Exploring the Hidden World of Microorganisms in Domestic Water and Sewage

All living organisms require water as a fundamental necessity. The absence of this vital element can cause some organisms to perish prematurely, while others may form resistant stages or eventually succumb. The human species is no exception to this need. The primary sources of water suitable for human consumption include lakes, rivers, and streams. Almost all natural water sources contain some microbes, while others are devoid of such organisms. These microbes can include benign water bacteria, such as nitrogen-fixing and nitrifying bacteria, as well as those derived from the atmosphere, soil, and even decaying bodies and waste matter from humans and animals. The type and quantity of microbes present in water depend on its origin, the addition of waste products from animals and humans, and other forms of contamination. Along with microbes, chemical substances, both organic and inorganic, may be present in water due to contamination from industrial and agricultural sources.

Wastewater, or sewage, is a liquid material that carries refuse and various other types of waste through a drainage system. The challenge of wastewater management has been a persistent problem since the early days of urbanization. Even civilizations such as Ancient Rome in Europe and Mohenjo-Daro in Pakistan had advanced drain sewer systems for the time. It's clear that even in the past, people recognized the importance of proper wastewater disposal for aesthetic and hygienic reasons. In recent times, with the advancement of microbiology in the applied field, we have gained a better understanding of the potential hazards associated with wastewater. This knowledge has led to the development of effective and efficient methods for its disposal.

SOURCE OF WATER

Water sources can be categorized into two main types: surface water and groundwater. Surface water can be obtained from ponds, lakes, rivers, shallow wells, and reservoirs created by damming. Due to its contact with humans, animals, and soil microorganisms, surface water tends to harbor a significant number of microbes. It may contain benign microbes originating from soil and often, in areas near cities and towns, it is contaminated with bacteria from wastewater. Soil microorganisms are primarily found in the top 6 inches of the Earth's crust. The composition of microbes in surface water can vary depending on the concentration of dissolved or suspended salts, minerals, and organic nutrients. Freshwater is predominantly inhabited by algae and protozoa, while bacteria, fungi, and viruses comprise a smaller proportion of the aquatic microbial population. Pathogens in surface water mostly originate from infected individuals or untreated wastewater that are discharged into rivers or lakes. The presence of organic nutrients and physical conditions in the surface water, such as temperature and pH, greatly support the growth and survival of microorganisms.

Groundwater is sourced from deep wells and springs and typically contains a low concentration of microorganisms (one or two per milliliter) as the water filters through multiple layers of the Earth's surface. However, microbial contamination may occur if the well is situated within 200 feet of the source of contamination, while chemical contamination may occur at a distance of 400 feet. The likelihood of contamination increases during wet weather conditions.

SANITARY CLASSIFICATION OF WATER

Water can be categorized as potable, contaminated, or polluted. Potable water is free from harmful agents such as pathogens and has a pleasant taste, making it safe for human consumption. Contaminated water contains dangerous microbial or chemical agents, although it may have a pleasing appearance, odor, and taste. Microbes in contaminated water make it unsafe for drinking or domestic use, while toxic chemical agents such as heavy metals, nitrates, chloramines, and organic compounds pose significant health risks. Polluted water is characterized by an unpleasant appearance, taste, or odor, and may contain enteric pathogens due to the addition of fecal matter or wastewater. Polluted water is not suitable for drinking or domestic use due to its harmful contents.

MICROBIOLOGICAL ASPECTS OF POLLUTION

The term pollution refers to the state of being impure or dirty and can result from physical, chemical, or biological pollutants. Microorganisms can also be pollutants, and many waterborne pathogens are transmitted from infected individuals to others through contaminated water. Microbial pollution of water supplies is a significant health concern, with numerous epidemics of various enteric diseases arising in communities due to drinking polluted water, leading to hundreds of deaths.

Moreover, the release of various chemical wastes into the water can create an artificial nutrient source that promotes microbial growth. The excessive growth of non-pathogenic microorganisms in water can cause severe pollution problems. This process of increasing nutrient concentration in water, known as eutrophication, can result in the overgrowth of algae and cyanobacteria. The addition of chemical waste that contains inorganic substances can stimulate the growth of nitrogen-fixing and carbon dioxide-fixing cyanobacteria.

WATERBORNE DISEASES

John Snow, a British physician in London, conducted studies that confirmed water as a source of various bacterial, protozoal, and viral diseases. In 1854, a cholera epidemic occurred in a particular area of the city where many victims had consumed water from the same well. Snow discovered that the well was contaminated by a faulty sewer pipe, and nearly 700 deaths occurred in the area. After the pump was removed, the epidemic dwindled rapidly. Snow's findings were described in a classical scientific paper, the Mode of Communication of Cholera, which represented the first clear-cut proof of water as a carrier of disease microorganisms. Subsequently, many other epidemics were reported, and the role of water as a carrier of pathogens was authenticated. Drinking contaminated water remains a significant public health concern today. His discovery that contaminated water was responsible for the spread of cholera helped to establish the germ theory of disease and paved the way for advances in public health and sanitation.

