RECTILINEAR FLOW EFFLUENT TANK WALL LAUNDER. LAUNDER INFLUENT D. Peripheral-Feed Settling Tank, Spiral Flow EFFLUENT DISTRIBUTION WELL INFLUENT

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1 7.17 SEDIMENTATION PARTIAL LIST OF SUPPLIERS Degremont-Cottrell Inc.; Dorr-Oliver Inc.; FMC Corp.; Dravo Corp.; Sybron Corp.; Edens Equipment Co.; Eimco Process Equipment; Envirex; General Filter Co.; Great Lakes Environmental; Lakeside Equipment Corp.; Komline-Sandersen Engineering Corp.; Neptune MicroFloc Inc.; Parkson Corp.; U.S. Filter Permutit Co.; Walker Process; Zimpro Environmental Inc.; Zurn Industries Inc. Sedimentation sometimes called clarification is generally used in combination with coagulation and flocculation to remove floc particles and improve subsequent filtration efficiency. Omitting sedimentation prior to filtration results in shorter filter runs poorer filtrate quality and dirtier filters that are more difficult to backwash. Sedimentation is particularly necessary for high-turbidity and highly colored water that generates substantial solids during the coagulation and flocculation processes. Sedimentation is sometimes unnecessary prior to filtration (direct filtration) when the production of flocculation solids is low and filtration can effectively handle solids loading. Sedimentation is sometimes used at the head of a water treatment plant in a presedimentation basin which allows gravity settling of denser solids that do not require coagulation and flocculation to promote solid separation. The application of a presedimentation basin is most common where surface water has a high silt or turbidity content. Some wastewater treatment plants use coagulation before presedimentation basins. Types of Clarifiers The design of most clarifiers falls into one of the following categories: horizontal flow solids contact or inclined surface. HORIZONTAL- CLARIFIERS In horizontal-flow clarifiers sedimentation occurs in specially designed basins. These basins are known as settling tanks settling basins sedimentation tanks sedimentation basins or clarifiers. They can be rectangular square or circular. The most common basins are rectangular tanks and circular basins with a center feed. In rectangular basins (see part A in Figure ) the flow is in one direction and is parallel to the basin s length. This is called rectilinear flow. In center-feed circular basins (see part B in Figure ) the water flows radially from the center to the outside edges. This is called radial flow. Both basins are designed to keep the velocity and flow distribution as uniform as possible so that currents and eddies do not form and keep the suspended material from RECTILINEAR SKIRT A. Rectangular Settling Tank Rectilinear Flow TANK WALL RADIAL D. Peripheral-Feed Settling Tank Spiral Flow B. Center-Feed Settling Tank Radial Flow DISTRIBUTION WELL C. Peripheral-Feed Settling Tank Radial Flow E. Square Settling Tank Radial Flow FIG Flow patterns in sedimentation basins.

2 settling. Other flow patterns are shown in parts C D and E in Figure Basins are usually made of steel or reinforced concrete. The bottom slopes slightly to make sludge removal easier. In rectangular tanks the bottom slopes toward the inlet end whereas in circular or square tanks the bottoms are conical and slope toward the center of the basin. The selection of any shape depends on the following factors: Size of installation Regulation preference of regulatory authorities Local site conditions Preference experience and engineering judgement of the designer and plant personnel The advantages and disadvantages of rectangular clarifiers over circular clarifiers follow. ADVANTAGES: Less area occupied when multiple units are used Economic use of common walls with multiple units Easy covering of units for odor control Less short circuiting Lower inlet outlet losses Less power consumption for sludge collection and removal mechanisms DISADVANTAGES: Possible dead spaces Sensitivity to flow surges Collection equipment restricted in width Multiple weirs required to maintain low-weir loading rates High upkeep and maintenance costs of sprockets chains and fliers used for sludge removal Square clarifiers combine the common-wall construction of rectangular basins with the simplicity of circular sludge collectors. These clarifiers have generally not been successful (Montgomery 1985). Because effluent launderers are constructed along the perimeter of basins the corners have more weir length per degree of radial arc. Thus the flow is not distributed equally resulting in large sludge WATER LEVEL BAFFLE BRIDGE ROTATING BAFFLE TORQUE CAGE DRIVE SKIMMING BLADE ON ONE TRUSS ONLY WEIR SCRAPER BOX PIPE SQUEEGEE PLOW PIPE FIG Parts of a circular basin. (Courtesy of the FMC Corp. Material Handling Systems Division) depositions in basin corners. Corner sweeps added to circular sludge collection mechanisms to remove sludge settling in the corners have been a source of mechanical difficulty. Because of these problems few square basins are constructed for water treatment. Circular settling tanks are often chosen because they use a trouble-free circular sludge removal mechanism and for small plants can be constructed at a lower capital cost per unit surface area. Figures and show the details of rectangular and circular horizontal flow clarifiers. SOLID-CONTACT CLARIFIERS Part A in Figure shows the operational principles of solid-contact clarifiers. Incoming solids are brought in contact with a suspended sludge layer near the bottom. This layer acts as a blanket and the incoming solids agglomerate and remain enmeshed within this blanket. The liquid rises upward while a distinct interface retains the solids below. These clarifiers have hydraulic performance and a reduced retention time for equivalent solids removal in horizontal flow clarifiers. INCLINED-SURFACE CLARIFIERS Inclined-surface basins also known as a high-rate settler use inclined trays to divide the depth into shallower sections. Thus the depth of all particles (and therefore the settling time) is significantly reduced. Wastewater treatment plants frequently use this concept to upgrade the existing overloaded primary and secondary clarifiers. Part B COLLECTOR DRIVE PIPE TRAVEL WATER LEVEL TROUGH FLIGHTS BAFFLE ADJUSTABLE WEIRS FIG Parts of a rectangular basin. (Courtesy of the FMC Corp. Material Handling Systems Division)

