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Chemical treatment is normally used in conjunction with flotation, although some vendors
combine it with gravity settling. These two forms of chemically-assisted treatment are
discussed in the following sections.
5.3.4.1 Chemically-Assisted Flotation
Chemical addition greatly improves the performance of DAF systems. The BOD reduction
efficiency of DAF systems without chemical addition range from 10 % to maximum of
50 % However, with pH adjustment and chemical addition efficiency of BOD removal
increases to up to 99 % European experience suggests that DAF with chemical addition
can be successfully used to treat fish processing plant effluents. The most widely used
operating scheme is pH adjustment to 4.2 -5.5 followed by ferric chloride and polymer
addition, BOD removal efficiency may reach 97 % (NovaTec, 1993a).
A summary of reported coagulant and DAF efficiencies in seafood effluent treatment is
presented in Table 5.6.
Only one fish processing facility in the Pacific Northwest, situated in Newport, Oregon
(USA), employs dissolved air flotation for effluent treatment. This plant has very high
contaminant loads (BOD, TSS and oil and grease) in the wastewater stream due to surimi
processing. The achieved BOD reduction varies from 45 to 80 % (Table 5.6). However,
effluent BOD concentrations can still be very high (e.g. 2,200 mg/L after 80 % reduction).
This is due to the inability of the DAF process to effectively remove dissolved organics.
The only treatment process that would be able to remove dissolved organics from the
wastewater streams to low levels is biological treatment (see Section 5.3.5). However,
biological treatment is generally not feasible in the fish processing industry due to the
industry’s seasonal nature and high capital and operating costs of biological treatment.
The sludge generated by the DAF system at the Newport facility is used in the facilities’
own reduction plant. However, reduction plants generally do not accept this type of
sludge, as its water content is very high and the material is of low quality which may
compromise the quality of the product. Also, chemicals used with DAF may result in
sludges containing unacceptably high levels of metals (i.e. iron or aluminum, see
Section 5.3.4).
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