm should cause few problems for most waste treatment plants. However, an ashfall of only 10 mm (following the may 1980 eruption of Mt St Helens volcano) on the city of Yakima, Washington was sufficient to cause $US 4 million (1980 value) damage to the city's wastewater treatment plant, primarily because of damage to the mechanically-cleaned bar screen and grit classifier.

Impacts of ashfall on wastewater treatment plant system component.

System Component Impacts of Volcanic Ashfall
Pre-treatment equipment
  • Ash will damage comminutors and grit classifiers
  • Coarse (>1 mm) ash will block mechanical screening equipment, overloading motors and gear boxes.
Primary settling tanks
  • Ash will increase the solid load arriving at the plant.
  • Coarse ash will increase the volume of sludge for disposal.

  • Ash will change the proportion of inorganic matter.
Secondary treatment
  • Ash will enter open reactors (biological reactors) and tanks from direct fallout, but the main ingress is likely to be through the sewer lines.
  • The main effect is likely to be a reduction in capacity (due to ash accumulation on tank floors) rather than interference with biological processes.

  • Cleaning reactors while the system is operational.
Tertiary treatment
  • Any residual very fine ash may increase the suspended solid load of the effluent, necessitating adjustments to disinfection treatment.
Sludge treatment
  • Ash will increase the mineral content of sludge.
General effects on plant
  • Ash will cause accelerated wear and tear on pumps and motors.