Performance regulation and forming mechanism of biomass ash filter material

In order to solve problems such as low pollutant concentrations, large treatment volumes and high purification costs in tailwater treatment of recirculating aquaculture systems, a new renewable biomass ash filter material was developed to remove nitrogen and phosphorus from tailwater, which is environment-friendly, economical and highly effective. Properties of biomass ash filter materials produced under different conditions were studied through orthogonal experiments with sintering temperature, keeping time and material trapped time as factors and the main control factors affecting the properties of filter materials were obtained. The pore evolution and forming principle of the filter material during the sintering process can be inferred by characterizing and analyzing the structural characteristics and microscopic morphology of biomass ash filter materials produced at different sintering temperatures. This helps to clarify the performance control mechanism of these filter materials via their different structures. Based on this, the biomass ash filter material that has the best performance could be selected as a test group. And commercial filter materials including biochar, ceramic granules and natural zeolite were used as control groups to conduct phosphate and nitrite adsorption tests to evaluate the adsorption performance of the biomass ash filter material. The results show that sintering temperature is the main control factor affecting the properties of biomass ash filter materials. As the sintering temperature increases, wollastonite, melilite and anorthite phases are generated sequentially in the filter materials, which leads to a series of microstructure transformations, such as crystallization, vitrification and precipitation. This is the reason for the formation of the different pore structures. In this process, water pores in the filter materials are gradually converted into closed pores. This causes positive effects of high sintering temperature on stomatal porosity, total porosity, and compressive strength. It also brings negative effects of high sintering temperature on permeable porosity, water pore rate, water absorption and permeability, pH and EC values. The performance control mechanism of these filter materials exists because of the two contrary effects. Without aging, the biomass ash filter material produced by sintering at 1170 ℃ for 40 minutes has better performance. This kind of filter material has a high saturation water absorption and average permeability rate, short water holding time, good formability and pore structure with a compressive strength of 0.74 MPa and total porosity of 65.49%. Its pH and EC values meet the demands of water purification. The unit adsorption capacity and removal rate of this filter material on phosphate can reach 0.996 mg/g and 2.767%, respectively. The phosphate adsorption effect is significantly better than that of biochar, ceramic granules and natural zeolite. The unit adsorption capacity and removal rate of this filter material on nitrite are 0.317 mg/g and 0.881%, respectively, which is equivalent to that of ceramic granules, significantly higher than natural zeolite and slightly inferior to biochar. The new renewable biomass ash filter material improves the defects of current mainstream filter materials for water purification. It is expected to meet the demand for nitrogen and phosphorus removal from circulating aquaculture tail water and can be used widely.