Chapter 22

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22.1 INTRODUCTION

Designers of water treatment plants and wastewater treatment plants are faced with the need to design treatment processes which must meet the following general hydraulic requirements:

• Water treatment plants. Provide the head required to allow the water to flow through the treatment processes and to be delivered to the transmission/distribution system in the flow rates and at the pressures required for delivery to the users.

• WastWW ewater treatment plants.rr Provide the head required to raise the flow of wastewater from the sewer system to a level which allows the flow to proceed through the treat-ment processes and be delivered to the receiving body of water.

The above requires knowledge of open-channel, closed-conduit, and hydraulic machine flow principles. It also requires an understanding of the interaction between these elements and their impact on the overall plant (site) hydraulics. Head is either available through the difference in elevation (gravity) or it has to be converted from mechanical energy using hydraulic machinery. Distribution of flows using open channels or closed conduit is critical for proper hydraulic loading and process performance.

This chapter brings together information on commonly used hydraulic elements and specific applications to water treatment plants and wastewater treatment plants. The devel-opment of hydraulic profiles through the entire treatment process with examples for water treatment plants and wastewater treatment is also presented.

Many processes and flow control devices are similar in both water treatment plants and wastewater treatment plants. Both types of plants require flow distribution devices, gates and valves, and flowmeters. These devices are discussed in Section 22.2. The development of water treatment plant hydraulics, including examples from in-place facilities, are pre-sented in Section 22.3. Wastewater treatment plant hydraulics are discussed in Section 22.4, and Section. 22.5 is devoted to non-Newtonian flow principles.

WATER AND WASTEWATER

TREATMENT PLANT

HYDRAULICS

Federico E. Maisch

Sharon L. Cole

David V. Hobbs

Frank J. Tantone

William L. Judy

Greeley and Hansen

Richmond, VA

22.1

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22.2 GENERAL

22.2.1 Introduction

This section addresses some elements which are common to both water treatment plants and wastewater treatment plants including:

• Flow distribution–manifolds • Gates and valves

• Flowmeters • Local losses

22.2.2 Flow distribution–manifolds

In the design of water and wastewater treatment plants, proper flow distribution can be as critical as process design considerations, which typically receive much more attention. Plant failures resulting from unequal and unmanageable flow distribution are possibly as common and as serious as those resulting from errors in process design.

Flow distribution devices, such as distribution channels, pipe manifolds or distribution boxes, are commonly used to distribute or equalize flow to parallel treatment units, such as flocculation tanks, sedimentation basins, aeration tanks, or filters.

22.2.2.1 Distribution boxes. The simplest of these devices, the distribution box, typical-ly consists of a structure arranged to provide a common water surface as the supptypical-ly to two or more outlets. The outlets are typically over weirs and the key to equal flow distribution is to provide independent hydraulic characteristics between the downstream system and the water level in the distribution box. In other words, provide a free discharge weir (non-submerged under all conditions) for each outlet to eliminate the impact of downstream physical system differences on the flow distribution. Velocity gradients across the distrib-ution box must be nearly zero to equalize flow conditions over each outfall weir. Weirs clearly should be of uniform design in terms of physical arrangement length and materi-als of construction. They should materi-also be adjustable to account for any minor flow differ-ences noted in actual operation. The same principles apply if the designer wishes to dis-tribute flows in specific proportions which are not necessarily equal. In this case the designer could control the proportions of flow distribution by varying the relative geom-etry of the weirs (i.e., change the width or invert of each weir to achieve a desired flow distibution). The specifics of weir hydraulics are covered in various texts in the literature. Attention should always be paid to the selection of the proper coefficients to model the specific weir geometry and the geometry of the approach flow.

22.2.2.2 Distribution channels and pipe manifolds. Distribution channels and manifolds are also common in plant design but a bit more complex in their function and design. The distribution of flow in these devices is impacted by the flow distribution itself. Since a por-tion of the flow leaves the channel or manifold along the length of the device, the veloci-ty of flow and, therefore, the relationship of energy grade line, velociveloci-ty head and hydraulic grade line varies along the length of the device. This is more clearly visible in a tion channel of uniform cross section, using side weirs along its length for flow distribu-tion. At each weir, flow leaves the channel, resulting in less velocity head in the channel

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and possibly a higher water surface at each ensuing weir. Chao and Trussell (1980), Camp and Graber (1968), and Yao (1972) have presented comprehensive approaches for the design of distribution channels and manifolds and should be reviewed for details of design.

As in distribution boxes, the most important consideration to achieving equalized flow distribution is to minimize the effects of unequal hydraulic conditions relative to each point of distribution. In channels this can be accomplished by tapering the channel cross section, varying weir elevations, making the channel large enough to cause velocity head changes to be insignificant or a combination of these. Similar considerations may be applied to manifolds with submerged orifice outlets. A reliable approach here is to pro-vide a large enough manifold, resulting in a total headloss along the length of the distrib-ution of less than one tenth the loss through any individual orifice. This approach essen-tially results in the orifices becoming the only hydraulic control and the accuracy of the flow distribution is then dependent on the uniformity of the orifices themselves.

22.2.3 Gates and Valves

Gates and valves generally serve to either control the rate of flow or to start/stop flow. Gates and valves in treatment plants are typically subjected to much lower pressures than those in water distribution systems or sewage force mains and can be of lighter construction.

22.2.3.1 Gates. Gates are typically used in channels or in structures to start and stop flow or to provide a hydraulic control point which is seldom adjusted. Because of the time and effort required to operate gates, they are not suited for controlling flow when rapid response, frequent variation, or delicate adjustments are needed. Primary design consid-erations when using gates are the type of gate fabrication and the installation conditions during construction.

There are many fabrication details including materials used, bottom arrangement, and stem arrangement. For instance, for solids bearing flows, a flush bottom, rising stem gate can be used to avoid creating a point of solids deposition and to minimize solids contact with the threaded stem. Gate manufacturers are a good source of information for gate fab-rication details and can assist with advice regarding specific applications.

Most commonly used gates are designed to stop flow in a single direction. They may use upstream water pressure to assist in achieving a seal (seating head), but typically also must be designed to resist static water pressure from downstream (unseating head). Both seating and unseating heads must be evaluated in design of a gate application. For most manufacturers, the seating or unseating head is expressed as the pressure relative to the center line of the gate.

22.2.3.2 Valves. Table 22.1 provides a summary of several types of valves and their applications. Valves are used to either throttle (control) flow or start/stop flow. Start/stop valves are intended to be fully open or fully closed and nonthrottling. They should present minimum resistance to flow when fully open and should be intended for infrequent operation.

Gate valves, plug valves, cone valves, ball valves, and butterfly valves are the most common start/stop valve selections. Butterfly valves have a center stem, are most common in clean water applications and should not be used in applications including materials that could hang-up on the stem. Therefore, they are seldom used at wastewater plants prior to achieving a filter effluent water quality.

Water and Wastewater Treatment Plant Hydraulics 22.3

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Check valves are a special case of a start/stop valve application. Check valves offer quick, automatic reaction to flow changes and are intended to stop flow direction rever-sal. Typical configurations include swing check, lift check, ball check and spring loaded. These valves are typically used on pump discharge piping and are opened by the pressure of the flowing liquid and close automatically if pressure drops and flow attempts to reverse direction. The rapid closure of these valves can result in unacceptable “water-hammer” pressures with the potential to damage the system. A detailed surge analysis may be required for many check valve applications (see Chapter. 12). At times, mechanically operating check valves should be avoided in favor of electrically or pneumatically operat-ed valves (typically plug, ball, or cone valves) to provide a mechanism to control time of closing and reduce surge pressure peaks.

