Performance evaluation of a novel anaerobic-anoxic sludge blanket reactor for biological nutrient removal treating municipal wastewater

Optimization of Simultaneous Nutrients and Chemical Oxygen Demand Removal from Anaerobically Digested Liquid Dairy Manure in a Two-Step Fed Sequencing Batch Reactor System Using Taguchi Method and Grey Relational Analysis

The technological development for efficient nutrient removal from liquid dairy manure is critical to a sustainable dairy industry. A nutrient removal process using a two-step fed sequencing batch reactor (SBR) system was developed in this study to achieve the applicability of simultaneous removal of phosphorus, nitrogen, and chemical oxygen demand from anaerobically digested liquid dairy manure (ADLDM). Three operating parameters, namely anaerobic time:aerobic time (min), anaerobic DO:aerobic DO (mg L-1), and hydraulic retention time (days), were systematically investigated and optimized using the Taguchi method and grey relational analysis for maximum removal efficiencies of total phosphorus (TP), ortho-phosphate (OP), ammonia-nitrogen (NH3-N), total nitrogen (TN), and chemical oxygen demand (COD) simultaneously. The results demonstrated that the optimal mean removal efficiencies of 91.21%, 92.63%, 91.82%, 88.61%, and 90.21% were achieved for TP, OP, NH3-N, TN, and COD at operating conditions, i.e., anaerobic:aerobic time of 90:90 min, anaerobic DO:aerobic DO of 0.4:2.4 mg L-1, and HRT of 3 days. Based on analysis of variance, the percentage contributions of these operating parameters towards the mean removal efficiencies of TP and COD were ranked in the order of anaerobic DO:aerobic DO > HRT > anaerobic time:aerobic time, while HRT was the most influential parameter for the mean removal efficiencies of OP, NH3-N, and TN followed by anaerobic time:aerobic time and anaerobic DO:aerobic DO. The optimal conditions obtained in this study are beneficial to the development of pilot and full-scale systems for simultaneous biological removal of phosphorus, nitrogen, and COD from ADLDM.

Comparative analysis of floc characteristics and microbial communities in anoxic and aerobic suspended growth processes

A fully anoxic suspended growth process is an appealing alternative to conventional activated sludge (AS) due to considerable aeration reduction and improved carbon processing efficiency for biological nutrient removal (BNR). With development of the hybrid membrane aerated biofilm reactor (MABR) technology, implementation of a fully anoxic suspended growth community in BNR facilities became practical. To better understand potential limitations with the elimination of aeration, we carried out microscopic examination and 16S rRNA gene-based microbial community profiling to determine how an anoxic suspended growth would differ from the conventional aerobic process in floc characteristics, microbial diversity, microbial temporal dynamics, and community assembly pattern. Fewer filamentous populations were found in the anoxic mixed liquor, suggesting easily sheared flocs. The anoxic microbial community had distinct composition and structure, but its diversity and temporal dynamics were similar to the conventional aerobic community. A variety of well-studied functional guilds were also identified in the anoxic community. The anoxic microbial community assembly was more stochastic than the conventional aerobic community, but deterministic assembly was still significant with a large core microbiome adapted to the anoxic condition. PRACTITIONER POINTS: Flocs developed under the anoxic conditions had less filamentous backbones, implying reduced flocculation capacity and easily sheared flocs. Knowledge about the ecophysiology of Thauera, Thiothrix, and Trichococcus can help achieve good properties of the anoxic flocs. A diverse microbial community sustainably adapted to the fully anoxic condition, containing a variety of filaments, denitrifiers, and PAOs. The anoxic microbial community displayed a similar degree of diversity and temporal dynamics compared to the aerobic counterpart. The anoxic community's assembly was more stochastic, so it may be less subject to changes in environmental variables.

