Predicting the potential of sludge dewatering liquors to recover nutrients as struvite biominerals

Abstract

Phosphorus and nutrient recovery from wastewater as mineral salts can support local replenishment of fertilisers and reduce mining, contributing to the circular economy. Wastewater and related streams are rich in nutrients, however; there is need to develop bio-based processes to recover them. This study investigates the fractions of phosphorus (P) used by Brevibacterium antiquum to form struvite biominerals (bio-struvite) in wastewater sludge dewatering liquors. After 72h of incubation, 25.6 mg P/L were recovered as bio-struvite from 12.4 mg P/L organic plus condensed P and 13.2 mg P/L of ortho-phosphate. The potential of sludge dewatering liquors to recover nutrients as struvite was investigated by characterising ten types of sludge liquors (originating from primary, secondary sludge, feed to anaerobic digester and digestate, from 3 types of wastewater treatment plants) for their P fractions together with other parameters relevant for B. antiquum growth. Results indicated that liquors obtained from primary sludge, feed to anaerobic digesters and digestate were the most suitable to produce bio-struvite, as these were found to frequently have a high content of organic and condensed P, between to 276-732 mg P/L. Liquors, from all the investigated sites, presented a higher potential for bio-struvite production than with conventional struvite precipitation. This study demonstrated that B. antiquum could convert organic and condensed P into bio-struvite, and this opens up a completely new way to recover forms of phosphorus that are not typically available for nutrient recovery in a single process.

Keywords: Acid-hydrolysable phosphorus; Biomineralization; Centrate; P-recovery; Sidestream.

Fig. 1

Flow diagram for sampling and analysis procedures to distinguish sources of phosphorus used in bio-struvite production.

Fig. 2

B. antiquum cell count (a) and phosphorus content per cell (b) when incubated in sludge dewatering liquors at room temperature for 72h. Inoculated experiments (♦, closed symbols), non-inoculated controls (□, open symbols); high P accumulation reference value (11 fg P/cell) (dashed line) for EBPR microorganisms with high phosphorus accumulation [25].

Fig. 3

Changes in P fractions when growing B. antiquum in sludge liquors from a BNR site, at room temperature for 72h. Errors bars show standard deviation of triplicate experiments.

Fig. 4

Proportion in percentage, of phosphorus fractions in sludge dewatering liquors (left side) and after incubating B. antiquum for 72h with the formation of bio-struvite (right side) and scanning electron microscope (SEM) pictures of the bio-struvite produced showing the orthorhombic crystal structure.

Fig. 5

Environmental electron scanning microscope photos, and energy-dispersive X-ray (EDX) spectra of selected areas highlighted in the photos, of bio-struvite: (a) 18x magnification, and (b) 100x magnification. And of chemical struvite: (c) 18x magnification, and (d) 100x magnification.