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If you have not already read Chapter 2 (pp. 21-97) of Crites and Tchobanoglous and Chapter 2 (pp. 25-34) of Burks and Minnis, I encourage you to do so.
Wastewater is sewage, stormwater, and water that has been used for various purposes around the community. Unless properly treated, wastewater can harm public health and the environment. Most communities generate wastewater from both residential and nonresidential sources
Residential wastewater
Although the word sewage usually brings toilets to mind, it actually
is used to describe all types of wastewater generated from every room in a house.
In the U.S., sewage varies regionally and from home to home based on such factors
as the number and type of water-using fixtures and appliances, the number of
occupants, their ages, and even their habits, such as the types of foods they
eat. However, when compared to the variety of wastewater flows generated by
different nonresidential sources, household water shares many similar characteristics
overall. Table 1 lists the composition of typical residential
untreated wastewater.
There are two types of domestic sewage: blackwater (wastewater from toilets) and graywater (wastewater from all sources except toilets). Blackwater and graywater have different characteristics, but both contain pollutants and disease-causing agents that require treatment. Some areas in the U.S., including Arizona (Direct Reuse of Reclaimed Water Rule, effective 01/16/01), permit the use of innovative systems that safely recycle household graywater for reuse in toilets or for irrigation to conserve water and reduce the flow to treatment systems.
Nonresidential wastewater
Nonresidential wastewater in small communities is generated by such diverse
sources as offices, businesses, department stores, restaurants, schools, hospitals,
farms, manufacturers, and other commercial, industrial, and institutional entities.
Stormwater is a nonresidential source and carries trash and other pollutants
from streets, as well as pesticides and fertilizers from yards and fields. Because
of the variety of nonresidential wastewater characteristics, communities need
to assess each source individually or compare similar types of nonresidential
sources to ensure that adequate treatment is provided. For example, public restrooms
may generate wastewater with some characteristics similar to 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.
Laundaries 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.
Wastewater from some nonresidential sources also may require additional treatment steps. For example, stormwater should be collected separately to prevent the flooding of treatment plants during wet weather. Trash and other large solids from storm sewers often are removed by screens.
In addition, many industries produce wastewater high in chemical and biological pollutants that can overburden onsite and community systems. Dairy farms and breweries are good examples -- communities may require these types of nonresidential sources to provide their own treatment or preliminary treatment to protect community systems and public health.
Wastewater is mostly water by weight. Other materials make up only a small portion of wastewater, but can be present in large enough quantities to endanger public health and the environment. Because practically anything that can be flushed down a toilet, drain, or sewer can be found in wastewater, even household sewage contains many potential pollutants. The wastewater components that most wastewater facilities are designed to remove are suspended solids, biodegradable organics, and pathogenic organisms.
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,
stormwater, and inflow and infiltration from cracked pipes and leaky manhole
covers. Most inorganic substances are relatively stable and cannot be broken
down easily by organisms in wastewater.
Large amounts of many inorganic substances 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. Heavy metals, for example, which are discharged with many types of industrial wastewaters, are difficult to remove by conventional treatment methods. Although acute poisonings from heavy metals in drinking water are rare in the U.S., potential long-term health effects of ingesting small amounts of some inorganic substances over an extended period of time are possible.
Inorganic ions in solution contribute to the conductivity (e.g., the ability to carry an electrical current) of the wastewater. The conductivity of domestic wastewater may be near that of the local water supply, although not necessarily. Backwash from water softeners contains high calcium chloride concentrations which will increase the conductivity.
Nutrients
Wastewater often contains large amounts of nitrogen and phosphorus in the form
of nitrate and phosphate, respectively -- nutrients that promote plant growth.
Organisms only require small amounts of nutrients in biological treatment, so
there normally is an excess of available nutrients in treated wastewater. In
severe cases, excess nitrogen and phosphorous can result in eutrophication,
the nutrient enrichment of water bodies causing excessive growth of aquatic
plants (algae, cyanobacteria, rooted aquatic vegetation, duckweed). The dissolved
oxygen in the water body becomes depleted when the aquatic plants die, fall
to the bottom, and are decomposed by aerobic bacteria. The oxygen depletion
can reduce the populations of indigenous fish and other oxygen-consuming organisms.
Nutrients from wastewater have also been linked to ocean "red tides" that poison fish and cause illness in humans. Nitrogen in drinking water may contribute to miscarriages and is the cause of a serious illness in infants called methemoglobinemia or "blue baby syndrome."
