This page contains a Flash digital edition of a book.

installation completed in 2002. Fundamental research was done on DAF at the University of Cape Town with an initial interest for treating sewage effluents and for thickening, but by the late 1970s DAF was examined to treat eutrophic water supplies, culminating with construction of several plants in the 1980s.

Powerful process DAF is effective in separating low density particles from waters, such as algae, natural colour, or low mineral water turbidities, and is less sensitive than settling in treating cold waters. Following coagulation and flocculation, the densities of the flocs containing algae and metal hydroxide precipitated particles are low, as are metal-humate precipitate and metal hydroxide precipitated particles formed from coagulating waters with natural colour and low turbidity. An important recent study examined 1,400

water plants in the USA and Canada and examined treatment processes used in the plants against raw water quality (Valade et al., 2009). From the data the authors proposed process selection guidelines with respect to raw water quality. For raw water turbidity, the authors distinguished between mineral and non-mineral turbidity, and recommended DAF for average raw water mineral turbidity supplies of 10 Nephelometric Turbidity Units (NTU) or less, usually river types. DAF is recommended for reservoir type supplies with non-mineral turbidity of 100 NTU or less. For maximum raw water quality, DAF is

recommended as long as the maximum mineral turbidity is 10

oocysts, DAF is more effective than sedimentation, since for conventional design hydraulic loadings DAF can achieve 2 to 3 log removals compared to removals by sedimentation of 2 log, decreasing to 1 log or less for winter water temperatures. An advantage

External view of Stamford, CT plant: New DAF addition is in the lower lefthand corner

for DAF plants is filtration serves as a polishing step since most of the pathogens are removed by DAF and leave the system in the floated sludge as opposed to leaving in waste filter backwash water which is often recycled. DAF can be more effective in removing

particles and turbidity than sedimentation processes, leading to lower particle counts, which at conventional rate filters (say 10 – 15m/h) will produce longer filter run times and less frequent backwashing. A measure of the filtered water production is the unit filter run volume (UFRV): the quantity of water in m3 produced in a run divided by the filter area in m2. Pilot studies done with DAF for Boston found UFRVs of 800m3/m2 or greater, similar to data collected in treating the Croton water supply for New York City. Pilot studies collected for Canada’s largest DAF

plant in Winnipeg found UFRVs of about 600m3/m2. An extensive evaluation over an entire year of the full-scale DAF plant in Fairfield, CT, showed DAF turbidities generally less than 0.4 NTU and filtered water at 0.07 NTU. High water production was obtained with an average UFRV for the year of 640m3/m2. Even greater UFRVs could be achieved by the plant if the filters were run longer, but filters are backwashed on a schedule basis at night when power costs are less.

Precious progress

With significant developments in the technology, driven by more fundamental understanding of the processes, DAF has now gained universal acceptance for large water supply plants. Pretreatment flocculation times have seen a major reduction from about 20-30 minutes, used early on and into the 1980s, to about 10 minutes now. In the NYC Croton water plant, the world´s largest DAF, the pilot facility with a total two-stage flocculation time of five minutes

42 Water & Wastewater Treatment September 2010

showed good performance and this design parameter was retained for full scale. At the same time, the conventional hydraulic loadings of 5-15m/h used exclusively until the late 1990s have been extended by alternative high rate DAF systems. This latest trend is to apply nominal loading rates of 15-30m/h and greater, many at 20-40m/h, based on the treated water flow (excluding recycle), and the gross footprint area. The system developed by Finnish company Rictor Oy is available around the world by license to Degrémont as Aqua-DAF. A large plant, Haworth in northern New Jersey, with a capacity of 760Ml/d, began operation in June 2009. A key feature of the Rictor DAF tank is the orifice plate floor at the bottom that produces good flow distribution in the separation zone and at the outlet. Other high rate DAF systems are available from ITT (Leopold Clari-DAF ), Purac (DAF-Rapide) and Enpure (Enflo-vite). Twenty-five years after the Lenox start-up and

my first encounter with DAF there are now in the USA and Canada an estimated total of around 150 DAF installations, 10% of them larger than 50Ml/d. Canada built its first DAF plant in 1996 at Port Hawkesbury, Nova Scotia, leading to some 30 DAF plants in the Maritime Provinces of Canada alone. A recent worldwide survey by Haarhoff found some 62 larger DAF plants with a capacity of 50Ml/d or greater in 18 countries. A large DAF plant for 400Ml/d plant for Winnipeg went on-line in late 2009, and the new Croton plant for New York City, also a combined flotation over filtration configuration, is under construction for a capacity of 1100Ml/d and scheduled to go on-line in 2011. ■■■

Professor James Edzwald’s original article can be found in Water Research journal (Volume 44, Issue 7)

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52
Produced with Yudu -