Abstract

Inadequate treatment of hospital wastewater could result in considerable risks to public health due to its macro- and micropollutant content. In order to eliminate this problem, a new nanoparticle composite was produced under laboratory conditions and a photocatalytic degradation approach was used. Chemical oxygen demand (COD), biological oxygen demand (BOD5), total suspended solids (TSS), total Kjeldahl nitrogen (TKN), total phosphorus (TP) (macro) and oflaxin (micro) pollutant removal were investigated with the nano graphene oxide magnetite (Nano-GO/M) particles by two different processes, namely adsorption and photodegradation. Low removal efficiencies (21–60%) were obtained in the adsorption process for the parameters given above, after 90 min contact time at a pH of 7.8 with 5 g/L Nano-GO/M composite. Using the photodegradation process, higher removal efficiencies were obtained with 2 g/L Nano-GO/M composite for COD (88%), TSS (82%), TKN (95%) and oflaxin (97%), at pH 7.8 after 60 min irradiation time at a UV power of 300 W. The synthesized nanoparticle was reused for two sequential treatments of pharmaceutical wastewater with no significant losses of removal efficiencies (for oflaxin 97%–90%). The quality of the treated hospital wastewater was first class according to the Turkish Water Pollution Control Regulations criteria. This water could also be used for irrigation purposes.

INTRODUCTION

Hospital wastewaters (HWWs) are complex mixtures capable of generating major environmental problems, since they have been estimated to be between 5 and 15 times more toxic than classic urban effluents (Mendoza et al. 2015). The micropollutants in the HWWs are directly discharged into the sewage system without treatment since the conventional sewage/urban treatment plants can only remove macro-pollutants such as biological oxygen demand (BOD5), chemical oxygen demand (COD), nitrogen and phosphorus (Guney & Sponza 2016). Many of the pharmaceuticals have low biodegradability and they cannot be transformed into ultimate inorganics. In European countries there are specific treatment methods for the direct discharge of HWWs in surface waters. In Turkey, there are no regulations yet for the discharge of HWW into sewage channels. Among widely used antibiotics, fluoroquinolones (FQs; ciprofloxacin, norfloxacin, levofloxacin, ofloxacin, clinafloxacin, etc.) are the most common ones. The selection of appropriate treatment technologies influences the effluent quality of HWW. The quality of reclaimed water for water reuse can also be determined based on the treated effluent limits. The utilization of treated wastewater as a water source should be highlighted for sustainable water management. Hospital discharges can contain disinfectants, detergents, contagious faeces/excreta, biological liquids, drug residues, metal radioelements, and many other chemicals (acids, alkalis, solvents, benzene, hydrocarbons, colorants, etc.) (Fallis 2013). These substances may exhibit different behavior in the wastewater treatment plants due to their different solubility, volatility, molecular weight, adsorbility and biodegradability. If they are not treated in the wastewater treatment, they are released in surface waters (Verlicchi et al. 2010). In İzmir (Turkey), water resources are not sufficient and the water demand has increased due to the increase in the urban population and expansion of industrial and agriculture activities. Therefore, treated water should be used again in various industries. Certain advanced technologies should be used to improve the quality of treated effluent for reusing the treated water in the industrial sectors (Al Aukidy et al. 2014).

Some advanced treatment processes have been reported about oflaxin (OFL) removal from HWWs in the recent literature: Peng et al. (2012) investigated the adsorption of OFL (initial concentration was 10 mg/L) onto carbon nanotubes (CNT; 200 mg/L) at different pHs (pH 2.0–12.0). Seventy six percent OFL removal was obtained at pH = 5 at 25 °C after 7.0 days. Wang et al. (2016), used magnetic chitosan grafted with graphene oxide (MCGO) for the removal of CIP water by adsorption. The maximum adsorption capacity of OFL was 282.9 mg g−1 using the Langmuir model.

Recent studies relevant to OFL removal via photodegradation from HWWs can be summarized as follows: Titouhi & Belgaied (2015), investigated the heterogeneous oxidation of OFL using a composite material synthesized from sodium alginate and cyclohexane dinitrilo tetraacetic acid (CDTA). The maximum removal yield was obtained as 94% at an initial OFL concentration of 10 mg/L at pH 3, after 2 hours of photooxidation. Zheng et al. (2017) used a mesoporous silicon supported Fe-Cu bimetallic catalyst (Fe-Cu@MPSi) for the photodegradation of 30 mg/L OFL. Eighty five percent OFL removal was obtained in 2 hours at 1 g/L catalyst and 2 g/L H2O2.Ding et al. (2017), prepared a magnetite nanoparticle (MNP) throughout activation of persulfate (PS) to degrade norfloxacin (NOR). Ninety percent NOR was degraded within 60 min at a NOR concentration of 15 μM and an MNP concentration of 0.3 g L−1 at pH 4.0.

Kong et al. (2017) and Nguyen et al. (2017) obtained approximately 69–74% OFL removal using an aerobic activated sludge sequencing batch reactor (SBR) at 52 days of hydraulic retention time and a membrane reactor using FS-Sponge. Kong et al. (2017) studied the OFL removal from aqueous solution using activated carbon (AC). Ninety six percent of OFL was adsorbed (132 mg/g) at pH 6 at a temperature of 293 K and at a AC dosage of 0.5 g/L.

Graphene exhibits an extremely high specific surface area (2,600 m2g−1) (Shahriary & Athawale 2014). This can serve as a good support to make the loaded nanoparticles (NPs) achieve uniform distribution without aggregation in the adsorption of pollutants, due to its mechanical, thermal, optical, and electrical properties. The presence of oxygen functionalities in graphene oxide (GO) allows interactions with the cations and provides reactive sites for the nucleation and growth of NPs (Guria et al. 2016). Magnetite (Fe3O4-M) NPs have promising applications in the treatment of antibiotics (Mostofizadeh et al. 2011). Additionally, Fe3O4 NPs are also known to have low cost and eco-friendliness when serving as a stabilizer (Dong et al. 2010). Magnetic Fe3O4 (M) has an advantage for usage as a support material for the Nano-GO/M composite because it can be easily separated by an external magnetic field (Dong et al. 2010). Iron oxide (IO) nanomaterials binding with GO as magnetic adsorbents were useful in the separation of treated wastewater from the GO since they do not need extra filtration or centrifugation (Mostofizadeh et al. 2011).

Recovery of nanocomposites constituted an eco-friendly approach and this decreased the treatment cost of the pollutants. On the other hand, HWW treatment with Nano-GO/M has not been attempted yet via adsorption and photooxidation. The recovery of Nano-GO/M composite and reuse of treated water have not been studied before for the removal of macropollutants and OFL in HWWs. For the reasons given above, in this study, the photocatalytic treatment and adsorption of pollutants in HWW was studied with the Nano-GO/M composite.

In this study, the adsorption and photocatalytic treatment of macropollutants (BOD5, COD, total suspended solids (TSS), total Kjeldahl nitrogen (TKN) and total phosphorus (TP)) and OFL antibiotic as a micropollutant from a real raw HWW were investigated using graphene oxide magnetite composite produced under laboratory conditions. The effects of increasing Nano-GO/M composite concentrations (0.5, 2, 5 and 10 mg/L) both for adsorption and photodegradation was studied. The effects of increasing irradiation times (30, 60, 90 and 120 min), increasing pH levels (4.0, 7.8 and 10.0) and UV power (100, 300, 400 and 600 W) on the photodegradation of the macropollutants mentioned above and the OFL in the raw HWW were investigated. For maximum adsorption and photodegradation of both macro and micropollutants in the HWW, the optimum Nano-GO/M composite dose, irradiation time and pH were investigated. In addition, a cost analysis was performed for optimum operational conditions. Moreover, the reusability of the treated HWW and recovery of Nano-GO/M composite were investigated.

