High Rate Microsand Enhanced Sedimentation Process is the Key
In response to a 25 percent increase in water demand, the North Table Mountain Water and Sanitation District, north of Golden, Colo., incorporated a high rate water clarification process to add capacity to its existing water treatment plant. As a result, the plant now can process a larger volume of water daily while filter backwash frequency and chemical consumption have decreased significantly.
High Rate Microsand Enhanced Sedimentation Process is the Key
The new Actiflo(r) system occupies a small area, and the indoor installation eliminates complications related to Colorados winter weather. Operators have found the process is easy to operate due to its flexibility, quick response and stability.
High Rate Microsand Enhanced Sedimentation Process is the Key
The 11 mgd North Table Mountain Water and Sanitation District plant, which started up in May 1998, serves a population of 9,000 and a light industrial park. The raw water comes from the Ralston Reservoir via a 5,000 foot pipeline on the western slope of North Table Mountain. The water production rate for 1998 was 635 million gallons and is expected to grow. When the raw water arrives at the plant, it has a turbidity range of 2-10 NTU and TOC levels of 2-6 mg/L.
High Rate Microsand Enhanced Sedimentation Process is the Key
Prior to the addition of the process, the plant relied on conventional treatment steps: coagulation, flocculation and sedimentation, followed by filtration and chlorination.
High Rate Microsand Enhanced Sedimentation Process is the Key
The Actiflo process, provided by Kr?ger Inc. of Cary, N.C., combines microsand enhanced flocculation with lamellar settling to perform the first three functions of coagulation, flocculation and sedimentation. The increased efficiency comes from injecting microsand, which acts as a seed for floc formation. The microsand ballasted floc accelerates its settling and allows design rise rates in drinking water treatment applications between 16 and 40 gpm/sf. The North Table Mountain plant achieves better removal rate with the process, thus reducing settled water turbidities, and lessening the workload for the plants filters.
Summer Peak Demand Strains Filters
With average settled water turbidities of 6 NTU, the old conventional sedimentation basin was not providing efficient solids removal, placing a strain on the filtration system particularly during peak summertime demand. As a result, each of the plants four filters had to be backwashed every 16-17 hours, and while one filter was being backwashed the remaining three had to process the extra water. During this procedure, the overall process required much more attention in order to achieve the desired water quality.
Summer Peak Demand Strains Filters
With the Actiflo system, average settled water turbidity dropped to 0.5 NTU, thus the time between backwashing filters during the summer has nearly doubled, increasing from approximately 16 hours to 28-30 hours. Due to increased turbidity removal, the filter loading rate can be increased from 2.3 gallons per minute per square foot (gpm/sf) to 3.7 gpm/sf without sacrificing water quality. The plant can now run on three filters, allowing operators to use the fourth as needed during peak demand times.
Summer Peak Demand Strains Filters
With the old system the sedimentation basin had to be taken out of service twice a year for cleaning and maintenance because it was not equipped with automatic scrapers. During the cleaning operation the solids-laden water had to be direct filtered. The plants new system includes continuous sand/sludge removal. The sand is separated from the sludge in hydrocyclones and the sand is reinjected into the system while the sludge is continually sent to the drying beds. This eliminates the need to drain and hose out the sedimentation basin, and has put an end to the procedure of direct filtration during basin cleaning.
Faster Processing, Consistent Water Quality
With the new system, the average finished water turbidity readings have dropped from .060 NTU to 0.036 NTU. Furthermore, more efficient organic removal has reduced the amount of chlorine needed to disinfect the finished water. Lower organic concentrations and chlorine dosages decrease the THM formation potential. Before installing the new process, the THM levels reached 70 ?g/l. They have now dropped below 50 ?g/l, and have been as low as 19 ?g/l. The TOC levels are easily maintained below 2 mg/l.
Faster Processing, Consistent Water Quality
The system in North Table Mountain consists of two trains with a capacity of 5.5 mgd each. Raw water enters the Actiflo system in the coagulation tank. A chemical coagulant is thoroughly mixed into the water to destabilize suspended solids and colloidal matter in the influent stream. The retention time at this stage is designed at approximately two minutes.
Faster Processing, Consistent Water Quality
The coagulated water then passes into the injection tank where the polymer and microsand are added to initiate floc formation. The combination of polymer and microsand serve as a "seed" for floc formation and development in the next step.
Faster Processing, Consistent Water Quality
Water moves from the injection tank into the maturation tank, where the formation of the ballasted floc is optimized. Gentle mixing is applied over a hydraulic retention time of approximately six minutes. The large surface area of the microsand provides an enhanced opportunity for polymer bridging and the enmeshment of microsand and floc already in suspension.
Faster Processing, Consistent Water Quality
The fully formed ballasted floc leaves the maturation tank and enters the settling tank, where rapid settling of the floc occurs. Clarified water exits via collection troughs for subsequent filtration and chlorination.
Faster Processing, Consistent Water Quality
The sand/sludge mixture is collected at the bottom of the settling tank and sent to hydrocyclones for separation. The microsand is discharged from the bottom of the hydrocyclones and re-injected into the system. The lighter density sludge is discharged from the top of the cyclones and sent to drying beds.
Faster Processing, Consistent Water Quality
The sand recovery in the hydrocyclones is efficient, with approximately eight pounds of sand lost per one million gallons of water treated. A $3,000 supply of the microsand used at North Table Mountain is expected to last six to eight years.
Faster Processing, Consistent Water Quality
The entire system at North Table Mountain, including the pipe gallery, requires about 2,200 square feet of space and is housed in a 5,800-square-foot self-contained building that also houses chemical feed and loading docks.
Conclusion
The switch to the new high rate microsand enhanced sedimentation process has significantly reduced chemical consumption at the North Table Mountain facility. In the summer months (June-Aug. 1997 compared to June-Aug. 1998), for example, alum consumption was reduced by 17 percent, soda ash by 40 percent, and chlorine by 32 percent. In total, chemical costs were reduced by 28 percent during these months, while average filter run hours increased by 45 percent.
Conclusion
Other benefits include faster startup. The process can reach steady state operation within 20-30 minutes. It also is simple to operate and closely follows familiar water treatment stages. As a result of the ease of operation, the evening shift length has been shortened by two hours.
About the Authors:
Rick Jeschke, PE, is Project Engineer, and Dave Hansen is Plant Supervisor at the North Table Mountain Water and Sanitation District.
