Hydrogen Sulphide Gas
Nathan Whitta and Cristine Janz
ENVE362
Macquarie University
25 October 2012
Executive summary
The intention of this report is to provide an overview of the issues related to the exposure of
humans to hydrogen sulphide gas (H2S). H2S is a naturally occurring atmospheric gas, although it is
generally found in extremely low concentrations. As concentrations rise above normal background
levels the gas becomes a nuisance at low levels, and an extremely toxic hazard as levels rise,
particularly above 100 parts per million (ppm). Although H2S can occur at high levels in the natural
environment, the hazard for humans primarily comes from H2S produced in providing waste and
industrial services for the benefit of society. As these services are essential they cannot be removed,
therefore management of the production of the gas and separation of people from the gas itself are
the key management techniques. Landfill waste generally produces H2S at lower concentrations, and
is generally considered more of a nuisance than a hazard. In this case landfill should be isolated from
residential areas where possible, otherwise there are identified techniques for suppressing odour. In
sewerage systems the goal is to minimise the production of H2S by providing an oxygenated
environment. This is made difficult by the age of much of the sewerage infrastructure in urban
environments. Similarly, the industrial sector requires that workers maintain equipment, such as
sour gas pipelines, which transport H2S. The corrosive nature of H2S can endanger workers by
damaging infrastructure. This in turn provides a contamination hazard for workers in this industrial
environment. Where humans must work in potentially contaminated environments they must have
detection equipment and breathing apparatus.
Contents
Executive summary ............................................................................................................................. 2
Introduction ........................................................................................................................................ 2
Issues related to H2S emissions........................................................................................................... 4
Management Approaches................................................................................................................... 5
Summary and Conclusions .................................................................................................................. 7
References .......................................................................................................................................... 7
Introduction
Hydrogen sulphide gas (H2S) can impact on humans and the environment in many ways over a
variety of scales, although it is at the local scale that is of most immediate concern for humans.
Properties
In low concentrations H2S is identifiable by its pungent odour, and is widely known as rotten egg gas.
The gas is toxic, colourless, and flammable and is heavier than air, which makes it most likely to
accumulate in confined spaces or low lying pits or wells (Snyder et. al. 1995, p. 200).
Production
The production of H2S occurs as sulphate is reduced to sulphide by bacteria in an anoxic
environment. This process occurs in the built environment in both industrial and waste services, and
is an unfortunate side effect of providing these essential services. Toxic H2S is also produced in the
natural environment, although there tend to be fewer interactions between humans and H2S in this
setting.
Occurrence
As a proportion of the atmosphere H2S makes up approximately 0.0001ppm (Li, Hsu & Moore 2009,
p. 387). In the natural environment H2S is produced in anoxic environments, such as enclosed
stagnant waters or in active geothermal zones.
Uses
H2S is a by-product of certain industrial and waste disposal processes, and is generally considered to
be an unwanted hazard. At the micro-scale, it is produced in the human body and researchers are
beginning to understand the benefits it provides to human health in low concentrations (Li, Hsu &
Moore 2009).
Toxicity
At high concentrations H2S becomes a health problem for people (Table 1). Safe Work Australia
guidelines limit workers to exposure of 10ppm averaged over the course of an eight hour day, with
an absolute limit of 15ppm at any time (Safe Work Australia 2011). Sustained exposure at these low
levels can lead to illness and irritation. Health impacts become increasingly more severe as
concentrations and exposure rise, with high concentrations at around 1000ppm leading to almost
immediate death (Yalamanchili & Smith 2008).
Table 1. Effects of H2S on humans (Nogue S et al. 2011)
Hydrogen sulphide levels
Effects
(parts per million)
0.003-0.002
Odour threshold
50
Eye and respiratory irritation
150
Olfactory nerve paralysis
250
Exposure may cause pulmonary oedema
500
Anxiety, headache, ataxia, dizziness, stimulation of respiration,
amnesia, unconsciousness
750
Quickly unconscious; death without rescue
1000
Rapid collapse; respiratory paralysis leading to death
5000
Immediate death
Waste produced by human society and the manner in which it is stored results in the production of
H2S. The odour, corrosive nature and toxicity of H2S poses a number of problems for waste disposal,
and these need to be managed for the maintenance of infrastructure and the protection of human
health. Several industrial activities also generate H2S as a by-product, ranging from petroleum
refineries to tanneries. Such large scale activities can potentially expose workers to high levels of
toxic waste, such as this lethal gas. In this paper we discuss the issues associated with H2S emissions
by human and industrial waste, as well as efficient management strategies.
Issues related to H2S emissions
According to the National Pollutant Inventory (NPI) (2010) exposure to levels from 5ppm up to
50ppm begins to cause irritation to the airways and the eyes, at 100ppm it becomes hazardous to
human health when exposed to a sustained dose for over an hour, potentially resulting in death.
