1. Impact on Residents due to Sewer Gas
Authors:
Mayank K. Sinha, Consultant, Environmental Management
Pinaki Dasgupta, Consultant, Hazardous Substances Management
Summary:
As per Water and Waste Magazine, (2000, e-edition, article by Tony Palmer, Paul
Lagasse and Maureen Ross “Sewers with low velocities encourage the growth of
anaerobic bacteria in a slime layer coating the sewer. These bacteria reduce sulfur
compounds such as sulfate (SO4), thereby producing sulfides (SO2). Under anaerobic
(septic) wastewater conditions, sulfides cannot be oxidized. Therefore, they combine with
hydrogen to produce hydrogen sulfide gas, creating the "rotten egg" odor associated with
septic wastewater. If the concrete sewer is normally only partly full, the damp surface
above the water line is home to aerobic bacteria that oxidize the H2S and produce
sulfuric acid (H2SO4)”
Preliminary Report for C-159, Defence Colony, New Delhi
Background
The defence colony drain (nallah) has been proposed to be developed under a
beautification project coined as a “park” by the developers, the Delhi Development
Authority (DDA). The vent of the nallah (& a proposed third ramp of the park) is less
than 10 m in front of C-159, which makes it a direct receptor of hazardous gases releases
due to micro-biological actions taking place as a result of substances present in the nallah
in absence of sunlight and aeration.
The present scenario
During a field visit, it was observed that the pungent smell of H2S (Hydrogen Sulfide)
and NH3 (Ammonia) was strongly present out of the open space of the drain left open in
front of the said premises. These gases are known for their toxicity to humans and
environment.
The entire nallah is covered by concrete structure without any gas collection system, as a
result of which, the gases emit continuously unabated causing various health problems to
the residents.
This gas can contain hydrogen sulfide, ammonia, carbon dioxide, methane, nitrogen, and
hydrogen. The odor threshold, the lowest concentration, that can be detected with the
nose, for hydrogen sulfide is somewhere between .001 and .01 parts per million (ppm), an
extremely low concentration.
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2. Consequence Analysis
The emissions of harmful gases are well known to cause severe health diseases like
asthma, skin irritation as well as nausea. Prima facie it seems that the mixture of gases
also have CO (carbon dioxide) which is equally toxic. As these are known to be present
in the air all time, the residents would have exposed well above the threshold limit of the
gases considered safe for human exposure as prescribed by WHO, AAH and CPCB
norms.
A detailed monitoring and test report is underway and results would be provided as
quantitative data for substantiating the above findings.
Hazardous Chemicals
Ammonia
Ammonia has a very strong odor that is irritating. Humans can smell when it is in the air
at a level higher than 50 ppm. Ammonia has a strong smell, similar to urine, which can be
detected by most people even in small amounts. It is a colorless gas with a pungent,
suffocating odor. Ammonia is an irritant and the solution and gas can cause burns of the
skin, eyes, mouth, and lungs. Chronic inhalation has been associated with increased
cough, phlegm production, wheeze and asthma. Ammonia and ammonia solutions are
irritant and corrosive and may be harmful by all routes of exposure
Ammonia at 50 ppm (35 mg/m3) can easily cause irritation to eyes over a two hours
exposure.
The recommended threshold quantities are provided below.
National Institute for Occupational
Safety and Health (NIOSH)
Recommended Exposure Limit (REL)
25 ppm, (18 mg/m3) TWA
35 ppm, (27 mg/m3) STEL
CPCB national ambient quality standards prescribe an annual limit of 100 micro
grams per cubic meter for Ammonia
Hydrogen Sulfide
Hydrogen Sulfide is a colorless gas with a strong odor of rotten eggs. It is a strong
oxidizer. The gas can be detected at a level of 2 parts per billion. The density of hydrogen
sulfide is 1.393 g/L at 25 oC and 1 atm which is 18% higher than that of air making it
heavier than air. The American Conference of Governmental Industrial Hygienists
(ACGIH) provides a Threshold Limit Value (TLV) of 1 ppm, (1.4 mg/m3) and TWA of 5
ppm with 7.0 mg/m3 STEL
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3. In the UK and EU, long-term exposure limits (LTEL) for hydrogen sulphide is 7 mg/m3
(8-hour time weighted average (TWA) exposure reference period). The short-term
exposure limit in the UK (STEL) is 14 mg/m3 (15-minute reference period)
Hydrogen sulfide at 2 ppm is known to cause bronchial constriction in asthmatic
individuals. Nasal lavage fluid can also occur through continuous exposure of the gas. In
these cases, the concentration generally found to occur is 14 mg/m3
Carbon Monoxide
Carbon monoxide is a colourless, tasteless, odourless, non-irritating gas produced during
incomplete combustion of gas or fossil fuels due to there being insufficient oxygen
present. Carbon monoxide has a slightly lower density than air. In the human body, it
reacts readily with haemoglobin to form carboxyhaemoglobin. Because carbon monoxide
is a rather stable gas in the atmosphere, the lungs are practically the only significant route
for environmental exposures. Global background concentrations of carbon monoxide
range between 0.06 mg/m3 and 0.14 mg/m3 (0.05–0.12 ppm). Air quality guidelines by
the WHO provides for the following threshold values such as COHb does not exceed
2.5% anytime
 30 mg/m3 (25 ppm) for 1 hour
 10 mg/m3 (10 ppm) for 8 hours.
CPCB national ambient quality standards prescribe an eight hours limit of 02 milli
grams per cubic meter for Carbon Monoxide
Further, HPA, UK, reports that exposure to lower concentrations of carbon monoxide for
a longer period may affect learning, manual dexterity, driving performance and attention
level.
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4. Abbreviations:
COHb: Carboxyhaemoglobin
Nallah: Sewer
ppm: Parts per million
ppb: Parts per billion (1 ppm = 1000 ppb)
mg m-3 = ppm x gram molecular weight/24.45 (molar volume of air at standard
temperature and pressure)
STEL: Short Term Exposure Limit
TWA: Time Weighted Average
Sources:
1. CPCB: Central Pollution Control Board, National Ambient Air Quality Standard,
18th November, 2009
2. US Environment Protection Agency
3. Health and Safety Executive (HSE), UK
4. Health Protection Agency, UK (From 1st April, 2013: Public Health England)
5. Ammonia:
http://www.hpa.org.uk/webc/HPAwebFile/HPAweb_C/1194947367219
6. Hydrogen Sulfide:
http://www.hpa.org.uk/webc/HPAwebFile/HPAweb_C/1246260030216
7. Carbon Monoxide:
http://www.hpa.org.uk/webc/HPAwebFile/HPAweb_C/1194947421806
8. WHO Air Quality Guidelines - Second Edition Chapter-5: Carbon monoxide
9. http://www.wwdmag.com/corrosion/hydrogen-sulfide-control-wastewatercollection-systems
10. http://www.marionohio.us/wpc/sewer_gas
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