The main problem in olive mill wastewater treatment are the vegetation waters witch have the appearance of a brown watery residue liquid. This effluent, which has relatively high organic matter content, constitutes a source of pollution which creates a serious disposal problem for olive industries.
Vegetation waters are nearly 50 percent by weight before the pressing of the olives.
Olive mill wastewater characteristic vary widely depending on the types of crushers (continuous or discontinuous). BOD concentration values range between 18,000 and 77,000 mg/l and COD values range between 60,000 and 180,000 mg/l.
The wastewater is poorly biodegradable: it has a BOD:N:P ratio that is not good for biological processes and as has great quantities of phenolic compounds content.
An typical analysis report of the olive mill wastewater discharged from some practical case follow (*).
- pH 5.4
- COD 89,800 mg/l
- BOD 24,000 mg/l
- Suspended solids 23,000 mg/l
- Dissolved solids 60,000 mg/l
- Ether extractable matter 520 mg/l
- Ammoniacal Nitrogen (N) 0.5
- Organic Nitrogen (N) 11 mg/l
- Nitric Nitrogen (N) 13 mg/l
- Phosphorus (P) 5 mg/l
- Total Phenols 11,000 mg/l
Production is seasonal and the discharge amount varies widely depending on the installation: it can be a small old craft mill or an industrial facilities.
On average, the production cycle is active for approximately 100 days a year.
Despite several technical books suggest to spread vegetation water in agricultural land even up to 150 m3 per hectare because contains some substances necessary for soil (nitrogen, phosphorus, potassium, magnesium) this disposal techique must be carefully adopted due to high phenolic concentrations that can cause phytotoxicity for plants and may pollute the aquifer.
Some national and regional regulations set values up to 50-80 m3 per hectare per year although strongly discouraged by some authors that indicate not to exceed the value of 0.4 m3 per hectare per year.
Conventional wastewater plant doest not work properly: active sludge systems, oxidation ponds and trickling filters are not suited for both long times needed, for odour problems and the risk of clogging.
Specific biological treatments with microbial cultures, selected and grown in laboratory, can increase treatment efficiency. A physical-chemical section (lime and ferric chloride) should be added to finalise purification process.
Some cultivation as water hyacinth gave good results because are well adapted to extreme conditions inside the ponds.
Anaerobic processes allow energy recovery due to the biogas production and heat recovery will further improve efficiency.
The best solution is the a vacuum evaporator, maybe heated by combustion of biogas and cooled by a cooling tower.
Liquid evaporation is carried out in a vacuum environment at low temperature, not altering characteristics but but greatly reducing in volume.
Membrane is a more recent technology but it has a problem with the quick clogging of the filtration system.
Ozone treatments are still in development and get good result due to high oxidizing power and reaction speed.
Micro filtration and reverse osmosis are very favourable methods because they allow to use olive byproduct opening a new logical but they always require primary treatment.
The olive mill waste (OMW) obtained from olive pressing process is a very complex waste disposal problem and it is not always possible to achieve a good purifying level.
If faced as purification and waste disposal problem olive vegetation water is a big cost but the the recent possibility to its important quality exploitation in various application modify the approach to the problem.
For example the polyphenols are employed in the food, pharmaceutical and cosmetic industry, for their biological, antioxidant, anti-inflammatory and bacteriostatic characteristic.
Even other elements contained in the vegetation water, such as phosphorus, potassium, nitrogen and boron, can be used as fertilizers.