hydraulics of chlorine contact tanks - aeesp foundation

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORS

AEESP CASE STUDIES COMPILATION 2006 58

Hydraulics of Chlorine Contact Tanks

Alok Bhandari

Department of Civil Engineering Kansas State University

Manhattan, KS66506-5000 Summary A chlorine contact tank was designed and constructed by an engineering firm at a rural water district’s surface water treatment facility. The initial contract with the engineering firm was implicit in designing a system that would meet state regulations. Since the proposed design of the contact tank was unconventional, the state regulatory agency mandated that tracer tests be conducted to confirm plug flow for adequate disinfection. Initial tracer tests found that the tank hydraulics did not meet the state criteria for chlorine-based disinfection. The tank was redesigned by the engineering firm, but the modified tank also failed the tracer test. A second redesign of the tank also failed to meet the regulatory requirements for residence time, and the engineers were directed to perform a third redesign of the basin. The water treatment plant was shut down until the disinfection criteria were fully satisfied. The bad designs cost the water utility thousands of dollars for every day that the plant sat idle. The costs were passed on to the engineering firm responsible for the design of the contact tank. This case study presents the different design variations along with tracer data. A complete tracer analysis is presented for the original design. Tracer data analyses for the first and second design modifications are left for the students to complete. This case study allows students to learn how to conduct tracer test analyses in plug flow systems with spike input, and put their knowledge to practice in the context of a “real-world” scenario. The case study also illustrates how an inappropriate design can prevent an engineering project from being realized in a timely manner. Keywords: Plug flow, tracer test, hydraulic design, disinfection, mean residence time



Context and Logistics Learning Objectives

The learning objectives of this exercise are to ensure that students: 1. understand the principles behind tracer tests for plug flow reactors

using a spike input; 2. are able to analyze and interpret tracer test data; 3. can use hydraulic data to analyze and evaluate alternative designs

of plug flow reactors; 4. can discuss key features of a well-designed chlorine contact tank;

and 5. are able to communicate the results of their exercise in the form of

technical reports. Accommodating Course(s) and Level

• junior or senior level courses in reactor engineering, environmental engineering, water and wastewater treatment

• senior level laboratory course in environmental engineering • graduate level courses in water treatment, wastewater treatment • senior or graduate level laboratory course in unit operations and

processes in environmental engineering Prerequisite Course(s)

• Fluid Mechanics Point of Use

• This case study can be used for illustrating the concept of plug flow in non-ideal reactors. Ideal points of use in a course would be immediately after a discussion of reactors, residence time distributions, and tracer tests. The case study can be used to illustrate disinfection processes in contact basins during water or wastewater treatment.

Type of Activity

• The activity in this case study involves analysis of “real world” tracer test data. Students will learn to develop residence time distribution curves from concentration versus time data obtained from slug-dose tracer tests. This activity can be conducted in the lecture



(demonstration) or laboratory (hands-on) setting. Alternately, the case study can be discussed in class and the activity assigned for homework. Students can work individually or in pairs.

Level of Effort by instructor

• About one hour to review the case study and supplemental material. This case study can be incorporated in a class or laboratory period dedicated to flow in reactors. Alternately, the case study and accompanying activity can be assigned for homework. It is recommended that the instructor review the case study in class. The solutions provided in the accompanying Excel worksheets should minimize grading time. The instructor should not make solution spreadsheets available to students.

Level of Effort by Individual Student

• One 50-minute class period or less may be used to discuss the topic of reactor hydraulics. Students may be encouraged to work in pairs. The deliverables can be assigned as a homework exercise. The activity will require 3 to 4 hours.

Suggested Assessment Methods

• The question set accompanying the case study can be used as a tool to assess learning effectiveness.

