Assessment of exposure to dust,gaseous pollutants and endotoxins in sewage treatment plants of Ahmedabad city,India

Abstract

BACKGROUND:

Sewage treatment plant workers (STPs) are exposed to gaseous pollutants (H₂S) and bioaerosols and their health is at risk.

OBJECTIVE:

The aim of the study was to evaluate exposures to dust, 1,3 Beta D Glucans, endotoxins and gaseous pollutants in different process plants and to provide suitable recommendations.

METHODS:

Gaseous pollutants and bioaerosols (inhalable dust, 1,3 beta D-glucans and endotoxins) were evaluated in two sewage treatment plants (STPs) of Ahmedabad city, India. The concentration of H₂S, CO, CH₄ and Cl₂ were monitored in two process areas of STPs using real-time gas detectors. The dust, 1,3 beta D-glucans and endotoxins were evaluated as per standard methods.

RESULTS:

The mean concentration of H₂S exceeded the permissible exposure limit of 10 ppm, whereas the concentration of other gaseous pollutants (CO, Cl₂ and CH₄) were below the permissible exposure limits of Indian Factories Act, 1948. The inhalable dust concentration was also within the permissible exposure limit of 10 mg/m³ as per Indian Factories Act, 1948.

CONCLUSIONS:

Significant exposures to gaseous and bioaerosols were found in the work environment of STPs. The paired t-test result showed a significant difference between two STPs for H₂S, 1,3 beta D-Glucans and endotoxins. STPs were advised to reduce the exposure to H₂S and bioaerosols as per CPCB guidelines applicable to India to prevent health effects.

Keywords

Wastewater plants inhalable dust hydrogen sulphide endotoxins 1 3 Beta D-Glucans

1 Introduction

The occupational environment of sewage treatment plants (STPs) are potential sources of hydrogen sulphide, ammonia, methane, mercaptans, endotoxins and microorganisms, which are associated with many adverse human health problems [1, 2]. The dust, odor, pathogenic microorganisms and endotoxins spread in the air due to movement of sewage in sewage treatment plants [3, 4]. Though earlier studies have also reported musculoskeletal disorders among solid waste collectors [5, 6] only respiratory and gastrointestinal problems are attributed to work in sewage treatment plants [7]. Some of the similar exposures in other occupations has also reported respiratory problems [8]. The personal exposure to endotoxins and bacteria exceeded 14% and 34%, respectively among workers in sewage treatment plants [7]. The extent of exposure to chemical and biological pollutants depends on exposure duration, capacity of STP plant, technology adopted and pollution control equipment in place. The corrosion caused due to airborne hydrogen sulphide in sewage treatment plants is responsible for increased cost in the management of sewers and sewage treatment plants [9].

The average concentrations of H₂S found in two sewage treatment plants of Brazil were beyond the US EPA guideline of 2μg/m³ [10]. The concentration of H₂S in air samples of 3 wastewater works and 3 rural areas adjacent to STP were above 10 ppm [11]. Fifteen volatile odorous compounds were detected and quantified by GC-MS from wastewater pump stations in China and the potential health effects evaluated were corresponded to four volatile odorous compounds namely chlorobenzene, dichloromethane, hydrogen sulfide, and carbon disulfide. The health risk has been quantified and reported hazardous quotients above 1 at all locations [12]. The concentrations of N-acetylhexoaminase (NAHA), bacterial hydralase (BH) and endotoxin in 120 work places in 10 sewage treatment plants of Sweden were measured and higher levels were observed at agitator and repairing of pump areas [13]. The largest abundance of bacteria and fungi were found at mechanical separation and reactors of STPs [14, 15]. The health risk among workers of sewage treatment plants (STPs) in Poland was in correspondence with higher concentrations of odorous compounds [16]. The average concentration of bioaerosols (bacteria and fungi) found at aeration tank of sewage treatment unit in Tehran was 1016 and 1973 in summer and winter, respectively [17]. The major sources of microorganisms were found at raw sewage inlet and biological reactors areas in sewage treatment plants [18]. Neurological and respiratory tract symptoms are observed among waste water treatment workers [11 , 19–21]. The workers handling dry sludge are more exposed to dust and endotoxins when compared to other category workers STPs [21]. The commonly observed health symptoms among waste water workers were headache, infrequent tiredness, concentration difficulties, increased respiratory and skin ailments [22]. The medical effects among sewage workers found to appear at endotoxin concentrations of 50–70 EU/m³ exceeding 15 minutes [13]. The symptoms of asthma in sewage workers were higher than those of other municipal workers. This might be due to the higher exposure to irritant or inflammatory substances. As per a recent review article, the reasons for increased respiratory morbidity among sewage workers are, exposure to noxious aerosols and endotoxins [23].

