Post-coagulation sludge

Key words Fish processing wastewater, post-coagulation sludge, fish silage. 

Fish silage is usually produced from fish offals (heads, intestines, skin, etc.) or waste fish. The enzymes present in fish viscera decompose these tissues and the liquid protein hydrolyzate is the final product [9-11]. To avoid bacteria growth a pH about 2 is required and a relevant amount of organic or inorganic acid is added, usually it is sulphuric acid. The idea was to supply a certain amount of post-coagulation sludge for silage production [12]. 

Table 3. Iron concentration in wastewater and in post-coagulation sludge...

When the post-coagulation sludge is added to hydrolyzate production, then the iron hydroxide is converted into ferric sulfate, which is well soluble in water. Too high content of soluble iron compounds, which are easily absorbable, should be avoided, as the excessive amount of the iron in the diet is harmful [13]. It leads to some disease like hemochromatosis or siderosis. Thus, the aim of our research was to find a method of decreasing the absorbable iron content of fish silage. 

In the next series the hydrolyzates were produced from fish or fish viscera with the addition of 20 % of post-coagulation sludge. The characteristic of hydrolyzates is presented in Table 7. 

Figure 2 shows that the content of absorbable iron was higher if hydrolyzates were produced with the addition of post-coagulation sludge. Very effective reduction of the soluble iron concentration has been achieved when phosphoric acid was used instead of sulphuric acid. 

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