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Physics Waste Treatment

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Flotation is used to reduce the content of suspended solids, oils and fats in wastewater. In building flotation the separation is carried out using air bubbles. Air bubbles are obtained by dispersing air into wastewater at a certain pressure. The main variables for flotation are pressure, recirculation ratio, concentration and residence time.

There are four methods of flotation including;

  1. Spontaneous Flotation

Flotation will occur spontaneously if the density of the particles is smaller than the density of water.

  1. Dispersed Air Flotation

Air bubbles are formed due to air pressure entering the liquid through a rotating diffuser or impeller.

  1. Vacuum Flotation

Dissolution of air in waste water at atmospheric pressure, then vacuum at a lower pressure will reduce the solubility of air in water, air will come out of the solution in the form of fine bubbles.

  1. Disolve Air Flotation (DAF)

Air is dissolved in wastewater under pressure from some of the atmosphere until it is saturated, to atmospheric pressure. Due to changes in pressure the dissolved air will release again in the form of very fine air bubbles.

Of the four methods above, the Disolves Air Flotation (DAF) method has been widely used for industrial wastewater treatment, because it is efficient for solid-liquid separation of materials with specific gravity <1 or high.

There are two types of Disolve Air Flotation (DAF) methods, namely:

  1. With recirculation
  2. Without recirculation.

Air-Solubility With Discharges.

The theoretical amount of air that will be released in solution when the pressure is reduced to 1 atm can be calculated by:


S = Air released at atmospheric pressure per unit volume at 100%, cm3/lt

Sa = The air is saturated at a certain pressure, cm3/ lt

P = absolute pressure

Pa = atmospheric pressure

The solubility of air in industrial waste water is smaller than water, correction from the above formula becomes:


f = fraction of air solubility in waste water in the tank (0.85 - 0.9)

For the results of flotation units related to effluent quality in the flotation process in relation to the air or solid ratio determined at the mass of air released per solid mass in the effluent wastewater can be calculated:


Q = wastewater discharge influent

R = waste water recirculation discharge

Sa = concentration of oil or fat or solid wastewater

The relationship between air / solid ratio and effluent quality is obtained in the graph below,

Figure 2.8 Relationship between air / solid ratio and effluent quality

To determine the characteristics of the flotation unit to be designed according to the characteristics of wastewater, it is needed laboratory test. If using a recirculation system the air / solid ratio can be calculated:


Sa = saturated air, cm3/ l

R = recirculation volume, l

P = absolute pressure (atm)

Q = Wastewater discharge (l)

Sa = Suspended solid influent (mg / l)

Surface area can be calculated:


A = Surface area

Q = Wastewater discharge

R = Discharge circulation

Apart from that, laboratory tests also obtained surface loading values at certain pressures plotted with effluent concentration. In order for the flotation process to take place perfectly, coagulant can be added.



Wastewater with a discharge of 150 gal/min (0.57 m3/min) with a temperature of 103oF (39.4oC) . The concentration of influent oil is 120 mg/e and the desired concentration is 20 mg/e. Laboratory results obtained:

Alum dose = 50 mg / e
Absolute pressure = 60 lb / in2 (414 kPa/4,1 atm)
Sludge production = 0.64 mg / mg alum
Sludge weight = 3%

The graph is obtained at a pressure of 60 lb / in2.

Surfase load (gal/min.ft2) Oil and fat effluent (Mg / l)
0,87 14,4
1,58 15
2,21 17,22
3 25


  1. Recirculation Discharge
  2. Flotation Unit Area
  3. Amount of Sludge Produced


The air / solid ratio for oil and fat effluent of 20 mg / e from the graph is obtained:

= 0.03 lb air release / lb solids applied

At 103oF (39.4oC) the water solubility in water is 18.6 3g/e with the f value of wastewater assumed to be 0.85.

  1. Recirculation Discharge

R   = 11,76 gal/min = 53,45 e/min

  1. From the lab results. in the graph obtained for effluent 20 mg / e, surface loading 2.6 gal / (min.ft2)

A = 62,22 ft2

  1. Amount of Sludge Produced

Oil sludge = (Sa - Se) x Q
= (120 – 20) mg/e x 150 gal/min
= 82 kg/day

Alum sludge = sludge production x alum dose x Q
= 0,64 mg sludge/mg alum x 50 mg/e alum x 150 gal/min
= 26 kg/day

Total sludge           = (82 + 26) kg/day
= 108 kg/day

Flotation Planning Criteria

  1. Disolved Air Flotation

Air pressure in the tank = 50 - 70 lb / in2 (3,4 – 4,8 atm)
Detention time in the tank = 1-3 minutes
Detention time like flotation tub = 20 -30 minutes
Surface loading = 1.5 - 4 gal / min ft2 (0.061 - 0.163 m3/min.m2)

  1. Dispersed Air Floation

Air pressure = 20 - 50 lb / in (1.36 - 34 atm)
The size of the air bubble is 30mm - 120 mm
Mixing speed of air - solid = 1-5 in / min (2.56 - 12.7 cm / min)

Hopefully the information we conveyed above can be useful for colleagues, so they can treat waste properly. Waste treatment that is carried out correctly can benefit the environment. so that the environment is kept clean. for further information contact us here