dissolved air flotation and cheap efficient

What is Dissolved Air Flotation Definition–DAF Unit
Dissolved Air Flotation(DAF)technology is very suitable for removal of floating particulate foulants such as algal cells,oil,grease or other contaminants that cannot be effectively removed by sedimentation or filtration.Dissolved Air Flotation DAF system can typically produce effluent turbidity of<0.5NTU and can be combined in one structure with dual-media gravity filters for sequential pretreatment of seawater.Dissolved Air Flotation(DAF)process uses very small air bubbles to float light particles and organic substances(oil,grease)contained in the seawater.The floated solids are collected at the top of the DAF tank and skimmed off for disposal,while the low turbidity seawater is collected near the bottom of the tank.The time(and therefore,the size of flocculation tank)needed for the light fine particulates contained in the seawater to form large flocs is usually 2to 3times shorter than that in conventional flocculation tanks,because the flocculation process is accelerated by the air bubbles released in the flocculation chamber of the DAF tanks.In addition,the surface loading rate for removal of light particulates and floatable substances by DAF is approximately 10times lower than that needed for conventional sedimentation.Another benefit of DAF as compared to conventional sedimentation is the higher density of the formed residuals( Sludge ).While residuals collected at the bottom of sedimentation basins typically have concentration of only 0.3to 0.5%solids,DAF residuals(which are skimmed off the surface of the DAF tank)contain solids concentration of 1to 3%.In some full-scale applications,the DAF process is combined with granular media filters to provide a compact and robust pretreatment of seawater with high algal and/or oil and grease content.Although this combined DAF/ Filter configuration is very compact and cost-competitive,it has three key disadvantages:
(1)complicates the design and operation of the pretreatment filters;
(2)DAF system loading is controlled by the filter loading rate and therefore,DAF tanks are typically oversized;
(3)Flocculation tanks must be coupled with individual filter cells. The feasibility of Dissolved Air Flotation DAF unit use for seawater pretreatment is determined by seawater quality and governed by source water turbidity and overall lifecycle pretreatment costs.
In flotation, the effects of gravity settling are offset by the buoyant forces of small air bubbles.These air bubbles are introduced to the flocculated water,where they attach to floc particles and then float to the surface.Flotation is typically sized at loading rates up to 10times that for conventional treatment.Higher rates may be possible on high-quality warm water.Dissolved air flotation(DAF)is an effective alternative to sedimentation or other clarification processes.

The DAF unit can handle source seawater with turbidity of up to 50NTU.Therefore,if the source seawater is impacted by high turbidity spikes or heavy solids(usually related to seasonal river discharges or surface runoff),then DAF system may not be a suitable pretreatment option.In most algal bloom events however,seawater turbidity almost never exceeds 30to 50NTU,so the DAF technology can handle practically any red tide event.Although a DAF system have much smaller footprint than the conventional flocculation and sedimentation facilities,it includes a number of additional equipment associated with air saturation and diffusion,and with recirculation of portion of the treated flow,and therefore,their construction costs are typically comparable to these of conventional sedimentation basins.Usually,the O&M costs of DAF system is higher than these of gravity sedimentation tanks due to the higher power use for the flocculation chamber mixers,air saturators,recycling pumps,and sludge skimmers.The total power use for a Dissolved Air Flotation–DAF system is usually 2.5to 3.0kWh/10,000m3/day of treated source seawater,which is Significantly higher than that for sedimentation systems(0.5to 0.7kWh/10000m3/day of treated seawater).

Theory and Operation of a Dissolved Air Flotation DAF Unit
Effective gravity settling of particles requires that they be destabilized,coagulated,and flocculated by using metal salts,polymers,or both.The same is true for DAF.In gravity settling the flocculation process must be designed to create large,heavy floc that settles to the bottom of the basin.In Dissolved Air Flotation DAF unit,flocculation is designed to create a large number of smaller floc particles that can be floated to the surface.For efficient flotation,flocculated particles must be in contact with a large number of air bubbles.
Three mechanisms are at work in this air/floc attachment process:
•Adhesion of air bubbles on the floc surface
•Entrapment of bubbles under the floc
•Absorption of bubbles into the floc structure
The size of air bubbles is important.If bubbles are too large,the resulting rapid rise rate will exceed the laminar flow requirements,causing poor performance.If bubbles are too small,a low rise rate will result and tank size may need to be increased.In a typical Dissolved Air Flotation tank,flocculated water is introduced uniformly across the end of the tank,near the bottom,into the recycle dispersion zone.Recycle is continuously introduced through a distribution system of proprietary nozzles,valves,or orifices.When the recycle flow pressure is suddenly decreased from its operating pressure of 60to 90psi(414to 620kPa)to atmospheric pressure,saturated air within the recycle stream is released in the form of microbubbles with a size range of 10to 100/xm,and averaging around 40to 50/xm.These microbubbles attach to flocculated material by the mechanisms described previously,causing flocculated material to float to the surface.At the surface,the bubble-floc forms a stable and continuously thickening layer of float,or sludge.If left at the surface,the float can thicken to as much as 3%to 6%dry solids.This can be an advantage if solids are to be mechanically dewatered,because solids may be suitable for dewatering without further thickening,or the thickening process can be reduced.Sludge thickness depends on the time it is allowed to remain on the surface and the type of removal system employed.
Dissolved Air Flotation–DAF System–Key Design Criteria
Dissolved Air Flotation DAF system include three key components:flocculation chamber;flotation tank and recycling system.The design criteria for these three components are presented below:

Sewage Treatment Equipment

Rotary Drumfilter

Dissolved Air Flotation Machine Sewage

High-efficiency Horizontal Dissolved Air Flotation

Horizontal Dissolved Air Flotation Machine Sewage

Flow Fine Grille

Sewage treatment equipment

Rotary drumfilter

Dissolved air flotation machine sewage

High-efficiency Horizontal Dissolved Air Flotation

Horizontal Dissolved Air Flotation Machine Sewage

Flow Fine Grille