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In-depth characterization of secondary effluent from a municipal wastewater treatment plant located in Northern China for advanced treatment Shutao Wang, Xingwen Zhang, Zhi-Wu Wang, Xiangkun Li and Jun Ma
ABSTRACT This study provided insight into the characterization of secondary effluent from a wastewater treatment plant located in northeastern China. The secondary effluent was separated into three fractions, the dissolved, the near-colloidal and the suspended, to study their individual characteristics. It revealed that most of the organics in the secondary effluent existed in the dissolved form, accounting for 78.1–86.5% of the total chemical oxygen demand and 82.6–86.6% of the total organic carbon. Results from the molecular weight distribution study further indicated that organics with MW < 1k Da constituted 56.3–62.7% of total organics. Moreover, the particle size distribution study suggested that particles between 2.0 and 6.8 μm in diameter made up 80.0% of the total suspended solids. Both biological oxygen demand/chemical oxygen demand and biological dissolved organic carbon/dissolved organic carbon were measured ranging from 0.2 to 0.3, suggesting the most secondary effluent organics were biologically refractory. This conclusion was further strengthened by the functional groups information obtained from the GC/MS (gas chromatography/mass spectrometry) analysis. The characteristics information revealed from this study will help the design and selection of water quality-specific tertiary treatment technologies for secondary effluent water purification and reuse. Key words
| biodegradability, fractionation, inorganic carbon, organic carbon, secondary effluent
Shutao Wang (corresponding author) Xiangkun Li Jun Ma State Key Laboratory of Urban Water Resource and Environment, and School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73, Huanghe Rd, Nangang Dist., Harbin 150090, China E-mail: [email protected] Xingwen Zhang Department of Chemistry, Harbin Institute of Technology, 92 West Dazhi Street, Nangang Dist., Harbin 150001, China Zhi-Wu Wang Agricultural Technical Institute, College of Food, Agricultural and Environmental Sciences, The Ohio State University, 1328 Dover Rd, Wooster, OH 44691, USA
INTRODUCTION In recent years, the development of technologies for water reuse has drawn increasing research and social interest in China due to the aggravated shortage of water resources. With steady supply and stable quality, secondary effluent from municipal wastewater treatment plants has shown the potential to become an alternative water resource upon tertiary purification (Acero et al. ). A variety of tertiary wastewater purification technologies have been proposed to process secondary effluent to the extent that meets specific water reuse standards. These technologies usually involve two or more of the following techniques, i.e., coagulation, sedimentation, dissolved-air flotation, media filtration, microfiltration, reverse osmosis (RO), chlorine disinfection/oxidation, advanced oxidation processes, and biological processes (Chuang et al. ; Tripathi et al. ; Üstün et al. ). Yet, the exact techniques to be employed primarily hinge on the secondary effluent quality, and thus should be tailored specifically according to the doi: 10.2166/wst.2014.040
characteristics of its remaining pollutants. Therefore, an indepth characterization of the secondary effluent will provide indispensable baseline information for the downstream process design and techniques integration. In the face of the demanding standards that are being implemented in the water reuse area, those traditional wastewater characteristic indices, e.g. chemical oxygen demand (COD), biological oxygen demand (BOD5), total suspended solids (TSS), NH4þ-N, total nitrogen (TN), and total phosphate (TP), may prove inadequate in the analysis of low strength secondary effluent. Hence, those more sensitive measurements that are commonly used for potable water analysis such as the total organic carbon (TOC), ultraviolet absorbance (UV254), and biological dissolved organic carbon (BDOC), have been exploited for the secondary effluent characterization (Dignac et al. ). Our previous research showed the utilization of these more insightful organic pollutant-targeting parameters has provided broader latitude in the
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characterization of secondary effluent for advanced treatment (Wang et al. ). To characterize the contaminants, we can separate the constituents of secondary effluent into three fractions: the dissolved, the suspended and the near-colloidal (the definitions of the three fractions are in the ‘Analytical methods and instruments’ section). Therefore, it is necessary to analyze the three fractions individually and be concerned with more thorough and concrete effluent characteristics. For example, the distribution of COD among different fractions, the biodegradability of the dissolved fraction, the classification of different types of organic matter, the molecular weight distribution (MWD) of the organics, the characteristics of the organics between different molecular weight ranges, and the particle size distribution are important for characterizing secondary effluent and for designing appropriate treatment technologies. Generally, the dissolved contaminants (including organic and inorganic matter) are given most attention and many studies have been carried out on the analysis of the dissolved fraction. For example, some separation methods, such as RO (Abdessemed et al. ; Imai et al. ) and resin adsorption (Hu et al. ), are used to separate secondary effluent. It was also reported that Pernet-coudrier (Pernet-coudrier et al. ) made a fractionation of dissolved organic matter (DOM) in secondary effluents using resin adsorbents and the organics were classified into three fractions: hydrophobic, transphilic and hydrophilic (HPI) accounting for 35, 20 and 45% of extracted carbon, respectively. The objective of this study was to offer in-depth assessment of secondary effluent from a northeastern China wastewater treatment plant, where the seasonal temperature varies remarkably. To this end, the composition of the secondary effluent was divided into dissolved, suspended and near-colloidal fractions based on their particle size. Results from this study may contribute to the selection and design of appropriate processes for advanced tertiary treatment.
