High pH (and not free ammonia) is responsible for Anammox inhibition in mildly alkaline solutions with excess of ammonium

Towards advanced simultaneous nitrogen removal and phosphorus recovery from digestion effluent based on anammox-hydroxyapatite (HAP) process: Focusing on a solution perspective

In this paper, the state-of-the-art information on the anammox-HAP process is summarized. The mechanism of this process is systematically expounded, the enhancement of anammox retention by HAP precipitation and the upgrade of phosphorus recovery by anammox process are clarified. However, this process still faces several challenges, especially how to deal with the ∼ 11% nitrogen residues and to purify the recovered HAP. For the first time, an anaerobic fermentation (AF) combined with partial denitrification (PD) and anammox-HAP (AF-PD-Anammox-HAP) process is proposed to overcome the challenges. By AF of the organic impurities of the anammox-HAP granular sludge, organic acid is produced to be used as carbon source for PD to remove the nitrogen residues. Simultaneously, pH of the solution drops, which promotes the dissolution of some inorganic purities such as CaCO₃. In this way, not only the inorganic impurities are removed, but the inorganic carbon is supplied for anammox bacteria.

A comprehensive root cause analysis of anammox bioreactor performance decline

The cultivation of anaerobic ammonia oxidizing bacteria (anammox) has gained enormous awareness over the last few decades. Although numerous studies focus massively on successfully growing these anammox to different enrichment environments, in reality, the failure rates are somewhat comparable to the reported success rates. This study combines a variety of measurement techniques to observe and monitor the sequence of a bioreactor performance decline following elevated influent substrate concentration. After attaining stable substrate removal throughout a nitrogen loading rate (NLR) range of 0.691 to 1.669 kg-N·m^-3·d^-1, the performance of the lab-scale anammox-sequencing batch reactor (SBR) abruptly broke down as the NLR reached 2.01 kg-N·m^-3·d^-1. The gathered information showed that the increased NLR firstly caused a significant and unfavorable change in the free ammonia (FA) and free nitrous acid (FNA) concentration in the bioreactor. A subsequent drop in N₂ production and a decline from a peak high of 0.381 to a low of 0.012 kg-N·kg-VSS^-3·d^-1 of the specific nitrogen removal rate (SNRR) led to an 82% absurd decline in microbial cellular energy production. Prior to these anammox switching to survival mode and secreting larger quantities (32% higher) of extracellular polymeric substances (EPS), the activity of syntrophic decomposers increased substantially leading to the internal production of excess CO₂ in the bioreactor and thereby diverging the bioreactor pH to lower levels. The purposes of this study are to understand the reason an anammox process shows different signals during a decline phase and to enable immediate response to performance deterioration.

Autotrophic nitrogen removal for decentralized treatment of ammonia-rich industrial textile wastewater: process assessment, stabilization and modelling

Digital textile printing (DTP) is a game-changer technology that is rapidly expanding worldwide. On the other hand, process wastewater is rich in ammoniacal and organic nitrogen, resulting in relevant issues for discharge into sewer system and treatment in centralized plants. The present research is focused on the assessment of the partial nitritation/anammox process in a single-stage granular sequencing batch reactor for on-site decentralized treatment. The technical feasibility of the process was assessed by treating wastewater from five DTP industries in a laboratory-scale reactor, in one case investigating long-term process stabilization. While experimental results indicated nitrogen removal efficiencies up to about 70%, complying with regulations on discharge in sewer system, these data were used as input for process modelling, whose successful parameter calibration was carried out. The model was applied to the simulation of two scenarios: (i) the current situation of a DTP company, in which wastewater is discharged into the sewer system and treated in a centralized plant, (ii) the modified situation in which on-site decentralized treatment for DTP wastewater is implemented. The second scenario resulted in significant improvements, including reduced energy consumption (- 15%), reduced greenhouse gases emission, elimination of external carbon source for completing denitrification at centralized WWTP and reduced sludge production (- 25%).

