Refinery Oily Effluent: Why Flocculant Choice Alone Will Not Fix the Emulsion

Effluent from synthetic rubber, synthetic plastics, fibre and detergent production - plus the benzene, naphthalene, methanol, glycerin and acetaldehyde streams behind them - carries raw material, product and by-product together. Organic content is high and the odour is unpleasant. Getting it to a reuse or discharge standard normally takes sedimentation, biochemical treatment, ozonation or activated-carbon adsorption, and tertiary treatment on top if the water is going back into production.

The hard part is almost never the dissolved organics. It is the emulsion.

The flocculant landscape

Inorganic polymer flocculants such as polyaluminium and polyferric are widely used and have largely displaced traditional inorganic-salt flocculants. They work, they are cheap, and they generate a great deal of sludge.

Organic polymer flocculants have a different set of properties: dose is 1/10-1/40 of inorganic flocculants, the applicable range spans pH 4-9, purification is better, sludge volume and water content are both lower, and neither the salt content of the water nor the metal-ion content of the sludge increases - which matters directly if the water is heading for reuse. Many refineries and petrochemical plants in the United States have fully replaced inorganic flocculants with organic ones.

The organic family splits three ways: anionic (polyacrylamide, sodium polyacrylate), cationic (polyamine-type, quaternary-ammonium-type, copolymer-type) and non-ionic (polyacrylamide, polyoxyethylene, water-soluble urea resin). For oily wastewater the cationic type is the better match, because oil droplets in water normally carry a negative surface charge.

The supply problem is real, though. Domestic production has historically concentrated on anionic and non-ionic types, and only a handful - polyacrylamide and carboxymethyl cellulose among them - reached commercial volume. Cationic polymer development started later, and while cationic acrylamide copolymers have entered organised production, most are aimed at high-suspended-solids streams and sludge dewatering rather than refinery oily water. As long as plants stay on inorganic flocculants, high sludge production and difficult sludge handling are unavoidable. Strengthening the R&D of organic flocculants and getting them into pilot production and field trials is the practical route forward, and because the right flocculant depends entirely on the target stream, field testing is usually decisive. Choosing a flocculant is, in that sense, closer to an art than a specification.

Coalescing filtration versus flotation

Coalescing filtration uses a bed packed with a coalescent material chosen for a rough surface, strong oil adhesion, moderate particle size and adequate strength. The process runs in three stages. In the oil-film initial stage, fine droplets passing through the bed are captured by the coalescent and spread across its surface as a film. In the oil-film thickening stage, continued capture thickens that film, which is retained in the bed voids. In the film-release stage, coalesced and agglomerated oil is dragged forward by the flowing water.

Coalescing oil removal relies on the first two stages. Once stage three begins, effluent oil content starts climbing, and that is the signal to stop and backwash - knocking adhered oil and suspended matter off the coalescent so they form larger particles that separate by gravity.

Operating conditions matter. With low-emulsification oily wastewater, no flocculant is needed at all. When the water carries many surface-active sites, a small flocculant dose is required for destabilisation and coalescence.

The measured comparison is worth quoting. In coalescing-filtration trials, oil droplets >15 um were basically removed, and droplets <10 um were still removed at 60%. After two-stage coalescing filtration, oil content dropped from a 25-142 mg/L inlet range to 6-32 mg/L. Flotation, by comparison, gave effluent oil content of 1-51 mg/L. On the other two counts that operators care about, coalescing filtration cut sludge by 70%, saved 30% on electricity and lowered water-treatment cost by 31%. Add a simple flow sheet, easy operation and management, a compact unit and small footprint, and the case for automatic control is straightforward.

When the emulsion is severe

Refineries generate genuinely difficult streams: diesel alkaline-refining wash water, heavy-oil and dirty-oil-tank drains, tank-washing-station wash water. Mixed into the general oily wastewater, these turn a lightly emulsified stream into a severely emulsified one and disrupt both oil separation and flotation.

The conventional demulsification answers each carry a penalty. Heating is energy-intensive. Acidification to pH below 3 consumes considerable chemical and often still demulsifies poorly. Dosing demulsifiers is expensive and inconsistent. Alternating asymmetric pulse electrocoagulation has tested well against emulsified oil and is worth evaluating where the conventional three have already failed.

Microwave radiation is the other option with a mechanism worth understanding. Under irradiation, ion movement in the emulsion intensifies and compresses the double electric layer, lowering the Zeta potential and achieving demulsification. It is effective, and the energy balance has to be checked against the specific stream.

High-concentration and refractory streams

For high-concentration petrochemical discharge, the split is straightforward: readily degradable streams go to anaerobic biological treatment, and streams that inhibit or resist biodegradation go to chemical or physical methods.

Anaerobic treatment has a genuine cost case - low energy consumption, biogas recoverable as energy, low inorganic-nutrient demand, low treatment cost and little excess sludge. The high-efficiency bioreactors developed from the late 1970s into the 1980s hold high biomass in the bed and suit high-concentration wastewater well. Perhaps more useful: the hydrolysis-fermentation stage of the anaerobic process is highly adaptable, and with appropriate cultivation and acclimatisation it achieves reasonable degradation even on refractory organic matter. That has made it increasingly relevant in petrochemical treatment.

Inhibitory or non-biodegradable high-concentration wastewater remains the central difficulty of the sector. Whether that fraction can be treated properly determines whether a plant meets its discharge standard at all. Both China and other countries have invested heavily here and developed working technologies, but the gap to a fundamental solution is still considerable, and further experimental work on new technology is needed.

What to do with this

Three conclusions hold across most refinery and petrochemical sites. Move from inorganic to organic polymeric flocculants where reuse is a goal, accepting that the selection has to be made on site rather than from a datasheet. Evaluate two-stage coalescing filtration against flotation on sludge disposal cost, not just on effluent oil - the 70% sludge reduction is usually worth more than the marginal difference in outlet concentration. And treat severe emulsions as a separate stream rather than mixing them into the general oily water, because that single mixing decision is what converts a manageable problem into an unmanageable one.

Where the emulsion is the constraint, no amount of flocculant optimisation will substitute for a demulsification step. The flocculant is downstream of that decision.