About dissolved oxygen in water – Part 1

dissolved oxygen

1.1 What is dissolved oxygen

Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water. It is expressed in milligrams of oxygen per liter of water. Dissolved oxygen exists in water in a molecular state. The amount of dissolved oxygen in water is one of the important indicators of water quality and one of the important factors for water purification.

1.1.1 Source of oxygen in water

Dissolved oxygen is an indispensable condition for the survival of aquatic organisms. One source is the infiltration of oxygen in the atmosphere into the water body when the dissolved oxygen in the water is not saturated; the other source is the oxygen released by the plants in the water through photosynthesis.

1. Dissolution of air

When the water surface is in contact with the air, the oxygen in the air will dissolve in the water. The rate of dissolution is proportional to the degree of unsaturation of the dissolved oxygen in the water. It is also related to the disturbance of the water surface and the surface area per unit volume, which is also related to the wind force and water depth. When the degree of unsaturation of oxygen in water is large, and the wind on the water surface is large and the water is shallow, the effect of air dissolution is large.

What causes an increase in dissolved oxygen levels in water

2,photosynthesis

The photosynthesis of aquatic plants and sunlight contained in the water body can release oxygen, which is another main source of oxygen in the water body.

3. Some ponds or reservoirs can increase the oxygen content of anoxic water while replenishing water.

Rehydration in factory running water fish farming is an oxygenation process. In ponds or reservoirs where fish are cultured in non-running water, the amount of water supplementation is small, and the direct oxygenation effect of supplementary water on fish ponds is not large.

1.1.2 What causes the depletion of oxygen in water

1. Respiration of fish, shrimp and other aquaculture organisms

The respiration oxygen consumption rate of fish and shrimp varies with fish and shrimp species, individual size, developmental stage, water temperature and other factors. Generally, the respiratory oxygen consumption rate of fish is 63.5-665 mg/kg·h, and it increases with the increase of the individual. The oxygen consumption rate (measured in milligrams of oxygen consumed per unit time) decreased with the increase of the individual. In a suitable temperature range, the water temperature increases, and the oxygen consumption rate of fish and shrimp increases, that is, water temperature and individual size have a great influence on the oxygen consumption rate of organisms.

2. Microorganisms consume oxygen in water

The oxygen consumption of micro-organisms in water mainly includes: zooplankton, phytoplankton, bacterial respiration and oxygen consumption by the decomposition of organic matter with the participation of bacteria. This part of the oxygen consumption is also related to the species of oxygen-consuming organisms, the size of the individual, the water temperature and the amount of organic matter in the water. Phytoplankton also respires and consumes oxygen, but the photosynthetic oxygen production during the day is much greater than its own respiratory oxygen consumption. According to research, in the rapid growth period of phytoplankton, the daily oxygen consumption of respiration accounts for 10-20% of its oxygen production. The oxygen consumption of organic matter mainly depends on the quantity and type of organic matter (whether it is easy to decompose at room temperature). This part of the oxygen consumption is usually called “water respiration” oxygen consumption.

The amount of dissolved oxygen consumed by microorganisms in water when decomposing organic matter is what we usually call biochemical oxygen demand (BOD5).

3. Substrate oxygen consumption

Substrate oxygen consumption is complex, mainly including: benthic organisms respiration oxygen consumption, organic matter decomposition oxygen consumption, inorganic chemical oxidation oxygen consumption in the reduced state, etc.

4. Escape

Oxygen escape occurs when the dissolved oxygen in the surface water is supersaturated. The escape rate is very slow under stationary conditions, and the disturbance of the water surface by the wind can accelerate this process.

5. Organic matter and chemical substances in water consume oxygen

Water contains various organics, nitrites, sulfides, ferrous salts and other reducing substances, which consume oxygen during the redox process. The aerobic decomposition of microorganisms will convert ammonia into nitrite and then into nitric acid, hydrogen sulfide into sulfate, and ferrous salt into ferric salt. The amount of oxygen consumed by reducing substances in water such as various organics, nitrites, sulfides, ferrous salts, etc. in the redox process is chemical oxygen demand (CODcr), and biochemical oxygen demand (BOD5) Similarly, the size of the chemical oxygen demand (CODcr) value can indicate the amount of reducing substances such as organic matter, nitrite, sulfide, and ferrous salt in the water, which is used to indicate the amount of organic matter, nitrite, sulfide, and ferrous salt in the water. The degree of contamination by reducing substances.

In the organic matter in the water, the carbon exists in different oxidation states, some carbon can be oxidized by biological utilization, we quantify it by the biochemical oxygen demand, and generally more carbon can be oxidized by chemical oxidants to form Carbon dioxide (CO2), we use chemical oxygen demand to quantify. However, there are parts of carbon that cannot be measured biologically or chemically.

Total organic carbon (TOC) is a more convenient, comprehensive and direct method of carbon expression than COD or BOD. It refers to the carbon combined with all organic matter. The content of all organic matter in the water body is expressed by the amount of carbon, the main element in the organic matter, and the carbon content is a comprehensive indicator of the total amount of all organic matter in the water body.

