Water & Wastewater

Water and wastewater sites generate their hazards biologically rather than industrially, which changes where the gas appears and how consistent it is. A sedimentation tank does not release hydrogen sulphide because something failed. It releases it because that is what happens when sulphides cannot oxidise.

Most of what follows comes from work our Technical Consultants have done on Australian treatment plants, networks and desalination facilities.

Hydrogen Sulphide, And Where It Forms

Treatment moves through several stages to remove solids, filter pollutants and purify water. Wastewater is collected in pools, settles in sedimentation tanks, and chemicals are applied to complete the process. Most of those stages release toxic and flammable gas.

Hydrogen sulphide is the one that dominates, and it is most commonly found in the degreasing and biological treatment stages. The mechanism is worth understanding because it explains why the release is continuous rather than occasional. Under septic wastewater conditions, sulphides cannot be oxidised, so they combine with hydrogen to produce hydrogen sulphide. That is the source of the rotten egg odour associated with wastewater, and it is a process outcome rather than a fault.

Chlorination can be used to reduce hydrogen sulphide levels, and on its own it is not sufficient. Detection is still needed to protect people from high level exposure.

Three problems follow from hydrogen sulphide generation, and only two of them are usually discussed. The toxicity is well known, and the odour is impossible to ignore. The third is corrosion. Hydrogen sulphide generates sulphuric acid, which attacks concrete and steel, so the gas is degrading the asset as well as threatening the people working on it. That gives the detection a maintenance justification alongside the safety one.

Inhalation is the primary route of exposure, and the lungs absorb hydrogen sulphide quickly. Symptoms run from nausea, headaches and eye and throat irritation through to death at high concentration. The workplace exposure limit is an 8 hour TWA of 10 ppm and a STEL of 15 ppm, and those figures don’t change when the Workplace Exposure Limits take effect on 1 December 2026.

Where fixed detection belongs is specific rather than general. Screening tanks, grit removal, the degreasing area, sedimentation tanks and biological treatment tanks are the five locations that account for most of the risk. Fixed single gas detection at those points, with portable instruments on the people moving between them, covers the plant properly. Typical ranges are 0 to 100 ppm for hydrogen sulphide on both fixed and portable, with area monitors where a work front needs covering, and a sampling probe for testing before anyone puts their head into a space.

Chlorine, And Why It Needs Testing More Often

Chlorine appears across the sector, in treatment plants, desalination and public facilities, and it behaves differently from other gases in a way that affects maintenance.

In a desalination plant the process runs seawater through pre-treatment filtration, then reverse osmosis under high pressure to strip minerals, then reintroduces fluoride and other minerals before chlorine is added to disinfect and kill bacteria. The chlorine arrives as a liquid in tanks held in a controlled environment, and the room typically carries no hazardous area classification because it is climate controlled with its own HVAC.

Detection is arranged in two tiers, and the second tier has serious consequences. Fixed detectors at 0 to 10 ppm near the tanks signal the control room. Separate high range detectors at 0 to 100 ppm are configured so that an alarm shunts the entire plant. Gas detection here is not advisory, it is an engineering control with the authority to stop production.

Which brings up the detail worth knowing. Chlorine is a sticky gas. It adsorbs onto surfaces including tubing, fittings and the inside of a sensor housing, which means it takes time to reach the sensor and produce a reading. A detector that appears unresponsive may be working correctly and simply waiting for the gas to arrive.

The practical consequence is that these systems need monthly functional testing with a chlorine generator rather than an annual calibration and an assumption. That is a service requirement most operators do not build into their maintenance schedule until somebody points it out.

Workers entering the room to offload or unload chlorine cylinders carry portable single gas instruments, typically 0 to 100 ppm with low and high alarms around 0.5 and 1.0 ppm.

Storm Water And Manholes

Network assets carry a risk that gets underestimated because of what people expect to find in them.

Manholes, storm water tunnels and access chambers are assumed to contain sewer gas, meaning hydrogen sulphide and methane from decomposition. They frequently contain something else. Fuel spillages reach the drainage network. Chemicals and solvents get dumped. Fertilisers, pesticides and factory waste all find their way in. Natural gas can enter from a damaged service.

So the flammable risk in a storm water system is often higher than the biological risk, and a crew equipped only for hydrogen sulphide is equipped for the wrong hazard. Detection for network entry needs LEL and oxygen alongside the toxics, which is a four or five gas instrument rather than a single gas monitor.

Digesters And Biogas

Anaerobic digestion is increasingly part of a treatment plant rather than a separate facility, and it produces methane deliberately.

That changes the gas profile from a toxic problem to a flammable one, with methane, carbon dioxide and hydrogen sulphide together. Digester roofs, gas holders, compression and flare or generation equipment all need coverage, and the requirement shifts from personal monitoring toward fixed detection with controllers capable of driving isolation and ventilation rather than only reporting.

Confined Space Entry

Wet wells, pump stations, digesters, tanks, sewers, manholes and storm water tunnels are all confined spaces, and this sector probably contains more of them per site than any other.

The hierarchy of control puts elimination first, and that is achievable more often than people assume. Gastech supplies gas detection for sewer tunnel flotation platforms, where a float travels at least 300 metres along a cleaned and ventilated tunnel carrying detection, thermal imaging and cameras, so operators can assess the asset from a non-hazardous area and decide whether anyone needs to enter at all.

Where entry does happen, pre-entry testing, continuous monitoring during occupancy, and the specific problem with catalytic bead LEL sensors are all covered on our confined space page.

Who Regulates It

Water and wastewater operations fall under general work health and safety law, so the regulator is WorkSafe WA, SafeWork NSW or Workplace Health and Safety Queensland depending on location. Safe Work Australia publishes the model laws and codes of practice but does not regulate or inspect.

Water quality itself is governed separately from worker safety, through the Australian Drinking Water Guidelines published by the National Health and Medical Research Council, which is where the chlorination requirement originates rather than from any gas detection standard.

Standards relevant to detection include AS 2865:2009 for confined space entry, which applies to most of the assets on a treatment site, and AS/NZS 60079.29.2 for the selection, installation, use and maintenance of gas detectors. Chlorine storage and handling also falls under state dangerous goods legislation.

What Gastech Supplies

Fixed gas detection for screening, grit removal, degreasing, sedimentation and biological treatment areas, chlorine storage rooms, digesters and pump stations.

Portable single gas and multi-gas instruments for plant work, network entry, chlorine handling and confined space entry, with area monitors for work fronts.

Accessories including sampling probes and tubing for testing a space before entering it.

Controllers with the relay capacity to drive ventilation, isolation and plant shutdown rather than only reporting a reading.

Behind them sit project engineering, NATA calibration and service, field service including the regular functional testing chlorine systems need, fleet management and hire, from WA, NSW and QLD.

If you are reviewing detection across a treatment plant or a network, talk to our Technical Consultants.