Wastewater—Sewage and Stormwater Management in Ashfall

Ash can quickly accumulate in stormwater and sewage pipelines to reduce and completely block the passage of water and debris. Once in the wastewater pipelines, it is difficult and expensive to remove. It can also damage or destroy mechanical components of wastewater systems. Top priority should be given to prevent as much ash as possible from entering stormwater and sewage pipelines.

Wastewater systems

Measures to protect stormwater and sewage systems should be implemented before or during ashfall, and coordinated with community residents and businesses during clean up operations after ashfall.

Suggested measures for reducing effects of ash to sewage and stormwater systems:
  • Have local ordinances in effect that ban connections of downpipes and roof drains to the sewer.
  • Instruct the public how to protect stormwater systems (protect manhole covers from ash, disconnect downpipes when possible, ash disposal instructions).
  • Instruct citizens where to deposit ash cleared from property.
  • Warn citizens against disposing ash down manhole access of both sewer and stormwater systems.
  • When hosing streets, place sand bags around or over manhole covers or avoid them entirely, because holes in the cover and the areas between the cover and the rings allow passage of ash.
  • When possible, disconnect downpipes from the stormwater system until ash is removed from the roofs of homes and buildings.
  • Closely monitor the cleanup activities of privately-owned parking areas.
  • Use dry methods, like hand sweeping, prior to flush cleaning with water when clearing streets and parking areas that are served by a free-discharging or dry-well stormwater system.

Note: Shallow deposits of ash in the stormwater or sewerage system will not reduce the hydraulic capacity of the pipes by a significant amount; thus expenditure of time and money in these circumstances to clean lines may not be warranted. Modified from, FEMA, 1984

Sewage treatment facilities

The most effective mitigation measure for treatment facilities is to reduce the input of ash. Sewage treatment facilities can be severely affected by either ashfalling directly on the facility or receipt of ash-laden sewage. Bypassing and/or shutting down parts of a plant may need to be considered to reduce the likelihood of damage. All sewage treatment facilities should have planned measures in place for dealing with volcanic ash, especially with regards to back-up power provisioning.

Suggested measures for protection of sewage treatment facilities from volcanic ash:
  • Temporarily cover all equipment (mechanical, biofilters), including ventilation intakes, that might be directly exposed to the ashfall before or during the ashfalls.
  • Shut down all equipment not absolutely necessary. Where possible, place sandbags or other devices at the entrance to the facilities to trap ash. This will require frequent checks due to normal settleable solids present in sanitary sewage.
  • Consider removing or bypassing the comminutor during the intitial heavy flows of ash into the facilities.
  • Frequently check the primary clarifiers to prevent (a) damage to the sludge collection mechanism and/or the digesters sludge pumps; and (b) the transference of ash to the digester. Depending on the type of mixing employed in the digester, further damage may occur in the sludge transfer pumps.
  • To clean ash from individual sections of the treatment facility, bypass individual units, or in extreme instances, make a complete plant bypass to a holding pond or lagoon.
Modified from, FEMA, 1984