6.salt

Various acids, alkalis, salts and other inorganic substances enter the water body (acids and alkalis are neutralized to form salts, and they interact with certain minerals in the water body to produce some salts), which increases the salinity and reduces the transparency of freshwater resources, and Squeeze and fill the gaps between water molecules, resulting in a decrease in dissolved oxygen in the water.

1.1.3 Saturation of dissolved oxygen

What is the saturation of dissolved oxygen in a saturated state? How is it called a saturated state?

Saturated dissolved oxygen refers to the amount of oxygen in water when the exchange of oxygen in the water and the atmosphere is in equilibrium. dissolved oxygen concentration.The saturation of dissolved oxygen is related to the size of the atmospheric pressure; exist standard atmospheric pressure, it only changes with the water temperature T.The saturation state refers to the maximum solubility of oxygen in air;

The general dissolved oxygen (DO) calculation formula:

Considering that pure water is used to dissolve oxygen, the empirical formula for calculating the dissolved amount DO (unit: mg/L) is as follows.

Among them, DO is the dissolved oxygen in water, the unit is mg/L

T is the temperature in °C

p(O2) is the partial pressure of oxygen, the unit is Pa. In a standard atmospheric pressure air, the oxygen content is 21 %, take p(O2) = 21270 Pa and the above formula is the experimental data obtained according to thermodynamic fitting. The freezing point, within a suitable range lower than the boiling point of water, can meet the application requirements near normal pressure, and the error is about 5%.

Dissolved oxygen influencing factors, dissolved oxygen distribution change law, dissolved oxygen daily change, vertical distribution law, horizontal distribution law.

The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of water.

Under natural conditions, the oxygen content in the air does not change much, so the water temperature is the main factor. The lower the water temperature, the higher the dissolved oxygen content in the water. Molecular oxygen dissolved in water, called dissolved oxygen, usually denoted as DO, expressed in milligrams of oxygen per liter of water.

The amount of dissolved oxygen in the water is a measure of the self-purification ability of the water body.

The oxygen content on land is 210 ml/l.

The amount of dissolved oxygen in water varies. For example, 8-12 mg/L in fresh water is less than 1/20 of that in air. There is less dissolved oxygen in seawater.

Direct or indirect hypoxia causes death, accounting for about 60% of the total death of farmed fish.[kacise1] 

Under certain other conditions, dissolved oxygen is related to temperature, oxygen partial pressure (or air pressure), salinity, water depth, activity of aquatic organisms and the concentration of oxygen-consuming organic matter.

1. The relationship between the solubility of oxygen in water and temperature[kacise2] 

Temperature (℃) Dissolved oxygen (mg/L) Temperature (℃) Dissolved oxygen (mg/L) Temperature (℃) Dissolved oxygen (mg/L) Temperature (℃) Dissolved oxygen (mg/L)
0 14.62 8 11.87 16 9.95 twenty four 8.53
1 14.23 9 11.59 17 9.74 25 8.38
2 13.84 10 11.33 18 9.54 26 8.22
3 13.48 11 11.08 19 9.35 27 8.07
4 13.13 12 10.83 20 9.17 28 7.92
5 12.80 13 10.60 twenty one 8.99 29 7.72
6 12.48 14 10.37 twenty two 8.83 30 7.63
7 12.17 15 10.15 twenty three 8.63    

According to the principle of thermal expansion and contraction of gas, when the temperature is high, the intermolecular spacing is large, and when the temperature is low, the intermolecular spacing is small, so the oxygen in the air is more soluble in water when the temperature is low (this is also different from solids. On the other hand, when the temperature increases, the solubility of the gas decreases. For example, when boiling water, many small bubbles can be seen, and some of the gas dissolved in the water escapes when heated.

When the temperature rises, the volume expands, and the density becomes relatively smaller; on the contrary, when the temperature drops, the volume shrinks and the density becomes larger. The mass of a substance is not affected by temperature, but its volume expands and contracts with heat. But there are also special ones, which are thermal contraction and cold expansion. For example, water has the highest density at 4 degrees, so its density decreases during the process from 0 degrees to 4 degrees.

Generally speaking, no matter what substance, no matter what state it is in, with the change of temperature and pressure, the volume or density will also change accordingly. The relationship between the three physical quantities of temperature T, pressure p and density ρ (or volume) is called the equation of state. The volume of a gas varies significantly with the pressure it is subjected to and the temperature it is exposed to.

The density of solid or liquid substances changes only slightly when temperature and pressure change. For example, in the vicinity of 0°C, the temperature coefficients of various metals (the rate of change of the volume of an object when the temperature increases by 1°C) are mostly around 10-9. The pressure in deep water and the pressure of underwater explosion can reach hundreds of atmospheres or even higher (1 atmosphere = 101325 Pa), and the change of density with pressure must be considered at this time.