MATERIALS AND METHODS Materials and chemicals Sep-pak C18 cartridge (Waters Corp., Milford, USA) and Solid phase extraction (SPE, Supelco Corp., Bellefonte, USA) were adopted for the enrichment of organics in water samples. Stirred Ultrafiltration Cell and Milli-Q membrane (with pore sizes of 0.45 μm, 1 k, 5 k, 10 k and 100 k Da) (Millipore Corp., Billerica, USA) were employed for MWD measurement. Chromatographic grade (>99.99%)
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methanol, acetone and dichloromethane (Sigma-Aldrich, Milwaukee, USA) as well as the ultrapure water (Milli-Q, Millipore, Billerica, USA) were used for organics analysis. Analytical methods and instruments The composition of secondary effluent was separated into the suspended, the near-colloidal and the dissolved fractions by sequential filtration: (1) that retained on 30–120 μm filter paper following first filtration was considered as the suspended fraction; (2) that filtered again and retained on the 0.45-μm membrane was considered as the near-colloidal fraction; and (3) that remaining in the filtrate passing through 0.45-μm membrane was considered as the dissolved fraction. Detailed characterizations were made toward these fractions with regard to their carbon composition, biodegradability, organics classification, and MWD. Measurements of the TOC, the ultraviolet (UV254) absorbance, and the BDOC were also employed to give insight into the characteristics of the secondary effluent. COD, NHþ 4 -N, NO3 -N, NO2 -N and TP were measured strictly according to American Public Health Association Standard Methods (APHA). A TN analyzer (Shimadzu 5000-TN, Tokyo, Japan) and Shimadzu UV-2550 spectrophotometer (Shimadzu, Tokyo, Japan) were used to measure TN and UV254, respectively. Both the nearcolloidal and the suspended organic carbons were analyzed using a Shimadzu-SSM-5000A Solid Sample Combustion Unit (Shimadzu, Tokyo, Japan) coupled with a Shimadzu 5000-TOC analyzer (Shimadzu, Tokyo, Japan). At least triplicate analysis was performed for each sample. The MWD was determined using a Stirred Ultrafiltration Cell coupled with a TOC analyzer. The sample was filtered through a series of Milli-Q membranes with pore sizes of 0.45 μm, 100 k Da, 10 k Da, 5 k Da and 1 k Da, respectively. TOCs retained by each of these membranes were measured to reflect the MWD. A Liquid Particle Counting System (Royco 9703, HIAC Corp., USA) was used to measure the particle size distribution. The BDOC was analyzed using an assay adopted from previous studies (Joret et al. ; Trulleyova & Rulik ). Briefly, the water sample was filtered through a 0.45 μm membrane, and then measured for the initial DOC as DOC0. Then, 100 ml filtrate was transferred into a 125 ml flask inoculated with a 1 ml original water sample. The flask was incubated for 5 days at 20 C in darkness. After that, the mixture was again filtered through 0.45 μm membrane and measured for the final DOC as DOC5. BDOC can thus be the calculated by BDOC ¼ DOC0-DOC5. W
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In-depth characterization of secondary effluent from MWTP
Water Science & Technology
