Mathematical modeling of simultaneous carbon-nitrogen-sulfur removal from industrial wastewater

The mixed/mixotrophic nitrogen removal for the effective and sustainable treatment of wastewater: From treatment process to microbial mechanism

Biological nitrogen removal (BNR) is one of the most important environmental concerns in the field of wastewater treatment. The conventional BNR process based on heterotrophic nitrogen removal (HeNR) is suffering from several limitations, including external carbon source dependence, excessive sludge production, and greenhouse gas emissions. Through the mediation of autotrophic nitrogen removal (AuNR), mixed/mixotrophic nitrogen removal (MixNR) offers a viable solution to the optimization of the BNR process. Here, the recent advance and characteristics of MixNR process guided by sulfur-driven autotrophic denitrification (SDAD) and anammox are summarized in this review. Additionally, we discuss the functional microorganisms in different MixNR systems, shedding light on metabolic mechanisms and microbial interactions. The significance of MixNR for carbon reduction in the BNR process has also been noted. The knowledge gaps and the future research directions that may facilitate the practical application of the MixNR process are highlighted. Overall, the prospect of the MixNR process is attractive, and this review will provide guidance for the future implementation of MixNR process as well as deciphering the microbially metabolic mechanisms.

Modelling the effect of SMP production and external carbon addition on S-driven autotrophic denitrification

The aim of this study was to develop a mathematical model to assess the effect of soluble microbial products production and external carbon source addition on the performance of a sulfur-driven autotrophic denitrification (SdAD) process. During SdAD, the growth of autotrophic biomass (AUT) was accompanied by the proliferation of heterotrophic biomass mainly consisting of heterotrophic denitrifiers (HD) and sulfate-reducing bacteria (SRB), which are able to grow on both the SMP derived from the microbial activities and on an external carbon source. The process was supposed to occur in a sequencing batch reactor to investigate the effects of the COD injection on both heterotrophic species and to enhance the production and consumption of SMP. The mathematical model was built on mass balance considerations and consists of a system of nonlinear impulsive differential equations, which have been solved numerically. Different simulation scenarios have been investigated by varying the main operational parameters: cycle duration, day of COD injection and quantity of COD injected. For cycle durations of more than 15 days and a COD injection after the half-cycle duration, SdAD represents the prevailing process and the SRB represent the main heterotrophic family. For shorter cycle duration and COD injections earlier than the middle of the cycle, the same performance can be achieved increasing the quantity of COD added, which results in an increased activity of HD. In all the performed simulation even in the case of COD addition, AUT remain the prevailing microbial family in the reactor.

Statistical methods for assessing water quality after treatment on a sequencing batch reactor

Given that water scarcity is a big issue nowadays, wastewater treatment is necessary for the future water use and the prevention of environmental contamination. In this context, this study presents the results of the statistical analysis of ten water parameters before and after the water cleaning in a wastewater treatment plant (WWTP). The influent biodegradability is assessed by computing the BOD₅/COD ratio. New momentary and global efficiency indices are introduced for the evaluation of the pollutants' removal efficiency. Five classes of efficiencies (excellent, good, fair, marginal, and poor) are also proposed for the evaluation of WWTP's performances. The study shows that the WWTP's efficiency is good - when working with six water parameters - and marginal - when working with ten parameters. Small efficiencies of nitrates, total dissolved solids, and chlorine removal are noticed. Statistical distributions of ten water parameters in the influent and effluent are fitted and tested by the Kolmogorov-Smirnov and Chi-squared tests. They are utilized for computing the probability of exceeding the admissible limits of water parameters in the effluent, which can be used as a warning tool for improving the WWTP's performance.

Continuous sulfur biotransformation in an anaerobic-anoxic sequential batch reactor involving sulfate reduction and denitrifying sulfide oxidization

The pathways and intermediates of continuous sulfur biotransformation in an anaerobic and anoxic sequential batch reactor (AA-SBR) involving sulfate reduction (SR) and denitrifying sulfide oxidization (DSO) were investigated. In the anoxic phase, DSO occurred in two sequential steps, the oxidation of sulfide (S^2-) to elemental sulfur (S⁰) and the oxidation of S⁰ to sulfate (SO₄^2-). The oxidation rate of S^2- to S⁰ was 3.31 times faster than that of S⁰ to SO₄^2-, resulting in the accumulation of S⁰ as a desired intermediate under S^2--S/NO₃^--N ratio (molar ratio) of 0.9:1. Although, approximately 60% of generated S⁰ suspended in the effluent, about 40% of S⁰ retained in the sludge, which could be further oxidized or reduced in anoxic or anaerobic phase. In anoxic, S⁰ was subsequently oxidized to SO₄^2- under S^2--S/NO₃^--N ratio of 0.5:1. In anaerobic, S⁰ coexist with SO₄^2- (in fresh wastewater) were simultaneously reduced to S^2-, and the reduction rate of SO₄^2- to S^2- was 3.17 times faster than that of S⁰ to S^2-, resulting in a higher production of S⁰ in subsequent anoxic phase. Microbial community analysis indicated that SO₄^2-/S⁰-reducing bacteria (e.g. Desulfomicrobium and Desulfuromonas) and S^2-/S⁰-oxidizing bacteria (e.g. Paracoccus and Thermothrix) co-participated in continuous sulfur biotransformation in the AA-SBR. A conceptual model was established to describe these main processes and key intermediates. The research offers a new insight into the reaction processes optimization for S⁰ recovery and simultaneous removal of SO₄^2- and NO₃^- in an AA-SBR.

