Most common cleaning process for eliminating solids in potable water treatment. Often preceded by a flocculation step to improve its efficiency.

Measurement of the turbidity value is applied here for two purposes: monitoring of the actual filtration operation for breakthroughs, and optimization of the sand purging process. The following Figure illustrates the change in turbidity during the cleaning cycle.

		Sand filter: turbidity monitoring of filtration and sand purging

At measurement position 1, the instrument measures the turbidity of the filtered water. Whenever purging of the sand layer is initiated because of the rising pressure drop or (even better) rising turbidity, sampling switches to measurement of the purge water (position 2). At the end of the purging process, its turbidity falls to the level of the pure water, thus signaling that purging is complete. Now the system is switched back to filtration and position 1. The turbidity reading will be high at first, until the filtering sand layer has settled again, and will then drop back to the normal value for the purified water. This turbidity monitoring makes an important contribution to efficient operation of the treatment plant: the purging time and the amount of pure water consumed are optimal, and the quality of the filtrate is assured.