Mainstream partial nitritation and anammox: long-term process stability and effluent quality at low temperatures

Abstract

The implementation of autotrophic anaerobic ammonium oxidation processes for the removal of nitrogen from municipal wastewater (known as "mainstream anammox") bears the potential to bring wastewater treatment plants close to energy autarky. The aim of the present work was to assess the long-term stability of partial nitritation/anammox (PN/A) processes operating at low temperatures and their reliability in meeting nitrogen concentrations in the range of typical discharge limits below 2 [Formula: see text] and 10 mgNtot·L(-1). Two main 12-L sequencing batch reactors were operated in parallel for PN/A on aerobically pre-treated municipal wastewater (21 ± 5 [Formula: see text] and residual 69 ± 19 mgCODtot·L(-1)) for more than one year, including over 5 months at 15 °C. The two systems consisted of a moving bed biofilm reactor (MBBR) and a hybrid MBBR (H-MBBR) with flocculent biomass. Operation at limiting oxygen concentrations (0.15-0.18 [Formula: see text] ) allowed stable suppression of the activity of nitrite-oxidizing bacteria at 15 °C with a production of nitrate over ammonium consumed as low as 16% in the MBBR. Promising nitrogen removal rates of 20-40 mgN·L(-1)·d(-1) were maintained at hydraulic retention times of 14 h. Stable ammonium and total nitrogen removal efficiencies over 90% and 70% respectively were achieved. Both reactors reached average concentrations of total nitrogen below 10 mgN·L(-1) in their effluents, even down to 6 mgN·L(-1) for the MBBR, with an ammonium concentration of 2 mgN·L(-1) (set as operational threshold to stop aeration). Furthermore, the two PN/A systems performed almost identically with respect to the biological removal of organic micropollutants and, importantly, to a similar extent as conventional treatments. A sudden temperature drop to 11 °C resulted in significant suppression of anammox activity, although this was rapidly recovered after the temperature was increased back to 15 °C. Analyses of 16S rRNA gene-targeted amplicon sequencing revealed that the anammox guild of the bacterial communities of the two systems was composed of the genus "Candidatus Brocadia". The potential of PN/A systems to compete with conventional treatments for biological nutrients removal both in terms of removal rates and overall effluent quality was proven.

Keywords: Effluent quality; Low temperature; Mainstream anammox; Micropollutants; Municipal wastewater; Nitrogen removal; Partial nitritation/anammox.

Graphical abstract

Fig. 1

Conditions and performance of the MBBR (a–c) and H-MBBR (d–f) reactors during the last 240 days of operation. The two reactors were inoculated independently and run in total for 400 and 360 days respectively (the full operational period is presented in Figs. S1, S2). Time series of temperature (a, d); maximum anammox activity (${\mathrm{mg}}_{({\mathrm{NH}}_4+{\mathrm{NO}}_2)-\mathrm{N}}·{\mathrm{L}}^{-1}$·d⁻¹), overall total nitrogen removal rate (mg_N·L⁻¹·d⁻¹), and rainfall (b, e); total and ammonium nitrogen removals, and yield of NO₃⁻ production over total nitrogen removed (c, f). Grey areas indicate operation at temperatures above 15 °C. Rainfall data source: Swiss National Air Pollution Monitoring Network (FOEN/NABEL).

