When oxygen is taken out of receiving streams by any demand leaving in the effluent of a wastewater treatment plant, serious problems can develop with aquatic plant life and fish & that is why either BOD or COD testing, and sometimes both of these parameter tests is required all around the Country. What is the difference between BOD and COD testing?
The main difference between BOD and COD is that BOD is the amount of oxygen that is consumed by bacteria while decomposing organic matter under aerobic conditions whereas COD is the amount of oxygen required for the chemical oxidation of total organic & inorganic matter in H2O.
Coming out of the ’70s from wastewater treatment training and eventually running my wastewater plant the most important training I received and passed on to new Operators was learning to run the BOD test twice a week for a sewage treatment plant. The more stringent you ran the tests the better results and control you had over operating a treatment plant.
Biochemical Oxygen Demand BOD Test
BOD refers to the biological oxygen demand, which measures the amount of dissolved oxygen (DO) required by aerobic organisms (oxygen demand) to break down organic waste material present in a given water sample at a given temperature and specified time.
The Biochemical Oxygen Demand (BOD) test is a critical procedure used in environmental science and water management to measure the amount of oxygen required by aerobic microorganisms to decompose organic matter in a water sample over a specified period, typically five days at 20°C (referred to as BOD₅). This test indicates the organic pollution level in water, which can help assess water quality and the effectiveness of wastewater treatment processes.
Purpose of the BOD Test
- Assess Water Quality: Determine the degree of pollution in natural water bodies and the impact of effluents.
- Monitor Wastewater Treatment: Evaluate the efficiency of wastewater treatment plants by comparing BOD levels before and after treatment.
- Environmental Compliance: Ensure that industries and municipalities comply with environmental regulations regarding wastewater discharge.
The procedure of the BOD Test
- Sample Collection: Water samples are collected in airtight BOD bottles to avoid exposure to air, which could alter the oxygen levels.
- Initial Dissolved Oxygen (DO) Measurement: The initial DO concentration of the sample is measured using a dissolved oxygen meter or Winkler titration method.
- Incubation: The sample is incubated in the dark at 20°C for five days to prevent photosynthesis, which could increase oxygen levels.
- Final DO Measurement: After five days, the DO concentration is measured again.
- BOD Calculation: The BOD₅ value is calculated by subtracting the final DO measurement from the initial DO measurement. The difference represents the oxygen consumed by microorganisms to decompose the organic matter.
- Calculate BOD:
- Calculate the BOD₅ value by subtracting the final DO reading from the initial DO reading.
- If the sample was diluted, multiply the difference by the dilution factor.
BOD5=(DOinitial−DOfinalP)×Dilution Factor\text{BOD}_5 = \left( \frac{\text{DO}_{\text{initial}} – \text{DO}_{\text{final}}}{\text{P}} \right) \times \text{Dilution Factor}BOD5=(PDOinitial−DOfinal)×Dilution Factor
Where:
- DOinitial\text{DO}_{\text{initial}}DOinitial = Initial dissolved oxygen concentration (mg/L)
- DOfinal\text{DO}_{\text{final}}DOfinal = Final dissolved oxygen concentration after 5 days (mg/L)
- PPP = Decimal dilution factor (volume of sample/total volume of sample + dilution water)
Factors Influencing BOD
- Temperature: Higher temperatures can increase microbial activity, leading to higher BOD values.
- pH Levels: Extreme pH levels can inhibit microbial activity, affecting BOD results.
- Presence of Toxic Substances: Chemicals toxic to microorganisms can suppress decomposition, leading to lower BOD readings.
- Sample Handling: Proper sample collection and handling are essential to avoid contamination or changes in DO levels.
Significance of BOD
- Environmental Indicator: High BOD values indicate high levels of organic pollution, which can deplete oxygen in water bodies, harming aquatic life.
- Wastewater Treatment Performance: A significant reduction in BOD after treatment indicates effective removal of organic matter.
