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Dissolved Air Flotation
Dissolved Air Flotation
Dissolved Air Flotation system is a kind of commonly used solid-liquid separation equipment in the sewage treatment industry, which can effectively remove the suspended matter, grease, rubber substances in the sewage, and is the main equipm

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Product introduction

Dissolved Air Flotation system is a kind of commonly used solid-liquid separation equipment in the sewage treatment industry, which can effectively remove the suspended matter, grease, rubber substances in the sewage, and is the main equipment of the sewage preliminary treatment.
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(I) structural features: the main body of the equipment is rectangular steel structure. The main components are dissolved air pump, air compressor, dissolved air tank, rectangular box, air flotation system, mud scraper system, etc.


1. The air dissolving tank produces small bubbles with a particle size of 20-40um, and adheres to the flocculate firmly, which can achieve good air flotation effect;


2. Less use of flocculant and lower cost;


3. The operation procedures are easy to master, the water quality and quantity are easy to control, and the management is simple.


4. Equipped with back flushing system, the releaser is not easy to block.


(II) working principle: the dissolved water is produced in the dissolved gas tank, and the dissolved water is released to the water to be treated through the releaser under reduced pressure. The air dissolved in the water is released from the water to form 20-40um micro bubbles. The micro bubbles combine with the suspended solids in the sewage to make the proportion of the suspended solids smaller than the water, and gradually float to the water surface to form scum. There is a scraper system on the water surface to scrape the scum into the sludge tank. Clear water enters the clean water tank through the overflow tank at the lower part.


(III) scope of use: 1. It is used to remove the solid suspended matter, oil and various colloidal substances in the sewage, such as the sewage treatment of petrochemical, coal mine, paper making, printing and dyeing, slaughtering, brewing and other industrial enterprises; 2. It is used to recover useful substances, such as the collection of small fibers in the white water of paper making.
 

What is a DAF ?

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewater by removing suspended solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it is then removed by a skimming device. Chemicals can be added to the feed water to improve solids removal.

Where and Why are DAFs used?

DAFs are widely used in treating industrial wastewater effluents from oil refineries, chemical plants and paper mills to the food & beverage industries. DAFs are used to removed suspended solids, such as  Total Suspended Solids (TSS), Fats, Oil & Grease (FOG) and other pollutants from wastewater so that the water is suitable for re-use or discharge to a wastewater treatment facility.

How does a DAF work?



Wastewater is fed into a DAF system and hit with a stream of “whitewater”, which is recirculated clarified water from the DAF that’s super saturated with dissolved air. As these two mixtures blend together, microscopic bubbles attach to solid particulates, giving them enough buoyancy to surface in the DAF tank.

As solids accumulate in a floating layer on the top of the DAF tank, a skimmer gently nudges the sludge toward a discharge hopper.

Any solids that don’t float will sink to the “V” bottom of the DAF tank. Settled solids are concentrated and discharged by an automatically controlled pneumatic drain valve.

The clarified water flows out via an under-over weir on either side of the DAF unit. Some of this water is used in the recirculation loop while the rest flows out of the vessel.

Which DAF Type is Right for My Application?

Generally speaking, a plate pack DAF unit is suited for high hydraulic and low solids loading rates. Open style DAF tanks are better suited for high solids loading rates.

But before we go and place plate pack DAFs and open tank DAFs into specific applications or industries, let’s understand one concept: anywhere you can use a plate pack DAF unit, you can also use an open tank DAF unit, but the opposite does not hold true. Similar to the rule that says, “a square is a rectangle, but a rectangle is not a square.” The trade off for always going with an open style tank is the amount of floor space they occupy, and their overall cost, especially as flow rates rise above a few hundred gallons per minute.

What are Common Components of a DAF?

Flotation Tank

Flotation Tank

The Flotation Tank is where the water separates from pollutants.  The water is flowing through the flotation tank at a slow speed, giving time for heavy objects to settle down and lighter objects to float up to the top with help from bubbles and flocculators.  The flotation tank can vary in depth, width and length based on the application and the time needed for pollutants to be removed.

Sludge Dewatering Grid 

Sludge Dewatering Grid

The Dewatering Grid is a rectangular framework of angular steel plates that lock sludge in place as it rises to the surface. Only when sludge has thickened enough to rise above the top edge of the grid can it be skimmed and pushed to the float hopper.

The Dewatering Grid helps:

  • Operators control sludge thickness
  • Eliminate pre-mature removal of solids
  • Reduce build-up and/or re-entrainment of sludge
  • Generate drier sludge

Skimmers

While many DAF system designs push sludge across the entire length of the tank in the same direction as the wastewater flow called a Co-Current Skimmer, another design uses a skimmer assembly that rotates against the hydraulic flow of the water called a Counter-Current Skimmer.  The Counter-Current Skimmer design shortens the sludge skimming distance and eliminates solids carry-over.

Flocculators

Flocculator

Flocculators are designed to provide the mixing action and retention time required to adequately coagulate and flocculate solids in wastewater to improve solids removal.

Recycle Pump

pump

 

DAF pumps are a key component of all DAF Systems. On it rides some of the largest capital, operations and maintenance expenses involved in wastewater pre-treatment systems.

DAF manufacturer’s approach recycle pumps in two different manners.

The first way is to provide a specialty white water pump.  This pump not only pumps the water but also dissolves the air into the water.  These types of pumps are often more difficult to find and are more expensive.  Also, with putting air in the pump, there is always a risk of cavitation, which causes internal damage and results in more-frequent-than-desired parts replacement.

The second way is a more efficient and cost effective approach.  It is to combine a standard ANSI pump with an angled air dissolving tube.  In this second case, the pump doesn’t do any air dissolving – it just pumps water.  No air in the pump, means very little risk for cavitation.  In this way, standard ANSI recycle pumps don’t do any air dissolving – it just pumps water. That means we can provide higher solids tolerances, use stronger pump materials, operate at lower pressures, and do it at a cost much lower than possible with a specialty whitewater pump.

Angled Air Dissolving Tube

Where many DAF system manufacturers use a mechanical means to dissolve air into water, i.e. a specialty “whitewater pump,” there is a more efficient and cost-effective approach.

The air dissolving tube is where whitewater is generated.  This short expansion in the recirculation piping allows clarified effluent and a small volume of compressed air to mix until saturation is achieved. The angled configuration allows for increased water and air interface so saturation occurs almost instantly.

This design works so well that often a customer will change from their specialty whitewater pump to an ANSI pump and angled air dissolving tube.

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