MATERIALS AND METHODS

Preparation of Nano-GO/M

In brief, 5.0 g of graphene was mixed with 2.5 g of NaNO3 and 200 mL of (180:20, 9:1) sulphuric acid and 15 g of KMnO4 mixture at an initial temperature of 18 °C and then heated to 50 °C for 12 hours. After the reaction time, 1.5 mL of hydrogen peroxide was added to the ice-cooled reaction mixture and washed successively using 30% HCl, water, ethanol to remove other reactants with the help of centrifugation. Finally the material was precipitated with diethylether and dried to obtain Nano-GO (Ai et al. 2011). The resulting material was washed with ultrapure water to remove the ions in order to attain neutral pH and then centrifuged at 10,000 rpm to obtain graphene oxide-sodium borohydride (GO-NaBH4). Synthesis of nano GO-Fe3O4 (Nano-GO/M composite) was performed in two steps: Initially, Nano-GO/M composite was prepared by using the one-step co-precipitation and reduction technique. 100 mL of an aqueous dispersion of GO (1 mg/mL) was subjected to sonication for 30 min at 70% amplitude. Initially, 100 mL of GO suspension was taken in a reaction flask with equal volumes of Fe3+and Fe2+ salts solution (molar ratio of 2:1) and the mixture was stirred in an N2 atmosphere for 5 min. The pH of the reaction mixture was adjusted to 10–11 with ammonia solution. Furthermore, 10 mL of hydrazine hydrate was added to the reaction mixture and the temperature of the solution was increased to 85–90 °C with stirring at 1,500 rpm for 3 hours. Then the reaction mixture was cooled to room temperature and magnetically decanted several times with milli-Q water to bring the pH to 7 (Ai et al. 2011). In the second step, Nano-GO/M composite dispersion was mixed with ammonia solution to increase the pH to 10 and heated at 80–85 °C for 24 hours. Synthesized Nano-GO/M composite dispersion was purified by magnetic decantation and was stored at 4 °C until further use (Ye et al. 2014).

Reactors used in the studies

Adsorption studies

Adsorption isotherms were obtained by equilibrium of 1 L solutions of raw HWW with the sorbing medium (Nano-GO/M composite). 0.5, 2, 5 and 10 g Nano-GO/M were added to 1 L of raw HWW and were shaken in an incubator for 30 min, 60 min, 90 min and 120 min. The Nano-GO/M composite particles were settled. Then, the BOD5, COD, TSS, TKN and TP and OFL analyses were performed in the supernatant samples. The most efficient doses used for different pHs (4.0, 7.8, and 10.0) and different reaction times (30 min, 60 min, 90 min and 120 min) were investigated. Equation (1) shows the maximum adsorption capacity:  
formula
(1)
where C0 and Ce (mg/L) are the initial and the equilibrium concentrations of OFL, respectively. V (L) is the volume of the solution and W (g) is the mass of the adsorbent. The maximum OFL adsorption was investigated using an optimum amount of Nano-GO/M composite and at optimum pH.

Photocatalytic studies

Photocatalytic experiments were conducted in an open batch system at a room temperature of 20–25 °C. Quartz glass reactors (with dimensions of 38 cm × 3.5 cm) and 10 UV lamps with a power of 30 W were placed in the closed stainless steel system for the photocatalytic experiments. The effects of Nano-GO/M composite concentrations (0.5, 2, 5 and 10 g/L) irradiation times (30 min, 60 min, 90 min and 120 min), pH (4.0, 7.8, 10.0) on the treatment of the HWW were investigated. After experiments the Nano-GO/M composite was separated magnetically, then analyzed. All the experimental data were found from the duplicates analysis and the results presented as the mean values of the duplicates samples.

Analytical procedure

Measurement of OFL in HPLC

Aqueous OFL stock solution was prepared from the ofloxacin standard (>98%, Bayer AG, Germany, high performance liquid chromatography (HPLC)). An HPLC degasser (Agilent 1100), an HPLC pump (Agilent 1100), an HPLC auto-sampler (Agilent 1100), an HPLC column oven (Agilent 1100) and an HPLC diode-array-detector (DAD) (Agilent 1100) and C-18 (5 μm, 4.6 mm 250 mm, Thermo Scientific) column were used. The analyte was separated at ambient temperature. The mobile phase consisted of acetonitrile (18:82, v/v). The aqueous component of the mobile phase was prepared by dissolving 1.0 g of ammonium acetate and 1.75 g of potassium perchlorate by ultrasonic treatment in 325 mL water and the pH was adjusted to 3.50 using 85% orthophosphoric acid. The column was equilibrated to a stable baseline at a flow rate of 1.0 mL min−1, maintaining the temperature of the column at 45 °C. Detection was at 294 nm (Zivanovic et al. 2006). Also, a C8 column was used for the analysis.

Macro-pollutant measurement methods

COD and BOD5 were measured according to Standard Methods APHA 5220A, APHA 5210A, respectively (APHA/AWWA/WEF 2012). Total nitrogen and total phosphorus were measured with reagent kits in a Photometer Nova 60/Spectroquant. pH was measured with WTW probes. The concentrations of Na, K, Ca and Mg ions were measured by an atomic absorption spectrophotometer (APHA 3111B method). Total dissolved solids (TDS), conductivity and salinity measurements were performed by a portable Mettler Toledo type conductivity meter (APHA 2540C). Bicarbonate was determined by titration method (APHA 2310B method). The total coliform number was enumerated using Standard Methods (APHA 9222 A). The sodium absorption ratio (SAR), residual sodium carbonate (RSC), soluble sodium percentage (SSP), exchangeable sodium percentage (ESP) values were calculated with the following equations:  
formula
(2)
 
formula
(3)
 
formula
(4)
 
formula
(5)

Recovery of Nano-GO/M composite and reusability of treated HWW

After the first use of the Nano-GOM composite, it was separated magnetically and then regenerated using ethanol (adjusted to pH 2.0 with 0.1 mol/L HCl) as eluent (Chowdhury & Balasubramanian 2014). Then the Nano-GO/M composite was dried under vacuum and the Nano-GO/M was used for the second treatment process of the HWW. For every new treatment step the same procedure was applied to the same Nano-GO/M composite.

In the investigation of the reusability of the treated wastewater, the HWW was treated sequentially twice with the new Nano-GO/M NPs to reach irrigation standards or to be used as cooling water, as process water and as cleaning water for toilets and other dirty places.

Characterization of the photocatalytic properties of Nano-GO/M composite

Fourier transform infrared (FT-IR). FT-IR spectra were carried out to identify the functional groups in the synthesized composites and to confirm the chemical bonding between Fe3O4 and graphene. The FT-IR spectra of the Fe3O4, synthesized GO and Nano-GO/M were measured with the Perkin Elmer FTIR Spectrum System using the BX and KBr method.

Scanning electron microscopy (SEM). The morphological and structural observation of the raw Nano-GO/M composite was made on a scanning electron microscope VegaII/LMU (Tescan, Czech Republic).

Scanning probe microscopy (SPM). The particle size of the Nano-GO/M was measured using SPM.

Statistical analysis

Regression analysis is widely used for prediction and forecasting, where its use has a substantial overlap with the field of machine learning. Regression analysis is used to understand which among the independent variables are related to the dependent variable, and to explore the forms of these relationships. Alpha (α) level is the significance of the analysis of variance (ANOVA) statistic. In the study α was accepted as 0.05. The F value of the analysis was obtained using the MS Office 2010 Excel program.

RESULTS AND DISCUSSION

Physicochemical properties of Nano-GO/M composite during adsorption process

FT-IR analysis of Nano-GO/M composite

FT-IR is a powerful tool for verifying the vibrational stretching frequency of GO sheets, Fe3O4 particles and Nano-GO/M composites and the peaks plotted between wavenumber (cm−1) and percentage of transmittance (T-%) (Figure 1). In the FT-IR spectra of GO samples, two peaks at 1734.86 cm−1 were observed, which were derived from the C = O stretching vibration of carboxylates and conjugated carbonyls. A peak at 118.79 cm−1 suggested the C-O stretching vibration of epoxy rings in the as-prepared GO. Note that the characteristic peak at 996.36 cm−1 was often attributed to the C-O stretching of alcohols. The peak at 590.32 cm−1 was the typical Fe-O stretching vibration of Fe3O4 (Zhou et al. 2017).

Figure 1

FT-IR analysis of raw Nano-GO/M (cm−1: wavenumber and T-%: percent transmittance).

Figure 1

FT-IR analysis of raw Nano-GO/M (cm−1: wavenumber and T-%: percent transmittance).

SEM analysis of Nano-GO/M composite

The SEM images of synthesized GO, Fe3O4 NPs and raw Nano-GO/M composites are shown in Figures 24. Figure 2 shows the GO structure which was in sheet form. Fe3O4 NPs can be seen as small dots on GO sheets (Figure 3). The Fe3O4 sample was composed of spherical particles with a size of approximately 20 nm and the GO was composed of wrinkled sheets (Zhang et al. 2015). Many Fe3O4 microspheres were firmly anchored on both sides of the wrinkled graphene sheets with a size of 48 nm (Figure 4). The graphene layers might play a part in hindering the Fe3O4 microspheres from forming aggregations as reported by (Tang et al. 2013). The Fe3O4 particles were dispersed on the GO and there were some interspaces among them.

