# Biological treatment of indoor air for VOC removal: Potential and challenges

Benoit Guieysse<sup>a,*</sup>, Cecile Hort<sup>b</sup>, Vincent Platel<sup>b</sup>, Raul Munoz<sup>c</sup>, Michel Ondarts<sup>b</sup>, Sergio Revah<sup>d</sup>

## ARTICLE INFO

### Article history:

Received 17 December 2007

Received in revised form 25 March 2008

Accepted 29 March 2008

Available online xxxx

### Keywords:

Biofiltration

Indoor air quality

Sick-building syndrome

Volatile organic compound

Green building

## ABSTRACT

There is nowadays no single fully satisfactory method for VOC removal from indoor air due to the difficulties linked to the very low concentration (µg m⁻³ range), diversity, and variability at which VOCs are typically found in the indoor environment. Although biological methods have shown a certain potential for this purpose, the specific characteristic of indoor air and the indoor air environment brings numerous challenges. In particular, new methods must be developed to inoculate, express, and maintain a suitable and diverse catabolic ability under conditions of trace substrate concentration which might not sustain microbial growth. In addition, the biological treatment of indoor air must be able to purify large amounts of air in confined environments with minimal nuisances and release of microorganisms. This requires technical innovations, the development of specific testing protocols and a deep understanding of microbial activities and the mechanisms of substrate uptake at trace concentrations.

© 2008 Published by Elsevier Inc.

## Contents

<table><tr><td>1. Introduction</td><td>0</td></tr><tr><td>2. Indoor air quality</td><td>0</td></tr><tr><td>3. VOCs and indoor air quality</td><td>0</td></tr><tr><td>4. Indoor air treatment</td><td>0</td></tr><tr><td>5. Biological treatment of indoor air</td><td>0</td></tr><tr><td>5.1. Influence of type of VOCs</td><td>0</td></tr><tr><td>5.2. Influence of low concentrations on biomass productivity and transfer rates</td><td>0</td></tr><tr><td>5.3. Impact of purification efficiency on design</td><td>0</td></tr><tr><td>5.4. Humidification and biohazards</td><td>0</td></tr><tr><td>5.5. Esthetic, noise, purification perception</td><td>0</td></tr><tr><td>5.6. Evaluating performance</td><td>0</td></tr><tr><td>6. Designing biological purifiers</td><td>0</td></tr><tr><td>7. Conclusions</td><td>0</td></tr><tr><td>8. Uncited references</td><td>0</td></tr><tr><td>Acknowledgments</td><td>0</td></tr><tr><td>References</td><td>0</td></tr></table>

## 1. Introduction

Indoor air contamination is a complex problem involving particles (such as dust and smoke), biological agents (molds, spores), radon,

asbestos, and gaseous contaminants such as CO, CO₂, NOₓ, SOₓ, aldehydes and VOCs (Volatile Organic Compounds) (Table 1). The latter are strongly suspected to cause many Indoor Air Quality (IAQ) associated health problems and “sick-building” symptoms (Wallace, 2001; Jones, 1999; Wieslander et al., 1997; Yu and Crump, 1998).

Singularly, despite the abundance of evidence linking the exposure to VOCs in indoor air with various health effects, only few reports evaluating the existing abatement technologies are currently

<footnote><sup>a</sup> School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, Nanyang Avenue, Singapore 639798, Singapore</footnote>

<footnote><sup>b</sup> Laboratoire de Thermique, Energétique et Procédés, Université de Pau et des Pays de l'Adour, Quartier Bastillac, 65000 Tarbes, France</footnote>

<footnote><sup>c</sup> Valladolid University, Department of Chemical Engineering and Environmental Technology, Paseo del Prado de la Magdalena, s/n, Valladolid, Spain</footnote>

<footnote><sup>d</sup> Universidad Autónoma Metropolitana-Cuajimalpa c/o UAM-I, Departamento de Ingeniería de Procesos e Hidraulica, Av. San Rafael Atlíxco No. 186, Col. Vicentina, C.P. 09340, México, Distrito Federal, Mexico</footnote>

<footnote>* Corresponding author. Tel.: +65 6790 5282; fax: +65 6791 0676. E-mail address: bjguieysse@ntu.edu.sg (B. Guieysse).</footnote>
<table><caption>t1.1 Table 1
Typical air pollutants in indoor air (US EPA)</caption><thead><tr><th></th><th>Group</th><th>Definition/example</th><th>Origin</th><th>Toxic effects</th></tr></thead><tbody><tr><td>t1.2</td><td></td><td></td><td></td><td></td></tr><tr><td>t1.3</td><td></td><td></td><td></td><td></td></tr><tr><td>t1.4</td><td>Particles</td><td>Very small liquid or solid substances in suspension in the air: mists, dust, pollen, cigarette smoke, viruses, bacteria, molds</td><td>Outdoor air, combustion, carpets, human activity, decaying building</td><td>Irritation to eyes and/or respiratory tissues, allergies, cancer, indirect effect through biological production of toxins.</td></tr><tr><td>t1.5</td><td>Gaseous pollutants</td><td>CO, CO<sub>2</sub>, NO<sub>x</sub>, formaldehyde, VOCs</td><td>Combustion, human activity, building materials, furniture, cleaning products, mold development etc.</td><td>Irritation to eyes and/or respiratory tissues, allergies, cancer, effects on the respiratory liver, immune, reproductive and/or nervous system</td></tr><tr><td>t1.6</td><td>Radon and its progeny</td><td>Radioactive gases</td><td>Rock, soil, groundwater, natural gas, mineral building materials</td><td>Lung cancer</td></tr></tbody></table>

available. Several studies have demonstrated the potential of biological methods to remove indoor VOCs (Wolverton et al., 1984; Wolverton et al., 1989; Darlington et al., 2000; Darlington et al., 2001; Chen et al., 2005; Orwell et al., 2006; Wood et al., 2006). However, there is still a lack of solid and relevant data available to understand the true removal mechanisms in these systems and the apparent mismatch between experimental observations and theoretical results from transfer-based models on air biological treatment, as demonstrated in this review. This paper presents a critical review on the potential of biological technologies for indoor air purification with the dual objective of providing a state of the art of the relevant literature and a roadmap for future research. For the latter purpose, a scaled-down approach was used in order to understand the consequences of indoor biological treatment on microbial growth and process design. Then, we identified current knowledge gaps hindering the proper understanding and development of biological processes for indoor air VOC removal. In the following, the term “biological purifier” is used to describe any device including a biological component (botanical or microbial) used for VOC removal; “botanical purifier” was used to specifically describe devices using plants and their associated microorganisms. Classical systems were named according to the conventional air treatment nomenclature (Revah and Morgan-Sagastume, 2005).

## 2. Indoor air quality

Because Americans spend nearly 90% of their time indoors and nearly 25% of US residents are affected by poor IAQ either at the workplace or at home, the US Environmental Protection Agency (EPA) ranks poor IAQ among its largest national environmental threats. Its counterpart, the European Environmental Agency (EEA) has pointed out IAQ as one of the priority concerns in children's health and similar issues are faced worldwide (Zhang and Smith, 2003; Observatory on Indoor Air Quality, 2006, Zumairi et al., 2006). In fact, some buildings contain such high levels of contaminants that they are qualified as “sick” because exposure to them results in multiple sickness symptoms (e.g. headache, fatigue, skin and eye irritations, or respiratory illness) commonly described as the “sick-building syndrome” (SBS) (Burge, 2004).

Data on IAQ-health related effects is still lacking and sometimes contradictory. For instance, Pilotto et al. (1997) found a strong link between exposure to peak NO<sub>2</sub> concentration and respiratory infections in children aged 6-10 years whereas other authors failed to establish this association (Brunekreef et al., 1990; Samet et al., 1993). Nevertheless, it has been suggested that indoor air pollution causes between 65,000 and 150,000 deaths per year in the US, which is comparable to outdoors pollution induced mortality (Lomborj, 2002). IAQ also impacts work productivity as for instance Wargocki et al. (1999) showed subjects exposed to a typical indoor pollution source (plastic carpet) typed 6.5% less than control subjects. Likewise, empirical studies have shown that the use of ventilation rates lower than 25 L s<sup>-1</sup> per person in commercial and institutional buildings was correlated to an increase in the number of short-term sick leaves

(Sundell, 2004). It has therefore been estimated that 40-200 billions 108 USD could be annually saved or gained by improving IAQ in the USA 109 only (in 1996 USD; Fisk, 2000). This problem is already driving an 110 important IAQ market that reached $5.6 billion in 2003 in the USA 111 where it was expected to rise up to $9.4 billion by 2008 (Market 112 report: indoor air quality, 2004). 113

