Primary sludge fermentate as carbon source for mainstream partial denitrification-anammox (PdNA)

Genome-centric metagenomics reveals the effect of organic carbon source on one-stage partial denitrification-anammox in biofilm reactors

Nitrogen removal from wastewater with anammox saves energy and resources. Partial denitrification-anammox (PDA) is a promising process alternative for municipal wastewater treatment, given that the understanding about how to control the microbiome and its activity reach sufficient level. Here, two moving bed biofilm reactors were fed with either acetate or propionate to study the role of organic carbon type for microbiome composition and nitrogen turnover during development of PDA. With acetate, 87 % of the removed nitrogen was converted via anammox during stable operation at a rate of 0.52 g N/(m2·d). With propionate, the anammox contribution was considerably lower (41 %), as was the rate of nitrogen removal (0.27 g N/(m2·d)). The microbiome composition in the acetate- and propionate-fed reactors was however similar, with an enrichment of metagenome assembled genomes (MAGs) having genes for nitrate reduction (narG, napA). A large fraction of these MAGs had the potential to accumulate nitrite since they lacked genes for nitrite reduction (nirS, nirK, nrfA). Genes for acetate utilization were common among these MAGs, but the necessary genes for propionate conversion were rare, suggesting that the genetic make-up of the individual denitrifiers had major influence on the nitrogen turnover. One anammox MAG (Ca. Brocadia sapporoensis), harboring genes for organic carbon utilization, prevailed in the PDA reactors. Another three anammox MAGs (Ca. B. fulgida, Ca. B. pituitae and a potentially new species within Ca. Brocadia), lacking genes for organic carbon utilization, decreased in abundance in the reactors, indicating the importance of metabolic versatility for anammox bacteria in PDA.

Pilot-scale implementation of mainstream anammox for municipal wastewater treatment against cold temperature

Applying anammox to municipal wastewater treatment promises enormous energy and resource savings; however, seasonally cold conditions pose a considerable challenge, impeding its future applications towards non-tropical regions. In this study, we establish a pilot-scale wastewater treatment plant (50 m3/d) in northern China and implement the partial denitrification coupling anammox process on actual municipal wastewater. Despite seasonal cooling, the nitrogen removal efficiency remains high, ranging from 75.0 ± 4.6% at 27.8-20.0 °C to 70.4 ± 4.5% at 10-7.5 °C. This process exhibits remarkable low-temperature tolerance, achieving an in-situ anammox rate of 32.7 ± 4.7 g-N/(m3·d) at 10-7.5 °C and contributing up to 39.7 ± 6.7% to nitrogen removal. Further 15N stable isotope tracing and kinetic tests reveal that the partial denitrification is capable of supplying increasingly abundant NO2- to anammox with decreasing temperature, enabling robust mainstream anammox against seasonal cooling. From 27.8 °C to 7.5 °C, anammox bacteria not only survive but thrive under mainstream conditions, with absolute and relative abundances increasing by 429.1% and 343.5%, respectively. This pilot-scale study sheds fresh light on extending mainstream anammox towards non-tropical regions, taking a necessary step forward toward the sustainability goals of the wastewater treatment sector.

Molecular evidence of internal carbon-driven partial denitrification in a mainstream pilot A-B system coupled with side-stream EBPR treating municipal wastewater