Waterborne diseases are commonly caused by pathogens that leave the body through the digestive system and are contracted by drinking contaminated water. These diseases tend to result in epidemics, with most cases appearing within a few days of infection. Typhoid fever, bacillary and amebic dysentery, giardiasis, balantidiasis, salmonellosis, viral hepatitis, and cholera are some of the most significant waterborne diseases. Extreme contamination of water with sewage often leads to epidemics of acute diarrhea. E. coli, rotavirus, and Norwalk virus are among the pathogens that cause serious intestinal infections. Other diseases, such as tularemia, trichinosis, anthrax, leptospirosis, poliomyelitis, and various worm and enterovirus infections, can also be transmitted through water. Bacteria such as Campylobacter, Yersinia, and Helicobacter can cause severe diarrheal diseases in humans. While pathogenic bacteria do not live long in water and do not multiply there, they may persist for a time in cool water with high organic matter concentrations. However, they die rapidly when exposed to ice.

In addition to the known waterborne diseases, certain opportunistic pathogens can survive and propagate in a wet and stagnant environment, such as many species of Pseudomonas. These bacteria can be found in sinks, drains, faucets, and oxygen therapy apparatus in hospitals, and can act as an unsuspected source of infection. Newborn infants, patients with surgical wounds, and individuals with chronic illnesses are especially vulnerable to these bacteria.

Typhoid Fever

Typhoid fever is a significant disease that occurs when an individual consumes food that has been contaminated with fecal matter. Its underlying cause is the bacterium Salmonella typhi, which is a short, non-spore-forming, non-capsulated, and motile Gram-negative rod. It can survive with or without oxygen. The human population, including patients and long-term carriers, serves as the primary natural reservoir for S. typhi. These individuals expel the pathogen through their feces.

Salmonellosis

Salmonellosis is a disease that can be caused by any of the various species within the Salmonella genus, except S. typhi (which causes typhoid fever), S. paratyphi A, and S. schottmuelleri (the agents behind paratyphoid fever). Among the most frequently implicated species in salmonellosis are S. enteritidis, S. typhimurium, S. Newport, and S. heidelberg.

E. coli Diarrhea

The bacterium most commonly found in the human gastrointestinal tract is Escherichia coli. While the strain that inhabits the human intestine is considered harmless and does not cause any illnesses, certain strains of E. coli can lead to diseases affecting the gastrointestinal tract, urinary tract, lungs, and gallbladder. These pathogenic strains are responsible for roughly 90% of acute urinary tract infections in non-hospitalized patients and 30% of urinary tract infections acquired in hospitals.

There are five strains of E. coli, which are transmitted through food and water and cause various forms of gastroenteritis.

1. Enteropathogenic E. coli (EPEC): It can cause severe diarrhea, particularly in developing countries, and is known to cause infantile diarrhea in newborn babies.
2. Enterotoxigenic E. coli (ETEC): This strain of E. coli responsible for infantile diarrhea produces two different exotoxins, which are released after the bacteria attaches to the intestinal epithelial cells in the small intestine. These exotoxins impact the intestinal mucosa, leading to a loss of fluid and resulting in severe watery diarrhea. This disease is often referred to as traveler’s diarrhea.
3. Enteroinvasive E. coli (EIEC): This strain of E. coli targets the epithelial cells in the large intestine by invading and reproducing within them, ultimately causing their death. This leads to patients experiencing diarrhea with mucus and blood in their stools.
4. Enterohemorrhagic E. coli (EHEC): This strain of E. coli is significant as it is responsible for numerous cases of severe diarrhea worldwide, often resulting in high mortality rates. It produces a potent exotoxin that kills intestinal epithelial cells, causing hemorrhagic colitis and leading to severe diarrhea and potential kidney damage. In some cases, the damage to the kidneys can be so severe that it results in acute renal failure, a life-threatening condition known as a hemolytic uremic syndrome.
5. Enteroaggregative E. coli (EAEC): This strain of E. coli produces both acute and chronic diarrhea, with the duration of symptoms sometimes lasting up to 14 days. This pathogenic strain can adhere to the epithelial cells of the intestinal tract, where it produces a potent heat-stable exotoxin and hemolysin.

