I have been intrigued with Kruger Inc.’s (part of Veolia Water) ACTIFLO® CARB process ever since it was selected as pretreatment to ceramic membranes for a new water treatment plant at Parker Water & Sanitation District (PWSD) in Colorado. ACTIFLO® CARB was selected based on what I thought was very shaky science – a ‘trial’ on a high total organic carbon (TOC) water source that consisted of dosing 4000 mg/L of virgin powdered activated carbon in front of the membranes operated in crossflow mode and recirculated for eighty hours. The results showed TOC removal starting at 90% and finishing at about 50% after the eighty hours. Somehow Kruger was able to convince the consulting engineer and PWSD that these results could be extrapolated to predict the performance of a continuously operating full-scale plant with a 25 mg/L fresh PAC make-up dose.The ACTIFLO® CARB process is an extension of Kruger’s ACTIFLO® process where powdered activated carbon (PAC) is added in a contact stage at the front of the process followed by coagulant addition and then microsand and polymer to provide ballasted flocculation/clarifiation (see Figure 1). Most of the PAC is recirculated while a portion is wasted, which allows more of the carbon adsorption sites to be utilized. Make-up PAC doses range from 15 to 40 mg/L. The addition of the “CARB” step to ACTIFLO® provides greater TOC removal when the coagulants used in the ACTIFLO® process cannot achieve treated water quality goals.
Figure 1: Schematic of the ACTIFLO® CARB Process
What was astounding to me was that the engineer and District accepted the manufacturer’s recommendation after such a short trial of a process that has no operating installations in the US, where the pilot plant did not even simulate how the full scale system would operate and where more proven treatment alternatives, such as ion exchange, tested over longer periods reliably demonstrated equivalent levels of TOC removal…. Such is the influencing power of a large established technology provider!
Finally a Fair Evaluation
In the past few months, more than two years after the PWSD trial, I finally had the opportunity to see the performance of ACTIFLO® CARB in a trial in Georgia operated for at least several weeks on a pilot plant representative of the full-scale process. The objective of the project was to improve TOC removal of the existing water treatment plant to meet tightening EPA standards for disinfection byproducts. As with the PWSD trial, ion exchange pre-treatment was evaluated in parallel but this time it was a fair side-by-side comparison over similar operating periods.
Lo and behold….under steady-state operating conditions the ACTIFLO® CARB process only achieved 52% TOC removal at very high PAC make-up doses and about 40% removal at economically realistic make-up doses. This compared to 66% TOC removal for the ion exchange pretreatment process. As the trial has only recently been completed, specific results will likely be publicly available in early 2011.
In this Georgia project the power and influence of the manufacturer could not overshadow the results of a well thought out and executed trial. While ACTIFLO® CARB has been shown to remove more TOC than coagulation alone, I can't see how the marginal improvement in removal justifies the considerable extra expense.
