Storage without nitrite or nitrate enables the long-term preservation of full-scale partial nitritation/anammox sludge

Efficient reactivation of anammox sludge after prolonged storage using a combination of batch and continuous reactors

Due to the slow growth rate of anammox bacteria, enriched sludge is required for the rapid start-up of anammox-based reactors. However, it is still unclear if long-term stored anammox sludge (SAS) is an effective source of inoculum to accelerate reactor start-up. This study explored the reactivation of long-term SAS and developed an efficient protocol to reduce the start-up period of an anammox reactor. Although stored for 13 months, a low level of the specific anammox activity of 28 mg N/g VSS/d was still detected. Experimental Phase 1 involved the direct application of SAS to an upflow sludge bed reactor (USB) operated for 90 d under varying conditions of hydraulic retention time and nitrogen concentrations. In Phase 2, batch runs were executed prior to the continuous operation of the USB reactor. The biomass reactivation in the continuous flow reactor was unsuccessful. However, the SAS was effectively reactivated through a combination of batch runs and continuous flow feed. Within 75 days, the anammox process achieved a stable rate of nitrogen removal of 1.3 g N/L/day and a high nitrogen removal efficiency of 84.1 ± 0.2%. Anammox bacteria (Ca. Brocadia) abundance was 37.8% after reactivation. These overall results indicate that SAS is a feasible seed sludge for faster start-up of high-rate mainstream anammox reactors.

Chameleon-like Anammox Bacteria for Surface Color Change after Suffering Starvation

Bacteria are often exposed to long-term starvation during transportation and storage, during which a series of enzymes and metabolic pathways are activated to ensure survival. However, why the surface color of the bacteria changes during starvation is still not well-known. In this study, we found black anammox consortia suffering from long-term starvation contained 0.86 mmol gVSS-1 cytochrome c, which had no significant discrepancy compared with the red anammox consortia (P > 0.05), indicating cytochrome c was not the key issue for chromaticity change. Conversely, we found that under starvation conditions cysteine degradation is an important metabolic pathway for the blackening of the anammox consortia for H2S production. In particular, anammox bacteria contain large amounts of iron-rich nanoparticles, cytochrome c, and other iron-sulfur clusters that are converted to produce free iron. H2S combines with free iron in bacteria to form Fe-S compounds, which eventually exist stably as FeS2, mainly in the extracellular space. Interestingly, FeS2 could be oxidized by air aeration, which makes the consortia turn red again. The unique self-protection mechanism makes the whole consortia appear black, avoiding inhibition by high concentrations of H2S and achieving Fe storage. This study expands the understanding of the metabolites of anammox bacteria as well as the bacterial survival mechanism during starvation.

Evaluation the role of soluble microbial products for denitrification sludge characteristic under starvation stress

Physiological changes with the assist role of soluble microbial products (SMP) of preserved denitrifying sludge (DS) undergoing long-term stress of starvation under different storage temperature is extremely important. In this study, SMP extracted from DS were added into DS in starvation condition under room temperature (15-20 °C), 4 °C and -20 °C with three different bio-augmentation phases of 10, 15 and 30 days. Experimental results showed that added SMP in room temperature was optimal for preservation of DS under starvation stress with optimized dosage of 2.0 mL mL^-1 sludge and bio-augmentation phase of 10 d. SMP was more effective in maintaining the specific denitrification activity of DS, and it was nearly boosted to 94.1 % of control one due to assist of 2 times SMP addition with 10 days interval of each. Under assist of SMP, extracellular polymeric substances (EPS) secretion was enhanced as the defense layer to withstand starvation stress, and the protein may be utilized as an alternative substrate to gain energy, accelerate electron transport and transfer during denitrification process. This investigation revealed the feasibility of SMP as an economical and robust strategy for preservation of DS.

Gel-immobilized partial nitritation/anammox achieves reliable nitrogen removal at different concentrations of nitrogen and reactivation processes

A two-stage partial nitritation/anammox process based on microbial encapsulation (PN/A-E) was established. The nitrogen removal characteristics of PN/A-E under high and low ammonia nitrogen and after reactivation following a long-term shutdown were comprehensively investigated and compared with anammox granular sludge (AnGS). The stable PN process did not depend on high ammonia nitrogen, and the nitrite accumulation rate reached 95.2 ± 0.7 %. The overall nitrogen removal rate of encapsulated anammox bacteria was twice that of the AnGS, and it was more tolerant to external interference. Moreover, PN/A-E showed good reactivation performance, and the total nitrogen in the effluent was 10.0 ± 1.4 mg·L^-1 when the final hydraulic retention time was 2.18 h. The immobilized fillers support an increase in ammonia-oxidizing bacteria under restricted conditions and were more conducive to the dominance of functional bacteria and the stability of microbial community under low ammonia nitrogen. This study provides a positive method to achieve a reliable PN/A.

Feeding strategy for single-stage deammonification to treat moderate-strength ammonium under low free ammonia conditions

Single-stage deammonification (SSD) processes have been successfully operated using the step-feeding strategy to treat high-strength NH₄⁺ (>300 mg/L), but often failed to treat moderate-strength NH₄⁺ (100-300 mg/L). Because it is hard to maintain the free ammonia (FA) above 1 mg/L, which is a concentration in which the activity of NO₂^- oxidizing bacteria (NOB) can be selectively suppressed. In this study, to evaluate the effectiveness of the step-feeding strategy on the long-term stability of treating moderate-strength NH₄⁺, two SSD sequential-batch reactors (SBRs) were operated under one-step feeding and multi-step feeding strategies. The one-step feeding SBR achieved a higher nitrogen removal efficiency (86 %), nitrogen removal rate (0.61 kg/m³/d), and COD removal efficiency (95 %) than the multi-step feeding SBR (73 %, 0.39 kg/m³/d, and 95 %, respectively). This means the appropriate FA to selectively suppress NOB activity was successfully maintained in the one-step feeding SBR (FA > 1 mg/L). Therefore, it the necessary to apply a step feed strategy that can be maintained above FA (1 mg/L) from the start-up of operation to treat moderate-strength NH₄⁺.

Towards mainstream partial nitritation/anammox in four seasons: Feasibility of bioaugmentation with stored summer sludge for winter anammox assistance

The strong effect of low temperatures on anammox challenges its mainstream application over the winter in temperate climates. Winter bioaugmentation with stored summer surplus sludge is a potential solution to guarantee sufficient nitrogen removal in winter. Firstly, the systems for which nitrogen removal deteriorated by the temperature decrease (25 °C → 20 °C) could be fully restored bioaugmenting with granules resp. flocs stored for 6 months at 118 resp. 220% of the initial biomass levels. Secondly, the reactivation of these stored sludges was tested in lower temperature systems (15.3 ± 0.4/10.4 ± 0.4 °C). Compared to the activity before storage, between 56% and 41% of the activity of granules was restored within one month, and 41%-32% for flocs. Additionally, 85-87% of granules and 50-53% of flocs were retained in the systems. After reactivation (15.3 ± 0.4/10.4 ± 0.4 °C), a more specialized community was formed (diversity decreased) with Candidatus Brocadia still dominant in terms of relative abundance. Capital and operating expenditures (CAPEX, OPEX) were negligible, representing only 0.19-0.36% of sewage treatment costs.