Knowledge of Biochemical oxygen demand of water

Knowledge of Biochemical oxygen demand of water

1. Definition of BOD.

Biochemical oxygen demand (often referred to as BOD) refers to the amount of dissolved oxygen consumed in the biochemical reaction of microorganisms decomposing biodegradable organic matter in water under certain conditions. It is expressed in mg/L or percentage, ppm. It is a comprehensive indicator reflecting the content of organic pollutants in water. If the biological oxidation time is five days, it is called five-day biochemical oxygen demand (BOD5), and there are BOD10 and BOD20 accordingly.

The decomposition of organic matter in water is carried out in two stages. The first stage is the carbon oxidation stage, and the second stage is the nitrification stage. The amount of oxidation consumed in the carbon oxidation stage is called carbonization biochemical oxygen demand (CBOD).

Microorganisms need to consume oxygen when decomposing organic compounds in water. If the dissolved oxygen in the water is not enough to supply the needs of microorganisms, the water body is in a polluted state. Therefore, BOD is an important indicator that indirectly indicates the degree of organic pollution in water. Through the determination of BOD, we can understand the biodegradability of sewage and the self-purification capacity of water bodies. The higher the value, the more organic pollutants there are in the water and the more serious the pollution.

Generally, the degradation process of organic matter under the metabolism of microorganisms can be divided into two stages. The first stage is the process of organic matter being converted into CO2, NH3, and H2O. The second stage is the nitrification process of NH3 further converted into nitrite and nitrate. Since NH3 is already an inorganic substance, the biochemical oxygen demand of sewage generally refers only to the amount of oxygen required by organic matter in the stage biochemical reaction. The degradation of organic matter by microorganisms is related to temperature, and 20°C is generally used as the standard temperature for measuring biochemical oxygen demand. Under the measurement conditions of sufficient oxygen and constant stirring, it usually takes 20 days for organic matter to basically complete the stage oxidation decomposition process, about 99%, and the 20-day BOD value is often regarded as the complete BOD value, that is, BOD20. However, 20 days is difficult to achieve in actual work. Therefore, a standard time is stipulated, generally 5 days, which is called the five-day biochemical oxygen demand, recorded as BOD5. BOD5 is about 70% of BOD20.

The difference between BOD and COD is that BOD is biochemical oxygen demand; COD is chemical oxygen demand, which refers to the amount of all pollutants (including organic and inorganic substances) in water that can be oxidized by strong oxidants under certain conditions, expressed in mg/L of oxygen required for oxidation. It can reflect the degree of water pollution by reducing substances. Generally speaking, the COD of sewage is greater than BOD. This is because the former is oxidized more thoroughly. Except for a few volatile organic compounds, aromatic organic compounds, and a few alkanes, they can generally be oxidized, and there is also a part of the amount of inorganic substances; while BOD only refers to organic matter that can be directly decomposed by microorganisms, and is easily interfered by toxic substances and bacteria in the water. The ratio of biochemical oxygen demand to chemical oxygen demand can indicate how much of the organic pollutants in the water are difficult for microorganisms to decompose. Organic pollutants that are difficult for microorganisms to decompose are more harmful to the environment.

The BOD5 of a general river does not exceed 2mg/L. If it is higher than 10mg/L, it will emit a foul odor. my country's comprehensive sewage discharge standard stipulates that at the factory outlet, the allowable concentration of the BOD secondary standard of wastewater is 60mg/L, and the surface water BOD shall not exceed 4mg/L.

The traditional test method for BOD5 is the inoculation dilution method. The specific method is to culture for 5 days at 20±1℃, and measure the dissolved oxygen of the sample before and after culture respectively. The difference between the two is the biochemical oxygen demand for 5 days. This is the method currently widely used.

The biochemical oxygen demand (BOD) analyzer provided by Lianhua Technology is designed based on the measurement principle of the differential pressure method. The instrument simulates the biodegradation process of organic matter in nature: the oxygen in the air above the test bottle continuously replenishes the dissolved oxygen consumed in the water, the CO2 produced during the degradation of organic matter is absorbed by the sodium hydroxide in the sealing cover, and the pressure sensor monitors the changes in the oxygen pressure in the test bottle at any time. A correlation is established between the biochemical oxygen demand BOD (i.e., the amount of oxygen consumed in the test bottle) and the gas pressure, and then the biochemical oxygen demand BOD value is directly displayed.

The traditional dilution inoculation method is cumbersome and time-consuming, and a dedicated person is required to supervise during the five-day culture process. In comparison, Lianhua Technology's BOD analyzer is easy to operate and convenient to test. When the set culture time (such as 5 days, 7 days or 30 days) is reached, the test system automatically shuts down and stores the measurement results. It can do 6 or 12 water samples at the same time, and no special person is needed to watch during the test. And it is faster than the dilution method. Keeping the bottle in a state of continuous stirring can provide additional oxygen for the water sample and allow bacteria to have more contact with organic matter. By accelerating the respiration and oxygen consumption process, the results can be obtained faster. The measurement results equivalent to the dilution culture method can be obtained within 2 to 3 days. These measurement results can be used for process control.