Between 100-150ppm olfactory senses become disabled, meaning the rotten egg smell is no longer
present (Yalamanchili & Smith 2008). Above this level even brief exposure becomes life threatening,
with time required to cause death reducing as the concentration rises. Knockout occurs immediately
with inhalation at around 800ppm and death follows shortly after (Yalamanchili & Smith 2008).
The toxicity of H2S at concentrations above ~ 1000ppm makes it a very efficient killer. This
characteristic of H2S has led to numerous instances where people intending to commit suicide
produce H2S for this purpose (Maebashi et. al. 2011, p. 91). On a larger scale, it has been theorised
that H2S has been partly responsible for the mass extinction event that occurred at the end of the
Permian period. H2S would have contributed in two ways, first in its role as an ozone depleting gas,
and secondly by poisoning the eukaryotic species as the H2S concentration multiplied (Knoll et. al.
2007).
Human waste
Human activities are responsible for the production of a significant quantity of waste. This waste
may occur as refuse, which is generally disposed of and stored as landfill; or alternatively, human
faecal waste, which is stored and often treated before being released into the environment. One of
the consequences of concentrating this waste is the production of H2S.
Landfill generates a number of environmental challenges, one of which is the production of gases
resulting from the decomposition of waste material. Amongst the hazardous gases produced by
landfill is H2S, which generally occurs in low concentrations (Heaney et. al. 2011, p. 847). The study
by Heaney et. al. (2011) suggests that people living in close proximity to landfill sites may suffer
negative physical effects from exposure to H2S and other landfill related gases. These are apart from
the general negative mental reactions to exposure to unpleasant odorous gases. Although, Heaney
et. al. (2011) do not establish whether the negative physical reactions are brought on by
psychological responses to the malodour. This experience suggests that although there is the
potential for illness from exposure to landfill generated H2S, most negative reactions are resulting
from a dislike of the pungent odour.
The presence of H2S occurring in response to the treatment of sewage is potentially far more serious
than in the landfill scenario. H2S forms during the treatment of waste in an anaerobic environment,
where oxygen phobic bacteria reduce sulphate to sulphide, producing the toxic gas (US EPA 1991, p.
5). Wastewater is transported and often stored in enclosed spaces, the resulting limited aeration
and the confinement of these spaces both encourage the production of H2S and allow its
concentration to multiply (US EPA 1991, p. 5). The corrosive nature of H2S damages sewerage
infrastructure and produces leakage, this results in significantly increased maintenance
requirements (US EPA 1991). There have been a number of incidents that have resulted in the
deaths of workers in the wastewater industry due to H2S exposure. In two instances the toxic
exposure occurred within the sewer line, and was worsened by the difficulties in extracting the
victim from the confined and toxic location (Yalamanchili & Smith 2008; Christia-Lotter et. al. 2007).
These deaths only highlight the difficulty in managing H2S emissions whilst providing essential waste
services.
Industrial waste
Due to the high toxicity of H2S, its release into the atmosphere is potentially life threatening. After
carbon monoxide, hydrogen sulphide is the second leading cause of deaths in the workplace related
to toxins (Gabbay D et al. 2001). The management of patients contaminated by toxic gases also
causes possible danger to the paramedics and emergency staff. Certain professions are especially at
risk of H2S exposure, such as pipe fitting, well drilling and servicing, pumping and gas refining.
Although fatal accidents are more common in confined spaces where H2S has accumulated over time
due to a lack of air exchange, they have also happened in unconfined spaces. In another account,
deaths occurred in an unconfined room containing a silo of sludge. The first worker was checking a
pump mechanism at the same time that a truck dumped several tons from water purification
stations, and lost consciousness. Two other workers tried to help him but also lost consciousness,
and all three died. The H2S accumulated inside the silo had spilled out after the dumping of more
sludge (Nogue S et al. 2011). Such accounts only reinforce the importance of effective management
of H2S emissions.
Management Approaches
Human waste
Waste is an inevitable by-product of human society, therefore dealing with toxic H2S will be an
ongoing issue. Management solutions for H2S will involve both separating people from the hazard
and attempting to minimise production of the gas. In the case of landfill, ideally communities will be
located with enough separation to avoid the odours from these sites. Where communities are
impacted by malodour this can be managed with liners, capture of leachate and regular cover with
new waste (Heaney et. al. 2011, p. 847). Ultimately whilst H2S is not attributed to significant
negative health effects there will be limits to preventative measures.