Achieving mainstream short-cut nitrogen removal via nitrite has become a carbon and energy efficient way, but still remains challenging for low-strength municipal wastewaters. This study integrated sidestream enhanced biological phosphorus removal system in a pilot-scale adsorption/bio-oxidation (A-B) process (named A-B-S2EBPR system) and nitrite accumulation was successfully achieved for treating the municipal wastewater. Nitrite could accumulate to 5.5 ± 0.3 mg N/L in the intermittently aerated tanks of B-stage with the nitrite accumulation ratio (NAR) of 79.1 ± 6.5 %. The final effluent concentration and removal efficiency of total inorganic nitrogen (TIN) were 4.6 ± 1.8 mg N/L and 84.9 ± 5.6 %, respectively. In-situ process performance of nitrogen conversions, routine batch nitrification/denitrification activity tests and functional gene abundance of nitrifiers collectively suggested that the nitrite accumulation was mainly caused by partial denitrification rather than out-selection of nitrite oxidizing bacteria (NOB). Moreover, the single-cell Raman spectroscopy analysis first demonstrated that there was a specific microbial population that could utilize polyhydroxyalkanoates (PHA) as the potential internal carbon source during the partial denitrification process. The integration of S2EBPR brings unique features to the conventional A-B process, such as extended anaerobic retention time, lower oxidation-reduction potential (ORP), much higher and complex volatile fatty acids (VFAs) etc., which can largely reshape the microbial communities. The dominant genera were Acinetobacter and Comamonadaceae, which accounted for (17.8 ± 15.5)% and (6.7 ± 3.4)%, respectively, while the relative abundance of conventional nitrifiers was less than 0.2%. This study provides insights into phylogenetic and phenotypic shifts of microbial communities when incorporating S2EBPR into the sustainable A-B process to achieve mainstream short-cut nitrogen removal.

Partial denitrification in rope-type biofilm reactors: Performance, kinetics, and microflora using internal vs. external carbon sources

Stable nitrite accumulation through partial denitrification (PDN) represents an efficient pathway to support the anammox process, but limited studies explored the internal wastewater carbon sources and biofilm processes. This study assessed the viability of the PDN process, biofilm community evolution, and functional enzyme formation in rope-type biofilm media reactors using primary effluent (PE) and anaerobically pretreated wastewater carbon sources for the first time. Comparison was made with external carbon (acetate) under varied pH and biofilm thicknesses, maintaining a favourable sCOD: NO3-N ratio of 3. The wastewater's internal carbon resulted in thinner biofilms; nevertheless, modest nitrite accumulation (0.24 g/m2/d) occurred only at elevated pH. The highest nitrite accumulation (0.79 g/m2/d) was exhibited in the biofilm thickness-controlled acetate-fed reactor, featuring porous biofilms dominated by denitrifier Thauera (10.24 %) and imbalance between Nar, Nap, and Nir reductases. Using internal wastewater carbon sources offers a sustainable avenue for adopting the PDN process in full-scale application.

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

Redirecting carbon to recover VFA to facilitate biological short-cut nitrogen removal in wastewater treatment: A critical review

Wastewater treatment plants (WWTPs) are facing a great challenge to transition from energy-intensive to carbon-neutral and energy-efficient systems. Biological nutrient removal (BNR) can be severely impacted by carbon limitation, particularly for wastewater with a low carbon-to-nitrogen (C/N) ratio, which can significantly increase the operational costs. Waste activated sludge (WAS) is a valuable byproduct of WWTPs, as it contains high levels of organic matter that can be utilized to improve BNR management by recovering and reusing the fermentative volatile fatty acids (VFAs). This review provides a comprehensive examination of the recovery and reuse of VFAs in wastewater management, with a particular focus on advancing the preferable biological short-cut nitrogen removal process for carbon-insufficient municipal wastewaters. First, the method of carbon redirection for recovering VFAs was reviewed. Carbon could be captured through the two-stage A/B process or via sludge fermentation with different sludge pretreatment and process control strategies to accelerate sludge hydrolysis and inhibit methanogens to enhance VFA production. Second, VFAs can support the metabolism of autotrophic N-cycling microorganisms involved in wastewater treatment, such as AOB, NOB, anammox, and comammox bacteria. However, VFAs can also cause inhibition at high concentrations, leading to the partition of AOB and NOB; and can promote partial denitrification as an efficient carbon source for heterotrophic denitrifiers. Third, the lab- and pilot-scale engineering practices with different configurations (i.e., A²O, SBR, UASB) were summarized that have shown the feasibility of utilizing the fermentate to achieve superior nitrogen removal performance without the need for external carbon addition. Lastly, the future perspectives on leveraging the relationships between mainstream and sidestream, nitrogen and phosphorus, autotrophs and heterotrophs were given for sustainable and efficient BNR management.

