Advanced treatment of biologically pretreated coal gasification wastewater using a novel expansive flow biological intermittent aerated filter process with a ceramic filler from reused coal fly ash

Abstract

A novel expansive flow biological intermittent aerated filter (BIAF) process was employed for the advanced treatment of real biologically pretreated coal gasification wastewater (CGW) which had poor biodegradability and a low carbon/nitrogen ratio. The results indicated that the expansive flow BIAF with reused coal fly ash and clay as a ceramic filler exhibited efficient performance for pollutant removal. The effluent concentrations of COD, total phenols, NH₄⁺-N, and total nitrogen (TN), were 39.5, 4.8, 6.3, and 18.8 mg L⁻¹ respectively, with a turbidity of 3 NTU and chromaticity of 26 degrees at the optimal hydraulic retention time of 9 h, meeting the standards for the reuse of water. Moreover, most toxic and refractory pollutants were also eliminated. Meanwhile, intermittent aeration not only significantly increased nitrate and nitrite reductases, but also successfully improved simultaneous nitrification and denitrification activity which facilitated TN removal. The results of high-throughput sequencing represented that the biological process included 28 major bacteria which were affiliated to 7 phyla, where Thermomonas, Methylococcus and Comamona were the most important functional genera. After stable operation for 192 h, the optimal backwashing time was 6 min when the air and water intensities were 8 and 2.5 L (s m²)⁻¹, respectively. These results demonstrated that the expansive flow BIAF process, with the advantages of efficient, economical and sustainable development, was beneficial to engineering applications.

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1. Introduction

Biologically pretreated coal gasification wastewater (CGW) could cause serious deterioration of the environment due to it containing a large number of various toxic and refractory compounds including phenolic compounds, long-chain hydrocarbons, nitrogenous heterocyclic compounds and ammonia.^1,2 What’s worse, this wastewater exhibits poor biodegradability (BOD₅/COD of 0.13), a low carbon/nitrogen ratio (COD/TN of 2.8) and toxicity to the microbes in the activated sludge process; the resulting water quality struggles to meet the standards for the reuse of water using conventional biological treatment methods.³ Wastewater control has become a bottleneck for the development of the coal gasification industry, which has played a key role in the new clean and renewable energy market in recent years.⁴ Thus, in order to meet the standards for the reuse of water (COD < 60 mg L⁻¹, NH₄⁺-N < 8 mg L⁻¹ and total phenols < 5 mg L⁻¹), some innovative processes should be developed.⁵

Biological aerated filters (BAFs) integrate the functions of physical filtration and biodegradation for organic carbon and nitrogen removal and have attracted great attention due to their high quality effluent, rapid start up, small size and low cost.⁶ Recently, expansive flow BAFs in a novel modified process show better removal efficiency and hydraulic conditions both in treatment and backwashing.⁷ Meanwhile, intermittent aeration is apparently the most practical strategy to shorten the duration of aeration, resulting in high TN removal.^8,9 Hence, there is a great potential advantage of integrating the expansive flow BAFs with intermittent aeration to attain a more efficient and cost-effective process, which would remarkably improve the performance of nitrogen removal and reduce consumption.¹⁰ However, to the best of our knowledge, no reports about using this novel process for the advanced treatment of real biologically pretreated CGW have been published. Meanwhile, coal fly ash (CFA) is a by-product of the process of burning coal, the annual production of which is huge and continuing to grow, posing many environmental problems from cases of inappropriate disposal. Moreover, the application of CFA as an attractive material to be used in adsorption and as a ceramic additive has drawn much attention.¹¹ For the purpose of treating waste with waste, recycling CFA and clay for a new application as a prepared ceramic filler can reduce the amount of solid waste produced and provide additional economic benefits, in an environmentally friendly and sustainable development method.

In this study, reused CFA and clay were used as raw materials for ceramic filler preparation, and the expansive flow biological intermittent aerated filter process (expansive flow BIAF process) with the prepared ceramic filler was adopted to evaluate its effectiveness and feasibility for the advanced treatment of real biologically pretreated CGW. The role of the hydraulic retention time (HRT) on the performance of the biological process was investigated, and the main toxic and refractory compounds were analyzed using GC-MS. Meanwhile, the effect of intermittent aeration on the enzymatic activity involved in nitrogen removal was evaluated, and high-throughput sequencing was used to reveal the main components of the microbial community structure. The novel process was investigated the feasibility of further engineering application.

