For almost a year [in 1998] we have been averaging one inquiry per month in regards to dewatering shrimp and crab waste. The material to be pressed includes shrimp heads; the shells from the shrimp tail; and both hard-shell and soft-shell crab waste.
Samples of these materials have been run in a variety of screw press configurations. The amount of dewatering that can be achieved is limited: the press will knock out the loose water, but most of the water does not separate mechanically because it is bound chemically in the organic material. Only heat will break it loose. Both the Series CP/VP and KP presses have proven successful, depending on the client’s objectives.
It is difficult to get representative samples of either inbound or pressed material. In the case of inbound material, the presence of flush water and/or ice varies greatly. With press cake, the amount of shell in the sample distorts the results. Typically we read 88% moisture in the press liquor and 78% to 80% in the press cake.
These results are measured even though we know that shrimp shells are 50% to 70% moisture to start with. On a dry basis they are 51% calcium, 30% protein, and 17% chiten.
More important is the separation: typically we can achieve a yield of up to 30% to 40% press liquor. We call the press liquor “strawberry milkshake". The color and consistency are very similar to the fountain drink.
To further research the subject, we have visited a local plant, Tampa Bay Fisheries, which uses Vincent equipment to process shrimp tail shells. They pump their waste to a static screen and then use a 6" screw press. The press dewaters the screen tailings ahead of a Vincent triple pass rotary drum dryer. This is an older installation, dating back to the late 1960’s.
The screw press is run without a discharge cone. Its function is to remove only the water that can be readily separated. This is because the “strawberry milkshake" is a pollutant stream that is so rich in organics that its disposal is expensive. The function of the press is to take some load off the dryer.
The press cake is dried to about 10% moisture in the dryer. Probably the most unique feature of the dryer is the way in which odor is controlled. (Fish drying plants are notorious for their foul odor). At Tampa Bay Fisheries our dryer does this with an afterburner furnace. The second furnace is mounted above the first furnace, ducted so that some of the hot gasses help dry the shrimp shells. Odor control is quite acceptable.
Tampa Bay Fisheries sells the dry shrimp waste as animal feed. It was used as cattle or poultry feed additive. It has little value because the shrimp heads, containing the protein, are no longer removed at this plant. In fact, there are immediate plans to shut down the drying plant as the local landfill will now accept the shells.
Other uses have come to our attention. There is a small market as feed for pet fish. A significant market is feed for pen salmon in order to add red color to the meat. The more profitable market involves extracting chitin from the dried shells. This is used to produce chitosan, the miracle weight-loss remedy.
One potential customer is exploring the possibility of processing the strawberry milkshake into a bisque base for human consumption. Protein assays of the liquid show excellent nutritional value.
Update August, 2013
In the intervening years since 1998 this market has gone nowhere. There was a period of strong interest in producing chitosan, but most of our customers went out of the business almost as fast as they got into it. The industry is dominated by suppliers in Asia.
The use of chitosan to prevent the body from digesting fats proved to be a total fantasy.
Technically we learned that shrimp waste has an extremely strong tendency to co-rotate with the screw of a press. That stops the press from working. The solution to this is to use an automatic-reversing feature in a single screw press, or to use a twin screw press. Both have proven successful with shrimp waste.