Introduction of dissolved oxygen sensor

dissolved oxygen At present, most of the biological treatment in sewage treatment adopts a combination of anaerobic . And aerobic treatment processes. Dissolved oxygen plays a pivotal role in the actual biological treatment of wastewater. The inappropriateness or excessive fluctuation of this indicator will quickly lead to Activated sludge systems are to impact, which in turn affects treatment efficiency. Therefore, in the actual biochemical treatment process, the content of dissolved oxygen needs to strictly controll.

An overview of dissolved oxygen

Dissolved oxygen (DO) is the abbreviation of Dissolved Oxygen.

The unit of dissolved oxygen is mg/L, expressed in milligrams of oxygen per liter of water. The amount of dissolved oxygen in the water is an indicator of the self-purification ability of the water body. High dissolved oxygen is conducive to the degradation of various pollutants in the water body, so that the water body can be purified quickly; on the contrary, low dissolved oxygen, the degradation of pollutants in the water body is slow.

2. Factors Affecting Dissolved Oxygen

The dissolved oxygen content in water is affected by two effects: one is the oxygen consumption effect that reduces DO. Including the oxygen consumption of aerobic organic matter degradation and advanced metabolic oxygen consumption. The other is also the reoxygenation effect that increases DO, mainly including: Dissolution of oxygen in the air. Means of aeration, etc. The mutual ebb and flow of these two effects makes the dissolved oxygen content in water show also spatiotemporal changes.

Environmental factors that affect the content of dissolved oxygen in water include water temperature, Oxygen partial pressure, salinity and other factors.

 Water temperature

When the partial pressure of oxygen and the salt content are constant, the saturated content of dissolved oxygen decreases with the increase of water temperature. The saturation content of dissolved oxygen changes more significantly with temperature at low temperature.

Salt content

When the water temperature and oxygen partial pressure are constant, the higher the salt content of the water. The smaller the saturated content of dissolved oxygen in the water. The salt content of seawater is much higher than that of fresh water. Under the same conditions, dissolved oxygen in seawater The saturation content is much lower than in fresh water. The change of the salt content in natural fresh water is very small. So the salt content has little effect on the saturated content of dissolved oxygen. Which can be approximated by the saturated content of pure water.

Partial pressure of oxygen

When the water temperature and salt content are constant. The saturated salt content of dissolved oxygen in the water increases. With the increase of the oxygen partial pressure on the liquid surface.

Monitoring of dissolved oxygen DO

Determination method of dissolved oxygen: (iodometric method)

1. Principle

Manganese sulfate and alkaline potassium iodide are added to the water sample. And the dissolved oxygen in the water oxidizes low-valent manganese to high-valent manganese. Resulting in a brown precipitate of tetravalent manganese hydroxide. After acid addition, the hydroxide precipitate dissolves and reacts with iodide ions to release free iodine. Using starch as an indicator, the released iodine was titrated with a standard solution of sodium thiosulfate. And the dissolved oxygen content was calculated according to the consumption of the titrated solution.

Reagents

Manganese sulfate solution: Weigh 480g of manganese sulfate (MnSO4·4H2O). Dissolve it in water, and dilute to 1000mL with water. This solution is added to the acidified potassium iodide solution. And it should not produce blue color in case of starch.

Alkaline potassium iodide solution: Weigh 500g of sodium hydroxide and dissolve it in 300-400mL of water. Weigh 150g of potassium iodide and dissolve it in 200mL of water. After the sodium hydroxide solution is cooled. Combine the two solutions, mix well, and dilute to 1000mL with water. If there is precipitation. Put it overnight, pour out the supernatant, store it in a brown bottle, close it with a rubber stopper. And store it in the dark. After this solution is acidified, it should not be blue in the presence of starch.

1+5 sulfuric acid solution.

1% (m/V) starch solution: Weigh 1 g of soluble starch, make a paste with a small amount of water, and then dilute to 100 mL with just boiled water. After cooling, add 0.1 g of salicylic acid or 0.4 g of zinc chloride for preservation.

0.02500mol/L (1/6K2Cr2O7) potassium dichromate standard solution: weigh 1.2258 g of potassium dichromate, which was dried at 105-110 for 2 hours, and cooled, dissolved in water, transferred to a 1000 mL volumetric flask, and diluted with water to the standard line, shake well.

Sodium thiosulfate solution: Weigh 3.2g of sodium thiosulfate (Na2S2O3 5H2O), dissolve it in boiled and chilled water, add 0.2g of sodium carbonate, dilute to 1000mL with water, store in a brown bottle, use 0.02500mol/ L potassium dichromate standard solution for calibration.

Sulfuric acid, ρ=1.84.

Assay Procedure

Fixing of dissolved oxygen: Insert a pipette under the liquid surface of the dissolved oxygen bottle, add 1 mL of manganese sulfate solution and 2 mL of alkaline potassium iodide solution, close the stopper, invert and mix several times, and let stand. Usually fixed at the sampling site.

Open the stopper, and immediately insert 2.0 mL of sulfuric acid into the liquid surface with a pipette. Close the bottle stopper, invert, mix and shake until all the precipitates dissolve, and leave it in the dark for 5 minutes.

