Environmental Engineering Reference
In-Depth Information
In processing plants that deal with animal tissue,
flotation cells
assist with the separation of
fats and oils from the wastewater. These cells operate by passing air bubbles through the water,
which make fats and oils to rise to the surface where they can be skimmed off.
Centrifugation
is used primarily for the separation of fats, oils, and particles from process water.
Evaporation
is employed for the separation and recovery of dissolved compounds such as proteins and
inorganic salts. Evaporation is seen frequently in the meat, poultry, and fish industries for the
separation of soluble protein from wastewater. In terms of operational costs, evaporation is the
most expensive separation technique in comparison to others because the high energy require-
ments as a result of phase change during the process.
Filtration
is used for the separation of
particles and insoluble compounds that are in suspension or as a slurry. For particles prone to
sink,
sedimentation
units provide the residence time necessary by the particles to sink to the
bottom of the tank by gravity.
Separation by membranes
In addition to conventional technologies,
membranes
allow the separation of solids from
wastewater streams effectively and with less energy requirements than other methods.
With the right combination of microfiltration, ultrafiltration, nanofiltration, and reverse
osmosis, membranes can clean virtually any water stream coming from food-processing
plants to the point that it can be reused in certain parts of the process, which is contingent
upon regulations.
Microfiltration membranes separate colloidal and suspended particles from the water
streams to diameters down to 0.05 microns. Ultrafiltration is used to separate large molecules
such as proteins and starch in a typical molecular weight range between 3,000 and 100,000
Daltons. Nanofiltration separates sugars, color compounds, and polyvalent ions in a range
between 100 and 1,000 Daltons. Reverse osmosis is capable of removing monovalent ions and
small organic molecules such as alcohols (“Ultrafiltration, nanofiltration, microfiltration, and
reverse osmosis for liquid separation,” n.d.).
A typical example of treatment of a liquid waste stream by membrane filtration is in the
cheese industry. Whey is the liquid that remains after milk has been curded and contains
mostly soluble proteins, lactose, minerals, and fat. A cascade system with membranes can
separate the different fractions and generate at the end a virtually pure water stream. The first
step is a microfiltration unit that separates fat globules and large protein aggregates and lets
permeate all the other compounds. Secondly, an ultrafiltration step fractionates the protein
compounds that are then used to produce whey protein concentrates. Then, a nanofiltration
membrane separates the lactose and all other solutes except monovalent charged ions. Finally,
a reverse osmosis step retains all the solids except water and compounds that mimic water
structure (“Dairy Applications,” n.d.).
Membrane separation is not only applicable to reducing the solid content of waste streams
but also in processing to replace or to make more efficient some energy intensive processes.
Reverse osmosis has proven effective to preconcentrate juices before an evaporator, thus
reducing thermal energy consumption at the evaporation step. Among other applications,
membranes can be used in the clarification and concentration of fruit and vegetable juices; in
the purification, concentration, and ash removal during gelatin production; in bacteria removal
from brines; in the removal of color, impurities, and salt of ethnic foods; in the clarification
of beer; in alcohol removal from wines and in egg white and whole egg concentration; in
sugar industry for the removal of waxes, dextrans, and gums before evaporation; in several
operations in the starch and sweetener industry, such as clarification of syrups, concentration
of starch, dextrose enrichment, and depyrogenation of dextrose syrup (“Food and Beverage
Applications,” n.d.).
