2-13. ION EXCHANGE
a. Types of Processes. Ion exchange operations are either batch or
continuous type. In the batch process, the reaction is confined to the contents of one
vessel at a time. In the continuous type, there is a constant feed and a constant drain.
In the batch process, the resin is stirred with the water to be treated until the equilibrium
reaction is complete. Used resin is removed by settling. In the continuous process, the
exchange material is placed in a bed or column and the water is passed through it.
b. Resins. The extent of removal of the ions in the water depends upon the
chemical equilibrium state between the ions in the liquid phase and those in the solid
resin material. Both natural and man-made ion-bearing resins are available.
Regenerant and separate restorant chemicals are used to remove the undesirable
inorganic ions and organic materials from the used resin in order to prepare the resin for
Section IV. SLUDGE
2-14. SLUDGE DIGESTION
a. General. For each miIIion gallons of wastewater passing through a treatment
plant, 5 to 10 thousand gallons of sludge are collected In the sedimentation units. This
sludge is about 95 percent water and about 5 percent (50,000 ppm) highly putrescible
organic matter. The sludge collected in settling tanks and not stabilized by return to the
influent is treated in a separate sludge digester. In the digester, the sludge is changed
into readily disposable products with minimum interference with other plant operations.
Organic matter in the sludge furnishes food for anaerobic bacteria. The bacteria breaks
the sludge down (digests it) into simple, more stable substances. Under optimum
conditions, welI-digested sludge is produced in 24 to 26 days. The entire process may
take place in one tank or it may be divided into two stages. In two-stage digestion, the
relatively violent initial digestion is separated from the slower final period. After the
partial digestion in the primary tank, heavier sludge which has settled to the bottom is
pumped into the secondary tank where gas evolution and resultant mixing is relatively
b. Types of Digesters. Sludge digesters installed as separate treatment units
normally are closed structures of sufficient capacity to hold a quantity of sludge
equivalent to 30 to 40 days accumulation as calculated from the number of people the
unit is expected to serve. Open digesters, such as hoppers and lower compartments of
lmhoff tanks, are not as efficient nor as free from odors as the closed digesters. Closed
digesters usually are constructed from concrete and steel and have a sloping bottom
and either a fixed or a movable top. Digesters of more than one type may be operated
at the same treatment plant. Two or more tanks provide flexibility of operation. They
can be installed independently as single-stage digesters in parallel or be installed as
two-stage digesters in series with one tank a primary and the other a secondary unit.