Introduction

Most aquatic organisms need dissolved oxygen (often abbreviated DO) to survive, but the source of the oxygen is not water molecules (H2O). DO is gaseous molecular oxygen that exists in the atmosphere as O2 or as a by-product of photosynthesis. Once dissolved in water, it is bioavailable and plays an important role in many chemical processes within the aquatic environment. This oxygen is no different from the oxygen people breathe, except that it dissolves in water.

In the natural state, the oxygen dissolved in water is the main source of oxygen for the respiration of aquatic organisms. Therefore, dissolved oxygen (DO) is the most important indicator of water. DO refers to the dissolved oxygen mass per unit volume of water, and the measurement unit is mg/L. In unpolluted water, dissolved oxygen is saturated. An appropriate amount of oxygen is the basic condition for the survival and reproduction of fish and aerobic bacteria. In fresh water with an atmospheric pressure and a temperature of 0 °C, when DO is saturated, the content is 10 mg/L, and when the dissolved oxygen is lower than 4 mg/L, it is difficult for fish to survive. After the water is polluted by organic matter, the organic matter is oxidized due to the action of aerobic bacteria, and the dissolved oxygen in the water is consumed. If the oxygen in the air is not replenished in time, the DO in the water will decrease, which will eventually lead to the deterioration of the water body, so DO is used as an indicator of the degree of water pollution. The less DO, the more serious the degree of pollution. Therefore, the accurate measurement of DO content in water is of great significance for environmental monitoring and environmental law enforcement.

DO is also a useful parameter to measure when conducting groundwater surveys. During subsurface drilling, DO can help determine when stable conditions have been reached and evaluate the configuration of the well. DO also plays an important role in chemical reactions that occur underground. It regulates the valence state of trace metals and inhibits microbial metabolism of dissolved organic compounds such as oils. Microbes, like other living things, need to respire (that is, take in oxygen). An electron acceptor is required for respiration, and since oxygen is preferred, DO is quickly depleted in the presence of pollution. Therefore, DO can only be found in uncontaminated groundwater. Once DO is depleted, other electron acceptors are used. After the oxygen is depleted, nitrate is also depleted, so nitrate can only be found further away from the mantle plume. The electron acceptor of last resort was carbon dioxide (CO2). The process that uses CO2 is the process that produces methane, and this process takes place closest to the source of the pollution.

During water treatment in sewage treatment plants, microorganisms are used to consume waste and transform it into harmless end products. DO plays a key role in this process because these microbes rely on it to break down pollutants in wastewater, such as organic matter or ammonia. In the activated sludge process (ASP) air is pumped into an aeration tank filled with microorganisms.

In order to better understand the importance and measurement technology of DO in water, the author will elaborate on the source of DO in water, the significance of DO measurement in different water bodies, and the selection of commonly used DO detection methods and measurement equipment.

DO unit of measure

DO has different measurement units, and the commonly used measurement units are mg/L or % saturation (DO%). mg/L is the number of milligrams of gaseous oxygen dissolved in 1 L of water. When interpreting % saturation, the air pressure at sea level needs to be considered, which is equivalent to 760 mmHg (13.595 1 g/cm3=1 mmHg=133.322 387 415 Pa). The part of the total pressure caused by oxygen (called the partial pressure) is equal to 160 mmHg (21% x 760 mmHg = 160 mmHg). If the DO sensor is calibrated at sea level, assuming the water and air are in equilibrium, the calibration can reach 100% saturation. But if the air pressure is less than 760 mmHg, and assume that the atmospheric pressure measured by the barometer is 750 mmHg. To determine the calibration value for the sensor, divide 750 mmHg by 760 mmHg, which is 98.68% (750 mmHg/760 mmHg=98.68%). Under this pressure, as long as the water and air are in equilibrium, the saturation cannot be greater than 98.68%. Therefore, the sensor will be calibrated to 98.68%. When it is necessary to obtain DO in local water, the calibration value is 100% regardless of the barometric pressure at the time of calibration. A 100% calibration value means that the calibration environment for that particular location is 100% oxygen pressure. Some DO instruments are capable of measuring DO in local water.

