Flocculation and Sedimentation

Preliminary purification by flocculation is necessary, if the untreated water has a high turbidity, particularly as a result of colloidal or soluble organic impurities. Iron or aluminum salts are added to the water, so that iron(III) or aluminum hydroxide is precipitated  

The optimum pH for flocculation is about 6.5 to 7.5 for aluminum salts and about 8.5 for iron salts. If the natural alkali content of the untreated water is insufficient to neutralize the acid formed, alkali has to be added (e.g. calcium hydroxide or sodium hydroxide). In addition flocculation aids such as poly(acrylamide) or starch derivatives may be added (not in the case of potable water production). When aluminum sulfate Al2(S04)3 I8H2O is used 10 to 30 g/m- is added. The very fine hydroxide flakes which precipitate are positively charged and adsorb the negatively charged colloidal organic materials and clay particles. 

A variety of industrial equipment has been used to carry out the flocculation process and the separation of the flocculated materials producing a well-defined sludge suspension layer, which can be removed. Some plant operates with sludge feedback to enable more efficient adsorption. Sludge flocks can also be separated by flotation. 

Flocculation affects filler packing and therefore it also affects surface roughness and gloss. The composition of fillers (pigments) can be changed by coflocculation. Special additives are used to promote this effect because co- 

The rheology of the suspension is affected through the particle interaction coefficient  

Opg / Dj, summation of all individual particle contributions to the particle interaction coefficient particle contribution constant number average particle size 

This equation has been confirmed by experimental results. These have shown that the interaction parameter increases as the particle size decreases. The particle interaction coefficient, a, in the following equation is required to describe the viscosity-concentration relationship of suspensions  

Filler particles can be modified to decrease flocculation. Kaolin particles modified by a graft of poly(ethylene oxide) showed an increase in the upper critical flocculation temperature. Stabilization of particle dispersion was due to an enhanced steric stabilization.  

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The trend in the use of deep bed filters in water treatment is to eliminate conventional flocculators and sedimentation tanks, and to employ the filter as a flocculation reactor for direct filtration of low turbidity waters. The constraints of batch operation can be removed by using one of the available continuous filters which provide continuous backwashing of a portion of the medium. Such systems include moving bed filters, radial flow filters, or traveling backwash filters. Further development of continuous deep bed filters is likely. Besides clarification of Hquids, which is the most frequent use, deep bed filters can also be used to concentrate soflds into a much smaller volume of backwash, or even to wash the soflds by using a different Hquid for the backwash. Deep bed filtration has a much more limited use in the chemical industry than cake filtration (see Water, Industrial water treatment Water, Municipal WATERTREATiffiNT Water Water, pollution and Water, reuse). 

Suspended matter in raw water suppHes is removed by various methods to provide a water suitable for domestic purposes and most industrial requirements. The suspended matter can consist of large soflds, settleable by gravity alone without any external aids, and nonsettleable material, often colloidal in nature. Removal is generally accompHshed by coagulation, flocculation, and sedimentation. The combination of these three processes is referred to as conventional clarification. 

These chemicals ensure the aggregation of the suspended solids during the next treatment step-flocculation. Sometimes polyclectrolytes (usually polyacrylamides) are also added after flocculation and sedimentation as an aid to the filtration step. 

Flocculation and sedimentation arc two processes used to separate waste streams that contain both a liquid and a solid phase. Both are well-developed, highly competitive processes, which arc oflcii used in the complete treatment of waste streams. They may also be used instead of, or in addition to, filtration. Some applications include the removal of suspended solid particles and soluble heavy metals from aqueous streams. Many industries use both processes in the rcmowal of pollutants from their wastewaters. These processes work best when the waste stream contains a low concentration of the contaminating solids. Although they are applicable to a wide variety of aqueous waste streams, these processes arc not generally used to treat nonaqueous or semisolid waste streams such as sludges and slurries. 

FIGURE 16.13 Representative configuration using precipitation, flocculation, and sedimentation. 

