Pure Dutch lime pellets are sustainable
Lime pellet reuse: a subject KWR has researched over the years for the drinking water companies Dunea, PWN and Waternet. Reststoffenunie and the drinking water company WML also carry out research into lime pellets. All the researchers involved are sharing their knowledge with each other. It has now been shown that lime pellets seeded on Dutch calcite can produce interesting sustainability benefits.
The process of water softening produces lime in the form of pellets as a residual. For water companies these pellets are by-products which can be put to good use. For a number of years now, the DPW companies (Dunea, PWN, Waternet) have been researching ways of improving the quality of their lime pellets so as to increase their added value and sustainability benefits. KWR and Reststoffenunie support them in this effort, and in 2014 WML is joining the research.
Production of lime pellets is more sustainable with Dutch calcite
Calcite as seeding material
Normally, the pellets are seeded with garnet sand, upon which the lime precipitates. Because clients, like those in the glass and paper industries, want the purest possible lime pellets, experiments have been carried out over the last few years with the aim of producing pure pellets, that is, lime pellets with calcite nuclei. The initial results were promising: technically the concept is very feasible. Previously the research was done using imported Austrian calcite. In 2013, the most cost-effective way of producing pure pellets is being studied. Pilot-scale research is employing lime pellets that have been ground and sieved as seeding material. The pellets can be ground into seeding material by putting them through a grinder with water. After grinding and sieving, about 40% of the volume is transformed into seeding material.
“Together with Delft University of Technology we’ve conducted a life-cycle analysis and a cost-benefit analysis for Dutch calcite, imported Italian calcite and garnet sand,” says researcher Luc Palmen. “For Waternet, the best solution is to use Dutch calcite, which is locally ground, sieved and used as seeding material. The sustainability of the water treatment at Weesperkarspel can be improved by about 10,600 ecopoints annually – i.e., around 5% – by switching to Dutch calcite. This is primarily because the calcite is transported over a shorter distance. For Waternet, and for a few other DPW sites, the cost-benefit analysis also produces a positive picture.” Microbiological analyses are also being carried out: the application of ground and sieved pellets must not produce any health hazards. The initial results show that the processing of the lime pellets into calcite is possible without micro-organism contamination. “However,” notes Palmen, “the legal and company standards concerning the use of seeding material are strict. That’s why the follow-up research is thoroughly examining, among others, the microbiological risks associated with the sieving and grinding of the pellets.”
Improve the sustainability of operations
Waternet has decided on the basis of this research to temporarily adapt the reactors in its softening plant on a large-scale, and to gradually replace the garnet sand in the softening process with calcite, so that the reactors ultimately produce purer lime pellets. “What fascinates me personally,” says Palmen, “is that a water company, in this case Waternet, is prepared to do research into modifying a primary process – the production of drinking water – so as to improve sustainability and the quality of a residual. In 2014 WML is to follow suit. This represents a completely new way of thinking for a water company. To me the vision behind it is wonderful.”
Lime pellets in TKI Water Technology
Further microbiological, chemical and physical analyses, and a practical test, form part of the TKI project “Marketing Dutch lime pellets”, that starts in 2014. KWR is conducting the research together with Reststoffenunie, Waternet, Dunea, PWN and WML.
© 2017 KWR Watercycle Research Institute
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Senior scientific researcher and Project Manager