Phosphate is an essential nutrient, and is necessary for example, in low concentrations in biological wastewater treatment, but excessive concentrations can cause significant ecological harm. It is therefore common for phosphate to be included in environmental permits at municipal and industrial wastewater treatment plants.
Phosphate can be removed from wastewater by dosing with precipitants such as Ferric Chloride. However, dosing to excess can be very costly and inefficient, so it is common practice to continuously monitor phosphate levels for dosing control, and for demonstrating discharge compliance. Swan’s online AMI Phosphate instruments combined with the Sample Filtration System, have been designed specifically for both applications.
Background.
Phosphate is a key nutrient in the maintenance of aquatic animal and plant life. However, it is also considered to be one of the most important pollutants in surface waters. Excessive quantities, through natural accumulation or derived from human activities such as wastewater treatment and agricultural runoff, can stimulate the rapid growth of algae, resulting in algal blooms. This reduces the amount of light available for plants and can lead to oxygen depletion, bacterial growth and eutrophication. In addition, some algal blooms produce toxins that are harmful to other organisms. High phosphate concentrations can therefore cause enormous ecological and aesthetic damage to streams, lakes, rivers and oceans.
In England, the Environment Act 2021 and the supplementary Environmental Targets Regulations 2023 have set targets for reducing the levels of phosphorus, and other contaminants, that are released into fresh and coastal waters. As a consequence, phosphate limits in treated wastewater discharged into water courses are being tightened.
Monitoring phosphorus – the challenge.
The most prevalent compounds of phosphorus are derivatives of the phosphate (PO4) anion, with orthophosphate being soluble and the only bioavailable form. Orthophosphate is therefore a key measurement parameter in wastewater process control.
Inlet variation of orthophosphate concentration is common in sewage and wastewater treatment plants, which increases the necessity for monitoring to inform dosing control. In addition, some of the organically bound phosphorus and polyphosphates are converted into orthophosphate during the wastewater treatment process, which can influence the total orthophosphate concentration over time. This conversion is specifically optimised by some wastewater treatment plants to improve the overall balance of phosphorus recovery. For these reasons, orthophosphate concentration is the appropriate control parameter for optimising precipitation treatment.
Precise dosing of precipitants is extremely important for phosphorus elimination/recovery to avoid over- or under-dosing. The key to cost-efficient precipitant dosing is robust online measurement analytics that can determine the orthophosphate parameter accurately and reproducibly.
Orthophosphate precipitation typically takes place in three process locations:
1. Pre-precipitation (before the biological stage) – early elimination of orthophosphate.
2. Simultaneous precipitation (in activated sludge) - combining biological and chemical processes.
3. Post-precipitation (after the biological stage) - reduces residual phosphate in effluent.
With high levels of contamination, these process sampling points are always very demanding. A highly effective filter mechanism is therefore necessary to ensure that there are no blockages and that samples are truly representative.
Monitoring phosphorus – the solution.
Swan’s Sample Filtration Cabinet is an all-in-one solution for orthophosphate analysis in wastewater, facilitating problem-free sampling and analysis in even the most challenging wastewater streams.
The system supports ISO 6878 or APHA 4500-P/C colorimetric methods, with model options offering ranges of either 0.01 – 10.0 mg/l, or 0.1 – 50 mg/l. The cabinet houses all of the key components of the measurement system, including reagents, pumps, compressor, photometer and transmitter/display unit.
The filtration head removes suspended solids from the sample, but a number of highly innovative features ensure sampling does not affect measurement uncertainty. For example, a built-in compressor automatically implements air-pulse cleaning to extend service intervals.
Typically, the filters are replaced every 3-4 weeks – a process that takes just a few minutes, and requires no special tools. Used filters can then be chemically cleaned, using a special cleaning apparatus that is supplied as standard. This means that filters can be expected to last for over 2 years.
The stability, repeatability and accuracy of measurements are enhanced by an overflow cell that eliminates pressure and flow fluctuations, and an auto-zero check is conducted before every measurement. In addition, the instrument conducts automatic self-diagnosis of sample flow and reagent levels.
Orthophosphate measurement is ideal for process control, but many plants also employ this technology for monitoring final effluent, correlating with laboratory results for Total Phosphorus (TP). This is because online TP takes longer due to the requirement for a digestion stage. Online TP instruments are also more expensive.
Optional extras… for even greater peace of mind An optional cleaning module is available to help keep the photometer and overflow cell free from biofilm, and heated sample lines can be added for particularly cold locations. In addition, many customers also purchase Swan’s solid-state photometer verification kit.
Summary.
Swan Analytical’s wastewater phosphate measurement solutions reflect the company’s key brand values. The AMI Phosphate instruments are designed to offer highly accurate, stable and repeatable measurements with a low maintenance requirement and a low lifetime cost, but as John Saxton emphasises: “This can only be achieved with the reliable sample quality that is provided by the new Sample Filtration Cabinet. With this technology, wastewater process managers can minimise precipitation costs, optimise performance and maintain regulatory compliance.”
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