3 Influent Circular-solids-contact clarifier Direction of flow To sludge collection Counter-current flow in tubes Influent Parallel-inclined-plates in a circular clarifier Tube settlers in a rectangular clarifier FIG Types of clarifiers. A. Circular-solids-contact clarifier. B. Parallel inclined plates in a circular clarifier. C. Tube settlers in a rectangular clarifier. D. Counter-current flow in tubes. in Figure shows the operating principles of inclined surface clarifiers. Inclined-surface clarifiers provide a large surface area reducing clarifier size. No wind effect exists and the flow is laminar. Many overloaded horizontal-flow clarifiers are upgraded with this concept. The major drawbacks of the inclined-surface clarifiers include: Long periods of sludge deposits on the inner walls can cause septic conditions. The effluent quality can deteriorate when sludge deposits slough off. Clogging of the inner tubes and channels can occur. Serious short-circuiting can occur when the influent is warmer than the basin temperature. Two design variations to the inclined-surface clarifiers are tube settlers and parallel-plate separators. Tube Settlers In these clarifiers the inclined trays are constructed with thin-wall tubes. These tubes are circular square hexagonal or any other geometric shape and are installed in an inclined position within the basin. The tubes are about 2 ft long and are produced in modules of about 750 tubes. The incoming flow enters these tubes and flows upward. Solids settle on the inside of the tube and slide down into a hopper. The most popular commercially available tube settler is the steeply inclined tube settler. The angle of inclination is steep enough so that the sludge flows in a countercurrent direction from the suspension flow passing upward through the tube. Thus solids drop to the bottom of the clarifier and are removed by conventional sludge removal mechanisms. Test results for alum-coagulated sludge indicate that solids remain deposited in the tubes until the angle of inclination increases to 60 or more from the horizontal. Parallel Plate Separators Parallel-plate separators have parallel trays covering the entire tank. The operational principles for these separators are the same as those for the tube settlers. Other Inclined-Surface Separators Another design of shallow-depth sedimentation uses lamella plates (see Figure ) which are installed parallel at a 45 angle. In this design water and sludge flow in the same direction. The clarified water is returned to the top of the unit by small tubes. Flow distribution orifices Overflow box Overflow (effluent) Discharge flumes Feed box Lamella Plates Flocculation tank Flash mix tank Coagulant aid Feed (influent) Picket fence sludge thickener Underflow (sludge) FIG Lamella plates. (Courtesy of Parkson Corp.)

4 Slotted distribution baffle Dead Space Density flow A. Good design B. Effect of density flow or thermal stratification Dead space Wind-driven circulation cell C. Effect of thermal stratification FIG Flow patterns in rectangular sedimentation tanks. D. Effect of wind in formation of circulation cell Design Factors The design objective of primary sedimentation is to produce settled water with the lowest possible turbidity. For effective filtration the turbidity of settled water should not exceed 10 NTU. Since effective sedimentation is closely linked with coagulation and flocculation the wastewater treatment plant must ensure that the best possible floc is formed. The flow should be distributed uniformly across the inlet of the basin (see Figure ). The solids removal efficiency of a clarifier is reduced by the following conditions: Eddy currents induced by the inertia of the incoming fluid Surface current produced by wind action (see part D in Figure ). The resulting circulating current can short-circuit the influent to the effluent weir and scour settled particles from the bottom. Vertical currents induced by the outlet structure Vertical convection currents induced by the temperature difference between the influent and the tank contents (see parts B and C in Figure ). Density currents causing cold or heavy water to underrun a basin and warm or light water to flow across its surface (see part B in Figure ). Currents induced by the sludge scraper and sludge removal system Therefore factors such as the overflow rate detention period weir-loading rate shape and dimensions of the basin inlet and outlet structures and sludge removal system affect the design of a sedimentation basin (Table ). DETENTION TIME The detention time depends on the purpose of the basin. In a mechanically cleaned presedimentation basin the de- TABLE TYPICAL WATER TREATMENT CLARIFIER DESIGN DETAILS Weir Overflow Rate Surface Overflow Rate Detention Type of Basin Time hr m 3 /(m day) gal/(ft day) m/day gal/(ft 2 day) Presedimentation 3 8 Standard basin following: Coagulation and flocculation Softening Upflow clarifier following: Coagulation and flocculation Softening Tube settler following: Coagulation and flocculation 0.2 Softening 0.2

