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Waste Water

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Waste water systems (sewage and storm water) are vulnerable to damage from volcanic ash because the tiny rock fragments can block pipes and canals, increase wear on pumps and other machinery, and interfere with water-treatment processes. When ash falls on impervious surfaces, such as roads, roofs, and other paved areas, the ash is easily washed into waste water systems by rain and water used during clean-up operations.

Removing ash from sewage and waste water systems is time-consuming and costly. Keeping as much ash as possible from entering a community's waste water system is a top priority for reducing the potential effects of ash fall—coordination of community-wide clean-up efforts is critical after ash stops falling, but especially if it is raining during ash falls to keep ash out of the wastewater and sewage collection systems.


Ash enters a waste water system just as water does—through drains along streets, access covers (for example, manholes covers), roof downspouts, canals, and illegal connections—and by falling directly on water-treatment facilities. Measures to protect stormwater and sewage systems should begin before and during ash fall events.

The degree to which sewage and storm-water drainage lines are interconnected will increase the detrimental effects of ash on a community's sewage treatment facilities and capability—where sewage and storm water are collected into a single pipe network, the consequences can be severe. For example, in Yakima, Washington, located 140 km east of Mount St. Helens volcano, water treatment equipment experienced failures within 3 days of a 1-cm ash fall, resulting in the release of raw sewage into a river for several days (see press release). About 5 percent of Yakima's sewage and storm water lines (446 km length) were combined.

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Sewage treatment facilities

Ash-laden sewage that enters a treatment plant may overload equipment and filters designed to trap solid debris at both the pre-treatment and primary treatment stages. Milliscreens, mechanical grit and sludge removal mechanisms, comminutors and other equipment may become damaged. Ash falling directly into sedimentation tanks will add to the volume of material which has to be removed. Low density pumice and finer pumice shards may float on the surface of ponds.

Volcanic ash that enters secondary treatment facilities, such as oxidation ponds or biofilters, will tend to reduce or halt the oxidation process until the ash settles out or is removed. Ash may also affect the acidity or toxicity level of effluent to such an extent that bacterial growth may be damaged or lost. If a treatment facility can no longer process the sewage or there is a deliberate shutdown to prevent or limit damage, untreated sewage may have to be released directly into rivers.

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Historical Examples  || Yakima, 1980 || Anchorage, 1992 ||

Yakima, Washington; U.S.; 1980

The 9-hour explosive eruption of Mount St. Helens in Washington State on 18 May 1980 spread volcanic ash over central Washington and a large part of the Western United States. Yakima, 140 km east of the volcano, received about 1 cm (0.4 in) of  volcanic ash from the eruption.

By the next day about 15 times the usual amount of solid matter was being removed from the pre-treatment processes at Yakima's sewage-treatment facility. Ash was also observed in the raw sludge in the primary clarifiers. Two days later, it was evident that the facility was suffering as vibrations were occurring in the grit classifier and the gear box of the mechanically cleaned bar screen. Raw sludge lines became plugged and pumping difficulties were experienced. On May 21, equipment failures and shutdowns occurred, and city officials issued the following press release (Blong, 1984):

Press Release
Office of City Manager, City of Yakima, May 21, 1980

City Manager's Office announced this morning a decision to bypass the Yakima Wastewater Treatment Plant with sewage flow to the Yakima River, due to the disaster conditions from the volcanic ash. This decision was arrived at through consultation with the Washington State Department of Ecology.

We anticipate the bypass to be effective May 22, 1980, and to last indefinitely.

The specific reasons for the bypass are as follows:

  • Volcanic ash and grit accumulations in the system form unpumpable masses forcing equipment and operational failures.
  • Trickling Filter rock media is stripped by grit which destroys its ability to remove pollutants.
  • Abrasive wear on equipment may put the plant out of operation indefinitely and also cause damage to a major investment in new equipment installed as part of the current plant upgrading.

The conditions of the bypass include the following:

  • Regular testing of influent to determine when the grit content is low enough to handle in the plant.
  • Lines and equipment will be cleaned and readied for service.
  • We will assess equipment damage and take any available measures to prevent additional damage.
  • We will monitor river conditions which at this time are most favorable with high flows.
  • We will provide full documentation of problems, discussions, tests, remedial efforts and other actions.

The total damage to the Yakima plant was estimated at US$4 million, and primary treatment of sewage resumed on May 25.

Lessons from Yakima. This experience led public works officials to complete list of things to do in order to protect waster water disposal equipment and help other cities that might experience similar ash fall (Blong, 1984):

  1. Cover all external equipment with plastic.
  2. Shut down all equipment not absolutely required.
  3. Shut down ventilation equipment where possible.
  4. Place all pre-treatment equipment into operation and adjust for maximum removal rates.
  5. Place all primary clarifiers into operation and increase pumping rates.
  6. Pump all grit and ash into one digester until conditions improve, then dump the contents.
  7. Shut down the biofilters and cover with plastic.
  8. Monitor all processes for introduction of grit and ash.
  9. Monitor torque or current on all motor driven devices.
  10. Grease everything subject to a dusty atmosphere, and those subject to pumping gritty material.
  11. Filter or change out gear lube weekly; flush with solvent recommended by the manufacturer.
  12. Alert the public to the consequences should they introduce this material into the sewer system.
  13. Notify regulatory agencies and public health officials.
  14. Formulate a plan to bring the plant processes and equipment back on line.
  15. Contact equipment manufacturers for their assistance.
  16. Step up preventive maintenance scheduling until the system is completely purged of contaminants.

Anchorage, Alaska; U.S.; 1992

The city of Anchorage was covered by 3 mm of volcanic ash from the August 1992 eruption of Mt. Spurr volcano, located 125 km to the west. Although care was taken to minimize the amount of ash entering the stormwater system, large amounts of ash still had to be removed the following spring. Ash did not move far into the system but settled out rapidly, eventually forming a hardened deposit. During the spring thaw some local flooding occurred due to pipe blockages. The ash was removed from the pipes using vacuum trucks.

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Keeping Ash Out  || wastewater systems || sewage treatment facilities ||

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 pipeplines.

Wastewater systems

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

Suggested measures for reducing effects of ash to sewage and stormwater systems
  • Have local ordinances in effect banning 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 systems.

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 ash falling 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.


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 ash fall before or during the ash falls.
  • 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

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Blong, R.J., 1984, Volcanic hazards: a sourcebook on the effects of eruptions: Academic Press, Australia, 424 p.

Federal Emergency Management Agency (FEMA), 1984, The mitigation of ashfall damage to public facilities: lessons learned from the 1980 eruption of Mount St. Helens, Washington. Region X, Wm. H. Mayer, Regional Director. 1984.

White, W.T., Day, T.G., Berteck, D., Fisher, J., 1980, Mt. St. Helens volcanic ash vs. Yakima wastewater treatment facility, Institute for Water Resources Newsletter, 6(2), p. 4-8.

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