Evaluating the removal of organic fraction of commingled chemical industrial wastewater by activated sludge process augmented with powdered activated carbon

2.1 Wastewater characteristics

Raw wastewater was obtained from the influent of a full-scale wastewater treatment plant in a chemical industrial park in Tianjin, China. More than 50 types of industrial plants are located in the area, including pharmaceutical, manufacturing, petroleum chemical, electronics, chemical reagent, household, and personal care product plants. The wastewater contains the following components: chemical oxygen demand (COD), 982–1602 mg/L; 5-day biochemical oxygen demand (BOD₅), 208–824 mg/L; total nitrogen, 24.5–30.8 mg/L; ammonia nitrogen, 17.9–20.6 mg/L; total phosphorus, 2.3–2.4 mg/L, and total dissolved solids, 4532–9670 mg/L.

2.2 Wastewater treatment process

Three sequencing batch reactors (SBR) were employed for PAC–AS process (1#) individual AS process (2#), and PAC process (3#) (Fig. 1). Each reactor had a working volume of 2 L and was operated one cycle per day at room temperature with the following sequence: filling (0.09 h), anoxic reaction with stirring (6 h), aerobic reaction (14 h), settling (1.5 h), decanting (0.5 h), and idleness (1.91 h). The inoculated sludge in reactors was obtained from a local municipal wastewater treatment plant and acclimated for 30 d. The approximate mixed liquor suspended solids of AS in reactors were kept at 3.6–4.2 g/L. The sludge was removed every day and the sludge retention time was kept at 30 d.

Close

Figure 1

Schematic layout of the reactor system: 1 – SBR reactor; 2 – magnetic stirrer; 3 – air vent; 4 – sampling port; 5 – air pump; 6 – timer controller; 7 – water bath; 8 – mass flow controller.

Export to PPT

The PAC, which was added to the reactors, was produced by the company of Zhejiang Rong Xing Activated Carbon and the product number was 785. The characteristics of PAC are depicted in Appendix A. Approximately 2 g/L of PAC was added to the first system initially and the system was replenished with 0.25 g/L of PAC per 4 d to maintain the removal performance of the system (PAC–AS system). The amount and the adding method of PAC were determined according to the optimization of the PAC adding dosage test. The second system was operated without PAC addition (AS system). In the third system, the same amount of PAC was added and replenished, but the AS was inactivated by 120 °C autoclave to prevent biodegradation (PAC system). And the PAC system was also aerated, because aeration could blow off a part of organic matters, especially the volatile organic matters. Maintaining PAC system aerated could eliminate the effect to make it as a control group to the other two groups.

2.3 Fractionation of wastewater organic matter

Organic matters in the wastewater were divided based on polarity into hydrophobic fractions (hydrophobic acid HPO-A and hydrophobic neutral HPO-N), transphilic fractions (transphilic acid TPI-A and transphilic neutral TPI-N), and hydrophilic fraction (HPI). According to the previous study (Bu et al., 2010), the transphilic fractions refer to the materials whose solubility is between hydrophobic fractions and hydrophilic fractions. Fractionation was conducted using the method of Bu et al. (2010). Wastewater was fractionated by two serial columns filled with Amberlite XAD-8 and XAD-4 resins. The resins were cleaned with methanol and acetonitrile by Soxhlet extraction method for 12 h before addition into the columns. The wastewater samples were first filtered through a 0.45 μm glass fiber filter to remove suspended solids. The filtrate was then acidified to a pH of 2 with 6 M HCl and passed through the XAD-8 and XAD-4 resin columns in series. Materials retained on XAD-8 were hydrophobic fractions, whereas materials retained on XAD-4 were transphilic fractions. The materials not retained on either resin comprised the HPI fraction. HPO-A and TPI-A were eluted by backwashing with 0.1 M NaOH from the XAD-8 and XAD-4 columns respectively. The resins were washed with distilled water and extracted with a solution of 75% acetonitrile and 25% ultrapure water (Millipore Milli-Q). Extracts from the XAD-8 and XAD-4 columns were condensed by rotary evaporator and then freeze-dried. Next, the extracts were separately redissolved in Milli-Q water with pH 7, which comprised the HPO-N for XAD-8 columns and TPI-N for XAD-4 columns.

