Aerobic Systems General Management SWMS
SWMS variants reference your state’s WHS legislation. Instant download after payment.
Aerobic assets invert the hazard profile, and crews trained on sewers get them wrong in both directions. Aeration strips dissolved gases and the presence of oxygen suppresses the sulfate reduction and methanogenesis that generate hydrogen sulphide and methane, so an aeration basin is not gas-dominant the way a wet well is. What replaces the gas hazard is machinery and electricity: surface and submerged aerators, mixers and impellers, rotating bridges and scrapers, blowers and diffusers, all electrically driven in a permanently wet environment, mounted over deep liquid. Aerated water is also a worse drowning medium than still water — entrained air lowers the liquid density, so buoyancy drops, a person sinks faster, and a lifebuoy thrown into an aerated zone underperforms badly.
This SWMS covers the non-entry management of aeration basins, oxidation ditches, IDEA and SBR reactors, aerobic digesters, clarifiers and aerated lagoons: inspection from walkways and bridges, aerator and mixer servicing, blower and diffuser operation, scraper and bridge maintenance, weir cleaning, sampling and process monitoring. Two conditions bring the gas back and both are built in. Stop the aeration and the tank turns anaerobic within hours, generating H2S and methane. And the settled sludge blanket at the base is anaerobic even under fully aerated water, so desludging or a scraper pass releases H2S from a tank reading clean at the surface. Authored for New South Wales. Regulator: SafeWork NSW.
Hazards identified
16 hazards covered, sorted by priority.
Rapid drowning — aerated water gives far less buoyancy, so a person sinks faster and a lifebuoy underperforms
Amputation or fatal entanglement in an aerator, mixer, scraper or bridge drive
Electrocution from motors and VSDs in a permanently wet environment
An asset believed gas-free begins generating H2S and methane within hours of aeration stopping
Respiratory exposure to pathogens and endotoxin on walkways over aerated zones
Fall into deep liquid from a corroded or unguarded walkway
Crushing or trapping by a bridge that moves slowly and silently and will not stop
Chemical burns, eye damage or respiratory injury from process chemicals
H2S released from the base of a tank that reads clean at the surface
Infection from contact, splash or ingestion
Noise-induced hearing loss in blower rooms and near aerators
Fall injury on algal-covered walkways over open tanks
Impact injury from a drive or air line releasing unexpectedly
Musculoskeletal injury handling heavy assemblies
Crush injury from a dropped assembly over open liquid
Heat illness and UV exposure on unshaded plant
Control measures
Hierarchy-of-controls order: elimination → substitution → isolation → engineering → administrative → PPE.
- 1Isolate and lock out every drive serving the tank before any work within reach — surface and submerged aerators, mixers and impellers, rotating bridges, scrapers, blowers and diffusers — including automatic, timed and remote/SCADA start, verified physically rather than from a control room indication.
- 2De-energise and prove dead before any electrical work — energised electrical work is prohibited under Part 4.7 Division 4, sections 154 and 157 unless de-energisation is not reasonably practicable, and convenience is never a valid reason.
- 3Guard every walkway, bridge, platform and tank edge, inspect walkways for corrosion before use, and permit no work at an unguarded edge over liquid.
- 4Shut the aeration down FIRST in any rescue — aerated water gives far less buoyancy than still water, so a person sinks faster and a lifebuoy is markedly less effective until the air is off.
- 5Treat any extended aeration outage as converting the asset to anaerobic — the tank turns septic within hours and generates H2S and methane, at which point gas monitoring is mandatory and the Anaerobic Systems General Management SWMS applies.
- 6Monitor for H2S whenever the sludge blanket is disturbed by desludging or a scraper pass — settled sludge is anaerobic regardless of the aerated water above it, so an aerated tank is never gas-free at the base.
- 7Work upwind of the aerosol plume, limit time on walkways over aerated zones, and use respiratory protection against bioaerosols carrying pathogens and endotoxin.
- 8Isolate and lock out the bridge drive before any work on or near a clarifier — bridges and scrapers travel slowly and silently and will not stop for a person.
