Autotrophic denitrification with sulphide in a sequencing batch reactor

Unraveling nitrogen removal performance during increasing loading rates in simultaneous nitrification and autotrophic denitrification: A functional and ecological analysis approach

Nitrogen contamination of water sources poses significant environmental and health risks. The sulfur-driven simultaneous nitrification and autotrophic denitrification (SNAD) process offers a cost-effective solution, as it operates in a single reactor, requires no organic carbon addition, and produces minimal sludge. However, this process remains underexplored, with microbial population dynamics, their interactions, and their implications for process efficiency not yet fully understood. To address this gap, this study analyzed microbial populations in a 0.8 L fluidized bed reactor performing sulfur-driven SNAD under increasing nitrogen loading rates (NLR), ranging from 11 to 105 g N/m3 d. The process achieved 93.5 % total nitrogen and 95.1 % ammonium removal at a hydraulic residence time (HRT) of 1.8 days. However, when the HRT was reduced to 0.96 days, nitrate removal instability occurred, reducing the nitrate removal efficiency to 42 %. Although increasing the HRT improved performance, two additional instability events were observed in subsequent stages at HRTs of 1.2 and 1.03 days, where nitrate removal efficiencies dropped to 11 % and 39 %, respectively. Functional analysis showed that NLR negatively impacted the proportion of sulfur-oxidizing bacteria, which was correlated with high nitrate levels in the effluent, although ammonium oxidation remained stable. Ecological network analysis revealed positive interactions between ammonia-oxidizing and heterotrophic bacteria, supporting nitrification stability. However, it also uncovered negative interactions between heterotrophic bacteria and sulfur-oxidizing denitrifiers, such as Dyella and Thiobacillus, suggesting these negative interactions contributed to temporary nitrogen removal problems in the system. This study highlights the importance of functional microbial and ecological network analyses over traditional metataxonomic approaches in understanding SNAD processes.

Advance in the sulfur-based electron donor autotrophic denitrification for nitrate nitrogen removal from wastewater

In the field of wastewater treatment, nitrate nitrogen (NO3--N) is one of the significant contaminants of concern. Sulfur autotrophic denitrification technology, which uses a variety of sulfur-based electron donors to reduce NO3--N to nitrogen (N2) through sulfur autotrophic denitrification bacteria, has emerged as a novel nitrogen removal technology to replace heterotrophic denitrification in the field of wastewater treatment due to its low cost, environmental friendliness, and high nitrogen removal efficiency. This paper reviews the advance of reduced sulfur compounds (such as elemental sulfur, sulfide, and thiosulfate) and iron sulfides (such as ferrous sulfide, pyrrhotite, and pyrite) electron donors for treating NO3--N in wastewater by sulfur autotrophic denitrification technology, including the dominant bacteria types and the sulfur autotrophic denitrification process based on various electron donors are introduced in detail, and their operating costs, nitrogen removal performance and impacts on the ecological environment are analyzed and compared. Moreover, the engineering applications of sulfur-based electron donor autotrophic denitrification technology were comprehensively summarized. According to the literature review, the focus of future industry research were discussed from several aspects as well, which would provide ideas for the application and optimization of the sulfur autotrophic denitrification process for deep and efficient removal of NO3--N in wastewater.

Comparative investigation on heterotrophic denitrification driven by different biodegradable polymers for nitrate removal in mariculture wastewater: Organic carbon release, denitrification performance, and microbial community

