By Dale Sukow, Darin St. Germain and William D. Sullivan
West Plains, MO, is a pleasant place to live, work and visit, but it's not an easy place to make clean drinking water.
The groundwater that feeds the city's six water wells recharges directly from the surface. Karst topography and numerous caves and sinkholes create channels by which surface water can reach the aquifer with little natural filtration. As it trickles down, the water collects fine particles of clay that for years caused turbidity in West Plains residents' tap water, especially after rainfalls.
But in August 2001, West Plains finally beat the turbidity problem with a new water treatment plant, built around a high-speed microsand settling process backed by dual-media gravity filtration. Valued at a little over $4 million, the plant consistently delivers water at 0.04 to 0.05 nephelometric turbidity units (NTU).
Growing Pains
Known as "The Heart of the Ozarks," West Plains has 10,800 residents. Located within a few hundred miles of four major cities in three states, West Plains serves as an economic hub for the region, as well as a tourist crossroads. From 1989 to 1996 alone, the city grew by 25 percent, which exacerbated the water turbidity problem.
For many years, West Plains had treated its water by using chlorination. Well department personnel controlled water quality by monitoring turbidity meters at each of the six 1,500-foot-deep wells and pumping from those with the lowest readings.
"That was an acceptable solution as long as we had substantial excess well capacity," said Jim Woodworth, water department superintendent. "But by the early 1990s, growth caught up with us, and we could no longer operate the wells selectively."
During dry weather, turbidity averaged between 5 and 6 NTU, but often spiked to more than 100 NTU after heavy rains.
In the mid-1990s, the city tried using a gravity sand filter on one well, but it couldn't capture the finer clay particles and thus wasn't able to produce water with less than 4 NTU. Exasperated, city residents voted in a referendum in 1996 to build the new water treatment plant.
Ahead of the Curve
In early 2000, the Missouri Department of Natural Resources reclassified West Plains' drinking water supply as groundwater under the direct influence of surface water and, as a result, the city needed to comply with a state consent decree to produce water with turbidity less than or equal to 0.5 NTU. Fortunately, the city's plans for the new treatment plant were by then well underway.
The city solicited bids for the project in March 2000, and in May 2000, put USFilter in charge of building the major process equipment for the plant. The plant, with a normal capacity of 3.7 million gallons per day (mgd) and a peak capacity of 5.0 mgd, was completed on August 31, 2001.
The plant uses USFilter's Actiflo® ballasted high-rate clarification process for rapid coagulation and settling of suspended materials, followed by a CenTrol® dual-media gravity filter for final polishing. Both processes are widely used, though West Plains is the first community to employ the two together.
Bench-scale testing performed in the plant's laboratory confirmed that using high-rate clarification with dual-media filtration would resolve the turbidity problem. As part of its contract with West Plains, USFilter provided a process performance guarantee of 0.3 NTU effluent turbidity.
The process works on the same principles as conventional water treatment technology, using coagulant for destabilization and flocculant-aid polymer to aggregate suspended materials, which are then removed by settling.
One of the distinguishing features of the process is microsand that's used to speed up floc formation and settling. The microsand aids development of chemical floc far denser and more durable than floc in a conventional clarifier. The sand also acts as ballast to promote rapid settling and thus allow shorter detention times, higher clarifier overflow rates and smaller system footprints. The footprint can be 5 to 50 times smaller than those of conventional clarification systems of similar capacity.
The microsand concentration in the process effectively dampens the effects of fluctuating raw water quality. Because the sand does not react with process chemistry, it can be removed from the system sludge and reused in the process.
Process at Work
West Plains installed two Actiflo microsand ballasted high-rate clarification processes. In the first rapid-mix stage of the process, alum coagulant is added to the raw water in the coagulation tank where it is mixed for approximately two minutes. Although the typical alum dose ranges between 8 and 12 mg/L, it may be increased slightly during wet weather due to greater raw water turbidity.
After passing through the coagulation tank, the coagulated water is sent to the injection tank where flocculant-aid polymer and microsand are added to initiate floc formation. Hydraulic detention time in the injection process is about two minutes, with a typical polymer dose of 0.08 to 0.1 mg/L. Mixing in the maturation tank provides ideal conditions for polymer bridges to form between the microsand and the destabilized suspended solids.
