The wastewater generated in the canning process has two specific characteristics compared to wastewater generated in other food sectors, which are:
- A great variability of flows and organic load, depending on the processes being carried out and the more or less seasonal operation of many factories.
- The high salt content of some discharges, especially regarding the cooking of tuna, as well as the brine from anchovy processing, which makes biological treatment difficult, due to the inhibition produced by the salt content and mainly its fluctuation in these systems.
As an example, the following effluents are generated in the manufacturing process of canned tuna, which is produced mostly in the Galicia-Northern Portugal Euroregion:
- Tuna cooking brines: These come from the emptying of the steam cookers, constituting the main source of organic contamination in this industrial sector. It is also worth highlighting their high salt content (21300 ppm). This emptying normally takes place at the end of the day and its volume reaches an average of 15%. Some companies carry out a separation of the fat released during cooking, thus reducing the final load that is discharged.
- Discharges from the dirty area: Includes water from defrosting, cutting and washing of tuna. Discharge is usually continuous during these operations, usually in the mornings. Its volume varies considerably from one factory to another, with an average value of 30% of the total discharge volume. This effluent stands out mainly for its high blood content, i.e. organic matter, and solids (chemical oxygen demand (COD) of 1463 ppm, total suspended solids (TSS) of 2800 ppm), as average values. Filters or decanters are usually available in the chambers to separate coarser solids.
- Can sterilisation water: This comes mainly from the cooling phase of the cans. These waters are characterised by a significant volume of water of around 3m³/Tonne per year processed, having a low organic load and a high final temperature. It is preferably discharged in the afternoon and accounts for 25% of the total water consumed in tuna processing.
- Other discharges: this section includes the cleaning of equipment and facilities, the washing of grills and baskets, cans, etc., as well as cooling and purging water, etc. It accounts for the remaining 30%, with cleaning water being the main point of consumption at the plant and having a variable pollutant load, which is higher during the first wash (COD 2,000 – 4,000 ppm, TSS 1,000 – 5,000 ppm)..
What improvement is proposed by Conserval?
The measures proposed to improve water management in seafood processing and canning companies are based on consumption optimisation and on measures aimed at managing effluents.
The CONSERVAL project is focused firstly on the study of the type of effluent (according to raw material, or processing point) in order to efficiently develop technologies for the targeted production of volatile fatty acids (VFA), as well as the optimisation of processes for the purification of VFA, so that a greater economic benefit can be achieved later on, as will be seen in activity 4 of this project.
Thus, a change of paradigm should be introduced in the management of effluents, going from being a mere cost for the company to a raw material for the production of a high added value bio-product.
The analysis conducted by Conserval reflected annual effluent treatment costs ranging between EUR 50,000 and EUR 200,000, which is a significant fixed annual cost.