Current through Register Vol. 50, No. 9, September 20, 2024
A.
Clarification is generally considered to consist of any process or combination
of processes which reduce the concentration of suspended matter in drinking
water prior to filtration.
B.
Plants designed to treat surface water, groundwater under the direct influence
of a surface water, or for the removal of a primary drinking water contaminant
shall have the ability to meet the plants average daily flow of the maximum
month with one unit out of service. Design of the clarification process shall:
1. be constructed to permit units to be taken
out of service without disrupting operation, and with drains or pumps sized to
allow dewatering in a reasonable period of time;
2. provide multiple-stage treatment
facilities when required by the state health officer; and
3. minimize hydraulic head losses between
units to allow future changes in processes without the need for
repumping.
C.
Presedimentation. Waters containing high turbidity may require pretreatment,
usually sedimentation, with or without the addition of coagulation chemicals.
1. Basin Design. Presedimentation basins
should have hopper bottoms or be equipped with continuous mechanical sludge
removal apparatus, and provide arrangements for dewatering.
2. Inlet. Incoming water shall be dispersed
across the full width of the line of travel as quickly as possible to prevent
short-circuiting.
3. Bypass.
Provisions for bypassing presedimentation basins shall be included.
4. Detention Time. Detention shall consider
removal requirements for the unit.
D. Coagulation. Coagulation refers to a
process using coagulant chemicals and mixing by which colloidal and suspended
material are destabilized and agglomerated into settleable or filterable flocs,
or both. The engineer shall submit the design basis for the velocity gradient
(G value) selected, considering the chemicals to be added and water
temperature, color and other related water quality parameters. For surface
water plants using direct or conventional filtration, the use of a primary
coagulant is required at all times.
1.
Mixing. The detention period should be instantaneous, but not longer than
thirty seconds with mixing equipment capable of imparting a minimum velocity
gradient (G) of at least 750 feet per second per feet (fps/ft). The design
engineer should determine the appropriate G value and detention time through
jar testing.
2. Equipment. Basins
should be equipped with devices capable of providing adequate mixing for all
treatment flow rates. Static mixing may be considered where the flow is
relatively constant and will be high enough to maintain the necessary
turbulence for complete chemical reactions.
3. Location. The coagulation and flocculation
basin shall be as close together as practical.
4. Flow shall be determined at the point of
coagulant dosing.
E.
Flocculation. Flocculation refers to a process to enhance agglomeration or
collection of smaller floc particles into larger, more easily settleable or
filterable particles through gentle stirring by hydraulic or mechanical means.
1. Basin Design. Inlet and outlet design
shall minimize short-circuiting and destruction of floc. Basins shall be
designed so that individual basins may be isolated without disrupting plant
operation. A drain and/or pumps shall be provided to handle dewatering and
sludge removal.
2. Detention.
Detention shall account for regulatory requirements for the plant.
3. Equipment. Agitators shall be designed to
provide variable peripheral speed of paddles ranging from 0.5 to 3.0 feet per
second.
4. Other Designs.
Variations or alternate designs can be submitted to the state health officer at
any time.
5. Piping. Flocculation
and sedimentation basins shall be as close together as practical. The velocity
of flocculated water through pipes or conduits to settling basins shall be no
less than 0.5 feet per second (fps) and no greater than 1.5 fps. Allowances
must be made to minimize turbulence at bends and changes in
direction.
F.
Sedimentation. Sedimentation refers to a process that allows particles to
settle by gravity and typically precedes filtration. The detention time for
effective clarification is dependent upon a number of factors related to basin
design and the nature of the raw water. The following criteria apply to the
design of conventional gravity sedimentation units.
1. A minimum of four hours of settling time
shall be provided. This may be reduced to two hours for lime-soda softening
facilities treating only groundwater. Reduced detention time may also be
approved when equivalent effective settling is demonstrated or when the
overflow rate is not more than 0.5 gallons per minute [gpm] per square foot
[sqft] (1.2 m/hr).