Upon leaving the maturation tank, the fully formed ballasted floc enters the settling tank, where laminar upflow through the lamellar settling zone provides rapid removal of the microsand/ sludge floc. The clarified water is then collected in a series of settled water troughs before it is sent to the gravity filter. Clarified water usually exits the Actiflo process with 0.8 to 1.0 NTU.
A rubber-lined centrifugal slurry pump withdraws the ballasted floc sand/sludge mixture from the bottom of the settling tank, sending it to a hydrocyclone that separates the sludge from the denser microsand. Once separated, the microsand is concentrated and discharged from the bottom of the hydrocyclone, and then re-injected into the process for re-use. The lighter density sludge is simultaneously expelled from the top of the hydrocyclone and sent for disposal.
Media Filtration Step
The clarified water flows through a series of collection troughs on its way to the gravity filter where it enters an inlet distribution device. Next, the water passes through filter cells clustered around a central control column. The filtered water flows into a common effluent chamber before passing over an effluent weir to the clearwell. As the effluent weir is higher than the filter wash troughs, the filter cells are always under a positive head, rendering rate-of-flow controllers unnecessary.
The CenTrol filter's small footprint makes every filter cell visible from the main operating platform. The filter system has two possible design configurations: the inlet flow splitting model, where each cell treats equal flow, and the declining rate model, which increases effective media solids storage capacity and minimizes breakthrough potential.
More than 50 declining rate CenTrol filters are currently in use, including the one installed at West Plains. The West Plains filter uses four filter cells, each 14 feet by 15 feet, 9 inches, with a total filter area of 882 square feet. Filter media consists of an upper layer of anthracite (1.0 mm to 1.2 mm particles) and a bottom layer of sand (0.45 mm to 0.55 mm).
As the West Plains filter is a declining rate model, the inlet valves are constantly submerged, providing an equal water level and equal filtering head in each cell. No mechanical flow-splitting device is used; rather, solids captured in the beds affect the flow through each filter cell. Cleaner filter cells have less total headloss and therefore treat more flow; likewise, the opposite is true of dirtier cells and headloss.
Simple Backwash
The system furnishes its own backwash water supply from in-service cells, reducing or eliminating the need for backwash supply pumps. West Plains uses a low profile Multiwash® baffle system that backwashes with a mixture of air and water, reducing backwash volume and increasing filter cleaning efficiency.
To initiate backwash, the filter cell inlet valve is closed and the cell continues to filter until the water drops to a level just over the effluent weir. At this point, the backwash waste valve is opened, and the water remaining over the wash troughs flows to waste. As the water in the cell drops to a level below the effluent weir, water from the in-service cells flows through the cell that's being backwashed.
The backwash flow gradually increases until the cell water level has lowered to the wash troughs, at which time the flow reaches a design rate. Any excess water produced by the in-service cells flows over the effluent weir to the clearwell.
Meanwhile, a blower activated by the control system forces air into the filter by way of an air distribution header beneath the underdrain. Backwash is followed by an air purging and media restratification cycle. From start to finish, the entire backwash process takes approximately 30 to 40 minutes.
Monitoring Performance
The West Plains Water Treatment Plant monitors system performance with inline turbidity meters before and after the clarification process and after the gravity filter. The plant has consistently delivered water better than the 0.3 NTU specified in USFilter's performance guarantee.
"An additional benefit has been reduced chlorine use," Woodworth said. "Previously, the high turbidity in the well water required high chlorine doses because pathogens could hide in the suspended particles. Now, instead of using 50 pounds of chlorine each day, we use only eight to 10 pounds to achieve the same result."
Moreover, about half the chlorine is added upstream of the media filters to retard algae growth and prevent media fouling. The remainder is added after filtration for final disinfection, where the chlorine must sit in the clearwell for at least an hour before it is distributed. The final chlorine concentration is 0.8 to 1.0 parts per million.
About the Authors: Dale Sukow, P.E., has been the operations manager of the West Plains Water Treatment Plant since the plant's start-up in August 2001. Prior to that, he was a contractor in commercial building and water treatment plant construction. Darin St. Germain, P.E., is a product manager for USFilter's Microfloc and General Filter Products equipment lines in Ames, IA. Since joining USFilter in 1991, Darin has helped develop several products and processes, including the low profile Multiwash® baffle system. William D. (Will) Sullivan is the Actiflo product manager at USFilter's Kruger Products in Cary, N.C. Prior to joining USFilter in 2000, he was general manager of the U.S. Region at EIMCO Process Equipment Co.
