Air Ionization Improves Indoor Air Quality

By Dennis Tulenko, Gayle Van Durme and Michael Carver

Polarized air ionizers produce positively and negatively charged oxygen molecules, or ions, with no ozone generated. The ionized air can be used to oxidize most air contaminants including hydrogen sulfide, ammonia, and other organic gases. The Denver Metro Wastewater Reclamation District (MWRD) recently installed such as system to handle odor control in its solids handling facilities.

MWRD Sludge Processing Building Basement Air Ionizing Plenum AIU-2.

This Swiss technology (Bentax-Air-Tec), now offered in the United States by Trans-Tech Energy and Environmental Inc., has been used at wastewater facilities in Europe for 20 years.

The ion level in the freshest mountain air environment averages about 900 to 1100 negative oxygen (O2-) ions and 1000 to 1200 positive oxygen (O2+) ions per cubic centimeter. This electrically charged air can oxidize odors and air borne bacteria. The Bentax™ ion tube produces a strong electromagnetic field or corona surrounding the ion tube which generates up to one million ions per cubic centimeter and saturates the air.

In the presence of ionized air, chemical contaminants are fully oxidized to a simpler form with no harmful compounds generated. Polarized ionization has been found to be effective on hydrogen sulfide, ammonia, methyl-mercaptans, formaldehyde, and many organic total reduced sulfur compounds. Benign end products such as carbon dioxide, nitrogen, sulfate ion and water vapor are formed. Hydrogen sulfide (H2S) is diluted to the non-toxic sulfate-ion.

Small portable ionizers can be installed as self contained recirculation units complete with fan. Alternately, modules can be installed in ductwork on the fresh air supply side of an existing ventilation system. Each duct mounted model 50F5 ionizer is 25 x 8.8 x 8.4 in and weighs only 16 lb. The number of modules required depends on both the volume of air treated and the contaminant concentration.

Cost comparison of polarized air ionization and "end of pipe" systems as applied to sludge dewatering buildings with a maximum of 10 ppm H2S.

The system has a minimal footprint and power requirements of 35 watts per module. The system has zero water usage and no chemical handling or storage. In addition to providing odor control and a safe work environment, air ionization prevents corrosion of electronics and equipment. Both capital and operating costs were found to be less than other options such as wet scrubbers, activated carbon, and biological treatment.

Case Study

The Denver Metro Wastewater Reclamation District installed air ionization supplied by Trans-Tech as part of the High Solids Cake Storage Improvements Project which included the new ionized air handling system for the sludge processing building basement, completed in September of 2003, and the cake storage building and conveyor section, completed in January 2004. The MWRD project involved a major upgrade to high solids centrifuges.

The dewatered sludge is dumped onto a serpentine conveyor belt and transported on open belts through a conveyor section to a cake storage building where it is dumped into large hoppers or tanks for storage and trucking off site. The main goal of air ionization was improved worker safety and corrosion protection along with odor control as an additional objective.

The success of a pilot test run in house by MWRD, led the district to request Black & Veatch to investigate all aspects of the technology, including safety, effectiveness on a variety of organic gases, previous testing for ozone, reliability, and operation and maintenance costs. After reviewing available literature, and other US installations, Black & Veatch approved the technology for application at MWRD's 26,800 cfm sludge processing building basement and the 16,700 cfm cake storage building.

System Results

For the final design, MAU-7, a 20,200 cfm air handler with high efficiency filters, was installed on the roof of the sludge processing building to provide supply air to the basement through ionization plenum AIU-2. For the sludge building basement, a total of 24 model 50F5 ionizer modules are installed in two separate plenums. Plenum AIU-2 houses 18 modules, and an additional plenum AIU-1, 6,600 cfm being supplied air by air handler MAU-1, houses 6 modules. Both plenums are positioned on the centrifuge room floor directly above the basement. The ionizer modules are flange mounted to the plenum and can be removed for routine maintenance in a few minutes.

Ionized air is discharged over the walkways on the north side of the basement via carefully designed and positioned air inlet registers, while spent air is exhausted from the south side of the basement. An air ventilation rate of 12 air changes per hour is maintained. Additional ionized air is discharged through ducts mounted directly at the sludge drop zones.

At the cake storage building, a single plenum of 24 50F5 modules is employed. For the conveyor section, four shelf mounted model 70E5 units with integral fan are positioned at intervals along the conveyor.

After startup, compliance testing for H2S levels was performed using a factory calibrated Industrial Scientific TMX412 for all air ionization systems based on achieving a requirement of less than 4 ppm in the walkways and general work areas. The compliance testing demonstrated that air ionization reduced H2S to non-detectable concentrations in all areas including the sludge processing building basement walkways, fresh sludge dump zones at the conveyor belt, cake storage building penthouse, and directly above the dump zone where sludge dumps from the conveyor belt into the cake storage tanks.

Subsequent to the compliance testing, additional testing was performed by Black and Veatch to measure odor for purposes of information. For odor sampling, the St. Croix Sensory field olfactometer or "Nasal Ranger" was used. This instrument measures fresh air dilutions to odor detection or D/T.

Odor levels were low in the walkways and work areas, 2-7 D/T. Odor six inches above the fresh sludge on the belt, 30 D/T. Stack outlet odor being discharged measured 15 D/T, which is considerably less the typical stack emissions limit of 300 d/t, which might be achieved by other "end of pipe" treatment devices such as wet scrubbers or biofilters. The added advantage of air ionization is the improved interior air quality, which would not be obtained with end of pipe treatment.

MWRD is pleased with the performance and operation of the air ionization system. The sludge building basement, which was previously very uncomfortable for operators, is now a pleasant workspace. Facility Engineer Mike Carver now includes the basement on plant tours, so that visitors can witness the superior air quality provided by air ionization.

About the Authors:
Dennis Tulenko, REM, is President of Trans-Tech Energy and Environmental. He is a Registered Environmental Manager and has worked in air pollution control for over 20 years. Gayle Van Durme, P.E., is a Senior Process Engineer for Black and Veatch in Kansas City, MO. Michael Carver, P.E., is Facility Manager for the Metro Wastewater Reclamation District in Denver. For more information on air ionization technology, visit Trans-Tech's web site at www.trans-tech.org.