عدد المساهمات : 3484
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: معالجة الصرف الصناعى(كيف تحدد معالحة مياه الصرف الصناعى) الأحد ديسمبر 12, 2010 12:24 pm|| |
the nature of the wastewater because the effluent quality depends upon the influent characteristics
The treatment capacity and treatment efficiency of systems are calculated based upon the
influent concentrations and the effluent requirements
Efficiency = [(C in - Cout)/Cin ] 100
Cin = Influent concentration (typically mg/L)
Cout = Effluent concentration (typically mg/L)
And Efficiency is expressed as a percentage(%)
Also, the treatment capacity over time for biochemical processes is usually modeled as a
first-order equation such that
Ct/C0 = e
( Ct = Concentration at time, t (typically in mg/L
C0 = Initial concentration at time = 0 (typically in mg/L)
k = reaction rate constant (typically in days)
t = time (typically in days)
Fundamental Concepts for Environmental Processes curriculum:-
the wastewater strength (concentration of contaminants), availability of the contaminants as a food source, the characteristic are of being easily metabolized or being difficult to metabolize.
designing the treatment processes.
Typical components of raw sewage entering the septic tank and their concentrations are:
Table 1: Raw Sewage Characteristics
Component Concentration Range Typical conc
Total Suspended Solids, TSS 155 – 330 mg/L 250 mg/L
155 – 286 mg/L 250 mg/L bod
pH 6 -9 s.u. 6.5 s.u
Total Coliform Bacteria 10(8) – 10(10) CFU/100mL 10(9) CFU/100mL
Fecal Coliform Bacteria 10(6) – 10(8) CFU/100mL 10(3) CFU/100mL
Ammonium-Nitrogen, 4 - 13 mg/L 10 mg/L
Nitrate-Nitrogen, NO3-N Less than 1 mg/L Less than 1 mg/L
Total Nitrogen 26 – 75 mg/L 60 mg/L
Total Phosphorus 6 - 12 mg/L 10 mg/L
mg/L = milligrams per liter
s.u. = standard units
CFU/100 mL = Colony-Forming Units per 100 milliliters
One of the most common onsite wastewater treatment systems is the traditional septic
tank followed by a soil absorption system. As the raw sewage flows through the
treatment process, some contaminants are removed, and some contaminants are
transformed. Figure 1 illustrates a traditional septic tank followed by a soil absorption
The Septic Tank
The septic tank, illustrated in figures 2 and 3, reduces the BOD5 slightly without an
effluent filter (screen). With an effluent screen, the BOD5 reduction and the TSS
reduction is enhanced. The septic tank processes convert the organic nitrogen
compounds into ammonium; however no conversion of ammonium to nitrate is expected
to occur in the septic tank. The septic tank alone is not expected to affect the phosphorus
concentrations in the wastewater. Some coliform bacteria removal and reduction occurs
in the septic tank.
Table 2: Septic Tank Effluent Characteristics
Component Concentration Range Typical conc
Total Suspended Solids, TSS 36 - 85 mg/L 60 mg/L
bod 118 - 189 mg/L 120 mg/L
pH 6.4 – 7.8 s.u. 6.5 s.u
Fecal Coliform Bacteria 10(6) – 10(7) CFU/100mL 10(6) CFU/100mL
Ammonium-Nitrogen, 30 – 50 mg/L 40 mg/L
Nitrate-Nitrogen, NO3-N 0 – 10 mg/L 0 mg/L
Total Nitrogen 29.5 – 63.4 mg/L 60 mg/L
Total Phosphorus 8.1 – 8.2 mg/L 8.1 mg/L
The leaching system
The leaching system – sometimes called the soil absorption system or dispersal system
also performs physical and biochemical treatment. The physical processes include
straining and filtration and detention or retardation of the wastewater in the soil
Designing the various hydraulic components.
