This Krofta Dissolved Air Flotation/Filtration Clarifier combines the typically separate components of flocculation, dissolved air flotation, dual media filtration, and continuous on-line backwash into a highly efficient, space-saving, single-equipment footprint. The advanced design of the Sandfloat is ideal for applications such as potable water, industrial pre-treatment, and wastewater polishing.

Raw water, combined with flocculation and coagulation agents, is introduced into the bottom center of the unit in a mixing chamber. Several directional nozzles and baffles within the chamber cause a slow blending of the raw water and chemicals. The chamber can be designed for extended flocculation times when necessary. As the raw water and flocculated particles rise out of the chamber, they are gently mixed with the aerated water at the surface. Typical retention time within the flocculator is 3-4 minutes.

Aerated water combines with the flocculated, raw water and flows under a baffle that slows horizontal velocity into the flotation zone. The aerated water is generated by recycling a portion of the clarified water through a centrifugal pump and, subsequently, through a Krofta Air Dissolving Tube (ADT). The pressurized flow is released to the atmosphere through a globe valve (or series of globe valves depending on the capacity of the unit). The flocculated solids attach to the 30-70 micron air bubbles, formed during the pressure release, and float to the surface. Clarified water then gravitates towards the media beds for filtration.


A portion of the clarified water, typically 20%, is recycled to feed the air dissolving system. A standard centrifugal pump draws clarified water from its source into the ADT, where it has an 8-12 second retention time. A uniquely designed influent nozzle causes the flow from the recycle pump to enter the ADT in a spiraling pattern. Compressed air, injected across the surface of a panel located inside the ADT, is dispersed evenly throughout the tube. The air mixes with the recycle water stream and is dissolved. A "bleed-off" mechanism prevents too much air from being introduced into the recycle stream, as this could cause air binding in the tube (the formation of coarse air/large air bubbles). The pressurized flow is fed through an inlet header, where it ultimately mixes with the raw water.

A globe valve, located on the discharge line, controls the flow. Flow through the tube can be readily gauged by the difference in pressure from the inlet nozzle to the center of the tube. Krofta provides the valves, gauges, rotometers, site tube, and safety relief valves for this system.

The ADT does not usually qualify as a pressure vessel so it is not subject to ASME coding or annual certification.
A spiral scoop assembly removes floated sludge from the clarifier. The scoop is programmed to precisely remove only the top layer of floated materials when used in conjunction with the Krofta Automatic Level Control System. This keeps the sludge consistency high, generally in excess of 3%. Sludge removed from the top of the flotation zone is deposited down the sludge cone located in the center of the unit.


The lower section of the Sandfloat is divided into individual filter bed segments or cells. Each cell has a flat plate bottom equipped with extraction nozzles or optional stainless wedge wire assemblies. Using gravity, the clarified water flows to the extraction point. Typically, there is a minimum of 2' of filtration media in each cell. The media generally consists of sand and anthracite; sand only, greensand, or some combination of these filtering materials. Depending on the application, a wide variety of filtering media may be used. Pea stone surrounds the extraction assemblies for support and protection. Clarified/filtered water is collected in a central header attached to each cell segment for discharge.


Without interrupting the filtration process, individual cells or segments are routinely taken off-line for backwashing. The pipe rings that surround the Sandfloat each contain one valve for effluent and another for the backwash line. During normal operation, the backwash valves are closed and the effluent valves open. The backwashing sequence for a cell activates when the unit's rotating carriage assembly and a single cell are aligned. A neoprene seal inflates, isolating the cell to be backwashed from the rest of the online cells. The positions of the two valves on the backwashing cell reverse, stopping effluent flow through the filtering media in that cell.

Water is drawn down through the backwash line (as a feed source to the air dissolving system) filling to a level approximately 4" to 6" above the media. Next, an air scour is injected into the cell's media, gently lifting it. This is followed by a partial flow backwash (~10 gpm / sf.), then a full flow backwash (~15 gpm / sf.), and lastly another partial flow to allow the media to re-stratify. Backwash water is channeled up into a backwash trough to be either recycled into the flocculator for re-use or directed out as waste. Subsequently, the hood seal is deflected and water drawn off again through the backwash ring. This process continues for a short period, until the filtered water returns to the required 'spec' (NTU). The valve positions then reverse and the carriage moves to the next cell, repeating the process. An optional monitoring system, which tracks the backwash procedure in each cell, is available (this ensures the media settles and limits any turbidity spikes).

• Reduces particles to 2 microns.
• Effectively filters common microbial contaminants such as giardia and cryptosporidium.
• First filtrate recycle capability for potable water applications.
• Viewing windows for flotation and backwash observation.
• Compact, high throughput design capable of processing up to 5gpm/sf.
• Dual media filtration.
• Flocculation, flotation, and filtration in one piece of equipment.
• Continuous backwash capability, backwash system cleans one filter segment while all others are in operation.
• PLC/DCS/SCADA compatible.
• Stainless steel construction - bolt together design.