In a broader perspective of cleaning, water is a very good solvent and is a major component of cleaning technology. However, there are a number of incidents where some parameters of water actually play a negative role in cleaning and hinder the process.
Water forms the most important form of the cleaning process; it removes the gross soils from various surfaces and in most cases, does 30-80% cleaning itself. However, it must be kept in mind that the composition of water changes from place-to-place and from source-to-source.
Natural pure water is very rare. Most of the time, water picks up various components from its surroundings, for example, rain water picks up the particles from the atmosphere, while river water picks up the organic and inorganic matters in it and sometimes even clays are picked up, making it look very different.
Taste or odours from water are normally due to the contents of microorganisms, minerals, presence of chlorine, hydrogen sulphide or other decomposed products. Foul smell indicates microbial contamination. This water cannot be used for cleaning. In fact it may soil the surface; already cleaned surfaces can be recontaminated if this water is used.
Presence of clays or organic matter in traces can result in turbidity in water. The smell of rotten eggs indicates presence of sulphur reducing bacteria. Many times water is seen slightly coloured due to the presence of iron or manganese.
The pH indicates the acidity or alkalinity of water. Presence of certain salts makes it more alkaline. However, in most of the cases it is between six-and-eight and will not affect the cleaning process if well within this range.
The hardness of water is due to the presence of calcium and magnesium salts; this is the single largest factor to interfere in the cleaning process. Temporary hardness is due to bicarbonates of calcium and magnesium and permanent hardness is due to the sulphates, nitrates and chlorides of calcium and magnesium.
The main drawback of hardness is that it precipitates on heating or when the pH is increased due to addition of alkalies. The calcium salts which are soluble in normal water become insoluble at higher pH or at elevated temperatures and are thrown out of solution. These precipitate on all available surfaces in the form of scales. Calcium or magnesium carbonates being insoluble in water, precipitate out and form scales on the surface. Some of the examples are scaling on heating surfaces, graying of white fabric and choking of pipes due to scales.
The disadvantages are that they react with detergent and a part of it gets consumed due to hardness, which results in excess use of detergent. Also, they form soap scum and prevent effective removal of fatty soil. The calcium and magnesium salts from water react with fatty acids from fatty soil to form Ca/Mg soap which is insoluble soap.
This not only is difficult to dissolve in water, but forms a viscous insoluble layer on the surface of fatty soil making it extremely difficult to remove it. Thus, removal of fatty soil is extremely difficult using hard water. Further, it suppresses foam; the scales create problems in terms of making heat transfer more difficult, reduce pipe diameters and consume more chemicals. Scales also pose a threat microbiologically as they harbour bacteria and make surfaces difficult to disinfect.
How can water quality be improved for better cleaning? First of all, water has to be microbiologically safe. Treatment with chlorine is the most cheap and convenient way of doing it. A quantity of two-ppm free available chlorine will almost make it fit for this job. There are other ways of doing this as well.
A simple filtration through fine filters will remove most of the suspended solids and possibly, the turbidity. Use of chelating agents will take care of hardness; however, it is one of the costliest ways of softening water. Conventionally, ion exchange technology is used to replace Ca and Mg by Na ions, thus converting hard into soft water. The softening process does not change total solids much as only Ca or Mg gets replaced by Na ions.
If total solids are also very high, a demineralisation process is used. Here, both anions and cations are replaced by H and OH ions respectively. Alternately, a reverse osmosis technique can be used to remove a large dissolved solids load. However, in a system where total solids are high, a softening treatment is preferred prior to RO filtration in order to avoid scaling in the membranes.
Managers in charge of cleaning processes need to understand critical facts such as that ion exchange is the simplest and most cost effective way to remove hardness and can result in saving huge costs in detergents, besides improving performances. In case of high dissolved solids, demineralisation or reverse osmosis are the only ways to reduce them. Further, the water used for rinsing has to be of good microbial quality else, already cleaned and sanitised surfaces can get recontaminated.
