Water contamination and wastewater treatment

The food processing industry generates large volumes of waste water and large amounts of solid waste that can produce environmental problems and whose management increases the cost of processes. It is normally necessary to adopt some measures to decrease the environmental impacts of the processes and reduce the costs that are incurred in the treatment and the disposal of the wastes generated. These measures should be focused on the reduction of the generation of wastes rather than their treatment. It is always more cost-effective to reduce the amount of wastes generated rather than treating them before discharge (minimisation at source is better then end-of-pipe treatment).

Water contamination by food processing

There are numerous methods of wastewater treatment and new ones are continuously being developed. There are many handbooks, for example Baruth published by AWWA (2004) and monographs (Kawamura, 2000; Crittenden and Montgomery, 2005) available, where it is possible to study the existing methodologies in detail.

As this field is developing rapidly, potential users will probably be most interested in practical applications from sources of continuously updated information or specialised company websites. For example a Dutch company Lenntech Water- & Luchtbeh. Holding b.v. (2005) offers:

• physical-chemical treatment systems;

• sludge treatment;

• anaerobic/aerobic treatment;

• drum and disc filtration;

• membrane filtration;

• advanced oxidation systems;

• equalisation tanks;

• biological excess sludge reduction.

As food processing has some specific contaminants, mostly biological, then appropriate specific wastewater treatment methodologies should be selected.

The final documentation of the EU AWARENET project (de las Fuentes et al, 2002a,b; AWARENET, 2006) provides the following overview of the main technologies for food processing:

• mechanical treatment (sedimentation, screening, degreasing, flotation, membrane separation);

• biological treatment (land application, activated sludge treatment, trickling filter and submerged contact aerator, constructed wetlands, anaerobic treatment);

• physical-chemical treatment (neutralisation, precipitation, flocculation, flotation, oxidation/reduction and disinfection, ion exchange, adsorption, incineration, NH3 stripping, evaporation, membrane separation).

Wastewater treatment

Methodologies for wastewater handling can also be subdivided into different levels of treatment as follows.

1 Pre-treatment: mechanical separation of coarse particles (e.g. sticks, plastics, etc.).

2 Primary treatment: removal of suspended solids by physical or physical-chemical treatment. This can consist of natural sedimentation or assisted sedimentation via coagulants and/or flocculants addition, or via centri-fugation. This step also includes neutralisation, stripping (elimination of NH3) and the removal of oils and grease by flotation.

3 Secondary treatment: this is used to eliminate colloids and similar matters from the waste water. With this treatment the organic load of the waste water is removed to a large extent. It can include chemical and biological processes. The most common processes are activated sludge treatment and anaerobic digestion which lead to important removals of chemical oxygen demand (COD), biological oxygen demand (BOD), phosphate and ammonia.

4 Tertiary treatment: this comprises physical and chemical processes to eliminate defined pollutants such as phosphate, ammonia, minerals, heavy metals, organic compounds, etc. These types of treatments are considered as a 'polishing phase' and are usually more expensive than conventional ones. The need of the application of this type of treatment is dictated mainly by two potential factors:

• The requirement to meet discharge conditions based on Environmental Quality Standards (ESQs) (Environmental Quality Standards, 2006), which may be stricter than the requirements of BAT. Relevant substances include ammonia, List I and List II substances and suspended solids.

Recycling of the waste water for its use in the factory, either as process water or washing water. These approaches can be also applied in sensitive areas where the effluent has to have a very low charge of N and P.

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