Olive Oil Mill Wastewater Treatment: OMWW Polyphenols, Seasonal Load and Disposal

Olive milling is one of the few industries where the entire annual wastewater volume arrives in about a hundred days. Between November and February a medium mill processes its whole year of fruit, and for those months it discharges an effluent with COD in the tens of grams per litre, an intense dark colour, and a phenolic content high enough to be phytotoxic. For the rest of the year the plant is dry. That seasonality shapes every treatment decision more than the chemistry does.

Three-Phase, Two-Phase and What Each Produces

The conventional three-phase decanter separates oil, a solid pomace and an aqueous phase. Per tonne of olives it produces roughly 0.8 to 1.4 tonnes of olive mill wastewater — OMWW — with COD typically between 40 and 150 g/L, BOD of 20 to 60 g/L, suspended solids around 10 to 30 g/L and pH between 4.5 and 5.5.

Two-phase decanters were introduced largely to cut water consumption. They discharge oil plus a wet semi-solid pomace known as alperujo or "two-phase olive mill waste", at 55 to 75 percent moisture. The liquid effluent almost disappears, but the wet pomace volume roughly doubles and it still has to be dealt with — usually by drying and solvent extraction of the residual pomace oil, or by composting.

There is no free lunch here. Two-phase extraction moves the problem from the water line to the solids handling line, and a site that switches process without planning for it discovers that too late in the season to fix.

Why OMWW Resists the Usual Biology

The organic load is mostly sugars, organic acids, pectin, tannins and lipids — all biodegradable in principle. What makes the stream awkward is the phenolic fraction: hydroxytyrosol, tyrosol, oleuropein derivatives and a family of polymeric tannins at concentrations from 0.5 to well over 10 g/L. We have written about the downstream implications separately in Acid Rinse, Dissolved Iron and Recovery.

Those compounds are antimicrobial. Dilute them enough and a conventional activated sludge plant will treat them; undiluted, they inhibit the biomass. Anaerobic treatment shows the same pattern, with methanogenesis noticeably more sensitive than acidogenesis, which is why an unacclimated digester fed raw OMWW acidifies and stalls rather than simply underperforming.

The standard responses are dilution, acclimation and polyphenol removal before biology. Dilution with 5 to 20 volumes of water is simple where water is available, and dilution with another organic stream — manure, whey, sewage sludge — is better still because it fixes the nutrient balance at the same time.

Storage and Evaporation Ponds

In hot, dry climates, open evaporation ponds remain the most common answer and the cheapest. Sizing is the whole problem: in a region with 1,000 to 1,400 mm of annual evaporation and a short wet season, a pond can be sized to dry out over summer and accept the next season without discharge. In a wet climate it never dries, it overflows, and the operator has an environmental incident.

The other issue is odour. A pond holding 15,000 mg/L of readily degradable organics will go anaerobic and generate volatile fatty acids and sulphur compounds. Sites near housing need covered or at least vegetated ponds, and a realistic buffer distance.

Ponds do have one underrated advantage: they store the entire season and release it slowly, which is exactly what a downstream biological plant wants. A pond followed by a small aerobic unit sized for a steady year-round load is often more practical than a biological plant sized for the peak.

Coagulation-flocculation with ferric chloride, aluminium sulphate or lime removes 40 to 70 percent of COD and most of the suspended solids, and it decolourises reasonably well. sludge volumes are large and the sludge itself is phytotoxic, so disposal is part of the cost.

What This Looks Like in Practice

For comparison, our 2021 project in Wenshui, Shanxi ran at 6 m³/day with influent COD near 172,000 mg/L and treated water around 5,000 mg/L. That is 97.1% removal in practice, not on paper. The train was hydrolysis acidification + DAF + oil separation + ultrafiltration.

Figures are site-measured values from commissioned plants, not modelled estimates.

Fenton and photo-Fenton oxidation are the most studied advanced options for this stream. At pH 3 with hydrogen peroxide and ferrous iron, COD removals of 60 to 90 percent have been reported, with most of the polyphenols destroyed rather than transferred. Reagent cost is the constraint — peroxide consumption on a 100 g/L COD stream is significant — so Fenton is usually applied as a pretreatment to improve biodegradability rather than as a standalone.

