A Microbiologically-based Rating System for In-duct Ultraviolet Germicidal Irradiation Air Disinfection Systems

Open Access
Author:
Martin, Stephen Benjamin
Graduate Program:
Architectural Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 06, 2014
Committee Members:
  • James Freihaut, Dissertation Advisor
  • William P Bahnfleth, Dissertation Advisor
  • James Freihaut, Committee Chair
  • William P Bahnfleth, Committee Chair
  • Richard George Mistrick, Committee Member
  • James Landis Rosenberger, Committee Member
Keywords:
  • Ultraviolet Germicidal Irradiation
  • UVGI
  • In-Duct
  • Biodosimeter
  • Air Disinfection
  • Collimated Beam
  • k-value
Abstract:
A new rating concept, termed the Dose Effectiveness Rating (DER) for in-duct ultraviolet germicidal irradiation devices is proposed and developed. The DER is a measure of device performance, under standard testing conditions, relating the effective UV germicidal dose delivered to an airborne population of microorganisms to the power input into the UV-producing device that permits estimation of performance under other conditions and comparison between devices. The DER is microbiologically-based in that the performance of an in-duct UV device is directly related to its ability to inactivate standard test organisms. For this project, Bacillus subtilis (bacterial endospore) and Aspergillus niger (fungal spore) were selected as the reference organisms. To establish the relationship between device performance and organism inactivation, accurate UV dose-response models and model parameters were established for each organism using novel dual-collimated-beam reactor designs. UV exposure systems typically used for microbial inactivation studies collimate UV energy from only one direction, resulting in two issues that can affect the accuracy of the results, leading to uncertainty in the observed organism response. First, the systems do not allow for UV irradiance measurements during microorganism exposure. An average of measurements taken before and after exposure is assumed to represent the irradiance incident on the microorganisms during an actual exposure test. Second, the systems require the determinations of numerous correction factors which are then included in estimations relating measured pre- and post-irradiance readings to the calculated irradiance levels experienced by the microbial population. Four correction factors are generally required for systems equipped with low-pressure mercury UV lamps. Including these correction factors in the UV dose calculation does not account for all aspects of system design or any lamp output variations that might occur during microbial exposures (when the radiometer sensor is not in place to record them). To address the above issues associated with single-collimation reactors, and to improve the accuracy of microbial inactivation rate constant (k-value) determinations, two new dual-collimated-beam UV reactors are developed and characterized in this project and then employed to establish UV dose-response models for two standard microorganisms. The first reactor allowed testing of aqueous microbial suspensions, while the second reactor allowed testing of microorganisms suspended in air. These new dual-collimated-beam reactor designs provide two distinct advantages over traditional single collimation systems. First, they allow real-time UV dose determination, which helps capture any UV lamp variations that may occur during the exposure tests, that allows microbial exposure tests to be terminated at directly-measured, desired dose levels. Secondly, the dual-collimated-beam design allows for simple actinometric determination of necessary correction factors relating the real-time radiometer readings of irradiance to the actual irradiance levels experienced by the microorganism population being exposed. Having determined the inactivation kinetics models for the two reference organisms using the new dual-collimated-beam reactors, the organisms were used as biodosimeters to measure the effective UV dose provided by full scale, in-duct UV devices. The effective UV dose is a key variable in calculating the proposed DER metric. To demonstrate the rating concept, the effective UV doses provided by five different in-duct UV lamp devices are determined by conducting tests with the reference organisms in a large, full-scale test rig over a range of air flow rates typical of most residential and commercial ventilation systems. The UV dose provided by each in-duct device was determined by comparing the microbial inactivation measured in the rig to the reference inactivation curves established in the dual-collimated-beam reactors for the two organisms. These UV dose values were then used to calculate the proposed DER metrics and demonstrate their usefulness to building owners and facility managers.