Application and process parameters of activated sludge

Multiplication of activated sludge and its application

The proliferation of microorganisms in activated sludge is an inevitable result of the reaction of activated sludge and the degradation of organic matter in the aeration tank, and the proliferation of microorganisms is the growth of activated sludge.

1. Activated sludge proliferation curve

  1. Adaptation period:
    The short-term adaptation process of activated sludge microorganisms to new environmental conditions and types of organic pollutants in sewage. After the adaptation period, the microorganism may not proliferate in large numbers, but some qualitative changes have occurred: a. The cell volume has increased; the enzyme system has also been adjusted accordingly; c. Some changes have occurred in order to adapt to the new environment; etc. . Various pollution indicators such as biochemical oxygen demand and chemical oxygen demand did not change much.
  2. Logarithmic growth period:
    The F / M value is very high (2.2, so the organic substrate is very rich, nutrition is not a factor that controls the proliferation of microorganisms; the growth rate of microorganisms has nothing to do with substrate concentration, and is a zero-order reaction. It is only affected by the microorganism’s own minimum generation time Control, that is, it is only limited by the physiological function of the microorganism itself. The microorganism takes up organic matter at the highest speed and reproduces at the highest speed to synthesize new cells. At this time, the activated sludge has a higher energy level, of which The microbial activity is very strong, resulting in loose sludge texture, unable to form better flocs, and poor sludge sedimentation performance. Activated sludge has a very high metabolic rate and a large oxygen demand. Generally, this stage is not used as the operating condition However, this stage is also used, such as the high-load activated sludge process.
  3. Slow growth period:
    When the F / M value drops to a certain level, the organic substrate concentration becomes a controlling factor for microbial proliferation. The proliferation rate of microorganisms is directly proportional to the residual organic substrate, which is a first-order reaction. The degradation rate of the organic matrix also began to decline. The proliferation rate of microorganisms gradually decreased until it reached zero in the final stage of this period, but the number of microorganisms was still increasing. The energy level of activated sludge is reduced, flocs begin to form, and activated sludge has good coagulation, adsorption and sedimentation properties. Due to the low concentration of residual organic matter, the quality of the effluent water has been greatly improved, and the entire system operates stably. Generally speaking, most activated sludge treatment plants control the operating conditions of the aeration tank within this range.
  4. Endogenous breathing cycle:
    At the beginning of this period, the rate of endogenous respiration exceeded the rate of synthesis for the first time, so overall, the amount of activated sludge is decreasing, and eventually all living cells will die, leaving only the endogenous respiratory residues. These residues are mainly cell walls that are difficult to degrade. The sludge has a high degree of mineralization, good sedimentation performance, but poor cohesion. Organic matter is almost depleted and the water quality is good after treatment. Generally, this stage is not used as an operating condition, but it is also used, such as extended aeration.

2. Application of activated sludge proliferation law;

  1. The proliferation of activated sludge is mainly controlled by the F / M value;
  2. Activated sludge with different value-added periods has different properties and effluent quality;
  3. By adjusting the F / M value, the operating conditions of the aeration tank can be adjusted and controlled to achieve different effluent water quality and activated sludge with different properties;
  4. The activated sludge method has different operation modes and different positions on the value-added curve.

3. Organic degradation and microbial proliferation:

The microbial proliferation of activated sludge is the combined result of microbial proliferation and auto-oxidation (endogenous respiration),
Daily net growth of activated sludge microorganisms in the aeration tank is 33,360

4. Organic degradation and oxygen demand:

The microorganisms in activated sludge require oxygen supply during metabolic activities. The main functions of oxygen are:

  • oxidatively decompose a part of organic matter;
  • auto-oxidize a part of the material of its own cells.

5. the actual process of activated sludge purification of wastewater:

In the activated sludge treatment system, the essence of removing organic pollutants from wastewater is the process by which organic substrates are taken up, metabolized and utilized by the activated sludge microorganisms as nutrients. As a result of this process, the sewage is purified. Microbes gain energy to synthesize new cells, and activated sludge grows. Generally, the entire purification reaction process is divided into three stages: ① initial adsorption; ② microbial metabolism; ③ condensation, precipitation and concentration of activated sludge.

The so-called “initial adsorption” refers to: in the activated sludge system, within a short time (10 ~ 30min) after the sewage starts to contact the activated sludge, the activated sludge has a strong adsorption due to its large surface area Capacity, so within a short period of time, a large amount of organic pollutants in suspended and colloidal state can be removed from the wastewater, and the BOD5 value (or COD value) of the wastewater is greatly reduced. However, this is not true degradation. Over time, the BOD5 value of the mixed solution will rise, and after that, the BOD5 value will gradually decrease.

The size of activated sludge adsorption capacity is related to many factors:

  • The nature and characteristics of wastewater: it has a good effect on wastewater containing higher concentrations of suspended or colloidal organic pollutants;
  • State of activated sludge: After the adsorption is saturated, sufficient regeneration aeration should be given to restore and enhance the adsorption function. Generally, activated sludge microorganisms should enter the endogenous metabolic period.

6. the basic process parameters of activated sludge process

  1. Volumetric Organic Loading Rate:
  2. Sludge Organic Loading Rate:
  3. Hydraulic Retention Time: (h)
  4. Sludge Age or Sludge Retention Time: (h or d)
  5. Reflow ratio:

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