The soil particles provide surfaces on which biochemical processes occur. As the
effluent moves through the soil infiltrative surface, microbial growth is stimulated and a
As the wastewater moves through the biomat and into and through the
soil, organic compounds are used as a source of food by the microbes and converted to
carbon dioxide and water
Organic Matter + O2 + microbes => CO2 + H2O + new microbes (Equation 1)
As shown in this equation, oxygen is required for the reaction to proceed to carbon
dioxide and water.
In well-sited and properly-functioning septic systems, this process
occurs within the boundaries of the site upon which the system is located, and the
wastewater is renovated within a distance that prevents contamination of groundwater or
Ammonium in the wastewater is converted to nitrite and then to nitrate by processes
using free oxygen (aerobic processes) called nitrification.
First, ammonium is converted t o Nitrite plus hydrogen ions (Equation 2).
Then, nitrite is converted to nitrate hydrogen ions (Equation 3). )
NH4 + O2 + microbes => NO2 + H (Equation 2))
(NO2 + O2 + microbes => NO3 + H (Equation 3)
As with microbial degradation of organic matter, the nitrification reaction requires the
presence free oxygen. Thus, the soil must be unsaturated to allow the wastewater to
move through the soil under unsaturated conditions.
If the nitrified wastewater encounters anoxic conditions and contains enough organic
compounds, the nitrate can be converted to nitrogen gas and other gases by anaerobic
processes called denitrification as shown in Equation 4
NO3 + Organic compound + microbes => N2 (gas) (Equation 4)
These processes are affected by the concentration of the wastewater applied to the soil.
if typical septic tank effluent is applied, the development of the biomat will be
from its development when advanced secondary quality effluent is applied, or if high
strength wastewater is applied
Typical values of contaminant concentrations at 0.6 meters and 1.2 meters (2 feet and 4
The Concept of Load
In addition to the daily flow variation, seasonal variations may also occur. Typically
wastewater treatment processes are sized to treat the maximum daily flow rather simply having the capacity to treat the average daily flow. than
The maximum daily flow is the maximum flow that occurs over the course of a single day, perhaps 450 gallons per day for a typical 3-bedroom home.
The average daily flow is the average of the flow may be 160 gallons per day
that occur during single days over the course of some period of time – perhaps years,
Philosophically (if not particularly statistically rigorous) designing the wastewater
treatment system performance based upon average daily flow would imply that 50
percent of the time, the system is in compliance, and 50 percent of the time the system is
out of compliance. For this reason, treatment systems are typically designed to produce
the required effluent quality when treating the maximum daily flow. This concept is the
hydraulic loading rate. In sizing for the hydraulic loading rate, the volume of water
flowing through the treatment process is the design parameter under consideration. For
the concept of mass loading rate, the idea of the mass or weight of a particular
contaminant flowing through the system over some time is considered. The “organic
loading rate,” the number of pounds or kilograms of BOD per day, or the “solids loading
rate,” the number of pounds or kilograms of TSS per day are common mass loading rates.
By using the wastewater characteristics determined by estimates from tables or typical
residential wastewater, or perhaps by sampling and analyzing a particular wastewater
stream, and combining this with the flow rate, the wastewater “load” may be calculated.
The calculation is the product of the flow rate and the concentration as follows
Load = Concentration X Flow
Typically, as shown in the tables provided in this document, the concentration is given in
units of mg/L and the flow rate is given in units of gallons per day.
consistent units is required to produce units of mg/day or lbs/day or other expressions of
weight (or mass) of contaminant per time.
Other materials make up only a small Wastewater is mostly water by weight
portion of wastewater, but can be presen 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 house hold sewage contains many potential pollutants.
The wastewater components that shoul be of most concern to homeowners and communities are those that have the potential to cause disease or detrimental environmental effects
Many different types of organisms live in wastewater and some are essential contributors to
treatment. A variety of bacteria, protozoa and worms work to break down certain carbon-based (organic) pollutants in wastewater by consuming them.
