Environmental Engineering Reference
In-Depth Information
•
Chorine may be applied as a gas, as a solid, or in liquid
hypochlorite
form.
•
Chorine is a very reactive substance. It has the potential to react
with many different chemicals (including ammonia), as well as with
organic matter. When chlorine is added to wastewater, several reac-
tions occur:
1. Chlorine will react with any reducing agent (e.g., sulfide, nitrite,
iron, thiosulfate) present in the wastewater. These reactions are
known as
chlorine demand.
The chlorine used for these reac-
tions is not available for disinfection.
2. Chlorine also reacts with organic compounds and ammonia com-
pounds to form chlororganics and chloramines. Chloramines are
part of the group of chlorine compounds that have disinfecting
properties and show up as part of the chlorine residual test.
3. After all of the chlorine demands are met, the addition of more
chlorine will produce free chlorine residual. Producing free
chlorine residual in wastewater requires very large additions of
chlorine.
9.2.2.2 Hypochlorite Facts
hypochlorite
is relatively safe to work with, although some minor
hazards are associated with its use (skin irritation, nose irritation, and
burning eyes). It is normally available in dry form as a white powder, pel-
let, or tablet or in liquid form. It can be added directly using a dry chemi-
cal feeder or dissolved and fed as a solution.
Note:
In most wastewater treatment systems, disinfection is accom-
plished by means of combined residual.
9.2.3 Wastewater Chlorination Process description
A very reactive substance, chlorine is added
to wastewater to satisfy all chemical demands—
that is, to react with certain chemicals (such as
sulfide, sulfite, or ferrous iron). When these ini-
tial chemical demands have been satisfied, chlo-
rine will react with substances such as ammonia
to produce chloramines and other substances
that, although not as effective as chlorine, have disinfecting capabil-
ity. This produces a combined residual, which can be measured using
chlorine residual test methods. If additional chlorine is added, free chlo-
rine residual can be produced. Due to the chemicals normally found
in wastewater, chlorine residuals are normally combined rather than
free residuals. Control of the disinfection process is normally based on
maintaining total chlorine residual of at least 1.0 mg/L for a contact
time of at least 30 minutes at design flow. Based on water quality stan-
dards, total residual limitations on chlorine are:
Key Point:
Residual level, contact time,
and effluent quality affect disinfection.
Failure to maintain the desired residual
levels for the required contact time will
result in lower efficiency and increased
probability that disease organisms will
be discharged.