The gas chromatography/mass spectrometry (GC/MS) analysis was carried out using a 6890/5973N apparatus (Agilent Corp., Santa Clara, USA) equipped with an Ion Trap detector as well as an online data collection and processing workstation. High purity helium was used as carrier gas at a flow rate of 2 ml/min. A HP-5 capillary column (60 m × 0.32 mm I.D., 0.25 μm) was used. Temperature at the injector port and detector were 270 and 320 C, respectively. The temperature program was 40 C for 5 min, 5 C/min to 200 C, isothermal at 200 C for 5 min, 20 C/min to 280 C, isothermal at 280 C for 5 min. For the dissolved fraction, 3.50 L filtrate from a 0.45 μm membrane was enriched using SPE cartridge. The flow rate for Sep-pak cartridge adsorption was kept at 15 ml/min. The trapped fraction was dissolved in methanol, acetone and dichloromethane in sequence, and then a sample of the combined extracts was submitted to GC/MS assays. For the near-colloidal and suspended fractions, a solid sample was dried at 70 C and then ground to powder. Thereafter, the organics were sequentially extracted using 1.50 ml methanol, acetone, and dichloromethane. The combined organic phase was filtered through a 0.45 μm membrane and concentrated to 1.00 ml prior to GC/MS analysis. W
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Sample source Secondary effluent was sampled from an activated sludge process (anaerobic/anoxic/aerobic, A2/O) in Wenchang
Table 1
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Wastewater Treatment Plant located in Harbin, Heilongjiang province, northeastern China. The treatment scale was 30,000 m3/d. All reactors were running in continuous stirred tank reactor mode, dissolved oxygen (DO) concentrations were 6 mg/L in the anaerobic, anoxic and aerobic units, respectively. The primary treatment units are grid and grit chamber. Samples were transported and stored at 4 C, and analyzed within 3 days, during which negligible sample changes can be detected. The typical qualities of these secondary effluent samples were measured as: dissolved COD: 40.0–60.0 mg/L; BOD5: 8.0–15.0 mg/L; NHþ 4 -N: 9.0–19.0 mg/L; TN: 15.0– 26.0 mg/L; TP: 0.6–1.6 mg/L; TSS: 9.0–15.9 mg/L and pH: 6.5–8.0. W
RESULTS AND DISCUSSION COD and carbon distributions The total COD of the secondary effluent was separated into dissolved COD, suspended COD, and near-colloidal COD in Table 1, respectively. Total carbon (TC) was divided into TOC and total inorganic carbon (TIC). Their hierarchical relationship can be found in Figure 1. As Table 1 shows, COD ranged from 58.2 to 66.4 mg/L over three seasons, accounting for 78.2–86.5% of the dissolved, 10.1–17.0% of the suspended, and 2.9–4.8% of the near-colloidal matter.
Distribution of COD and carbon in the secondary effluent in different forms
Winter Value (mg/L)
Spring Percentage (%)
Value (mg/L)
Summer Percentage (%)
Value (mg/L)
Percentage (%)
Dissolved COD
51.9 ± 5.2
78.2
48.3 ± 2.3
83.02
52.4 ± 4.2
86.5
Suspended COD
11.3 ± 2.5
17.02
8.2 ± 1.7
14.1
6.1 ± 1.0
10.1
3.2 ± 0.5
4.8
1.7 ± 0.5
2.9
2.03 ± 0.5
3.4
Total COD
66.4 ± 8.2
100.0
58.2 ± 4.5
100.0
60.6 ± 5.7
100.0
Dissolved OC
10.1 ± 0.5
82.6
9.2 ± 1.1
85.7
8.1 ± 0.3
86.6
Suspended OC
1.8 ± 0.1
14.9
1.3 ± 0.1
11.7
1.07 ± 0.05
11.4
Near-colloidal COD
0.3 ± 0.2
2.6
0.3 ± 0.2
2.6
12.2 ± 0.8
100.0
10.8 ± 1.4
100.0
Dissolved IC
25.2 ± 2.1
99.9
20.9 ± 1.7
99.9
Suspended IC
0.007 ± 0.005
Near-colloidal OC TOC
0.03
0.01 ± 0.01
–
0.002 ± 0.002
–
25.2 ± 2.2
100.0
20.9 ± 1.8
100.0
TC
37.47 ± 2.9
100.0
31.62 ± 3.1
100.0
ND
2.03 100.0
19.8 ± 2.6
99.9
0.006 ± 0.006
0.06
TIC
Near-colloidal IC
0.2 ± 0.05 9.4 ± 0.4
0.03 –
ND 19.8 ± 2.6
100.0
29.15 ± 3.0
100.0
Note: the percentage was the mean value of three to five repetitions; ND: no detected. Temperature: spring: 0 to 8 C; summer: 20 to 34 C; winter: 10 to 35 C. W
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Figure 1
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In-depth characterization of secondary effluent from MWTP
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The hierarchal structure of carbon (a) and COD (b) in the secondary wastewater effluent.
