Modern cities generate enormous volumes of wastewater every day from homes, industries, hospitals, restaurants, and commercial buildings.
If this sewage were discharged directly into rivers or lakes without treatment, it would quickly pollute water bodies, damage ecosystems, and create severe public health risks.
This is where Sewage Treatment Plants (STPs) play a critical role. These facilities treat contaminated wastewater through a series of mechanical, biological, and chemical processes to remove pollutants before releasing the water safely into the environment or reusing it for purposes like irrigation, cooling systems, or groundwater recharge.
In recent years, advanced treatment technologies have replaced many traditional wastewater treatment systems.
Among the most efficient modern solutions is the Moving Bed Biofilm Reactor (MBBR) system combined with an anoxic treatment stage, which significantly improves nitrogen removal and organic matter degradation.
In this detailed guide, we will explain the entire sewage treatment process step-by-step, focusing particularly on four essential components:
Bar Screen
Anoxic Tank
Moving Bed Biofilm Reactor (MBBR)
Secondary Clarifier
By the end of this article, you will have a clear understanding of how these units work together to transform polluted sewage into environmentally safe water.
Overview of the Sewage Treatment Plant Process
A typical sewage treatment plant operates in multiple stages. Each stage is designed to remove a specific type of contaminant from wastewater.
The treatment process usually includes the following phases:
1. Preliminary Treatment
The first stage removes large debris and heavy materials from incoming sewage. Equipment like bar screens and grit chambers protect downstream pumps and treatment units.
2. Primary Treatment
During this stage, heavy suspended particles settle at the bottom of a sedimentation tank while oils and grease float on the surface. This reduces the overall pollution load.
3. Secondary (Biological) Treatment
Microorganisms break down dissolved organic matter and nutrients such as nitrogen and phosphorus. Advanced plants often use MBBR reactors and anoxic tanks in this phase.
4. Tertiary Treatment and Disinfection
In the final stage, the treated water is filtered and disinfected to remove pathogens before discharge or reuse.
Sludge produced during treatment is processed separately through thickening, digestion, and dewatering before disposal or reuse.
Step 1: Preliminary Treatment Bar Screen (First Protection Unit)
The treatment process begins as soon as raw sewage enters the plant. Wastewater at this stage contains a mixture of water, organic waste, plastic bags, rags, food scraps, wood pieces, and other debris.
The bar screen is the first and most important piece of equipment used to remove these large solid materials.
What is a Bar Screen?
A bar screen is a filtering device consisting of a series of vertical metal bars installed at the entrance of the sewage treatment plant. These bars allow wastewater to pass through while trapping solid objects.
Bar screens are typically made from stainless steel or galvanized metal to resist corrosion caused by wastewater.
There are two main types of bar screens used in STPs:
Coarse Screens
Bar spacing: 10–20 mm
Remove large debris like plastics, cloth pieces, and sticks
Fine Screens
Bar spacing: 3–6 mm
Capture smaller waste particles
Working Principle
As sewage flows toward the plant, it passes through the bar screen. The water flows between the bars, but solid waste gets stuck on the surface.
In modern treatment plants, mechanical cleaning systems automatically remove the trapped debris using a rotating rake or chain mechanism. The collected waste is then transported to a bin for disposal.
In smaller plants, manual cleaning may be used, but this requires regular operator attention.
Importance of Bar Screens
Without proper screening.
Pumps may get clogged.
Pipes and valves could become blocked.
Downstream treatment equipment might get damaged Bar screens therefore protect the entire treatment system from operational failures.
Typical Operating Conditions
Flow velocity: 0.3 – 0.6 m/s
Head loss when clean: about 0.15 – 0.3 meters.
Screenings produced: around 0.03 – 0.1 cubic meters per 1000 cubic meters of wastewater.
After screening, sewage typically passes through a grit chamber, which removes sand and small stones that could cause wear in pumps and pipelines.
Step 2: Primary Treatment Sedimentation of Solids
Following preliminary treatment, wastewater enters the primary clarifier, a large tank designed for gravity based separation.
In this tank.
Heavy suspended solids settle at the bottom.
Oils and grease float to the top.
Clarified water flows out from the middle layer.
The settled material is called primary sludge, which is pumped to sludge treatment units.
Primary treatment typically removes.
50–70% of suspended solids.
25–40% of biochemical oxygen demand(BOD) Reducing this load helps biological treatment systems operate more efficiently.
Step 3: Biological Treatment Anoxic Tank and MBBR Reactor
Biological treatment is the most important phase in modern sewage treatment plants. In this stage, microorganisms break down dissolved pollutants present in wastewater.
In advanced treatment plants, this stage includes two major components.
Anoxic Tank
MBBR Reactor
These units work together to remove organic pollutants and nitrogen compounds.
The Anoxic Tank Removing Nitrogen Through Denitrification
Nitrogen pollution is a serious environmental issue. Wastewater contains high levels of ammonia and nitrogen compounds from human waste, detergents, and industrial activities.
If these nutrients enter rivers or lakes untreated, they cause algal blooms and oxygen depletion, which can harm aquatic life.
