TECHNICALLY SPEAKING Taking the DAF approach
Aqualia’s Frank Rogalla calls on the expertise of Professor Jim Edzwald of the University of Massachusetts to review the development of DAF treatment
M
y first job after finishing graduate school while on a Fulbright Scholarship in the US was at the
Lenox Institute of Research (LIR) in Western Massachusetts, starting in the autumn of 1985. Lenox has a population of only 5,000 people, but is well known as the summer home of the Boston Symphony Orchestra with the Tanglewood Music Festival, and for multi- millionaire mega-mansions built as summer homes at the beginning of the 20th century. It was here that the application of dissolved air floatation (DAF) to drinking water began in the USA, when a first demonstration plant went on- line in 1982, replaced by a new and permanent facility in 1994. Those DAFs were supplied by my employer, Krofta Engineering Corporation, who owned the LIR, and had developed a specific treatment configuration with all processes packaged into one circular unit – flocculation, clarification, and filtration – and using low loading rates and high recycle rates. Two similar plants were commissioned in the winter of 1986-87 in the neighbouring town of Pittsfield, MA; one of the plants has a fairly large capacity of 90Ml/d. General opinion in the US at the time was that flotation had no application to clarification of drinking waters, and its applications were limited to the mining industry and to certain industrial wastewater treatment applications – despite DAF being used in Scandinavia for drinking water treatment since the 1960s. Therefore, driven by the interest of Massachusetts in whether DAF was a viable treatment process, a research project on DAF was sponsored in 1985 by the US Environmental Protection Agency (USEPA). Professor Jim Edzwald of the University of Massachusetts in Amherst, with his PhD student, James P Malley, Jr, was commissioned for this three year study to develop fundamental process principles for DAF and to examine the process for treating supplies containing algae and humic substances (natural colour).
European experience This work was later followed up by a research project funded by the Water Research Foundation (then AWWA RF), which involved examination of the performance and costs of several DAF plants in Norway, Sweden, and England. Further work was later sponsored by Purac in Sweden and by Anglian Water, with pilot-scale facilities set-up in Newport News, VA
Graphic showing the stacked DAF and filter in Stamford, Connecticut
and Fairfield, CT, to examine high rate flotation and integration with flocculation and filtration. The first DAF plant to use UK based design and operating concepts was commissioned in 1993 – Millwood plant at New Castle, NY. Research was continued in Connecticut,
where the Aquarion Water Company later commissioned four full-scale DAF plants, among them: ■ BA 190Ml/d plant in Fairfield, CT, commissioned in 1997
■ BA 115 Ml/d plant in Stamford, CT, commissioned in 2007
Fairfield, with a nominal design DAF and filter loading of 15m/h and 30 dual media filters, has the originality of combining flotation over filtration, in a vertical arrangement that allows a much smaller plant footprint and significant construction cost savings. This process variation was first developed by in Sweden in the 1960s for small package plants with capacities of up to 4Ml/d. Elsewhere, DAF as a drinking water clarification
method began much earlier, with vacuum systems used during the 1920s – at least two of these plants were still operating in Sweden in the
“In the Netherlands, several DAF plants are used primarily in treatment of algal-laden waters. UK designs have been applied as far as Hong Kong”
1970s. In the 1960s DAF as we know it, with pressurised recycle for production of air bubbles, was examined in Finland and Sweden. These DAF systems used higher hydraulic loading rates of 5-10m/h and deeper tanks than the vacuum systems. Sweden had pressurized type DAF plants in operation beginning in the 1960s, with numerous plants built beginning in the late 1960s and continuing since then in Sweden, Finland, and Norway. Finland had its first plant in 1965 and by the 1970s DAF was the primary clarification method for treating surface waters rather than settling. The Water Research Centre in the UK began
extensive laboratory and pilot plant studies on DAF in the early 1970s, showcased at the first international conference on flotation was held in the UK in 1976. These practical studies demonstrated the efficiency of DAF to treat algal- laden waters and low turbidity waters containing natural colour, compared to settling; and the developed design and operating criteria were incorporated into DAF plants in Great Britain in the 1970s and 1980s. In the Netherlands, several DAF plants are used primarily in treatment of algal-laden waters, with the first installation dating from 1979. UK designs have been applied as far as Hong Kong for the Tai-Po DAF plant, designed for 250Ml/d in the first stage, and Singapore, where pre- treatment for the 136Ml/d desalination plant in Tuas is by a combined flotation and filtration process. Already in the 1960s there were extensive studies done on DAF in Namibia and South Africa, with a wastewater reclamation plant completed in 1968 in Windhoek using DAF for potable water reuse, replaced by a new DAF
September 2010 Water & Wastewater Treatment 41
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