LOW PRESSURE FORMATION OF STRUVITE

Struvite, MgNH4PO4•6H2O, precipitation causes operational problems in plants handling anaerobically digested biosolids. The typical method of struvite inhibition is adding ferric chloride, FeCl3. Ferric chloride is extremely corrosive and is thus difficult to handle. Also the addition of ferric chloride can lead to another precipitate-vivianite, Fe3(PO4)2. This paper proposes exposing the biosolids to low pressure, causing carbon dioxide (CO2) to be released from solution, thereby decreasing the carbonate concentration. Removing carbon dioxide from solution would affect struvite solubility in two ways: 1) Raise the pH of the solution; 2) Magnesium forms a ion pair with the carbonates, by removing the carbonates magnesium is made available for struvite formation. Both of these would decrease struvite solubility. This paper will present research showing that when anaerobic biosolids are exposed to low pressure the pH did rise and the concentration of all struvite components decreased. Using this process would allow struvite to be formed under controlled conditions. The advantage of forming struvite under these conditions is it will prevent struvite from forming on piping, pumps and equipment. In addition by not using ferric chloride the possibility of vivianite formation is eliminated.