Wisconsin Administrative Code
Department of Natural Resources
NR 800 - Environmental Protection - Water Supply
Chapter NR 811 - Requirements For The Operation And Design Of Community Water Systems
Subchapter VII - Treatment
Section NR 811.47 - Clarification

Universal Citation: WI Admin Code ยง NR 811.47

Current through August 26, 2024

Clarification plants prior to filtration shall meet all of the following requirements unless other design criteria are justified through pilot testing under s. NR 811.44 or other specific information provided to the department supports the proposed design, as determined by the department:

(1) Provide a minimum of 2 units each for rapid mix, flocculation and sedimentation.

(2) Permit operation of the units either in series or parallel.

(3) Be constructed to permit units to be taken out of service without disrupting operation with drains or pumps sized to allow dewatering in a reasonable period of time.

(4) Provide multiple-stage treatment facilities if required by the department.

(5) Be started manually following shutdown.

(6) Minimize hydraulic head losses between units to allow future changes in processes without the need for repumping.

(7) Meet the following specific requirements:

(a) Presedimentation. Waters containing high turbidity or having unusual treatment requirements may require pretreatment, usually sedimentation or detention either with or without the addition of coagulation chemicals.
1. 'Basin design.' Presedimentation basins shall have the following:
a. Hopper bottoms or be equipped with continuous mechanical sludge removal apparatus, and provide arrangements for dewatering.

b. Cover or superstructure.

2. 'Inlet.' Incoming water shall be dispersed across the full width of the line of travel as quickly as possible. Short circuiting shall be prevented.

3. 'Bypass.' Provisions for bypassing presedimentation basins shall be included.

4. 'Detention time.' Three hours detention is the minimum period required for presedimentation. Greater detention may be required in individual cases of chemical pretreatment.

5. 'Raw water samples.' A means for collecting raw water samples prior to any chemical addition shall be provided.

(b) Rapid mix. Mixing shall mean the rapid dispersion of chemicals throughout the water to be treated, usually by violent agitation. 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 shall not be more than 30 seconds with mixing equipment capable of imparting a minimum velocity gradient (G) of at least 750 feet per second per foot or as determined through jar testing. The appropriate G value and detention time shall be determined through jar testing.

2. 'Equipment.' Basins shall be equipped with mechanical mixing devices unless other methods, such as baffling, or injection of chemicals at a point of high velocity, are approved by the department after determining that the other requirements of this chapter will be met. Variable speed drive equipment is recommended.

3. 'Location.' The rapid mix and flocculation basin shall be as close together as possible.

(c) Flocculation - slow mixing. Flocculation installations shall meet all of the following requirements:
1. 'Basin design.' Inlet and outlet design shall prevent short circuiting and destruction of floc. Series compartments shall be provided to minimize short-circuiting and to provide decreasing mixing energy with time. Basins shall be designed so that individual basins may be isolated without disrupting plant operation. A drain or pumps or both shall be provided to allow dewatering and sludge removal.

2. 'Detention.' Flow-through velocity may be not less than 0.5 nor greater than 1.5 feet per minute with a detention time for floc formation of at least 30 minutes. Tapered energy with diminishing velocity gradient shall be considered in the design of the flocculation basin.

3. 'Equipment.' Agitators shall be driven by variable speed drives or other means which vary the peripheral speed of paddles in the range of 0.5 to 3.0 feet per second and the tip speed of vertical shaft impellors in the range of 6 to 10 feet per second. Uniform mixing shall be provided to prevent settling in the flocculation basin.

4. 'Piping.' Flocculation and sedimentation basins shall be as close together as possible. The velocity of flocculated water through pipes or conduits to settling basins may not be less than 0.5 nor greater than 1.5 feet per second. Allowances shall be made to minimize turbulence at bends and changes in direction.

5. 'Other designs.' Baffling may be used to provide flocculation only after approval by the department. The design shall be such that the velocities and flows in this section shall be maintained.

6. 'Superstructure.' A superstructure shall be provided over the flocculation basins.

