Physical, chemical and Biological characteristics of sewage

Sewage, also called wastewater, is the used water that comes from homes, businesses, and industries. It contains a wide variety of substances, including human waste, food scraps, oils, soaps, chemicals, and anything else that goes down drains. To manage and treat wastewater properly, it’s important to understand its characteristics. These characteristics are typically divided into three categories: physical, chemical, and biological.

In this simplified and easy-to-understand explanation, we’ll explore each category in detail, focusing on what makes wastewater challenging to treat and why understanding these traits is important for public health and environmental protection.

Physical Characteristics of Sewage

The physical characteristics of wastewater are those that you can observe or measure without using chemicals. These include factors like appearance, temperature, solids content, and odor.

a) Appearance and Color

Fresh sewage is usually a grayish or light brown color. As it ages and undergoes changes (like decomposition of organic matter), it can turn darker, becoming more of a blackish or greenish color. The color gives a basic indication of how long the wastewater has been standing or how much organic material it contains.

The appearance of sewage also depends on its source. For example, wastewater from households may look different from sewage coming from factories due to the types of wastes they contain.

b) Odor

Sewage typically has an unpleasant odor due to the decomposition of organic materials, like food waste and human waste. The more decayed the wastewater, the stronger and worse the smell becomes. The presence of gases like hydrogen sulfide (which smells like rotten eggs) can make the smell even more pungent.

Odor is not just unpleasant; it also indicates the presence of harmful substances, including bacteria and gases that could affect human health if not properly managed.

c) Solids Content

Sewage contains both suspended solids and dissolved solids:

Suspended solids are particles that float in the water and can be seen, like bits of food, paper, or other debris. These solids are often removed during the primary stages of wastewater treatment through processes like sedimentation, where the heavy particles sink to the bottom.

Dissolved solids are particles that have dissolved in the water, such as salts and organic matter, which are not visible to the naked eye. These are typically removed during later stages of treatment, often through chemical processes or filtration.

d) Temperature

The temperature of sewage varies depending on the environment and the type of wastewater. Warmer sewage is often seen in warmer climates or industries, while cooler sewage might come from areas with cold water usage. Higher temperatures can speed up the breakdown of organic matter, but they can also encourage the growth of bacteria and other microorganisms, making the sewage more challenging to treat.

e) Turbidity

Turbidity refers to how clear or cloudy the water is. Cloudiness is caused by suspended particles in the sewage, like dirt, organic matter, and microorganisms. The more turbid the sewage, the more difficult it is to treat, as those particles need to be filtered out before the water can be safely released back into the environment.

Chemical Characteristics of Sewage

The chemical characteristics of sewage involve the substances that are dissolved in the wastewater. These include organic and inorganic chemicals, nutrients, and gases.

a) pH (Acidity or Alkalinity)

The pH of sewage measures how acidic or alkaline (basic) it is. Most sewage tends to have a pH between 6.5 and 8.5, making it either neutral or slightly alkaline. However, industrial wastewater can sometimes be highly acidic or highly alkaline, depending on the chemicals used in the process.

Maintaining the proper pH level is crucial in sewage treatment because too high or too low pH can interfere with the treatment processes and harm the bacteria that break down organic matter.

b) Organic Matter (Biochemical Oxygen Demand – BOD)

Organic matter in sewage mainly comes from human waste, food particles, and natural debris. The amount of organic matter is often measured using Biochemical Oxygen Demand (BOD), which is the amount of oxygen needed by microorganisms to break down the organic material in the water.

Higher BOD levels mean the sewage is loaded with organic material, which requires more oxygen for decomposition. If untreated sewage with a high BOD is released into a body of water, it can deplete oxygen levels, harming fish and other aquatic life.

c) Chemical Oxygen Demand (COD)

Chemical Oxygen Demand (COD) is similar to BOD but measures all organic compounds, including those that microorganisms cannot break down easily. It gives a broader picture of the pollution level in sewage.

