Phosphorus and alginate-like polymer recovery from municipal granular and activated sludge

Aerobic granular sludge technology offers distinct benefits over conventional activated sludge in the wastewater industry. However, managing excess sludge from both conventional and granular systems presents serious challenges. While it indeed offers opportunities for the recovery of valuable by-products, such as phosphorus and alginate-like polymers. This investigation focuses on the recovery potential of phosphorus and alginate from excess granular sludge of a municipal wastewater treatment reactor, and on a synergistic strategy to recover both compounds. Activated and granular sludge demonstrated both high potential for phosphorus recovery in form of struvite, with similar phosphorus content of the recovered struvite (120 mg P/g stv). The leaching and phosphorus recovery efficiency was higher in the case of activated sludge ash (98% and 83%, respectively) compared to granular sludge ash (82% and 77%, respectively). However, concerns emerged regarding the purity of sludge-derived struvite. Granular sludge exhibited higher alginate like yields (267 ± 21 mg/g VSS) compared to activated sludge (221 ± 11 mg/g VSS). Despite this, no significant differences were observed in the characteristics of the extracted alginate from either system. Although sludge represents a potential source of alginate-like polymers, the extracted alginate still contained microbial byproducts, unlike commercially produced alginate. Overall, 88% to 98% of the phosphorus content remained extractable from the sludge after alginate-like pre-extraction. However, the alginate-like extraction process may affect the efficiency of phosphorus recovery and induce additional costs. Further research is required to fully assess potential economic implications, especially at full-scale applications. • Activated and granular sludge are promising sources of P and ALE. • Activated sludge showed slightly higher P recovery (83%) than granular sludge (77%). • Granular sludge contained higher ALE than activated sludge. • Extracted ALE from municipal sludge have lower purity than commercial alginate. • After alginate extraction, 88–98% of P remains recoverable from sludge.