In India there are 816 STPs having capacity of 23277 MLD in 28 States/UTs in India. Out of 816 STPs, 522 STPS are operational, 79 STPs are non-operational, 145 STPs are under construction and 70 STPs are planned for construction [24]. The occupational health studies among sewage workers are very scarce in India. These studies are limited to evaluation of DNA damage with reference to Pb and Cd exposure among sewage workers [25, 26]. Sewage treatment plants does not come under the Indian Factories Act (1948) and therefore workers may not get essential safety and health care from concerned stakeholders and majority of them working on contact basis [27]. The present study has been carried out to evaluate exposures to dust, 1,3 Beta D Glucans, endotoxins and gaseous pollutants in different process plants and to provide suitable recommendations.

2 Methods

2.1 Air monitoring for gaseous pollutants and bioaerosols

The required permission to conduct the study were obtained from the management of two sewage treatment plants before the study. The institutional ethical committee clearance was also obtained. The study was conducted at two STPs of 106 and 180 MLD plants located in Ahmedabad city, Western India. Air monitoring was carried out for 8 hours (8 AM to 4 PM) at the breathing zone of workers for quantification of concentrations of gaseous pollutants (carbon monoxide (CO), Chlorine (Cl), Methane (CH₄) and hydrogen sulphide (H₂S)); inhalable dust for evaluation of 1–3 β-D-Glucans and endotoxins at two process areas- (pretreatment unit and aerator areas) in STP-1 (106 MLD) and STP-2 (180 MLD). In primary treatment unit of STP-1 seven samples for H₂S were collected but for all other pollutants (inhalable dust, 1,3 Beta D-Glucans, endotoxins including H₂S) five samples were collected in each process plants in both the STPs.

The airborne concentrations of gaseous pollutants (CO, H₂S and Methane) were determined using pre-calibrated real-time portable gas detectors Gas Alert Micro-5 IR/PID; Entry and Ultra RAE; X-am 7000 manufactured by BW Technologies-Canada; RAE systems-USA and Draeger Germany respectively. The air monitoring was carried out at 1.5 meters height in different process plants in each of the STPs from 8 am to 4 pm for 4–5 days. The concentrations of air pollutants were measured at two process areas namely pretreatment unit (Inlet chamber, distribution tank, dirt remover area), and aerator areas. The real-time gas detectors directly give time weighted average concentration for the monitoring period, entire work-shift duration.

Respirable dust samples, each from primary treatment unit (inlet-distribution chambers and grit remover), and Aerator of STP-1 (106 MLD) and STP-2 (180 MLD) were collected using vertical elutriator sampler fitted filter cassette with 37 mm micro cellulose acetate (MCE) paper 0.4 porosity at the flow rate of 7.4 litres/min for 5 hours duration. Filters are loaded, weighted and wrapped in aluminium foil and put in a clean petri dish and again with aluminium foil and brown paper. The filter cassette is sterilized with 70% alcohol and filter paper is loaded and the cassette is covered. Then sample number is written. Then it is autoclaved for 10-ton pressure for 15 minutes. The elutriator is also sterilized with 70% alcohol. The dust concentration was estimated by gravimetric method. Endotoxins were estimated by kinetic limulus amoebocyte lysate (LAL) gel clot technique while 1–3 beta glucan was measured by Glucatell assay kit supplied by Kan Healthcare Ltd. and Charles Diagnostics.

2.1 Data analysis

Statistical Package for Social Sciences (SPSS version 20) was used for the data analysis. Student’s t-test was used to compare the means between the two groups. A p-value <0.05 was considered significant at 95% confidence interval.

3 Results

3.1 Gaseous pollutants

The concentrations of gaseous pollutants CO, H₂S and Methane were measured in all the three STPs.

The time weighed average (TWA) concentrations of H₂S concentrations measured in two process areas in two STPs is shown in Table 1. The mean TWA concentrations of H₂S at two work areas namely primary treatment unit (Inlet/Distribution chambers and Grit Remover) and Aerator in STP-1 and 2 were 82.01 and 20.82, and 27.32 and 20.74 respectively. The paired t-test value showed that the TWA concentration of H₂S in primary treatment unit area between STP-1 and STP-2 was statistically significant, whereas in the aerator process it was not significant. The mean concentrations of H₂S in other areas such as gas holder system and outlet lagoons were below detection limit of 0.1 ppm and less than PEL of 10 ppm [27].

