Detrimental Effects of Feeding Lime in Wastewater for pH Adjustment

Feeding lime to raise pH in wastewater can have detrimental effects to the treatment process and the wastewater plant itself. Lime (CaO or CaOH2) raises pH and precipitates calcium carbonate (CaCO3) scale onto pipe walls. The scale coating on pipes can build up so much it reduces pump efficiencies. The carbon needed to precipitate calcium carbonate comes from the same inorganic carbon needed by nitrifying bacteria. Simply stated: lime reduces carbonate alkalinity.

 

Let's look at a few application examples:

 

If a wastewater plant needs pH to be raised for biological activity support, magnesium hydroxide and lime will raise pH. However, if a wastewater plant needs alkalinity for nitrification they should not feed these products because neither increase the carbonate portion of alkalinity, which in most pH ranges is HCO3-, carbonic acid.  Carbonic acid converts to CO2 below pH 4.3 which is why total alkalinity is titrated to this pH. If a wastewater plant needs alkalinity for nitrification, they need to raise the carbonate portion because the nitrifying bacteria utilize inorganic carbon (HCO3-)in the process. Sodium bicarbonate, or baking soda, would be an appropriate addition to raise carbonate alkalinity for alkalinity adjustment.

For more information on wastewater nitrification, call Aulick today or check out this primer here: http://www.ecos.ie/wastewater-nitrification-how-it-works/