- 9Fit fixed guarding to all rotating parts, provide emergency stops accessible from the walkway, and release stored energy in drives, actuators and air lines before work.
- 10Provide RCD protection and inspect cabling for damage; electrical equipment lives permanently in a wet environment and any live work is to AS/NZS 4836.
- 11Use closed dosing systems, bunding, containment, and eyewash and safety shower to AS 4775 for alum, polymer, chlorine, caustic and other process chemicals, isolating and depressurising before breaking a dosing line.
- 12Never enter a tank, reactor or aerobic digester under this SWMS — any entry is confined space work requiring a permit, monitoring, a standby person and rescue arrangements under Part 4.3, and is covered by the Wastewater Confined Space Entry SWMS.
- 13Ensure all workers hold a current White Card (CPCCWHS1001) where on a construction site, together with electrical licensing and plant isolation competencies as applicable.
- 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 plant, process, method or equipment changes, after any incident, or at minimum every 12 months.
Applicable Codes of Practice
The benchmark for guarding, isolation and lock-out of aerators, mixers, scrapers, rotating bridges and drives.
The benchmark for de-energisation, isolation and proving dead on motors and VSDs in a permanently wet environment.
The benchmark for guardrails and edge protection on walkways, bridges and tank edges over deep liquid.
The technical standard governing any entry into a tank, reactor or aerobic digester.
The technical standard for any electrical work that cannot be carried out de-energised.
The H2S values applying whenever aeration stops or the anaerobic sludge blanket is disturbed.
High-Risk Construction Work triggered
Walkways, bridges, platforms and tank edges sit well above the liquid surface, and corroded walkways are common on older plants.
Aerator and blower motors, variable speed drives and control circuits are worked on in a permanently wet environment.
If aeration stops the tank turns anaerobic within hours and generates H2S and methane; the settled sludge blanket at the base is anaerobic even under fully aerated water.
Aerator, mixer and diffuser assemblies are lifted over open liquid by crane or hoist, so there is movement of powered mobile plant in the work area.
Aeration basins, ditches, reactors and clarifiers hold deep liquid, and aerated water gives markedly reduced buoyancy — a person who falls in sinks faster than in still water.
General management of aerobic wastewater assets is high risk construction work under Section 291 of the WHS Regulation 2025 (NSW) — it is carried out in or near water or other liquid involving a risk of drowning, on or near energised electrical installations or services, and can involve a risk of a person falling more than 2 m from a walkway, bridge or tank edge, with powered mobile plant moving where aerator and mixer assemblies are lifted. A SWMS must therefore be prepared before work commences (Section 299), kept readily accessible and reviewed as necessary (Section 302). Energised electrical work is separately prohibited under Part 4.7 Division 4, sections 154 and 157 unless de-energisation is not reasonably practicable. Plant must be guarded and isolated under Chapter 5. Although this SWMS is non-entry, an aeration basin, reactor, aerobic digester or clarifier is a confined space and any entry engages the Part 4.3 duties in full. A drowning, an electric shock, an entanglement, an amputation or a fall causing death or serious injury is prosecuted as a Category 1 or Category 2 offence under the WHS Act 2011 (NSW), with the most serious breaches carrying imprisonment for individuals.
Who this is for
- →Local councils and water utilities operating aerated treatment plants — aeration basins, oxidation ditches, IDEA and SBR reactors, aerobic digesters, clarifiers and aerated lagoons.
- →Treatment plant operators and process controllers monitoring, sampling and operating aerators, blowers and scrapers.
- →Mechanical fitters servicing aerators, mixers, scrapers, rotating bridges and drives.
- →Licensed electrical workers maintaining aerator and blower motors, variable speed drives and control circuits in wet environments.
- →WHS managers and HSE advisors responsible for machine safety, drowning risk and bioaerosol exposure across an aerated treatment 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 aeration-outage rule — an aerobic tank is only conditionally gas-free, and any extended outage turns it anaerobic within hours and hands over to the Anaerobic Systems General Management SWMS.
- ✓The anaerobic sludge blanket hazard — settled sludge is septic even under fully aerated water, so an aerated tank is never gas-free at the base.
