The reason for me doing this report is because I could not attend class enoughto grasp the concept of Wastewater Treatment. This report is an overview of eachstage of the treatment of sewage. I have included a diagram of a typical sewageplant. A) Primary Treatment The wastewater that enters a treatment plantcontains debris that might clog or damage the pumps and machinery.
The materialis removed by screens, and is burned or buried. The wastewater then passesthrough a comminutor (grinder), where all the organic material such as leavesare mushed smaller so that they can be removed later. 1) Grit Chamber Back inthe day, long narrow channel-shaped settling tanks, known as grit chambers, wereused to remove all the inorganic substances like sand, silt, gravel, andcinders. These chambers were made to allow inorganic particles 0.008 in. orbigger to settle at the bottom while the smaller particles and most of theorganic material that remain in suspension pass through.
Today, spiral-flowaerated grit chambers with hopper bottoms, or clarifiers with automatic scrapperarms are used. The grit is removed and disposed of as sanitary landfill. Gritbuild 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 sedimentationtank, where the organic materials removed.
The method of sedimentation canremove about 20 to 40 percent of the biochemical oxygen demand and 40 to 60percent of the suspended solids. The big boys in the industry use a chemicalprocess known as coagulation and flocculation in the sedimentation tank. Ireally dont know much about this subject so Im going to move on. 3)Flotation The alternative to sedimentation is a treatment called flotation, inwhich 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 thesuspended solids. 4) Digestion Digestion is a microbiological process thatchanges the chemically complex sludge to methane, carbon dioxide, and a harmlessfertilizer. The reactions occur in a closed tank or digestor that is oxygendeficient.
The transformation happens after a series of reactions. First thesolid matter is made soluble by enzymes, then the substance is fermented by agroup of acid-producing bacteria, reducing it to simple organic acids such asacetic acid. The organic acids are then resolved to methane and carbon dioxideby bacteria. The sludge that is to thick is heated and added to the digester asmany 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 digestedsludge is place on sand beds for air drying. Air drying needs dry, warm weatherfor it to work. Some plants have shelters over the sand beds. Dried sludge inmost cases is used as a fertilizer because of the 2 percent nitrogen and 1percent phosphorus content. B) Secondary Treatment After removing 40 to 60percent of the suspended solids and 20 to 40 percent of the BOD5 in the primarystage by physical resources, the secondary treatment biologically reduces theorganic material that stayed in the liquid stream. Secondary treatment containskeeping and speeding up natures process of waste disposal.
Aerobic bacteriain 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 indirectresult of biological treatment processes, and is removed before the wastewateris dumped into the streams. 1) Trickling Filter In this process, a waste streamis sent over a bed or column of some type of porous medium. A sticky film ofmicroorganisms coats the medium and acts as the removal agent. The organicmatter in the waste stream is absorbed by the film and changed to carbon dioxideand water.
If the trickling filter step comes before the sedimentation stage itcan remove about 85 percent of the BOD entering the plant. 2) Activated SludgeThis stage is an aerobic process that adds sticky sludge particles that havemillions of of actively growing bacteria stuck together by a gelatinous slime.Organic matter is assimilated by the floc and changed to aerobic output. Thereduction of BOD varies between 60 to