Towards a water-wise world Annual report 2014

Less waste product and smaller impact on nature and environment 

Iron sludge: from by-product to a resource for phosphorus removal

16 April 2014BTO

The process of producing drinking water releases (wet) iron sludge as a waste product. KWR conducts research into using this wet iron sludge to bind phosphorus in the soil. Researchers have also developed a way of transforming iron sludge into pellets with the help of a binding agent. The pellets are easy to produce, strong and adsorb the phosphorus well.

Drinking water production in the Netherlands releases over 75,000 tons of iron sludge every year. Groundwater contains iron which, during the aeration step in drinking water treatment, is transformed into ferric (hydr)oxide and settles. During the treatment of surface water, iron is actually often added as a coagulation agent, usually in the form of ferric chloride, which settles as ferric (hydr)oxide during the treatment.

Injection of liquid iron sludge

Injection of liquid iron sludge

Useful application of iron sludge to bind phosphorus

The iron oxide in iron sludge can be usefully employed to bind the phosphorus in the soil or to filter it out of surface water. An excessive amount of phosphorus in the soil or in surface water contributes to over-fertilisation and is harmful to nature and the environment. The blue-green algae that appear in the summer are for instance often the result of an excess of phosphorus in the water. The use of iron sludge to lower phosphorus concentrations thus kills 2 birds with 1 stone: the former iron sludge “waste product” acquires a useful application and the available phosphorus content is reduced. Research within the water sector’s joint research programme (BTO), with partners including Brabant Water, Waterbedrijf Groningen, Vitens, Reststoffenunie, Alterra and B-Ware, focuses on soil treatment using (wet) iron sludge, as a cheaper alternative to the digging up of phosphorus-rich ground to promote sustainable nature development. The iron forms a strong chemical bond with the phosphorus, so that the latter is no longer available for the vegetation. In this project’s field trials, the sludge has been used at a close distance from the drinking water treatment operations. Starting in 2015, this research will be continued within TKI Water Technology. Other subjects of TKI research include the use of iron sludge to remove sulphur from gases.

“KWR combines the scientific and practical knowledge that is needed to develop innovative products”

Making it dry enough for other applications

To increase the applications of iron sludge, and extend the distance from the treatment plant that it can be used – for instance, for the removal of phosphorus during water treatment – it must be made sufficiently dry and permeable to enable its storage, transport and application. Approximately 27,000 tons of the annual production of iron sludge has a dry-matter content of about 30 percent (“dewatered”), while the dry-matter content of the remaining 50,000 tons is only about 10 percent, which is too wet and impermeable for use farther away or in water treatment.
KWR has researched, both within BTO and TKI, how one can make pellets out of (wet) iron sludge that are relatively easy to produce, sufficiently strong as to not disintegrate during transport and use, and, most of all, that adsorb phosphorus well. Using a carefully selected binding agent and drying temperatures of a little more than 100 °C, this work has found that it is possible to produce pellets that meet these requirements. The temperature cannot be too high since this can trigger crystallisation which decreases the adsorption surface. Aging can also lead to a decrease in the adsorption surface. The pellets produced are strong enough to serve as column material for the adsorption of phosphorus, taking up the phosphorus both quickly and in large quantities.

Retain markets through innovation

Olaf van der Kolk, Commercial Manager at Reststoffenunie and closely involved in this and other research projects into the reuse of drinking water production residuals, has this to say: “Reststoffenunie assists water companies in bringing their by-products to market for reuse by other parties, preferably at the highest added value possible. This reduces costs and increases sustainability: for example, these residuals can frequently replace other chemical products that have a considerable CO2 footprint. But you don’t acquire and retain a position in the market just like that. You have to be innovative and always offer the markets the best products. For us, KWR combines the scientific and practical knowledge that is needed to develop innovative products for our clients. This helps the Dutch drinking water sector to play an international pioneering role in the field of the reuse of residuals.”

© 2018 KWR Watercycle Research Institute

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