In the process of water treatment, especially in the process of sewage treatment, in order to determine whether the treated water quality meets the discharge standard, it is necessary to test the water quality through various testing equipment, among which the two most common indicators are COD (chemical oxygen demand) and BOD (biochemical oxygen demand).
In the process of sewage treatment, there are many types of organic matter, which is different from the single analysis of inorganic salt ions, and the analysis of organic substances one by one is time-consuming and consuming of drugs. After research, it has been found that all organic substances have two commonalities, one is that they are composed of hydrocarbon, and the other is that most organic substances can be chemically oxidized or oxidized by microorganisms, and their carbon and hydrogen form non-toxic and harmless carbon dioxide and water with oxygen respectively. The organic matter in the sewage consumes oxygen in both the chemical oxidation process and the biological oxidation process, and the more organic matter in the wastewater, the more oxygen is consumed, and the relationship between the two is proportional. Therefore, the amount of oxygen consumed by the oxidation of sewage with chemical agents is called COD (chemical oxygen demand), and the amount of oxygen consumed by microbial oxidation in sewage is called BOD (biochemical oxygen demand).
The basic concepts and measurement methods of COD and BOD
COD (Chemical Oxygen Demand).: It is the oxidation dose consumed when a certain strong oxidant is used to treat a water sample under certain conditions. COD is a metric that characterizes the amount of reducing substances, especially organic matter, in milligrams of liters.
COD assay method: The determination methods of COD are divided into potassium dichromate method and potassium permanganate method. The former is generally called chromium CODCR, and the COD indicator without remarks defaults to CODCR. The potassium permanganate method, referred to as the manganese method, is professionally called the potassium permanganate index (CODMN or LM). The permanganate index is subdivided into two types: acidic and alkaline, the former is used when the chloride ion content is low, and the latter is used when the chloride ion content is high (such as seawater, salt lake water, etc.).
Main Differences Between Chromium and Manganese:
Different oxidizing capacity: potassium dichromate is more oxidizing than potassium permanganate, and can oxidize most organic matter in the presence of silver salts.
The oxidation rate is different: potassium dichromate has an oxidation rate of more than 80% in the presence of silver salts, while potassium permanganate has only about 50%.
Different application scopes: It is difficult to find an obvious pattern in the correlation between CODCR and CODMN.
Relatively speaking, the CODCR method is widely used and is suitable for various types of wastewater, especially industrial wastewater with relatively complex water quality. The CODMN method is only suitable for the determination of surface water, drinking water and domestic sewage. It is not suitable for water with high organic content (containing aromatic compounds, benzene rings, heterocyclic rings, heteropolycyclics, etc.) such as printing and dyeing, electroplating, and petrochemical industries. Therefore, the wastewater discharge standard is based on the CODCR results, and the surface water quality evaluation is based on the CODMN results.
BOD (Biochemical Oxygen Demand).: refers to the mass concentration of dissolved oxygen required in the biochemical process of microbial decomposition of organic matter in water under aerobic conditions. In order to make the BOD detection value comparable, it is generally specified at a certain temperature, a time period, and the dissolved oxygen consumption in the water is measured, and a five-day time is generally used, which is called the five-day biochemical oxygen demand, which is recorded as BOD5, and the five-day biochemical oxygen demand is often used. BOD is an indicator that characterizes the amount of organic matter degraded by microorganisms in water, measured in milligrams of liters.
BOD assay method: The mass concentration of dissolved oxygen in the water samples before and after incubation was measured by the difference between the mass concentrations of dissolved oxygen before and after incubation, and the amount of dissolved oxygen consumed per liter of sample was calculated from the difference between the mass concentrations of dissolved oxygen before and after incubation, which was expressed as BOD5.
Difference Between COD and BOD
COD not only reacts organic matter in water, but also represents inorganic substances with reducing properties in water, such as sulfide, ferrous ions, sodium sulfite, etc. For example, if the ferrous ions in the sewage are not completely removed in the neutralization tank, there are ferrous ions in the biochemical treatment effluent, and the COD in the effluent may exceed the standard.
Some of the organic matter in the sewage can be biooxidized (such as glucose and ethanol), some can only be partially biooxidized and degraded (such as methanol), and some organic matter cannot be biooxidized and degraded, and there is also a certain toxicity (some surfactants). In this way, the organic matter in the sewage can be divided into two parts, biodegradable and non-biodegradable organic matter.
