Understanding the biochemical characteristics of struvite bio-mineralising microorganisms and their future in nutrient recovery.

The biochemical properties of selected microorganisms (Bacillus pumilus, Brevibacterium antiquum, Myxococcus xanthus, Halobacterium salinarum and Idiomarina loihiensis), known for their ability to produce struvite through biomineralisation, were investigated. All five microorganisms grew at mesophilic temperature ranges (22-34 °C), produced urease (except I. loihiensis) and used bovine serum albumin as a carbon source. I. loihiensis was characterised as a facultative anaerobe able to use O2 and NO3 as an electron acceptor. A growth rate of 0.15 1/h was estimated for I. loihiensis at pH 8.0 and NaCl 3.5% w/v. The growth rates for the other microorganisms tested were 0.14-0.43 1/h at pH 7-7.3 and NaCl ≤1% w/v. All the microorganisms produced struvite, as identified by morphological and X-ray Powder Diffraction (XRD) analysis, under aerobic conditions. The biological struvite yield was between 1.5 and 1.7 g/L of media, the ortho-phosphate removal and recovery were 55-76% and 46-54%, respectively, the Mg2+ removal and recovery was 92-98% and 83-95%, respectively. Large crystals (>300 µm) were observed, with coffin-lid and long-bar shapes being the dominant morphology of biological struvite crystals. The characterisation of the biochemical properties of the studied microorganisms is critical for reactor and process design, as well as operational conditions, to promote phosphorus recovery from waste streams.

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

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.