Oil refining converts crude feedstock into usable products such as petrol, diesel, aviation fuel, LPG, bitumen and sulphur. Every stage uses water for desalting, cooling, steam generation, equipment cleaning and product handling. That water leaves the site carrying oil droplets, suspended solids, dissolved hydrocarbons, sulphides, ammonia and other contaminants.
Effective wastewater treatment protects rivers, coastal waters, groundwater and refinery equipment. It also helps operators meet discharge conditions, reduce freshwater demand and recover valuable hydrocarbons. For an integrated producer such as OrdaSintez Gaz, treatment is part of a broader chain involving crude processing, product storage, logistics and industrial supply.
Australian operators face particular pressure because many facilities are close to urban areas, ports or sensitive waterways. Water scarcity around Perth, Adelaide and parts of regional Queensland makes reuse attractive, while strict environmental licences and community expectations require reliable process control rather than occasional corrective action.
| Treatment stage | Main purpose | Typical contaminants addressed | Key operating concern |
|---|---|---|---|
| Equalisation and preliminary screening | Balance flow and remove debris | Solids, rags, large particles | Hydraulic surges and maintenance |
| API or corrugated-plate separator | Recover free oil and settle solids | Free oil, grease, grit, suspended solids | Droplet size, temperature and sludge removal |
| Dissolved air flotation | Remove fine oil and light solids | Dispersed oil, emulsified material, floc | Chemical dosing and skimmer performance |
| Biological treatment | Degrade biodegradable dissolved pollutants | Phenols, residual hydrocarbons, ammonia and sulphides | Toxic shocks, oxygen demand and nutrient balance |
| Tertiary polishing | Prepare water for discharge or reuse | Fine solids, colour, residual organics, nutrients | Membrane fouling and operating cost |
Refinery effluent changes throughout the day. Desalter blowdown may contain high salinity and oil, while sour-water streams can contain hydrogen sulphide, ammonia and phenolic compounds. Stormwater from loading bays may be relatively clean during dry weather but heavily contaminated after a spill or washdown.
A single treatment unit cannot handle this variability consistently. A well-designed wastewater treatment plant separates clean stormwater from process drainage, contains accidental releases and balances fluctuating flows before physical and biological treatment. This reduces the risk that a sudden hydrocarbon load will damage downstream microorganisms.
The design should begin with a complete water balance. Engineers assess flow rates, pH, temperature, salinity, chemical oxygen demand, biological oxygen demand, total petroleum hydrocarbons, sulphide, ammonia, phenols and suspended solids. Sampling across operating conditions is more useful than relying on one laboratory result.
An API separator is a gravity-based oil-water separator designed to remove free oil and settleable solids. Wastewater enters a long, relatively calm chamber, where heavier particles sink and oil droplets rise. Flights, scrapers or chains move settled sludge and floating oil towards collection points.
The system works best when oil is present as sufficiently large, unbroken droplets. Flow velocity, retention time, temperature and density difference between oil and water all influence separation. Warm wastewater generally supports better oil removal because viscosity falls, while turbulence, detergents and pumps can break oil into small droplets that pass through the unit.
API separators are common because they are simple, robust and suitable for large refinery flows. Their performance is limited when the wastewater contains stable emulsions, surfactants or dissolved hydrocarbons. Regular removal of skimmed oil and bottom sludge is essential; accumulated material reduces effective volume and can create odours or secondary contamination.
Corrugated-plate interceptors use inclined plates to shorten the distance oil droplets and solids must travel. The plates provide a large effective separation area in a compact footprint, making them useful where land is expensive or an existing plant needs greater capacity. They require careful cleaning because wax, sludge and sticky hydrocarbons can block the channels.
Dissolved air flotation, commonly called DAF, is often installed after primary gravity separation. Recycled water is saturated with air under pressure and released into the treatment stream. Tiny bubbles attach to oil and floc particles, lifting them to the surface for removal. Coagulants and polymers may improve the capture of emulsified oil and fine solids.
These physical processes protect biological reactors from excessive oil loading. They also produce recoverable hydrocarbon streams, although recovered material must be assessed before returning it to refinery processing. Sludge classification, storage and disposal need controls because it may contain concentrated petroleum residues and hazardous substances.
Biological treatment uses microorganisms to break down biodegradable organic compounds that remain after oil separation. Aerobic activated sludge systems supply oxygen to a mixed microbial population, while moving-bed biofilm reactors grow microorganisms on plastic carriers. Sequencing batch reactors combine aeration and settling in timed cycles and can suit variable flows.
