White, J.; Stewart, C.; Wareham, D.; Wilson, T. 2011 Treatment of volcanic ash-contaminated surface waters through the optimisation of physical and chemical processes. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/35 30 p.
Abstract: Safe drinking-water, available to everyone, is a fundamental requirement for public health. A safe water supply is also essential for hygiene and sanitation. Volcanic activity can damage or disrupt water supplies in a number of ways. Experience from previous eruptions has shown that in general, for large municipal water treatment systems fed from surface waters such as lakes or reservoirs, the physical impacts of a volcanic ashfall will tend to overwhelm problems caused by chemical contamination resulting from the release of soluble components from the surface of the ash. A particularly important problem is that ash suspended in water increases the turbidity of the raw water supply. Turbidity is a critically important parameter for water treatment, and effective treatment depends on its control. The overall objective of the research described in this report was to investigate a physical (plain settling) and chemical (addition of coagulant) method for removing suspended ash from natural waters. For this work we used a range of natural and synthetic volcanic ash samples with varying characteristics. The first phase of our investigation was a study of the settling rates through water of different volcanic ashes using plain sedimentation alone. Ash/water suspensions will undergo a rapid initial reduction in turbidity, but there may be a level of residual turbidity that is not amenable to further reductions by plain settling alone. The level of residual turbidity depends on the grain size distribution and surface chemistry of the ash, the initial concentration of ash in water and the chemical composition of the water. Grain size distribution is a major control on settleability of volcanic ash. Ashes from Ruapehu volcano (1996 eruption) and Pacaya volcano (2010 eruption) were both coarse-grained and had high settleability, with turbidity reducing from initial values of approximately 10-30 NTU at the start of the trial to