As discussed previously (see industrial water), the agri-food industry is a substantial water consumer (e.g. from 1 to 5 L of effluent per litre of beer, milk, carbonated drink, wine type beverage); this water is utilised for:
- washing and transporting matter;
- generating steam;
- raw material (e.g.: beverages);
- washing reactors, equipment and pipes, floors, etc.;
These different uses produce different wastewater characterisitics. However, they do have properties that can be summarised as follows:
- an essentially organic and easily biodegradable organic content identified by COD/BOD5 ratios < 2;
- a general tendency to acidification and rapid fermentation.
Therefore, the wastewater treatment process is usually based on biological treatment (for the principles of this treatment, please refer to fundamental biological engineering processes applicable to water treatment, and for the relevant technologies, to chapter biological processes), preceded, more often than not, by specific pre-treatment appropriate to the specific industry.
One of the particular features of many agri-food industries is the seasonal nature of their output due to:
- the need to process the agricultural raw material as soon as it has been harvested (viticulture, fruit and vegetables or sugar mills);
- seasonal demands made by the consumer (beverage industry).
This creates a substantially variable pollution volume and load that has to be processed; in particular, this pollution can be very high during a short period, thus requiring the treatment plant to be designed for two operating modes, one “peak processing season”, and the other “non-seasonal”. In this case, the most suitable treatment is often a dual biological process (high-load + low-load) treatment.
Finally, we should note that in many cases with discharge requirements becoming increasingly stringent, consideration has to be given to supplementary treatments (see “tertiary treatments”) subsequent to the biological treatment, and it is becoming increasingly relevant to design these supplementary treatments not just for discharging water into the natural environment but above all so that all or part of this water can be reused.