Enhancement of anaerobic digestion sludge dewatering performance using in-situ crystallization in combination with cationic organic polymers flocculation

Anaerobic digestion (AD) has been widely used in sludge treatment for biogas recovery, organic fraction stabilization, and sludge reduction. However, after AD, sludge filterability is extremely deteriorated due to the release of biopolymers and the formation of fine particles. AD sludge is generally rich in nutrients, mainly ammonium nitrogen and phosphates, that result from biopolymer degradation. We designed a conditioning process that combines the in-situ crystallization of magnesium ammonium phosphate (MAP), as a skeleton builder, with organic polymer flocculation. We show that crystallized MAP can bind with extracellular polymeric substance fractions to increase sludge floc density. The molecular structure and electrical charge of organic polymers importantly influence sludge particle flocculation and aggregation. We found that cationic polyacrylamides form flocs that aggregate with branching structures which are characterized by a larger size and a more compact structure. Simultaneous crystallization and flocculation produced by a magnesium–organic polymer gel improved AD dewaterability more than the separate addition of magnesium ions and organic polymers. The method of sludge conditioning that we propose was tested by extensive use in different AD sludge conditioning protocols. The method reduces the ammonium nitrogen load in AD liquor and increases the suitability of the biosolids for use as land fertilizer.