(d) Sedimentation. Sedimentation shall follow flocculation. The detention time for effective clarification is dependent upon factors related to basin design as well as the nature of the raw water, such as turbidity, color and colloidal matter, and taste and odor causing compounds.
1. 'Detention time.' Plants with conventional sedimentation shall provide a minimum of 4 hours of settling time. This may be reduced to 2 hours for lime-soda softening facilities treating only groundwater. Also, reduced sedimentation time may be approved when equivalent effective settling is demonstrated or when overflow rate is not more than 0.5 gallons per minute per square foot.

2. 'Inlet devices.' Inlets shall be designed to distribute the water equally and at uniform velocities. Open ports, submerged ports, and similar entrance arrangements are required. A baffle shall be constructed across the basin, close to the inlet end, and project several feet below the water surface to dissipate inlet velocities and provide uniform flows across the basin.

3. 'Outlet devices.' Outlet devices shall be designed to maintain velocities suitable for settling in the basin and to minimize short circuiting. The use of submerged orifices is recommended in order to provide volume above the orifices for storage when there are fluctuations in flow.

4. 'Weir overflow rate.' The rate of flow over the outlet weir may not exceed 20,000 gallons per day per foot of weir length. If submerged ports or orifices are used as an alternate for overflow weirs, they may not be lower than 3 feet below the flow line with flow rates equivalent to weir loadings. The entrance velocity through the submerged orifices shall not exceed 0.5 feet per second.

5. 'Drainage.' Basins shall be provided with a means for dewatering. Basin bottoms shall slope toward the drain not less than one foot in 12 feet where mechanical sludge collection is not provided.

6. 'Covers.' Covers or superstructures are required at all plants. Where covers are used, access hatches shall be provided as well as drop light connections so that observation of the floc can take place at the inlet, midpoint and outlet of the basin.

7. 'Velocity.' The velocity through settling basins may 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.

8. 'Overflow.' An overflow weir or pipe shall be installed, which will establish the maximum water level desired on top of the filters. It shall discharge by gravity with a downturned pipe elbow a minimum of one foot above a concrete splash pad and shall be covered with 4-mesh corrosion resistant screen at a location where the discharge is visible and where the water can be appropriately drained.

9. 'Safety.' Guard rails shall be installed around openings which may be hazardous to maintenance personnel. Permanent holders or handholds shall be provided on the inside walls of basins above the water level.

10. 'Sludge collection.' Mechanical sludge collection equipment may be provided.

11. 'Sludge removal.' Facilities for disposal of sludge are required by the department. Sludge removal design shall provide:
a. Sludge pipes not less than 3 inches in diameter and so arranged as to facilitate cleaning.

b. Entrance to sludge withdrawal piping to prevent clogging.

c. Valves located outside the tank for accessibility.

d. Provisions for the operator to observe and sample sludge being withdrawn from the unit.

12. 'Sludge disposal.' Sections NR 811.858 and 811.861 contain additional specific requirements for sludge disposal. Flushing lines or hydrants shall be provided to backflush sludge lines and basins or for other purposes. Protection shall be provided for all potable water lines used if potable water could become contaminated by nonpotable water.

(e) Solids contact unit. Units designed for combined softening and clarification, if water characteristics, especially temperature, do not fluctuate rapidly and flow rates are uniform and operation is continuous, may be used if specifically approved by the department. Units shall be designed for the maximum uniform rate and be adjustable to changes in flow, which are less than the design rate and for changes in water characteristics. A minimum of 2 units are required unless the department waives this requirement. For plants with multiple units, the rated capacity of the plant shall be available with one unit out of service.
1. 'Installation of equipment.' Supervision by a representative of the manufacturer shall be provided whenever mechanical equipment is installed and at the time of initial operation.

2. 'Operating equipment.' A complete outfit of tools and accessories shall be provided. Laboratory equipment to control the treatment process shall be provided at all waterworks. In addition, sampling taps with adequate piping located to permit the collection of samples of water from critical portions of the units shall be provided.

3. 'Chemical feed.' Chemicals shall be applied at points and by means as to ensure satisfactory mixing of the chemicals with the water.