2. Inlet
Devices. Inlets shall be designed to distribute the water equally and at
uniform velocities. A baffle should be constructed across the basin close to
the inlet end and should project several feet below the water surface to
dissipate inlet velocities and provide uniform flows across the
basin.
3. If flow is split, a means
of measuring the flow to each train or unit shall be provided.
4. Velocity. The velocity through a
sedimentation basin should not exceed 0.5 feet per minute. The basins shall be
designed to minimize short-circuiting. Fixed or adjustable baffles shall be
provided as necessary to achieve the maximum potential for
clarification.
5. If flow is split,
it is recommended that a means of modifying the flow to each train or unit be
provided.
6. Outlet Devices. Outlet
weirs or submerged orifices shall maintain velocities suitable for settling in
the basin and minimize short-circuiting. The use of submerged orifices is
recommended in order to provide a volume above the orifices for storage when
there are fluctuations in flow. Outlet weirs and submerged orifices shall be
designed as follows.
a. The rate of flow over
the outlet weirs or through the submerged orifices shall not exceed 20,000
gallons per day per foot (250 m3/day/m) of the outlet launder or orifice
circumference.
b. Submerged
orifices located greater than three feet below the flow line shall be
justified.
c. The entrance velocity
through the submerged orifices shall not exceed 0.5 feet per second.
7. Overflow. An overflow weir or
pipe designed to establish the maximum water level desired on top of the
filters shall be provided. The overflow shall discharge by gravity with a free
fall. The discharge shall be equipped with monitoring equipment to annunciate
the overflow or be installed at a location where the discharge can be
observed.
8. Drainage.
Sedimentation basins shall be provided with a means for dewatering. Basin
bottoms shall slope toward the drain where mechanical sludge collection
equipment is not required.
9.
Flushing lines or hydrants shall be provided and shall be equipped with
backflow prevention devices acceptable to the state health officer.
10. Sludge collection system shall be
designed to ensure the collection of sludge from throughout the
basin.
11. Sludge removal design
shall provide that:
a. sludge pipes shall be
not less than three inches in diameter and arranged to facilitate
cleaning;
b. entrance to sludge
withdrawal piping shall prevent clogging;
c. valves shall be operable from outside the
tank;
d. the operator can observe
and sample sludge being withdrawn from the unit.
G. Solids Contact Unit. Plants
designed to treat surface water, groundwater under the direct influence of
surface water or required to meet primary drinking water standards using solids
contact shall have a minimum of two units. The clarifiers shall be designed for
the average daily flow of the maximum month such that the plants design
capacity can be met with one unit out of service.
1. Operating equipment shall include:
a. adequate piping with suitable sampling
taps or other means to sample sludge located to permit the collection of
samples from various depths of the units; and
b. if flow is split, a means of measuring and
modifying the flow to each unit.
2. Consideration shall be given to chemical
feed location to ensure proper dosing and application.
3. A rapid mix device or chamber ahead of
solids contact units may be required by the state health officer to assure
proper mixing of the chemicals applied. Mixing devices within the unit shall be
constructed to:
a. provide good mixing of the
raw water with previously formed sludge particles; and
b. prevent deposition of solids in the mixing
zone.
4. Flocculation.
Flocculation equipment:
a. shall be adjustable
(speed and/or pitch);
b. shall
provide for coagulation in a separate chamber or baffled zone within the
unit;
c. should provide a
flocculation and mixing period of at least 30 minutes.
5. Sludge Concentrators. Large basins should
have at least two sumps for collecting sludge located in the central
flocculation zone.
6. Sludge
removal design shall provide that:
a. sludge
pipes are not less than three inches in diameter and so arranged as to
facilitate cleaning;
b. entrance to
sludge withdrawal piping shall prevent clogging;
c. valves shall be located outside the tank
for accessibility, and
d. the
operator may observe and sample sludge being withdrawn from the unit.
