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Activated sludge process (activated sludge process)

An aerobic biological treatment method for treating organic wastewater by using suspended microbial flocs.

Activated sludge refers to microbial flocs composed of aerobic microorganisms (including bacteria, fungi, protozoa and metazoa) and organic and inorganic substances metabolized and adsorbed by them. In activated sludge process, the main body of pollutant degradation process is microorganisms in activated sludge. Soluble organic matter can be directly utilized and decomposed by bacteria and fungi. As a nutrient, but not as a direct source of nutrition for micro-animals. Saprophytic microorganisms such as bacteria play a major role. In addition, there are microorganisms that are completely nutritious to animals, such as protozoa and micro-metazoa.

Bacteria forming activated sludge flocs

Micellar bacteria

The main components of activated sludge floc have strong adsorption and oxidation ability to organic matter. The formation of floc can prevent bacteria from being swallowed by micro-animals, which is related to sludge sedimentation and effective separation of sludge and water in the secondary sedimentation tank.

Formation mechanism of bacterial micelle

Alternating matrix theory

In the aging stage of cells, nitrogen limitation occurs and the secretion of extracellular polymers increases. These bacterial polysaccharides can aggregate bacteria.

Cellulose theory

Bacterial cells secrete a lot of mucus or cellulose, which makes cells aggregate to form flocs.

Filamentous bacteria in activated sludge

Filamentous bacteria are also an important part of activated sludge.

Cross-woven in bacterial micelles, or epiphytic on the surface of flocs, a few are free.

Strong ability to oxidize and decompose organic matter, and can purify sewage.

Filamentous bacteria and sludge bulking in activated sludge

When the number of filamentous bacteria exceeds the number of bacterial micelles, the settling performance of sludge flocs becomes worse, which seriously leads to the expansion of activated sludge and the decline of effluent quality.

There are mainly Chlamydomonas planktonic, Thiobalite and sulfur-producing bacteria.

The reason of activated sludge bulking: non-filamentous bacteria bulking. Filamentous bacteria swell.

Degradation process of activated sludge process

Adsorption stage

In the process of growth and reproduction, microorganisms form micelles with large surface area, flocculate and adsorb a large amount of wastewater, and most of the organic pollutants in wastewater are removed by adsorption.

Absorption and decomposition stage

Bacteria absorb adsorbed pollutants into cells and metabolize them. Some of them are transformed into structural components of bacteria and new cells, and the other part is completely oxidized into carbon dioxide and water.

Basic principle of activated sludge process

19 14 British Aden and Lokot created this law.

The first sewage treatment plant was built in Britain.

Basic characteristics of activated sludge process

Using biological floc as the main body of biochemical reaction;

Using aeration equipment to disperse air or oxygen into the biochemical reaction system to provide oxygen source for microorganisms;

Mixing and stirring system, increasing contact and accelerating the mass transfer process of biochemical reaction;

Removing organic matter by precipitation to reduce the microbial solid content in effluent;

The concentrated microbial floc in the sedimentation tank returns to the reaction system through reflux;

In order to ensure the stability of the average residence time of biological cells in the system, some biological solids are often discharged.

Main types of activated sludge process:

According to the entry mode of wastewater and return sludge and their mixing mode in aeration tank:

Push flow: several long and narrow flow pools, where wastewater enters from one end and flows out from the other. With the process of water flow, the matrix degrades and microorganisms grow.

Complete mixing type: After the wastewater enters the aeration tank, it is immediately mixed with the mixed solution of activated sludge in the tank under stirring, so that the influent is well diluted, and the sludge and wastewater are fully mixed to bear the impact of the change of wastewater quality to the maximum extent.

Push-flow activated sludge process

Wastewater and return sludge enter the reaction system from one end of the aeration tank at the same time, and the water flow is plug flow.

Comprises a primary sedimentation tank, an aeration tank, a secondary sedimentation tank and a sludge reflux device.

In the aeration tank, the ratio F/M of pollutant concentration (f) to microbial biomass (m) decreases with the process.

Short-time aeration

Improve the aeration mode: increase the ventilation at the entrance, and then reduce the ventilation with the gradual decrease of organic matter concentration. Also known as gradually decreasing aeration method.

Stage aeration method

On the basis of ordinary plug-flow aeration method, the water inlet point is adjusted to make the wastewater flow into several points along the length of the tank.

Also known as multi-point water inflow method. Advantages: it can reduce the oxygen consumption rate at the front end of the aeration tank, avoid hypoxia, and improve the air utilization rate and the working capacity of the aeration tank. The volume of aeration tank can be reduced by about 30%.

Biological adsorption method (regenerative adsorption aeration method)

Features: wastewater adsorption and sludge regeneration, that is, the adsorption stage and oxidation decomposition stage of activated sludge purification wastewater are carried out in two ponds or two parts of a pond respectively.

Advantages: It is an ideal choice for treating colloidal pollutants in wastewater.

The total volume of the adsorption regeneration aeration tank can be reduced by more than 50%.

Disadvantages: Due to the limited adsorption of soluble organic matter by activated sludge in a short time, the treatment effect will be slightly reduced.

Fully mixed activated sludge process

After the primary sewage and return sludge enter the aeration tank, they are completely mixed with the original mixed liquid in the tank immediately, so that the concentrated wastewater is well diluted.

Advantages: it can bear large impact load and aerate evenly.

Shortcomings: The wastewater stays in the pond for a short time, and the bacteria are always in logarithmic growth period, so the treatment effect is generally worse than that of plug flow treatment.

In the fully mixed aeration tank, the aeration zone is stirred by the impeller, which plays the role of oxygenation and improving the mixing of sludge and muddy water.

Sequencing batch reactor

New process of activated sludge process

The biochemical treatment of wastewater is realized by automatic control of five stages: water inflow, reaction, sedimentation, drainage and sludge discharge.

During the operation, the control time and total hydraulic retention time of each stage are determined according to the test, and the corresponding automatic control is carried out.

When complete aeration is used, aerobic process takes place in the reactor. Under the condition of limited aeration, anoxic or anaerobic environment appears in the reactor.

Advantages of SBR process:

1. Activated sludge with good settling performance can be obtained.

2. The concentration of activated sludge can be greatly increased.

3. The activity of activated sludge is obviously improved.

Its biological reproduction rate is very fast.

5. Complete the decomposition of refractory organic matter through anoxic-anaerobic-aerobic process.

Deep-water aeration activated sludge process

Features: The aeration tank is deep, which improves the saturated dissolved oxygen concentration of the mixed solution, accelerates the introduction of oxygen into the mixed solution, and is beneficial to the degradation and removal of organic pollutants.

Advantages: The aeration tank is deeply developed, with small floor space, power saving and less excess sludge. Due to the strong oxygen supply capacity formed by water pressure, it can run at high load.

oxidation ditch

Double-ditch oxidation ditch: the whole operation process is alternately carried out through double ditches, denitrification is carried out when the brush rotates at low speed, nitrification is carried out at high speed, and ditch 1 and ditch 2 alternately discharge water.

Advantages: Compared with the conventional activated sludge process, the sludge in the oxidation ditch has a long residence time and the nitrification reaction is easy to carry out. By adjusting the oxygen supply, higher nitrogen removal efficiency can be obtained.