Louisiana Administrative Code › Title 51 - PUBLIC HEALTH-SANITARY CODE › Part XII - Water Supplies › Chapter 1 - General › Subchapter D - Treatment › Section XII-177 - Filtration Design

Current through Register Vol. 50, No. 6, June 20, 2024
A.
				  Rapid Rate Gravity Filters. General design criteria for rapid rate gravity
				  filters is as follows.
1. Pretreatment. The
				  use of rapid rate gravity filters shall require pretreatment.
2. Rate of Filtration. The rate of filtration
				  shall be determined through consideration of such factors as raw water quality,
				  degree of pretreatment provided, filter media, water quality control
				  parameters, and competency of operating personnel. Typical filtration rates
				  range from 2 to 4 gpm/sqft. Maximum filtration rates for plants treating
				  surface waters or ground water under the influence of surface water shall not
				  exceed 3.0 gpm/sqft. For surface water treatment plants or GUISW with proposed
				  filtration rates above 3.0 gpm/sqft, data from pilot testing shall be submitted
				  to the state health officer for consideration and approval.
3. Number. Plants employing rapid rate
				  gravity filters shall provide at least two filter units. The filters shall be
				  capable of meeting the plant design capacity at the plants average daily flow
				  of the maximum month with one filter unit removed from service. Where declining
				  rate filtration is provided, the variable aspect of filtration rates, and the
				  number of filters must be considered when determining the design capacity for
				  the filters.
4. Structural Details
				  and Hydraulics. The filter structure shall be designed to provide for:
a. vertical walls within the
				  filter;
b. no protrusion of the
				  filter walls into the filter media;
c. head room to permit normal inspection and
				  operation;
d. minimum depth of
				  filter box of 8.5 feet;
e. minimum
				  water depth over the surface of the filter media of three feet;
f. trapped effluent to prevent backflow of
				  air to the bottom of the filters;
g. prevention of floor drainage in to the
				  filter;
h. prevention of flooding
				  by providing overflow;
i. maximum
				  velocity of treated water in pipe and conduits to filters of two feet per
				  second;
j. cleanouts and straight
				  alignment for influent pipes or conduits where solids loading is heavy, or
				  following lime-soda softening;
k.
				  washwater drain capacity to carry maximum flow;
l. handrails or walls around filter banks
				  adjacent to normal walkways; and
m.
				  construction to prevent cross connections and common walls between potable and
				  non-potable water.
5.
				  Washwater troughs should be constructed to have:
a. the bottom elevation above the maximum
				  level of expanded media during washing;
b. a two-inch freeboard at the maximum rate
				  of wash;
c. the top edge level and
				  all at the same elevation;
d.
				  spacing so that each trough serves the same number of square feet of filter
				  area;
e. maximum horizontal travel
				  of suspended particles to reach the trough not to exceed three feet;
f. means to exclude the loss of media when
				  providing for concurrent air/high rate water backwashing; and
g. a two-inch freeboard at the main wash
				  water gullet at the maximum rate of wash.
6. Filter Material. The granular filter media
				  shall be in accordance with AWWA B100 and have the following characteristics:
a. a total depth of not less than 24 inches
				  and generally not more than 30 inches;
b. a uniformity coefficient of the smallest
				  material not greater than 1.65;
c.
				  a minimum of 12 inches of media with an effective size range no greater than
				  0.45 mm to 0.55 mm unless specified otherwise per the following.
i. Anthracite shall have:
(a). an effective size of 0.45 mm - 0.55 mm
				  with uniformity coefficient not greater than 1.65 when used alone;
(b). an effective size of 0.8 mm - 1.2 mm
				  with a uniformity coefficient not greater than 1.7 when used as a cap;
				  and
(c). an effective size for
				  anthracite used as a single media on potable groundwater for iron and manganese
				  removal only shall be a maximum of 0.8 mm (effective sizes greater than 0.8 mm
				  may be approved based upon onsite pilot plant studies or other demonstration
				  acceptable to the state health officer).
ii. Sand shall have:
(a). an effective size of 0.45 mm to 0.55 mm;
				  and.
(b). a uniformity coefficient
				  of not greater than 1.65.
iii. High density sand shall have:
(a). an effective size of 0.2 to 0.3
				  mm;
(b). a uniformity coefficient
				  of not greater than 1.65.
iv. Granular activated carbon (GAC) shall be
				  in accordance with AWWA B604 and the design of shall meet the following:
(a). There shall be provisions for a free
				  chlorine residual and adequate contact time in the water following the filters
				  and prior to distribution (See §177.C and §177 D)
(b). There shall be means for periodic
				  treatment of filter material for control of bacterial and other
				  growth.
(c). Provisions shall be
				  made for frequent replacement or regeneration. Regeneration of GAC shall be in
				  accordance with AWWA B604.
v. Other Media. Other media will be
				  considered based on experimental data and operating
				  experience.
d.
				  Characteristics of support media shall include the following.
i. Torpedo Sand. A three-inch layer of
				  torpedo sand shall be used as a supporting media for filter sand where
				  supporting gravel is used, and shall have:
(a). effective size of 0.8 mm to 2.0 mm;
				  and
(b). uniformity coefficient not
				  greater than 1.7.
ii.
				  Gravel, when used as the supporting media shall consist of cleaned and washed,
				  hard, durable, rounded silica particles and shall not include flat or elongated
				  particles. The coarsest gravel shall be 2.5 inches in size when the gravel
				  rests directly on a lateral system, and shall extend above the top of the
				  perforated laterals. Not less than four layers of gravel shall be provided in
				  accordance with the following size and depth distribution.
