La. Admin. Code tit. 51 § XII-177

Current through Register Vol. 50, No. 11, November 20, 2024

Section XII-177 - Filtration Design

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.

B. Rapid Rate Pressure Filters. The normal use of these filters is for iron and manganese removal. For raw water with iron concentration of 2 mg/L or greater consideration should be given to pretreatment prior to filtration. Pressure filters shall not be used in the filtration of surface or other polluted waters or following lime-soda softening.

1. Minimum criteria relative to rate of filtration, structural details and hydraulics, filter media, etc., provided for rapid rate gravity filters also apply to pressure filters where appropriate. At least two filter units shall be provided. The filters shall be capable of meeting the average daily flow of the maximum month with one filter unit removed from service.

2. Rate of Filtration. The rate shall not exceed six gallons per minute per square foot of filter area except where manufacturers performance studies of the unit have demonstrated to the satisfaction of the state health officer that higher filtration rates are achievable. Consideration shall be given to backwash frequency and deteriorating water quality when selecting the filtration rate.

3. The filters shall be designed to provide for:

a. loss of head gauges on the inlet and outlet pipes of each filter;

b. an easily readable meter or flow indicator on each battery of filters;

c. filtration and backwashing of each filter individually;

d. minimum side wall shell height of five feet for vertical filters. A corresponding reduction in side wall height is acceptable where proprietary bottoms permit reduction of the gravel depth;

e. the top of the washwater collectors to be at least 18 inches above the surface of the media;

f. the underdrain system to efficiently collect the filtered water and to uniformly distribute the backwash water at a rate not less than 15 gallons per minute per square foot of filter area;

g. backwash flow indicators and controls that are easily readable while operating the control valves;

h. an air release valve on the highest point of each filter;

i. an accessible manhole of adequate size to facilitate inspection and repairs for filters 36 inches or more in diameter. Manholes should be at least 24 inches in diameter where feasible;

j. means to observe the wastewater during backwashing; and

k. construction to prevent cross-connection.

C. Diatomaceous Earth Filtration. The use of these filters may be considered for application to surface waters with low turbidity and low bacterial contamination.

1. Conditions of Use. Diatomaceous earth filters are expressly excluded from consideration for the following conditions:

a. bacteria removal;

b. color removal;

c. turbidity removal where either the gross quantity of turbidity is high or the turbidity exhibits poor filterability characteristics; and

d. filtration of waters with high algae counts.

2. Pilot Plant Study. Installation of a diatomaceous earth filtration system shall be preceded by a pilot plant study on the water to be treated.

a. Conditions of the study such as duration, filter rates, head loss accumulation, slurry feed rates, turbidity removal, bacteria removal, etc., must be approved by the state health officer prior to the study.

b. Satisfactory pilot plant results must be obtained prior to preparation of final construction plans and specifications.

c. The pilot plant study must demonstrate the ability of the system to meet applicable drinking water standards at all times.

3. Types of Filters. Pressure or vacuum diatomaceous earth filtration units will be considered for approval. However, the vacuum type is preferred for its ability to accommodate a design which permits observation of the filter surfaces to determine proper cleaning, damage to a filter element, and adequate coating over the entire filter area.

4. Treated water storage capacity in excess of normal requirements shall be provided to:

a. allow operation of the filters at a uniform rate during all conditions of system demand at or below the approved filtration rate, and

b. guarantee continuity of service during adverse raw water conditions without by-passing the system.

5. Number of Units. At least two units shall be provided. Where only two units are provided, each shall be capable of meeting the plant design capacity (normally the projected maximum daily demand) at the approved filtration rate. Where more than two filter units are provided, the filters shall be capable of meeting the plant design capacity at the approved filtration rate with one filter removed from service.

6. Pre-coating criteria includes the following.

a. Application. A uniform precoat shall be applied hydraulically to each septum by introducing a slurry to the tank influent line and employing a filter-to-waste or recirculation system.

b. Quantity. Diatomaceous earth in the amount of 0.2 pounds per square foot of filter area (0.98 kg/m2) or an amount sufficient to apply a 1/8 inch coating should be used with recirculation.

7. A body feed system to apply additional amounts of diatomaceous earth slurry during the filter run is required to avoid short filter runs or excessive head losses.

a. Rate of body feed is dependent on raw water quality and characteristics and shall be determined in the pilot plant study.

b. Operation and maintenance can be simplified by providing accessibility to the feed system and slurry lines.

c. Continuous mixing of the body feed slurry is required.

