W.D. Griffiths

Computational fluid dynamics (CFD) and empirical modelling of the performance of a number of cyclone samplers

Abstract An investigation has been carried out to examine the usefulness of a computational fluid dynamics (CFD) package to model the collection performance of a range of small sampling cyclones. To further assess the effectiveness and usefulness of CFD. A number of semi- empirical theories were also used to model cyclone sampler performa nce. The cyclones considered in this study could be used to collect samples of aerosol particles, including airborne microorganisms (bioaerosols). The work described here was carried out as an integral part of a study to select and develop a number of techniques for assessing airborne microorganisms. It has been found necessary to characterise bioaerosol samplers with a view that the selection of analysis methods and samplers are interdependent. This publication describes a study carried out to compare the performance of three types of small cyclone samplers using data from CFD, experiments, and three empirical theories. CFD package Fluent 3D has been used satisfactorily to model the performance of three types of small cyclone aerosol samplers. Performance curves were produced with the same shape and 50% cut-off diameter and approximately the same gradients as those obtained by experiment. The predicted pressure drops were also in excellent agreement with the measured data. The CFD model is able to predict the salient features of the cyclone flow fields in great detail, thus providing a better understanding of the fluid dynamics of these devices. Specifically, results obtained from the computer modelling exercise have demonstrated that CFD is a reliable method of modelling the performance of three types of small sampling cyclones. It has also been shown to be useful for examining the effects of a number of design changes have on their performance. This method of analysis is almost certainly less expensive than experiment, and represents a cost-effective route for design optimisation. Of the three empirical theories considered in this work, only the Barth (1956) theory accurately reflects the performance of the two smaller types of cyclones, provided the design geometries and sampling rates of the cyclone do not deviate too much from the values allied to the “standard” version of the sampler. For the larger of the three cyclones considered, only the empirical theory of Iozia and Leith (1989) has been shown to be useful in mirroring experimental performance. The application of empirical performance modelling on the same types of cyclone sampler was not as successful. Performance curves from the Barth (1956) theory matched experimentally derived data for two of the three cyclone designs under consideration. However, for the Stairmand design, the model tended to overestimate the 50% cut-off diameters by between a factor of 2 and 4; and the inclination of the performance curves were reproduced in all cases. This CFD model is a reliable and relatively inexpensive method of examining the effects of design changes on sampler performance of any of these three types of sampling cyclone. It will almost certainly be less expensive than experimental characterisation studies.

Effect of aerosolisation, growth phase and residence time in spray and collection fluids on the culturability of cells and spores

Abstract Work has been carried out in an initiative to establish a U.K.-based measurement infrastructure for aerosol and bioaerosols. The initiative is supported by the National Measurement System Policy Unit (NMSPU) of the U.K. Government Department of Trade and Industry (DTI) under the Valid Analytical Measurement System (VAM) scheme. The overall aim of the work is to examine the appropriateness and practicality of validating and certifying microbiological reference materials for bioaerosol standards. One of the prime uses of such bioaerosol standards is the characterisation of samplers and assay methods. Candidate microbiological materials were selected from a survey of mainly U.K. industrial (and others) user needs. The microorganisms selected are Escherichia coli (National Collection of Industrial and Marine Bacteria, NCIMB 86), Saccharomyces cerevisiae (National Collection of Yeast Cultures, NCYC 1337) and Penicillium expansum spores (Rothamsted isolate no. C2636). Culturing techniques are often used in conventional aerobiology to enumerate the numbers of microorganisms. This publication reports on the programme of work to examine the effects that aerosolisation parameters and residence time in the spray and collection liquids can have on bioaerosol culturability. Data showed that suspensions of S. cerevisiae (NCYC 1337) cells can be maintained at an average culturable fraction of 0.7 for a period of 3 h in a suitable buffer solution maintained at the ambient room conditions prevailing in the studies (20°C and 30% RH). This suspension can be aerosolised to produce a bioaerosol with a culturable fraction that depends on spray suspension age and collected aerosol age. The culturable fraction of this bioaerosol was found to decrease with spray suspension age and with collected aerosol age. This strain of the species may not be suitable as a microbiological reference material. The results of the work with E. coli (NCIMB 86) showed that aerosolisation reduced the culturable fraction in the bioaerosol to close to zero, although the culturable fraction could be maintained at approximately 0.24 (stationary phase) and 0.33 (log-phase) for cells maintained in the spray suspension for up to 3 h. It is concluded that this strain of E. coli (although defined to be a fairly robust strain of this species) is not suitable as a standard bioaerosol or microbiological reference material. The results of the studies with P. expansum (Rothamsted isolate No. C2636) spores showed that aerosolisation had little general effect on the culturability of the bioaerosol (average culturable fraction of 0.25). This observation is to be expected with fairly robust microorganisms such as fungal spores. In view of their robustness, P. expansum spores are recommended as a microbiological reference material, and that a standardised aerosol of this test microorganism should be prepared according to the methodology described in this paper for culturing, preparing the spray suspension, aerosolisation under the correct ambient conditions, collection and assay. The results of the tests indicate that traditional culturing techniques underestimate cell/spore numbers in the collected bioaerosols of the microorganisms used. Total count techniques are therefore recommended for the determination of cell/spore numbers in collected aerosols of the microorganisms used in the tests. The culturability of P. expansum spores is unaffected by aerosolisation, and total number may be determined either by total counts or by culture counting, taking into account the initial culturable fraction in the spray suspension. It should be noted that different methods for counting total cell numbers may give different results.

