Environ Monit Assess (2013) 185:59–72 DOI 10.1007/s10661-012-2533-0
Assessment of indoor airborne contamination in a wastewater treatment plant Juliana V. Teixeira & Sandra Miranda & Ricardo A. R. Monteiro & Filipe V. S. Lopes & Joana Madureira & Gabriela V. Silva & Nazaré Pestana & Eugénia Pinto & Vítor J. P. Vilar & Rui A. R. Boaventura
Received: 7 September 2011 / Accepted: 11 January 2012 / Published online: 10 February 2012 # Springer Science+Business Media B.V. 2012
Abstract The main objective of this work was to quantify and characterize the major indoor air contaminants present in different stages of a municipal WWTP, including microorganisms (bacteria and fungi), carbon dioxide, carbon monoxide, hydrogen sulfide ammonia, formaldehyde, and volatile organic compounds (VOCs). In general, the total bacteria concentration was found to vary from 60 to >52,560 colony-forming units (CFU)/m3, and the total fungi concentration ranged from J. V. Teixeira : S. Miranda : R. A. R. Monteiro : F. V. S. Lopes : V. J. P. Vilar (*) : R. A. R. Boaventura LSRE-Laboratory of Separation and Reaction Engineering-Associate Laboratory LSRE/LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal e-mail:
[email protected] J. V. Teixeira : N. Pestana REQUIMTE, Porto, Portugal E. Pinto CEQUIMED, Microbiology Service, Biological Sciences Department, Faculty of Pharmacy, University of Porto (FFUP), Rua Aníbal Cunha, 4050-047 Porto, Portugal J. Madureira : G. V. Silva IDMEC, Institute of Mechanical Engineering, Faculty of Engineering, University of Porto (FEUP), Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
369 to 14,068 CFU/m3. Generally, Gram-positive bacteria were observed in higher number than Gramnegative bacteria. CO2 concentration ranged from 251 to 9,710 ppm, and CO concentration was either not detected or presented a level of 1 ppm. H2S concentration ranged from 0.1 to 6.0 ppm. NH3 concentration was <2 ppm in most samples. Formaldehyde was <0.01 ppm at all sampling sites. The total VOC concentration ranged from 36 to 1,724 μg/m3. Among the VOCs, toluene presented the highest concentration. Results point to indoor/outdoor ratios higher than one. In general, the highest levels of airborne contaminants were detected at the primary treatment (SEDIPAC 3D), secondary sedimentation, and sludge dehydration. At most sampling sites, the concentrations of airborne contaminants were below the occupational exposure limits (OELs) for all the campaigns. However, a few contaminants were above OELs in some sampling sites. Keywords Wastewater treatment plant . Indoor airborne . Volatile organic compounds . Bioaerosols
Introduction Wastewaters contain a large number and great diversity of microorganisms such as viruses, bacteria, fungi, and protozoans which become aerosolized during different stages of the wastewater and sludge treatment process at the wastewater treatment plants (WWTPs)
60
(Gangamma et al. 2011; Heinonen-Tanski et al. 2009; Li et al. 2011). The stages that involve aeration and mechanical agitation represent one of the main sources of bioaerosols which may contain potentially pathogenic microorganisms (Sanchez-Monedero et al. 2008; Li et al. 2011). Odor emissions from WWTPs are characterized by a large number of volatile compounds (mainly sulfur and organic based). Generally, malodorous compounds are emitted from wastewater in each step of the treatment process, when volatile compounds are transferred from the aqueous phase to the atmosphere (Canela and Jardim 2008). The primary treatments (e.g., grit removal or primary sedimentation) and sludge handling activities constitute the two major contributors to odor impact in most WWTPs (Lebrero et al. 2011). The degradation of wastewater and sludge results in the release of several gasses, including ammonia, carbon dioxide, carbon monoxide, hydrogen sulfide, and methane (Gerardi and Zimmerman 2005). H2S and NH3, strongly malodorous and toxic, are the two major inorganic odorous compounds emitted from wastewater, resulting from the anaerobic decomposition of organic matter containing nitrogen and sulfur (Canela and Jardim 2008; Dincer and Muezzinoglu 2008). Other volatile organic compounds (VOCs) are also generated in the anaerobic digestion as well as when chemical reagents are used during the treatment stages (Lebrero et al. 2011; Liu et al. 2004). VOCs, such as benzene, chloroform, ethylbenzene, toluene, m-xylene, and oxylene, are found in some industrial wastewaters as well as in many municipal wastewaters in significant amounts (Hamoda 2006). Trichloroethylene, tetrachloroethylene, carbon tetrachloride, limonene, dimethyl sulfide, and dimethyl disulfide have been also detected in WWTPs (Lebrero et al. 2011; Liu et al. 2004; Wu et al. 2006). The type and quantity of airborne contaminants vary from plant to plant depending on the type of wastewater, operational characteristics, and environmental conditions (Sree et al. 2000; Gangamma et al. 2011; Li et al. 2011). Airborne contaminants generated during wastewater treatment may cause adverse health effects in workers and affect the population in surrounding areas (Heinonen-Tanski et al. 2009; Lee et al. 2006; Thorn and Kerekes 2001). The major occupational health problems encountered in workers are pulmonary diseases, gastrointestinal and skin problems, irritation of the eyes and mucous membranes, allergic reactions,
