Tackling foul play

1st September 2016


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Rosa Richards reports on how to ensure water sensors are kept clean from fouling and remain monitoring effectively

Sensors are the ‘eyes and ears’ of industry to check that processes are running smoothly and efficiently. Many industries – among them water, power, oil and gas, and food and beverage – use sensors to routinely monitor water, wastewater and effluent. The reasons for monitoring include process control, quality control and environmental management, as well as regulatory compliance.

In the past, sensor stability was the biggest issue affecting the accuracy and reliability of monitoring data, and the equipment had to be recalibrated regularly. Nowadays, fouling by organisms or inert materials is the biggest hindrance to obtaining long-term, reliable data.

Sensors are affected over time by fouling and there are different ways to combat this but the best methods will depend on their application. This may be in surface water or drinking water, seawater, wastewater or effluent. With the correct specification, installation and maintenance, water sensors can monitor effectively for many years.

Significant challenge

Biofouling is a significant challenge to obtaining reliable long-term water monitoring data and disrupts industrial processes if feedback from sensors is critical to controlling the process efficiently. Wesley Irving, instrument engineer at the Environment Agency, outlines the main issues: ‘In a river estuary, the environment is ever-changing, giving rise to a wide range of fouling, which leads to impaired data quality. Biofouling can be the most difficult for probe manufacturers to address, as the fauna and flora in an estuary are adapted to the harsh conditions and extremely resilient.

‘Effective handling of biofouling is important to us as it leads to longer and more cost-effective deployments, and also lowers the cost of ownership of sondes [multiple sensors bundled together]. Long-term datasets allow the agency to see subtle shifts in the environment that may cause concerns in the future.’

Bio-fouling progress:

AQF: Aquaculture affected; ME: Marine energy projects affected; Sens: Sensors affected; T&S: Transport and shipping affected

Thousands of aquatic organisms cause biofouling. The progression of biofouling follows an established pattern in seawater, but the initial stages are the same in freshwater (see diagram above). Sensors suffer from biofouling from minutes to months after deployment (industrial activities are affected later on). The rate and extent of biofouling will depend on the monitoring site. Warm, marine waters are ideal environments for biofouling, where organisms will quickly colonise the surfaces of sensors and hinder their functioning.

Inert materials, including sewage sludge, sediments and rags or other rubbish in wastewater, can also cause fouling. Paul Norman, an engineer at measurement instruments business Partech, says fouling is a barrier to obtaining good monitoring data from sensors. ‘One particular challenge is the heavy fouling present in the clay mining industry. The settlement tanks in this industry are some of the harshest fouling applications we encounter. One client needed to monitor water quality in their settlement tank but their monitoring probes became silted up rapidly to the extent where the probe could not function at all.’

Partech’s solution was to deploy a retractable probe in the china clay quarry sludge settlement tank.

Anti-fouling methods

Fouling can be limited by using anti-fouling surfaces. The design of the sensor probe is central to preventing fouling, as are the coatings applied to their surfaces. These could take the form of a bleach that is injected onto the surface or the use of copper alloy (copper has natural anti-microbial properties). However, these coatings and even the copper alloy will degrade over time.

A probe can be designed to be streamlined to prevent ragging, or it may have a wire mesh over the sensor to allow water to flow. Although a mesh may help keep the sensor head clean, it provides a surface for colonisation and build-up of solid matter so it must be cleaned regularly. Other methods include production of chlorine on the surface of the sensor – for example, using electro-chlorination to produce chlorine from seawater or incorporating an LED to generate UV radiation to prevent biofilms forming. Production of chlorine can result in trapped bubbles of gas, which must be wiped away from the sensor. UV radiation works by disrupting DNA, but LEDs need to be replaced regularly, despite the constantly improving technology.

Fouling can be removed using mechanical or chemical methods. These include: mechanical cleaning with wipers or brushes; retractable sensors; air or water cleaning using jets blowing across the sensor head; or ultrasonic cleaning. Although effective, these methods can damage the sensor over time. Chemicals can be used to clean sensors automatically (with a wash wired in) or manually but there are issues with health and safety and disposal of the wastewater.

The anti-fouling methods mentioned are suitable and effective in the correct applications only (see panel, below). Anti-fouling techniques in drinking water are limited because they must not affect quality, but fouling will be minimal in this clean water environment. The methods in surface water or marine water applications may need to be powered by battery in some remote locations. However, in wastewater anti-fouling methods generally require power, so the probe must be installed with a supply.

The importance of maintenance

Each anti-fouling technique has its own benefits but is bound by its limitations, among them the operational life. There is therefore no such thing as a ‘fit and forget’ sensor. Regular maintenance of sensors will always be required so they should be installed in such a way to make this possible, says Chris Jones, research and development manager at Northumbrian Water Group.

‘Every sensor will be subject to fouling and we need to deal with it, otherwise we risk not delivering the levels of customer service or environmental performance that we’re aiming for,’ he says. ‘Although auto-clean and anti-fouling systems can help reduce the need for manual cleaning they’re not 100% effective, and some can promote more rapid fouling.