Table 1
Mean concentration of hydrogen sulphide in two sewage treatment plants

Area/Location TWA concentration of H₂S (ppm) p value

STP-1 (106 MLD) STP-2 (180 MLD)

Primary Treatment Unit (Inlet/Distribution Chambers and Grit Remover) 82.01^±15.05 20.82^±6.75 p < 0.01

(55.00–98.70) (10.80–28.80)

7 5

Aerator 27.32^±5.24 20.74^±4.68 NS

(22.00–35.70) (14.70–26.50)

5 5

Area/Location	TWA concentration of H₂S (ppm)	p value
Primary Treatment Unit (Inlet/Distribution Chambers and Grit Remover)	82.01^*±15.05	20.82^*±6.75	p < 0.01
	(55.00–98.70)	(10.80–28.80)
	7	5
Aerator	27.32^*±5.24	20.74^*±4.68	NS
	(22.00–35.70)	(14.70–26.50)
	5	5

^*Exceeding permissible exposure limit of 10 ppm.

The carbon monoxide levels determined were also below detection limit of 0.1 ppm and were below permissible exposure limit of 50 ppm at only one STP (180 MLD capacity) [27].

The time weighted average concentration of methane was below 1% (BDL) at all process areas, and the work environment can be considered safer as methane concentration was less than Lowest Explosive Limit of 5% in both the STPs [28].

The chlorine levels measured at all points were below the detection limit of 0.1 ppm and the permissible exposure limit of 1 ppm as per Indian Factories Act 1948.

3.2 Dust, 3 β-D Glucan and endotoxins

The mean concentration of inhalable dust, 1, 3 β-D Glucan and endotoxins measured are shown in Table 2, 3 and 4 respectively.

Table 2
Mean concentration of inhalable dust in two sewage treatment plants

Area/Location Inhalable dust (mg/m³) p value

STP-1 (106 MLD) STP-2 (108 MLD)

Primary Treatment Unit (Inlet Chamber, Distribution Chambers and Grit Remover) 0.62±0.03 0.58±0.08 NS

(0.55–0.71) (0.35–0.80)

5 5

Aerator 0.50±0.08 0.48±0.04 NS

(0.33–0.77) (0.35–0.58)

5 5

Area/Location	Inhalable dust (mg/m³)	p value
Primary Treatment Unit (Inlet Chamber, Distribution Chambers and Grit Remover)	0.62±0.03	0.58±0.08	NS
	(0.55–0.71)	(0.35–0.80)
	5	5
Aerator	0.50±0.08	0.48±0.04	NS
	(0.33–0.77)	(0.35–0.58)
	5	5

Table 3

Mean concentration of 1, 3 β-D Glucan in two sewage treatment plants

Area/Location	1, 3 β-D Glucan (ng/m³)		p value
	STP-1	STP-2
	(106 MLD)	(180 MLD)
Primary Treatment Unit (Inlet/Distribution Chambers and Grit Remover)	0.19±0.08	0.40±0.02	p < 0.05
	(0.05–0.22)	(0.35–0.45)
	5	5
Aerator	0.34±0.17	0.42±0.05	NS
	(0.05–0.88)	(0.28–0.55)
	5	5

Table 4

Mean concentration of endotoxins in two sewage treatment plants

Area/Location	Endotoxins (ng/m³)	p value
	STP-1	STP-2
	(106 MLD)	(180 MLD)
Primary Treatment Unit (Inlet/Distribution Chambers and Grit Remover)	0.43±0.15	0.19±0.08	p < 0.05
	(0.15–0.96)	(0.05–0.22)
	5	5
Aerator	0.39±0.14	0.34±0.17	NS
	(0.11–0.92)	(0.05–0.88)
	5	5

In the present study it was observed that both the STPs were outdoor, STP-2 (180 MLD plant) was fully automated while STP-1 (106 MLD plant) was semi-automated. The inhalable dust concentration found in primary treatment unit (Inlet/Distribution chambers and Grit Remover) and Aerator were 0.62 and 0.58, and 0.50 and 0.48 mg/m³ in STP-1 and 2, respectively. The paired t-test value showed that the concentration of dust in two process plants of two STPs was not statistically significant.

The mean concentration of 1,3 β-D Glucan found in pretreatment and aerator process areas were 0.19 and 0.40, and 0.34 and 0.42 in STP-1 and STP-2, respectively. The paired t-test result showed that there was significant difference between two STPs at only pretreatment unit (p < 0.05), but not in aerator area.

