7. Stickwater

7.1. Definition
7.2. Composition
7.3. Assumptions on volume

Fish entering the process are cooked and pressed to separate the solids from the liquids. The solids eventually become fishmeal while the liquids undergo further processing first to recover suspended solids that might have escaped the press and then to separate and recover the oil. The water that is left after the fish oil is recovered is called stickwater.

7.1 Definition

Stickwater simply is the water present in the fish plus some of the blood water and a small amount of seawater mixed with some oil, suspended solids and dissolved salts and solids. The content of the total solids (8-10%), proteins, vitamins, minerals and fat present in the stickwater makes the recovery critical from a technical, environmental and economical point of view. As a general rule, about 60% of the fish weight will be generated as stickwater with about 8% total solids. It is easy to see that for plants that do not recover the stickwater by evaporation, the losses of solids will be approximately 48 kg/ton of fish.

There are several different types of evaporators that are used in the fishmeal industry. These are described as follows:

  1. The rising film evaporator is a multi effect system with 3 or 4 effects to save energy. The saturated boiler steam enters in the first stage and the subsequent steam generated by evaporation of the stickwater is used to heat the next succeeding stages. This type of evaporator cannot be heated with waste steam from the steam dryers because there is not sufficient heat generated to run the evaporator.
  2. The falling film evaporator requires less input heat, normally has 2 or 3 effects and can be operated with the waste heat from the steam dryers. This type of dryer can be balanced with the rest of the plant so that the production of stickwater concentrate is balanced with the dryer capacity to utilize all of the solubles.
  3. Mechanical Recompression evaporators contain a mechanical compressor to recompress steam produced in the concentration process, which are recirculated to the evaporator thus reducing energy consumption.
  4. Flash evaporators involve heating the stickwater without allowing it to evaporate and then expanding it in a vacuum chamber. The amount of evaporated water is proportional to the energy difference of the liquid before and after expansion divided by the liquid's latent heat. This system can be used with steam from direct heat dryers.

Waste heat evaporators operate by pumping hot water through a heat exchanger in which the energy is transferred to a circulating flow of stickwater. The stick water is thereby heated and then flashed into a vacuum chamber in which the absorbed energy is flashed off as water vapor. In a single effect plant the vapors are condensed in a sea water cooled condenser. The cooled water is recycled to the condensation tower and again reheated. A single effect wast heat evaporator will generally take about 40-50% of the required water evaporation from the stick water. This reduces the load on the stickwater plant and allows the operator to raise the temperature of the last effect which is necessary if these vapors are to be used in waste cookers. Waste heat evaporators might be sufficient for small operations that cannot afford a multi-stage evaporator. In Figure 41 we have outlined the flow for the liquid phase of the fishmeal process through the production of stickwater concentrate.

7.2 Composition

Stickwater is a mixture of water, suspended and dissolved solids, salts and fat. Generally, stickwater will contain about 8-10% total solids made up of approximately 5.6% protein, %0.6 fat, 1.8% ash, 92% moisture.

7.3 Assumptions on volume

As an assumption for this project, about 60% of the fish weight will be generated as stickwater. In Figure 14 we outlined what we considered the theoretical material balance for an anchovy fishmeal plant. We used 200 kg/ton for dry solids and 90 kg/ton for fat. The theoretical yields would be 23.1% fishmeal and 7.6% oil. For our average plant of 50 tons/hour, about 30 tons of stickwater would be produced per hour or 60,000 tons in a 2000 hour season.

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