Environ Monit Assess (2013) 185:59–72
fatigue, and headache (EASHW 2007; Gangamma et al. 2011; Lebrero et al. 2011; Thorn and Kerekes 2001; Zarra et al. 2008). Human response to exposure to biological agents depends on the specific substance involved, the level of exposure, and the individual susceptibility of the exposed worker (EASHW 2007). In contrast to chemical hazards, biological agents are living organisms that are able to grow and multiply in the workplace if they encounter the right living conditions. The assessment of the infection risks associated with WWTPs takes on new significance in the context of more stringent regulations for workplace safety (Korzeniewska 2011). This concern is within the scope of Directive 2000/54/EC (EC 2000b) to determine and assess the risks posed by biological agents in the workplace. However, the assessment of biological risks is seriously hampered as, contrary to the majority of chemical and physical factors, commonly approved criteria are not yet available for assessing exposure to biological factors, for dose–effect relationships (EASHW 2007), and for occupational exposure limits (OELs) (EASHW 2007; Korzeniewska 2011). Nevertheless, some papers, particularly related with WWTPs, have indicating OELs for biological agents (Oppliger et al. 2005; Korzeniewska 2011). The aim of the present study was to assess and quantify the major indoor air contaminants present at different stages of a WWTP operating near Oporto (Portugal), including microorganisms (bacteria and fungi), CO2, CO, H2S, NH3, formaldehyde, and VOCs.
Material and methods Plant description and sampling locations The study was performed in a WWTP located in the north of Portugal, which serves a population of 300,000 habitants, corresponding to a treatment capacity of 66,718 m³/day with an annual average biochemical oxygen demand (BOD5) load of 16,352 kg/day. Figure 1 presents a schematic diagram of the WWTP processes, and their general description is presented in Table 1. The facilities of the WWTP are totally covered. Samples were collected in September 2010 (first campaign), December 2010 (second campaign), and February 2011 (third campaign). Each campaign took place in a single day and with the WWTP in normal
Environ Monit Assess (2013) 185:59–72 Fig. 1 Schematic diagram of the wastewater treatment plant and sampling sites. 1 bar rack, 2 SEDIPAC, 3/4 secondary sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control
61 Sea
State Road 4
3
Biological Reactor
Deodorization
2
1
6
7
8
Anaerobic Digester
Flotator
5
Anaerobic Digester
Gasometer
Legend: Tree
operating conditions. Seven sampling locations were established at the WWTP for assessing the indoor air contamination: (1) bar rack chamber, (2) (SEDIPAC 3D), (3 and 4) two locations at secondary sedimentation tanks, (5) sludge thickener, (6) sludge dehydration chamber, (7) and sludge disposal area. An (8) outdoor control sampling point was also included (outdoor WWTP facilities) (Fig. 1).
Microbiological analysis The evaluation of indoor microbiological contamination comprised the measurement of total bacteria, Gram-positive and Gram-negative bacteria, total fungi, and Aspergillus fumigatus. Air samples were collected by impaction, i.e., by forced deposition of airborne particles on a solid surface. A MAS 100 (Merck) air
62 Table 1 Description of the treatment process used at WWTP
Environ Monit Assess (2013) 185:59–72
Treatment processes
Process description
Preliminary treatment (bar rack)
The removal of coarse solids and other large materials commonly found in raw wastewater.
Primary treatment (SEDIPAC 3D) Secondary treatment
Grit removal, degreasing, and primary sedimentation.
Tertiary treatment
Filtration and disinfection by open-type UV system which allows the utilization of treated effluent for irrigation and washing.
Sludge treatment
Primary sludge thickening (thickener with a diameter of 15 m), biological flotation (flotator with a diameter of 13 m), mixture, anaerobic digestion (2 anaerobic digesters with a capacity of 4,000 m³ each), and dewatering by centrifuge. Alternatively, sludge can be treated chemically or by aerobic digestion.
