| (a) General policy. Facilities for disinfection shall
be provided to protect the public health and as an aid to plant operation.
(b) Chlorination facilities.
(1) Chlorination equipment. Chlorination equipment
shall be selected and installed which is capable of applying desired
amounts of chlorine continuously to the effluent. Chlorination equipment
may also be installed to control odors and generally assist treatment.
To accomplish these objectives, points of chlorine application may
be established at the head of the plant for prechlorination, in the
effluent chlorine contact chamber, or other suitable locations.
(A) Capacity. Chlorination equipment shall have a capacity
greater than the highest expected dosage to be applied. Chlorination
systems shall be capable of operating under all design hydraulic conditions.
Duplicate equipment with automatic switchover should be considered
for standby service, so that continuous chlorination can be provided.
(B) Controls. Means for automatic proportioning of
the chlorine amount to be applied in accordance with the rate of effluent
being treated is encouraged for all plants and may be required if
a maximum chlorine residual is required in the applicable discharge
permit. Manual control will be permitted where the rate of effluent
flow is relatively constant and for prechlorination applications.
Consideration shall also be given to controlling chlorine feed by
use of demand.
(C) Measurements. A scale for determining the amount
of chlorine used daily, as well as the amount of chlorine remaining
in the container, shall be provided.
(D) Safety equipment. Self-contained breathing apparatus
shall be available for use by plant personnel. The equipment should
be located at a safe distance from the chlorine facilities to insure
accessibility. Self-contained breathing apparatus shall be located
outside the entrance to the chlorine facility.
(E) Housing. Housing of chlorination equipment and
cylinders of chlorine shall be in separate rooms above ground level,
with the door opening to the outside, as a measure of safety. Doors
should be equipped with panic hardware. The chlorination room should
be separated from other rooms by gas-tight partitions and should be
equipped with a clear glass, gas-tight window which permits the chlorinator
to be viewed without entering the room. Forced mechanical ventilation
shall be included in chlorination rooms which will provide a complete
air change a minimum of every three minutes. The exhaust equipment
should be automatically activated by external light switches and gas
detectors that are provided with contact closures or relays. No other
equipment shall be installed or stored in the chlorinator room. Vents
from chlorinators, vaporizers, and pressure reducing values should
be piped to the outdoors at a point not frequented by personnel, nor
near a fresh air intake. Detectors and alarms should be located in
each area containing chlorine gas under pressure. If gas withdrawal
chlorine storage cylinders are subjected to direct sun, pressure reducing
devices must be provided at the cylinders. Fire protection devices
and fireproof construction is required for all chlorine storage areas.
Electrical controls in chlorine facilities must be replaceable or
protected against corrosion. Separate, trapless floor drains or a
drain to an ample dilution point shall be provided from the chlorine
storage room and from liquid feed chlorinator rooms.
(F) Emergency chlorination. Emergency power should
be provided for chlorination facilities.
(G) Other. Chlorine rooms shall maintain a minimum
temperature of 65 degrees Fahrenheit. Chlorinate solution should be
prepared using treated effluent. If potable water is used, the potable
water supply system must be protected by an adequate backflow prevention
device. When a booster pump is required, duplicate equipment should
be provided.
(2) Pellets. The use of pellet systems will be considered
for approval on a case-by-case basis.
(3) Chlorine contact chamber design criteria.
(A) Initial mixing. Rapid initial mixing of the chlorine
solution and wastewater is essential for effective disinfection. Effective
initial mixing can be accomplished by applying the chlorine solution
in a highly turbulent flow regime created by in-line diffusers, submerged
hydraulic structures, mechanical mixers, or jet mixers. The mean velocity
gradient in the area of turbulent flow, or G value, shall exceed 500
sec.-1 with residence times of three to 15 seconds. Calculations supporting
the design G value shall be presented in the engineering report. Mixing
devices for which the mean velocity gradient is difficult to verify
shall be justified by pilot or full-scale performance data.