Introduction

The Interim Enhanced Surface Water Treatment Rule (IESWTR) applies to public water systems that serve at least 10,000 people and use surface water or ground water under the direct influence of surface water (CFR40, 1998a). Among other things, IESWTR requires disinfection profiling and benchmarking of water treatment facilities. These procedures ensure that microbial inactivation is significantly reduced due to implementation of the Stage 1 Disinfection and Disinfectant Byproduct Rule (DBPR). Stage 1 DBPR sets maximum residual disinfectant levels (MRDLs) and maximum residual disinfectant level goals (MRDLGs) for product water that are analogous to the maximum contaminant level (MCL) and maximum contaminant level goals (MCLGs). DBPR allows for modification of the disinfectant dose or the disinfection process to meet MRDLs (CFR40, 1998b).



The CT method is commonly used to evaluate compliance of a disinfection process to IESWTR (EPA 1999). A CT value is a product of the residual disinfectant concentration, C (mg/L) and the time of contact between the disinfectant and water, T (min). CT values are expressed in mg-min/L. IESWTR mandates 99.9% (3-logs) inactivation or removal for Giardia and 99.99% (4-logs) disinfection for viruses. As an example, the CT values for inactivation of Giardia cysts by free chlorine at 20ºC and a pH of 7.0 are summarized in Table 1.

Table 1. CT values for log inactivation of Giardia cysts at different residual chlorine concentrations at pH 7.0 and 20ºC (EPA 1999).

CT Values (mg-min/L) for Different Log Inactivation Residual Chlorine (mg/L) 0.5 1.0 1.5 2.0 2.5 3.0 ≤ 0.4 9 17 26 35 43 52 0.6 9 18 27 36 45 54 0.8 9 18 28 37 46 55 1.0 9 19 28 37 47 56 1.2 10 19 29 38 48 57 1.4 10 19 29 39 48 58 1.6 10 20 30 39 49 59 1.8 10 20 31 41 51 61 2.0 10 21 31 41 52 62 2.2 11 21 32 42 53 63 2.4 11 22 33 43 54 65 2.6 11 22 33 44 55 66 2.8 11 22 34 45 56 67 3.0 11 23 34 45 57 68

When conducting tracer studies is impractical or prohibitively

expensive, the contact time of a chlorine contact tank may be estimated using the hydraulic characteristics of the tank and baffling factors. For further discussion on this method, the reader is referred to EPA’s Disinfection Profiling and Benchmarking Guidance Manual (EPA 1999). However, wherever possible, T should be determined by conducting a tracer study. In a simple pipe flow or an ideal tubular (plug-flow) reactor, the contact time can be calculated simply as:

T = V/Q



where V is the volume of the pipe section and Q is the volumetric flow rate of water in the pipe.

In most “real-world” (non-ideal) reactors, water may either short-circuit the desired flow pattern or accumulate in “dead zones” in the reactor resulting in a distribution of potential residence times in the basin. This residence time distribution can be determined by a tracer test where a conservative (non-reacting) chemical is introduced with the flow at the inlet of the reactor and the concentration of the chemical is monitored as a function of time at the outlet. Plotting the effluent tracer concentration against time yields tracer response curves like those illustrated in Figure 1. Under ideal conditions, a slug input would yield a spike output at the theoretical residence time (t) of the reactor while a continuous input would result in a step output. In a non-ideal plug flow reactor (PFR), dispersion processes result in a bell-shaped curve when a slug of tracer is introduced at the reactor inlet and an ‘S’ shaped curve when a continuous flow of tracer is used.

IESWTR specifies that the contact time used in determining CT should

be the minimum detention time experienced by 90% of the water passing through the disinfection basin (T10) (EPA 1999). EPA suggests that tracer studies be conducted at two to four different flow rates that span the range of expected flows in the system. At the minimum, tracer tests should be conducted at the average flow rate and at 91% of the maximum flow rate expected to ever occur in the system. Flow variations during a tracer test should be minimized.

Figure 1. Tracer response curves for slug input (left) and step input (right) in ideal and non-ideal plug flow reactors.