Figure 2

SEM image of GO.

Figure 2

SEM image of GO.

Figure 3

SEM image of nano Fe3O4.

Figure 3

SEM image of nano Fe3O4.

Figure 4

SEM imagine of raw Nano-GO/M (1 μm).

Figure 4

SEM imagine of raw Nano-GO/M (1 μm).

Characterization of raw HWWs

The characterization of the raw HWW taken from the influent of the Dokuz Eylül University HWW is illustrated in Table 1 with mean and standard deviation values. The COD and BOD5 concentrations were not high (625 ± 12.3 mg/L and 259 ± 10.1 mg/L, respectively) while the OFL concentration varied between 7 and 10 mg/L. The mean TKN and TP concentrations were 13.5 ± 2.4 mg/L and 2.8 ± 1.1 mg/L, respectively, with a low TSS concentration of 197 ± 10.1 mg/L.

Table 1

Characterization of HWW

Macro-/micro pollutant pH BOD5 (mg/L) COD (mg/L) TSS (mg/L) TKN (mg/L) TP (mg/L) OFL concentration (μg/L) 
Raw HWW 7.8 ± 0.2 259 ± 101 625 ± 123 197 ± 101 13.5 ± 2.4 2.8 ± 1.1 7–10 
Macro-/micro pollutant pH BOD5 (mg/L) COD (mg/L) TSS (mg/L) TKN (mg/L) TP (mg/L) OFL concentration (μg/L) 
Raw HWW 7.8 ± 0.2 259 ± 101 625 ± 123 197 ± 101 13.5 ± 2.4 2.8 ± 1.1 7–10 

Effects of Nano-GO/M composite concentrations on the treatment of HWWs by adsorption process

In order to determine the effects of increasing Nano-GO/M composite concentrations (0.5, 2, 5 and 10 g/L) on the adsorption process of some pollutants in the raw HWWs, the adsorption studies were performed at a pH of 7.8 and at a temperature of 21 °C. Preliminary experiments showed that among the adsorption times that were tested, the maximum removal yields were obtained after 90 min adsorption time (data not shown). Therefore, all experiments were performed at 90 min adsorption time. Increasing the Nano-GO/M concentration from 0.5 g/L to 5 g/L significantly increased the OFL and COD adsorption yields from 20% and 40% to 39% and 60% at a temperature of 21 °C and a pH of 7.8 (Figure 5). For the other pollutant parameters in the HWWs (BOD5, TSS, TKN and TP) the adsorption yields increased from 30%, 17%, 30%, 32% and 10% to 42%, 29%, 42% and 19% as the Nano-GO/M was increased from 0.5 g/L to 5 g/L. A multiple linear regression between maximum adsorption efficiencies of all pollutant parameters and Nano-GO/M composite concentrations was obtained (R = 0.99) and this regression was significant (ANOVA p = 0.06 < α (0.05) and F = 100.92) as the Nano-GO/M was increased from 0.5 g/L to 5 g/L. Any further increase of Nano-GO/M from 5 g/L to 10 g/L did not significantly affect the adsorption yields of all pollutants (Figure 5). The multiple regression analysis between all pollutant adsorptions and 10 g/L Nano-GO/M concentration showed no linear regression (R = 0.09) and the correlation was not significant (ANOVA, p = 0.71>α (0.05) and F = 65.07). The results reveal that the removal of pollutants by adsorption from HWW increases up to a certain Nano-GO/M limit (5 g/L) and then it remains almost constant. This can be attributed to an increase in the surface area and the availability of more adsorption sites up to this Nano-GO/M concentration as reported by Hameed (2009). The reason for the same adsorption yields at high Nano-GO/M (10 g/L) levels compared to 5 g/L can be attributed to limited numbers of active sites on the surface of adsorbent in contact with the pollutants in the HWW. As a result, the optimum Nano-GO/M concentration was chosen as 5 g/L for maximum adsorption of COD, BOD5, TSS, TKN, TP and OFL in order to reduce the operational cost of the adsorption process. The optimum amount of Nano-GO/M increases the number of active sites on the adsorbent surface, which causes the adsorption of the pollutants increase (COD, OFL, TSS). Excess catalyst causes turbidity and prevents the adsorption of pollutants onto the surface of Nano-GO/M. At a high catalyst concentration the active sites on the surface of the Nano-GO/M were low and partially exposed to COD and OFL. Therefore, low adsorption yields were obtained at high Nano-GO/M concentrations as reported by Li et al. (2013). The proposed mechanisms for the adsorption of COD, OFL and of the other pollutants in HWW on Nano-GO/M materials mainly involve van der Waals forces (permanent dipole–induced dipole forces and London dispersion forces), hydrophobic interaction, π–π interaction (π was considered as one of the predominant driving forces due to the aromatic rings of OFL), electrostatic interaction and hydrogen bonds (Chen et al. 2015). The π−π interaction has always been applied to explain the binding mechanism of OFL with C = C double bonds adsorbed on the surface of graphene. On the other hand, graphene also contains π electrons and may interact with the π electrons of benzene rings of OFL by means of π−π electron coupling (Chen et al. 2015). Generally, the intensity of van der Waals forces between an adsorbed OFL and Nano-GO/M composite is related to the contacted surface area of them and to the van der Waals index, which is specific to the adsorbent surface (Teixidó et al. 2011). The graphene surface of carbonaceous adsorbents has a very high van der Waals index, and the OFL molecule has a planar ring. However, no significant adsorption yields were obtained between the Nano-GO/M composite and OFL since no strong van der Waals forces occurred between OFL and the Nano-GO/M composite (Chen et al. 2015).

Figure 5

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to Nano-GO/M concentration (T: 21 °C, pH: 7.8, contact time: 90 min) by adsorption process.

Figure 5

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to Nano-GO/M concentration (T: 21 °C, pH: 7.8, contact time: 90 min) by adsorption process.

The maximum adsorption yields with this Nano-GO/M concentration were 39%, 42%, 30%, 45%, 21% and 60% for COD, BOD5 TSS, TKN, TP and OFL after 90 min contact time at a pH of 7.8 and at a temperature of 21 °C (Figure 5). The effluent concentrations of COD, BOD5 TSS, TKN, TP and OFL were obtained as 384.3 mg/L, 150.8 mg/L, 140 mg/L, 7.645 mg/L, 2.291 mg/L and 3.28 μg/L, respectively, after the adsorption process. Since low adsorption yields were obtained for all the pollutant parameters in the HWW, the effects of some operational conditions such as pH, temperature and pollutant concentrations on the adsorption were not studied.

Effects of Nano-GO/M composite concentrations on the treatment of HWWs by photocatalytic process