2.4 Molecular weight and polydispersity

The apparent molecular weight (MW) distribution of organic matter was characterized by a dead-end batch ultrafiltration apparatus. The experimental procedure was conducted by Zhao et al. (2012). Wastewater samples were first filtered through a 0.45 μm cellulose acetate fiber filter to remove suspended solids. Then, about 240 ml of wastewater sample was pressed through the membrane disk (63.5 mm) driven by pressure (120 kPa) from an N₂ tank. The MW cutoffs of membrane disks used in the present study were 0.5, 1, 10, and 100 kDa. Wastewater samples were collected in glass bottles and stored at 4 °C for further analysis.

2.5 Molecular spectrum analyses

Ultraviolet (UV) absorbance was measured with a Cary 100 UV–vis spectrometer at 200–500 nm using a quartz cell with 1 cm path length. All samples were adjusted to pH 7 with HCl prior to measurement. Milli-Q water was used as a blank.

For FT-IR analysis, the wastewater samples were freeze-dried under −75 °C and 3 kPa pressure. A 3 mg sample was thoroughly mixed with about 100 mg of KBr desiccated under infrared radiation. The mixture was pressed into a tablet, and infrared spectrum was recorded on a Nicolet 6700 FT-IR spectrometer. The FT-IR spectra of solid samples were obtained in a wave number range of 4000–400 cm⁻¹ and baseline-corrected for analysis. The absorbance bands at 3400 cm⁻¹ and 1640 cm⁻¹ were the absorbance of H₂O, which could not be avoided in KBr tablets.

2.6 GC–MS analysis

Samples were collected from the influents and effluents in the three reactors. Aqueous samples were filtered with 0.45 μm glass filters and stored at 4 °C. Based on the contents of organic compounds, 200 ml of raw wastewater and treated effluents were extracted with CH₂Cl₂ under acidic, neutral, and alkaline conditions using the extraction procedure proposed by Lai et al. (2008). The compositions of organic compounds in the wastewater were analyzed via GC–MS. Then, 1 μL pretreated extracted samples were injected into the GC–MS system (Agilent, USA). The carrier gas was pure helium at a flow rate of 1 ml/min. A 30 m-long HP-5MS capillary column with an inner diameter of 0.25 mm was used in the separation system. The temperature control program was implemented as follows. The initial temperature of 40 °C was maintained for 4 min. Afterward, the oven temperature was increased to 250 °C at a rate of 5 °C/min, to 300 °C at a rate of 10 °C/min, and then maintained for 10 min. Organic compound analysis was conducted according to the National Institute of Standards and Technology 05 mass spectral library database.

2.7 Methods for analyzing wastewater quality

The parameters of COD, BOD₅, pH, ammonium nitrogen (NH₄⁺–N), total nitrogen (TN), total phosphorus (TP), and total dissolved solids (TDS) were measured according to standard methods (EPA of China, 2002). Total organic carbon (TOC) was determined by a fully automated TOC-V_CPH analyzer.

3.1 Comparison of organic matter removal by different technologies

Fig. 2 shows the performance of organic matter removal in terms of COD and TOC by the three systems. The standalone AS system removed 40% of COD and 45% of TOC from the wastewater. The standalone PAC system removed 37% of COD and 38% of TOC. Results also suggested that about 40–45% of organic matter in the wastewater could be biodegradable, whereas residual organic matter could be hard-biodegradable. However, only 37–38% of organic matters were adsorbed by PAC, which indicated that some organic matters were not easily removed by PAC adsorption because of molecular characteristics.

Close

Figure 2

COD and TOC concentrations of the raw wastewater and the effluent samples (values are given as average value with n = 3).

Export to PPT

In the combined PAC–AS system, higher removal efficiencies of COD (64%) and TOC (68%) were obtained, which took advantages of both AS biodegradation and PAC adsorption. Some non-biodegradable organic matters were removed by adsorption. The removal of organic matter by the PAC–AS system involved complex processes. Hence, multiplicate analytical methods such as UV–visible, FT-IR spectroscopy, and GC–MS, were combined to characterize organics in wastewater. Insights into the removal characteristics of organic matters during treatment were thus provided.