Fig. 2

Removal of the studied organic micropollutants at 15 °C in the treatment schemes comprising the A-stage followed by PN/A systems in comparison to a conventional activated sludge reactor. A-stage: pre-treatment for COD removal only; A-stage + MBBR and A-stage + H-MBBR: full treatment schemes; Nitrification: reference reactor for oxidation of organic matter and nitrification (12 h HRT, 15 d SRT); Nitrification/denitrification: literature values from a nitrifying and denitrifying reactor with an HRT of 12 h and an SRT of 10 d (Falås et al., 2016). All removals were calculated for the same time period (sampling campaign days 326–333 MBBR, and 255–262 H-MBBR). The concentrations detected in the MWW served as the initial concentrations C₀ for calculating the removal (C/C₀) in the different systems. Error bars display standard deviations of 48-h composite samples (n = 3). Compounds displaying removals in the range 0 ± 25% are here considered as persistent. The compounds have been ordered according to their removals in the reference nitrification reactor. Reference removals are connected with a dashed line to facilitate visual comparison. Compounds with removals below −25% (e.g. due to deconjugation during biological treatment) are not visualized on the graph. Micropollutants acronyms: DHH- Carbamazepine: 10,11-dihydro-10-hydroxy-carbamazepine; DHDH-Carbamazepine: 10,11-dihydro-10,11-dihydroxy-carbamazepine; SMX + Ac-SMX: sum of sulfamethoxazole and N₄-acetylsulfamethoxazole.

Fig. 3

Evolution of the concentrations of nitrogen species (NH₄⁺, NO₂⁻, NO₃⁻) and dissolved organic matter (COD_sol) during representative SBR cycles at 15 °C (dotted area: initial settling + feeding phase; white areas: pre- and post-anoxic phases; grey area: aeration phase at 0.18 ${\mathrm{mg}}_{{\mathrm{O}}_2}·{\mathrm{L}}^{-1}$) (a). Nitrogen species, COD_sol and DO set-point during in situ batch tests conducted by spiking acetate as a representative readily biodegradable organic compound under different DO conditions (set at 1.5, 0.2 and 0 ${\mathrm{mg}}_{{\mathrm{O}}_2}·{\mathrm{L}}^{-1}$), at 15 °C and in absence of ammonium (b).

Fig. 4

Preferential localization of “Ca. Brocadia”-related AMX, Nitrosomonas-related AOB, and Nitrospira-related NOB guilds over the four biomass samples collected along the operation at 15 °C in the MBBR (biofilm carriers only – a) and in the H-MBBR (biofilm carriers and flocs – b and c respectively) examined by 16S rRNA gene-based amplicon sequencing analyses (error bars display the standard deviation of biological technical triplicates). The sequencing results are only qualitatively displayed as read counts (out of 25,000 reads per sample), whereas the relative abundances of these guilds were estimated by qFISH-CLSM (Table 2).

Fig. 5

Representative FISH-CLSM digital images illustrating the distribution of AMX, AOB and NOB in the different biomass fractions, namely MBBR biofilm (a), H-MBBR biofilm (b) and flocs (c) at the end of the experimental period at 15 °C. Anammox populations (AMX; Amx820 + Bfu613 oligonucleotides labeled with the fluorescent probe Cy5) are displayed with purple color allocation, aerobic ammonium-oxidizing bacteria (AOB; AOB-mix, Cy3) in white, aerobic nitrite-oxidizing bacteria (NOB; NOB-mix, FLUOS) in green, and DAPI stain in blue. Each image is the maximum intensity projection of a single z-stack. Biomasses were homogenized prior to imaging (scale bars: 20 μm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Temporal evolution of the maximum volumetric anammox activity (expressed as the sum of NH₄⁺, NO₂⁻ consumption) in response to the temperature step variation during the operation for PN/A in MBBR-2 (a). Online NH₄⁺, O₂ and temperature signals of MBBR-2 between day 192 and 201 (b). For completeness, three normal SBR cycles are included before and after the in situ anammox activity performed on days 194–199 (grey area; non-limiting nitrite concentration 10–15 mg_N·L⁻¹; test stripes). The white arrow highlights the beginning of the anammox activity on day 194, namely when SBR operation was stopped, the reactor was set to mixing mode and non-limiting concentrations of NH₄⁺ and NO₂⁻ were added. Note: the minor temperature fluctuations around the set value in (b) are due to the feeding events (the influent tank was not temperature controlled) or to the diurnal ambient temperature variations when the reactor was not fed (i.e. during the batch anammox test).