5 tention time can be sufficient to remove only coarse sand and silt. In a plain sedimentation basin which depends on removing fine SS the detention time must be long since small particles settle very slowly. SURFACE OVER RATE The surface overflow rate is an important parameter for basins clarifying flocculent solids. It is expressed in cubic meters per day per square meter of the surface area of the tank or gal/ft 2 -day. The optimum surface overflow rate depends on the settling velocity of the floc particles. If the floc is heavy (as with lime softening) the overflow rate can be higher than with lighter alum floc. A typical overflow for alum floc is 500 gpd/sq ft (AWWA 1990). The surface overflow rate can be determined by jar test studies in which the best coagulant optimum dosage and best flocculation are used. However the environmental engineer must usually rely on past empirical experience and estimate a safe basin overflow rate based on representative water analyses and estimated coagulant use. Changing seasons and changing water quality pose additional problems. WEIR-OVER RATE Weir-loading rates have some effect on the removal efficiency of sedimentation basins. These rates are expressed in cu m/m or gal/ft length of the weir. The higher the weir overflow rate the more influence the outlet zone can have on the settling zone. To minimize this impact environmental engineers should not use a rate exceeding gpd/ft. For light alum floc the rate may have to be decreased to gpd/ft or 10 gpm/ft. Typical weirs consist of 90 V notches approximately 50 mm (2 in) deep placed from 100 to 300 mm (4 to 12 in) on the center. TABLE DIMENSIONS OF RECTANGULAR AND CIRCULAR BASINS Clarifier Range Typical Rectangular Length m Length-to-width ratio Length-to-depth ratio Sidewater depth m Width m a Bottom slope % 1 1 Circular Diameter m b Side depth m Bottom slope % 8 8 a Most manufacturers build equipment in width increments of 61 cm (2 ft). If the width is greater than 6 m (20 ft) multiple bays may be necessary. b Most manufacturers build equipment in 1.5-m (5-ft) increments of diameter. The length calculated from the weir overflow rate is the total length not the length over which flow occurs. Table is a compilation of typical surface overflow rates weir overflow rates and detention times used in water treatment. These values are provided for comparison purposes not as recommended standards. Individual states normally establish recommended design criteria that environmental engineers can alter by demonstrating that they do not apply to the water being treated or the process being used. The design of water treatment systems should be based on a laboratory evaluation of the proposed system. DIMENSIONS The dimensions of a sedimentation basin must accommodate standard equipment supplied by the manufacturer. Also environmental engineers must consider the size of the installation local site conditions regulations of local water pollution control agencies the experience and judgment of the designer and the economics of the system. Table summarizes the basic dimensions of rectangular and circular clarifiers. For any wastewater supply that requires coagulation and filtration to produce safe water a minimum of two basins should be provided. INLET STRUCTURE Water that by-passes the normal flow path through the basin and reaches the outlet in less than normal detention time occurs to some extent in every basin. It is a serious problem causing floc to be carried out of the basin due to the shortened sedimentation time. The major cause of short-circuiting is poor inlet baffling. If the influent enters the basin and hits a solid baffle a strong current and short-circuit result. The ideal inlet reduces entrance velocity to prevent development of currents toward the outlet distribute water uniformly across the basin and mixes it with water already in the tank to prevent density current. A near-perfect inlet consists of several small openings ( mm diameter circular [4 8-in or equivalent]) distributed through the width and depth of the basin. In these openings the head loss is large compared to the variation in head between the dif- Inlet Baffle pipe Multiple openings Tank bottom Inlet pipe Multiple openings Baffle FIG Typical sedimentation tank inlets. Inlet pipe Multiple openings