Pipette 100.00 mL of the above solution into a 250 mL conical flask, titrate with sodium thiosulfate standard solution until the solution is pale yellow, add 1 mL of starch solution, continue to titrate until the blue color just subsides, and record the amount of sodium thiosulfate solution.

 Calculation

Dissolved oxygen (O2, mg/L)=M*V*8000/100

where:

M–concentration of sodium thiosulfate standard solution (mol/L);
V–titration consumes the volume (mL) of standard solution of sodium thiosulfate.

Precautions

When the water sample contains nitrite, it will interfere with the determination. Sodium azide can be added to decompose the nitrite in the water to eliminate the interference. The adding method is to add sodium azide to the alkaline potassium iodide solution in advance.

If the Fe3 content in the water sample reaches 100-200mg/L, 1mL of 40% potassium fluoride solution can be added to eliminate the interference.

If the water sample contains oxidizing substances (such as free chlorine, etc.), a considerable amount of sodium thiosulfate should be added in advance to remove

4. The relationship between dissolved oxygen and other control indicators

Relationship between dissolved oxygen and raw water components

The relationship between dissolved oxygen and raw water components focuses on the relationship between organic matter content and dissolved oxygen in raw water components. The specific performance is that the more organic matter content in the raw water, the more dissolved oxygen that microorganisms need to metabolize and decompose these organic matter, and on the contrary, the less. Therefore, when controlling aeration, it is necessary to pay attention to the matching of the amount of water and the content of organic matter in the wastewater.

When the water intake is 1.5 times the usual amount, if the aeration volume is not adjusted, the dissolved oxygen in the effluent of the aeration tank will be too low, sometimes even lower than 0.5mg/L, which is not conducive to the efficient treatment of activated sludge. If the influent flow does not increase, but the concentration of organic matter in the wastewater is too high, the demand for dissolved oxygen will also increase, and then the dissolved oxygen in the effluent of the aeration tank will be too low. The presence of some special components in the raw water will also affect the oxygenation effect. For example, the presence of detergent in the water makes the liquid surface of the aeration tank have an isolation layer that isolates the atmosphere, which in turn affects the improvement of the aeration effect.

The relationship between DO and activated sludge concentration

The relationship between DO and activated sludge concentration is still relatively close. It is usually to see that the demand for DO at high activated sludge concentration is significantly higher than the demand for dissolved oxygen at low activated sludge concentration. Therefore, in order to remove pollutants and achieve the discharge concentration, it is necessary to reduce the concentration of activated sludge as much as possible, which is very beneficial to reduce the amount of aeration and reduce power consumption.

At the same time, in the case of low activated sludge concentration, care should take not to over-aerate, so as to avoid excessive DO. And over-oxidation of the only activated sludge. Which is detrimental to the effluent of the secondary sedimentation tank.

Usually, it can see that more unsettled particles mixed in the effluent of the secondary sedimentation tank,. Which is why the oxidized activated sludge is Tto decompose in the effluent after disintegration. Similarly, the high activated sludge concentration has a very high demand for DO. And the activated sludge concentration cannot increase without control. So that the oxygen supply cannot keep up and anoxic phenomenon occurs. Naturally, the activated sludge The processing effect is also suppress.

The relationship between dissolved oxygen and activated sludge sedimentation ratio

The relationship between DO and activated sludge sedimentation ratio can be understood as the effect of DO on activated sludge sedimentation. There are mainly two situations:

Excessive aeration tends to make fine air bubbles adhere to the bacterial micelles of the activated sludge, causing the activated sludge to float to the liquid surface and produce scum.

The compressibility of activated sludge becomes poor, especially when the activated sludge expands with filamentous bacteria, it is more likely to cause the aeration of fine air bubbles to adhere to the bacterial micelles, resulting in a large amount of scum on the liquid surface.

Control basis and optimization of DO

Mainly based on: raw water quality (organic matter, nitrogen, phosphorus). Concentration of activated sludge, sludge settling ratio, pH, temperature, food to microratio (F/M), etc. to control.

Of course, the theoretical value given in writing: the DO concentration is generally ≥2.0 mg/L under aerobic conditions, the DO concentration under anaerobic conditions is ≤0.2 mg/L, and the DO concentration under anoxic conditions is 0.2-0.5 mg. /L. The specifics still have to grasp according to the actual situation.

In order to reduce the DO content of anaerobic or anoxic pools, work can do from the following aspects.

Water intake

Sewage generally has very little dissolved oxygen. But if it goes through the aeration grit chamber or falls and oxygenates before entering the water. It is necessary to consider reducing the amount of gas or reducing the drop to reduce oxygenation.

2. Return sludge

The DO in the inlet water of the sedimentation tank is good enough. As long as denitrification does not occur in the sedimentation tank. Too much DO will make the DO in the return sludge too high.

3. Internal reflow

Both AO/AAO are to design with internal return flow. By controlling the aeration near the internal return pump. The air volume in this section of the aeration tank is less than other sections. And the DO  brought back by the internal return flow will also be less.

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