The role of pH on the biological struvite production in digested sludge dewatering liquors

Abstract

Struvite production mediated by bacteria has opened up a new route for phosphorus recovery from wastewater streams but its application to digested sludge dewatering liquors is not yet well understood. This study investigates the growth and biological struvite production of selected bacteria in wastewater liquors with pHs between 5.7 to 9.1. The bacterial growth was assessed through flow cytometry. Bacillus pumilus, Halobacterium salinarum and Brevibacterium antiquum remained viable at pHs between 5.7 to 9.1 but B. antiquum was able to grow at pHs between 7.3 to 7.8. Further analysis allowed the identification of crystals as struvite in tests between pH 7.3 to 8.3. All strains were capable of producing struvite at a range of pHs, but the highest production of 135-198 mg/L was observed for pHs between 7.3 to 8.3. At pHs > 8.3, precipitation of struvite and calcium compounds was observed in inoculated and non-inoculated tests. This study demonstrates that biological struvite production can occur at a wide range of pHs, hence significantly different from chemical struvite precipitation that occurs at pH > 8.3, making it a potentially viable process for phosphorus recovery as struvite from wastewater streams and sludge liquors without strict pH control.

Figure 1

Electron microscope images of the solid precipitate collected at the end of the incubation period of sludge dewatering liquors without pH control at 2000× magnification: (a) B. antiquum; (b) non-inoculated control.

Figure 2

Flow cytometry live cell counts after incubation of the selected bacteria at different pH values.

Figure 3

Electron scanning microscope images of the solid precipitate collected at the end of the 10 days of incubation for B. pumilus (a), H. salinarum (b), and B. antiquum (c) at pH 7.3 in sludge dewatering liquors at 300× magnification, and (d) example of an energy dispersive x-ray (EDX) microanalysis of the crystals for B. antiquum (c) with weight percentage of the elemental components of the analysed crystal that allowed the identification of the crystal as bio-struvite. EDX was also completed for the crystals produced by B. pumilus (a), H. salinarum (b) and present overlapping results to figure (d).

Figure 4

Solid precipitate collected after 6 days of incubation of pure cultures of the selected bacteria in magnesium supplemented sludge dewatering liquors at different initial pH values. Error bars represent standard deviation obtained from triplicate tests.

Figure 5

Solid precipitate production when incubating the selected bacteria in: (a) pH controlled through acid addition (Experiment A) and (b) pH controlled by limiting degassing in closed serum bottles (Experiment B). Controls were not inoculated with bacteria. Error bars represent standard deviation obtained from triplicate tests.