Secondary Treatment of Sewage

No.1 Method: Biological treatment and some chemical methods

1. Overview of biological treatment for sewage

The biological treatment of sewage is to use the oxidative decomposition and transformation function of microorganisms, to the organic matter of sewage (a few inorganic substances) as the nutrients of microorganisms, to take certain artificial measures to create a controllable environment, through the metabolism of microorganisms, so that the pollutants in the sewage are degraded and transformed, and the sewage can be purified.

2. Classification of biological treatment of sewage

(1)Aerobic biological treatment

A. Theory

Under the condition of sufficient oxygen supply, the use of aerobic microbial life activity process, the oxidation of organic pollutants into more stable inorganic matter treatment method, in engineering known as sewage aerobic biological treatment.

B. Notice

a. In the aerobic treatment of sewage, dissolved oxygen must be supplied continuously. Because oxygen is the final hydrogen acceptor of organic matter, it is due to this transfer of hydrogen that energy is released to become the energy source for bacterial life activities and the synthesis of new cellular materials.

b. To ensure the effect of sewage treatment, there must first be a sufficient number of microorganisms, at the same time, there must also be a sufficient number of nutrients.

C. Aerobic biological treatment

a. Activated sludge process

· Concept

Activated sludge (activesludge) is a general term for the microbial community and the organic and inorganic materials to which they are attached. The microbial community mainly includes bacteria, protozoa and algae. Among them, bacteria and protozoa are the two main groups. Activated sludge is mainly used to treat sewage.

· Traditional activated sludge process

oxidation ditch, sequencing batch activated sludge process.

b. Aeration tank

· Concept

Aeration tank (aeration tank) is a structure that uses activated sludge process for wastewater treatment. A certain residence time of sewage is provided in the tank to meet the oxygen required by aerobic microorganisms and the mixing conditions of full contact between sewage and activated sludge. The aeration tank is mainly composed of three parts: the tank body, the aeration system and the inlet and outlet. The pool body is generally made of reinforced concrete, and the plane shape is rectangular, square and round.

· Classification

Air blast aeration, also known as compressed air aeration, is mainly composed of aeration fans and special aerators. The aeration pool using this method is mostly rectangular concrete pool, and the pool is divided into several separate water compartments with partition walls, and each compartment is divided into several corridors. After entering the tank, the sewage flows in the gallery successively and is discharged at the other end. Air is transported by air compressor through the pipeline to the air diffusion device located at the bottom of the pool, and becomes a bubble dispersion and escape, and the oxygen is dissolved into the water at the gas-liquid interface. The diffusion device has four different types: porous tube, fixed spiral aerator, water ejector and microporous diffusion plate.

Mechanical aeration，the mechanical impeller installed in the aeration tank is generally used to rotate and violently agitate the wastewater in the tank so that the oxygen in the air dissolves into the water. The impeller installed on the surface of the wastewater in the pool for aeration is called surface aeration. This device, through the water lifting effect of the impeller, promotes the continuous circulation of the wastewater in the pool, and constantly updates the gas-liquid contact surface to increase the oxygen intake. When the impeller rotates, it forms a hydraulic jump at the periphery, which can effectively wrap the air; Negative pressure is generated at the back of the blade, and air can be inhaled, so the inflation effect is better. The depth and speed of the impeller can be adjusted to ensure the best results. Typical mechanical aeration tanks include circular surface accelerated aeration tank, standard accelerated aeration tank, IO accelerated aeration tank and square accelerated aeration tank. The two methods of blast aeration and mechanical aeration can sometimes be combined to improve the oxygen capacity, which is suitable for sewage with a high concentration of organic matter.

· Aeration theory

Aeration is a means of bringing air into strong contact with water, with the purpose of dissolving oxygen from the air into the water, or expelling unwanted gases and volatile substances from the water into the air. In other words, it is a means of facilitating the exchange of substances between gases and liquids. It also has some other important roles, such as mixing and stirring. The oxygen in the air is transferred to the water by aeration, and the oxygen flows from the gas to the liquid phase is the double film theory proposed by Lewis and Whitman.

· Process flow

Aeration tank and sedimentation tank generally form a joint process flow. The one set in front of the aeration tank is called the primary sedimentation tank, and the one set behind the aeration tank is called the secondary sedimentation tank.

c. Biofilm process

· Concept

Biofilm method is a method of organic wastewater treatment by attaching microorganisms (i.e. biofilms) that grow on the surface of some solid objects.

· Composition structure

Biofilm is an ecosystem composed of highly dense aerobic bacteria, anaerobic bacteria, facultative bacteria, fungi, protozoa and algae, and the solid medium attached to it is called filter material or carrier.The biofilm self-filtration material can be divided into anaerobic layer, good gas layer, attached water layer and moving water layer.

· Theory of biofilm method

The principle of the biofilm method is that the biofilm first adsorbs and adheres to the organic matter in the water layer, which is decomposed by the aerobic bacteria in the aerobic layer, and then enters the anaerobic layer for anaerobic decomposition, and the flowing water layer flushes the aging biofilm to grow a new biofilm, and so on to achieve the purpose of purifying sewage.Biological filter, biological turntable; Fluidized bed, Air stripping Reactor (BAS).

(2)Anaerobic biological treatment

A. Summary

Anaerobic biological treatment refers to the use of anaerobic microbial metabolism process, in the anaerobic condition of the organic pollutants in the sewage into inorganic substances and a small amount of cellular matter sewage treatment method.

B. Defect

a. The COD and BOD values of the effluent after anaerobic treatment are high, which is difficult to reach the standard (aerobic treatment is required as post-treatment).

b. Anaerobic hydraulic retention time is generally longer, and anaerobic start-up time is generally longer.

c. Affected by temperature, odor.

No.2  Target pollutants: soluble organic matter and some colloidal pollutants in wastewater.

No.3 Effect

After secondary treatment, the removal rate of BOD in wastewater can reach 80%~90%, that is, the BOD content can be less than 30mg/L.The water after secondary treatment can generally meet the standards of agricultural irrigation and wastewater discharge, so secondary treatment is the main body of wastewater treatment.

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