the 1st stage of sewage treatment where large debris, such as sticks, grit, and rags pas through a screen followed by a grit chamber, where the flow of water is slowed enough to allow for sedimentation.

Primary treatment in sewage treatment involves physical removal of particles (large and small) from the sewage through filtration and sedimentation. Initially floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation.

There are three main stages of the wastewater treatment process, aptly known as primary, secondary and tertiary water treatment.

It is sludge formed during primary sewage treatment. It is sludge formed during secondary sewage treatment. A lot of decomposition occurs during the formation of primary sludge. Very little decomposition occurs during the formation of activated sludge.

sewage treatment

Treatment Steps

• Step 1: Screening and Pumping.
• Step 2: Grit Removal.
• Step 3: Primary Settling.
• Step 4: Aeration / Activated Sludge.
• Step 5: Secondary Settling.
• Step 6: Filtration.
• Step 7: Disinfection.
• Step 8: Oxygen Uptake.

There are two types of sewage: treated and untreated. Treated sewage: Treated sewage refers to wastewater or sewage which has passed through a treatment plant. Sewage goes through several stages in the treatment process ensuring that all harmful bacteria, pollutants and contaminants are eliminated.

Four common ways to treat wastewater include physical water treatment, biological water treatment, chemical treatment, and sludge treatment. Let us learn about these processes in detail. In this stage, physical methods are used for cleaning the wastewater.

Types of Sewage Treatment Plants

• Activated sludge plant (ASP)
• Rotating disc system.
• Submerged aerated filter (SAF)
• Suspended Media Filters (SMF)
• Sequencing batch reactor (SBR)
• Non-electric filter.
• Trickling filter.

When wastewater arrives at the treatment plant, it contains many solids that cannot be removed by the wastewater treatment process. This can include rags, paper, wood, food particles, egg shells, plastic, and even toys and money.

Coagulation, flocculation, sedimentation – elimination of turbidity and colloid substances. Filtration – mechanical filters to remove rough particles, sand filters.

These include: (1) Collection ; (2) Screening and Straining ; (3) Chemical Addition ; (4) Coagulation and Flocculation ; (5) Sedimentation and Clarification ; (6) Filtration ; (7) Disinfection ; (8) Storage ; (9) and finally Distribution.

Primary treatment removes material that will either float or readily settle out by gravity. It includes the physical processes of screening, comminution, grit removal, and sedimentation.

Carbon filtering removes remaining contaminants and impurities by chemical absorption onto activated carbon. Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.

Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater (or treated effluent) that is safe enough for release into the environment. A by-product of sewage treatment is a semi-solid waste or slurry, called sewage sludge.

New research shows that wastewater treatment plants that employ a combination of purifying techniques followed by reverse osmosis – a process by which water is forced through a barrier that only water can pass – do a good job of removing chemicals that may elicit health effects.

Bacillus is an excellent treatment of bacteria in wastewater but is best suited for treating fats, oils, greases, and proteins. That is why they are primarily used in wastewater treatment plants.

The main role of these bacteria in sewage treatment is to reduce the volume of sludge and produce methane gas from it. Unlike aerobic bacteria, this type of bacteria is able to get more than enough oxygen from its food source and will not require adding oxygen to help do its job.

Furthermore, anaerobic bacteria are an important element in the wastewater treatment processes. They are responsible for methane fermentation of sewage sludge, facilitating decomposition of macromolecular organic matter into simpler compounds.

Protozoa play a critical role in the treatment process by removing and digesting free swimming dispersed bacteria and other suspended particles. This improves the clarity of the wastewater effluent. Like bacteria, some protozoa need oxygen, some require very little oxygen, and a few can survive without oxygen.

The activated sludge process is a multi-chamber reactor unit that uses highly concentrated microorganisms to degrade organics and remove nutrients from wastewater, producing quality effluent. The goal is to maintain aerobic conditions and to keep the activated sludge suspended.

The combination of wastewater and biological mass is commonly known as mixed liquor. In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge.

The activated sludge process in the treatment of wastewater involves blowing oxygen or air into raw, unsettled sewage. This process smashes the solids. The sewage is bubbled, and sewage liquor is discharged into an activated sludge chamber.

In activated sludge process wastewater containing organic matter is aerated in an aeration basin in which micro-organisms metabolize the suspended and soluble organic matter. Part of organic matter is synthesized into new cells and part is oxidized to CO2 and water to derive energy.

Dark brown/earthy odor indicates old sludge with high solids (increase wasting). Black color/rotten-egg odor indicates septic conditions, low dissolved oxygen concentration, and too low of an airflow rate (increase aeration). A light chocolate brown mixed liquor color indicates a well-operated activated sludge process.

The purpose of Return Activated Sludge (RAS) is to prevent loss of microorganisms from the aeration tank and maintain an adequate population for treatment of the wastewater. As microbes metabolize their substrate, they grow and multiply in number.