Effect of Solids Retention Time on Effluent Dissolved Organic Nitrogen

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Effect of Solids Retention Time on Effluent Dissolved Organic Nitrogen in the Activated Sludge Process: Studies on Bioavailability, Fluorescent Components, and Molecular Characteristics Haidong Hu, Kewei Liao, Yuanji Shi, Lili Ding, Yan Zhang, and Hong-qiang Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05309 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Environmental Science & Technology

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Effect of Solids Retention Time on Effluent Dissolved Organic Nitrogen in the Activated

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Sludge Process: Studies on Bioavailability, Fluorescent Components, and Molecular

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Characteristics

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Haidong Hu, Kewei Liao, Yuanji Shi, Lili Ding, Yan Zhang, and Hongqiang Ren*

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State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment,

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Nanjing University, Nanjing 210023, Jiangsu, PR China

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*Corresponding author.

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Tel.: +86 25 89680512; fax: +86 25 89680569.

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E-mail address: [email protected] (H. Hu); [email protected] (H. Ren).

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Notes

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The authors declare no competing financial interest.

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ACS Paragon Plus Environment


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ABSTRACT: Wastewater-derived dissolved organic nitrogen (DON) should be minimized by

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municipal wastewater treatment plants (MWWTPs) to reduce its potential impact on receiving

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waters. Solids retention time (SRT) is a key control parameter for the activated sludge (AS)

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process; however, knowledge of its impact on effluent DON is limited. This study investigated the

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effect of SRT on the bioavailability, fluorescent components, and molecular characteristics of

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effluent DON in the AS process. Four lab-scale AS reactors were operated in parallel at different

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SRTs (5, 13, 26, and 40 days) for treatment of primary treated wastewater collected from an

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MWWTP. Results showed the positive effect of prolonged SRT on DON removal. AS reactors

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during longer SRTs, however, cannot sequester the bioavailable DON (ABDON) and occasionally

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contribute to greater amounts of ABDON in the effluents. Consequently, effluent DON

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bioavailability increased with SRT (R2 = 0.619, p ˂ 0.05, ANOVA). Analysis of effluent DON

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fluorescent components and molecular characteristics indicated that the high effluent DON

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bioavailability observed at long SRTs is contributed by the production of microbially-derived

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nitrogenous organics. The results presented herein indicate that operating an AS process with a

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longer SRT cannot control the DON forms that readily stimulate algal growth.

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INTRODUCTION

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With recent advances in biological nutrient removal technologies, many municipal

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wastewater treatment plants (MWWTPs) can achieve highly dissolved inorganic nitrogen removal

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(˃ 95%), leading to dissolved organic nitrogen (DON) becoming a major nitrogen form (˃ 65%)

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of the total effluent dissolved nitrogen.1 The amount of DON in effluents ranges from 0.76 to 6.46

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mg/L.2-5 Previous studies indicate that some portions of effluent DON can be utilized by natural

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algae and plankton, which would increase the risk of eutrophication and have a negative impact on

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water quality.6-9 Two different bioassay procedures are used to measure DON utilization. The two

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bioassays are the bioavailable DON (ABDON) approach (algae seeds and bacteria seeds)

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developed by Pehlivanoglu and Sedlak10 and the biodegradable DON (BDON) approach (only

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bacteria seeds) developed by Khan et al.11 Attempts to imitate real environmental conditions when

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effluent is discharged into receiving waters fits the ABDON protocol better due to the presence of

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algae in natural waters.12

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Activated sludge (AS) is the most common biological technology used to treat municipal

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wastewater. The sources of effluent DON in the AS processes include influent-derived nitrogenous

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compounds, i.e., natural organic matter derived from drinking water sources and trace organic

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compounds originating from industrial or residential sources, and microorganism-derived

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nitrogenous compounds produced during biotreatment, i.e., soluble microbial products (SMPs).1,12

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Solids retention time (SRT), a key operational control parameter for the AS process, is known to

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affect the removal and characteristics of trace organic compounds13-15 and SMPs.16-18 To date, 3



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however, only limited studies have investigated the effect of SRT on the characteristics of

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DON.19-21 In a very interesting paper, Simsek et al.20 conducted laboratory scale chemostat

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experiments to investigate the effect of SRT on DON, BDON, and biodegradability (BDON/DON)

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levels in treated wastewater. They found no trend between effluent DON and SRTs, yet, effluent

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DON biodegradability generally decreased with SRT. Unfortunately, the study did not determine

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the effluent ABDON and bioavailability (ABDON/DON) levels.