In this step of this study the HWW containing COD, BOD5 TSS, TKN, TP and OFL was treated via photodegradation under UV light power in the presence of a nanocomposite (Nano-GO/M) generated under laboratory conditions. Nanoparticle concentration is an important parameter for the photo-treatment of pollutants. In order to determine the effects of increasing Nano-GO/M concentrations on the photocatalytic treatment of COD, BOD5 TSS, TKN, TP and OFL' the effects of 0.5 g/L, 2 g/L, 5 g/L and 10 g/L Nano-GO/M composite concentrations were investigated. Preliminary experiments showed that among the irradiation times tested, the maximum OFL removal was obtained after 60 min irradiation time (data not shown). Figure 6 summarizes the photocatalytic treatment efficiencies of COD, BOD5 TSS, TKN, TP and OFL at increasing Nano-GO/M concentrations (0.5, 2, 5 and 10 g/L) at an UV power of 300 W. The photodegradation removal of each pollutant (COD, BOD5, TSS, TKN, TP and OFL) increased significantly with an increase of the Nano-GO/M concentration from 0.5 g/L to 2 g/L. The photo-removal of the pollutants mentioned above increased from 80–90% up to 88–98%, respectively. These yields were also the maximum photodegradation efficiencies. The photocatalytic treatment efficiencies of COD, BOD5 TSS, TKN, TP and OFL (89%, 91%, 84%, 97%, 81%, and 99%, respectively) increased slightly as the Nano-GO/M concentration was increased to 5 g/L. Any further increase of Nano-GO/M from 5 g/L to 10 g/L slightly decreased the photo-removal of all pollutants (Figure 6). At the optimum catalyst concentration (2 g/L) more active sites and hydroxyl radicals in its surface lead to increase in the photodegradation extent of the pollutant. But excessive dosage of the catalyst led to an increase in the suspension turbidity and light scattering (Arabpour & Nezamzadeh-Ejhieh 2015). Consequently, a screening effect of excess particles occurred, which masked a part of the photosensitive surface and hence obstructed the penetration of photons in the solid phase decreases in the activated semiconductors (Arabpour & Nezamzadeh-Ejhieh 2015). Finally, the number of hydroxyl radicals generated and consequently the photodegradation rate tend to decrease. Our results showed that, by increasing the Nano-GO/M concentration from 0.5 to 2 g/L, the surface area of the Nano-GO/M particle was increased, leading to an increase in the production of reactive species as reported by Pelaez et al. (2012). Further increasing the Nano-GO/M composite concentration from 2 to 5 and 10 g/L resulted in a slight decrease of the photodegradation efficiencies of the pollutant parameters (COD, BOD5 TSS, TKN, TP and OFL) in the HWW. A high concentration of the catalyst led to a more turbid solution, thus obstructing the penetration of incident UV radiation and impairing the effectiveness of the photocatalytic process (Bhatia et al. 2016). Therefore, 2 g/L Nano-GO/M composite was used as the optimum to maintain maximum photodegradation yields for all pollutants in the HWW. Furthermore, this optimum Nano-GO/M concentration was used in all experimental studies to reduce the operational cost of the photooxidation process.

Figure 6

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to Nano-GO/M concentration (T: 21 °C, pH: 7.8, irradiation time: 60 min, UV power: 300 W) by photocatalytic process.

Figure 6

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to Nano-GO/M concentration (T: 21 °C, pH: 7.8, irradiation time: 60 min, UV power: 300 W) by photocatalytic process.

The photocatalytic process is initiated by the illumination of a semiconductor catalyst with radiation of an energy higher than the band gap energy of the semiconductor. This irradiation generates electrons (e) and holes (h+) in the conduction band (CB) and valence band (VB) respectively, as given by Equation (6) (Rajamanickam & Shanthi 2016).  
formula
(6)
The electron-hole pair formed may recombine in the bulk lattice or migrate to the surface where its can react with the adsorbents (Rajamanickam & Shanthi 2016). The trapping reaction in the holes proceeds with the formation of hydroxyl radicals as given by Equation (7).  
formula
(7)
 
formula
(8)
The electrons are trapped by dissolved oxygen resulting in the formation of a superoxide ion.  
formula
(9)
The photocatalytic degradability by Nano-GO/M involved the hydroxy radical and holes for oxidation of organic molecules (OM) like COD and BOD5. The reactions are given below.  
formula
(10)
 
formula
(11)
 
formula
(12)
Small molecules such as water and carbon dioxide are eliminated by OFL when this compound is subjected to photocatalysis, as reported by Calza et al. (2008). Subsequent attacks by hydroxyl radicals can lead to the formation of bi- or tri-hydroxylated products or dimerization of the ofloxacin molecule, resulting in by-products (data not shown). Peres et al. (2015) suggested that there are two main routes for the generation of OFL by-products by photocatalytic reactions: dealkylation of the piperazine ring and decarboxylation (removal of the carboxyl group, –COOH) (Peres et al. 2015). Studies have shown that the principal photolytic reactions that occur are that FQs lose fluoride (F−), followed by decarboxylation (Peres et al. 2015).

Photocatalytic removal of dissolved organic TN and TP compounds release inorganic nutrient compounds such as nitrite, ammonia and phosphate as well as CO2 in the presence of nano-TiO2 and nano-Fe2O3 (Helbling & Horacio 2003). Photocatalytic oxidation is proposed for the conversion of ammonia and N-organic into nitrogen gas or nitrate while it was reported that free hydroxyl radicals generated during photodegradation create the possibility of ammonia oxidation to N2 (Maroneze et al. 2014).

Bhatia et al. (2016), studied the photodegradation of 25 mg/L OFL antibiotic using TiO2 (0.5–2 g/L) as a catalyst. After 6 hours' irradiation the maximum OFL photodegradation yield was found to be 72% with 1.5 g/L TiO2 NP concentration. A further increase of TiO2 concentration to 2 g/L decreased the photo-removal efficiency to 56%. This can be explained as follows: the increase in the turbidity of the solution, decreased the degree of light penetration through the solution. Arabpour & Nezamzadeh-Ejhieh (2015) studied the photodegradation of cotrimoxazole antibiotic using IO-supported clinoptilolite NPs and an Hg lamp as the radiation source. They found that increasing the catalyst concentration from 0.25 to 0.5 g/L increased the removal efficiency slightly from 50% to 55%. A further increase in the catalyst dosage from 0.5 g/L to 4 g/L decreased the removal efficiency of cotrimoxazole antibiotic from 55% to almost 35%. Our results were compatible with the studies mentioned above (Nezamzadeh-Ejhieh & Moazzeni 2013; Arabpour & Nezamzadeh-Ejhieh 2015; Bhatia et al. 2016). Rodriguez et al. (2014), found 76% photodegradation yields for 1 mg/L OFL with 4 g/L TiO2 at 360 W UV power after 120 min.

A multiple linear relationship between maximum COD, BOD5 TSS, TKN, TP and OFL photodegradation efficiencies and Nano-GO/M composite concentration (from 0.5 g/L to 2 g/L Nano-GO/M) was obtained (R = 0.89) and this regression was significant (ANOVA p = 0.0009 < α (0.05) and F = 1.10). However, the multiple linear relationship between the maximum photodegradation efficiencies of all pollutants and the Nano-GO/M composite concentration at between 5 and 10 g/L was not obtained (R = 0.16) and this regression was not significant (ANOVA p = 0.21 > α (0.05) and F = 19.08).

Effects of UV power on the treatment of HWWs by the photocatalytic process

UV power is an important parameter for the yield of the photocatalytic process in order to degrade the pollutants. To investigate the effect of a large range UV power, COD, BOD5 TSS, TKN, TP and OFL removal were studied under increasing UV power (100 W, 300 W, 400 W and 600 W) at a pH of 7.8 and a temperature of 21 °C after 60 min irradiation. For maximum removal efficiencies of each pollutant the optimum Nano-GO/M concentration (2 g/L) and the optimum irradiation time (60 min) had been determined in previous stages of the study as mentioned above. As the UV power increased from 100 W to 600 W, the removal yields also increased (Figure 7). However, this increase was not significant in the range of 300–400 W. The removal efficiencies were in the range of 81–88% and 83–89% for COD and BOD5, respectively as the UV power was increased from 100 W to 300 W. TSS and TKN removal efficiencies varied between 72%–82% and 89%–95%, respectively, for UV power between 100 W and 300 W (Figure 7). The TP and OFL photo-removal efficiencies were obtained as 71–79% and 88–97%, respectively, for the same UV power. In the present study, 300 W UV light power was determined as the optimum UV level for the maximum photooxidation yield of the pollutants in the HWW. The UV power determines the amount of photons absorbed by the catalyst. In the photocatalytic treatment process with Nano-GO/M, at increased light power electron-hole pair separation competes with recombination, causing less of an effect on the reaction efficiency (Konstantinou & Albanis 2004). With the increase of the UV power to the optimum, the catalyst absorbs more photons, producing more electron-hole pairs in the catalyst surface, and this increases the concentration of hydroxyl radicals and consequently increases the removal percentage. Earlier studies on the effect of UV light power on the photodegradation of different pollutants, such as dye, have shown almost the same results. Muruganandham & Swaminathan (2006) researched the influence of UV power on the decolorisation of reactive yellow 14 (RY14) at varying UV power (from 16 W to 62 and 97 W). An increase of UV power from 16 W to 62 W increased the decolourisation efficiency from 35.9% to 87.9% at 20 min irradiation for an initial RY14 concentration of 5 × 10−4M. A further increase of UV power decreased the dye photodegradation yield. The solar TiO2 process yielded 65% degradation of the examined substrate and DOC reduction of about 50% in 120 min of the photocatalytic treatment. Michael et al. (2010) found 78% OFL photodegradation yield for 12 mg/L OFL concentration after 89 min irradiation time at an UV power of 445 W using 12 g/L TİO2. A further increase of UV power to 167 W did not significantly affect the OFL oxidation under UV. In our study the OFL yields were higher and the irradiation time and the UV power used were lower than in the study of Michael et al. (Michael et al. 2010). This can be attributed to the high activity of Nano-GO/M, to the type of HWW and to some operational conditions such as temperature and pH.