Throttling valves are used to control rate of flow and are designed for frequent or near-ly continuous operation depending on whether they are manualnear-ly operated or electroni-cally controlled. Typical throttling valve types include globe valves, needle valves, and angle valves in smaller sizes, and ball, plug, cone, butterfly, and pinch/diaphragm valves in larger sizes. Throttling valves are typically most effective in the mid-range of loose line open/close travel and for best flow control should not be routinely operated nearly fully closed or nearly fully open.

22.2.4 Flow meters

The most common types of flow meters used in water and wastewater treatment plants are summarized in Table 22.2 and fall into the following categories:

TABLE 22.1 Typical Valves and Their Application*

Type Open/Close Throttling Water Wastewater

Sluice gate X X X Slide gate X X X Gate valve X X X Plug valve X X X X Cone valve X X X X Ball valve X X X X Butterfly valve X X X Swing check X X X Lift check X X Ball check X X X Spring check X X X Globe valve X X Needle valve X X Angle valve X X Pinch/diaphragm X X X X *

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• Pressure differential/pressure measuring meters (e.g., Venturi, orifice plate, pitot tube, and Parshall flume meters)

• Magnetic meters

• Doppler (ultrasonic) meters

• Mechanical meters (e.g., propeller and turbine meters)

Accurate flow measurements require uniform flow patterns. Most meters are significantly impacted by adjacent piping configurations. Typically a specific number of straight pipe diameters is required both upstream and downstream of a meter to obtain reliable measurements. In some cases, 15 straight pipe diameters upstream and 5 straight pipe diameters downstream are recommended. However, different types of meters have varying levels of susceptibility to the uniformity of the flow pattern. Meter manufacturers should be consulted.

22.2.4.1 Pressure differential/pressure measuring meters. Pressure differential/pressure measuring flow meters include Venturi meters, orifice plates, averaging pitot meters, and Parshall flumes. These meters measure the change in pressure through a known flow cross section–or in the case of the pitot meter, measure the difference in pressure at a point in the flow versus static pressure just downstream in a uniform section of pipe.

Venturi meters and orifice plates are commonly used in water and wastewater. Solids in wastewater could plug the openings of a pitot tube meter-limiting their use to relative-ly clean liquids. The Venturi meter and orifice plate meter use pressure taps at the wall of the device and can be arranged to minimize potential for debris from clogging the taps. The Parshall flume can be arranged with a side stilling well and level measuring float sys-tem or an ultrasonic level sensing device to measure water level.

22.2.4.2 Magnetic meters. In a magnetic flowmeter, a magnetic field is generated around a section of pipe. Water passing through the field induces a small electric current propor-tional to the velocity of flow. Because a magnetic meter imposes no obstruction to the flow, it is well suited to measuring solids bearing liquids as well as clean liquids and pro-duces no headloss in addition to the normal pipe loss. Magnetic meters are among the least susceptible to the uniformity of the stream lines in the approaching flow.

Water and Wastewater Treatment Plant Hydraulics 22.5

TABLE 22.2 Common Types of Flow Meters

Type Typical Accuracy Size Range Headloss Cost W WW

Venturi 0.75% of rate 1–120 in Low Medium X X

Orifice plate 2% of scale Any size Medium Low X X

Pitot tube 0.5–5% of scale 1/2–96 in Low Low X

Parshall flume 5% of rate Wide range Low Medium X X

Magnetic 0.5% of rate 1/10–120 in None High X X

Doppler 1–2.5% of rate 1/8–120 in None High X X

Propeller 2% of rate Up to 24 in High High X

Turbine 0.5–2% of rate Up to 24 in High High X

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22.2.4.3 Ultrasonic meters. In an ultrasonic flow meter, a pair of transceivers (transmit-ter/receiver) are positioned diagonally across from each other on the pipe wall. The transmitter sends out a signal which is affected by the speed of the flow. The receiver mea-sures the difference between the speed of the signal when directed counter to the flow (slowed by the flow) and when directed with the flow (speeded up by the flow). The time difference is a function of fluid velocity, which is used to compute the flow. As with mag-netic meters, no flow obstruction is imposed resulting in no headloss in addition to the normal pipe loss. Ultrasonic meters are also available for open-channel applications. 22.2.4.4 Mechanical meters. Mechanical meters include propeller and turbine-type

equipment. The two meters are similar in function in that in each a device is inserted into the flowpath. The device is rotated by the flow and the speed of rotation is used to com-pute rate of flow. These devices impose an obstruction to flow, are recommended for clean water only, and generally result in significant headloss.

22.2.5 Local Losses

In any piping system as flow travels along the pipe, pressure drops as a result of headloss due to friction along the pipe and local losses at bends, fittings, and valves. The local losses at bends, fittings, and valves are least significant in long, straight piping systems and most significant at treatment plants where the length of straight pipe is relatively short and therefore, the frictional pipe losses comprise a smaller fraction of the total losses when compared to the summation of all local losses. A term often used to refer to local losses is “minor losses,” however, because of the later consideration the term “minor loss-es” can be misleading.

Traditionally, local losses have been computed in terms of “equivalent length” of straight pipe or in terms of multiples of velocity head. The “equivalent length” or loss fac-tor K methods attempt to estimate the local losses based on the characteristic of the spe-cific bend, fitting or valve. The K loss factor method is discussed here. Essentially, a local loss is computed as follows:

hL KKK 2 V g 2 V V (22.1)

where hL local loss, K loss factor, V velocity, g gravitational acceleration. The values for K reported by various sources vary considerably for some local losses and are relatively consistent for others. See references. There are many literature sources for K values. The Bureau of Reclamation (1992) is one such source of information regard-ing energy loss equations. Table 22.3 shows a range of K factors from additional sources as well as a typically used value for each. Judgment must be applied in computing local losses, taking into account any unique system conditions. Throughout this chapter K val-ues were obtained from equipment manufacturers when available. Valval-ues from Table 22.3 were used only as an approximation when more specific data were unavailable. The read-er is cautioned that thread-ere are application-specific charactread-eristics which have significant influence on the K factors. One of these characteristics, for example, is size. A K value of 0.6 is often encountered in literature to characterize the losses associated with flow through the run of a tee. However, for flow past tees in large pipes this factor can be very small and nearly zero.

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Water and Wastewater Treatment Plant Hydraulics 22.7 W alski (1992 ) Cr ane Co. (1987 ) T en-State Standar ds (1978 ) Bulletin No. 2552, Univer sity o f W isconsi n Dau g hert y (1977 ) Camer on H ydr aulic Dat a Simon (1986 ) Sanks (1989 )

Committee on Pipeline Plannin

g (1975 ) Ty picall y Used V alu e

TABLE 22.3 Typical K Factors for Computing Local Losses

Valve and Fitting Types

Gate valve 100% open 0.39 0.19 0.19 0.1–0.3 0.2 0.2 75% open 1.1 1.15 1.2 1.2 50% open 4.8 5.6 5.6 5.6 25% open 27 24 24 25 Globe valve–open 10 10 10 4.0–6.0 10 10 Angle valve–open 4.3 5 2.1–3.1 5 1.8–2.9 2.5 5 Check valve–ball 4.5 65–70 5 Swing check 0.6–2.3 06–2.2 0.6–2.5 2.5 Butterfly valve–open 1.2 0.16–0.35 0.5 Foot valve–hinged 2.2 1.0–1.4 2.2 Foot valve–poppet 12.5 5.0–14.0 14 Elbows 45° regular 0.30–0.42 0.42 0.42 45° long radius 0.18–0.20 0.18 0.5 0.2 90° regular 0.21–0.3 0.25 0.7 0.25 90° long radius 0.14–0.23 0.18 0.6 0.19 180° regular 0.38 0.38

