Vivianite Occurrence and Remediation Techniques in Biosolids Pre-treatment Process

Iron salt is often used in wastewater treatment to precipitate phosphorus from liquid stream into iron-phosphate minerals. The precipitated phosphate compounds are subsequently removed during solids settling and filtration processes and remain with the sludge solids. Under anaerobic conditions, insoluble ferric iron in the sludge solids is reduced to more soluble ferrous iron which is then released into the liquid; dissolution of this iron causes release of the phosphorus that was associated with it. Ferrous ions can combine with phosphate to form a new mineral, vivianite (Fe3(PO4)2). Vivianite is a hard scale and has the potential to accumulate in piping and on equipment, creating problems for plant operation. The aim of this study was to investigate vivianite remediation techniques, including prevention and removal. Qualitative X-ray diffraction (XRD) analysis and geochemical modeling were utilized to identify and predict the vivianite formation. Lab scale tests were performed to assess the capability of sodium hypochlorite, chelating agent, crystal inhibitors and aluminum sulfate (alum) to prevent vivianite scale formation. The bench scale study revealed that chelating agent, crystal inhibitor and alum had reasonable potential to control vivianite formation. Full-scale alum addition to the first of two in-series solids holding tanks (prior to screening) was not as effective to prevent scale buildup in the centrifuge as anticipated from the bench-scale tests because of continued fermentation and ferrous ion release in the second holding tank which contained screened solids. Future plans include modifications to allow addition of alum to the second sludge blend tank as well as testing of chelating agents or crystal inhibitors on centrifuge feed solids.