Achieving robust and highly efficient nitrogen removal in a mainstream anammox reactor by introducing low concentrations of readily biodegradable organics

In this study, an anammox reactor was operated to treat low-strength (NH₄⁺ + NO₂^-, 25-35  mg/L) wastewater without (phase I) or with (phase II) readily biodegradable chemical oxygen demand (rbCOD). In phase I, although efficient nitrogen removal was achieved at the beginning, nitrate accumulated in the effluent after long-term operation (75  days), resulting in a decrease in the nitrogen removal efficiency to 30%. Microbial analysis revealed that the abundance of anammox bacteria decreased from 2.15 to 1.78%, whereas that of nitrite-oxidizing bacteria (NOB) increased from 0.14 to 0.56%. In phase II, rbCOD, in terms of acetate, was introduced into the reactor with a carbon/nitrogen ratio of 0.9. The nitrate concentration in the effluent decreased within 2 days. Advanced nitrogen removal was achieved in the following operation, with an average effluent total nitrogen of 3.4 mg/L. Despite the introduction of rbCOD, anammox pathway still dominated to the nitrogen loss. High-throughput sequencing indicated that high anammox abundance (2.48%) further supports its dominant position. The improvement in nitrogen removal was attributed to the enhanced suppression of NOB activity, simultaneous nitrate polishing through partial denitrification and anammox, and promotion of sludge granulation. Overall, the introduction of low concentrations of rbCOD is a feasible strategy for achieving robust and efficient nitrogen removal in mainstream anammox reactors.

Novel insights into aerobic duration control-based partial nitritation in source-separated blackwater treatment: Growth type, inoculation source, and comammox threat

Aerobic duration control (ADC), whereby aeration is terminated before nitrite is extremely oxidized during the nitrification process, is an effective strategy to achieve partial nitritation (PN) for blackwater. This study evaluated the effects of microbial growth type, influent ammonia-oxidizing organisms (AOO), and comammox bacteria from seeding sludge to ADC-based PN. The long-term operation of lab-scale reactors and model simulations were implemented to select the best growth type. The biofilm formed on the inner wall of the activated sludge reactor decreased the nitrite accumulation ratio (NAR) from 99.2% to 77.2%. Meanwhile, the NAR of the pure-biofilm reactor decreased from 95.9% to 47.8%. The deteriorated PN of the biofilm-related reactors was due to the extended solid retention time and increased substrate saturation constants of AOOs compared with those of nitrite-oxidizing organisms (NOO). Periodic biofilm carrier regeneration and biofilm thickness control can recover PN performance but are difficult to implement. In contrast, the optimized activated sludge reactor exhibited high (NAR >94%) and stable (>3 months) PN performance when treating real blackwater. Nitrifiers were found in blackwater, and chemically enhanced high-rate activated sludge pretreatment removed more NOOs than AOOs (41.8% vs. 24.3%) and increased the influent AOO/NOO ratio. Interestingly, the influent AOOs supported fast PN start-up in the moving-bed biofilm reactor without the initial inoculation of activated sludge. Moreover, model simulations verified that high and stable PN could also be realized in an activated sludge reactor by the continuous inoculation of influent AOOs, which is a novel PN start-up strategy. Metagenomic analyses showed that the comammox bacteria from the seeding sludge eventually disappeared owing to their intrinsic specific growth rates and free ammonia inhibition. The findings of this study will provide insightful guidelines for PN application in decentralized and semi-centralized wastewater treatment systems.

