Why can’t dissolved oxygen be too high in wastewater treatment?
The process principle of aerobic treatment system is to utilize the metabolism of aerobic microorganisms to convert organic pollutants in wastewater into harmless carbon dioxide and water as well as the energy for their own survival, and oxygen is necessary for them to maintain the normal life activities of microorganisms. So the higher the dissolved oxygen, the better the aerobic system treatment effect will be?
Before answering this question, first understand the concept of food and micro-ratio in aerobic system. Take the commonly used activated sludge system as an example, the ratio of the total amount of BOD supplied to the aeration tank and the total amount of activated sludge in the aeration tank every day is the ratio of food to microorganisms (in which the supplied BOD can be regarded as the food provided to microorganisms).
The formula for calculating the food-microbe ratio is as follows:
F/M=Q*BOD5/(MLVSS*Va)
F: Food stands for food, the amount of food entering the system (BOD)
M: Microorganism stands for the amount of active matter (sludge volume)
Q: water volume, BOD5: the value of influent BOD5
MLVSS: activated sludge concentration
Va: volume of aeration tank
Usually the appropriate range of food and micro-ratio is between 0.1-0.25kgBOD5/kgMLSS.d. Too high a food and micro-ratio indicates that the microorganisms have too much food and the aeration tank is in a high load operation state, while too low a food and micro-ratio means that the aeration tank is in a low load operation state.
Food micro-ratio is too high and too low will appear what results?
1. When the aeration tank is in the appropriate range of food micro-ratio operation, the activated sludge floc structure is good, good settlement performance, clear and transparent water;
2. When the aeration tank is in high food micro ratio operation state, even overload operation, due to excess food, activated sludge settling performance deterioration, turbid water, wastewater in the BOD is difficult to be completely degraded;
3. When the aeration tank is in low food micro ratio operation state, due to insufficient food, activated sludge is easy to appear aging phenomenon.
Long-term low-food micro-ratio operation may lead to unflocculation of sludge, and even induce the expansion of activated sludge filamentous bacteria.
When the aging phenomenon of activated sludge occurs and triggers the deflocculation of sludge, the floc structure of activated sludge will become looser, and the effluent will carry a lot of fine sludge fragments, resulting in a decline in the clarity of the effluent and a deterioration of water quality.
After understanding the food micro ratio, we look at the impact of dissolved oxygen on the treatment effect.
When the aeration tank is in a high food micro-ratio operation, maintaining relatively high dissolved oxygen is favorable, can accelerate the degradation rate of organic matter in wastewater.
When the aeration tank is in the low food micro ratio operation state, if still maintain high dissolved oxygen, due to food shortage, will promote the activated sludge endogenous metabolism to accelerate the occurrence of the activated sludge flocculation phenomenon, that is, usually referred to as the phenomenon of over-exposure. High dissolved oxygen will accelerate the metabolism of microorganisms, you can give an image of some examples, it is like a person, in the case of not eating enough food, you also let him work hard, can only accelerate his form of thinning, until the demise.
Therefore, in the operation of aerobic system, the control of dissolved oxygen concentration should be closely related to the control of food micro-ratio, high food micro-ratio can control the higher concentration of dissolved oxygen, promote the effective degradation of organic pollutants. On the contrary, when the food micro ratio is insufficient, the dissolved oxygen concentration should be controlled relatively low, reduce the rate of endogenous metabolism, in order to avoid sludge aging and sludge deflocculation phenomenon, but also can reduce power consumption and save operating costs. In practice, we can control the dissolved oxygen of aerobic tank by controlling the frequency of fan, running time or adjusting the size of air release valve.
Wastewater treatment in the principle of evaporation crystallization, process knowledge is what?
In the chemical industry, industrial production industry evaporation, evaporation and concentration, evaporation and crystallization are common processes, evaporation and crystallization is currently more widely used in industrial wastewater treatment, the principle of evaporation and crystallization is what?
The principle of evaporation
The principle of evaporation is to make the solution containing non-volatile solutes boiling vaporization, and move out of the steam, so that the concentration of solutes in the solution to increase the unit operation, evaporation operations are widely used in the chemical industry, petrochemical industry, evaporation crystallization, evaporation and concentration is a common type of process.
The principle of evaporation crystallization
Evaporation crystallization is through the process of evaporation, with the volatilization of the solvent, the original unsaturated solution gradually become saturated solution, saturated solution and then gradually become supersaturated solution, then the solute will begin to precipitate from the supersaturated solution. Many solutes can be precipitated in the form of crystals (also in the form of amorphous precipitation), which is the crystallization process.
For evaporation operations, evaporative crystallization is carried out to remove the solvent, increase the solution to saturation, and subsequently heat or cool it to precipitate a solid product to obtain a solid solute.
How evaporation crystallization works
Evaporation crystallization operation, the need for a constant supply of heat energy, the heat source used in industry is usually water vapor, and evaporation of most of the material is an aqueous solution, evaporation of steam is also produced by water vapor, in order to easily distinguish between the former is called heating steam or raw steam, the latter is known as the secondary steam.
Take evaporation crystallization, the operation mode has: atmospheric pressure, pressurization, decompression (vacuum) evaporation.
Evaporation crystallization process
In the process of evaporative crystallization, the flash mode of evaporation (flash evaporation) will be commonly used: this is a special decompression evaporation, the pressure of the hot solution will be reduced to a lower pressure than the saturation pressure at the temperature of the solution, then part of the water will be boiled in the instant the pressure is reduced to vaporize. The advantage of flash evaporation is to avoid generating a scale layer on the heat transfer surface, flash evaporation does not need to be heated, the heat comes from their own excretion of sensible heat.
Heat pump evaporation is also one of the evaporation crystallization process, increase the pressure and temperature of the secondary steam, re-used as evaporation of the heating steam, called heat pump evaporation or steam recompression evaporation.
For heat pump evaporation is to consume part of the high quality energy (mechanical energy, electrical energy) or high temperature thermal energy at the expense of the thermal cycle, the heat will be transferred from the low temperature object to the high temperature object of the energy utilization device.
When carrying out the evaporation crystallization process, we also need to consider how to pick the right evaporation crystallization equipment.
How to select the appropriate evaporation crystallization equipment
According to the situation, for the evaporation of salts, the forced circulation type evaporator is preferred. If the concentration of salts is low, the front falling film evaporator + forced circulation evaporator can also be used to reduce the operation and initial investment. For the evaporation of other non-salt type, the falling film evaporator is preferred.
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