The F.B. Leopold Company, Inc.
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Dissolved Air Flotation to Remove Algae and Manganese in Potable Water Treatment James E. Farmerie, Product Manager The F. B. Leopold Co., Inc. 227 South Division Street Zelienople, Pennsylvania 16063
The Evitts Creek Water Treatment Plant that is owned and operated by the City of Cumberland, Maryland is located on the border of Pennsylvania near the Maryland panhandle. It provides potable water for Cumberland as well as other surrounding smaller communities in Maryland, West Virginia, and Pennsylvania. Both the state of Maryland and Pennsylvania have regulatory responsibility for this facility. The raw water source for the treatment plant influent is Lake Koon with 2.2 billion gallon capacity that feeds the 1.3 billion gallon Lake Gordon reservoir where the intake structure is located. The original designed 12 MGD plant had a treatment scheme with rapid mix, upflow clarifiers, filters, clearwell, and a contact tank along with vacuum drying beds for sludge dewatering. Flow through the plant averaged 9 MGD, with peaks of 13 MGD. The influent turbidity averaged 3 NTU with influent soluble manganese levels averaging 0.1 mg/L, but peaking at 0.172 mg/L. Potassium permanganate (KMnO4) was fed regularly at the intake for taste and odor control as well as to oxidize the manganese for removal in the clarification stage of the treatment process. The plant fed inorganic flocculant and polymer for clarification, caustic for corrosion control, fluoride, chlorine for disinfection, and ammonia to form chloramines to carry the residual throughout the large distribution system. In 1998, in addition to the soluble manganese problem, increased agricultural run-off and a reduction in the surrounding forestry were causing progressively higher algae blooms during the summer months. These conditions were creating severe taste and odor problems, shortened filter run times, and discoloration and staining in the distribution system consumers homes due to manganese. After investigating alternative clarification technologies, the city’s consulting engineer recommended to install a Dissolved Air Flotation (DAF) clarification system as well as to replace 5 filters as the solution to the problem of increased algae and also improve manganese removal. In the DAF process, raw water particles are flocculated and separated out of the water by floating them to the surface, rather than settling them to the bottom of a basin. The process introduces micro-sized air bubbles through diffusers at the bottom of the contactor to float the floc. The air bubbles are produced by recycling a portion of the effluent through a tank where air is introduced and the water saturated, and then reduced to ambient pressure, thus creating the pressurized flow. The floated sludge is removed from the top of the basin by mechanical or hydraulic means, while the clarified water is removed through laterals in the bottom of the basin.
DAF is particularly effective in removing low-density solids such as turbidity, color, algae, and precipitated iron and/or manganese. These are all contaminants that do not settle well, but tend to float or hover in the water column. Since particle removal is by floatation, rather than sedimentation, both the flocculation and clarification detention times are less than for conventional treatment. The particle size for removal in flotation can be as low as tens of microns in size rather than the hundreds of micron size required for sedimentation. DAF will also handle rapid changes in temperature and water quality provided that the coagulation chemistry is optimized. In the summer of 1999, a Leopold state-of-the art DAF mobile pilot plant was brought in to determine the process effectiveness and develop design criteria. The six-week pilot study optimized the inorganic coagulant doses at 15 to 20 mg/L, and was able to achieve an operating loading rate of 7 gpm/ft² with an efficient recycle flow of 7.8%. The pilot DAF indicated that it could effectively remove algae and manganese, as well as produce a consistent effluent quality for filter loading, thus maximizing the filter run times between backwashes. It would also produce 2% to 5% sludge solids in the float, which would reduce the sludge volume to handle and the cost of further processing by dewatering and hauling away the sludge solids. In addition, with the smaller footprint, it allowed the water utility to handle a proposed 36% increase in ultimate plant capacity to 15 MGD in the same area for the proposed new facility. After evaluating the success of the pilot study and visiting several existing dissolved air flotation installations, Cumberland officials were convinced that a dissolved air flotation system was the best alternative for the Evitts Creek treatment scheme. A design/build team formed between the city; the equipment supplier; a local contractor; and the engineering firm accomplished the final design and installation in June 2002. The project was completed ahead of schedule and under budget. It consisted of three DAF systems (designed at 5 MGD apiece), each with dual flocculators, basins that are 22 ft wide x 32 ft long x 10 ft deep, and with all the required pumps, saturation tank, and controls. In ® ® addition it involved replacing five of the existing filters with Leopold Type S underdrain, ® ® Leopold I.M.S. (Integral Media Support) cap, backwash troughs, sand and anthracite media, and three dual filter control consoles to provide automatic control and monitoring of the filter operation backwash sequence. The new filter system also included the addition of air scour to the backwash procedure. Before the system start-up, the utility added aeration in the reservoir to help oxidize the manganese so that it precipitates and is easily removed in the DAF process. This aeration has reduced the KMnO4 feed from 9 months per year to only a couple of weeks per year during the summer. The inorganic coagulant chemical feed has been reduced to 8 to 15 mg/L. Some of the reduction is due to the operators gaining experience as well as the fact that DAF is more efficient in removing smaller diameter particles by floatation than by clarifiers attempting to settle them. The DAF effluent turbidity is consistently below 0.2 NTU and has increased filter runs to the Pennsylvania state regulatory recommended maximum of 72 hours. Previously, with the upflow clarifier, the filter runs were at 24 hours between backwashes. The taste and odor problems have been resolved, soluble manganese in the finished water is now below 0.02 mg/L, and the process obtains a minimum of 85% removal of the incoming algae. With the current plant design and an average flow at 9 MGD, the loading rate in the DAF units is 3.6 gpm/ft² and the pressurized recycle flow is 10% while maintaining removals of turbidity, algae, and particulate manganese.
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One of the big benefits from the DAF operation is the high solids content of the floated sludge. As it comes off the top of the unit, it is actually diluted to 2% for pumping purposes, and then sent to the belt press that has replaced the vacuum drying beds. The belt press, using anionic polymer as a sludge conditioner, produces sludge cake ranging from 16% to 22% solids. This compares favorably to the drying beds that produced 5% cake solids and required sawdust filler to be added to handle it for hauling to the landfill. The belt press processes a one-week accumulation of sludge in 10 to 14 man-hours rather than the 48 to 60 man-hours previously required using the drying bed. This has reduced the overall operating costs of the solids handling and disposal operation by greater than 60%. The table below compares the actual performance that has been achieved with the two clarification processes at the Evitts Creek Treatment plant.
Parameter
Upflow Unit
DAF
2.0 - 5.0 NTU
2.0 - 5.0 NTU
0.1 - 0.17 mg/L
0.1 - 0.17 mg/L
1.0 NTU
0.2 NTU
Effluent Manganese
0.05 mg/L
0.02 mg/L
Filter Run Length
24 hours
72 hours
Filter Effluent Turbidity
0.1 NTU
0.04 NTU
0.05 mg/L
0.02 mg/L
Raw Water Turbidity Raw Water Manganese Effluent Turbidity
Filter Effluent Manganese
Today, the DAF system’s effluent water quality and subsequent filter run times are exceeding expectations. Also, the maintenance and operations requirements of the DAF units are less than anticipated, allowing plant operators extra time to accomplish more tasks during their regular shift.
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