Like BOD, higher COD levels indicate more pollution in the wastewater, meaning more treatment will be needed before the sewage can be safely released.

d) Nutrients (Nitrogen and Phosphorus)

Sewage contains important nutrients like nitrogen and phosphorus. These come from human waste, detergents, and other household chemicals. While these nutrients are essential for plant growth, too much of them can cause problems when they are released into rivers or lakes.

Excess nutrients can lead to eutrophication, a condition where water bodies become overloaded with nutrients, causing excessive growth of algae. This can deplete oxygen levels in the water, leading to the death of fish and other aquatic organisms.

e) Toxic Chemicals

Sewage can contain a variety of toxic chemicals, especially if it comes from industrial sources. These chemicals might include heavy metals (like lead, mercury, or cadmium), pesticides, pharmaceuticals, and other harmful substances.

Toxic chemicals are dangerous because they can harm both the environment and human health if they are not properly removed during treatment. These chemicals often require specialized processes to remove them from the wastewater.

f) Chlorides and Sulfates

Chlorides (from salts) and sulfates (from soaps and detergents) are common inorganic chemicals in sewage. While they are not harmful in small amounts, high levels can disrupt the sewage treatment process and harm the environment.

Chlorides, in particular, can make treated water unsafe for irrigation, while sulfates can contribute to the production of hydrogen sulfide gas, which smells like rotten eggs and is toxic at high levels.

Biological Characteristics of Sewage

The biological characteristics of sewage refer to the living organisms present in the wastewater, such as bacteria, viruses, and parasites. Some of these microorganisms are beneficial for breaking down waste, while others can cause diseases if not properly treated.

a) Pathogens

Pathogens are harmful microorganisms that can cause diseases in humans and animals. These include bacteria, viruses, parasites, and fungi. Sewage is a significant source of pathogens, especially if it contains human waste. Common diseases associated with pathogens in sewage include cholera, typhoid, dysentery, and hepatitis.

Treating sewage to remove or kill these pathogens is one of the most critical steps in wastewater treatment, ensuring that the water is safe for the environment and public health.

b) Beneficial Microorganisms

Not all microorganisms in sewage are harmful. Many bacteria, for example, play a crucial role in breaking down organic matter. In sewage treatment plants, these microorganisms are encouraged to grow and thrive, as they help decompose waste.

Aerobic bacteria require oxygen to survive and are used in treatment processes like activated sludge, where they break down organic matter in the presence of oxygen.

Anaerobic bacteria do not need oxygen and are used in processes like sludge digestion, where they break down organic material in the absence of oxygen, producing methane gas as a byproduct.

c) Indicator Organisms

Sometimes, instead of testing directly for pathogens, treatment plants look for indicator organisms like E. coli. These bacteria are commonly found in human waste and are used as a signal that pathogens might be present.

If large numbers of E. coli are found in sewage, it suggests that the wastewater has been contaminated with fecal matter and may contain harmful pathogens.

d) Biomass and Sludge

During the treatment process, the biological activity of microorganisms generates biomass, which is a combination of organic matter and microbial cells. This biomass is often referred to as sludge, and managing this sludge is a major part of sewage treatment.

Sludge can be treated further to reduce its volume, remove harmful organisms, and even produce useful byproducts like biogas (methane) for energy.

Importance of Understanding Sewage Characteristics

Understanding the physical, chemical, and biological characteristics of sewage is crucial for designing and operating effective wastewater treatment systems. By knowing what’s in the sewage, engineers can create processes that remove harmful substances, break down organic matter, and return clean water to the environment.

If sewage isn’t properly treated, it can lead to significant environmental problems, like water pollution, fish kills, and the spread of diseases. That’s why it’s so important to treat sewage before releasing it into rivers, lakes, or oceans.

Conclusion

Sewage is a complex mix of physical, chemical, and biological components. Its characteristics vary depending on where it comes from—households, industries, or businesses. Properly understanding these characteristics helps in the design and operation of sewage treatment systems, ensuring that wastewater can be cleaned and returned safely to the environment.

The physical characteristics include factors like color, odor, and solids, while the chemical characteristics involve substances like organic matter, nutrients, and toxic chemicals.

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