Principles of Food Sanitation

●Grease and solid materials from plant

and animal products frequently have a

good market value. Demand from soap

plants, feed plants, and other industries

can make a recovery of waste solids a

profitable operation. Such operations

also reduce the amount of wastewater

treatment.

●If municipal charges and surcharges are

high, additional pretreatment can be eco-

nomically advantageous because better

pretreatment will reduce these charges.

●Municipality complaints can be reduced

through additional treatment responsi-

bilities assumed by the food processor.

The following disadvantages can discour-
age pretreatment of wastewater:

●Pretreatment facilities are expensive and

increase the complexity of the process-

ing operation.

●Maintenance costs, monitoring costs,

and record keeping of a wastewater

treatment operation can be expensive.

●Pretreatment facilities are placed on the

property tax roll unless state regulations

permit tax-free waste treatment.

If pretreatment is conducted, this process
should be based on facts revealed from the
waste disposal survey. Results from the plant
survey and review of viable waste conserva-
tion and water reuse systems are essential for
identification, design, and cost estimates of a
pretreatment system. Cost estimates should
include those parts of the pretreatment
attributable to flow, such as dissolved air
flotation and grease basins. Thus, major in-
plant expenses for waste conservation and
water recycling can be determined based on
the estimated reduction in flow, BOD, sus-
pended solids, and grease.
Most common pretreatment processes
include flow equalization and the separation
of floatable matter and SS. Separation is fre-

quently increased by the addition of lime

and alum, ferric chloride (FeCl 3 ), or a

selected polymer. Paddle flocculation may

follow alum and lime, and lime or ferric chlo-

ride additions, to assist in coagulation of the

suspended solids. Separation is usually

accomplished by gravity or by air flotation.

Screening by vibrating, rotary, or static-type

screens is a step that precedes the separation

process and concentrates the separated float-

ables and settled solids.

Flow Equalization

Flow equalization and neutralization are

used to reduce hydraulic loading in the

waste stream. Facilities required are a hold-

ing device and pumping equipment designed

to reduce the fluctuation of effluent dis-

charge. This operation can be economically

advantageous, whether processing firms

treat their own wastewater or discharge into

a municipal sewage treatment facility after

pretreatment. An equalizing tank has the

capacity to store wastewater for recycling

or reuse, or to feed the flow uniformly to

the treatment facility day and night. This

unit is characterized by a varying flow into

and a constant flow from the tank. Equaliz-

ing tanks can be lagoons, steel construc-

tion tanks, or concrete tanks, often without

a cover. It is important to integrate the dis-

card flow of the process to the normal capac-

ity of the treatment equipment that has been

installed.

Screening

The most frequently used process for pre-

treatment is screening, which normally

employs vibrating screens, static screens, or a

rotary screen. Vibrating and rotary screens

are more frequently used because they can

permit pretreatment of a larger quantity of

wastewater that contains more organic mat-

ter. These screening devices are well adapted

to a flow-away (water in forward flow and

Waste Product Handling 221