Simply synthesized sodium alginate/zirconium hydrogel as adsorbent for phosphate adsorption from aqueous solution: Performance and mechanisms

The traditional zirconium hydrogel beads were synthesized by multi-step method, which was comparatively complex. In this study, a high phosphate removal efficient sodium alginate/zirconium (SA/Zr) hydrogel was synthesized by a simple method, with the phosphate adsorption performance and mechanism be explored. The results showed that the adsorption capacity of SA/Zr hydrogel to phosphate was greatly affected by pH. With the increase of initial pH (3-11), the adsorption capacity of SA/Zr for phosphate descended. The phosphate adsorption capacity of SA/Zr hydrogel exceeded 120 mg PO₄^3-/g at pH 2-7, while reaching the maximum adsorption capacity at pH 3 (256.79 mg PO₄^3-/g). The process of adsorption kinetics was well fitted by intraparticle diffusion model, indicating that there was chemical adsorption during the adsorption process. The Redlich-Peterson isotherm model can well accord with isotherm data. In addition, the material showed high selectivity to phosphate. Besides, combining X-ray photoelectron spectroscopy with Zeta potential results suggested that when the pH value was less than 4.19, SA/Zr hydrogel adsorbed phosphate by electrostatic attraction and hydrogen bonding while the adsorption was made mainly through ligand exchange when pH value was higher than 4.19.

Enhancement of simultaneous nitrogen and phosphorus removal using intermittent aeration mechanism

Biological nutrient removal grows into complicated scenario due to the microbial consortium shift and kinetic competition between phosphorus (P)-accumulating and nitrogen (N)-removing microorganisms. In this study, three sequential batch reactors with constant operational conditions except aeration patterns at 6 h cycle periods were tested. Intermittent aeration was applied to develop a robust nutrient removal system aimed to achieve high energy saving and removal efficiency. The results showed higher correspondence of P-uptake, polymeric substance synthesis and glycogen degradation in intermittent-aeration with longer interval periods compared to continuous-aeration. Increasing the intermittent-aeration duration from 25 to 50 min, resulted in higher process performance where the system exhibited approximately 30% higher nutrient removal. This study indicated that nutrient removal strongly depends on reaction phase configuration representing the importance of aeration pattern. The microbial community examined the variation in abundance of bacterial groups in suspended sludge, where the 50 min intermittent aeration, favored the growth of P-accumulating organisms and nitrogen removal microbial groups, indicating the complications related to nutrient removal systems. Successful intermittently aerated process with high capability of simple implementation to conventional systems by elemental retrofitting, is applicable for upgrading wastewater treatment plants. With aeration as a major operational cost, this process is a promising approach to potentially remove nutrients in high competence, in distinction to optimizing cost-efficacy of the system.

Recent progress using membrane aerated biofilm reactors for wastewater treatment

The membrane biofilm reactor (MBfR), which is based on the counter diffusion of the electron donors and acceptors into the biofilm, represents a novel technology for wastewater treatment. When process air or oxygen is supplied, the MBfR is known as the membrane aerated biofilm reactor (MABR), which has high oxygen transfer rate and efficiency, promoting microbial growth and activity within the biofilm. Over the past few decades, laboratory-scale studies have helped researchers and practitioners understand the relevance of influencing factors and biological transformations in MABRs. In recent years, pilot- to full-scale installations are increasing along with process modeling. The resulting accumulated knowledge has greatly improved understanding of the counter-diffusional biological process, with new challenges and opportunities arising. Therefore, it is crucial to provide new insights by conducting this review. This paper reviews wastewater treatment advancements using MABR technology, including design and operational considerations, microbial community ecology, and process modeling. Treatment performance of pilot- to full-scale MABRs for process intensification in existing facilities is assessed. This paper also reviews other emerging applications of MABRs, including sulfur recovery, industrial wastewater, and xenobiotics bioremediation, space-based wastewater treatment, and autotrophic nitrogen removal. In conclusion, commercial applications demonstrate that MABR technology is beneficial for pollutants (COD, N, P, xenobiotics) removal, resource recovery (e.g., sulfur), and N₂O mitigation. Further research is needed to increase packing density while retaining efficient external mass transfer, understand the microbial interactions occurring, address existing assumptions to improve process modeling and control, and optimize the operational conditions with site-specific considerations.