2. Experimental

2.1 Materials

The ceramic granules used in the biofilter consisted of 30% of CFA, 65% of clay and 5% of sodium silicate as an adhesive. Firstly, CFA and clay were stirred in a dry powder stirrer (B10-20B, China) for about 30 min and the mixture was poured into a pelletizer (DZ-20, China) to produce pellets (with an added 20 wt% of tap water) with diameters of 5–6 mm. Then, the samples were diverted to the front of a rotary kiln to finish desiccation at a temperature range of 100 to 400 °C for 3 h and fired at a high temperature of 1000 °C for 20 min under anoxic conditions. Finally, the prepared ceramic filler was washed with 3.0 mol L⁻¹ HCl then thoroughly washed with Milli-Q water until the pH of the rinsed water remained constant, and was then dried at 50 °C in an oven until use. The expansive flow BIAF was a circular truncated cone shaped Plexiglas reactor (Fig. 1), with an effective volume of 16 L (a bottom diameter of 80 mm, a top diameter of 240 mm, a height of 120 cm and a media depth of 100 cm). The biological reactor was packed with the prepared ceramic filler which had an average diameter of 0.5 cm, the density of the filter element was 1.46 g cm⁻³, with a bulk density of 0.84 g cm⁻³, a porosity of 42.5% and a specific surface area of 10.5 m² g⁻¹ (Table 1). Air was supplied to the BIAF with an optimal air–liquid ratio of 10 through diffusers at the bottom of the column.¹² CFA was supplied by Donghua thermal power plant in Baotou city, China. Seed sludge was obtained from a full-scale CGW facility in Harbin, China, which had been operating for over 3 years. The seed sludge was 3 g MLSS L⁻¹, with good settling characteristics and a SVI of 80. The real biologically pretreated CGW used in the experiment was collected from the effluent of a secondary settling tank of the aforementioned wastewater treatment plant which had been treated with a series of biological treatments (mainly in an upflow anaerobic sludge bed reactor and a A²/O process) after ammonia stripping and phenol solvent extraction processes. The characteristics of the wastewater were as follows (in mg L⁻¹): the COD = 112 ± 15, the BOD₅ = 15 ± 3, total phenols (TPh) = 35 ± 4, NH₄⁺-N = 28 ± 3, total nitrogen (TN) = 40 ± 4, turbidity = 185 ± 12 NTU, chromaticity = 314 ± 24 degree and pH = 7.5 ± 0.5.

Fig. 1 Schematic diagram of expansive flow BIAF.

Table 1 Summary of physicochemical properties of the prepared ceramic filler

Parameters	Clay	CFA	Ceramic filler
SiO2 (wt%)	55.59	30.57	42.49
Al2O3 (wt%)	17.29	22.25	16.86
Fe2O3 (wt%)	5.28	8.45	5.45
CaO (wt%)	2.51	10.56	4.51
MgO (wt%)	2.55	2.85	2.38
ZnO (wt%)	1.51	1.56	1.26
K2O (wt%)	0.45	0.85	0.52
Average diameter (mm)	—	—	0.5
Apparent density (g cm^−3)	—	—	1.46
Bulk density (g cm^−3)	—	—	0.84
Porosity (%)	—	—	42.5
Specific surface area (m^2 g^−1)		1.6	10.5

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2.2 Experimental procedures

Firstly, there was a 30 day batch cultivation process before experimental operation began. Then, the system was operated at 15–20 °C and the HRT was set to 6, 9 and 12 h by changing the influent flow rate using peristaltic pumps (Longer pump, China) with continuous operation for 90 days. Every operational process was divided into 2 phases using a time controller, i.e. I) aerobic, 4 h, anoxic, 2 h; II) aerobic, 6 h, anoxic, 3 h; III) aerobic, 8 h, anoxic, 4 h. The concentrations of the COD, TPh, NH₄⁺-N and TN were analyzed every two days. The BOD₅, turbidity and chromaticity were analyzed every five days. Moreover, attached growth activated sludge samples were collected for the analysis of enzyme activity and the microbial community on days 30–60. Ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO) were the key enzymes responsible for ammonia oxidation that catalyzed the oxidation of NH₄⁺-N to nitrite. The enzymatic activity was calculated by the relative oxygen uptake rate (OUR) involved in reduced nitrogen oxidation with AMO and HAO, as described explicitly in eqn (1). Each batch of respirometric tests was accompanied by a positive control (e.g., untreated nitrifying biomass only).¹³

Simultaneous nitrification and denitrification (SND) activity was defined as the loss of nitrogen (eqn (2)).¹⁴

where NO_x⁻ accumulated was the amount of NO₂⁻ and NO₃⁻ accumulated.

