1、ASCE-1 8-96 American Society of Civil Engineers Standard Guidelines for In-Process Oxygen Transfer Testing ASCE ASCE-I 8-96 American Society of Civil Engineers Standard Guidelines for In-Process Oxygen Transfer Test i ng Published by 345 East 47th Street New York, NY 1 O01 7-2398 American Society AS
2、CE of Civil Engineers Abstract: This Standard Guidelines for In-Process Oxygen Transfer Testing describes several proven techniques for measuring oxygen transfer under process conditions. Nonsteady state, offgas, and inert gas tracer methods are detailed in the body of this standard, which is follow
3、ed by a brief discussion of comparisons among methods. It is intended that these guidelines be used by engineers, owners, and manufacturers in evaluating the performance of aeration devices under process conditions. Library of Congress Cataloging-in-Publication Data Standard guidelines for in-proces
4、s oxygen transfer testing. P. cm. Prepared by the ASCE Oxygen Transfer Standards Subcommittee. 1. Aeration tanks-Testing. 2. Water-Aeration-Evaluation. I. American Society of Civil Engineers. Oxygen Transfer Standards Subcommittee. TD758.S73 1997 97-8344 628.165-dc21 CIP ISBN 0-7844-0 1 14-4 Photoco
5、pies. Authorization to photocopy material for internal or personal use under circumstances not falling within the fair use provisions of the Copyright Act is granted by ASCE to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service, provided th
6、at the base fee of $4.00 per article plus $SO per page is paid directly to CCC, 222 Rosewood, Drive, Danvers, MA O1 923. The identification for ASCE Books is 0-7844-01 14-4/97/$4.00 + $SO per page. Requests for special permission or bulk copying should be addressed to Permissions Mueller, 1985). A d
7、ual non- steady state method has been used to estimate accuracy of the nonsteady state test (Mueller and Rysinger, 1981; Mueller et al., 1983a) however it requires a significant amount of process control to ensure constant conditions during testing. 2.3 Assumptions and limitations The assumptions ma
8、de in the non-steady state analysis are: 1. a completely mixed system. 2. constant 02 uptake rate and KLaf during the du- ration of the study. 3. probes are located so that they sense equal tank volumes. These assumptions require reasonable time periods, approximately 4/Kh, over which to conduct the
9、 tests and relatively constant process conditions. No dissolved I -I w w -I w o n i I i/ CONSTANTR TIhlE Figure 2: Non-Steady State DO Curve for (a) Changing Power Levels and (b) H202 Addition. or atmospheric oxygen limitation can exist in any por- tion of the tank immediately prior to or during the
10、 test, because changing oxygen uptake rates will invalidate the results. This may require that the test be conducted on only a portion of the flow, so that conditions can be properly controlled. A change in the power level to obtain the perturbation from steady state conditions must be made quickly
11、to provide a response to a theo- retically instantaneous change. This can be obtained by changing the number of blowers on line or by changing the speed of surface aerators. It cannot be obtained by changing the blade submergence of a surface aerator by effluent weir level control, because of the si
12、gnificant time required to change the tank volume. 2.4 Procedure To obtain non-steady state curves un- der process conditions, either of two approaches is taken, as shown in Figure 2. The first approach (1) is to change power levels from either a lower to a higher level or vice versa to obtain a cha
13、nge in oxygen concentration with time and a new steady state DO concentration, CR. The alternative (2) is to add hydrogen peroxide to increase the DO to well above saturation, while maintaining a constant power level. The oxygen will then be stripped 2 out of solution until the CR value is again obt
14、ained. Both curves will yield the same KLaf and CR values, providing that the uptake rate remains constant over the study. A nonlinear regression (NLR) technique, which is similar to the data analysis technique for clean water, as well as a log deficit approach, is used to estimate CR, Co, and K for
15、 analysis of the process data. The former provides not only best estimates for both CR and K, but also the standard deviation of each in fitting the non-steady state data. Constant load and oxygen uptake conditions should be maintained either by diverting some flow from the test basin or testing dur
16、ing periods of approximately constant loading. Initially, DO concentrations should be monitored at several points throughout the test basin to define mixing patterns and ensure that the entire basin is aerobic. A minimum of four DO probes should be installed strategically within the aeration basin b
17、oth vertically and horizontally to best represent the tank contents. Each probe should represent approximately equal tank volumes. The recommended test procedure for non-steady state testing is: 1. Calibrate the DO probes in tap water at the same temperature as the test basin. 2. Place the probes at
