Wiley Wastewater Quality Monitoring and Treatment_8 pdf

There are basically two basic types of flow measurement techniques: 1 those that rely on a relation between stage and discharge, e.g., Manning’s equation and flumes; and 2 those that est

sediment and debris deposition, turbulence, confined space/hazardous conditions is-sues, access, variable pipe slope along a reach resulting from differential settlement

of individual pipes, and different pipe sizes Nevertheless, continually increasing environmental concerns and the need to more optimally manage stormwater and wastewater flows have increased the need to accurately monitor flows in storm, sani-tary and combined sewers These concerns are not new, e.g., North Rhine-Westphalia, Germany, issued a decree that the most important detention facilities of the com-bined sewer network were to be equipped with continuous monitoring devices more than 20 years ago (Weyand, 1996) Fortunately, as the need for measurement devices capable of high accuracy has increased new and improved measurement techniques also have been developed over the last 20 years This chapter attempts to summarize the accuracy, advantages, and disadvantages of the available techniques

There are basically two basic types of flow measurement techniques: (1) those that rely on a relation between stage and discharge, e.g., Manning’s equation and flumes; and (2) those that estimate average velocity by acoustic or electromagnetic means and multiply this by the cross-sectional area obtained through a depth measurement device and known conduit geometry Most of these devices have two parts: (1) a primary device that directly interacts with or controls the flowing water; and (2) a

secondary device for measuring water depth (Church et al., 1999).

This chapter focuses on the general characteristics of the various measurement techniques of types 1 and 2 and does not provide a direct comparison of the com-mercially available equipment for flow measurement in sewers that apply the various measurement techniques Due to the limited space available and the limited num-ber of independent evaluations of measurement equipment, a proper comparison of the equipment cannot be done here Further, it is not the purpose of this chapter

to advocate or criticize any particular device, but rather to give the readers basic information on the measurement techniques to aid in the selection of the appropriate technique When purchasing equipment readers should carefully review the liter-ature provided by the manufacturers, discuss experience with the equipment with

professional colleagues, and apply the time-honoured principle of caveat emptor (let

the buyer beware)

2.2.1.1 Purposes of Flow Monitoring

There many reasons for flow monitoring, among the most common are:

(1) Real-time control (RTC) of the sewer system Existing large sewers can be controlled by gates, e.g., to increase storage capacity and prevent overburdening

of treatment plants (Curling et al., 2003) RTC also can optimize treatment

plant operation to ensure consent standards are met and to minimize the total pollutant load reaching the environment (Watt and Jeffries, 1996) or improve plant efficiency in order to provide capacity for future sewer extensions (Anon., 1996)

Introduction 121

(2) Sewerage system operational considerations Information about storage and dis-charge conditions can, for example, be the basis for optimizing cleaning and maintenance work (Weyand, 1996)

(3) In regional sewerage networks, flow monitoring can equitably allocate costs among communities

(4) Compliance with regulatory requirements

(5) Provide data for calibration and verification of numerical models (Baughen and Eadon, 1983)

(6) Identify inflow and infiltration (I/I) problems

(7) Performance evaluations of pumps and hydraulic structures (e.g., overflow structures)

Items (1)–(4) typically require long-term, effectively permanent, monitoring, whereas items (5)–(7) generally require only temporary monitoring On the basis

of 10 years of sewer monitoring experience in Germany, Weyand (1996) made two very important, related observations: (1) it is important to start the planning of monitoring systems with the formulation of necessary demands; and (2) experience shows that the requirements of monitoring systems rise with their use Thus, it is quite possible that sites that originally were established for a temporary study may become long-term sites, and careful selection of equipment and sites is necessary

2.2.1.2 Equipment Selection Considerations

Huth (1998) prepared a list of considerations for selection of flow measurement equipment Six of his eight issues are:

(1) Know the relative strengths and weaknesses of the available equipment (2) Buy the level of accuracy required for the application For example, high accu-racy is needed for RTC, cost allocation, and model calibration and verification; whereas lesser accuracy may be required for I/I studies, basic sewerage system operation, and performance evaluations of hydraulic structures However, it must

be remembered that requirements of monitoring systems rise with their use (3) Know your flow rate Sanitary sewers may have fairly constant flows, whereas storm and combined sewers have wider flow ranges and require equipment that

is accurate over a wide range of flow conditions Curling et al (2003) stressed

the importance of having high accuracy over the full range of flows noting that varying accuracy will increase variation in modelling results and lead to poor understanding of problem sites, improper estimation of capacity, and improper allocation of capital improvement funds

(4) Learn what is in the water Debris may clog some equipment (flumes) and reduce the performance of other equipment (acoustic transducers) requiring frequent maintenance, also high particulate loads may affect the ability of sound waves

to penetrate the flow

(5) Location, location, location (discussed in detail in Section 2.2.1.3)

(6) Make sure there is power

Similarly, Church et al (1999) noted that selection of the most appropriate method

for collection of accurate flow data that are representative of a particular site re-quires knowledge of the flow regime(s), range of flow rate and depth, rapidity of flow changes, channel geometry, and the capabilities and accuracies of the methods available for measuring flow

2.2.1.3 Monitoring Locations

The importance of proper site selection cannot be overstated (Church et al., 1999).