180° long radius (flanged) 0.25 0.25

Tees Std. teee–flowthrough run 0.6 0.6 0.6 0.3 1.8 0.6 Std. teee–flow-through branch1.8 1.8 1.8 1.8 0.75 1.8 Return bend 1.5 2.2 2.2 0.4 2.2 Mitre bend 90° 1.8 1.129–1.265 0.8 1.3 60° 0.75 0.471–0.684 0.35 0.6 30° 0.25 0.130–0.165 0.1 0.16 Expansion d/D = 0.75 0.18 0.19 0.2 0.2 d/D = 0.5 0.55 0.56 0.6 0.6 d/D = 0.25 0.88 0.92 0.9 0.9 Contraction d/D = 0.75 0.18 0.19 0.2 0.2 d/D = 0.5 0.33 0.33 0.3 0.33 d/D = 0.25 0.43 0.42 0.4 0.43 Entrancee–projecting 0.78 0.78 0.83 0.8 0.8 0.78 0.8 Entrancee–sharp 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Entrancee–well rounded 0.04 0.04 0.04 0.04 0.25 0.04 0.04 Exit 1.0 1.0 1.0 1.0 1.0

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22.3 HYDRAULICS OF WATER TREATMENT PLANTS

22.3.1 Introduction

Water treatment comprises the withdrawal of water from a source of supply and the treatment of raw water through a series of unit processes for the beneficial use of the system customers. Raw water quality can vary widely. The ultimate uses of water by the system customer (e.g., drinking, fire protection, irrigation, aquifer recharge, etc.) can also vary and be subject to different treatment level requirements and regulations. Therefore, the selected treatment processes vary widely over a multitude of treatment technologies in use. Water treatment consists of a series of chemical, biological, and physical processes connected by channels and pipelines. Figures 22.1 and 22.2 illustrate process flow diagrams (flowsheets) for typical surface water and groundwater treatment plants, respectively. The designer of the water treatment process must carefully evaluate source water characteristics and desired water quality characteristics of the treated water to design treatment processes capable of purifying the source water to water suitable for the system customers. The objective of this chapter is to review the hydraulic considerations required to convey water through the treatment process.

Design of a plant’s treatment process is closely linked with the hydraulic design of the treatment plant. This chapter presumes that the designer has evaluated and selected treat-ment processes for the water treattreat-ment plant. Although design flows are discussed below, we have also assumed that the designer has chosen a design flow requirement for the treat-ment process. For municipal treattreat-ment plants, design flows are based on the service area

FIGURE 22.1 Typical surface water treatment plant process flow diagram.

FIGURE 22.2 Typical ground water treatment plant process flow diagram with dual trains (#1 and #2).

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population and the per capita use of water by the population served. The per capita use of water can be obtained from literature sources as an initial approximation. However, these initial estimations must be corroborated with actual site specific population counts and water usage. For nonmunicipal treatment facilities, treated water needs of the service area must be individually evaluated.

22.3.1.1 Sources of supply. Natural sources of supply include groundwater and surface water supplies. Groundwater supplies typically are smaller in daily delivery but serve more systems than surface water supplies. Groundwater supplies normally come from wells, springs, or infiltration galleries.

Wells constitute the largest source of groundwater. Except in rare circumstances of artesian wells (wells under the influence of a confined aquifer) and springs, groundwater collection involves pumping facilities. Hydraulics of groundwater treatment plants are fre-quently based on hydraulics of conduits under pressure, such as pipelines, pressure filters, and pressure tanks. Raw water characteristics of groundwaters are uniform in quality compared with surface supplies.

Surface water supplies are normally larger in daily delivery. Surface supplies are used to service larger population centers and industrial centers. In areas where groundwater supplies are limited in yield or where groundwater supplies contain undesirable chemical characteristics, smaller surface water treatment plants may be utilized. Surface water sources of supply include rivers, lakes, impoundments, streams, and ponds. The treatment processes chosen in plants treating surface water favor nonpressurized systems such as gravity sedimentation. The larger flow volumes characteristic of surface water supplies also favor open channel hydraulic structures for conveying water through the treatment process. Raw water characteristics of surface supplies can vary rapidly over short periods of time and also experience seasonal variation.

22.3.1.2 Treatment requirements. Treatment requirements for municipal water treat-ment plants are normally defined by regulatory agencies having authority over the plant’s service area. In the United States, regulatory agencies include national government regu-lations promulgated through the Environmental Protection Agency and state government regulations. Water treatment plants are designed to meet these regulations. Treatment reg-ulations change through improved knowledge of health effects of water constituents and through identification of possible new water-borne threats. The designer therefore should attempt to select treatment processes which will also meet treatment requirements which are expected to be promulgated over the next few years. To the extent possible, treatment plant process design should provide flexibility for future plant expansions or for possible additional treatment processes. Because hydraulic design of plants must go hand-in-hand with the process selection, plant hydraulic design should provide for the flexibility to add future plant facilities.

Treatment requirements for industrial water treatment plants are dictated by process needs of the industry and less by regulatory agency requirements. Industrial water treat-ment plants that result in contact between or ingestion of the treated water by humans must conform to the local regulatory requirements.

22.3.1.3 General design philosophy. Effective design of water treatment plant hydraulics requires that the hydraulic designer have a thorough knowledge of all aspects of the water system. The overall treatment system hydraulic design must be integrated and coordinat-ed including the treatment plant, the raw water intake and pumping facilities, the treatcoordinat-ed water storage, and treated water pressure/head requirements. The design within the water treatment plant must also be integrated between the various treatment processes.

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Additionally, design considerations must address the availability of operating personnel and hours of operation such that the process and hydraulic requirements conform to avail-able resources.

22.3.2 Hydraulic Design Considerations in Process Selection

Water treatment plant process selections are controlled principally by characteristics of the raw water and by the desired water quality characteristics of the finished water. Flow through each unit process and each conduit connecting processes results in loss of hydraulic head. Most treatment plants have limited head available.

The selection of a particular unit process will include evaluation of numerous criteria including costs, operability, performance, energy use and similar items. One criteria which must also be evaluated for each process is the hydraulic head requirements of the process.

22.3.2.1 Head available. For the design flow to pass through a water treatment plant, the total available head must exceed the head requirements of the unit processes and con-necting conduits. The head available is the difference in energy grade line (EGL) in the hydraulic profile between the head works of the plant and the end of the plant. Additional head may be provided by pumping or by lowering the elevation of treatment units at the end of the plant. See Figure 22.3 for a typical water treatment plant hydraulic profile.

For most surface water plants, the hydraulic profile at the head of the plant is controlled by raw water pumps pumping from the intake facilities. The hydraulic profile at the head of a plant in a groundwater system is typically determined by the well pumps serving the plant.

22.3.2.2 Typical unit process head requirements. Following below is a table of typical head requirements for water treatment plant processes. This table may be used for initial evaluation of unit processes. More detailed hydraulic evaluations must be performed after plant operating modes and design flows are determined. Detailed hydraulic evaluations must also include headlosses in connecting conduits.