## 3. VOCs and indoor air quality

Interestingly, there is no clear or unanimous definition of what is a VOC: The US EPA defines VOCs as substances with vapor pressure 115 greater than 0.1 mm Hg, the Australian National Pollutant Inventory as 116 any chemical based on carbon chains or rings with a vapor pressure 117 greater than 2 mm Hg at 25 °C, and the EU as chemicals with a vapor 118 pressure greater than 0.074 mm Hg at 20 °C. Chemicals such as CO, 120 CO<sub>2</sub>, CH<sub>4</sub>, and sometimes aldehydes, are often excluded. In addition, 121 sub-classifications such as Very Volatile Organic Compounds (VVOCs) 122 or Semi Volatile Organic Compounds (SVOCs) have been used in the 123 context of IAQ (Crump, 2001; Ayoko, 2004). 124

Several organizations such as the World Health Organization 125 (WHO), the US EPA, or the OQAI (French Indoor Air Quality 126 Observatory), have established lists of priority indoor air pollutants 127 (WHO, 2000; Johnston et al., 2002; Mosqueron and Nedellec, 2002, 128 OQAI) based on the ubiquity, concentration and toxic effect of the substances involved. These lists are relatively similar and systematically include aldehydes, aromatics, and halogenates as well as 130 biocides. Differences are due to the type of pollution taken into 131 account (only chemicals for the EPA, no mixtures such as tobacco 132 smoke for the OQAI) and the geographic specificities of indoor air 133 pollution. Indeed, variations in the building materials, cleaning 134 products, or type of ventilation used generate differences in the 135 indoor air pollution (Sakai et al., 2004). These priority lists will most 136 likely evolve upon new analytical and toxicological findings more 137 relevant to IAQ such as the health effects of chronic exposure to 138 multiple pollutants at low concentration (Mosqueron and Nedellec, 139 2002). This lack of relevant knowledge probably explains why there 140 are only few guidelines for VOC indoor air concentration currently 141 available (WHO, 2000; Canada, 1987). 142

Hundreds of VOCs can be simultaneously found in indoor air. These 144 compounds exhibit very large variations in concentration as well as 145 physical, chemical, and biological properties. Furthermore, the 146 composition of the mixture greatly varies in time as the concentration 147 of VOCs released from coating and furniture generally decreases in 148 time whereas the release of certain substances depends on punctual 149 human activities or even human breathing (Ekberg, 1994; Phillips, 150 1997; Miekisch et al., 2004). Primary emissions of VOCs constitute the 151 major source in new or renovated dwellings during the first months, 152 while physical and chemical deterioration of buildings material 153 (named secondary emission) later becomes the main mechanisms of 154 VOC release (Wolkoff and Nielsen, 2001; Yu and Crump, 1998). Indoor 155 VOC concentrations depend on the total space volume, the pollutant 156 production and removal rates, the air exchange rate with the outside 157 atmosphere, and the outdoor VOCs concentrations (Salthammer, 158
<table><caption>t2.1 Table 2
Example of VOCs found in indoor air</caption><thead><tr><th rowspan="2">Compound</th><th rowspan="2">CAS<sup>a</sup></th><th rowspan="2">Priority list</th><th colspan="3">Indoor concentration (µg m<sup>-3</sup>)</th><th rowspan="2">Recommended values (µg m<sup>-3</sup>)</th><th rowspan="2">IARC<sup>c</sup></th><th rowspan="2">Health effects</th><th rowspan="2">References</th></tr><tr><th>EPA OQA<sup>b</sup></th><th>WHO</th><th>Min</th><th>Max</th><th>Average</th></tr></thead><tbody><tr><td>Acetaldehyde</td><td>75-07-0</td><td>HP</td><td></td><td></td><td>3.24</td><td>119</td><td>18.9</td><td>LFC<sup>d</sup></td><td>2B</td><td>Respiratory disorders, irritation of the eyes</td><td>Weisel et al. (2005)<br>Mosqueron<sup>A</sup> (2002)</td></tr><tr><td>Benzene</td><td>71-43-2</td><td>HP</td><td>HP</td><td>0.48</td><td>364</td><td>78.0</td><td>12.0</td><td>0.17</td><td>1</td><td>Immunological disorders, leukemia,<br>neurological effect</td><td>Weisel et al. (2005)<br>Edwards et al. (2001)</td></tr><tr><td>Dieldrin</td><td>60-57-1</td><td>HP</td><td>HP</td><td></td><td>141</td><td>1.57</td><td>1.57</td><td>LFC<sup>d</sup></td><td>3</td><td>Neurological effect, cancer of the liver</td><td>Mosqueron and<br>Nedellec (2002)</td></tr><tr><td>Dichlorvos</td><td>62-73-7</td><td>HP</td><td></td><td></td><td>2.24</td><td>6.00×10<sup>-4</sup></td><td>7.00×10<sup>-5</sup></td><td>–</td><td>2B</td><td>Neurological effect, cancer of the liver</td><td>Mosqueron and<br>Nedellec (2002)</td></tr><tr><td>Formaldehyde</td><td>50-00-0</td><td>HP</td><td>HP</td><td>11.2</td><td>53.8</td><td>62.3</td><td>33.0</td><td>1000 (1 h)<br>60<sup>e</sup></td><td>2A</td><td>Respiratory disorders, irritation<br>of the eyes</td><td>Weisel et al. (2005)<br>Edwards et al. (2001)</td></tr><tr><td>Naphthalene</td><td>91-20-3</td><td>HP</td><td>HP</td><td>2.20</td><td>90.1</td><td>2.20</td><td>90.1</td><td>–</td><td>2B</td><td>Respiratory disorders, irritation<br>of the eyes</td><td>Mosqueron and<br>Nedellec (2004)</td></tr><tr><td>Tetrachlorethylene</td><td>127-18-4</td><td>HP</td><td>HP</td><td>0.10</td><td>20.9</td><td>73.6</td><td>0.56</td><td>250</td><td>2A</td><td>Neurological effect, renal disorder</td><td>Weisel et al. (2005)<br>Mosqueron and<br>Nedellec (2002)</td></tr><tr><td>Toluene</td><td>108-88-33</td><td>HP</td><td>HP</td><td>2.83</td><td>122</td><td>247</td><td>10.1</td><td>260</td><td>3</td><td>Neurological effect</td><td>Weisel et al. (2005)<br>Edwards et al. (2001)</td></tr><tr><td>Trichlorethylene</td><td>79-01-6</td><td>HP</td><td>HP</td><td>0.04</td><td>7.84</td><td>41.8</td><td>0.12</td><td>2.3</td><td>2A</td><td>Neurological effect, cancer<br>of the testicles</td><td>Weisel et al. (2005)<br>Mosqueron and<br>Nedellec (2002)</td></tr></tbody></table>

a Chemical Abstract Service.

b HP: Highly Priority, VP: Very Priority.

c International Agency for Research on Cancer (IARC) classification: Group 1: agent carcinogenic to humans, group 2A: agent probably carcinogenic to humans, group 2B: agent possibly carcinogenic to humans, Group 3: agent not classifiable as to humans, group 4: agent not carcinogenic to humans, ns: non-study.

d NIOSH recommended for the carcinogenic Lowest Feasible Concentration.

e Canada (1987).

1997). Typical air exchange rates in rooms without mechanical ventilation systems range from 0.1 to 0.4 h⁻¹, as stated above. Indoor VOCs concentrations are generally higher than outdoor concentrations because VOCs can be released from human activities and a wide variety of materials such as floorings, linoleum, carpets, paints, surface coatings, furniture etc (Yu and Crump, 1998). Salthammer (1997) for instance showed furniture coatings could release 150 VOCs (mainly aliphatic and aromatic aldehydes, aromatic hydrocarbons, ketones, esters and glycols) at Total VOC (TVOC) concentrations up to 1288 µg m⁻³ in test chamber studies and TVOCs emission rates as high as 22,280 µg m⁻² h⁻¹ have been recorded from vinyl/pvc flooring (Yu and Crump, 1998). Molds and bacteria can contribute significantly to the presence of particles (spores) or VOCs in indoor pollution (Schleibinger et al., 2004). Microbial development in buildings can be found in places where humidity accumulates including defective heating and air conditioning systems, garbage disposal, bathrooms, water leaks etc. and has been shown to provoke toxic and allergenic responses. Thus, although the individual concentrations of each contaminants are generally low (µg m⁻³), several hundreds contaminants can be found simultaneously, resulting in significant TVOC levels. Kostiainen (1995) found that individual concentrations of selected pollutants were 5–1000 times higher in 38 Finish sick-houses (defined as houses in which people experienced symptoms associated with SBS) than their mean concentrations in 50 normal houses used as reference, with over 200 VOCs being simultaneously detected in 26 houses. The same study also reported a maximal TVOC concentration of 9538 µg m⁻³ in one sick house compared to the mean concentration of 121 µg m⁻³ recorded in the normal houses. Likewise, Brown and Crump (1996) recorded TVOC concentrations up to 11,401 µg m⁻³ in UK homes and Daisey et al. (1994) reported indoor TVOC concentrations of 230–700 µg m⁻³ (geometric mean of 510 µg m⁻³) in 12 Californian office buildings. It is not easy to correlate TVOC with health effects because this generic parameter does not reflect the individual differences in toxicities found among indoor air VOCs; however, experience of eye, nose or mouth irritation has been reported at 5000–25,000 µg TVOC m⁻³ (Andersson et al., 1997).