2.3 Analysis methods

During the experiment, samples were taken from the expansive flow BIAF and immediately analyzed after being filtered through a cellulose acetate membrane with a thickness of 0.45 μm. COD, TPh, NH₄⁺-N, NO₂⁻-N, NO₃⁻-N, TN, turbidity and chromaticity were measured in accordance with standard methods.¹⁵ Dissolved oxygen (DO) and pH values were determined using a DO meter (HACH 30d) and a pH meter (pHS-3C, Leici, China). The specific surface area was obtained according to the Brunauer–Emmet–Teller (BET) method from the nitrogen adsorption at liquid nitrogen temperature using a Micromeritics ASAP 2020. The percentage content of major elements was determined using X-ray fluorescence (XRF) with a X-ray spectrometer (AXIOS-PW4400, Holland). Heavy metal leaching tests of the ceramic filler at different pH values were conducted under the following conditions: a liquid/solid ratio of 14 : 1, a temperature of 30 °C and mechanical agitation for 48 h (150 rpm). The leached concentrations of metal irons were analyzed using ICP-AES (Optima 5300DU, Perkin Elmer Inc). The morphologies were examined using a Hitachi S-3400 scanning electron microscope. The organic compositions of the wastewater were analyzed using GC-MS according to the previous literature.¹⁶ The acute toxicity of treated wastewater was assessed by the Daphnia magna test following the National Standards of China (Water Quality-Determination of the Acute Toxicity of Substance to Daphnia, GB/T13266-91). Nitrate reductase (NR) and nitrite reductase (NIR) activities of the biofilm were measured according to the method published by Li et al.¹⁷ Genomic DNA was extracted by a previous study’s method, and the concentration was determined using a NanoDrop (Nano Drop Technologies Inc., Wilmington, DE, USA).¹⁸ Primers used in this study were the same with those in the previous report,⁵ and the PCR products were determined by pyrosequencing using a Miseq Illumina by Sangon Biotech (Shanghai, China) Co., Ltd. to obtain the effective sequencing data. All of the experiments were repeated three times, and the results were the average of at least three measurements with an accuracy of ±5%.

3. Results and discussion

3.1 Characteristics of the prepared ceramic filler

The SEM micrograph of the prepared ceramic filler showed the high specific surface area (Table 1) and many pores randomly distributed in the surface (Fig. S1†) which were formed from gas escaping and organic loss during the firing process, further providing space for the growth of microorganisms and enhancing water mass transfer. Meanwhile, the ceramic filler was primarily made up of Al₂O₃, SiO₂, Fe₂O₃, and so on which was consistent with the raw material components (Table 1). As shown in Table S1,† when increasing the pH from 2 to 10, there were no marked differences in the leaching of major metals of the ceramic filler throughout 48 h of mechanical agitation. The leached concentrations of main heavy metals were all lower than the national standards for the Emission Standard of Pollutants for Petroleum Chemistry Industry (GB31571-2015) which is the latest national authority standard for the petroleum chemistry industry and most suitable for CGW. This finding was probably because the heavy metals of CFA formed more stable metal oxidation states with high temperature calcination and were immobilized in the ceramic filler matrix. Moreover, the partial inorganic fractions were effectively removed by HCl washing which further limited the mobility of heavy metals. Thus, the prepared ceramic filler was without toxicity and secondary pollution to the water environment when applied as a filler for BAFs.