Head Requirement

Unit Process at Rated Capacity, m (ft)

Intakes, including bar screens 0.3–0.9 (1–3)

Rapid mixing 0.15–0.30 (0.5–1) Flocculation 0.06–0.15 (0.2–0.5) Sedimentation 0.6–2.4 (2–8) Filtration – Gravity 3–4.6 (10–15) – Pressure 3–7.6 (10–25) Disinfection 0.15–0.6 (0.5–2) Aeration – Spray 3–4.6 (10–15) – Cascade 3–4.6 (10–15) – Compressed air 0.15–0.6 (0.5–2) Softening 0.15–0.6 (0.5–2)

Ion exchange softening 0.6–1.5 (2–5)

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Water and Wastewater Treatment Plant Hydraulics 22.11 FI GU RE 22. 3 H y draulic p rof ile .

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22.3.3 Hydraulic Design Considerations in Plant Siting

Plant sites are normally selected before the hydraulic designer initiates design of the treat-ment system. If a plant site has not been selected, the designer should be aware of hydraulic considerations which may influence site selection.

Site elevation has the most significant impact on plant hydraulics. A plant site located above the service area will eliminate or reduce pumping requirements from the plant to the service areas. Typical municipal distribution system pressures are 40–70 psi, therefore the elevation of the treatment plant should be at least 100 ft above the service area to elim-inate finished water pumping. Similarly, plant sites which permit gravity intake of the source water may reduce or eliminate raw water pumping. Few plants are able to meet these optimal conditions.

The typical surface water plant must pump both raw and finished water. Raw water (low-lift) pumps are used to pump water from the water source into the treatment facilities and finished water (high-lift) pumps are used to pump from the treatment plant into the service area distribution system.

22.3.4 Hydraulic Design Consideration in Plant Layout

After the plant site has been identified, the plant design may be arranged for optimal hydraulic benefit. In particular, arrangement of treatment processes to allow flow to move down gradient minimizes excavation needs for structures. Arrangements which are designed for future expansion should consider the hydraulic needs of the expanded plant as well as the process needs. Grouping of processes together facilitates movement of water through the treatment process train.

The designer should also consider secondary hydraulic systems for optimal design. Chemical feed systems and dewatering systems are examples of secondary hydraulic systems which must be coordinated with the treatment flow system. Normally it is desirable to minimize the length of chemical piping systems. Dewatering systems are usu-ally based on gravity drainage of basins and conduits.

22.3.5 Bases for Design

After evaluation and selection of a source of supply and development of the treatment plant process train, the designer is prepared to develop the plant Bases for Design. The Bases for Design is a summary of design flow and capacity, and proposed treatment processes, including the chemical storage and feed facilities.

22.3.5.1 Design flows. Design flows for water treatment plants serving municipalities are typically based on the projected population within the water service area for the design life of the treatment facilities. Population data is normally determined from census records, land use zoning information, and studies of existing and projected population densities. Service area per capita demands are affected by the mix of domestic, commercial, and industrial water users which are unique to each service area.Typically water consumption records are available for water service areas. For new facilities, the use of generalized water consumption data may be needed. In the United States, water consumption varies widely but generally ranges between 100–200 gal-lons per capita per day.

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From studies of projected population and per capita demand, planned design flows for the water treatment facilities may be developed. These demands include the following: • Annual average demand. The average daily water consumption for the water service

areas, generally computed by multiplying the average daily consumption (gallons per capita) by the projected population of the service area.

• Maximum demand. Maximum demand experienced by the water plant throughout its service life. The maximum hour demand is generally 200 to 300 percent of the aver-age demand but numerous factors affect the peak demand experienced by water treat-ment plants. These factors include seasonal demands (particularly for plants where ser-vice areas are located in extremes of hot and cold temperatures), normal daily flow variations, the community size, industrial usage, and system storage. Normally system storage is provided to service peak hour demands, allowing the treatment facilities to be designed on peak day demands. Peak day demands generally range between 125 and 200 percent of the average demand.

• Minimum flow. As the name suggests, the minimum flow expected to be processed through the treatment facilities. Minimum flow depends upon system operations. In general, minimum flows for municipal plants may be estimated as 50 percent of the average demand, but range between 25 and 75 percent of the average demand. 22.3.5.2 Rated treatment capacity. The rated treatment capacity of a plant is that capac-ity for which each of the unit processes are designed. For municipal treatment plants with adequate system storage, the rated treatment capacity is the system’s maximum day demand. Where storage is limited, the rated treatment capacity may be greater, for exam-ple, the system maximum hour demand or greater. Smaller systems may be designed to produce the rated treatment capacity in one or two 8-h shifts rather than over the entire 24-h day.

22.3.5.3 Hydraulic treatment capacity. Treatment plants are normally designed for a hydraulic capacity greater than the rated treatment capacity. Hydraulic treatment capaci-ties are normally equal to 125 to 150 percent of the rated treatment capacity. The hydraulic treatment capacity provides flexibility for future process changes or alternative flow rout-ings through the plant. Hydraulic capacities in excess of the rated treatment capacity pro-vide some margin of safety for operations which may not be optimal (e.g., control gates inadvertently left partially open).

22.3.5.4 Treatment process bases for design. The development of the water treatment plant’s “Bases for Design” is a key step in establishing the criteria to which the plant will be designed. This document must be reviewed carefully with the water treatment plant owner representatives and understood and agreed to by all before the final design pro-ceeds. The Bases for Design presents a summary of each treatment process including design flows (minimum, average, rated capacity), specification of dimension of major ele-ments (e.g., tanks, pumps), both hydraulic and process loading characteristics, required performance, and design data for the chemical storage and feed system. Table 22.4 pre-sents an example of the bases for design for sedimentation basins (one of the many unit processes in a water treatment plant).

22.3.6 Plant Hydraulic Design

As noted above, a water treatment plant consists of a series of treatment processes connected by free surface flow channels and pipelines. During development of the plant’s

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TABLE 22.4 Treatment Process Bases for Design—Sedimentation Basins

Item Stage I Stage II StageIII

Maxi– Maxi–

Maxi-Annual mum Annual mum Annual mum

Average Day Average Day Average Day

Number of basins 4 4 8 8 12 12

Basin characteristics

Plan–75 ft 230–6 in

Nominal side water depth–12 ft (SWD) Surface area/basin–17,288 ft2 Volume/basin–207,456 f_ff3 Channels/basin–2 L:W ratio–6.1:1 Displacement time (h) 3.17 1.99 3.17 1.99 3.17 1.99

Surface loading [(galⴢm)/ft2_] _0.47 _0.75 _0.47 _0.75 _0.47 _0.75

Flowthrough velocity (ft/min) 1.21 1.93 1.21 1.93 1.21 1.93

Sludge collectors Longitudinal collectors

Type: chain flight

Number per basin 8 8 8 8 8 8

Cross collectors Type: chain flight

Number per basin 1 1 1 1 1 1

Settled sludge pumps Type: progressive cavity Number: 100 gal/min capacity 4 4 4 4 4 4 400 gal/min capacity 4 4 4 4 4 4 200 gal/min capacity — — 8 8 16 16 Capacity (gal/min) Installed 2000 2000 3600 3600 5200 5200 Firm 1600 1600 3200 3200 4800 4800

Bases for Design, the designer determines the rated treatment capacity, average flow, minimum flow and hydraulic capacity of the plant.

Following development of the Bases for Design, the designer must evaluate plant operating modes to develop a detailed plant flow diagram and hydraulic profile through the plant.

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22.3.6.1 Plant operating modes. Operating modes describe the sequence of treatment processes the water goes through to achieve the required level of purification. Operational modes are normally presented in the form of simplified block diagrams which illustrate the flow path through the plant from one process to the next. These operational mode block diagrams are useful in visualizing stages during construction, future planned plant expansions or simply alternative operating modes.

