project

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.

Challenge

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.

Development Timeline and Related Research Activitie

In light of the Water Framework Directive (WFD) objectives for surface water quality, the application of Polishing Pellets for phosphate removal from surface water and/or treated effluent is a logical next step. Phosphate removal from treated wastewater was tested through a six-month column test at a water treatment plant of the Hollands Noorderkwartier Water Board. Pellets originating from various types of iron sludge were included in this test.

The final pellet recipe will be used to produce several hundred to ultimately (tens of) thousands of kilograms of pellets annually. Follow-up research focuses on i) scaling up pellet production, including optimization of the pellet recipe, ii) demonstrating pellet application on a practical scale, iii) regeneration of the loaded pellets, enabling a sustainable, multi-cycle application, and iv) further development and optimization of the pellet washing process. The recovery and reuse of phosphate, an increasingly scarce raw material, offers additional opportunities towards the circular economy.