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Design Considerations When Adding
Chemicals To Water Systems
The following discussion is intended to identify various considerations
when installing chemical feed equipment. Installation of a chemical feed
system to a public water system also requires operation by a certified
treatment operator for community and non-transient non-community systems.
General Considerations
1. The rate of chemical feed must
be precisely proportioned to the production flow of each well(s).
a. Thus where there are multiple wells a separate chemical
feed pump is needed or the chemical feed pump must be of the type that
automatically adjusts to the rate of flow.
b. Ideally the chemical feed pump system would be paced by continually
measuring one or more water quality parameters and adjusting the chemical
feed rate accordingly.
2. The piping materials must be compatible with the chemical types and
strengths being used. Clearly label all chemical storage tanks, connecting
piping, and fill spouts.
3. There should be sufficient
field or laboratory monitoring equipment available to allow the accurate
determination of the concentration of chemicals in the treated water.
4. Add oxidizer type chemicals
(oxygen, chlorine, KMnO4) noticeable after such additives as
polyphosphates (for iron and manganese sequestering).
5. Where feeding chemicals
into water with a very low ambient pressure (0-20 psi), a backpressure
control valve is necessary to ensure that the discharge check valve on
the chemical feed pump has sufficient minimum pressure to work against.
6. Pulsation dampers
are available that provide for more even flow of the chemicals than from
a chemical feed pump.
7. The operator should develop
a record keeping system that allows the amount of chemicals actually used
to be compared to the value predicted based on the residual measurements
and the volume of water used.
8. Obtain and follow the
manufacture 's
instructions for the maintenance of the chemical feed pump.
Chemical Makeup Solution
- Replacing the Chemicals That Have Been Used Up
1. Have proper safety
equipment for personnel. This equipment should be matched to the danger
level of the chemicals being used. Examples include:
Rubber groves and
apron;
Dust and fumes control;
Self contained air breathing
apparatus (SCABA); or
Water spray eye wash.
2. A day tank is
a small volume tank that allows more careful tracking of the amount of
chemicals used each day or each week. A day tank should be install between
large bulk storage tanks and chemical feed pump(s).
3. When using dusty chemicals,
provide a dedicated room for chemical mix. Install a vacuum fan in the
area to remove dust. The fan may need special design considerations concerning
explosive condition and corrosive atmospheres. Purchase chemical handling
equipment that controls dust, include covered storage bin; air suction
vents on mixing equipment.
4. Some chemicals, such as
soda ash, dissolve slowly in water. To improve this situation the makeup
water could be heated before reaching the chemical mix activity.
5. Any liquid chemical should
be stored within a "containment area" that is water tight and has sufficient
volume to handle any possible spill volume.
6. The foot valve in the
chemical feed tank should be 3-6 inch off the bottom of day tank.
7. Provide a strainer
on suction side of the chemical feed pump.
The Chemical Injection System
1. Always inject
chemicals on the discharge side of a pump. Where there is an atmospheric
(non-pressurized) storage tank, it may be preferred to inject chemicals
into the feed line before that tank. This allows better chemical mixing
and lowers the detention time necessary for the chemical reaction(s) to
occur.
2. Mounting location. Attached
chemical feed pumps to a heavy object and rigid surfaces to prevent vibration.
Mounted the chemical feed pump above the floor. The area must have adequate
heat, light and ventilation.
3. Install unions
to facilitate repairs and replacement on the chemical feed tubing.
4. Duplicate chemical
feed pumps are required when the chemical being added is in response to
an acute contaminant; for example the addition of chlorine to kill bacterial.
Duplication is optional for non acute treatment such as corrosion control.
5. The discharge line from
a chemical feed pump requires an anti-siphon valve.
6. Mineral deposits often
occur at the injection point on the water main when feeding chemicals.
Good design could include:
a. The injection
nozzle should be easy to disassemble so as to allow periodic cleaning.
b. Consider having the ability
to feed fresh water throughout the entire chemical feed system so that
any chemical deposits can be dissolved and then flushed away. This flushing
system would be manually activated.
c. Develop a formal maintenance
schedule for all chemicals.
7. It is helpful to be able
to identify precisely the amount of chemicals being feed by chemical feed
pumps. To achieve this, a "calibration
tube
"
should be installed. This tube would be isolated by gate valves.
8. Air bubbles sometimes
occur in either the suction or discharge line of a chemical feed pump.
The following can minimize this condition.
a. Use a chemical
day tank that has a bottom discharge connection. Elevate this tank above
the chemical feed pump to maintain positive pressure on the suction side
of the pump.
b. Have the discharge line
of the chemical feed line grade continuously upward to the injection point
in water main. If this is not possible, have a
"tee
"
fitting and an accumulation air space, on the side branch, with a manually
operated air release valve at the high point of the chemical feed discharge
line.
FOR ADDITIONAL INFORMATION
For additional information
please call the DES's
Water Supply Engineering Bureau at 271-3139. For a complete list of fact
sheets please see WD-WSEB-15-2. We would appreciate your comments concerning
this document. Drinking water fact sheets are available through the DES
web site at: http://www.des.state.nh.us then select: publications,
fact sheets, water division, water supply.
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