Online Optical DO Analyser - Online Dissolved Oxygen Meter – OxySense

As described elsewhere, the ORPSense can come equipped to automatically clean itself at user defined intervals with all the benefits of no operator intervention. The AutoFlush is particularly useful in food preparation, pulp and paper, and many applications where there is likely to be a build up of solids in the sample.

The whole range of ORPSense ORP meters can be fitted with additional sensors such as chlorine or pH. Please ask your local distributor for more details.

It is probably the most reliable and cost effective DO sensor available as it;

• is extremely resistant making it last longer in the harsh environment of an aeration lane.
• can self clean and self check its response every time it cleans.

Pi has developed a special end cap that screws onto the end of the sensor. It is smooth and low profile to reduce the possibility of ragging. An air blast cuts across the flat sensor end helping it clean.

When the sensor is being cleaned the design of the end cap means that the water is excluded from the end cap by the force and volume of air. This cleans the end cap (lumiphore) and exposes the sensor to air. The sensor then reads the DO in air (which is much greater than in water) and that response is measured. If the response is not what is expected an error is flagged for the operator to intervene.

How often and for how long the sensor is cleaned is set on the simple controls on the analyser itself. It is also possible to set a ‘hold’ for the output from the analyser to ensure that the control is not affected by the self clean. This ‘hold’ starts at the beginning of the cleaning cycle and continues for a user defined time after the cleaning cycle.

Sensor Fouling

What is the solution?

Process Instruments’ AutoClean and AutoFlush Systems

A solution for each application

AutoClean

AutoVerify

AutoFlush

Introduction

Northern Ireland Water (NIW) recently trialled three dissolved oxygen analysers (DO meters) in parallel. These were installed at Culmor Waste Water Treatment Plant side by side in the same aeration lane, where measurements were recorded every minute for 6 months.

After processing more than a million points of data, the OxySense from Process Instruments (Pi) was shown by NIW to be the best solution.

The graph shows a three hour snapshot of some of the results that led to NIW deciding that Pi’s DO meter performed the best overall. From looking at this graph it is clear why Pi was preferred: Competitor A’s sensor displays a low, inaccurate reading and Competitor B’s sensor is afflicted by noise. These results are discussed in more details later in this Focus On.

The Plant

The trial plant had used Competitor B’s DO meters for some time in all their aeration lanes, however they found that the sensing end cap required replacing quite frequently (every 6 months). The replacement costs of £90 ($120, €105) per sensor started to accumulate and the plant decided to seek an alternative.

Auto-Cleaning

Dissolved Oxygen Sensor – OxySense S

Pi’s sensor was equipped with its Autoclean end cap (see image, right), which actively cleans the sensor with compressed air (or clean water) to remove sludge and debris from the sensor tip on a regular basis.

Competitor A’s sensor was attached to a flexible mounting which allowed the sensor to move with the sludge and aeration bubbles, theoretically cleaning the sensor passively.

Competitor B’s sensor did not have any automatic cleaning functionality.

All three sensors periodically received a manual clean as part of the plant’s regular maintenance schedule, ensuring the test was fair for the sensors without a cleaning system.

Results

Measurements from the 25th of May 2015 have been used to demonstrate the variation between the different analysers. This date set was chosen to be representative of the whole trial.

Process Instruments

The trial found that the CRIUS^® OxySense from Pi provided the most accurate and reliable DO meter. The sensor was kept clean by Pi’s Autoclean system, and reported accurate and precise readings with the lowest noise. A gradual increase in dissolved oxygen content is easily observed during aeration, followed by a steady decline as the air blowers are turned off.

Competitor A

Competitor A’s sensor struggled to keep itself clean between the regular maintenance events, as the passive self cleaning system was ineffective. The instrument appears to inaccurately report lower readings as a result of sludge and debris coating the sensor. The DO sensor detects the increased oxygen dissolved during aeration, but then falls to a reading of zero for long periods of time.

Competitor B

Readings from Competitor B’s sensor appear to lack precision and contain noise. This is most likely due to fouling of the sensor head (see image, right). The results had large variations between measurements, often making it difficult to distinguish the peaks during aeration. By the end of the six months, Competitor B’s sensor was found to be worn and required a replacement lumiphore end cap.

You probably know that online dissolved oxygen monitors are crucial for controlling the oxygen available to the microbes in an aeration lane or oxidation ditch but did you know that…
…typically up to 50% of the cost of running a waste water treatment plant with aeration is the cost of the electricity to run the blowers?
…often the cost of purchasing many online DO meters is less than 10% of its 10 year total cost of ownership?
…an online DO meter that is reading 0.1ppm low could cost more than its purchase and installation costs in increased electricity costs in just a few months?

Galvanic vs Optical DO measurements

Early measurement of online dissolved oxygen in waste water was made by electrochemical cells that needed sacrificial electrodes, electrolyte, membranes and a lot of calibration. Whilst there are some companies still advocating the use of this technology it was largely replaced in the 1990s and 2000s with online optical measurement. The vast majority of online DO systems used today are optically based.

Why is controlling the blowers so important?

Simply put, the microbes need enough oxygen to do the job. More oxygen than this, and we are wasting energy and promoting the growth of unwanted biologies in the process. Many plants are estimated to waste significant energy by over aerating their process.

What do you need from an online DO meter?

Reliability, Dependability and Reproducibility are the three most important things that a plant needs if it is to optimise its treatment (i.e. control the DO in the aeration lanes to a precise and accurate level).

Typically, an aeration basin will have ‘lanes’ with more than one DO sensor. If these DO meters are reading very different levels and do not track each other it is very difficult for a plant to know how to implement a control strategy. DO meters fail low, an unfortunate consequence of the technology. A reliable, dependable and reproducible DO meter will be consistent and will check its own responsiveness so an operator doesn’t need to worry.

Reliability means Robust

The Process Instruments sensor has what is probably the most robust lumiphore (sensing material) of any available DO meter. The lumiphore is the disc of material that is used to sense the amount of oxygen present and it needs to be tough to withstand the high energy, aggressive environment that they work in. 

Dependability means Clean

If you ask operators how often they maintain a DO meter (clean/calibrate etc.) you often get the answer “every week”. On a large plant that can mean an operator spends half their time doing nothing but maintaining the DO sensors!

The Pi online DO analyser has an optional cleaning system that can clean the lumiphore and keep it clean potentially for the entire life of the sensor. Utilising compressed air from site or a small robust compressor available from Pi, the Autoclean keeps the sensor clean and that’s not all…

Dependability means Self-Checking

If you ask the operator why they clean/calibrate the sensor weekly you will often get the answer “so we know it’s working”.

In modern plants the DO levels are kept low to reduce the energy required for the blowers. This means that the DO sensors can often read close to 0mg/l.

How do we know that the sensors are reading correctly and aren’t faulty?

The traditional solution is for an operator to pull the sensor out and watch it respond to air, and when it’s out the operator may as well clean it and calibrate it.

What if there was a way to check the response to air without taking it out of the aeration lane?

Well there is! The Pi Autoclean system not only automatically cleans the system but whilst doing so, also checks the response to air. Any issues are reported to the operator who no longer has to maintain the sensors every week.

How long can they last without any operator interference?

On a recent trial in a UK Utility the Pi OxySense was installed for 13 months alongside their existing dissolved oxygen analyser and during that time the sensor was removed once for manual cleaning when the power to the Autoclean system was inadvertently off for a week. That was the only time the sensor was removed from its position in the aeration lane and throughout the entire trial the instrument was reliable, dependable, and consistently accurate.