Recirculating powdered activated carbon (PAC) is a water treatment process directed towards removing organic and emerging contaminants. It doesn't generate a brine waste stream and uses less power than reverse osmosis (RO). While the process is relatively new to the U.S., successful pilot testing resulted in the process' selection for a new water treatment plant in Parker, Colorado, scheduled to begin operation in 2014. The source water could be heavily impacted by wastewater treatment effluent and high phosphorus levels in the area's native soils, which contribute to reservoir eutrophication.

What Is Recirculating PAC?

Recirculating PAC is a ballasted sedimentation process that utilizes microsand to settle and capture PAC particles for recirculation, achieving a very high PAC utilization efficiency compared to conventional single contact PAC addition. For those familiar with activated sludge wastewater treatment, the recirculating PAC process is similar in principal to an activated sludge system:

Activated Sludge Wastewater Treatment

• Level of treatment is a function of the concentration and residence time of the treatment mass (mixed liquor solids)
• Residence time of the treatment mass can be controlled by adjusting the "sludge" wasting rate
• New treatment organisms grow to replace those that die and are wasted

Recirculating PAC Process

• Level of treatment is a function of the concentration and residence time of the treatment mass (PAC)
• Residence time of the treatment mass can be controlled by adjusting the "sludge" wasting rate
• New PAC is added to replace the PAC that is exhausted and wasted

Advantages of Recirculating PAC

Recirculating PAC can:

• Operate at very high PAC concentrations (up to approximately 5,000 mg/l)
• Achieve excellent organic contaminant removal. Pilot testing in Parker, Colorado, achieved:
• Removing 70 percent of dissolved organic carbon with a 22 mg/L fresh PAC dose at an operating contact tank concentration of 4,200 mg/l
• Removing 80 percent of DOC with a 50 mg/L fresh PAC dose at 4,200 mg/l operating concentration
• Adjustable to achieve target organic removal rates
• Recirculate for a controllable amount of time to ensure full utilization of PAC adsorptive capacity

Providing Operational Flexibility

The recirculating PAC configuration offers a high level of operational flexibility to achieve a broad spectrum of removal depending on the PAC dose and type, as well as wasting rate. Flexibility is particularly important for projects with potentially significant source water quality variability, but the inherent operational flexibility of this process lends itself to any treatment facility that experiences a broad seasonal fluctuation in raw water organic levels.

An Alternative to RO

The combination of microfiltration, RO, and ultraviolet treatment processes is frequently considered a standard process for emerging contaminant removal. However, this approach is energy intensive and large-scale disposal of the brine waste stream can be difficult in locations without ocean disposal options. The recirculating PAC process maximizes PAC treatment efficiency for a cost-effective and flexible treatment approach without RO. The physical facilities required for a PAC recirculation system are similar to those for a ballasted sedimentation system, and thus tankage requirements are relatively low. The adjustability of the system lends itself to applications with high seasonal variability in organic levels.