Development of a Struvite Crystallizer for Reducing Phosphorus in Effluent from Livestock Waste Lagoons

Abstract

The research successfully met its objectives of designing and characterizing a crystallizer for removing phosphorus from livestock lagoon effluent in North Carolina. A design was created for a struvite crystallizer utilizing a cone-shaped continuous-mode fluidized bed. A laboratory-scale system operating at 49.2 L/h of total liquid flow achieved near-steady state operation at two sets of operating conditions. The first used ammonia addition of 100 ppm and no magnesium (Mg) addition. The second used ammonia addition of 100 ppm and Mg addition of 30 ppm. Estimated reduction in orthophosphate phosphorus (OP) and total phosphorus (TP) was 36 percent and 24 percent, respectively, over the first condition set and 74 percent and 58 percent, respectively, over the second. Three models were developed to describe crystallizer behavior. The MLMB model assumed perfect mixing of both liquid and bed. The PLCB model assumed plug flow of liquid and perfect classification of the bed. The PLMB model, which best predicted behavior, assumed plug flow of liquid and perfect mixing of the bed. Factorial experiments were conducted to test the effects of Mg addition, ammonia addition, and flow rate on phosphorus removal. Increasing Mg addition in the range tested (0 to 60 ppm) and increasing ammonia addition in the range tested (0 to 200 ppm) were significantly associated with increasing phosphorus removal. The effect of flow rate was insignificant between the values tested (41.2 and 56.8 L/h). A field-scale system was assembled at a lagoon and used to conduct factorial experiments testing the same effects as those in the laboratory. Increasing Mg addition in the range tested (0 to 60 ppm), increasing ammonia addition in the range tested (from no pH enhancement to 1 pH point enhancement), and decreasing flow rate between the values tested (341 and 568 L/h) were all significantly associated with increasing phosphorus removal. Reduction in TP and OP averaged 70.2 percent and 77.3 percent, respectively, when ammonia and Mg were being added. Peak reductions in TP and OP were 81.9 percent and 87.1 percent, respectively.