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The potential for public health risks associated with intrusion of contaminants into water supply distribution systems resulting from transient low or negative pressures is assessed. It is shown that transient pressure events occur in distribution systems; that during these negative pressure events pipeline leaks provide a potential portal for entry of groundwater into treated drinking water; and that faecal indicators and culturable human viruses are present in the soil and water exterior to the distribution system. To date, all observed negative pressure events have been related to power outages or other pump shutdowns. Although there are insufficient data to indicate whether pressure transients are a substantial source of risk to water quality in the distribution system, mitigation techniques can be implemented, principally the maintenance of an effective disinfectant residual throughout the distribution system, leak control, redesign of air relief venting, and more rigorous application of existing engineering standards. Use of high-speed pressure data loggers and surge modelling may have some merit, but more research is needed.

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An analytical basis defining the steady-state conditions occurring in an electrolysis cell is stated. Several assumptions are made which permit a theoretical determination of the void fraction and current density distributions. The effect of void fraction and inlet velocity on cell performance is shown. Experimental data for the void fraction and slip ratio occurring in an electrolysis cell are presented. It is found that the slip ratio is near unity. This result is of importance in defining the cell void fraction distributions and is prerequisite to an analysis of pressure drop in electrolysis cells.