On the one hand, the increase of water temperature will lead to the reduction of dissolved oxygen in the water, on the other hand, the increase of water temperature will accelerate the oxygen consumption reaction, which will eventually lead to the lack of oxygen in the water body or the deterioration of the water quality. Solubility varies with temperature and the partial pressure of oxygen in the atmosphere, following Henry’s Law, as follows:

[O2]=Kh×Po2[kacise3] 

In the formula, [O2] is the balance of oxygen in the aqueous solution, Po2 is the partial pressure of oxygen, and Kh is the Henry’s constant, which varies with temperature. When the temperature is high, the Kh value is low, and when the temperature is low, the Kh value is high. Therefore, when the temperature is high in summer, the dissolved oxygen value in the water is low, and when the temperature is low in winter, the dissolved oxygen value in the water is high.

According to relevant statistics, under one atmospheric pressure, the water temperature rises from 10°C to 35°C, and the solubility of oxygen in pure water drops from 11.27mg/L to 6.93mg/L. The saturated dissolved amount in the water body is 9.17-11.33 mg/L at 20-10 °C, and the saturated dissolved oxygen content in pure water is about 9 mg/L at 101.325 KPa and 20 °C. For humans, the dissolved oxygen content in healthy drinking water should not be less than 6mg/L.[kacise4] 

2. The relationship between the solubility of oxygen in water and air pressure

In everyday life, we know that liquids can evaporate into gases, such as water vapor, and gases can also condense into liquids. At a certain temperature, the two can reach a balance, that is, the evaporation rate of the liquid is equal to the condensation rate of the vapor. When this equilibrium is reached, the vapor has a certain pressure, and this pressure is called the saturated vapor pressure of the liquid (referred to as vapor pressure). The vapor pressure is related to the temperature. The higher the temperature, the greater the kinetic energy of the molecules, the faster the evaporation rate, and the greater the vapor pressure.

Vapor pressure is related to pressure, according to Henry’s Law: At a certain temperature, the solubility of a gas in a liquid is proportional to the equilibrium partial pressure of the gas. That is, the solubility changes with temperature and the partial pressure of oxygen in the atmosphere, following Henry’s Law, as shown in the following formula:

[O2]=Kh×Po2

In the formula, [O2] is the balance of oxygen in the aqueous solution, and Po2 is the partial pressure of oxygen. From Henry’s law, we can see that when the temperature is constant, that is, when Kh is a fixed value, the greater the pressure, the greater the gas solubility. For example, the carbon dioxide in the carbonated drinks we drink is dissolved into the water by pressurization; on the contrary, the lower the pressure, the lower the gas solubility. It reflects the principle that the lower the pressure, the lower the gas solubility.

3. The relationship between the solubility of oxygen in water and the salt content

The total amount of dissolved salts in water is called salinity or salinity. Salinity refers to the total amount of major ions in a water body.

High levels of mineralization in water can cause:

One is to make the fish grow slower. The higher salinity of the water quality will destroy the acid-base balance of the blood of the fish, and the ability of hemoglobin to bind oxygen will decrease. The relative reduction of nutrients affects the normal growth of fish.

The second is to reduce the disease resistance of fish. High salinity water quality is more destructive to the epidermal tissue of fish gills, the gas exchange inside and outside the fish is affected, the digestion and absorption rate of feed is reduced, the fish body constitution is weak, the disease resistance is reduced, and the disease is prone to disease;

The third is that when the water body is polluted by inorganic and organic reducing substances, the oxygen consumption rate of its oxidative decomposition exceeds the rate of supplementing oxygen from the air, and the dissolved oxygen in the water body will decrease;

Fourth, the transparency of the water body is reduced, making it difficult for sunlight to penetrate the water layer, affecting the photosynthesis of plants in the water body, and causing the dissolved oxygen in the water body to be supersaturated;

Fifth, there are other substances such as salts in the voids of water molecules, which squeeze and fill the voids between water molecules, so that the voids between water molecules are reduced, resulting in the lack of oxygen in the water body, or no oxygen can be added. The higher the salinity, the lower the dissolved oxygen.

somefishFish can only grow in high and cold mountainous areas with low water temperature and high dissolved oxygen. Some fish can only slowly adapt to changes in water temperature and dissolved oxygen. If their living environment is suddenly changed, they will die due to lack of dissolved oxygen. Among them, the sudden change of water temperature is the “culprit”, and the increase of salt content is the “accomplice”.

4. The relationship between the solubility of oxygen in water and the diurnal variation

The amount of dissolved oxygen in water varies depending on temperature and water quality. in normal

Under the conditions of clear weather, the diurnal variation of dissolved oxygen in water is as follows: daytime is higher than nighttime, the highest time is from afternoon to evening, it begins to decrease after nightfall, and the lowest in the early morning. This is because the sunshine time is long on sunny days, and the plants in the water release more oxygen due to photosynthesis. After dark, plants do not have photosynthesis and release less oxygen than during the day.

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