Chlorine Inhibition of BOD on
A Pecan Waste
PAUL E. McGINNIS, Graduate Student
WILLIAM S. MIDKIFF, Assistant Professor
Department of Civil Engineering
New Mexico State University
Las Cruces, New Mexico 88001
INTRODUCTION
At New Mexico State University, a methodology has been developed for
characterizing the effects of chlorine used in the processing of pecans. This methodology
should be applicable to the study of chlorination effects on other wastes as well. With this
method, the inhibitory effects of chlorine on BOD exertion can be studied and the possible
mechanisms by which inhibition occurs can be defined. The methodology involves a
modified amperometric titration determination of chlorine and a continuously monitored
electrolysis method of BOD determination.
BACKGROUND
Stahmann Farms is located in the Mesilla Valley, part of the Lower Rio Grande Valley
in New Mexico. It is the major producer of in-shell pecans in the United States and is among
the larger pecan shellers.
Pecans brought to the processing plant receive an external cleaning treatment designed
to free the shell of its husk, dirt, and debris lodged between the husk and shell.
Before the pecans are cracked, they are carried through a 3 minute chlorinated rinse
followed by a 30 minute chlorine soak. The chlorine residual of the water used in these two
operations is between 100 and 120 mg/l. From the soaking stage, the pecans are fed to the
crackers and then the shellers that attempt to separate the meat from the shell through
tumbling action.
Large shell particles are separated from the meat by passing the meat and shell
fragments through blowers. The light shells are lifted away while heavier particles (meat and
meat-shell combinations) fall into vibrating trays which help free additional meat from shell
fragments. These particles pass through three "floats" connected in series. The shell is
separated by gravity and the meat floats to an overflow grid. Meat and shell particles that
remain attached to each other in the first float settle and are recycled successively to the next
two floats. The water carrying the meat passes through the grid to a recirculation tank
equipped with an overflow tube. The meat from each float vibrates across the grid, falls into
vibrating trays and is carried to dryers. Blowers carry residual shell particles upward while
meats fall into trays to be carried to color sorters, visual inspection by operators, and then to
cold storage to await packaging.
The water supplied to the first and last floats is a chlorine solution having a residual of
75-140 mg/l. The second float receives only well water. The purpose of the chlorine is to
retard the growth of bacteria which might enter the system during routine operator
functions around the float.
Waste water from the washing, soaking, and flotation operations is combined in a
single sewer and conveyed to the treatment site. Pretreatment consists of screening by a 60
mesh screen in lieu of primary sedimentation. Treatment itself is provided by a completely
mixed activated sludge reactor utilizing a mechanical surface aerator. The reactor does not
employ sludge recycling. The detention time of the waste in this reactor is approximately
five days.
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