Introduction to activated sludge-process flow and performance indicators

Activated sludge can be divided into aerobic activated sludge and anaerobic granular activated sludge. Either way, activated sludge is a complex macroflocculation microbial symbiote composed of various microorganisms, organic and inorganic colloids, and suspended matter. This symbiote has strong adsorption and degradation capabilities, can adsorb and degrade a large number of pollutants, and achieves the purpose of treating and purifying sewage. Activated sludge is the most commonly used method for treating sewage biomass. After preliminary precipitation, the sewage enters the aerobic reaction tank, removes various large particles, and supplies oxygen through aeration or stirring in the tank. In the activated sludge process, most of the activated sludge in the treated effluent is precipitated and returned to the reaction tank, thereby maintaining a high microbial density and activity. When the sewage stays in the aerobic reaction tank, part of the organic matter is processed into inorganic matter, that is, mineralization; the other part is transformed into microbial cell material. In activated sludge process, the sedimentation performance of sludge is a factor that seriously affects the treatment effect. If the sedimentation performance of activated sludge is poor, excessive breeding of filamentous fungi and fungi will cause swelling of activated sludge. Although the swelling mechanism of activated sludge is not completely clear, activated sludge is usually easy to produce under conditions of high carbon-nitrogen ratio (carbon-nitrogen ratio) and carbon-phosphorus ratio (carbon-phosphorus ratio) and low dissolved oxygen concentration in water . In order to maintain good treatment results, sludge swelling should be avoided. Therefore, the activated sludge method should strictly control the carbon-to-nitrogen ratio and carbon-to-phosphorus ratio in the system wastewater inlet, and maintain a high level of dissolved oxygen to maintain a good operating state. Except for a part of the activated sludge generated, which is recycled, other surplus sludge requires additional treatment. The treatment method is anaerobic digestion, landfill or drying. The dried processed products can be used as agricultural fertilizers.

Activated sludge process is a continuous process, so it is easy to use computer control to realize monitoring automation. In addition, the efficiency of the activated sludge treatment of wastewater can be greatly adjusted according to specific conditions, so it can be used to treat wastewater with large changes in flow rate and concentration. Activated sludge process is widely used to treat urban domestic sewage and various industrial wastewater.

1. Basic process flow of activated sludge process

1. The basic composition of activated sludge process

  1. Aeration tank: reaction body
  2. Second settling pond: 1) Separation of mud and water to ensure the quality of the effluent; 2) Ensure the return of sludge and maintain the sludge concentration in the aeration tank
  3. Backflow system: 1) maintain the sludge concentration in the aeration tank; 2) change the backflow ratio and the operating conditions of the aeration tank.
  4. Excess sludge discharge system: 1) It is one of the ways to remove organic matter; 2) Keep the system running stably.
  5. Oxygen supply system: provide enough dissolved oxygen

2. The basic conditions for the effective operation of the activated sludge system are:

  1. The wastewater contains sufficient soluble and easily degradable organic matter;
  2. the mixed solution contains sufficient dissolved oxygen;
  3. Activated sludge is suspended in the pond;
  4. The activated sludge is continuously refluxed, and the remaining sludge is removed in time, so that the mixed liquid maintains a certain concentration of activated sludge;
  5. No toxic or harmful substances flowed in.

2, the properties and performance indicators of activated sludge

1. Basic properties of activated sludge

  1. Physical properties: “Bacillin”, “Biofloc”:
    Color: brown, (earth) yellow, iron red;
    Odor: earthy smell (city sewage);
    Specific gravity: slightly greater than 1, (1.002 ~ 1.006);
    Particle size: 0.02 ~ 0.2 mm;
    Specific surface area: 20 ~ 100cm2 / ml.
  2. Biochemical properties:
    1) Moisture content of activated sludge: 99.2 ~ 99.8%;
    Composition of solid matter: living cells (Ma), microbial endogenous metabolism residues (Me), organic substances (Mi), inorganic substances (Mii) that are difficult to biodegrade in the adsorbed raw wastewater.

2. Microorganisms in activated sludge:

  1. Bacteria: It is the most active ingredient in the activated sludge purification function.
    The main strains are: Genobacterium, Pseudomonas, Micrococcus, Flavobacterium, Bacillus, Alcaligenes, Achromobacter, etc
    Basic characteristics: 1) Most are aerobic or facultative heterotrophic protrophic bacteria;
    2) Under aerobic conditions, it has a strong function of decomposing organic matter;
    3) It has a high proliferation rate, and the generation time is only 20 ~ 30 minutes;
    4) Among them, Gliocladium has the function of combining a large number of bacteria into a “bacteria micelle”.
  2. Other microorganisms — protozoa, metazoa-about 103 per ml in activated sludge

3. Performance index of activated sludge:

  1. Mixed Liquor Suspended Solids (MLSS):
    MLSS = Ma + Me + Mi + Mii Unit: mg / l g / m3
  2. Mixed liquid volatile suspended solids concentration (MLVSS) (Mixed Volatile Liquor Suspended Solids):
    MLVSS = Ma + Me + Mi;
    Under certain conditions, MLVSS / MLSS is relatively stable. For urban sewage, it is generally 0.75 ~ 0.85.
  3. Sludge Volume (SV) (Sludge Volume):
    It means that the mixed liquid in the aeration tank is left for 30 minutes in a graduated cylinder, and the volume ratio of the precipitated sludge to the original mixed liquid is generally expressed in%;
    It can relatively reflect the amount of sludge and the sludge condensation and sedimentation performance, which can be used to control the sludge discharge and detect the early sludge expansion in time;
    The normal value is 20 ~ 30%.
  4. Sludge Volume Index (SVI):
    The volume of sludge formed by 1g of dry sludge after the mixed solution at the outlet of the aeration tank is allowed to settle for 30 minutes, the unit is ml / g.

It can more accurately evaluate the sludge agglomeration and sedimentation properties. If its value is too low, it means that the mud particles are small, dense, and there are many inorganic components. If its value is too high, it means that its sedimentation performance is not good, and the swelling phenomenon will or will occur.
The SVI of urban sewage is generally 50 ~ 150 ml / g;

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Further reading:

Introduction to activated sludge-process flow and performance indicators

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