Combined Partial-Nitrification and Phosphorus Removal with the co-Existence of Nitrite-resistant phosphorous accumulating organisms (PAOs) and nitrifiers in the treatment of high-strength manure digestate

Concurrent biological phosphorus (P) recovery and nitrogen (N) removal in treating high-strength wastewater (such as anaerobic digestate) has been considered incompatible due to presumed conflicts in the conflicting optimum conditions required by phosphorous accumulating organisms (PAO) and nitrifiers. However, this study achieved a stable nitrite accumulation while still maintained PAO activities in one sequencing batch reactor for treating the manure digestate under two aeration schemes (continuous versus intermittent aeration). Nitrite accumulated up to 80.5 ± 21.1 mg-N/L under continuous aeration (6 h) mode. Switching to intermittent aeration (equivalent to 3 h) halved nitrite accumulation but increased total nitrogen removal efficiency from 53.5 ± 12.2% to 84.7 ± 9.4%. Mass balance analysis indicates that nearly all ammonia was removed as N2O. Both Enhanced Biological Phosphorus Removal (EBPR) activity assessment and phenotypic trait detection via single cell Raman spectrum (SCRS) confirmed the existence of yet to be identified PAOs that are resistant to high nitrite inhibition in our system. Visual Minteq calculation indicates that high concentrations of Ca in manure digestate may form precipitates and influence the bioavailability of P forms. Therefore, both biotic and abiotic pathways lead to a total P removal rate around 61.0 ± 6.8%. This study highlights new opportunities to combine short-cut nitrogen removal via partial nitrification, nitrous oxide (N2O) collection, and EBPR in commercial farm-collected digested manure wastewater. Higher N and P removal efficiency could potentially be achieved by tuning aeration schemes in combination with down-stream anammox process. Synopsis Concurrent partial nitrification, N2O accumulation, and EBPR activity were found, leading to the exploration of novel nitrite-resistant PAOs, simultaneously N/P recovery, and waste-energy conversion in treating high strength wastewater. ### Competing Interest Statement The authors have declared no competing interest.