Coagulants in Water and Wastewater Treatment

The commonly used metal coagulants fall into two general categories: those based on aluminum and those based on iron. The aluminum coagulants include aluminum sulfate, aluminum chloride and sodium aluminate. The iron coagulants include ferric sulfate, ferrous sulfate, ferric chloride and ferric chloride sulfate. Other chemicals used as coagulants include hydrated lime and magnesium carbonate.

The effectiveness of aluminum and iron coagulants arises principally from their ability to form multi-charged polynuclear complexes with enhanced adsorption characteristics. The nature of the complexes formed may be controlled by the pH of the system.

When metal coagulants are added to water the metal ions (Al and Fe) hydrolyze rapidly but in a somewhat uncontrolled manner, forming a series of metal hydrolysis species. The efficiency of rapid mixing, the pH, and the coagulant dosage determine which hydrolysis species is effective for treatment.

There has been considerable development of pre-hydrolyzed inorganic coagulants, based on both aluminum and iron to produce the correct hydrolysis species regardless of the process conditions during treatment. These include aluminum chlorohydrate, polyaluminum chloride, polyaluminum sulfate chloride, polyaluminum silicate chloride and forms of polyaluminum chloride with organic polymers. Iron forms include polyferric sulfate and ferric salts with polymers. There are also polymerized aluminum-iron blends.

The principal advantages of pre-polymerized inorganic coagulants are that they are able to function efficiently over wide ranges of pH and raw water temperatures. They are less sensitive to low water temperatures; lower dosages are required to achieve water treatment goals; less chemical residuals are produced; and lower chloride or sulfate residuals are produced, resulting in lower final water TDS. They also produce lower metal residuals.

Pre-polymerized inorganic coagulants are prepared with varying basicity ratios, base concentrations, base addition rates, initial metal concentrations, ageing time, and ageing temperature. Because of the highly specific nature of these products, the best formulation for a particular water is case specific, and needs to be determined by jar testing. For example, in some applications alum may outperform some of the polyaluminum chloride formulations.

PoIymers are a large range of natural or synthetic, water soluble, macromolecular compounds that have the ability to destabilize or enhance flocculation of the constituents of a body of water.

Natural polymers have long been used as flocculants. For example, Sanskrit literature from around 2000 BC mentions the use of crushed nuts from the Nirmali tree (Strychnos potatorum) for clarifying water – a practice still alive today in parts of Tamil Nadu, where the plant is known as Therran and cultivated also for its medicinal properties. In general, the advantages of natural polymers are that they are virtually free of toxins, biodegradable in the environment and the raw products are often locally available. However, the use of synthetic polymers is more widespread. They are, in general, more effective as flocculants because of the level of control made possible during manufacture. Important mechanisms relating to polymers during treatment include electrostatic and bridging effects.

The figure below shows schematic stages in the bridging mechanism. Polymers are available in various forms including solutions, powders or beads, oil or water-based emulsions, and the Mannich types. The polymer charge density influences the configuration in solution: for a given molecular weight, increasing charge density stretches the polymer chains through increasing electrostatic repulsion between charged units, thereby increasing the viscosity of the polymer solution.

Flocculation

Figure 1. Stages in the bridging mechanism: (i) Dispersion; (ii) Adsorption; (iii) Compression or settling down (see inset); (iv) Collision (Reference 2)

One concern with synthetic polymers relates to potential toxicity issues, generally arising from residual unreacted monomers. However, the proportion of unreacted monomers can be controlled during manufacture, and the quantities present in treated waters are generally low.

The above is an excerpt for the following article:  Coagulation and Flocculation in Water and Wastewater Treatment – Article… http://iwawaterwiki.org/xwiki/bin/view/Articles/CoagulationandFlocculation