Types of Waste Water Treatment Plants

The simple definition of waste water is water that has been used. Waste water also includes water that has pollutants that have adversely affected it, such as toxic chemicals, acids, heavy metals, and/or body waste. Waste water can be categorized into two types: domestic and industrial. Domestic industrial waste water includes sewage.

Primary Treatment

• During primary treatment, any solid matter that is suspended in the water or floating on it is settled to the bottom and removed. First the waste water passes through a screen to strain out large matter, then it goes through a smaller process where material like sand is removed. This is called grit removal. The next step is a sedimentation tank or clarifier where suspended solids are settled out. The sludge that accumulates at the bottom of the clarifier is pumped out, and water is added to it for use as fertilizer, dumped in a landfill or incinerated. To kill bacteria, the remaining water is normally treated with chlorine. After treatment, the excess chlorine is taken out. Removing chlorine is necessary so it won't kill fish if it is dumped into a lake or river. Some treatment plants use ultraviolet light or ozone as a disinfectant.

Secondary Treatment

• After water has been through the first treatment, it is a lot cleaner but is still not up to the Environmental Protection Agency's standards. Next, it needs biological treatment to reduce its BOD (Biochemical oxygen demand) content. In this stage, bacteria is introduced that will eat the organic material in the waste water. This changes the organic material into a mass that can be settled out to the bottom of the settling tank or as slime that will cling to solid surfaces or vegetation. This kind of treatment can take place at any temperature that saves energy. A lot of oxygen is required in this stage for the bacteria to do their work, so more space is needed. Secondary treatment is usually done outside for these reasons.

Tertiary Treatment

• Tertiary treatment is also called advanced treatment. This treatment will eliminate the final impurities in the water. The need for this arose as science identified more elements that were unhealthy in water. These include heavy metals and toxins that are not easily removed by the other two treatments. Technologies used in this stage include evaporation and distillation, microfiltration, carbon adsorption and chemical precipitation. This may also include dechlorination, disinfection and ion exchange. In ion exchange, the magnesium, calcium and carbonate ions are removed and replaced with hydrogen and hydroxyl ions.

Industrial Treatments

• The treatment used for cleaning industrial waste water will depend on the kind of industry and what chemicals, heavy metals, or other pollutants need to be removed. Most industries are encouraged to treat and use the same water over and over. That way, they are not disposing of it into lakes and rivers or the sewer system. In industrial treatment plants, solid pollutants are removed with settling techniques like in primary treatment. Filtration can be used for smaller solids. Oils can be removed by skimming them off the top. Also, an oil-water separator may be used. Organic materials, like paint, pesticides, and solvents, can be removed with distillation, advanced oxidation, adsorption, incineration, vitrification, landfill disposal or chemical immobilization. Biodegradable organic material can usually be removed with sludge or filtering processes. Acids and alkalis need to be neutralized. Other toxic materials like metals (zinc and thallium, for example) and elements (arsenic or selenium, for example) can be made into precipitates by changing the pH or by adding certain chemicals. They may also be incinerated with advanced oxidation.