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9420.pdf
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‘ DAF in fish<br />
year-round<br />
processing plants in Europe is generally restricted to plants operating on a<br />
basis (NovaTec, 1993a).<br />
5.3.4 Chemical Treatment<br />
Chemical treatment generally refers to the addition of chemicals to screened effluent. This<br />
is required as emulsified oil and particulate matter suspended in wastewater are generally<br />
negatively charged. The resulting repulsion substantially impedes certain particle removal<br />
processes such as gravity settling, flotation and hydrocyclone separation. Consequently,<br />
coagulant which reduce or eliminate the repulsive forces between particles are often<br />
added to wastewaters prior to employing these processes. Coagulant are generally<br />
used in conjunction with flocculants which are long chain synthetic or natural polymers<br />
and cause the agglomeration of the now neutral particles.<br />
Both coagulant and flocculants require rapid mixing with the wastewater in order to be<br />
effective. Metal salts are the most common coagulant and flocculants in use at full scale<br />
facilities practicing chemical aided treatment. The most widely used coagulant and<br />
coagulant combinations are: ferric chloride and ferric sulphate, aluminum chloride and<br />
aluminum sulphate, sodium hexametaphosphate, anionic polyelectrolyte, and calcium<br />
chloride. Accurate pH control is generally required, as the efficiency of coagulant and<br />
.<br />
flocculants is pH dependent.<br />
A disadvantage of using iron or aluminum salts as coagulant is the increase in sludge<br />
volume, contamination of the resulting sludge with metals which may make the product<br />
unsuitable for reuse, reduce its value, or cause the sludge not to be accepted at landfills.<br />
Water soluble organic polyelectrolytes avoid these problems and have the added<br />
advantages of requiring concentrations of only a few milligrams per Iitre and not<br />
generating extra quantities of waste for disposal (Steiner and Gee, 1992). Nonetheless,<br />
there are very few full scale installations using only organic polyelectrolytes. The major<br />
detriment appears to be lower removal efficiencies (NovaTec, 1993a).<br />
Results of selected jar-test and bench scale studies are reported below. The economics,<br />
however, are unknown.<br />
Numerous processes using coagulation for treating fish processing wastewater have been<br />
reported. Ohhashi (1974) described a process involving heating effluent to 70-100 “C<br />
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