Bacillary Dysentery

Shigellosis, also known as bacillary dysentery, is a prevalent gastrointestinal disease in underdeveloped nations with inadequate hygiene practices. Shigellosis is caused by one of four Shigella species, namely Shigella dysenteriae, S. boydii, S. sonnei, and S. flexneri. These gram-negative, nonmotile, short rod-shaped bacteria are typically spread through the consumption of contaminated food or water. Upon entering the intestinal tract, they infiltrate the large intestine's mucosa and invade the epithelial cells, causing extensive damage that leads to severe inflammation and ulceration. Symptoms of the disease include fever, abdominal cramps, and frequent, intense diarrhea with blood and mucus present in the stool. S. dysenteriae is particularly dangerous as it produces a potent exotoxin known as Shiga toxin, which can cause more severe illness compared to other members of the genus.

Cholera

Cholera is a disease that frequently afflicts individuals living in developing countries. The disease is caused by the bacterium Vibrio cholera, a Gram-negative, comma-shaped, curved bacillus that actively moves with a polar flagellum. Vibrios are transmitted from one individual to another through contaminated food and water. In the intestinal tract, the bacteria attach to the brush border of epithelial cells' microvilli, where they multiply and produce heat-labile enterotoxin and endotoxin. Cholera toxin affects the intestinal mucosal lining, leading to altered permeability and prolonged hypersecretion of water and electrolytes. This results in severe, copious diarrhea that includes mucus, epithelial cells, and a large number of vibrios, resembling “rice water”. The significant loss of water and electrolytes in stool causes dehydration, shock, anuria, acidosis, and circulatory collapse, and excessive fluid and electrolyte loss may result in death.

Giardiasis

Giardiasis is a type of dysentery caused by the protozoan parasite Giardia lamblia. The parasite exists in two forms: trophozoite and cyst. The trophozoite is the vegetative form that is often attached to the epithelium in the small intestine. The cyst is the resistant form found in the large intestine and the formed stool. The parasite can infect directly via contaminated food or water in the same host or a different individual, and person-to-person transmission is also common. After ingestion, the cysts release two trophozoites in the small intestine, which play a crucial role in the clinical manifestation of the disease.

Giardiasis often presents as an asymptomatic infection. However, in cases where symptoms do occur, they typically appear after an incubation period of 7 to 10 days. The symptoms of giardiasis can include abdominal pain, abdominal cramps, chronic diarrhea, loss of appetite, weight loss, and in severe cases, jaundice. The stool of an infected individual may have a foul smell.

Amoebic Dysentery

Amoebic dysentery is a persistent disease resulting from the protozoan parasite Entamoeba histolytica. This parasitic amoeba occurs in two morphological forms, namely the trophozoite, which is found in uniformed stools or tissues, and the cyst, which is found in formed stools. Like G. lamblia, this parasite is transmitted via water, food, cockroaches, flies, or contact with objects contaminated with feces. The amoebas convert to trophozoites in the small intestine and then migrate to the large intestine, which is the primary site of infection.

Amoebic dysentery has an incubation period ranging from one to four months. During this time, trophozoites multiply slowly by binary fission and invade the intestinal epithelium, leading to ulceration. Symptoms gradually appear as episodes of diarrhea, nausea, abdominal cramps, vomiting, and malaise. Intestinal perforation, localized or generalized peritonitis, amoebic hepatitis, pericarditis, and abscesses in the lungs, brain, skin, and other organs are the most prevalent complications. Some of the trophozoites in the large intestine convert to the cyst form and are excreted in the stool, which can cause a fresh infection in a new host.

Hepatitis A

Hepatitis A is caused by the hepatitis A virus (HAV), which is a type of unenveloped icosahedral virus with a linear single-stranded RNA genome. HAV exhibits remarkable stability when exposed to heat and acid; it can remain stable at 60°C for 1 hour and pH 1.0 for 2 hours. Additionally, it can withstand 180°C in a hot-air oven for 1 hour. However, boiling water for 5 minutes is enough to destroy HAV. HAV can be inactivated by 10-15 ppm of chlorine in 30 minutes.

Hepatitis A Virus (HAV) is excreted in the feces of infected individuals. The primary mode of transmission is through the fecal-oral route, particularly in areas with inadequate sanitation. While water contaminated with feces is the leading cause of infection, there have been several documented cases of foodborne outbreaks.

Hepatitis A has an incubation period of 10-15 days on average, and up to 25-30 days. Symptoms can range from no symptoms at all to severe hepatitis which may lead to death. The disease is characterized by a sudden onset of fatigue, fever, nausea, and vomiting, accompanied by severe pain in the right upper abdomen. Yellowish skin and whites of the eyes (jaundice) follow, along with an enlarged and tender liver. Most patients recover within 1-2 weeks as the disease is self-limiting. Vaccination against nonreplicative hepatitis A is now available and recommended for people of all ages.

WATER PURIFICATION METHODS

Artificial water purification serves the primary goal of eliminating large pollutants and eliminating harmful bacteria and microorganisms. This is accomplished by implementing a combination of physical and chemical methods, including sedimentation and filtration, along with chemical processes to clarify and soften the water. Chlorination is also used to disinfect the water and eliminate any remaining harmful bacteria.

