Civil Engineering Reference
In-Depth Information
Operating Conditions
Suggested conditions for each of the operating phases are
shown in Table 18-3 for three different resins. The four operations (treatment, back-
wash, regeneration, and rinsing) are discussed below.
Treatment.
Treatment may be either upflow or downflow, although downflow treat-
ment is the most common in water treatment applications. The service flow rate will
vary between 2 and 5 gpm / cu ft, depending on the type of resin used (see Table
18-3) and the raw-water hardness. The length of the service cycle is a function of the
hardness concentration in the raw water and the exchange capacity of the resin. Man-
ufacturers typically express resin capacity in terms of kilograins of hardness (as
CaCO
3
) per cubic foot of resin. The conversion factors for hardness ions are listed in
Table 18-4.
Amberlite IR-120 Plus has an exchange capacity between 17.5 and 34.5 kilograins
of hardness (as CaCO
3
) per cubic foot of resin. The actual capacity depends upon the
regenerant level used. The more regenerant used per cubic foot of resin, the greater
the exchange capacity of the resin. However, the overall regenerant cost per unit of
hardness removed also increases. The relationship between regenerant level and
exchange capacity is shown in Table 18-5. In water treatment applications, 10 lb NaCl/
cu ft of resin represents a reasonable compromise between regenerant usage and re-
generation efficiency.
Pretreatment is required in many situations. Suspended solids should be low (zero
if possible), and turbidity should be less than 1 NTU, to prevent bed plugging. Ferrous
iron should be removed prior to ion-exchange treatment, as it can oxidize to the ferric
form within the bed if oxygen is present.
If water to be treated is a well water containing no dissolved oxygen, the resin can
effectively remove ferrous and manganous ions. Conversely, if oxygen is present, or
if iron or manganese is present in the oxidized form, it should be removed prior to
ion-exchange treatment, to avoid resin fouling. Chlorine should not be present in the
feedwater.
During conventional downflow treatment, there is a pressure drop across the resin,
which is a function of the service flow rate, water temperature, and amount of sus-
pended material in the raw water. When suspended matter is present, it is frequently
filtered out by the resin, thereby reducing the void volume in the upper portion of the
bed and causing increased resistance to flow. In extreme cases, resin particles can
shatter, producing fine particles that are lost during backwash. Although some sus-
pended material may be tolerated, it is wise to employ a resin bed as an ion-exchange
medium and not as a filtering mechanism.
Organic material in the feedwater can also result in resin fouling. The fouling is
caused by deposition within the resin, as well as by bacterial growth within the resin
bed.
Backwash.
Following completion of the treatment or operation cycle, the resin bed
should be backwashed for approximately 10 minutes at a rate that will cause a bed
expansion of between 50 and 75 percent. The goal of backwashing is to remove
material that was filtered out by the bed in its upper layers. If this material is not
removed during backwashing, it will lead to channeled water flow through the bed,
premature leakage of hardness, and reduced exchange capacity.
Regeneration.
Regenerant brine concentration has little effect on resin-exchange
capacity. For example, Amberlite IR-120 resin shows only a 7 percent decrease in
