CHM 331 INTRODUCTION TO INDUSTRIAL CHEMISTRY LECTURE NOTES
WASTEWATER TREATMENT
The principal objective of wastewater treatment is generally to allow human and industrial effluents to be
disposed of without danger to human health or unacceptable damage to the natural environment. Irrigation with...
chm 331 introduction to industrial chemistry lecture notes wastewater treatment the principal objective of wastewater treatment is generally to allow human and industrial effluents to be disposed of w
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INTRODUCTION TO INDUSTRIAL CHEMISTRY
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CHM 331 INTRODUCTION TO INDUSTRIAL CHEMISTRY LECTURE NOTES
WASTEWATER TREATMENT
The principal objective of wastewater treatment is generally to allow human and industrial effluents to be
disposed of without danger to human health or unacceptable damage to the natural environment. Irrigation with
wastewater is both disposal and utilization and indeed is an effective form of wastewater disposal (as in slow-
rate land treatment). However, some degree of treatment must normally be provided to raw municipal
wastewater before it can be used for agricultural or landscape irrigation or for aquaculture. The quality of
treated effluent used in agriculture has a great influence on the operation and performance of the wastewater-
soil-plant or aquaculture system.
Wastewater treatment is the process of converting wastewater – water that is no longer needed or is no longer
suitable for use – into bilge water that can be discharged back into the environment. It is formed by a number of
activities including bathing, washing, using the toilet, and rainwater runoff. Wastewater is full of contaminants
including bacteria, chemicals and other toxins. Its treatment aims at reducing the contaminants to acceptable
levels to make the water safe for discharge back into the environment.
There are two wastewater treatment plants namely chemical or physical treatment plant, and biological
wastewater treatment plant. Biological waste treatment plants use biological matter and bacteria to break down
waste matter. Physical waste treatment plants use chemical reactions as well as physical processes to treat
wastewater. Biological treatment systems are ideal for treating wastewater from households and business
premises. Physical wastewater treatment plants are mostly used to treat wastewater from industries, factories
and manufacturing firms. This is because most of the wastewater from these industries contains chemicals
and other toxins that can largely harm the environment.
Step by Step Wastewater Treatment Process
The following is a step by step process of how wastewater is treated:
1. Wastewater Collection
This is the first step in waste water treatment process. Collection systems are put in place by municipal
administration, home owners as well as business owners to ensure that all the wastewater is collected and
directed to a central point. This water is then directed to a treatment plant using underground drainage systems
or by exhauster tracks owned and operated by business people. The transportation of wastewater should
however be done under hygienic conditions. The pipes or tracks should be leak proof and the people offering
the exhausting services should wear protective clothing.
2. Odour Control
At the treatment plant, odour control is very important. Wastewater contains a lot of dirty substances that cause
a foul smell over time. To ensure that the surrounding areas are free of the foul smell, odour treatment processes
are initiated at the treatment plant. All odour sources are contained and treated using chemicals to neutralize the
foul smell producing elements. It is the first wastewater treatment plant process and it is very important.
3. Screening
This is the next step in wastewater treatment process. Screening involves the removal of large objects for
example nappies, cotton buds, plastics, diapers, rags, sanitary items, face wipes, broken bottles or bottle tops
that in one way or another may damage the equipment. Failure to observe this step, results in constant machine
and equipment problems. Specially designed equipment is used to get rid of grit that is usually washed down
into the sewer lines by rainwater. The solid wastes removed from the wastewater are then transported
and disposed off in landfills.
4. Primary Treatment
This process involves the separation of macrobiotic solid matter from the wastewater. Primary treatment is done
by pouring the wastewater into big tanks for the solid matter to settle at the surface of the tanks. The sludge, the
solid waste that settles at the surface of the tanks, is removed by large scrappers and is pushed to the center of
the cylindrical tanks and later pumped out of the tanks for further treatment. The remaining water is then
pumped for secondary treatment.
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,5. Secondary Treatment
Also known as the activated sludge process, the secondary treatment stage involves adding seed sludge to the
wastewater to ensure that is broken down further. Air is first pumped into huge aeration tanks which mix the
wastewater with the seed sludge which is basically small amount of sludge, which fuels the growth of bacteria
that uses oxygen and the growth of other small microorganisms that consume the remaining organic matter. This
process leads to the production of large particles that settle down at the bottom of the huge tanks. The
wastewater passes through the large tanks for a period of 3-6 hours.
6. Bio-solids handling
The solid matter that settle out after the primary and secondary treatment stages are directed to digesters. The
digesters are heated at room temperature. The solid wastes are then treated for a month where they undergo
anaerobic digestion. During this process, methane gases are produced and there is a formation of nutrient rich
bio-solids which are recycled and dewatered into local firms. The methane gas formed is usually used as a
source of energy at the treatment plants. It can be used to produce electricity in engines or to simply drive plant
equipment. This gas can also be used in boilers to generate heat for digesters. Here, bacteria break down (digest)
the material, reducing its volume, odours, and getting rid of organisms that can cause disease. The finished
product is mainly sent to landfills, but sometimes can be used as fertilizer.
