dissolved oxygen control by pressurized
TRANSCRIPT
Dissolved Oxygen Control by Pressurized Side Stream Ozone Contacting and Degassing
Presented at International Ozone Association
World Congress 1999 Annual Conference
Dearborn, Michigan, USA
August 24, 1999
Courtesy of:
Mazzei Injector Corporation | 500 Rooster Dr. | Bakersfield, CA 93307 USA | Phone: 661-363-6500
Visit us online: http://www.mazzei.net
Dissolved Oxygen Control by Pressurized Side Stream Ozone Contacting and Degassing
James R. Jackson, Paul K. Overbeck, John M. Overby GDT Water Process Corporation
20805 North 19th Avenue, Suite #1 Phoenix, Arizona 85027 USA
ABSTRACT
The introduction and utilization of high concentration (10-15% wt.) oxygen fed ozone generators have been a break through in providing affordable ozone treatment for municipal and industrial water treatment applications. The dissolution of ozone in water by fine bubble diffusers, static mixers or venturi injectors is aided by the high gas phase ozone concentration following Henry’s Law. Unfortunately, the oxygen carrier gas present in high concentrations from any concentrated oxygen fed ozone generator is also transferred readily to solution at levels well above those typically found under ambient air conditions. The result can be oxygen supersaturation (15-50 mg/L) resulting in operational problems for down stream treatment processes and possible corrosion in distribution and at point-of-entry and point-of-use locations in homes and industry. Side stream pressurized contacting for high ozone mass transfer followed by entrained, undissolved oxygen gas removal, before combination with the main flow, can provide a viable means to control dissolved oxygen levels. Examples of side stream ozone contacting and degassing systems relative to flow rate, side stream ratio, applied ozone dosage, ozone and oxygen concentrations will be reviewed including field performance data.
Introduction Potable water systems consist of small and large treatment plants typically employing multiple treatment processes to reduce contaminants present in raw source waters to levels meeting local, national or international standards. These treatment systems may include thousands of feet of pipe within the treatment building and thousands of miles of piping in the distribution system to homes and businesses. These domestic water systems often have deterioration in the piping system due to the corrosive tendency of the potable water source or treatment processes employed. Oxygen induced corrosion, coupled with the effects of water hardness, salinity and pH on iron and copper piping materials have been studied extensively1. Most utilities have taken specific measures to minimize the impact of what have previously been typical dissolved oxygen (DO) levels in distribution systems and in the home. Industry has also
established added water treatment designs to further improve typical potable waters entering their facilities to quality levels specific for the applications intended. Over the past 20 years in the United States and 90 years in Europe and around the world, ozone has been added to potable water treatment plants to take advantage of its oxidation, disinfection and microflocculation properties. Until recently, these plants generated ozone from the oxygen present in dried ambient air (20.9% O2). Over the past 10 years, the majority of ozone systems have been designed and operated using concentrated oxygen feed gas from on-site air separation equipment or liquid oxygen (LOX)
sources. The development of ozone generators capable of high concentration (10-15% wt.) ozone production has helped position ozone as a more affordable technology for municipal and industrial water, wastewater and chemical process applications. Figure 1 shows the impact of ozone gas concentration on the unit cost ($ per pound) of ozone produced.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13Ozone Concentration (%wt)
0.00.10.20.30.40.50.60.70.80.91.01.11.21.31.4
Unit
Ozo
ne C
ost (
$/lb
)
Measured During OFEPerformance Test CurvePerformance Test Data7 Month Average
Energy @ $0.08 / kWhLOX @ $0.03 / lb
Figure 1
Unit Ozone Cost per Ozone gas Phase Concentration 2
These data, for the specific ozone generator evaluated is representative of the direction the industry is moving. The envelope for generator performance has been pushed even further since these performance data were developed with a reasonable assumption being, the curve will continue to shift further to the right in the future. While the performance of oxygen fed ozone generators makes ozone’s benefits more attractive, the potential for negative effects such as accelerated corrosion rates and consumer aesthetics (milky water at the tap) complaints, due to exceptionally high DO levels, increased piping pressure loss must be considered. Several utilities have heard the concerns of their customers and operating staff after installing oxygen fed ozone