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What you should know about biochemical oxygen demand (BOD), total organic carbon (TOC) and total oxygen demand (TOD)!



Biochemical oxygen demand (BOD), total organic carbon (TOC), total oxygen demand (TOD), dissolved oxygen (DO) of water, resistivity of water, acidity of water, alkalinity of water and other knowledge points related to water, come and understand!

What is Total Organic Carbon (TOC)?
The content of organic matter in water, expressed by the amount of one carbon, the main element in organic matter, is called total organic carbon. The determination of TOC is similar to that of TOD. Under the high temperature of 950 ℃, the organic matter in the water sample is gasified and burned to generate CO2. The total organic carbon can be known by measuring the amount of CO2 generated through the infrared analyzer. Inorganic carbon compounds in the water, such as carbonate and bicarbonate, will also generate CO2 during the determination process, which should be measured separately and deducted. If the water sample passes 0.2 μ After m microporous membrane filtration, the measured carbon is dissolved organic carbon (DOC). TOC and DOC are commonly used water quality indicators.

What is Total Oxygen Demand (TOD)?
The determination of total oxygen demand is the amount of oxygen consumed by burning and oxidizing organic matters at 900 ℃ in a special burner with platinum as catalyst. The determination result is closer to the theoretical oxygen demand than COD. It only takes about 3 minutes for TOD to be determined by instrument. Therefore, it has the advantages of fast analysis speed, simple method, small interference and high precision, and has been paid attention to. If the correlation coefficient between TOD and BOD5 can be determined, it is more practical to guide production with TOD indicators.

What is biochemical oxygen demand (BOD)? How to judge by biochemical oxygen demand (BOD)
The so-called biochemical oxygen demand (BOD) refers to the amount of oxygen consumed when organic substances that can be decomposed in water are completely oxidized and decomposed under aerobic conditions due to the role of microorganisms, which is called biochemical oxygen demand (BOD) for short.

It is expressed by the amount (mg/L) of dissolved oxygen of water sample stored in a closed container for a certain time at a certain temperature (such as 20 ℃). When the temperature is at 20 ℃, it takes about 20 days for ordinary organic substances to complete the oxidative decomposition process, and it takes 100 days to complete this decomposition process. However, such a long time has lost practical value for actual production control. Therefore, it is currently stipulated that 5 days of incubation at 20 ℃ is the standard for determining BOD. At this time, the measured BOD is called five-day BOD5. If 20 days of culture is used as the standard for determining BOD, the measured BOD is called 20 days of BOD ­ express. The amount of biochemical oxygen demand (BOD) indicates the degree of organic pollution of the water body and reflects the quality of the water quality.

What is Chemical Oxygen Demand (COD)?
The so-called chemical oxygen demand (COD) is the amount of oxidation consumed when certain strong oxidants are used to treat water samples under certain conditions.

It is an indicator of the amount of reducing substances in water. The reducing substances in water include various organic substances, nitrite, sulfide, ferrous salt, etc. But the main thing is organic matter. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the content of organic substances in water. The greater the chemical oxygen demand is, the more seriously the water is polluted by organic matters.
The determination of chemical oxygen demand (COD) varies with the determination of reducing substances in water samples and different determination methods. At present, acid potassium permanganate oxidation method and potassium dichromate oxidation method are widely used. The potassium permanganate (KmnO4) method has low oxidation rate, but it is relatively simple. It can be used to determine the relative comparative value of organic content in water samples. Potassium dichromate method, with high oxidation rate and good reproducibility, is suitable for determining the total amount of organic matter in water samples. Organic matter is very harmful to industrial water system. When water containing a large amount of organic matter passes through the desalination system, it will pollute the ion exchange resin, especially the anion exchange resin, reducing the resin exchange capacity.

During pretreatment (coagulation, clarification and filtration), the organic matter can be reduced by about 50%, but it cannot be removed in the desalination system, so it is often brought into the boiler through make-up water to reduce the pH value of boiler water. Sometimes, organic matters may also be brought into the steam system and condensate, which will reduce the pH and cause system corrosion. The high content of organic matter in the circulating water system will promote microbial reproduction. Therefore, no matter for desalination, boiler water or circulating water system, the lower the COD is, the better, but there is no unified limit index. When COD (DmnO4 method) in the circulating cooling water system is more than 5mg/L, the water quality has started to deteriorate.


What is dissolved oxygen (DO) in water?
The free oxygen dissolved in water is called dissolved oxygen (expressed in DO), usually expressed in O2mg/L, mL/L and other units. The main source of oxygen in natural water is atmospheric oxygen dissolved in water, and its dissolved amount is closely related to temperature and pressure. The solubility of oxygen decreases with the increase of temperature, and increases with the increase of pressure.

