What is the PACT method?

PACT method: Powdered Activated Carbon Treatment Process. In Chinese, it is known as the biochar method; this method is also referred to as the PACT method or PACSBR biochemical method. It is considered by foreign experts to be the most promising new biochemical treatment process for wastewater. Powdered activated carbon is added to the biochemical influent (or within the aeration tank), where it mixes with the recycled carbon-containing sludge in the aeration tank; the excess sludge discharged from the sludge thickening tank is then sent to a sludge dewatering unit. In the aeration tank, activated sludge adheres to the surface of powdered activated carbon. Due to the large specific surface area and strong adsorption capacity of powdered activated carbon, the adsorption capacity of the sludge is enhanced; in particular, the concentrations of dissolved oxygen and degradation substrates at the interface between the activated sludge and powdered activated carbon increase significantly, which in turn improves the rate of COD degradation and removal. Generally, in PACT systems, the dynamic adsorption capacity of activated carbon for removing COD is between 100–350% (on a weight percentage basis); that is, one kilogram of powdered activated carbon can absorb and remove 1.0–3.5 kilograms of COD. Moreover, the PACT method can handle toxic and harmful organic pollutants that are difficult for organisms to degrade.

Let’s talk again about the advantages and applications of the PACT method. Reduce investment costs ； Flexible and easy to operate ； High utilization rate of activated carbon ； It overcomes the drawback of granular activated carbon, where biofilms tend to form, leading to clogging and affecting the effluent flow rate. In the powder activated carbon–activated sludge system, the activated sludge adheres to the surface of the powder activated carbon. Thanks to the large specific surface area of the powder activated carbon and its strong adsorption capacity, the concentrations of dissolved oxygen and degradation substrates at the interface between the activated sludge and the powder activated carbon increase significantly, thereby enhancing the rate of COD degradation and removal. Generally, COD removal (depending on the type of wastewater) can be increased by 10-40% ； Since the toxic and harmful organic substances in the wastewater are adsorbed by powdered activated carbon, the concentration of such substances in the wastewater can be maintained at a low level, thereby ensuring the proper operation of the biochemical treatment system ； It is highly effective in preventing the rebound of ammonia nitrogen levels and ensuring that the ammonia nitrogen levels in the effluent meet the specified standards. It can be used for pharmaceutical wastewater that is difficult to biodegrade. It is quite challenging to reduce the COD level in the effluent from biochemical treatment to below the **first-class discharge standard (100 mg/L); therefore, it is essential to employ granular activated carbon adsorption technology on this effluent to ensure that it meets the required standards. However, the granular activated carbon adsorption treatment method has a fatal weakness: its high treatment cost. The underlying reason for this is that the dynamic adsorption capacity of granular activated carbon for COD is around 10% (on a weight basis); in other words, one ton of activated carbon can only absorb approximately 100 kilograms of COD from wastewater. Due to the difficulty in regenerating granular activated carbon and the high treatment costs, the application of granular activated carbon treatment technology is not yet widespread in China. The PACT method was developed by DuPont in the United States.