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Water utilities’ plans to make water soft no simple feat

Posted October 17, 2017

Danish drinking water is famous for its high quality, but in several parts of the country—not least in the Greater Copenhagen area—the water is also very hard (high in calcium and magnesium). The calcium is a major inconvenience to consumers, as it builds up in boilers and water taps, on tiles and heating coils, and the hard water also means greater energy and soap consumption.

There is therefore major interest in removing the calcium before the water reaches the taps. This is now becoming a reality. After several years of pilot projects and pre-analyses, Greater Copenhagen Utility (HOFOR) is commencing water decalcification in Brøndby Municipality.

Denmark lags far behind when it comes to centralized water softening. Countries such as Germany, Sweden, and the Netherlands have had water softening initiatives in place for some time. Based on these countries’ experiences, HOFOR has opted for a technology known as a pellet reactor. This involves adding sand and a base, causing the calcium to deposit on the sand grains, which can then be removed in the form of calcium pellets (please see fact box below).

A pilot study has shown that the process can soften hard Danish groundwater by as much as 71 per cent. The water is not completely calcium-free, but this is not the aim, as calcium-free water is corrosive and would therefore damage water pipes.

Pros and cons

Calcium is a nuisance when it gets deposited everywhere we use water, but it is also an important mineral for human bones and teeth. So the first question asked in connection with the prospect of reduced calcium in the water was whether the calcium levels had any impact on health.

“We’ve been involved in some desk studies on the possible effect of calcium in relation to consumers’ teeth, but the studies showed no significant correlation between the number of tooth cavities and calcium in the water. There are also indications that removing magnesium from the water supply increases the risk of cardiovascular disease. However, the pellet reactor is excellent in this regard, as it only removes calcium,” explains Professor Hans-Jørgen Albrechtsen, DTU Environment.

There is no purely medically reason for water utilities not to soften water, and on the positive side, consumers will not have to use as much soap, detergent, and shampoo. With fewer limescale deposits, appliances will also consume less energy and will require less frequent descaling.

“You could say that the consumption of chemicals is transferred from the consumer to the central waterworks, which may be better equipped to deal with this in a safe and controlled manner. And even though decalcification at the waterworks means slightly more expensive water, consumers can still save money if they reduce their soap consumption correspondingly,” says Hans-Jørgen Albrechtsen.

Several technologies to choose from

The pellet method is not the only technology option for water softening. Levels of iron and heavy metals in water vary a lot from location to location, so research is needed into links between technology and water quality.

For example, Hans-Jørgen Albrechtsen notes that water with high iron or phosphate levels causes the pellets produced during decalcification to be more airy and flaky. They do not settle as easily, making them more difficult to separate from the water. However, this did not happen for any of the waterworks and water types the technology has been tested with.

If a bore is close to the coast, the water can contain so much sodium that the limit is exceeded when sodium hydroxide is added as the base to start the calcium precipitation. In such situations it may be necessary to use another base, such as calcium hydroxide, but this means adding additional calcium and producing even more pellets from the process.

Alternative technologies include ion exchange, as used in dishwashers, where calcium and magnesium are swapped with sodium and the water is then regenerated using salt. Or membrane treatment, where the water is filtered so thoroughly that only H2O comes through. This is how sea water is desalinated.

Membrane technology may be appropriate if there are unusually high levels of heavy metals, pesticides, or even solvents from industrial processes in the water. The method uses a lot of energy, but is effective and may save an additional step using active carbon, which might otherwise be required to remove the unwanted substances. But the membrane also removes the minerals you actually want in the water, such as magnesium, and they must then be re-added.

Action and reaction

In parallel with this evaluation of methods, research is also needed into how the new softening technology interacts with the normal water treatment for heavy metals etc.

Many waterworks pass water through a sand filter. This serves as a strainer to remove naturally occurring substances, such as iron and other metals, and also as a biological reactor, because the bacteria in the sand convert substances such as ammonium and manganese.

But the new softening technology may affect these processes. The pellet reactor removes some of the iron in the water. Since the iron is what draws heavy metals such as nickel and arsenic through the sand filter, removing it may cause problems. The bacteria’s biological conversion of ammonium etc. may also be reduced if there is insufficient copper in the water.

On the other hand, less iron means less sludge in the sand filters. This means less water and energy will be required to rinse them, and less backwashing will mean less interference with the bacteria populations.

Researchers will take samples and measure the water during the first year to get to the bottom of these issues.

How to use the calcium

Calcium pellets are the last link in the chain of events centralized water softening initiates. Hans-Jørgen Albrechtsen estimates that when decalcification has been introduced at all HOFOR’s waterworks, around 14,000 tonnes of pellets will be produced each year. What can be done with them?

They could be used for soil improvement in agriculture. But again, thorough analysis is necessary before taking action. DTU Environment, HOFOR, and Niras are therefore jointly examining the physical and chemical properties of the pellets.

Calcium pellets are not a simple substitute for agricultural lime, as they are as hard as marbles and will take longer to dissolve and have an effect in a field. They can be crushed mechanically, but that requires energy and the original sand grains may frustrate the process.

An alternative solution where the calcium is not precipitated onto sand, but onto crushed pellets, would make the final product more pure and hence more useful. But there are other possible applications for the calcium pellets, such as a substitute for sand in the construction sector, for glass production, and as a filler in carpet squares.

Source: DTU

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