SEDIMENTATION

Sedimentation is a process that aims to remove larger contaminants and microorganisms that may be attached to their surfaces from water. This is achieved by holding water in settling basins for several hours, allowing the large particles to settle out. To make the process more effective, flocculants, which are chemicals that cause particles to clump together, are added to the water. When alum or ferric sulfate is added to alkaline water, it forms jelly-like masses called floccules. This process is called flocculation. The insoluble flocculants settle to the bottom of the sedimentation basin and are removed mechanically along with the sediments.

FILTRATION

Filtration is a crucial process in water purification, as it removes any remaining suspended particles in the water after sedimentation. There are two types of sand filters used for water filtration: slow sand filters and rapid sand filters. Slow sand filters have a thick layer of sand on top of a gravel bed, and water is forced through the sand at a slow rate, allowing for the biological filtration of pathogens. Rapid sand filters, on the other hand, have a thinner layer of sand and use a faster rate of water flow to achieve physical filtration of particles. Both types of filters are effective in removing impurities from water.

(A) Slow Sand Filter

A slow sand filter typically consists of a large tank with an area of one or more acres. The bottom of the tank contains a network of drain pipes that transport the filtered water outside the filter bed. A layer of sand and gravel is placed over the drain pipes, starting with coarse gravel with an average size of 5 cm at the bottom and graduating in size to fine sand with an average size of 0.25 to 0.35 mm. The water is poured onto the surface of the filter bed and allowed to trickle slowly through the sand particles. The name “slow sand filter” comes from the fact that the water passes very slowly through the layers of sand and gravel.

(B) Rapid Sand Filter

The rapid sand filter utilizes the same fundamental principles as the slow sand filter, but there are two primary distinctions: first, the filter bed requires a smaller area, and second, a schmutzdecke does not develop during filtration. The filter is constructed with a water-resistant floor, featuring grooves and tiles to drain the filtered water. A 12 to 18-inch thick layer of gravel is laid on top of the filter, over which a layer of sand (0.35 to 0.55 mm in contrast to 0.25 to 0.35 mm for the slow sand filter) is applied. The gravel at the bottom of the filter is sized to pass through a 2-inch screen mesh, and as the surface of the layer where the gravel will pass through a 1/16-inch mesh is approached, the size of the gravel gradually decreases. The sand layer has a thickness of approximately 2.5 feet.

CHLORINATION

Chlorination is a method of disinfecting drinking water by introducing gaseous chlorine to eliminate non-sporing microorganisms, particularly harmful bacteria that may have survived the filtration process. While it may not eliminate all types of microorganisms, it significantly reduces their populations to a safe level. The use of chlorine in water treatment has saved countless lives by preventing waterborne illnesses such as typhoid and cholera. It is widely used globally as a cost-effective disinfectant for water purification, with the added benefit of being safe for human tissues even at low concentrations. Moreover, chlorine also eliminates unpleasant odors and tastes that may remain in the water after treatment.

Methods of Chlorination

There are three methods of chlorination used for water treatment. The first method is simple chlorination, where a predetermined amount of chlorine is added to the water. When chlorine gas reacts with water, it produces hypochlorous acid (HClO), which is responsible for the antimicrobial action of chlorine.

Cl₂ + H₂O → HCl + HClO

Hypochlorous acid is an unstable compound that frequently undergoes further changes, releasing nascent oxygen.

HClO → HCl + O

Cellular components, such as proteins, can be affected by nascent oxygen, which is a powerful oxidizing agent. This interaction can cause the denaturation of cell enzymes, ultimately leading to the inhibition of metabolic activities.

Chloramines are a type of organic compound that is formed by mixing ammonia and chlorine. These compounds replace one or more hydrogen atoms in ammonia (NH₃) or the ammonia group (^–NH₂) with chlorine atoms. Examples of chloramines include monochloramine, chloramines-T, and azochloramide. Although they are less effective as germicidal agents compared to free chlorine, they are more stable. When added to water, chloramines undergo hydrolysis, producing hypochlorous acid, which is then transformed into nascent oxygen and hydrochloric acid.

Superchlorination is a method in which a significantly higher amount of chlorine is added to the water compared to simple chlorination, and then the excess chlorine is removed.

MICROBIOLOGY OF WASTEWATER

Wastewater is a hazardous and unwanted substance that can spread diseases and contaminate clean water resources. As urban populations continue to grow, the amount of domestic sewage being discharged has also increased exponentially.

CLASSES OF WASTEWATER

There are two major types of wastewater: (a) domestic wastewater and (b) industrial wastewater.