equipment. The new ozone treatment trains installed at two Milwaukee, Wisconsin plants are producing treated water with DO above 20 mg/L identified at outer points in the distribution system3. The City of Valdosta, Georgia is another example that will be discussed in this paper. Other utilities including the Orlando Utility Commission (OUC), Metropolitan Water District of Southern California (MWD) and Southern Nevada Water Authority (SNWA) 4 have recognized the potential and are including methods to reduce DO levels after formation in conventional fine bubble diffusion ozone contacting systems. Various alternatives exist for DO reduction after ozonation. These include diffused air/nitrogen stripping, isolated contactor vacuum degasification, packed tower stripping and the new VacGDT™ process. These methods require mechanical and structural additions to the effluent end of conventional ozone contactors. Each operating system
has specific capital and operating cost impacts. Examples included added space, electrical and maintenance costs for process components and larger ozone off gas destruct equipment to handle added gas volumes. Rather than reduce the DO after it is created, this paper will detail a method to control the increase in DO levels in the ozone contacting process. The use of side stream venturi injector contacting has received significant interest due the efficiencies demonstrated when applying high concentration ozone to solution. However, side stream ozone contacting by itself will not reduce the levels of DO in the full flow. The excess, entrained oxygen present in the side stream must be removed before this highly ozonated flow is returned to the main flow to have an impact on DO levels. The performance of the GDT™ process for gas to liquid mass transfer in side stream contacting is discussed. A preliminary pilot evaluation for process improvement conducted by the City of Valdosta, Georgia will be reviewed in addition to research conducted by GDT Corporation.
Background Dissolved Gas Solubility Henry’s law can describe the solubility of a gas in water. Henry's law states that the solubility of a gas is almost directly proportional to its partial pressure in the gas phase5, 6. Strictly speaking, this law applies only to gases that do not undergo chemical reaction with water during the mass transfer. One form of Henry's law that expresses dissolved gas concentration in units of mg/l is as follows6:
CS = B x M x P CS = Dissolved Gas Concentration, mg/L M = Gas Phase Density, mg/L B = Bunsen Absorption Coefficient P = Partial Pressure In Atmospheres
A number of parameters influence the solubility of certain gases, such as ozone in water. They include system pressure, temperature, pH, solution ionic strength, the decomposition of the gas if any and concentration in gas-liquid phases. Figure 2 illustrates the relationship between oxygen solubility (air 21% O2), salinity and temperature7. Figure 3 illustrates the effect of partial pressure (concentration) on calculated oxygen solubility.8
Oxygen Solubility vs. Temperature
0 5 10 15 20 25 30 35 40
Water Temperature, C
5
6
7
8
9
10
11
12
13
14
15Dissolved Oxygen, mg/l
Chlorinity 0 Chlorinity 5 Chlorinity 10
note: Salinity = 1.806 x Chlorinity
Figure 2
OXYGEN SOLUBILITYRELATIVE TO GAS PHASE CONCENTRATION, @ 1 Atm(A)
mazzei injector corp
0 10 20 30 40 50 60 70 80 90 100
Gas Phase Oxygen Concentration, %
0
10
20
30
40
50
60
70
Dis
solv
ed O
xyge
n, m
g/L
0 DEGREES C10 DEGREES C20 DEGREES C30 DEGREES C40 DEGREE C50 DEGREES C
Figure 3
Even with the use of advanced ozone generators, gas phase oxygen concentration is above 80% by weight. The dissolved gas level in the finished water will be a function of the gas phase concentration applied and the specific mass transfer capability of the contacting system. It can be seen in Figure 3 that increasing the partial pressure increases the
calculated oxygen solubility dramatically. Therefore, the DO levels exiting a full flow FBD ozone contactor will range between 16 and 30 mg/L or 150% to well above 200% of typical atmospheric solubility. GDT™ Ozone Contacting Process The GDT™ process provides rapid and efficient ozone mass transfer under pressurized contacting conditions. Pressurized contacting allows for greater gas solubility as described in Dalton's law which states that the partial pressure of a gas is equivalent to its volumetric concentration in the gas phase multiplied by the absolute pressure of the system.5, 6. In the GDT™ process as shown in Figure 4, high efficiency Mazzei® injectors aspirate ozone under vacuum conditions. In this way, the gases added expand into the liquid, resulting in violent small bubble gas liquid mixing. The dynamic mixing and mass transfer that occurs at the injector can be enhanced in the reaction vessel, which is specifically designed for each application based on gas concentration, applied ozone dosage, reaction rates and desired transfer efficiency.
