Elsevier

Chemical Engineering Journal

Volume 338, 15 April 2018, Pages 680-687
Chemical Engineering Journal

Mathematical modeling on the nitrogen removal inside the membrane-aerated biofilm dominated by ammonia-oxidizing archaea (AOA): Effects of temperature, aeration pressure and COD/N ratio

https://doi.org/10.1016/j.cej.2018.01.040Get rights and content

Highlights

  • A multipopulation model was constructed for the AOA-dominating MAB by AQUASIM.

  • Effects of temperature, aeration pressure and COD/N ratio were evaluated for the TN removal performance.

  • Aeration pressure should be negatively-regulated with the varied temperature.

  • NOB could be effectively inhibited through enhancing the aerobic HB activity with ammonia oxidization little affected.

  • Hydrolysis of slowly-degradable organic substrate would provide the carbon source for anaerobic denitrification.

Abstract

One-dimensional multispecies model on membrane-aerated biofilm (MAB) containing ammonia-oxidizing archaea (AOA), nitrite-oxidizing bacteria (NOB) and heterotrophic bacteria (HB), the initial fraction of which were 55%, 15% and 30%, respectively, was successfully developed and simulated by AQUASIM 2.1 to comprehend effects of temperature, aeration pressure and influent COD/N ratio on the nitrogen removal performance for the wastewater treatment of low-leveled ammonia nitrogen (5 gN/m3). Results indicated that it’s applicable to decrease the aeration pressure for inhibiting the NOB activity and maintaining the total nitrogen (TN) removal efficiency under higher temperatures. Microbial distribution inside the MAB revealed that through moderately increasing the COD/N ratio at 293 K and the aeration pressure of 0.1 atm, the oxygen competition among AOA, NOB and aerobic HB could be better balanced with TN removal efficiency improved, demonstrating the feasibility of simultaneous nitrification and denitrification via nitrite. It’s been evaluated that with increased aeration pressures, higher TN removal efficiency could be achieved through the short-cut denitrification by improving the COD/N ratio to enhance the outperformance of aerobic HB over NOB for oxygen while substantially maintaining the AOA activity. And the denitrification process could be better performed in the biofilm adjacent to the bulk liquid by anaerobic HB utilizing soluble organics from the hydrolysis of slowly-degradable particulates to reduce the nitrite. The simulation results would be of great importance for the design, operation and optimization of AOA-dominating MAB applied in the nitrogen removal from micro-polluted wastewater.

Introduction

For the conventional biological nitrogen removal (BNR) in the biofilm, nitrite oxidizing bacteria (NOB) outcompetes heterotrophic bacteria (HB) for the nitrite, increasing the consumption of oxygen in the nitrification and carbonaceous substance in the denitrification. The nitrogen removal via the nitrite-pathway, comparatively, exhibits lower resource consumption and higher removal efficiency [1]. For mainstream during the municipal wastewater treatment, autotrophic nitrogen removal, combining the partial nitrification and anaerobic ammonia oxidization (ANAMMOX), is more effective, which excludes the supplement for carbon source and reduces 60% for oxygen demand [2], [3]. HB growth, however, is identified on the soluble microbial product (SMP) and decay released substrate from the ANAMMOX bacteria metabolism [4], [5], not even during the advanced treatment of wastewater with organic substance in low amount, impacting the systematic performance. Hence it’s practical to incorporate the HB in the wastewater treatment for enhancement of carbonaceous substance removal. Simultaneous nitrification and denitrification (SND) is feasible to be effectively implemented to achieve the high-efficient removal of nitrogenous and carbonaceous substances during the BNR. Furthermore, short-cut pathway, viz. the partial nitrification and the denitrification via nitrite, would improve the nitrogen removal by reducing the oxygen demand during the controlled nitrite oxidation and the consumption of organic matter for the subsequent denitrification process [6].

Characterized by the biofilm attached on the gas-permeable membrane, membrane-aerated biofilm (MAB) presents great technological advantages on the high-efficient removal of organic and nitrogen substances [7]. It’s been widely demonstrated that controllable nitrogen removal inside the counter-diffusional biofilm system exhibits higher total nitrogen (TN) removal efficiency and lower energy consumption than the conventional nitrification–denitrification process within the co-diffusional biofilm. Counter-diffusion of electron donors and electron acceptors inside the MAB [8] promote the formation of redox-stratified of metabolic processes [9], [10], [11]. Then the SND process is feasible within the distinct microbial niche [12], [13], particularly the short-cut pathway via nitrite through well-regulating oxygen surface loading, based on the aeration pressure in the membrane lumen [14] and the air flowrate. In essence, nitrite accumulation is involved in the aforementioned partial nitrification/nitritation process, generally realized by the physiological distinctions between ammonia and nitrite oxidizing microbes. Investigations reported that multiple operational factors, including the oxygen [15], temperature [16], pH/alkalinity [17], salinity [18] and free ammonium and nitrous acid [19], [20], [21], are extensively utilized to regulate the growth competition between the nitrifiers [22]. The denitrifying microbes are mainly categorized to the facultative anaerobes, with diversified metabolic patterns, which are insensitive under most of the above conditions. Slow-growing ammonia-oxidizing bacteria (AOB), however, are susceptible to the implemented parameter ranges, particularly inhibited by the low oxygen and ammonium availability and the temperature stress.