Although indoor VOCs such as benzene or some polycyclic aromatic hydrocarbons are recognized as human carcinogens, a direct association between VOCs exposure and SBS symptoms or cancer has not been fully established at typical indoor air concentrations (Wallace, 2001). Several studies however correlated exposure to low concentrations of these pollutants with increased risks of cancer or eye and airways irritations (Table 2) (Vaughan et al., 1986, Wallace, 2001, 1991, Wolkoff and Nielsen, 2001). Symptoms such as headache, drowsiness, fatigue and confusion have been recorded in subjects exposed to 22 VOCs at 25 µg m⁻³ (Hudnell et al., 1992). Likewise, exposure to 1000 µg m⁻³ of formaldehyde causes coughing and eye irritation. In addition, many harmless VOCs can react with oxidants such as ozone, producing highly reactive compounds that can be more harmful than their precursors, some of which are sensory irritants (Sundell, 2004; Wolkoff et al., 1997; Wolkoff and Nielsen, 2001). Finally, most of the reported concentrations based on stationary measurement might lead to an underestimation of the real exposure dose of the subjects evaluated in epidemiological studies because concentrations in the breathing zone could be 2 to 4 times higher than those recorded with traditional methods (Rodes et al., 1991; Wallace, 2001, 1991; Wolkoff and Nielsen, 2001).

## 4. Indoor air treatment 216

Existing solutions to poor indoor air quality include combination of actions such as removing the pollutant sources; increasing ventilation rates and improving air distribution; and cleaning the indoor air (US EPA). Although certain furniture or appliance-manufacturers are already phasing out the use of formaldehyde, removing the pollutant sources is only possible when these are known and control is technically or economically feasible, which is seldom the case. New substances are constantly detected and classified as hazardous and many sources can release compounds for years. In addition, there are fears that many air pollutants are yet to be discovered (Otake et al., 2001; Carlsson et al., 2000; Muir and Howard, 2006) and preventive approaches might therefore be needed to ensure indoor air
<table><caption>**Table 3**
Current and emerging indoor air treatment methods: principle, examples and limitations</caption><thead><tr><th></th><th>Method</th><th>Principle</th></tr></thead><tbody><tr><td>t3.1</td><td>Current methods</td><td></td></tr><tr><td>t3.2</td><td>Filtration</td><td>Air is passed through a fibrous material (often coated with a viscous substance), which is efficient for particle removal but not gases. Filters are compact and commonly used but their efficiency decreases as they become saturated (fouling). Microorganisms can also develop in filters and particles reemission might occur.</td></tr><tr><td>t3.3</td><td>Electrostatic precipitator with ionization</td><td>An electric field is generated to trap charged particles. Electrostatic precipitators are often combined with ion generators that charge particles. Remove efficiently particles, are, compact, commonly used but can generate hazardous charged particles.</td></tr><tr><td>t3.7</td><td>Adsorption</td><td>Air pollutants are adsorbed onto activated carbon or zeolites, often as filtration post-treatment. The adsorbent might be too specific and might saturate fast because the pollutant are not destructed. There is therefore a potential risk of pollutant reemission.</td></tr><tr><td>t3.8</td><td>Ozonation</td><td>Ozone is generated to oxidize pollutants. Only remove some fumes and certain gaseous pollutants and might generate unhealthy ozone and degradation products. Ozone-based purifiers are not recommended by the American Lung Association.</td></tr><tr><td>t3.9</td><td>Photolysis</td><td>High energy ultra violet radiation oxidizes air pollutants and kills pathogens. It can however only remove some fumes and some gaseous pollutants and might release toxic photoproducts. Accidental exposure to UV light is harmful and UV irradiation is energy consuming.</td></tr><tr><td>t3.10</td><td>Photocatalysis</td><td>High energy ultra violet radiation is used in combination with a photocatalyst (TiO<sub>2</sub>) to generate highly reactive hydroxyl radicals that can oxidize most pollutants and kill pathogens. This energy-intensive method is increasingly popular and suitable for a broad range of organic pollutants.</td></tr><tr><td>t3.11</td><td>Emerging methods</td><td></td></tr><tr><td>t3.13</td><td>Membrane separation</td><td>Pollutants are passed through a membrane into another fluid by affinity separation. This method is normally recommended for highly loaded streams and has yet to be proven at low VOC levels. If the separated VOC are not reused, membrane filtration must be completed with a destruction step.</td></tr><tr><td>t3.14</td><td>Enzymatic oxidation</td><td>Air pollutants are transferred into an aqueous phase where they are degraded by suitable enzymes. Little information is however available concerning the efficiency of the commercial system (Air and Water Solution Inc., USA, http://www.srebiotech.com/) and new enzymes must be supplied periodically.</td></tr><tr><td>t3.15</td><td>Botanical purification</td><td>Air is passed through a planted soil or directly on the plants. The contaminants are then degraded by microorganisms and/or plants, the precise mechanisms being unclear. Although the efficiency of botanical purification has not been fully proven, a number of devices have been patented and several commercial products are available.</td></tr><tr><td>t3.16</td><td></td><td>US patent 6,676,091 for instance discloses a device where air is forced directly through a vertical (or slightly inclined) porous material serving as support for hydroponic plants, the plant's main purpose being to support the activity of pollutant degrading microorganisms in their rizosphere. This device is rather large in regards to other technology but can be use for interior design purposes.</td></tr><tr><td>t3.17</td><td>Biofilters and biotrickling filter</td><td>Air is passed through a packed bed of a solid support colonized by attached microorganisms that biodegrade the VOCs. In one configuration, air was purified through lava rocks covered with a geotextile cloth supporting mosses (Darlington et al., 2001).</td></tr></tbody></table>

climate control, or noise. Periodical air refreshing is often not efficient 233 because many indoor air pollutants are constantly released. Hence, 234 forced ventilation is still one the most common methods used for air 235 treatment (Wargocki et al., 2002). The improvement of indoor air 236 quality and energy savings are encouraged in the EU and by 237 movements such as the "Green Building" (US Green Building Council), 238 which means that forced ventilation should be reduced at the same 239 time as IAQ should be improved. In a worst case scenario (no heat 240 recovery from ventilation) the energy requirements for heating a 241 commercial office of 100 m² and 2.5 m of height at 23 °C ventilated 242 with outdoor air (4 °C) at an air exchange rate of 3 room volumes per 243 hour is approx. 3420 kWh/month (or about 340 USD, based on 244 residential electricity prices in 2006). Consequently, there are few 245 alternatives left than purifying the air inside the building. 246

Current methods for air purification include combinations of air 247 filtration, ionization, activated carbon adsorption, ozonation, and 248 photocatalysis (Table 3). These processes can be integrated into the 249 central ventilation system (in duct) or used in portable air purifiers (or 250 air cleaner) designed for limited spaces. Efficient strategies for particle 251 removal are now well established and include combinations of 252 filtration and electrostatic precipitation. The situation is however 253 very different for VOC removal. For instance, in a study conducted to 254 compare several commercial air purifiers, Shaugnessy et al. (1994) 255 concluded that, although high efficiency particles air filters (HEPA 256 filters) and electrostatic precipitators were highly efficient for particle 257 removal, none of the techniques tested (HEPA filtration, electrostatic 258 precipitation, ionization, ozonation, activated carbon adsorption) 259 could significantly remove formaldehyde. A similar study was recently 260 conducted to compare 15 air cleaners with a mixture of 16 261 representative VOCs (Chen et al., 2005). The technologies evaluated 262 included sorption filtration, ultraviolet-photocatalytic oxidation (UV- 263 PCO), ozone oxidation, air ionization and a botanical purifier 264 prototype (where contaminated air was blown through the rizosphere 265 of plants and contaminants were in principle removed by soil 266 microorganisms, the plants or their enzymes through various 267 mechanisms). The results were: 268

1. Among the 7 air cleaners using activated carbon in combination 269 with HEPA filter and/or ionizer, the best single pass removal 270 efficiencies achieved for formaldehyde, toluene and dodecane were 271 4, 32 and 39%, respectively (these contaminants were selected 272 for being representative of soluble, semi-soluble and unpolar 273 substances). 274

2. The commercial UV-PCO purifier did not effectively remove any of 275 the tested VOCs, although a "properly designed" device was 276 effective for certain VOCs (no data presented). 277

