A Practical Approach To pH Control: Observations …A Practical Approach to pH Control Brice Poston 9/1/13 4:50 PM Formatted... [1] A Practical Approach To pH Control: Observations

A Practical Approach to pH Control

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A Practical Approach To pH Control:

Observations From Different Neutralization System Startups

Robert L. Rebodos, PhD, Samanth E. Dawson, PE E. Matthew Fiss, PhD, PE and Edward C. Fiss, Jr., PE

Fiss Environmental Solutions, Inc.

ABSTRACT For many industrial facilities, the only pretreatment of process wastewater necessary prior to discharging to a municipal sewer system is simply pH neutralization. In practice, however, a pH control system can be highly complex. All individual components need to perform effectively at all times. The pH probe, considered the weakest link in a pH control system, is prone to failure and needs to be checked and calibrated regularly. More importantly, the control system should be designed properly to provide efficient neutralization even in extreme conditions. A review of the fundamental pH concept including an overview of unit operations, common problems and system design pitfalls encountered in the field are discussed in this paper. Results of pH probe testing comparing bulb-type and flat-type pH probes are also presented. Issues observed from start-ups of pH control systems from different design companies over the past year at various industrial facilities are enumerated. Solutions to these problems are provided to avoid costly, and sometimes catastrophic, system failure.

Keywords: pH control systems, wastewater neutralization, pH probes

INTRODUCTION For many industrial facilities, the only pretreatment of process wastewater necessary prior to discharging to a municipal sewer system is simply neutralization to achieve necessary pH discharge requirement. pH, which is generally use to define the acidity or basicity of a solution, is the negative logarithm of the hydrogen ion activity as shown in the equation below:

pH = - log aH

Ion activity (a) can be mathematically expressed as:

a = γC

where γ is the activity coefficient and C is the hydrogen ion concentration.

pH is usually measured using a combine pH glass electrode made of doped glass membrane sensitive to hydrogen ion and a built-in reference electrode. Oftenly, the glass electrodes are either bulb-type or flat-type, depending on the structure of the electrode tip. In general, the pH probe is oftenly considered the weakest link in a pH control system since it is prone to failure and requires frequent checking and calibration.

Aside from the pH electrode and transmitter that relays the signal, a typical pH control system consists of a feedback controller, a control valve, and a mixing equipment. In general, wastewater pH control system include an equalization tank and chemical tanks as source of reagent feed. Despite this relatively simple system, pH control can be a complex process. One reason is that in water applications, the system deals with a wide range of hydrogen concentration, that is from 10-14 to 1 M. No other common wastewater quality parameter is measured at such wide range. Another reason for the complexity of pH control system is that for the system to work effectively, proper design and installation of all components are extremely necessary. Correct functioning of the pH electrodes, transmitter, controller, control valves, piping and mixing equipment should be maintained at all times. Neutralization reactions with an acid or base occur instantaneously in most cases and therefore requires control systems to react accordingly.

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In this work, different pH control system designs were evaluated to determine potential issues that may be encountered in the plant. Common problems and design pitfalls observed during system startups are described in the discussion below. By presenting these observations from different pH control systems over the past year at various softdrink facilities, system failure which could be costly, and sometimes catastrophic, may be prevented.

METHODOLOGY Evaluation of commonly used pH probes was performed using three commercially available pH electrodes. Two bulb type sensors and one flat-type electrode were used. All probes were calibrated on a daily basis prior to testing. The pH probe testing was performed by submerging all electrodes in a synthetic softdrink plant wastewater. Response due to acid and base addition was recorded. A comparison of each probe’s response time, steadiness of reading, noise, and slope of pH change over time was conducted to determine the ideal electrode for the softdrink plant wastewater pH control system.

Designs of pH control systems by two different companies were reviewed and implemented in a number of softdrink plants. These softdrink plants were located in various sites in Northern America. Merits and pitfalls for each design were evaluated and confirmed on-site through a series of Site Acceptance Tests (SAT).

RESULTS AND DISCUSSION

pH Probe Comparison The pH response for each probe during chemical adjustment was monitored and recorded. These data were analyzed to compare probe performance. Each probe was evaluated for steadiness of the probe reading, presence of noise, and slope of pH change per time.

As expected, proximity to the chemical feed point affected the performance of the electrodes. It was observed that the closer the electrode is to the chemical feed point, the more variable are the recorded pH readings (up to ±1.5 pH units variability), which would be expected due to concentration gradients of the chemical as it mixed into the wastewater. This erratic readings were particularly observed for Probe 2, a flat tip sensor. The bulb-type electrodes also exhibited some erratic readings but were minimal. Results also indicate that the flat tip pH probe is more prone to unstable measurements. In general, flat tip probes have higher impedance and slower speed of response while hemispherical or spherical bulb probes have lower impedance and faster response speed. However, for certain applications, flat tip probes may be more appropriate because the protruding tip in bulb type probes makes the sensor more mechanically fragile.

