Environmental Engineering Reference
In-Depth Information
•
Freshwater—Less than 11 ppb
total chlorine residual
•
Estuaries—Less that 7.5 ppb for
halogen-produced oxidants
•
Endangered species—Use of
chlorine is prohibited
9.2.4 Chlorination equipment
9.2.4.1 Hypochlorite Systems
Depending on the form of hypochlorite selected for use, special
equipment to control the addition of hypochlorite to the wastewater is
required. Liquid forms require the use of metering pumps, which can
deliver varying flows of hypochlorite solution. Dry chemicals require
the use of a feed system designed to provide variable doses of the form
used. The tablet form of hypochlorite requires the use of a tablet chlori-
nator designed specifically to provide the desired dose of chlorine. The
hypochlorite solution or dry feed systems dispenses the hypochlorite,
which is then mixed with the flow. The treated wastewater then enters
the contact tank to provide the required contact time.
9.2.4.2 Chlorine Systems
Because of the potential hazards associated with the use of chlo-
rine, the equipment requirements are significantly greater than those
associated with hypochlorite use. The system most widely used is a solu-
tion feed system. In this system, chlorine is removed from the container
at a flow rate controlled by a variable orifice. Water moving through the
chlorine injector creates a vacuum, which draws the chlorine gas to the
injector and mixes it with the water. The chlorine gas reacts with the
water to form hypochlorous and hydrochloric acid. The solution is then
piped to the chlorine contact tank and dispersed into the wastewater
through a diffuser. Larger facilities may withdraw the liquid form of
chlorine and use evaporators (heaters) to convert to the gas form. Small
facilities will normally draw the gas form of chlorine from the cylinder.
As the gas is withdrawn, the liquid will be converted to the gas form.
This requires heat energy and may result in chlorine line freeze-up if the
withdrawal rate exceeds the available energy levels.
9.2.5 Chlorination operation
In either type of system, normal operation requires adjustment of
feed rates to ensure that the required residual levels are maintained.
This normally requires chlorine residual testing and adjustment based
on the results of the test. Other activities include removal of accumu-
lated solids from the contact tank, collection of bacteriological samples
to evaluate process performance and maintenance of safety equipment
(e.g., respirator/air pack, safety lines). Hypochlorite operation may also
include make-up solution (solution feed systems) or adding powder or
pellets to the dry chemical feeder or tablets to the tablet chlorinator.
