11. Other processing methods

11.1. Fish silage

11.2. Composting

11.3. Extruded feeds

11.4. Bio-gas production

11.5. Discharge into the sea

For the small seafood processing plant whose main product is edible fish, the generation of waste streams both solid and liquid present very large problems. If the processing plant does not have or cannot deliver the solid waste (cuttings) to a fishmeal plant, it must make other provisions for disposal of the wastes. Unfortunately, this might involve discharge into the sea or burial in a landfill if one is available. This is a global problem for the small seafood processor. The lucky ones have fishmeal plants that are neighbors and are willing to take the solid waste. The liquid waste from the edible seafood processing plants usually does not contain a large load of nutrients and screening usually removes most of the nutrients in the water.

Fish silage is liquified fish stabilized against bacterial decomposition by an acid. The process involves (Figure 56) mincing of the fish followed by the addition of an acid for preservation. The enzymes in the fish break down the fish proteins into smaller soluble units and acid helps to speed up their activity while preventing bacterial spoilage. Formic, propionic, sulfuric and phosphoric acids have been used. Normally, about 3-4% of acid is added so that the pH remains near 4.0. Strong mineral acids require neutralization before feeding the final product. Silage might be defined as a crude form of hydrolizate.

Silage made from white fish offal does not contain much oil, but when made from fatty fish such as herring it is necessary to remove the oil. The composition of the silage will be very similar to the material from which it is made. Fish silage of the correct acidity is stable at room temperature for at least two years without decomposition. The protein becomes more soluble, and the amount of free fatty acids increases in any fish oil present during storage. Silage production offers a solution to the handling of fish waste, when the logistics of delivering to a fish reduction plant are not economical. Silage can be produced in large or small containers both on the vessel and onshore.

One such operation uses structures (Figure 57) that are 4 feet high x 5 feet wide x 16 feet long using wood and wire fencing. The structures have a 6 inch base of gravel and 8 x 5 foot drainage pipes laid widthwise to provide bottom aeration during the composting process. A 6 inch layer of peat moss was then put down as a base for the compost pile. The peat moss used throughout the pile proved helpful in eliminating odor and rodents. An alternating layered approach was used in setting up the compost piles building 6 inches of fish waste then 6 inches of peat moss. To speed the composting process a commercial compost starter was added along with water to each layer of waste. The completed piles each contained about 5000 pounds of fish waste and cost about US$510 to produce. Wood chips can be used to replace the peat moss and this will reduce the cost considerably. Almost any waste vegetable material can be used.

To control a healthy compost pile you must be aware of the odor (Figure 58 and Figure 59), temperature and composition (Figure 60 and Figure 61).