3.2 Molecular weight distribution of raw wastewater and treated effluents

Fig. 3 shows the MW distribution of organic matters in raw wastewater and treated effluents. In raw wastewater, the organic matter with MW less than 500 Da accounted for 69% of TOC. Low-MW organic compounds were the main components in the commingled chemical industrial wastewater. The low-MW fraction (<500 Da) was effectively removed in all the treatment systems, whereas the other MW fraction (>500 Da) presented low removal efficiencies. Results suggested that both AS biodegradation and PAC adsorption demonstrated good removal performance on the low-MW fraction (<500 Da). The low-MW organics were easier to move into the microbial cell and micropores inside the activated carbon (Antony et al., 2012; Newcombe, 2002). The fraction with MW of 1–0.5 kDa was reduced from 51 mg/L-TOC to 23 mg/L-TOC by the AS system. However, TOC values with the same MW fraction decreased to 38 mg/L in the PAC system. Organic matter in the fraction of 1 k to 0.5 kDa was more significantly biodegraded compared with PAC adsorption. Organic matter with MW more than 1 kDa was removed insignificantly in the PAC system. The lack of capacity to high MW organics removal of PAC was revealed, which was related to the pore structure of PAC (Ho et al., 2013).

Close

Figure 3

Molecular weight distribution of organic matters in the raw wastewater and treated effluents (values are given as average value with n = 3).

Export to PPT

In PAC–AS SBR system effluent, organic compounds in all size fractions, especially in the <500 Da size fraction, were significantly removed according to the TOC value in Fig. 3. The organic matter removal efficiency in <500 Da size fraction was 86%. An additive effect was shown because of the combination of AS transformation and PAC adsorption.

3.3 Removal characteristics of various organic matter fractions

Fig. 4 shows the removal characteristics of various organic matter fractions by different technologies. In raw wastewater, the organic matters in hydrophobic fractions (including HPO-A and HPO-N) were dominant, accounting for more than 48% of TOC. The compounds in transphilic fractions accounted for 29% and 22% for HPI of the TOC. The results revealed the predominance of hydrophobic organic compounds over the transphilic and hydrophilic compounds in the raw commingled chemical industrial wastewater (Greenwood et al., 2012). These hydrophobic organic compounds may have high persistence properties in aquatic ecosystem (Waller et al., 1996).

Close

Figure 4

TOC of the raw wastewater and the effluents from various systems (upper); TOC removal efficiencies of various organic matter fractions by various treatment systems (middle); Polarity fraction distribution of various samples (lower) (values are given as average value with n = 3).

Export to PPT

In the standalone AS system, the organic matters in TPI-N and HPI fractions showed the highest removal efficiencies of 73% and 60% respectively. However, the removal efficiencies of TPI-N and HPI fractions were 44% and 21% in the PAC system respectively. The PAC system preferred to remove organic matters in HPO-A and HPO-N fractions with removal efficiencies of 70% and 63% respectively, while the AS system removed 47% of organic matters in HPO-A fraction and 53% of compounds in HPO-N fraction. Organic matters in TPI-A fraction were removed inefficiently in the AS and PAC systems, which indicated organic matters in TPI-A fraction were refractory to biodegradation and PAC adsorption.

Biological technologies prefer to remove hydrophilic fractions and transphilic neutral fractions in wastewater, whereas activated carbon prefers to remove hydrophobic fractions (including HPO-A and HPO-N). Previous research reported that hydrophobic organics are apt to be adsorbed and have their final destination on the surface of porous material, such as activated carbon or soil, because of their strong hydrophobicity (Cui et al., 2013). On the contrary, hydrophilic organics can be utilized easier by microorganisms. A similar conclusion was also represented by Wang et al. (2004), who showed that activated carbon can adsorb hydrophobic compounds, whereas microorganisms probably remove hydrophilic biodegradable matters.