- Regulatory Compliance: Many regulatory agencies set BOD limits for wastewater discharge to protect water quality.
Limitations of the BOD Test
- Time-Consuming: The standard BOD test takes five days, which may not be suitable for real-time monitoring.
- Does Not Identify Pollutants: BOD indicates the presence of organic pollution but does not identify specific pollutants.
- Interference: The presence of nitrifying bacteria can affect BOD results by consuming oxygen for nitrification.
The BOD test is a fundamental tool in water quality management, providing vital information on the organic pollution levels in water. Despite its limitations, it remains widely used due to its simplicity and relevance in assessing the health of aquatic environments and the performance of wastewater treatment processes.
What is a Trickling Filter in Wastewater Treatment?
Trickling filters are used to remove organic matter from wastewater an aerobic treatment system that uses microbes attached to a filter medium rock, gravel, or plastic that removes organic matter from wastewater sprayed on it, followed by a recirculation tank It can handle peak shock loads to loads. ..…………………………………………………………………………… Read more
Chemical Oxygen Demand COD Wastewater Test
When treated wastewater is discharged into the environment through effluent, it can introduce pollution in the form of organic content to receiving waters. High levels of wastewater COD indicate concentrations of organics that can deplete dissolved oxygen in the water, leading to negative environmental and regulatory consequences. To help determine the impact and ultimately limit the amount of organic pollution in water, oxygen demand is an essential measurement.
Chemical oxygen demand (COD) is the amount of dissolved oxygen that must be present in water to oxidize chemical organic materials, like petroleum. COD is used to gauge the short-term impact wastewater effluents will have on the oxygen levels of receiving waters.
BOD: BOD is a biological oxidation process. COD: COD is a chemical oxidation process and COD describes the amount of oxygen required to chemically break down pollutants, while BOD indicates the amount of oxygen required to break down organic pollutants biologically with microorganisms.
There is a correlation between COD and BOD, however, it must be experimentally established before using one parameter to express another. If you run both tests you will come up with a ratio by dividing the BOD results by the COD results. Make sure to watch the video above. Usually, COD analysis (which is a much faster and more accurate method) is used to estimate BOD using the established correlation.
The Chemical Oxygen Demand (COD) test is a crucial analytical procedure used in environmental science and wastewater treatment to measure the amount of oxygen required to chemically oxidize organic and inorganic matter in a water sample. Unlike the Biochemical Oxygen Demand (BOD) test, which relies on biological processes, the COD test uses a strong chemical oxidant under acidic conditions to achieve oxidation, providing a faster and often more complete measure of the total oxidizable substances in the sample.
Purpose of the COD Test
- Assess Water Quality: Determine the extent of organic and inorganic pollution in natural water bodies and effluents.
- Monitor Wastewater Treatment: Evaluate the effectiveness of wastewater treatment processes by comparing COD levels before and after treatment.
- Environmental Compliance: Ensure compliance with environmental regulations regarding wastewater discharge.
The procedure of the COD Test
- Sample Collection: Water samples are collected and preserved to prevent changes in their composition before analysis.
- Addition of Oxidant: The sample is treated with a known excess of a strong oxidizing agent, typically potassium dichromate (K₂Cr₂O₇), in the presence of sulfuric acid.
- Heating: The mixture is heated to reflux, usually at 150°C, for a specified period (commonly 2 hours). This process ensures complete oxidation of the sample’s oxidizable materials.
- Addition of Catalyst: A silver sulfate catalyst is often added to enhance the oxidation of certain organic compounds.
- Titration: After cooling, the amount of oxidant consumed is determined by titration with a reducing agent, such as ferrous ammonium sulfate (FAS). The change in the oxidant concentration is proportional to the COD of the sample.
- Calculation: The COD value is calculated based on the amount of oxidant consumed during the reaction.