It is clear that the DOM has dominated the secondary effluent and should be regarded as the main target of tertiary treatment. Traditionally, the coagulation-clarification-filtration process has been widely used as a simple and economical process for the advanced treatment of secondary effluent in developing countries. The analytical results above explained the limitation of this combined process in COD removal for secondary effluent as it only worked on the suspended and the near-colloidal components of the wastewater (Zhou et al. ). Table 1 also shows that the TOC of the secondary effluent were approximately 12.2, 10.8 and 9.4 mg/L in winter, spring and summer, of which 82.6, 85.7 and 86.6% was identified as DOC, respectively. These results suggest that the majority of organics in secondary effluent was actually in dissolved form, which was consistent with the distribution of COD. In the tertiary treatment, the sequential coagulation-flocculation-clarification-filtration process has been commonly applied to remove suspended and near-colloidal fractions that are mainly composed of microorganism bodies, extracellular polymeric substances (EPSs) and adsorbed organic matter (e.g. protein and glucide) (Citulski et al. ). However, the results in Table 1 indicate that it is soluble rather than insoluble organics that are the major pollutants to remove in tertiary treatment. Subsequent long-term biodegradability tests showed that BDOC ranged from 1.0 to 1.9 mg/L, making up only 15–26% of DOC (Table 3), which indicates that most DOC was actually refractory to biological treatment. Therefore, an effective chemical pre-treatment, e.g. chemical oxidation, for enhancing the biodegradability of secondary effluent might become necessary for downstream biological treatment.
13.0, 8.7 and 9.2 mg/L were organic components. That means organic contents accounted for 75.5–89.9% of the suspended and the near-colloidal matters in secondary effluent. It is generally believed that substances like bacteria metabolites, EPSs, soluble microorganism products (SMPs), adsorbed protein, and some inorganic matters are the main constituents of those suspended and near-colloidal matters (Citulski et al. ).
Particle size distribution The removal of the suspended and the near-colloidal substances from secondary effluent generally relies on the application of coagulation and sedimentation processes, the performance of which is actually closely associated with particle size distribution (Liu et al. ). Particle size distribution of the secondary effluent is shown in Figure 2. As can be seen, particles ranging from 2.0 to 6.8 μm in diameter have made up 80.0% of the total particulate solid. In contrast, particles >8.8 μm in diameter have only made up 7.8%, indicating that most particulate matter actually fell the in near-colloidal range. Our laboratory test showed that, for different ranges of particle size, the removal
Organic and inorganic components in the mixture of suspended and near-colloidal matters As shown in Table 1, the sum of the solids concentrations of both the suspended and the near-colloidal were 15.9, 11.5 and 10.2 mg/L in winter, spring and summer, of which
Figure 2
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Size distribution of particles in the secondary effluent. TSS concentration: 12 ± 2.7 mg/L; temperature: 19 ± 1 C. W
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efficiency of coagulation-sedimentation-filtration varied significantly. For example, particles ranging from 8.8 to 29.9 μm had greater than 90% removal efficiency, while particles smaller than 6.8 μm only had less than 30%.
become necessary if biological processing is to be pursued in tertiary treatment.