2. How BOD is produced

BOD mainly comes from biodegradable organic matter in water. ‌

Biochemical oxygen demand (BOD) refers to the amount of dissolved oxygen consumed in the biochemical reaction process of microorganisms decomposing biodegradable organic matter in water under certain conditions. These organic matter can be human and animal excrement, food and industrial waste, etc. They are decomposed in water by the action of microorganisms, thereby consuming dissolved oxygen in water. BOD is usually measured in milligrams per liter or expressed as a percentage or ppm. It is an important water quality indicator used to assess the degree of organic pollution in water bodies. Most pollutants in sewage are organic matter, including tens of millions of known species and countless unknown species. BOD and another indicator, chemical oxygen demand (COD), are used together to assess the pollution status of water bodies. BOD focuses on measuring the amount of organic matter that can be decomposed by microorganisms, while COD includes the oxidation of all forms of organic and inorganic matter. In summary, BOD mainly comes from biodegradable organic matter in water. These organic matter are decomposed in water by microorganisms, thus affecting the self-purification capacity and ecological balance of water bodies. Biochemical oxygen demand is an important water quality pollution parameter. In wastewater, effluent from wastewater treatment plants and contaminated water, the amount of oxygen required for microorganisms to grow and reproduce using organic matter is the oxygen equivalent of degradable (microorganism-usable) organic matter. Pollutants in surface water consume dissolved oxygen in the process of oxidation mediated by microorganisms. The amount of dissolved oxygen consumed is called biochemical oxygen demand, which indirectly reflects the amount of biodegradable organic matter in the water. It indicates the total amount of dissolved oxygen consumed in the water when the organic matter in the water is oxidized and decomposed by the biochemical action of microorganisms to make it inorganic or gaseous. The higher the value, the more organic pollutants there are in the water, and the more serious the pollution. Hydrocarbons, proteins, oils, lignin, etc. that exist in suspended or dissolved states in domestic sewage and industrial wastewater such as sugar, food, papermaking, and fiber are all organic pollutants, which can be decomposed by the biochemical action of aerobic bacteria. Since oxygen is consumed during the decomposition process, they are also called aerobic pollutants. If too much of this type of pollutant is discharged into the water body, it will cause a lack of dissolved oxygen in the water. At the same time, the organic matter will cause corruption through the decomposition of anaerobic bacteria in the water, producing foul-smelling gases such as methane, hydrogen sulfide, mercaptan, and ammonia, causing the water body to deteriorate and stink.

It takes about 100 days for all organic matter in sewage to be completely oxidized and decomposed. In order to shorten the detection time, the biochemical oxygen demand is generally represented by the oxygen consumption of the tested water sample at 20°C within five days, which is called the five-day biochemical oxygen demand, referred to as BOD5. For domestic sewage, it is approximately equal to 70% of the oxygen consumption for complete oxidation and decomposition.

3. The impact of BOD.

Water quality detection BOD is the abbreviation of biochemical oxygen demand meter, which is a comprehensive indicator of the content of oxygen-consuming pollutants in water. The hazards of excessive BOD are mainly manifested in the following aspects:

1. Consumption of dissolved oxygen in water: Excessive BOD content will accelerate the reproduction rate of aerobic bacteria and aerobic organisms, causing the oxygen in the water to be rapidly consumed, thereby leading to the death of aquatic organisms.

2. Water quality deterioration: The reproduction of a large number of oxygen-consuming microorganisms in the water body will consume dissolved oxygen and synthesize organic pollution into its own life components. This is the self-purification characteristic of the water body. Too high BOD will cause aerobic bacteria, aerobic protozoa, and aerobic protophytes to multiply in large numbers, quickly consume oxygen, cause the death of fish and shrimp, and cause a large number of anaerobic bacteria to multiply.

3. Affect the self-purification ability of water bodies: The content of dissolved oxygen in water bodies is closely related to the self-purification ability of water bodies. The lower the dissolved oxygen content, the weaker the self-purification ability of water bodies.

4. Produce odor: Too high BOD content will cause odor in water bodies, which will not only affect water quality, but also threaten the surrounding environment and human health.

5. Cause red tide and algal bloom: Excessive BOD will cause eutrophication of water bodies, trigger red tide and algal bloom, which will destroy the aquatic ecological balance and threaten human health and drinking water.

Therefore, excessive BOD is a very important water pollution parameter, which can indirectly reflect the content of biodegradable organic matter in water. If sewage with excessive BOD is discharged into natural water bodies such as rivers and oceans, it will not only cause the death of organisms in the water, but also accumulate in the food chain and enter the human body, causing chronic poisoning, affecting the nervous system, and destroying the function of the liver. Therefore, it is necessary to purchase a Shenchanghong BOD meter for measurement. Only after passing the test can the sewage be discharged into the water body.

