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How to Lower and Raise BOD in Wastewater

By: Tom Frankel
Post Date: January 3rd 2024

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How to Lower and Raise BOD in Wastewater
 

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Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are harmful to the environment, aquatic life and human health. Understanding what raises BOD and how to lower it is crucial for effective wastewater treatment. Using the right treatment methods and quality equipment helps wastewater treatment plant operators reduce organic matter in wastewater, releasing safe, clean water back into the environment.

Importance of Proper BOD and COD

BOD indicates the amount of dissolved oxygen (DO) biological organisms consume in water when decomposing organic matter. COD indicates how much oxygen biological organisms consume in a chemically-oxidized water sample.

Maintaining proper BOD and COD is important because it prevents harm to the environment and aquatic life. High BOD and COD can reduce a lake, river or coastal waterway’s DO and cause eutrophication. Eutrophication increases nutrients and minerals such as phosphorus and nitrogen, advancing algae and plant growth and decomposition. Plant decomposition increases carbon dioxide, destroying natural habitats and harming aquatic life.

Proper wastewater treatment helps control BOD and COD, preserving waterway health. Low BOD levels ensure water is safe and clean when it leaves a treatment facility. Municipalities also test BOD levels to determine if drinking water is safe for consumption.

Maintaining BOD levels helps treatment plants comply with state regulations. Each state determines a permissible BOD level, so it’s essential for plant operators to understand their states’ specific regulations and BOD wastewater standards.

What Causes High BOD in Wastewater?

Understanding how BOD increases in wastewater can help treatment plant operators learn how to reduce it. High BOD occurs when sewage treatment plants release wastewater containing organic materials. Wastewater treatment plants release organic materials into the environment when they inadequately filter contaminants from the water.

How to Lower BOD in Wastewater

How to Lower BOD in Wastewater

Certain equipment considerations and chemical and physical processes can lower wastewater BOD. Consider the following ways to lower BOD:

1. Decrease Total Suspended Solids

Lowering total suspended solids (TSS) makes it easier to lower BOD. While knowing how to reduce TSS in wastewater is most important during the primary filtration phase, it’s also crucial to lower TSS following the primary phase — even the smallest suspended particles can affect BOD measurements. Consider these TSS reduction methods:

2. Increase Aeration

Activated sludge is a common wastewater treatment method, and increasing aeration is essential for lowering BOD during this process. BOD reduction in an aeration tank depends on air diffusers that feed oxygen to beneficial bacteria.

The air diffusers keep the beneficial bacteria alive in the activated sludge long enough to break down waste. The activated sludge process mixes beneficial bacteria and waste sludge to transform pollutants into waste sludge that the secondary clarifier can filter and dispose of.

3. Use the Correct Size Equalization Tank

A treatment plant’s equalization (EQ) tank can significantly impact wastewater BOD. An EQ tank’s size affects flow fluctuations, changing aeration within the water and lowering and raising BOD depending on how the plant controls it.

A treatment plant’s operators must understand their treated water’s ideal loading rate to determine what size EQ tank works best. Plant operators can typically determine the ideal loading rate based on the type of water the plant treats and the organic substrates they add to treatment processes.

When an EQ tank has the proper volume, it balances the loading rate and flow fluctuations. The correct volume reduces BOD by ensuring water moves as efficiently as possible.

4. Maintain Ideal Temperatures

Dissolved oxygen levels impact BOD because DO helps beneficial bacteria break down the organic solids that cause high BOD. Treatment plants strive to maintain a high DO, which results in lower BOD. Determining an effective, moderate temperature helps the wastewater treatment process go smoothly.

High water temperatures can decrease DO levels, so lowering the water temperature can increase BOD faster. However, it’s important to avoid lowering the temperature too much. Near-freezing temperatures can slow down the activated sludge process, so it’s best to use a temperature that’s warm enough to maintain a quick, efficient activated sludge process but low enough to maintain high DO levels.

5. Consider Alternative Water Treatment Methods

Using the correct treatment method can help plant operators reduce BOD efficiently. The best water treatment method depends on the type of water a facility treats. Some treatment plants benefit from the activated sludge method, while others can reduce BOD better using the moving bed biofilm reactor (MBBR) method, wastewater clarification or coagulation and flocculation.

MBBR Method

For some types of wastewater, the MBBR method can oxygenate and disinfect more effectively than the activated sludge method, helping to lower BOD. The MBBR method uses fine bubble diffusers to oxygenate water efficiently, and it allows plant operators to install ultraviolet systems on tertiary filters for additional disinfection.

An MBBR system also eliminates the need to recycle sludge and send it through the EQ tank more than once. Instead, an MBBR system uses a sieve to trap microorganisms in the basin while effluent flows out.

Wastewater Clarification

Wastewater clarification uses gravity to remove organic solids from water. Heavier particles settle to the bottom of a tank, eliminating them before a chemical process further removes lighter particles.

Wastewater Separation

The wastewater separation method is also known as coagulation and flocculation, and it uses an agglomerating agent to clump suspended particles together so a filtration system can easily remove them. This method precipitates organic particles using a chemical polymer, forming them into larger particles known as flocs. The system can then deposit the flocs into a sedimentation tank for further treatment use.

6. Use High-Quality Equipment

High-quality equipment facilitates effective filtration. No matter which method a treatment plant implements, quality equipment is essential. Using the right fine bubble disc diffusers, coarse bubble diffusers, fine bubble plate diffusers, tube diffusers and aeration systems can significantly impact how much BOD a plant removes from the water before releasing it.

BOD Removal Calculation

Wastewater treatment plants can calculate how efficiently they remove BOD from wastewater using a BOD removal efficiency equation. To determine BOD removal efficiency, subtract the effluent BOD volume from the influent BOD volume. Next, divide the result by the influent BOD volume and multiply this result by 100 to convert the decimal to a percentage. Completing this equation indicates what percentage of BOD your treatment plant removes from wastewater.

How to Reduce COD in Wastewater

Wastewater COD levels are similar to wastewater BOD levels. Both metrics measure water contamination. However, BOD only measures organic compounds, while COD measures all oxidizable materials. Wastewater treatment plants can reduce COD using wastewater separation or the MBBR method.

Reduce BOD With High-Quality Wastewater Treatment Equipment

Wastewater treatment plants can improve water safety and cleanliness by reducing BOD. Plants can lower BOD by using the right equipment and the most effective treatment methods for the type of water they treat.

SSI Aeration, Inc. offers high-quality water treatment equipment such as fine bubble diffusers, coarse bubble diffusers, tube diffusers and MBBR treatment systems to help you remove contaminants effectively. Contact us to learn more about how our products can help you reduce BOD in wastewater.

Reduce BOD With High-Quality Wastewater Treatment Equipment