Material Solutions

Water, the essential element

12 February 2008



A water supply that is contaminated or that contains natural impurities can cause big problems for laundries if it is not treated correctly, Ian Harris explains


Water is the basis for all life forms and essential in almost all aspects of industry. It covers 71% of the earth surface but only a small proportion is available for domestic and industrial use.

The impurities in the laundry’s water determine whether it needs treating before use, and if so the type of treatment required.

To be suitable for laundry use water should be “soft”; free from iron, manganese or other heavy metal contamination; colourless and free from odour, bacteria or particulate soiling. It should be neutral, neither very acidic nor very alkaline.

Hard water has a high mineral content usually including calcium and magnesium salts and often other dissolved metal salts, bicarbonates, silicates and sulphates.

Although generally harmless, hard water presents the launderer with a big problem. If the hardness is not removed, it will lead to greying of processed textiles and to increased use of chemicals. Hard water will also leave lime-scale deposits on washing and finishing equipment, causing production problems. Deposits can also build up inside the water lines, restricting the flow. Acid de-scaler will remove hard water deposits on machinery and in the pipes.

The greying on textiles will need special treatment. It is caused by the redeposition of soiling from the wash liquor and is made worse by the chemical reaction between the detergent and the hardness salts, which produces a grey scum.

Water-hardness testing usually only measures the total amount of calcium and magnesium present. However, in some areas there may be other contaminants that can cause additional problems.

Water hardness in the UK is recorded as degrees Clark, but many countries have their own local scale. (see table p26). The current trend is to measure hardness in parts per million (ppm). Most launderers and chemicals’ suppliers would class 0 – 100ppm as “soft”; between 101– 250ppm as “medium”; and over 251ppm as hard. (Other industry classifications may be slightly different.) Unless the laundry is in a naturally “soft” water area, it must soften its water to an acceptable level before use. This is generally done via the use of an ion exchange resin-based water softener.

When buying and using such a system, the laundry must ensure that the system is correctly sized to ensure its softening capacity is greater than the volume of water at any given hardness. The softener must also be regularly tested and regenerated to prevent hard water entering the laundry processes.

Launderers should remember that the harder the water is before it is softened, the higher will be its natural alkalinity. Softening only removes the hardness ions, not the alkalis. Indeed, softening frequently increases the alkalinity and this increases the risk of yellowing (galling).

It is difficult to achieve good rinsing with highly alkaline water and so in some very hard water areas, the raw water’s alkalinity will need to be “soured” or neutralised, by adding a small amount of acid to the final rinse.

Hard water causes other problems. The hardness breaks down in the boiler to form carbon dioxide, which dissolves in the liquid condensate to form carbonic acid. This causes corrosion of all the pipework and fittings in the condensate system (including the boiler feed tank).

So the level of hardness must be constantly monitored and controlled. Sloppy management in this respect will lead to the breakdown of boilers and equipment, to excessive fuel costs, to lost production and to greying as unremoved hardness will neutralise the detergent.

Iron

Raw water taken from the mains or from a bore hole will contain amounts of iron that will vary according to the geological area and water source.

Iron from bore-holes is normally present in the ferrous or bivalent form [Fe++]. This form is soluble in water but if it is exposed to air it readily oxidises to ferric iron [Fe+++], which is insoluble and forms an orange-coloured precipitate that will often colour the water supply. The precipitate can often readily be filtered from the water – but if the water supply has become orange in colour through oxidation of the ferric irons it is highly likely that there will also be a high dissolved-ferrous [Fe++] content which will lead to quality problems on processed articles.

Iron can often be “tasted” in the water, and as little as 0.1mg/litre of ferrous (soluble) iron or 0.2 mg/litre ferric (insoluble) iron will give a bitter or an astringent taste.

If the water contains appreciable hardness, the installation of a lime-soda softening plant will automatically remove the iron during the softening process.

However, naturally soft water is quite frequently contaminated with iron, and then other treatments may be needed.

Carbon filtration will remove particles of ferric iron (rust) but dissolved iron (ferrous) or a colloidal suspension, in the absence of hardness, requires a special treatment of aeration and filtration, possibly preceded by chemical dosing.

Filtration through a bed of material specially prepared for the purpose will sometimes be sufficient. This equipment is similar to that using a Zeolite softener, but has a different resin. The iron which accumulates in the bed is removed by "back-washing"; chemical regeneration is not normally required.

The use of a Zeolite base exchange softener for the removal of small amounts of hardness from water that is contaminated with ferrous iron is a costly procedure; iron is capable of causing serious deterioration to many Zeolite and other base exchange materials, which will result either in the need to replace the resin prematurely, or in hard water being supplied to the plant.