Biological systems may reduce BOD, COD, phenols and some residual hydrocarbons. Nitrification can convert ammonia into nitrate, followed by denitrification under low-oxygen conditions where nitrate is converted into nitrogen gas. These processes require suitable alkalinity, temperature, dissolved oxygen and nutrient balance.
Refinery wastewater can be toxic to microorganisms when it contains high concentrations of sulphide, solvents, metals, salinity or sudden hydrocarbon loads. Equalisation tanks, pH correction and upstream oil removal are therefore central to biological reliability. Operators should monitor oxygen uptake, sludge settleability, ammonia, oxidation-reduction potential and microscopic biomass condition.
Aerobic systems are widely used because they can deliver dependable removal of biodegradable contaminants and cope with moderate flow changes. Their drawbacks include energy use for blowers, production of biological sludge and sensitivity to toxic influent. Fine-bubble aeration can improve oxygen transfer, but diffusers require inspection and cleaning.
Anaerobic treatment operates without oxygen and can generate biogas from concentrated organic wastewater. It may be attractive for selected high-strength streams, though refinery effluent often contains inhibitory compounds that require pretreatment and careful acclimatisation. Anaerobic reactors are usually considered as part of a wider treatment train rather than a universal replacement for aerobic processes.
Hybrid systems combine attached-growth and suspended-growth biology, enabling a smaller footprint and greater resistance to short-term load changes. Membrane bioreactors can produce high-quality effluent for reuse, but membrane fouling, cleaning chemicals and concentrate management increase complexity. The right choice depends on discharge limits, land availability, energy prices and the intended water reuse.
In Australia, environmental approval is generally administered through state and territory regulators rather than one national refinery discharge permit. Facilities may require an environment protection licence under state legislation, with conditions covering effluent quality, monitoring, incidents, odour and reporting. The Environment Protection and Biodiversity Conservation Act 1999 can also apply where nationally protected matters may be affected.
A refinery near Brisbane, Geelong or Adelaide may discharge under conditions set by the relevant state EPA, while sewer discharge is usually controlled by a water utility or local authority. Trade-waste agreements can specify limits for oil, pH, temperature, suspended solids and toxic substances. Operators should therefore check both environmental licence conditions and sewer acceptance requirements.
Water reuse is increasingly relevant in Australia because drought, restrictions and rising utility costs affect industrial sites. Treated effluent may be reused for cooling-tower makeup, washdown, dust suppression or firefighting reserves, subject to risk assessment and appropriate polishing. In Western Australia, where Perth has experienced prolonged water stress, reducing potable or scheme-water demand can strengthen the business case for advanced treatment.
A treatment plant needs continuous attention to flow, pH, temperature, oil content and dissolved oxygen. Online analysers can identify process drift, while laboratory testing confirms hydrocarbons, COD, BOD, ammonia, phenols, sulphide, salinity and toxicity. Automatic alarms should direct operators to equalisation or emergency retention capacity before non-compliant water leaves the site.
API separators require inspection of inlet distribution, oil skimmers, sludge scrapers, coalescing media and level controls. Biological units need checks on aeration equipment, return sludge, foam, settling behaviour and microbial activity. In hot Australian summers, evaporation can concentrate salts and increase biological stress, particularly at inland facilities far from the coast.
Stormwater management deserves equal attention. Bunds around tanks, segregated drainage, shut-off valves and spill response equipment prevent contaminated runoff from bypassing treatment. At a busy port or fuel terminal, truck loading, ship movements and heavy rainfall can create rapid changes in drainage quality, so containment capacity must reflect credible worst-case events.
The strongest design links source control, physical separation, biological treatment and final polishing. Clean roof water should remain separate from oily drainage wherever practical. High-strength or toxic streams should be isolated, characterised and metered before entering the central plant. This approach lowers treatment costs and makes process failures easier to diagnose.
Engineers should consider peak hourly flow, rainfall intensity, planned maintenance, power interruptions and product changeovers. Australian sites may need to account for intense summer storms in Queensland, bushfire-related runoff risks in parts of Victoria and New South Wales, and water restrictions affecting reuse targets. Backup pumps, standby blowers and emergency storage provide protection when essential equipment is unavailable.
The final treatment objective should be defined clearly: compliant discharge, partial industrial reuse, or high-grade recycled water. API separators remain valuable for removing free oil and solids, while biological systems address dissolved biodegradable pollutants. Together with flotation, filtration, activated carbon or membranes where necessary, they form a flexible treatment strategy suited to modern petroleum refining.