Using Industrial Hydraulics |
Applications of Computer-Aided Manufacturing
Articles in this series:
Before about 1850, human and liquid industrial wastes were typically dumped in the street or alley or conveyed to the nearest body of water without treatment. Groundwater and other sources for drinking and bathing were regularly contaminated with raw sewage. Epidemics of cholera, typhoid, dysentery, and other waterborne diseases killed thousands. Outbreaks of these diseases were especially devastating in densely populated areas. Bathing was not accepted as a common practice and was viewed as a health hazard.
About 1854, the connection between cholera and sewage contaminated water was first discovered. Better attempts were made to treat and dispose of sewage separately from drinking water. However, until the latter part of the 1800s, most U.S. communities still allowed discharges of untreated or inadequately treated wastewater from homes and industries through combined storm and sanitary sewers into lakes, bays, oceans, streams, and rivers. Treatment standards varied from municipality to municipality. As population increased, damage to the environment and risks to public health reached dangerous levels.
Because of the vast land and water resources available for dumping wastes and wastewater, treatment of wastewater in the United States received little attention until the early 1900s. By this time health problems associated with improper disposal of waste and wastewater were causing real problems, especially in larger cities. Cities were having trouble obtaining areas for disposal, which led to more intensive methods of treatment.
In the 1970s, the U.S. Congress passed the Clean Water Act. This legislation led to the establishment of national water quality standards and limits for the discharge of pollutants. In recent years, amendments to this legislation have transferred implementation of pollution control to individual state governments. However, a few U.S. communities were still permitted to release untreated or inadequately treated wastewater into streams and rivers through the late 1900s.
Today, governmental leaders and health authorities are responsible for ensuring that state standards for wastewater treatment and water quality are met consistently-not only at inspection time, but always to protect public health and the environment.
In some cases, treatment plant operators and community leaders may be held personally liable for noncompliance. Homeowners also are personally liable for malfunctioning on-site systems. On site systems must be properly operated and maintained to protect groundwater and other drinking water sources, as well as the health of family and neighbors.
As described in the previous section, wastewater (sewage) is "used" water. It contains the waste products, excrement, or other discharge from the bodies of human beings or animals, and other noxious or poisonous substances that are harmful to the public health, or to animal or aquatic life, or to the use of water for domestic water supply or for recreation. Wastewater is a combination of the liquid and water-carried wastes from residences, commercial buildings, industrial plants, and institutions, together with any groundwater, surface water, and storm water that has infiltrated the public sewage system. Although the terms wastewater and sewage are used interchangeably, sewage technically contains waste products or excrement from human beings or animals and wastewater may not. Buildings nearly always generate sewage, and thus wastewater.
Wastewater from residences, apartments, motels, office buildings, and other similar types of buildings is referred to as domestic wastewater. There are two types of domestic waste water: gray water and black water.
Gray water is wastewater that typically contains the residues of washing processes. It’s generated in the bathtub, shower, sink, lavatory, and clothes washing machine. Gray water accounts for about two-thirds of the wastewater produced in a typical residence. Black water is wastewater that contains fecal matter and urine. It’s produced in water closets (toilets), urinals, and bidets. Black water and gray water have different characteristics, but both contain pollutants and disease-causing agents that require treatment. Some areas in the United States permit the use of innovative gray water reuse systems that safely recycle household gray water for reuse in toilets or for irrigation to conserve water and reduce the flow to treatment systems. These systems are discussed later in this section.
Commercial wastewater is nontoxic, nonhazardous wastewater from commercial and institutional food service operations and beauty salons. It’s usually similar in composition to domestic wastewater, but may occasionally have one or more of its constituents exceed typical domestic ranges.
Industrial wastewater is process and non-process wastewater from manufacturing, commercial, laboratory, and mining operations, including the runoff from areas that receive pollutants associated with industrial or commercial storage, handling, or processing. Industrial wastewater requires special handling, and such treatment is not discussed in detail here. It’s important to note that such wastes should not be put into a community wastewater system without first receiving approval from the governing authorities.
Because of the variety of characteristics tied to commercial and industrial wastewater, communities need to assess each source individually to ensure that adequate treatment is provided.
For example, public restrooms may generate wastewater with some characteristics similar to domestic sewage, but usually at higher volumes and at different peak hours. The volume and pattern of wastewater flows from rental properties, hotels, and recreation areas often vary seasonally as well. Laundries differ from many other nonresidential sources because they produce high volumes of wastewater containing lint fibers. Restaurants typically generate a lot of oil and grease. It may be necessary to provide pretreatment of oil and grease from restaurants or to collect it prior to treatment, For example, by adding grease traps to septic tanks.