Traditionally, COD (Chemical Oxygen Demand) basically represents all organic matter in sewage, and BOD (Biochemical Oxygen Demand) is the embodiment of the content of biodegradable organic matter in sewage, so the difference between COD and BOD can represent the non-biodegradable organic matter in sewage.
The relationship between COD and BOD
Both COD and BOD are indicators that characterize the concentration of organic pollutants in wastewater. COD basically reflects the concentration of all organic pollutants in the water. The BOD reaction is the concentration of easily degradable organic pollutants in the water, and the ratio of BOD COD can reflect the biodegradability of the sewage. The larger the BOD5 COD value, the higher the biodegradability of sewage, and the corresponding value can be used as an index of the biodegradability of traditional sewage, which is expressed as:
bod5/cod=(1-α)k/v)
It is the ratio of CODNB to COD that is biochemically difficult to degrade; K is the ratio of BOD5 to final biochemical oxygen demand (BOD). The COD value of BOD5 decreases with the increase, so this ratio can reflect the function of biodegradability of wastewater.
b/c＞0.58 fully biodegradable;
b/c=0.45-0.58. Good biodegradability;
b/c=0.30-0.45 biodegradable;
b/c=0.10-0.30. Refractory to biodegradation;
b/c＜0.1. Non-biodegradable.
In conventional wastewater treatment, it is usually used as BOD5 COD=03 is the lower limit of biochemical degradation of sewage.
There have been people (**: The daily life of an environmental engineerThe organic matter in some non-biochemical wastewater is a visual analogy, in which COD is the whole peanut (total organic matter concentration), BOD is peanut (edible part is biochemical part), and peanut shell is non-biochemical COD (i.e., COD-BOD). The non-biochemical COD is also variable, for example, if the residence time is longer (or the abundance and concentration of microbial populations are increased), the non-biochemical part of the COD can be further degraded by microorganisms (peanut shells are inedible, but they can be further powdered for preparation, but it takes longer to digest).
Appendix: Requirements for COD and BOD in common water quality standards
Requirements for surface water quality standards for categories I-IV
According to the "Environmental Quality Standards of Surface Water of the People's Republic of China", according to the environmental functions and protection objectives of surface water waters, China's water quality is divided into five categories according to the level of function:
Category I is mainly applicable to source water and national nature reserves.
Class II is mainly suitable for first-class protection areas of centralized surface water sources of domestic drinking water, habitats of rare aquatic organisms, spawning grounds for fish and shrimp, and feeding grounds for juvenile fish.
Class III is mainly applicable to fishery waters and swimming areas such as secondary protection areas for centralized surface water sources of domestic drinking water, fish and shrimp wintering grounds, migratory channels, and aquaculture areas.
Class IV is mainly applicable to general industrial water areas and recreational water areas that are not in direct contact with the human body.
Category I is mainly applicable to agricultural water use areas and waters with general landscape requirements.
Among them, the water quality of Class I is good, the groundwater only needs to be disinfected, and the surface water can be used for domestic drinking after simple purification treatment (such as filtration) and disinfection.
Class II water is mildly polluted and can be used for domestic consumption after conventional purification (e.g. flocculation, sedimentation, filtration, disinfection, etc.).
Class III water can also be used for domestic drinking after treatment.
Class IV and below water quality is poor and cannot be used as a source of drinking water.
The sewage treatment plant meets the requirements of the discharge water quality standard
According to the environmental function and protection objectives of the water area into which it is discharged, as well as the different treatment processes, the standard values of conventional pollutants of the basic control items are divided into first-class standards, second-level standards, and first-class standards. The specific criteria are as follows:
The first-level standard is divided into A standard and B standard
When the effluent of the sewage treatment plant is introduced into rivers and lakes with small dilution capacity for urban landscape water and general reuse water, the A standard of the first-class standard shall be implemented;
When discharged into GB3838 surface water functional waters (except for the designated drinking water source protection areas and swimming areas) and GB3097 seawater class II functional waters, the first-class standard B standard shall be implemented.
Secondary standards: When discharged into GB3838 surface water, functional water or GB3097 seawater.
In the case of Class III and IV functional sea areas, the second-level standard shall be implemented.
Standard: The sewage treatment plant of the town in the non-key control basin and the non-water source protection area shall implement the first-class enhanced treatment process according to the local economic conditions and water pollution control requirements.
Note: When the specific local standard is higher than the national standard, the high standard requirements will be implemented.