C: N: P ratio
The biodegradability of organic substances is closely associated with its molecular structure and is further related to its MWD. The results in Table 2 showed that organics with MW < 1 k Da were 56.3, 62.1 and 62.8% of the total in winter, spring and summer. In comparison, organics with 1 k < MW
© IWA Publishing 2014 Water Science & Technology
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69.7
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2014
In-depth characterization of secondary effluent from a municipal wastewater treatment plant located in Northern China for advanced treatment Shutao Wang, Xingwen Zhang, Zhi-Wu Wang, Xiangkun Li and Jun Ma
ABSTRACT This study provided insight into the characterization of secondary effluent from a wastewater treatment plant located in northeastern China. The secondary effluent was separated into three fractions, the dissolved, the near-colloidal and the suspended, to study their individual characteristics. It revealed that most of the organics in the secondary effluent existed in the dissolved form, accounting for 78.1–86.5% of the total chemical oxygen demand and 82.6–86.6% of the total organic carbon. Results from the molecular weight distribution study further indicated that organics with MW < 1k Da constituted 56.3–62.7% of total organics. Moreover, the particle size distribution study suggested that particles between 2.0 and 6.8 μm in diameter made up 80.0% of the total suspended solids. Both biological oxygen demand/chemical oxygen demand and biological dissolved organic carbon/dissolved organic carbon were measured ranging from 0.2 to 0.3, suggesting the most secondary effluent organics were biologically refractory. This conclusion was further strengthened by the functional groups information obtained from the GC/MS (gas chromatography/mass spectrometry) analysis. The characteristics information revealed from this study will help the design and selection of water quality-specific tertiary treatment technologies for secondary effluent water purification and reuse. Key words
| biodegradability, fractionation, inorganic carbon, organic carbon, secondary effluent
Shutao Wang (corresponding author) Xiangkun Li Jun Ma State Key Laboratory of Urban Water Resource and Environment, and School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73, Huanghe Rd, Nangang Dist., Harbin 150090, China E-mail: [email protected] Xingwen Zhang Department of Chemistry, Harbin Institute of Technology, 92 West Dazhi Street, Nangang Dist., Harbin 150001, China Zhi-Wu Wang Agricultural Technical Institute, College of Food, Agricultural and Environmental Sciences, The Ohio State University, 1328 Dover Rd, Wooster, OH 44691, USA
INTRODUCTION In recent years, the development of technologies for water reuse has drawn increasing research and social interest in China due to the aggravated shortage of water resources. With steady supply and stable quality, secondary effluent from municipal wastewater treatment plants has shown the potential to become an alternative water resource upon tertiary purification (Acero et al. ). A variety of tertiary wastewater purification technologies have been proposed to process secondary effluent to the extent that meets specific water reuse standards. These technologies usually involve two or more of the following techniques, i.e., coagulation, sedimentation, dissolved-air flotation, media filtration, microfiltration, reverse osmosis (RO), chlorine disinfection/oxidation, advanced oxidation processes, and biological processes (Chuang et al. ; Tripathi et al. ; Üstün et al. ). Yet, the exact techniques to be employed primarily hinge on the secondary effluent quality, and thus should be tailored specifically according to the doi: 10.2166/wst.2014.040
characteristics of its remaining pollutants. Therefore, an indepth characterization of the secondary effluent will provide indispensable baseline information for the downstream process design and techniques integration. In the face of the demanding standards that are being implemented in the water reuse area, those traditional wastewater characteristic indices, e.g. chemical oxygen demand (COD), biological oxygen demand (BOD5), total suspended solids (TSS), NH4þ-N, total nitrogen (TN), and total phosphate (TP), may prove inadequate in the analysis of low strength secondary effluent. Hence, those more sensitive measurements that are commonly used for potable water analysis such as the total organic carbon (TOC), ultraviolet absorbance (UV254), and biological dissolved organic carbon (BDOC), have been exploited for the secondary effluent characterization (Dignac et al. ). Our previous research showed the utilization of these more insightful organic pollutant-targeting parameters has provided broader latitude in the
1483
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In-depth characterization of secondary effluent from MWTP
characterization of secondary effluent for advanced treatment (Wang et al. ). To characterize the contaminants, we can separate the constituents of secondary effluent into three fractions: the dissolved, the suspended and the near-colloidal (the definitions of the three fractions are in the ‘Analytical methods and instruments’ section). Therefore, it is necessary to analyze the three fractions individually and be concerned with more thorough and concrete effluent characteristics. For example, the distribution of COD among different fractions, the biodegradability of the dissolved fraction, the classification of different types of organic matter, the molecular weight distribution (MWD) of the organics, the characteristics of the organics between different molecular weight ranges, and the particle size distribution are important for characterizing secondary effluent and for designing appropriate treatment technologies. Generally, the dissolved contaminants (including organic and inorganic matter) are given most attention and many studies have been carried out on the analysis of the dissolved fraction. For example, some separation methods, such as RO (Abdessemed et al. ; Imai et al. ) and resin adsorption (Hu et al. ), are used to separate secondary effluent. It was also reported that Pernet-coudrier (Pernet-coudrier et al. ) made a fractionation of dissolved organic matter (DOM) in secondary effluents using resin adsorbents and the organics were classified into three fractions: hydrophobic, transphilic and hydrophilic (HPI) accounting for 35, 20 and 45% of extracted carbon, respectively. The objective of this study was to offer in-depth assessment of secondary effluent from a northeastern China wastewater treatment plant, where the seasonal temperature varies remarkably. To this end, the composition of the secondary effluent was divided into dissolved, suspended and near-colloidal fractions based on their particle size. Results from this study may contribute to the selection and design of appropriate processes for advanced tertiary treatment.