In this treatment process, unit operations such as chemical coagulation, flocculation, and sedimentation followed by filtration, activated carbon, ion exchange, and reverse osmosis are employed to remove significant amounts of nitrogen, phosphorus, heavy metals, organic matters, bacteria, and viruses present in wastewater.2 It is always the last process step in the wastewater treatment plant that finally renders the treated wastewater reusable and disposable into the environment without any adverse effect (Figure 22.1). 

The mechanism for sedimentation (clarification) is based on the density difference between SS and liquid. In addition, SS with larger particle sizes can settle down more easily. Rectangular tanks, circular tanks, combination flocculator-clarifiers, and stacked multilevel clarifiers can be used.14 Oliveira et al.15 reported that flocculation and sedimentation were conducted in the cassava meal industry and reduced the effluent concentration of organics from 14,000 to 2000 mg/L in the bench-scale reactor, with a hydraulic retention time (HRT) of 37 min. 

In the flocculated state for all suspensions, immediately after mixing, no transmission of light could be detected through the cuvette used and the sedimentation behaviour was different. Thus, instead of the parameters used for the pyrogenic silica systems we used the following characteristics for the precipitated silica mixtures to describe the flocculation and sedimentation behaviour ... 

Concentration of pesticide after chemical flocculation and sedimentation (primary treatment). 

Metallic ions in soluble form are commonly removed from wastewater by conversion to an insoluble form followed by separation processes such as flocculation, sedimentation, and filtration. Chemicals such as lime, caustic soda, sulfides, and ferrous or ferric compounds have been used for metals separation. Polymer is usually added to aid in flocculation and sedimentation. 

Phosphate removal processes from wastewater have been studied by many workers, in order to protect stagnant water area, such as lakes and coastal region from eutrophication. Among conventional phosphate removal processes, the representative one was flocculation and sedimentation process, which was based on precipitation of insoluble metal phosphate or hydroxide. However, the main problem with this process, is to produce large amounts of sludge, which is difficult to dehydrate. 

Table 1 shows the performance of fixed bed type process, in application to various wastewaters. The merit of this process is stability in ability of phosphate removal and low sludge production. Sludge production of this process is from 1/5 to 1/10 lower than that of the conventional flocculation and sedimentation process. 

Xue, J. Huang, P.M. (1995) Zinc adsorption-desorption on short-range ordered iron oxide as influenced by citric add during its formation. Geoderma 64 343—356 Yamada, K. Harato,T. Shiozaki,Y. (1980) Flocculation and sedimentation of red mud. Light Metals 39-50... 

Particle density is the specific gravity of a soil, which is important in soil washing and in the determination of the setting velocity of suspended soil in flocculation and sedimentation processes. 

In practice, two types of plants are generally used for chemical precipitation hardness removal One type uses a sludge blanket contact mechanism to facilitate the precipitation reaction. The second type consists of a flash mix, a flocculation basin, and a sedimentation basin. The former is called a solids-contact clarifier. The latter arrangement of flash mix, flocculation, and sedimentation were discussed in previous chapters on unit operations. 

Not only are accurate data for trace metals in rivers sparse, there are complications that exist at the river-sea interface. The increase in salinity occurring at the river-sea water interface, with its concomitant increase in the concentrations of the major seawater cations, can lead to flocculation and sedimentation of trace metals such as iron (Boyle et al., 1978 Sholkovitz and Copeland, 1983) or to desorption from suspended riverine particles of trace metals such as barium (Edmond et al., 1978). In organic-rich rivers a major fraction of dissolved trace metals can exist in physiochemical association with colloidal humic acids. Sholkovitz and Copeland (1983) used product-mode mixing experiments on filtered Scottish river water, and observed that iron removal was almost complete due to the flocculation of strongly associated iron-humic acid colloids in the presence of the increased... 

T = 100 g m ) involved flocculation and sedimentation of colloidal suspensions with precipitated calcium carbonate. The samples became clear after 30 minutes of sedimentation at a coagulant dose (both A and B) of 500 g m. There was no significant difference in the final removal efficiency between the two... 

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