Heterotrophic denitrification is widely studied to purify freshwater wastewater, but its application to seawater wastewater is rarely reported. In this study, two types of agricultural wastes and two types of synthetic polymers were selected as solid carbon sources in denitrification process to explore their effects on the purification capacity of low-C/N marine recirculating aquaculture wastewater (NO3 --N 30 mg/L, salinity 32‰). The surface properties of reed straw (RS), corn cob (CC), polycaprolactone (PCL) and poly3-hydroxybutyrate-hydroxypropionate (PHBV) were evaluated by Brunauer-Emmett-Teller, Scanning electron microscope and Fourier-transform infrared spectroscopy. Short-chain fatty acids, dissolved organic carbon (DOC), and chemical oxygen demand (COD) equivalents were used to analyze the carbon release capacity. Results showed that agricultural waste had higher carbon release capacity than PCL and PHBV. The cumulative DOC and COD of agricultural waste were 0.56-12.65 and 1.15-18.75 mg/g, respectively, while those for synthetic polymers were 0.07-1.473 and 0.045-1.425 mg/g, respectively. The removal efficiency of nitrate nitrogen (NO3 --N) was CC 70.80%, PCL 53.64%, RS 42.51%, and PHBV 41.35%. Microbial community analysis showed that Proteobacteria and Firmicutes were the most abundant phyla in agricultural wastes and biodegradable natural or synthetic polymers. Quantitative real-time PCR indicated the conversion from nitrate to nitrogen was achieved in all four carbon source systems, and all six genes had the highest copy number in CC. The contents of medium nitrate reductase, nitrite reductase and nitrous oxide reductase genes in agricultural wastes were higher than those in synthetic polymers. In summary, CC is an ideal carbon source for denitrification technology to purify low C/N recirculating mariculture wastewater.

Efficient nitrite accumulation and elemental sulfur recovery in partial sulfide autotrophic denitrification system: Insights of seeding sludge, S/N ratio and flocculation strategy

Partial sulfide autotrophic denitrification (PSAD) has been proposed as a promising process to achieve elemental sulfur recovery and nitrite accumulation, which is required for anaerobic ammonium oxidation reaction. This study investigated the effect of seeding sludge on the start-up performance of PSAD process, with different sludge taken from the oxidation zone (S-o) of wastewater treatment plants, partial denitrification reactor (S-_PD), and anoxic/oxic reactor (S-_A/O). The results showed that the PSAD process could be achieved rapidly in three systems on day 22, 29 and 26, respectively. In particular, the S-_O system completed the start-up in the shortest time of 22 d, with NO₃^--N and S^2- removal efficiency of 85.3% and 99.3%, respectively. Selected the S-_O system to operate long term, the nitrite (NO₂^--N) and biological elemental sulfur (S⁰) accumulation efficiencies were systematically investigated under different S/N ratios (in a range of 0.71-1.2). The maximum NO₂^--N and S⁰ accumulation efficiencies were 85.2% and 73.5%, respectively, at the S/N ratio of 1.1. In addition, the separation and recovery of S⁰ in effluent was achieved by employing polyaluminum chloride (PAC) as a flocculant. Using 2D Gaussian function as quadratic model for the maximizing of S⁰ flocculant efficiency (SFR), an optimal condition of PAC dosage 7.92 mL/L and pH 5.14 was obtained, and the SFR reached 94.1%, under such conditions. The findings offered useful information to facilitate the application of the PSAD process.

Integration of a nitrification bioreactor and an anoxic biotrickling filter for simultaneous ammonium-rich water treatment and biogas desulfurization

A preliminary assessment has been carried out on the integration of an anoxic biotrickling filter and a nitrification bioreactor for the simultaneous treatment of ammonium-rich water and H₂S contained in a biogas stream. The nutrient consumption in the biotrickling filter was as follows (mol^-1 NO₃^--N): 6.3·10^-4 ± 1.2·10^-4 mol PO₄^3--P, 0.04 ± 0.05 mol NH₄⁺-N and 0.04 ± 0.03 mol K⁺-K. Furthermore, it was possible to supply a mixture of biogenic NO₃^- and NO₂^- into the biotrickling filter from the nitrification bioreactor to obtain a maximum elimination capacity of 152 gH₂S-S m^-3 h^-1. The equivalence between the two compounds was 1 mol NO₃^--N equal to 1.6 mol NO₂^--N. The biotrickling filter was also operated under a stepped variable inlet load (30-100 gH₂S-S m^-3 h^-1) and outlet H₂S concentrations of less than 150 ppm_V were obtained. It was also possible to maintain the outlet H₂S concentration close to 15 ppm_V with a feedback controller by manipulating the feed flow (in the nitrification bioreactor). Two stepped variable inlet loads were tested (60-111 and 16-102 gH₂S-S m^-3 h^-1) under this type of control. The implementation of feedback control could enable the exploitation of biogas in a fuel cell, since the H₂S concentrations were 15.1 ± 4.3 and 15.0 ± 3.4 ppm_V. Finally, the anoxic biotrickling filter experienced partial denitrification and this implied a loss of the desulfurization effectiveness related to SO₄^2- production.