Figures 22.4 through 22.9 show an example of a sequence of plant operating modes for a surface water treatment plant which illustrate three stages of a plant expansion program with alternatives for the flocculation and sedimentation basins to work in series or in par-allel. Plant processes proposed include raw water control chambers, rapid mix chambers, flocculation/sedimentation basins, ozone contact chambers, and filters. In this example, the raw water control chambers are used to split flow between plant process groups and also as a rapid mix chamber for chemical addition.

FIGURE 22.4 Stage I—operational mode diagram.

FIGURE 22.5 Stage II—parallel operational mode diagram.

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The Stage I facilities including raw water control chamber, flocculation/sedimentation basins and filters are depicted in Fig. 22.4. Operational modes for a proposed plant expan-sion to double the plant capacity (Stage II) are shown in Figs. 22.5 through 22.7 and oper-ating modes for a second plant expansion to triple the plant capacity (Stage III) are shown in Figs. 22.8 and 22.9. Settled water ozone contact chambers were added to the expanded plant, which illustrates treatment upgrades.

Operational modes for the Stage II treatment plant include parallel and series floc-culation/sedimentation. When the plant is operated in the parallel mode, influent raw water for each set of sedimentation basins flows by gravity from the raw water control chamber serving the basin set. Raw water flow is divided between each sedimentation basin in service at the raw water control chamber. Settled water from each set of basins is routed to an ozone contact chamber. Ozonated settled water is then combined prior to flowing to the filters.

FIGURE 22.6 Stage II—series flocculation/sedimentation basin operational mode diagram.

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The Stage III split parallel operational mode is similar to the parallel operational mode except that the ozonated settled water from each set of basins is not combined prior to flowing to the filters. Side-by-side plant scale treatment studies are possible with the future split parallel mode since part of the flocculation/ sedimentation/filtration processes can be operated as a “control” while the remainder of the plant can be operated in a controlled experimental mode.

The series flocculation/sedimentation operational mode is designed to permit opera-tion of the sedimentaopera-tion basins in two stages in lieu of the single–stage parallel mode. Under certain raw water conditions, operation of the basins in series may enhance perfor-mance of the basins. Chemical feed for the first and second sedimentation stages may be adjusted to respond to raw water conditions and settled water quality after the first–stage sedimentation. Series flocculation/sedimentation increases hydraulic losses through the

FIGURE 22.8 Stage III—parallel operational mode diagram.

FIGURE 22.9 Stage III—split parallel operational mode diagram.

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plant. Under this mode, twice as much flow is routed to each basin and the flow pattern is longer, since the settled water from the first sedimentation stage must be returned to the influent of the second sedimentation stage.

Operational mode block diagrams are also a convenient means to illustrate the effect of side stream flows which may impact the overall plant flow. For example, removal of sludge from the sedimentation basins is accompanied by a decrease in flow leaving the basins compared with flow entering the basins. In a similar manner, filter backwash water removes a certain amount of flow. A plant designed to produce a certain rated capacity may have to treat more than the rated capacity through certain processes. The impact of these side stream flows must be evaluated on an individual basis. In many treatment plants, backwash water treatment facilities are installed to recycle backwash water to the head of the plant.

22.3.6.2 Plant flow diagrams. After establishing plant operating modes, more detailed flow diagrams are developed by the designer. The diagrams normally start with possible valving and gating arrangements and are then expanded with tentative valve, sluice gate, pipeline, and conduit sizes.

Valving arrangements are designed to enable any of the major operational units (e.g., sedimentation basin, ozone contact chamber) to be removed from service. The arrange-ment may include design of temporary flow stop devices, such as stop logs (sectional bar-riers which were originally constructed of logs but are now commonly metal plates). The arrangement should be designed to permit maintenance work on major valves and sluice gates while minimizing the impact on plant process. Major channel sections should be designed so they can be removed from service and dewatered while minimizing impacts on the rest of the plant.

The designer should distinguish between units taken out of service frequently (such as filters), periodically (such as sedimentation basins), or rarely (such as conduits). Filter backwashing occurs so frequently that the rated treatment capacity can be met with one filter out for backwashing. Sedimentation basins may be removed from service once or twice per year for equipment maintenance. Since the basins outages occur at widely scattered intervals, it is reasonable to design the units to be removed from ser-vice during lower flow periods. Conduits and pipelines are rarely removed from serser-vice, but the hydraulic impacts can be significant. Depending on the conduit location, removal of a conduit can remove a portion of the plant from service. Effective design will provide redundant conduits so that a portion of the plant can remain in service dur-ing conduit dewaterdur-ing.

The focus of this section has been on the main plant hydraulics, but the hydraulic designer must also design for hydraulic subsystems. An important group of these subsys-tems include dewatering of all basins and conduits. Where plant elevations will allow, gravity dewatering is recommended. In most cases, dewatering pumps are necessary. These pumps may be located in the unit being dewatered or may be located in a separate structure connected to the process unit by dewatering pipelines.

22.3.6.3 Hydraulic Profile. One of the most important tools in the hydraulic design of a water treatment plant is the development of a hydraulic profile. The hydraulic profile is a diagram showing the energy grade line (EGL) at each unit process. For open tanks with flows at minimal velocities, which is the case in most water treatment plants, the velocity head is negligible and the hydraulic grade line (HGL) or water surface elevation (WSEL) provide an adequate representation of the EGL. Profiles normally include critical struc-tural elevations of processes and conduits. The profile may also include ground surface profiles and other site information.

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Hydraulic profiles are developed for each of the design flows. In the case of water treatment plants, the design flows may include rated treatment capacity, hydraulic capac-ity, average flow, and minimum flow. Hydraulic profiles should also take into considera-tion unit processes or conduits which may be taken out of service. Hydraulic profiles are valuable design and operational tools to assist in scheduling routine maintenance activi-ties and for evaluating the impact to the treatment plant capacity during outages of process units or conduits.

Computations of hydraulic profiles begin at control points where there is a definite relationship between the plant flow and water surface depth. For gravity flow plants, the most common forms of control points are weirs and tank water surface elevations (e.g., clear well water surface elevations), but other types of control points may be used. From each control point, head losses associated with local losses, plant piping, and open chan-nel flow are added to the control water surface. Since flows in water treatment plant’s are mostly in the subcritical regime (Froude number 1), most hydraulic designers will work upstream from the control point. For pressure plants, control points are typically pressure regulating or pressure control points, frequently in the service area distribution system. From these control points and knowledge of the flow velocity, both the EGL and HGL may be computed back to the treatment facilities.

Hydraulic profiles are valuable design tools to identify overall losses through the plant. Profiles are also valuable to identify units with excessive losses. Since total head available is normally limited, units with excessive losses should be considered for redesign to reduce local loss coefficients or to reduce velocities.

Figure 22.3 is an example hydraulic profile for a gravity surface water treatment plant with conventional treatment processes. The method of computing headlosses is presented in Section 22.3.7.

22.3.7 Water Treatment Plant Process Hydraulics

In this section calculations required to establish the WSEL through a medium-sized water treatment plant will be presented. A schematic of the water treatment plant is shown in Fig. 22.10. Notice that future growth has been considered in the initial design. Three examples are included which illustrate typical hydraulic calculations. The first example calculates the WSEL from the sedimentation basin effluent chamber back through the flocculation/sedimentation basins to the Raw Water Control Chamber. The second follows the flow from the clear well back through the filters. Filter hydraulics are illustrated in the third example. All examples are presented in a spreadsheet format which is designed to facilitate calculating the EGL, HGL, and WSEL at various points through the treatment process and for multiple flow rates (i.e., minimum, daily average, peak hour, future conditions).