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Recent findings indicate that the variability in the bioavailability of DON is related to

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differences in the fluorescent components and molecular characteristics of DON.7,22,23 Liu et al.,7

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characterized the effluent-derived DON by utilizing a three-dimensional excitation-emission matrix

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(EEM) fluorescence and found that DON fractions with a higher fluorescence ratio of tryptophan to

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humic substance was more bioavailable and, thus, more likely to stimulate phytoplankton growth. A

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highly sensitive technique which can provide molecular information about DON is the

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Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS). FTICR-MS allows

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for precise molecular formulae relating to thousands of measured individual organic compounds,

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which could offer novel insight and foster greater recognition of DON’s complexity.24 FTICR-MS

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has been widely used to characterize natural DON,25-29 but the application of this technique to

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wastewater-derived DON is limited in number.24,30

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Since MWWTPs are one of the main DON suppliers to surface waters, knowledge of the

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bioavailability of effluent DON at various SRTs provides useful information for minimizing their

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adverse effects on receiving water quality. Accordingly, the main objective of this study was to 4


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investigate the effect of SRT on the bioavailability of effluent DON in the AS process. The

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fluorescent components and molecular characteristics of DON in effluent at different SRTs were

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also investigated with the purpose of facilitating a better understanding of the impact of SRT on

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DON bioavailability.

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MATERIALS AND METHODS

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Experimental Setup and Operation. Four laboratory-scale conventional AS reactors were

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operated in parallel at four different SRTs. Characteristic SRTs for the AS process (i.e., 5, 13, 26,

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and 40 days) were chosen.14,31 The SRTs were maintained by wasting solids from the mixed liquor

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in the reactors at different rates according to Jarusutthirak and Amy.32 Primary treated wastewater

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was collected from a MWWTP (DC WWTP, Nanjing, China), and was used as influent for the

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laboratory-scale AS reactors. Information on the collection and characteristics of the substrate and a

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detailed description of the experimental setup and operation are provided in the Supporting

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Information. Each bioreactor was run for at least three SRTs prior to monitoring to ensure the

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establishment of steady state conditions. Effluent samples were collected and analyzed for DON

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and ABDON every five days for 30 days after each reactor became stable. Among them, triplicate

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samples collected from three representative sampling periods were also used for DON fluorescent

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components and molecular characterization analyses.

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DON and its Bioavailability Determination. In this study, DON is designated as organic

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nitrogen passing through a 0.45 µm membrane filter. It is important to note that the 0.45 µm filtrate 5



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still contains a small quantity of colloidal materials.33,34 However, since this pore size is widely used

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for wastewater characterization purposes,35,36 it was considered adequate for the quantification of

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the wastewater-derived DON.6,22,37,38 The concentration of DON was calculated as the difference

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between total dissolved nitrogen (TDN) and the sum of inorganic nitrogen species (eq 1) after

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dialysis pretreatment.39 The bioavailability of DON was determined with a 14−day algal growth

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bioassay.7 To start the bioassay, 1.5 mL of algal seeds (Selenastrum capricornutum, obtained from

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the FACHB-collection, Chinese Academy of Sciences) and 1 mL mixed culture bacteria collected

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from the DC WWTP effluent were added to 100 mL DON samples in a 250 mL Erlenmeyer flask.

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The fate of background DON introduced by inoculation was evaluated with the Milli-Q water

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control.7 The ABDON concentrations relied on the change of DON in the sample before and after

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the incubation period (eq 2),7 where, DONi and DONf represent DON before and after incubation

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for wastewater samples, and DONbi and DONbf represent DON before and after incubation for

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Milli-Q water samples. DON bioavailability was calculated according to eq 3.4

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DON (mg/L) = TDN – NH4-N – NO2-N – NO3-N

(1)

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ABDON (mg/L) = (DONi – DONf) – (DONbi – DONbf)

(2)

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DON bioavailability (%) =

୅୆ୈ୓୒ ୈ୓୒

×100%

(3)

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DON Fluorescent Components Measurement. DON fluorescent components were analyzed

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by three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy and collected

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using a FluoroMax−4 fluorescence spectrophotometer (HORIBA, France). Details of the EEM

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measurement and the parallel factor analysis (PARAFAC) are provided in the Supporting 6


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Information.

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DON Molecular Characteristics Measurement. DON was extracted by the SPE cartridge

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(functionalized styrene-divinyl-benzene polymer resin (PPL), Supelco, USA). A detailed

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description of the SPE extraction is given in the Supporting Information. SPE-extracted DON was

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analyzed using a 9.4 T FTICR-MS (Bruker, Germany). A standard Bruker ESI source was used to

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generate negatively charged molecular ions. A sample flow rate was maintained at 180 µL/h, and

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the ESI needle voltage was set to 4.0 kV. Ions were accumulated in the hexapole for 0.1 s before

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being transferred to the ICR cell. All SPE-extracted DON samples were run in 100% LC-MS

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methanol (Merck Lichrosolv, Germany).40 Masses within the mass range of 200−700 Da were