Table 175.A.6.e
Size
Depth
3/32 to 3/16 inches
2 to 3 inches
3/16 to 1/2 inches
2 to 3 inches
1/2 to 3/4 inches
3 to 5 inches
3/4 to 1 1/2 inches
3 to 5 inches
1 1/2 to 2 1/2 inches
5 to 8 inches
Reduction of gravel depths and other size gradations may be
					 considered upon justification to the state health officer.
7. Filter
				  bottoms and strainer systems. Departures from these standards may be acceptable
				  for high rate filters and for proprietary bottoms. Porous plate bottoms shall
				  not be used where iron or manganese may clog them or with waters softened by
				  lime. The design of manifold-type collection systems shall:
a. ensure even distribution of washwater and
				  even rate of filtration over the entire area of the filter;
b. provide the ratio of the area of the final
				  openings of the strainer systems to the area of the filter at 0.003;
c. provide the total cross-sectional area of
				  the laterals at twice the total area of the final openings;
d. provide the cross-sectional area of the
				  manifold at 1.5 to 2 times the total area of the laterals;
e. lateral perforations without strainers
				  shall be directed downward.
8. Filter media wash facilities are required
				  except for filters used exclusively for iron, radionuclides, arsenic or
				  manganese removal. Wash water systems shall be designed with:
a. water pressure per manufacturers
				  requirements;
b. a properly
				  installed vacuum breaker or other approved device to prevent back siphonage if
				  connected to the filtered or finished water system;
c. rate of flow of 2.0 gallons per minute per
				  square foot of filter area (4.9 m/hr) with fixed nozzles or 0.5 gallons per
				  minute per square foot (1.2 m/hr) with revolving arms if provided.
d. Air scouring. When provided, general
				  design criteria for air scouring is as follows.
i. Air flow for air scouring the filter shall
				  be 3-5 standard cubic feet per minute square foot of filter area (0.9-1.5
				  m3/min/m2) when the air is introduced in the underdrain; a lower air rate shall
				  be used when the air scour distribution system is placed above the
				  underdrains.
ii. When employing
				  concurrent air scour and water back wash a method for avoiding excessive loss
				  of the filter media during backwashing shall be provided.
iii. Air scouring shall be followed by a
				  fluidization wash sufficient to re-stratify the media.
iv. Air shall be free from
				  contamination.
v. Air scour
				  distribution systems should be placed below the media and supporting bed
				  interface; if placed at the interface the air scour nozzles shall be designed
				  to prevent media from clogging the nozzles or entering the air distribution
				  system.
vi. Piping for the air
				  distribution system shall not be flexible hose which will collapse when not
				  under air pressure and shall not be a relatively soft material which may erode
				  at the orifice opening with the passage of air at high velocity.
vii. Air delivery piping shall not pass down
				  through the filter media nor shall there be any arrangement in the filter
				  design which would allow short circuiting between the applied unfiltered water
				  and the filtered water.
viii. The
				  backwash water delivery system must be capable of 15 gallons per minute per
				  square foot of filter surface area (37 m/hr); however, when air scour is
				  provided the backwash water rate must be variable and should not exceed 8
				  gallons per minute per square foot (20 m/hr) unless operating experience shows
				  that a higher rate is necessary to remove scoured particles from filter media
				  surfaces.
ix. The filter
				  underdrains shall be designed to accommodate air scour piping when the piping
				  is installed in the underdrain.
9. Appurtenances. The following shall be
				  provided for every filter:
a. a means of
				  sampling influent and effluent water sampling taps;
b. a meter indicating the instantaneous
				  effluent rate of flow;
c. where
				  used for surface water, provisions for filtering to waste with appropriate
				  measures for cross connection control;
d. a flow rate controller capable of
				  providing gradual rate increases when placing the filters back into operation;
				  and
e. for surface water or systems
				  using ground water under the direct influence of surface water with three or
				  more filters, on-line turbidimeters shall be installed on the effluent line
				  from each filter. All turbidimeters shall consistently determine and indicate
				  the turbidity of the water in NTUs. Each turbidimeter shall report to a
				  recorder that is designed and operated to allow the operator to accurately
				  determine the turbidity at least once every 15 minutes. Turbidimeters on
				  individual filters should be designed to accurately measure low-range
				  turbidities and have an alarm that will sound when the effluent level exceeds
				  regulatory turbidity limits. It is recommended that turbidimeters be placed in
				  a location that also allows measurement of turbidity during filter to
				  waste.
10. Backwash.
				  Provisions shall be made for washing filters as follows.
a. a minimum rate necessary to provide for a
				  50 percent expansion of the filter bed shall be provided with a minimum of 15
				  gpm/sqft. A reduced rate of 10 gallons per minute per square foot (24 m/hr) may
				  be acceptable for full depth anthracite or granular activated carbon
				  filters;
b. filtered water shall be
				  used for backwashing filters except in the application of GAC filters for
				  removing formed disinfection by-products;
c. washwater pumps shall be in duplicate
				  except in the application of GAC filters or an alternate means of obtaining
				  washwater is available;
d. a
				  washwater regulator or valve on the main washwater line to obtain the desired
				  rate of filter wash with the washwater valves on the individual filters open
				  wide;
e. a flow meter, preferably
				  with a totalizer, on the main washwater line located so that it can be easily
				  read by the operator during the washing process;
f. design to prevent rapid changes in
				  backwash water flow;
g. automated
				  systems shall be adjustable; and
h.
				  appropriate measures for cross-connection control.