Full-scale transition from denitrification to partial denitrification-anammox (PdNA) in deep-bed filters: Operational strategies for and benefits of PdNA implementation

This study shows for the first time more than 2 years of operation of a mainstream anammox application at full-scale under temperate climate. This implementation of partial denitrification-anammox (PdNA) in deep bed filters at the HRSD York River treatment plant was demonstrated to achieve the benefits of shortcut nitrogen removal without nitrite oxidizing bacteria (NOB) out-selection. The transition from denitrification to PdNA filters required bleeding ammonium to the filters using an optimized ammonium versus NOx (AvN) control in the upstream aeration tanks and maintaining a nitrate residual in the filter effluent through feedforward/feedback control. The latter actions led to savings of 85% in methanol, 100% in alkalinity, and 35% in capacity enhancement. Up to 6 mg NH₄ ⁺ -N/L with an average of 2.2 ± 0.98 mg NH₄ ⁺ -N/L was removed through the anammox pathway, which accounted for about 15% of the overall plant nitrogen removal. Anammox enrichment was confirmed by activity testing and molecular analysis. The large excess of AnAOB capacity present in the filters (5-10 times more than normal operation) resulted in stable and reliable operation through winter conditions and showed potential for further intensification. PRACTITIONER POINTS: For the first time, long-term mainstream anammox was established full-scale through PdNA implementation in deep-bed filters. PdNA implementation required upstream aeration control optimization to provide a blend of ammonium and nitrate to the filters. Efficient anammox enrichment and retention resulted in reliable PdNA performance under different seasonal and influent conditions. PdNA implementation resulted in significant methanol and alkalinity savings and upstream capacity enhancement as ammonia removal depended less on aerobic nitrification. In the event of NOB out-selection and presence of nitrite, carbon savings in PdNA polishing filters can be enhanced via partial nitritation-anammox.

Mainstream partial denitrification-anammox in sand and expanded clay deep-bed polishing filters under practical loading rates and backwashing conditions

This study focused on evaluating the feasibility of expanded clay and sand as media types for mainstream partial denitrification-anammox (PdNA) in deep-bed single-media polishing filters under nitrogen and solids loading rates as well as backwash conditions similar to conventional denitrification filters. The surface roughness and iron content of the expanded clay were hypothesized to allow for enhanced anammox retention, nitrogen removal rates, and runtimes. However, under the tested loading rates and backwash conditions, no clear benefit of expanded clay was observed compared with conventional sand. This study showed the feasibility of PdNA in filters with both sand and expanded clay with PdN efficiencies of 76% and 77%, PdNA rates of 840 and 843 g N/m³ /d and TIN removal rates of 960 and 964 g N/m³ /d, respectively. Glycerol demands were 1.5-1.6 g COD _added per g TIN _removed , thus indicating potential carbon savings up to 75% compared with conventional denitrification. Overall, this study showed for the first time PdNA filters performing at nitrogen removal rates double that of previous PdNA studies under realistic conditions while providing insights into the media choice and backwashing conditions. Future research on expanded clay backwash conditions is needed to provide its full potential in PdNA filters. PRACTITIONER POINTS: Hydraulic and TSS loading rates similar to conventional denitrification can be applied in PdNA filters. Conventional sand can be used when retrofitting conventional denitrification filters into PdNA filters. Carbon savings up to 75% can be achieved with glycerol when retrofitting conventional filters into PdNA filters.

Partial denitrification-anammox (PdNA) application in mainstream IFAS configuration using raw fermentate as carbon source

