Tractament del centrat d'un digestor anaerobi mitjançant el procés anammox combinant amb la precipitació de fòsfor bioinduïda i en forma d'estruvita i K-estruvita

Abstract

Anaerobic ammonium oxidation (anammox) bacteria oxidize ammonium to nitrogen gas under lack of oxygen, using nitrite as electron acceptor. The combined partial nitritation – anammox (PNA) process is fully autotrophic and results in an advantageous alternative to remove nitrogen (N) from wastewater in comparison to the conventional nitrification-denitrification process. This study focused on the treatment of the centrate obtained after dewatering the digestate from an anaerobic digester in a wastewater treatment plant (Terri WWTP). The centrate was treated by PNA in a 10-L single-stage sequencing batch reactor (SBR), also looking for the simultaneous precipitation of phosphorus (P) through biologically induced mineralization (BIM). The PNA SBR was operated for 200 days involving different operational phases: inoculation, stabilization and regular performance. Autotrophic N-removal from centrate (1100 mg N/L, 15 mg P/L) was successfully achieved reaching a final ammonium removal rate of 206 mg N/L·d (equivalent to a loading rate of 260 mg N/L·d with a removal efficiency of 81%). The suspended solids content in the SBR increased by a factor of 3,5 approximately, with a final value of 3180 mg/L (86% volatile) and very good settling properties according to the granular shape of the sludge. Phosphate BIM was assumed to took place under appropriate reactor operation (days 125-169), resulting in the formation of calcium phosphate (Ca/P molar ratio of 1,56). Supplementation of the centrate with phosphate favoured mineralization. Average removals of 20% phosphate and 34% calcium were measured in the liquid phase. It was not possible to confirm phosphate precipitation as struvite or K-struvite inside the reactor. Effluent from the PNA SBR could be additionally post-treated by chemical precipitation to produce struvite and K-struvite with the consideration of a reduced concentration of ammonium-N (150-200 mg/L) and alkalinity (≈340 mg/L), as well as a remaining concentration of phosphate-P (50-100 mg/L) and potassium (200 mg/L). Phosphate precipitation during or after biological N-removal is interesting because it implies a reduction in the use of chemicals to increase the pH due to a decrease in the alkalinity of the wastewater