Simultaneous removal of nitric oxide and sulfur dioxide in a biofilter under micro-oxygen thermophilic conditions: Removal performance, competitive relationship and bacterial community structure

The efficiency of a biofilter to simultaneously remove nitric oxide (NO) and sulfur dioxide (SO₂) was investigated under thermophilic (48 ± 2 °C) micro-oxygen (3 vol%) conditions. After the start-up stage (Days 0-14), the stable operation period was divided into three stages. SO₂ inlet concentration remained 500 mg/m³, NO inlet concentrations were 300 mg/m³ (Days 15-40), 500 mg/m³ (Days 41-70) and 700 mg/m³ (Days 71-100). In each stable stage, the removal efficiency of NO and SO₂ exceeded 90%, the maximum removal rates of NO and SO₂ were 98.08% and 99.61%, respectively. The final products of SO₂ were mostly sulphur. Nitrate-reducing bacteria inhibited sulphate-reducing bacteria. Illumina high-throughput sequencing confirmed that the relative abundance of nitrate-reducing bacteria was positively correlated with NO removal efficiency, the relative abundance of sulphate-reducing bacteria was related to the conversion rate of sulphur.

Competitive dynamics of anaerobes during long-term biological sulfate reduction process in a UASB reactor

To reveal the long-term competitive dynamics of anaerobes in anaerobic bioreactors with sulfate reduction, a comprehensive structured mathematical model was designed for an extension of the Anaerobic Digestion Model No. 1 (ADM1). Sulfate reduction bacteria (SRB) were categorized into acetogenic-likewise SRB (ASRB) and methanogenic-likewise SRB (MSRB). Experimental data from 329 days of continuous operation of a laboratory-scale upflow anaerobic sludge bed (UASB) reactor was used for model calibration and validation. Results show that the model has a good agreement with experimental data and that three stages including the MPA dominant, stalemate and SRB dominant stages were clearly appeared throughout the whole competition period. The model was capable of predicting the long-term dynamic competition of sulfidogens and methanogens for electrons. This could explain a long-term of over 200 days needed for the SRB out-competing the MPA, and support speculation that the SRB could finally out-compete both the AcB and the MPA.

Response of the reactor performance and microbial community to a shift of ISDD process from micro-aerobic to anoxic condition

Micro-aerobic condition has proven to be effective in enhancing sulfide oxidation to elemental sulfur (S⁰) during integrated simultaneous desulfurization and denitrification process (ISDD). In this study we investigated and compared the performance and microbial community of ISDD process operating under initially anoxic, then micro-aerobic and finally switch back to anoxic condition. For all the three tested scenarios, comparable bioreactor performance in terms of sulfate (95.0 ± 4.4%, 90.6 ± 3.8%, 89.8 ± 3.5%) and nitrate (∼100%) removal was achieved. However, a shift of ISDD bioreactor from micro-aerobic to anoxic environment clearly increased the S⁰ production (30.6%), relative to that at initial anoxic condition (14.2%). Further anoxic bioreactor operation with different influent nitrate concentrations also obtained satisfactory performance particularly in terms of S⁰ production. Microbial community analysis results showed that functional microorganisms selectively enriched at micro-aerobic condition, particularly sulfide-oxidizing bacteria (SOB), could also function well and enhance S⁰ production when bioreactor switching from micro-aerobic to anoxic environment. We proposed that micro-aerobic strategy could function as a bio-selector and provide a new idea in functional microorganisms selectively enrichment for wastewater treatment.

Simultaneous autotrophic removal of sulphate and nitrate at different voltages in a bioelectrochemical reactor (BER): Evaluation of degradation efficiency and characterization of microbial communities

The autotrophic removal of sulphate and nitrate in bioelectrochemical reactors was investigated at different external voltages (0.2, 0.4, 0.6, 0.8 and 1.0 V) under anaerobic conditions. Sulphate and nitrate removal, nitrite accumulation, reduction trend of nitrate and sulphate and microbial community structure were explored. Results indicate the highest removal efficiencies of nitrate and sulphate at 43.3 ± 2.8 and 7.1 ± 0.2 mg·l^-1·d^-1 when the voltage is 0.6 V. Moreover, nitrite accumulation decreases with increased voltage from 0.2 V to 1.0 V. Illumina high-throughput sequencing results show similar richness and diversity of bacterial species with increased voltage from 0.2 V to 0.8 V. However, with further increased voltage to 1.0 V, bacterial diversity and richness decrease significantly. Overall, significant differences in community compositions are observed at different voltages.