Most of the flow measurement techniques described in this chapter work best at sites where fully developed, uniform, open channel flow not subject to backwater effects

is present composing optimal hydraulic conditions Fully developed, uniform open channel flow usually requires many diameters of straight, uniform, undisturbed pipe upstream and downstream of the measurement location For example, Johnson (1995) notes that the British Standard 1042 recommends that upstream from the measurement point a straight length of pipe equal to 30 to 50 diameters is sufficient depending on the type of turbulence causing device, whereas downstream from the measurement point 5 diameters of straight pipe should be present Shorter sections of straight pipe could affect flow measurement accuracy The accuracy

of some methods also may decrease due to backwater effects and transitions from open-channel to pressurized pipe-full flow

Practical considerations may make it necessary to place a monitor at a location with nonoptimal hydraulic conditions For example, important locations, such as over-flows, bifurcations, and known flooding points, may require individual monitoring irrespective of hydraulic conditions (Baughen and Eadon, 1983) Borders between communities may require monitoring irrespective of hydraulic conditions for ‘po-litical reasons’ in cost allocation Accurate model calibration and verification may require monitoring of each subcatchment (Baughen and Eadon, 1983) Finally, local constraints such as accessibility, power supply, and nonhydraulic goals of monitoring may also necessitate using nonhydraulically optimal sites For example, monitoring locations might be selected for ease of pollutant sampling regardless of hydraulic conditions, as was the case of combined sewer monitoring in the Chicago, USA,

area reported by Waite et al (2002).

Some of the techniques discussed in this chapter are better at measuring flow under nonhydraulically optimal conditions than others Thus, once the monitoring

Introduction 123

locations are selected the following questions (after Church et al., 1999) must be

considered:

(1) Is the flow measuring technique applicable to the flow and channel characteristics

at the site?

(2) Is the flow measuring technique capable of measuring the full range of flows? (3) Will the flow measurements be of sufficient accuracy to meet the objectives of the study?

2.2.1.4 Characteristics of Ideal Sewer Flow Measurement Equipment

In order to deal with the complex hydraulic environment of sewer systems, Wenzel (1975) recommended that the ideal device for flow measurement should have the following characteristics:

(1) capability to operate under both open channel and full flow conditions;

(2) a known accuracy throughout the range of measurement;

(3) a minimum disturbance to the flow or reduction in pipe capacity;

(4) a minimum of field maintenance;

(5) compatibility with real-time remote data transmission;

(6) reasonable construction and installation costs

Drake (1994) further suggested that the equipment must provide reliable and accurate level and/or flow measurements within dynamic conditions, withstand a corrosive environment, overcome turbulence, and resist entanglement with floating matter

2.2.1.5 Quality Assurance and Quality Control

For any flow monitoring, but particularly for sewer flow, detailed quality assurance

and quality control (QA/QC) programmes are necessary Church et al (1999)

de-scribe in detail the key components of a QA/QC programme for flow monitoring,

and their main QA/QC components are summarized as follows:

(1) Frequent and routine site visits by trained/experienced personnel to maintain equipment and keep the site clean

(2) Redundant methods for measuring flow

(3) Technical training of project personnel Weyand (1996) also stressed that it is necessary to have specially trained and qualified staff for operating and calibrat-ing the sewer flow meters

(4) Frequent review by project personnel of data collected Weyand (1996) also noted that data quality must be continually checked to detect equipment malfunctions

(5) Quality audits, in the form of periodic internal reviews

(6) Quality audits, in the form of periodic external reviews

Church et al (1999) noted that frequent calibration of equipment is necessary

because of the difficult monitoring environment, and that the difficulties of measuring

in this environment result in a high probability of incomplete record, even when stations are well maintained and properly calibrated

2.2.2 MANNING’S EQUATION

The simplest form of stage-discharge relation is obtained by assuming that Manning’s equation is valid for the selected monitoring location Using Manning’s equation

discharge, Q, is calculated as

Q= 1

n A(h)R(h)

where n is Manning’s roughness coefficient, A(h) is the cross-sectional area of flow, R(h) is the hydraulic radius of the flow, h is the depth (or pressure head for full-pipe flow) of flow, and S is the energy slope of the flow In this technique, h is measured using a pressure transducer or bubbler system, A and R are calculated as a function

of h from the known conduit geometry, S is approximated as the pipe slope, and n is

estimated from standard tables on the basis of pipe material and condition Soroko (1973) noted that Manning’s equation may be appropriate for discharge calculation

in channels with a straight course of at least 61 m, preferably longer, the course being free of rapids, abrupt falls, and sudden contractions or expansions