3. None of the ozone generator systems significantly removed any of 278 the VOCs tested; some even released ozone to concentrations much 279 higher than the safety limit set by the OSHA (Occupational Safety 280 and Health Administration). 281

4. The botanical purifier was able to remove around 20% of 282 formaldehyde (single pass) but no toluene and only 4% of 283 dodecane. 284

From this data, the authors concluded that only the biological 285 system significantly removed very volatile organic compounds, such 286 as formaldehyde, in contrast to the adsorption processes that 287 generally only satisfactorily removed the poorly soluble contaminants. 288 The overall elimination capacity (g VOC removed $d_{1/2}$) of the botanical 289 purifier for formaldehyde was however lower than the best activated 290 carbon based device because the biological purifier could not be 291 operated at high air flows. An air exchange rate of 3 room volumes per 292 hour is generally recommended for indoor air treatment, which 293 means that very large amounts of air must be treated into relatively 294 small units (for suitable use in the indoor space without visual or noise 295 nuisances). This might be difficult to achieve in botanical purifiers 296 where air is ventilated into the soil through the roots. 297

contaminants are maintained below satisfactory levels at all times. Natural aeration is the easiest alternative but it is often not possible because of outdoor weather, external pollution conditions (Ekberg, 1994; Daisey et al., 1994) or issues of security, safety in high buildings,
<table><caption>t4.1
**Table 4**
Biodegradability of typical indoor VOCs</caption><thead><tr><th rowspan="2">Substance</th><th rowspan="2"></th><th rowspan="2">Biodegradability<sup>a</sup></th><th colspan="2">Henry's law constants</th><th colspan="3">Biological treatment</th><th rowspan="2">References</th></tr><tr><th>H<sup>b</sup> (atm m<sup>3</sup> mol<sup>-1</sup>)</th><th>References</th><th>Inlet concentration<sup>c</sup> (mg m<sup>-3</sup>)</th><th>Removal efficiency (%)</th><th>Biological treatment<sup>d</sup></th></tr></thead><tbody><tr><td rowspan="3">Acetaldehyde (Ethanal; CH<sub>3</sub>CHO)</td><td rowspan="3">3</td><td>5.88 10<sup>-5</sup></td><td>Zhou and Mopper (1990)</td><td rowspan="3">18.1-180.1<sup>e</sup></td><td rowspan="3">40-80</td><td rowspan="3">B</td><td rowspan="3">Mohd Adly et al. (2001)</td></tr><tr><td>7.69 10<sup>-5</sup></td><td>Sander (1999)</td></tr><tr><td>5.88 10<sup>-5</sup></td><td>US EPA (1982)</td></tr><tr><td rowspan="3">Benzene (C<sub>6</sub>H<sub>6</sub>)</td><td rowspan="3">2</td><td>6.25 10<sup>-3</sup></td><td>Staudinger and Roberts (1996)</td><td rowspan="3">1.6<sup>e</sup></td><td rowspan="3">9-77</td><td rowspan="3">B</td><td rowspan="3">Ergas et al. (1992)</td></tr><tr><td>5.55 10<sup>-3</sup></td><td>US EPA (1982)</td></tr><tr><td>4.76 10<sup>-3</sup></td><td>Sander (1999)</td></tr><tr><td rowspan="3">Formaldehyde (Methanal; HCHO)</td><td rowspan="3">3</td><td>3.33 10<sup>-7</sup></td><td>Sander (1999)</td><td>0.12-0.49<sup>e</sup></td><td>50 to 60</td><td>BF</td><td>Wolverton et al. (1989)</td></tr><tr><td>3.23 10<sup>-7</sup></td><td>Zhou and Mopper (1990)</td><td rowspan="2">0.018-0.18<sup>e</sup></td><td rowspan="2">90</td><td rowspan="2">BF</td><td rowspan="2">Darlington (2004)</td></tr><tr><td>3.13 10<sup>-7</sup></td><td>Staudinger and Roberts (1996)</td></tr><tr><td rowspan="2">Naphthalene (C<sub>10</sub>H<sub>8</sub>)</td><td rowspan="2">1</td><td>4.76 10<sup>-4</sup></td><td>Sander (1999)</td><td rowspan="2">0.494<sup>e</sup></td><td rowspan="2">75</td><td rowspan="2">TPPB</td><td rowspan="2">MacLeod and Daugulis (2003)</td></tr><tr><td>4.76 10<sup>-4</sup></td><td>US EPA (1982)</td></tr><tr><td rowspan="2">Tetrachlorethylene (Tetrachloroethene; C<sub>2</sub>Cl<sub>4</sub>)</td><td rowspan="2">1</td><td>2.78 10<sup>-2</sup></td><td>US EPA (1982)</td><td rowspan="2">0.678<sup>e</sup></td><td rowspan="2">12-49</td><td rowspan="2">B</td><td rowspan="2">Ergas et al. (1992)</td></tr><tr><td>1.69 10<sup>-2</sup></td><td>Staudinger and Roberts (1996)</td></tr><tr><td rowspan="2">Toluene (Methylbenzene; C<sub>6</sub>H<sub>5</sub>CH<sub>3</sub>)</td><td rowspan="2">2</td><td>1.56 10<sup>-2</sup></td><td>Sander (1999)</td><td rowspan="2">1.88<sup>e</sup></td><td rowspan="2">14-78</td><td rowspan="2">B</td><td rowspan="2">Ergas et al. (1992)</td></tr><tr><td>6.67 10<sup>-3</sup></td><td>US EPA (1982)</td></tr><tr><td rowspan="3">Trichlorethylene (Trichloroethene; C<sub>2</sub>HCl<sub>3</sub>)</td><td rowspan="3">1</td><td>6.67 10<sup>-3</sup></td><td>Staudinger and Roberts (1996)</td><td rowspan="3">753.5</td><td rowspan="3">50</td><td rowspan="3">MS</td><td rowspan="3">Ergas<sub>5</sub> (1999)</td></tr><tr><td>9.09 10<sup>-3</sup></td><td>Sander (1999)</td><td>0.226-0.301<sup>e</sup></td></tr><tr><td>1.12 10<sup>-2</sup></td><td>US EPA (1982)</td><td>0.057-0.57<sup>e</sup></td></tr><tr><td rowspan="2">Trichlorethylene (Trichloroethene; C<sub>2</sub>HCl<sub>3</sub>)</td><td rowspan="2">1</td><td>1.00 10<sup>-2</sup></td><td>Staudinger and Roberts (1996)</td><td rowspan="2">107.44</td><td rowspan="2">30</td><td rowspan="2">MS</td><td rowspan="2">Darlington (2001)</td></tr><tr><td>0.01-0.04<sup>e</sup></td><td>0.054-2.149<sup>e</sup></td></tr></tbody></table>

a 1=low biodegradability, 2=moderate biodegradability, 3=good biodegradability (Shareefdeen and Singh, 2005; Devinny et al., 1999).

b Under standard conditions.

c Concentrations close to the average concentration observed in indoor air.

d B = Biofiltration; MS = Membrane Separation; BF = Botanical Filter; TPPB = Two-Phase Partitioning Bioreactor; BTr = Biotrickling Filter.

e In mixture with other compounds.

Membrane mediated VOCs removal in indoors air has been also recently evaluated. Formaldehyde and benzene were significantly removed at concentrations of 11,500 and 6500 µg m⁻³, respectively using a zeolite membrane operated at permeation fluxes of 0.01 and 0.0056 g m⁻² h⁻¹, respectively (Aguado et al., 2004). Further studies must however demonstrate pollutant removal at concentrations relevant to indoors environments (approx. one order of magnitude lower than those tested) and provide solutions to destroy the VOCs following separation.

