An understanding of the composition of wastewater can provide a better understanding to why certain types of concrete work more efficiently in sanitary sewer systems. A sanitary sewer system is an expensive investment. If the wrong type of concrete is used, the system can experience premature deterioration, weakening, etc. This can cause many problems. For example, if a concrete pipe in a sewer line were to deteriorate and break, the pipe would have to be repaired. Not only does this cause problems within the sewer line, additional problems such as traffic problems can occur if the sewer line is located on a major road or highway. Some sewer lines are four hundred (400) feet in length between manholes! This can be a lot of line to excavate if the whole section of piping needs to be replaced.
· Physical Characteristics
Color: Most fresh domestic wastewater is gray in color and stale wastewater has a black appearance. This is caused by the presence of dissolved suspended and colloidal matter within the wastewater. These are not the only colors of wastewater. Other variations of colors can be caused by the textile industry (i.e. textile, leather, meat processing, etc.)
Odor: Odor generally depends on an individual’s sense of smell. Fresh domestic waste has a musty, stale, unpleasant odor. If the wastewater contains hydrogen sulfide there is often a rotten egg odor. This is caused by the decay of organic matter under anaerobic conditions.
Solids: A common misconception of wastewater involves the consistency of wastewater. Many people are lead to believe that wastewater is full of large floating solids. In actuality, typical wastewater is approximately 99.9% water, has a biochemical oxygen demand (BOD) of 110-400 mg/L, and contains 250-850 mg/L of Total Dissolved Solids (TDS). Solids are typically classified as floating, settleable, suspended, or dissolved. Floating solids are from gross populations and can be removed by the simplest forms of treatment. Settleable solids are those that will settle out due to the presence of gravity. Suspended solids (SS) are the material that can be filtered out of the wastewater. Dissolved solids are those that cannot be removed through filtration. They remain in the wastewater following the filtration that removes the suspended solids.
Temperature: This is a very important characteristic because it plays a vital role in the biological, physical, and chemical activities in the wastewater. The temperature of wastewater will typically be higher than that of the water supply. This is caused by the addition of warm water from domestic, commercial, and industrial sources. Depending on location, most wastewater will vary from approximately 508 to 778 F (108 to 258 C). These values can vary. For example, inflow and infiltration of surface water into a sewer system can cause a decrease in temperature.
Variations in Flow: In general, the hourly flow rate for a community is at its maximum between 7:00 a.m. and 3:00 p.m. and minimum after midnight. Wastewater treatment plants will typically see their largest flow values in the late morning or early afternoon. The hourly flow variation can range from 50 to 200 percent of the average flow. As the number of people served increases, the variation in flow decreases. It is always good to keep in mind that the flow can be influenced by the arrangement, design, and quality of the system; the number, location, and design of pumping stations, etc.
· Chemical Characteristics
Inorganic Compounds: Inorganic compounds are defined to be all organizations of elements that do not contain carbon. Sand, grit, and mineral matter are the most common occurring inorganic compounds in wastewater. Chloride, hydrogen ions, phosphorus, nitrogen, heavy metals, and alkalinity-causing compounds are all very common in wastewater.
Other elements such as copper, lead, zinc, silver, chromium, arsenic, boron, fluoride, manganese, iron, cadmium, and mercury are can occupy biological systems, allowing them to occur within wastewater.
Gases are found in wastewater also. Some of the more common gases are carbon dioxide, hydrogen sulfide, nitrogen, oxygen, ammonia, sulfide, and methane. Oxygen, hydrogen sulfide, and methane generally have much attention made to them. For example, methane gas obtain from the treatment process can be extracted and used as a heat or power source for parts of the treatment plant.
Organic Compounds: Organic compounds are defined to be those compounds that contain carbon in combination with one or more other elements. There is a large number of organic compounds that have been classified and this number increases yearly. This is due to the result of unique bonding properties between carbon atoms.
