Flocculation units

Points of Chemical Addition In independent physical-chemical treatment or in phosphate removal in the primary clarifier ahead of biological treatment, chemicals are added to raw sewage. In tertiary treatment for phosphate removal and suspended solids (SS) reduction, they are added to secondary effluent. In both cases, proper mixing and flocculation units are needed. For phosphate removal or improvement of SS capmre in biological secondary treatment, chemicals are often added directly to aeration units or prior to secondary settling units, without separate mixing and flocculation. In some phosphate removal applications coagulants are added at... 

Flocculation units should have multiple compartments and should be equipped with adjustable speed mechanical stirring devices to permit meeting changed conditions. In spite of simplicity and low maintenance, non-mechanical, baffled basins are undesirable because of inflexibility, high head losses, and large space requirements. Mechanical flocculators may consist of rotary, horizontal-shaft reel units as shown in Figure 9. 

Td = Tetanus and diphtheria toxoids, adsorbed (for adult use), which is a combined preparation containing <2 flocculation units of diphtheria toxoid... 

Toxoids are usually expressed in terms of flocculating units (Lf), which represent the smallest amount of toxin that would flocculate one unit of standard antitoxin (within mixtures of varying amounts of toxin and constant amounts of antitoxin). 

Diphtheria vaccine contains diphtheria toxoid carried on aluminium hydroxide or calcium phosphate. Single antigen products are available only for cases in which combined antigens should not be used. The formulations used in most countries are a childhood formulation containing 25-30 Lf (flocculating units) of diphtheria toxoid (D) and an adult formulation containing 2 Lf of diphtheria toxoid (d). The formulations of choice in routine immunization are DTP (diphtheria and tetanus toxoids combined with pertussis vaccine), DT (diphtheria and tetanus toxoids) for pediatric use, and Td (tetanus and diphtheria toxoids with a limited amount of diphtheria antigen) for use in older children and adults. 

Where air stripping is used in conjunction with other water treatment processes such as flocculation, the positioning of the air stripper in the supply flow stream may be important. Positioning of the stripping tower downstream of the flocculation unit has been accomplished successfully where strict control over the quantities of floe particles released into the water stream has been maintained. 

In flow-through flocculation units (particularly paddle stirrers in series) where the concentration of suspended particles is low (typically 1000 p.p.m. v/v = 10 ) the flocculation is characterized by the dimensionless product Gt (sometimes called the Camp number). Arising from observational data from waterworks in the USA, the optimum value of Gt is set between the limits 10" and 10 (see Camp ). 

Waste water flocculants United States 3,697,420 1972 D. S. Blaisdell... 

Drilling mud flocculants United States 3,297,569 1967 Pan American Petroleum... 

Because of the complexity of bioassays, the expenses for animals and facilities, and ethical considerations, many investigators have sought in vitro methods for detection and quantitation of botulism toxins and antitoxins. One of the earliest was the Ramon flocculation test (57), in which the reaction between a toxin and its homologous antitoxin standard was visualized by a precipitation reaction. This was advantageous in vaccine production because the toxoid, which cannot be measured by bioassay can be quantitated in a comparable manner as the toxin by this test. The flocculation unit (Lf) was used as the antigenic unit employed in the formulation of botulism toxoids used for human immunization (7). 

The presence of the large repulsive potential barrier between the secondary minimum and contact prevents flocculation. One can thus see why increasing ionic strength of a solution promotes flocculation. The net potential per unit area between two planar surfaces is given approximately by the combination of Eqs. V-31 and VI-22 ... 

The carboxylated units, ionized, decrease adsorption on subterranean substrates (23), ia proportion to the number of units, an important parameter ia petroleum recovery processes. In waste treatment processes cationic acrylamide comonomer units are often used (31) to iacrease adsorption and thereby flocculation of soHds ia wastewater (see Acrylamide POLYMERS Flocculating agents). The favorable and characteristics of acrylamide facilitate the... 

Fig. 5. Effect of polymer dosage on different observed properties of flocculated slurry (40). Comparison of five parameters in a flocculation system (8%fluorite suspension + polyacrylamide Cyanamer P250). A, Rate of settling of floe boundary, in cm/s B, height of settled bed, cm C, height of consoHdated filter-cake, cm D, refiltration rate, arbitrary units and E, clarification, % optical transmission of 1 cm of supernatant Hquid after 3 min settling...

The conventional one-pass clarifier is designed for the lowest specific overflow rate (flow per unit area of Hquid surface), which is usually 1—3 m/h depending on the degree of flocculation. These clarifiers can be started and stopped without difficulty. 

Additions of new flocculants after conventional thickening produce further dewatering of mineral slimes. A clay flocculated with polyacrylamides and rotated in a dmm can produce a growth of compact kaolin pellets (84), which can easily be wet-screened and dewatered. A device called a Dehydmm, which flocculates and pelletizes thickened sludges into round, 3-mm pellets, was developed for this purpose. Several units reported in commercial operation in Japan thicken fine refuse from coal-preparation plants. The product contains 50% moisture, compared with 3% soflds fed into the Dehydmm from the thickener underflow (85). In Poland, commercial use of the process to treat coal fines has been reported (86), and is said to compare favorably both economically and technically to thickening and vacuum filtration. 

Elocculants and coagulants are sometimes used as pretreatments before dewatering. The market for flocculants and coagulants for water and wastewater treatment in the United States for 1989 was about 250 million, or 68,000 t, provided by 18 companies. In Europe, 22,000 t of flocculants and coagulants, made by 15 companies, had a market value of 115 million. In Japan, 12 companies made 23,000 t, valued at 184 million (150). 

The method is apphcable for unflocculated pulps or those in which the ionic characteristics of the solution produce a flocculent structure. If polymeric flocculants are used, the floccule size will be highly dependent on the feed concentration, and an approach based on the Kynch theoiy is preferred. In this method, the test is carried out at the expec ted feed solids concentration and is continued until underflow concentration is achieved in the cyhnder. To determine the unit area, Talmage and Fitch (op. cit.) proposed an equation derived from a relationship equivalent to that shown in Eq. (18-45) ... 

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