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close this bookSmall-Scale Processing of Fish (ILO - WEP, 1982, 140 p.)
View the document(introduction...)
View the documentI. FISH RESOURCES
View the documentII. WASTE DISPOSAL
View the documentIII. AIR POLLUTION


All fish processing operations require that waste water and pieces of fish be discarded. Often, the reservoir from which water is obtained for use in processing is also the ultimate destination of water borne wastes. If the wastes are not treated properly before disposal, they may cause pollution of the water body and make it unsuitable for use. It can also result in an upset in the biological balance in the water body causing a change in the animal and plant life.

All contaminated water discharged from fish processing operations should be treated so as to keep pollution of natural waters to a minimum. The types of waste water produced can be divided as follows:

(i) Blood water and contaminated process water: this water results mainly from the washing of fish and contains fish blood and some fish protein;

(ii) Ice melt water and used ice containing fish protein, blood and bacteria;

(iii) Wash-down water containing larger pieces of fish and fish protein;

(iv) Domestic sewage from toilets and urinals; and

(v) Specialised waste waters such as press liquor and stick water from fish meal operations and cooling water from canning operations, etc.

The sort of treatment required depends on the type of waste water and the quantity involved. Screening, filtration, flotation, sedimentation and/or centrifugal methods may be necessary to remove solid particles. This may be preceded by chemical treatment with alums and lime which will control coagulation and pH. The use of coagulating agents such as these prior to physical removal of solids can enhance the subsequent treatment. In many industries a crude filtering process is used which will remove most solids.

The Canadian Department of Fisheries (Blackwood, 1978) recommends that:

- The treatment of waste water for solids removal should be equivalent to that achieved by a screen with an opening size of 0.7 mm. Slightly contaminated process water may be discharged directly into the receiving water.

- Domestic sewage must be treated by the plant or the municipality prior to discharge.

- Blood water, stick water, press liquor, etc. should not be discharged directly into the receiving water because of their high organic material content which would result in a high biological oxygen demand (BOD) in the receiving waters. Solids removal by screening and scum and oil removal by flotation are minimum requirements for treatment of these effluents.

- Storm water not contaminated by fish or its constituents may be discharged directly into the receiving water, otherwise it must be treated as other wastes.

In most fish processing operations, there will be a certain amount of solid waste. This may be in the form of fish offal, heads and trimmings from cutting operations, or waste fish which may be too small to use. These wastes must be treated properly if they are not to become a nuisance and a hazard to public health.

Where small quantities of waste are produced in a small scale operation, then the most feasible way of disposing of them may be to bury them in a pit. The pit should be at least 6 ft. deep and offals, once put into the pit, should be buried under at least 6 inches of earth. This will stop the residues becoming infested with flies, etc. and also accelerate the rotting process.

Small quantities of fish waste can be also used for feeding animals such as ducks and chickens. Alternatively, the offal can be used fresh as a manure for plants adding valuable nitrogen and minerals to the soil. These possibilities are only feasible where relatively small quantities are involved as, without some form of preservation, the waste will become spoilt before it is eaten or is absorbed into the soil.

Where larger quantities of waste are being produced, then it may be feasible to produce a by-product from the main fish processing operation. In Japan, for instance, cod stomach, gills and gullets are salted for preservation and human consumption. In many countries, viscera are fermented to produce sauces and pastes while in other countries the viscera can be sold on the market at very low prices thus providing protein and food for the less well-off members of the community.

It may be possible to produce animal feedstuffs from larger quantities of offals. The acidification of fish offals with mineral and organic acids can produce a product known as fish silage. Much interest has been shown recently in silage production which represents a simple, cheap yet very effective alternative to the large scale production of fish meal. Briefly, silage is prepared by adding acid (usually formic acid) to pieces of fish waste and letting the whole mass break down to a liquid. This liquid can then be dried for inclusion into chicken feeds or can be fed as a liquid to pigs. Fish silages can also be produced by the fermentation of the fish offals using lactic acid producing bacteria such as Lactobacillus sp.

Where very large quantities of offal are generated by a fish processing plant or where a number of smaller plants are in close proximity, then fish meal production may be feasible. However, fish meal equipment and the running of a fish meal plant require high capital investments and a large amount of continuous supply of raw material which may not be available to the small-scale producer.

Fish glue can be made from skins and heads of fish by steaming fresh material over a perforated screen within a steam-jacketed vessel for about 8 hours. Fish glues were once used extensively in furniture making, book-binding, leathergoods, etc. However, with the advent of synthetic glue from petro-chemicals, fish glues have gone out of favour.

Filleting waste or small trash fish or shrimp waste can be sun dried and used as a fertiliser to improve crop production on the land.

Fish oils may also constitute an important byproduct from fish wastes processing. These are often derived from the fish meal process. Good quality fish meal must not have an oil content of more than 10%. Where oily fish are used in the production of fish meal, there may be a need for the extraction and removal of oil from the meal. This oil can be used for either human consumption in the production of margarine and cooking fats, or it can be used for the production of various compounds such as paints and varnishes. Oils can also be extracted from the livers of fish. Liver oils are often high in vitamins A and D and were of economic importance until fairly recently. However, in recent years, these vitamins have been produced synthetically.