In the PAC–AS system, the removal performance of organic matter in various fractions was superior to these in the individual AS and PAC system. The hydrophobic compounds could be biodegraded by AS and adsorbed by PAC (Imai et al., 2001). The organic matters in TPI-N and HPI fractions were mainly transformed by AS, whereas the TPI-A fraction, refractory to biodegradation, was partially adsorbed by PAC. Hence, the PAC–AS system can enhance overall removal performance owing to the additive effects of biodegradation and PAC adsorption.

3.4 UV absorbance analysis of raw wastewater and treated effluents

UV absorbance analysis can reveal preliminary information on organic matter molecule structures and achieve quantitative analysis. Fig. 5 shows the UV scanning spectra of various organic matter fractions in both raw wastewater and treated effluents. The wavelength for UV scanning ranged from 200 to 350 nm, since there is no absorption in the UV region of 350–500 nm. For all the samples (including overall organic matter and various fractions), the treated effluents presented similar but lower absorbance curve compared with the raw wastewater. The organic matters decreased after the three treatment systems. Absorbance curves of the PAC–AS system also decreased significantly compared with that of the individual AS and PAC systems, which were consistent with the results in Figs. 2 and 4.

Close

Figure 5

UV absorption spectra of overall organic matters and various fractions in the raw wastewater and treated effluents.

Export to PPT

In the absorbance curves, the absorption bands were observed in the UV region of 220–240 nm and 250–300 nm, which indicated conjugated C⚌C or C⚌O structure and aromatic rings respectively according to Appendix B. The fractions of HPO-A, HPO-N, TPI-A, and HPI might contain carboxylic acids, esters, phenol, and unsaturated hydrocarbons. HPO-N fractions also contained aromatic ring compounds.

For UV absorbance analysis of treated effluents, the organic matter in various fractions showed similar degrees of removal during the individual AS and PAC system treatments. However, the absorbance of organic matter in HPO-N fraction significantly decreased in the PAC–AS system. The organic matter removal in HPO-N and HPI fractions also showed an additive effect of AS biodegradation and PAC adsorption, which might contain conjugated C⚌C or C⚌O structure and aromatic rings. The organic matters of HPO-A, TPI-A, TPI-N, and HPI fractions during the PAC–AS system treatment also showed similar degrees of removal as the AS and PAC system treatment effluents.

3.5 Functional group investigation of raw wastewater and treated effluent by FT-IR

FT-IR analysis can provide insights into the information of organic compounds molecule structures, which are difficult to find using qualitative and quantitative analysis. FT-IR analysis can also reveal the removal characteristics of organic matter during treatment. Fig. 6 shows the FT-IR spectra of various fractions in the raw wastewater and treated effluents.

Close

Figure 6

FT-IR spectra of organic matter fractions in the raw wastewater and treated effluents.

Export to PPT

In the HPO-A fraction of raw wastewater, the main absorbance bands of organic matter were at 2923, 1625, 1374, 1240–1015, 750, and 650 cm⁻¹, which were indicative of the aliphatic group (—CH₃/—CH₂—), carboxylic acid group (—COO⁻), C—O in carboxylic acid, —(CH₂)_n –(n ⩾ 4) and —OH group respectively. The results suggested that the fraction organic matter might contain fatty carboxylic acids, fatty hydrocarbon, and phenol among others. Similar functional group bands, except the acid group (—COO⁻), were found in the HPO-N fraction because the pH was adjusted to neutral. However, distinct functional groups, including C≡C and the substituent group out of plane aromatic band, were detected in the TPI-A fraction, which indicated a great difference of the organic composition to other fractions. In both TPI-N and HPI fractions, the functional groups of aromatic structure, C—O—C, substituent group out of plane aromatic band and —OH were observed.

For the FT-IR analysis of treated effluent, —OH structure in HPO-A fraction and C—O—C in HPO-N fraction were reduced significantly during treatment. The compounds contained in both structures were easily removed by both AS and PAC systems. However, C—O—C structures in hydrophilic and transphilic fractions were reduced insignificantly. It indicated that esters in hydrophobic fractions were easily removed by AS degradation and PAC adsorption, whereas these compounds in hydrophilic and transphilic fractions were difficultly removed. Other fraction groups might be partially removed because the FT-IR spectroscopic analysis was difficult to conduct to achieve the quantitative analysis by KBr tablet samples.

