Dissolved Air Flotation
DAF system proves effective in membrane pretreatment
It has been known for hundreds of years that membranes can be used for solids removal and concentration in potable water treatment schemes. In recent years, new low-pressure membranes have greatly enhanced effluent quality to meet increasingly strict regulations. In addition, water treatment professionals have discovered that the performance of these membranes can be improved when a pretreatment system is utilized to reduce the contaminants that cause fouling.
A new dissolved air flotation (DAF) system, the Leopold Clari-DAF system, has proven to be an effective membrane pretreatment system, providing a multi-barrier approach to the treatment scheme, increased membrane flux rate, reduced membrane cleanings and prolonged membrane life. In addition to improving membrane performance, the Leopold Clari-DAF system is cost-effective, and its footprint is much smaller than conventional settling basins.
Need for membrane pretreatment
As the use of membrane treatment systems becomes more widespread, there is increasing agreement among consultants, engineers and membrane technology providers alike that membrane pretreatment is necessary to help ensure reliable and troublefree membrane filter performance.
According to Aqueous Solutions, Inc., “All membrane systems, specifically RO, require proper pretreatment to ensure reliable and trouble-free operation.” In November 2003, Black & Veatch stated:
“When membranes were initially used to treat surface water in the mid- 1990s, a simple membrane filtration approach with minimal pretreatment was generally applied. Pretreatment typically consisted solely of a protective backwashable screen with a rating in the 200- to 500-micron range to prevent comparatively large particles from plugging the membrane unit.
In many cases, however, simple membrane filtration is inadequate to fully meet the requirements of the Safe Drinking Water Act because many water supplies contain dissolved contaminants that exceed regulatory levels. Although they offer many benefits, MF/UF systems alone do not effectively control dissolved contaminants.
Treatment process designers today are likely to combine membranes with a variety of pretreatment technologies to address a broad range of water quality problems. In addition to expanding plant treatment capabilities, these pretreatment processes have been found to optimize membrane performance and reduce capital and whole-life costs,” according to Balck & Veatch.
An effective pretreatment choice
The Leopold Clari-DAF system, which utilizes a high-rate dissolved air flotation clarification process, provides one of the best membrane pretreatment alternatives. The process is designed with the concept that it is easier to float lowdensity solids than to settle them.
Color, organics, soluble metals or colloidal solids that are removed by adding inorganic chemicals to form aluminum or iron hydroxides have low density and require excess chemicals and mixing time to create particles large enough to settle.
Normally, solids hundreds of microns in size are required to settle, while particles tens of microns in size can be floated. It is also recognized and well documented that algae in the source water that will foul membranes is best removed by flotation.
In the Clari-DAF system 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 where it mixes with the coagulated solids and floats 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 in the reaction zone of the DAF cell, thus creating the pressurized flow.
The floated sludge is removed from the top of the basin by mechanical or hydraulic means, while laterals from the bottom of the basin remove the clarified water. The Clari-DAF system provides a small footprint with its typical flocculation mixing time of 10 to 12 minutes and a loading rate of 12 to 16 gpm/ft2 based on collection area.
The benefits of providing Clari- DAF systems for pretreatment are: a multi-barrier approach to the treatment scheme; increased membrane flux rate; reduced membrane cleanings; and prolonged membrane life.
Recently, Black & Veatch reported on their investigation of DAF for membrane pretreatment versus other pretreatment methods for a project with the South San Joaquin Irrigation District in California:
“On a case-by-case basis, alternative clarification methods, such as plate settlers, dissolved air flotation and ballasted flocculation, may be more costeffective than conventional sedimentation. A new 40-mgd UF membrane filtration plant was designed for the South San Joaquin Irrigation District and partner cities of Manteca, Lathrop, Escalon, and Tracy, Calif.
Due to the colloidal nature of the turbidity-causing suspended particles present in the source water—which resist settling without significant polymer addition—the project team evaluated DAF as well as conventional coagulation-sedimentation with a plate settler in a three-month pilot study.
The pilot study revealed that the application of DAF for pretreatment provided superior turbidity reduction, resulting in turbidity of 0.5 NTU compared with 1.5 NTU using conventional coagulation-sedimentation pretreatment. In addition, UF performance was more stable with longer intervals between cleanings following DAF pretreatment. UF following DAF was selected for full-scale treatment,” according to Balck & Veatch.
A second example pilot study was conducted in September 2002 at a water utility in Rhode Island on a water source with turbidity <2 NTU, true color <20 Pt-Co, total iron levels from 0.10 to 0.30 mg/L and total manganese levels from 0.02 to 0.20 mg/L. The data obtained by the pilot test at this site demonstrated optimum performance, as shown in Table 1.
The testing indicated very little organics fouling of the membranes following the Clari-DAF system, thus prolonging the membrane life. Table 2 provides a 20-year capital cost analysis based on a design flow of 8 mgd; a membrane replacement of five years for the process without pretreatment with a Clari-DAF system; and seven years for the process with pretreatment with a Clari-DAF system.
The total savings is $600,000 (16%) using the Clari-DAF system for membrane pretreatment. With the longer time between backwashes and chemically enhanced backwashes, the membranes will have less fatigue due to cleanings, which will result in longer times between membrane replacements (reducing capital cost) and lower chemical cost (reducing operational cost).
Increased flux rates
The third pilot study, completed for a California utility, compared conventional settling with plate settlers and the Clari-DAF system. The pilot study established the highest flux rate that could be achieved before the transmembrane pressure accelerated. The optimum flux rate for the plate settlers was 45 gfd, while optimum flux rate for the Clari-DAF system was 60 gfd. The Clari-DAF system will allow the membrane system to operate at a 33% higher flux rate, resulting in fewer membrane modules required for the treatment process.
In addition to the effluent water quality that will improve membrane performance, the Clari-DAF system produces a high sludge solids content to reduce the solids handling portion of the utilities operation (both dewatering and disposing cost). The operation of the Clari-DAF system can occur without the use of polymers or fine ballasted materials that can carry over in competing clarification processes and foul the membranes. It is a simple system to operate and typically can be started up and optimized within 45 minutes. The footprint compared to conventional settling basins will be one-sixth to one-tenth the size.
The Clari-DAF system high rate flotation process provides the most cost-effective membrane pretreatment process. It effectively removes potential fouling contaminants from the raw water source, does not use potentially fouling flocculating materials, and provides the highest solids content of sludge removed from the clarification process—all with one of the smallest footprints for a clarification treatment process.