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considered.28 All mass spectra were calibrated and assigned molecular formulas to peaks

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following Chen et al.41

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Wastewater Analysis. A Shimadzu 5000−A total organic carbon analyzer (Shimadzu, Japan)

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was used to measure the dissolved organic carbon (DOC). NH4-N was determined by the

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salicylate method.36 TDN, NO3-N, and NO2-N were analyzed by ion chromatography (Dionex

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ICS−1100, USA). More details of inorganic nitrogen measurements including the method

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detection limit (MDL) were included in our previous study.4 Mixed liquor volatile suspended

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solids (MLVSS) were analyzed according to Standard Methods.36

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Statistical Analyses. SPSS 19.0 (IBM, Armonk, New York) was used for Student's t test

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(t-test) comparison and analysis of variance (ANOVA). A p-value of < 0.05 was accepted as 7



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indicating significance. Principal component analysis (PCA) was conducted according to the

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matrix of distance using the PAleontological STatistics software (PAST, version 3.01).

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RESULTS AND DISCUSSION

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Performance of the AS System under Different SRTs. Basic performance parameters of

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the AS reactors at four different SRTs (5, 13, 26, and 40 days) are summarized in Table S2. The

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mixed liquor volatile suspended solids (MLVSS) increased as the SRT increased. This finding is

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consistent with previous studies,18,31 implying that sludge wasting successfully developed in all

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AS reactors in terms of the amount of MLVSS and SRT. Complete dissolved organic carbon (DOC)

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degradation was achieved in bioreactors under all SRT conditions, in which effluent DOC

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concentrations were less than 6.0 mg/L. There was no NO2-N accumulation in any of the

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bioreactors (0.0−0.3 mg/L). With respect to NH4-N, average removal efficiencies were greater

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than 88% for all SRTs tested, indicating that full nitrification of NH4-N had occurred in all

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bioreactors. Indeed, SRTs as low as two days were able to establish full nitrification.20

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Effect of SRT on Effluent DON and its Bioavailability. Effluent DON and ABDON of the

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AS reactors at different SRTs are presented in Figure 1. Effluent DON gradually decreased

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between SRTs of 5 days and 26 days. When the SRT was increased from 26 days to 40 days, the

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effluent DON maintained at a similar level. This finding is different from Simsek et al.,20 who

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investigated the effect of SRT (0.3−13 days) on the effluent DON in a laboratory-scale chemostat

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system, and reported that there was no trend between DON removal efficiency and SRT. This 8


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discrepancy might be due to the different wastewater characteristics used as the reactor influent.

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Simsek et al.,20 concluded that the lack of a trend or consistent trend for effluent DON versus SRT

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could be because of the fluctuation in influent total nitrogen concentration (interval range greater

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than 20 mg/L). In this study, the fluctuation of the influent total nitrogen concentration was

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relatively small (interval range less than 6 mg/L, Table S1). Extended SRT results in the enrichment

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of slow-growing microorganisms, which have proven favorable towards biotransformation of

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influent refractory nitrogenous organic compounds.13 Thus, even after 13 days, a decreasing

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tendency of effluent DON would still appear (Figure 1).

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Effluent ABDON was comparable for SRTs of 5, 13, and 26 days. Interestingly, a higher

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effluent ABDON was observed in the 40−day SRT bioreactor than that in the 5−day SRT and

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13−day SRT bioreactors (p < 0.05 for both, t-test, Figure 1). These observations indicate that at

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longer SRTs, AS reactors cannot control the ABDON and occasionally contribute to greater

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amounts of ABDON in the effluent. Effluent ABDON is an important issue and desirable to be

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removed b y WWTPs because bioavailable nitrogenous organic compounds can be bioavailable to

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natural algae and thus can lead to oxygen consumption and support eutrophication in effluent

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receiving waters.6,7

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Effluent DON bioavailability (ABDON/DON) increased as SRT increased (R2 = 0.619, p
1.5 regions are likely to be microbially derived.53 Thus, the effluent DON for longer SRTs

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which is more proteins/amino sugars- (H/C = 1.5−2.2) and lipids-like (H/C = 1.7−2.2) in character,

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were mainly associated with the production of more microbially-derived nitrogenous organic

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compounds. Compared to the influent-derived DON, microorganisms-derived DON are more

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bioavailable.7 This is likely the reason why the highly effluent DON bioavailability was observed

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at longer SRTs (Figure 2).