It’s been investigated that, under certain extreme environmental habitats, as the high temperature and salinity [23], low ammonium [24] and oxygen, ammonia-oxidizing archaea (AOA) is more active than AOB [25], [26], [27], which might depend on the lower biokinetic parameters of AOA compared with AOB, including the affinity constants for ammonium [28] and oxygen [29]. Consequently, AOA is more competitive than AOB under low ammonium [30] and oxygen [31] for the partial nitrification as indicated in the autotrophic nitrogen removal system [32], [33]. It might be feasible and effective to include AOA as an alternative to AOB-mediating BNR system [32], [33], [34]; few researches, however, have been conducted on the relevant heterotrophic biofilm system to elucidate the possible mechanism and identify regulatory measures for the practical application.

Performance dynamics could be explicitly comprehended through the microbial interactions for the complicated BNR systems. Generally, it’s time-consuming and effort-demanding [35] to conduct and stabilize the BNR system based on experimental evaluation of operational parameters. Over the years, mathematical modeling of wastewater treatment process, particularly the BNR process, has been extensively conducted and proposed as an important tool to comprehensively interpret the complex aquatic system, predict systematic performance and improve the design and optimization of high-efficient BNR process for the practical application after evaluating the impacts of different practical strategies on the emerging techniques.[35], [36], [37], [38], [39], [40], [41], [42], [43], [44] Though incompletely reflecting the actual system, the modeling results could be conducive to evaluating the experimental process and outlining the feasibility research on the critical aspects of the process [45].

For counter- and/or co-diffusional biofilms, several models are available for the autotrophic nitrogen removal via nitritation dominated by AOB and/or AOA and the subsequent ANAMMOX [34], which might be affected by the presentence of HB growing on the biomass decay products [5] or SMP and decay released substrate from the metabolism of the ANAMMOX bacteria [4]. As mentioned above, it might be practical to include the HB within the BNR process, indicating the relative-significance of necessitating SND process for the heterotrophic biological systems during the wastewater treatment irrespective of the presence of organics. Based on one-dimensional multi-population biofilm model, Matsumoto et al. [13] identified that SND via nitrite was feasible in MAB through suitably adjusting the ratios between oxygen and ammonia surface loading and between the COD and nitrogen. Landes et al. [46] illustrated the distinctions among different SND biopathways with the substrate removal characteristics, process contribution and substrate and biomass profiles inside the simulated biofilm by using one-dimensional biofilm models. Systematic performance of AOA-mediating autotrophic MAB has been explored through the mathematical modeling [34]; nonetheless few model-based investigations are available for assessing the nitrogen removal of heterotrophic MAB including AOA transforming ammonia into nitrite and, particularly, elucidating the synergistic and competitive interactions among microbes.

Overall, modeling investigations are extensively around the nitritation/partial nitrification process closely related with the changes of nitrogen forms. But nitrogen is substantially removed through the denitrification. Hence insufficient researches on the denitrification kinetics and the dynamic microbial interactions would hinder the further development and application of SND process. The well-established stoichiometric relationships and major kinetic parameters utilized for mathematical models has been widely adopted from Activated Sludge Models recommended by International Water Association (IWA) [36], mostly determined at temperature of 293 K and neutral pH. Enzyme-catalyzed reactions are intrinsically involved in the BNR process. Consequently, it’s indispensable to identify effects of temperature on the biokinetic process and physiological diversities among microbial communities, which is more valuable in the practical application [47].

This study was aimed to develop a modeling framework to systematically and quantitatively investigate the SND process and microbial interactions within the AOA (55%)-dominating MAB containing certain NOB (15%) and HB (30%) for the treatment of simulated wastewater with low-leveled influent ammonia and soluble organics under different temperatures, aeration pressures and COD/N ratios. The simulation results would preliminarily contribute to directing the AOA-mediating MAB implemented for the enhanced treatment of low strength mainstream in WWTPs and the remediation of micro-polluted surface water and supporting the process operation and optimization of potential biopathways for the proposed BNR system.

Section snippets

Model development

One-dimensional multi-population biofilm model was conducted to simulate bio-conversion processes and dynamic changes of microbial communities during the SND process inside the MAB with the aid of AQUASIM software [48]. The model described the interaction among six soluble components, i.e. degradable COD (SS), ammonia (SNH4), nitrite (SNO2), nitrate (SNO3), nitrogen (SN2) and dissolved oxygen (SO2), and five particulate components, including AOA (XAOA), NOB (XNOB), heterotrophic bacteria (XHB),

Temperature and aeration pressure effects on the nitrogen removal

Effects of temperature on the nitrogen removal of AOA-dominating biofilm in the MAB system was investigated with aeration pressure varied from 0.05 atm to 0.3 atm. According to available report [55], AOA is insensitive to the temperature range in this work, further demonstrated by the ammonia removal efficiency in Table 1. The ammonia concentration in the bulk liquid was maintained at 5.0 gN/m3 (ASL 2 gN/m2·d) to manifest the higher ammonia affinity of AOA. Hence the performance of biofilm with

Conclusion

In this work, the model-based evaluation on the SND process of MAB constituted by AOA (55%), NOB (15%),and HB (30%) for the low-leveled influent ammonia (5 gN/m3) were performed under varied temperatures, aeration pressures and COD/N ratios. Reduced aeration pressures should be applied for higher temperatures to counterbalance the oxygen consumption between the three microbes and achieve higher TN removal performance. Under 293 K and aeration pressure of 0.1 atm, the nitrogen removal was

Acknowledgement

This work was supported by the National Natural Science Foundation of China [Grant Number 51478304].

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