Precipitation of Struvite from Supernatants Separated from Enzymatically Disintegrated Digested Sewage Sludge

This paper presents the results of a study on the precipitation of struvite from filtrate liquids obtained from fermented sewage sludge subjected to prior disintegration with the enzyme papain. The methane fermentation process was carried out for sewage sludge with the addition of four different doses of papain: 0, 1, 2, and 3% (w/w) with respect to the dry weight of sludge sent for fermentation. After 20 days of methane digestion, struvite precipitation was carried out from the obtained supernatants using different pH values (9, 10, and 11) and a PO43−:NH4+:Mg2+ molar ratio of 1:1:1. The results showed that the efficiency of removing phosphorus, ammonium nitrogen, and organic compounds from the supernatant after the enzymatic disintegration of sludge was high under all analyzed conditions. The highest efficiency of the precipitation of phosphorus (93.1%), ammonium nitrogen (59.8%), and organic compounds (35.0%) in the form of struvite was obtained for filtrate supernatants derived from sludge disintegrated with the addition of 3% (w/w) enzyme at pH = 11. The purity of precipitated struvite was determined by thermal analysis methods, and an appropriate theoretical model was used to describe the crystallization process in batch reactors.