FUNDAMENTALS OF THE MULTI-TECH FILTRATION PROCESS
Improving the clarity of surface water has always presented a challenge because source quality varies. Traditional treatments rely on expensive, construction-intensive processes with lengthy times.
Culligan developed the Multi-Tech process through a 2 1/2 year R&D program that started in 1976. The result is a chemical treatment, clarification and filtration process that uses less expensive standard components, shortened contact time and reduced operator attention.
Suspended particles carry an electrical charge which causes them to repel one another. The Multi-Tech process uses alum (aluminum sulfate) and cationic polymer to neutralize the charge. That allows suspended particles to clump together to form more easily filtered particles. Alum combines with alkalinity in the raw water to form a white precipitate that neutralizes suspended particles' electrical charge and forms a base for coagulating those particles. Conventional technology uses a 30 to 50 mg/L alum dosage to form a large floc that requires extensive retention time to permit settling. The Multi-Tech process uses 5 to 15 mg/L of alum to form a smaller "pin floc". This floc also helps absorb organic contaminants. The addition of cationic polymer also helps neutralize particle charge. The polymer's long chain molecules also strengthen the floc as the suspended particles gather together.
The chemically treated water flows downward through the clarifier tank. Trubidity clings to the surface of the media as it passes through the clarifier's 42-inch (1100 mm) deep bed of coarse sand. Coagulation and flocculation continue as the water travels this tortuous path. Water clarity improves 50 to 85 percent, depending on raw water conditions. Then clarified water flows downward into the three-media depth filter tank. Clarity improvement after filtration is commonly 98 to 99.5 percent, unless the influent turbidity is quite low; below 5 NTU.
The Multi-Tech process improves surface water clarity to comply with the new EPA turbidity drinking water standard of 0.5 NTU. When used for potable water, the filtered water is chlorinated before use. The Multi-Tech process also effectively removes Giardia cysts. University studies have demonstrated that on cold water with less than 1 NTU turbidity, the process can reduce Giardia by as much as 99 percent.
When effluent water quality is no longer satisfactory, or when the pressure loss through the system increases by 5 to 10 psi (34 to 68 kPa), the system requires reconditioning. Control valves direct water upflow through the depth filter to backwash the bed. The filter backwash water then passes upflow through the clarifier, backwashing it. Before entering the clarifier, backwash water passes through an eductor, which is capable of drawing air. After a few minutes of backwashing with water only, the eductor valve opens to admit air, which scours the clarifier bed. The high-velocity air-water mixture carries out heavy turbidity. The air valve closes to allow continued backwashing with water only. That purges the clarifier of residual air.
The entire upflow backwash and air scour cycle lasts about 10 minutes. A downflow rinse follows to settle the bed and "ripen" the media with chemically treated water. Water passes through the clarifier and depth filter tanks in series, then to drain. When the filtered water reaches acceptable quality, the system returns to service. The rinse time can vary from 2 to 20 minutes, but typically runs about 5 minutes. The entire reconditioning cycle takes 15 to 20 minutes.
