Wastewater Treatment Basics
Posted: Tuesday, January 19, 2010
by Jack H. Schick
How many of you have thought about your sewer plant today? Probably not many. How many of you have taken advantage of your sewer plant's services today? I hope, most of you. You probably only think about it when the bill arrives or when your service is disrupted. That's the way it should be. Most American homes are connected to a wastewater (WW), treatment facility. If you have to think about it very often, someone is not doing their job well.
No two sewer plants are identical, but the principles of operation are generally the same. They all take "dirty water" and make it clean. For the vast majority of human history sewage would simply soak into the ground beneath the privy, be tossed into a ditch, or, if a drainage system was in place, be conveyed to the nearest body of water. Today the process is much more complicated.
1. The Collection System is the first part of the sewer system.
A. Laterals are pipes that exit from a building and connect to the public sewer system. The lateral usually has a Clean Out where plumbers can access it for service. The lateral belongs to the property owner who has the responsibility of keeping it working properly.
B. Sewer Mains are the larger pipes that collect WW from laterals and convey it to the sewer plant. The water flows by gravity. The mains are gently sloped toward the plant. Man Holes are installed every few hundred feet so the pipes can be inspected, cleaned or repaired. If there is higher terrain along the route a Pump Station is installed. Pumps lift the WW over the hill and it continues to the plant by gravity.
2. Preliminary Treatment is the first phase of WW treatment.
A. Screening is often the first process. A Bar Screen is frequently used. The water flows through a rack of closely spaced bars. Rakes with teeth that fit between the bars are dragged upward across the rack bringing along anything too large to pass through: paper, rubber and plastic products; sticks and boards; underware-anything that found its way into the system. The material is either removed (see Solids Handling), or ground up and returned to the water stream.
B. Grit Removal is another preliminary treatment. The WW flows through an aerated channel. Blowers distribute air through diffusers. Buoyancy is reduced in aerated water so grit and gravel sinks more quickly. A collector mechanism drags buckets across the floor of the channel and brings the grit to the surface. It is removed (see Solids Handling), to prevent damage to pumps and other equipment downstream in the process.
C. Some sewer plants who have a lot of restaruants in the collection system install equipment to remove grease. It is usually removed from the system (see Solids Handling).
3. Primary Clarification is the next phase of WW treatment. Sedimentation and Flotation is the main method of removing solids from WW. The water flow is slowed down by distributing it into large tanks. Heavier solids sink to the bottom and lighter materials float. Clarifiers have mechanisms enable the Primary Sludge to be removed. The Skim on top is returned to Head of the Plant (preliminary treatment unit).
4. Biological Treatment is a very important part of the process. There are billions of micro-organisms (Bugs), in each gallon of WW. Two types of bugs, oxygen using (Aerobic), and non-oxygen using (Anaerobic), through their metabolic processes, consume or break down pollutants like ammonia, organic solids and other detrimental compounds in the WW. The WW, mixed with a population of bugs grown over time in the sewer plant process, remain in contact for period of time and effect the biological treatment. This process is the heart of modern WW treatment. Trickling filters and Activated Sludge are two common methods of biological treatment.
5. Secondary Clarification is next. The WW and bugs that leave the biological process units looks like chocolate milk and is called Mixed Liquor. It is distributed into tanks similar to in primary clarification. In the Secondary Clarifiers solids settle and are removed from the system for further processing (see Solids Handling). Some is mixed with the incoming WW to perpetuate the biological process. The WW is now very clear and clean. It flows over notched weirs to the final processes.
6. Disinfection is the final step. The clarified WW flows through a Contact Tank where it either has Chlorine added or is exposed to Ultraviolet Light. Chlorine kills micro-organisms while UV exposure renders them sterile. Chlorine is neutralized with chemicals. The WW is now discharged to the Receiving Waters.
7. Final Effluent is what the treated WW is called as it leaves the sewer plant. The water flow is constantly measured. It is tested regularly for dissolved oxygen (DO); pH; Biochemical Oxygen Demand (BOD), which is the tendency to consume oxygen in the receiving waters; ammonia; Fecal Coliform, which show how many pathogenic (harmful to man), bugs have survived; solids, and many other components as required by the NPDES permit.
There are many variations of this process. There may be many othere treatment processes depending on WW and receiving waters characteristics. Professional, licensed people are being held accountable to see that the CWA standards are met. Most wastewater treatment plants are proud of what they do for their community. They are eager to show "rate payers" where the money goes. The next time you think about your sewer plant, give them a call. They'll gladly give you a tour.
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Top-level comments on this article: (1 total)now i know ... i never thought about it before. thanks for sharing and informing.Thanks for reading. Some parts of my life are boring.
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