The mean concentration of endotoxins found in pretreatment and aerator process areas were 0.43 and 0.19, and 0.39 and 0.34 in STP-1 and STP-2, respectively. The paired t-test result showed that there was significant difference between two STPs at only pretreatment unit (p < 0.05), but not in aerator area. There are no permissible exposure limits for airborne concentrations of endotoxin, and 1,3β-D-glucan in the workplace as per Indian Factories Act, 1948 and hence we could not compare.

4 Discussion

In the study by Austigard et al., the concentration of H₂S exceeded 10 ppm at sewage pumping stations, small plants and sewage inlet pools [11]. In the present study also the H₂S concentration exceeded the permissible exposure limit of 10 ppm at primary treatment unit (including sewage inlet, distribution chambers and grit removers) and aerators in both the two STPs. Godoi et al. determined the outdoor/indoor concentrations of H₂S at 11 sampling points and 15 sampling points in the neighbourhood of two sewage plants in Curitiba and it was found that average H₂S concentration exceeded the WHO recommended limit of 10 ppm and USPEA guidelines of 2μg/m³ [10]. In the study by Lehtinen and Veijanen, the concentration of H₂S exceeded the Finnish occupational exposure limits in two waste water plants in Finland [29] whereas in the study by Lee et al., the concentration of H₂S found were less than 1 ppm [19]. In the present study the time weighted average concentration of H₂S found was above the permissible exposure limit of 10 ppm as per Indian Factories Act at pre-treatment unit and aerator areas in both the two STPs and individual mean concentration of varied H₂S from 10.80 to 98.70 [27].

In an earlier study, the exposure to dust found in eight sewage treatment plants varied from 0.02 to 9.3 mg/m³ [30]. In the present study the exposure to dust was comparatively less, varied from 0.03 to 0.62 mg/m³. The possible reason for lower dust exposures in the present study may be due to outdoor work operations in both the STPs.

A study conducted by Niazi et al. did not find 1,3 β-D Glucan in the sewage treatment plant of Tehran, Iran [17] whereas in the present study the exposure range of 1,3 β-D Glucan was 0.05–0.88 ng/m³, very less, may be attributed to outdoor working environment.

In the study by Cyprowski et al., the range of endotoxins found in inhalable dust was 0.68–214 EU/m³ [31]. Similarly, the range of endotoxins found by other studies varied from 1 to 3,160 EU/m³, 9.3 to 48.7, 0.2 to 1397 and 4 to 887 EU/m³, respectively [13 , 31]. In the present study the range of endotoxins found were 0.05–0.96 ng/m³ that is equivalent to 0.5–9.6 EU/m³ (after converting ng to EU unit, multiplying minimum and maximum values by 10), which may be attributed to outdoor working environment and nature of sewage composition. However, as per Rylander and Calo, an exposure level for endotoxin exceeding 50–70 EU/m³ for more than 15 minutes may implicate a risk of medical effects [13]. But in the present study we found the endotoxin concentration much below this range.

5 Conclusions

The present study estimated hydrogen sulphide, CO, Chlorine and dust concentration in two STPs and found that only the time weighted average concentration of H₂S exceeded the permissible exposure limit (PEL) of 10 ppm at pre-treatment unit and aerator area of two STPs while the concentration of CO, Chlorine and dust were below the permissible exposure limits of 30ppm, 1ppm and 10mg/m³ respectively as per the Indian Factories Act, 1948 [27]. The difference between mean concentration of H₂S, 1,3 β–D-glucans and endotoxins showed significant difference among both the STPs studied. The methane gas concentration was also below detection limit of 0.1%, below lowest detection limit (LDL 5%) [27]. The reason might be due to efficient digestion system and the working of capturing system for collection of methane (CH₄) below the UASB reactors that is transported and stored in gasholders.

The management of STPs were advised to reduce the emissions of H₂S and bioaerosols as per Central Pollution Control Board guidelines [32]. The present study confirmed exposure to H₂S, 1,3 β-D Glucan and endotoxins among STP workers. Further studies targeting other odorous organic compounds and its impact on health outcomes among STP workers are warranted.

Conflict of interest

None to report.

Footnotes

Acknowledgments

The authors are grateful to the Director, ICMR-NIOH, Ahmedabad for the financial support, guidance and encouragement during the study. The authors are also thankful to the subjects who participated in the study and the management of sewage treatment plants who gave permission and cooperation for the study.

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