Lime is extensively used to treat municipal wastewaters and industrial liquid wastes.
Municipal Wastewater Treatment
In advanced wastewater treatment plants, lime precipitation is used in tertiary processes in which phosphorus is precipitated as complex calcium phosphates along with other suspended and dissolved solids. Due to the high pH of wastewater treated with lime (10.5 to 11.0), the stripping of nitrogen, another nutrient, is facilitated. Removal of phosphorus and nitrogen helps prevent eutrophication (algae build-up) in surface waters. When alum and ferric chloride are used to coagulate suspended matter, lime is added to counteract the low pH induced by these acidic salts and to provide the necessary alkalinity for efficient nitrogen removal. When sewage sludge is removed by vacuum or pressure filtration, lime and ferric chloride are used as filter aids to condition sludge and for final clarification of the effluent.
Lime has numerous applications in treating industrial wastewaters, especially where neutralization of acidic wastes is required.
- In steel plants, sulfuric acid-based waste pickle liquors are neutralized with lime while iron salts are precipitated. Lime is also a neutralizer and precipitant of chrome, copper, and heavy metals in wastewater discharges from plating plants.
- In rayon plants, lime is used to neutralize sulfuric acid wastes.
- In cotton textile finishing plants (dye works), lime neutralizes and precipitates dissolved solids from wastewater.
- In vegetable and fruit canning operations, wastewater can be clarified with lime alone or with supporting coagulants as an alternate to placing the liquid waste in a lagoon. In citrus canning operations, lime can help clarify wastewaters and aid in the processing of citrus pulp by-products.
See the fact sheet on the use of lime to neutralize acidic wastewaters.
Acid Mining Waste
Highly acidic drainage (known as acid mine drainage) from active or abandoned mines is frequently neutralized with lime. Further clarification of the discharged water is achieved by using lime to precipitate iron contained in this pyritic leachate. Coal washing plants use lime to neutralize the acidic waste or process water to reduce corrosion on steel equipment and to recover the water for reuse.