and volumes of products processed in the
plant vary widely with the season of year, as
is typical in many food plants, especially fruit
and vegetable processing plants. These steps
must be part of the survey: determination of
the water balance, sampling of wastewater,
and determination of extent of pollution.
Determination of Water Balance
Wastewater volume and flow rates from all
sources should be measured through meters
placed on all incoming water lines. Suitable
measuring devices are Parshall flumes, rec-
tangular and triangular weirs, and Venturi
tubes and orifices. Through calculation of
the water balance for an entire plant, the
quantity of water in the waste effluent,
together with the quantities lost through
steam leaks, evaporation, and other losses,
and the amounts used in the products of the
plant in a given period of time can be deter-
mined. All of these quantities should equal
the amount of water supplied to the plant
during a given period. This calculation can be
used to identify hidden water losses or major
leaks, which can affect the sanitation pro-
gram and cause increased waste, additional
effluent discharge, and reduced profits.
Sampling of Wastewater
Samples of the wastes should be obtained in
proportion to flow rates. Random “grab”
samples—taken by collecting a given quantity
of the effluent discharge in a container without
consideration of variations in volumes and
flow rates and changes in plant operations—
are of limited value for determining the true
characteristics of wastes and can provide mis-
leading results. Statistical sampling at planned
times during the operating and non-operating
periods, and in proportion to flow, can provide
valid data related to the characteristics of the
wastewater effluent from a plant.
The sampling device should be located in
the wastewater discharge system to obtain a
representative sample. Samples should be col-
lected where wastes are homogeneous-
perhaps below a weir or flume. Caution
should be exercised to avoid sampling errors
resulting from a deposition of solids upstream
from a weir or from accumulation of grease
immediately downstream. The sample should
be collected near the center of the channel
and at 20 to 30% of the depth below the sur-
face, where the velocity is sufficient to pre-
vent deposition of solids. Sewers, and deep,
narrow channels should be sampled at 33%
of the water depth from the bottom to the
surface, with the collection point rotated
across wide channels. During sample collec-
tion and handling, agitation should be
avoided for dissolved oxygen determination.
Food plant wastes readily decompose at
room temperature; thus, it is important to
chill samples promptly to 0 to 5ºC if they are
not analyzed immediately after sampling.Determination of Extent of Pollution
A large percentage of the waste discharged
in fruit and vegetable waters, wash water
from animal slaughter, and cleanup water
discharge are, product pieces (larger pieces
can be removed by screening). Finer solids,
which pass through a screen, and organic
matter in colloidal and true solution usually
have an oxygen demand in excess of the dis-
solved oxygen content of the water.
Biochemical Oxygen Demand. A fre-
quently used method of measuring pollution
strength is the 5-day BOD test. The BOD of
sewage, sewage effluents, and waters of
industrial wastes is the oxygen (in ppm)
required during stabilization of the decom-
posable organic matter by action of aerobic
microorganisms. The sample is stored in an
airtight container for a specified period of
time and temperature. Complete stabiliza-
tion can require more than 100 days at 20ºC.
Because such long periods of incubation are
impractical for routine determinations, theWaste Product Handling 215