Phosphorus Monitoring in Water and Phosphorus Removal

What is Phosphorus?

Phosphorus, one of the five main elements of living organisms, is essential to organic life. Although elemental phosphorus rarely exists naturally, it does occur in several other forms, mainly as phosphates. Orthophosphate (PO₄^3-) is the soluble form of phosphate and is a naturally occuring ion within water. This form is readily available for uptake by photosynthetic organisms but is most commonly found in low concentrations in natural waterways. Other types of phosphate are bound in living or decaying organic material or within sediments and soils.

Many biological processes rely on the availability of phosphorus. At a cellular level, phosphorus is a critical component to DNA and RNA, energy transfer with adenosine triphosphate, and structuring cell walls. In the human body, phosphorus is a major component of bones and teeth. There are several practical applications for phosphorus as well. Since phosphorus is critical for photosynthesis, it is a primary component in fertilizers. Some phosphates are common additives in foods, detergents, and even in our drinking water.

Phosphorus in the Environment

Phosphorus is vital within our freshwater ecosystems because it is the limiting nutrient for algae. Eutrophication, or excess nutrients within a body of water, can be harmful due to the potential for an explosion of plant life. This excessive algae growth, called "algal blooms", can deplete oxygen within a water body, causing fish kills. Also some algae can produce toxins that can be harmful to humans and animals.

There are several sources of phosphorus in our aquatic ecosystems. Non-point sources, such as fertilizer runoff, are the most significant contributor. However, municipal and industrial wastewater facilities (point sources) are another significant contributor of phosphorus. These facilities remove organics and solids from wastewater but, more recently, are required to remove nutrients like phosphorus and nitrogen. Phosphorus removal has become an essential priority for facilities discharging to freshwater systems or other at-risk environments. In the US, effluent limits for total phosphorus (TP) are commonly below 1 mg P/L, with critical regions implementing "ultra-low" limits below 0.1 mg P/L.

Learn more about phosphorus regulations, monitoring and removal. Check out our blog post, The Science of Phosphorus.

How to Remove Phosphorus in Wastewater

There are two primary mechanisms for removing phosphorus from wastewater, chemical removal, and biological removal. Chemical phosphorus removal requires the addition of metal salt to the water. Soluble phosphorus (orthophosphate) reacts with the metal salt to form a solid precipitate and then removed from the water within clarifier or filtration processes. Alum (aluminum sulfate) or ferric (ferric chloride) are common chemicals dosed in wastewater to achieve this effect. A common method is to add alum or ferric to the process near the end of the aeration basins within an activated sludge plant, giving it ample time to react with the soluble phosphorus before entering the clarifier. The precipitated phosphorus is then removed by sludge wasting. Chemical dosing can be automated using an orthophosphate analyzer by proving a control setpoint and a feedback orthophosphate signal to the dosing pump.

Learn more about phosphorus removal in wastewater and check out our blog posts, How Automated Orthophosphate Monitoring Cut Costs by 25 Percent and Monitoring Orthophosphate for Reduced Chemical Costs in Water Resource Recovery Facilities.

Similarly, biological phosphorus removal also requires turning soluble phosphorus into a solid form, followed by gravity separation within a clarifier. Biological phosphorus removal involves cultivating bacteria (phosphorus accumulating organisms, or PAOs) that take up phosphorus within their bodies. By providing the right environment, these PAOs will take up more phosphorus than usual to maximize removal.

Want to learn more about phosphorus removal?

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Measuring Phosphorus

Since phosphorus occurs in many forms, there are different approaches to measurement. Orthophosphate analysis is the most common because it is quick and straitforward. Alternatively, measuring total phosphorus requires more sample preparation and specialized equipment for the digestion of the sample.

Measuring orthophosphate has become essential to effectively monitoring and removing phosphorus in wastewater. Orthophosphate concentration is measured using wet chemistry analysis with reagents and colorimetry, traditionally conducted in a laboratory setting. However, online analyzers for orthophosphate are becoming increasingly popular because of the potential for chemical cost savings and the value of continuous data.

The YSI Alyza PO4 analyzer measures orthophosphate according to the yellow method (Molybdovanadate) and is designed to be as simple and accurate as possible. The analyzer uses a state-of-the-art multi-port mixing valve to minimize reagent volume, simplify tubing, and reduce maintenance. To further simplify maintenance, new IV-style reagent bags are smaller and easier to replace, and automatic cleaning functions keep the analyzer operating smoothly. For maximum accuracy, the analyzer features automatic 2-point calibration and precise temperature control at each measurement stage.

Orthophosphate Analyzer Applications

Improved dependability and ease of use have undoubtedly contributed to the higher popularity of orthophosphate analyzers. However, the demand for orthophosphate analyzers across various applications has also increased. Wastewater continues to be an essential application for these analyzers. Chemical phosphorus removal control is the leading application in which an orthophosphate analyzer can improve dosing efficiency and save money. Biological phosphorus removal is becoming more common, and an orthophosphate analyzer can monitor the effectiveness of your activated sludge process. Finally, wastewater operators are ultimately concerned with the total phosphorus concentration leaving their facility. Orthophosphate provides a valuable indicator of the total phosphorus at this stage of the treatment system since solids are typically low at the effluent.

The use of orthophosphate analyzers has extended past wastewater treatment and into other applications as well. Phosphate is a common additive in drinking water distribution systems and helps protect piping from corrosion. Orthophosphate analyzers are used to monitor phosphate levels throughout the distribution system and automate phosphate dosing. Due to eutrophication concerns in freshwater ecosystems, orthophosphate analyzers have been adapted for the environmental monitoring of lakes and rivers.

To learn more about orthophosphate analyzer applications in wastewater, download our Case Studies and Solutions for Wastewater Process Monitoring and Control. To learn more about orthophosphate analyzer applications in drinking water, download our latest technical note, How Online Orthophosphate Analyzers Can Help Control Pipe Corrosion in Drinking Water.