Water Supply

A volcanic ashfall event can cause a range of operational problems for water treatment plants. In general, ashfall is unlikely to cause service interruptions, but a great deal of increased maintenance can be expected. Ash-induced electricity outages are the most common cause of disruptions to water production after an eruption.

Effects of ashfall on water treatment plants:
  • Turbidity may be satisfactorily removed by normal coagulation/flocculation treatment.
  • If turbidity exceeds the normal operating range of the plant for storm flows, suspended.
  • ash may penetrate further into the treatment train and block filtration equipment.
  • Ash can also enter open air filter beds by direct fallout.
  • Ash is highly abrasive and likely to cause accelerated wear on pump impellers.
  • Ash can penetrate bearings and seals and overload motors.

System design

The design of water treatment systems is critically important in determining their vulnerability to a volcanic ashfall event. In general, the more 'closed' the system, the more resilient it will be. Important factors contributing to vulnerability of water supply systems are discussed further below.

Water source

  • Groundwater-fed systems are generally very resilient to volcanic ashfalls, although pumping equipment may be vulnerable to airborne ash penetrating motors and to power outages.
  • Systems with intakes in lakes/reservoirs will experience increased turbidity but other water quality changes are likely to be minor.
  • Systems with intakes on rivers or streams are highly vulnerable because of large catchment area to volume ratio; however design features can reduce vulnerability.

Gravity feed versus electrical pumping

Pump impellers are highly vulnerable to accelerated wear and tear from ash-laden intake water. Systems that utilise gravity feed rather than electrical pumping are less vulnerable to this mode of damage.

Automated shutdown at intake

Systems with automated shutdown of intake if a defined turbidity threshold is exceeded are able to limit the exposure of pumping equipment and treatment plant filtration equipment to suspended ash.

Uncovered plant (e.g. sand filter beds)

Ash can enter open sand filter beds by direct fallout, and its removal will necessitate a greatly increased level of maintenance.

Redundancy in equipment

Equipment (e.g. pressure sand filters) operating in parallel provide the ability to maintain production while closing down filters alternately for maintenance.

Storage capacity

The greater the storage capacity of the system, the more flexibility it will have to allow production to stop temporarily for testing of raw water sources or preventive maintenance.

Backup power generation

Ensure that this is in place.