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.
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.
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.
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.
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 they are whisked away. Flotation can remove more than 75 percent of the suspended solids.
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 too 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.
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 mostcases is used as a fertilizer because of the 2 percent nitrogen and 1 percent phosphorus content.
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.
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.
This stage is an aerobic process that adds sticky sludge particles that have millions 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 85 percent.
Another way of biological treatment is the stabilization pond or lagoon. Facultative lagoons are the most common, being 2 to 5 ft deep, with a surface area of several acres. Anaerobic conditions succeed in the bottom area, where the solids are decomposed. The area near the surface is aerobic, allowing the oxidation of dissolved and homogenous mixture of organic matter. A decrease in BOD of 75 to 85 percent can be accomplished.
There are many other ways and stages of wastewater treatment but these are the basic processes. I learned a lot about wastewater treatment and it is a good thing we have it so the lakes and streams are not as dirty.