4. 'Mixing.' Mixing devices employed shall be constructed to provide adequate mixing of the raw water with previously formed sludge particles and to prevent deposition of solids in the mixing zone. A rapid mix device or chamber ahead of the solids contact unit may be required by the department.

5. 'Flocculation.' Flocculation equipment shall be adjustable by speed, or pitch or both, provide for coagulation to occur in a separate chamber or baffled zone within the unit, and provide a flocculation and mixing period of not less than 30 minutes.

6. 'Sludge concentrators.' Sludge concentrators shall provide either internal or external concentrators in order to obtain a concentrated sludge with a minimum of wastewater.

7. 'Sludge removal.' Sludge removal design shall provide all of the following:
a. Sludge pipes not less than 3 inches in diameter, arranged to facilitate cleaning.

b. Entrance to sludge withdrawal piping to prevent clogging.

c. Valves located outside the tank for accessibility.

d. Facilities for an operator to observe or sample sludge being withdrawn from the unit.

8. 'Cross-connections.' Sludge blow-off outlets and drains shall terminate and discharge at places approved by the department. Cross-connection control shall be included for all potable water lines such as those used to backflush sludge lines or flush basins if potable water could become contaminated by nonpotable water.

9. 'Detention period.' The detention time shall be established on the basis of the raw water characteristics and local conditions that affect the operation of the unit. Based on design flow rates, the minimum detention time shall be 2 to 4 hours for suspended solids contact clarifiers and softeners treating surface waters, and one to 2 hours for the suspended solids contact softeners treating only groundwater.

10. 'Suspended slurry concentrate.' Softening units shall be designed so that continuous slurry concentrates of 1% or more, by weight, can be effectively maintained.

11. 'Water losses.'
a. Units shall be provided with suitable controls for sludge withdrawal.

b. Total water loss may not exceed 5% for clarifiers or 3% for softening units.

c. Solids concentration of sludge discharged to waste shall be at least 3% by weight for clarifiers and 5% by weight for softeners.

12. 'Weir or orifices.' The units shall be equipped with either overflow weirs or orifices. Weirs shall be adjustable, at least equivalent in length to the perimeter of the tank, and constructed so that surface water does not travel over 10 feet horizontally to the collection trough.

13. 'Weir or orifice loading.' Weir loading may not exceed 20 gallons per minute per foot of weir length for units used for softeners, or 10 gallons per minute per foot of weir length for units used for clarifiers. Where orifices are used, the loading rate per foot shall be equivalent to weir loadings. Orifices or weirs shall produce uniform rising rates over the entire area of the tank.

14. 'Upflow rates.' Unless supporting data is submitted to the department and the department grants an exception, the following rates may not be exceeded:
a. 1.75 gallons per minute per square foot of area at the slurry separation line if units are used for softeners.

b. 1.0 gallon per minute per square foot of area at the sludge separation line if units are used for clarifiers.

(f) Tube or plate settlers. Proposals for settler unit clarification shall include pilot plant or a full scale demonstration or both satisfactory to the department prior to the preparation of final plans and specifications for approval. 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. Tube or plate settler installations shall meet the following:
1. 'Inlet and outlet considerations.' Design the inlets and outlets to maintain velocities suitable for settling in the basin and to minimize short-circuiting.

2. 'Drainage.' 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.

3. 'Protection from freezing.' Units shall be located within a plant or within a covered basin.

4. 'Application rate for tubes.' A maximum application rate of 2 gallons per minute per square foot of cross-sectional area, unless higher rates are successfully shown through pilot plant or in-plant demonstration studies and are approved by the department.

5. 'Application rate for plates.' A maximum plate loading rate of 0.5 gallons per minute per square foot, based on 80 percent of the projected horizontal plate area.

6. 'Flushing lines.' Flushing lines shall be provided to facilitate maintenance and shall be properly protected against backflow or back siphonage.

7. 'Placement.' Modules shall be placed in zones of stable hydraulic conditions and in areas nearest effluent launders for basins not completely covered by the modules.

8. 'Inlets and outlets.' Inlets and outlets shall conform with par. (d) 2. and 3.

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