Peracetic Acid Analyser – PeraSense

Membraned sensors come with many advantages over non-membraned sensors such as higher resolutions, fewer interferents and a greatly reduced effect of flow rate changes. These advantages can make a huge difference to the bottom line, particularly if the cost of the chemical being dosed is quite high. For free chlorine sensors, using a membrane can make your measurement much less dependent on pH (if you are using sensors from Pi), meaning your measurement is a more accurate reflection of chlorine residual.

As such, membraned sensors are now largely the norm in residual chlorine measurement and are also prevalent for chlorine dioxide and ozone monitors, but did you know that…
…membraned sensors are sensitive to changes in pressure?
…flow cell outlets can airlock even when water is flowing through them?
…membraned sensors can still be used when the outlet does not go to drain?
…Pi has engineered and designed solutions to all of these potential issues?

Sensitivity to pressure

Membraned sensors do have one property that needs to be carefully managed; they are sensitive to pressure. Pi was an early adopter of membrane technology, so we know that the installation of these sensors is just as important as the sensor itself. In fact, the same sensors in different flow cells can give very different results.

In order to prevent pressure variations affecting the probe, Pi typically uses open flow cells which eliminate variability in pressure before it reaches the probe.

Flow

Whether the sample to the cell is pumped, gravity fed, or comes from a pressurised line, it is important that the flow is controlled to within a range of 350-1000ml per minute, to ensure that sufficient flow is reaching the sensor and to prevent the flow cell overflowing.

If the flow to the cell is variable, Pi can provide a dole valve which controls the flow to approximately 500ml per minute, which prevents the cell from overflowing when pressure variations mean more flow than the cell can handle, while also ensuring adequate flow when the sample line flow/pressure reduces.

Airlocks

The outlet of the flow cell needs to be open to atmosphere, and completely unobstructed. Any system with a long outlet line (particularly flexible pipe) is prone to get airlocks, which will cause the cell to overflow. Outlets which are visually clear and even have water flowing through them, can be partially airlocked which causes backpressure to overflow the cell. This is very easy to diagnose as if you see the cell overflowing and remove the outlet pipe, you will see the cell go back to normal operations within approximately 10 seconds. If this is a persistent problem, consider putting in an air break using a commercially available tundish.

Outlets that do not go to drain

The water from a membraned sensor doesn’t have to go to drain. For processes where saving water is a high priority, a simple tank and pump system that will pump sample water back into your main process line will allow water losses to be reduced to almost zero. Pi’s CRIUS^®4.0 controller can be used to control this return process and ensure that this tank never overflows and can automatically drain itself periodically to avoid sediment build up.

What if things go wrong?

As any water engineer can tell you, no matter how well the system is designed, lines can clog, pumps can break and someone on site could fiddle with the settings. Pi recognises these challenges and has engineered solutions into our systems. All Pi membraned sensors have the option to be able to:

• Have a flow switch to detect sample flow loss.
• Use dosing overfeed protection to protect against clogged dosing lines or pump failure.
• Have remote access for SMS or email alarms.
• Use relays to trigger beacons or sirens for alarms or control valves and pumps.
• Customise user security levels to control who can change what settings.
• Use status logs that show what happened to the system and when.

Closed Flow Cells

For membraned sensors, the best way of housing a membraned sensor is with an open flow cell. There are some occasions where this solution just isn’t practicable and in those instances the closed flow cells from Pi, which can take an overpressure of up to 3 bar, are the best solution.

Why is there a requirement for peracetic acid (PAA) online monitoring?

Common disinfectants such as chlorine and chlorine dioxide have been used effectively in water applications for over a century. Some industries however have looked to use other alternatives which have fewer or no considerations of safety and which don’t require residual or by-product removal. One of these alternatives, which has found increasing popularity across many industries, is peracetic acid (PAA). As more applications turn to PAA disinfection there has been an increase in demand for a reliable and accurate monitoring and dosing analyser.

Process Instruments’ (Pi) PeraSense is a robust online analyser capable of continually monitoring and optimising the levels of PAA, providing you with the peace of mind that chemicals won’t be wasted and the water quality will never be compromised!

Did you know that…
…Pi’s PeraSense requires approximately ten minutes of maintenance, once every six months?
…Pi’s PeraSense is suitable for all potable, process and salt waters?
…Pi’s PeraSense amperometric sensor has up to a 15 year life span?

What is peracetic acid and why is it used?

PAA is a powerful oxidant with an oxidation potential, used as a measure of disinfection effectiveness, greater than that of common disinfectants such as chlorine and chlorine dioxide. It is produced by the reaction between acetic acid and hydrogen peroxide when dissolved in water, and degrades over time forming non-toxic water soluble products. The free radical degradants (bug killing species); hydrogen peroxyl and hydroxyl have high oxidising capacities and are generally believed to destroy bacteria via the protoplasmic oxidation mechanism resulting in bacterial cell wall disintegration. The safe degradation pathway and high oxidation properties make PAA a popular disinfectant for many applications and industries.

Not only does PAA have greater disinfection power when compared to other alternatives but there are also implications of using chlorine which PAA users do not experience.

Chlorination of water can be achieved using three chemical mediums; chlorine gas, calcium hypochlorite and sodium hypochlorite, all of which have proved useful in many applications.

There are, however, several factors which affect their viability and all three mediums have safety concerns which must be addressed, including; potential gas release, corrosive properties and stability under heat and sunlight. As a result of this, sites often spend considerable amounts of time and money implementing safety precautions such as the safe generation and storage of these chemicals.