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 biological processes.
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 diseaseFor 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 Some illnesses from wastewater-related plants. sources are relatively common.
Gastroen teritis can result from a variety of pathogens in wastewater, and cases of illnesses caused by the parasitic protozoa, Giardia lambia and Cryptosporidium.
. 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.
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 fat 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 developing for agriculture and industry-
In addition, certain synthetic organics are highly toxic. Pesticides and herbicides are toxic to humans, fish, and aquatic plants and often are disposed of improperly in drains or carried in storm water
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
Oil and Grease
Fatty organic materials from animals, vegetables, and petroleum also 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 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, attract ing 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.
Onsite systems also can be harmed by too much oil and grease, which can clog onsite system drain 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 waste oils used for motors and industry are considered hazarous waste and should be collected and disposed of separately from wastewater
What is in wastewater
Inorganic minerals, metals, and com pounds, 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 leak manhole covers.
Most inorganic substances are relatively stable, and cannot be broken down easily by organisms in wastewater
Dispose of Household Hazardous Wastes Safely Many household
products are potentially hazardous to people and the environment and never should be flushed down drains, toilets, or storm sewers.
Treatment plant workers can be injured and wastewater systems can be damaged as a result of improper disposal of hazardous materials
Other hazardous chemicals cannot be treated effectively by municipal wastewater systems and may reach local drinking water sources.
When flushed into septic systems and other onsite systems, they can temporarily disrupt the biological processes in the tank and soil absorption field, allowing hazardous chemicals and untreated wastewater to reach groundwater.
Some examples of hazardous household materials include motor oil, transmission fluid, antifreeze, paint, paint thinner varnish, polish, wax, solvents, pesticides rat poison, oven cleaner, and battery fluid.
Many of these materials can be recycled or safely disposed of at community recycling centers.
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 potential long-term health effects of ingesting small amounts of some inorganic substances over an extended period of time are possible.
Wastewater often contains large amounts of the nutrients nitrogen and phosphorus in the form of nitrate and phosphate, which promote plant growth.
Organisms only require small amounts of nutrients in biological treatment, so there normally is an 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, emitting foul odors 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.
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 disinfection
They also can clog soil absorption fields in onsite systems.
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.
Materials that resist settling may remain suspended in wastewater must be treated, or they will clog soil absorption systems or reduce the effectiveness of disinfection systems.
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 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 and is highly combustible.
Special precautions need to be taken near septic tanks manholes, treatment plants, and other areas.
where wastewater gases can collect The gases hydrogen sulfide and ammonia 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.
For example, the color, odor, and turbidity of wastewater give clues about the amount and type of pollutants present and treatment necessary as well as the design, cost, and effectiveness of treatment.
The best temperatures for wastewater treatment probably range from 77 to 95 degrees Fahrenheit.
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 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
The acidity or alkalinity of wastewater affects both treatment and the environments.
Low pH indicates increasing acidity while a high pH indicates increasing alkalinity(a pH of 7 is neutral).
The pH of wastewater needs to remain between 6 and9 to protect organisms
Acids and other substances that alter pH can inactivate treatment processes when they enter waste water from industrial or commercial sources
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 various sources (peak) amount of flows generated by
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 is 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.
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
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 sample is measured, and the samples are degrees Celsius (68 degrees Fahrenhite)
Then incubated in commen incubation periods are (5/7/20 days)
is the most common. At the end of the incubation period, the dissolved oxygen is measured The amount that was used expressed in milligrams per liter) is indication of wastewater strength.
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 larger 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 below for some typical TSS amounts in municipal wastewater.
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 patho gens
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) tech nique 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
Typical Municipal Wastewater Characteristics (in milligrams per liter)
weak medium strong minimum treatment req.
BOD 110 220 400 30
TSS 100 220 350 30
(N) 20 40 85 variable
(P) 4 8 15 variable
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