Struvite is magnesium ammonium phosphate, a precipitate formed in digester systems. It is caused largely by environmental factors unique to anaerobic digestion. Ammonium and phosphate ions are released continuously by the digesting biomass, adding to those ions already in the sludge. Solvated magnesium ions also exist in the sludge. Thus, the digesting biomass can sometimes become supersaturated with magnesium ammonium phosphate. Precipitation of the compound occurs when the concentration exceeds the conditional solubility product of magnesium ammonium phosphate at the temperature and pH conditions in the digester. Conditional solubility refers to the specific set of actual conditions under which a given metal-salt compound may or may not be soluble.
Formation of struvite does not adversely affect the anaerobic digestion process. However, struvite scale deposits in piping and heat exchangers will cause maintenance problems. Scale deposits can also occur on parts of equipment that dewater digested sludge, especially the rollers of belt filter presses. Struvite particles in sludge are extremely hard and have been reported to damage pumps, especially progressive cavity and other positive-displacement pumps that pump anaerobically digested dewatered sludge.
Struvite scale deposits in pipelines can be controlled by frequent pigging of pipelines. Acid washing can remove the deposits, but it can be costly. A preventive solution to struvite formation is to precipitate the phosphorus in the digesting sludge by adding an iron compound such as ferrous chloride. An added benefit to adding ferrous chloride is that it controls hydrogen sulfide generation. Care should be taken to ensure that sufficient phosphorus remains to meet the nutritional needs of the anaerobic bacteria.
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