Wastewater treatment works benefit from Sonarflex

Hycontrol's Sonarflex sludge blanket level and interface monitoring system is benefiting wastewater treatment and allied industries, providing complex process control and automation functions.

The company claims that Sonarflex can penetrate deeper into the sludge column than any other traditional sonar, offering a number of advantages.

By definition, wastewater treatment works are high energy users and their efficient running requires a fine balance of biological and hydraulic parameters throughout the process.

However, maintaining that balance can be a difficult task, especially if operational management does not have access to continuing reliable and meaningful data on interface levels in critical parts of the process, such as primary and secondary clarifiers and thickeners.

The company claims that Sonarflex has a proven track record in providing this information to significantly reduce the energy consumption of treatment works.

The accurate measurement of interface levels is a complex problem in the murky, turbid settling tank environment and, without extensive sample extraction and subsequent laboratory analysis, it is said to be very difficult to obtain a clear picture of the density profile.

The sludge within the tank decreases in density from the bottom of the tank towards the top water level.

The densest sludge sits at the bottom of the tank and can range from 3,000mg/litre to 6,000mg/litre or more.

In a stable tank, the sludge will gradually decrease in density to around 200mg/litre at the top of the column.

Generally, treatment works are interested in 'quality' sludge, which has a density greater than 2,500mg/litre.

This sludge at the bottom of the tank is referred to as returned activated sludge (RAS).

Sludge at this density is heavy enough not to move hydraulically up the tank when process problems occur and is also dense enough to be termed 'good-quality' biomass, which can be returned to the aeration lanes for pre-treatment or diverted to waste.

However, when a change occurs to the site loading process, problems can occur and operators need to know the dynamics of the different interfaces to assess and effectively control the continuing process.

Many sonar systems can struggle to provide comprehensive and reliable information under difficult conditions as they do not have the power or the correct frequency to penetrate through the suspended solids.

Hycontrol claims that, in the absence of anything better, the only other way of gaining a full 'top-to-bottom' picture has been to use manual dipping products such as a 'Sludge Judge' or a gap sensor.

However, these labour-intensive devices do not provide a continuous output for trending and control and only give a visual snapshot of the interface layers in the tanks, while having associated and undesirable health-and-safety issues.

As outlined above, traditional sonar interface monitoring systems fall short of the necessary requirements.

Their frequency range and lack of power means that they cannot penetrate much further than densities of approximately 1,200mg/litre to 1,500mg/litre, thereby only allowing them to identify the upper FLOC interface with any level of certainty.

Based on this information, an assumption is then made that the corresponding denser RAS interface tracks the FLOC interface under all conditions.

The company claims that this is not always the case and, when imbalances occur owing to changes in site loading, continuing with this assumption makes matters even worse.

To run a plant at its most efficient, it is essential to monitor the 'good-quality' biomass at 3,000mg/litre to 6,000mg/litre.

Monitoring this interface allows for control of the RAS pumps/bell mouth to ensure that the process is optimised by returning only 'good-quality' biomass back to aeration or to the thickener for wasting.

If the site is using an instrument that can only monitor the lighter-density FLOC layer as a basis to control the RAS pump, when the FLOC layer rises owing to an imbalance, they are automatically assuming the denser RAS layer is also rising.

As a result, the site will either increase the RAS pumping rate or drop the bell mouth in an attempt to bring the rising blanket back down the tank.

However, what is actually happening is that the denser 'good-quality' biomass has remained at the bottom of the tank and it is only the lighter FLOC layer that has lifted.

Increasing the pumping rate or dropping the bell mouth will have very little effect on the lighter FLOC layer, which has risen up the tank, and these actions will quickly remove all of the 'good-quality' biomass from the tank and will then begin to pump back a lighter-density 'poor-quality' biomass, which could have very little suspended solids at all.

This will increase the problem by having a negative effect on the F:M ratio (food or CBOD entering the process) and mixed liquor suspended solids (MLSS) in aeration - and, therefore, the dissolved oxygen (DO) levels.

On some sites, it could take weeks to fully rectify the situation and, during this time, increased aeration may have been required, increasing energy consumption at the plant and, therefore, energy costs.

Sonarflex is intended to ensure that such situations cannot occur by simultaneously monitoring both interfaces.

The submerged high-power transducer sends ultrasonic pulses through the liquid, which are then reflected back from the different-density interfaces and are even powerful enough to penetrate densities in excess of 6,000mg/litre and to detect the tank floor.

These signals are processed by the specialist software to provide outputs relating to both the FLOC and RAS levels within the tank.

This information forms the basis of improved process and control to enable the site to optimise energy consumption and site operations, according to the company.

Alarm levels can be set so that, in the event of the FLOC level lifting, operators can make the necessary process changes in enough time to prevent the problem continuing and to avert a breach of consent.

Sonarflex features a range of available transducers, with frequencies ranging from 30kHz to 700kHz.

Comparing the theory for 'through-air' ultrasonics (for level measurement), it is possible to understand the need for multiple frequencies in sonar applications.

Measuring the level of a simple liquid in a vessel 10m deep is said to be very straightforward and almost any high-frequency transducer (40-50kHz) will give reliable and repeatable results.