3.2 The pollutant removal performance of the expansive flow BIAF process

The concentrations of COD and TPh in the influent and effluent at different HRTs are illustrated in Fig. 2. The effluent concentration of COD was 48.7 mg L⁻¹ on average with a removal efficiency of 56.3% when the HRT was 6 h. When the HRT increased to 9 and 12 h, the average removal efficiencies were 64.7 and 66.7%, and the corresponding concentrations were 39.5 and 37.2 mg L⁻¹ respectively. The removal tendency of TPh was close to that of the COD, and the effluent concentrations were 5.6, 4.8 and 3.4 mg L⁻¹ respectively, with an average removal efficiency of 84, 86.3 and 90.3%. The results indicated that the longer HRT of 12 h had a stabilized efficiency similar to that of 9 h, since the COD removal did not significantly improve. It was detected that the effluent BOD₅ value was only about 4 mg L⁻¹ at a HRT of 9 h (Table 2), thus considering the complex composition of the refractory pollutants in biologically pretreated CGW, the COD could be degraded to a higher level, difficult to achieve, only by extending the HRT. Meanwhile, the concentration of NH₄⁺-N was 5.3 mg L⁻¹ in the effluent, with a removal efficiency of 81.1% when the HRT was 6 h (Fig. 3). The removal efficiency dropped to 77.5 and 68.6%, respectively at HRTs of 9 and 12 h, the corresponding effluent concentrations were 6.3 and 8.9 mg L⁻¹. TN removal was highly correlated with NH₄⁺-N removal efficiency, and the corresponding effluent concentrations were 21.6, 18.8 and 23.6 mg L⁻¹ at HRTs of 6, 9 and 12 h, respectively. Obviously, the removal efficiency of NH₄⁺-N and TN declined with the increase in HRT. Although intermittent aeration simultaneously achieved a high COD and NH₄⁺-N and TN removal, longer HRTs further declined the COD/TN ratio and increased oxygen quenching which could cause the inhibition of denitrification bacteria and insufficient nitrogen removal.^19,20 Meanwhile, the concentrations of the attached growth biomass increased from 1.4 to 1.8 g L⁻¹ when the HRT increased from 6 to 9 h, and then slightly decreased to 1.6 g L⁻¹ with a HRT of 12 h. The results indicated that the shorter HRT of 6 h sped up the detachment of biofilm from the carriers and caused insufficient contact time for denitrification which would induce deterioration of the effluent quality. Thus, a HRT of 9 h was found to be optimal for simultaneous carbon and nitrogen removal.

Fig. 2 The performance of the expansive flow BIAF process for the removal of pollutants with variation of the HRT, (a) COD removal, (b) TPh removal, (c) NH₄⁺-N removal, and (d) TN removal.

Table 2 The performance of the expansive flow BIAF for the removal of pollutants (days 30–60)

Parameters	Influent	Effluent	Removal efficiency (%)
a Values represent the average values ± standard error from the steady operation stage.
Turbidity/NTU	185 ± 12^a	3 ± 0.2	98.4
Chromaticity/degree	314 ± 24	26 ± 3	91.7
COD (mg L^−1)	112 ± 15	39.5 ± 4.5	64.7
TPh (mg L^−1)	35 ± 4	4.8 ± 0.5	86.3
NH4^+-N (mg L^−1)	28 ± 3	6.3 ± 0.5	77.5
TN (mg L^−1)	40 ± 4	18.8 ± 2.1	53.5
BOD5 (mg L^−1)	15.5 ± 3	4.4 ± 0.5	70.7
Acute toxicity (%)	66.7 ± 5	30 ± 2	55.0

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Fig. 3 Removal of toxic and refractory compounds (days 30–60). (I) Phenolic compounds; (II) long-chain hydrocarbons; (III) aromatic hydrocarbons; (IV) olefin hydrocarbons; (V) alcohols; (VI) nitrogenous heterocyclic compounds; (VII) polycyclic aromatic hydrocarbons; (VIII) others.

Notably, the acute toxicity inhibition of biologically pretreated CGW remarkably declined by 55.0% (Table 2) which suggested that highly toxic pollutants were degraded into less toxic or even non-toxic substances using the expansive flow BIAF, and the removal of the toxic and refractory compounds was analyzed in Fig. 3. The major toxic and refractory compounds of biologically pretreated CGW were phenols and their derivatives, nitrogenous heterocyclic compounds (NHCs) and long-chain hydrocarbons, which were the most difficult to break down using microorganisms. The relative percentage of total peak area of the phenolic compounds reached about 40%, however, a dramatic decrement was obtained using the biological process, giving a value of 1%, which indicated that the expansive flow BIAF played an important role in the removal of phenolic compounds. In addition, NHCs were remarkably reduced, which was consistent with TN removal, suggesting that simultaneous nitrification and denitrification might be happening with intermittent aeration. The chromaticity might be associated with the NHCs such as indole, quinoline, pyridine and so on, which have moieties such as –NH=, –CO–NH, etc. which can carry a chromophore, and could hardly be detected in the effluent.²¹ Finally, the main organic compounds of the treated effluent were long-chain hydrocarbons, and due to their stable hydrocarbon bonds, they were difficult to remove using the biological process.