MATERIALS AND METHODS Materials and chemicals Sep-pak C18 cartridge (Waters Corp., Milford, USA) and Solid phase extraction (SPE, Supelco Corp., Bellefonte, USA) were adopted for the enrichment of organics in water samples. Stirred Ultrafiltration Cell and Milli-Q membrane (with pore sizes of 0.45 μm, 1 k, 5 k, 10 k and 100 k Da) (Millipore Corp., Billerica, USA) were employed for MWD measurement. Chromatographic grade (>99.99%)
Water Science & Technology
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methanol, acetone and dichloromethane (Sigma-Aldrich, Milwaukee, USA) as well as the ultrapure water (Milli-Q, Millipore, Billerica, USA) were used for organics analysis. Analytical methods and instruments The composition of secondary effluent was separated into the suspended, the near-colloidal and the dissolved fractions by sequential filtration: (1) that retained on 30–120 μm filter paper following first filtration was considered as the suspended fraction; (2) that filtered again and retained on the 0.45-μm membrane was considered as the near-colloidal fraction; and (3) that remaining in the filtrate passing through 0.45-μm membrane was considered as the dissolved fraction. Detailed characterizations were made toward these fractions with regard to their carbon composition, biodegradability, organics classification, and MWD. Measurements of the TOC, the ultraviolet (UV254) absorbance, and the BDOC were also employed to give insight into the characteristics of the secondary effluent. COD, NHþ 4 -N, NO3 -N, NO2 -N and TP were measured strictly according to American Public Health Association Standard Methods (APHA). A TN analyzer (Shimadzu 5000-TN, Tokyo, Japan) and Shimadzu UV-2550 spectrophotometer (Shimadzu, Tokyo, Japan) were used to measure TN and UV254, respectively. Both the nearcolloidal and the suspended organic carbons were analyzed using a Shimadzu-SSM-5000A Solid Sample Combustion Unit (Shimadzu, Tokyo, Japan) coupled with a Shimadzu 5000-TOC analyzer (Shimadzu, Tokyo, Japan). At least triplicate analysis was performed for each sample. The MWD was determined using a Stirred Ultrafiltration Cell coupled with a TOC analyzer. The sample was filtered through a series of Milli-Q membranes with pore sizes of 0.45 μm, 100 k Da, 10 k Da, 5 k Da and 1 k Da, respectively. TOCs retained by each of these membranes were measured to reflect the MWD. A Liquid Particle Counting System (Royco 9703, HIAC Corp., USA) was used to measure the particle size distribution. The BDOC was analyzed using an assay adopted from previous studies (Joret et al. ; Trulleyova & Rulik ). Briefly, the water sample was filtered through a 0.45 μm membrane, and then measured for the initial DOC as DOC0. Then, 100 ml filtrate was transferred into a 125 ml flask inoculated with a 1 ml original water sample. The flask was incubated for 5 days at 20 C in darkness. After that, the mixture was again filtered through 0.45 μm membrane and measured for the final DOC as DOC5. BDOC can thus be the calculated by BDOC ¼ DOC0-DOC5. W
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In-depth characterization of secondary effluent from MWTP
Water Science & Technology