THE GDT PROCESS
5. Degas Separator & Relief Valve: Additional Mixing, Detention &Gas Removal
Off G as
4. Reaction Vessel Detention Time
1. Ozone G enerator: Gas Concentration
2. M azzei Injector: Bubble Size & Mixing
3. BPCV: Injector O utlet Pressure
Figure 4
The contacted two phase flow then travels to the degas separator (DS) for additional mixing and entrained gas removal. As the entrained gas/water mixture enters the degassing separator, it is accelerated to a velocity that exerts 4-10 times gravity in a lateral force creating a water film at the separator wall and a gas vortex at the central, gas extraction core. This journey of a few seconds has the ability to extract 98% of the entrained gases within the water stream resulting in virtual plug flow conditions within the pipeline after the separator. A degas relief valve discharges the separated entrained gases under pressure for ozone destruct processing.
O ZO NE T RANS F ER E F FICIE NCYRE LA TIVE TO G A S P HAS E O ZO NE C ONC ENTR ATIO N
G DT Cor po ra ti on
2 4 6 8 10 12 14 16GA S PH AS E O Z O N E CO NC EN T R AT IO N , (% w t) IN
60
65
70
75
80
85
90
95
100
TR
AN
SF
ER
EF
FIC
IEN
CY
, %
A p plie d O zon e D o s e5 m g/l 10 m g/l 14-16 m g/l
A pp lie d O zon e Do se: 5- 16 m g/l, In je cto r O utle t P re ssur e :25 P S IG
T emp era tu re 6 3 F (1 7 C )D e te ntion T im e 12 Sec onds
Pow er Reg ressio n L ine F it
F igure 5
Figure 5 illustrates the system Mass Transfer Efficiency (MTE) as a function of ozone gas phase concentration and applied dosage with minimal detention time at 25 psig.9
Figure 6 illustrates the affects of detention time on process MTE. 9
TRANSFER EFFICIENCY vs DETENTION TIMEApplied Ozone Dosage: 15-18 mg/l, Injector Outlet Pressure: 25 PSIG
mazzei injector corp
0 10 20 30 40 50 60 70 80 90 100 110 12086
88
90
92
94
96
98
100
Temperature 53 F (12 C)Figure 6Ozone Conc.: 9.8 - 12.2% wt Detention Time, Seconds
Tran
sfer
Eff
icie
ncy,
%
TRANSFER EFFICIENCY vs INJECTOR OUTLET PRESSUREVg/Vl: 0.1, Applied Ozone Dose: 15 mg/l, Ozone Concentration: 10% wt
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 3586
88
90
92
94
96
Detention Time 12 SecondsTemperature 46 F (8 C )
Figure 7Injector Outlet Pressure, PSIG
Mas
s Tra
nsfe
r Effi
ienc
y, %
Figure 7 illustrates the affect of pressure on process MTE. 9
The importance of high ozone gas concentration is reflected in low Volumetric Gas/Liquid ratios (Vg/Vl) on MTE in Figure 8. 9
M A S S T R A N S F E R E F F IC IE N C Y v s G A S / L IQ U ID R A T IOA p p l ie d O z o n e D o s e : 1 0 - 1 4 m g /l , G a s C o n c e n tra t io n : 1 1 % w t
0 .06 0 .07 0 .08 0 .09 0 .1 0 .118 8
8 9
9 0
9 1
9 2
9 3
9 4
9 5
In jec to r O u tle t P re s s ure1 0 P SI G 2 0 P SI G 2 5 P SI G
D e te n tio n Tim e 12 se co n dsTe m p e rat u re 43 F (6 C )
F ig u re 8G D T C o rp o rat io n
V g/V l ( G a s/L iqu id R a t io n)
The Side Stream GDT™ Process Ozone is not the only gas transferred efficiently to solution with the GDT™ process. Oxygen is also transferred. Figure 9 illustrates typical DO levels developed in raw well water with 7.3 mg/L DO during GDT testing discussed later in the paper.
