Water waste disposal dates to ancient times. Sewers have been found in the ruins of Crete, Assyria, and storm drains built by ancient Romans are still in use today. Wastewater is carried by a pipe system from its source to the treatment facility. The type of pipe used depends on the type of waste moving through the system. A system that carries both domestic and storm water waste are a combined system and are typically used in older urban systems. Newer sewer systems have separated sewer systems, which is a more efficient system as it prevents combined sewer overflow. Combined sewer overflow is caused when a sewer is not big enough to handle storm water and domestic sewage. Overflow can contaminate drinking water supplies. Household sewer pipes are typically made of clay, cast-iron, or polyvinyl chloride and sewer mains are typically made of concrete, clay or asbestos cement. Waste water flow through to the treatment plant through gravity so the pipe must be sloped to permit the waste to move at a minimum velocity of 0.46 m/s. (19)
Wastewater originates from domestic sewage, industrial waste, groundwater (infiltration), and storm water sources. Domestic sewage results from day to day household activities like bathing, food preparation, recreation, and toilets. An average household has about 60 gallons of waste per person. Industrial wastewater varies from the type of industry, management, and treatment prior to sewage. Infiltration occurs when the sewage pipe is below the water table. Storm water waste depends on the amount of rainfall and its composition varies specific to the watershed. Storm sewers also collect water used for washing cars, watering lawns, and anything dumped into the sewers. Typical wastewater for an urban area is about 60- 80% of its daily water requirements. (20)
Primary wastewater treatment involves water passing through a screen to remove debris that is later burnt or buried. Screened water is then moved through a comminutor where leaves and other organic materials are ground to reduce its size for treatment. The next step is a grit chamber also known as settling chambers. Grit chambers are used to eliminate sand, silt, gravel, and cinders from the wastewater. Inorganic particles larger than 0.2 mm settle to the bottom of the chamber and are removed to sanitary landfills. Once the grit is removed, the water then moves to a sedimentation tank where organic materials settle out and are removed. 20-40% of microbes are removed and 40-60% of suspended solids are removed. To accelerate this process some plants induce chemical coagulation and flocculation. Aluminum sulfate, ferric chloride, or polyelectrolytes are added to the wastewater in coagulation. The chemicals cause a change in the surface of the water that makes the suspended particles attach to one another and precipitate. They then fall to the bottom of the tank. Flocculation is similar to coagulation in that it causes the particles to coalesce and fall to the bottom of the tank. Flotation is an alternative to sedimentation. Air is forced into the wastewater and then the water is discharged into an open tank. The rising air bubbles cause particles to rise to the surface where they are removed. Another alternative to sedimentation is digestion of suspended particles by microbes that convert the organic sludge to methane, carbon dioxide, and humus like material. The reaction occurs in an anaerobic digestor tank. Digested sludge is then placed on sand beds for air-drying. Dried sludge is often used as fertilizer because of its high nitrogen and phosphorus content. (21)
Secondary wastewater treatment involves biologically reducing the organic material that remains in the water. This treatment accelerated natures waste disposal with aerobic bacteria converting the organic material into carbon dioxide, water, nitrates, and phosphates. These by products must be removed before the water is returned to a receiving stream. Trickling filters are used in secondary treatment. The wastewater is distributed over a bed of porous medium where a film of microorganisms covers the water. Activated sludge is also a treatment method used in secondary treatment. Activated sludge particles, known as floc, are millions of active bacteria that cover the water. Stabilization pools are used in treatment plants with available land and anaerobic and aerobic bacteria are utilized to reduce the organic particles in the water. (22)
Tertiary treatment, also known as advanced wastewater treatment is used to further purify wastewater for release into bodies of water or for reuse. Phosphorus is removed in this step. Refractory pollutants are also removed at this step by a variety of methods including reverse osmosis, electro dialysis, ammonia stripping, denitrification, and phosphate precipitation. If the water is to be reused then a disinfectant such as chlorine is used. (23)
Septic tanks are a common treatment process used for domestic waste. A septic tank is composed of a concrete, cinder block, or metal tank where solids settle and floatable materials rise. The liquid stream flows from an outlet that is submerged into a subsurface rock filled trench. Wastewater flows and percolates through the rock filled trench into the soil where it is oxidized aerobically. Solids and floating matter may remain in the septic tank for 6 months to years where it is decomposed by anaerobic bacteria. (24)
After wastewater has been treated it is disposed of into a receiving stream or lake. In areas of the United States that face water shortages, water is reused for both domestic and agricultural uses. Treated wastewater can be used for groundwater recharge, irrigation of nonedible crops, industrial processing, and recreational uses. (25)