The main problem associated with iron in the water is the reaction between the iron and chlorine bleach. When iron levels exceed 0.1mg/litre, such a reaction can cause severe fabric damage and the higher the iron content the greater the damage.

Iron contaminated water can also cause greying on whitework. Even relatively low concentrations can cause the white fabric to turn creamy grey over a period.

The reason for this is that the bright whiteness is typically produced by the optical brightening agents (OBAs) that are in most fully-built synthetic detergents. Most of the OBAs used have a sodium (Na) head on the molecule and this is readily exchanged with the Iron (Fe) in the water. As iron is not removed during the wash process, even small amounts in the water supply will gradually build up on the fabric surface gradually changing the appearance from a bright white to a dingy yellow/grey. The problem applies particularly to any fabric with a cotton content, including blends and 100% cottons.

Try stacking a pile of sheets taken at random and look at the folded edges under good lighting conditions. If you have a “zebra” effect – various shades of white and grey – this could indicate a problem with iron contamination in the water.

Rust

Ferric iron particles in the water can often make it turn orange. This problem often arises from the corrosion of iron pipe lines and storage tanks and is especially likely to occur with soft water that is slightly acidic, such as moorland water.

The problem can be avoided by the use of polythene pipes.

Rusty water can produce local speckles or cause an overall discolouration. In serious cases, especially if the water is very acidic, the storage/holding tanks and the pipe-work will have to be renewed. It may be worthwhile having the water analysed to see whether it is possible to adopt some form of regular treatment which will reduce the water’s corrosive properties.

In less severe cases, sodium silicate (water-glass) may be used to control further corrosion and to prevent existing rust flakes inside tanks and pipe-work from breaking off into the water supply.

The treatment involves filling the tanks and pipes with a solution containing sodium silicate (or sodium metasilicate) at the rate of 25g/1,000 litres and allowing the solution to stand in them over the weekend.

This can be done by making-up the solution in the main water storage tank and then opening the water inlet valves on each machine so that the silicate reaches all parts of the system before the plant finally shuts down for the weekend.

This treatment may prevent further rust problems for a period but it will probably be necessary to repeat it every month or so.

As an alternative, a solution of 12.5g/1,000litre can be added to the main water storage tank on a continuous basis. This addition has no appreciable effect on the water’s use in the washroom and so may therefore be continued for a week or more at a time, to effect a longer lasting result.

The lasting effects of such treatment will vary and this will determine the frequency.

When it is used, it is essential that none of the treated water enters the boiler feed system as the presence of silicates in the boilers will result in extremely hard scale formations.

Particulate contamination

Particulate contamination in the raw water frequently comes from clay, silica and other insoluble minerals. This can occur when either a bore-hole is running low, especially during times of drought, or the local water authority has been working on the water supply lines in the immediate vicinity of the laundry.

Most of this contamination can be readily removed by filtration, but it often produces a temporary problem that can lead to items laundered that day becoming discoloured. This discolouration can be difficult, if not impossible, to remove.

The problem occurs more frequently in laundries that do not have a large storage tank.

With a large storage tank and the water take-off line located some way from the bottom of the tank, the particulate soiling will settle into the bottom of the tank and avoid being fed to the wash-house.

It is essential to ensure that the holding tank never runs low as the incoming water supply will then stir up the sediment in the bottom of the storage tank and contaminate the whole system.

Ideally water storage tanks should be completely drained every 3-4 months and washed out to remove any sediment that accumulates.

Avoiding bacteria

One of the main purposes of laundry is not only to remove the soiling but to also return the textiles in a hygienically clean and bug-free state. If the water supply is contaminated with micro-organisms, all processed textiles will be re-contaminated during the rinse stage.

The bacteria can come from several different sources. For example, if you use a bore-hole and you are in a rural area, there is a risk that your water will be periodically contaminated with urine and faecal matter from the local farms. The rain irrigates the waste from the animals through the soil which then enters the water table only to re-appear in your laundry.

Bacteria can also enter the laundry’s water supply if the storage tanks are not properly sealed, wild-life, birds and vermin looking for shelter, fall into the tank and cannot escape so they drown and decay contaminating the water supply.

It is always worthwhile sampling the water supply routinely and having it checked not only for bacteria but also for all the other contaminants that could lead to big quality and production problems.

Water is essential to any laundry and contaminated water will affect the laundry’s ability to produce satisfactory work.


Zebra effect Zebra effect
Borehole water Borehole water


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