The gas chromatography/mass spectrometry (GC/MS) analysis was carried out using a 6890/5973N apparatus (Agilent Corp., Santa Clara, USA) equipped with an Ion Trap detector as well as an online data collection and processing workstation. High purity helium was used as carrier gas at a flow rate of 2 ml/min. A HP-5 capillary column (60 m × 0.32 mm I.D., 0.25 μm) was used. Temperature at the injector port and detector were 270 and 320 C, respectively. The temperature program was 40 C for 5 min, 5 C/min to 200 C, isothermal at 200 C for 5 min, 20 C/min to 280 C, isothermal at 280 C for 5 min. For the dissolved fraction, 3.50 L filtrate from a 0.45 μm membrane was enriched using SPE cartridge. The flow rate for Sep-pak cartridge adsorption was kept at 15 ml/min. The trapped fraction was dissolved in methanol, acetone and dichloromethane in sequence, and then a sample of the combined extracts was submitted to GC/MS assays. For the near-colloidal and suspended fractions, a solid sample was dried at 70 C and then ground to powder. Thereafter, the organics were sequentially extracted using 1.50 ml methanol, acetone, and dichloromethane. The combined organic phase was filtered through a 0.45 μm membrane and concentrated to 1.00 ml prior to GC/MS analysis. W
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Sample source Secondary effluent was sampled from an activated sludge process (anaerobic/anoxic/aerobic, A2/O) in Wenchang
Table 1
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Wastewater Treatment Plant located in Harbin, Heilongjiang province, northeastern China. The treatment scale was 30,000 m3/d. All reactors were running in continuous stirred tank reactor mode, dissolved oxygen (DO) concentrations were 6 mg/L in the anaerobic, anoxic and aerobic units, respectively. The primary treatment units are grid and grit chamber. Samples were transported and stored at 4 C, and analyzed within 3 days, during which negligible sample changes can be detected. The typical qualities of these secondary effluent samples were measured as: dissolved COD: 40.0–60.0 mg/L; BOD5: 8.0–15.0 mg/L; NHþ 4 -N: 9.0–19.0 mg/L; TN: 15.0– 26.0 mg/L; TP: 0.6–1.6 mg/L; TSS: 9.0–15.9 mg/L and pH: 6.5–8.0. W
RESULTS AND DISCUSSION COD and carbon distributions The total COD of the secondary effluent was separated into dissolved COD, suspended COD, and near-colloidal COD in Table 1, respectively. Total carbon (TC) was divided into TOC and total inorganic carbon (TIC). Their hierarchical relationship can be found in Figure 1. As Table 1 shows, COD ranged from 58.2 to 66.4 mg/L over three seasons, accounting for 78.2–86.5% of the dissolved, 10.1–17.0% of the suspended, and 2.9–4.8% of the near-colloidal matter.
Distribution of COD and carbon in the secondary effluent in different forms
Winter Value (mg/L)
Spring Percentage (%)
Value (mg/L)
Summer Percentage (%)
Value (mg/L)
Percentage (%)
Dissolved COD
51.9 ± 5.2
78.2
48.3 ± 2.3
83.02
52.4 ± 4.2
86.5
Suspended COD
11.3 ± 2.5
17.02
8.2 ± 1.7
14.1
6.1 ± 1.0
10.1
3.2 ± 0.5
4.8
1.7 ± 0.5
2.9
2.03 ± 0.5
3.4
Total COD
66.4 ± 8.2
100.0
58.2 ± 4.5
100.0
60.6 ± 5.7
100.0
Dissolved OC
10.1 ± 0.5
82.6
9.2 ± 1.1
85.7
8.1 ± 0.3
86.6
Suspended OC
1.8 ± 0.1
14.9
1.3 ± 0.1
11.7
1.07 ± 0.05
11.4
Near-colloidal COD
0.3 ± 0.2
2.6
0.3 ± 0.2
2.6
12.2 ± 0.8
100.0
10.8 ± 1.4
100.0
Dissolved IC
25.2 ± 2.1
99.9
20.9 ± 1.7
99.9
Suspended IC
0.007 ± 0.005
Near-colloidal OC TOC
0.03
0.01 ± 0.01
–
0.002 ± 0.002
–
25.2 ± 2.2
100.0
20.9 ± 1.8
100.0
TC
37.47 ± 2.9
100.0
31.62 ± 3.1
100.0
ND
2.03 100.0
19.8 ± 2.6
99.9
0.006 ± 0.006
0.06
TIC
Near-colloidal IC
0.2 ± 0.05 9.4 ± 0.4
0.03 –
ND 19.8 ± 2.6
100.0
29.15 ± 3.0
100.0
Note: the percentage was the mean value of three to five repetitions; ND: no detected. Temperature: spring: 0 to 8 C; summer: 20 to 34 C; winter: 10 to 35 C. W
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Figure 1
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In-depth characterization of secondary effluent from MWTP
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The hierarchal structure of carbon (a) and COD (b) in the secondary wastewater effluent.
