TECHNOLOGY
THE STREAMING CURRENT DETECTOR IN WATER TREATMENT by D. R. DIXON, W. BARRON, T. W. HEALY, M. PASCOE and P. J. SCALES ABSTRACT The use of streaming current detectors (SCD) as a method of controlling coagulant dosing in water treatment plants has been reviewed in a project funded by the Urban Water Research Association of Australia (UWRAA) and jointly carried out by the CSIRO Division of Chemicals and Polymers, the Advanced Mineral Products Research Centre at the University of Melbourne and the Brisbane City Council. It involved theoretical and practical studies that included laboratory tests, pilot plant trials and full scale plant operation. A laboratory examination of several electrolytes and standard suspensions showed excellent correlation between data obtained from SCD measurements and from conventional electrokinetic techniques, namely electrophoretic mobility and streaming potential and led to the development of an improved theory of SCD measurement. An SCD was used in extensive pilot plant trials at the North Pine Water Treatment Works in Brisbane. SCD signals responded to changes in coagulant dose and pH for alum, ferric chloride, polyaluminium chloride and two cationic polyelectrolytes. Full scale trials at Westbank Water Treatment Plant, Brisbane, confirmed these data as raw water characteristics varied during several rainfall events. The Westbank Water Treatment Plant at Mt Crosby was the site of a full-scale plant investigation of the SCDs capacity to control alum dose. Plant parameters including alum dose, pH, raw water turbidity, conductivity and colour and SCD response were monitored. Several high turbidity and high colour events occurred during an unusually wet summer period providing an opportunity to study the proficiency of the instrument over a wide range of conditions. Finally, the SCD was used to control alum dose on the full scale plant for a short period. It is concluded that an SCD has the capability of controlling coagulation processes, provided plant operators have sufficient working knowledge of the instrument. Practical recommendations are made to both the manufacturers of the instruments and to operators.
INTRODUCTION Most modern water treatment plants use a coagulation stage to ach ieve clarification. Coagulants such as aluminium sulfate (alum) or ferric chloride are added and the pH adj usted to promote the formation of metal hydroxide floes which in turn result in the removal of unwanted colour bodies and turbidity particles. Unfortunately, inefficient method s of determining the optimal coagulant concentration often result in overdosing, which increases operating expenses not only because of higher than necessary chemical costs, but also because of increased sludge handling and disposal costs. Incorrect coagulant dosing can lead to an increased residual of dissolved coagulant in the filtered water and possible precipitation within the distribution system . Although the SCD was invented by Gerdes (1966), it has only recently been considered as an alternative to the traditional jar test method of determining optimal coagulant dose (Dentel 1988) . The aim of the present work was to examine the mechanism of operation of SCDs with a view to understanding its capability and capacity in controlling coagulant dosage in water treatment processes.
JAR TESTING Oft quoted criticisms of jar testing include (i) Changes in raw water conditions can occur more rapidly than the jar test can be performed. Or in other words, the most useful time to do jar tests is when the raw water is changing rapidly, precisely when an operator can least afford the time to do jar tests.
Dr. David Dixon is a Senior Principal Research Scientist with the CS/RO Watertek program in the Division of Chemicals and Polymers. He has a PhD from Melbourne University, in colloid and surface chemistry, and over twenty years's experience in the fields of water, wastewater and industrial effluents.
Wendy Barron is a Chemist in the Environment and Wastewater Laboratory in the Government Chemical Laboratories in Brisbane. She graduated in Applied Chemistry from the University of Central Queensland in 1989. For the duration of this project she worked at the University of Melbourne and with the Brisbane City Council.
Thomas Healy is Director of the Advanced Mineral Products Centre and Professor of Physical Chemistry at the University of Melbourne. His research interests include colloid and surface chem istry, aque ous interfaces, interfacial spectroscopy, ultrasmall co lloidal particles, stability of colloidal dispersions and the chemistry of mineral processing.
Mark Pascoe is Chemist-in-Charge, Water, Treatment, with Brisbane City Council, where he has worked for 20 years. He now manages the operation of the Brisbane Water Treatment Plants. He has postgraduate qualifications in environmental engineering and is interested in most areas of water management.
Peter Scales graduated BSc(Hons) and Ph.D from Melbourne University in 1981 and 1988 respectively. He then worked in the coal industry in colloid chemistry related areas and in the UK with JC/ Corporate Colloid Science Group. He is now managing Advanced Mineral Products Research Centre and working on a range of research topics in surface chemistry.
T his paper is a short ve rsion of th e fin al report to U WR AA , No. 44, {Barron, 1992) which co mains the complete se1 of experimental data and a more detailed interpretati on. Papers concent rati ng on the more fundamental aspects of strea mi ng cu rrent measure men ts are still to be writte n. A fu n her paper is contained in the Proceedings of the AW\VA 15th Federal Convent ion.
WATER April 1993
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