Metagenomic analysis revealed the sulfur- and iron- oxidation capabilities of heterotrophic denitrifying sludge

Heterotrophic denitrification is widely applied in wastewater treatment processes to remove nitrate. However, the ability of the heterotrophic denitrifying sludge to use inorganic matter as electron donors to perform autotrophic denitrification has rarely been investigated. In this study, we enriched heterotrophic denitrifying sludge and demonstrated its sulfur- and iron- oxidizing abilities and denitrification performance with batch experiments. Based on high-throughput sequencing of 16S rRNA genes, high diversity and abundance of sulfur-oxidizing bacteria (SOB) (e.g., Sulfuritalea, Thiobacillus, and Thiothrix) and iron (II)-oxidizing bacteria (FeOB) (e.g., Azospira and Thiobacillus) were observed. Metagenomic sequencing and genome binning results further suggested that the SOB in the heterotrophic denitrifying sludge were mainly Alphaproteobacteria and Betaproteobacteria instead of Gammaproteobacteria and Epsilonproteobacteria. The similarities of potential iron-oxidizing genes with known sequences were very low (32-51%), indicating potentially novel FeOB species in this system. The findings of this study suggested that the heterotrophic denitrifying sludge harbors diverse mixotrophic denitrifying bacterial species, and based on this finding, we proposed that organic carbon and inorganic electron donors (e.g., sulfur, thiosulfate, and iron) could be jointly used in engineering practices according to the quality and quantity of wastewater to balance the cost and efficiency of the denitrification process.

Anoxic biogas biodesulfurization promoting elemental sulfur production in a Continuous Stirred Tank Bioreactor

Biological desulfurization of biogas has been extensively studied using biotrickling filters (BTFs). However, the accumulation of elemental sulfur (S°) on the packing material limits the use of this technology. To overcome this issue, the use of a continuous stirred tank bioreactor (CSTBR) under anoxic conditions for biogas desulfurization and S° production is proposed in the present study. The effect of the main parameters (stirring speed, N/S molar ratio, hydraulic residence time (HRT) and gas residence time (GRT)) on the bioreactor performance was studied. Under an inlet load (IL) of 100 g S-H₂S m^-3 h^-1 and a GRT of 119 s, the CSTBR optimal operating conditions were 60 rpm, N/S molar ratio of 1.1 and a HRT of 42 h, in which a removal efficiency (RE) and S° production of 98.6 ± 0.4 % and 88 % were obtained, respectively. Under a GRT of 41 s and an IL of 232 g S-H₂S m^-3 h^-1 the maximum elimination capacity (EC) of 166.0 ± 7.2 g S-H₂S m^-3 h^-1 (RE = 71.7 ± 3.1 %) was obtained. A proportional-integral feedback control strategy was successfully applied to the bioreactor operated under a stepped variable IL.

Optimizing granulation of a sulfide-based autotrophic denitrification (SOAD) sludge: Reactor configuration and mixing mode

Challenges such as long-term cultivation and sludge floatation are common in flocculent sulfide-oxidizing autotrophic denitrification (SOAD) systems. The present study aims to optimize the granulation of SOAD sludge by mainly adjusting the reactor configuration and mixing mode. Three liquid-lift upflow reactors viz. a reactor equipped with a three-phase separator (Reactor A), a modified version of Reactor A equipped with a hydraulic regulator (Reactor B), and a reactor with a mounted baffle and intermittent mechanical mixing (Reactor C). These reactors were operated for more than 160 days. The results showed that dense and compact granules with 200 μm of diameter developed within 40 days and gradually increased to approximately 400 μm in Reactor C, which had a volatile suspended solids (VSS) concentration of 7500 mg VSS/L of sludge concentration; this Reactor C was also subject to modified reactor configuration and operating conditions. In comparison, filamentous granules formed in Reactor A due to a low substrate loading and granules formed in Reactor B but with significant biomass loss caused by sludge flotation. Both of the reactors only have ≤1000 mg VSS/L VS 7500 mg VSS/L in Reactor C. Also, Reactor C having 0.77 h of hydraulic retention time (HRT) and 0.94 kgNO₃^--N/m³ d & 1.87 kgS^2--S/m³ d of nitrogen and sulfide loading rate, respectively, showed a better performance in terms of nitrate removal (89%) and sulfur conversion (above 70%) due to its enrichment by the typical autotrophic denitrifiers (39.0% of Thiobacillus, 22.4% of Sulfurimonas) in the granules. Our findings provide a method to optimize the design and operation of granulation reactors that can be extended to similar processes treating organic-deficient wastewaters.