In the slug input method of determining the residence time distribution

(RTD) of a tracer, a concentrated pulse of tracer is introduce into the influent

time

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water and effluent tracer concentration is monitored over time. The step-dose method involves the introduction of a constant dose of the tracer until the effluent tracer concentration reaches a steady-state level. Both methods are theoretically equivalent and equally acceptable for the determination of the reactor’s mean hydraulic residence time. The duration of tracer measurement is a function of the basin’s theoretical residence time. EPA recommends that monitoring of effluent tracer concentrations be conducted for a duration of 2t to 3t.

For a slug-dose input, the theoretical mean residence time (tbar) is

estimated from the residence time distribution curve using the following equation (Metcalf and Eddy, 2003):

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where, ti and Ci are the time and corresponding concentration at the ith measurement and Dt is the time increment between effluent tracer measurements.

The spread of the residence time distribution curve or its variance (s2) is calculated using the following equation (Metcalf and Eddy, 2003):

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When the area under the RTD curve for a slug input is normalized, the

normalized curve is known as an E curve (Metcalf and Eddy, 2003). The peak concentration of the E curve is 1.0. The variance of the E curve is called the normalized variance (snorm2).

The reactor dispersion index, also known as the Morrill dispersion index

(MDI) is given by:

MDI = t90/t10



where t10 and t90 refer to the times at which 10% and 90% of the tracer has passed through the reactor. The parameters t10 and t90 are determined from the RTD curve by estimating the time interval corresponding to 10% and 90% of the area under the curve. The value of MDI for an ideal PFR is 1.0 and a PFR with MDI ≤ 2.0 is considered to be an effective PFR by the EPA (EPA, 1986). The reciprocal of MDI describes the volumetric efficiency of the reactor, which for an ideal PFR is 100%. The State regulated T10 parameter is calculated as:

T10 = t10/t Case Study Evaluating the Hydraulics of a Chlorine Contact Tank using the Slug-Test Method

A consultant was hired by the state regulatory agency to analyze data

obtained from a tracer test conducted on a uniquely designed 170,000-gallon chlorine contact tank located at a new 2 MGD surface water treatment facility. Rhodamine WT, a fluorescent dye was used as a conservative tracer. Approximately 1 lb of dye was dissolved in 4 gallons of water and injected into the water line connecting the membrane filter units to the contact tank (Figure 2). A submersible pump was used to inject the dye at a flow rate > 3 gpm. Effluent samples were collected from the residual chlorine sampling line. A spectrophotometer was used to measure the concentration of the dye in the effluent samples. A rough sketch of the original design of the tank is given in Figure 3.

Figure 2. Schematic showing approximate tracer injection and sampling locations.

Dye Injection

Membrane

Filters Contact Tank

Effluent

Sampling

Dye Injection

Membrane


Effluent

Sampling

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Sampling Point

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The tracer test data (time versus effluent dye concentration) are provided to you in an Excel file labeled “Tracer Study.xls”. The state regulatory agency requires reporting of the following parameters from the RTD data:

• Mean residence time (tbar) • Variance (σ2) • Normalized variance (σnorm2) • MDI • Volumetric efficiency • T10

Figure 3. Elevation, plan and 3-D representation of the original design of the chlorine contact basin. Arrows indicate general direction of water flow.



State regulations state that the tank should have a T10 value of at least 0.30 and 0.50 for tests performed at flows of ~1 MGD and ~2 MGD, respectively.

Tracer tests were conducted at flow rates of 1.08 MGD and 2.3 MGD.

Effluent dye concentrations were recorded against the time elapsed after dye injection. These data are summarized in the Excel spreadsheet labeled “Original Design”. The data were used to plot the RTD curves for the two flow rates (Figures 4 and 5). These plots were used to determine the tbar, σ2, and σnorm2. Exponential curve fitting was utilized to extrapolate data beyond the observation period. The total area under the effluent tracer curve was determined using a trapezoidal numerical integration technique. The cumulative area at the end of each sampling interval was determined similarly. Theoretical detention time (t) was calculated from the flow rate and CT volume. The parameters t10 and t90 were determined by interpolating for the time intervals corresponding to 10% and 90% of the cumulative area. Finally, MDI, volumetric efficiency and T10 parameters were determined from t10, t90, and t values.