Industrial wastewater also may require additional treatment steps. For example, storm water should be collected separately to prevent the flooding of treatment plants during wet weather. Screens often remove trash and other large solids from storm sewers. In addition, many industries produce wastewater high in chemical and biological pollutants that can overburden on-site and community systems. Additionally, with dairy farms and breweries, communities may require that these types of nonresidential sources provide their own treatment or preliminary treatment to protect community systems and public health.
Wastewater is mostly water by weight. Wastewater released by residences, businesses, and industries is approximately 99.94% water. Only about 0.06% of the wastewater is dissolved and suspended solid material. Other matter makes up only a small portion of wastewater, but can be present in large enough quantities to endanger public health and the environment. Practically any thing that can be flushed down a toilet, drain, or sewer can be found in wastewater. Even household wastewater contains many potential pollutants. As a result, wastewater treatment is as important to public health as drinking water treatment.
The wastewater constituents of most concern are those that have the potential to cause disease or detrimental environ mental effects. A wide range of unhealthy constituents, with widely ranging problems, may be found in the wastewater of residences and commercial and industrial facilities. These include the following:
Organisms ---Many different types of organisms live in wastewater and some are essential contributors to treatment. Bacteria, protozoa, and worms work to break down certain carbon-based (organic) pollutants in wastewater.
Through this process, organisms turn wastes into carbon dioxide, water, or new cell growth. Bacteria and other microorganisms are particularly plentiful in wastewater and accomplish most of the treatment. Most wastewater treatment systems are designed to rely in large part on these biological processes.
Pathogens--- Many disease-causing viruses, parasites, and bacteria are present in wastewater. These pathogens often originate from people and animals that are infected with or are carriers of a disease. Gray water and black water can contain enough pathogens to pose a sufficient risk to public health from sources in communities that include residences, hospitals, schools, farms, and food processing plants. In addition, municipal drinking water sources are vulnerable to health risks from wastewater pathogens if they are contaminated.
Some illnesses from wastewater-related sources are quite common. Gastroenteritis (from parasitic protozoa Giardia lambia and Cryptosporidium) is not unusual in the United States. Other diseases transmitted in waste water include hepatitis A, typhoid, polio, cholera, and dysentery. Outbreaks of these diseases can occur when drinking water from wells is polluted by wastewater or there is exposure from recreational activities in polluted waters. Animals and insects that come in contact with wastewater can spread some illnesses.
Organic Matter ---Organic materials that originate from plants, animals, or synthetic organic compounds are found everywhere in the environment. They enter wastewater in the form of human wastes, paper products, detergents, cosmetics, foods, and other agricultural, commercial, and industrial sources. Biodegradable waste is easily broken down by aerobic bacteria, which are natural organisms.
Waste that cannot be broken down by other living organ isms is non-biodegradable. Many organics are biodegradable (e.g., proteins, carbohydrates, or fats).
Large quantities of biodegradable materials are dangerous, because the bacteria consume large amounts of dissolved oxygen in water, which robs other aquatic life of the dissolved oxygen they need to live. Biochemical or biological oxygen demand (BOD) is a measurement procedure used to assess how fast biological organisms are depleting dissolved oxygen in a body of water, as measured in milligrams per liter (mg/L) over a 5-day period. High BOD levels indicate that much of the available dissolved oxygen has been consumed by aerobic bacteria. Pristine waterways have a 5-day BOD level of less than 1 mg/L, while moderately polluted waters are typically in the range of 2 to 8 mg/L. Effectively treated municipal sewage has a 5-day BOD level of 20 mg/L or less. Untreated sewage varies, but is typically around 200 mg/L in the United States.
Many organic compounds developed for agriculture and industry are non-biodegradable. In addition, certain synthetic organics are highly toxic. Pesticides and herbicides are frequently disposed of improperly or are leached from soil and carried by storm water. In receiving waters, they kill or contaminate fish, making them unfit for human consumption. They also can damage processes in treatment plants.
Oil and Grease--- Bacteria don’t quickly break down fatty organic materials from animals, vegetables, and petroleum. When large amounts of oils and greases are discharged to receiving waters from community systems, they increase BOD and may float to the surface and solidify, causing aesthetically unpleasing conditions. Oils and greases also can trap trash, plants, and other materials, causing foul odors, attracting flies and mosquitoes. On site sewage treatment systems can be harmed by too much oil and grease, which can clog on-site system drainage field pipes and soils, adding to the risk of system failure. Excessive grease also adds to the septic tank scum layer, causing more frequent tank pumping to be required. Petroleum-based oils used for vehicle motors are considered hazardous waste and should be collected and disposed of separately from wastewater.
