More efficient alkalisation with ground limestone in groundwater treatment

Most Finnish groundwater must be alkalised before use as municipal drinking water because it is corrosive. The corrosiveness is due to low pH and softness of water. Groundwater also often contains aggressive carbon dioxide (CO2), so there is more free CO2 than is required to keep calcium and hydrogen carbonate in solution. The most popular alkalisation method in the past was addition of sodium hydroxide, but use of limestone (calcium carbonate) for alkalisation is becoming increasingly popular, the main reasons being that the overdosing risk is avoided and that water hardness is increased by limestone. The most common method of alkalisation using limestone is through limestone filters, but these are quite large and the empty bed contact time needed often ranges from about 40 minutes to more than 2 h. The investment cost and size of plant required can be substantially reduced by using mechanically ground limestone instead of limestone filters. Incorporation of mechanical grinders into two commercial-scale water treatment plants in Finland greatly reduced the space and treatment time requirements, but some problems were observed by consumers because the harder water increased limescale formation.