Coagulation Controller – CoagSense

Many water treatment plants still use manual control for coagulant dosing (usually alum or ferric). Whilst the raw water quality is steady this is an effective method to get a coagulant dose, however when the raw water changes then manual control of the coagulant dose breaks down, particularly when plants are unmanned overnight or over the weekend.

When the raw water is likely to be of proper quality in the future (e.g. a weather forecast predicts a rain event) operators often increase the coagulant dose in anticipation of a rainfall event. This can lead to poor coagulation prior to, during, and after the rainfall event (if it comes at all) and also can introduce longer term issues such as filter blinding, shorter filter run times and increased aluminum residuals (in the case of alum).

For many plants it is possible to automate coagulant control such that a streaming current monitor can effectively increase and decrease the dose of coagulant automatically in response to changing water quality. This is particularly common in areas of low alkalinity raw water.

Yes. CoagSense is suitable for any water source although payback will be better on water sources with high and variable turbidity and organics – flashy rivers for example. CoagSense controls all the variables that affect your coagulant including pH.

Yes, CoagSense works with any coagulant.

Yes, CoagSense works with any coagulant.

Yes, CoagSense works with any coagulant.

Yes, CoagSense works with any coagulant.

The dosing levels of coagulant can be controlled by Pi’s CoagSense. CoagSense measures and controls all the variables required to achieve optimum coagulant dosage rates, including pH and streaming current.

There are only two ways to directly measure the amount of coagulant required in a water treatment process. Streaming Current is the most widely used and internationally accepted way of doing this. Streaming Current can be used on its own to control coagulation but is usually more effective if used as part of a larger control system such as CoagSense.
The other method is using zeta potential. Zeta potential is more typically used offline.

Automation of coagulant dosing levels is possible using a variety of different sensors and the best one to use varies from site to site. Coagulation controllers that can be adapted to suit each site and are able to control dosing based off many different parameters are the most useful. The key is to ensure that the dosing can first be controlled manually to produce the desired water quality. Then an automatic dosing controller, such as the CoagSense, can be used to automate this.

Coagulant dosing can be controlled using feed forward control from UV254 acting as a surrogate measure of organics in water. A turbidity meter can be used in the same way, and streaming current can be used as a feedback control. The most suitable measurement for primary control varies from site to site and often a single parameter may not be enough. Turbidity, UV254 and Streaming Current are the most common parameters to use.

There are a number of coagulants which can be used depending on site conditions, budget, water conditions etc.. The CoagSense is an essential tool which can be used to automatically control this coagulant dosing. For more specific advice on the best coagulant, please contact Pi.

Typically the pumps are controlled using a PID driven 4-20mA output or Modbus link, or pulse control (either variable frequency or variable width).

UV254 is a measurement of absorbance in water, used as a surrogate to measure organics levels in water. Typically the organics are removed from drinking water using a coagulation process because they cause color in the water and react with chlorine to form Disinfection By-Products (DBPs) which can be hazardous to health.

Typically organics are removed using a coagulation process. In order to get the best results the post coagulation pH and the alkalinity need to be controlled along with the coagulation dose. Often improvements in organics removal can be made by lowering the pH of coagulation or by swapping the addition points for pH corrections and coagulant. For specific advice, please contact Pi.

This depends on the coagulant, the temperature, the chemical makeup of the water. In all cases the best results are obtained if the pH is controlled.

Yes, one of our customers filmed a testimonial for us. Click here to view the video. If you would like to talk with a customer who has a CoagSense, please contact Process Instruments.

Contact Pi to start the process which is likely to include a site visit from an expert in coagulation control.

Contact Pi and a coagulation control expert will talk you through the process.

Yes. CoagSense typically runs a primary control algorithm that controls the dosing pumps with at least one secondary algorithm received from a different parameter. If these algorithms call for significantly different dosage rates then the system alarms.

Yes. CoagSense can use new or existing sensors.

Other than the maintenance associated with the instruments used for measurement, the sample line, dosage pump used, there is no additional maintenance.

Many! The CoagSense is fully protected with the intelligence you would expect on a cutting edge controller, e.g. failsafe on process variable alarm, on pump failure, on the control algorithm going out of spec etc..

Yes. Each CoagSense coagulation controller comes internet ready with full remote access reporting and control from any browser, both desktop/laptop and mobile.

Aluminum is generally introduced into drinking water through the addition of a coagulant. If the coagulant is overdosed there can be too much aluminum in the water as it leaves the plant. This can be prevented by having good and appropriate coagulation control to reduce the amount of coagulant used.

CoagSense uses digital comms e.g. Modbus to provide full remote control and data monitoring from a site DCS, SCADA and PLC.

That depends on what is required by each site. Generally coagulation can be optimized for somewhere between £15,000 and £50,000. Many plants achieve payback in under 12 months from the chemical saving.

Yes. CoagSense has been specifically designed to be used with highly variable raw water sources.