An overall comparison of the pH probes based on the test results is presented in Table 1. Each probe was rated for each specified criterion. The bases for the ratings are discussed below.

a) Response Time

During this test, 15 readings per second were recorded, where voltage was measured across a 250 ohm resistor in the 4-20ma loop. The voltage was converted into pH using the following formula:

pH = (mV/0.25) - 4)(0.875)

For this study, a pH probe response time is defined as the amount of lapsed time (in seconds) after chemical addition before a stable reading is attained. A reading was defined as being stable if the change in the average of 10 readings (over 0.67 seconds) is less than 0.01 pH units after filtering out noise.


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Table 2. Comparison of Four Different pH Probes Tested

In each of the four trials performed, pH changes were recorded during both acid addition and caustic addition resulting in pH swings from approximately pH 11 s.u. to pH 2 s.u. and back. During acid addition, Probe 1 and Probe 3 took turns stabilizing first, while Probe 2 stabilized after an average of 6 to 8 seconds later. During base addition, Probe 1 stabilized first, followed by the other probes in various order.

Of all the probes tested, the readings from Probe 2 were observed to have wider range (max + 1.5 pH unit), as compared to + 0.5 pH unit for Probe 1 and + 0.3 pH unit for Probe 3. From these observations, Probe 1 had the fastest response time followed by Probe 3 while Probe 2 had the slowest response time.

b) Signal Noise

Signal Noise is defined as the lack of stability in the pH reading during pH adjustment. Since the instantaneous reading is used to control chemical pump speeds, a signal with less noise allows the chemical feed rates to decrease more uniformly as the solution pH approaches the pH setpoint. A signal with higher noise will result to jumpy output causing the feed pumps to overfeed and/or underfeed chemicals. Since permit pH limits are intended to maintain sewer discharges at near-neutral pH and since a small amount of reagent can cause a large pH change at circumneutral pH, signal noise could lead to a significant overshooting of the pH setpoint.

Of all probes tested, Probe 3 had the least amount of signal noise while Probe 2 had the most signal noise and gave the most erratic readings.

c) Calibration Ease

Calibration is required to obtain accurate pH readings. Calibration is initiated by switching from measurement mode into calibration mode and submerging the probes in different buffers. Following the necessary steps shown on each respective transmitter, Probe 1 was the easiest to calibrate displaying the calibration slope (mV/pH unit) at the end of the process. Calibrating the other probes involved additional steps that were a little more complex to go through. Probe 3 calibration program has more built-in safeties that prevent operation when the slope is said to be out of range.

d) Probe Installation/Insertion Method

Criteria Criterion Weight

Probe 1 Probe 2 Probe 3 Bulb-type

Titanium shaft Flat-type Bulb-type

Polypropy-lene body

a) Response time 4 10 2 9

b) Signal Noise 4 4 1 9

c) Calibration ease 4 10 3 8

d) Probe insertion method 4 6 4 8

e) Probe tip replacement 3 5 10 0

f) Whole probe replacement 3 5 5 10

Total Weighted Grade 40 25 44

Note: Grade is on 1-10 scale where: 10 - Excellent, 8 - Very Good, 6 - Good, 4 - Fair, 2 - Poor, 0 - Fail/Unacceptable

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Probe 3 insertion method seemed to be better than the other probes. The probe can be installed directly through a weldalet or in a pipe tee or “Y” using a ball valve assembly kit. Closing off with a stainless ball valve is better than the PVC assembly used for the other probes since it is expected to be operated regularly during probe replacement or maintenance.

e) Sensor Part Replacement

Electrode replacement and preamplifier replacement are most convenient for Probes 2 since the preamplifiers are separate from the electrodes themselves. Probe 1 has built-in preamplifier, so the entire probe needs to be replaced if there are preamplifier issues. The double junction salt bridge for Probe 3 is field replaceable, but like Probe 1, preamplifier issues mean that the entire probe needs to be replaced as well.

f) Whole probe replacement

In terms of whole probe replacement, Probe 3 stands out with a quick connect capability and can be easily dismantled because of the Variopol connector attached to the probe cable. The other probes have cables that must to be re-pulled back to the analyzer or junction box and will need to be rewired each time the probe is replaced.

Overall, the results of the pH probe testing suggest that the bulb-type probes were more superior than the flat-type probe in terms of response time, steadiness of reading, noise, and slope of pH change over time.

