Hazardous Atmosphere in Sewers

Hazardous Atmosphere in Sewers

Important Information About VOC's In Melbourne's Sewer Network

In 2014, Melbourne Water undertook research into hazardous atmospheres in sewers and has identified the presence of Volatile Organic Compounds (VOC’s) in Melbourne’s sewer network. As a result, most Victorian water authorities including Melbourne Water, Yarra Valley and South East Water now require that anyone coming into contact with sewer headspace gas to have in addition to their traditional four gas sensors, a VOC sensor and an ammonia sensor as part of their gas detection equipment.  

What are VOCs and what is Ammonia?

VOC’s are a large group of carbon-based chemicals that easily evaporate at room temperature.  While most people can smell high levels of some VOC’s, other VOC’s have no odour. Odour does not indicate the level of risk from inhalation of this group of chemicals. There are thousands of different VOC’s produced and used in our daily lives including petrochemical compounds, refrigerants, acetone, cleaning and disinfecting chemicals and more. In most cases, water authorities accept these compounds from industrial customers however VOCs can also find their way into the sewer system via contaminated ground water infiltration, illegal discharge and domestic sources. 
Ammonia on the other hand is a compound that is commonly found in sewage. It is formed from the breakdown of urine and other nitrogenous biological compounds. 
Known Health Effects
The known health effects of VOCs vary depending on the chemical, duration of exposure and whether the person has existing health conditions such as asthma that may render this person particularly sensitive to the chemicals. VOCs might be highly toxic, carcinogenic or have little impact at all. Effects can be acute or chronic. Acute affects include damage to both the central and peripheral nervous system e.g. impaired judgment, memory loss or permanent loss of feeling in the hands. Chronic impacts include cancer or damage to the liver and/or kidneys. Chronic symptoms may be the result of low concentration exposures over an extended period and may be slow to develop.
Melbourne Water Research Results
After recognising the potential risk to worker health posed by VOCs, Melbourne Water started collecting data on VOC levels. A 6 gas detector was given to their sewer samplers and they recorded any gas readings that they experienced over a 3 ½ month period. The results are summarised:
  • 209 Entries
  • 37 VOC readings of over 10 ppm (Melbourne Water set their alarms for VOC at 10 ppm)
  • 11 VOC readings of over 100 ppm
  • The highest reading was 776  ppm
  • Only 3 of these readings also exceeded the alarm for one of the standard four gases e.g. without the photoionisation detector (PID) only three of these events would have been detected

Changes in Melbourne Water's Confined Space Entry Atmospheric Monitoring Requirements

As a result of the results collected from the study conducted, Melbourne water has changed the confined space entry atmospheric monitoring requirements for sewerage transfer work. Melbourne Water now require that anyone coming into contact with sewer headspace gas have a Volatile Organic Compound (VOC) sensor and an ammonia sensor as part of their gas detection equipment. These two sensors are in addition to the traditional four gas sensors required for sewer entry. See below table. Similarly, other water authorities such as Yarra Valley Water and South East Water have also issued similar changes to their requirements for confined entry in sewer networks. 

Sensors RequiredLow AlarmHigh Alarm
LEL (Lower Explosive Limit) 5% 10%
O2 (Oxygen) 19.5% 23.5%
CO (Carbon  Monoxide) 30 ppm 35 ppm
H2S (Hydrogen Sulphide) 10 ppm 15 ppm
NH(Ammonia) 15 ppm 25 ppm
VOC (Volatile Organic Compounds via a PID) 10 ppm 20 ppm

Table 1. Sensors and alarm limits for entry into MW's Sewers 

Air-Met Scientific encourages all parties who enter the sewer network to contact the specific water authority to ensure that they are equipped with the correct safety equipment and meet the relevant safety requirements for confined space entry in sewers. 


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Date and Time

Wed. 28 Oct 2015

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Hazardous Atmosphere in Sewers