## 5. Biological treatment of indoor air

There is little data available on the biological removal of VOCs from indoor air and all the studies hitherto conducted have, to the best of our knowledge, focused on botanical purifiers. In a pioneer study supported by the NASA, Wolverton and co-authors demonstrated the potential of plants (and their rizosphere) to remove indoor VOCs in sealed chamber. In their earliest study (Wolverton et al., 1984), the authors found that several plants could remove formaldehyde at 19,000-46,000 µg m⁻³ to levels lower than 2500 µg m⁻³ (detection limit) in 24 h. Similar studies were conducted with benzene and trichloroethylene at more relevant concentrations of 325-2190 µg m⁻³ (Wolverton et al., 1989). It was then found that the 8 plants tested could remove benzene by 47-90% in 24 h compared to 5-10% in the control tests, and that the rizosphere zone was the most effective area for removal. Orwell et al. (2004) later investigated the potential of indoor plants for removing benzene in sealed chamber (0.216 m³) and

found that microorganisms of the plant rizosphere were mainly responsible for benzene removal (40-80 mg m⁻³ d⁻¹). These results were obtained at high initial benzene concentrations (81,000-163,000 µg m⁻³) and benzene removal rate increased linearly with the dose concentration, suggesting the system might be inefficient under typical indoor air conditions. However, the same team more recently demonstrated that plants significantly reduced toluene and xylene at indoor air concentrations of 768-887 µg m⁻³ (Orwell et al., 2006) and even the TVOC concentration in office buildings during field testing under real conditions (Wood et al., 2006). Unfortunately, the divergences in toluene removal reported in the studies of Chen et al. (2005) and Orwell et al. (2006) cannot be explained, especially as the prototype used in the earlier study was not fully described. Many parameters such as the interfacial areas, the moisture content, and the type (hydrophobicity) of the biomass used can influence pollutant removal in biological purifiers. There is therefore a need for a more coordinated research in the area. Various botanical purifiers have also been patented (i.e. US5407470, US5277877) but such devices have not reached a broad market and no data on pollutant removal under relevant conditions is available. Research on the development of a commercial biological purifier has been carried out at the University of Guelph, Canada (Darlington et al., 2000; Air Quality Solution Ltd). In one configuration, air was purified through lava rocks covered with a geotextile cloth supporting mosses (Darlington et al., 2001). This device was operated at relevant influent levels equal or lower than 300 µg m⁻³ and displayed a purification efficiency of 30% at the lowest
air flow treated. Water was also added to the filter to compensate for water losses through evaporation (approx. 20 L d⁻¹ in 120 m² and 640 m³ room). In a second configuration, disclosed in US patent 6,676,091 from the same author, air is forced directly through a vertical (or slightly inclined) porous material serving as support for hydroponic plants which its main purpose is to support the activity of pollutant degrading microorganisms in the rizosphere.

From the studies herein presented, it appears that the role of plants in botanical purifier is often suspected to support a microbial activity that is responsible for pollutant removal. Direct pollutant accumulation or degradation by plants is however known to occur during phytoremediation of contaminated soils (Newman and Reynolds, 2004) and the ability of plant leaves to directly take up and remove pollutant during air treatment is still debated (Wolverton et al., 1984; Schmitz et al., 2000; Schäffner et al., 2002). A recent study has suggested that bacteria growing on plant leaves could also contribute to VOC biodegradation (Sandhu et al., 2007). More generally, there is growing evidence of the complexity, and importance of interactions between plants and bacteria (Dudler and Eberl, 2006) and research in this area is of utmost importance for IAQ. There is a lack of peer-reviewed data available in the literature and an urgent need to improve our understanding of the fundamental mechanisms of VOC uptake or release by plants and their microbial hosts (Kesselmeier and Staudt, 1999). The following discussion will therefore focus on the more established microbial degradation mechanisms.

## 5.1. Influence of type of VOCs

The biological treatment of organic compounds is based upon the capability of microorganisms to use these molecules as sources of carbon, nutrients and/or energy or to degrade them metacabolically using unspecific enzymes. The intrinsic biodegradability of an organic compound depends on many factors such as its hydrophobicity to the microbial population, the most soluble being generally the most biodegradable, or its toxicity. Toxicity effects, which sometimes limit the biological treatment of industrial air, are likely not a problem at the concentrations found in indoor air and this will not be discussed further in this review. Many VOCs are rather small molecules that are moderately soluble and in fact, are biodegradable (Table 4) although certain xenobiotic compounds, such as chlorinated compounds (i.e. tetrachloroethylene), may be recalcitrant. Synergetic or negative interacting effects within pollutant mixture should also be taken into consideration (Yu et al., 2001). Given the high number of VOC simultaneous found in indoor air, and the huge variations in structures and properties, a biological process suitable for indoor air treatment should rely on diverse, versatile and adaptive microbial communities to ensure all pollutants are removed. This can be achieved in fixed biofilm-based reactors where high microbial diversity and cell proximity favour cellular exchanges (Molin and Tolker-Nielsen, 2003; Singh et al., 2006), acclimation (long cell residence time) and synergetic effects under various growth conditions by the establishment of substrate concentration gradients thorough the biofilm (Beveridge et al., 1997; Marshall, 1994). Completing or combining biodegradation with a physicochemical post-treatment is also possible to ensure the complete removal of all pollutants. Finally, great variations in total and individual pollutant concentrations leading, for instance, to long periods of time when a given compound is not found in the indoor air could lead to permanent or momentary losses in catabolic ability. Such effects need to be further studied and possibly prevented as discussed below.

## 5.2. Influence of low concentrations on biomass productivity and transfer rates

During the biodegradation process, the concentration of an organic pollutant in the micro-environment where the microorganisms are found has a profound impact on microbial activity and ultimately on the pollutant removal rate. At reasonably high substrate concentrations, the

organic pollutant can be metabolized and used to synthesize more 410 biomass in a process that self-regenerates the biocatalyst. When the 411 concentration is decreased further, a critical level is reached below 412 which new cells are no longer produced. Nevertheless, the contaminant 413 can still be biodegraded if a significant active biomass is available and if 414 the gene(s) responsible for the production of the enzyme(s) required for 415 the pollutant degradation is (are) still expressed (Kovárová-Kovar and 416 Egli, 1998). Gene expression depends, among other factors, on the 417 concentration of the enzyme substrate which can be the pollutant or 418 other molecules. In this context, it is crucial to compare the low 419 concentrations at which indoor VOCs are typically found with known 420 threshold for microbial growth and biodegradation. 421

Toluene indoor air concentrations of 0.58-17 µg m⁻³ have been 422 reported in Californian office buildings (Daisey et al., 1994). Assuming 423 toluene must first transfer into an aqueous phase before being 424 biodegraded, the maximum aqueous toluene concentration (Cₐq*) at 425 which microorganisms will be exposed to can be calculated from the 426 Henry's law constant (H) coefficient: 427

$$C_{aq}^{*} = \frac{P_{i}}{H_{i}} \qquad (1)$$

Where Pᵢ is the partial pressure of the target contaminant in the 430 gas phase and Hᵢ its Henry constant. For toluene (H=6.67 10⁻³ atm m³ 431 mol⁻¹; Table 4), this will result in a Cₐq* of 2-60 ng L⁻¹ under normal 432 conditions of temperature and pressure. If toluene is removed by 90%, 433 microorganisms would actually be exposed to concentrations of 0.2- 434 6 ng L⁻¹ (under continuous treatment under a steady state). At such 435 concentration, toluene can be reasonably considered as the limiting 436 substrate if it is the only carbon source available. By comparison, the 437 threshold growth concentration of bacteria from drinking-water 438 biofilm has been estimated to about 0.1 µg L⁻¹ (van der Kooij et al., 439 1995) which is in the same range of reported toluene mineralization at 440 aqueous concentrations of 0.9 µg L⁻¹ with active bacteria (Roch and 441 Alexander, 1997). Hence, from the data currently available, it seems 442 unlikely that indoor air VOC can support growth. However, micro- 443 organisms might still be capable to use certain VOCs as energy source 444 for maintenance or meteorologically biodegrade them by using another 445 substrate for growth. 446

The specific biomass growth rate (μ, h⁻¹) at sub-inhibitory 447 substrate concentration can often be expressed as: 448

$$\mu = \frac{\mu_{\max}S}{K_s + S} - k_d \qquad (2)$$

Where μmax (h⁻¹) is the maximum specific growth rate, Ks is the 451 saturation constant (mg L⁻¹), kd is the endogenous decay coefficient 452 (h⁻¹) and S is the limiting substrate concentration (mg L⁻¹). The 453 substrate uptake volumetric rate is equivalent to: 454

$$r_{su} = \frac{\mu X}{Y} + mX \qquad (3)$$

where X is the biomass concentration, Y is the true biomass yield (g 456 biomass g substrate⁻¹), and m is the maintenance coefficient. This 457 equation shows that substrate consumption for maintenance can 458 occur even under no growth conditions (μmaxS/(Ks+S)<kd), although 459 cell decay would cause a continuous decrease in catabolic activity. 460

