Bittern-impregnated sisal: An alternative magnesium source for phosphorus recovery through struvite precipitation?

Abstract

Looking for cheaper and more transportable alternative magnesium(Mg2+) sources to envisage the full-scale application of downstream transfer of P recovery through struvite precipitation, sisal fibres were impregnated with bittern:water (B:W) ratio solutions 1:4; 1:5; 1:7 and 1:10. Two methods were attempted: the one-step method with a simultaneous Mg2+ release and struvite precipitation, and the two-step method with release and precipitation made separately. Sisal1:5 showed the highest recovery yield regardless of the method applied. The issue with the one-step method is that part of the crystals formed directly on the fibre surface, making their recovery harder. The two-step method using the sample impregnated with a 1:5 ratio resulted in highly pure struvite crystals, which are larger than those obtained with MgCl2·6H2O. Moreover, this method allows the retention of part of Cl− in the fibres instead of releasing it into the solution, and crystals do not form over the fibres. This work shows that the use of Mg2+-impregnated materials is a poor alternative for urine with a high concentration of P (>600 mg P L−1) because it shows a poorer performance, 50 % P recovery, when compared to the direct use of bittern (>99 % P recovery yield). However, the higher struvite purity and the retention of part of the Cl− make Mg2+-impregnated materials a potential solution to test in lower P concentrations (<300 mg P L−1). This study also opens several research opportunities regarding the application of Mg2+-impregnated material in P recovery through struvite precipitation.