Armed with the ozone and oxygen process performance data above, it can be seen that attaining high residual dissolved ozone and oxygen levels are possible. This stream can be mixed with a raw water stream to achieve the transferred ozone level necessary to accomplish particular treatment goals. The side stream ratio to full flow mass balance achieved can provide sufficient ozone transfer to the bulk fluid
Side Stream Ozone Contacting Test Results
while limiting DO increase if the entrained, non-dissolved gases are removed before side stream combination with the raw water main flow. If the entrained gases remain in the side stream they will continue to dissolve when mixed in the full flow. City of Valdosta Georgia Pilot Study The City of Valdosta, Georgia owns and operates a 15 million gallons per day (MGD) Water Treatment Plant (WTP). This water treatment plant obtains raw water from eight (8) deep wells that are under direct influence (UDI) from surface waters. Table 1 shows the typical ground water supply with low DO and relatively high total sulfides. Existing treatment consists of three induced draft packed tower strippers followed by a fine bubble diffusion (FBD) ozone contact basin. Ozone is generated at 6% wt. from vaporized liquid oxygen (LOX) fed ozone generators. Ozone treatment is used to oxidize the remaining sulfide ions after stripping, trace levels of iron and manganese, and for primary disinfection. The Valdosta WTP has been in operation since November 1992 and was one of the first facilities to employ oxygen fed ozone generators in the southeast United States. In recent years the City of Valdosta has become concerned with high dissolved oxygen levels in the finished water and the resultant corrosive effects on their City’s distribution system and the internal water loops at the water treatment plant site. Finished water dissolved oxygen levels have ranged from 20 to 25 mg/l. High service pumps have experienced accelerated impeller wear due to cavitation, and plant water circulation loops and cooling water loops have excessive internal pipe corrosion. In 1998, Jones, Edmunds and Associates (JEA), an industrial and municipal water and wastewater consulting engineering firm located in Gainesville, Florida, was contracted to develop a cost effective solution to control finished water DO levels. The GDT™ process in a side stream configuration was evaluated as one possible solution to reduce high DO level developed in the combined stripping and ozone treatment process.
Dissolved Oxygen Concentration vs. Gas/Liquid Ratio
GDT Corporation
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20
5
10
15
20
25
30
35
40
Operating Pressure10 PSI 20 PSI 30 PSI
80 GPM, 2 ft/s
30 second contact timebefore GDT Degasser
Figure 9Gas/Liquid Ratio Vg/Vl
Dissolved Oxygen Concentraion mg/L
The utility also wanted to explore the possible elimination of the sulfide stripping towers to reduce operating costs associated with packing fouling and blower maintenance. Additionally, the packed towers contributed to the finished water's DO levels by elevating the raw water to atmospheric saturation before ozonation. It was thought that the elimination of the packed tower and the use of a GDT side stream would allow the plant to reach its finished water goal of less than 10 mg/L DO. Pilot Study The focus of the Valdosta demonstration pilot study by JEA was process optimization to provide a treated side stream with sufficient dissolved ozone to treat the raw water flow, maintain a residual for disinfection and minimize finished water DO levels. The pilot study examined key parameters such as the impact of gas to liquid ratio (Vg/Vl), ozone gas concentration, side stream gas to liquid contact (detention) time, and process pressure. The side stream with high dissolved ozone (DO3) and DO, but without entrained gas would then be blended with the raw water streams in order to achieve the lowest dissolved oxygen levels while satisfying ozone demands for oxidation and disinfection. The side stream pilot plant equipment consisted of a 3” water supply line feeding a 1” GDT contacting skid. The combined side stream and main flows entered a 12” diameter pipeline contactor sized to provide minimum of 15 minutes of contact time. Instrumentation included an In-USA model H1-X high concentration ozone feed gas analyzer and ATI dissolved ozone and oxygen analyzers for determination of applied ozone dosage and the resultant dissolved gas concentrations. Gas to liquid contact (hydraulic) time was defined as the time the gas and water mixture remains in contact between the Mazzei® injector outlet and the GDT™ degas separator outlet. In order to readily vary the contact time, various lengths of clear 2 inch diameter PVC hose were utilized as a pressurize reaction pipe between the injector outlet and degas separator inlet. The use of clear hose allowed observation of the two-phase flow mixing activity. Ozone feed gas at approximately 6% wt. was provided by existing medium frequency Emery ozone generators. Higher concentration testing at 10-11% wt. was conducted using an Ozonia CFS-2 ozone generator. Dissolved oxygen and dissolved ozone instrument readings were confirmed or corrected utilizing the modified azide Winkler method (DO), colorimetric indigo trisulfonate method (DO3), and amperometric titration (DO3). Raw well water dissolved oxygen concentrations were measured to be less than 0.2 mg/l by the Winker titration method. The ATI dissolved oxygen sensor and upflow gravity cell were placed immediately down stream of the degas separator. Oxygen Injection JEA ran a series of base line pure oxygen tests by injecting O2 into the raw (unstripped) well water. The first series of tests injected pure oxygen at increasing gas to liquid ratios into a Mazzei injector at injector outlet pressures of 12, 17, and 25 psig.