The anoxic tank is designed specifically to remove nitrogen through a biological process called denitrification.
How the Anoxic Tank Works
Unlike aeration tanks, the anoxic tank operates in low oxygen conditions.
Dissolved oxygen is maintained below 0.5 mg/L.
Mechanical mixers keep wastewater circulating.
No air is supplied to the tank.
Under these conditions, special bacteria called denitrifying bacteria convert nitrate into nitrogen gas.
The simplified reaction is-
Organic carbon + Nitrate → Nitrogen gas + Carbon dioxide + Water
The nitrogen gas escapes into the atmosphere, effectively removing nitrogen from wastewater.
To support denitrification, water from the aerobic tank (which contains nitrates) is recirculated back to the anoxic tank.
Typical recirculation ratio: 2:1 to 4:1 of influent flow.
Benefits of Anoxic Tanks
Significant nitrogen removal (70–90%).
Reduces environmental pollution.
Improves biological treatment efficiency.
Prevents excessive algae growth in receiving water bodies.
MBBR Reactor High Efficiency Biological Treatment
The Moving Bed Biofilm Reactor (MBBR) is one of the most advanced technologies used in sewage treatment today.
Unlike traditional activated sludge systems where microorganisms float in water, MBBR technology allows bacteria to grow on small floating plastic carriers.
Structure of MBBR Media
The reactor contains thousands of plastic carrier elements made from polyethylene or polypropylene.
Key characteristics:
Size: around 10–20 mm.
Density slightly less than water.
Surface area: 500–1200 m² per cubic meter.
These carriers provide a large surface area for microbial growth.
Working Mechanism
Air diffusers installed at the bottom of the tank provide oxygen and keep the carriers in constant motion.
The moving carriers create a biofilm layer on their surfaces where bacteria grow and consume pollutants.
Different microbial layers perform different functions:
Outer layer
Removes organic matter and converts ammonia to nitrate.
Inner layer
Can support limited denitrification due to lower oxygen levels.
Operational Conditions
Typical operating parameters include.
Media filling ratio: 40–60% of reactor volume.
Hydraulic retention time: 4–8 hours
Dissolved oxygen: 2–4 mg/L
Organic loading rate: up to 10 kg BOD per cubic meter per day
Advantages of MBBR Technology
MBBR systems offer several benefits compared to traditional treatment processes.
Requires less space.
Handles shock loads effectively.
Produces less sludge.
Simple operation and maintenance.
Suitable for upgrading existing treatment plants.
Because of these advantages, many modern STPs across India use MBBR technology.
Step 4: Secondary Clarifier Solid Liquid Separation
After biological treatment, wastewater still contains suspended biomass particles. These solids must be removed before final discharge.
This is the role of the secondary clarifier, also known as the final settling tank.
How the Secondary Clarifier Works
Wastewater enters the clarifier slowly to prevent turbulence.
Inside the tank:
Suspended solids settle at the bottom due to gravity.
Clear water rises and flows over outlet weirs.
Sludge accumulates in the bottom hopper.
Rotating scraper arms slowly push the sludge toward a central collection point where it is removed.
Typical Design Parameters
Surface loading rate: 20–40 m³/m²/day.
Tank depth: 3.5–5 meters.
Detention time: 2–4 hours.
Proper clarification ensures the final effluent meets environmental standards for suspended solids.
Operational Challenges
Some issues that may occur include.
Sludge bulking due to filamentous bacteria.
Rising sludge caused by nitrogen gas formation.
Hydraulic overload during heavy rainfall.
Operators monitor sludge quality regularly to maintain stable plant performance.
Final Treatment and Sludge Management
After clarification, the treated water may undergo additional polishing steps such as.
Sand filtration
Activated carbon filtration
Disinfection using chlorine or UV
These steps remove remaining particles and harmful microorganisms.
Meanwhile, sludge generated during treatment undergoes further processing.
Thickening.
Anaerobic digestion (biogas production).
Dewatering using centrifuges or filter presses.
The final sludge may be used as fertilizer or disposed of safely.
Why MBBR Based STPs Are Becoming Popular
Modern sewage treatment plants increasingly use MBBR combined with anoxic treatment because this configuration offers excellent performance.
Key advantages include:
High removal efficiency for BOD and nitrogen.
Smaller plant footprint.
Reduced sludge generation.
Stable operation under fluctuating loads.
Lower operational costs.
Because of these benefits, many municipal wastewater projects now use this technology.
Conclusion
Sewage treatment plants are essential infrastructure for protecting public health and the environment. Every stage of treatment, from screening large debris to biological nutrient removal, plays a critical role in cleaning wastewater.
Key units such as the bar screen, anoxic tank, MBBR reactor, and secondary clarifier work together to remove pollutants, nutrients, and suspended solids efficiently.
With growing urban populations and increasing environmental regulations, advanced treatment systems like MBBR will continue to play a vital role in sustainable water management.
Properly designed and operated STPs ensure that wastewater is transformed into a valuable resource rather than an environmental hazard.
Clean water begins with effective wastewater treatment.