3.6 Organic matter identification

The organic matters of various fractions in the raw wastewater sample were further identified using the GC–MS technique, as shown in Table 1. The identified compounds confirmed the functional group detected by the previous FT-IR analysis. The organic matters in HPO-A fraction mainly contained hexadecanoic acid, dichlorobenzoic acid, and hydroxybenzal dehyde, which were consistent with the functional groups of the aromatic structure and fatty acid group in Fig. 6. In TPI-A fraction, the compound of benzoic acid was identified, which also contained the functional group of aromatic structure and acid group. Experimental results also indicated that the hydrophobic fraction (HPO-A and HPO-N) mainly contained bicyclic aromatic compounds and heterocyclic ring structures, which were difficult to biodegrade. Most aromatic acid-like compounds were identified in TPI-A fraction.

Table 1 Organic matter identification of various fractions in raw wastewater sample using GC–MS technique.

Fraction	Name	Molecular formula	Molecular structure
HPO-A	n-Hexadecanoic acid	C16H32O2
	2, 4-dichloro-Benzoic acid	C7H4O2Cl2
	3,5-di-tert-butyl-4-hydroxybenzal dehyde	C15H22O2
HPO-N	Dibutyl phthalate	C16H22O4
	Dimethyl phthalate	C10H10O4
	Diisobutyl phthalate	C16H22O4
	Neneicosane	C20H42
	Naphthalene	C10H8
	Methylnaphthalene	C11H10
	Acenaphthene	C12H10
TPI-A	Benzenecarboxylic acid	C7H6O2
	2-methyl-Benzoic acid	C8H8O2
	2, 4-dichloro-Benzoic acid	C7H4O2Cl2
	4-tert-Butyl-2,6-diisopropylphenol	C16H26O
	2,6-dibutyl-nitrophonel	C14H21O3N
TPI-N	2,4-di-tert-butyl-phenol	C14H22O
	2,6-di-tert-butyl-4-methylphenol	C15H24O
	Diethyltoluamide	C12H17NO
	Quinline	C9H7N
	2,4,6-tribromophenol	C6H3OBr3

According to the results of spectrum analysis (Fig. 5 and 6), esters in HPO-N, including phthalate esters, were easily removed by biodegradation and adsorption (Huang et al., 2008, 2010). Phenols in neutral fractions preferred to be utilized by microorganisms (Baird et al., 1974; Dignac et al., 2000). On the contrary, the aromatic compounds and carboxylic acid in hydrophobic and transphilic fractions showed refractory characteristics during treatment.

3.7 Influence mechanism of PAC on activated sludge system

The water used in the experiment was refractory comprehensive industrial wastewater, which had the characteristic of poor biodegradability. Results showed that the removal efficiencies of COD and TOC in PAC–AS process were higher than those in AS and PAC system, which was mainly because of the additive effects of biodegradation and PAC adsorption.

Further analysis found that the addition of PAC in AS system (PAC–AS system) could not only improve the removal of organic matter in low-MW fraction (<500 Da), but also removed the organic matter with MW more than 500 Da. Moreover, PAC had a strong adsorption capacity for hydrophobic substances and absorbed TPI-A fraction partially. The removal of hydrophobic refractory organic compounds and TPI-A fraction was enhanced after the addition of PAC to the AS system.

In addition to the above reasons, the addition of PAC might enhance the biomass and the biological stability, because PAC had large surface area, acting as a supporting medium and supplying bacteria with suitable living micro-environment (Ma et al., 2013, 2012). More biomass could adhere to the surface of PAC and degraded the components adsorbed to the PAC as them being released slowly. The addition of PAC in AS system might also improve the metabolic activity (Hu et al., 2015) of microorganisms and the settleability performance of AS (Aziz et al., 2011). Therefore, in the PAC–AS system, PAC also plays a role in the reinforcement of biodegradation except to the adsorption effect.