Wastewater Characterization of Fish Processing Plant Effluents

A more extreme version of offal separation from wastewater prior to pumping includes
screening of all, or major sources of, wastewater prior to pumping. Such a setup requires
sufficient clearance below the floor flumes to install a screen and to collect screenings.
None of the major fish processing facilities visited screened the wastewater prior to
pumping. The smaller processors (New West Net, Shearer and S.M. Products) discharged their
wastewater by gravity flow, but used coarse screens only.
3.3 Screening
All of the large processing facilities visited screen their effluent before discharge. Screening
is a physical wastewater treatment process and removes solids which cannot pass through the
openings of the screen. Solids removal is an important step in wastewater treatment, as solids
of organic origin contribute to the BOD of a wastewater. However, a substantial fraction of the
BOD of wastewaters is due to dissolved substances (such as blood) which, together with particles
smaller than the screen openings, cannot be removed by screening.
Dissolved BOD cannot be removed by simple physical means, but must be removed by a combination
of chemical and/or biological treatment. Therefore, the separation of waste material from water,
as outlined above, is an important means of reducing contaminant loadings if only physical
treatment processes are employed.
All of the large fish processors use rotary screens, although a sidehill screen is used at Ocean.
The rotary screens are available in two configurations. In one configuration the untreated
wastewater is delivered into a headbox which distributes the flow evenly across the rear, upper
surface of a horizontal, rotating cylindrical screen. Effluent passes through the screen twice.
First, through the top of the screen where the removal of solids takes place. Second, through the
bottom of the screen in order to drain away. This second step also causes the screen to be
backwashed as a result of the cascading action of the screened water. Retained particles are
transported by the rotation of the screen to a doctor blade which scrapes off screenings. The
screenings are generally collected in a bin or hopper. Internal high pressure sprays (spraying
from the inside of the screen) may be installed for additional backwashing of the screen.
FREMP 03/21/1994
Fish Processing Plant Effluent 21