Olive Oil Mill Wastewater

A  relief carved in marble  in 1982 by sculptor, Aristeidis Metallinos

A relief carved in marble in 1982 by sculptor, Aristeidis Metallinos

When you dip your fresh baguette in a plate of olive oil or add some to your green salad for your dinner, the last thing you probably think is the amount of waste that was left behind until that bottle made its way to your home. This is normally the point we miss the most. The dirty side of food production normally does not get much attention or interest. We are interested in the quality of flavors, texture and smell. We may ask ten thousand questions around GMO, organic and pesticides, or locality but very seldom we ask “Does the production facility treat the environment in a best way possible when they are processing the harvest for consumption as well?” “How much water was wasted throughout the chain?” or “What happened to the solid waste?” Maybe we think we do not have much control over the production side and try to vote with our money on the farm side by choosing non-GMO, local or organic labels. But we need to evaluate if a product has high environmental cost throughout the whole value chain. People are normally are taken aback once they hear how much water is consumed in the value chain or if the waste is handled appropriately. 

Olive oil production is one of these industries. Each olive tree produces 15 to 40 kg of olives a year depending on the cultivar and conditions. On average, the oil yield is 1 kg of olive oil from 5 kg of olives. Normally the general composition is 18-28% oil, 40-50% vegetation water/stone, and  30-35% olive pulp.  

Main processing steps needed to obtain olive oil are: leaf removal&washing, crushing, mixing, separating the oil and centrifuging the oil. There are three types of extraction processes that are being used currently at the olive oil mills:  
-    Traditional: Olives are washed, crushed and kneaded with warm water. Paste then pressed to drain oil. Liquid waste is a mixture of olive juice, added water and residual oil. Oil is separated by decanting or vertical centrifuging. This method is normally employed by small mills. 
-    2 phase and 3 phase processes uses either 2 or 3 decanting steps to separate the oil. 2-phase does not demand as much water addition as 3-phase. In 3-phase, hot water is added to the decanters. 

Depending on the mill type, 50% (traditional) to 80 to 110%  (3-phase) of olive oil waste is produced relative to the initial weight of the olives, or as another reference notes 0.5 m3 to 1.5 m3 per 1000 kg of olives depending on the process. The wastewater obtained from the process is a heavy organic polluter: High in phenolic compounds (up to 80 gr/L) which are toxic to microorganisms and not easily biodegradable, high biological and chemical oxygen demand (up to 220 g/L COD and 100 g/L BOD), low pH (3-5.9), high odor and high solids concentration (up to 20 g/L total solids). Due to the oxidation and polymerization of tannins, wastewater is also discolored. What happens when this type of wastewater is released to the water bodies without any treatment is that the high oxygen demand eats up the available oxygen in the water, compete with other aquatic life. Heavy metals can leach and found in the river bed sediments. When mixed into the domestic wastewater treatment in low dilution ratios, it can upset the treatment since the bacteria needs time to acclimate it. 

The paste or cake obtained might be treated further and used as fertilizer/soil mulch or animal feed. Due to high energy content, it has been used as fuel as well. Lately, there has been some research on the suitability of this cake for food and pharmaceutical industry due to high anti-oxidant content of it.  

Wastewater has been a bigger issue. Due to the drastic oil production increase (from 1.4 million tons in 1990/91 to 2.48 million tons in 2014/15 per IOOC), moving from traditional mills to 3-phase continuous process, the wastewater volume has also increased considerably. Normally, evaporation ponds are being used to accumulate the water, reduce the water content. It is low cost and the seasonal production nature makes it preferable but odor development and leaking of wastewater into surface and ground water could be a problem depending on the proximity and groundwater table. Then the sediment collected can be used as fuel or land applied as fertilizer due to high organic and nutrient content. When it is land applied as fertilizer, the chemical composition of the waste can affect the germination of the plants in the land they are applied. 

One study focused on treating the wastewater by constructed wetlands. Wastewater was applied to the wetland and the effluent was recycled to the inlet of the wetland. Although the removal of COD was achieved, it was still higher than the region’s acceptable limits for stream discharge. But it has a potential as a pretreatment step. In various researches, anaerobic, aerobic, advanced oxidation and physicochemical treatment steps have been studied either as sole treatments or combined. A combination of physicochemical treatments along with anaerobic treatment achieved up to 80% COD removal. Anaerobic digestion is normally less energy intensive, produces lesser sludge than aerobic one and energy recovery can be done by the methane gas produced. However, as always, the olive oil wastewater needs conditioning: dilution, nutrient addition and alkalinity adjustment to make the characteristics right for the treatment environment. Aerobic treatments also require a long acclimation period (up to 25 days retention time in a completely mixed batch activated sludge process) and high dilutions ratios, and even with 90% COD removal, the effluent can still have substantial levels of COD and color to safely discharge. Advanced oxidation techniques with ozone or UV with hydrogen peroxide have also been tested with some mixed results. For further reading, below I have listed the references I have used to obtain the information. 

There are 12000 olive oil mills of which 80% uses 3-phase approach. They are spread out through the regions. Besides the geographical spread, the seasonal production nature is making centralized treatment very difficult. As with all pollution problems we face, there is no one magic silver bullet due to the complexity of the problem. The options to treat water with various treatment processes for each mill becomes costly due to operational expenses. As the complexity of the treatment equipment increases, the need for expertise and staff also increase. Doing more researches on how to treat this type of wastewater should come hand in hand with also the researches on new techniques on olive mills where we aim to reduce the water used or recycle within the process. 


Bhatnagar A. et.al.; Valorization of Solid Waste Products From Olive Oil Industry as Potential Adsorbents For Water Pollution Control- A Review; Environmental Science and Pollution Research (2014), Volume 21(1)

Cedola A. et. al.; Fish Burger Enriched by Olive Oil Industrial By-Product; Food Science and Nutrition (2017), Volume 5(4)

Coz A. et.al.; Management Scenarios for Olive Oil Mill Waste Based on Characterization and Leaching Tests; Journal of Chemical Technology and Biotechnology (2011), Volume 86(12)

Elhag M. et.al.; Stream Network Pollution by Olive Oil WastewaterRisk Assessment in Crete, Greece; Environmental Earth Sciences (2017), Volume 76 (7)  

Kapellakis I.E., PAranychianakis N.V., Tsagarakis K.P., and Angelakis A.N.; Treatment of Olive Mill Wastewater with Constructed Wetlands, Water (2012) , Volume 4 

Paraskeva P., and Diamadopoulos E.; Technologies for OMWW Treatment: A Review;  Journal of Chemical Technology and Biotechnology (2006) Volume 81(9)

Tsagaraki E., Lazarides H.N., and Petrotos K.B.; Olive Mill Wastewater Treatment, Chapter 8 in “Utilization of By-Products and Treatment of Waste in the Food Industry (2006)

Ilke McAlileyComment