Recent advancements in the biological treatment of high strength ammonia wastewater

The estimated global population growth of 81 million people per year, combined with increased rates of urbanization and associated industrial processes, result in volumes of high strength ammonia wastewater that cannot be treated in a cost-effective or sustainable manner using the floc-based conventional activated sludge approach of nitrification and denitrification. Biofilm and aerobic granular sludge technologies have shown promise to significantly improve the performance of biological nitrogen removal systems treating high strength wastewater. This is partly due to enhanced biomass retention and their ability to sustain diverse microbial populations with juxtaposing growth requirements. Recent research has also demonstrated the value of hybrid systems with heterogeneous bioaggregates to mitigate biofilm and granule instability during long-term operation. In the context of high strength ammonia wastewater treatment, conventional nitrification-denitrification is hampered by high energy costs and greenhouse gas emissions. Anammox-based processes such as partial nitritation-anammox and partial denitrification-anammox represent more cost-effective and sustainable methods of removing reactive nitrogen from wastewater. There is also growing interest in the use of photosynthetic bacteria for ammonia recovery from high strength waste streams, such that nitrogen can be captured and concentrated in its reactive form and recycled into high value products. The purpose of this review is to explore recent advancements and emerging approaches related to high strength ammonia wastewater treatment.

Earthworm gut: An overlooked niche for anaerobic ammonium oxidation in agricultural soil

Soil fauna takes an active part in accelerating turnover of nutrients in terrestrial ecosystems. Anaerobic ammonium oxidation (anammox) has been widely characterized, however, whether anammox is active in earthworm gut and the effect of earthworm on anammox in soil remain unknown. In this study, the activity, abundance and community of anammox bacteria in earthworm guts and soils from microcosms were determined using a ¹⁵N-tracing technique, quantitative PCR, and anammox bacterial 16S rRNA gene amplicon sequencing. Results showed that anammox rates in guts ranged between 5.81 and 14.19 nmol N g^-1 dw gut content h^-1, which were significantly (P < 0.01) higher than that in their surrounding soils during 30 day incubation. On the contrary, abundances of hzsB genes encoding subunit B hydrazine synthase in guts were significantly (P < 0.05) lower than those in their surrounding soils. Anammox rates, denitrification N₂ production rates and hzsB genes in soils with earthworms were significantly (P < 0.05) lower than those in control soils. Anammox bacterial compositions differed significantly (P < 0.05) between gut and soil, and earthworm altered anammox bacterial communities in soils. Brocadia, Kuenenia and abundant unclassified anammox bacteria were detected in collected soils and gut contents, in which Brocadia was only detected in guts. These results suggested that microbes in earthworm gut increase, but present of earthworm reduces anammox and denitrification associated N loss by altering the anammox bacterial community compositions in soils.

Importance of denitrification driven by the relative abundances of microbial communities in coastal wetlands

Excessive nitrogen (N) loadings from human activities have led to increased eutrophication and associated water quality impacts in China's coastal wetlands. Denitrification accounts for significant reduction of inorganic N to nitrous oxide (N₂O) or dinitrogen gas (N₂), and thereby curtails harmful effects of N pollution in coastal and marine ecosystems. However, the molecular drivers and limiting steps of denitrification in coastal wetlands are not well understood. Here, we quantified the abundances of functional genes involved in N cycling and determined denitrification rates using ¹⁵N paring technique in the coastal wetland sediments of Bohai Economic Rim in eastern China. Denitrification accounting for 80.7 ± 12.6% of N removal was the dominant pathway for N removal in the coastal wetlands. In comparison, anaerobic ammonium oxidation (ANAMMOX) removed up to 36.9 ± 7.3% of inorganic N. Structural equation modeling analysis indicated that the effects of ammonium on denitrification potential were mainly mediated by the relative abundances of nosZ/nirS, nirS/(narG + napA) and amoA/nirK. Denitrification was limited by the relative strength of two steps, namely N₂O reduction to N₂ and nitrite (NO₂^-) reduction to nitric oxide (NO). Our results suggest that the relative abundances of functional genes which are more stable than sediment chemical compounds in the context of environmental changes are indictive of denitrification potential in coastal wetlands.

Approaches and processes for ammonia removal from side-streams of municipal effluent treatment plants

The main objective of this review article is to provide a comprehensive view on various conventional and emerging side-stream ammonia removal treatment options for municipal wastewater treatment plants (WWTPs). Optimization of wastewater treatment facilities from an energy and emissions stand-point necessitates consideration of the impact of the various internal side-streams. Side-streams from anaerobic sludge digesters in particular have the potential to be a significant ammonium load to the mainstream treatment process. However, the literature suggests that managing side-streams through their treatment in the mainstream process is not the most energy efficient approach, nor does it allow for practical recovery of nutrients. Furthermore, as effluent criteria become more stringent in some jurisdictions and sludge hydrolysis pre-treatment for digesters more common, an understanding of treatment options for ammonia in digester supernatant becomes more important. Given these considerations, a variety of side-stream treatment processes described in the literature are reviewed.

