What is the BOD level in Sewage Treatment Plant?

Understanding BOD in Sewage Treatment Plants: A Comprehensive Guide to Effective Wastewater Treatment

Biochemical Oxygen Demand (BOD) is a critical parameter used to assess the performance of a sewage treatment plant (STP). It serves as a key indicator of how much organic matter is present in wastewater and how efficiently the treatment processes are breaking down these organic pollutants. The proper management of BOD is essential for ensuring that treated wastewater meets environmental standards, preventing pollution in water bodies, and protecting aquatic ecosystems.

In this article, we will explore what BOD is, how it is measured, its sources, and its impact on wastewater treatment. We will also look into the various treatment stages, methods used to reduce BOD, and monitoring strategies to maintain treatment effectiveness. Whether you are an industry professional or someone interested in understanding the importance of BOD in sewage treatment, this article will guide you through every aspect of BOD management in STPs.

What is BOD (Biochemical Oxygen Demand)?

Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen required by microorganisms to break down organic matter in water over a specific period (usually 5 days at 20°C). It reflects the level of organic pollution in wastewater. When sewage or industrial wastewater is treated biologically, microorganisms such as bacteria consume the organic waste present in the water, using oxygen in the process. The more organic matter in the water, the higher the BOD, as more oxygen is required for the breakdown process.

The main purpose of measuring BOD is to determine the amount of biodegradable organic matter present in the wastewater. High BOD levels indicate the presence of a significant amount of organic pollutants, while low BOD levels suggest that the water is relatively clean. BOD is one of the primary indicators used by environmental agencies to regulate and assess the quality of effluent being discharged from treatment plants.

The BOD Measurement Process

BOD testing involves measuring the reduction in dissolved oxygen (DO) levels over a specified period. The standard method used to determine BOD is a 5-day test (BOD5), which allows operators to observe how much oxygen is consumed by microorganisms over five days as they break down organic matter.

Steps involved in BOD measurement:

1. Sample Collection: Water samples are collected from different stages of the treatment process, such as influent (incoming sewage), effluent (treated water), and intermediate stages.
2. Initial DO Measurement: The dissolved oxygen levels in the sample are measured at the beginning of the test (Day 0).
3. Incubation: The sample is placed in a sealed container and incubated at 20°C for five days. During this time, microorganisms in the sample consume oxygen as they break down the organic waste.
4. Final DO Measurement: After five days, the dissolved oxygen levels are measured again (Day 5).
5. BOD Calculation: The BOD is calculated by subtracting the final DO from the initial DO. The result gives the amount of oxygen consumed by microorganisms to degrade organic matter in the water sample.

The BOD value is typically expressed in milligrams per liter (mg/L), and lower BOD values indicate cleaner water with less organic pollution.

Sources of BOD in Sewage Treatment Plants

BOD levels vary based on the type and source of wastewater. Some common sources of BOD include:

1. Domestic Wastewater

Domestic sewage, which contains human waste, food residues, detergents, and other organic materials, typically has high BOD levels. This is one of the most significant contributors to BOD in sewage treatment plants.

2. Industrial Wastewater

Certain industries, such as food processing, paper manufacturing, and chemical production, generate wastewater with high BOD concentrations due to the presence of organic materials, chemicals, and by-products. For example, the wastewater from food processing industries is often rich in starches, sugars, fats, and proteins, all of which contribute to high BOD.

3. Commercial Wastewater

Wastewater from commercial establishments like restaurants, hotels, and laundromats can also contribute to high BOD levels. These establishments generate wastewater with a moderate to high concentration of organic materials, especially from food scraps, oils, and detergents.

4. Agricultural Runoff

Agricultural runoff, which includes animal waste, fertilizers, and pesticides, can also elevate BOD levels in wastewater. Although this is not directly a part of sewage treatment plants, agricultural runoff can contribute to high BOD in nearby water bodies, which may require additional treatment.

The Impact of High BOD Levels on the Environment

When wastewater with high BOD is released into water bodies without proper treatment, it can have detrimental effects on the environment. Some of the major impacts include:

1. Depletion of Dissolved Oxygen

High BOD levels consume large amounts of oxygen from the water as microorganisms break down the organic matter. This can lead to a significant depletion of dissolved oxygen (DO), which is essential for the survival of aquatic life such as fish, algae, and other organisms. When the oxygen levels drop too low, it creates dead zones where aquatic life cannot survive.

