Minimizing heavy metals in recovered struvite from swine wastewater after anaerobic biochemical treatment: Reaction mechanisms and pilot test

Abstract

Struvite crystallization is a chemicophysical process for nitrogen and phosphorus recovery, and its product can be used as a slow-release fertilizer. Heavy metals (mainly Cu and Zn) present in swine wastewater not only affect the crystallization kinetics, but also increase the product’s ecological risk and make it inadequate for agricultural applications. However, there is currently little research focusing on heavy metal minimization during struvite recovery. This paper is to develop a cost-effective operation strategy, in which minimum process modification and optimum phosphorus recovery could be achieved. The results indicated that the Cu and Zn co-precipitation was mainly controlled by the chemical reaction rather than adsorption during struvite crystallization, making the pre-removal of heavy metals possible. Increasing the ammonia nitrogen concentration could effectively inhibit the co-precipitation of Cu and Zn. When struvite crystallization was performed in synthetic swine wastewater, it was found that the pre-removal of Cu/Zn by chemical precipitation, followed by the increase in initial solution pH, could markedly reduce the Cu and Zn co-precipitation and their contents in the recovered struvite. A pilot-scale test validated such a recommendation. Compared to the conventional struvite crystallization process, the proposed approach could reduce 43.5% Cu and 77.6% Zn in the recovered struvite.