The content of dissolved oxygen in natural water is about 8-14 mg/L, and that in open circulating cooling water is generally about 6-8 mg/L. The amount of dissolved oxygen in the water also reflects the degree of water pollution. When the water body is polluted by organic substances, the dissolved oxygen in the water is gradually reduced due to the consumption of oxygen by oxidizing pollutants. When the pollution is serious, the dissolved oxygen will be close to zero. At this time, anaerobic bacteria will grow and multiply, and the organic pollutants will decay and stink. Therefore, dissolved oxygen is also an important indicator to measure the degree of water pollution.

What is the resistivity of water?
When measuring the conductivity of water, it is related to the resistance value of water. If the resistance value is large, the conductivity is poor, and if the resistance value is small, the conductivity is good.
According to Ohm's law, when the water temperature is constant, the resistance value R of water is inversely proportional to the vertical sectional area F of the electrode, and is proportional to the distance L between the electrodes, as shown in the following formula: R= ρ Where ρ—— Resistivity, or specific resistance. The unit of resistance is ohm (ohm, code ohm), or micro ohm( μ Ω), 1 Ω equals 106 μ Ω; The International System (SI) unit of resistivity is ohm meter (Ω • m). If the cross section product F of the electrode is made into 1cm2, the distance L between the two electrodes is 1cm, and the unit of resistivity is Ω • cm, then the resistance value is equal to the resistivity value.

The resistivity of water is related to the amount of salt in the water, the content of ions in the water, the charge number of ions and the movement speed of ions. Therefore, the pure water resistivity is large, and the ultra pure water resistivity is even greater. The purer the water, the greater the resistivity.

What is the acidity of water?
The acidity of water refers to the total amount of substances in the water that can provide H ions and strong alkalis (such as NaOH, KOH, etc.) for neutralization reaction. These substances can release H, or they can produce H after hydrolysis.

The substances that form acidity in water have three parts:

(1) Strong acids in water can dissociate H completely, such as sulfuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3), etc;

(2) Weak acid substances in water, such as free carbon dioxide (CO2), carbonic acid (H2CO3), hydrogen sulfide (H2S), acetic acid (CH3COOH) and various organic acids;

(3) The existing salts consist of strong acids and weak bases, such as aluminum, iron, ammonium plasma and

strong acids. In natural water, the composition of acidity is mainly weak acid, that is, carbonic acid. In general, natural water does not contain strong acid acidity. The determination of acidity in water is to titrate the water with standard solution of strong base (such as 0.1mol/LNaOH). For example, the acidity measured with methyl orange indicator refers to the acidity of strong acid and strong acid and weak base forming salts; The acidity measured with phenolphthalein indicator includes the above three parts, which is called total acidity.


How to know the water quality according to the relationship between hardness and alkalinity?
The hardness of natural water mainly refers to metal ions such as Ca2+and Mg2+, and the alkalinity of water mainly refers to the alkalinity of hydrogen carbonate HCO3 -. The main ions in the water are Ca2+, Mg2+, Na, K, HCO3 -, SO42 -, Cl -, etc.

The relationship between hardness and hardness in water can be divided into three cases.

(1) Alkalinity>hardness (in mol/L) The hardness (Ca2+, Mg2+) of HCO3>(Ca2+, Mg2+) water becomes bicarbonate, and there are also bicarbonate of Na and K, but there is no non carbonate hardness. At this time, the difference between alkalinity and hardness is equal to the bicarbonate of Na and K. The alkalinity of hydrogen carbonate of this part of excess Na and K, namely the so-called excess alkalinity, is also called negative hardness.

(2) Alkalinity=hardness (in mol/L), that is, HCO3 -=(Ca2+, Mg2+). At this time, only the hardness of Ca2+, Mg2+and the alkalinity of hydrogen carbonate are available. There is neither non carbonate hardness nor Na, K bicarbonate.

(3) Alkalinity<hardness (in mol/L), i.e. HCO3 -<(Ca2+, Mg2+). At this time, there are two cases. One is calcium hard water with Ca2+>HCO3 -. At this time, there are non carbonate hardness CaSO4 and MgSO4 in the water, but there is no carbonate hardness Mg (HCO3 -) 2 of magnesium. The other case is magnesium hard water with Ca2+>HCO3 -, in which there is magnesium carbonate hardness Mg (HCO3 -) 2, but no non carbonate hardness of calcium, while there is magnesium non carbonate hardness MgSO4.