In summary, at the optimal HRT of 9 h, the removal efficiencies of the COD, TPh, NH₄⁺-N, TN, turbidity and chromaticity were 64.7, 86.3, 77.5, 53.5, 98.4 and 91.7% after the expansive flow BIAF process, with corresponding effluent concentrations of 39.5, 4.8, 6.3, and 18.8 mg L⁻¹, respectively and with a turbidity of 3 NTU and chromaticity of 26 degrees (Table 2), which all met the standards for the reuse of water and the national standards for the Emission Standard of Pollutants for Petroleum Chemistry Industry. Meanwhile, most of the toxic and refractory pollutants were also eliminated. These results showed that the expansive flow BIAF demonstrated efficient and stable performance for the removal of pollutants when applied to the advanced treatment of real biologically pretreated CGW, especially for nitrogen removal.

3.3 The mechanism of the expansive flow BIAF process for treating biologically pretreated CGW

3.3.1 Analysis of enzymes and SND activities. In order to further analyze the role of intermittent aeration, the continuous aeration expansive flow BAF was investigated for another 30 days at a HRT of 9 h (Fig. S2†), and the COD, TPh and NH₄⁺-N removal efficiencies showed no obvious variations, however, the TN removal significantly declined by 45.2%, indicating a strong inhibitory effect of DO on denitrification. As shown in Fig. S3,† the DO concentrations could be as high as 5.2 (upper) and 4.5 mg L⁻¹ (bottom) on average during the aeration periods and can maintain complete nitrification in biological treatment. When aeration was turned off, the DO concentrations were reduced to 1.2 (upper) and 0.7 mg L⁻¹ (bottom) on average which was beneficial for distributing DO well and for creating an alternate anaerobic and aerobic microenvironment. Fig. 4a shows that simultaneous nitrification and denitrification activity (SND) with intermittent aeration was 4.6 times that of continuous aeration and could achieve efficient TN removal at low DO concentrations of 0.5–1.5 mg L⁻¹.²² However, there was no significant difference in AMO and HAO activity between intermittent and continuous aeration. These were key enzymes for catalyzing the oxidation of NH₄⁺-N to nitrite, so this activity may be ascribed to sufficient DO concentrations and quantities of NH₄⁺-N. Meanwhile, NR and NIR activities were significantly increased by 89.1 and 93.7% with intermittent aeration which indicated anaerobic nitrate reduction with denitrification (Fig. 4b), and this was in accordance with SND and TN removal.¹⁷ Thus, intermittent aeration can strengthen the denitrifying enzymatic activity, simultaneously enhancing the performance of the biofilm in the biodegradation of organic carbon and nitrogen.

Fig. 4 Activities of enzymes and SND using the intermittent aeration (IA) and continuous aeration (CA) expansive flow BAF process. Error bars represent standard deviation of triplicate tests (AMO, ammonia monooxygenase; HAO, hydroxylamine oxidoreductase; SND, simultaneous nitrification and denitrification activity; NRA, nitrate reductase activity; NIRA, nitrite reductase activity).

3.3.2 Analysis of the microbial community. It is known that environmental conditions have a great impact on the microbe community structure, and the relative abundances of the dominant phyla and genera identified in the expansive flow BIAF are shown in Table 3. The results of the high throughput sequencing revealed the biological process included 28 major bacteria which were affiliated to 7 phyla. Proteobacteria (35.62%) were the most abundant phylum, and the dominant subgroups were Chlamydiae (3.34%), followed by Firmicutes (2.71%). Proteobacteria were the major group of bacteria which were able to degrade a broad spectrum of organic pollutants and enable biological nitrogen removal. Meanwhile, Firmicutes were dominant in the treatment of municipal solid waste under anaerobic and microaerobic conditions.²³ Particularly, the core genus Thermomonas was the typical denitrifying bacteria under the anoxic conditions, and the relative abundance reached 5.57% which was related to reduction of TN. The core genus Methylococcus, with a 3.28% relative abundance, indicated that a micro anaerobic environment had been formed in the biofilm on the carriers.¹⁸ The core genus Comamonas (1.52%) was the versatile aromatic degrader for phenolics, polycyclic aromatic hydrocarbons and heterocyclic aromatics, such as indole, quinoline and carbazole.²⁴ Thus, efficient performance of the core genera in the expansive flow BIAF should facilitate the removal of pollutants, especially nitrogen removal.

