Alkalinity Consumption during Water Coagulation

At water treatment plants which practice coagulation using alum, alkalinity in the form of lime (Ca(OH)₂) is almost always added either prior to dosing with alum (pre-liming) or after (post-liming). Most water works which use alum in Nigeria practice post-liming. Some problems have been observed to be associated with this mode of operation; namely, corrosive (low pH) water continuously passes through the slow mix, sedimentation and filter units and tends to shorten their service lives, and lime sludge is deposited in the clear well or underground storage tank where its removal poses health problems. Laboratory studies relating to alkalinity consumption and turbidity removal show that although post-liming consumes more lime than pre-liming, the latter enhances settleability, thus reducing required detention time. Pre-liming may thus translate into savings in capital or operational costs or both, in addition to eliminating unsanitary conditions in underground storage tanks.