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A Midwestern water treatment plant with a 20 MGD capacity uses Lake Erie as the water source. Lake water turbidity ranges from 3 to 150 NTU and the pH from 7.4 to 8.4. The parameters for finished water are turbidity of 0.1 NTU or less, pH 8.1 to 8.3, chlorine residual 1.9 to 2.3 mg/L, no taste and odor(T&O) problems and no algae breakthrough in its multimedia filters.
At headworks a traveling screen removes debris and chlorine dioxide. The plant's 7,500-gallon rapid mix tanks are used for dosing coagulant aid, caustic or chlorine. The plant runs six 100,000 gallon setting basins and two 50,000 gallon sludge thickening tanks.
After sedimentation, water from basins flows through a flume to the multimedia filters. The filter media consist of layers of anthracite coal, sand and gravel. Filter beds are cleaned periodically by a standard backwashing process at 13.5 gpm/sq. ft. Wash water is pumped back to the rapid mix tanks. Powdered activated carbon, fluoride, chlorine or a filter aid are added to the water prior to filtration.
After filtration, water passes through storage tanks, clearwells and a chlorination chamber before it is pumped to the distribution system.
Alum was the only coagulant aid used during the initial years of operation. Alum was added continuously to the rapid mix tanks at a dose of ~20 ppm. Operating inefficiencies and cost concerns consistent with alum caused the plant to investigate alum replacements. Alum-related problems included:
1. High volume of sludge production of low TSS
2. High filterability index of 1.5 on settled water
3. Corrosivity index range of -15 to -21
To remedy these problems and enhance plant performance, the plant discontinued alum use and converted to a Tramfloc coagulant. This coagulant neutralizes the negative charges on suspended particles, effectively coagulates fine colloidal turbidity and quickly forms a dense and rapidly settling floc. Without requiring any charges in the existing alum storage and feed system, the plant began use of Tramfloc® 620 series coagulant at a dose of 6-7 mg/L, less than half the alum dosage.
Improved Plant Performance
Sludge Reduction. The water plant has reduced its sludge production from ~190,000 gpd to 50,000 or about by 67% since the coagulant was introduced. Sludge hauling costs were cut by more than 50%.
Corrosion Control Improvement. Because alum reduces normal alkalinity of water, the metal salt increases water's corrosivity. To correct this untoward condition, the water district adjusted pH with caustic soda(NaOH) for corrosion control. Since the coagulant does not demand alkalinity as does alum, 67% less caustic is required now that the plant applies the Tramfloc 620 coagulant. The plant now generates a non corrosive product with a corrosivity index of 0 to 2. The district has saved $21,000 annually on caustic alone.
Filtration Improvements. The plant's filterability of settled water decreased from an average of 1.4 to less than 1.1. The plant has been able to increase filter runs from 48 hours with alum to 80 hours with Tramfloc coagulant. Much less backwashing is required, and more finished water is sent to distribution. A 17 percent reduction in filter washes was achieved. The water district was able to reduce annual expenses by more than $40,000.
Sludge Reduction........... $14,400
Filtration Operation....... $14,000
Corrosion Control.......... $21,000
Total Cost Reduction.......$49,000
Cost of New Programs......$-8,200
Net Annual Savings $41,200