Parker (1976) Stockton, California
Lubbock, Texas Komline-Sanderson (1972) El Dorado, Arkansas Bore et al. (1975) Logan, UT Stone et al. (1975) Sunnyvale, California
Coagulant and Dose (mg/L)
NA 4.0c 1.3-2.4d 2.0e a Including 33% pressurized (35-60 psig) recycle. b Including 30% pressurized (50 psig) recycle.
c Including 100% pressurized recycle. d Including 25% pressurized (45 psig) recycle.
e Including 27% pressurized (55-70 psig) influent.
Detention Time (min)
12b 8c NA 11e
280-450 93 NA NA
240-360 450 125 150
These three types of dissolved-air flotation are illustrated by flow diagrams in Figure 5.6. In the total pressurization system, the entire wastewater stream is injected with air and pressurized and held in a retention tank before entering the flotation cell. The flow is direct, and all recycled effluent is repressurized. In partial pressurization, only part of the wastewater stream is pressurized, and the remainder of the flow bypasses the air dissolution system and enters the separator directly. Recycling serves to protect the pump during periods of low flow, but it does hydraulically load the separator. Partial pressurization requires a smaller pump and a smaller pressurization system. In recycle pressurization, clarified effluent is recycled for the purpose of adding air and then is injected into the raw wastewater. Approximately 20 to 50% of the effluent is pressurized in this system. The recycle flow is blended with the raw water flow in the flotation cell or in an inlet manifold.
Important parameters in the design of a flotation system are hydraulic loading rate, including recycle, concentration of suspended solids contained within the flow, coagulant dosage, and the air-to-solids ratio required to effect efficient removal. Pilot-plant studies by Stone et al. (1975), Bare (1971), and Snider (1976) reported maximum hydraulic loading rates that ranged between 2 and 2.5 gpm/ft2 (81.5 and 101.8 L/min-m2). A most efficient air-to-solids ratio was found to be 0.019 to 1.0 by Bare (1971). Solids concentrations during Bare's studies were 125 mg/L. Experimental results with the removal of algae indicate that lower hydraulic rates and air-to-solids ratios than those recommended by the manufacturers of industrial equipment should be utilized when attempting to remove algae.
In combined sedimentation flotation pilot-plant studies at Windhoek, Southwest Africa, van Vuuren and van Duuren (1965) reported effective hydraulic loading rates that ranged between 0.275 and 0.75 gpm/ft2 (11.2 and 30.5 L/min-m2), with flotation provided by the naturally dissolved gases. Because air was not added, the air-to-solids ratios were not reported. They also noted that it was necessary to use from 125 to 175 mg/L of aluminum sulfate to flocculate the effluent containing from 25 to 40 mg/L of algae. Subsequent reports on a total flotation system by van Vuuren et al. (1965) stated that a dose of 400 mg/L of aluminum sulfate was required to flocculate a 110-mg/L algal suspension sufficiently to obtain a removal that was satisfactory for consumptive reuse of the water. Based on data provided by Parker et al. (1973), Stone et al. (1975), Bare (1971), and Snider (1976), it appears that a much lower dose of alum would be required to produce a satisfactory effluent to meet present discharge standards.
Dissolved-air flotation with the application of coagulants performs essentially the same function as coagulation-flocculation-sedimentation, except that a much smaller system is required with the flotation device. Flotation will occur in shallow tanks with hydraulic residence times of 7 to 20 min, compared with hours in deep sedimentation tanks. Overflow rates of 2 to 2.5 gpm/ft2 (81.5 to 101.8 L/min-m2) can be employed with flotation, whereas a value of less than 1 gpm/ft2 (40.7 L/min-m2) is recommended with sedimentation. However, it must be pointed out that the sedimentation process is much simpler than the flotation process, and, when applied to small systems, consideration must be given to this factor.
The flotation process does not require a separate flocculation unit, and this has definite advantages. It has been shown that the introduction of a flocculation step after chemical addition in the flotation system is detrimental. It is best to add alum at the point of pressure release where mixing occurs and a good dispersion of the chemicals occurs. Brown and Caldwell (1976) have designed two tertiary treatment plants that employ flotation, and they have developed design considerations that should be applied when employing flotation. These features are not included in standard flotation units and should be incorporated to ensure good algae removal (Parker, 1976). In addition to incorporating various mechanical improvements, the Brown and Caldwell study recommended that the tank surface be protected from excessive wind currents to prevent float movement to one side of the tank. They found that the relatively light float is easily moved across the water surface by wind action. It was also recommended that the flotation tank be covered in rainy climates to prevent breakdown of the floc by rain. Another alternative proposed has been to store the wastewater in stabilization ponds during the rainy season and then operate the flotation process at a higher rate during dry weather.
Alum-algae sludge was returned to the wastewater stabilization ponds for over 3 years at Sunnyvale, California, with no apparent detrimental effect (Farn-ham, 1981). Sludge banks, floating mats of material, and increased TSS concentrations in the pond effluent were not observed. Return of the float to the pond system is an alternative at least for a few years. Most estimates of a period of time that sludge can be returned range from 10 to 20 years.
Sludge disposal from a dissolved-air flotation system can impose considerable difficulties. Alum-algae sludge is very difficult to dewater and discard. Centrif-ugation and vacuum filtration of unconditioned algae-alum sludge have produced marginal results. Indications are that lime coagulation may prove to be as effective as alum and produce sludge more easily dewatered.
Brown and Caldwell (1976) evaluated heat treatment of alum-algae sludges using the Porteous, Zimpro® low-oxidation, and Zimpro® high-oxidation processes and found relatively inefficient results. The Purifa process, using chlorine to stabilize the sludge, produced a sludge dewaterable on sand beds or in a lagoon; however, the high cost of chlorine eliminates this alternative. If algae are killed before entering an anaerobic digester, volatile matter destruction and dewatering results are reasonable. But, as with the other sludge treatment and disposal processes, additional operations and costs are incurred, and the option of dis-solved-air flotation loses its competitive position.
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