5. Methods for treating BOD

To treat the problem of excessive BOD (biochemical oxygen demand) in water, it is necessary to use a variety of methods such as physical, biological and chemical methods. The following are some effective methods:

1. Physical method:

A. Pre-treat wastewater to remove suspended solids and sediments, usually using physical methods such as sedimentation, filtration or centrifugation.

B. Screening and sedimentation. Remove suspended solids in sewage through physical screening and sedimentation. These solids usually contain high BOD.

2. Biological method:

A. Biological treatment is one of the key steps to remove BOD in wastewater. It uses the metabolic capacity of microorganisms to decompose organic matter and reduce BOD content. Common methods include activated sludge method and biofilm method.

B. Activated sludge method: Create suitable environmental conditions through stirring, aeration and other methods to enable microorganisms to decompose organic matter.

C. Biofilm method: Attach microorganisms to a fixed membrane, and the organic matter in the wastewater is removed by microorganisms when it passes through the membrane.

D. Adjust pH value: pH value in wastewater has a certain influence on the activity of microorganisms and BOD removal effect, and needs to be adjusted according to the characteristics of specific wastewater.

E. Aeration to increase dissolved oxygen: By increasing the oxygen supply, the activity of microorganisms and the removal efficiency of BOD in wastewater are improved.

F. Residual sludge treatment: During the biological treatment process, the sludge produced needs to be further treated, including anaerobic digestion, aerobic digestion, dehydration, drying, etc.

3. Chemical method:

A. Chemical oxidation: Use oxidants such as ozone, chlorine or persulfate to oxidize organic matter in sewage and reduce BOD.

B. Flocculation and flotation: Add flocculants to make suspended particles and organic matter condense into larger flocs, and then remove them by flotation.

4. Advanced treatment technology:

A. Anaerobic ammonia oxidation technology: Under specific conditions, anaerobic ammonia oxidation bacteria are used to remove ammonia nitrogen in sewage and reduce BOD at the same time.

B. Constructed wetland system: Through the synergistic effect of plants and microorganisms in constructed wetlands, pollutants such as organic matter, nitrogen and phosphorus are removed.

5. Process optimization:

A. SBR (Sequencing Batch Activated Sludge Process): Improve the efficiency of sewage treatment through periodic water filling, aeration, sedimentation and drainage processes.

B. CAST (Circulating Activated Sludge Process): Combines the periodic operation of aeration and stirring to improve the removal efficiency of organic matter.

6. Pretreatment and post-treatment:

A. Pretreatment such as coarse screens, fine screens and grit chambers remove large particles of organic matter and reduce the burden of subsequent biological treatment.

B. Post-treatment: After biological treatment, BOD is further reduced by filtration, adsorption and other methods.

In summary, the problem of excessive BOD in treated water needs to comprehensively consider factors such as the nature of the wastewater, treatment requirements and economic conditions, select appropriate treatment methods, and pay attention to energy consumption and emissions during the treatment process to ensure that the treatment process meets environmental protection requirements.

5. BOD analysis method.

The analysis methods of BOD mainly include five-day culture method, pressure measurement method, microbial electrode method, BOD5 method, BOD20 method, biosensor method, optical oxygen sensor method, chemical analysis method, etc. 1, ‌ The five -day training method is a commonly used BOD measurement method. It calculates the BOD value by changing the water samples at (20 ± 1 ° C) conditions for 5 days, and then determining the changes in the oxygen content in the water sample before and after the water sample. It is to calculate the BOD value by measuring changes in the closed system by measuring changes in the closed system. The electrical signal changes caused by microbial metabolic activities to determine the BOD value. This method has high sensitivity and accuracy. The BOD5 method is simple and economical, and is widely used in the field of water quality monitoring, while the BOD20 rule can more comprehensively evaluate the degradation of organic matter in the water body, and it is suitable for occasions that require more accurately evaluating BOD. There are advantages of fast response, simple operation and high sensitivity. The reaction between chemical reagents and organic matter is calculated to calculate the BOD value. This method usually requires a longer operation time and complex experimental steps, but in some specific cases, it is still a effective method for determining the BOD value. In addition, different countries and regions may have different standards and requirements. Therefore, when performing BOD, it is necessary to refer to the relevant methods and standards applicable to the area to ensure the accuracy and comparability of the measurement results.

Lianhua Technology's biochemical oxygen demand (BOD5) analyzer is designed based on the differential pressure measurement principle. It simulates the biodegradation process of organic matter in nature. In a sealed culture bottle, the oxygen in the air above the culture bottle continuously replenishes the dissolved oxygen consumed by the decomposition of organic matter in the sample. The CO2 produced during the degradation of organic matter is removed, causing the air pressure in the culture bottle to change. By detecting the change in the air pressure in the culture bottle, the biochemical oxygen demand (BOD) value of the sample is calculated. Wide detection range, direct testing below 4000mg/L, automatic printing of results, optional measurement cycle of 1-30 days, simple operation.