Deodorization
The WWTP is fully equipped with a deodorization system using a chemical washing, with a total ventilation flow of 60,000 m³/h.
sampler comprising a single-stage impactor aspiring air through a 400-hole perforated plate was deployed at the selected wastewater sampling locations. The resulting air stream, which contained particles with a diameter equal to or larger than 1 mm, was directed onto 90-mmdiameter agar Petri dishes. The impaction flow rate was 100 L/min. The volume of the collected air samples was set up at 50–1,000 L, depending on the sampling site and on the expected concentration of airborne microorganisms (Table 2). For total bacteria and total fungi, samples were collected in duplicate, and for Grampositive bacteria, Gram-negative bacteria, and A. fumigatus, only one sample was collected at each location. After sample collection, the agar plates were transported to the laboratory in a thermal bag and incubated until the formation of colonies was clearly shown. Detailed information about the media used in microbiological
The biological treatment by activated sludge and secondary sedimentation. The facilities have 4 aeration tanks with a capacity of 7,500 m³ each and 4 secondary sedimentation tanks with a capacity of 3,740 m³ each.
analyses and the conditions of incubation can be found in Table 2. After incubation, the colonies were counted, and the results were expressed as colony-forming units (CFU) per cubic meter of air after applying the correction factor recommended by the sampler manufacturer. Chemical analysis The levels of CO2, CO, H2S, NH3, formaldehyde, and VOCs were determined in air samples collected during approximately 30 min. CO2 was measured using a directreading instrument (Testo 535) with a non-dispersive infrared sensor. According to the manufacturer, the accuracy at the levels of 0 to 5,000 ppm is ±(50 ppm of CO2 + 2% of the measured value) and at levels 5,001 to 9,999 ppm±(100 ppm of CO2 +3% of the measured value). CO was also determined using a direct-reading
Table 2 The media used in microbiological analyses, conditions of microorganism incubation, and volume of the collected samples Groups of microorganisms
Media used in microbiological analyses
Temperature of incubation (°C)
Time of incubation
Volume (L) of air samples (impact method)
Total bacteria
TSA (Oxoid)
37
48 h
50–250a
Gram-positive bacteria
Mannitol salt agar (Liofilchem)
37
48 h
250
Gram-negative bacteria
MacConkey Agar (Liofilchem)
37
48 h
250
Total fungi
DG18 (Liofilchem)
25
5 days
50–250a
Aspergillus fumigatus
DG18 (Liofilchem)
45
5 days
1,000
a
Depends on WWTP sampling site and the expected microorganism concentration
TSA tryptone soya agar, DG18 dichloran–glycerol agar base with chloramphenicol
Environ Monit Assess (2013) 185:59–72
instrument (Testo 315–2) based on an electrochemical sensor with an accuracy of ±10% in the 0 to 100-ppm range and of 10% of the measured value from 100 to 2,000 ppm. H2S was determined with a portable directreading instrument (Graywolf) with a multi-gas detector. NH3 concentration was measured with Dräger tubes. Ambient air was aspirated through the tube using a pump (Dräger Gasspürgerät Multi-Gas Detector Mod. 21/31, made in Germany). The results were read in the scale marks printed on the tube wall. Formaldehyde was determined using a portable detector (RKI Instruments FP-30) with an accuracy of ±10%. For determination of VOCs, air samples were collected in stainless steel tubes with Tenax TA 60/80 mesh (Supelco). Personal air pumps (Casella), with flow rates in the range of 60–135 mL/min, provided with online flow meters (Cole-Parmer), were used to collect the air samples. The sampled volume varied between 4.0 and 5.0 L. The Tenax tubes were closed immediately after sampling and VOCs analyzed within a week’s time. A thermal desorption system (Dani, model SDT 33.50) online with a gas chromatograph (Agilent Technologies, model 6890N) coupled to a mass spectrometer detector (Agilent Technologies, model 5973) was used for VOC identification and quantification (GC/MSD). Total volatile organic compound (TVOC) concentration was calculated for all compounds eluted between hexane and hexadecane, using the response factor of toluene for all the compounds. The analytical procedure was carried out according to standard ISO 16000–6 (ISO 2004). Temperature and humidity determinations Temperature and humidity were monitored using a direct-reading thermohygrometer (Testo 175-H2). The accuracy of the temperature and humidity reading were ±0.5°C and ±3%, respectively.
