Automated High Rate Filters Improve Performance, Reduce Costs
By replacing microscreen filters with high-rate gravity filters as part of an $11 million modernization program, the Philadelphia Suburban Water Company has been able to reduce backwash water consumption by 23 percent while also upgrading its 25-year-old Upper Merion treatment plant to meet the stringent Federal and State drinking water standards.
The plant, located on a five-acre site near Norristown, about 15 miles northwest of Philadelphia, can supply up to 15 mgd for customers in that area and throughout Upper Merion Township. It is a key part of the private water company's system, which provides water for 248,000 residential, commercial and industrial users in 72 municipalities encompassing over 380 square miles in southeastern Pennsylvania.
The Upper Merion plant is unique in that it draws raw water from an abandoned lime quarry. Originally constructed in the 1930s by removing 17 million cubic yards of rock, the quarry was purchased in 1967 by Philadelphia Suburban and modified into a source of supply and storage reservoir. The quarry holds 800 mil gal within a surface area of 45 acres. It is 400 ft deep and is fed by underground springs that provide an average daily yield of over 7 mil gal.
To utilize this high quality ground water, the company bored a 1,300-ft tunnel from the quarry floor to the site of the planned water treatment plant. Vertical shafts then were drilled into the tunnel and four turbine pumps, each rated at over five mgd, were installed to draw water into the plant. Since this raw water had an average turbidity of less than 0.5 NTU, conventional tanks for coagulation and settling were not required in the initial facility. Three cylindrical stainless steel micro-strainers were added to filter out any foreign materials that might have fallen onto the quarry's open surface. Facilities for the storage and feeding of chlorine, ammonia and sodium phosphate also were included in the original plant, which had an area of a little over 6,000 sq ft.
This initial facility, which went on-line in 1969, was operated on a semi-automatic basis. Although personnel periodically toured the plant to assure its safe operation, it was continuously monitored from a centralized control center at company headquarters in Bryn Mawr. Pumps and other equipment could be started or stopped from the center, and instruments for measuring such parameters as chemical dosages or finished water quality were programmed to shut the plant down if necessary. The plant was incrementally upgraded in 1983 with the addition of two aeration towers to eliminate volatile organic compounds (VOCs) and the replacement of the original ammonia gas system with a liquid feed arrangement.
With the advent of more stringent federal standards in recent years, the Upper Merion's quarry water was reclassified from groundwater to surface water. This change called for additional treatment, triggering the Philadelphia Suburban Water Company to implement the modernization program that was completed in October, 1993. The project included the addition of eight gravity filters with air/water backwash, two elevated tanks to store clear water used in backwashing, two sludge drying lagoons and a fully automatic computerized control system for the entire facility. The filters are housed in a new 12,000-sq ft brick and glass structure that was added to the plant's original building.
As currently operating, raw water continues to be drawn from the quarry at rates that vary from five mgd in the winter up to 15 mgd during peak summer months. Four high-lift turbine pumps raise the water to the top of the aeration towers and into diffusers that allow it to cascade down through 14 ft of internal packing. Blowers piped into the base of each tower force some 1,400 cfm of air up through the packing and down-flowing water, stripping away over 96 percent of the contained VOCs. A slurry of powdered activated carbon is fed into the water being discharged from the towers for taste and odor control.
Next the water flows into a sump from which low lift pumps carry it into a 60-ft long flume leading to the new line of eight filters, each with a surface area of just under 300 sq ft. As the water enters the pumps, poly aluminum chloride and a polymer are injected to induce coagulation and remove turbidity. Chlorine is added where the pumps discharge into the inlet flume.
The water flows equally into the filters, each rated at two mgd, and down through 32 in. of dual media beds. These consist of 22 in. of anthracite followed by 10 in. of sand, which rests on a 12-in. base of gravel laid atop the air/water underdrain system. With a uniformity coefficient of 1.6, the media is designed to remove fine particles or contaminants such as asbestos fibers, Giardia cysts or Cryptosporidium. From the filters, the clarified water enters the plant's 80,000-gal underground clearwell. After a final addition of chlorine to ensure adequate residual and bi-metallic phosphate for corrosion control, the water is pumped into Philadelphia Suburban's distribution system.
The operation of each filter is automatically monitored by its own AFC 5000 control system integrated with electronic Valve PACs located in the plant's pipe gallery. The individual control systems contain programmable hardware that is housed in corrosion-free fiberglass consoles located beside each filter. The systems continuously monitor and feed flow and backwash data to Upper Merion's new programmable logic controller (PLC) system and automatically initiate and control backwashing on the basis of pre-set parameters. The individual filter controllers also trigger an alarm in the case of malfunction.
When the water flowing through a given filter has exceeded the plant's normal production standards to a pre-set maximum of 0.35 NTU, five feet of head loss, or 50 hours of run time, the control system initiates the backwash sequence. This starts with a signal to close the inlet flow valve so the filter water can draw down to within 12 in. of the media, and is followed by the closing of the filter effluent valve. The individual controller, operating through the plant's PLC system, simultaneously increases the flow to the other seven filters to maintain a constant total flow through the inlet flume. The Valve PACs automatically position the valves to maintain a set flow rate, briefly reverse direction if an obstruction lodges in a valve, or trigger an alarm if an obstruction doesn't clear.
Backwashing then starts with the introduction of air through the underdrain at an initial rate of four scfm. The air, which is supplied by twin compressors rated at 1,400 cfm each, flows up through orifices in the top of the underdrain to agitate the media. After approximately four minutes of air flow, water is added at the rate of five gpm/sq ft of filter surface area to fluidize the media and scour away any collected solids.
The backwashing cycle continues for another eight minutes with only water being pumped up through the underdrain and media at a rate of 17 gpm/sq ft. Water flow finally is increased to 20.7 gpm/sq ft for the final few minutes of backwashing. The complete cycle takes about 18 minutes and expands the media bed some 30p;40 percent.
The 30,000 gal required for each backwash are pumped to the sludge drying beds. After the solids have settled out, the water is recycled to the aeration towers and back through the treatment process. Dried sludge is trucked to a company-owned impoundment area.
Upper Merion's new filters have been consistently producing finished water of less than 0.05 NTU compared to the 1.0 level of the microscreen filters they replaced. The staff has found an additional benefit in the fact that with the air wash, the eight new filters require only 115,000 gal of water a day compared to the 150,000 gal needed by the three microscreens. This 23 percent reduction means less wear on pumps and other system components, and lower operating costs overall.
The automatic filter operation, like all of the plant's other functions, is monitored by operators through the new PLC system. The system virtually runs the plant with Square D programmable controllers and a network of interface stations that are in constant communication with each other. It is tied into a master control center at the company's Pickering West water treatment facility and to headquarters in Bryn Mawr through modems and dedicated phone lines. Color CRTs at Upper Merion and the other locations facilitate operational interaction.
In the case of filter operation, for example, the control center screen and individual CRTs display over a dozen different parameters. These include backwash rates, air scrubber performance, valve positions, flow rates, water levels, head loss and total filter run time. Operators can use the CRTs to modify any function if they deem it appropriate for optimum performance.
The AFC 5000 Filter Control Systems, Valve PACs and air/water underdrain system at Upper Merion were supplied by The F. B. Leopold Company of Zelienople, Pennsylvania. The overall modernization project was designed by Philadelphia Suburban Water Company's Production and Internal Engineering departments. John McQuade Company, of Philadelphia, was the prime contractor.