Oil/Water Interceptor Installation SWMS
SWMS variants reference your state’s WHS legislation. Instant download after payment.
Oil/water interceptor installation is high risk construction work in New South Wales on several counts at once: an excavation deeper than 1.5 metres, a confined space, a contaminated and flammable atmosphere, fuel lines in a forecourt, and mobile plant working at the crest. Section 291 of the Work Health and Safety Regulation 2025 (NSW) captures all of them, and a safe work method statement is required under section 299. SafeWork NSW is the regulator. AS 1940 governs the hydrocarbon vapour control and AS 2865 the confined space regime.
The dangerous half of this job is almost never the new tank — it is the old one. A new interceptor lowered into a hole is a lifting and excavation job. An existing interceptor is a below-ground vessel that has spent years accumulating a hydrocarbon layer, a water layer and a sludge layer, and it is three hazards stacked in one confined space: hydrocarbon vapour in the flammable range above the liquid, hydrogen sulphide generated by sulfate-reducing bacteria in the sludge, and oxygen deficiency because the vapour has displaced the air. None of them is visible. An existing interceptor is never opened as though it were a pit.
The second thing that catches crews is buoyancy. An empty interceptor is a sealed void. Sitting in an excavation with a high water table, or one that fills with rain overnight, it will float out of the ground — lifting, cracking connections and shifting under anybody standing on it. Anti-flotation is a design requirement, not an afterthought.
Hazards identified
14 hazards covered, sorted by priority.
Fatality — vapour, H2S and oxygen deficiency in one below-ground vessel, and the rescuer dies second
Fire or explosion — an 'empty' vessel has a vapour space, and a vapour space is the flammable range
Fatality — burial, and a face that has failed once will fail again on the rescuer
Fatality — electrocution, explosion or fuel release from a service the plans did not show
Fatality — crush injury from the unit during placement into the excavation
Fatality — fall into the open excavation or the open interceptor, by a worker or a member of the public
Serious injury and structural damage — the unit floats, cracks connections and shifts underfoot
Fatality — struck by a vehicle in a live carpark, forecourt or road reserve
Chemical and biological exposure from hydrocarbon sludge and contaminated water
Excavation instability and collapse as saturated ground loses the strength it had dry
Fatality — crush injury from excavator, truck or agitator movement at the crest
Serious injury — a cap or plug released as a projectile during pressure test
Slips and falls on a hydrocarbon-contaminated wet surface
Musculoskeletal injury from handling covers and grates that are far heavier than they look
Control measures
Hierarchy-of-controls order: elimination → substitution → isolation → engineering → administrative → PPE.
- 1Clean, pump out and decommission the existing unit from outside using a licensed liquid waste contractor and remote equipment — an existing interceptor is never opened as though it were a pit.
- 2Test the atmosphere before and continuously during entry for oxygen, LEL and H2S, force ventilate, and isolate every inflow so nothing can enter the space while a person is in it.
- 3Require a confined space entry permit under AS 2865 with a trained standby person outside who does not enter under any circumstances, and rescue equipment and a plan in place before entry.
- 4Recognise that H2S deadens the sense of smell at roughly 100–150 ppm — odour is never a control, and the smell going away means it is getting worse.
- 5Prohibit all hot work on or within the assessed zone of a vessel that has contained hydrocarbons until it is cleaned, gas freed and tested by a competent person, with intrinsically safe lighting and instruments only.
- 6Bench, batter or shore the excavation to a competent person's design for the actual ground, actual depth and the surcharge from plant, spoil and the tank itself, installed as the excavation is dug.
- 7Obtain and interpret all service plans and then prove every service by potholing or vacuum excavation — a forecourt carries fuel lines and vapour recovery that are not always plotted.
- 8Place the unit with nobody in the excavation and nobody within the swing path, using the manufacturer's lifting points, rated gear to AS 4991 and tag lines from outside the zone.