6 Influent A B Influent ferent openings. Figure shows some typical designs that compromise between simplicity and function. Based on inlet structures circular clarifiers are classified as center- and peripheral-feed. In center-feed circular clarifiers the inlet is at the center and the outlet is along the periphery. A concentric baffle distributes the flow equally in radial directions. The advantages of center-feed clarifiers are low upkeep cost and ease of design and construction. The disadvantages include short-circuiting low detention efficiency lack of scum control and loss of sludge into the effluent. Part A in Figure shows the flow scheme of a center-feed circular clarifier. In peripheral-feed clarifiers the flow enters along the periphery. These clarifiers are considerably more efficient and have less short-circuiting than center-feed clarifiers. Peripheral-feed clarifiers have two major variations. These variations are shown in parts B and C in Figure OUTLET STRUCTURES structures are designed to do the following: Provide uniform distribution of flow over a large area C FIG Influent and effluent structures for circular clarifiers. (Reprinted with permission from Envirex Inc. a Rexnord Company) Minimize lifting of the particles and their escape into the effluent Reduce floating matter from escaping into the effluent The most common effluent structures for rectangular and circular tanks are weirs that are adjustable for leveling. These weir plates are long enough to avoid the high heads that can result in updraft currents and particle lifting Ḃoth straight-edge and V notches on either one or both sides of the trough have been used in rectangular and circular tanks. V notches provide uniform distribution at low flows. A baffle in front of the weir stops floating matter from escaping into the effluent. Normally weirs in rec- Weir plate Outlet flume Outlet pipe Multiple openings Outlet pipe FIG Outlet details of sedimentation tanks.

7 TRAVELING BRIDGE COLLECTING BRIDGE TRAVEL SKIMMING TROUGH WATER LEVEL SKIMMING POSITION DRAWOFF COLLECTION POSITION HOPPER SCREW CROSS COLLECTOR FIG Traveling-bridge collector. WEIR RETURN INDIVIDUAL SIPHON VALVES SIPHON PIPES FLOATS FLOATING BRIDGE GUIDE WHEEL COLLECTION HEADERS DRIVE FIG Floating-bridge siphon collector. (Courtesy of Leopold Co. Division of Sybron Corp.) tangular tanks are on the opposite end of the inlet structure. Environmental engineers can use different weir configurations in rectangular basins to obtain a beneficial weir length. Figure shows typical sedimentation tank outlets. In circular clarifiers the outlet weir can be near the center of the clarifier or along the periphery as shown in Figure The center weir generally provides a high-velocity gradient that can result in solids carryover. REMOVAL As solids settle to the bottom of a basin a sludge layer develops. This layer must be removed because the solids can become resuspended or tastes or odors can develop. Wastewater treatment plants can manually remove the sludge by periodically draining basins and flushing the sludge to a hopper and drawoff pipe. This practice is recommended only for small installations or installations where not much sludge is formed. Mechanical removal is usually warranted. For rectangular basins sludge removal equipment is usually one of the following mechanisms (AWWA 1990): A chain and flight collector (see Figure ) consisting of a steel or plastic chain and redwood- or fiberglassreinforced plastic flights (scrapers). A traveling-bridge collector (see Figure ) consisting of a moving bridge which spans one or more basins. The mechanism has wheels that travel along rails mounted on the basin s edge. In one direction the scraper blade moves the sludge to a hopper. In the other direction the scraper retracts and the mechanism skims any scum from the water s surface. A floating-bridge siphon collector (see Figure ) using suction pipes to withdraw the sludge from the basin. The pipes are supported by foam plastic floats and the entire unit is drawn up and down the basin by a motor-driven cable system. For suction sludge removal the velocity can be 1 m/min (3 fpm) because the main concern is not the resuspension of settled sludge but the disruption of the settling process. To keep solids from returning to the cleaned liquid scrapers should operate at velocities below 1 fpm. The power requirements are about 1 hp per sq ft of tank area but straight-line collectors must have motors about ten times that strong to master the starting load (Fair Geyer and Okun 1968). Circular basins are usually equipped with scrapers or plows as shown in Figure These slant toward the center of the basin and sweep sludge toward the center of the basin then to the effluent hopper or pipe. The bridge can be fixed as illustrated or it can move with the truss. Regardless of the collection method the sludge is washed or scraped into a hopper. It is then pumped to sludge discharge treatment facilities. David H.F. Liu References American Water Works Association (AWWA) Water quality and treatment: A handbook of community water supplies. American Water Works Association McGraw Hill Inc. Fair G.M. J.C. Geyer and D.A. Okun Water and water engineering. New York: John Wiley & Sons. Montgomery J. McKee Water treatment principles and design. John Wiley & Sons Inc.