(B) Contact time. Contact chambers shall be designed
to provide a minimum average hydraulic residence time (chamber volume
divided by flow) of 20 minutes at the design peak hydraulic flow.
(C) Contact chamber configuration. Pipe contact chambers
shall be sized so that a scour velocity of at least one foot per second
will be obtained at the existing maximum daily dry weather flow rate.
If adequate initial mixing is not provided, contact chambers shall
have a flow pathway length-to-width ratio of at least 40 and a maximum
depth-to-width ratio of no greater than 1.0. This length-to-width
ratio may be accomplished by baffling.
(D) Sludge and scum removal. Contact chambers shall
either be provided with a means to remove sludge and scum, such as
a small hydraulic dredge and skimmers, without taking the contact
tank out of service, or shall be configured so that one-half of the
contact chamber can be drained for cleaning without interrupting flow
through the other half.
(c) Other means of disinfection.
(1) Chemical disinfection is not normally required
when the total residence time in the wastewater treatment system (based
on design flow) is at least 21 days.
(2) Ultraviolet light (UV) disinfection.
(A) General. Ultraviolet disinfection systems are considered
applicable to treated wastewaters with daily average five-day biochemical
oxygen demand (BOD5 ) and total suspended
solids (TSS) concentrations consistently less than 20 milligrams per
liter (mg/liter).
(B) Definitions.
(i) Ultraviolet module--A grouping of UV germicidal
lamps of a specified arc length in a quartz or teflon sleeve, sealed
and supported in a single stainless steel or some other noncorrosive
frame.
(ii) Ultraviolet bank--A grouping of UV modules which
span the entire width and depth (of flow) of the reactor.
(C) Sizing, configuration, and required dosage. Ultraviolet
disinfection units will be designed in accordance with methodologies
presented in the United States Environmental Protection Agency Design
Manual, Municipal Disinfection, EPA/625/1-86/021. Turbulent flow is
necessary due to non-uniform intensity fields in an ultraviolet reactor.
The proposed design shall have a Reynolds' number of greater than
6,000 at average design flows. Disinfection systems shall consist
of a minimum of two ultraviolet banks in series and shall be capable
of providing disinfection to permitted fecal coliform levels at the
design daily average flow with the largest bank out of service.
(D) System details. The ultraviolet unit shall be configured
so that there is adequate space for the removal and maintenance of
lamps. One person should be able to replace lamps without the aid
of mechanical lifting devices, special tools, or equipment. Drains
shall be provided to completely drain the ultraviolet reactor unless
the equipment can be easily removed from the effluent channel, but
lamps shall be replaceable without draining the unit. The materials
used to construct the reactor shall be resistant to ultraviolet light.
Ballasts and other electrical components shall be consistent with
the ultraviolet lamp manufacturer's recommendations. Temporary screens
shall be installed to protect the lamps and other fragile components
from construction debris.
(E) Controls. Each individual ultraviolet lamp shall
be provided with a remote operation indicator. Lamp failure alarms
shall also be provided for a predetermined number of lamp failures.
Techniques that result in nonirradiated flow pathways are prohibited.
Each ultraviolet bank shall be equipped with at least one ultraviolet
intensity meter or some means to monitor changes in ultraviolet dosage;
however, intensity meters shall not be relied upon to automatically
control system operation. A flow control device, such as an automatic
level control, shall be provided to ensure that the lamps are submerged
in the effluent at all times regardless of flow rate. The automatic
level control shall be arranged so that it will allow suspended solids,
which may settle, to be washed out of the area of UV disinfection.
Proper heating and ventilation are critical to ultraviolet system
operation. Cabinets containing ballasts and or transformers shall
be provided with positive filtered air ventilation and automatic shutdown
alarms at high temperatures. Provisions shall also be made to maintain
the ultraviolet lamps at or near their optimum operating temperature
and to filter ventilating air so as to limit ultraviolet Cont'd... |
