Chemical Precipitation -- Basics

It can be used on a small or large scale. A beaker full of waste, a 50,000 tank, a 1,000,000 gallon lagoon or a lake can be batch treated with chemicals. Chemical precipitation can be used in a continuous treatment system on flows ranging from a trickle to 1 gallon/minute, 1,000 gallons/minute and more.

Chemical precipitation can be accomplished with very little equipment. For example, a 55 gallon drum and a mixing paddle can be used by a small discharger to treat wastewater with little capital investment. For larger volumes, a tank with a mixer and chemical feed pumps will suffice. For even larger volumes a continuous system with metering pumps, mixing tanks, a clarifier and control instrumentation can be employed.

A variety of recipes can be used perhaps the most common being the addition of lime, NaOH, ferric chloride + lime or NaOH, ferrous sulfate + lime or NaOH, or alum + lime or NaOH, followed by high molecular weight anionic polymer addition to aid the flocculation of the particles. Other recipes involve the use of sodium borohydride, sulfides, sodium carbonate, thiourea, thioacetamide, xanthate, and other agents.

When colloidal matter such as emulsified oil or metal bearing particles are treated with metal salts and lime or NaOH, the metal salts act as primary coagulants. The positively charged metal ions combine with the negative colloid particles and neutralize their charge. The particles then repel each other less strongly and tend to coagulate or collect into larger particles. Lowering the pH to between about 3 to 5 at this stage, before or as a result of adding the metal salts may result in more effective treatment. When the pH is subsequently adjusted to between 8 and 11 the soluble metal salts and other soluble metals in the solution form insoluble hydroxide particles that are large enough to settle. These hydroxide particles coprecipitate other contaminants in the solution, including oil particles and other colloids. Addition of an anionic polymer after pH adjustment and formation of hydroxide particles usually results in the rapid growth of large flocculant which sweeps smaller particles out of solution and quickly settles. Anionic polymers are available from a number of suppliers. Bench testing is used to determine the most effective combination and dose of chemicals, optimum pH, and type of polymer.

Polymers alone can be used to treat wastewater in some cases. Cationic polymers can be used as primary coagulants, at pH less than about 6, followed by pH adjustment to about 8 and the addition of anionic polymer. Other recipes may also be used.

Take a look at the references listed in the link in the Wastewater Treatment Information page for books providing details about chemical precipitation.