La. Admin. Code tit. 51 § XII-175

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

Section XII-175 - Clarification Design

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.

La. Admin. Code tit. 51, § XII-175

Promulgated by the Department of Health, Office of Public Health, LR 44310 (2/1/2018), effective 8/1/2018.

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).