Oil and grease
Oil and grease is the term given to the combination of fats, oils, waxes, and
other related constituents found in wastewater. Fatty organic materials from
animals, vegetables, and petroleum are not quickly broken down by bacteria and
can cause pollution in receiving environments. When large amounts of oils and
greases are discharged to receiving waters from community systems, they increase
BOD levels, and they may float to the surface and harden, causing aesthetically
unpleasing conditions. They also can trap trash, plants, and other materials
causing foul odors and attracting flies and mosquitoes and other disease vectors.
In some cases, too much oil and grease causes septic conditions in ponds and
lakes by preventing oxygen from the atmosphere from reaching the water.
If the wastewater is warm and greasy, the grease cannot quickly separate from the water, so the grease flows through the septic tank and into the soil where it solidifies upon cooling. The grease can clog the gravel and soil pores ruining the drainage system. Excessive grease also adds to the septic tank scum layer, requiring more frequent tank pumping. Both possibilities can result in significant costs to homeowners and are the reasons why grease traps are manditory for restaurants and food service facilities.
Petroleum-based waste oils used for motors and industry are considered hazardous waste and should be collected and disposed of separately from wastewater.
Organic Matter
Organic materials are found everywhere in the environment. They are composed
of the carbon-based chemicals that are the building blocks of most living things.
Organic materials in wastewater originate from plants, animals, or synthetic
organic compounds, and enter wastewater in human wastes, paper products, detergents,
cosmetics, foods, and from agricultural, commercial, and industrial sources.
Organic compounds normally are some combination of carbon, hydrogen, oxygen,
nitrogen, and other elements. Many organics are proteins, carbohydrates, or
fats and are biodegradable, which means they can be consumed and broken down
by organisms. However, even biodegradable materials can cause pollution. In
fact, too much organic matter in wastewater can be devastating to receiving
waters.
Large amounts of biodegradable materials are dangerous to lakes, streams, and oceans because organisms use dissolved oxygen in the water to break down the wastes. This can reduce or deplete the supply of oxygen in the water needed by aquatic life, resulting in fish kills, odors, and overall degradation of water quality. The amount of oxygen organisms need to break down wastes in wastewater is referred to as the biochemical oxygen demand (BOD) and is one of the measurements used to assess overall wastewater strength.
Some organic compounds are more stable than others and cannot be quickly broken down by organisms, posing an additional challenge for treatment. This is true of many synthetic organic compounds developed for agriculture and industry. In addition, certain synthetic organics are highly toxic. Insesticides and herbicides are toxic to humans, fish, and aquatic plants and often are disposed of improperly in drains or carried in stormwater. In receiving waters, they kill or contaminate fish, making them unfit to eat. They also can damage processes in treatment plants. Benzene and toluene are two toxic organic compounds found in some solvents, pesticides, and other products. New synthetic organic compounds are being developed all the time, which can complicate treatment efforts.
Pathogens
Many disease-causing viruses, parasites, and bacteria also are present in wastewater
and enter from almost anywhere in the community. These pathogens often originate
from people and animals who are infected with or are carriers of a disease.
For example, graywater and blackwater from typical homes contain enough pathogens
to pose a risk to public health. Other likely sources in communities include
hospitals, schools, farms, and food processing plants.
Some illnesses from wastewater-related sources are relatively common. Gastroenteritis
can result from a variety of pathogens in wastewater, and cases of illnesses
caused by the parasitic protozoa Giardia lambia and Cryptosporidium are not
unusual in the U.S. Other important wastewater-related diseases include hepatitis
A, typhoid, polio, cholera, and dysentery. Outbreaks of these diseases can occur
as a result of drinking water from wells polluted by wastewater, eating contaminated
fish, or recreational activities in polluted waters. Some illnesses can be spread
by animals and insects that come in contact with wastewater.
Even municipal drinking water sources are not completely immune to health risks
from wastewater pathogens. Drinking water treatment efforts can become over-whelmed
when water resources are heavily polluted by wastewater. For this reason, wastewater
treatment is as important to public health as drinking water treatment. For
a detailed discussion of the health risks associated with wastewater, refer
to the Summer 1996 issue (vol. 7, no. 3) of Pipeline, National Small
Flows Clearinghouse, 800-624-8301, Item#SFPLNL06.
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 levels when discharged to receiving waters and provide places
for microorganisms to escape disinfection. They also can clog soil absorption
fields in onsite systems. The amount of solids in the wastewater affects the
septic tank size and the frequency for pumping out the septic tank. Proper solids
analysis is important for the control of biological and physical wastewater
treatment processes and assess compliance with effluent quality limits.
• total solids – The term applied to the residue left after evaporation; it includes the total suspended soilds and total dissolved solids.