Furthermore, chlorine reacts with natural organic compounds found in the water supply forming potentially harmful disinfectant by-products such as Trihalomethanes (THMs) and Haloacetic acids (HAAs). The potential health hazards associated with exposure to these by-products, particularly THMs, have resulted in extensive drinking water regulations across the developed world with a stipulation on regular monitoring of these compounds across the water distribution system. As PAA usage only results in non-toxic degredants, no further costs are incurred on instrumentation to monitor potentially harmful residuals or by-products.

Although PAA does still have some safety considerations, as with all disinfectants, it is widely accepted that storage, generation and stability is superior to using chlorine.

What are the typical applications PAA is used for?

PAA has been primarily used as a cleanser and disinfectant in the food industry since the 1950s, particularly for fruit and vegetable washing due to its highly effective anti-microbial properties. All disinfectants applied directly to food must not leave harmful residual by-products, which is why PAA is so widely used in this industry around the world. Other industries in which the water cleanliness is of paramount importance have also turned to PAA for disinfection including; agricultural, dairy processing, wineries, breweries and even cooling towers.

Pi’s work with H&M Disinfection – Food and Beverage Disinfection

H&M Disinfection have designed, manufactured and installed cleaning and disinfection systems for the Food, Dairy and Beverage industries since the late 1980’s, providing their customers with bespoke cleaning solutions specific to their clients needs. More recently, Pi and H&M Disinfection combined their experience and expertise in water quality monitoring instrumentation and disinfection process engineering respectively to supply a UK food processing plant with an optimum water treatment solution, capable of handling their peak process water demand to ensure the sites final water meets the industry’s strict regulations. Pi’s peracetic acid analyser made up a small but integral part of the water treatment system provided by H&M Disinfection and continues to ensure the customer has optimum water quality.

Fundamentally, an effective PAA chemical dosing controller should allow the user to find the perfect balance between saving PAA and dosing enough for optimum disinfection and this relies on the quality of the instrumentation. Pi’s PeraSense is the complete PAA dosing controller which uses a state of the art two electrode amperometric sensor to provide the highest levels of accuracy for process PAA monitoring.

The controller provides the flexibility to ensure you have all the functionality you need and nothing which you don’t. The CRONOS^® and CRIUS^®4.0 controllers have up to two and four sensor inputs respectively, with the option of up to 16 sensor inputs with expansions on the CRIUS^®4.0 meaning you can monitor multiple parameters from a single controller.

The PeraSense has the option to come with full PID and output controls, which are completely user configurable to provide the site with the ability to make the instrumentation suit their needs. Pi’s controllers can also offer the option for remote access, which provides site or servicing contractors the ability to remotely monitor the live levels of PAA, trend historic data, view system alarms, remotely configure settings and much more.

Many different sites ranging across the whole water industry have a daily struggle to keep instrumentation functioning correctly due to fouling. However, did you know that…
…self cleaning and self flushing systems are now available from Process Instruments for most types of sensors?
…these fouling removal systems can extend the life of sensors and drastically reduce maintenance regimes?
…Pi’s self cleaning/flushing systems are affordable, simple and trouble free by design?

Sensor Fouling

Whatever the process being monitored is, there is often something in the sample water capable of fouling a sensor, and therefore causing erroneous results. The obvious solution to this problem is to clean the sensor, but how regular should inspection and cleaning programs be for each piece of instrumentation? Too regular and the inspection and cleaning regime is time consuming and unnecessarily costly. Not often enough and the instrumentation will give false results and probably fail prematurely.

What is the solution?

Process Instruments’ AutoClean and AutoFlush Systems

Simple, reliable and easy to maintain, Process Instruments’ AutoClean/AutoFlush systems are an alternative to mechanical cleaning mechanisms which can clog and break. By regularly spraying the sensor/probe with clean water or air, the sensor remains clean and free from fouling for extended periods of time. The sensor cleaning cycle is activated by Pi’s controller for a user selectable length of time and frequency so that no matter how dirty the application, the probe remains clean. With no moving parts in the sensor body or in the cleaning attachment there is nothing to replace or check other than a simple valve positioned in an easy to reach location.

Pi’s AutoClean and AutoFlush systems can give trouble free and fouling free functioning of sensors for weeks, if not months, at a time.

A solution for each application

AutoClean

This option can be added to our pH, ORP, Turbidity, Suspended Solids and Dissolved Oxygen (DO) sensors. Consisting of an end cap to direct the flow of clean water (or air for a DO sensor) across the face of the sensor blasting any dirt away. The cleaning is controlled by a single valve positioned in an easily accessible location.

AutoVerify

If using air to clean a DO sensor the system can also automatically verify that the sensor is still responding correctly, removing any need to remove the sensor from the sample for months at a time.

AutoFlush

For sensors that require flow cell mounting like Chlorine, Ozone and Chlorine Dioxide, an AutoFlush system has inbuilt valves which automatically start/stop the sample flow and control the flow of clean water past the probe. The user can set the flushing interval and duration to keep the flow cell and sensor clear from fouling. For particularly dirty or stubborn contaminants, warm water can be used as the flush water to aid cleaning.

With the above options, whatever the application or parameter being measured, Process Instruments will be able to provide a monitoring system that will not only be accurate, precise and long lasting but that will also remain free from fouling and save the operator both time and money.