Figure 10 illustrates side stream DO at injector outlet pressures of 12 and 25 psig.
GDT Side Stream Ozone Injection
Figure 11 shows the increase in dissolved oxygen levels as the result of increased gas to liquid contact times and gas to liquid ratios (Vg/Vl). These data suggest minimizing finished water dissolved oxygen concentrations requires a side stream ozone gas to liquid ratio at or below 0.1 and a side stream flow that is 20 percent or less of the total treated water flow rate (≤ 20% side stream ratio).
Ozone Injection The pilot testing then moved to the side stream injection of ozone and the determination of the percent side stream required to provide sufficient dissolved ozone to meet the raw water demand of the combined mixed flow. The quality of the plants raw water varied with the well sites utilized to meet plant flow demands. Through out the pilot study, an attempt was made to utilize the same blended well sites, which provided an average raw water quality shown below.
S2-
Fe
Mn
pH
Color(cu)
Temp (°C)
Turbidity (NTU)
DO (mg/L)
Blended Well: 1,2,8
1.57
0.03
0.03
7.97
11
22
0.01
0.2
It was determined, early in the ozone pilot, that the raw water had an ozone demand between 5 and 6 mg/L. To develop sufficient side stream dissolved ozone residuals utilizing a 6% wt. ozone gas, required both a high side stream ratio (50%) and high Vg/Vl ratio (≥ 0.20:1) at 25 psig and a hydraulic detention time of 38 seconds. These operating parameters created excessive side stream dissolved oxygen; with resulting full flow DO levels of 16 to 24 mg/L range, consistent with the FBD contactor.
Oxygen Saturation Vs Pressure
GDT Corporation
0 0.1 0.2 0.3 0.40
10
20
30
40
50
60
70
80
Operating ConditionsRaw Water 12 PSIG 25 PSIG
Power Regression Line Fit
Contact Time 38 seconds
Figure 10
GDT Side Stream Ozone InjectionOxygen Saturation vs. Contact Time
GDT Corporation
0 0.1 0.2 0.30
10
20
30
40
50
60
Contact TimeRaw Water 9 Seconds17 Seconds 38 Seconds
12 PSIG Operating Pressure
Power Regression Line Fit
Figure 11
A second series of ozone tests where conducted using 10 and 11 % wt. ozone from the Ozonia CFS-2 generator in an effort to minimize DO levels. The decision was made to operate the side stream at an injector outlet pressure of 12 psig. This low injector
outlet pressure limited the dissolved ozone residual obtainable, requiring side stream to main flow ratios of 0.29 to 0.45 to meet inorganic oxidation demand and achieve a 0.7 mg/L DO3 residual for disinfection. The DO levels ranged from 12.5 to 13.2 mg/L and 11.6 to 12.5 mg/L for 10 % and 11 % wt. ozone respectively. Figure 12 details the side stream DO and DO3 test results.