• suspended solids – The portion of wastewater that, when passed through a filter, remains on the filter. Materials that resist settling may remain suspended in wastewater. Suspended solids in wastewater must be treated, or they will clog soil absorption systems or reduce the effectiveness of disinfection systems.
• dissolved solids – The portion of wastewater that, when passed through a filter, does not remain on the filter. Small particles of certain wastewater materials can dissolve like salt in water. Some dissolved materials are consumed by microorganisms in wastewater, 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.
• setteable solids – Certain substances, such as sand, grit, and heavier organic and inorganic materials, settle out from the rest of the wastewater stream during the preliminary stages of treatment. On the bottom of settling tanks and ponds, organic material makes up a biologically active layer of sludge that aids in treatment. Knowing the amount of setteable solids in the wastewater provides information on how much sludge will be created in the septic tank.
• fixed solids – The term applied for the residue (total, suspended, or dissolved) that is left after igniting a dried sample. The weight loss on ignition is called volatile solids.
Each type of solids is processed differently, so it is essential to understand the distinction amongst the various types. Setteable and some suspended solids are usually removed by filtration or settling. Other suspended solids and dissolved solids are treated by biological processes or chemical precipitation. Some dissolved solids may pass through an onsite wastewater treatment system intact. Dissolved solids such as metals and chloride can only be removed by distillation or reverse osmosis.
|
Constituent
|
Unit
|
Range
|
Typical
|
| Total Solids |
mg/L
|
300-1200
|
700
|
| Dissolved |
mg/L
|
250-850
|
500
|
| Fixed |
mg/L
|
150-550
|
150
|
| Volatile |
mg/L
|
100-300
|
150
|
| Suspended |
mg/L
|
100-400
|
220
|
| Fixed |
mg/L
|
30-100
|
70
|
| Volatile |
mg/L
|
70-300
|
150
|
| Setteable |
mg/L
|
50-200
|
100
|
| BOD5 |
mg/L
|
100-400
|
250
|
| TOC |
mg/L
|
100-400
|
250
|
| COD |
mg/L
|
200-1,000
|
500
|
| Total Nitrogen |
mg/L
|
15-90
|
40
|
| Organic |
mg/L
|
5-40
|
25
|
| Ammonia |
mg/L
|
10-50
|
25
|
| Nitrite |
mg/L
|
0
|
0
|
| Nitrate |
mg/L
|
0
|
0
|
| Total Phosphorous |
mg/L
|
5-20
|
12
|
| Organic |
mg/L
|
1-5
|
2
|
| Inorganic |
mg/L
|
5-15
|
10
|
| Chloride |
mg/L
|
30-85
|
50
|
| Sulfate |
mg/L
|
20-60
|
15
|
| Alkalinity |
mg/L
|
50-200
|
100
|
| Grease |
mg/L
|
50-150
|
100
|
| Total Coliform |
colonies/100 mL
|
106-108
|
107
|
| VOCs |
µg/L
|
100-400
|
250
|
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, color, odor, and turbidity of wastewater give clues about the amount and type of pollutants present and 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 temperature of wastewater is usually higher than that of the water supply
because of the addition of warm water from domestic use. Wastewater temperature
is important for two reasons: (1) biological processes are temperature dependent
and (2) chemical reactions and reaction rates and aquatic life are all temperature
sensitive.
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. For example, aerobic digestion and nitrification stop when temperatures rise to 122 °F; and when temperatures fall below 36 °F, virtually all biological activity becomes dormant. 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 byproduct 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.
The pH is actually the measure of the inverse concentration of hydrogen ions
and is a logrithmic scale. As pH values lower, it indicates increasing acidity,
while a high pH indicates alkalinity (a pH of 7 is neutral). The pH of wastewater
needs to remain between 6 and 9 to protect beneficial organisms. Acids, cleaning
agents, and other substances that alter pH can inactivate treatment processes
when they enter wastewater.
Gases
Certain gases in wastewater can cause odors, affect treatment, or are potentially
dangerous. Methane gas, for example, is a byproduct of anaerobic biological
treatment (found in septic tanks) and is highly combustible. Special precautions
need to be taken near septic tanks, manholes, treatment plants, and other areas
where wastewater gases can collect. Hydrogen sulfide and ammonia gases can be
toxic and pose asphyxiation hazards. Ammonia as a dissolved gas in wastewater
also is dangerous to fish. Both gases emit odors, which can be a serious nuisance.
Unless effectively contained or minimized by design and location, wastewater
odors can affect the mental well-being and quality of life of residents. In
some cases, odors can even lower property values and affect the local economy.
Flow
Whether a system serves a single home or an entire community, it must be able
to handle fluctuations in the quantity and quality of wastewater it receives
to ensure proper treatment is provided at all times. Systems that are inadequately
designed or hydraulically overloaded may fail to provide 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 during different times of the day and on different days of the week, 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 several or all water-using appliances or fixtures being used at once also is taken into account.