(A) DOMESTIC WASTEWATER

Wastewater that originates from household activities and contains human waste is commonly referred to as domestic wastewater. This type of wastewater is typically disposed of through household drains and eventually makes its way to the sewers of a city. Domestic wastewater poses a significant health risk due to the potential presence of pathogenic bacteria, protozoa, and viruses. It is largely composed of water, with solids accounting for less than 1% of its overall composition. The solid portion primarily consists of household garbage, such as tin containers, wooden materials, bottles, papers, tissue papers, cardboard, and other similar materials. The presence of such materials complicates the process of wastewater treatment. Additionally, detergents in domestic wastewater can interfere with certain biological reactions involved in the treatment process, making their removal a challenging task. Another crucial component of domestic wastewater is human or animal excreta, which is a major source of microorganisms, both pathogenic and nonpathogenic.

(B) INDUSTRIAL WASTEWATER

Chemical discharges originating from factories are commonly known as industrial wastes. While industrial waste is generally not a significant contributor to pathogenic organisms, it often contains a relatively high concentration of organic matter that can be decomposed by microorganisms. This decomposable matter is commonly referred to as putrescible material, and its decomposability is primarily due to a high content of sugars, proteins, lipids, and other organic compounds. The chemical composition of industrial waste, like domestic waste, varies significantly between different industries, communities, and over time. It's worth noting that the amount of waste produced by a single factory can often be equivalent in terms of decomposable matter to that produced by a relatively large city. For example, a single oil refinery can discharge as much putrescible material into the sewer system as an entire city like Karachi.

Furthermore, industrial operations generate a substantial amount of wastewater. The production of one ton of rayon, for instance, requires the use of approximately 250,000 gallons of water. Similarly, the leather, paper, and textile industries consume millions of gallons of water to produce their products. Additionally, for every gallon of gasoline produced, approximately 1000 gallons of water are used.

MICROORGANISMS IN WASTEWATER

The physical conditions of wastewater provide an ideal environment for the growth and reproduction of microorganisms. The pH levels in wastewater typically range from 6.8 to 8.5, with an average of 7.0. The temperature of wastewater fluctuates with seasonal changes and can range from 5°C to 30°C. Additionally, the nutritional conditions in wastewater are highly favorable for microorganisms. Consequently, wastewater supports the growth of a wide range of microorganisms, including bacteria, fungi, algae, protozoa, and viruses. Among these microorganisms, bacteria are the most dominant group, with millions of bacteria per milliliter of wastewater. The bacterial population in wastewater includes bacilli, cocci, spiral, spore-formers and non-spore-formers, Gram-positive and Gram-negative, as well as both pathogenic and non-pathogenic species.

Most of the bacteria present in wastewater come from the human and animal intestinal tract, while others may enter the wastewater from surface, ground, and atmospheric water or from industrial wastes. The number of coliform bacteria, which are commonly found in the intestinal tract and discharged in fecal material in large quantities, outnumber enteric pathogens. Microbes in wastewater are metabolically active and effectively participate in the degradation of various chemical complexes in the wastewater.

MAJOR PROBLEMS WITH WASTEWATER

Although domestic and industrial wastewater may differ in the number of pathogenic microbes they contain, both types of wastewater present a significant problem due to the substantial amounts of organic matter they contain. The organic matter in wastewater is readily attacked by microorganisms, leading to various environmental and health issues. The biological activities in wastewater are the primary cause of these problems.

• Foul Odor: Wastewater can be likened to a crude culture medium where a highly diverse and active population of microorganisms thrive. These microorganisms aggressively attack a variety of organic compounds and break them down into different products. Unfortunately, this process also produces malodorous substances such as hydrogen sulfide, leading to unpleasant smells.
• Objectionable taste and colors: The degradation products resulting from the activity of microorganisms on chemical compounds often have unpleasant tastes and colors. These undesirable tastes and colors can manifest in the water that carries the wastewater.
• Spoilage of shoreline: As organic matter undergoes microbial decomposition, partially decomposed organic solids may accumulate along the stream or shoreline, forming filthy gray-black sludge bars that emit large quantities of methane gas. Swimming beaches may be adversely affected by the accumulation of greasy substances and partially decomposed chemical matter, leading to their deterioration.
• The killing of marine life: Discharging wastewater into natural bodies of water can have severe consequences for marine life. The toxic chemicals and byproducts resulting from microbial degradation can cause significant harm to fish and vegetation, resulting in substantial economic losses.
• Damage to buildings: The chemicals and byproducts produced through microbial activities in wastewater can be highly damaging to buildings and concrete structures. They can lead to extensive corrosion and gradual deterioration of such structures. Metallic structures can also be destroyed, and contact with hydrogen sulfide can cause paint to chip and peel.
• Dissemination of diseases: Due to the presence of enteric pathogens, wastewater is a major source of disease-causing microorganisms. This can pose a significant risk to human health, as the water can become highly hazardous for drinking, while swimming and boating can become health hazards.
• Depletion of oxygen: Freshwater typically contains 8 to 12 parts per million (ppm) of dissolved oxygen. However, when wastewater is introduced into a freshwater stream, the readily decomposable organic matter is quickly consumed by microorganisms, leading to a depletion of dissolved oxygen. When the oxygen levels drop to around 4 ppm, aquatic life begins to die off. Further reductions in oxygen give rise to stale wastewater, which takes on a milky appearance. Once all the oxygen is consumed, the wastewater becomes septic, turns dirty gray in color, and emits a foul odor. During this process, microorganisms break down carbohydrates to produce carbon dioxide and water, while proteins yield ammonia, nitrates, sulfates, and phosphorous. These molecules are relatively stable, and as the decomposition of putrescible materials comes to an end, the water will eventually return to its original state concerning oxygen content.