‘Elbow grease is the only really foolproof approach and instruments must be designed to be manually cleaned without having to work around a complex auto-cleaning system. For example, for drinking water intake monitoring, we have found that loosely mounting water quality sensors over a trough arrangement, rather than being “plumbed in”, reduces biofilm growth on and around the sensors and makes the whole arrangement much easier to clean and maintain.’

A similarly designed set-up of flow through cells combined with a pump system is used by the Environment Agency to undertake intermittent monitoring. This massively reduces fouling by limiting the time the sensors are in contact with the wastewater and final effluent. Water quality can then be tested every 15 minutes – although hourly is often enough – at sewage treatment works where conditions do not change rapidly.

Norman stresses the importance of a properly planned and executed maintenance regime: ‘The operator should design installations with maintenance in mind, train staff to ensure they know how the sensors work and build regular checks and maintenance into operating costs. Engagement with manufacturers for advice on the correct installation and maintenance regime is also crucial. Manufacturers have a responsibility to maintain regular dialogue with clients to ensure the correct advice and support is provided.’

The maintenance regime will depend on the application and on the sensor type and design, and the conditions of the monitoring site. Some ‘ballpark’ figures on frequency of maintenance are provided in the panel, but it is always best to seek manufacturers’ advice.

Norman says good maintenance pays off in the long term, saving the need to install new sensors. ‘I know of one wastewater treatment plant where sensors have been so well looked after by well-trained staff with advice and spare parts (seal sets and wiper rings) that they have lasted for more than 20 years.’

In the future, there may be less need for routine maintenance due to the general move in industry towards smart systems. There are already smart sensors that signal to the control system whether they are working properly by comparing direct measurements with an expected range of results to check for drift in performance. If the process and normal range of measurements are well understood, the sensor can recognise the anomalies. Supervisory control and data acquisition or SCADA systems can take in the diagnostics and produce an alarm for the operator to respond.

Future developments

Until now, the focus of anti-fouling has been on physical methods to remove muck, but there may be a move towards using advanced material science applications. To add to the biocides, copper alloys and various surface coatings, more sophisticated techniques are being developed that use nanoparticles or electrochemistry.

Dublin City University is developing silica-based coatings for a sensor’s optical windows, using the sol-gel process. The sol-gels can be ‘doped’ using anti-microbial metal nanoparticles such as copper to prevent biofilm growth. An added benefit is that the final coating is anti-corrosive. Sol-gels have also been used to produce self-cleaning solar panels and even waterproof fabrics.

Warwick University is working on highly durable synthetic diamond electrodes. These have great potential due to the intrinsic properties of diamond, which has a high thermal conductivity and is resistant to corrosion or abrasion, making it fairly resistant to fouling. ‘Man-made’ diamonds can be doped with boron to make them conductive, and can be surrounded by insulation. UV light can then be applied through the back face of the diamond to provide an optical sensor, while the front face in contact with the solution hosts the anti-fouling properties. A pure diamond electrode can hydrolyse water to produce hydroxyl radicals, which will oxidise anything in contact and can kill biofilms.

A diamond pH sensor is being developed for the oil and gas industry, which can operate at high pressures and temperatures and can withstand cleaning using bleach or wipers due to its resistance to corrosion or abrasion. Man-made diamond is tenable in terms of price and becomes cheaper with mass production.

However, there is no magic solution to biofouling, notes Darren Hanson, general manager at Xylem Analytics UK. ‘The combination of anti-fouling measures chosen will depend on the location, power availability and nature of monitoring required – be that in freshwater, seawater, drinking water or wastewater. There are off-the-shelf sensors available but the best solution is to discuss your needs with a supplier directly to gain expert advice.’

Anti-fouling methods

Drinking water

Surface water or marine water

Wastewater

Ultrasonics

Air jet

Biocide coatings

Material coatings

Copper alloy

Chlorine production

Wipers

Ultrasonic

UV radiation

Bleach injection

Air cleaning

Water jet

Ultrasonic

Wipers

Retractable sensors

Anti-fouling maintenance programme

Deployment

Sensor type/design

Frequency of maintenance

Points to note

Surface water, that is rivers and lakes

Upland rivers (high water quality, clear water)

NH3

Other sensors, such as DO, turbidity

Four weeks to two months

Three months

Recalibration is usually needed after four weeks to address drift in readings. Sometimes this can be extended to two months depending on conditions.

In clear waters, biofouling is minimal.

Estuaries

NH3

Four to eight weeks

See above, anti-fouling methods become more important for longer deployment.

Coastal water

Wiper mechanism

Four times a year

As long as good anti-biofouling measures, such as wipers, are installed.

Drinking water

Wiper mechanism

More than three years

Low rate of fouling.

Wipers and seal set replaced.

Wastewater

Wiper mechanism

Every two to three years

Based on self-clean every six to eight hours.

Wipers and seal set replaced.

Septic tank/ Aeration tank

Wiper mechanism

Every four to five months

Based on self-clean every hour.

Very high rate of fouling.

Wipers and seal set replaced.

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