Key internal factors leading to the variability in CO2 fixation efficiency of different sulfur-oxidizing bacteria during autotrophic cultivation

Variability in the apparent CO₂ fixation yield of four aerobic sulfur-oxidizing bacteria (Halothiobacillus neapolitanus DSM 15147, Thiobacillus thioparus DSM 505, Thiomonas intermedia DSM 18155, and Starkeya novella DSM 506) in autotrophic culturing was studied, and mutual effects of key intrinsic factors on CO₂ fixation were explored. DSM 15147 and DSM 505 exhibited much higher CO₂ fixation yields than DSM 18155 and DSM 506. The differences in CO₂ fixation yield were determined not only by cbb gene transcription, but also by cell synthesis rate, which was determined by rRNA gene copy number; the rRNA gene copy number had a more significant effect than cbb gene transcription on the apparent CO₂ fixation yield. Moreover, accumulation of EDOC was observed in all four strains during chemoautotrophic cultivation, and the proportion of EDOC accounting for total fixed organic carbon (TOC; EDOC/TOC ratio) was much higher in DSM 18155 and DSM 506 than in DSM 15147 and DSM 505. The accumulation of EDOC led to a significant decrease in the cbb gene transcription efficiency during cultivation, and a further feedback inhibitory effect on CO₂ fixation.

Feasibility test of autotrophic denitrification of industrial wastewater in sequencing batch and static granular bed reactors

In order to evaluate the efficacy of using reduced sulfur species in lieu of conventional substrates, a sequencing batch reactor (SBR) was used to develop an autotrophic denitrifying culture which in turn was used to seed a static granular bed reactor (SGBR) for continuous flow treatment. Both bioreactors were able to quickly acclimate to the anoxic environment and achieve stable autotrophic denitrification within several weeks of being placed in operation. The seed for the SBR was obtained from operating basins at the Cedar Rapids plant. MiSeq analysis showed the presence of the autotrophic denitrifier Thiobacillus in the seed from the sulfur oxidation basin; however, Shinella and Sulfurovum became the dominant autotrophic denitrifiers in the SBR. Both the SBR and SGBR achieved excellent nitrate removal (i.e., >95%) with stoichiometric amounts of thiosulfate added to the synthetic influent. The results of this feasibility study suggest that anaerobic granules from the UASB at the plant serve as good seed biomass for autotrophic denitrification when augmented by sulfur oxidation basin and sulfide scrubber biomass, and that reduced sulfur species at the plant (or augmented with an external sulfur source) can serve as electron donors for nearly complete denitrification. PRACTITIONER POINTS: Autotrophic denitrification of industrial wastewater was investigated to evaluate reduced sulfur species as electron donor for nitrogen removal. An autotrophic denitrifying culture was cultivated in an SBR, and continuous autotrophic denitrification was accomplished in an SGBR. No increase in head loss was observed in the SGBR, and it was able to operate without the need for backwashing in more than 200 days of operation. Reduced sulfur was demonstrated to be a sufficient electron donor for nearly complete denitrification. MiSeq analysis resolved primary species responsible for autotrophic denitrification in this study.

Predicting Accumulation of Intermediate Compounds in Nitrification and Autotrophic Denitrification Processes: A Chemical Approach

Nitrification and sulfur-based autotrophic denitrification processes can be used to remove ammonia from wastewater in an economical way. However, under certain operational conditions, these processes accumulate intermediate compounds, such as elemental sulphur, nitrite, and nitrous oxide, that are noxious for the environment. In order to predict the generation of these compounds, an analysis based on the Gibbs free energy of the possible reactions and on the oxidative capacity of the bulk liquid was done on case study systems. Results indicate that the Gibbs free energy is not a useful parameter to predict the generation of intermediate products in nitrification and autotrophic denitrification processes. Nevertheless, we show that the specific productions of nitrous oxide during nitrification, and of elemental sulphur and nitrite during autotrophic denitrification, are well related to the oxidative capacity of the bulk liquid.

