WATER REGENERABLE CATALYTIC CARBON ADSORPTION: A CASE STUDY AT THE BROADWATER WATER RECLAMATION FACILITY

The Anne Arundel County Department of Public Works in Annapolis, Maryland has implemented an odor control program for their sewage collection systems and water reclamation facilities. As part of that program, Parsons Engineering Science, Inc. (Parsons) evaluated treatment technologies for odors emanating from the headworks of the Broadwater Water Reclamation Facility (WRF). Wet scrubbing, carbon adsorption, and bio-filtration were evaluated based upon technical and economic criteria.The evaluation was based upon enclosing the existing headwork structure and ventilating it at 12 air changes per hour or 3,300 cfm. Average and peak levels of 10 and 15 ppm of hydrogen sulfide (H2S) were used in the evaluation.The evaluation team concluded that a carbon adsorption system using a water-regenerable, catalytic carbon was the most viable alternative considering site space restrictions, degree of odor reduction, safety concerns, net present cost, and required permitting for construction. The design resulted in selection of a Calgon PhoenixTM 4000 carbon adsorption unit for odor control.During periodic water-regeneration of the carbon, a dilute sulfuric acid waste stream is formed with resulting pH values below 2.0. In most applications, this intermittent waste stream is simply discharged and combined with the influent wastewater stream. At the Broadwater WRF, however, average daily flows can fall below 0.8 mgd and the influent alkalinity is often below 140 mg/l. Addition of the acid waste stream to the influent wastewater created the potential for destruction of most of the alkalinity and hence, pH depression, which could inhibit the downstream biological nutrient removal (BNR) process. To protect the downstream biological process, the acid waste stream was contacted in a separate, in-ground, neutralization tank containing high calcium limestone prior to discharge.The odor control system was put into service in November 1999. In the initial 3 months of operation, the unit has achieved an estimated 99 percent or greater reduction of H2S. Furthermore, odors are not perceptible at the unit outlet. The neutralization tank has also been effective in converting the acid waste stream from a pH of 2.0 to pH values of approximately 7.8. No odors have been reported in the vicinity of the headworks and after several regeneration cycles, there have been no detrimental impacts from the neutralized acid waste stream upon the downstream biological processes. As a result of the success of this installation, Parsons will be applying waterregenerable, catalytic carbon adsorption technology at additional client locations.