In industrial wastewater treatment, chemical precipitation is one of the most common methods for removing metals. In this process, dissolved metals react with added chemicals and turn into a solid form. This solid fraction, the precipitate, is separated from the purified water, for example by sedimentation or filtration.
Many people imagine that the process ends here, but in reality, water purification is only the first step. The metals do not disappear, but are transferred from the water to the precipitate. If the precipitate ends up in a disposal area without proper treatment and is chemically unstable, rain or groundwater can dissolve the metals back into the environment. Therefore, the long-term safety of the precipitate is just as important as the purity of the water.
More information about chemical precipitation is available in our last week’s article: Precipitation of metals from industrial wastewater.
What is precipitate?
The composition of the produced precipitate, also known as sludge, varies depending on the water being treated. The quantity and quality of the precipitate in chemical precipitation depends on the quality of the water being treated, the metal concentrations, and the chemicals used in the treatment. Typically, precipitate produced in the treatment of industrial wastewater contains metals removed from the water, sulfate compounds, and small amounts of neutralization chemical residues. (Chen et al. 2021)
Once the behavior of the precipitate is known, its quality and quantity can be controlled in an environmentally safe manner.
The quality and separability of the precipitate depends not only on the impurities in the water, but also on the process technology—such as mixing and reaction time—which have a decisive influence on its properties. The EPSE™ method produces a controlled and chemically stable precipitate, as its quality is determined by both controlled reactions and an optimized process. Once the behavior of the precipitate is known, its quality and quantity can be controlled in an environmentally safe manner.
How is the quality of EPSE precipitate assessed?
The precipitate produced by the EPSE™ Method undergoes laboratory testing during the Lab PoC phase to assess its chemical stability.
The chemical stability and environmental safety of the sludge are assessed in Europe using a solubility test in accordance with the SFS-EN 12457-2 standard. This test indicates how much metal can be released from the sludge if it is placed in a landfill, for example, and comes into contact with water. This is not only good practice for process control – it is also a regulatory requirement for the classification and management of waste fractions. This supports compliance with regulatory requirements in different countries, whether it is the EU’s SFS-EN standard or the US EPA’s TCLP test. The solubility of metals in EPSE sludge is typically very low, which supports safe disposal or possible reuse as part of the circular economy.
The EPSE Lab PoC stage can be used to determine the dry matter content (TS) of the treated sample, which can be used to estimate the amount of precipitate generated in the process. This gives the customer an estimate of how much solid waste will be generated during treatment, which facilitates process dimensioning, logistics planning, and the assessment of possible beneficial use.
How are the precipitate and water separated?
Separating precipitate from water is a critical part of a successful precipitation process. The faster and more clearly the precipitate settles, the more efficiently clean water can be separated and recovered. Many factors affect the settling rate, particularly the particle size, shape, and density, which are determined by the chemicals used and the process conditions (Mancell-Egala et al. 2016; Mullins et al. 2018). The properties of the liquid, such as temperature, viscosity, solids content, and surface tension, also affect settling behavior (Davis 2019).
In the EPSE Lab PoC phase, precipitate separability is assessed using the SSR (Sludge Settling Rate) test, which measures the settling rate and clarification of the supernatant as a function of time. The test provides valuable information on how effectively the precipitate separates and helps to optimize the industrial-scale treatment process. However, on a large scale, the settling rate is also affected by conditions such as flow and temperature differences (Davis 2019).
In industry, separation is usually carried out in settling tanks or lamella clarifiers, which use gravity to settle the sludge before the clarified water is recovered.
Where does the precipitate end up and why does it matter?
Once the precipitate has been separated from the water, a safe and compliant final disposal site must be found for it. If reuse is not possible, it is sent to an approved waste treatment plant or landfill, where it is classified based on the results of solubility tests. EPSE precipitate often meets the criteria for permanent waste, which facilitates safe final disposal if reuse is not possible.
Heavy metals are persistent and harmful to the environment, but they are also often valuable raw materials. Recovering them from the precipitate can be profitable. Even if a company does not have its own metal recovery process, the precipitate can be delivered to a metal recycling plant, for example. In this case, it is transformed from waste into a valuable raw material and is no longer the company’s responsibility to dispose of. This is part of the circular economy, in which waste streams are managed as a resource.
The EPSE™ Method not only purifies water, but also manages and stabilizes the waste generated, reducing the environmental burden, facilitating waste management, and promoting the circular economy. The end result is permanently controlled precipitate – and permanently clean water.
This article is written by
Anette Anttonen
Laboratory Engineer
anette.anttonen(a)epse.fi
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References in order of appearance:
Blais, J.-F., Djedidi, Z., Ben Cheikh, R., Tyagi, R. D., & Mercier, G. 2008. Metals Precipitation from Effluents: Review. Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management 12(3). Accessed January 3, 2025. Access restricted. https://doi.org/10.1061/(ASCE)1090-025X(2008)12:3(135)
Chen, T, C., Wing, Y, A., Stijn, E., Amitava, R. & Stegemann, J. 2021. Elemental and mineralogical composition of metal-bearing neutralisation sludges, and zinc speciation – A review. Journal of Hazardous Materials (416). Accessed July 11, 2025. Access restricted.https://doi.org/10.1016/j.jhazmat.2021.125676
Mullins, D., Coburn, D., Hannon, L., Jones, E., Clifford, E., & Glavin, M. 2018. Using image processing for determination of settled sludge volume. Water Sci Technol. Accessed March 18, 2025. Access restricted. https://doi.org/10.2166/wst.2018.315
Mancell-Egala, W., Kinnear, D., Jones, K., De Clippeleir, H., Takács, I., & Murty, S. 2016. Limit of Stokesian settling concentration characterizes sludge settling velocity. Water Research (90). Accessed March 18, 2025. Access restricted. https://doi.org/10.1016/j.watres.2015.12.007
Davis, M.L., 2019. Water and Wastewater Engineering: Design Principles and Practice. 2nd ed. New York: McGraw-Hill.