18、 preselected vertical and horizon- tal positions in the aeration basin so that each probe senses an equal portion of the test volume. Stirred probes should be employed, but if they are unavail- able, locate the probes so that sufficient and regular velocity of mixed liquor across each is obtained. 3
19、. If recorders are used to continually monitor DO, calibrate the recorders to probe readings. Ensure that no change in probe signal occurs when probes are connected to recorders. This sometimes occurs when recorders are grounded. 4. For diffused air systems, measure, at regular in- tervals, gas flow
20、 parameters, differential pressure readings, gas temperature, and pressure at the mea- suring device. For mechanical systems, measure power drawn. 5. Measure all influent, wastewater and return sludge flow rates, and DO concentrations. 6. Before and after tests, measure aeration basin tem- perature,
21、 air temperature, and barometric pressure -the last two by calling to the weather bureau. 7. Change DO in basin by either (a) or (b): a. Change the power level by changing air flow dis- tribution, turning a blower on or off, or changing power input to the mechanical aerator. A min- imum DO differenc
22、e of 2 m should occur. b. Add H202 to tank. This is the recommended procedure, because a large difference in DO can be obtained; 10 mg/L is recommended. The H202 is,dumped in by 4 to 8 buckets, equally distributed around the tank. The full procedure is described in Kayser and Dernback (1980). 8. Mon
23、itor DO by recorder continuously during the test or at preselected time intervals to provide between 20 and 30 points during each test. The test duration should be N 4/Kaf, as recommended in the clean water standard. 9. Analyze DO versus time data with the nonlinear regression program to obtain the
24、best estimate of KLaf and CR. 10. Calculate average KLaf for the tank as follows: or for equal volumes: where a = each probe location; n = number of probe locations; r/; = volume of probe location i; V = total volume. 11. Calculate OTRfZo, for the test as follows: (9) 2.4.1 The HzOz technique Kayser
25、 (1979) indicated a very convenient technique to raise the dissolved oxy- gen concentration in an aeration tank without changing power levels by using hydrogen peroxide. In the pres- ence of a suitable catalyst - a reducing agent such as iron Fet+ - hydrogen peroxide will dissociate to water and oxy
26、gen as given in Equation 10. Mixed liquor in an aeration tank contains significant amounts of reducing substances. They act as catalyzing agents, which allow the above reaction to take place within one to two minutes. Because mixing of aeration tank contents generally takes two to five minutes, this
27、 can be considered as a slug dose of oxygen into the system. Both laboratory and field studies have used an 3 indicator to ensure that the peroxide did dissociate within one to two minutes. This technique cannot be used in clean water because dissociation generally does not occur or occurs over long
28、 time periods or in wastewaters, where enough catalyst may not be present. However, for aeration tanks with a significant amount of mixed liquor suspended solids ( 1000 mg/L), the technique is highly successful. The volume of peroxide required in an aeration tank to obtain an incremental change in d
29、issolved oxy- gen concentration can be calculated as follows. The stoichiometry in Equation 10 indicates that 2.13 mg of H202 will be required for every mg of oxygen produced. At 25C pure hydrogen peroxide has a density of 1.44 kg per liter (CRC, 1973). For field use, 35% by weight hydrogen peroxide
30、 drums are normally available. They yield 0.504 kg HzO2A 35% solution. Using the above values, the hydrogen peroxide dose, V, can be obtained as given in Equation li. V, = Co1.2077 x 10-4(ADO)V (11) where: V, = volume of H202, L; ADO = theoretical DO increase in aeration tank, m; V = aeration tank v
31、olume, m3; cH,O, - hydrogen peroxide, wt %. The actual increase in DO will be less than that cal- culated using the H202 dose in Equation 11, because of the oxygen uptake rate and the stripping that occurs during the time required to completely mix the tank contents. This difference is typically bet
32、ween 1 and 5 In field studies, hydrogen peroxide addition is straightforward. A drum is placed horizontally on a drum rack, and a spigot used to fill the buckets. Four to eight 18 L (5 gal) buckets are filled from a drum of peroxide and placed on two sides of the aeration tank opposite each other. A
33、t a signal the peroxide is immedi- ately dumped into the tank, allowing the aerators to mix it thoroughly with the tank contents. After five minutes, recording of data for analysis is generally begun. When using HzOz, eye protection must be provided and plastic gloves are typically used to protect t