Results and discussion Indoor airborne levels of biological contamination For the three different campaigns performed in the WWTP, the concentrations of total airborne bacteria and fungi at each sampling site are shown in Figs. 2 and 3, respectively. Concentrations ranged from 60 to >52,560 CFU/m3 for total cultivable bacteria. Total
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bacteria concentration exceeded the instrumental upper detection limit (>52,560 CFU/m3) in the first campaign at the secondary sedimentation compartment and in the second campaign at the sludge dehydration chamber. These results were probably related to the operating conditions at the moment of sampling. In the first campaign, a failure in the ventilation system at secondary sedimentation was detected. In the second campaign, the operational conditions at the sludge dehydration chamber differed from the other campaigns, particularly regarding the centrifuge operation time before taking the sample (higher in the second campaign), since it is not continuously running. In the WWTP stages where the treatment processes occur within enclosed facilities, insufficient ventilation and reduced die-off rates associated to the lack of solar radiation can lead to the presence of a high number of microorganisms in the air. Adequate ventilation is therefore needed in enclosed spaces (Heinonen-Tanski et al. 2009). Generally, Gram-positive bacteria were observed in higher number than Gram-negative bacteria (Table 3). An identical situation was also reported by Prazmo et al. (2003) that also detected a clear dominance of Gram-positive bacteria in the airborne microflora of the examined sewage treatment plant. The same type of results was also reported by Cyprowski et al. (2006). The total fungi concentration ranged from 369 to 14,068 CFU/m3 (Fig. 3). The highest concentration of total fungi was detected in the third campaign at the SEDIPAC, but significant concentrations were also detected at secondary sedimentation, particularly in the first campaign. In this study, the most representative genera of fungi were Aspergillus, Penicillium, Cladosporium, and Alternaria. Li et al. (2011) reported that Penicillium and Aspergillus were among the most often isolated fungi, being potentially hazardous to humans in high concentrations owing to their ability to produce mycotoxins. A. fumigatus is a pathogenic fungus commonly found in wastewater, and it is capable of causing several respiratory diseases such as allergic bronchopulmonary aspergillosis and aspergilloma (Gerardi and Zimmerman 2005). In this study, the highest concentration of A. fumigatus was detected in the first campaign at the bar rack chamber (19 CFU/m3) and was not detected at some locations (Table 3). Generally, total airborne bacteria and fungi concentrations were highest in the first campaign, probably
64
Environ Monit Assess (2013) 185:59–72 16000 *
16000
16000 *
16000*
14000
Total Bacteria (CFU/m3)
12000
10000
8000
6000
4000 2700
2000
1620
1410
1705 1000 1400
1690
1400 1470
1290 975
760 750
565
1025 570
85
60
224 242 125
0 1
2
3
4 5 Sampling sites
7
8
(2005). *Concentration above >52,560 CFU/m3. 1 bar rack, 2 SEDIPAC, 3/4 secondary sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control
Fig. 2 Total concentration of airborne bacteria at each sampling site in the first (dark gray bars), second (light gray bars), and third (white bars) campaigns. The horizontal dashed line indicates the occupational exposure limit value reported by Oppliger et al.
16000
6
14068
14000
Total Fungi (CFU/m3)
12000 9609
10000
8000
6000 4677 3667
4000
3151 2489 1944
2462
2000
2093 1333
516 534
2384
2174
1371 437
437 369
518
528
393 623
17779
0 1
2
3
Fig. 3 Total concentration of airborne fungi at each sampling site in the first (dark gray bars), second (light gray bars), and third (white bars) campaigns. The horizontal dashed line indicates the occupational exposure limit value reported by Oppliger et al.
4 5 Sampling sites
6
7
8
(2005). 1 bar rack, 2 SEDIPAC, 3/4 secondary sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control
Environ Monit Assess (2013) 185:59–72 Table 3 Concentrations (CFU per cubic meter) of Gram-positive bacteria, Gram-negative bacteria, and A. fumigatus
Sampling site
C
1
1st
72
4
19
2nd
128
44
0
3rd
204
24
8
1st
156
140
8
2nd
532
272
1
3rd
108
28
0
1st
36
460
13
2nd
16
76
0
3rd
280
20
0
1st
52
20
0
2nd
20
40
0
3rd
116
8
1
1st
328
4
2
2nd
8
32
1
3rd
68
0
0
1st
ND
ND
0
2
3
4
5
6
7 ND not determined, C campaign, 1 bar rack, 2 SEDIPAC, 3/4 secondary sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control
65
8
Gram-positive bacteria
Gram-negative bacteria
Aspergillus fumigatus
2nd
2,396
76
3
3rd
440
216
2
1st
ND
ND
4
2nd
360
24
1
3rd
8
0
1
1st
8
8
6
2nd
24
24
0
3rd
36
0
1
due to higher temperature (around 27°C), compared to the other campaigns (13–18°C). High temperatures in indoor facilities may lead to high levels of microorganisms associated with increasing microbial growth rate. The high ambient temperature can contribute to a rise in the emission of potentially pathogenic microorganisms (Paluszak and BrezaBoruta 2007). Gangamma et al. (2011) and Li et al. (2011) reported concentrations of airborne bacteria, at three WWTPs stages, between 249 and 41,837 CFU/m3, and 459 and 5,565 CFU/m3, respectively. Li et al. (2011) reported concentrations of total fungi from 141 to 1,590 CFU/m3 and Prazmo et al. (2003) from 0.24–140 CFU/m3. The highest microbiological air contaminations were found in pre-treatment stages (Pascual et al. 2003; Sanchez-Monedero et al. 2008; Heinonen-Tanski et al. 2009; Li et al. 2011), primary clarifiers (Pascual et al. 2003), sludge processing
(sludge dewatering house and sludge thickening) (Sanchez-Monedero et al. 2008; Heinonen-Tanski et al. 2009; Li et al. 2011), and biological treatment (Sanchez-Monedero et al. 2008). In our study, SEDIPAC, secondary sedimentation compartment and sludge dewatering chamber were the stages where the highest indoor air microbiological contamination was found. However, the concentration of microorganisms in bioaerosols may be affected by variables such as operational activities, the flow rate and characteristics of the wastewater to be treated, the time of day, the distance from the aerosol source, and local temperature or other local environmental conditions (Gangamma et al. 2011; Li et al. 2011). Moreover, the concentrations of airborne microorganisms found in different studies vary widely due to differences in sampling and analytical techniques (Heinonen-Tanski et al. 2009). Results point to indoor/outdoor ratios higher than one.