Performance and bacterial diversity of bioreactors used for simultaneous removal of sulfide, solids and organic matter from UASB reactor effluents

Two bioreactors were investigated as an alternative to post-treatment of effluent from an upflow anaerobic sludge blanket (UASB) reactor treating domestic sewage, with an aim of oxidizing sulfide into elemental sulfur, and removal of solid and organic material. The bioreactors were operated at different hydraulic retention times (HRTs) (6, 4, and 2 h) and in the presence or absence (control) of packing material (polypropylene rings). Greater sulfide removal efficiencies - 75% (control reactor) and 92% (packed reactor) - were achieved in both reactors for an HRT of 6 h. Higher organic matter (COD) and solid (TSS) removal levels were observed in the packed reactor, which produced effluent with low COD (100 mg CODL^-1) and TSS concentrations (30 mg TSSL^-1). Denaturing gradient gel electrophoresis results revealed that a metabolically diverse bacterial community was present in both bioreactors, with sequences related to heterotrophic bacteria, sulfur bacteria (Thiocapsa, Sulfurimonas sp., Chlorobaculum sp., Chromatiales and Sulfuricellales), phototrophic purple non-sulfur bacteria (Rhodopseudomonas, Rhodocyclus sp.) and cyanobacteria. The packed reactor presented higher extracellular sulfur formation and potential for elemental sulfur recovery was seen. Higher efficiencies related to the packed reactor were attributed to the presence of packing material and higher cell retention time. The studied bioreactors seemed to be a simple and low-cost alternative for the post-treatment of anaerobic effluent.

Simultaneous nitrite and ammonium production in an autotrophic partial denitrification and ammonification of wastewaters containing thiocyanate

Various products are observed in biological oxidation and reduction of molecules containing elements of variable valence. The variability is caused by the diversity of microorganisms and their metabolic enzymes, which may develop into novel processes in wastewater treatment. The study aimed to develop a novel denitrification process forming nitrite and ammonium in wastewaters containing thiocyanate. High-efficiency nitrite and ammonium production was observed due to autotrophic partial denitrification and ammonification as a result of nitrate and thiocyanate removal. Nitrite, ammonium and sulfate were observed as the ultimate products. The increased NO₃^--N/SCN^--N ratio in the treated wastewater resulted in the decreased removal efficiency of nitrate, and the increased nitrate-to-nitrite transformation ratio and the ratio of NO₂^--N to NH₄⁺-N. Thiocyanate sulfur was oxidized to sulfate via intermediate elementary sulfur providing electron to nitrate or nitrite. The Thiobacillus genus dominated in the sludge providing ammonium and nitrite as substrate for the potentially anammox process.

Effects of salinity, C/S ratio, S/N ratio on the BESI process, and treatment of nanofiltration concentrate

A laboratory-scale biodegradation and electron transfer based on the sulfur metabolism in the integrated (BESI^®) process was used to treat a saline petrochemical nanofiltration concentrate (NFC). The integrated process consisted of activated sludge sulfate reduction (SR), and sulfide oxidation (SO) reactors, and a biofilm nitrification reactor. During the process, the total removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen, and total nitrogen (TN) were 76.2, 83.8, and 73.1%, respectively. In the SR reactor, most of the organic degradation occurred and approximately 70% COD were removed by the sulfate-reducing bacteria (SRB). In the SO reactor, both the autotrophic and heterotrophic denitrifications were observed to take place. In parallel, batch experiments were conducted to detect the effects of different C/S and S/N ratios on COD removal and denitrification efficiency. The batch experiments were also conducted to detect the effects of salinity on COD and sulfate reduction. The composition of pollutants in the wastewater was complex, and some existing organics were not degraded by the SRB. The non-SRB groups also played important roles in the reactor. Under salinity-induced stress, the metabolisms of the SRBs and non-SRB groups were both inhibited. However, 6 g/L NaCl did not have much effect on the final COD removal efficiency. In the batch experiments, the added sulfide served as the electron donor for autotrophic denitrification. The added organics provided substance for heterotrophic denitrification.

