Earth, Climate & Environment

Wastewater Treatment

The processes involved in municipal wastewater treatment plants are usually classified as being part of primary, secondary, or tertiary treatment.

Primary Treatment
The wastewater that enters a treatment plant contains debris that might clog or damage the pumps and machinery. Such materials are removed by screens or vertical bars, and the debris is burned or buried after manual or mechanical removal. The wastewater then passes through a comminutor (grinder), where leaves and other organic materials are reduced in size for efficient treatment and removal later.

Secondary Treatment
Having removed 40 to 60 percent of the suspended solids and 20 to 40 percent of the BOD5 in primary treatment by physical means, the secondary treatment biologically reduces the organic material that remains in the liquid stream. Usually the microbial processes employed are aerobic—that is, the organisms function in the presence of dissolved oxygen. Secondary treatment actually involves harnessing and accelerating nature's process of waste disposal. Aerobic bacteria in the presence of oxygen convert organic matter to stable forms such as carbon dioxide, water, nitrates, and phosphates, as well as other organic materials. The production of new organic matter is an indirect result of biological treatment processes, and this matter must be removed before the wastewater is discharged into the receiving stream.

Advanced Wastewater Treatment
If the receiving body of water requires a higher degree of treatment than the secondary process can provide, or if the final effluent is intended for reuse, advanced wastewater treatment is necessary. The term tertiary treatment is often used as a synonym for advanced treatment, but the two methods are not exactly the same. Tertiary, or third-stage, treatment is generally used to remove phosphorus, while advanced treatment might include additional steps to improve effluent quality by removing refractory pollutants. Processes are available to remove more than 99 percent of the suspended solids and BOD5. Dissolved solids are reduced by processes such as reverse osmosis and electrodialysis. Ammonia stripping, denitrification, and phosphate precipitation can remove nutrients. If the wastewater is to be reused, disinfection by ozone treatment is considered the most reliable method other than breakpoint chlorination. Application of these and other advanced waste-treatment methods is likely to become widespread in the future in view of new efforts to conserve water through reuse.

Liquid Disposal
The ultimate disposal of the treated liquid stream is accomplished in several ways. Direct discharge into a receiving stream or lake is the most commonly practiced means of disposal.

The treatment process involves conventional primary and secondary treatment followed by lime clarification to remove suspended organic compounds. During this process, an alkaline (high-pH) condition is created to improve the process. In the next step, recarbonation is used to bring the pH level to neutral. Then the water is filtered through multiple layers of sand and charcoal, and ammonia is removed by ionization. Pesticides and any other dissolved organic materials still present are absorbed by a granular, activated-carbon filter. Viruses and bacteria are then killed by ozonization. At this stage the water should be cleansed of all contaminants, but, for added reliability, second-stage carbon adsorption and reverse osmosis are used, and chlorine dioxide is added to attain the highest possible water standard.

Septic Tank
A sewage treatment process commonly used to treat domestic wastes is the septic tank: a concrete, cinder block or metal tank where the solids settle and the floatable materials rise. The partly clarified liquid stream flows from a submerged outlet into subsurface rock-filled trenches through which the wastewater can flow and percolate into the soil where it is oxidized aerobically. The floating matter and settled solids can be held from six months to several years, during which they are decomposed anaerobically.

See also: Solid Waste Disposal