Table 3 Abundance of the major microbial phyla and core genera (days 30–60)

Phylum	Genus	Abundance (%)	Phylum	Genus	Abundance (%)
Proteobacteria	Blastopirellula	6.55	Proteobacteria	Denitratisoma	0.65
Proteobacteria	Thermomonas	5.77	Deinococcus-Thermus	Truepera	0.62
Proteobacteria	Roseomonas	5.56	Proteobacteria	Caldimonas	0.55
Proteobacteria	Planctomyces	3.38	Proteobacteria	Byssovorax	0.44
Proteobacteria	Methylococcus	3.28	Proteobacteria	Oligotropha	0.35
Firmicutes	Pasteuria	2.47	Acidobacteria	Gp3	0.33
Proteobacteria	Isosphaera	2.35	Proteobacteria	Parvularcula	0.31
Chlamydiae	Parachlamydia	2.18	Proteobacteria	Oceanibaculum	0.29
Planctomycetes	Rhodobacter	1.89	Acidobacteria	Gp16	0.28
Proteobacteria	Comamonas	1.52	Proteobacteria	Diaphorobacter	0.26
Proteobacteria	Singulisphaera	1.37	Firmicutes	Saccharofermentans	0.24
Chlamydiae	Neochlamydia	1.16	Nitrospira	Nitrospira	0.15
Gemmatimonadetes	Gemmatimonas	1.05	Proteobacteria	Hyphomicrobium	0.11
Proteobacteria	Phycisphaera	0.76		Unclassified	22.92

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3.4 Backwashing

Backwashing was conducted after the expansive flow BIAF had been in continuous operation for 192 h. Pulse backwashing was used with combination of pulse air and continuous water backwashing, where the air and water intensities were 8 and 2.5 L (s m²)⁻¹, respectively. The suspended solids in the backwashing liquor rose faster in the first 8 min than that in the later 2 min (Fig. S4†). An appropriate backwashing was not only to purify the filter layer, but also to regenerate the biofilm, guaranteeing the activity of biofilm. The recovery time was about 5 h when the backwashing time was 6, 7 and 8 min (Fig. S5†). The recovery time rose to 10 and 15 h respectively when the backwashing time was prolonged to 9 and 10 min. Thus, the appropriate backwashing time was 6 minutes, with air and water intensities of 8 and of 2.5 L (s m²)⁻¹ in a pulsed manner. These results indicated the expansive flow BIAF was able to remove pollutants with an efficient and stable performance for the advanced treatment of real biologically pretreated CGW with low cost and sustainable advantages, facilitating engineering applications.

4. Conclusions

The preparation and application of a ceramic filler from reused CFA and clay in an expansive flow BIAF was investigated for the advanced treatment of biologically pretreated CGW. The prepared ceramic filler had a high specific surface area and porous structure without toxicity to microorganisms, suitable for biofilters. Results indicated that the HRT significantly affected the pollutant removal performance, and the treated effluent concentrations of COD, TPh, NH₄⁺-N, and TN, and the turbidity and chromaticity were 39.5, 4.8, 6.3, and 18.8 mg L⁻¹, and 3 NTU and 26 degrees, respectively at the optimal HRT of 9 h, all of which met the standards for the reuse of water. In addition, most toxic and refractory compounds were eliminated after the expansive flow BIAF process. Meanwhile, intermittent aeration not only significantly increased NR and NIR, but also successfully improved SND which facilitated TN removal. The results of high-throughput sequencing revealed the biological process included 28 major bacteria which were affiliated to 7 phyla. The Thermomonas, Methylococcus and Comamona were the most important functional genera. After stable operation for 192 h, the optimal backwashing time was 6 min when the air and water intensities were 8 and 2.5 L (s m²)⁻¹. Thus, the novel process demonstrated economical, efficient and sustainable advantages and was beneficial for engineering applications.

Acknowledgements

This work was supported by the International Scientific and Technological Cooperation Program of China (No.2014DFE90040) and the Public welfare Technology Application Research Project of Zhejiang province (No. 2016C33108) and the Innovative Team Foundation of Zhejiang Province (2013TD12).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01638c

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