BIOLOGICAL OXYGEN DEMAND

The degradation and stabilization of unstable organic substances by the aerobic activities of bacteria and microbes result in the consumption of oxygen, which is referred to as Biological Oxygen Demand (BOD). During the process, aerobic bacteria oxidize organic and oxidizable inorganic substances and consume oxygen. A high concentration of unstable complexes in wastewater leads to the rapid decomposition of these substances and a subsequent increase in BOD. Conversely, a decrease in the number of unstable complexes reduces BOD.

BOD is a reliable indicator of the efficacy of wastewater treatment methods. A method is deemed efficient if it reduces BOD levels rapidly. Therefore, a stable effluent does not deplete oxygen levels when released into a water body.

CHEMICAL OXYGEN DEMAND

Chemical oxygen demand (COD) is a measure of the amount of organic and inorganic substances in a sample of water that can be oxidized by a strong chemical oxidizing agent, such as potassium dichromate, under specific conditions.

COD is often used as an indicator of water quality and can be used to estimate the level of pollution in water bodies. It is an important parameter for wastewater treatment plants and other industries that discharge their effluent into the environment.

The COD test involves adding a measured amount of the oxidizing agent to a sample of water and then heating it under specific conditions. The amount of oxidizing agent consumed is then measured, which is proportional to the amount of organic and inorganic substances in the water sample. The COD value is expressed in milligrams of oxygen consumed per liter of water (mg/L).

COD is different from biochemical oxygen demand (BOD), which measures the amount of oxygen consumed by microorganisms as they break down organic matter in water over some time. While BOD is a measure of the potential impact of organic matter on the environment, COD is a measure of the actual amount of organic matter present in the water sample.

WASTEWATER DISPOSAL METHODS

It is now widely accepted that wastewater must be treated effectively before being discharged into receiving bodies of water. Wastewater can be disposed of with or without treatment.

DISPOSAL OF WASTEWATER BY TREATMENT METHODS

Various methods are currently available to eliminate microorganisms and stabilize putrescible organic and inorganic chemicals in wastewater, collectively known as wastewater treatment methods. Wastewater treatment plants utilize a combination of physical, chemical, and microbiological techniques to address the diverse challenges posed by wastewater.

Once it arrives at the treatment plant, the wastewater undergoes four distinct types of treatment, each serving a specific purpose. Each treatment process aims to remove different types of materials and reduce the microbial load in the wastewater.

(A) Primary Treatment

The primary objective of wastewater treatment is to either reduce the concentration of solids through sedimentation or eliminate pathogenic bacteria through chlorination, or both. Initially, bar screens are installed to intercept large objects that could enter the sewer system. While screens with smaller mesh sizes are available to remove even finer particles, the collected debris is typically ground up and reintroduced into the wastewater stream.

After passing through the bar screens, the wastewater proceeds to a series of large primary settling chambers. These chambers are designed to remove most of the dense inorganic particles and organic matter present in the wastewater. Typically, there are two types of chambers: a grit chamber and a sedimentation tank (also known as a quiescent settling chamber).

In the grit chamber, the wastewater flows slowly, allowing large, heavy particulate matter or grit to settle out. This is followed by the sedimentation tank, where the water remains for a period of 1 to 3 hours, during which most of the suspended organic matter settles to the bottom. The sedimented material forms a semisolid mass called sludge.

To improve the efficiency of sludge formation, various chemicals can be added to the wastewater to coagulate the suspended particles, which will then settle more rapidly. This process can significantly enhance the performance of the sedimentation tank.

The sedimentation tanks are fitted with scraper mechanisms that slowly move the settled sludge toward a collection hopper. This sludge is considered a byproduct of the treatment process and is typically discarded. The remaining liquid portion of the wastewater, which exits the sedimentation tank, is referred to as effluent.