Figure 4. RTD curve for chlorine contact tank at a flow rate of 1.08 MGD.

The hydraulic design of the chlorine contact basin failed to meet the State mandated values of T10 ≥ 0.30 and T10 ≥ 0.50 for flow rates of 1 and 2 MGD, respectively. The engineering firm was directed to produce an improved design of the tank. The tank geometry was modified as illustrated in Figure 6 and a slug-dose tracer test was repeated at flow rates of approximately 1 and 2 MGD. The t vs C data for the second test are summarized in an Excel spreadsheet labeled “First Modification”.



Figure 5. RTD curve for chlorine contact tank at a flow rate of 2.3 MGD.

Figure 6. First design modification of the chlorine contact basin. Arrows indicate direction of flow.

The engineering firm was asked to further improve the hydraulic design of the tank. The tank geometry was modified as illustrated in Figure 7 and a slug-dose tracer test was performed at a flow rate of approximately 2 MGD. The t vs C data for the third tracer test are summarized in an Excel spreadsheet labeled “Second Modification”.

y = 0.3805e-0.0121x

R2 = 0.9262

0

0.05

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0.15

0.2

0.25

0.3

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0 60 120 180 240 300 360 420 480

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t90 = 190 min

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2 = 0.666

MDI = 10.0

Volumetric Efficiency = 0.1

T10 = 0.179

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Figure 7. Second design modification of the chlorine contact basin.

Supporting Information Students: Excel file labeled “Student Workbook” Students: Word file labeled “Original Design Sample Spreadsheet” Instructor: Excel file labeled “Instructor Workbook” Instructor: Word file labeled “Final Design Details” References CFR40 (1998a) Code of Federal Regulations 40, Parts 9, 141 and 142. U.S.

Environmental Protection Agency National Primary Drinking Water Regulations: Interim Enhanced Surface Water Treatment Rule. Federal Register, Vol. 63, No. 241.

CFR40 (1998a) Code of Federal Regulations 40, Parts 9, 141 and 142. U.S. Environmental Protection Agency National Primary Drinking Water Regulations: Disinfectants and Disinfection Byproducts. Federal Register, Vol. 63, No. 241.

EPA (1986) Design Manual, Municipal Wastewater Disinfection. U.S. Environmental Protection Agency. Pub No. EPA 625-1-86-021, Cincinnati, OH.

EPA (1999) Disinfection Profiling and Benchmarking Guidance Manual. U.S. Environmental Protection Agency, Office of Water. Pub No. EPA 815-R-99-013.

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Metcalf and Eddy (2003) Wastewater Engineering Treatment and Reuse. McGraw Hill, New York, NY.

Analysis Students: See “Original Design Sample Spreadsheet” Instructor: See “Instructor Workbook”

Learning Assessment Questions 1. List operational factors that can affect the determination of hydraulic

detention time of a contact basin using tracers? 2. What other types of chemicals can be used to conduct tracer tests? What

are their advantages and limitations? 3. What are some disadvantages of the slug-dose tracer test method? What

are some advantages compared to the step-dose method? 4. Plot the RTD curves for the first and second modifications of the contact

tank design. Determine the values of tbar, σ2, dispersion number, t10, and T10 for each case and compare these values with the original design of the basin. Discuss the observed changes in these values?

5. The T10 criteria remained unmet after design modifications and the

engineering firm was directed to conduct major design revisions. Discuss why the tank repeatedly failed to meet the State’s requirements. Suggest possible modifications of the tank design to allow it to meet performance criteria.

6. Plot the RTD curves and determine the values of tbar, σ2, dispersion

number, t10, and T10 for the final design of the contact basin. 7. Check if the final hydraulic design of the chlorine contact tank meets

EPA’s CT requirements for (a) 90% (b) 99% and (c) 99.9% inactivation of Giardia cysts at a residual chlorine concentration of 1.0 mg/L, pH 7.0 and water temperature of 20°C. Check for both flow rates of 1 and 2 MGD.

hydraulics of chlorine contact tanks - aeesp foundation

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