The number, type, and efficiency of all water-using fixtures and appliances at the source are factored into the estimate (for example, the number and amount of water normally used by faucets, toilets, and washing machines), as is the number of possible users or units that can affect the amount of water used (for example, the number of residents, bedrooms, customers, students, patients, seats, or meals served).
According to studies, water use in many homes is lowest from about midnight to 5 a.m., averaging less than one gallon per person per hour, but then rises sharply in the morning around 6 a.m. to a little over 3 gallons per person per hour. During the day, water use drops off moderately and rises again in the early evening hours.
Weekly peak flows may occur in some homes on weekends, especially when all adults work during the week. In U.S. homes, average water use is approximately 45 gallons per person per day, but may range from 35 to 60 gallons or more.
Peak flows at stores and other businesses typically occur during business hours and during meal times at restaurants. Rental properties, resorts, and commercial establishments in tourist areas may have extreme flow variations seasonally.
Estimating flow volumes for centralized treatment systems is a complicated task, especially when designing a new treatement plant in a community where one has never existed previously. Engineers must allow for additional flows during wet weather due to inflow and infiltration of extra water into sewers. Excess water can enter sewers through leaky manhole covers and cracked pipes and pipe joints, diluting wastewater, which affects its overall characteristics, and increasing flows to treatment plants sometimes by as much as three or four times the original design load.
Testing and Measuring Wastewater
System operators, designers, and regulatory agencies use tests to evaluate the strength of wastewater and the amount of treatment required, the quality of effluent at different stages of treatment, and the quality of receiving waters at the point of discharge. Tests also determine whether treatment is in compliance with state, local, and federal regulations. In small communities, operators and health officials often are trained to collect samples and perform some or all wastewater tests themselves. An option that sometimes is more economical for small systems is to send samples away to a lab for testing. The following are a few important tests:
BOD – biochemical oxygen demand
The BOD test measures the amount of dissolved oxygen organisms are likely to
need to degrade wastes in wastewater. This test is important for evaluating
both how much treatment wastewater is likely to require and the potential impact
that it can have on receiving waters.
To perform the test, wastewater samples are placed in BOD bottles and are diluted with specially prepared water containing dissolved oxygen. The dilution water is also "seeded" with bacteria when treated wastewater is being tested. The amount of dissolved oxygen in the diluted samples is measured using a dissolved oxygen meter, and the samples are then stored at a constant temperature of 20 degrees Celsius (68 degrees Fahrenheit). Common incubation periods are five, seven, or twenty days; five days (or BOD5) is the most common. At the end of the incubation period, the dissolved oxygen is measured again. The amount that was used (expressed in milligrams per liter) is an indication of wastewater strength. Refer to the Table 2 for some typical BOD amounts. For an in-depth explanation of BOD and testing wastewater, refer to the module Fundamental Microbiology of Sewage.
TSS – total suspended solids
In addition to BOD, estimating the amount of suspended solids in wastewater
helps to complete an overall picture of how much secondary treatment is likely
to be required. It also indicates wastewater clarity and is important for assessing
the potential impact of wastewater on the environment.
After large solids are removed in primary treatment, TSS is measured as the portion of solids retained by a 2.0-micron filter. Refer to the Table 2 for some typical TSS amounts.
TC and FC – total coliforms and fecal coliforms
Coliform tests are useful for determining whether wastewater has been adequately
treated and whether water quality is suitable for drinking and recreation.
Because they are very abundant in human wastes, coliform bacteria are much easier
to locate and identify in wastewater than viruses and other pathogens that cause
severe diseases. For this reason, coliform bacteria are used as indicator organisms
for the presence of other, more serious pathogens. Some coliforms are found
in soil, so tests for fecal coliforms are considered to be the most reliable.
However, tests for both total coliforms and fecal coliforms are commonly used.
There are two methods for determining the presence and density of coliform bacteria.
The membrane filter (MF) technique provides a direct count of colonies trapped
and then cultured. The multiple tube fermentation method provides an estimate
of the most probable number (MPN) per 100 milliliters from the number of test
tubes in which gas bubbles form after incubation.
|
Constituent
|
weak
|
medium
|
strong
|
minimum treatment requirements
|
| BOD5, mg/L |
110
|
220
|
400
|
30
|
| TSS, mg/L |
100
|
220
|
350
|
30
|
| Nitrogen (N), mg/L |
20
|
40
|
85
|
variable
|
| Phosphorus (P), mg/L |
4
|
8
|
15
|
variable
|