Practical Considerations for pH Control System Operation and maintenance of pH control system can sometimes be tiresome because of its complexity. Common operational issues involve elaborate start-up procedures, rapid deterioration of components particularly when dealing with extreme conditions, and tedious maintenance of pH probes. Proper design of pH control system could help avoid these issues. Some practical considerations observed during pH system startups are listed below to deal with some of these problems.

1. pH Electrode

As discussed above, performance of pH electrodes varies depending on type and material of construction. It is important to make sure that the pH electrode chosen is compatible with the influent wastewater to be treated. For example, when using glass electrode, make sure that glass attacking chemicals (e.g. hydrofluoric acid) that can cause probe malfunctioning are not present in the wastewater.

Aside from choosing the right electrode, the pH sensor should be properly installed. The placement of the pH electrode in the reaction tank or effluent discharge piping could majorly affect the performance of the system. In addition, pH probes in general have high impedance (~109 ohms). This impedance is compounded by noise (electric interference) from other components in the plant resulting to erroneous pH output. One simple way to avoid electrical interference is to minimize distance between the electrode and controller.

pH electrodes need to be regularly cleaned and calibrated to maintain accurate readings particularly those that are applied in aggressive environments. In somes cases, cleaning and calibration requires removing the pH electrode from the reaction tank requiring process shutdown. Whenever possible, in-line probe washing/flushing and calibration should be preferred because both allow pH electrode maintenance without causing disruption of process.

2. Equalization (EQ) and Reaction Tanks

The characteristics and variability of wastewater often dictate how difficult it is to control the wastewater pH. Installing an equalization tank prior to the reaction tanks could minimized variability in wastewater characteristics making treatment more predictable. The reaction tank should be sized accordingly to allow sufficient reaction time for neutralization. As observed in the facilities evaluated in this study,

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installing more than one reaction tank provides better process control. It is important that the mixing unit in the reaction tanks should be properly designed to provide adequate mixing minimizing overfeeding or underfeeding of chemical reagents.

3. Chemical Feed The choice of chemicals for pH neutralization usually depends on the influent wastewater characteristics and the desired effluent pH range. In most of facilities evaluated, 25% or 50% caustic solution is fed to raise the pH and either sulfuric acid or carbon dioxide is used to lower the wastewater pH. For this specific application, since softdrink plant wastewater are mostly acidic, caustic solution is used to neutralize the influent wastewater. In cases when the desired pH is exceeded, it was usually sufficient to lower the pH using carbon dioxide (CO2) rather than using an acid. CO2 is readily available in these facilities as part of the beverage manufacturing process. CO2 is also safer to handle and cannot lower the wastewater pH below 5.5-6 s.u. which is usually the lower limit of the discharge in most of the plants. By using CO2, accidental discharge of wastewater with pH lower than the pH limit, due to malfunctioning equipment for example, could be avoided.

Another operation to keep in mind is the control of chemical delivery to the reaction tank. When using limit control, chemical dosing is at a constant rate which may result to overshooting or undershooting of feed. To avoid such occurrences, proportional control should be used instead. The rate of chemical feed would be proportional to the deviation of the pH from the desired set point. Proportional control can be achieved by two modes namely pulse length and pulse frequency. Using pulse length mode, the total time of the pulse can be fixed by the user, assigning longer times for higher pH deviation from setpoint. For pulse frequency mode of operation, the frequency is set by the user such that the further away from the pH setpoint, the higher the frequency and consequently higher chemical dosing is achieved.

Furthermore, one thing to be avoided in controlling chemical feed is hunting of the relay which could potentially cause breakdown of pumps (and/or solenoid at times). This is experienced when at a given setpoint, the chemical feed stops once a specific setpoint is reached. However, after sufficient mixing, the actual pH drops again re-intitiating the chemical feed. This on-off-on-off cycle could eventually lead to pump malfunction. To correct for this problem, it is sometimes necessary to adjust the set point at a higher (or lower) value than the actual desired value. Consequently, it would be best to initiate feed and alarm relay circuit timer first when the pH set point is exceeded. By doing so, only after a user-set period in which the desired pH is not attained, would a system alarm be triggered.