Percent Dissolved Oxygen (DO%) can be thought of as a unit of measure for DO that is directly determined by any instrument using electrochemical or optical sensors. The only variable affecting DO% is air pressure.

Table 1 DO% under different atmospheric pressure

The DO in mg/L was calculated by the instrument based on DO%, temperature and salinity. Table 2 shows the influence of different temperature and salinity on DO.

Table 2 DO at different temperature and salinity

Supersaturation of DO

The percentage value of dissolved oxygen in the natural environment can reach more than 100%. Photosynthesis may be an important driver of supersaturation because this process produces pure oxygen. Sometimes it can even make DO% values as high as 500%. Another factor is rapid temperature changes. Although the process of equilibrating the water with the air above is slow, the temperature of the water body can change rapidly. So, assuming the temperature of a stagnant lake rises rapidly by 5 K when the sun starts to shine, the DO content in the water should decrease as the temperature increases. However, if the balance between air and water does not change as rapidly as the temperature, the lake will technically become supersaturated with dissolved oxygen until a state of equilibrium is established again. Additional causes of supersaturation include turbulent flow conditions or any other substance that causes air and water to mix (e.g. air stones, rapids).

Environment variables that affect DO

Temperature

The most important variable is temperature, so it must be measured together with DO. The solubility of oxygen in water is inversely proportional to temperature, the higher the temperature, the lower the solubility. Therefore, with other variables held constant, DO concentrations in winter water bodies will be higher than those in summer water bodies. The same holds true at night, when bodies of water cool down at night, more oxygen is dissolved.

Salinity

Like temperature, the solubility of oxygen in water is inversely proportional to salinity—as salinity increases, solubility decreases. For example, seawater contains 20% less oxygen than freshwater at the same temperature and pressure. Therefore, when collecting DO data in estuaries, wetlands, coastal areas, aquaculture, or any other application where salinity may vary, it is critical to measure salinity (which can be done with conductivity sensors). Most modern DO measuring instruments, such as the SUP-DO700, can provide real-time salinity-compensated DO measurements as long as the conductivity and DO sensors are connected. Otherwise, salinity must be entered into the meter for compensation.

Air pressure

Unlike temperature and salinity, there is a direct relationship between air pressure and DO levels in water—as pressure decreases, DO levels decrease. In addition to altitude, air pressure also changes with the weather. A rapid drop in air pressure indicates that a storm is approaching. Most modern DO instruments have built-in barometric pressure sensors that automatically compensate for DO indications as barometric pressure changes.

DO measurement method

Colorimetry

A colorimeter, also known as a filter photometer, is an instrument that measures the intensity of color. When using these instruments, chemical reagents are mixed with the sample. If the parameter of interest is present, the solution has a color whose intensity is proportional to the concentration of the parameter being measured. After passing through the test tube containing the sample solution, the light passes through the color filter to the photodetector. Filters allow specific wavelengths of light to be selected. When the solution is colorless, all light passes through. Colored samples absorb light, and a corresponding reduction of light passes through the sample. There are two different colorimetric methods that can be used to measure DO: the indigo carmine reagent method and the Rhodazine D method. These two methods react with dissolved oxygen to produce blue and pink compounds, respectively.

Winkler titration

The Winkler titration method for the determination of DO concentration also requires the use of reagents [9]. In this method, the reagent reacts with DO to form an acidic compound, followed by a neutralization titration. Also, the color of the solution in this method will change, and the concentration value of DO can be determined by observing the critical point at which this color change occurs. Many standard operating procedures (SOPs) still use Winkler titration. For example, when performing biological oxygen demand (BOD) determination in a sewage treatment laboratory, the Winkles experiment needs to be performed in triplicate, and the average value is taken as the final experimental result.

Electrochemical sensors

Unlike Winkler titration or colorimetric measurements of DO, electrochemical sensors, also known as membrane-covered DO sensors, do not require reagents. These sensors measure quickly and over a wide range, but consume oxygen during the measurement process, and water must constantly pass through the membrane.

There are two types of electrochemical sensors: polarographic and amperometric. Polarographic sensors are called polarographic sensors, and amperometric sensors are called galvanic sensors.