- 9Install the anti-flotation anchorage, ballast slab or strapping specified by the manufacturer and engineer before the excavation can take water, and keep the excavation dewatered until backfill is complete.
- 10Never leave the unit empty in an excavation overnight or into forecast rain — an empty interceptor is a sealed void and it will float.
- 11Hard-barricade the full perimeter of the excavation and fix a secured, load-rated cover to every interceptor opening, checked at every break and at handover.
- 12Install a traffic guidance scheme to AS 1742.3 with hard barriers rather than cones, and schedule outside trading hours where the site is a service station or workshop.
- 13Track and lawfully dispose of all waste through a licensed contractor — a discharge of hydrocarbon-contaminated water or sludge to stormwater or sewer is an offence under the POEO Act 1997 (NSW).
- 14Test hydrostatically rather than with air wherever AS/NZS 3500 and the authority permit, with the area cleared and nobody in line with a cap or plug.
Applicable Codes of Practice
The benchmark for entry permits, atmospheric testing, ventilation, standby and non-entry rescue — the existing interceptor is the archetypal case.
The benchmark for ground stability, the location and protection of underground essential services, spoil placement and access.
The benchmark for hydrocarbon vapour and H2S as hazardous chemicals, including the workplace exposure standards and the control of ignition.
The benchmark for hydrocarbon vapour control, ignition source elimination, hot work near a vessel that has contained hydrocarbons, and separation distances.
Confined space classification, atmospheric testing, entry permits, standby arrangements and non-entry rescue.
The installation, connection, venting and testing requirements for the interceptor and its drainage, and the trade waste connection the water authority must approve.
High-Risk Construction Work triggered
An existing interceptor being decommissioned or replaced is a below-ground vessel with restricted egress containing hydrocarbon vapour, H2S from microbial sludge, and an oxygen-deficient atmosphere. The new unit is also entered for connection and inspection.
An interceptor pit routinely exceeds 1.5 metres, and the surcharge from the tank itself, the spoil and the plant working at the crest all bear on a face a worker stands beside.
Interceptors are installed in service station forecourts, workshops and wash bays where fuel lines and vapour recovery run underground and are not reliably shown on plans.
Hydrocarbon vapour above the liquid layer sits in the flammable range, and the sludge generates hydrogen sulphide. Both are present in and around any existing unit.
The work is routinely in a live carpark, forecourt or road reserve with vehicles — including vehicles carrying fuel — moving around the excavation.
Excavators, trucks, cranes and agitators work at the crest of the excavation and around the unit throughout the installation.
Carrying out high risk construction work without a compliant SWMS is an offence under the Work Health and Safety Regulation 2025 (NSW). A confined space fatality in an interceptor attracts immediate SafeWork NSW attention and, because these incidents almost always take a rescuer as well, they are investigated as multiple-fatality events. Separately, a discharge of hydrocarbon-contaminated water or sludge is an offence under the POEO Act 1997 (NSW) and carries its own penalties.