However, if the same unit was to be used on a similar-size silo that contains a solid such as cement, with high airborne dust concentrations, the results would be far from successful.

It would struggle to penetrate more than a few metres and would be unstable in fill conditions.

This is because of the high-frequency/short-wavelength range at 40-50kHz.

The suspended particles will attenuate the high frequency/short wavelength.

If a lower frequency (5-10kHz) is used with a longer wavelength, the sound wave can pass through the suspended particles more easily.

An example of this is the use of a foghorn.

In bad weather conditions, visibility is poor because the air is saturated with moisture.

A high frequency/short wavelength would be far less effective in this scenario as the sound would be attenuated by the moisture particles and would only travel a short distance.

Foghorns use a low frequency/long wavelength to project the sound through the moisture particles miles out to sea to warn ships.

This is known as the 'Foghorn Principle'.

The same analogy remains true for sonar.

While competitive designs adopt a 'one-size-fits-all' philosophy for their sludge blanket systems (adopting a range around 600-700kHz), Hycontrol engineers select the optimum transducer for each application, according to the company.

Sonarflex uses a different frequency transducer for primary sedimentation, primary and secondary clarifiers, sludge thickeners, lamella clarifiers and sequential batch reactors (SBRs).

SBRs are typically installed where space or cost are at a premium.

They combine the primary sedimentation tank, the aeration process and the final/secondary settlement all in one tank.

By the nature of the principle of their operation, the liquid levels change within the tanks and a traditional fixed transducer cannot cater for these changes.

To overcome this, Hycontrol offers a floating transducer, enabling it to track the settling blanket interface as decant levels change.

As a result, settling times can be monitored far more accurately and the improved batch times can increase throughput by up to 20 per cent.

The ultrasonic transducers (either submerged or floating) need regular cleaning to avoid unreliable performance as a result of signal attenuation, caused by the build-up of scum, scaling, air bubbles or fats.

However, mechanical cleaning systems, such as wipers, have a finite life and require constant maintenance.

Additionally, components may need changing as often as every few weeks.

To overcome this, Sonarflex uses a patented actuator lever arm system, which carries a five-year warranty.

The automatic cleaning cycle is triggered on a time basis or by a predetermined reduction in signal level.

When this occurs, the actuator pushes the transducer support arm through the water to an angle of 45 degrees and then returns it to the vertical.

This sharp shearing action through the water removes any debris or scum from the front face and ensures optimum performance, without the need for any operator involvement.

The attention to detail in the design of Sonarflex covers both electronic and mechanical operational features.

There is a hazardous-area ATEX version, which can be used for the growing number of enclosed settling tanks, built to minimise odour release to the atmosphere or to capture and reuse the methane gases.

This option provides transducers and cleaning mechanisms suitable for use in the hazardous-area zone.

A range of communication protocols, including Fieldbus, Profibus, Hart and Devicenet, are intended to ensure seamless integration with modern plant instrumentation and DCSs.

The transducer can be located up to 500m from the control unit and a robust wireless link option can provide communication for rotating bridge fitted units, while GSM connectivity provides instant access to all parameters for servicing, technical support and commissioning by Hycontrol engineers.

Multiple outputs and relays can be used for alarm and control functions, as well as cleaner arm actuation.

Sonarflex can help to maximise the efficiency of the process using the two-analogue-outputs option or the four-output option when utilising a bus communications protocol.

When using the analogue version of the instrument, two 4-20mA outputs are available for monitoring the different densities within the tank.

On a primary tank, the interface can be monitored using one output, while suspended solids between the transducer face and the interface can be monitored using the clarity output, providing an indication of how well the tank is settling.

This second output can be used to provide control over dosing by only triggering the dosing mechanism when settlement is seen to be degrading and reducing dosing when the settlement improves.

By dosing only when needed and not on a timed basis, the amount of flocculent or coagulant used can be reduced and cost savings can be made.

On a secondary tank, the two outputs can be used to monitor the RAS layer and the FLOC layer, providing control of the RAS pumps or the bell mouth to optimise the density being returned to aeration and to ensure that a consistent density is wasted to the thickener.

The FLOC output of the lighter density can provide an indication of process problems and can also give an early warning of a possible breach of consent.

In a thickener, the two outputs can be used to monitor BED level and water clarity.

Monitoring the BED level is said to ensure that the digester or filter presses receive sludge of consistent density with low water content from the underflow pumps.

This reduces foaming and mechanical wear and tear, making the process more efficient.

Monitoring clarity (suspended solids between the transducer face and the BED level) provides a control for dosing, whereby the instrument will provide an output indicating the concentration of suspended solids.

As suspended solids increase, dosing can be increased; as they reduce, the dosing can also be reduced, maximising the dosing process and reducing waste from overdosing.

Alternatively, if the instrument is utilised with the communications options, the plant PLC can receive four outputs with any combination of RAS, FLOC, BED (level), clarity and temperature being available.

Whether utilising the instrument with two analogue outputs or the four communications outputs, the information provided can improve control through the works, provide rapid indication of process problems, prevent breaches of consent, control dosing in primary tanks and thickeners and reduce wear and tear on filter presses.

These can all help to reduce energy consumption, maintenance and chemical costs on sites, according to the company.

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