The primary advantage of this technique is that only a stage measurement device

is needed to estimate flow The primary disadvantages of this technique are that

proper estimates of S and n are difficult to obtain For steady, uniform flow in a

channel as specified by Soroko (1973) the bed slope equals the energy slope, how-ever, for unsteady, nonuniform flow common in storm and combined sewers the bed slope and energy slope diverge Further, even in cases where the bed slope approxi-mates the energy slope well, determination of the bed slope is difficult Most often the bed slope is estimated from design plans, but this can be substantially different from the actual pipe slope For example, Melching and Yen (1986) compared ‘as built’ measurements of pipe slopes between manholes with the slope indicated on

Manning’s Equation 125

the plans for 80 storm sewers in Tempe, (AZ, USA) and found a standard construc-tion error of 0.0008 m/m Given that slopes of these sewers ranged from 0.001 to 0.0055 m/m, the standard construction error represented a substantial portion of the design slope in this case Even when the pipe slope between manholes at the mea-surement location has been measured in the field there may be inaccuracies in the estimated slope because differential settlement and/or sag of pipes in the measure-ment reach between manholes cause the measured slope to not be representative of the energy slope

Determination of Manning’s n from tables for sewer pipes is at best a

‘guessti-mate’ (Soroko, 1973) Slime, debris, deposition, and decay of the pipes may cause

Manning’s n for a pipe to be substantially different from values in the standard tables Further, in pipes Manning’s n is a function of depth not a constant Lanfear and Coll (1978) state that at depths of 5 to 70 % of pipe diameter, n is 20 to 30 %

higher than the value for full pipe flow obtained from standard tables, failure to account for this phenomena will cause flows to be overestimated by more than

20 % For improved estimates of n, Wright (1991) recommended that Camp’s dis-tribution of n as a function of depth (Figure 2.2.1) be used to adjust the value of Manning’s n.

Wright (1991) presented the results of a field study for 22 sites in Grand Rapids (MI, USA) that illustrates the accuracy of the typical application of Manning’s equa-tion for estimaequa-tion of flow in storm sewers The pipes ranged in diameter from 45 to

243 cm, and in 5 of the 22 cases slopes were estimated from field measurements while the remaining slopes were based on design drawings The actual flow rate was esti-mated using a hand-held electromagnetic velocity meter to measure the maximum

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

n//n(full)

1 1.1 1.2 1.3 1.4

Figure 2.2.1 Camp’s normalized distribution of Manning’s n versus relative depth in a circular

section

velocity and assuming that the mean velocity was 0.9 times the maximum velocity.

At the 22 sites discharge was measured an average of nine times, and the measured

discharge was used to calculate values of S1/2 /n for each measurement Average val-ues of S1/2 /n were determined for each site, and compared with the values of S1/2 /n for each site estimated from field conditions including the variation of Manning’s n

with flow depth (as would be done in the typical application of Manning’s equation) The mean percent error was 28.9 % In 50 % of the cases the errors were greater than

25 %, and in 27 % of the cases errors were greater than 50 % Wright (1991) also presented a case for sewers in Mobile, (AL, USA) that illustrated the even poorer results obtained with Manning’s equation in pipes subject to surcharge and backwa-ter If a site is subject of surcharge and backwater, two stage gauges should be used, and the water-surface slope should be used to approximate the energy slope This approach is rarely applied in practice

Lanfear and Coll (1978) found that a ‘fitted’ Manning’s equation, calibrated by

a single flow measurement, provided good agreement with observed flow data and eliminated the need to measure slope A single discharge measurement is used to

calculate S1/2 /n, which then is applied to all other flows This approach was

illus-trated for multiple flow measurements in three 122-cm diameter brick sewers with

‘as built’ slopes between 0.00044 and 0.0144 in Washington (DC, USA) Lanfear

and Coll (1978) stated that if S1/2 /n is determined for those flows of most concern, most of the error caused by variable Manning’s n is eliminated This implies that

if a wide range of flows are of interest, the value of S1/2 /n may need to be

cali-brated throughout the flow range Marsalek (1973) stated that under conditions of unsteady, nonuniform flow in pipes, Manning’s equation underestimates flows in the rising stage and overestimates flows in the falling stage Finally, Alley (1977) reported that the accuracy of the Manning’s equation technique is, at best, about