7. Criteria for backflow
protection from cross-connections shall be as follows.
a. Blow-off outlets and drains shall
terminate in a location with an acceptable air gap for backflow
protection.
b. A backflow
prevention device shall be included on potable water lines used to back flush
sludge lines.
8.
Detention Period. The detention time shall be established on the basis of the
raw water characteristics, regulatory requirements and other local conditions
that affect the operation of the unit.
9. Water Losses. Units shall be provided with
controls to allow for adjusting the rate or frequency of sludge
withdrawal.
10. Weirs or orifices.
The units should be equipped with either overflow weirs or orifices constructed
so that water at the surface of the unit does not travel over 10 feet
horizontally to the collection trough.
a.
Weirs shall be adjustable, and at least equivalent in length to the perimeter
of the tank.
b. Weir loading shall
not exceed:
i. 10 gpm per foot of weir length
(120 L/min/m) for clarifiers;
ii.
20 gpm per foot of weir length (240 L/min/m) for softeners.
c. Where orifices are used the
loading rates per foot of launder rates should be equivalent to weir loadings.
Either shall produce uniform rising rates over the entire area of the
tank.
11. Upflow Rates.
Unless supporting data is submitted to the State Health Officer to justify
rates exceeding the following, rates shall not exceed:
a. 1.0 gpm/sqft (2.4 m/hr) at the sludge
separation line for units used for clarifiers;
b. 1.75 gpm/sqft (4.2 m/hr) at the slurry
separation line, for units used for softeners.
H. Tube or Plate Settlers. Settler units
consisting of variously shaped tubes or plates which are installed in multiple
layers and at an angle to the flow may be used for sedimentation, following
flocculation. Proposals for settler unit clarification must demonstrate
satisfactory performance under on-site pilot plant conditions or documentation
of full scale plant operation with similar raw water quality conditions as
allowed by the state health officer prior to the preparation of final plans and
specifications for approval.
1. General
design criteria for tube or plate settlers is as follows.
a. Inlet and Outlet Considerations. Design to
maintain velocities suitable for settling in the basin and to minimize
short-circuiting. Plate units shall be designed to minimize maldistribution
across the units.
b. Protection
from Freezing. In areas where freezing occurs, consideration shall be given
regarding sufficient freeboard.
c.
Application Rate for Tubes. A maximum rate of 2 gallon per minute per square
foot [gpm/sqft] of cross-sectional area (4.8 m/hr) for tube settlers, unless
higher rates are successfully shown through pilot plant or in-plant
demonstration studies.
d.
Application Rates for Plates. A maximum plate loading rate of 0.5 gpm/sqft (1.2
m/hr), based on 80 percent of the projected horizontal plate area.
e. Flushing lines shall be provided to
facilitate maintenance and must be properly protected against backflow or back
siphonage.
f. Drain piping from the
settler units shall be sized to facilitate a quick flush of the settler units
and to prevent flooding other portions of the plant.
g. Placement. Modules shall be placed:
i. in zones of stable hydraulic conditions;
and
ii. in areas nearest effluent
launders for basins not completely covered by the modules.
h. Inlets and outlets shall conform to §175.
F.2 and §175. F.6 of this Part.
i.
The support system shall be able to carry the weight of the modules when the
basin is drained plus any additional weight to support maintenance.
j. Provisions shall be made to allow the
water level to be dropped, and a water or air jet system for cleaning the
modules.
I.
High Rate Clarification Processes. High rate clarification processes may be
approved upon demonstrating satisfactory performance under on-site pilot plant
conditions or documentation of full scale plant operation with similar raw
water quality conditions as allowed by the state health officer.
AUTHORITY NOTE:
Promulgated in accordance with the provisions of
R.S.
40:4.A.(8), 40:4.13.D.(1)(2) and
40:5.A.(2)(3)(5)(6)(7)(17).
