VIVIANITE: NUTRIENT REMOVAL's HIDDEN PROBLEM?

Struvite, which is a magnesium ammonia phosphate precipitate [MgNH4PO4 · 6H2O] compound, can cause operational problems in wastewater treatment plants that stabilize their waste sludge using the anaerobic digestion process. A typical method for controlling Struvite precipitation is to add iron in the form of ferric chloride (FeCl3) to the digesters. The iron reacts with the phosphorus to form ferrous phosphate precipitate, which is also known as Vivianite [Fe3(PO4)2 · 8H2O]. This paper presents a case study wherein Vivianite, not Struvite, precipitation caused operational problems in the anaerobic digestion process even though no iron was fed directly to the anaerobic digestion process vessels.Baltimore's Back River wastewater treatment plant uses iron to precipitate phosphorus in the activated sludge secondary treatment process. The problem with Vivianite precipitation was investigated as part of a preliminary design effort to expand and rehabilitate the existing anaerobic digestion facilities. The investigation concluded that the deposits were likely caused by adding the iron-rich raw sludge to hot digested sludge containing high concentrations of phosphorus and then passing the mixture through numerous pipe fittings causing turbulence and potential loss of CO2. Stripping CO2 would lower the pH and result in precipitation of Vivianite on the walls of the pipes. The study report recommended designing the new digestion facilities similar to the existing ESD system. In addition, the sludge lines downstream of the heat injection should be designed to minimize turbulence and should be designed to facilitate cleaning, chemical injection, or replacement if required in the future.