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Environ Monit Assess (2013) 185:59–72
Indoor airborne levels of chemical contamination The concentrations of inorganic gasses such as CO2, CO, H2S, and NH3 determined in this study are presented in Table 4. CO2 concentration ranged from 251 to 9,710 ppm. The highest concentration was measured at the secondary sedimentation compartment in the first campaign. The CO2 concentration was also high at the SEDIPAC and sludge dehydration chamber. At most sampling sites, no CO concentration was detected, but in some sites, the concentration was 1 ppm, and in the first campaign, the concentration was 4 ppm at the sludge disposal area. H2S concentration ranged from 0.1 to 6.0 ppm, with the highest concentration detected at the sludge dehydration chamber. In the second campaign, the H 2 S
concentration was also high (around 2 ppm) at the secondary sedimentation zone. Lee et al. (2006) reported H2S concentrations below 1 ppm in four WWTPs. Dincer and Muezzinoglu (2007) reported a maximum concentration of 328 μg H2S/m3 at the primary sedimentation facility. NH3 was only determined in the third campaign, and NH3 levels below 2 ppm were found in most samples. Formaldehyde was less than 0.01 ppm at all sampling sites. The total VOC concentrations (including unidentified) in the three campaigns are presented in Fig. 4 for each location at the WWTP. The total VOC concentration in the three campaigns ranged from 36 to 1,724 μg/m3, depending on the sampling site. The total VOC concentration was higher in the first campaign at most sampling sites, similarly to the
Table 4 Concentrations of CO2, CO, H2S, NH3, and formaldehyde Sampling site
C
CO2 (ppm)
CO (ppm)
H2S (ppm)
NH3 (ppm)
Formaldehyde (ppm)
1
1st
474±7
1
ND
ND
<0.01
2nd
329±36
1
0.83±0.01
ND
<0.01
3rd
349±17
nd
0.58±0.05
<2
ND
1st
475±7
1
97.4
ND
<0.01
2nd
836±112
nd
1.04±0.04
ND
<0.01
3rd
2,409±300
1
1.7±0.2
<2
ND
2
3
4
5
6
7
8
1st
9,204±419
nd
ND
ND
<0.01
2nd
3,567±287
nd
2.3±0.1
ND
<0.01
3rd
1,594±65
1
0.63±0.06
<2
ND
1st
9,710±154
nd
ND
ND
<0.01
2nd
2,840±98
nd
1.8±0.2
ND
<0.01
3rd
1,458±70
1
0.76±0.08
<2
ND
1st
1,137±49
nd
ND
ND
<0.01
2nd
515±38
nd
0.77±0.04
ND
<0.01
3rd
251±5
1
0.14±0.03
<2
ND
1st
634±14
nd
ND
ND
<0.01
2nd
918±134
1
5.1±0.8
ND
<0.01
3rd
1,033±356
nd
6±2
2
ND
1st
260±16
4
ND
ND
<0.01
2nd
263±9
nd
0.68±0.09
ND
<0.01
3rd
434±55
nd
0.5±0.1
2
ND
1st
281±19
nd
ND
ND
<0.01
2nd
218±4
1
1.3±0.3
ND
<0.01
3rd
214±4
1
0.28±0.01
<2
ND
C campaign, nd not detected, ND not determined, 1 bar rack, 2 SEDIPAC, 3/4 secondary sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control The highlighted values refers to the highest values observed
Environ Monit Assess (2013) 185:59–72 1800
67
1724
1479
1600
1170
1400 1188
Total VOCs (µg/m3)
1200 1000
800 641
600
397
217
400
425
502
526
260
200
209
200
169
153
99
142129130
120
58 73 44
36
0 1
2
3
4
5
6
7
8
Sampling sites
Fig. 4 Total concentration of VOCs at each sampling site in the first (dark gray bars), second (light gray bars), and third (white bars) campaigns. 1 bar rack, 2 SEDIPAC, 3/4 secondary
sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control
microorganism’s concentrations, probably due to the higher temperature, which probably raised the volatilization rate of the organic compounds. The higher total VOC concentration was detected at the SEDIPAC. In the second campaign, the total VOC concentration at the sludge dewatering chamber was higher than in the first and third campaigns, as also observed for microorganisms in the three campaigns. Dincer and Muezzinoglu (2008) reported that the total VOC concentration varied from 292 to 4,406 μg/m3 at the sludge basin. In the three campaigns, about 42 VOCs were identified at the different sites. They comprise several compounds such as alkanes (e.g., decane), organic acids (e.g., acetic acid), halogenated hydrocarbons (e.g., chloroform), sulfur compounds (e.g., dimethyl disulphide), and aromatics (e.g., toluene). Table 5 shows common VOCs at WWTP for the three campaigns. These VOCs represent 27–88% of the total VOC concentrations. Toluene showed the highest concentration, and it was present at all sampling sites in the three campaigns. The greatest concentration of toluene was measured in the first campaign at the second point of the secondary sedimentation compartment (801 μg/ m3). Generally, the samples from the SEDIPAC and secondary sedimentation zone exhibit higher concentrations of toluene. Other VOCs, such as dimethyl disulphide, tetrachloroethylene, and decane, were in
significant concentrations at most sampling sites in the three campaigns. Dincer and Muezzinoglu (2008) reported that the most abundant VOCs in WWTP units were monoaromatics, and toluene presented the highest concentration in the group. Zarra et al. (2008) reported that the dimethyl disulphide was the most detected VOC in WWTP. The types and levels of airborne chemical contaminants are dependent on the type of wastewater, operational characteristics of the treatment plant, and environmental conditions (Sree et al. 2000). In general, the concentrations of chemical contaminants in the air were lower at the outdoor control than inside the treatment facilities, as expected. Indoor and outdoor air temperature and humidity The temperature and humidity ranged from 13 to 28°C and 44% to 91%, respectively (Table 6). The temperatures recorded in the first campaign (summer season) were higher than in the second and third campaigns (winter season), which may be the reason for the higher levels of microorganisms and VOCs, as reported earlier. Occupational exposure limit values Bioaerosols generated during wastewater treatment represent a potential health hazard to workers. Directive
Acetic acid
Benzaldehyde
Chloroform
Undecane
Limonene
Decane
m,p-Xylene
Tetrachloroethene
2.3
8.3
2nd
3rd
<0.4
3rd
9±2
3.4
1st
7±1
1.5
3rd
2nd
<0.4
1st
10±1
2nd
5.0
3rd
1st
7.2
2nd
16.2
3rd
<0.4
20.9
2nd
1st
9±1
7.5
3rd
1st
9.8
2nd
9.0
3rd
5.4±0.6
3.9
1st
9±1
2nd
3.3
3rd
1st
8.0
2nd
14.0
3rd
2.50±0.03
10.4
2nd
1st
23±2
2.3
3rd
1st
4.4
2nd
Toluene
33.3±0.6
1st
Dimethyl disulfide
1
C
VOCs
Sampling site
5.9±0.4
53±36
4.6
<0.4
11±1
<0.4
11.5±0.6
5±1
9.3
13.57±0.04
27±7
2.7
53.2±0.7
95±28
84.7
11.66±0.08
30±7
304.8
5.2±0.4
12±3
20.1
24.5±0.5
44±15
83.5
138±2
140±45
481.5
22±2
12±2
35.6
2
12±2
4.4±0.6
31.7
4.0±0.8
3±2
<0.4
3.04±0.06
<0.4
5.2
<0.4
<0.4
<0.4
2.1±0.3
4.4±0.7
<0.4
2.4±0.3
3±0
8.3
2.8±0.4
8.2±0.1
5.7
5.2±0.3
17.4±0.4
16.0
104±5
215±3
172.6
13±2
3±2
5.7
3
6±2
19±15
10±2
3.0±0.2
5.5±0.7
10.5±0.5
3.12
<0.4
7±3
<0.4
3.6±0.9
<0.4
1.66±0.04
8.5±0.9
<0.4
1.8±0.4
6.2±0.6
9±1
2.8±0.3
<0.4
8±2
5.61±0.01
12.5±0.1
17±2
40±1
25.2±0.4
801±51
11±1
4.8±0.5
8.8±0.1
4
Table 5 Concentrations of VOCs (micrograms per cubic meter) (value±standard deviations)
5.3±0.8
12±2
27±18
3±1
5±2
5.3±0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
5.1±0.6
1.6±0.6
<0.4
10.6±0.9
<0.4
<0.4
4.20±0.02
5.1±0.6
9.2±0.9
232±15
<0.4
6.6±1.2
8±2
5
4±2
11±7
35±21
<0.4
4.4±0.3
9.21±0.04
1.8
<0.4
28.8±0.5
7.7±0.8
28±3
5.70±0.05
8±2
5.8±0.1
<0.4
7±1
399±9
10.1±0.3
12±1
16.6±0.7
8±2
2.5±0.2
2.3±0.1
<0.4
43±2
254±6
22±2
13±1
13.6±0.6
15±4
6
4.3
14±9
31.2
7.0
4.0±0.1
3.2
<0.4
<0.4
<0.4
2
<0.4
<0.4
1.6
<0.4
<0.4
3.9
6.9±0.1
1.98
5.1
<0.4
4.24
<0.4
<0.4
<0.4
9.1
5.7±0.3
7.98
7.4
20±0
8.62
7
7.9±0.3
3±0
24±12
2.7±0.2
2.3±0.1
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
<0.4
26±8
<0.4
1.16±0.02
<0.4
<0.4
2.3±0.2
<0.4
<0.4
<0.4
<0.4
<0.4
4.5±0.2
<0.4
2.2
<0.4
<0.4
<0.4a
8
68 Environ Monit Assess (2013) 185:59–72
18.6±0.6 (42%)
69 Table 6 Values of temperature and humidity (value±standard deviations) Sampling site
C
Temperature (°C)
Humidity (%)
1
1st
25.6±0.2
52.0±0.8
2nd
16.3±0.4
84±1
3rd
14.6±0.6
61±4
1st
26.4±0.2
65±5
2nd
16.0±0.1
86±1
3rd
14.8±0.1
81±1
1st
28.0±0.2
80±4
2
3
4
5
6
7
8
2nd