22.3.7.1 Coagulation. Process criteria and key hydraulic design parameters. The coag-ulation process, used to reduce particulates and turbidity, is carried out in three steps: mix-ing (often referred to as rapid or flash mixmix-ing), flocculation, and sedimentation. Each of these steps is briefly discussed below.

Rapid mixing. The mixing process imparts energy to increase contact between existing solids and added coagulants. Possible mixer types include turbine, propeller, pneumatic, and hydraulic. Headloss that occurs in mixing chambers depends on the cho-sen mixing device. Most mechanical mixers do not create significant head losses. The headloss coefficient (K) associated with a specific mixer can be obtained from the manu-KK facturer. Pneumatic mixing, which is not common, has associated losses similar to those

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for aeration (see table in Section 22.3.2.2, above). Hydraulic mixing takes place using weirs, swirl chambers, throttled valves, Parshall flumes, or other devices to induce turbu-lence. Head loss coefficients for these devices can be obtained from the manufacturer. Important considerations during the initial design of a mixing chamber include:

• Velocity gradient. This is mixer—specific information and can be obtained from the manufacturer. The system should be designed to provide a velocity gradient that is optimal for the coagulation process taking place.

• Dead spots and short circuiting. An ideal mixing system will have minimal dead spots and short circuiting. These can be avoided with proper sizing and placement of mixers.

Flocculation. Coagulated particles form larger particles (flocs) during the gentle mix-ing of flocculation, where the flow travels slowly through a series of flocculator paddles, baffles, or conduits. Inlets and weirs are designed to provide low turbulence for protection of the flocs. The energy provided to the system by the flocculators (manufacturer-specif-ic) or baffling is decreased as the flow approaches the sedimentation basins.

Sedimentation. Gravity sedimentation removes coagulated solids prior to filtration. There are four zones in a clarifier as shown in Fig. 22.11 and listed below:

• Inlet zone—where upstream flow conditions transition smoothly to uniform flow set-tling conditions

• Sedimentation zone—where sedimentation takes place • Sludge zone—where solids collect and are removed

• Outlet zone—where settling conditions smoothly transition to downstream flow conditions

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Each of the zones is designed to minimize turbulence and avoid short circuiting. The velocity in the sedimentation zone is limited to 0.3 m/s (1 ft/s) for average flow. Sludge removal equipment moves slowly so that settling patterns are not disturbed. Because the process is designed for smooth flow and minimal turbulence, very little head loss occurs in sedimentation basins. Ports at the inlet and outlet produce the greatest head losses in this process.

Hydraulic design example. Table 22.5 illustrates the calculation of the WSEL, using metric units, through the coagulation process at the medium-sized water treatment plant shown in Fig. 22.10. Figs. 22.12 through 22.14 show plan views and details of the

FIGURE 22.11 Hypothetical zones in a rectangular sedimentation basin.

TABLE 22.5 Hydraulic Calculations of a Typical Coagulation Process, SI Units

Initial Operation Design Operation

Parameter Min. Day. Avg. Day Avg Day Max. Hour

1. Plant Flow (m3_/s) _2.19 _3.06 _3.28 _4.38

Note: For Points 1 through 8, see Fig. 22.12

2. WSEL at Point 1 (Calculation done in Table 22.6) (m) 109.73 109.73 109.74 109.74 3. Point 1 to Point 2

Average flow 21Q/32 (m3_/s) _1.44 _2.01 _2.15 _2.87

Flow depth WSEL @ 1 – invert (106.60 m) (m) 3.13 3.13 3.13 3.14 Flow area 5.13 m width depth (m2₎ _16.05 _16.06 _16.07 _16.10

Velocity flow/area (m/s) 0.09 0.13 0.13 0.18

Hydraulic Radius r A/P/ (P w 2d) (m) 1.41 1.41 1.41 1.41

Conduit loss [(V n)/(r_{rr )]}2/3 2_{L (m)}

where Manning’s n 0.014 and L 28.96 m 0.00 0.00 0.00 0.00

WSEL at Point 2 (m) 109.73 109.73 109.74 109.74

4. Point 2 to Point 3

Average Flow 5Q/16 (m3_/s) _0.68 _0.96 _1.03 _1.37

Flow depth WSEL @ 2 invert (106.60 m) (m) 3.13 3.13 3.13 3.14 Flow area 5.13 m width depth (m3₎ _16.05 _16.06 _16.07 _16.10

r A/P// (P w 2d) (m) 1.41 1.41 1.41 1.41

Conduit loss [(V n)/(r_{rr )]}2/3 2_{L (m)}

where Manning’s n 0.014 and L 14.63 m 0.00 0.00 0.00 0.00

WSEL at Point 3 (m) 109.73 109.73 109.74 109.74

Average flow Q/8 (m3_/s) _0.27 _0.38 _0.41 _0.55

Flow depth WSEL @ 3—invert (106.60 m) (m) 3.13 3.13 3.13 3.14 Flow area 5.13 m width depth (m3₎ _16.05 _16.06 _16.07 _16.10

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TABLE 22.5 (Continued)

Parameter Min. Day. Avg. Day Avg. Day Max. Hour

Velocity flow/area (m/s) 0.02 0.02 0.03 0.03 r = A/P// (P w 2d) (m) 1.41 1.41 1.41 1.41 Conduit loss [(V n)/(r_{rr )]}2/3 2_{L (m)} where n 0.014 and L 21.95 m 0.00 0.00 0.00 0.00 WSEL at Point 4 (m) 109.73 109.73 109.74 109.74 6. Point 4 to Point 5 Flow Q/32 (m3_/s) _0.07 _0.10 _0.10 _0.14

Port area 0.30 m deep 0.76 m wide (m2₎ _0.23 _0.23 _0.23 _0.23

Submerged entrance loss 0.8 VVV /2g (m)2 0.00 0.01 0.01 0.01 WSEL at Point 5 (in Sedimentation Tank) (m) 109.73 109.74 109.74 109.76 7. Point 5 to Point 6

Width of sedimentation basin (W) (m)WW 23.16 23.16 23.16 23.16

Flow (Q/4) (m3_/s) _0.55 _0.77 _0.82 _1.09

Invert elevation of sedimentation baffles (m) 105.97 105.97 105.97 105.97 Flow depth (H) (WSEL at Point 5—baffle invert) (m)HH 3.76 3.77 3.77 3.79 Area downstreams of baffle (W H) (mHH 2₎ _87.21 _87.36 _87.41 _87.68 Horizontal openings in baffle, 2.54 cm wide

spaced every 22.86 cm. Area of

openings A W .0254 H/.2286 (m2₎ _9.69 _9.71 _9.71 _9.74 Velocity of downstream baffle (V downstream) 0.01 0.01 0.01 0.01

(Q/A) (m/s)

Velocity of 2.54 cm opening section (V1) (Q/A// ) (m/s) 0.06 0.08 0.08 0.11 Local losses sudden expansion (1.0 (V downstream)2_/2g)

 sudden contraction (0.36 (VI)VV 2

/ 2g) (m) 0.00 0.00 0.00 0.00

WSEL at Point 6 (Upstream of sedimentation baffles) (m) 109.73 109.74 109.74 109.76 8. Point 6 to Point 7