2. Eutrophication

Excessive organic matter and nutrient loading (such as nitrogen and phosphorus) from high BOD can lead to eutrophication, a process in which water bodies become enriched with nutrients, causing excessive algae growth. As algae die and decompose, they further consume oxygen, exacerbating the problem and leading to the creation of hypoxic (low oxygen) or anoxic (no oxygen) conditions.

3. Disruption of Aquatic Ecosystems

High BOD in receiving water bodies can disrupt the balance of aquatic ecosystems. The lack of oxygen can lead to the death of fish and other oxygen-dependent organisms. Additionally, the decomposition of organic matter can lead to the production of harmful gases like methane and hydrogen sulfide, further polluting the water.

Stages of BOD Removal in Sewage Treatment Plants

Sewage treatment plants use multiple stages to reduce BOD levels, aiming to meet regulatory discharge requirements. These stages include primary, secondary, and advanced treatment processes.

1. Primary Treatment

The primary treatment stage is designed to remove large, easily separable solids from the wastewater, which reduces the overall BOD by about 20-30%. In this stage:

• Screening removes large debris such as plastics, rags, and other solid waste.
• Sedimentation tanks allow suspended solids to settle by gravity, while floating materials such as oils and greases are skimmed off the surface.

While primary treatment removes solid particles, it does not significantly reduce BOD as most organic waste remains dissolved in the water.

2. Secondary Treatment (Biological Treatment)

Secondary treatment is the most critical stage for reducing BOD. This stage uses biological processes to remove dissolved organic matter from the wastewater. The primary method used is the activated sludge process, where air (oxygen) is introduced into aeration tanks to encourage the growth of bacteria that feed on organic pollutants.

• Aeration tanks: Air is supplied to promote the growth of bacteria and other microorganisms that consume organic matter, significantly reducing BOD levels.
• Clarifiers: After the aeration process, the water flows to settling tanks (clarifiers) where bacteria and other solids are allowed to settle, separating clean water from sludge.

In well-functioning secondary treatment systems, BOD can be reduced by 85-95%.

3. Advanced Treatment

In some cases, especially for discharge into sensitive environments, advanced treatment methods are employed to further reduce BOD and other pollutants. These methods may include:

• Chemical coagulation and flocculation to remove additional dissolved solids.
• Membrane filtration (e.g., reverse osmosis) to remove fine particles and dissolved contaminants.
• Activated carbon adsorption to remove residual organic compounds.

Advanced treatment can reduce BOD levels to very low concentrations, making the water suitable for reuse or discharge into sensitive ecosystems.

Monitoring and Controlling BOD Levels

To ensure that treatment processes are effectively reducing BOD levels, continuous monitoring and control measures are necessary.

1. Regular BOD Testing

BOD levels are monitored at various stages of treatment to assess the effectiveness of the processes. Typical testing frequencies include:

• Daily tests to track short-term fluctuations.
• Weekly tests to evaluate process performance over time.
• Monthly or quarterly tests to ensure compliance with regulatory standards.

Operators use BOD data to adjust aeration rates, sludge removal, and chemical dosing to optimize the treatment process.

2. Process Adjustments

BOD testing results guide operational adjustments to improve treatment efficiency. For example:

• Aeration rates can be increased to promote bacterial activity when BOD levels are high.
• Sludge wasting can be adjusted to manage bacterial populations and maintain proper microbial activity.
• Chemical dosing can be optimized to address specific pollutants that contribute to high BOD levels.

3. Performance Tracking

Over time, the BOD removal performance of the treatment plant is tracked to ensure that treatment objectives are met. Long-term data can reveal trends and help in predictive maintenance planning, identifying areas where process improvements or upgrades are needed.

Conclusion

Biochemical Oxygen Demand (BOD) is a fundamental parameter that reflects the organic pollution levels in wastewater. Proper management of BOD levels in sewage treatment plants is crucial for achieving high-quality effluent, protecting aquatic ecosystems, and meeting regulatory standards. The treatment process, which involves primary, secondary, and advanced treatment stages, is designed to reduce BOD efficiently, ensuring that the treated water is safe for discharge or reuse.

For industries and municipalities looking to optimize their sewage treatment processes, understanding and managing BOD is essential. At 3D Aqua Water Treatment Company, we provide expert solutions to help you manage BOD levels and ensure that your treatment plant performs at its best.

Contact 3D Aqua Water Treatment Company

For customized solutions for your commercial or industrial wastewater treatment needs, including STP and ETP systems, contact us today:

• Phone: +91-89630-89630
• Email: info@3daqua.in

Let us help you optimize your treatment plant for better BOD removal and wastewater management efficiency!