It's important to note that sedimentation tanks are designed to separate particulate matter based on differences in density or specific gravity between the particles and the suspended fluid. Additionally, some tanks may function as flotation tanks for materials with a specific gravity lower than that of the wastewater, such as grease. Surface-skimming devices are used to collect this material, which adds to the number of waste products generated during treatment but enhances treatment efficiency.

In certain regions, a two-compartment tank, known as the Imhoff Tank, is utilized for primary treatment. First developed by Karl Imhoff in 1907, the tank comprises an upper settling basin with a long ‘V’-shaped trough that receives fresh wastewater and a lower sludge digestion tank. The wastewater enters the upper compartment, where the sludge settles to the bottom and the effluent slowly flows along the trough.

A septic tank, similar to the Imhoff tank, allows sewage to flow through it slowly, depositing a portion of its solids during the process. The remaining sludge undergoes anaerobic decomposition.

The efficiency of the septic tank depends on several factors, including the type of wastewater, the amount of particulate matter, the depth of the tank, and the retention time. Primary treatment can remove between 40 to 85% of suspended particles, 25 to 50% of organic matter, and 30% of BOD from domestic wastewater. Most industrial wastes do not contain significant amounts of settleable solids, and primary treatment does not apply to them.

(B) Secondary Treatment

After primary treatment, wastewater may still require further treatment to ensure safety and prevent stream pollution. Additionally, the settled sludge still contains a significant amount of organic matter and needs further stabilization. The objective of secondary treatment is to remove all soluble and deposited organic matter from the effluent and sludge. There are several common methods used for secondary wastewater treatment. The following are given common methods employed for secondary wastewater treatment.

Sludge Digestion

Environmental pollution control technologists face a costly and challenging issue when it comes to disposing of sludge. The main objectives of sludge treatment include reducing the sludge volume, which requires the removal of water that makes up 97 to 98% of the sludge, reducing the volatile organic content, and disposing of the residue safely. Sludge digestion involves pumping it into specialized digesters designed for this purpose. These large digesters can hold thousands of gallons of sludge. Mechanical thickening is used to reduce the water content of the sludge by slowly rotating rakes through it. This process promotes the flocculation of sludge particles, allowing for further compaction and decanting of the released water. Sludge filtration can also remove water from the sludge. Before filtration, sludge may undergo heat treatment to coagulate the particles or the addition of coagulating agents like ferric chloride and lime.

Once the sludge has been concentrated, it can undergo biological decay or anaerobic digestion. Anaerobic bacteria are responsible for converting the organic matter into various soluble organic compounds, such as low-molecular-weight fatty acids and methane, in the absence of oxygen. This process is relatively slow and can take anywhere from 2 to 4 weeks. However, during this period, anaerobic degradation can remove up to 65-75% of the total organic matter present in the sludge.

Once digestion is complete, 75% of the digested sludge is removed from the digestor for final disposal. Fresh sludge is continuously added to the remaining sludge, which helps to maintain a constant rate of digestion by adding a metabolically active bacterial population to the incoming sludge. The digested sludge has a thick consistency and little or no unpleasant odor. It is then dehydrated by pumping it onto drying beds and can either be used as fertilizer or disposed of by dumping it into a sanitary landfill.

Activated Sludge Process (Aerobic Decomposition)

The activated sludge process was first developed in the USA in 1912 and is a powerful method of using oxygen for the aerobic breakdown of sludge. This technique is suitable for treating both raw sewage and effluent from primary treatment. The process involves pumping the effluent into large open aeration tanks where a significant amount of air under pressure is intermittently blown through the effluent from vents at the bottom of the tanks. The aeration typically lasts for 4 to 8 hours, during which time most of the odor is eliminated, and the putrescible material is actively aerobically decomposed. The fine particles in the effluent clump together to form flocs or masses that quickly settle out when the effluent is transferred to a final sedimentation tank after aeration. The process by which flocs are formed is known as flocculation. The flocs contain a vast number of aerobic bacteria, as well as a small number of protozoa, fungi, and algae. These microorganisms are close to organic matter, resulting in their aerobic decomposition. The sedimented sludge is referred to as activated sludge since it contains a high concentration of metabolically active bacteria and other microorganisms.

The activated sludge process requires filling an aeration tank with activated sludge, which constitutes around one-third of the tank's volume, and then pumping the effluent into it. This introduction of activated sludge acts as an inoculant, initiating the process of sludge digestion. The flocs of sludge are then dehydrated and disposed of in a sanitary landfill. The effluent undergoes chlorination before being discharged into a watercourse.

Trickling Filter

The trickling filter is a process that utilizes aerobic conditions to facilitate microbial decomposition, resulting in a significant reduction of the organic content in the effluent. The filter consists of a large bed filled with coarse materials such as stone, slag, cinders, gravel, or sand, which creates ample space for air penetration. The bed is typically about 6 feet deep and may be circular with a diameter of approximately 100 feet.