Loss per stage (provided by flocculator manufacturer) (m) 0.01 0.01 0.03 0.05

Total loss (three stages) (m) 0.04 0.04 0.09 0.15

WSEL at Point 7 (m) 109.77 109.78 109.83 109.91

Flow Q/24 (m3_/s) _0.09 _0.13 _0.14 _0.18

Port area 0.30 m deep 0.46 m wide (m2₎ _0.14 _0.14 _0.14 _0.14

Velocity flow / area (m/s) 0.65 0.92 0.98 1.31

Entrance loss 1.25 V_V_{V /2g (m)}2 _0.03 _0.05 _0.06 _0.11 WSEL at Point 8 (inlet port) (m) 109.80 109.83 109.89 110.02

Average flow Q/24 (m3_/s) _0.09 _0.13 _0.14 _0.18

Velocity flow/area (m/s) 0.15 0.19 0.19 0.22 r = A/P (P w 2d) (m) 0.27 0.28 0.29 0.30 Conduit loss [(V n)/(r_{rr )]}2/3 2_{L (m)} where n 0.014 and L 3.86 m 0.00 0.00 0.00 0.00 WSEL at Point 9 (m) 109.80 109.83 109.89 110.02 11. Point 9 to Point 10 Average flow Q/12 (m3_/s) _0.18 _0.26 _0.27 _0.36

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Velocity flow/area (m/s) 0.29 0.39 0.39 0.44 r = A/P// (P w 2d) (m) 0.27 0.28 0.29 0.30 Conduit loss [(V n)/(rrr2/3)]2_{L (m)} where n 0.014 and L 3.86 m 0.00 0.00 0.00 0.00 WSEL at Point 10 (m) 109.80 109.84 109.89 110.02 12. Point 10 to Point 11 Flow Q/8, m3_/s _0.27 _0.38 _0.41 _0.55

Flow depth WSEL @ 10 invert (109.12 m) (m) 97.34 97.38 97.44 97.56 Flow area 0.91 width depth (m2₎ _89.01 _89.04 _89.09 _89.21

Loss at two 45° bends 2 0.2 V2/2g (m) 0.00 0.00 0.00 0.00

WSEL at Point 11 (m) 109.80 109.84 109.89 110.02

Flow Q/4 (m3_/s) _0.55 _0.77 _0.82 _1.09

Flow depth WSEL @ 11 invert (109.12 m) (m) 0.68 0.72 0.78 0.90 Flow area 1.52 m width depth (m2₎ _1.04 _1.09 _1.18 _1.37

Loss at two 45° bends 2 0.2 V2_{/2g (m)} _0.00 _0.00 _0.00 _0.00

r = A/P// (P w 2d) (m) 0.36 0.37 0.38 0.41 Conduit loss [(V n)/(r_{rr )]2}2/3 L (m) where n 0.014 and L 9.75 m 0.00 0.00 0.00 0.00 WSEL at Point 12 (m) 109.81 109.84 109.90 110.03 14. Point 12 to Point 13 Flow Q/4, (m3_/s) _0.55 _0.77 _0.82 _1.09

Flow depth WSEL @ 12 invert (109.12 m) (m) 0.69 0.72 0.78 0.91 Inlet area 1.52 m width depth (m2₎ _1.05 _1.10 _1.19 _1.38

Inlet loss 1 V2_{/2g (m)} _0.01 _0.02 _0.02 _0.03

WSEL at Point 13 (Mixing Chamber No. 2 outlet) (m) 109.82 109.87 109.92 110.06 15. Point 13 to Point 14

Note: Mixers provide negligible head loss

Flow Q/4 (m3_/s) _0.55 _0.77 _0.82 _1.09 Chamber area 1.83 m 1.83 m (m2₎ _3.34 _3.34 _3.34 _3.34 Velocity flow/area (m/s) 0.16 0.23 0.25 0.33 Losses Mixer (1 V2 /2g) Sharp bend (1.8 V2 /2g) (m) 0.00 0.01 0.01 0.02 WSEL at Point 14 (Mixing Chamber No. 2 inlet) (m) 109.82 109.87 109.93 110.07

Flow Q/2 (m3_/s) _1.09 _1.53 _1.64 _2.19

Conduit area 2.29 m wide 1.22 m deep (m2) 2.79 2.79 2.79 2.79

Velocity flow/area ( m/s) 0.39 0.55 0.59 0.78

R = A/P// (P 2w 2d) (m) 0.40 0.40 0.40 0.40

Conduit losses L [V/(0.849VV _{C R}0.63_{)] 1/0.54 (m)}

where L 47.24 m and Hazen-Williams C 120 0.00 0.01 0.01 0.02 Local losses flow split (0.6 V2

/2g) contraction (0.07 V2

/2g) 0.67 V2

/2g (m) 0.01 0.01 0.01 0.02

WSEL at Point 15 (at Mixing Chamber No. 1) (m) 109.83 109.89 109.95 110.11

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17. The above calculations (for Points 1 through 15) have been for flow routed through Tank No. 4. When the

flow is routed through Tank No. 1. the WSEL (m) is: 109.82 109.88 109.94 110.08 In reality, the headloss through each basin is equal.

The flow through the basin naturally adusts to equalize headlosses, i. e. flow through Tank No. 1 is greater than Q/4 and flow through Tank No. 4 is less than Q/4. The actual headloss through each basin can be estimated as the average of: Losses through Tank No’s. 1 and 4

and the WSEL (m) at Point 15 is: 109.83 109.89 109.95 110.10 18. Point 15 to Point 16

Flow Q (m3_/s) _2.19 _3.06 _3.28 _4.38

Conduit area 2.29 m wide 1.22 m deep (m2₎ _2.79 _2.79 _2.79 _2.79

Velocity flow/area (m/s) 0.78 1.10 1.18 1.57 R A/P// (P 2w 2d) (m) 0.40 0.40 0.40 0.40 Conduit losses L [V/(0.849VV C R0.63_)] 1/0.54 (m) where L 125.58 m and Hazen-Williams C 120 0.04 0.08 0.10 0.16 WSEL at Point 16 (m) 109.87 109.97 110.04 110.26 19. Point 16 to Point 17 Flow Q (m3_/s) _2.19 _3.06 _3.28 _4.38

Conduit area @ 16 2.29 m wide 1.22 m deep (m2₎ _2.79 _2.79 _2.79 _2.79 Conduit area @ 17 1.68 m wide 1.68 m deep (m2₎ _2.81 _2.81 _2.81 _2.81

Average area (m2₎ _2.80 _2.80 _2.80 _2.80

Velocity flow / Area (m/s) 0.78 1.09 1.17 1.56

R @ 16 A16/ (2 (2.29 m 1.22 m)) (m) 0.40 0.40 0.40 0.40 R @ 17 A17/ (2 (1.68 m 1.68 m)) (m) 0.42 0.42 0.42 0.42 Average R, (m) 0.41 0.41 0.41 0.41 Conduit losses L [V/(0.849VV C R0.63_{)]1/0.54 (m) where L} 9.14 m and Hazen-Williams C 120 0.00 0.01 0.01 0.01 WSEL at Point 17 (m) 109.88 109.98 110.05 110.27 20. Point 17 to Point 18 Flow Q (m3_/s) _2.19 _3.06 _3.28 _4.38