Figure 7

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to UW power (T: 21 °C, irradiation time: 60 min, Nano-GO/M concentration: 2 g/L) by the photocatalytic process.

Figure 7

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to UW power (T: 21 °C, irradiation time: 60 min, Nano-GO/M concentration: 2 g/L) by the photocatalytic process.

A multiple linear regression between maximum COD, BOD5 TSS, TKN, TP and OFL photodegradation efficiencies and increasing UV power was obtained up to 400 W (R = 0.99) and this regression was significant (ANOVA p = 0.007 < α (0.05) and F = 68.48).

Effects of irradiation time on the treatment of HWWs by photocatalytic process

Although in the previous section it was mentioned that 60 min irradiation was found to be optimum for maximum photodegradation of all pollutants in HWW, in this step a large irradiation interval (30, 60, 90 and 120 min) was studied to determine maximum pollutant yields. The concentration of Nano-GO/M composite was selected as 2 g/L as mentioned previously. The irradiation time experiments were performed at a pH of 7.8 and at 21 °C under 300 W UV light. The maximum removal efficiencies of COD, BOD5 TSS, TKN, TP and OFL were obtained as 88%, 89%, 82%, 95%, 79%, and 97% respectively, after 60 min irradiation time (Figure 8). As the irradiation time was increased from 30 min to 60 min and then to 90 min the removal efficiencies of each pollutant increased. A further increase in the irradiation time from 90 min to 120 min did not affect the removal efficiencies and a saturation plateau was reached. A plausible explanation would be the generation of oxidation products, which would compete with the parent compound for the oxidizing species and for the available radiation, and thus, the photodegradation yield after 120 min remained as it was at 60 min. The removal of pollutants was found to increase linearly with an increase in the retention time from 30 min up to 90 min. The removal efficiencies of pollutant parameters decreased for photooxidation time >90 min since at long irradiation times the surface energy of Nano/GO-M decreases (Upadhyay et al. 2014). Photooxidation can form small molecules such as H2O, CO2 and benzene etc. after long irradiation; it will lead to the decrease of polar groups and the oxygen content of the pollutant surface. Therefore the dispersivity decreases, resulting in low photooxidation yields. The dispersive component of the surface energy, and the density of the GO surface has a great influence on the dispersivity of pollutants in the HWW (Li et al. 2013). Aromatic metabolites of OFL which would adsorb strongly onto the Nano/GO-M surface block a significant part of photo-reactive sites (Li et al. 2013). Kaur et al. (2017) found that the irradiation time did not significantly increase OFL photodegradation using 0.25 g/L silver-modified ZnO after 150 min irradiation at pH 7 (Kaur et al. 2016).

Figure 8

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to irradiation time (T: 21 °C, pH: 7.8, Nano-GO/M concentration: 2 g/L, UV power: 300 W) by photocatalytic process.

Figure 8

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to irradiation time (T: 21 °C, pH: 7.8, Nano-GO/M concentration: 2 g/L, UV power: 300 W) by photocatalytic process.

A linear relationship between maximum COD, BOD5 TSS, TKN, TP and OFL photodegradation efficiencies and irradiation time was obtained (R = 0.95) and this regression was significant (ANOVA p = 0.007 < α (0.05) and F = 4.5).

Effects of pH on the treatment of HWWs by photocatalytic process

The pH of HWW is an important parameter for the treatment mechanism. In this study, the effect of acidic, near-neutral and alkaline pH levels (4.0, 7.8 and 10.0) on the treatment efficiency of the HWW with the Nano-GO/M composite was investigated. All experiments were performed with 2 g/L Nano-GO/M composite at 60 min retention times at 21 °C and under 300 W UV light. The photo-removal efficiencies of COD and BOD5 were obtained as 70%, 88%, 65% and 72%, 89%, 68%, respectively at pH 4.0, 7.8 and 10.0. The TSS and TKN removal efficiencies were obtained as 67%, 82%, 60% and 90%, 95%, 88%, respectively at pH levels of 4.0, 7.8 and 10.0 (Figure 9). The TP photodegradation yields were 70%, 79%, 63% while the OFL removal efficiencies were 89%, 97%, 80% at pH 4.0, 7.8 and 10.0, respectively. Maximum pollutant photodegradation yields were obtained at near-neutral pH levels (7.8). This decreases the treatment cost of the HWW since no chemical was added to adjust the pH for maximum pollutant removal. As a result, it can be seen that as the pH increased from 4.0 to 7.8, the photodegradation yields of all pollutants increased while under alkaline conditions (pH = 10.0) the photo-removal of each pollutant decreased. The zero charge point of (pHpzc) Nano-GO/M is at pH ≈ 5.5 (Liu et al. 2015). Thus, the surface of Nano-GO/M was positively charged when pH < pHpzc, and become negatively charged when pH > pHpzc. OFL was positively charged at pH values lower than the pKa1 (6.05) while it was negatively charged at pH values above the pKa2 (8.11), and neutral at pH values between pKa1 and pKa2 (Peres et al. 2015). The low photodegradation efficiency at more acidic pH (for example, at 4) may be related to the decomposition and corrosion of the catalyst in the acidic medium. By increasing the pH levels, both Nano-GO/M and OFL, and other organic (BOD5, COD) and inorganic (TSS, TKN and TN) molecules, are becoming negatively charged and electronic repulsion occurs between them. The decrease in the photodegradation below a certain pH value can also be explained by both the surface chemical state of Nano-GO/M and the ionization state of ionizable organic molecules at this pH. It is very well-known that for pH values higher than the point of zero charge (pzc) which is pH 6.8 for Nano-GO/M, the surface becomes neutral and it is the opposite for pH < pHpzc. The OFL molecule ionizes easily in neutral media and becomes a soluble OFL anion. Therefore, in neutral solutions, OFL anions are easily adsorbed to Nano-GO/M particles with a positive surface charge. These OFL anions can be oxidized directly by oxygen under visible radiation (Li et al. 2013). That is why high degradation ratios were achieved in neutral pH regions. However, at higher pH values, OFL anions generally move away from the negatively charged surface of Nano-GO/M particles, so the photodegradation ratio decreases. The reason for the decrease in photodegradation ratio in the alkaline medium may be the occurrence of partial radical oxidation degradation (Kaur et al. 2016). Lower degradation rates at acidic pH have also been reported for Nano-GO/M because of an inefficient electron transfer process due to low surface complex bond formation (Kaur et al. 2016). As a consequence, photodegradation yields decreased at acidic and basic conditions. Kaur et al. (2017) investigated the photodegradation efficiency of OFL in the reaction time of 80 min under acidic and alkaline conditions. For maximum OFL (79%) photodegradation efficiency the optimum pH was found to be 7.9 (Kaur et al. 2017).

Figure 9

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to pH levels (T: 21 °C, irradiation time: 60 min, Nano-GO/M concentration: 2 g/L, UV power: 300 W) by photocatalytic process.

Figure 9

The comparison of the efficiencies of COD, BOD5, TSS, TKN, TP and OFL according to pH levels (T: 21 °C, irradiation time: 60 min, Nano-GO/M concentration: 2 g/L, UV power: 300 W) by photocatalytic process.

A multiple linear relationship between maximum COD, BOD5 TSS, TKN, TP and OFL photodegradation efficiencies and pH levels was not obtained (R = 0.53) and this regression was not significant (ANOVA p = 0.11 > α (0.05) and F = 0.39).

