Flocculation with organic coagulants

Coagulation followed by flocculation with a metal-based coagulant such as aluminum sulphate or ferric chloride. This process removes most particulate materials and some dissolved organic compounds. 

Coagulation-flocculation with FeCl3 only enables the elimination of a rather small part of the organic load. This part is nevertheless more important in acidic conditions as shown for Grospierres leachate in Fig. 5. COD data before and after treatment are given in Table 4 for Grospierres leachate. The results show that a pH decrease to 5, in the presence of FeCl3, leads to the removal of about 30% of the COD, probably explained by the precipitation of humic-like substances. 

The organic coagulant (type 2 cationic polymer) with a strong cationic nature provides a coagulation mechanism fairly close to the metallic coagulants mechanism but does not include the subsequent flocculation stage. 

However, in the absence of this highly adsorbent hydroxide gel, fine colloids (with FeS among them) do not flocculate as well and water clarified by an organic coagulant alone does not exhibit the limpidity and shininess given by metallic salts. Turbidity may be from 2 to 3 NTU, often 0.5 to 1 in the second instance. 

Organic coagulants are used mainly in low solids environment such as potable water and industrial effluent treatment. They can also be used in combination with high molecular weight flocculants. In particular, the addition of a coagulant improves filtrate and centrate clarities, when the flocculant treatment alone is imable to attain acceptable levels. 

There are a number of major, international manufacturers of coagulant and flocculant polymers whose primary markets are high-volume users (i.e., cities, states, and national governments). There are also many smaller regional manufacturers who tend to specialize in niche markets and produce various polymer blends (organic polymers blended with various ratios of inorganic coagulants such as ACH, PAC, and alum). These polymer blends are particularly useful in industrial facilities where process contamination and difficult clarification problems may exist. 

Chemical addition is typically used with clarification to improve both the utility and performance of the unit operation. Coagulants and flocculants are generally used to improve the ability to settle particles in the clarifier. Jar tests are used to determine the proper dosage of chemicals and streaming current detectors or turbidity monitors are used to monitor performance and control chemical dosage. Chlorine is often used to improve the removal of organics and color in the clarifier. Chlorine also provides disinfection of the make-up water to prevent the clarifiers from going septic. 

The electrochemical coagulation is a complex process that can be used to reduce the organic content of many types of industrial wastewaters including those polluted with colloidal particles, macromolecules, or O/W emulsions. This process consists of two sequential processes the dose of the reagent, which is really the more important electrochemical process, and the subsequent coagulation/flocculation processes. In principle, these later processes are not electrochemical but chemical or physical processes. However, the mobility of the pollutants can be greatly improved due to electrophoretic or the electromigration processes, and consequently electrochemistry should also be considered in the description of these processes. 

Coagulants may be either organic such as polyelectrolytes or inorganic such as alum. Coagulants can be used alone or in conjunction with flocculants to improve the performance of the flocculant or reduce the quantity oF the flocculant required. In some systems, where a flocculant has been used in an upstream process, a coagulant may be needed to allow additional flocculant to be effective. 

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