An Efficient Solution for Sewage Treatment and Its Challenges

This process adds powdered activated carbon (hereinafter referred to as PAC), which is a type of extruded activated carbon, to the activated sludge reactor. By leveraging the adsorption properties of activated carbon and the degradation ability of microorganisms, it can remove compounds that are difficult to be biodegraded, and achieve in-depth treatment results. Granular activated charcoal water filter, another form of activated carbon, has also shown effectiveness in water purification, but in the PACT process, extruded activated carbon proves to be more suitable for integration with the sludge system.

 

Powdered activated carbon, or extruded activated carbon, can serve as a carrier for biofilm attachment. This can maintain a relatively high sludge concentration and greatly improve the system's ability to withstand shock loads. Compared to columnar activated carbon, which is often used in fixed - bed reactors, the extruded form provides a larger surface area in the PACT process for microbial colonization.

1,PACT Process Flow and Mechanism of Action

PACT Process Flow

Since the emergence of the PACT process, discussions about its mechanism of action have never stopped. There are mainly two representative theories:

Adsorption - Degradation - Biological Regeneration - Re-adsorption: PAC, an extruded activated carbon, and microorganisms work together to remove organic matter.
Activated carbon, including extruded activated carbon, has a large surface area and can adsorb organic matter and oxygen in water. Microorganisms attach to the surface of PAC. The extracellular enzymes they secrete, and the active centers of some enzymes, can enter the micropores of the activated carbon. They break down the adsorbed refractory organic matter into small molecules, regenerate the adsorption sites, and restore the adsorption function of PAC. The decomposed small molecular organic matter can provide nutrients and energy for microorganisms, increasing the activity of microorganisms. This makes it more conducive to the removal of organic matter.
It is worth mentioning that even if the enzyme molecules cannot enter the micropores, after the organic matter in the macropores and mesopores is degraded, due to the concentration difference, the organic matter in the micropores will diffuse into the macropores and mesopores and can still be degraded by microorganisms.

 

Simple Superposition of Microbial Degradation and Activated Carbon Adsorption
Obviously, this theory is completely opposite to the first one. After several adsorption cycles, the extruded activated carbon (PAC) will gradually become saturated, its adsorption capacity will decrease, and the removal rate of organic matter will also drop. This shows that the surface of PAC is saturated, and there is no biological regeneration phenomenon.

The PACT method is better than the single activated sludge method. Columnar activated carbon, if used in a similar process, might face challenges in maintaining a uniform distribution and effective contact with microorganisms, while extruded activated carbon in PACT has proven to be more efficient.
In fact, whether it is the first theory or the second theory, and whether there is biological regeneration of PAC or not, the PACT method is better than the single activated sludge method:
The oxidation of microorganisms depends on the concentration of organic matter. Adsorption by extruded activated carbon increases the concentration of organic matter fixed on the surface of carbon particles, making the reaction more thorough.
PAC and activated sludge stay in the aeration tank together for a time similar to the sludge age. In this way, refractory organic matter has more opportunities to be degraded.
Because the extruded activated carbon adsorbs refractory organic matter and microorganisms at the same time, it extends the contact time between organisms and organic matter. Moreover, the adsorption of extracellular enzymes by PAC also helps microorganisms degrade organic matter. Granular activated charcoal water filter, on the other hand, is mainly used for simple filtration processes rather than the complex biological - adsorption 协同 process in PACT.

2,Application of the PACT Process and Points for Attention

As soon as the PACT method emerged, due to its advantages in terms of economy and treatment efficiency, it has been widely applied to the treatment of industrial wastewater, such as the treatment of wastewater from oil refining, petrochemical, printing and dyeing, coking, and organic chemical industries. The use of extruded activated carbon in this process has shown better adaptability compared to other forms like columnar activated carbon or granular activated charcoal used in water filters.

Moreover, when the PACT method is used for urban sewage treatment, it can significantly improve the nitrification effect. It is better than the activated sludge method, can increase the overall removal efficiency of the system, and greatly improve the quality of the effluent.

 

Application of the PACT Process in Industrial Wastewater
In China, the PACT process is mainly applied to the actual treatment of industrial wastewater.
For example, using the PACT process with extruded activated carbon to treat the pretreated PAM production wastewater can improve the oxygen utilization efficiency, enhance the settling performance of the sludge, and the removal rate of organic matter can reach over 80%. Compared to traditional water treatment methods using granular activated charcoal water filters, PACT offers more comprehensive treatment.
A certain chemical industrial park uses the method of PAC - strengthened activated sludge, with extruded activated carbon being the key component, to treat the "three highs" refractory comprehensive chemical wastewater. The removal effect of COD has almost doubled, and the chromaticity removal rate reaches 60% to 70%. Columnar activated carbon, due to its shape and structure, might not be as effective in this sludge - based system as extruded activated carbon.
Using the A2/O(PACT) process to treat the hydrolyzed and acidified printing and dyeing wastewater can effectively improve the operation effect of the system in winter. It has a better treatment effect on characteristic organic pollutants such as benzene ring, polycyclic aromatic, and heterocyclic compounds, and also has a more obvious promoting effect on the removal of macromolecular substances.
For another example, a domestic pharmaceutical company uses the PACT process with extruded activated carbon to treat pharmaceutical wastewater containing benzothiazole. Compared with the activated sludge method without adding activated carbon, the removal rates of TOC and COD by the PACT process have increased from 54.8% and 45.6% to 86.13% and 76.34% respectively. It has improved the system's tolerance to shock loads and has a screening effect on the benzothiazole - degrading bacteria in the system.

 

Points for Attention in Applying the PACT Process:
(1) When choosing materials for sludge disposal equipment, more considerations are needed. PACT adds extruded activated carbon to the activated sludge aeration tank. The discharged excess sludge is PAC - biological sludge, which is abrasive. So, the pump body, tank body, the sludge scraping machinery of the secondary sedimentation tank, and the sludge disposal equipment all have relatively high requirements for wear resistance. This is different from the milder operation environment of granular activated charcoal water filters.
(2) Because the sludge produced by this process has a relatively high density, when designing the sludge scraping machinery of the secondary sedimentation tank and the sludge disposal equipment, a relatively high torque limit value should be used.
(3) When a large amount of extruded activated carbon is added, there will be many PAC particles in the effluent. To improve this situation, it is best to use an SBR system, or add a tertiary filter. A membrane separation unit can also be used to replace the secondary sedimentation tank.
(4) The adsorption capacity of extruded activated carbon in the PACT system is different from the values predicted by the intermittent isothermal adsorption test. Continuous flow treatment tests should be carried out to obtain relevant data for design. This is also a significant difference from the performance evaluation of columnar activated carbon or granular activated charcoal in water filters.
(5) Because extruded activated carbon has a strong adsorption capacity, if it is directly exposed to the air, it will easily adsorb substances in the surrounding environment, occupying the adsorption sites and making PAC ineffective. So, during production or testing, proper storage is crucial, which is also applicable to other forms of activated carbon like granular activated charcoal water filters and columnar activated carbon.