Start-up of a full-scale two-stage partial nitritation/anammox (PN/A) process treating reject water from high solid anaerobic sludge digestion (HSAD)

Abstract

High solid anaerobic digestion (HSAD) achieves the benefits of high volumetric loading rates and lower reject water production, which, however, results in much more concentrated reject water with a remarkable increase in organics and nitrogen compared with that from conventional AD with low solid content. The high concentrations of ammonium (2000-3500 mg/L) and COD (3000-4000 mg/L) were reported to exert inhibition on anammox bacteria (AnAOB), posing challenges to the application of the partial nitritation/anammox (PN/A). To date, no cases of PN/A process start-up for sludge HSAD reject water were reported. This study demonstrated the start-up process of a 480 m³/d PN/A project without anammox sludge inoculation and treating HSAD reject water from a centralized dewatered sludge treatment plant. The project did not construct new infrastructures but utilized previously constructed tanks to upgrade the process from existing short-cut nitrification-denitrification to a two-stage PN/A process. Although no external anammox sludge inoculation was performed to save seeding sludge cost, the start-up was successfully achieved in about 9 months (273 days) based on a three-step method of "AnAOB enrichment - sludge acclimation - capacity doubling". During start-up, the relative abundance of AnAOB (Candidatus_Kuenenia) increased from near zero to 12.0%. After start-up, the total inorganic nitrogen (TIN) removal load reached 0.74 kgN/(m³•d), with a total nitrogen removal efficiency of over 90%. Compared to the traditional nitrification-denitrification process, the PN/A process remarkably reduces the addition of organic chemicals and aeration energy consumption, saving approximately 4.2 million yuan (RMB) in operational costs annually. In summary, this research provides a full-scale reference for the start-up of the PN/A process treating sludge HSAD reject water.

Keywords: Full-scale; High solid anaerobic digestion; Partial nitritation/anammox; Start-up; Two-stage.

Graphical abstract

Fig. 1

Schematic diagram of the three-step method of “AnAOB enrichment (synthetic solution) - sludge acclimation (reject water) - capacity doubling (reject water)”. In Step I, AnAOB was enriched by feeding synthetic solution in a single anammox reactor, with the other reactor idle, at a working volume of 400 m³; In Step II, the feeding solution gradually shifted from synthetic solution to reject water, achieving stable anammox nitrogen removal, with the working volume remaining unchanged; In Step III, full reject water is used and the capacity was doubled by dividing anammox sludge into two respective tanks, expanding the total working volume to 800 m³.

Fig. 2

Performance during three-step start-up of two-stage PN/A. Nitrogen removal performance (a); nitrogen removal load (b). Step I: 93 days; Step II: 101 days; Step III: 79 days (273d in total).

Fig. 3

Three-step nitrogen removal load and treatment capacity for a two-stage PN/A (a) and the general situation of global sludge reject water PN/A projects (b).

Fig. 4

Microbial community relative abundance at the genus level in the two-stage PN/A reactor during step I.

Fig. 5

Microbial community relative abundance at the genus level in the two-stage PN/A reactor during step II.

Fig. 6

Microbial community relative abundance at the genus level in the two-stage PN/A reactor during step III.

Fig. 7

The centralized HSAD sludge treatment project. (a) Schematic diagram of the centralized sludge treatment mode; (b) Actual photo and process flow diagram of the upgraded two-stage PN/A process.