in the sense that is the water which has been used for washing

http://www.degremont-technologies.com/IMG/pdf/BIOFOR_US_Infi...

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Note added at 3 hrs (2010-06-01 22:07:33 GMT)
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Penso che si tratta del lavaggio dei filtri

"vasca di accumulo acqua ex lavaggio filtri"
http://www.onofrj.com/media/7511/schedaj088-montalbano.pdf

To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backflushing or backwashing) to remove embedded particles. Prior to this, compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process; this is known as air scouring. This contaminated water can be disposed of, along with the sludge from the sedimentation basin, or it can be recycled by mixing with the raw water entering the plant