Selection and evaluation of water pretreatment technologies for managed aquifer recharge (MAR) with reclaimed water

Managed aquifer recharge with reclaimed water is a promising strategy for indirect potable reuse. However, residual contaminants in the treated wastewater effluent could potentially have adverse effects on human health. Hence, adequate water pretreatment is required. A multi-criteria approach was used to select and evaluate suitable water pretreatment technologies that can remove these critical contaminants in wastewater effluent for MAR identified in a previous study (Yuan et al., 2017). The treatment efficiency targets were calculated based on the concentrations and the suggested limits of critical contaminants. Treatment efficiency credits were then assigned to each treatment option for the removal of critical contaminants based on literature data. Treatment units that resulted in the highest efficiency credit scores were selected and combined into treatment train options, which were evaluated in terms of treatability, cost, and sustainability. This paper proposes an approach for the selection and evaluation of water treatment options, which will be helpful to guide the future implementation of MAR projects with reclaimed water.

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.

Model-based evaluation of a trickling filter facility upgrade to biological nutrient removal

This article presents the feasibility evaluation and preliminary design of a wastewater treatment plant upgrade supported by simulation. The existing facility was based on trickling filters, and the objective of the upgrade was to achieve nutrients removal. The proposed solution modifies the existing primary clarifier to host an anaerobic-anoxic suspended growth reactor, which is an alternative that, to our knowledge, has not been proposed or explored so far. The trickling filters would remain as aerobic reactors. In this study, the novel treatment scheme has been assessed for the first time, through model simulations. The modified treatment train was simulated, showing that the anoxic zone is able to denitrify satisfactorily achieving the required effluent nitrogen concentration. However, to promote biological phosphorus removal, an additional aerobic zone combined with a bypass of activated sludge from the anoxic zone to the first trickling filter is needed, in order to provide aerobic conditions to the phosphate accumulating organisms. Several combinations of additional aerobic volume and sludge bypass flowrate were found to successfully achieve both nitrogen and phosphorus removal, using the existing facilities without the need for new reactors neither implementing modifications that could put the trickling filters' physical integrity at risk. The novel treatment scheme could be applied in other cases with similar flowsheet in the same context.

Entrapped biomass for removal of organics and total nitrogen from anaerobic reactor effluents

Anaerobic processes have been applied to treat low-strength domestic wastewaters with significant energy saving. However, anaerobic process effluents must be further removed of residual organics and total nitrogen before discharge. Reported here are an aerobic entrapped bio-technology (EBT) system and an EBT coupled with activated sludge (EBT + AS) system being tested as a post-anaerobic treatment. Both systems have been operated under aerobic condition to provide organics and total nitrogen removal, achieving COD removal by 74-88% and TN removal by 58-65% at hydraulic retention times of 8-24 h. ΔCOD/ΔNO₃ ratios that represent the carbon usage efficiency as electron donors for denitrification were 1.82-1.93 in the EBT and 2.01-2.02 in the EBT + AS systems, with both ratios being lower (i.e. more efficient) than 6 typically required in traditional activated sludge bioreactors. Both systems demonstrate promise for polishing removal of COD and TN.

Process bioengineering applied to BTEX degradation in microaerobic treatment systems

The effect of different microaeration flow rates and dosing points, and of effluent recirculation, on microaerobic BTEX degradation in an anaerobic bioreactor was assessed. Additionally, a sensitivity and recovery analysis for this system was performed during microaeration failure simulations. Under anaerobic conditions, BTEX removal efficiencies between 55 and 82% were achieved depending on the compound, being benzene the most recalcitrant one. Microaeration (0.5-2.0 mL air min^-1) ensured high removal efficiencies (>83%) for all compounds, and the best results were obtained for the flow rate of 1.0 mL air min^-1, particularly for benzene, with a 30% increase in its removal efficiency. Effluent recirculation showed to be an important factor to improve mass transfer and, consequently, BTEX removal. Volatilization was negligible even under microaerobic conditions, suggesting that microbial activity was the main removal mechanism. Finally, after microaeration shutdown periods, the bioreactor could recover its prior performance within up to 2 days.