Effect of temperature on the treatment of HWWs by the photocatalytic process

Temperature is an important parameter for the photocatalytic process. To investigate the effect of temperature, HWW photodegradation was studied under different temperatures (20 °C, 40 °C, 60 °C and 120 °C). For maximum HWW photodegradation the optimum Nano-GO/M concentration (2 g/L) and optimum irradiation time (60 min) were determined in the preliminary studies as mentioned above. The photocatalytic treatment efficiency of HWWs increased (from 56% to 96% for OFL) with an increase in the temperature from 20 °C to 60 °C (data not shown). Increasing the temperature from 60 °C to 120 °C did not increase the photocatalytic treatment efficiency as reported by Titouhi & Belgaied (2015). At 20 °C, photooxidation of the antibiotic was not complete. The free radical concentration usually increases with increasing reaction temperature, as reported by Bautista et al. (2010), allowing an improvement of molecule degradation. Kondru et al. (2009), observed similar results in their research. It is established that increasing the working temperature allows reactant molecules to overcome the activation energy barrier by providing more energy which makes the oxidative reaction easy (Hassan & Hameed 2011). The OFL concentration did not decrease at higher temperature (120 °C) but reached a minimum at 60 °C. At high temperatures a photochemical disaggregation of OFL was not recorded due to the binding sites of Nano-GO/M on OFL being destroyed and insoluble hydrolysis products being produced documented by Porcal et al. (2015).

Recovery of Nano-GO/M NPs for their reutilization

Nano-GO/M NPs could be reused to create a cost-effective process. Magnetic separation of this nanocomposite provides a very convenient approach for removing and recycling magnetic particles (such as magnetite) by applying external magnetic fields. Most water or wastewater treatment systems require settling, filtration or centrifuge processes to separate the solids from the treated water. However, the magnetic NPs used in our study can be separated and recovered with an external magnetic field due to the intrinsic magnetic characteristic of the Nano-GO/M NPs, helping to achieve a significant recovery of the NPs without a filtration process (Tang & Lo 2013). Catalyst cost usually dominates the photocatalytic processes. As a consequence, the reusability of the catalyst becomes important. In this study, two sequential photo-treatment steps were applied in order to determine the reusability of the Nano-GO/M composite, the same 2 g/L Nano-GO/M composite that was used for two sequential treatments. The photo-reactor was operated under the same operational conditions mentioned above (irradiation time: 60 min, UV power: 300 W, pH: 7.8 at room temperature). During each sequential new photo-treatment step the Nano-GO/M composite was cleaned and used again. From Figure 10, it can be clearly seen that with the utilization of the Nano-GO/M composite after the second sequential treatment, the photo-removal efficiencies of COD and BOD5 decreased slightly from 88% and 89% to 82% and 83%. After the second sequential treatment the photo-removal efficiencies of TSS and TKN decreased to 82%–75% and 95%–90%, respectively, while the photodegradation yields of TP and OFL decreased from 79% to 71% and from 97% to 94%, respectively (Figure 10). As shown, the yields did not decrease significantly (ANOVA R = 0.98 p = 0.008 < α (0.05) and F = 96.33). Slight decreases in pollutant yields are not important since only 2 g/L Nano-GO/M needs to be used during the two sequential treatment steps with high pollutant yields (in the range of 97%–71%) to treat the HWW. This decreases the cost of producing the nanocomposite.

Figure 10

COD, BOD5, TSS, TKN, TP and OFL measurement by recovery of Nano-GO/M composite (T: room temperature, Nano-GO/M composite concentration: 2 g/L, UV irradiation time: 60 min, UV power: 300 W, pH: 7.8).

Figure 10

COD, BOD5, TSS, TKN, TP and OFL measurement by recovery of Nano-GO/M composite (T: room temperature, Nano-GO/M composite concentration: 2 g/L, UV irradiation time: 60 min, UV power: 300 W, pH: 7.8).

Reusing treated HWW: water quality evaluation

Treated wastewater can be effectively used for many useful urban, agriculture and industrial applications as long as it are treated adequately. The treated water can be used for irrigation, landscaping and recreational irrigation purposes where significant savings can be achieved by reducing the purchase of fresh water. The water quality requirement of each end-user determines the type of wastewater and its degree of treatment (Vergine et al. 2016; Rekik et al. 2017; Urbano et al. 2017). The treated water can be used in urban areas for irrigation and landscaping of public parks, recreational fields, school yards, golf courses, highway medians and residential areas, fire protection, toilet flushing in commercial and industrial buildings and vehicle washing (Vergine et al. 2016; Rekik et al. 2017; Urbano et al. 2017). Treated water can be used in agriculture for the irrigation of non-food crops (seed crops, industrial crops, processed food crops, fodder crops, orchard crops, etc.), in commercial nurseries, as irrigation for food crops and for the watering of livestock. Treated HWW can be used in recreational impoundments such as artificial lakes and ponds by creating artificial wetlands and enhancing natural wetlands, sustaining and augmenting river or stream flows and for non-potable reuse like groundwater recharge/recovery of treated water for subsequent reuse or discharge and recharge of adjacent surface streams (Vergine et al. 2016; Rekik et al. 2017; Urbano et al. 2017). Furthermore, it can be used as industrial reuse, for example for process cooling, cooling towers and boiler water.

In this study, in order to reuse the treated wastewater; the HWW was sequentially treated twice with new Nano-GO/M NPs. After the first and second treatments of the HWW with 2 g/L Nano-GO/M composite, high removal efficiencies were obtained for COD, BOD5, TSS, TKN, TP and OFL. Table 2 shows the removal and effluent concentrations of the aforementioned pollutants after the first and second sequential treatment steps. The initial 8.2 mg/L OFL; reduced to 0.246 and 0.00246 mg/L after first and second sequential treatments.630 and 260 mg/L COD and BOD5 reduced to 75.6 and 28.6 mg/L after the first treatment while their concentrations decreased to 7.76 and 2.28 mg/L after the second treatment. Electrical conductivity (ECw) and RSC levels decreased from an initial 15.61 and 3.2 to 3.12 and 0.34, respectively, then to 0.43 and 0.00352 after the second treatment. All the pollutant parameters given in Table 2 were removed with yields varying between 80% and 97%.

Table 2

Raw HWW and effluent concentrations of each pollutant in HWW at maximum removal efficiencies after second treatment (T: room temperature, Nano-GO/M composite concentration: 2 g/L, UV irradiation time: 60 min, UV power: 300 W, pH: 7.8)

Parameters Raw HWW After second treatment Removal efficiency (%) 
COD (mg/L) 630 7.56 98.8 
BOD5 (mg/L) 260 2.288 99 
TSS (mg/L) 200 5.4 97 
TKN (mg/L) 13.9 0.02085 100 
TP (mg/L) 2.9 0.10353 96 
OFL (μg/L) 8.2 0.00246 99 
Ca (mg/L) 269 3.497 98 
Mg (mg/L) 174 3.8976 98 
ECw 15.61 0.43708 97 
TDS (mg/L) 3,985 135.49 97 
SAR – 4.68 – 
Na (mg/L) 1,213 9.704 99 
K (mg/L) 92.3 2.98129 97 
Bicarbonate 421 8.2095 98 
RSC 3.2 0.00352 99 
SSP  48.32695797  
ESP  5.341574927  
pH 7.8 ± 0.2 7.8 ± 0.2 7.8 ± 0.2 
Coliform (colony number/100 mL)   
Parameters Raw HWW After second treatment Removal efficiency (%) 
COD (mg/L) 630 7.56 98.8 
BOD5 (mg/L) 260 2.288 99 
TSS (mg/L) 200 5.4 97 
TKN (mg/L) 13.9 0.02085 100 
TP (mg/L) 2.9 0.10353 96 
OFL (μg/L) 8.2 0.00246 99 
Ca (mg/L) 269 3.497 98 
Mg (mg/L) 174 3.8976 98 
ECw 15.61 0.43708 97 
TDS (mg/L) 3,985 135.49 97 
SAR – 4.68 – 
Na (mg/L) 1,213 9.704 99 
K (mg/L) 92.3 2.98129 97 
Bicarbonate 421 8.2095 98 
RSC 3.2 0.00352 99 
SSP  48.32695797  
ESP  5.341574927  
pH 7.8 ± 0.2 7.8 ± 0.2 7.8 ± 0.2 
Coliform (colony number/100 mL)   

Tables 36 show the guidelines for interpretation of irrigation water quality, limits for chemical quality of treated wastewater for utilization in irrigation, limits for maximum allowable heavy metal and toxic element concentrations in irrigation and quality criteria according to the class of continuous water resources, respectively (WPCR 2010; SWQMR 2012). Table 3 shows the limits given for irrigation using treated HWW. In other words, the effluent after the second treatment (Table 2) was evaluated for irrigation purposes. Based on the evaluation of salinity, infiltration, toxicity and miscellaneous hazards for the suitability of treated hospital water it was found that it was suitable for agricultural irrigation. Also, electrical conductivity (ECw) and its specific ion toxicities, Na+, Ca2+ and Mg2+, which impact on the SAR values, were compared to the irrigation water guidelines. The treated hospital water gives a SAR value of 4.68 with an ECw 0.43708 dS/m. Tables 2 and 3 show the limits for SAR and ECw are 3–6 and >1.2, respectively). Results after the second treatment exhibited a good-quality effluent suitable for agricultural irrigation. The water quality assessment based on SAR, SSP, ESP and Na+ concentration in the above suggested blending ratio results in the reduction of sodium hazard and specific ionic toxicity and miscellaneous hazards (Tables 2 and 3). The sodium hazard related to RSC is completely resolved by this blending strategy. RSC values become negative, which is required for agricultural irrigation water (Tables 2 and 3).