A feasibility study on biological nitrogen removal (BNR) via integrated thiosulfate-driven denitratation with anammox

To exploit the advantages of less electron donor consumptions in partial-denitrification (denitratation, NO₃^- → NO₂^-) as well as less sludge production in autotrophic denitrification (AD) and anammox, a novel biological nitrogen removal (BNR) process through combined anammox and thiosulfate-driven denitratation was proposed here. In this study, the ratio of S₂O₃^2--S/NO₃^--N and pH are confirmed to be two key factors affecting the thiosulfate-driven denitratation activity and nitrite accumulation. Simultaneous high denitratation activity and substantial nitrite accumulation were observed at initial S₂O₃^2--S/NO₃^--N ratio of 1.5:1 and pH of 8.0. The optimal pH for the anammox reaction is determined to be 8.0. A sequential batch reactor (SBR) and an up-flow anaerobic sludge blanket (UASB) reactor were established to proceed the anammox and the high-rate thiosulfate-driven denitratation, respectively. Under the ambient temperature of 35 °C, the total nitrogen removal efficiency and capacity are 73% and 0.35 kg N/day/m³ in the anammox-SBR. At HRT of 30 min, the NO₃^- removal efficiency could achieve above 90% with the nitrate-to-nitrite transformation ratio of 0.8, implying the great potential to apply the thiosulfate-driven denitratation & anammox system for BNR with minimal sludge production. Without the occurrence of denitritation (NO₂^- → N₂O → N₂), theoretically no N₂O could be emitted from this BNR system. This study could shed light on how to operate a high rate BNR system targeting to electron donor and energy savings as well as biowastes minimization and greenhouse gas reductions.

The role of COD/N ratio on the start-up performance and microbial mechanism of an upflow microaerobic reactor treating piggery wastewater

This study investigated the role of COD/N ratio on the start-up and performance of an upflow microaerobic sludge reactor (UMSR) treating piggery wastewater at 0.5 mgO₂/L. At high COD/N ratio (6.24 and 4.52), results showed that the competition for oxygen between ammonia-oxidizing bacteria, nitrite-oxidizing bacteria and heterotrophic bacteria limited the removal of nitrogen. Nitrogen removal efficiency was below 40% in both scenarios. Decreasing the influent COD/N ratio to 0.88 allowed achieving high removal efficiencies for COD (∼75%) and nitrogen (∼85%) due to the lower oxygen consumption for COD mineralization. Molecular biology techniques showed that nitrogen conversion at a COD/N ratio 0.88 was dominated by the anammox pathway and that Candidatus Brocadia sp. was the most important anammox bacteria in the reactor with a relative abundance of 58.5% among the anammox bacteria. Molecular techniques also showed that Nitrosomonas spp. was the major ammonia-oxidiser bacteria (relative abundance of 86.3%) and that denitrification via NO₃^- and NO₂^- also contributed to remove nitrogen from the system.

Significance of pH control in anammox process performance at low temperature

Anaerobic ammonium oxidation (anammox) is an efficient process for biological nitrogen removal from wastewater. Common use of this technology is still limited by relatively high optimal temperature. Temperature and pH influence on the anammox process was widely studied, but the significance of pH control in the anammox performance at low temperature was omitted. Moreover up to now, these two parameters were analyzed separately without looking into the composite effects. Statistical approach was used to conduct an in-depth study of the individual and interactive influence of pH and low temperature on the anammox activity. Optimal pH was observed between 7.0 and 7.5, but results indicate that there is no statistically significant interaction between pH and temperature. However, it was observed that the optimal pH range narrows along with the temperature decrease, which means that the efficiency of the anammox process at low temperatures can be improved by correction and adequate control of pH.

Influence of temperature and pH on the anammox process: A review and meta-analysis

The anammox (anaerobic ammonium oxidation) process was considered a very efficient and economic wastewater treatment technology immediately after its discovery in 1995, thus research in this field was intensified. The anammox process is characterised by a high temperature optimum and is very sensitive to both temperature and pH fluctuations. The process can also be inhibited by many factors, including by its substrates, i.e. nitrite and ammonium (or its unionised forms: free ammonia and free nitrous acid). This paper presents a comprehensive study of the most important and recent findings on the influence of two parameters that are crucial in wastewater treatment, i.e. temperature and pH. Because both parameters may influence the anammox process simultaneously, a meta-analysis was conducted of the data from the literature. Although meta-analysis is commonly used in medical research, mathematical analysis of the literature data has become an interesting and important step in the environmental sciences. This paper presents information on the influence of both temperature and pH on process efficiency and microbial composition. Additionally, the responses of different operating systems on both temperature and pH changes are described. Moreover, the role of both adaptation to changed conditions and of pH control as well as indicated areas of process operation are discussed.