If coagulation isn’t controlled well, overdosing causes:

• Increased chemical costs
• High aluminum carryover
• Increased sludge values and disposal costs
• Premature filter blinding

And under dosing causes:

• Failure of coagulation

Most people in the water treatment industry know that achieving the correct coagulant dosing level can be challenging at times. Maintaining an optimum dosage is crucial to achieving the best water quality possible, which includes an optimum reduction in organics for disinfection by-product (DBP) control. A plant operator may use historical data, jar tests, or experience to decide on their dosing level. To avoid the possibility of under dosing and releasing poorly treated water, many water treatment plants end up over dosing which results in large unnecessary chemical costs, reduced filter run times, reduced organics removal, aluminum carry over, increased sludge treatment costs etc. However, did you know that…

… no single online water quality measurement stands out as always being the best to monitor or control coagulation in all situations (as there are multiple factors that can affect coagulation)?
… some of these factors, like organics, are very critical to coagulation but are not commonly measured online and so are often overlooked?
… Process Instruments controllers can monitor multiple parameters simultaneously including charge (streaming current), organics (UV254), pH, flow rate and turbidity to provide a complete overview of water characteristics minute by minute?

What is coagulation control?

The aim of water treatment is to remove soluble (e.g. organics) and insoluble (e.g.colloidal particulate) contaminants. These contaminants often carry an anionic charge which keeps them in suspension or in the soluble phase. Coagulants are added to neutralize the charge of colloidal particles and to also “complex” with organic molecules which ultimately results in their precipitation. Reducing the charge of the particles and precipitating the organics means they will no longer be able to remain in a stabilized suspension. Once the repulsive charge component is removed these contaminants will naturally start to clump together due to attraction forces called Van Der Waals forces. The removal of these contaminants is aided by the formation of chemical floc (resulting from the hydrolyzation of the coagulant) that both enmeshes and sweeps up the smaller agglomerations, so they can be removed more easily by sedimentation or flotation.

If the coagulant dosing is too high, the unnecessary additional coagulant can cause increased amounts of sludge to be generated, shorter filter run times, aluminum carry over, and other unwanted side effects.

This situation is made considerably more complex because the amount of coagulant required and the effectiveness of coagulation can vary greatly throughout the day or even hour by hour due to changes in turbidity, pH, temperature or the levels of natural organic material (NOM) of the incoming water.

What happens to the raw water entering a plant during a rainfall event?

The obvious answer is that incoming water gets more turbid and the coagulant dose goes up. Turbidity is used as an online measurement of water quality in virtually all WTP’s and also when jar tests are carried out, however turbidity is not the only water quality parameter that is changing. There are other factors which are critical to coagulation that are likely to be changing at the same time:

• Depending on the nature of the river the pH can change which in turn can increase or decrease coagulation efficiency. Each coagulant has an optimum pH range where it can achieve the highest efficiency in terms of turbidity and organics removal. Thus, changes in pH can have a considerable impact on coagulation and filtration performance, and resulting turbidity and DBP reduction.
• The alkalinity of the water can change. If this drops below around 10ppm then effective coagulation cannot occur because the hydrolysis reaction that is essential for coagulation cannot occur. This situation can only be rectified by adding alkalinity.
• The level of dissolved organics (NOM) can change. Unless monitored with appropriate instrumentation (preferably online), initial changes in organics often go unseen and under-treated as a result. The actual demand on a coagulant from organics can in some cases appear before an increase in turbidity is seen and can also remain elevated after the turbidity has returned to normal. Lack of proper instrumentation to monitor changes in organics as they occur leads to a reduced removal efficiency of NOM which can lead to the formation of Trihalomethanes and other unwanted disinfection by products.

So what is the best measurement to optimize and control coagulation?

The answer is that no one single water quality measurement provides the complete picture on its own. As has been discussed here, there are multiple factors that need to be taken into account and ideally monitored on a continuous and online basis in order to most effectively control coagulation. Expecting a single measurement parameter to be effective is simply not very practical because different water treatment plants all have different raw waters, which each have different challenges to overcome when controlling their coagulation dosing. This means that for any system to be most effective it needs to be flexible and modular, allowing different parameters to be measured based on what is needed at different water treatments plants, which depends greatly on the individual characteristics of the raw water. Even if a particular water treatment plant doesn’t currently face an issue like DBP formation due to elevated levels of organics, we have lots of examples where water quality has changed in unpredictable ways for a good many WTP’s. So, having a flexible, modular, and well rounded approach to online coagulation control, versus a single measurement approach, makes the most obvious sense.

The solution to this problem?

Not a ‘one size fits all’ product but a modular, customizable system designed to be flexible enough for all applications.

How we design your individual coagulation monitoring/control system.

At the heart of Pi’s coagulation monitoring/control systems is Pi’s CRIUS^® CoagSense controller.

• First we will ask you for details about your particular application and water treatment challenges and from this discuss the parameters you may wish to monitor. These parameters will include one or more of the following: pH, temperature, conductivity, streaming current, UV254 and turbidity.
• We will then discuss what control and communication requirements you might need. How many dosing pumps, alarms or signal outputs are needed? Is flow pacing going to be required? Does the system run continuously or only certain hours of the day? Do you require remote access or integration with telemetry systems? If your SCADA will be handling control, then what sort of communication protocol is needed to get the measurement data integrated?
• Finally, we will discuss the ideal system installation to get the most reliable performance out of your online coagulation control system.

Once this process is complete you will have a custom designed system, created by you, specifically for your needs with all the necessary features for effective control and no unnecessary extras. You will also have a system that can be easily expanded upon should it prove necessary at a later time, or as budgets allow.

Process Instruments Coagulation Controllers – truly bespoke systems.