Dissolved Air Flotation for Membrane Pretreatment
Many references can be found in water industry literature to support the need for membrane pretreatment.
By J.E. Farmerie
Many references can be found in water industry literature to support the need for membrane pretreatment. This is due to the fact that the dissolved contaminants in non seawater sources may not allow the system to use microfiltration and ultrafiltration (MF/UF) membranes alone to meet the requirements of the Safe Drinking Water Act.
Color, organics, iron and/or manganese compounds are contaminants that will create a need for some sort of pretreatment process. Algae and excess particles can lead to higher capital cost due to designing systems at lower flux rates, and higher operational cost as a result of more frequent backwashes and chemical cleanings. The performance of MF/UF membranes can be improved when a pretreatment system is used to reduce the potential contaminants that can foul the membrane.
Considering the source water contaminants listed above which foul membranes, the high rate dissolved air flotation (DAF) clarification process provides one of the best membrane pretreatment alternatives. Color, organics, soluble metals, or colloidal solids that are removed by adding inorganic chemicals to form aluminum or iron hydroxide flocs create low density floc and require excess chemicals and mixing time to create particles large enough to settle. Normally, solids with hundreds of microns in size are required to settle, while particles of 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 taking advantage of algae buoyancy and flotation technology.
Figure 1: Pilot Schematic
In the dissolved air flotation 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 micron-sized air bubbles through diffusers that span the width of the tank at the bottom of the contactor where they mix with the coagulated solids and float the floc.
The air bubbles are produced by recycling a portion of the effluent through a tank where 80 psi (5.5 Bar) air is introduced causing the water to become saturated, and then reduced to ambient pressure at the nozzles in the header of 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 collect the clarified water. Mechanical desludging will provide a floated sludge with 2% to 5% solids content resulting in significantly less sludge volume to process in the solids handling portion of the treatment plant. A high rate DAF system provides a small footprint with its typical flocculation mixing time of 10-12 minutes and a loading rate of 12 to16 gpm/ft2 based on collector area.
The benefits of providing high rate dissolved air flotation for pretreatment are: 1.) to increase the membrane flux rate, 2.) to reduce backwashing and chemical cleaning of the membranes, and 3.) to prolong the membrane life and increase the time between membrane replacements.
As an example, ITT Water & Wastewater Leopold conducted a pilot plant study with membranes and DAF at a Rhode Island water utility. Figure 1 contains the Leopold DAF pilot unit, a state-of-the-art, self-contained potable water treatment system. It consists of two static mixers for coagulation, two-stage flocculation, and dissolved air flotation followed by filtration. The unit has a fully functional SCADA system that monitors and datalogs all onboard instruments for the various parameters that are selected. The flocculation tanks are adjustable in order to optimize the flocculation time and energy input. The pilot plant has a design flow of 40-120 gpm that gives a DAF loading of 4-16 gpm/ft2 based on total basin area. The recycle rate is fully adjustable to give full process optimization capability.
Rather than use the gravity filters that were available on the pilot trailer, a pressurized Ultrafiltration Membrane was used in the evaluation. It had a 0.01 micron nominal pore size (approximately 150,000 MWCO), Modified Polyethersulfone Hollow fiber, 500 Square Feet, Inside Out Flow Path, and Permeate Backwash with NaOCl augmented Chemically Enhanced Backwash.
The water source contained turbidity < 2 NTU, true color < 20 Pt-Co, total iron levels from 0.10 mg/L to 0.30 mg/L, and total manganese levels from 0.02 mg/L to 0.20 mg/L. The membranes were operated without pretreatment followed by replacing the membranes and operating with DAF pretreatment. The pilot test data at this site yielded the following optimum performance in Chart 1.
After pilot testing, an autopsy of both sets of membranes revealed that very little organic fouling occurred in those membranes following the dissolved air flotation pretreatment. With the longer time between backwashes and chemically enhanced backwashes (CEBW), the membranes will have less fatigue due to cleanings and will result in longer times between membrane replacements (reducing capital cost) and lower chemical cost (reducing operational cost).
Another example is from a pilot study completed for a California utility. Data was collected from pilot studies that compared inclined plate settlers and DAF for pretreatment for the same membrane.
Inclined plate technology takes advantage of the fact that separation of suspended solids from a liquid by gravity is primarily a function of the size of the available sedimentation area. By installing parallel plates inclined to the horizontal in a given basin volume, the sedimentation area will increase considerably. By the inclination, the separated particles that settled on the plates will slide down and fall into the sludge space below the plates. This results in allowing higher loading (1 – 2 gpm/ft2) rates and substantial savings in volume of construction over a conventional sedimentation basin (0.5 gpm/ft2).
The pilot study used the Leopold Pilot DAF system outlined above followed by a Microza microfiltration Hollow Fiber Module. This membrane is a 0.1 µm rated PDVF membrane with filtration from outside fiber to inside of the fiber. The goal of the pilot study was to establish the highest flux rate that could be achieved before the transmembrane pressure (TMP) accelerated.
For both the plate settlers and DAF pilot units, powdered activated carbon (PAC) was fed from 2 to 14 mg/L, potassium permanganate (KMnO4) was fed at 0.2 mg/L, Sumachlor 50 coagulant (polyaluminum chloride) was fed from 20 mg/L to 45 mg/L and chlorine dioxide (ClO2) was fed at 0.75 mg/L. Backwash frequency/duration was 20 min/60 sec for all evaluations. Pilot data is listed below in Chart 2.
The optimum flux rate for the plate settlers was 45 gfd, while optimum flux rate for the DAF system was 60 gfd. In this study, the DAF system allowed 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, a DAF system produces a high sludge solids content to reduce the solids handling portion of a utility's membrane pretreatment operation (both dewatering and disposing cost). The operation of the DAF 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 is one sixth to one tenth the size. --m
About the Author:
Jim Farmerie is Product Manager for ITT Water & Wastewater Leopold in Zelienople, PA. He has over 40 years experience in the municipal water and wastewater market providing chemicals and equipment for treatment processes. He has a Bachelor of Science degree from the University of Pittsburgh and has been active in the AWWA and WEF organizations.