The electrochemical dissolved oxygen sensor consists of two parts, an anode and a cathode, which are confined in an electrolyte solution by an oxygen-permeable membrane. Oxygen molecules dissolved in the sample diffuse through the membrane before being reduced (i.e. consumed) at the cathode. This reaction produces an electrical signal that travels from the cathode to the anode and finally to the instrument/gauge. The amount of oxygen diffused through the membrane is proportional to the partial pressure and concentration of oxygen outside the membrane. As the oxygen concentration changes, so does the oxygen diffused through the membrane, which also causes a proportional change in the probe current.

Polarographic sensors have a silver anode and a gold cathode. These materials require the probe to be warmed up or polarized for approximately 10 minutes before use. Polarographic sensors last longer than galvanic sensors because polarographic sensors are not always on (i.e. not always polarized).

Galvanic sensors have a zinc anode and a silver cathode. These materials allow for continuous polarization of the sensor, even when the meter is off, so no warm-up time is required. There is a downside to being polarized all the time - these sensors have a shorter lifespan than polarographic sensors.

Optical Sensors

Optical sensors and electrochemical sensors have some similarities. First, these sensors measure the pressure of oxygen dissolved in the sample. The "raw" indication is expressed as DO%, the only variable affecting DO% is barometric pressure. The higher the air pressure, the more oxygen is injected into the water. It should be noted that the mg/L of DO is calculated based on DO%, temperature and salinity. As with electrochemical sensors, no reagents are required when using optical sensors. Both types of sensors are also placed directly in the sample when measuring.

Optical DO sensors have several key structures. The sensor cap of an optical dissolved oxygen sensor contains a diffusion layer through which dissolved oxygen is constantly moving. Unlike electrochemical sensors, optical sensors do not consume oxygen during the measurement process, therefore, water does not need to flow continuously through the sensor cap. When oxygen passes through the diffusion layer, it affects the luminescence of the dye layer. The amount of oxygen passing through the sensing layer is inversely proportional to the lifetime of the excitation light in the sensing layer, which is measured by the sensor and compared with a reference to determine DO.

Selection of measurement methods and equipment

Choice of measurement method

There are several methods for measuring DO in water, and it is necessary to choose correctly.

When the only parameter being measured is DO, a colorimeter is generally not used because of the time required to mix the reagents and solutions. In addition, the measurement range of colorimeters is strictly limited.

Winkler titration is time consuming and complex to perform. If the standard operating procedure (SOP) is to follow ISO 5813 or ASTM D888, then Winkler titration must be used. In this case, an automatic titrator is recommended rather than manual titration. For cases where DO needs to be measured on site or where the number of samples is large, electrochemical or optical sensors are recommended for DO measurement.

Electrochemical and optical sensors are currently the most commonly used tools for measuring DO. Unlike other water quality sensors (such as nitrate) which are usually designed for specific applications, DO sensors can be used in various scenarios: surface water, aquaculture, groundwater, wastewater, etc.

Table 3 Comparison of common DO determination methods

In order to better compare the actual measurement results of the three methods, saturated DO water samples at 10°C, 20°C, and 30°C were prepared in the experiment, and the above methods were used to measure them respectively. It can be seen from Table 4 that the above methods There was no significant difference in the determination results under this condition.

Table 4 DO at different temperatures of 1 013 hPa

Selection of instruments and equipment

Equipment used for aquaculture monitoring such as SUP-DC2000 and SUP-DM2800 are also continuous monitoring facilities, but the system requires an external power supply, which is usually fixed. These facilities can be connected to an app, allowing users to monitor water quality conditions on the app, send alerts if there are problems, etc.

The IQ sensor network system is ideal for wastewater monitoring and control. Operators use such systems to see processes in real time and react accordingly. A wide range of controllers, modules and sensors (e.g. SUP-DO 700 optical sensor) allow for on-demand setup.

Summary

This article introduces the sources, influencing factors, measurement significance and methods of DO in water. By analyzing and comparing colorimetric methods, titration methods, electrochemical sensors and optical sensor methods, the corresponding applicable occasions are given; at the same time, several different models of DO measuring instruments produced by Supmea are briefly introduced and compared.