Anaerobic Systems General Management SWMS
SWMS variants reference your state’s WHS legislation. Instant download after payment.
Anaerobic means two gases, and they sit at opposite ends of the space. Wherever wastewater goes septic — which is the normal state of any sewer with real detention time — the absence of oxygen generates hydrogen sulphide, acutely toxic with a relative density of about 1.19 so it sinks and pools at the invert, and methane, flammable with a lower explosive limit near 5% and a relative density of about 0.55 so it rises and collects at the crown. Carbon dioxide sinks with the H2S and displaces oxygen. A single-point gas test is therefore close to worthless: test at the opening and you miss the H2S at the base; test low and you miss the methane at the roof.
This SWMS covers the daily non-entry work on anaerobic assets — sewers, rising mains, maintenance holes, wet wells, anaerobic digesters, sludge holding and thickening, gas holders and biogas pipework. Jetting and hydro cleaning, CCTV, cover removal, blockage clearing, surface and walkway inspection, sampling, and gas system operation and monitoring. This is where most routine H2S exposure actually happens: entry is rare and heavily controlled, while lifting a cover happens many times a week and is often treated casually. No person enters a space under this SWMS — any entry is covered by the Wastewater Confined Space Entry SWMS. Where the asset is a digester or biogas system, the gas is a process stream and the plant is a hazardous area. Authored for New South Wales. Regulator: SafeWork NSW.
Hazards identified
16 hazards covered, sorted by priority.
Acute poisoning of a worker standing over the release path; odour disappears as concentration rises
Catastrophic explosion — methane collects at the crown where nobody tests
A false-negative gas test: methane sits at the roof, H2S and CO2 at the base, so one reading clears neither
Acute poisoning or explosion from a process gas stream at extreme concentration
Fatal or serious injury to a worker struck by a vehicle at an open chamber in a live carriageway
A surgical emergency from a wound that looks trivial; amputation or death if untreated
An unpermitted, unmonitored entry into a lethal atmosphere to retrieve equipment or clear a blockage
Explosion from a phone, torch, vehicle or hot work in a flammable atmosphere
Fall into an open, gas-filled chamber — including by a member of the public
Respiratory infection or illness from an aerosol plume carrying pathogens and endotoxin
A worker in the release path struck or engulfed by a sudden discharge
Explosion from a non-rated ignition source, or shock from wet cabling at gas plant
Loss of containment, gas release, or damage to the relief path
Injury to a member of the public at an unguarded opening in a public street
Musculoskeletal injury lifting heavy cast covers
Fall injury on wet surfaces and hearing loss from jetting plant
Control measures
Hierarchy-of-controls order: elimination → substitution → isolation → engineering → administrative → PPE.
- 1Test at the top, middle and base of every space using a sample line — methane rises to the crown while H2S and CO2 sink to the base, so a single-point reading is a false negative for one gas or the other.
- 2Never rely on odour: at roughly 100-150 ppm H2S deadens the sense of smell, so the smell disappearing signals greater danger, not less.
- 3Stand upwind and never lean over an open chamber; ventilate before approaching, carry continuous personal gas monitoring, and withdraw on any alarm.
- 4Enforce the no-entry rule as a control in its own right — no ladder or steps deployed, dropped equipment abandoned rather than retrieved, and any entry escalated to the Confined Space Entry SWMS and a permit.
- 5Install a traffic guidance scheme to AS 1742.3 before the cover is lifted wherever the asset is in or beside a live carriageway, with barriers, buffer and taper.
- 6Guard every open chamber at all times it is open, against workers and the public alike, and never leave an open chamber unattended.
- 7Treat digester gas plant as a hazardous area — gas holders, relief valves, flame arrestors, condensate traps and gas mixing systems require explosion-protected (Ex) equipment to the AS/NZS 60079 series and strict ignition control.
- 8Eliminate ignition sources around any open chamber and from the classified area: no phones, no non-rated torches, no idling vehicles over an open space, and hot work only under a separate permit after the atmosphere is proven safe.
- 9Isolate and prove de-energised before any electrical work on gas plant, use a licensed electrical worker, and inspect cabling for damage with RCD protection in wet areas.
- 10Never handle a pressurised jetting hose at the nozzle, use a dead-man control that stops flow on release, maintain an exclusion zone, and depressurise before any adjustment.