4. System Startup

Despite their complexity, pH neutralization systems, by nature of their operation, are mostly straighforward in both design and control logic and because of these, wastewater treatment systems are frequently cookie cutter in design. This could lead to errors from carryover in coding or logic such that rigorous stress testing of the neutralizations system is critical during startup and system commissioning. Most issues identified during the Site Acceptance Tests (SAT) performed for the different facilities were not catastrophic for plant operation but could have led to plant downtime due to, for example, false alarm conditions being triggered when left unchecked. In rare cases, system startup commissioning failed due to inadequate hydraulic design and/or faulty construction or inappropriate material of construction. System commissioning should involve hydraulic and chemical stress testing along with a thorough check of all alarm conditions listed in a system alarm matrix. Startup commissioning should always take place with clean water before introduction of wastewater to ensure all system components are operating as required and there are no leaks etc. in the system. Although pH probes are usually calibrated at the factory before shipment, probes should always be field calibrated before the testing phase. One of the most important hydraulic design checks is to ensure that the system can push flows higher than the rated design without any overflow. Usually this involves filling up the EQ tank to the primary overflow point (if EQ tank is aboveground) and then operating the influent pump(s) at full capacity. The system should have adequate capacity to handle this peak flow and, in most cases, the system should be designed to handle 2 to 3 times the rated design flow. As a rule of thumb, the system should be

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hydraulically designed to withstand flows when all influent pumps are running at capacity with a full or nearly full EQ tank (when present) to simulate a worst case scenario for system hydraulics. False alarms can usually be rectified on-site unless there is a structural element to them. For example, a neutralization system manufactured by a certain vendor was repeatedly triggering the overflow alarm from the EQ tank at or below the system design flows. This was a result of wastewater backsplash from the flow in the normal discharge pipe triggering the overflow sensor located at a slightly higher elevation in the EQ tank primary overflow pipe. The short term solution was to pull the sensor tip higher in the overflow pipe, with additional follow up testing to ensure that the alarm triggered normally as required during an overflow event. The system design was modified during subsequent installations where the EQ overflow pipe was located a higher elevation from the discharge pipe to avoid this false alarm. Other instances of false alarms that were encountered during system commissioning were during the pH probe wash operations. At sites where the incoming potable water had a high pH, the momentary injection of a high pH water stream on the probes triggered a pH deviation alarm. Usually the pH deviation alarms are timed to trigger at 10 seconds. In this case the trigger time was set at a longer duration to allow the probes to read the correct pH of the wastewater after the probe flush.

CONCLUSIONS For many industrial facilities, a simple pH adjustment for neutralization of their wastewater will suffice as a pretreatment method prior to discharging to a municipal sewer system. Yet, based on experiences from a number of system startups and commissioning, wastewater pH control is anything but simple. Proper design begins by characterizing wastewater and choosing the correct components needed to achieve the required neutralization. This include determining the appropriate pH electrode, choosing the correct chemical reagents, sizing the EQ and reaction tanks properly and designing efficient control system.

Performing a thorough Site Acceptance Tests (SAT) during startup and commissioning provided valuable insights to issues oftenly experienced in pH control systems. Specific procedures that involved hydraulic and chemical stress testing along with a thorough check of all alarm conditions listed in a system alarm matrix proved to be valuable tools in preventing unnecessary plant downtime and avoiding costly and potentially catastrophic system failures. REFERENCES

McMillan, Gregory K., 1994, pH Measurement and Control, , Instrument Society of America. Patterson, James W., 1985. Industrial Wastewater Treatment Technology, James W. Patterson, Butterworth Publishers, p 361-368. EUTECH Instruments pH Control, reprinted from Asian Environmental Technology, Vo. 8, Issue 5 and Volume 9 Issue 1 (1998).

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Brice Poston � 9/1/13 2:54 PMFormatted: Font:(Default) Arial, 10 ptBrice Poston � 9/1/13 2:54 PMFormatted: Font:(Default) Arial, 10 ptRob � 8/30/13 4:49 PMFormatted: Font:12 pt, BoldBrice Poston � 9/1/13 10:05 PMFormatted: Justified, Line spacing: singleBrice Poston � 9/1/13 10:06 PMFormatted: Font:10 pt, Not BoldBrice Poston � 9/1/13 10:06 PMFormatted: Justified, Space After: 0 pt,Line spacing: singleBrice Poston � 9/1/13 5:43 PMFormatted: Font:14 pt, BoldBrice Poston � 9/1/13 5:05 PMDeleted: Based on the criteria evaluated in this study, the HACH/GLI pHD probe and the Rosemount 396RVP probe had the highest ratings (199 and 197, respectively) among the probes tested during this study . Rosemount 396RVP gave the most stable signal and had a similar response time compared to the HACH/GLI probe. However, because of the higher replacement cost and limited availability of this specific titanium-shaft model, the HACH/GLI pHD probe edges-out the Rosemount 396RVP for most of our intended applications. The polypropylene Rosemount probes should provide a similar performance to the 396RVP model, but are cheaper and more readily available. Since the Rosemount probes gave the most stable readings, it may be desirable to perform similar testing and evaluation using these other Rosemount probes in order to compare their performance to the HACH/GLI pHD probe. ... [199]

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A Practical Approach To pH Control: Observations …A Practical Approach to pH Control Brice Poston 9/1/13 4:50 PM Formatted... [1] A Practical Approach To pH Control: Observations