COD=(V(a−b)×M×8000)
Where:
- aaa = volume of titrant used for the blank (mL)
- bbb = volume of titrant used for the sample (mL)
- MMM = molarity of the titrant (mol/L)
- VVV = volume of the sample (mL)
Factors Influencing COD
- Sample Composition: Presence of certain substances (e.g., chloride ions) can interfere with the test, leading to erroneous results.
- Oxidation Efficiency: The efficiency of oxidation can vary based on the types of organic and inorganic compounds present.
- Temperature and Time: Proper control of heating temperature and reflux time is essential for consistent results.
Significance of COD
- Comprehensive Pollution Indicator: COD measures both biodegradable and non-biodegradable substances, providing a more complete assessment of water pollution.
- Rapid Analysis: COD test results can be obtained much faster than BOD test results, enabling quicker decision-making.
- Regulatory Compliance: Many environmental regulations specify maximum allowable COD levels for wastewater discharge to protect water quality.
Limitations of the COD Test
- Inability to Distinguish Pollutants: COD does not differentiate between types of pollutants, such as specific organic or inorganic compounds.
- Interferences: Chlorides and other substances can interfere with the test, necessitating additional steps or corrections.
- Not Biologically Relevant: COD does not provide information on the potential biological impacts of the pollutants, unlike the BOD test.
The COD test is a vital tool in water quality management and wastewater treatment, offering a rapid and comprehensive measure of water pollution. Its ability to quickly assess both organic and inorganic pollutants makes it indispensable for monitoring and regulatory compliance, despite its limitations and potential interference
What is the Difference Between BOD and COD Testing
The biochemical oxygen demand (BOD) represents the amount of dissolved oxygen (DO) consumed by biological organisms when they decompose organic matter in water. COD is the only method to measure the amount of industrial waste in water, which cannot be measured under BOD. The plants that treat wastewater from commercial operations measure COD.
The chemical oxygen demand (COD) is the amount of oxygen consumed when the water sample is chemically oxidized. BOD and COD can reduce the DO of lakes and rivers, and low concentrations can cause eutrophication and harm aquatic life. 
Oxygen demand measurements are used to estimate water pollution levels entering and leaving wastewater treatment and industrial facilities. Particularly close attention is paid to effluents as high oxygen demand levels indicate a danger to aquatic life.
Since treatment facilities are considered point sources, they are regulated under the Clean Water Act (CWA) which in turn sets facility-specific pollutant release targets as part of the National Permit Discharge Elimination System (NPDES) permitting process.
NPDES permits usually specify limits related to oxygen demand based on either the Chemical Oxygen Demand test (COD) test or Biochemical Oxygen Demand (BOD) testing methods.
Of the two tests, COD test analysis is more commonly performed as it takes less time and is more replicable. Furthermore, treatment operators can quickly react to changes in oxygen demand and modify treatment processes appropriately.
COD test measurements will always be higher than BOD measurements because COD measures everything that can be oxidized in the sample, whereas BOD measures only what can be oxidized through biological processes.
Plant engineers may establish a statistically validated COD/BOD test ratio to demonstrate their ability to reliably predict a BOD test value based solely on a COD measurement to their permitting authority. The COD test value may be used as justification for BOD removal from a plant permit. These test methods are designed to indicate the amount of contamination in a water sample.
Almost all wastewater treatment plants are required to measure one of these three items as a measure of the pollution value in the water. COD should always measure higher than TOC and then BOD.
COD or Chemical Oxygen Demand is the total measurement of all chemicals in the water that can be oxidized. TOC or Total Organic Carbon is the measurement of organic carbons. BOD- Biochemical Oxygen Demand is supposed to measure the amount of food (or organic carbons) that bacteria can oxidize.
Municipal wastewater discharge and industrial processes can create water high in COD/BOD, requiring careful treatment before discharge to preserve the health of waterways. Many people tend to confuse these two metrics since they are used for the same purpose to find out the degree of water contamination. The permissible limit of BOD biochemical oxygen demand is 30 mg/L.