Ozonation works well on colour and phenolics and produces no sludge, but the energy cost per kilogram of COD removed is high. Electrochemical oxidation has been demonstrated at pilot scale with good results on both COD and colour, though electrode life and passivation in a stream this dirty remain the practical barrier. For the reuse angle specifically, Cosmetics and Personal Care Manufacturing Wastewater Treatment goes further than we can here.

Anaerobic Digestion and Land Application

Straight anaerobic digestion of raw OMWW is difficult. Co-digestion with manure, poultry litter or sewage sludge is the version that actually runs: it dilutes the phenolics, supplies alkalinity and trace elements, balances the carbon to nitrogen ratio, and produces a digestate that is far easier to spread than the raw effluent.

Land application is permitted in several producing countries at controlled rates, typically 50 to 120 m³/ha·year, and it works because soil microbes handle the phenolics and the potassium and organic matter are genuinely valuable. The limits are real, though — salinity accumulation, transient phytotoxicity if applied immediately before sowing, and the risk of surface runoff into watercourses. Application timing relative to the crop calendar is not a detail.

Composting OMWW with pruning waste, olive leaves or two-phase pomace is the other route with a genuine product at the end. It needs a bulking agent, turned windrows or in-vessel aeration, and four to six months of curing — which means land area and a schedule that runs past the end of the milling season.

Combining Treatment Stages

The process logic sits close to what is described in Sugar Refinery and Starch Processing Wastewater Treatment, particularly on the pretreatment side. Most facilities do not run a single clean stream, and a shared equalisation and biological stage is usually the economical answer once the streams are chemically compatible.

Why Choose Baihuipu as Your Manufacturer

Choosing a manufacturer here comes down to one question: who is still accountable once the commissioning team flies home. Our answer is that we build and programme our own equipment, then stand behind it for the life of the plant.

Manufacturing Base in Guangdong

Owned production lines mean we control tolerances, materials and delivery. Standard modular units and bespoke skids are both built by our own people, so schedule commitments are ours to keep.

Documentation You Can Actually Audit

Each system ships with as-built drawings, material certificates, welding records and a functional specification. If a client's own consultant wants to review the process calculation basis, we hand it over rather than treating it as proprietary.

20+ Years Across Dozens of Industries

Experience matters most when your waste stream behaves nothing like the textbook example. We have dealt with toxic organics, heavy metals, extreme salinity and refractory COD in food processing, chemical manufacturing, electroplating, mining and municipal projects.

Certification and Export Practice

CE marking is standard and we work to ISO 9001 procedures. Where a project calls for specific material grades, pressure vessel certification or ATEX-rated equipment, we build to that standard and ship the full documentation pack.

Complete Units and Turnkey Plants

Whether you need a single package unit or a turnkey installation with civil works, we deliver the entire treatment scheme. Our engineering team covers process, mechanical, electrical and automation so one party owns the outcome.

Spare Parts and After-Sales Support

We carry membranes, pumps, blowers, diffusers and instruments for every system we supply. Remote support through the PLC is standard on most builds, and on-site service covers commissioning, training and urgent callouts.

Talk to Our Engineers

Send through your raw water data and we will advise on the viable process route and realistic operating cost before you commit to anything. We can be reached on WhatsApp: +86 13631765076 or through hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

Which discharge standard will the treated water meet?

We design against whatever consent applies at your boundary — GB 8978, local sewer limits, or a ZLD target with no liquid discharge at all. The design basis is agreed in writing before fabrication, so there is no argument later about what "compliant" meant.

Can you work with our existing equipment?

Often yes. We regularly integrate into existing civil structures where the concrete is sound, replacing internals and controls rather than starting over. We will tell you honestly when legacy kit is beyond economic repair.

What does it cost to run, per cubic metre?

Energy dominates, then chemicals, then sludge disposal. We give you a per-cubic-metre breakdown during proposal so you can compare it against your sewer charge — treating too aggressively is a common and expensive mistake.