GDT Side Stream Ozone Injection
A final series of ozone testing was done, utilizing packed tower stripper effluent (PTSE) to simulate current plant operation. Although the stripping towers reduced ozone demand by stripping and microbial digestion of sulfide ion, the towers contained a significant amount of biological matter. The shedding of microbes resulted in a high organic load from the strippers, producing higher ozone demand than in the raw water. The required side stream ozone dose coupled with the typical DO level of 8.4 mg/L after the strippers, produced finished water results similar to that achieved with the 6% wt. FBD contactor. It was concluded, from the pilot data, that under current plant operating conditions (6%), a GDT™ side stream would not allow the Valdosta plant to eliminate its packed tower and produce finished water with less than 10 mg/L of dissolved oxygen. Further review and extrapolation of the pilot data and plant conditions predicts that a side stream GDT contact train may be effective at the Valdosta WTP, utilizing 12% wt. ozone gas and increased contacting pressure to facilitate a reduced side stream ratio. It was also proposed that elimination of the packed towers’ biological fouling and adjusting the raw water to 6.0 pH for increased sulfide (H2S) removal would drop ozone demand to a level that would allow the side stream process to be of significant benefit. GDT Side Stream Research The data generated and questions raised by the Valdosta pilot study encouraged GDT Corporation to pursue additional research on the relationship of ozone contacting and oxygen saturation. Figures 4 through 8 show the GDT™ process is capable of high efficiency mass transfer of ozone to liquid under dynamic pressurized conditions 9. Figure 9 illustrates that DO levels are also increased dramatically in the side stream. This study evaluated the effects of gas concentration, Vg/Vl, side stream ratio, contact time and pressure operating parameters on oxygen and ozone levels in the combined bulk water flow.
Dissolved Oxygen & Ozone vs. Gas/Lquid Ratio
GDT Corporation
0 0.05 0.1 0.15 0.20
10
20
30
40
10% Ozone 11% Ozone Dissolved O2
12 PSIG Operating Pressure17 Second Contact Time
Power Regression Line Fit
Figure 12
Side stream contacting of ozone gas provides many advantages. In side stream contacting, a portion of the full water flow is pumped through GDT™ process which, transfers a high percentage of the ozone gas to solution and removes the entrained gases under pressure. The resulting treated side stream with high dissolved ozone is efficiently mixed with the main flow using MTM™ mixing nozzles. The nozzles also provide a controlled pressure on the GDT™ process. Among the advantages are conservation of
full flow line pressure, low energy requirements for pumping of the side stream and limited spacial requirement. Removal of entrained gas from the side stream combined with high energy mixing with main flow establishes virtual plug flow in the pipeline after recombination. This provides a positive impact on C·T credit calculation.
Side Stream Contacting
295 gpm (66.9 m3/h)at 14.5 psig (1.0 bar)
DS-200-W Degas Vessel& Relief Valve
Pipe Lineor TankContactor
16.4 mg/L dissolvedozone less demand
Isolation Valve
50.7 gpm (11.5 m3/h)at 40 psig (2.8 bar)
1584 Injector
10.5 ppd (200 g/h)Ozone
at 10% wt (138.89 g/m3)
50.9 scfh (1.44 m3/h) at 20C
Vg/Vl: 0.13
Design Transfer: 95%
Applied Dosage: 3.0 mg/L
Side Stream Ratio: 17%
Side Stream Dosage: 17.3 mg/L
Side Stream Pump: 0.74 WHP
0.56 WkW
Contact Vessel
GDT CorporationFigure 13
Mass TransferMultiplier
Figure 13 depicts the typical side stream GDT™ process.
Experimental Procedure The experimental system for this paper is designed to test the effect of operating parameters on full flow DO and DO3 at flow rates of 80 to180 GPM. The raw well water quality used for the experiments was as follows:
pH
Ca
(mg/L)
Mg
(mg/L)
Total Hardness
(mg/L as CaCO3)
Alkalinity
(mg/L as CaCO3)
OrganicCarbon (mg/L)
Fe
(mg/L)
Mn
DO
(mg/L)
7.6
71
16
244
208
1.2
0.82
ND
7.3
The experimental system is shown in Figure 14. The water entered the system through a 4” flowmeter. The piping that served as the ozone pipeline contactor consisted of about 90 feet of 4 inch Schedule 40 PVC arranged in a serpentine layout. The first 20 feet of pipe were clear PVC in order to facilitate visual observation.