Based on the toluene biodegradation kinetics reported by 461 Alagappan and Cowan (2003) in *Pseudomonas putida* F1 cultures 462 (μmax=0.37 h⁻¹ and Ks=0.44 mg L⁻¹), the specific cell production rate 463 under indoor conditions should range from 5 × 10⁻⁵-1.7 × 10⁻⁶ h⁻¹. 464 These values are far below the cellular maintenance rates reported in 465 the literature for aromatic compounds (0.019 h⁻¹ for ethylbenzene 466 and 0.016 h⁻¹ for benzene), and the death cells coefficients for *P. putida* F1 during the degradation of toluene (0.06 h⁻¹; Alagappan and 467 Cowan, 2003). Thus, in this particular situation, neither would 468 pollutant supply meet maintenance requirements nor would the 469 specific growth rate meet the cellular decay rate. 470
Although these kinetics parameters were obtained at pollutant concentrations higher than those found in indoor air, this simple calculation shows that indoor air biological treatment will likely require the development of specific methods to provide and maintain a suitable catabolic activity. First, due to the complexity and variability of indoor air, an inoculum that possesses the suitable catabolic ability might be difficult to obtain. These microorganisms would also likely need to be pre-cultivated at higher VOC concentration to obtain a significant cell number in a relative short time, which might impair their ability to take up substrates at trace levels (microorganisms can lose selective traits when the corresponding selection pressure is released). Attached growth should therefore be recommended for inoculum preparation to combine, through the establishment of substrate concentration gradients inside the biofilm, microbial growth at the liquid-biofilm interface with continuous selection of microorganisms acclimated to low-substrate concentration inside the biofilm (Beveridge et al., 1997). Second, maintaining catabolic activity (and not only cell mass or cellular activity) could be challenging as microorganisms can lose their ability to biodegrade certain substrates when deprived from them during long periods of time. Biofilm systems could ensure constant performance under fluctuating operating conditions by allowing substrate accumulation at the biofilm interface until degradation becomes possible (Singh et al., 2006). As mentioned above, attached microorganisms also form diverse and dynamic communities (cell communication and cellular exchanges such as horizontal gene transfer being favoured by the cell proximity) that can respond more quickly to changes, which in return favours functional redundancy (Rittmann and McCarty, 2001). Biofilms are in fact ubiquitous and prevail under diluted environment (Beveridge et al., 1997). Finally, even under conditions when suitable degradation-enzymes are expressed, microbial activity must be capable to reduce the contaminant at concentration low enough to permit significant mass transfer. Roch and Alexander (1997) showed toluene mineralization at 0.9 µg L⁻¹ but the pollutant still remained at 79 ng L⁻¹ after 8 days of incubation. Similar findings were reported by Pahm and Alexander (1993) when studying the biodegradation of p-nitrophenol at trace concentration although addition of a secondary carbon source was capable to trigger pollutant removal at concentrations of 1 µg L⁻¹. However, the feasibility of removing estrogens at 100 ng L⁻¹ to below 2.58 ng L⁻¹ (detection limit) with pure laccase

from *T. versicolor* was recently demonstrated (Auriol et al., 2007), showing biological systems should be able to perform at indoor air concentrations.

Clearly, the development of biological methods for indoor air filtration faces several challenges and requires more research on the microbial mechanisms of acclimation, survival, substrate recognition, accumulation and uptake at trace concentration. Most published biodegradation studies have been conducted under irrelevant conditions (single substrate, high substrate concentration, suspended growth) using isolates cultivated in the laboratory. Current kinetics data might therefore underestimate the capacity of natural strains to take up trace pollutants (Subba-Rao et al., 1982; Pahm and Alexander, 1993). Low concentrations are common in the environment and certain microorganisms have developed original survival strategies under such conditions by for instance accumulating limiting substrate before starting to growth (Singh et al., 2006). New models to correlate growth with substrate concentration are therefore needed at trace concentration, as suggested by Butterfield et al. (2002) in a study on drinking-water biofilm formation under carbon-limited conditions (<2 mg L⁻¹).

The simultaneous presence of many contaminants in indoor air might sustain microbial growth or, at least, induce pollutant mineralization, as suggested by the experience of Pahm and Alexander (1993) described above. Furthermore, under starving conditions, certain microorganisms are capable to quickly increase their affinity for the limiting substrates or acclimate to simultaneously mineralize a high number of carbon sources (Kovárová-Kovar and Egli, 1998, Tros et al., 1996). Particles and other air macropollutants (carbon dioxide, carbon monoxide, ammonia, etc) or plants might also provide enough energy and carbon substrates for growth, as evidenced by the microbial colonization of indoor surfaces and air handling systems or the microbial activity recorded in the rizosphere of botanical purifier. In addition, certain microorganisms are able to grow both heterotrophically and autotrophically (Larimer et al., 2003) or on myriads of different organic compounds (Chain et al., 2006). Such metabolic versatility would give obvious advantages under conditions where numerous potential carbon and energy sources are simultaneously found at very low concentrations and would greatly enhance the treatment of indoor air. The question is therefore not if microbial growth would occur, but if it will cause VOC reduction. For instance,

<figure><img src="image_1.png" /><figcaption>**Fig. 1.** Schematic presentation of a simple steady state mass balance analysis of indoor air treatment.</figcaption></figure>
from field studies on the efficiency of plants to purify air from office buildings, Wood et al. (2006) suggested that a TVOC concentration of 100 ppb was sufficient to induce a biological response that could reduce the TVOC concentration up to 75%. In addition, during laboratory studies on VOC removal by potted plants in sealed chamber, Orwell et al. (2006) observed the presence of toluene accelerated the removal of x-xylene, although the reciprocal was not true.

Several authors have also challenged the mass transfer and microbial uptake theories use to predict the effect of substrate concentration in biological purifiers. Active transfer by enzymatic transformation has for instance been reported and mechanisms of direct uptake at the air-cell interface have been suggested. For instance, Miller and Allen (2005) reported that direct pollutant diffusion through the aqueous layer surrounding the biofilm could not explain the surprisingly high performances of biological systems treating the highly hydrophobic alpha-pinene. Likewise, it has been suggested that the aerial mycelia of fungi, which are in direct contact with the gas phase, might promote the direct uptake of VOC from the gas phase. This uptake is faster than if a flat biofilm of bacteria directly contacts the gas phase because of a high gas-mycelium interfacial area of the fungal mat and the highly hydrophobic nature of the fungal cell wall (Arriaga and Revah, 2005; Kennes and Veiga, 2004; Van Groenestijn and Kraakman, 2005, Vergara et al., 2006).

### 5.3. Impact of purification efficiency on design

A simple steady state mass balance analysis can be used to design a device for indoor air purification (Fig. 1) based on the following equation:

$$V \frac{dC}{dt} = QC_{\text{in}} - QC_{\text{out}} + R_p \quad (4)$$

where V is the room volume (m³); C the TVOC concentration in the room (µg m⁻³); Q is the refreshment flow rate (m³ h⁻¹) through the room, C<sub>in</sub> is the TVOC concentration of the air entering the room (µg m⁻³), C<sub>out</sub> is the TVOC concentration of the air leaving the room (µg m⁻³), and the R<sub>p</sub> is the TVOC production rate inside the room (µg h⁻¹).

At steady state under completely mixed conditions within the control volume analyzed dC/dt=0 and C=C<sub>out</sub>. Assuming that C<sub>in</sub> is negligible compared to C<sub>out</sub> (indoor air concentrations are usually higher than outdoor), Eq. (5) gives:

$$R_p = QC_0 \quad (5)$$

where C₀=C<sub>out</sub> represents the initial pollutant concentration in the room, before air is being cleaned. When an air treatment unit (i.e. a biological purifier) is started in the room the mass balance analysis becomes:

$$V \frac{dC}{dt} = QC_{\text{in}} - QC_{\text{out}} + R_p - R_b \quad (6)$$

Where, for a biological purifier, R<sub>b</sub> is the TVOC biological removal rate (µg h⁻¹).

If we assume that R<sub>p</sub> is constant (which is unlikely as the rate of VOC evaporation from coatings depends on the concentration of the VOC in the room and the concentration in the material) and C<sub>in</sub>≠0, the rate of TVOC biologically removed under steady state conditions can be expressed as:

$$R_b = Q(C_0 - C) \quad (7)$$

If we now consider the biological purifier, R<sub>b</sub> can also be expressed as:

$$R_b = Q_b (C_{\text{bin}} - C_{\text{bout}}) \quad (8)$$

Where C<sub>bin</sub> and C<sub>bout</sub> are the TVOC concentration in the air entering 608 and leaving the biological purifier, respectively; and Q<sub>b</sub> is the air flow 609 treated by the biological filter. 610