Conduit area @ 17 1.68 m wide 1.68 m

deep (m2₎ _2.81 _2.81 _2.81 _2.81 Velocity 17 flow/area 17 (m/s) 0.78 1.09 1.17 1.56 Pipe area @ 18 (D 4) 2 (m) where D 1.68 m 2.21 2.21 2.21 2.21 Velocity 18 flow/area 18 (m) 0.99 1.39 1.49 1.98 Exit losses V182_{/2g – V17}2_{/2g (m/s)} _0.02 _0.04 _0.04 _0.8 WSEL at Point 18 (m) 109.90 110.01 110.09 110.35 21. Point 18 to Point 19 R = A/P// (P d ) (m) 0.42 0.42 0.42 0.42

Local losses 3 elbows (3 0.25V2_/2g) entrance (0.5 V2 /2g) 1.25 V2 /2g (m) 0.06 0.12 0.14 0.25 Conduit losses L [V/(0.849VV C R0.63_{)]1/0.54 (m) where L} 138.68 m and Hazen-Williams C 120 0.07 0.13 0.15 0.26

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Weir elevation (m) 109.73 109.73 109.73 109.73

Depth of flow over weir (WSEL @

19 – weir elevation), (m) 0.30 0.54 0.66 1.13

Length of weir, L, (m) 2.74 2.74 2.74 2.74

Flow over weir q 1.71 h3/2_{[ 1 (d / n)}3/2

]0.385

L

Note: Rather than solve for h, find an h by trial

and error that gives a q equal to the flow for the given flow scenarios (given in Item 1)

assume h (m) 0.60 0.90 0.95 1.35 then q (m3_/s) _1.84 _3.14 _3.12 _4.21 assume h (m) 0.66 0.89 0.97 1.37 then q (m3_/s) _2.18 _3.07 _3.27 _4.42

Note: These q’s equal the flows for the given

scerios (Item 1)

h (m) 0.66 0.89 0.97 1.37

WSEL at Point 20 (h WSEL @ Point 19) (m) 110.39 110.62 110.70 111.10 23. Point 20 to Point 21

Flow Q (m3_/s) _2.19 _3.06 _3.28 _4.38

Sluice gate area 1.37 m 1.37 m (m2₎ _1.88 _1.88 _1.88 _1.88

Velocity Flow/Area (m/s) 1.16 1.63 1.74 2.33

Gate Losses 1.5 V2_{/2g (m)} _0.10 _0.20 _0.23 _0.41 WSEL at Point 21 (Raw Water Control

Chamber) (m) 110.49 110.82 110.93 111.51

The overflow weir in the Raw Water Control Chamber is 3.05 m long and is sharp crested

Q = 1.82 L h3/2_{so h}_(Q/1.82L)2/3_(m) _0.54 _0.67 _0.70 _0.85

The water surface must not rise above elevation 112.78 m The overflow weir elevation may be safely set at 111.86 m

hydraulic reaches analyzed in the example. The circled numbers indicate points at which the WSEL is calculated. Hydraulic calculations start downstream of the sedimentation basins (Fig. 22.12) and proceed upstream through the mixing chamber (Fig. 22.13) and the Raw Water Control Chamber (Fig. 22.14). Mechanical mixers and mechanical floccu-lators are used. Conduit losses between the rapid mix chambers and the Raw Water Control Chamber are also calculated in the example. Three different flow rates (i.e., min-imum day, average day, and, maxmin-imum hour) are used in the calculations. This is a range of design flow conditions that a design engineer would typically take into consideration.

The longest path through the flocculation and sedimentation processes, through Basin No. 4, is followed (Points 1 through 15). Although not shown, losses along the shortest path have also been calculated. As would be expected, the calculated head loss is smaller for the shorter path. The actual losses are equal for each path. The flows through each path naturally adjust to equalize losses. The flow through the longest path is slightly smaller than the flow through the shortest path. In the example, the WSEL at Point 15 is adjusted to reflect the average losses through the basins. The WSEL calculations upstream of Point

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15 are based on the adjusted WSEL. Alternatively the weirs or ports feeding flow into each basin may be adjusted to create an equal distribution of flows in all basins as discussed in Sec. 22.2.1.

22.3.7.2.2 Filtration. Process criteria. Suspended solids are removed from the water as it passes through a porous medium during filtration. Filters operate under either gravity or pressure. Filters also differ in the type and distribution of the media used (fine, course, uniformly graded, graded coarse to fine, etc.) and the direction of flow through the media (upflow, downflow, and biflow). Pressure filter hydraulics information is very product specific and should be obtained from the manufacturer. The design engineer using pres-sure filters should then apply this information to the project using project–specific hydraulic considerations. This section presents information on gravity filters.

Key hydraulic design parameters. The headloss through a filter increases with use as the voids become filled with solid particles. When the headloss reaches a certain point (terminal headloss), the filter is backwashed to remove the solids. The rate of headloss buildup is dependent on several factors, including how the filter is graded (the arrange-ment of media particle sizes). The rate of headloss buildup is reduced (and filtration is more effective) when the flow first goes through the coarse media and then the fine media.

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FIGURE 22.13 Mixing chamber

FIGURE 22.14 Raw water control chamber

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However, during backwash, the high rate of flow expands the filter bed and, over time, the media are regraded so that the more coarsely graded grains are located at the bottom and the fines are located at the top. To benefit from the coarse-to-fine grading, an upward flow pattern can be used, but is very uncommon. More often the filter media are selected such that the fine media have a higher specific gravity than the coarse media to maintain the course-to-fine gradation during backwash. The most commonly used filter media are nat-ural silica sand and crushed anthracite coal; however garnet and ilmenite are used in mixed media beds. Granular carbon is often used if taste and odor control is desired.

The terminal headloss is determined by a combination of factors including filter break-through (when the filter bed loses its adsorptive capacity), available static head, and out-let pressure required. The filter should be designed so that the headloss in any level of the filter bed does not exceed the static pressure. A negative head can result in air binding in the filter which will, in turn, further increase headloss.

Filter influent piping is sized to limit velocities to about (0.6 m/s). Wash-water and effluent piping flow velocities are kept below (1.8 m/s) so that hydraulic transients(waterhammer) and excessive headlosses are minimized and controlled to within tolerable limits.

Hydraulic design example. Table 22.6 illustrates the calculation of the WSEL from the clear well back upstream to the Sedimentation Basin effluent at the medium-sized water treatment plant shown in Fig. 22.10. Figures 22.15 and 22.16 show details of the hydraulic reaches analyzed in the example. Table 22.7 illustrates the filter hydraulic calculation, the details of which are shown in Figs. 22.17 and 22.18.

The hydraulic profile of the plant (based on hydraulic calculations done in Tables 22.5, 22.6 and 22.7) is shown in Figure 22.3.

TABLE 22.6 Hydraulic Calculations in a Medium–Sized Water Treatment Plant from the Filter Effluent to the Effluent Clearwell

Parameter Min Day Avg Day Avg Day Max Hour

1. Flow (m3_s) _2.19 _3.06 _3.28 _4.38

Note: for Points 22 through 28, see Figure 22.15

Maximum water level in Clearwell (Point 22) (m) 105.16 105.16 105.16 105.16 Invert in Clearwell (m) 101.50 101.50 101.50 101.50

Flow Q/2 (m3_/s) _1.09 _1.53 _1.64 _2.19

Stop logs @ A

Flow area (2 openings, 1.52 m wide,

3.66 m deep) (m2₎ _11.15 _11.15 _11.15 _11.15

Loss 0.5 V2_{/2g (m)} _0.00 _0.00 _0.00 _0.00

Baffles

Flow area (3.05 m wide, 3.66 m deep) (m2₎ _11.15 _11.15 _11.15 _11.15

Loss 1.0 V2_{/2g (m)} _0.00 _0.00 _0.00 _0.01

Stop logs @ B and C