FLOTASI

Contoh modern unit DAF

Pengertian DAF Proses pengolahan air limbah atau air lainnnya dengan cara memisahkan endapan seperti minyak atau solid Pemisahan tercapai oleh udara yang terlarut di dalam air di bawah tekanan, lalu udara dilepaskan pada tekanan atmosferik didalam tangki flotasi

How DAF Works Air under pressure is dissolved into water according to Henry’s Law of Dissolution. Releasing the pressure back to standard temperature and pressure via a special device creates millions of “microbubbles” approximately 30 to 100 microns in diameter. At an 8 percent recycle flow at 70 psig saturation pressure, the number of 40-micron-diameter bubbles created at 40°F is 681 million per gallon. At 68°F the number of bubbles per gallon is 530 million

How DAF Works

Benefits of DAF

Benefits of DAF Small footprint Short star-tup time Minimal mechanical equipment No submerged moving parts Algae floats better than settles Sed. Basin: 75-1 gpm/ft /ft2 DAF: 4-6 gpm/ft /ft2

How DAF Works

DAF System Air Supply Pressurizing Pump Retention Tank Flotation Chamber

SKEMATIK PROSES DAF

DAF Configuration Full Flow Pressurization Partial Flow Pressurization Recycle Flow Pressurization

Full Flow Pressurization The entire influent feed stream is pressurized by a pressurizing pump and held in the retention tank The system is usually applicable to the feed stream with suspended solids exceeding 800 mg/L in concentration It is not susceptible to the shearing effects caused by the pressurizing pump and the high pressure drop at the pressure release valve Suspended solids will flocculate rapidly with the addition of chemical coagulants in the inlet compartment in the presence of the released air

Partial Flow Pressurization Only about 30–50% of the influent feed stream is pressurized by a high- pressure pump and held in the retention tank. Materials with low specific gravity can be removed with the partial flow pressurization system. The increased hydraulic flow on the flotation chamber due to the flow recirculation must be taken into account in the flotation chamber design.

Recycle Flow Pressurization A portion (15–50%) of the clarified effluent from the flotation chamber is recycled, pressurized, and semisaturated with air in the retention tank. The system is usually employed in applications where preliminary chemical addition and flocculation are necessary ahead of flotation. This system is not recommended for use when the suspended solids are susceptible to the shearing effects of the pressurizing pump and the high- pressure drop at the pressure release valve. The suspended solids concentrations are low.

Faktor-Faktor Yang Mempengaruhi Kinerja DAF Sifat Partikel Ukuran Partikel Dispersing Agents Komposisi dan sifat Influent Arus Cairan Rasio A/S Float Removal

Sifat Partikel The specific gravity is a characteristic of the particle or liquid to be abated or separated. It can easily be accepted that sand, for example, cannot be floated while voluminous material, such as activated sludge, or a water immiscible liquid such as oil, can be floated.

Ukuran Partikel Generally, floatability increases with the size of the particle. In many cases,the size of particles can be increased by flocculation with various chemical coagulants.

Dispersing Agents Certain wastewaters and liquids contain unusual concentrations of various chemicals, resulting in specific flotation problems or advantages. Surfactants, such as detergents, tend to alter the physical properties of the sludge particle surface to be floated.

Komposisi dan sifat Influent The composition and nature of the influent is most important. Equalization of composition and flow improves the performance of the flotation unit.

Arus Cairan The liquid currents are governed by the physical design and hydraulics of the flotation unit. This becomes a consideration in the design of the tank and hydraulic loadings of the flotation unit.

Rasio A/S The amount of air and the method of mixing the air with the material to be floated are functions of the design of a particular flotation unit. For a specific application, a definite amount of air is necessary for flotation. In thickening applications it has been shown that increased performance is obtained at higher A/S ratios.

Air to Solids Ratio

Float Removal A float-removal mechanism must be designed to have adequate capacity to remove water carryover. Various items to be considered in this design are the depth of submergence of the scooping mechanism and the speed of scoop operation.

Gas to Solids Ratio : Full Flow Pressurized System Gin Gout Q Cf X Q Cr Q Ce G Correction factor, F or f, because complete gas saturation of liquid is often not achieved in a pressurized retention tank P ≥ 2 atm F = 0,5 – 1,0 P < 2 atm f = 0,167 – 1,0 Henry’s law

Gas to Solids Ratio : Partial Flow Pressurized System Q Cf X G Qn Cf Gin Gout Q Ce Qp Cr G P ≥ 2 atm F = 0,5 – 1,0 P < 2 atm f = 0,167 – 1,0 The Qp/Q ratio ranges between 0.3 and 0.5

Gas to Solids Ratio : Recycle Flow Pressurized System Qr Cr Q Ce G Cf X Gin Gout P ≥ 2 atm F = 0,5 – 1,0 R = Qr/Q P < 2 atm f = 0,167 – 1,0

Air Solubility in Water at 1 Atm (a) Temp. oC Air Sol. (mL/L) 28,8 10 23,5 20 20,1 30 17,9

Air Characteristics and Solubilities

Pressure Calculations

Design Parameters Hydraulic loading rate Solids loading rate Air to Solids ratio Retention Tank Pressure

Basic Design Concept The ratio of Q/As is also defined as the hydraulic loading rate VH = horizontal velocity (m/s), Q = influent flow rate (m3/s), Ac = cross-sectional area of a flotation chamber (m2) VT = vertical rise rate of suspended solids (m/s), D = effective depth of the flotation chamber (m), T = detention time (s), Q = influent flow rate (m3/s), AS = surface area of flotation chamber (m2)

Basic Design Concept The D/W ratio is usually between 0.3 and 0.5 D = effective depth (m) W = effective width (m) L = effective length (m) The D/W ratio is usually between 0.3 and 0.5 F′= factor for short circuiting and turbulence, assumed as 1.4.

The influence of Loading Rate

DAF Operation and Performance

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