High nitrate removal by starch-stabilized Fe0 nanoparticles in aqueous solution in a controlled system

This study was conducted to investigate biodenitrification efficiency with starch-stabilized nano zero valent iron (S-nZVI) as the additional electron donor in the presence of S₂O₃ in aqueous solutions, under anaerobic conditions. The main challenge for nZVI application is their tendency to agglomeration, thereby resulting in loss of reactivity that necessitates the use of stabilizers to improve their stability. In this study, S-nZVI was synthesized by chemical reduction method with starch as a stabilizer. The synthesized nanoparticles were characterized by TEM, XRD, and FTIR. Transmission electron microscopy (TEM) image shows S-nZVI has a size in the range of 5-27.5 nanometer. Temperature and S-nZVI concentration were the important factors affecting nitrate removal. Biodenitrification increased at 35°C and 500 mg/L of S-nZVI, in these conditions, biodenitrification efficiency increased from 40.45 to 78.84%. Experimental results suggested that biodenitrification increased by decreasing initial nitrate concentration. In the bioreactor biodenitrification rate was 94.07% in the presence of S-nZVI. This study indicated that, Fe²⁺ could be used as the only electron donor or as the additional electron donor in the presence of S₂O₃ to increase denitrification efficiency.

Effects of hydraulic retention time and [Formula: see text] ratio on thiosulfate-driven autotrophic denitrification for nitrate removal from micro-polluted surface water

This study was carried out to investigate the possibility of a thiosulfate-driven autotrophic denitrification for nitrate-N removal from micro-polluted surface water. The aim was to study the effects of [Formula: see text] ratio (S/N molar ratio) and hydraulic retention time (HRT) on the autotrophic denitrification performance. Besides, utilization efficiencies of [Formula: see text] along the biofilter and the restart-up of the bioreactor were also investigated. Autotrophic denitrification using thiosulfate as an electron donor for treating micro-polluted surface water without the addition of external alkalinity proved to be feasible and the biofilter could be readied in two weeks. Average nitrate-N removal efficiencies at HRTs of 0.5, 1 and 2 h were 78.7%, 87.8% and 97.4%, respectively, and corresponding removal rates were 186.24, 103.92 and 58.56 g [Formula: see text], respectively. When water temperature was in the range of 8-12°C and HRT was 1 h, average nitrate-N removal efficiencies of 41.9%, 97.1% and 97.0%, nitrite accumulation concentrations of 1.45, 0.46 and 0.22 mg/L and thiosulfate utilization efficiencies of 100%, 98.8% and 92.1% were obtained at S/N ratios of 1.0, 1.2 and 1.5, respectively. Besides, the autotrophic denitrification rate in the filtration media layer was the highest along the biofilter at an S/N ratio of 1.5. Finally, after a one-month period of starvation, the biofilter could be restarted successfully in three weeks without inoculation of seed sludge.

Evaluation of Nitrous Oxide Emission from Sulfide- and Sulfur-Based Autotrophic Denitrification Processes

Recent studies have shown that sulfide- and sulfur-based autotrophic denitrification (AD) processes play an important role in contributing to nitrous oxide (N2O) production and emissions. However, N2O production is not recognized in the current AD models, limiting their ability to predict N2O accumulation during AD. In this work, a mathematical model is developed to describe N2O dynamics during sulfide- and sulfur-based AD processes for the first time. The model is successfully calibrated and validated using N2O data from two independent experimental systems with sulfide or sulfur as electron donors for AD. The model satisfactorily describes nitrogen reductions, sulfide/sulfur oxidation, and N2O accumulation in both systems. Modeling results revealed substantial N2O accumulation due to the relatively low N2O reduction rate during both sulfide- and sulfur-based AD processes. Application of the model to simulate long-term operations of activated sludge systems performing sulfide- and sulfur-based AD processes indicates longer sludge retention time reduced N2O emission. For sulfide-based AD process, higher initial S/N ratio also decreased N2O emission but with a higher operational cost. This model can be a useful tool to support process operation optimization for N2O mitigation during AD with sulfide or sulfur as electron donor.