Ozone Side Stream Contacting W/ GDT
Model 1583Injector
15 GalContact
Tank
DS200Degasser
AirReliefValve
InjectorPumpSide Stream
InjectionPoints
MazzeiInjector
Corporation
Figure 14
At the inlet of the contactor piping a 2” sidestream was taken off to feed a centrifugal pump which boost the raw water pressure and fed it through the ozone injector. A valve was installed on the discharge of the pump to control injector inlet pressure and therefore sidestream flow rate. Mazzei injector models 1078, 1583 and 1584 were utilized in the testing. The outlet of the injector could be connected to a 16 gallon reaction vessel to provide additional contact time prior to entrained gas removal or directly to a DS-200-SS degas separator. A Mazzei MTM™ nozzle was used in the reaction vessel to shear coalesced bubbles and aid in tank mixing. With the tank in place, detention times ranged from 15 to 60 seconds. Please see Figure 15 for layout details. Injector inlet pressure
adjustment allowed evaluation of the effect of side stream to total flow ratio. The degas separator outlet was piped to MTM™ nozzles installed at a 90° inlet into the main flow pipeline. The nozzles were sized to provide a backpressure of 4.0 to 6.0 psig, depending on the side stream injector and operating pressure.
SidestreamPump
Injector/NozzleManifold
OzoneGenerator
Sample 1
Contactor PipingDS300Degasser
Ozone Side Stream Contacting Test Apparatus
Mazzei Injector CorpFeb 1999
Sample 2
Sample 3
Sample 0
Figure 15
Sample points for DO and DO3 measurements were placed at 9’, 29’, 62’ and 93’ downstream from the side stream reintroduction point. Depending on the water flow rate, these sample points represented contact times ranging from about 2 seconds up to about 45 seconds. Bottled oxygen was used to feed an Ozonia Model CFS-3 ozone generator. Oxygen flow rate was measured using a Sierra Instruments Top-Trak Model 821S2-H-3-V4 mass flow meter. Gas phase ozone concentration was measured with an Orbisphere Model 3600 high concentration ozone monitor. Dissolved ozone concentration was measured with an Orbisphere Model 26504 ozone monitor with a Model 2301 sensor. An Orion model 840 meter was used for dissolved oxygen measurement. All monitors were calibrated vs. air at the beginning of each test session. The gas phase ozone monitor was checked for accuracy vs. potassium iodide wet titration method 4. Titration results correlated with monitor values within a range of 2.2% with a standard deviation of 2.8%. The dissolved ozone monitor was checked for accuracy with a Hach indigo method test kit with results correlating within 0.02 mg/l. The system was operated at full flow rates of 80, 120, 160 and 180 gpm. These flow rates corresponded to linear flow velocities of 2 to 4.5 ft/s.
Results As stated previously, the most important factors that affect ozone mass transfer are gas phase ozone concentration, gas/liquid ratio (Vg/Vl), system pressure, gas/water mixing and contact time. These variables are not independent from each other. The 10% wt. gas volume (Vg) for the same full flow applied O3 dosage (mg/L) will have a significantly lower side steam Vg/Vl than if 6% wt. ozone is applied to the side stream flow (Vl). To maintain a set Vg/Vl relative to the side stream flow, the gas injection rate must remain constant. At a constant gas injection rate, the only way to change the applied ozone dose is to change the ozone gas concentration. This difficulty in isolating variables is addressed by spelling out the operating conditions under which the results were obtained. Figures reflecting performance utilize the DO3 readings at the last sample point. It was noted that DO3 readings decreased slightly from the first to the last sample point. This is assumed to be due to decay and utilization. The Effect of Ozone Concentration and Detention Time The ozone concentration and ozone dosage can not be varied independently under constant Vg/Vl ratio conditions.
GDT Side Stream Ozone Contacting Test Results
In Figure 16 we see the affect on the full flow DO and DO3 as the applied dosage and Vg/Vl increase into a 16% side stream. Figure 16 also show the affect of added side stream detention time on the full flow results.