By definition, the single pass efficiency (%) of the biological purifier 611 is expressed as:

$$\eta = \frac{C_{\text{bin}} - C_{\text{bout}}}{C_{\text{bin}}} \quad (9)$$

Based on the assumption of completely mixed conditions in the 615 room C<sub>bin</sub>=C, then:

$$R_b = \eta C Q_b \quad (10)$$

Combining Eqs. (8) and (11) gives the following expression for Q<sub>b</sub>: 619

$$Q_b = \frac{C_0 - C}{\eta C} Q \quad (11)$$

The required biological purifier air flow is therefore a function of 622 the single pass biofiltration efficiency, the initial and required TVOC 623 concentrations, and the refreshment rate. It can also be expressed as a 624 function of the ratio between initial and final concentrations in the 625 room (A=C<sub>0</sub>/C), which is another expression of the removal efficiency 626 according to:

$$Q_b = \frac{A-1}{\eta} Q \quad (12)$$

The overall performance of the purification device can then be 630 expressed as its effectiveness (R, %) according to:

$$R = \frac{C_0 - C_1}{C_0} \quad (13)$$

Based on this equation, a simulation was made to determine Q<sub>b</sub> 634 under varying R and η values at C₀=500 µg m⁻³ (Fig. 2) as typical 635 TVOC values in sick houses range from 100 to 1000 µg m⁻³. The 636 refreshment rate was assumed to 0.35 h⁻¹ in a room of 100 m³ 637 (Q=35 m³ h⁻¹) as described by Wolkoff et al. (1991). This simulation 638 shows that a purification flow (Q<sub>b</sub>) of approx. 400 m³ h⁻¹ is required to 639 achieve a satisfactory overall purification effectiveness of 90% with 640 η=80%. This is rather logical since the removal efficiency of any 641 system is intrinsically limited by the low concentration of the 642 pollutants. These values are in accordance with the recommended 643 air cleaner true effectiveness of 80% for particle removal, which is 644 equivalent to providing 4-5 room volumes of clean air per hour 645 (Shaughnessy and Sextro, 2006). Hence, it is not only the single pass 646 purification efficiency of the biofiltration device but the overall 647 purification capacity (ηQ<sub>b</sub>) that is important, explaining why the 648 concept of Clean Air Delivery Rate (CADR=ηQ<sub>b</sub>, the amount of purified 649 air delivered per unit or time) was introduced to evaluate and 650 compare the various devices proposed for air removal (Shaughnessy 651 and Sextro, 2006). Interestingly, at equivalent CADR, purification 652 devices with high single pass efficiencies should be preferred because 653 of their lower energy requirement (lower required flow rate). 654

The biological purifier can be designed using another engineering 655 approach based on the gas residence time (t<sub>res</sub>) in the purifier and the 656 purifier refreshment capacity (α): 657

$$t_{\text{res}} = \frac{V_b}{Q_b} \quad (14)$$

where V<sub>b</sub> is the volume of the biological purifier. 659

$$\alpha = \frac{\eta \cdot Q_b}{V} = \frac{\text{CADR}}{V} \quad (15)$$
<figure><img src="image_2.png" /><figcaption>**Fig. 2.** Simulated performance of the effect of the biological purifier single pass efficiency ($\eta$, %) on the required biofiltration volumetric flow rate ($Q_b$, m³ h⁻¹) to achieve an overall effectiveness $R=80-95\%$ (with $R=(C_o-C_0-C)/C_0$ where $C$ and $C_0=500$ µg m⁻³ are the steady state TVOCs concentrations before and during the biological purifier operation, respectively) in a 100 m³ room with a refreshment rate of 0.35 h⁻¹. The air cleaner effectiveness can also be expressed as the ratio $A=C/C_0$.</figcaption></figure>

Thus, the biological purifier volume required can be expressed as:

(17) $V_b = \frac{\alpha t_{max} V}{\eta}$

Assuming a typical gas residence time (industrial applications) of 30 s, a purifier refreshment capacity of 4 h⁻¹, and a 80% single pass efficiency, the biological purifier volume required for a 100 m³ room would be 4.2 m³, which is a prohibitive volume in indoor environment. Even at such a short residence time as 2 s, the volume of the bioreactor needed in our simulation would be 278 L. By comparison, the biological purifier developed by Darlington et al. (2001) was tested at surface loading rates of 90-720 m³ m⁻² h⁻¹, which is within the range of operation of industrial biotrickling filter and bioscrubbers (100-1000 m³ m⁻² h⁻¹; Van Groenestijn and Hesselink, 1993). The 1920 L (9.6 m²) bioscrubber (not including plants and aquarium used for water circulation) was also operated in a 640 m³ room at influent toluene, ethylbenzene and xylene concentrations of 0-300 µg m⁻³. Single pass efficiencies of approx. 10-30% were achieved depending on the flow (the higher the flow, the lower the single pass efficiency) and temperature, which accounted for a CADR of 720 m³ h⁻¹ equivalent to a refreshment rate of 1.1 h⁻¹. Although it is difficult to draw conclusions from this single study, the data provided seems consistent with our simulation. It clearly shows that the engineering of compact biological purifiers with high effectiveness will be an important challenge.

The model described above can also be used to simulate the dynamic changes in pollutant concentration in a room where air purification is started by using Eq. (4) expressed as:

$$V \frac{dC}{dt} = QC_0 - QC - \eta Q_b C \qquad (16)$$

Which can then be integrated as:

$$C_t = C_0 \exp(-Bt) + \frac{A}{B}(1 - \exp(-Bt)) \qquad (17)$$

With

$$B = \frac{Q + \eta Q_b}{V} = \frac{Q + CADR}{V} \qquad (18)$$

And

$$A = \frac{QC_0}{V} \qquad (19)$$

In Eq. (17), $C_0\exp(-Bt)$ represents the combined VOC removal from 699 refreshment and biofiltration and $A/B(1-\exp(-Bt))$ represents the 700 production within the room. Using the example described above 701 ($V=100$ m³, $Q=35$ m³ h⁻¹, $C_0=500$ µg m⁻³), Fig. 3 shows that at the 702 range of purification flows required for such application, steady states 703 conditions are achieved rather quickly (1-2 h). Similar models are 704 used to estimate the single pass efficiency of purification devices in 705 sealed chamber test where pollutant are introduced at a certain 706 amount but where there is no production (Chen et al., 2005). Thus, 707 Wolverton et al. (1989) reported a decreased in benzene concentration 708 from 765 to 78 µg m⁻³ in 24 h in a sealed chamber containing a plant, 709 which resulted in a B coefficient in Eq. (18) of 0.1 h⁻¹. The B coefficient 710 is composed of the pollutant leakage rate from the system ($Q/V$) and 711 the pollutant removal in the air purifier ($CADR/V=\text{purifier refreshment capacity}$). The same author conducted a leak experiment which 713 allowed calculating the leak contribution to approx. 0.01 h⁻¹. Hence, 714 the biological purifier used in this study generated an amount of 715 purified air equivalent to 0.09 room volume per hour (CADR of 716 0.075 m³ h⁻¹) and would not significantly improve IAQ under realistic 717 conditions. Low refreshment rates of 0.02-0.3 h⁻¹ were also achieved 718 by Orwell et al. (2006) in sealed chambers containing potted plants 719 and initially supplied with 768-886 µg m⁻³ of m-xylene or toluene, 720 based on VOC exponential removal rate constants of 0.52-7.44 d⁻¹ 721 Likewise, Chen et al. (2006) achieved the highest CADR of 8.3 m³ h⁻¹ 722 (refreshment rate of 0.15 h⁻¹) with the biological purifier compared to 723
<figure><img src="image_3.png" /><figcaption>**Fig. 3.** Simulated changes in TVOC concentration (C, µg m⁻³) in a 100 m³ room where air purification is started at t=0 at a flow of 500 (dashed line), 300 (plain line) or 100 (circles) m³ h⁻¹ in a biological purifier with 90% single pass efficiency. The refreshment rate in the room is 0.35 h⁻¹ and the initial TVOC concentration is 500 µg m⁻³.</figcaption></figure>

values above 200 m³ h⁻¹ with other portable devices. Despite this, a significant TVOC removal was recorded when using potted plants during field testing in office (Wood et al., 2006) and even if such results should be reproduced under better controlled conditions, they might indicate that our current evaluation models are inadequate.

## 5.4. Humidification and biohazards

Since biological purifiers are typically saturated with water and since indoor air treatment requires high flows, indoor biological purification might increase the moisture content in the room or building where it is used. This beneficial effect when indoor air is too dry (moisture contents of 30-60% are generally recommended for comfort) could also trigger to the excessive growth of fungi with negative impact on IAQ (Schleibinger et al., 2004), although these effects are still uncertain (Robbins et al., 2000; Pasanen, 2001). Darlington et al. (2000) for instance reported that the use of an indoor biological purifier significantly increased the concentrations of total suspended spores, although these values were similar to concentrations found in flats containing house plants, and still remained within healthy levels (100-200 CFU m⁻³). In addition, none of the 17 fungal species identified was known pathogenic. Likewise, Ottengraf and Konings (1991) reported that the concentration of microbial germs (mainly bacteria) in the outlet of full scale industrial biofilters was within the range of typical indoor air concentration, and only slightly higher than typical outdoor air concentrations, which was more recently confirmed by Zilli et al. (2005). There is however too little data available and the potential release of microorganisms from indoor biological purifiers (especially in the case of faulty equipment or accidents) should be better studied and prevented if necessary.

## 5.5. Esthetic, noise, purification perception

Besides being efficient for pollutant removal, indoor air purifiers must be esthetic (unless it is integrated into the ventilation system) and silent, which is rather challenging considering the flow required and the volumetric constrains. As mentioned above, indoor air pollution involves many types of pollutants (particulates, inorganic etc), which concentrations must all be reduced below a certain "perception" level for users to feel the improvement of air quality. Especially, the effects of macropollutants such as CO₂ and H₂O should also be considered.

