DAF & Ozone Eliminate Quality Woes, Meet New Regs
A New York State town developed a water treatment plant which uses dissolved air flotation and ozone disinfection to ensure meeting anticipated tighter regulations
When a town meeting in New Castle, New York, prompted local government officials to take citizen complaints about water quality seriously, they initiated a lengthy multi-step project to fix the problems. They could not have known then, in the early '80s, that their decision to proceed was timely. The water system improvement program they embarked upon anticipated some of the key regulatory requirements put in place years later, and solved problems of quality and quantity that arose as a result of the growth and development of the town and its surroundings.
The citizen complaints were about cloudy water, brown water, water with an unpleasant taste and an odor that wasn't too good either. Now New Castle's supply is treated in the up-to-date Millwood plant, which can handle the variable incoming raw water quality that seasonal fluctuations produce, and deliver a finished product which meets current and expected regulatory limits.
Developed about 60 years ago, the town's water supply was obtained from New York City's Catskill Aqueduct through an aqueduct connection and a pumping station. Later, in the '50s, a second source was tapped-the City's Croton Aqueduct. The water from each was disinfected with chlorine but no filtration was carried out.
Then came the Safe Drinking Water Act of 1974, and the Amendments to it in 1986. The Amendments called for the USEPA to develop national primary drinking water regulations. This led to the 1989 Surface Water Treatment Rule, which said surface supplies should be filtered. These supplies, such as the New York City aqueduct system water delivered to New Castle, are subject to, for instance, contamination by animals and birds. Of particular concern are Cryptosporidia, Giardia lamblia, Legionella, viruses, and certain bacteria.
For the last four decades the New York-based engineering firm of Hazen and Sawyer has served as consultant to New Castle. When it became clear a dozen years ago that the town had to undertake a water supply upgrade and expansion program, the engineers were asked to recommend solutions to the growing number of problems. After a study the firm proposed completely relining the old and deteriorated water mains, building a new pumping station to boost supply to the distribution system, and constructing a brand new water treatment plant around a process scheme that would be based upon the latest technology.
Hazen and Sawyer provided planning, design, construction administration, start-up and operator training services for the Millwood treatment plant and pumping station project. The firm was awarded the first prize in the New York Association of Consulting Engineers' 1994 Engineering Excellence Award for its significant role in the Millwood project, which qualified the entry for the national award program conducted annually by the American Consulting Engineers Council.
The task of relining the water mains was started in 1984, the pumping station construction work began in 1988, and the treatment plant project got underway in 1990. The new pumping station, along with an additional connection to the Catskill Aqueduct, went into service early in 1992, and the water treatment plant was started up in August of 1993. Cost of the project was $26 million-$5 million for the pump station and $21 million for the plant. Water rates were raised to pay the debt on the bonds issued for the project, but a number of ways to save money were recommended by a special Water Advisory Committee. One of these was to engage the services of a contract operations firm. As a result, the facility is being operated for New Castle by the firm of Mid-Hudson Pollution Control, Inc., of Wappingers Falls, New York, under a five-year renewable contract.
Advanced Processes Incorporated
The 7.5 mgd Millwood Treatment Plant uses several up-to-date processes to produce finished water that more than meets federal water quality standards. In fact, it is the first full scale installation in the U.S. of a European technology, dissolved air flotation (DAF), to accomplish clarification is to be found on the site. In conjunction with this is the use of ozone as the primary disinfectant in the process train.
Pilot studies had determined that DAF would be a cost-effective way to clarify the raw water from the Catskill Aqueduct. This water supply exhibits reasonably low turbidity much of the year, but during some months the water becomes more turbid and contains significant quantities of entrained air. In the spring, when the reservoirs usually have been drawn down substantially, heavy rains in the watershed area can cause turbidity levels to increase to unacceptable levels. When this occurs, New York City reduces the flow in the Catskill Aqueduct from about 600 mgd to half of that figure, and makes up for the decrease by withdrawing more from other sources.
One effect of the lower flow is to cause large quantities of air to become entrained at the numerous inverted siphons along the course of the aqueduct. The result is that the water withdrawn from the system has become supersaturated with air, a condition which affects the performance of conventional settling clarifiers adversely. A large stilling basin could have been installed to allow release of the supersaturated air, but space was not available on the plant site for such a facility. It was clear that the treatment process selected for New Castle would have to handle high turbidities and supersaturation simultaneously. The DAF approach appeared to be the answer. Its clarification performance was proven and the supersaturation phenomenon presented no problem, since the process is dependent on the introduction of air into the water in treatment.