8. Filtration criteria includes the following.

a. Rate of Filtration. The recommended nominal rate is 1.0 gallon per minute per square foot of filter area (2.4 m/hr) with a recommended maximum of 1.5 gallons per minute per square foot (3.7 m/hr). The filtration rate shall be controlled by a positive means.

b. Head Loss. The head loss shall not exceed 30 psi (210 kPa) for pressure diatomaceous earth filters, or a vacuum of 15 inches of mercury (^-51 kPa) for a vacuum system.

c. Recirculation. A recirculation or holding pump shall be employed to maintain differential pressure across the filter when the unit is not in operation in order to prevent the filter cake from dropping off the filter elements. A minimum recirculation rate of 0.1 gallon per minute per square foot of filter area (0.24 m/hr) shall be provided.

d. Septum or Filter Element. The filter elements shall be structurally capable of withstanding maximum pressure and velocity variations during filtration and backwash cycles, and shall be spaced such that no less than one inch is provided between elements or between any element and a wall.

e. Inlet Design. The filter influent shall be designed to prevent scour of the diatomaceous earth from the filter element.

9. Backwash. A satisfactory method to thoroughly remove and dispose of spent filter cake shall be provided (see Subchapter F. §257-275 of this Part).

10. The following appurtenances shall be provided for every filter:

a. a means of sampling for raw and filtered water;

b. loss of head or differential pressure gauge;

c. rate-of-flow indicator, preferably with totalizer;

d. a throttling valve used to reduce rates below normal during adverse raw water conditions;

e. evaluation of the need for body feed, recirculation, and any other pumps, in accordance with §217 of this Part; and

f. provisions for filtering to waste with appropriate measures for backflow prevention.

D. Slow Sand Filters. The use of these filters shall require prior engineering studies to demonstrate the adequacy and suitability of this method of filtration for the specific raw water supply.

1. Quality of Raw Water. Slow rate gravity filtration shall be limited to waters having maximum turbidities of 10 units and maximum color of 15 units; such turbidity shall not be attributable to colloidal clay. Microscopic examination of the raw water shall be made to determine the nature and extent of algae growths and their potential adverse impact on filter operations.

2. Number. At least two units shall be provided. Where only two units are provided, each shall be capable of meeting the plant design capacity (normally the projected maximum daily demand) at the approved filtration rate. Where more than two filter units are provided, the filters shall be capable of meeting the plant design capacity at the approved filtration rate with one filter removed from service.

3. Structural Details and Hydraulics. Slow rate gravity filters shall be so designed as to provide:

a. headroom to permit normal movement by operating personnel for scraping and sand removal operations;

b. adequate access hatches and access ports for handling of sand and for ventilation; and

c. an overflow at the maximum filter water level.

4. Rates of Filtration. The permissible rates of filtration shall be determined by the quality of the raw water and shall be on the basis of experimental data derived from the water to be treated. The nominal rate may be 45 to 150 gallons per day per square foot of sand area (1.8 - 6.1 m/day), with somewhat higher rates acceptable when demonstrated to the satisfaction of the approving authority.

5. Underdrains. Each filter unit shall be equipped with a main drain and an adequate number of lateral underdrains to collect the filtered water. The underdrains shall be placed as close to the floor as possible and spaced so that the maximum velocity of the water flow in the underdrain will not exceed 0.75 feet per second. The maximum spacing of laterals shall not exceed 3 feet if pipe laterals are used.

6. Filter material criteria shall be as follows.

a. Filter sand shall be placed on graded gravel layers for a minimum depth of 30 inches.

b. The effective size shall be between 0.15 mm and 0.30 mm. Larger sizes may be considered by the state health officer.

c. The uniformity coefficient shall not exceed 2.5.

d. The sand shall be cleaned and washed free from foreign matter.

e. The sand shall be rebedded when scraping has reduced the bed depth to no less than 19 inches. Where sand is to be reused in order to provide biological seeding and shortening of the ripening process, rebedding shall utilize a "throw over" technique whereby new sand is placed on the support gravel and existing sand is replaced on top of the new sand.

7. Filter Gravel. The supporting gravel should be similar to the size and depth distribution provided for rapid rate gravity filters (see §177.A.6.d ii of this Part).

8. Depth of Water on Filter Beds. Design shall provide a depth of at least three to six feet of water over the sand. Influent water shall not scour the sand surface.

9. Control Appurtenances. Each filter shall be equipped with:

a. means of sampling influent and effluent water;

b. an indicating loss of head gauge or other means to measure head loss;

c. an indicating rate-of-flow meter. A means of controlling the rate of filtration and limiting the rate of filtration to a maximum rate shall be provided;

d. provisions for filtering to waste with appropriate measures for cross connection control; and

e. an effluent pipe designed to maintain the water level above the top of the filter sand.

10. [Ripening] Slow sand filters shall be operated to waste after scraping or rebedding during a ripening period until the filter effluent turbidity falls to consistently below the regulated drinking water standard established for the system.

E. Direct Filtration. Direct filtration, as used herein, refers to the filtration of a surface water following chemical coagulation and possibly flocculation but without prior settling. The nature of the treatment process will depend upon the raw water quality. A full scale direct filtration plant shall not be constructed without prior pilot studies which are acceptable to the state health officer. In-plant demonstration studies may be appropriate where conventional treatment plants are converted to direct filtration. Where direct filtration is proposed, an engineering report shall be submitted prior to conducting pilot plant or in-plant demonstration studies.

