Krofta Engineering Limited, India was started in 1983 with technical collaboration with Krofta Technologies Corporation, USA. Krofta is the world leader in Dissolved Air Floatation clarifiers ( DAF) and the technology was developed and patented worldwide by its founder Dr. Milos Krofta.Krofta clarifiers have retention times as low as 3 minutes and with a possibility of resource recovery in some cases as opposed to the sedimentation methods characterised by very long retention times. Krofta, is the world leader in this field having more than 3500 installations running successfully all over the world.The advanced DAF system offered by Krofta does not warrant a separate flocculation tank, flocculation mixer, flash tank and flash mixer. The Air Saturation Tank of yesteryears is replaced by the patented Krofta Air Dissolving Tube (ADT) which provides effluent saturated with air in residence time of 10 seconds. Air requirement is also very low as compared to the DAFs of earlier designs.The polyelectrolyte is mixed with the air-saturated effluent on line, just before entering the Supracell. Flocculation is achieved due to the superior fluid dynamics design of the unit ensuring a virtual zero-velocity zone at the point of entrance.In the Krofta supracell, 25% of the exit stream is pressurised to about 6 kg/cm2 pressure and saturated with air in the patented air dissolving tube. This pressurised, air saturated stream is then mixed with the incoming effluent. Polymer is dosed just before this combined stream enters the supracell. The fluid dynamics design of the unit enables zero turbulence at the point of entrance. As soon as the high-pressure stream is released into the supracell, the dissolved air comes out in the form a stream of very fine bubbles. This causes a gentle mixing action to take place and helps in the growth of the flocs. The air bubbles attach themselves to these flocs thereby reducing their apparent density and taking them to the top surface where these flocs form a sludge blanket. The sludge collected at the top is continuously removed with the help of a revolving scoop system and flows by gravity to the sludge drying bed or any other collection/storage system. The sludge consistency is of the order or 3%. Clarified water is collected from the lower portion of the supracell with the help of slit-pipes, which revolve round the supracell, and flows by gravity to storage/discharge.The polymer dosage required for a particular duty is optimised during commissioning of the system and is changed only in case the characteristics of the incoming water change. It is thus advisable to have an equalisation tank prior to the DAF unit in case wide fluctuations in the water characteristics are expected.Krofta DAF are also have provision for an auto-purging system to take care of any solids which tend to settle down. All settled solids, if any, are collected in a sump provided in the supracell tank with the help of a bottom scrapper attached to the movable arm of the DAF (this incorporates the water entry portion, spiral scoop for sludge removal and the pipes for clarified water removal). A timer system actuates a pneumatic valve for a few seconds to enable the settled sludge to flow out of the system.For a given set of inlet water charateristics and polymer dosing, the DAF will give consistent water quality at the outlet. This can be monitored by measuring the TSS content of the clarified water either manually or by instrumentation. An audio alarm may be incorporated in case TSS is measured by on-line. No other control instrumentation is warranted.