- ✓The reduced-buoyancy drowning control set, including the requirement to shut aeration down first in a rescue because aerated water gives far less buoyancy than still water.
- ✓In-tank and on-tank plant isolation named in full — surface and submerged aerators, mixers and impellers, rotating bridges, scrapers, blowers and diffusers — including automatic, timed and remote/SCADA start.
- ✓The full high risk construction work breakdown — drowning, energised electrical, falls over 2 m, contaminated atmosphere and powered mobile plant — 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 fitter is sent to a clarifier to free a jammed scraper. The plant is aerated, the water looks clean, and there is no gas smell — so he treats it as a mechanical job, which it mostly is. He walks the bridge, leans over the rail, and works the rake with a bar. Three things are wrong. The bridge drive is not locked out, only stopped at the panel, and a clarifier bridge starts on a timer, travels slowly and makes almost no noise — it will simply keep going with him on it. He is leaning over deep aerated liquid with no edge protection, and if he goes in, the entrained air means he sinks rather than floats and the lifebuoy on the rail is close to decorative unless someone kills the aeration first. And the thing he is levering at is the sludge blanket, which is anaerobic no matter how clean the water above it looks, so he is about to release H2S from the base of a tank nobody is monitoring. This SWMS controls all three: physical isolation of every drive including timed starts, edge protection over aerated liquid with aeration shutdown as the first rescue step, and gas monitoring whenever the blanket is disturbed.
Related legislation
- Work Health and Safety Act 2011 (NSW) — Section 19 primary duty of care; 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) — Part 4.7 (electrical), including the prohibition on energised electrical work at Division 4, sections 154 and 157, with any permitted live work to AS/NZS 4836 and wiring to AS/NZS 3000.
- Work Health and Safety Regulation 2025 (NSW) — Chapter 5 (plant) for guarding and isolation of aerators, mixers, scrapers and bridges, and Part 4.4 (falls) for walkways, bridges and tank edges.
- Work Health and Safety Regulation 2025 (NSW) — Part 4.3 (confined spaces) governing any entry into a tank, reactor or aerobic digester; AS 2865, AS/NZS 1891.1 (fall arrest) and AS 4775 (eyewash and shower).
Frequently asked questions
Our plant is aerated. Why would we need gas documents at all?
Two reasons, and both are built into this SWMS. First, stop the aeration and the tank turns anaerobic within hours — a blower trip, a power failure or a planned outage removes the oxygen supply and the tank goes septic and starts generating H2S and methane. An aerated tank is only conditionally gas-free. Second, the settled sludge blanket at the base is anaerobic regardless of what is happening above it, so desludging or a scraper pass releases H2S from a tank that reads perfectly clean at the surface. An aerated tank is never gas-free at the base.
Why is drowning rated so highly when the plant has walkways and rails?
Because aerated water is a markedly worse drowning medium than still water, and most rescue thinking is calibrated on still water. Entrained air lowers the density of the liquid, so buoyancy is reduced — a person floats less, sinks faster, and a lifebuoy thrown into an aerated zone is far less effective than it looks. That is why the first step in the rescue procedure in this SWMS is not to throw anything, it is to shut the aeration down. Walkways and rails are the control; the rating reflects what happens when they fail.
Do we need the anaerobic SWMS as well?
Almost certainly yes, if you operate a reticulated network. Your collection system — sewers, rising mains, maintenance holes, wet wells and pump stations — is anaerobic regardless of how your plant treats, and generates H2S and methane continuously. This SWMS covers your aerated treatment assets; the companion Anaerobic Systems General Management SWMS covers everything upstream of them, plus anaerobic digesters and biogas systems. Note that an aerobic digester belongs in this document, not that one.
Isn't stopping at the panel the same as isolating?
No, and this is the most common fatal assumption around aerated plant. A stop is a command; an isolation is a physical break in the energy supply, locked with a personal lock by the person doing the work, with stored energy released and the isolation verified at the switchboard rather than from a control room indication. Aerators, mixers, scrapers and clarifier bridges start automatically, on a timer, or remotely from SCADA — and a bridge moves slowly and silently and will not stop for a person on it.