## 5.6. Evaluating performance

Setting-up realistic purification goals is difficult as there are only few guidelines on indoor VOC concentration. Evaluating performance

is no less challenging due to the analytical difficulty to detect numerous substances at very low concentrations, the lack of knowl-edge about contaminants and their effects, and because of the number of associated effects of air purification (i.e. moisture, temperature, physiological impacts on user). Ultimately, blind testing in sick-buildings that could correlate pollutant removal with customer/user satisfaction (by survey, measurement of productivity, etc) and health improvements would be necessary. Unfortunately, such methods cannot be used during phases of design and optimization for economical reasons and because of the need for well-defined and reproducible testing conditions, replicate and control. Hence, the efficiency of air purifiers is generally evaluated by either direct measuring of the single pass efficiency or by using test-chambers. In the first case, an artificially contaminated air stream is passed at a certain flow through the purification device and the concentrations of the target contaminants are measured at the inlet and outlet of the purifier (Howard-Reed et al., 2002). This method is rather simple and the influence of parameters such as the effluent composition or the treatment flow can easily be tested under continuous inlet air composition. Test-chamber assays are however often preferred, perhaps because they offer more flexibility and better simulate the indoor environment. Here, pollutants are injected in a hermetical chamber equipped with the air purifier as well as various sampling analysis, and air conditioning devices and their concentrations are measured over time. The purifier efficiency can then be evaluated using the basic model described above. Depending on the size and complexity of the chamber, the pollutant can be replaced by polluting material and treatment efficiency can be evaluated, for instance, in terms of work productivity by monitoring the activity of human test-workers operating inside the chamber (Wargocki et al., 1999). The use of artificially contaminated air also brings its own challenges as it is very difficult to determine a universal "model indoor air" due to the diversity and variability of pollutants concerned (Ekberg, 1994; Yu and Crump, 1998; Otson et al., 1994). However, recent progresses have been made in that direction (Ondarts et al., 2007).

VOC analysis from indoor air normally requires large air samples that are passed through a solid or liquid absorbent that serves to concentrate the contaminants, followed by further extraction/separation and analysis (Crump, 2001). However, many pollutants are not known or cannot be detected at indoor air concentration levels (Ondarts et al., 2007) and the costs of monitoring all known compounds would be prohibitive. In such cases, the TVOC (for details on protocols see Crump, 2001) can be measured although this parameter can be exclusive and does not take into consideration the different intrinsic toxicities of each compounds and interaction effects between the pollutants (Wolkoff, 2003; Wolkoff and Nielsen, 2001; Molhave, 2003).

# 6. Designing biological purifiers

Common biological processes for VOCs abatement include bio-scrubbers, biotrickling filter, and biofilters (Iranpour et al., 2005; Burgess et al., 2001; Delhoménie and Heitz, 2005; Revah and Morgan-Sagastume, 2005). In bioscrubbers, the air is washed with an aqueous phase into which the pollutants transfer, and the aqueous phase is transferred into a bioreactor where the pollutants are biodegraded. In biotrickling filters, microorganisms are grown on an inert material (plastics resins, ceramics etc). An aqueous solution containing the nutrients required for microbial growth is continuously distributed and recirculated at the top of the reactor and percolates by gravity, thus covering the biofilm with an aqueous layer. Contaminated air is introduced as counter current and the contaminants diffuse into the aqueous phase where they are biodegraded. The purpose of the packing material is to facilitate the gas and liquid flows and enhance gas/liquid contact, to offer a surface for microbial growth, and to resist crushing and compaction. In biofilters, air is passed through a moist

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porous material which supports microbial growth. Water remains within the packing material and is added intermittently to maintain humidity and microbial viability. The packing material is generally a natural material (peat, compost, wood shavings, etc.) which is biodegradable and provides nutrients to the microorganisms although intensive research has been done to use synthetic materials (Jin et al., 2006).

Because they provide large interfacial areas for exchange, biofilters are typically recommended for low-substrate concentrations and poorly soluble substances. On the other hand, biotrickling filters and bioscrubbers allow higher surface volumetric loading rates and are more suitable for conditions of fast transfer (high pollutant concentration) or when pH needs to be controlled. As indoor air biofiltration implies low transfer and high volumetric flow, it is not clear which of the configurations described above will be the best. However, biofilm-based technologies offer many advantages in regards of the microbial properties required and the utilization of a biodegradable (i.e. compost) or bioactive (i.e. soil, plants roots, plant leaves) support for growth might allow microbial activity at low-substrate concentration. It is therefore not surprising that botanical purifiers, which are based on configurations taking advantage of air exchange through the plant root-soil or plant-foliage areas, have been more extensively studied for indoor air purification so far.

Emerging technologies could provide a more suitable platform for indoor air biological purification. Biological indoor air treatment can potentially release dust, microorganisms, and water. These problems can be simultaneously solved by using membrane bioreactors which physically disconnect the sorption step (air-water exchange) from the biodegradation step. The use of membrane bioreactors for VOC removal is nowadays only established at high pollutant loads (Ergas et al., 1999) but recent studies have demonstrated that such systems could be efficient at indoor air concentration levels (Llewellyn and Dixon, 2006, Ramis et al., 2007). The use of membrane could allow more compact designs for indoor air treatment. This is clearly a very promising technology which should be further investigated.

An additional common limitation to all biological air treatment processes is the need to transfer contaminants into an aqueous phase prior to their biodegradation, which is especially problematic in the case of hydrophobic pollutants such as hexane. The addition of a hydrophobic organic phase into the bioreactors (two liquid phase partitioning bioreactors) could significantly enhance the transfer of the pollutants to the microorganisms and thereby, their removal (Muñoz et al., 2007). Other possibilities include the addition of activated carbon or other adsorbents in combination with the biological system. Such approaches should be investigated in the case of indoor air treatment as they could also concentrate the contaminants to levels suitable for growth.

## 7. Conclusions

Poor indoor air quality is a worldwide problem with tremendous human health and economical consequences. Although technologies for particle removal are rather well established, there are nowadays no satisfactory methods for VOC control because removing indoor VOC sources or increasing ventilation rates is often not feasible or economical. There is a therefore a need for designing specific air purifying devices to clean and circulate the air inside affected buildings. Among the technology potentially suitable for this purpose are biological systems replicating upon the ability of plants and/or organisms to detoxify organic compounds. However, a critical review of the existing literature in regards to biological and engineering constrains reveals numerous problems that must be solved before biologically-based air purifiers can be designed and implemented.

First, our current knowledge on microbial kinetics and the thresholds for substrate uptake, consumption and gene expression raise serious doubt concerning the feasibility of microbial degradation of VOCs at indoor air concentrations. Yet, we also know biological systems (in a

broad sense) respond to indoor VOCs because of the linkages between VOC occurrence and SBS. In addition, there is experimental evidence that VOCs can be biologically removed at indoor concentration even if the precise mechanisms are unknown. This apparent contradiction is perhaps explained by the fact that our current knowledge was derived from studies conducted under conditions (single strains with single substrate at high concentration) irrelevant to the indoor air environment (diverse communities exposed to multiple substrates at low concentrations and direct pollutant uptake). Clearly, there is a need for fundamental research under indoor relevant conditions. This would not only help to design and optimize indoor air biological purifiers but also to solve growing environmental issues linked to trace contaminants in water resources. One of the most interesting areas of research is perhaps the study of heterotroph-phototroph relationships such as those observed in plants. This could explain why plants "appear" to remove VOC at trace concentration when this hardly benefit them (from a detoxification point of view) and why microorganisms could degrade indoor VOCs and survive starvation.

Second, the design of biological air purifier requires the development of new technologies for highly efficient pollutant transfer (from air to the biological catalyst) in order to allow high volumetric treatment flows while maintaining high treatment efficiencies. Current biological purifiers have shown some potential but are all limited by their low treatment capacity. Solutions could be found among technologies (i.e. membrane bioreactors) that liberate from the need to transfer VOC to an aqueous phase. This opens interesting possibilities for cross-disciplinary research initiatives.

Finally, as IAQ is linked to the presence of pollutants other than VOC and as biological methods might always be limited in the cases of poorly soluble or recalcitrant substances, there is a need to develop combined physicochemical-biological methods. This is especially necessary to eliminate potential nuisances from the biological purifier itself.

## 8. Uncited references

US EPA

US Green Building Council

## Acknowledgments

The "Université de Pau et de Pays de l'Adour" is gratefully acknowledged for supporting the visit of Dr Guieysse at its "Laboratoire de Thermique, Energétique et Procédés". We finally would like to thank supports from the French-Mexican exchange program PCP 07-05 "Evaluation of biofiltration supports for indoor air purification" (Etude des performances d'un nouveau support de biofiltration appliqué à la dépollution de l'air intérieur) and from the Spanish Ministry of Education and Science (RYC-2007-01667 contracts).

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