TABLE OF CONTENTS Absolutely pure water is never found in nature and it is increasingly rare to encounter a source of water that requires no treatment before being used for potable-water supply. Water contains both biological and inorganic matter. It is normal to classify the impurities found in water in one of three progressively finer states suspended, colloidal, and dissolved. The method of treatment required for the removal of impurities, or their reduction to acceptable limits, depends in part on the fineness of the material. While the matter found in raw water may render it unfit for human consumption, treatment can also adversely affect water quality by introducing pollutants, or by modifying chemicals that are harmless prior to water treatment.
Running water obviously will carry floating debris, but it also has the capacity to pick up and transport solid particles of greater density than water; the higher the velocity the bigger the particle that can be transported. Rivers are normally at their most turbid during flood, because of the increased water velocity. Table 2.1 indicates the sizes of solids that are transported at different velocities. Large rivers in flood often run at velocities in excess of the maximum shown in Table 2.1 and are capable of carrying high quantities of suspended material.
| Material | Diameter of particle (mm) | Velocity of water (m/s) |
|---|---|---|
| Fine sand | 0.4 | 0.15 |
| Medium sand | 1.1 | 0.23 |
| Coarse sand | 2.5 | 0.3 |
| Gravel | 2.5 25 | 0.76 |