Table 3

Guidelines for interpretation of irrigation water quality (WPCR 2010)

Potential irrigation problem Units Degree of restriction on use
 
None Slight to moderate Severe 
Salinity (affects crop water availability
 ECw dS/m <0.7 0.7–3.0 >3.0 
 TDS mg/L <450 450–2,000 >2,000 
Infiltration (affects infiltration rate of water into the soil. Evaluate using ECw and SAR together
 SAR =0–3 ECw  >0.7 0.7–0.2 <0.2 
 =3–6   >1.2 1.2–0.3 <0.3 
 =6–12   >1.9 1.9–0.5 <0.5 
 =12–20   >2.9 2.9–1.3 <1.3 
 =20–40   >5.0 5.0–2.9 <2.9 
Specific ion toxicity (affects sensitive crops
 Sodium (Na)        
  Surface irrigation    SAR <3 3–9 >9 
  Sprinkler irrigation    me/L <3 >3  
 Chloride (Cl)        
  Surface irrigation    mg/L <140 140–350 >350 
  Sprinkler irrigation    mg/L <100 >100  
 Boron (B)    mg/L <0.7 0.7–3.0 >3.0 
Miscellaneous effects (affects susceptible crops
 Nitrogen (NO3-N)    mg/L <5 5–30 >30 
 Bicarbonate (HCO3) (overhead sprinkling only)    me/L <1.5 1.5–8.5 m > 8.5 
 Other parameters for sodium toxicity        
 RSC     <0 0–1 >1 
 SSP     <60 60–80 >80 
 ESP     2–10 10–40 >40 
 pH     Normal range 6.5–8.4 
Potential irrigation problem Units Degree of restriction on use
 
None Slight to moderate Severe 
Salinity (affects crop water availability
 ECw dS/m <0.7 0.7–3.0 >3.0 
 TDS mg/L <450 450–2,000 >2,000 
Infiltration (affects infiltration rate of water into the soil. Evaluate using ECw and SAR together
 SAR =0–3 ECw  >0.7 0.7–0.2 <0.2 
 =3–6   >1.2 1.2–0.3 <0.3 
 =6–12   >1.9 1.9–0.5 <0.5 
 =12–20   >2.9 2.9–1.3 <1.3 
 =20–40   >5.0 5.0–2.9 <2.9 
Specific ion toxicity (affects sensitive crops
 Sodium (Na)        
  Surface irrigation    SAR <3 3–9 >9 
  Sprinkler irrigation    me/L <3 >3  
 Chloride (Cl)        
  Surface irrigation    mg/L <140 140–350 >350 
  Sprinkler irrigation    mg/L <100 >100  
 Boron (B)    mg/L <0.7 0.7–3.0 >3.0 
Miscellaneous effects (affects susceptible crops
 Nitrogen (NO3-N)    mg/L <5 5–30 >30 
 Bicarbonate (HCO3) (overhead sprinkling only)    me/L <1.5 1.5–8.5 m > 8.5 
 Other parameters for sodium toxicity        
 RSC     <0 0–1 >1 
 SSP     <60 60–80 >80 
 ESP     2–10 10–40 >40 
 pH     Normal range 6.5–8.4 
Table 4

Limits for chemical quality of treated wastewater for utilization as irrigation purpose (WPCR 2010)

Recovery type Treatment type Quality of recovered watera Monitoring period Application distanceb 
Class A 
  • a) Agricultural irrigation: commercially unprocessed foodl

  • b) Urban Area Irrigation

 
  • a) Superficial and sprinkler irrigation were used on foods that are eaten raw

  • b) Every kind of green field irrigation (parks, golf courses etc.)

 
  • - Secondary treatmentc

  • - Filtrationd

  • - Disinfectione

 
  • - pH = 6-9

  • - BOD5 < 20 mg/L

  • - Turbidity <2 NTUf

  • - Fecal coliform: 0/100 mLg,h

  • - In some cases, specific virus, protozoa and helminth analysis might be needed

  • - Residual chlorine >1 mg/Li

 
  • - pH: weekly

  • - BOD5: weekly

  • - Turbidity: constant

  • - Coliform: daily

  • - Residual Chlorine: constant

 
At least 50 m away from  fresh water wells. 
Class B 
  • a) Agricultural irrigation: commercially processed foodm

  • b) Access restricted irrigation sites

  • c) Agricultural irrigation: non-edible plants

 
  • a) Using surface irrigation on harvests such as fruit orchards and vineyards

  • b) Places such as grass production and cultural agriculture where public entrance is restricted

  • c) Pasture irrigation for grass-fed animals

 
  • - Secondary treatmentc

  • - Disinfectione

 
  • - pH = 6-9

  • - BOD5 < 30 mg/L

  • - TSS <30 mg/L

  • - Fecal coliform <200 number/100 mLg,j,k

  • - In some cases, specific virus, protozoa and helminth analysis might be needed.

  • - Residual chlorine >1 mg/Li

 
  • - pH: weekly

  • - BOD5: weekly

  • - TSS: daily

  • - Coliform: daily

  • - Residual chlorine: constant

 
  • - At least 90 m away from fresh water wells.

  • - Sprinkler irrigation is used, at least 30 m away from the residential area.

 
Recovery type Treatment type Quality of recovered watera Monitoring period Application distanceb 
Class A 
  • a) Agricultural irrigation: commercially unprocessed foodl

  • b) Urban Area Irrigation

 
  • a) Superficial and sprinkler irrigation were used on foods that are eaten raw

  • b) Every kind of green field irrigation (parks, golf courses etc.)

 
  • - Secondary treatmentc

  • - Filtrationd

  • - Disinfectione

 
  • - pH = 6-9

  • - BOD5 < 20 mg/L

  • - Turbidity <2 NTUf

  • - Fecal coliform: 0/100 mLg,h

  • - In some cases, specific virus, protozoa and helminth analysis might be needed

  • - Residual chlorine >1 mg/Li

 
  • - pH: weekly

  • - BOD5: weekly

  • - Turbidity: constant

  • - Coliform: daily

  • - Residual Chlorine: constant

 
At least 50 m away from  fresh water wells. 
Class B 
  • a) Agricultural irrigation: commercially processed foodm

  • b) Access restricted irrigation sites

  • c) Agricultural irrigation: non-edible plants

 
  • a) Using surface irrigation on harvests such as fruit orchards and vineyards

  • b) Places such as grass production and cultural agriculture where public entrance is restricted

  • c) Pasture irrigation for grass-fed animals

 
  • - Secondary treatmentc

  • - Disinfectione

 
  • - pH = 6-9

  • - BOD5 < 30 mg/L

  • - TSS <30 mg/L

  • - Fecal coliform <200 number/100 mLg,j,k

  • - In some cases, specific virus, protozoa and helminth analysis might be needed.

  • - Residual chlorine >1 mg/Li

 
  • - pH: weekly

  • - BOD5: weekly

  • - TSS: daily

  • - Coliform: daily

  • - Residual chlorine: constant

 
  • - At least 90 m away from fresh water wells.

  • - Sprinkler irrigation is used, at least 30 m away from the residential area.

 

aUnless otherwise stated, indicates the quality of treated wastewater.

bLimitation to protect water resources and therefore people from the effects of treated wastewater.

cSecondary treatment may include activated sludge systems, biodiscs, trickling filters, stabilization basins, aerated lagoons, etc.

dFiltration can be through a membrane, such as ultrafiltration with sand filters or microfiltration.

eClarification, as a disinfectant does not restrict the use of other disinfection methods.

fThe turbidity value (maximum 5 NTU) must be provided before disinfection. Suspended solid material (5 mg/L) can be used instead of turbidity.

g7 days.

hMaximum fecal coliform value: 14 colony/100 mL.

iBalance the chlorine value after 30 min contact time.

jMaximum fecal coliform value: 800 colony/100 mL.

kStabilization basins can provide fecal coliform value for disinfection.

lAdvanced treatment should be applied.

mCommercially processed food products are undergo a physical or chemical process to kill pathogenic microorganisms before being sold to the public.