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Implications. An optimal strategy for abatement of eutrophication aims at minimizing

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bioavailable nitrogen rather than total nitrogen since only bioavailable nitrogen drives

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phytoplankton primary production, which is also known as the process initiating eutrophication in

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aquatic environments.54,55 Our results indicated that AS reactors during longer SRTs cannot

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control the ABDON and will in the end contribute to ABDON in effluents, causing effluent DON

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bioavailability to increase with SRT. In addition, the results of DON fluorescent components and

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molecular characterization analysis indicated that the high effluent DON bioavailability observed

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at longer SRTs is contributed by SMP-based nitrogenous organic compounds. Compared to the

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influent-derived natural organic matter, microorganisms-derived organic matter is more difficult to 12


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remove by conventional post-treatment processes, e.g., coagulation.56 Consequently, it is expected

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that the newly created DON that is more bioavailable in character tend to enter into receiving

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waters.

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Taken together, operating an AS with a longer SRT, as is favored under many conditions to

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achieve inorganic nitrogen removal, cannot sequester the ABDON and even comes at the cost of a

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higher final effluent ABDON. A recent study has reported that a considerable fraction of DON has

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an even greater potential to stimulate phytoplankton biomass than inorganic nitrogen.57 A more

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holistic approach towards selecting operating SRT is, therefore, required with taking into

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consideration minimizing the potential eutrophication risks posed by wastewater-derived nitrogen.

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Since Selenastrum capricornutum has been used widely as a standard test species for

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eutrophication,10

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algae.2-4,7,10,12,48,58,59 Algal capabilities to uptake DON are not only related to the characteristics of

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DON but also to algal species.60 To further understand the role of effluent DON in eutrophication

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potential, further studies are needed to assess uptake of effluent DON by mixed-algal cultures

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which occur in the receiving waters.

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ASSOCIATED CONTENTS

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Supporting Information

ABDON

determination

experiments

were

conducted

with

this

246

The Supporting Information is available free of charge on the ACS Publications website.

247

Experimental setup and operation (Method S1); fluorescence EEM and PARAFAC modeling 13



248

(Method S2); solid phase extraction of DON (Method S3); batch control experiments (Method S4);

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characterization of the influent (Table S1) and effluent (Table S2); DON concentrations in batch

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control experiments (Figure S1); residual analysis and split half analysis (Figure S2); fluorescence

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spectra of EEM-PARAFAC components (Figure S3); PCA plots and PC1 and PC2 values (Figure

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S4) (PDF)

253

ACKNOWLEDGMENTS

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We would like to acknowledge the crew from the FACHB-collection, Chinese Academy of

255

Sciences for assisting with algal bioassay tests. We also gratefully acknowledge financial support

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by the National Science and Technology Major Project (2017ZX07202003).

257

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436

model. Environ. Sci. Technol. 2011, 45 (7), 2909−2916.

19



437

Page 20 of 26

Table 1–Description of the identified EEM-PARAFAC components. Component

Ex/Em maxima (nm)

Description

References

C1

345/426

Humic-like; Terrestrial delivered

46, 47, 61

C2

395/470


47, 61, 62

C3

310/422


45, 47

C4

325/378

Humic-like; Microbial delivered

45, 46

C5

280/314

Protein-like; Microbial delivered

45, 47, 48

20


Page 21 of 26


(a)

(b)

438 439

Figure 1. (a) DON and ABDON in bioreactor effluent at different SRTs under steady-state

440

conditions (n = 7). (b) Variation of effluent (AB)DON at different SRTs under steady-state

441

conditions. The difference of (AB)DON between each SRT was calculated by the t-test.

21



442 443

Figure 2. The relationship of SRTs and DON bioavailability (ABDON/DON) values in bioreactor

444

effluent under steady-state conditions. The box plot represents the fifth (lower whisker), 95th

445

(upper whisker), 25th (bottom edge of the box), and 75th (top edge of the box) percentiles. The

446

horizontal line and rectangle in the box indicate the median and mean, respectively. Linear

447

regression fitting (blue line) is based on the 28 sample values (blue circle) in the bioreactor

448

effluent.

22


Page 22 of 26

Page 23 of 26


449

450 451

Figure 3. Fluorescence intensity of the EEM-PARAFAC C4 and C5 components of influent DON

452

and effluent DON from bioreactors with different SRTs under steady-state conditions. The

453

description of the EEM-PARAFAC components is presented in Table 1.

23



454 455

Figure 4. Van Krevelen diagram shows the produced and consumed nitrogen-containing formulas

456

in wastewater after biotreatment with different SRTs. The types of molecules include52: (1) lipids

457

(O/C = 0−0.2, H/C = 1.7−2.2), (2) proteins/amino sugars (O/C = 0.2−0.6, H/C = 1.5−2.2, N/C ≥

458

0.05), (3) carbohydrates (O/C = 0.6−1, H/C = 1.5−2.2), (4) unsaturated hydrocarbons (O/C = 0−0.1,

459

H/C = 0.7−1.5), (5) lignin (O/C = 0.1−0.6, H/C = 0.6−1.7, modified aromaticity index (AImod)

Effect of Solids Retention Time on Effluent Dissolved Organic Nitrogen

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