The development of sampling methods for the assessment of indoor bioaerosols

Abstract Airborne bioaerosols form a large component of the indoor domestic environment and their presence, growth and dispersion are often influence by heating, ventilation, air conditioning and humidifier systems. The most common microorganisms may cause respiratory disease and allergic reactions in susceptible individuals. Sampling methods amenable to standardisation have been developed for the characterisation of indoor bioaerosols. A major objective of this study was to develop cyclone samplers that collect bioaerosols in size ranges that reflect those collected in the human respiratory system, and have minimal effect on their culturability. The physical collection characteristics and bioefficiency of the sampler systems were assessed. A induced-size version of a well-characterised cyclone sampler, with a 50% collection (d50) of 0.8 μm aerodynamic diameter, was identified as the basis for a sampling and collection system suitable for characterising indoor bioaerosols. A series of single-stage inertial impactors was designed to size-separate the aerosol entering the cyclone. Additional wind tunnel efficiency tests showed that d50 values for these size-separators were 3.5 and 4.7 μm, respectively. Five species of microorganisms were used to determine the bioefficiency of (a) the cyclone sampler and (b) the cyclone sampler coupled with a pre-separator impactor. All but Penicillium expansum spores suffered damage during aerosolisation decreasing their culturability by several orders of magnitude, compared with culturability prior to aerosolisation. The reduced-sized cyclone sampler had satisfactory sampling and collection efficiencies, and could “gently” collect different commonly found indoor bioaerosols for subsequent analysis.

Performance of bioaerosol samplers used by the UK biotechnology industry

Abstract The quantitative assessment of bioaerosols is important in a number of industrial and health-care applications. Measurements to aid the control of airborne infection in hospitals, detection of the release of potentially harmful microorganisms from bio-processing equipment and monitoring the exposure of workers and animals to bioaerosols are examples of situations where different concentrations of microorganisms and varying ambient conditions may be expected. Microorganisms are notoriously difficult to assess accurately under such variable conditions and no single assay method is suitable for all applications; rather the method needs to be tailored to the application of interest. Problems are compounded by the differences in assay method (such as the type of media used for culturable counts) or sampler type selected, making the interpretation of the results difficult. An understanding of the airborne behaviour of microorganisms over a range of environmental conditions is vital if procedures are to be defined and recommended for the handling, sampling and assessment of bioaerosols. Microorganisms that are robust over a wide range of conditions are ideal as tracer particles. Unfortunately, the large majority of non-fungal bioaerosols are susceptible to damage. A predictable sampling procedure is required which will not affect the viability of the collected sample. Studies, reported on the development of procedures to characterise airborne biological particles, included tests carried out under controlled environmental conditions to compare the performance of five industrially important bioaerosol samplers with that of a reference glass wet-walled, cyclone sampler, using test aerosols of Saccharomyces cerevisiae cells and Penicillium expansum spores. The samplers tested were Andersen Microbial Sampler, Biotest RCS, Biotest RCS Plus, MicroBio MB1, and MicroBio MB2. The cyclone sampler, Andersen Microbial Sampler and the MicroBio MB1 and MB2 meet the basic criteria for a suitable reference sampler, except that the last three systems do not provide total counts. It will be important to investigate in the future how well they perform under the challenge of “real-life” conditions.

Procedures for the characterisation of bioaerosol particles. Part I: aerosolisation and recovery agent effects

The sampling and assay of bioaerosols are important ina number of industrial and health-care applications. Airborne microorganisms are notoriously difficult toenumerate accurately under such conditions and nosingle procedure is suitable for all applications. Problems are compounded by the differences in assaymethod or sampler type selected, making theinterpretation of results difficult.Understanding the airborne behaviour of microorganismsover a range of environmental conditions is vital ifprocedures are to be defined and recommended for theassessment of bioaerosols. Microorganisms that arerobust over a wide range of conditions are ideal astracer particles. Unfortunately, the large majorityof non-fungal bioaerosols are susceptible to damage. A predictable assessment procedure is required whichwill not affect the viability of the collectedsample.This paper examines how aerosolisation may affect the characteristics of two speciesof microorganism (Pseudomonas fluorescens andMS2 coliphage). It forms part of a larger programmeto develop standards for the assessment of biologicalparticles. The aim of the work was to develop procedures toexamine the effects of aerosolisation onmicroorganisms, with particular reference topre-aerosolisation protocol (spray suspension age) andpost-sampling handling protocol (aerosol age incollection solution). These procedures were then usedto examine the effect of recovery agents, addedto the spray suspension prior to aerosolisation, onthe culturability of E.coli.Aerosolisation reduces the culturability of P. fluorescensand the viability of viability of MS2coliphage. Pre-sampling and post-collection handlingand storage of these aerosolised microorganisms werealso found to have an effect. This and earlierstudies have shown that the culturable fraction ofmicroorganisms can be affected by the same factorsdescribed above. Of five microorganisms tested so farin the main programme, only Penicillium expansumspores were shown to be robust and stable with aconstant culturable fraction. Therefore, recommendinga particular microorganism (apart from P. expansum) as an airborne biological standard foraerosol studies is not advised. It is recommendedthat a microorganism, representative of the envisagedapplication, be characterised it in terms of theaerosolisation parameters, storage time and conditionsin the manner reported in this study. This can beachieved using the experimental equipment described.The addition of 0.1 mM concentrations of the sugarsinositol, trehalose and raffinose to spray suspensionsof Escherichia coli, prior to aerosolisation,made no significant difference to the culturablefraction of the aerosol.