Where is the best quality of raw water generally found?

To understand why we need different treatment regimes for different sources of water we need to look at the differences in quality of the three main sources.

Raw water quality can be split into three source quality bands.

Upland surface impounded water, which is water that has fallen on high ground and run off and collected in reservoirs and natural lakes or lochs.

Lowland surface water, which is water in rivers, streams and includes springs.

Underground water, which is water in wells and boreholes

The quality of these waters will vary depending on the nature of the soil and rock strata through which they have passed and the extent to which pollutants have passed into them.

In quality terms, groundwater sources are normally the highest quality source, the next best being upland surface sources with lowland river supplies being poor or very poor since the water will very likely have had a degree of re-use and include, agricultural and urban run-off and sewage and industrial effluents.

The poorer the raw water quality the more complex and extensive will be the treatment required to bring it to acceptable quality.

Thus lowland surface water will generally require complex and extensive treatment in order to produce acceptable final drinking water quality.

Whilst upland sources can present quite specific problems, in general the degree of treatment required is relatively little to produce acceptable final drinking water quality.

By contrast underground waters can require very little in the way of treatment with often simple disinfection being the only treatment necessary.