Langenau drinking water production more sustainable thanks to pellet softening

Dutch know-how combined with optimal hydraulic mixing with limewater

Langenau has switched from a precipitation process (cold lime softening) to a crystallisation process (pellet softening) at its Landeswasserversorgung Baden-Württemberg production site. This means that the residual generated is no longer lime sludge but lime pellets, which have better sales prospects. A relatively large volume of limewater is added to the reactors instead of the smaller volumes of sodium hydroxide, as is usual in the Netherlands. In developing the process, the German water company drew on the know-how of AquaMinerals and KWR. The switchover to the new process offers financial and sustainability benefits.

Watery lime sludge for paper production

Since the 1990s, the German water company Landeswasserversorgung Baden-Württemberg  has used flocculation to soften the highly calcareous groundwater at its Langenau production site. Every year, the process generated about 30,000 tons of watery lime sludge which, because of its whiteness, was highly suitable for use as a (secondary) raw material in the production of high-grade paper. The lime sludge was taken up by a paper factory in the vicinity of the production site. For Landeswasserversorgung this involved investments in, for example, the centrifuges for the mechanical sludge dewatering and the grinding equipment needed to achieve the right grain size.

From sludge to pellets

Since the demand for (high-grade) paper was, and is, decreasing, the sales prospects of this lime sludge have become poorer. Moreover, the sludge processing and transport costs are very high. For these reasons, in 2012, Landeswasserversorgung began looking for an alternative. Over the past few years, the German water company has turned for advice to the Dutch AquaMinerals (formerly Reststoffenunie), the Dutch water companies and KWR. This led it to decide to make a transition to the softening of its hard groundwater using a crystallisation process in pellet reactors; that is, to switchover from a precipitation process (cold lime softening) to a crystallisation process (pellet softening). This means that the residual generated is no longer lime sludge but lime pellets, which have better sales prospects.

Extensive hydraulic modelling

During the pilot research, the German water company directed a lot of time and attention to (hydraulic) modelling and combining the analyses with tests on a (semi-) practical scale. In 2014, it visited some Dutch water distribution companies, including Dunea, Waternet and Brabant Water, to learn about the Dutch expertise and experience and incorporate it into the design of the pellet reactors.

Seven softening reactors operate with limewater

In 2016, Landeswasserversorgung started up a total of seven softening reactors at the Langenau site, with an annual capacity of about 30 million m3. This pellet softening has a special design: the groundwater to be softened is mixed with limewater at about one-quarter of its volume. In the Netherlands, a much smaller volume of sodium hydroxide is added for the softening. The large volume was opted for because of its favourable effect on the hydraulics of the process. In addition, the reactors use seeding material based on the lime pellets of the own process, which results in ‘ultra-pure calcite’!

20,000 tons of lime pellets annually

The new softening installation generates about 20,000 tons of lime pellets every year. Following the Dutch example, the Landeswasserversorgung Baden-Württemberg lime pellets – a first for a German water company – have achieved the status of a ‘by-product’ of drinking water production. The pellets have also been registered in conformity with the REACH regulations. By using its ‘own’ lime pellets as its seeding material, the installation produces calcite that is ultra pure. This calcite also has a high purity level because it contains no (heavy) metals like iron, manganese and nickel, and it is very white, which increases its sales opportunities. Unprocessed pellets are in turn sold to an Austrian animal feed producer and a German manufacturer of special (white) glass, and a portion of the pellets are processed into seeding material for the own softening process.

Following the conduct of extensive market studies into sales opportunities for the ‘ultra-pure calcite’, Landeswasserversorgung Baden-Württemberg is currently examining, along with German industrial partners, the possibilities of upcycling the ‘in-company’ lime pellets into a high added-value product, for example, as a raw material/filler in the production of a number of types of plastic.

Financial and sustainability benefits

By generating sufficient revenues from the sale of lime pellets, Landeswasserversorgung Baden-Württemberg would like to cover the costs of the softening process even more. The switchover to this new process is however already producing financial and sustainability benefits. The financial benefits are: the residuals transport volumes have been cut by a third; the mechanical dewatering and fine grinding of the sludge is no longer necessary; and the lime pellets fetch better prices than lime sludge. The sustainability benefits are the decreased use of chemicals and a lower consumption of (electric) energy, which means reduced CO2 emissions.