A two-stage partial nitritation-denitritation/anammox (PN-DN/A) process to treat high-solid anaerobic digestion (HSAD) reject water: Verification based on pilot-scale and full-scale projects

Abstract

High-solid anaerobic digestion (HSAD) reject water, distinguished by elevated levels of chemical oxygen demand (COD), NH₄⁺-N and an imbalanced COD/TIN, presents a significant challenge for treatment through conventional partial nitritation/ anammox (PN/A) process. This study introduced a revised two-stage PN/A process, namely partial nitritation/denitritation-anammox (PN-DN/A) process. Its effectiveness was investigated through both pilot-scale (12 t/d) and full-scale (400 t/d) operations, showcasing stable operation with an impressive total removal rate of over 90 % for total inorganic nitrogen (TIN) and exceeding 60 % for COD. Notably, 30 % of TIN was eliminated through heterotrophic denitritation in partial nitritation-denitritation (PN-DN) stage, while approximately 55 % of TIN removal occurred in the anammox stage with anaerobic ammonium oxidizing bacteria (AnAOB) enrichment (Candidatus Kuenenia, 25.9 % and 26.6 % relative abundance for pilot and full scale). In the PN-DN stage, aerobic-anaerobic alternation promoted organics elimination (around 50 % COD) and balanced nitrogen species. Microbial and metagenomic analysis verified the coupling between autotrophic and heterotrophic denitritation and demonstrated that PN-DN stage acted as a protective buffer for anammox stage. This comprehensive study highlights the PN-DN/A process's efficacy in stably treating HSAD reject water.

Keywords: AD reject water; Anammox; Full-scale; High solid anaerobic digestion; PN/A.

Graphical abstract

Fig. 1

(a) SAA of A/O-treated HASD reject water with different dilution ratios (1.00–2.00); (b) COD removal rates of various COD removal methods (R1, anoxic-aerobic alternation; R2, low-intensity aeration; R3, high-intensity aeration; R4, anoxic-aerobic alternation with alkalinity addition).

Fig. 2

COD variations (a) and N variations (b) during the PN-DN and the anammox stages of PN-DN/A process at pilot-scale and full-scale.

Fig. 3

(a) COD transformations of PN-DN/A process in pilot scale and full scale; (b) N transformations of PN-DN/A process in pilot scale and full scale.

Fig. 4

(a) Bubble plot of microbial abundance at the genus level of four reactors; (b) Venn diagram of microbial distribution at the genus level; (c) Microbial clustering at the genus level; Dumbbell plot of abundance of the main microorganisms at the phylum level in the PN-DN stages (d) and the anammox stages (e) at pilot-scale and full-scale. PN-DN: PN-DN stage, A: anammox stage.

Fig. 5

The optimal pH range in (a) PN-DN stages and (b) anammox stages affected by the concentrations of NO₂⁻-N and NH₄⁺-N. A solid line with a double arrow indicates the intersection of the two ranges. A dotted line with a double arrow indicates that the intersection of the two ranges does not exist.

Fig. 6

(a) Abundance of genes involved in nitrogen metabolism processes. PP: pilot-scale PN-DN stage, PA: pilot-scale anammox stage, FP: full-scale PN-DN stage, FA: full-scale anammox stage; (b) Heat map of abundance and clustering of genes related to nitrogen metabolism; (c) Clustering map of samples at the gene level. PN-DN: PN-DN stage, A: anammox stage.

Fig. 7

(a) Full process of sludge HSAD and reject water PN-DN/A treatment; (b) Comparison of COD and NH₄⁺-N of conventional sludge AD reject water (each blue triangle represents the annual average water quality of an actual AD reject water. A total of 22 full-scale projects were collected, mainly distributed in the Netherlands, Germany, and other countries (Ali and Okabe, 2015). Details were shown in Table S1 in supplement materials) and HSAD reject water in this study (each orange circle represents the reject water quality of a certain day, showing a total of about two months of water quality); The actual photos of pilot-scale (c) and full-scale (d) PN-DN/A reactors.