This research examined the feasibility of raw fermentate for mainstream partial denitrification-anammox (PdNA) in a pre-anoxic integrated fixed-film activated sludge (IFAS) process. Fermentate quality sampled from a full-scale facility was highly dynamic, with 360-940 mg VFA-COD/L and VFA/soluble COD ratios ranging from 24% to 48%. This study showed that PdNA selection could be achieved even when using low quality fermentate. Nitrate residual was identified as the main factor driving the PdN efficiency, while management of nitrate conversion rates was required to maximize overall PdNA rates. AnAOB limitation was never observed in the IFAS system. Overall, this study showed PdN efficiencies up to 38% and PdNA rates up to 1.2 ± 0.7 g TIN/m² /d with further potential for improvements. As a result of both PdNA and full denitrification, this concept showed the potential to save 48-89% methanol and decrease the carbon footprint of water resource recovery facilities (WRRF) by 9-15%. PRACTITIONER POINTS: Application of PdNA with variable quality fermentate is feasible when the nitrate residual concentration is increased to enhance PdN selection. To maximize nitrogen removed through PdNA, nitrate conversion rates need enhancement through optimization of upstream aeration and PdN control setpoints. The IFAS PdNA process was never anammox limited; success depended on the degree of PdN achieved to make nitrite available. Application of PdNA with fermentate can yield 48-89% savings in methanol or other carbon compared with conventional nitrification and denitrification. Integrating PdNA upstream from polishing aeration and anoxic zones guarantees that stringent limits can be met (<5 mg N/L).

A review on the integration of mainstream P-recovery strategies with enhanced biological phosphorus removal

Phosphorus (P), an essential nutrient for all organisms, urgently needs to be recovered due to the increasing demand and scarcity of this natural resource. Recovering P from wastewater is a feasible and promising way widely studied nowadays due to the need to remove P in wastewater treatment plants (WWTPs). When enhanced biological P removal (EBPR) is implemented, an innovative option is to recover P from the supernatant streams obtained in the mainstream water line, and then combine it with liquor-crystallisation recovery processes, being the final recovered product struvite, vivianite or hydroxyapatite. The basic idea of these mainstream P-recovery strategies is to take advantage of the ability of polyphosphate accumulating organisms (PAO) to increase P concentration under anaerobic conditions when some carbon source is available. This work shows the mainstream P-recovery technologies reported so far, both in continuous and sequenced batch reactors (SBR) based configurations. The amount of extraction, as a key parameter to balance the recovery efficiency and the maintenance of the EBPR of the system, should be the first design criterion. The maximum value of P-recovery efficiency for long-term operation with an adequate extraction ratio would be around 60%. Other relevant factors (e.g. COD/P ratio of the influent, need for an additional carbon source) and operational parameters (e.g. aeration, SRT, HRT) are also reported and discussed.

Sustainable upgrading of biological municipal wastewater treatment based on anammox: From microbial understanding to engineering application

Anaerobic ammonium oxidation (anammox) has drawn increasing attention as a promising option to energy-neutral wastewater treatment. While anammox process still faces challenges in the low-strength and organics-contained municipal wastewater due to its susceptibility and the technical gaps in substrate supply. Effective strategies for extensive implementation of anammox in municipal wastewater treatment plants (WWTPs) remain poorly summarized. In view of the significance and necessity of introducing anammox into mainstream treatment, the growing understanding not only at level of microbial interactions but also on view of upgrading municipal WWTPs with anammox-based processes need to be considered urgently. In this review, the critical view and comprehensive analysis were offered from the perspective of microbial interactions within partial nitrification- and partial denitrification-based anammox processes. To minimize the microbial competition and enhance the cooperation among anammox bacteria and other functional bacteria, targeted control strategies were systematically evaluated. Based on the comprehensive overview of recent advances, the combination of flexible regulation of input organic carbon with anaerobic/oxic/anoxic process and the integration of sludge fermentation with anoxic biofilms in anaerobic/anoxic/oxic process were proposed as promising solutions to upgrade municipal WWTPs with anammox technology. Furthermore, a new perspective of coupling anammox with denitrifying dephosphatation was proposed as a promising method for in-depth nutrients removal from carbon-limit municipal wastewater in this study. This review provides the critical and comprehensive viewpoints on anammox engineering in municipal wastewater and paves the way for the anammox-based upgrading of municipal WWTPs.