The permissible limit of COD is 250 to 500 ppm. These limits are measured in the Discharge Parameter test samples in the effluent of the plant and reported for State compliance.
However, unlike the BOD test, the COD test measures all oxidizable matter in a sample of water, and not just organic compounds. Thus, BOD is typically lower than COD for the same sample. Moreover, a standard COD test can be done in a few hours, unlike a BOD test which requires a standard 5-day incubation period.
BOD and COD test measurements are taken to determine the pollution level of wastewater. COD value is always greater than the BOD value of a particular water body. BOD measures the oxygen demand for the decomposition of organic material by the microbes in the wastewater. COD measures the oxygen demand for the decomposition of both organic and inorganic waste in wastewater. This is the difference between BOD and COD.
Conclusion:
The Chemical Oxygen Demand (COD) test is an essential procedure in water quality management and wastewater treatment. It provides a rapid and comprehensive measure of the total amount of oxidizable organic and inorganic substances in a water sample. This test is invaluable for assessing pollution levels, evaluating the effectiveness of wastewater treatment processes, and ensuring compliance with environmental regulations.
Compared to the Biochemical Oxygen Demand (BOD) test, the COD test offers a faster and more complete analysis of water pollution, making it particularly useful for timely monitoring and decision-making. Despite its inability to distinguish between specific pollutants and potential interferences from certain substances, the COD test remains a critical tool in maintaining water quality and protecting aquatic environments.
In summary, the COD test plays a crucial role in environmental science and wastewater management, providing essential data for safeguarding water resources and ensuring public health and environmental sustainability.
What are the 3 Stages of Wastewater Treatment?
- Primary Stage-separates settable organic solids & inorganic material that won’t degrade
- Secondary Stage-removes suspended & soluble solids converting them to settable solids using Biological Oxidation
- The tertiary stage uses chemical & physical treatment to create H2O closer to potable quality ..………………………………………………………………………………. Read more
*How many times have you driven past the Wastewater Treatment Plant and held your breath and said to yourself how can anybody work there, well, I did. Wastewater treatment has come a long way from where I started back in the ’70s.
Jim has been a Consulting/Operator and Superintendent in Pennsylvania-held a Class A type 1 for Biological Treatment
- Conventional Activated Sludge with Anaerobic Digestors-4 million gals. per/day,
- Tertiary Treatment with Phosphorus and Denitrification Removal
- Numerous Extended Aeration Treatment Plants
- Contact Stabilization Treatment Plants
References: University of Georgia Wastewater Lab Testing
FAQ’s
- What is the purpose of the COD test?
The COD test measures the amount of oxygen required to chemically oxidize organic and inorganic substances in a water sample. It is used to assess the level of pollution, monitor the effectiveness of wastewater treatment processes, and ensure compliance with environmental regulations.
- How does the COD test differ from the BOD test?
The COD test uses a chemical oxidant to measure the oxygen demand, providing results within a few hours. In contrast, the BOD test relies on biological oxidation by microorganisms over five days. COD results are typically higher and include both biodegradable and non-biodegradable substances, while BOD primarily measures biodegradable organic matter.
- Why is the COD test preferred for rapid analysis?
The COD test is preferred for rapid analysis because it provides results within a few hours, allowing for quick assessment and decision-making. This is particularly useful for real-time monitoring and immediate corrective actions in wastewater treatment processes. The COD test does not differentiate between types of pollutants and may be affected by the presence of certain substances such as chlorides, which can interfere with the results. It also does not provide information on the biological impact of the pollutants, unlike the BOD test.
- Can the COD test be used for all types of water samples?
While the COD test is versatile and can be used for a wide range of water samples, certain high-chloride samples may require pretreatment to prevent interference. It is widely used for industrial wastewater, municipal wastewater, and natural water bodies.