There are a number of approaches for managing the hazard created by the transport and storage of
sewage. In existing wastewater infrastructure managing the production of H2S may involve chemical
measures or aeration. There are a number of chemical procedures for removing H2S from a
sewerage system, these may include Hydrogen Peroxide or Chlorine (US EPA 1991, p. 4-6). In these
cases the sulphide component of the H2S is oxidised by the chemicals, removing the gas. The US EPA
(1991, p. 4-6) also suggests the use of iron salts to react with the H2S to produce a precipitate that is
insoluble. Alternately, measures could be taken to aerate the sewer, removing the anoxic
environment that is essential for the H2S to form. Aeration could occur by injecting air or oxygen into
the sewer (US EPA 1991 p. 4-6). These methods of reducing the production of H2S would depend on
expense and the available infrastructure.
Building new infrastructure provides an opportunity to mitigate the impacts of H2S on the sewerage
system. Once again, the problem may require a solution that involves adding chemicals to counter
H2S production. The sewerage system can be designed taking account of this and be constructed
with infrastructure such as chemical addition stations, allowing the most efficient delivery of
suppressing chemicals (US EPA 1991, p. 4-7). Alternatively, the design can ensure the sewerage
system remains oxygenated. The methods predominantly used include ensuring high velocities in
the sewer, providing natural ventilation and creating an environment that encourages turbulence
(US EPA 1991, p. 4-10). Whilst this infrastructure may be beneficial in new developments, the
heaviest populations are concentrated in existing centres, meaning there is less opportunity to build
entirely new sewerage infrastructure. Ultimately, personnel working potentially contaminated
environments will rely on gas monitoring equipment and self contained breathing apparatus for
personal protection.
Industrial waste
Different means for H2S control have been used, such as biofilters, biotrickling filters and
bioscrubbers (referred to as organic perfusion columns); adsorption using activated carbon,
molecular sieves and silica gel; metal oxides; aqueous solutions; and also a combination of activated
carbon and oxidation, followed by chemical precipitation (Soreanu G et al. 2010; Oviedo E et al.
2011; McNevi D & Barford J 2000; Sahu R et al. 2001). The efficiency of each technique depends on
the gas concentration.
The traditional methods for removal are absorption and adsorption, but the high cost of equipment,
toxic chemical usage and secondary contaminations pose problems (Chang & Shin 1993, cited in Oh
K et al. 1999). Currently, chemicals are added to the water to eliminate sulphide by chemical
precipitation - using iron or other metal salts - or oxidation - using hydrogen peroxide, oxygen or
chlorine (Oviedo E et al. 2011).
In the last decade, extensive research has been conducted in applied biotechnology for the
treatment of gas fluxes, and biological treatments were recognised as environmentally-friendly and
cost-effective alternatives to traditional technologies (Ahn 1993, cited in Oh K et al. 1999). Removal
using microbes is generally divided into aerobic and anaerobic. Aerobic microbes used to oxidise H2S
are Thiobacillus, Pseudomonas, Beggiatoa and Thiotrix (Tanji et al. 1989, cited in Oh K et al. 1999),
while phototrophic bacteria such as Chlorobium and Chromatium are used in anaerobic processes to
convert H2S to elemental sulphur and sulphate, depending on the intensity of light (Kusai and
Yamanaka 1973, cited in Oh K et al. 1999).
A different control strategy also using biotechnology is preventing the formation of sulphides, either
by the addition of biocides to inhibit the activity of sulphate-reducing bacteria (SRB) or by the
addition of sodium hydroxide or calcium hydroxide to elevate the pH to inactivate SRB activity
(Oviedo E et al. 2011).
Another promising option is the use of red mud, a mining waste material. This caustic by-product of
the alumina industry currently has no economical value and can be used at ambient conditions. In
addition to effectively removing H2S from industrial emission and significantly reducing industrial air
pollution, this method also has the benefits of solving the problem of another industry’s waste with
no additional manufacturing cost (Sahu R et al. 2001).
In addition to emission control methods, H2S alarms are used in industrial facilities. Casualties must
be removed from the area into fresh air, and rescue staff must wear breathing apparatus.
Supplemental oxygen or hyberbaric oxygen should be administered to the victim (Vale A 2011).
Summary and Conclusions
Exposure to hydrogen sulphide pose many problems due to its high toxicity. Whether a by-product
of human or industrial waste, H2S emissions must be controlled to avoid fatalities.
Management strategies such as chemical procedures to remove the gas, aeration to avoid anoxic
environments and improving the design of sewerage system infrastructure have been successfully
used to deal with emissions from human waste. In the industrial setting, different biotechnologies
are replacing more traditional methods such as absorption and adsorption, the addition of chemicals
to oxidate and precipitate. The use of red mud offers the most advantages, since it uses the waste of
alumina mining.
The effective control of hydrogen sulphide emissions must be ensured to decrease air pollution and
avoid fatal accidents.
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