air relief valve

reflux valve

pressure relief valve

sluice valve

5 litres/sec

50 litres/sec

500 litres/sec

1000 litres/see

more depth

less depth

more discharge

less discharge

suspended impurities

dissolved impurities

both suspended and dissolved impuri¬ties

none of the above

contains pathogenic bacteria

consists of undesirable substances rendering it unfit for drinking and domestic use

is safe and suitable for drinking and domestic use

is contaminated