Specialty Chemicals in Water Treatment

Water (and wastewater) treatment plants are most often the highest volume chemical-processing facilities in a community. These plants take the local source water, whether it be lakes, rivers, reservoirs (surface water) or wells/aquifers (ground water), and make it safe for drinking. On the other end, the wastewater plants combine the sewage, industrial and storm water sources and treat it for safe return to rivers and lakes. In each case, the condition of the incoming water will determine the chemicals needed to make it safe for public use or to return to downstream users.

Water Treatment Processes

• The most common sequence for processing drinking water is to mix incoming water with coagulants to separate particles in settling tanks, then the top of the settled water is skimmed off to filtration and disinfection.

  Wastewater receives a similar sequence, with suspended solids being settled out and the clearest portion disinfected and filtered before being passed downstream.

  In each case, the process is a mix of mechanical (settling solids) and chemical. The chemicals used will be driven by cost and the condition of the water being treated.

Settling and Clarifying Chemicals

• In this step, particles suspended in water have found a balance between their weight and their bouyancy. Coagulant chemicals make these suspended particles "sticky" and cause them to form clumps of heavier masses. These masses no longer have that weight/bouyancy balance and tend to settle to the bottom of a still pool, sweeping similar particles to adhere to themselves as they drift down.

  The chemicals chosen to cause this precipitation are tailored to the water condition (pH, temperature and types of particulates) and cost. In the United States, the most common coagulant/precipitant is alum. Because of alum's sensitivity to pH (alkalinity), water conditions will push treatment plants to use lime (calcium carbonate), phosphates and polymers, either separately or in combinations to achieve the same results.

Disinfection

• By far, the most common water treatment disinfectant in the nation is chlorine. It is used in powdered, liquid and gaseous forms. Water conditions such as temperature and pH as well as cost generally drive which form is used to deliver the chlorine.

  Very popular in Europe and growing in American use is ozone as the primary disinfectant. Ozone is an unstable gaseous substance formed by passing high purity oxygen through an electric corona to break up oxygen and reform ozone. Ozone has a half-life of around 20 minutes, so the ozone must be produced near the point where it is diffused into the water for treatment. It oxidizes waterborne organics as well as many colloidal metals. As the oxidizing process is a chemical process, using ozone for water treatment is most often and correctly considered a chemical treatment step.

  Chlorine and ozone account for the majority of disinfection processes. Other disinfection steps include reverse osmosis and ultraviolet light sterilization, neither of which is a chemical process.

Flouridation

• U.S. water treatment requires the addition of flouridation chemicals to drinking water. This is thought to improve the dental health of consumers, particularly children. The flouride chemicals added to the drinking water process is driven by the water pH and cost.

Other Treatments

• Since water treatment is heavily dependent upon the source water conditions, a vast array of algaecides, bacteria treatments, metal scouring and targeted anti-protozoaic chemicals may be added to treat specific conditions. In most cases, the treatment process must include a succeeding step to remove the specific chemical.

  Common among these cases is the use of copper sulfate to kill algae from lakes and reservoirs, dissolved oxygen to precipitate colloidal metals and fungicides to keep piping clear of growth buildups.
  Area chemical suppliers are most familiar with local water conditions and treatments.

Non-chemical Treatments

• When water conditions are difficult or expensive to treat using specialty chemicals, the most common alternative is to use high-purity membrane filtration. Micro- or ultra-filtration will remove bacteria, colloidal metals and viruses from water.

  Extreme conditions may make reverse osmosis (RO) filtration a cost-effective alternative to chemical treatment. Unlike chemical treatments, these are not as sensitive to water conditions.