In order to achieve a finished DO level less than 11 mg/L in ground water with 7.3 mg/L DO, it would be necessary to operate the side stream ozone contactor without added detention time. This results in a lower DO3 transfer rate as indicated by the difference in DO3 as a percent of applied DO3 applied. This represents an 8% decrease in DO3 vs. a 35% reduction in DO present in the bulk full flow at a 0.1 Vg/Vl side stream.
Dissolved Ozone & Oxygen vs. Side Stream Gas/Liquuid Ratio
GDT Corporation
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20
2
4
6
8
10
12
14
16
18
20
50
55
60
65
70
75
80
85
90
95
100
Dissolved O2, W/ TankDissolved O3, W/ Tank Dissolved O2, W/O TankDissolved O3, W/O Tank
30-35 PSI, O3 Gas Conc 108-158 g/m3Main Flow Applied O3 Dose, 2.5-5.6 mg/l
Power Regression Line Fit
Figure 16
The effect of Pressure and Detention Time Figure 17 illustrates the affect of pressure and detention time on DO and DO3 in the full flow (15 psig) with a 12% side stream and ozone applied dosage of 2.4-2.6 mg/L. It is important to note that the side stream applied dosage is equal to the full flow dosage divided by the side stream ratio. Therefore a 2.4 mg/L full flow design dose is 20 mg/l to a 12% side stream.
GDT Side Stream Ozone Contacting Test Results
Higher side stream operating pressures provide increasing dissolved gas levels in the full flow at the Vg/Vl and ozone gas concentrations specified. The difference in DO level with and without added detention time of 45 seconds, narrowed as side stream pressure increased. The effect of detention time on DO3 appears to be negligible at the conditions specified. Therefore, operation without a tank would be recommended to control DO at the lowest level in this example. Conclusions Side steam ozone contacting can provide a method to control dissolved oxygen
levels while meeting ozone application requirements in full flow finished water. Side stream gas to liquid ratio Vg/Vl, ozone gas concentration and applied
dosage and ratio of full flow are critical design parameters. Detention time is a process variable to be minimized in side stream contacting to
control DO increase.
Dissolved Ozone & Oxygen vs. Side Stream Pressure
GDT Corporation
0 5 10 15 20 25 30 35 40 45 500
2
4
6
8
10
12
14
16
18
20
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Dissolved O2, w/ TankDissolved O3 w/ TankDissolved O2, w/o TankDissolved O3, w/o Tank
Power Regression Line Fit
Side Stream Vg/Vl 0.154-0.155Full Flow Applied O3 2.40-2.63 mg/l
O3 Gas Concentration 147.8-163.4 g/m3
Figure 17
References 1. Betz Handbook of Industrial Water Conditioning, 8th Edition, Pittsburgh 1980 2. K. L. Rakness, L. D. DeMers: Process Applications, Inc., E. Sedeno: Southern
California Edison, K. Ozekin: AWWARF, Draft: “Results of the Ozone Facility Evaluation at the Mesa Consolidated Water District”, Costa Mesa, California Funding Agencies: Ozone Energy Efficiency Project-Phase II AWWARF Project Number 284-95
3. Personal communication with John Gavre, Milwaukee Department of Public Works,
07 April 1999. 4. Personal communication with Orlando Utility Commission, Metropolitan Water
district of southern California and Southern Nevada Water Authority Staff, 1999 5. CRC Handbook of Chemistry and Physics, 58th Edition 1977-78 6. Ozone In Water Treatment, Applications and Engineering AWWA Research
Foundation, 1991, Lewis Publishers 7. Benson, B. B., Krause, D. Jr., “The Concentration & Isotopic Fractionation of
Oxygen Dissolved in Fresh Water and Seawater in Equilibrium with the Atmosphere”, Limnology & Oceanography (1984)
8. Mazzei Injector Corporation Calculated Data, January 1998 9. Personal communication with the City of Valdosta Georgia, Water and Wastewater
Department and Jones, Edmunds & Associates, Side Steam Ozone Addition System with Dissolved Oxygen Control Pilot Demonstration Data Summary, 30 March 1999
Key Words ozone, GDT™ process, side stream, dissolved ozone, dissolved oxygen, corrosion, entrained gas, milky water