The organics in wastewater are usually of animal or vegetable origin and include dead animal and plant matter. Living organisms and their waste products also contribute to the organics of wastewater. Other examples of organic compounds are oils, fats, starch, vegetable fibers, and sugars. Aside from these, the interaction of microorganisms and organic material can cause organic acids, alcohols, and antibiotics.
· Biological Characteristics
There are many different types of microorganisms that exist in wastewater. The most common type are free-living or clustered cells that are able to complete growth, metabolism, and reproduction independently.
The two most common types of microorganisms plant and animals. Due to the fact that the distinction between the two is sometimes hard to make, plant and animal microorganisms are classified in the same group with Protista. Protista are of equal status with plants and animals.
The bacteria in the wastewater are very important for the biological process of treatment. They are key to breaking down the BOD and Suspended Solids so that the amount of the two meets the Clean Water Act of 1972’s standards. The bacteria help convert the soluble organic material in the wastewater into new cells and inorganic elements. These new cells and inorganic elements then help provide food for the other organisms.
· Commonly Measured Wastewater Characteristics
Alkalinity: Alkalinity is the ability of a substance to resist a change in ph or to neutralize itself. The bicarbonate, carbonate, and hydroxide ions are the main contributors to alkalinity while borates, silicates, and phosphates can also contribute.
pH: The ph is a method used to determine if a substance is acidic, basic, or neutral. A scale known as the pH scale is used to determine what the material actually is. The scale ranges from 1 to 14 where 1 to 7 is acidic, 7 is neutral, and 7 to 14 is basic.
Biochemical Oxygen Demand (BOD): This is defined to be the amount of oxygen required by microorganisms to degrade the waste biologically. It can be considered a measure of the pollutional strength of the waste is the sense that it yields the amount of oxygen required to stabilize waste after discharge to a receiving body. Due to the fact that a test to determine how much oxygen is required to completely stabilize a sample of taste would take a long time (at least several weeks), a 5-day test is performed in which a BOD5 is yielded. This value is known as the five day BOD and is the total amount of oxygen consumed by microorganisms in the first five days of biodegradation. Typical values of BOD5 are generally 100 – 300 mg/L.
Carbonaceous Oxygen Demand (CBOD): This is the amount of oxygen required to oxidize organic carbon to carbon dioxide.
Chemical Oxygen Demand (COD): This is defined to be the amount of oxygen required to oxidize the waste chemically. This is another method for determining the pollutional strength of wastewater. The values of COD are generally higher than the BOD values because the BOD only measures the quantity of organic material oxidized by microbial action, while the COD test yields a more complete oxidation. This test generally only takes a few hours. Typical COD values range between 200 and 500 mg/L.
Nitrogenous Oxygen Demand (NBOD): This is defined to be the amount of oxygen required to convert ammonia to nitrate.
Theoretical Oxygen Demand (ThOD): This is the amount of oxygen required to oxidize a chemical to carbon dioxide and water.
Nitrogen: Nitrogen appears in several different forms in wastewater. It can occur as organic nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen. Fresh, cold wastewater is characterized by high amounts of organic nitrogen and small amounts of ammonia nitrogen. Warm wastewater is characterized by high amounts of ammonia nitrogen and low amounts of organic nitrogen. Fresh domestic wastewater will have nitrates and nitrites but in low amounts. When the wastewater is treated, the amounts of nitrates and nitrites will generally be higher. Nitrites will eventually be turned into ammonia or oxidized into nitrates.
Phosphorous: Phosphorus is very important to the metabolism of biological organisms. A small amount of phosphorous is needed to accomplish optimum operation of biological treatment systems. About 30 percent of the phosphorous in municipal wastewater is removed during the primary and biological treatments. It is often removed by the addition of a coagulant.
The different forms of phosphorous that exist in wastewater are orthophosphates, pyrophosphates, polyphosphates, metaphosphates, and organic phosphorous. The orthophosphates are important to note because they are freely available for biological metabolism.