- 11Stand clear of the chamber and downstream discharge points when a blockage releases, and coordinate upstream control with the network controller.
- 12Work upwind of the jetting plume, maintain hygiene and washing facilities, cover cuts, and use respiratory protection against bioaerosols.
- 13Ensure all workers hold a current White Card (CPCCWHS1001) where on a construction site, together with gas detection and, where required, jetting, traffic control and electrical competencies.
- 14Consult workers on WHS matters affecting them per Section 47 of the WHS Act 2011 (NSW), record the consultation, and review this SWMS whenever the assets, process, method or exposure standards change, after any incident, or at minimum every 12 months.
Applicable Codes of Practice
The benchmark for the no-entry rule, atmospheric testing at open spaces and the rescue arrangements required if entry ever becomes necessary.
The management of hydrogen sulphide and methane as hazardous chemicals, including the duty to keep exposure below the workplace exposure standard.
The technical standard governing any entry, and the basis for treating every chamber and tank as a confined space even when working from outside.
The traffic guidance scheme required before a cover is lifted in a live carriageway.
Hazardous area classification and explosion-protected equipment for digester gas holders, biogas pipework and gas mixing systems.
The H2S values of 10 ppm TWA and 15 ppm STEL against which atmospheres and exposures are assessed.
High-Risk Construction Work triggered
An open maintenance hole or chamber presents a fall risk exceeding 2 m to workers and to the public.
Although this SWMS is non-entry, the assets are confined spaces and the risk of an inadvertent entry to clear a blockage or retrieve equipment is a recognised fatality pathway.
Anaerobic conditions generate hydrogen sulphide and methane continuously; opening a cover or jetting releases both, and biogas plant carries them at process concentration.
Sewers and maintenance holes sit in road reserves, and covers are opened from within a live carriageway.
General management of anaerobic wastewater systems is high risk construction work under Section 291 of the WHS Regulation 2025 (NSW) — it is carried out in an area that may have a contaminated or flammable atmosphere, on or adjacent to a road or traffic corridor in use by traffic, and can involve a risk of a person falling more than 2 m into an open chamber. A SWMS must therefore be prepared before work commences (Section 299), kept readily accessible and reviewed as necessary (Section 302). Although this SWMS is non-entry, the assets are confined spaces: the moment anyone enters, the Part 4.3 duties apply in full — permit, atmospheric monitoring, standby person and rescue arrangements established before entry — whether or not the entry is construction work. Hydrogen sulphide is a hazardous chemical and a PCBU must ensure no person is exposed above the workplace exposure standard. An H2S exposure, an asphyxiation, a biogas fire or explosion, a traffic strike or a fall causing death or serious injury is prosecuted as a Category 1 or Category 2 offence under the WHS Act 2011 (NSW), and the duty of care extends to members of the public near an open chamber in a public street.
Who this is for
- →Local councils and water utilities operating sewer networks, rising mains, pump stations and anaerobic treatment assets.
- →Network maintenance crews carrying out jetting, hydro cleaning, CCTV and blockage clearing.
- →Treatment plant operators running anaerobic digesters, gas holders and biogas systems.
- →Drainage and pipeline contractors maintaining reticulated wastewater networks under contract to a utility.
- →WHS managers and HSE advisors responsible for H2S and methane exposure across a collection network and anaerobic plant.
What you receive
- ✓A complete, editable Safe Work Method Statement authored for New South Wales — the WHS Act 2011 (NSW), the WHS Regulation 2025 (NSW) and SafeWork NSW as regulator.
- ✓16 identified hazards with initial and residual risk ratings on a 5x5 matrix, each with controls ordered through the full hierarchy — eliminate, engineer, administrative, PPE.
- ✓The gas stratification control set — a dedicated hazard and multi-depth sample-line testing, because methane collects at the crown while H2S and CO2 pool at the base and one reading clears neither.
- ✓The hazardous area control set for digester gas plant — gas holders, relief valves, flame arrestors, condensate traps and gas mixing — with explosion-protected equipment and ignition control.
- ✓An explicit no-entry rule treated as a control: no ladder deployed, dropped equipment abandoned rather than retrieved, inadvertent entry recognised as a fatality pathway.
- ✓High-pressure jetting controls including injection injury — a surgical emergency from a wound that looks trivial — named explicitly in the emergency procedures.
- ✓The full high risk construction work breakdown — contaminated or flammable atmosphere, traffic corridor, falls over 2 m and confined space — with the reason each category applies.