15 to 20 %

2.2.3 FLUMES

Flumes have been used to measure open channel flows in small streams, irrigation canals, water and wastewater treatment plants, and sewers for more than 50 years Flumes are flow-constriction structures that control the flow hydraulics such that

flow is directly related to head (Church et al., 1999) The most common type of

flume constricts the flow such that critical flow results somewhere in the constricted section, which results in a unique relation between head and discharge as detailed later These flumes are known as critical-flow flumes Flumes work best at sites where the potential for surcharge, full-pipe pressurized flow, and backwater effects are expected to be negligible Flume measurements are reliable for both uniform and

nonuniform flow unless the sewer becomes surcharged (Parr et al., 1981) Baughen

and Eadon (1983) noted that flumes give misleading results if they are surcharged and this condition is not suspected

Flumes 127

The flow computation principle applied for critical-flow flumes may be derived

as follows (Wenzel, 1975) The energy conservation equation is applied between a reference section 1 located immediately upstream of the flow constriction (flume)

and section 2 is located in the constriction a distance L downstream from section

(1) resulting in:

h1+ α1 Q2

2g A2 1

+ z1 = h2 + α2 Q2

2g A2 2

whereα is the kinetic energy correction factor, g is the acceleration of gravity, z is the vertical distance from some datum, and h L is the head loss between sections 1 and 2 In the application of Equation (2.2.2) the following assumptions are made: (1) steady flow; (2) hydrostatic pressure distribution at section 1; (3) small slope such

that the flow depth h approximately equals the vertical component of depth; and (4)

two- and three-dimensional effects are negligible or accounted for as coefficients or energy loss terms (Wenzel, 1975) Equation (2.2.2) can be solved for discharge if all other terms are measured or evaluated as follows:



2g(h1− h2 + LS0 − h L)

α2

A2 − α1

A2

1/2

(2.2.3)

where S0is the bed slope If open channel flow is present and A2is sufficiently small, critical flow will occur at some point in the constriction If section 2 is defined as the point of critical flow the following relation is derived from the fact that the Froude number equals 1 for critical flow:

Q2B2

where B2is the width of the free surface at section 2 Substitution of Equation (2.2.4)

into Equation (2.2.3) and using known relations between A, h, and B, the discharge can be implicitly determined by measuring only h1 and evaluating h L, since all other terms are known The head loss can be determined from boundary layer theory (Wenzel, 1975), but typically the relation between flow and discharge for a flume is determined by laboratory ratings

The Palmer–Bowlus flume was first proposed in the 1930s (Palmer and Bowlus, 1936), was extensively tested in the 1950s (Wells and Gotaas, 1958), and has be-come the most commonly used critical-flow flume in sewer systems Palmer–Bowlus flumes have low head loss and can be installed in manholes where there is a stan-dard, straight-through design, or they can be installed in the half section of the sewer conduit (Soroko, 1973) Palmer–Bowlus flumes can be permanently installed, or be portable devices which can be inserted in the downstream pipe of a manhole using

a pneumatic seal (Baughen and Eadon, 1983) Wells and Gotaas (1958) extensive laboratory experiments on the Palmer–Bowlus flume indicated that accuracy within

3 % of the theoretical discharge is readily attainable at depths up to 0.9D (where D

is the upstream pipe diameter) for flumes installed in circular conduits However,

Hunter et al (1991) indicated that Palmer–Bowlus flumes typically are inaccurate

at depths greater than 0.75D.

Figures 2.2.2 and 2.2.3 show two standardized trapezoidal Palmer–Bowlus flume sections for which a rating table is presented in Ludwig and Parkhurst (1974) Ludwig and Parkhurst (1974) noted that it is believed that the typical trapezoidal sections offer advantages regarding flow range and the provision of more accurate mea-surements at low flow values Figure 2.2.4 shows a standardized rectangular throat Palmer–Bowlus flume for which a rating table is presented in Ludwig and Parkhurst

(1974) Ludwig and Parkhurst (1974) noted that a value of D/10 represents a

desir-able rise in the base of rectangular flumes installed within circular conduits Standard Palmer–Bowlus flumes only have a stage measurement device in the approach sec-tion To measure pressurized full-pipe flow, pressure should be measured at both sections 1 and 2 (approach and throat, respectively) Flumes with such two pressure sensor designs are known as Venturi flumes, which are discussed in the following paragraphs

1

2

D/2

D/10

hc hu D

Figure 2.2.2 Standardized Palmer–Bowlus trapezoidal flume with a bottom width of one-half of

the pipe diameter

Flumes 129

1

2

D/3

D/10

hc hu D

Figure 2.2.3 Standardized Palmer–Bowlus trapezoidal flume with a bottom width of one-third

of the pipe diameter

hu hc

D

BT

t = D/10

Figure 2.2.4 Standardized Palmer–Bowlus rectangular flume