It is clear that the DOM has dominated the secondary effluent and should be regarded as the main target of tertiary treatment. Traditionally, the coagulation-clarification-filtration process has been widely used as a simple and economical process for the advanced treatment of secondary effluent in developing countries. The analytical results above explained the limitation of this combined process in COD removal for secondary effluent as it only worked on the suspended and the near-colloidal components of the wastewater (Zhou et al. ). Table 1 also shows that the TOC of the secondary effluent were approximately 12.2, 10.8 and 9.4 mg/L in winter, spring and summer, of which 82.6, 85.7 and 86.6% was identified as DOC, respectively. These results suggest that the majority of organics in secondary effluent was actually in dissolved form, which was consistent with the distribution of COD. In the tertiary treatment, the sequential coagulation-flocculation-clarification-filtration process has been commonly applied to remove suspended and near-colloidal fractions that are mainly composed of microorganism bodies, extracellular polymeric substances (EPSs) and adsorbed organic matter (e.g. protein and glucide) (Citulski et al. ). However, the results in Table 1 indicate that it is soluble rather than insoluble organics that are the major pollutants to remove in tertiary treatment. Subsequent long-term biodegradability tests showed that BDOC ranged from 1.0 to 1.9 mg/L, making up only 15–26% of DOC (Table 3), which indicates that most DOC was actually refractory to biological treatment. Therefore, an effective chemical pre-treatment, e.g. chemical oxidation, for enhancing the biodegradability of secondary effluent might become necessary for downstream biological treatment.
13.0, 8.7 and 9.2 mg/L were organic components. That means organic contents accounted for 75.5–89.9% of the suspended and the near-colloidal matters in secondary effluent. It is generally believed that substances like bacteria metabolites, EPSs, soluble microorganism products (SMPs), adsorbed protein, and some inorganic matters are the main constituents of those suspended and near-colloidal matters (Citulski et al. ).
Particle size distribution The removal of the suspended and the near-colloidal substances from secondary effluent generally relies on the application of coagulation and sedimentation processes, the performance of which is actually closely associated with particle size distribution (Liu et al. ). Particle size distribution of the secondary effluent is shown in Figure 2. As can be seen, particles ranging from 2.0 to 6.8 μm in diameter have made up 80.0% of the total particulate solid. In contrast, particles >8.8 μm in diameter have only made up 7.8%, indicating that most particulate matter actually fell the in near-colloidal range. Our laboratory test showed that, for different ranges of particle size, the removal
Organic and inorganic components in the mixture of suspended and near-colloidal matters As shown in Table 1, the sum of the solids concentrations of both the suspended and the near-colloidal were 15.9, 11.5 and 10.2 mg/L in winter, spring and summer, of which
Figure 2
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Size distribution of particles in the secondary effluent. TSS concentration: 12 ± 2.7 mg/L; temperature: 19 ± 1 C. W
S. Wang et al.
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In-depth characterization of secondary effluent from MWTP
Water Science & Technology
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efficiency of coagulation-sedimentation-filtration varied significantly. For example, particles ranging from 8.8 to 29.9 μm had greater than 90% removal efficiency, while particles smaller than 6.8 μm only had less than 30%.
become necessary if biological processing is to be pursued in tertiary treatment.
C: N: P ratio
The biodegradability of organic substances is closely associated with its molecular structure and is further related to its MWD. The results in Table 2 showed that organics with MW < 1 k Da were 56.3, 62.1 and 62.8% of the total in winter, spring and summer. In comparison, organics with 1 k < MW