7. Tertiary treatment
This stage is similar to the one used by drinking water treatment plants which clean raw water for drinking
purposes. The tertiary treatment stage has the ability to remove up to 99% of the impurities from the
wastewater. This produces effluent water that is close to drinking water quality. Unfortunately, this process
tends to be a bit expensive as it requires special equipment, well trained and highly skilled equipment operators,
chemicals and a steady energy supply. All these are not readily available.
8. Disinfection
After the primary treatment stage and the secondary treatment process, there are still some diseases causing
organisms in the remaining treated wastewater. To eliminate them, the wastewater must be disinfected.
The process of destroying all the bacteria (either harmful or harmless) is known as sterilization. But in a water
supply scheme, we require only the removal of harmful bacteria (pathogenic bacteria) which may cause water-
borne diseases like cholera, dysentery, typhoid, etc.
The process of destroying harmful bacteria from water and making it safe for drinking is known as disinfection.
Common methods of disinfection include using ozone, chlorine, iodine or bromine, excess lime, potassium permanganate,
silver, or ultraviolet light.
Chlorination remains the most common form of wastewater disinfection in some places due to its low , reliability and
long-term history of effectiveness. The water is held for at least 20-25 minutes in tanks that contain a mixture of chlorine
and sodium hypochlorite. Chlorine contact basins are usually rectangular channels, with baffles to prevent short-
circuiting, designed to provide a contact time of about 30 minutes. However, to meet advanced wastewater treatment
requirements, a chlorine contact time of as long as 120 minutes is sometimes required for specific irrigation uses of
reclaimed wastewater. The bactericidal effects of chlorine and other disinfectants are dependent upon pH, contact time,
organic content, and effluent temperature. One disadvantage is that chlorination of residual organic material can generate
chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines
may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine
is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of
treatment. Chloramine, which is used for drinking water, is not used in wastewater treatment because of its persistence.
Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are
used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other
methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making
them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp
maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms
are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect
microorganisms from the UV light). In some countries, light is becoming the most common means of
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, disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the
wastewater and in chlorinating organics in the receiving water.
Ozone O3 is generated by passing oxygen O2 through a high voltage potential resulting in a third oxygen atom
becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it
comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer
than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an
accidental release), ozone is generated onsite as needed. Ozonation also produces fewer disinfection by-
products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation
equipment and the requirements for special operators.
The disinfection process is an integral part of the treatment process because it guards the health of the animals
and the local people who use the water for other purposes. The effluent (treated waste water) is later released or
discharged into the environment (local river or ocean) through the local water ways.
9. Sludge Treatment
The sludge that is produced and collected during the primary and secondary treatment processes requires
concentration and thickening to enable further processing. It is put into thickening tanks that allow it to settle
down and later separates from the water. This process can take up to 24 hours. The remaining water is collected
and sent back to the huge aeration tanks for further treatment. The sludge is then treated and sent back into the
environment and can be used for agricultural use.
Wastewater treatment has a number of benefits. For example, wastewater treatment ensures that the
environment is kept clean, there is no water pollution, makes use of the most important natural resource; water,
the treated water can be used for cooling machines in factories and industries, prevents the outbreak of
waterborne diseases and most importantly, it ensures that there is adequate water for other purposes like
irrigation.
Conclusion
Wastewater treatment process is one of the most important environmental conservation processes that should be
encouraged worldwide. Most wastewater treatment plants treat wastewater from homes and business places.
Industrial plant, refineries and manufacturing plants wastewater is usually treated at the onsite facilities. These
facilities are designed to ensure that the wastewater is treated before it can be released to the local environment.
Some of the water is used for cooling the machines within the plants and treated again. They try to ensure that
nothing is lost. It is illegal for disposing untreated wastewater into rivers, lakes, oceans or into the environment
and if found culpable one can be prosecuted according to the Laws of all countries including the Gambia.
In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure
the amount of organic compounds in water.
Most applications of COD determine the amount of organic pollutants found in surface
water (e.g. lakes and rivers) or wastewater, making COD a useful measure of water quality. It is expressed in
milligrams per litre (mg/L) also referred to as ppm (parts per million), which indicates the mass of oxygen
consumed per litre of solution.
Biochemical oxygen demand is a measure of the quantity of oxygen used by microorganisms (e.g., aerobic
bacteria) in the oxidation of organic matter. Natural sources of organic matter include plant decay and leaf fall.
Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed by
aerobic biological organisms in a body of water to break down organic material present in a given water sample
at certain temperature over a specific time period.
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