30) If the total hardness of water is greater than its total alkalinity, the carbonate hardness will be equal to

31) The amount of residual chlorine left in public water supply for safety against pathogenic bacteria is about

32) The dissolved oxygen level in natural unpolluted waters at normal temperature is found to be of the order of

33) The velocity of flow of water in a sedimentation tank is about

34) The length of rectangular sedimentation tank should not be more than [where B is the width of the tank]