16.6±0.2
90±1
3rd
14.9±0.3
86±3
1st
27.9±0.2
84±2
2nd
16.7±0.1
89.2±0.7
3rd
17.9±0.6
78±2
1st
26.8±0.2
55±4
2nd
15.9±0.2
91±2
3rd
16±1
71±4
1st
27.6±0.5
53±4
2nd
16.8±0.2
90±1
3rd
12.9±0.1
69±2
1st
27.8±0.6
46±3
2nd
16.3±0.5
87±2
3rd
13.9±0.9
70±3
1st
27.7±0.7
44±1
2nd
18.0±0.3
84±2
3rd
14.7±0.3
74±2
The highlighted values refers to the highest values observed
Below detection limit (<0.4 μg/m3 )
C campaign, 1 bar rack, 2 SEDIPAC, 3/4 secondary sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control
a
C campaign, 1 bar rack, 2 SEDIPAC, 3/4 secondary sedimentation, 5 sludge thickening, 6 sludge dehydration chamber, 7 sludge disposal area, 8 outdoor control
68.3 (52%) 98±7 (38%) 15±3 (42%) 149±10 (88%) 63.2 (41%) 3rd
286±1 (54%)
73±3 (74%)
25±11 (43%) 57.31 (40%)
49±7 (38%) 736±16 (62%)
134±29 (32%) 299±38 (60%)
32±4 (27%) 85±18 (42%) 259.0±0.5 (65%)
872±57 (58%) 245.1 (38%)
429±141 (37%) 70.3 (32%) 2nd
1047.6 (61%) 109±5 (52%) 1st ∑ VOCs
Sampling site
Table 5 (continued)
31±8 (42%)
Environ Monit Assess (2013) 185:59–72
2000/54/EC (EC 2000b) lays down the principles for the management of biological risks and assigns to employers the duty of assessing the risks posed by biological agents in the workplace. However, proper assessment of biological risks is difficult in practice (EASHW 2007). Risk assessment is difficult because OELs for airborne microorganisms have not been established yet (Korzeniewska 2011; EASHW 2007). Oppliger et al. (2005) reported recommended Swiss OELs of 104 CFU/m3 for total cultivable bacteria, 103 CFU/m3 for Gram-negative bacteria, and 103 CFU/m3 for total fungi. The levels of total bacteria at the secondary sedimentation compartment, in the first campaign, and at the sludge dehydration chamber, in the second campaign, exceeded the exposure limit value of 104 CFU/ m3 (Fig. 2). Gram-negative bacteria concentrations
70
Environ Monit Assess (2013) 185:59–72
were always below 103 CFU/m3 at all sampling sites. In the first campaign, six of the seven sampling sites exceeded the limit of 103 CFU/m3 of fungi. However, in the second and third campaigns, only three sites exceeded this limit (Fig. 3). A Polish proposal concerning OELs for bioaerosols at industrial settings polluted with organic dust indicates threshold limit values of 1.0×105, 2.0×104, and 5.0×104 CFU/m3 for mesophilic bacteria, Gram-negative bacteria, and fungi,
respectively (Korzeniewska 2011). The levels of indoor airborne contaminants (biological and chemical) detected at the WWTP are compared with different OELs in Table 7. The European Union approved legislation for the protection of the health and safety of workers from the risks related to chemical agents at work. According to Directive 98/24/EC (EC 1998), the employer must determine whether any hazardous chemical agents
Table 7 Comparison of the indoor airborne contaminant levels at WWTP with different OELs 1 Swiss OELs (Oppliger et al. 2005) Polish OELs (Korzeniewska 2011)
Total bacteria Gram-negative Total fungi Total bacteria Gram-negative Total fungi CO2 H2S
Directives 2000/39/EC 2006/15/EC 2009/161/EC DL 305/2007
NH3 Toluene chloroform m,p-xylene Acetic acid CO2 CO H2S NH3
NP 1796:2007
Formaldehyde Toluene Tetrachloroethene m,p-xylene Acetic acid
× not in agreement; a b
2
3 ×
Sampling sites 4 5 ×
×××
×××
× ×
6 ×
7
×
TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb TWAa STELb
---
---
---
---
-
×
×
---
---
×
×
---
---
×
---
---
×
×
-
-
8
×
-
-
-×× ---
---
---
---
---
---
-
-
-
in agreement; - values not measured
In relation to a reference period of 8 h time-weighted average
Short-term exposure limit. A limit value above which exposure should not occur and which is related to a 15-min period unless otherwise specified (each symbol represents one of the three sampling campaigns)
Environ Monit Assess (2013) 185:59–72
