Lime Recovery and Reuse in Primary Treatment

Dosing lime to municipal wastewater prior to primary sedimentation tanks and the subsequent recovery, classification and recalcining of the primary sludge which is rich in calcium carbonate, was studied in detail by monitoring a 1,500,000 gpd test facility in Contra Costa County, California. A definite improvement in primary treatment performance occurs when the recalcined lime is introduced to the sewage flow, compared to the performance achieved when only virgin lime is used in the process. Primary phosphorus removals averaged 96% when steady-state recycle was established, with average BOD removals of 73% and suspended solids removals of 92%. Continued recycle of the lime causes a buildup in inert fractions such as silica, which are partially removed by a dry classification of the recalcined material. Close agreement was obtained between the observed solids constituent levels and the levels calculated by a specially designed mathematical model of the lime recycling process.