To aerobically decompose the organic content of the effluent, the trickling filter sprays the effluent over a large bed of coarse pieces of stone, slag, cinders, gravel, or sand. The filter bed, about 6 feet deep and often circular with a diameter of about 100 feet, has many open spaces allowing air penetration. The effluent is either sprayed over the filter bed from fixed or rotating metal pipes with holes along them, which oxygenates the water. As the oxygenated effluent trickles over the surface, it percolates down through the filter bed, allowing the aerobic bacteria and other microbes to decompose the organic matter into carbon dioxide and water. The finely divided material of the filter bed surface forms a thick slimy biofilm called zoogloea, which serves as a site for bacteria to actively degrade the organic substances. To maintain aerobic conditions, the effluent is sprayed for a few minutes and then turned off. Saturation of the filter bed with effluent is avoided.

(C) Tertiary Treatment

Tertiary treatment, also known as an advanced treatment, is a sewage treatment process that aims to remove organic substances, phosphorous, and nitrogen compounds that were not removed during secondary treatment. This process involves the use of chemical methods, such as adding calcium carbonate, to create a chemical reaction that forms precipitates of undecomposed substances. These precipitates can then be collected and removed, effectively reducing the number of harmful substances present in the effluent. The primary goal of tertiary treatment is to improve the quality of the treated water before it is released into the environment.

In addition to the use of chemicals, tertiary treatment of sewage also involves the application of activated charcoal to remove synthetic organic substances that may be present in the primary or secondary effluent. The process involves the passage of the effluent through large silos filled with activated charcoal. During percolation, the synthetic organic substances are adsorbed onto the surface of the charcoal particles. Periodically, the charcoal particles are heated to high temperatures to remove the adsorbed substances and cleanse the particles, making them suitable for reuse in the process. This method is effective in removing a wide range of pollutants and can significantly improve the quality of treated effluent.

To ensure that the effluent is safe for disposal, it is crucial to eliminate any pathogenic bacteria that may pose a risk to human health. Chlorination is the commonly used method to remove these bacteria from the effluent. The amount of chlorine required for this purpose is determined based on the level of coliform MPN present in the effluent, and the goal is to reduce it to 100 per 100 ml. To achieve this, the effluent is treated with 0.5 ppm chlorine, which is allowed to remain in the effluent for 15 minutes. After chlorination, the effluent is dechlorinated before being discharged into natural water bodies or land.

DISPOSAL OF A SMALL QUANTITY OF WASTEWATER

Disposal of wastewater in rural areas and some parts of urban areas often does not require treatment methods due to the small quantities involved. This wastewater is typically drained from individual families, shopping centers, or industrial units. Below are the disposal methods for such a small amount of wastewater.

Septic Tank

A septic tank is a common method for treating small quantities of wastewater in rural and some urban areas. It consists of an underground concrete tank located near the dwelling or unit. Wastewater flows into the tank where it undergoes two operations. Firstly, heavy solid materials settle at the bottom and undergo anaerobic degradation by bacteria. Organic acids, hydrogen sulfide, and other soluble end products move into the effluent. The effluent is then drained continuously through an outlet drainage pipe and taken to an absorption field where aerobic bacteria further degrade the organic substances.

The sediment is periodically extracted from the septic tank every four to five years, dried on sand beds, and either incinerated or deposited in landfills. As septic tanks do not employ any chemical disinfectant, there is no guarantee of effective removal of pathogens. Thus, it is essential to prevent the effluent from seeping into the drinking water supply.

Lagoonization

Lagoonization is a cost-effective and straightforward method for improving the quality of wastewater before its discharge into a water body. The process involves depositing the wastewater into a large lagoon or pond where it undergoes a natural purification process for over 30 days. This method is very simple and requires minimal equipment and expenses to purify a large amount of wastewater. Bacterial action mineralizes or stabilizes virtually all of the putrescible material in the wastewater by the end of the lagoonization period. In some cases, the addition of inorganic nitrogenous fertilizer to the water may encourage the heavy growth of algae, but it is often unnecessary.

The presence of algae plays a crucial role in the natural purification process during lagoonization. The process of active photosynthesis by algae results in the production of significant amounts of oxygen, which is dissolved in the water. This increased oxygen content greatly aids in the decomposition of organic matter by aerobic bacteria. Additionally, the growing algae absorb a substantial amount of nitrates, sulfates, and phosphates in their cells, leading to a reduction in the mineral salt content of the wastewater. However, one significant drawback of this method is the need to remove the algae before they die, as the dead algae can become putrescible material. To address this issue, aerated lagoons equipped with an aerating device, known as the aerator, are now used. The aerator pumps air into the pond, agitating and aerating the water and encouraging the growth of aerobic bacteria to decompose organic materials.