are present at the workplace, assess any risk to safety and health arising from their presence, and take the necessary preventive measures. Successive lists of indicative occupational exposure limit values were established by Directives 2000/39/EC (EC 2000a, 2006/15/EC (EC 2006, and 2009/161/EC (2009). In Portugal, DR no. 305/2007 (DR 2007) and Portuguese Standard NP 1796:2007 (NP 2007) established OELs for chemical agents. Only in the first campaign, at the secondary sedimentation zone, the CO2 concentration exceed the OEL for 8-h exposure of 5,000 ppm. These results are probably related with failures in the ventilation system. However, it is important to note that CO2 can be an asphyxiant, and it can cause narcosis (Gerardi and Zimmerman 2005). CO and NH3 concentrations were below the OEL. The levels of H2S only exceeded the OEL for 8 h of exposure of 5 ppm, according to the European Directive 2009/161/EU (EU 2009), at the sludge dehydration chamber. H2S is not only malodorous and corrosive but it is also markedly toxic. Longterm exposure to H2S concentrations of around 10– 30 μg/m3 in the air can cause eye and respiratory system irritations and headache. Thus, the H2S guideline value of 150 μg/m3 for 24 h recommended by WHO seems to be too high to prevent both odor complaints and adverse health effects (Dincer and Muezzinoglu 2007, 2008). The concentration of each VOC, individually, did not exceed the respective Portuguese OEL. Workers in the WWTPs are exposed to complex mixtures of bioaerosols, gasses, and VOCs. Synergistic interactions among the chemical and biological agents may therefore be taken into account. As the range of potential subsequent health effects is wide, it is difficult to determine which of the constituents primarily accounts for which health effects (EASHW 2007, 2009). It is evident that exposure to these agents should be minimized. The use of personal protective equipment, thorough hand washing, and changing clothes after the work shifts must be followed as policy. In addition, the efficient ventilation in WWTPs with enclosed facilities is very important to remove the contaminants from the air (EASHW 2007; HeinonenTanski et al. 2009; Kiviranta et al. 1999).
Conclusion Wastewater treatment plants are important sources of airborne contaminants, including bioaerosols, inorganic
71
gasses, and VOCs. In this study, the highest levels of biological and chemical contaminants were detected at the degritting/degreasing/primary sedimentation facility (SEDIPAC), secondary sedimentation compartment, and sludge dehydration stage. The results suggest the need for efficient ventilation in enclosed WWTP facilities, since failures can lead to high concentrations of indoor airborne contaminants such as microorganisms, CO2, and VOCs. Although the concentrations of air contaminants have often been below OELs, high levels of different types of microorganisms and chemicals to which workers are potentially exposed were detected. Since it is not possible to completely eliminate the risks inherent to WWTP management, the best efficient prevention measures must be implemented to minimize the generation of airborne contaminants at the workplace. Adequate ventilation, optimization of the process operations, as centrifuge working time, use of personal protective equipment, thorough hand washing, and changing clothes after work shifts are essential for minimizing the exposure of workers to biological and chemical agents. Acknowledgments Financial support for this work was provided by a FCT project (PTDC/EQU-EQU/100554/2008) and by project PEst-C/EQB/LA0020/2011, financed by FEDER through COMPETE–Programa Operacional Factores de Competitividade and by FCT–Fundação para a Ciência e a Tecnologia for which the authors are thankful. R.A.R. Monteiro and F.V. S. Lopes acknowledge their PhD and post-doc scholarships (SFRH/BD/69323/2010 and SFRH/BPD/73894/2010, respectively), both supported by FCT.
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