Inorganics---Inorganic minerals, metals, and compounds, such as sodium, potassium, calcium, magnesium, cadmium, copper, lead, nickel, and zinc, are common in wastewater from both residential and nonresidential sources. They can originate from a variety of sources in the community, including industrial and commercial sources, storm water, and inflow and infiltration from cracked pipes and leaky manhole covers. Large amounts of many inorganic sub stances can contaminate soil and water. Some are toxic to animals and humans and may accumulate in the environment. For this reason, extra treatment steps are often required to remove inorganic materials from industrial wastewater sources. For example, heavy metals that are discharged with many types of industrial wastewaters are difficult to remove by conventional treatment methods. Poisonings from heavy metals in drinking water are possible.
In receiving waters, they kill or contaminate aquatic life.
Nutrients ---Wastewater often contains large amounts of the nutrients nitrogen and phosphorus, which promote plant growth. Organisms only require small amounts of nutrients in biological treatment, so there normally is excess available in treated wastewater. In severe cases, excessive nutrients in receiving waters cause algae and other plants to grow quickly, depleting oxygen in the water. Deprived of oxygen, fish and other aquatic life die.
Nutrients from wastewater have also linked to ocean "red tides" that poison fish and cause illness in humans.
Solids ---Solid materials in wastewater can consist of organic and/or inorganic materials and organisms. The solids must be significantly reduced by treatment or they can increase BOD when discharged to receiving waters and provide places for microorganisms to escape, reducing the effectiveness of disinfection system. Suspended solids can also clog soil absorption fields in on-site sewage treatment systems. Small particles of certain waste water materials can dissolve like salt in water. Micro organisms in wastewater consume some dissolved materials, but others, such as heavy metals, are difficult to remove by conventional treatment. Excessive amounts of dissolved solids in wastewater can have adverse effects on the environment.
Gases ---Many gases in wastewater can cause odors or are dangerous. For example, methane gas, a by-product of anaerobic biological treatment, is highly combustible.
The gases hydrogen sulfide and ammonia can be toxic and pose asphyxiation hazards. Ammonia as a dissolved gas in wastewater is dangerous to fish. Many gases emit odors, which can be a serious nuisance and, in severe cases, lower property values and affect the local economy. Unless effectively contained or minimized by de sign and location, wastewater odors can affect quality of life. Special precautions need to be taken near septic tanks, manholes, treatment plants, and other areas where wastewater gases can collect.
Other Important Wastewater Characteristics
In addition to the many substances found in wastewater, there are other characteristics that system designers and operators use to evaluate wastewater. For example, the color, odor, and turbidity of wastewater give clues about the amount and type of pollutants present and the treatment necessary. The following are some other important wastewater characteristics that can affect public health and the environment, as well as the design, cost, and effectiveness of treatment.
Temperature ---The best temperatures for wastewater treatment range from 77° to 95°F. In general, biological treatment activity accelerates in warm temperatures and slows in cool temperatures, but extreme hot or cold can stop treatment processes altogether. Therefore, some systems are less effective during cold weather and some may not be appropriate for very cold climates. Wastewater temperature also affects receiving waters. Hot water, For example, which is a by-product of many manufacturing processes, can be a pollutant. When discharged in large quantities, it can raise the temperature of receiving streams locally and disrupt the natural balance of aquatic life.
pH --- The acidity or alkalinity of wastewater affects both treatment and the environment. Low pH indicates in creasing acidity, while a high pH indicates increasing alkalinity (a pH of 7 is neutral). The pH of wastewater needs to remain between 6 and 9 to protect organisms.
Acids and other substances that alter pH can inactivate treatment processes when they enter wastewater from industrial or commercial sources.
Flow Whether a system serves a single residence or an entire community, it must be able to handle fluctuations in the quantity and quality of wastewater it receives to ensure proper continuous treatment. Systems that are under-designed or that are overloaded may fail to pro vide treatment and allow the release of pollutants to the environment.
To design systems that are both as safe and as cost-effective as possible, engineers must estimate the average and maximum (peak) amount of flows generated by various sources. Because extreme fluctuations in flow can occur, estimates are based on observations of the minimum and maximum amounts of water used on an hourly, daily, weekly, and seasonal basis. The possibility of instantaneous peak flow events that result from fixtures being used at once is also taken into account. The number of possible users or units and the number, type, and efficiency of water using fixtures and appliances at the source are considered in design. Estimating flow volumes for centralized treatment systems is a complicated task, especially when designing a new treatment plant in a community where one has never existed previously.COMMUNITY WASTEWATER TREATMENT AND DISPOSAL