PACT - Powder Activated Carbon Treatment of Activated Sludge Process - Series One

Powdered Activated Carbon technology (PAC), referred to as the PACT process, is a process that removes difficult biodegradable compounds by adding powdered Activated Carbon (PAC) to activated sludge reactors. By adding powdered Activated Carbon (hereinafter referred to as PAC) to the activated sludge reactor, the process can remove difficult biodegradable compounds by using the adsorption characteristics of Activated Carbon and the degradation ability of microorganisms, and can achieve the effect of deep treatment.

Powdered Activated Carbon as a carrier to attach biofilm can maintain a high sludge concentration and make the system much more resistant to shock loading.

Fig. 1: Schematic  of  a  typical powdered  activated  carbon  treatment  (PACT) process  (modified  from Meidl,  1992; 1997; IPIECA, 2010; Jafarinejad, 2017a).

The process is essentially Activated Carbon adsorption bio-oxidation in the form of activated sludge. By combining the advantages of both Activated Carbon adsorption and biochemical action, it solves the cost problem of expensive Activated Carbon alone and breaks the limitation that the biological method alone is only applicable to the removal of organic pollutants.

Regarding the mechanism of PACT action, which involves a complex process of simultaneous adsorption and biodegradation, there are two main representative theories.

1. Absorption-degradation-biological regeneration- re-adsorption, PAC and microorganisms strengthen each other

The huge surface area provided by Activated Carbon can adsorb organic matter and oxygen in water. Microorganisms attach to the surface of PAC, and the secreted extracellular enzymes and some of the active centers of enzymes can enter the micro-pores of Activated Carbon to decompose the adsorbed hard-to-degrade organic matter into small molecules, which regenerates the adsorption sites and restores the adsorption function of PAC. And the small molecules of organic matter after decomposition can provide nutrients and energy for microorganisms, which improves the activity of microorganisms and thus is more conducive to the removal of organic matter.

It is worth mentioning that even though the enzyme molecules cannot enter the micropores, when the organic matter in the large and medium pores is degraded, due to the existence of the concentration difference, the organic matter diffuses from the micropores to the large and medium pores and can also be degraded by microorganisms.

2. Simple superposition of microbial degradation and Activated Carbon adsorption

This theory is the opposite of theory 1. Because after several adsorption cycles, the Activated Carbon gradually saturates, the adsorption capacity decreases and the removal of organic matter shows a decreasing trend, which indicates that the PAC surface is saturated and there is no biological regeneration phenomenon.

In fact, regardless of theory 1 or theory 2, the PACT method is superior to the Activated sludge method alone, regardless of the presence of biological regeneration of PAC.

Microbial oxidation depends on the concentration of organic matter, and adsorption increases the concentration of organic matter fixed on the surface of the carbon particles and allows the reaction to proceed more completely.

The PAC and activated sludge remain together in the aeration tank for a period of time equivalent to the sludge age, and the refractory organic matter has a greater chance of being degraded.

Since the Coconut Carbon adsorbs the microorganisms while adsorbing the hard-to-degrade organic matter, thus prolonging the contact time between organisms and organic matter Moreover, the adsorption of extracellular enzymes by PAC also facilitates the degradation of organic matter by microorganisms.