Mechanisms and Control of NO2- Inhibition of Anaerobic Ammonium Oxidation (Anammox)

Nitrite (NO2-), one of the main substrates in the anaerobic ammonium oxidation (anammox) process, has the potential to inhibit anammox bacteria. The sensitivity of anammox cells with different energy status to NO2- was evaluated, and addition of nitrate (NO3-) inhibition on the basis of narK gene with the putative function of facilitating NO3-/NO2- antiporter. The results showed that the resistance of anammox bacteria to NO2- inhibition follows the order: active-cells > starved-cells > resting-cells > starved-/resting-cells. Anammmox resting cells have increasing tolerance to NO2- in the pH range from 7.0 to 7.5. Dissipating the proton gradient by using carbonyl cyanide m-chlorophenyl hydrazine (CCCP) caused severe inhibition at all pH values including pH = 7.5. Addition of NO3- enabled activity recovery of NO2--inhibited anammox bacteria regardless of whether the proton gradient was disrupted or not, supporting the hypothesis of NO3--dependent detoxification via a secondary transport system.

Modeling the pH effects on nitrogen removal in the anammox-enriched granular sludge

The aim of the study was to determine the pH effects on nitrogen removal in the anammox-enriched granular sludge. The experimental data were extracted from a 4 L completely-mixed batch reactor with the granular sludge at different initial pH values (6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5) and constant temperature T = 30 °C. Simulations were run in GPS-X 6.4 using a comprehensive mechanistic model Mantis2. Two kinetic parameters, the maximum specific growth rates of ammonia oxidizing bacteria (AOB) and anammox bacteria, were optimized at different pH scenarios. The inhibitory effects of the pH extremes on the anammox-enriched sludge were discussed in terms of the inhibition of free nitrous acid and free ammonia and metabolic mechanisms. Two different pH functions were used to examine the pH effects on the nitrogen removal kinetics. The pH optima for AOB and anammox bacteria were 7.4 and 7.6, respectively. The maximum specific growth rates of AOB and anammox bacteria at the pH optima were 0.81-0.85 d^-1 and 0.36-0.38 d^-1 (at T = 30 °C). The measured specific anammox activities (SAAs), predicted SAAs by Mantis2 and fitted SAAs by the Michaelis pH function at the pH optima were 0.895, 0.858 and 0.831 gN/(gVSS·d), respectively (VSS: volatile suspended solids).

Process stability and the recovery control associated with inhibition factors in a UASB-anammox reactor with a long-term operation

A UASB-anammox reactor was operated for 900 days to study its process stability. The negative effects of free ammonia (FA) and free nitrous acid (FNA) were investigated over three separate inhibitions and recoveries. The IC10, IC50 and IC90 (inhibitory concentration/a 10%, 50% and 90% activity loss) of FNA and FA responding to the NH4(+)-N, NO2(-)-N and TN removal efficiency were evaluated. In the 1st inhibition, the average FNA-IC10 observed was 0.67 μg L(-1) and the FA-IC10 for TN removal was 4.85 mg L(-1). In the 2nd inhibition, an FNA-IC10 of 0.44μ g L(-1) and an FA-IC10 of 3.56 were found. In the 3rd inhibition, however, both the FNA-IC10 and FA-IC10 were found to have increased, with values of 0.50 μg L(-1) and 4.42 mg L(-1), respectively. A clear control region was established for multiple inhibitions and the recoveries, which followed (pH 7.5-8.5, FA below 10mg/100mg NH4(+)-N and an FNA below 0.005 mg/100 mg NO2(-)-N) for the purpose of optimizing the operation conditions of the UASB-anammox reactor.

Long-term performance of side-stream deammonification in a continuous flow granular-activated sludge process for nitrogen removal from high ammonium wastewater

An innovative granular sludge deammonification system was incorporated into a conventional-activated sludge process. The process incorporated an internal baffle in the bioreactor for continuous separation of granular biomass from flocculent biomass, which allowed for controlling the solids retention time of flocculent sludge. The process was evaluated for ammonium removal from municipal digested sludge dewatering centrate under various operating conditions lasting over 450 days. The process successfully removed, on average, 90% of the ammonium from centrate at various ammonium loading reaching 1.4 kg/m³d at 20 hours hydraulic retention time. Controlling the retention time of the flocculent biomass and maintaining low nitrite concentration were both found to be effective for nitrite oxidizing bacteria management, resulting in a low nitrate concentration (below 50 mg/L) over a wide range of flocculent biomass concentration in the bioreactor.