Dissolved Air Flotation (DAF) is a process that removes solids by injecting pressurized air into wastewater, where micro-bubbles attach to suspended solid particles, and cause them to float to the surface of a flotation tank where they are skimmed and removed.
DAF is a highly efficient method for removal of turbidity, color, suspended solids and other contaminants from water. A recirculation loop of pressurized water is saturated with dissolved air using a multistage impeller pump. The product of this process is termed “whitewater.” Whitewater discharges into calculated points along the pipe flocculator. Microscopic air bubbles become entrained in the floc particles. When the floc particles leave the pipe flocculator and enter the body of the DAF, they rise to the surface and form a sludge blanket. As the sludge blanket thickens, a skimmer drive transfers sludge to the DAF sludge sump. From the sump, sludge is pumped to dewatering equipment. The treated water gravity flows from the DAF vessel to a clear well or city discharge point.
Flocculation Mixing Tubes
The flocculator is designed to thoroughly mix raw influent with water treatment polymers and dissolved air. It consists of a spiraled mixing tube with strategically placed polymer injection ports, highly aerated/pressurized water, and sample ports. A metered control valve manifold ensures accurate flow of pressurized, highly aerated water into the mixing pipe flocculator to enhance coalescing of microscopic air bubbles and wastewater mixing. Polymer injection ports are quick-connected, allowing easy removal and re-insertion during system operation. The Dissolved Air Flotation process utilizes design principles unique to our DAF system. The design allows for highly effective flocculator units to be installed in a footprint significantly smaller than that of other systems. Plant wastewater enters the DAF system via an ascending pipe flocculator which acts as both a flash mixer (lower section) and a gentle mixer (upper section) for superior flocculation.
Pressurized Water Recirculation
Clean, treated water removed from the wastewater discharge cell is pressurized and fed back into the system via inlet ports on the flocculator. Additionally, pressurized water is injected into the flotation cell. The Pressurized Recirculation Loop Creates Whitewater, which is a key factor to water treatment success. The Pressurization Pump is sized to provide adequate recirculation water flow at 400’ TDH. The pump is installed on the DAF flotation cell skid. The Air Dissolving Tank provides the retention time necessary to allow injected air to dissolve, creating “whitewater”. The tank is supplied complete with internal piping, inlet/outlet flanges, pressure gauge, pressure bleed-off valve, and air control assembly (solenoid valve, filter/ regulator/gauge, rotometer, and check valve). The pumps we use are side-channel multi-stage pumps, which is a unique product with characteristics of both displacement pumps and centrifugal pumps. At low flow and high pressures, our self-priming pumps have a noticeable advantage over centrifugal pumps. It allows trouble free handling of Maximum gas contents of 50% and attains pump heads which are many times greater per stage than that generated by radial flow centrifugal pumps. These pumps are essentially immune to cavitation at variable vapor pressure and therefore flow is not interrupted during partial degassing. The pressure rate is up to 10 times higher than that generated by normal pump impellers rotating at the same speed. The best part is that we can generate more air with extremely small bubble sizes of 20 microns and smaller that remain in solution for over 30 minutes, resulting in extremely efficient separation of solids from the water and very dry sludge.
Dual Chamber Chemical Reaction Tank
The Reaction tank is designed specifically as an option to allow for longer chemical contact time. The tank allows for 5 minutes of chemical reaction time per chamber. The Dual Chamber Reaction Tank features plate mounted agitators and typically utilizes a Walchem dual probe pH controller to aid in automated chemical dosage and operator monitoring. The unit can also be outfitted with a stainless steel chemical metering pump stand for application of sulfuric acid, acidic coagulants or caustic soda. This feature allows for short distance pipe runs of hazardous chemicals and minimizes operator exposure.
The flotation cell is a rectangular vessel designed to provide controlled influent wastewater velocity and optimized surface area. It contains an inlet diffusion pipe, sludge rake, and one or more blow-down valves. The blow-down valve permits removal of the small portion of sludge that settles in the flotation cell.
The sludge rake assembly consists of a stainless steel chain and a three-blade scraper arrangement. The sludge scrapers skim the surface of the sludge in the flotation cell and transfer the floating solids to the sludge sump. From the sludge sump a pump removes the sludge to the Thickening/Sludge Dewatering Tank.
Thickened sludge skimmed from the DAF discharges into a sludge sump. The sump has a hopper bottom with inclined sides to allow sludge to gravity feed to the suction side of the sludge transfer pump.
The discharge cell is designed to allow clean, treated water to flow downward and away from rising sludge, and then upward through a clear-well section. It contains a wastewater baffle, adjustable weir, pressurized water diffusion pipe, and blow-down valve. Clean, treated effluent, flows down and under the flotation cell, upward through a clear-well and then over an adjustable weir into a collection launder. The weir is made to be adjustable with threaded bolts. A portion of the clean, treated water, is removed from this cell via a recirculation water header piped to the suction inlet of the pressurized water recirculation pump. A portion of the pressurized water is piped into the inclined section of the flotation cell. A blow-down valve permits removal of the small portion of sludge that settles in the discharge cell. As a result of these extra clarification steps our clean water is commonly as much as 5-6 times purer than other competitive DAF systems.
Clear Well and Dewatering
Some systems employ a clear well for finished clean water inspection, post pH adjustment, or as a short term holding reservoir when the water needs to be transferred to another location for discharge or reuse. The clear well is constructed as a low side wall stainless steel tank for easy visual reference of effluent quality.
The DAF vessel is constructed of 304L Stainless Steel with suitable stiffeners to prevent deformation under all normal stresses incurred during erection and operation. The unit can also be skid-mounted for mobility. All joints are double welded and leak tested prior to shipment. A DAF-mounted catwalk and ladder are provided for operator access to the top of the DAF vessel on large units. Handrails are painted yellow per OSHA regulations. The DAF mixing-pipe flocculator is fabricated out of 304L Stainless Steel (316SS is optional). The flocculator is mounted on legs, and connects to the main DAF unit via a 150lb flange. The flocculator and DAF system are pressure tested prior to shipment.
Automatic Control Systems
The control panel for each system can be designed to include as much or as little automation as desired by the operator and project engineers. They are designed to provide fully automatic operation utilizing a PLC (Programmable Logic Controller). User interface with the control system is provided in the form of panel-mounted selector switches, Allen-Bradley AC variable drive terminals, a pH controller, alarm with lights/horn and a panel-mounted process display terminal. Process variables and chemical dosage set points are monitored and changed via the panel-mounted display terminal. Chemical pump rates follow process flow which is monitored and controlled by a magmeter and proportional valve. Polymer usage is automatically displayed in ml/min and lbs/day on the panel-mounted display. System kilowatt-hour usage is also monitored and displayed. A fail-safe operation mode is provided that will allow the system to be operated manually in the event of a computer malfunction. A semi-automatic mode is also designed into the Ecologix control system to provide maintenance and operational flexibility.
Sludge Holding/ Dewatering
The sludge that has been removed from the wastewater by the scraper is pumped into a sludge holding tank for further thickening. From there it is pumped into a dewatering system such as Filter Press or Belt Press for final dewatering. The end result is a 25%-50% dry sludge by weight that is solid to the touch and can be safely disposed of to a land fill.