34、he skin. Immediate removal of HzOz from the skin by washing will prevent damage. 2.5 Sample calculation An example test run using batch dumps of H202 is given for Test #8 conducted mg/L. I I - A7 _) t t t TuTt t t t - X 012x #5. 10“ 15 0 #3.6 i- X I * I 0 #2,6 +I 5 TANK #7 PROBE #1 ,6DEPTH Primary W
35、ATER DEPTH = 12.5 X: H,O, ADDITION LOCATIONS 0: PROBE LOCATIONS PROBE No. AND DEPTH in fi. from LIQUID SURFACE) Figure 3: Dissolved Oxygen Probe Location During Haverstraw Test. Table 1: Test Conditions for Haverstraw Test #8 Surface Aerator = 22.4 kw (30 hp nominal) Tank Volume = 783 m3 (0.207 MG)
36、Water Depth = 3.8 m (12.5 ft) Power measurement = 3 phase; assumed power factor = 90% Voltage = 200 volts Amperage = 70 amps Wire hp = 29.3 hp Average Qp = 6283 m3/d (1.66 MGD) Average QX = 3444 m3/d (0.91 MGD) (from mass balance or steady state) Average Q = 9727 m3/d C, = 2.3 mg/L (flow weighted av
37、erage of return sludge and primaq effluent) Measured average oxygen uptake rate = 19.4 mg/L-hr Mixed liquor temp = 20.00 C Mixed liquor VSS = 2900 mgL Influent BOD,5 = 90 mg/L FIM = 0.25 dav- 4 o. O u I 1.5 ! 1.0 0.5 2 I n - 2.0 8 c; c 6 3 w a K u - Hz02 ADDITION I I l 10.0 O 11:OOAM 11:30AM NOON 12
38、:30PM TIME. 24 JUNE 1982 3 z u g 5- O Figure 4: Flow and Oxygen Uptake Rate Variability During Haverstraw Test #8. at Haverstraw, New York, on 24 June 1982 (see Figure 3). The test conditions for this run are given in Table 1. Five DO probes were used in the tank, as shown in Figure 3, while the mea
39、sured O2 uptake rates and primary effluent flow rates are given in Figure 4. Table 2 and Figure 5 show the recorder data and calculated nonlinear regression results for probe #2 of Test #8. Table 3 gives the average KLaf and Cn values for each probe. The data from this study were excellent, with the
40、 coefficient of variation of KLaf generally less than 2% and the standard deviation on c, = Kaj(Ckf - C)V (14) The fraction of oxygen transferred to the liquid may be determined without knowledge of the gas flow rate, but the rate of air supplied must be known to determine the mass of oxygen transfe
41、rred. i Symbols and nomenclature are defined in Appendix D. OUTLET GAS INLET GAS Figure 7: Gas Phase Mass Balance. 9 The value of Kaf may be estimated from Equation 14 provided measurements are made of the inlet and outlet mole fractions of oxygen (x,Ye), the total gas flow rate (g), and the tank DO
42、 (c). In addition, an estimate must be made of C that is, there is negligible net transfer of these constituents; (2) process onditions at the point of sampling are not changing rapidly with respect to the gas sample residence time; (3) the concentration of dissolved oxygen in the liquid remains rea
43、sonably constant during the period that gas analysis is carried out; and (4) the difference in oxygen transferred at the liquid surface under the hood and on the tank surface is small with respect to the transfer beneath the surface and may be ignored. The principal limitation of the off-gas method
44、is that it is not applicable to mechanical surface aeration systems. Mechanical aerators, including sparged turbine aerators with and without draft tubes, and aspirating propeller pump aerators, however, have been success- fully tested by this technique. Other constraints of the method are that (I)
45、tanks must be accessible to personnel; (2) severe foaming can complicate gas sampling; (3) severe turbulence can cause difficulty in hood placement; and (4) very high off-gas flux rates may require special provisions for gas collection and flow measurement. 3.4 Apparatus and supplies The primary com
46、po- nents required in performing the off-gas method are an off-gas analyzer, a gas capture hood, and a means of conveying the sampled gas to the analytical instrument. In addition, a dissolved oxygen meter is required to measure mixed liquor DO in the vicinity of the off-gas collection hood. 3.4.1 O
47、ff-gas analyzer (see details, Appendix A) As used in this document, an off-gas analyzer is an ap- paratus with two main functions: (I) accurate measure- ment of the difference in the mole fraction of oxygen, or mole ratio of oxygen to inerts, between ambient air and off-gas, and (2) measurement of t
48、he rate of gas flow exiting the liquid surface beneath the collection hood. Temporal changes in response occur due to changes in pressure, temperature, specimen flow rate, and instru- ment drift, to name a few factors. As a consequence, it is believed that to obtain the required degree of accuracy i
49、n measuring the comparative oxygen content in off-gas and in ambient air, sequential measurements of the two must be obtained within a few minutes (e.g., 15 minutes or less) of one another. Preferably each is bracketed by two of the other, and all occur under equal conditions of flow, temperature, and pressure. An apparatus that accomplishes this is described in 3.5. 10 3.4.2 Off-gas collection system A wide variety of collection hoods have been successfully employed, in- cluding large fixed hoods and small portable hoods, the dimensions of which are influence
