Polishing pellets

During the production process of drinking water (dissolved) iron is removed from the water. Sand filtration is a treatment technique commonly used for this purpose. The flushing of the sand filters then generates aquafer, which is a sludge that contains relatively high levels of oxidised iron. Because of the high levels and the chemical properties of freshly-formed iron oxides, this sludge is able to adsorb hydrogen sulphide, phosphate and arsenic. However, when the aquafer is generated, it still has a high-water content, which limits its practical application and makes its environmentally-friendly transport difficult. In the present project pellets are being made from the oxidised iron, with a view to using them as a circular adsorption material.

Three properties of these Polishing Pellets are especially important for their successful application:

  1. The maximum adsorption capacity, which indicates how much material can be adsorbed, and is primarily a function of the available adsorption surface.
  2. The mechanical strength, which largely determines whether the pellets are suitable for use in, for example, flue gas treatment, filters, surface water, digesters and/or agriculture.
  3. The pellets must not release any unwanted materials, depending on the specific location or process conditions.


The key to having porous, stable pellets, with a large internal surface, lies in avoiding high mechanical pressure and temperatures during the production process. A ‘pellet recipe’ has been developed in the project for the small-scale production from aquafer of (a few dozen kilogrammes) of high-value pellets with a high phosphate adsorption capacity. Numerous binders, including their variations, have been tested with a view to improving the pellets’ mechanical strength. In addition, a study was made of a thermal post-treatment stage that possibly strengthens the pellets. The current production recipe was defined on the basis of i) the mechanical strength and ii) the maximum phosphate-adsorption capacity, which were determined under controlled conditions.


The further development for the purpose of the practical application of the Polishing Pellets still faces three main challenges:

  1. Translation and validation of the current production recipe for a larger production scale.
  2. Investigation of long-term (>1 month) application, with mechanically and chemically stable operations under practice-relevant process conditions and loading.
  3. Development of a washing procedure that succeeds in limiting the release of natural organic matter in particular, and makes the pellets potentially usable in drinking water applications.

Current research activities and timeline development

The existing experience with pellet washing will be further developed and optimised. With WFD surface-water quality objectives in mind, the application of the Polishing Pellets for phosphate removal from surface water and/or treated effluent is self-evident. The removal of phosphate from treated wastewater will be assessed in a six-month column test at a treatment plant of the Hollands Noorderkwartier Water Authority. This field test will include pellets from several types of aquafer. In the simultaneous scaling-up initiative, the recipe for the Polishing Pellets will be further optimised through, among others, the testing of alternative, green binders.

An effort will be made to develop a definitive pellet recipe, for the production of a few hundred kilogrammes, by the third quarter of 2024. The ambition is, in 2025, to achieve a one-off production of ten thousand kilogrammes of pellets. Follow-up research will focus on the regeneration of the loaded pellets, to enable their sustainable, multi-cycle application. The recovery and reuse of phosphate, an increasingly scarce raw material, opens up additional possibilities with regard to the circular economy.