1. Engineering Report

a. In addition to the items considered in §113 of this Part, "Engineering Report", the report shall include a historical summary of meteorological conditions and of raw water quality with special reference to fluctuations in quality, and possible sources of contamination. The following raw water parameters shall be evaluated in the report:

i. color;

ii. turbidity;

iii. bacterial concentration;

iv. microscopic biological organisms;

v. temperature;

vi. total solids;

vii. general inorganic chemical characteristics; and

viii. additional parameters as required by the state health officer.

b. The report shall also include a description of methods and work to be done during a pilot plant study or, where appropriate, an in-plant demonstration study.

2. Pilot Plant Studies. After approval of the engineering report and pilot plant protocol, a pilot study or in-plant demonstration study shall be conducted. The study must be conducted over a sufficient time to treat all expected raw water conditions throughout the year. The pilot plant filter must be of a similar type and operated in the same manner as proposed for full scale operation. The pilot study must determine the contact time necessary for optimum filtration for each coagulant proposed. The study shall emphasize but not be limited to, the following items:

a. chemical mixing conditions including shear gradients and detention periods;

b. chemical feed rates;

c. use of various coagulants and coagulant aids;

d. flocculation conditions;

e. filtration rates;

f. filter gradation, types of media and depth of media;

g. filter breakthrough conditions;

h. adverse impact of recycling backwash water due to solids, algae, trihalomethane formation and similar problems;

i. length of filter runs;

j. length of backwash cycles;

k. quantities and make-up of the wastewater. Prior to the initiation of design plans and specifications, a final report including the engineer's design recommendations shall be submitted to the state health officer.

3. Pretreatment. The final coagulation and flocculation basin design should be based on the pilot plant or in-plant demonstration studies augmented with applicable portions of §175 D, "Coagulation" and §175 E, "Flocculation" of this Part.

4. Filtration. Filters shall be rapid rate gravity filters with dual or mixed media. The final filter design shall be based on the pilot plant or in-plant demonstration studies and all portions of§177 A"Rapid rate gravity filters" of this Part. Pressure filters or single media sand filters shall not be used.

5. Appurtenances. The following shall be provided for every filter:

a. influent and effluent sampling taps;

b. an indicating loss of head gauge;

c. a meter indicating instantaneous rate of flow;

d. where used for surface water, provisions for filtering to waste with appropriate measures for cross connection control;

e. measures for providing gradual rate increases when placing the filters back into operation; and

f. for systems 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 0.3 NTU.

F. Deep Bed Rapid Rate Gravity Filters. Deep bed rapid rate gravity filters, as used herein, generally refers to rapid rate gravity filters with filter material depths equal to or greater than 48 inches. Filter media sizes are typically larger than those listed in §177.A.6.d of this Part.

1. Deep bed rapid rate filters may be considered based on pilot studies pre-approved by the state health officer.

2. The final filter design shall be based on the pilot plant studies and shall comply with all applicable portions of §177.A of this Part. Careful attention shall be paid to the design of the backwash system which usually includes simultaneous air scour and water backwash at subfluidization velocities.

G. Biologically Active Filters. Biologically active filtration, as used herein, refers to the filtration of surface water (or a ground water with iron, manganese, ammonia or significant natural organic material) which includes the establishment and maintenance of biological activity within the filter media.

1. Objectives of biologically active filtration may include control of disinfection byproduct precursors, increased disinfectant stability, reduction of substrates for microbial regrowth, breakdown of small quantities of synthetic organic chemicals, reduction of ammonia-nitrogen, and oxidation of iron and manganese. Biological activity can have an adverse impact on turbidity, particle and microbial pathogen removal, disinfection practices; head loss development; filter run times and distribution system corrosion. Design and operation should ensure that aerobic conditions are maintained at all times. Biologically active filtration often includes the use of ozone as a pre-oxidant/disinfectant which breaks down natural organic materials into biodegradable organic matter and granular activated carbon filter media which may promote denser biofilms.

2. Biologically active filters may be considered based on pilot studies pre-approved by the state health officer. The study objectives must be clearly defined and must ensure the microbial quality of the filtered water under all anticipated conditions of operation.

a. The pilot study shall be of sufficient duration to ensure establishment of full biological activity. The pilot study shall establish empty bed contact time, biomass loading, and/or other parameters necessary for successful operation as required by the state health officer.

3. The final filter design shall be based on the pilot plant studies and shall comply with all applicable portions of §177.A of this Part.

La. Admin. Code tit. 51, § XII-177

Promulgated by the Department of Health, Office of Public Health, LR 44313 (2/1/2018), effective 8/1/2018, Amended LR 481102 (4/1/2022).

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).