Nitrite and nitrate as electron acceptors for biological sulphide oxidation

Autotrophic denitrification with sulphide using nitrate (R1) and nitrite (R2) as electron acceptor was investigated at bench scale. Different solids retention times (SRT) (5 and 20 d) have been tested in R1 while R2 was operated at SRT=13 d. The results indicated that the process allows complete sulphide removal to be achieved in all tested conditions. Tested sulphide loads were estimated from the H2S produced in a pilot-scale anaerobic digester treating vegetable tannery primary sludge; nitrogen loads originated from the nitrification of the supernatant. Average nitrogen removal efficiencies higher than 80% were observed in all the tested conditions once steady state was reached. A maximum specific nitrate removal rate equal to 0.35 g N-NO3- g VSS(-1) d(-1) was reached in R1. Due to sulphide limitation, incomplete denitrification was observed and nitrite and thiosulphate tend to accumulate especially in the presence of variable environmental conditions in both R1 and R2. Lower SRT caused higher NO2accumulated/NO3reduced ratios (0.22 and 0.24, with SRT of 5 d and 20 d, respectively) using nitrate as electron acceptor in steady-state condition. Temperature decrease caused sudden NO2accumulated/NO3reduced ratio increase in R1 and NO2- removal decrease in R2.

Influence of denitrification reactor retention time distribution (RTD) on dissolved oxygen control and nitrogen removal efficiency

Low concentrations of dissolved oxygen (DO) are usually found in biological anoxic pre-denitrification reactors, causing a reduction in nitrogen removal efficiency. Therefore, the reduction of DO in such reactors is fundamental for achieving good nutrient removal. The article shows the results of an experimental study carried out to evaluate the effect of the anoxic reactor hydrodynamic model on both residual DO concentration and nitrogen removal efficiency. In particular, two hydrodynamic models were considered: the single completely mixed reactor and a series of four reactors that resemble plug-flow behaviour. The latter prove to be more effective in oxygen consumption, allowing a lower residual DO concentration than the former. The series of reactors also achieves better specific denitrification rates and higher denitrification efficiency. Moreover, the denitrification food to microrganism (F:M) ratio (F:MDEN) demonstrates a relevant synergic action in both controlling residual DO and improving the denitrification performance.

Cross effect of temperature, pH and free ammonia on autotrophic denitrification process with sulphide as electron donor

Autotrophic denitrification is a suitable technology to simultaneously remove oxidised nitrogen compounds and reduced sulphur compounds yielding nitrogen gas, sulphur and sulphate as the main products. In this work, several batch tests were conducted to investigate the cross effect of temperature, pH and free ammonia on the autotrophic denitrification. Denitrification efficiencies above 95% were achieved at 35°C and pH 7.5-8.0 with maximum specific autotrophic denitrifying activities up to 188mgN2g(-1)VSSd(-1). Free ammonia did not show any effect on denitrification at concentrations up to 53mg NH3-NL(-1). Different sulphide concentrations were also tested with stoichiometric nitrite and nitrate concentrations. Sulphide inhibited denitrification at concentrations higher than 200mgS(2-)L(-1). A 50% inhibition was also found at nitrite concentrations above 48mg NO2(-)-NL(-1). The maximum specific activity decreased until a value of 25mgN2g(-1) VSSd(-1) at 232mg NO2(-)-NL(-1). The Haldane model was used to describe denitrification inhibition caused by nitrite. Kinetic parameters determined from the fitting of experimental data were rmax=176mgN2g(-1)VSSd(-1), Ks=10.7mg NO2(-)-NL(-1) and Ki=34.7mg NO2(-)-NL(-1). The obtained model allowed optimising an autotrophic denitrification process by avoiding situations of inhibition and thus obtaining higher denitrification efficiencies.

Phosphorus removal in a sulfur-limestone autotrophic denitrification (SLAD) biofilter

The sulfur-limestone autotrophic denitrification (SLAD) biofilter was able to remove phosphorous from wastewater during autotrophic denitrification. Parameters influencing autotrophic denitrification in the SLAD biofilter, such as hydraulic retention time (HRT), influent nitrate (NO3(-)), and influent PO4(3-) concentrations, had significant effects on P removal. P removal was well correlated with total oxidized nitrogen (TON) removed in the SLAD biofilter; the more TON removed, the more efficient P removal was achieved. When treating the synthetic wastewater containing NO3(-)-N of 30 mg L(-1) and PO4(3-)-P of 15 mg L(-1), the SLAD biofilter removed phosphorus of 45% when the HRT was 6 h, in addition with TN removal of nearly 100%. The optimal phosphorus removal in the SLAD biofilter was around 60%. For the synthetic wastewater containing a PO4(3-)-P concentration of 15 mg L(-1), the main mechanism of phosphorus removal was the formation of calcium phosphate precipitates.