After several years' operation and, say, twenty or thirty scrapings the depth of filtering material will have dropped to its minimum designed level
(usually about 0.5-0.8 m above the supporting gravel, according to the grain size of the medium). In the original construction of a marker, such as a concrete block or a step in the filter box wall, is sometimes set in the structure to serve as an indication that this level has been reached and that resanding has become due.
During the long operation of the filter some of the raw water impurities and some products of biochemical degradation will have been carried into the sand-bed to a depth of some 0.3-0.5 m, according to the grain size of the sand. To prevent cumulative fouling and increased resistance, this depth of sand should be removed before resanding takes place, but it is neither necessary nor desirable that it should be discarded.
Instead it is moved to one side, the new sand is added, and the old sand replaced on top of the new, thus retaining much of the active material to enable the resanded filter to become operational with the minimum of re-ripening.
This process, known as "throwing over", is carried out in strips. Excavation is carried out on each strip in turn, making sure that it is not dug so deeply as to disturb the supporting gravel layers below. The removed material from the first strip is stacked to one side in a long ridge, the excavated trench is filled with new sand, and the adjacent strip is excavated, throwing the removed material from the second trench to cover the new sand in the first. The operation is illustrated in Fig. 26 and 27. When the whole of the bed has been resanded, the material in the ridge from the first trench is used to cover the new sand in the last strip.
In areas where the sand is expensive or difficult to obtain, the surface scrapings from regular cleanings may be washed, stored, and used for resanding at some future date.
These scrapings must be washed as soon as they are taken from the filter, otherwise, being full of organic matter, the material will continue to consume oxygen, quickly become anaerobic, and putrefy, yielding taste- and odour-producing substances that are virtually impossible to remove during any later washing process.
It must also be remembered that filter sand, when washed, loses its finer particles, so that the effective diameter is increased.
This is likely to result in deeper penetration of impurities into the bed during subsequent filter runs.