Phosphorus removal of wastewater: How to reduce chemical dose for treatment plants?

1/ What is phosphorus removal of wastewater in a treatment plant?

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Phosphorus removal in a treatment plant is a wastewater treatment process aimed at reducing the concentration of phosphorus in water before discharge into the natural environment. Phosphorus is an essential nutrient for plants, but in excess in waterways, it can cause an overabundance of nutrients and lead to excessive proliferation of algae and aquatic plants. This can cause ecological imbalances, a decrease in water quality and the death of fish and other aquatic organisms due to lack of oxygen.

By retaining phosphorus, phosphate removal makes it possible to limit the risks for fauna and flora in aquatic environments linked to the discharge of treated wastewater into watercourses, and therefore contribute to the preservation of biodiversity. Many environmental regulations impose strict limits on phosphorus concentrations in effluents released from treatment plants.

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2/ Biological or chemical phosphate removal?

In order to comply with regulatory constraints, two main methods of treating phosphorus exist, alone or in combination:

• Chemical phophorus removal : Consists of adding reagents (ferric chloride or other), which precipitate phosphorus in the form of insoluble salts which are then removed with the sewage sludge.
• Biological phophorus removal: Uses specific bacteria capable of absorbing large amounts of phosphorus. These bacteria are integrated into biological treatment processes.

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Their effectiveness can be very variable, as the performance of the treatment also depends on the variation of wastewater loads, and on external conditions (temperature, rainfall, pH...). Most often, a combination of these two methods is used to improve the efficiency of phosphate removal. Nevertheless, regulatory effluent concentrations are becoming more and more stringent and the stability of phosphorus removal poses problems for operators of treatment plants. Even effective biological phosphate removal systems sometimes need to use chemicals to meet effluent limits. However, the majority of the costs of the physicochemical removal of phosphorus come from the reagents used, and a third of the costs are linked to the excessive production of sludge*.

In addition to the significant cost, the chemical removal of phosphorus represents a major source of indirect greenhouse gas emissions. Thus, the global warming potential of an installation with purely chemical removal of phosphorus is 5.2% higher than a treatment plant with simple biological removal**, in the case of an effluent concentration of 0.5 mg P/l (effluent limit proposed in the future directive on the treatment of urban waste water of the European Commission of 2022***).

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3/ Reduce the use of chemical products through real-time management

To reduce these operating costs and the associated climate impact of the phosphate removal process, solutions exist to reduce the use of treatment products, while complying with current waste standards. This is the case of the Purecontrol solution, which has developed a new strategy for dosing chemical products for optimal physicochemical elimination of phosphorus. By using an artificial intelligence (AI) algorithm that controls chemical injection pumps in real time, the Purecontrol solution makes it possible to stabilize effluent concentrations and minimize chemical doses.

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Based on a modeling of the incoming wastewater load, and the historical analysis of phosphorus removal efficiency, the solution simulates scenarios for optimizing chemical injection. Thus, the solution sends automatic commands in real time to the injection pumps in order to apply the most effective strategy: the necessary and sufficient dosage injected at the optimum moment. By predicting the level of phosphorus at the outlet of the treatment plant, Purecontrol adjusts the phosphorus removal treatment of wastewater to achieve product reduction objectives while ensuring the compliance of the discharges. In fact, this is what has been put in place Grand Besancon Metropole for 6 months, and the result is there: 30% gain on ferric chloride.

In some cases, the reduction in the volume of reagents used may even reach - 40%.

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This solution, which uses reinforcement learning, will be able to continuously adjust to variations in parameters, and will even adapt to regulatory changes. During the various projects to optimize phosphate removal with the Purecontrol solution, the use of chemical products has been considerably reduced, while performance has been stabilized.