DAF Process Solves Problems for New York State Water Utility

While the process has been used by the industrial sector for many years, including in wastewater treatment applications, dissolved air flotation has not been applied very often in municipal water or wastewater situations. However, one quite recent example is to be found in New Castle, New York. The DAF approach was taken at the 7.5 million gallon per day (mgd) Millwood Water Treatment Plant serving that community to solve a turbidity problem. The upgrade project also brought ozone disinfection and an advanced settling system for cleaning the filter backwash waste into the facility's process train. The result is that Millwood now is said to be well prepared to handle the water treatment requirements of the town for the foreseeable future.

The New Castle water system, located in Westchester County, was started up in the 1930s. Its source of supply was New York City's Catskill Aqueduct. Twenty five years later an expansion required a tap in to the Croton Aqueduct, and by the 80s another expansion of capacity was necessary. And by that time the finished water standards called for by the Safe Drinking Water Act and its amendments meant that process performance had to be upgraded to produce the higher quality levels required.

Raw water quality is a determinant in the design of any water treatment facility. In New Castle's case, two main factors had to be considered. First, the incoming raw water exhibited wide seasonal turbidity variations. Spring rains combine with low reservoir levels to raise the turbidity of the water in the Catskill Aqueduct to 100 NTUs or more. To counteract this to some degree, the flow in this aqueduct is reduced to roughly half the peak rate of about 600 mgd, and extraction from the secondary supply is increased to compensate.

This strategy has a drawback however. Lowering the flow rate through the inverted siphons in the aqueduct system causes large volumes of air to be entrained. This tends to produce supersaturated conditions in the water supply reaching the treatment plant. In many cases this problem can be resolved by constructing a stilling basin to allow escape of the air. However, the Millwood facility's situation precluded this. There were cost and space limitations. Consequently, the upgrade project's design team were faced with finding a process arrangement which would deal successfully with a raw water supply that was both high in turbidity and entrained air.

The design study led to the selection of the dissolved air flotation concept, and the system subsequently acquired for the New Castle upgrade was supplied by Purac Engineering, Inc., of Wilmington, Delaware. This firm has roots in the United Kingdom, where its parent has retrofitted a number of conventional sedimentation systems in British water utility plants with DAF technology.

Millwood's treatment process is illustrated in the schematic drawing. It starts with two-stage rapid mixing of seven chemicals. Two basins in series provide one minute each of mixing. The process stream then is distributed to five identical treatment units, each with three flocculators and one 29 by 15 ft DAF clarifier. Design flow detention time is 30 minutes in the flocculators followed by 20 minutes in the clarifier.

After the clarification step the water is sent to two ozone contactors for primary disinfection. Then it passes through six gravity flow filters with anthracite over sand as the filter media. Finally, before being pumped into the town's distribution system, the water is finished with an injection of a small quantity of chlorine to ensure a disinfected condition in the distribution network.