Wastewater Treatment Essays - Sewerage, Environmental Engineering

Wastewater Treatment The reason for me doing this report is because I could not attend class enough to grasp the concept of Wastewater Treatment. This report is an overview of each stage of the treatment of sewage. I have included a diagram of a typical sewage plant. A) Primary Treatment The wastewater that enters a treatment plant contains debris that might clog or damage the pumps and machinery. The material is removed by screens, and is burned or buried. The wastewater then passes through a comminutor (grinder), where all the organic material such as leaves are mushed smaller so that they can be removed later. 1) Grit Chamber Back in the day, long narrow channel-shaped settling tanks, known as grit chambers, were used to remove all the inorganic substances like sand, silt, gravel, and cinders. These chambers were made to allow inorganic particles 0.008 in. or bigger to settle at the bottom while the smaller particles and most of the organic material that remain in suspension pass through. Today, spiral-flow aerated grit chambers with hopper bottoms, or clarifiers with automatic scrapper arms are used. The grit is removed and disposed of as sanitary landfill. Grit build up can reach from 3 to 8 cubic feet per1 million gallons of wastewater. 2) Sedimentation With the grit removed, the wastewater goes into a sedimentation tank, where the organic materials removed. The method of sedimentation can remove about 20 to 40 percent of the biochemical oxygen demand and 40 to 60 percent of the suspended solids. The big boys in the industry use a chemical process known as coagulation and flocculation in the sedimentation tank. I really don't know much about this subject so I'm going to move on. 3) Flotation The alternative to sedimentation is a treatment called flotation, in which air is forced into the wastewater under pressures of 25 to 50 lbs per sq. in. The wastewater, is compressed with air, is then released into an open tank ; there the rising air bubbles cause the suspended solids to rise to the surface, where the are wisked away. Flotation can remove more than 75 percent of the suspended solids. 4) Digestion Digestion is a microbiological process that changes the chemically complex sludge to methane, carbon dioxide, and a harmless fertilizer. The reactions occur in a closed tank or digestor that is oxygen deficient. The transformation happens after a series of reactions. First the solid matter is made soluble by enzymes, then the substance is fermented by a group of acid-producing bacteria, reducing it to simple organic acids such as acetic acid. The organic acids are then resolved to methane and carbon dioxide by bacteria. The sludge that is to thick is heated and added to the digester as many times as possible, where it sits for 10 to 30 days and is decomposed. Digestion reduces organic matter by 45 to 60 percent. 5) Drying The digested sludge is place on sand beds for air drying. Air drying needs dry, warm weather for it to work. Some plants have shelters over the sand beds. Dried sludge in most cases is used as a fertilizer because of the 2 percent nitrogen and 1 percent phosphorus content. B) Secondary Treatment After removing 40 to 60 percent of the suspended solids and 20 to 40 percent of the BOD5 in the primary stage by physical resources, the secondary treatment biologically reduces the organic material that stayed in the liquid stream. Secondary treatment contains keeping and speeding up nature's process of waste disposal. Aerobic bacteria in the oxygen change the organic matter to stable forms such as CO2 , water, nitrates, and phosphates. The new organic material that is made is an indirect result of biological treatment processes, and is removed before the wastewater is dumped into the streams. 1) Trickling Filter In this process, a waste stream is sent over a bed or column of some type of porous medium. A sticky film of microorganisms coats the medium and acts as the removal agent. The organic matter in the waste stream is absorbed by the film and changed to carbon dioxide and water. If the trickling filter step comes before the sedimentation stage it can remove about 85 percent of the BOD entering the plant. 2) Activated Sludge This stage is an aerobic process that adds sticky sludge particles that have millions of of actively growing bacteria stuck together by a gelatinous slime. Organic matter is assimilated by the floc and changed to aerobic output. The reduction of BOD varies between 60 to