However, in either case, the hardness of non carbonate exists in the water, but there is no bicarbonate of Na and K.

What is the relationship between alkalinity in water?
The alkalinity of water is determined by neutralization with hydrochloric acid. When titrating the alkalinity of water, two indicators are used to indicate the end point of titration. When phenolphthalein is used as indicator, the titration end point is pH 8.2~8.4, which is called phenolphthalein alkalinity or P alkalinity. At this time, all hydroxides in the water are neutralized, and the carbonate is converted into bicarbonate, which means that the carbonate is neutralized in half. That is, P alkalinity=CO32 - all OH -. When methyl orange is used as indicator, the end point of titration is pH 4.3~4.5, which is called methyl orange alkalinity or M alkalinity.

At this time, the hydroxides, carbonates and bicarbonate in the water are all neutralized. The sum of various weak acid salts in the water measured is also called total alkalinity. That is, M alkalinity=all HCO3 - all CO32 - all OH -. If OH alkalinity exists in water alone, pH of water is>11.0; When OH - and CO32 - exist in the water at the same time, pH9.4~11.0; If only CO32 - exists in the water, pH=9.4; When CO32 - and HCO3 - exist together, pH8.3 ~ 9.4; In the presence of single HCO3 -, pH=8.3; However, when pH<8.3, only HCO3 - exists in the water, and the change of pH value at this time is only related to the content of HCO3 - and free CO2.

What is the alkalinity of water? What are the forms of alkalinity in water?
The alkalinity of water refers to the content of substances in water that can receive H ions and strong acids for neutralization reaction. The substances producing alkalinity in water are mainly carbonate alkalinity produced by carbonate, hydrogen carbonate alkalinity produced by bicarbonate, and hydroxide alkalinity produced by the presence of hydroxide and hydrolysis of strong alkali weak acid salt. Therefore, alkalinity is the sum of CO32 -, HCO3 -, OH - and other weak acid salts in water. The aqueous solutions of these salts are alkaline and can be neutralized with acids.

However, in natural water, alkalinity is mainly composed of HCO3 - salts. It can be considered that when the total alkalinity M=[HCO3 -] 2 [CO32 -] [OH -] - [H+] is greater than 7.0, [H] can be omitted. Therefore, when M=c (Bx -)=[HCO3 -] [2CO32 -] [OH -] mol/L, carbonate and bicarbonate can coexist, as well as carbonate and hydroxide can coexist.

However, bicarbonate and hydroxide cannot exist at the same time. They can react as follows in water: HCO3+OH -==CO32 H2O. It can be seen that carbonate, bicarbonate and hydroxide can exist alone in water. In addition, there are two alkalinity combinations. Therefore, alkalinity in water exists in five forms, namely: (1) hydrogen carbonate alkalinity HCO3 -; (2) Carbonate alkalinity CO32 -; (3) Alkalinity of hydroxide OH -; (4) Bicarbonate and carbonate alkalinity HCO3 - CO32 -; (5) Carbonate and hydroxide alkalinity CO32 - OH -.


What harm does hard water do to industrial production?
As the cooling water for industrial production, hard water will make the heat exchanger scale, which will not only block the water flow channel seriously, but also greatly reduce the heat exchange effect, affect the smooth progress of production, and even forced to stop production. The scaling will also cause corrosion under the scale, which will make the heat exchanger perforated and damaged, not only material leakage, but also increase the equipment investment cost and waste steel. Hard water used for washing also often affects the quality of products. For example, textile printing and dyeing will cause spots on the fabric, which not only affects the appearance, but also affects the strength.

As boiler water, hard water is heated in the boiler and then evaporated and concentrated, which makes the heating surface of the boiler scale, and the thermal conductivity of the scale is very poor. The thermal conductivity of scale is only a few hundred times that of steel. After scaling in the boiler, if the same boiler water temperature without scale is still required, the wall temperature of the heating surface must be increased. For example, for a 1.01Mpa (10atm) boiler, the wall temperature is 280 ℃. When the silicate scale reaches 1mm thick, the same boiler water temperature must be reached, and the wall temperature must be increased to 680 ℃. At this time, the strength of the steel plate is reduced from 3.92Mpa (40kgf/cm2) to 0.98Mpa (10kgf/cm2), which will cause a serious explosion accident.

