Adsorption-biodegradation method in wastewater treatment

Adsorption -Biodegradation process

The adsorption-biodegradation method refers to a two-stage activated sludge process connected in series. These two stages each have a sedimentation tank, and activated sludge in different growth stages is returned to their respective aeration tanks. It is also known as the A-B method.

1. There is no primary sedimentation tank. The reason is that the adsorption tank needs to utilize bacteria screened from the drainage pipes. Setting up a primary sedimentation tank would cause the loss of these bacteria.

2. There is pretreatment at the front end, including bar screens and grit chambers. The difference between pretreatment and primary treatment should be noted here. Primary treatment refers to the primary sedimentation tank; all processes before the primary sedimentation tank are considered pretreatment, generally including bar screens and grit chambers.

For the A section of the A-B process, it not only removes most of the organic matter but also plays a regulating and buffering role, ensuring the shock resistance, toxicity resistance, and stability of the entire treatment system. In summary, there are the following 7 characteristics:

1. It has a strong ability to flocculate, adsorb, and degrade organic matter;

2. It has a high degradation rate of COD, degrading it into easily biodegradable BOD5 substances;

3. High sludge yield and high organic matter content are beneficial for biogas production and energy generation;

4. Strong adaptability, resistant to changes in influent water quantity, quality, and pH value, and has the ability to resist shock loads;

5. It can tolerate the effects of toxic compounds and has toxicity resistance;

6. No primary sedimentation tank is needed;

7. Once the operating system is damaged, it can restore its original treatment efficiency within a few hours.

For the B section of the A-B process, due to the regulation and buffering of the A section, the influent water quality of the B section is quite stable, giving full play to its organic matter degradation function, and the load is relatively low (0.15~0.3) kgBOD/(kgMLSS▪d), with a long sludge age of 15~20d, providing conditions for nitrification in the B section. Denitrification in the A-B method mainly occurs in the B section, and only under certain special circumstances can it occur in the A section.

A-B process denitrification process

A-B process phosphorus removal process

A-B process denitrification and phosphorus removal process

Function and design operating parameters of Section A

1. Section A continuously receives wastewater from the drainage system and is also inoculated with microbial populations that survive in the drainage system, meaning the drainage system acts as a "microbial selector." Processes of microbial population adaptation, elimination, selection, and proliferation continuously occur here. This allows for the cultivation, domestication, and induction of microbial populations adapted to the raw wastewater. Because this process does not have a primary sedimentation tank, Section A can fully utilize the microbial populations selected by the drainage system, allowing Section A to form an open biological dynamic system.

2. Section A has a high load, providing good environmental conditions for rapidly proliferating microbial populations. The microbial populations that can survive in Section A can only be prokaryotic bacteria with strong shock load resistance; protozoa and metazoa are difficult to survive.

3. Section A has a high sludge yield and a certain adsorption capacity. The removal of pollutants in Section A mainly relies on the adsorption of biological sludge. In this way, certain heavy metals and microorganisms degrade organic matter, as well as nitrogen and phosphorus, can be removed to a certain extent through Section A, thus greatly reducing the load on Section B. The BOD removal rate of Section A is roughly between 40% and 70%, but the biodegradability of the wastewater after treatment by Section A will be improved, which is beneficial for the subsequent biodegradation of Section B.

4. Since the removal of pollutants in Section A is mainly dominated by the adsorption function of physicochemical action, it has a certain adaptability to load, temperature, pH, and toxicity.

5. For the treatment of urban wastewater, the recommended design and operating parameters for Section A are:

1) BOD-sludge load (LS) 2~6 kgBOD/(kgMLSS·d), which is 10~20 times that of ordinary activated sludge treatment systems;

2) Sludge age (θC) 0.3~0.5d;

3) Hydraulic retention time (t) 30min;

4) Dissolved oxygen (DO) concentration in the adsorption tank 0.2~0.7mg/L.

B Section Function and Design and Operating Parameters

First, it should be stated that the performance of various functions of Section B is conditional on the normal operation of Section A.

1. Section B receives the treated water from Section A, with stable water quality and quantity. Shock loads no longer affect Section B, and the purification function of Section B is fully utilized.

2. Removal of organic pollutants is the main purification function of Section B.

3. The sludge age in section B is longer, and nitrogen is partially removed in section A, resulting in a lower BOD/N ratio. Therefore, section B has the conditions for nitrification.

4. Section B bears 30%~60% of the total load. Compared with ordinary activated sludge treatment systems, the volume of the aeration bioreactor can be reduced by about 40%.

5. The recommended design and operating parameters for section B in treating urban wastewater are:

1) BOD-sludge load (LS) 0.15~0.3 kgBOD/(kgMLSS·d);

2) Sludge age (θC) 15~20d;

3) Hydraulic retention time (t) 2~3h;

4) Dissolved oxygen content (DO) in the aeration tank 1~2mg/L.