The New Castle plant contains five flocculation/dissolved air flotation flow-trains, one of which is depicted in the accompanying schematic drawing. Flocculated solids formed in the three-stage flocculators are floated to the surface by introducing a recycled pressurized stream of air-saturated water into the bottom of the DAF units just ahead of an inclined baffle. When the pressure of the recycle stream approaches atmospheric across sets of globe valves, supersaturation occurs and air is released from solution. The microscopic bubbles formed by the pressure release attach to floc particles, causing them to rise to the surface of the liquid. A scraper mechanism in the top of the tank transfers the floated solids into a DAF solids receiver, while clarified water is taken off through a perforated pipe arrangement at the bottom of the DAF tanks.
A system containing five fixed-speed centrifugal pumps is used to generate the air-saturated water needed for the flotation process. Recycled water (6 to 12 percent of the DAF-clarified water) is pressurized to over 100 psig. Each pump is connected to a 2.5-in. stainless steel eductor, which entrains air into the recycle flow. Air dissolution occurs rapidly, and excess undissolved air is released in pressurized separation tanks and returned to the eductor. Two compressor/receiver assemblies provide high pressure make-up air to the recycle system.
Ozone for Primary Disinfection
Located immediately after the DAF section of the plant's process train are the ozone contactors. As noted earlier, ozone was selected as the primary disinfectant for this new facility in anticipation of forthcoming tighter regulations governing control of disinfection by-products. While ozone-generated by-products are less understood than those produced by the traditional chlorine disinfection process, there are fewer concerns about them at this time. Ozone also produces a finished water which some believe has better aesthetic qualities-taste, odor and appearance. But it does not provide a residual in distribution systems, and for that reason a relatively low dose of chlorine is added to the New Castle supply before it leaves the Millwood plant. The goal is to maintain a minimum residual concentration of 0.5 ppm of free chlorine in the distribution system.
Ozone is generated on-site and injected into the clarified water in the contact chambers, which provide five to six minutes of contact time. By applying the ozone after clarification, but before filtration, the ozone demand is minimized, and oxidized organic or inorganic materials can be removed by the filters, which are placed next in the process scheme. The ozonation system comprises air compressors, air dryers, ozone generators, and ozone destructors to prevent the escape of excess ozone into the plant atmosphere.
The Rest of the Process
Raw water from the aqueduct is fed into rapid-mix basins where high-intensity dispersion of a variety of chemicals can be accomplished. These include alum, polyaluminum chloride, chlorine, caustic soda, polymer, and potassium permanganate. Flocculation follows the rapid mixers and precedes the DAF. The flocculators provide about 30 minutes of gentle mixing in three stages to develop the floc.
Later, clarified, ozonated water is fed to the declining-rate, rapid gravity filters containing 24 in. of anthracite over 12 in. of sand. Fifteen to 20 minutes of backwashing are required every 72 to 120 hours of operation. Waste backwash water is pumped into a balancing tank for temporary storage before it is transferred at a relatively low rate to an inclined plate/thickener unit. Solids are drawn off at the bottom and pumped to two drying lagoons, where they remain for at least a year. Subjected to the freeze-thaw cycles of a typical New York State cold season, the solids become an easy-to-dry crumbly cake which finds an end-use as inert fill.
The filtered water gets its low-dose shot of chlorine, to ensure a disinfecting residual is present, before being sent on its way to the utility's customers. In addition, caustic soda is fed to raise the pH to above 7.0 so that the water is slightly basic. This condition optimizes the effectiveness of the orthophosphate which also is added, its job being to act as corrosion inhibitor and to reduce the introduction of lead and copper by leaching from pipes and fittings, particularly from the piping beyond the customer connection to the utility's mains. These chemical additions are made at the Millwood Pumping Station, as is the injection of fluoride solution, before the finished water enters the distribution system.
Millwood Pumping Station
This 7.5-mgd facility was completed in 1992 and has been pumping filtered water since August of 1993. The pumps can lift water by more than 400 ft so that an adequate level can be maintained during peak demand periods in the 2 million gallon Alpine Lane storage tank. Four single-stage, 300 hp, centrifugal pumps with variable speed electric motor drives are installed in the station. The drives provide "soft" start and stop conditions, which keep pressure surges to a minimum in the system. Pump output is automatically varied by speed adjustments made to match demand, a feature which helps to minimize power consumption.
Nominal capacity of the station can be increased to 10 mgd by the addition of another pump, for which space was allowed in the design. Water leaves the station through a new 20-in. transmission main which extends about 2500 ft to meet the existing piping system.