Treatment of wastewater and olive paste from oil mills
Olive oil is a must in your kitchen, but remains from olive oil mills are very contaminant.
The olive oil sector is an enormous economic, cultural and environmental asset in some countries, such as Spain, Italy or Greece. As a result, improving the treatment of the waste obtained after olive oil production and attempting to revalue it is crucial for improving the competitiveness and profitability of the sector.
The production process usually follows this sequence:
- Horizontal centrifugation
- Vertical centrifugation
- Storage and packaging
Once the fruit has been gathered and transported, the production process begins in the oil mill. Currently, there are two production systems: the traditional or three-stage system, that produces three types of product in addition to the oil: olive juice, pomace and wastewater. And the two-stage system, that aside from the oil generates wastewater and olive paste (mixture of pomace and juice). This new two-stage system is more efficient, generates less waste and consumes less water, and therefore generates less wastewater.
The two-stage system generates two types of waste: wastewater and olive paste. The wastewater from the cleaning and vertical centrifugation processes, and from the cleaning of the tanks, hoppers and other elements. This waste does not comply with the regulations for discharge into public waterways, nor can it be used for irrigation due to its high pollution loads. Traditionally this waste has been stored in pools without treatment, which causes serious problems for the sector, since the surface are occupied must increase, generating bad odors, overflowing, sanctions, stalling activity, plagues of insects, etc.
The wastewater generated by the activity of the oil mills, commonly known as “alpechín” or juice, contains a wide variety of waste such as: dust, earth, oil and greases, sugars, nitrogenated substances, organic acid, polyalcohols, polyphenols, etc. Polyphenols are a large problem as they inhibit the bacterial activity in the soil. For this reason, this water has to be treated to be able to be reused for irrigation. The treatment to eliminate this pollutant consists of a physical-chemical purification due to its inhibiting action on microbiological processes.
This wastewater, or alpechín, before being treated, is characterized by its dark color and strong odor. It has a high level of organic pollution with a COD/BOD5 ratio between 2.5 and 5, and a high content of polyphenols and solid material. The pH is slightly acid, easily fermented, with a high electrical conductivity and it contains emulsifying fats. There are a variety of techniques for treating wastewater from oil mills so that it meets the legal standards: Physical-chemical methods (coagulation-flocculation, oxidation and electrochemical processes), biological treatment (activated sludge, anaerobic treatment, processes based on membrane biological reactors). Each method has its advantages and disadvantages in terms of costs and efficacy, so it is common to combine several technological solutions.
As we have commented previously, once this wastewater has already been treated it may be reused for irrigation or other uses such as cooling for boilers and topping up aquifers. In fact this is a practice recommended by the public administrations and international organisms. Nevertheless, this treated water must be subjected to monitoring of its use and quality in order for it to be used as a hydric resource that is safe for health and the environment.
The treatment of olive paste is also extremely important, as its uncontrolled discharge causes problems with the water color and entails a threat to aquatic biodiversity, damage to soils, phytotoxicity and odors. Moreover the pomace producers have adapted to the reception of this product from which they can extract pomace oil using a physical or chemical process. After obtaining the pomace, by-products can be obtained from the olive paste. After an energy cogeneration or composting process for the production of biomass, the production of PHB for the manufacture of bioplastics, the production of enzymes and pectins, production of dyes and antioxidants, the production of polysaccharides of commercial interest for the food and cosmetics industry and, also, as agricultural fertilizers.
Therefore, olive paste is a highly pollutant product, but may be used as fuel on one hand (once the residual oil has been extracted) or to manufacture compost on the other. This latter option is ideal for oil mills that are not near any pomace treatment plants. Thus it is used as a resource which in principle would be waste. When the olive paste is mixed with olive leaves and manure, a compost of excellent quality is obtained.
For organic matter to be converted into compost, aerobic fermentation must take place. The quality of the product will depend on the following parameters: carbon and nitrogen ratio (from 25/1 to 45/1), the moisture content of the initial material (from 30% to 80%), the pH (there is no cause for concern if the C/N ratio is suitable), the oxygenation and the temperature.