The increase of metal temperature will also make the metal stretch. The 1m long steel plate will stretch 1.2mm for every 100 ℃ increase, which will increase the material stress and cause damage. In addition, after scaling, the heat transfer of the heating surface will deteriorate, and the combustion heat cannot be transferred to the water supply well, reducing the thermal efficiency of the boiler, thus wasting fuel. If 1.5mm thick sulfate scale is formed, more than 10% of the fuel will be wasted, and the output of the boiler will be greatly reduced. After scaling, it must be cleaned frequently, which not only affects production, but also reduces the service life of the boiler, and also consumes manpower and material resources. Therefore, hard water is very harmful to industrial production. It must be softened, desalted or other effective water treatment according to the requirements of products or equipment for water quality.

How is the unit of hardness expressed?
The common unit of hardness is mmol/L or mg/L. The normality concentration N commonly used in the past has been discontinued. During conversion, 1N=0.5mol/L. Since hardness is not formed by a single metal ion or salt, it is necessary to convert it to another salt in order to have a unified comparison standard. It is usually expressed by the mass concentration of CaO or CaCO3. When the hardness is 0.5mmol/L, it is equal to 28mg/L CaO or 50mg/L CaCO3. In addition, some countries also use German and French degrees to express hardness. 1 German degree is equal to 10mg/L CaO, 1 French degree is equal to 10mg/L CaCO3. 0.5mmol/L is equivalent to 2.8 German countries and 5.0 French countries.

What are the total solids, dissolved solids and suspended solids of water?
All impurities in water except dissolved gases are called solids. The solids in water can be divided into dissolved solids and suspended solids. The sum of these two is called the total solid of water. Dissolved solids refer to various inorganic salts and organic substances that are still dissolved in the water after the water is filtered. Suspended solids refer to the suspended substances such as sand, clay, organic matter and microorganism that can be filtered out and are insoluble in water. The determination of total solids is obtained by evaporation and weighing. Therefore, the temperature for steam drying has a great relationship, and is generally controlled at 105~110 ℃.

Why does some water stink?
Clean water is odorless, tasteless, colorless and transparent liquid. But polluted water often makes people feel abnormal gas. What you smell with your nose is called your nose, and what you taste with your mouth is called your taste.

The main sources of water odor are:
(1) The stink from the reproduction and decay of aquatic animals, plants or microorganisms in the water;
(2) Odor from the decomposition of organic substances in water;
(3) Odor of dissolved gases in water, such as SO2, H2S and NH3;
(4) Smell of dissolved salt or soil;
(5) The industrial wastewater discharged into the water body contains impurities such as oil, phenol, etc;
(6) The smell of chlorine gas is added in the process of disinfecting water.

Because of the above reasons, some water will have a bad smell. For example, the lake and marsh water has fishy smell and musty smell due to algae reproduction or excessive organic matter; Turbid river water often has earthy smell or astringent taste; Hot spring water often has sulfur smell; Groundwater sometimes has hydrogen sulfide smell; Water with more oxygen, calcium sulfate, organic matter or NO2 - often has abnormal sweet taste; The water contains sodium chloride and has a salty taste; The water contains magnesium sulfate, which has a bitter taste; Hot metal with astringent taste; The smell of domestic sewage and industrial wastewater is more diverse.

What are the effects of the main anions and cations in the water on the water quality?
The main anions in water are Cl -, SO42 -, HCO3 -, CO32 -, OH -, etc. HCO3 -, CO32 -, OH - often form hardness and alkalinity with cations K, Na, Ca2+, Mg2+, etc. The change of their amount will affect the change of pH value of water. From this change, it can be known that the nature of water is corrosive or scaling. Therefore, they are the main ions that affect the properties of water. Cl - is the most common anion in water and the catalyst that causes corrosivity of water quality. It can strongly promote and promote the exchange reaction of electrons on metal surfaces. Especially for stainless steel materials in the water system, stress concentration (such as thermal stress, shock stress, etc.) will cause Cl - enrichment and accelerate the electrochemical corrosion process. SO42 - is also a common corrosive anion in the water, which increases the conductivity of the water. At the same time, it can form CaSO4 precipitation with cation Ca2+and scale. It is not the nutrient source of sulfate reducing bacteria in the water.

The main cations in the water are K, Na, Ca2+, Mg2+, Fe3+, Mn2+, etc. Among them, Na is the most common cation in the water. The presence of Na and K increases the conductivity of the water and increases the instability tendency of the water; Among them, Ca2+and Mg2+are the main ions that make up the hardness of water. Under certain conditions, they often scale on the surface of heating equipment, affecting the heat transfer effect. Fe3+and Mn2+are easy to form the precipitation of Fe (OH) 3 and Mn (OH) 2 to form scale, thus causing corrosion under the scale, which is also a promoter for the growth of iron bacteria.


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