- ✓A PPE matrix mapping each task to the required equipment and Australian Standard, plus emergency procedures and a worker sign-on table.
- ✓Microsoft Word (.docx) format, unbranded, editable fields for PCBU, ABN, site, prepared by, reviewed by, approved by and review date.
Worked example
A crew jets a blocked main in a residential street. Nobody enters anything, so the job feels routine. They lift the cover, drop the hose, and run the jetter. What the jetting is doing is stripping dissolved sulfide out of the flow and driving it into the air at the open chamber, where the offsider is leaning over to feed the hose. His meter is clipped to his collar, at chest height, above a chamber where the H2S is pooling at the base and rolling out at ground level. Downstream, at a maintenance hole two hundred metres away, the surge from the clearing blockage pushes a slug of gas up through a second open cover where nobody is monitoring at all. Meanwhile the methane the same decomposition produced sits at the crown of the main, above every probe on site, waiting for the grinder someone is about to use on the cover frame. Not one of these is an entry. All of them are in this SWMS: multi-depth testing, upwind positioning, downstream warning before a blockage releases, ignition control at open chambers, and the recognition that the most dangerous work on a sewer is the work nobody calls dangerous.
Related legislation
- Work Health and Safety Act 2011 (NSW) — Section 19 primary duty of care, owed to workers and the public; Section 47 consultation; Sections 35-38 notifiable incidents.
- Work Health and Safety Regulation 2025 (NSW) — Section 291 (high risk construction work) and Section 299 (preparation and content of a SWMS), with review under Section 302.
- Work Health and Safety Regulation 2025 (NSW) — confined spaces (Part 4.3): although this SWMS is non-entry, any entry requires a permit, atmospheric monitoring, a standby person and rescue arrangements, and is covered by the Wastewater Confined Space Entry SWMS.
- Work Health and Safety Regulation 2025 (NSW) — hazardous chemicals (Part 7.1) and the workplace exposure standard for hydrogen sulphide (10 ppm TWA / 15 ppm STEL), transitioning to Workplace Exposure Limits from 1 December 2026.
- AS 2865 (Confined spaces), AS 1742.3 (Traffic control for works on roads), the AS/NZS 60079 series (explosive atmospheres) for digester gas plant, and AS/NZS 60079.29.2 (gas detectors for flammable gases and oxygen).
Frequently asked questions
Why split anaerobic and aerobic into separate SWMS?
Because the process chemistry drives the hazard profile, and the two are opposites. Anaerobic conditions generate H2S and methane continuously, so anaerobic assets are gas-dominant: toxic atmosphere, explosion risk, hazardous area, stratification. Aerobic assets are aerated, which strips the gases and suppresses their generation, so their hazards become machinery, electricity, drowning and bioaerosol. One document covering both would either bury the gas controls under the machine ones or import gas framing into tanks that do not have a gas problem. The companion Aerobic Systems General Management SWMS covers aerated assets.
Everyone has an aerobic plant now. Do we still need the anaerobic SWMS?
Yes, and this is the most commonly missed point in wastewater safety documentation. Your treatment process does not change your collection network. Sewers, rising mains, maintenance holes and wet wells are anaerobic regardless of what happens at the plant — they generate H2S and methane continuously, and the highest H2S concentrations in many systems are at rising main discharge points where turbulence strips dissolved sulfide out of solution. Every wastewater utility needs this document; the aerobic one is the addition if your plant is aerated.
This is non-entry work. Why is a SWMS required at all?
Because high risk construction work is defined by the conditions of the work, not by whether anyone enters a space. Maintenance and cleaning of a structure is construction work, and this work is carried out in an area that may have a contaminated or flammable atmosphere, on or adjacent to a live traffic corridor, and where a person can fall more than 2 m into an open chamber. Any one of those engages Section 291, and Section 299 then requires a SWMS before work commences.
Why does the SWMS make so much of testing at multiple depths?
Because it is the difference between a gas test and a false sense of security. H2S has a relative density of about 1.19 and CO2 about 1.53, so they sink and pool at the base. Methane is about 0.55, so it rises to the crown. A probe at the opening reads the layer that happens to be at the opening — which may be neither. Crews have been poisoned holding meters that read clear all day, and structures have exploded with detectors on site. Test top, middle and base with a sample line, and treat any single-point reading as unproven.