Sedimentation or clarification of the wastewater, usually achieved via sedimentation tank and settling chambers, reduces the amount of suspended materials in the water. Screening occurs via the use of screens of desirable mesh sizes to prevent larger particles from entering into the equipments. Aeration involves contacting and mixing the air with wastewater through aerators to reduce the biochemical oxygen demand --- the amount of oxygen required by micro-organisms to break down the organic material in the wastewater. Filtration rectifies the wastewater and reduce its total suspended solids content. Degassification removes the dissolved carbon dioxide and other gases from the wastewater through degassers.
Chlorination, the most commonly used tool for disinfection process for wastewater, generally consists of passing chlorine gas through the wastewater. Ozonation disinfects wastewater, eliminating bacteria, protozoa and viruses through the application of ozone gas. Coagulation treatment of wastewater results in removal of sludge and suspended solids from the wastewater. Coagulation consists of treating the wastewater in special tanks with extensive use of coagulants and precipitating chemicals. Aeration often accompanies coagulation to improve the treatment. Adsorption by activated charcoal and activated carbon removes gases through adsorption columns. Ion exchange is also a widely used chemical treatment tool for reducing the hardness (mineral salts of alkali metals) of the water with ion exchange resins. Commonly used ion exchange resins include cation exchange resin and anion exchange resins used selectively for removing the mineral content.
Biological treatment tools involve the action of micro-organisms on wastewater. Common aerobic biological treatment tools include activated sludge treatment, trickling filters and aerobic digestors. Activated sludge treatment employs the addition of micro-organisms to the wastewater sludge contained in a chamber. The process is usually accompanied by aeration to increase the treatment rate. A trickling filter slowly trickles the wastewater onto a bed of micro-organisms contained in organic matter as a sheet or a layer attached to porous packings of wood or plastic. As the water trickles down through the packings, the micro-organisms attached to the packings treat it. Anaerobic tools include anaerobic digestion and septic tanks. The only major difference between aerobic and anaerobic tools lies in the absence of air in the latter, a result of the type of micro-organism used for the treatment. Aerobic treatment requires micro-organisms that can perform in presence of oxygen, while anaerobic treatment requires micro-organisms that operate in the absence of oxygen.
Physical tools, including ultrafiltration and reverse osmosis, separate the dissolved contaminants from wastewater by passing it through extremely fine membranes with a mesh size of less than 0.005 microns. Other biological tools used include rotating biological contactors, oxidation ponds and aerobic/anaerobic lagoons.