Effective and precise control of phosphorus can improve compliance and save money. It starts with smart application of phosphorus analyzers.

It’s no secret that effluent nutrient limits are getting stricter: It’s more essential than ever to minimize nitrogen and phosphorus releases that can pollute lakes and streams.

For phosphorus in particular, some clean-water plants are seeing permit limits for the first time; others are seeing their limits made stricter. In either case, operators need to know how to meet the new limits consistently. Will it take a facility upgrade? A simple process adjustment? Something in between?

A good place to start is by measuring phosphorus levels accurately and at the most appropriate points in the process. The resulting information can then help guide a control strategy that helps achieve compliance at a reasonable cost to implement and sustain. But what kinds of analyzers should a given plant use? Where should they be placed?

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As national product marketing manager for analysis products with Endress+Hauser, a supplier of process measurement and automation, Tracy Doane-Weideman visits many treatment plants and helps operators deploy instrumentation for maximum benefit. She talked about approaches to phosphorus measurement in an interview with Treatment Plant Operator.

TPO: Why is phosphorus measurement so important?

Doane-Weideman: Many communities can’t afford to upgrade or expand their plants to comply with new regulations. They need to make their processes more efficient. That means figuring out how to measure and control phosphorus by using online analyzers. Besides ensuring compliance with an NPDES permit, an optimized phosphorus removal strategy can provide a relatively quick return on investment.

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TPO: When dealing with phosphorus, what exactly are facilities measuring?

Doane-Weideman: There are two forms of phosphorus we hear discussed in the field: Total phosphorus, which includes all forms, and orthophosphate, which is the reactive portion. Plants in many states have total phosphorus on their permits. Others have orthophosphate.

TPO: How are these forms of phosphorus measured in the laboratory?

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Doane-Weideman: In the test for total phosphorus, a grab sample is digested to break all the phosphorus components down into the reactive form. Then that reactive form is measured in a colorimeter. For orthophosphate, the sample is filtered to remove the phosphorus bound in solids, and then the colorimetric measurement is taken. These are both Standard Methods and U.S. EPA-approved methods.

TPO: Are the measurement methods in online instruments similar to those in the laboratory?

Doane-Weideman: Yes. The challenge with the total phosphorus measurement is that the digestion step can take as long as two hours. For the online methodology, we have to shorten that step. So we take the digestion to 90 percent and then correlate that back to the laboratory measurement. That allows us to shorten the process.

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TPO: What is the role of the orthophosphate analyzer?

Doane-Weideman: When treating for phosphorus, the only thing they’re really treating is the reactive portion. So at the end of the day, they really want to know how much reactive phosphorus, or orthophosphate, they have. If they have total phosphorus on their permit, what they need to do is measure the orthophosphate and do a correlation.

TPO: What are the most typical applications for total phosphorus analyzers?

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Doane-Weideman: The usual application is in facilities that have a lot of combined sewers or have large industrial applications upstream, so that they can experience very large swings in the phosphorus concentration. Most of these plants deploy the analyzer at the headworks before the primary clarifier, or both before and after the primary clarifiers.

TPO: Where are orthophosphate analyzers usually deployed?

Doane-Weideman: Most often at the aeration basins or the final clarifiers, or in both places.

TPO: Can orthophosphate analyzers help with biological phosphorus removal?

Doane-Weideman: Some facilities are equipped to do biological removal by maintaining and controlling certain bacteria in the aeration process. It’s difficult, and the most they can usually remove is about 2 mg/L. Phosphorus is necessary for the biology to function properly, so they need to be sure they have enough, and yet not too much to meet the discharge permit. That’s where a balancing act comes into play. The best control strategy at that point is to have measuring points pre-aeration and post-aeration, so they can have feed-forward and feedback.

TPO: How does the control loop actually work?

Doane-Weideman: It’s flow paced. The pre-aeration measurement is to check and balance the load coming into the facility to determine how much is actually in there and make sure they’re returning enough activated sludge and other nutrients to support the biology.

TPO: What happens at facilities that can’t achieve enough removal biologically?

Doane-Weideman: They’re going to do chemical addition with ferric chloride or aluminum chloride before the final clarifier, or in a side basin. Most plants take a single measurement pre-clarifier, dose the chemicals, and assume everything is fine. It’s a fairly straightforward calculation. If they want to be exact — did we really get it all? — then they will measure before and after the clarifier. And then some plants will put one analyzer in the final effluent and feed it forward.

The point is that they want to save cost and still maintain the effluent quality they need. If they’re just doing grab samples, they are likely over-dosing or under-dosing. If they’re over-dosing they waste a lot of money because the chemicals are not cheap, and they also produce excess sludge. If they under-dose and feed the water back to aeration to re-treat, they get chemicals in the basin that can influence the performance of the biology.

TPO: How much impact can be achieved simply by using the proper instruments and with the optimum control strategies?

Doane-Weideman: The biggest savings we’ve quantified are with customers who are over-dosing chemicals. For example, one facility was dosing ferric chloride with simple flow pacing at a cost of $100,000 per year. Their average effluent orthophosphate was 1 mg/L, but the permit limit was 2 mg/L. By installing a dual-channel orthophosphate analyzer and implementing an advanced control strategy to operate closer to the permit limit, they reduced their chemical and sludge management costs by $40,000 per year, while still maintaining compliance.

TPO: What kinds of plants can gain the most from online measurement and control?

Doane-Weideman: The best opportunity is for plants that are not controlling for phosphorus now but are being forced to comply with a new permit limit. They have the best opportunity to start measuring and determine what they need to do next.

TPO: How do facilities interpret the information they collect through monitoring?

Doane-Weideman: There’s a lot more to it than installing analyzers. They’re great, but they’re just tools. It’s about looking at the data and figuring out what they need to next, with outside help if need be. My company, for example, does plant walkthroughs to find out what measurements they have, what flows they have, what treatment they’re using, what has and hasn’t worked, what their problems and concerns are. Sometimes with education they can make their own decisions. Some are already very sophisticated. We can consult with them and help them, work with their engineering firm, provide project management, and be there after they put a measurement in to make sure it’s optimized.

TPO: What are some qualities plant teams should look for in analyzers?

Doane-Weideman: First they need to evaluate how much maintenance the analyzer requires and whether they have the capability to maintain it. Ideally there should be no tools required on any standard item that has to be touched during maintenance. They need to evaluate the volume and cost of the reagents required. They also need to know whether the device has to be placed in a building or can sit outside next to a basin. Finally, they need to know the precision and accuracy of the analyzer itself and make sure it meets the criteria for where they plan to place it and how they intend to use it.

TPO: Do the phosphorus analyzers tie in the SCADA systems?

Doane-Weideman: The orthophosphate can be fed to the plant control system. However, sophisticated analyzers have a PID controller built in. In that case there is no need to do the engineering into the SCADA system — just send the output to the SCADA so the operators know what they’ve got. Analyzers today are a lot more than chemists in a box. They can be actual control systems.

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