Who this is for
- →Plumbing and drainage contractors installing or replacing oil/water interceptors and separators
- →Civil contractors carrying out forecourt, wash bay and workshop drainage works
- →Service station and fuel infrastructure contractors replacing existing separators
- →Principal contractors required to obtain and review a SWMS before interceptor works start
- →Facility managers and site occupiers whose carpark or forecourt remains in use during the work
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, with correct section numbers throughout
- ✓14 identified hazards with initial and residual risk ratings on a 5x5 matrix, each with the full hierarchy of control from elimination through to PPE
- ✓The existing-unit control set — the recognition that the old interceptor, not the new tank, carries vapour, H2S and oxygen deficiency stacked in one confined space
- ✓The H2S control set including the rule that odour is never a control, because H2S deadens the sense of smell at roughly 100–150 ppm
- ✓The anti-flotation control set — anchorage installed before the excavation can take water, and the unit never left empty in a wet excavation overnight
- ✓The gas-free hot work regime for any vessel that has contained hydrocarbons, built on AS 1940
- ✓The full high risk construction work breakdown — confined space, excavation, contaminated atmosphere, fuel lines, traffic corridor and mobile plant — with the reason each category applies
- ✓A PPE matrix mapping each task to the required equipment and Australian Standard, including multi-gas monitoring that includes H2S
- ✓Microsoft Word (.docx) format, unbranded, editable fields for PCBU, ABN, site, dates and worker sign-on
Worked example
A crew is replacing a separator in a workshop wash bay. The old unit has been pumped out by a waste contractor and the lid is off. The plumber needs to check the outlet invert before setting the new tank, and the hole is only 1.4 metres deep — shallow enough to step into. He leans in, then climbs down. He does not come back up. His offside sees him drop and goes in after him. Both die. The pump-out removed the liquid. It did not remove the sludge, and the sludge is where the sulfate-reducing bacteria live. Disturbing it releases H2S into a space that is also oxygen-deficient because hydrocarbon vapour displaced the air. Neither man smelled anything meaningful, because at the concentration present H2S deadens the sense of smell within a breath or two. The controls in this SWMS break that chain at every step. The invert is checked from outside. If entry is genuinely required it proceeds only under permit, with the atmosphere tested for oxygen, LEL and H2S — not oxygen and LEL alone — force ventilation running, and a standby person who does not enter under any circumstances. Rescue equipment and a plan are in place before anybody goes in, because the retrieval line is the only thing that makes the second death avoidable. And the document states plainly that a pumped-out interceptor is not an empty pit.
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; Section 299 SWMS required and content prescribed; Section 302 review
- Work Health and Safety Regulation 2025 (NSW) — Part 4.3 Division 2: confined spaces, and Division 3: excavation, including the control of an excavation deeper than 1.5 metres
- Protection of the Environment Operations Act 1997 (NSW) — discharge of hydrocarbon-contaminated water or sludge to stormwater or sewer is an offence; waste is tracked and disposed of through a licensed contractor
- Work Health and Safety Regulation 2025 (NSW) — Chapter 5, plant: the duty to manage risks from plant working at the crest and placing the unit
Frequently asked questions
The old interceptor has been pumped out. Is it still a confined space?
Yes, and this is the assumption that kills people. Pumping out removes the liquid. It does not remove the sludge, and the sludge is where the sulfate-reducing bacteria that generate H2S live — disturbing it releases them. The vapour that displaced the oxygen is also still there. A pumped-out interceptor is a confined space with a contaminated, oxygen-deficient atmosphere, and the SWMS treats it as one.
Why does the SWMS insist on testing for H2S specifically?
Because a crew testing a drainage vessel will often test for oxygen and flammables and stop there, and H2S is the thing that drops them. It is generated biologically in the sludge layer, it is not what anyone expects from a separator, and it deadens the sense of smell at roughly 100–150 ppm — so the warning disappears exactly as the concentration becomes lethal. Odour is never treated as a control in this document.
What is tank flotation and why is it in a safety document?
An empty interceptor is a sealed void. In an excavation with a high water table, or one that fills with rain overnight, it floats — lifting out of the ground, cracking the connections and shifting under anybody standing on or in it. It is a safety issue as much as a defects issue. The SWMS requires the anti-flotation anchorage or ballast installed before the excavation can take water, dewatering maintained until backfill is complete, and the unit never left empty in a wet hole.
Can we cut or weld on the old unit to get it out?
Not until it has been cleaned, gas freed and tested by a competent person. A vessel that has contained hydrocarbons has a vapour space, and a vapour space sits in the flammable range — an 'empty' tank is the most dangerous state it will ever be in. The SWMS applies AS 1940: a hot work permit requiring a gas test immediately before, continuous monitoring during, retesting after any break, and work stopped on any LEL reading.