Table 5

Limits for maximum allowable heavy metal and toxic element concentrations in irrigation (WPCR 2010) and their removal

Elements Maximum total quantities that can be given to the unit area, kg/ha Maximum permissible concentrations
 
Influent concentrations of raw HWW Removal efficiencies of treated HWW (%) Effluent concentrations of HWW after second treatment 
Limit values for continuous irrigation on all types of ground mg/L When irrigating less than 24 years on clayey soil with pH value between 6.0–8.5, mg/L 
Chromium (Cr) 90 0.1 1.0 0.06 99 0.0006 
Cobalt (Co) 45 0.05 5.0 0.09 99 0.0009 
Copper (Cu) 190 0.2 5.0 0.25 99 0.0025 
Iron (Fe) 4,600 5.0 20.0 0.18 99 0.0018 
Lead (Pb) 4,600 5.0 10.0 1.11 99 0.0111 
Lithium (Li) – 2.5 2.5 0.02 99 0.0002 
Nickel (Ni) 920 0.2 2.0 0.08 99 0.0008 
Zinc (Zn) 1,840 2.0 10.0 0.45 99 0.0045 
Elements Maximum total quantities that can be given to the unit area, kg/ha Maximum permissible concentrations
 
Influent concentrations of raw HWW Removal efficiencies of treated HWW (%) Effluent concentrations of HWW after second treatment 
Limit values for continuous irrigation on all types of ground mg/L When irrigating less than 24 years on clayey soil with pH value between 6.0–8.5, mg/L 
Chromium (Cr) 90 0.1 1.0 0.06 99 0.0006 
Cobalt (Co) 45 0.05 5.0 0.09 99 0.0009 
Copper (Cu) 190 0.2 5.0 0.25 99 0.0025 
Iron (Fe) 4,600 5.0 20.0 0.18 99 0.0018 
Lead (Pb) 4,600 5.0 10.0 1.11 99 0.0111 
Lithium (Li) – 2.5 2.5 0.02 99 0.0002 
Nickel (Ni) 920 0.2 2.0 0.08 99 0.0008 
Zinc (Zn) 1,840 2.0 10.0 0.45 99 0.0045 
Table 6

Quality criteria according to the class of continuous water resources (SWQMR 2012)

  First class Second class Third class Fourth class Our results after first treatment Our results after second treatment 
COD (mg/L) 25 50 70 >70 75.6 7.56 
BOD5 (mg/L) 20 >20 28.6 2.228 
TKN (mg/L) 0.5 1.5 >5 0.695 0.02085 
pH 6.5–8.5 6.5–8.5 6.0–9.0 6.0–9.0 7.8 7.8 
  First class Second class Third class Fourth class Our results after first treatment Our results after second treatment 
COD (mg/L) 25 50 70 >70 75.6 7.56 
BOD5 (mg/L) 20 >20 28.6 2.228 
TKN (mg/L) 0.5 1.5 >5 0.695 0.02085 
pH 6.5–8.5 6.5–8.5 6.0–9.0 6.0–9.0 7.8 7.8 

The limits for conductivity and total dissolved solid are <0.7 dS/cm and <450 mg/L while for sodium adsorption rate and sodium they are >0.7 meq/L <3 mg/L, respectively (Table 3). For chlorine and boron, the limit values are <140 mg/L, <0.7 mg/L, respectively (Table 3; WPCR 2010). Limit values of reuse for pH, BOD5 and coliform are 6–9, <30 mg/L, <200 number/100 mL, respectively. According to the limits given above, pH, salt, conductivity, ions (Ca+2 and Mg+2), oil and grease, pathogen microorganism, and heavy metal concentration in the HWW were measured before being used for irrigation water (Table 2). After the second sequential treatment the effluent concentrations of the parameters given above were suitable for the limits given by the irrigation parameters (Tables 2 and 3). Table 4 shows the limits given for the chemical quality of the treated water and the class of the wastewater. Table 5 shows the maximum allowable limits for heavy metals and toxic element concentrations necessary in the irrigation of treated water and the treatment results of HWW after first and second treatments. Table 6 illustrates the quality criteria according to the class of continuous water resources and the quality of the treated HWW after first and second treatments. According to Turkish Water Pollution Control Regulations after second treatment of the HWW, the effluent values of the treated HWW could be classified as first-class quality according to the parameters studied in this work (Tables 26; SWQMR 2012). Furthermore, the treated HWW could be used as cooling water, and also for cleaning water for toilets and other dirty places after disinfection as mentioned in the above section. The use of wastewater for irrigation is regarded as a way to address the imbalance between water demand and water supply.

COST ANALYSIS

Cost analysis was carried out for the photodegradation of 1L HWW using 2 g/L Nano-GO/M composite under 300 W UV light. Electricity, 10 UV lamps and Nano-GO/M composite cost were 0.014 €, 76.20 € and 4.40 €. All the consumptions figures were calculated and are given in Table 7. The total cost for the photodegradation of the HWW was 80.65 €. A study including the cost for the photooxidation of OFL with Nano-GO/M was not found in recent literature. Therefore, the cost analysis in our study was compared with some other NPs. Rodríguez-Mozaz et al. (2015) calculated a total cost of 98 € to photodegrade 76% of 1 mg/L OFL with 4 g/L TiO2 at 360 W UV power after 120 min. In a recent study, performed by Ahmadzadeh et al. (2017) the total cost was calculated as 102 € to photodegrade 79% of 32.5 mg/L FQs from HWW using electrocoagulation (EC) by aluminum electrodes under optimal operating conditions (at pH 7.78, at reaction time 20 min, a current density 12.5 mA/cm2 and at an electrolyte dose of 0.07 M NaCl with an electrical energy consumption cost of 0.613 kWh/min). Ge et al. (2015) found a total cost of 89 € for the photooxidation of 19 mg/L FQs using 290 W UV power after 78 min irradiation time using 37 mg/L Nano ZnO. These costs are slightly higher than our study (Ge et al. 2015).

Table 7

Cost analysis for photocatalytic treatment of HWW under UV light

Cost analysis Treatment of HWW under UV light 
UV 1 UV lamp: 7.62 €
10 UV lamps: 76.20 € 
Electricity consumption 60 min UV irradiation: 0.014 € 
Chemicals Magnetite (Fe3O4) (1 kg): 12.99 €
Graphene (1 kg): 151.95 € 
Cost analysis for treatment of 1 L HWW:
2 g Nano-GO/M was used for treatment of 1 L HWW under UV light. 100 mg magnetite, 20 g graphene was used for prepare 2 g Nano-GO/M. 
For 2 g Nano-GO/M:
100 mg magnetite: 1.40 €
20 g: 3.04 € 
Total cost for treatment of 1 L HWW: 76.20 + 0.014 + 1.4 + 3.04 = 80.65 € 
Cost analysis Treatment of HWW under UV light 
UV 1 UV lamp: 7.62 €
10 UV lamps: 76.20 € 
Electricity consumption 60 min UV irradiation: 0.014 € 
Chemicals Magnetite (Fe3O4) (1 kg): 12.99 €
Graphene (1 kg): 151.95 € 
Cost analysis for treatment of 1 L HWW:
2 g Nano-GO/M was used for treatment of 1 L HWW under UV light. 100 mg magnetite, 20 g graphene was used for prepare 2 g Nano-GO/M. 
For 2 g Nano-GO/M:
100 mg magnetite: 1.40 €
20 g: 3.04 € 
Total cost for treatment of 1 L HWW: 76.20 + 0.014 + 1.4 + 3.04 = 80.65 € 

The cost of chemicals is calculated according to market prices. Electricity costs are calculated according to consumer price industrial units.

CONCLUSION

The adsorption and the photocatalytic degradation of HWW were investigated in the presence of Nano-GO/M NPs. Low adsorption yields were obtained for pollutant removal (39%, 42%, 30%, 45%, 21% and 60% for COD, BOD5 TSS, TKN, TP and OFL, respectively) in the HWW with 5 g/L Nano-GO/M after 90 min contact time at a pH of 7.8, compared to the photodegradation process. The maximum pollutant photodegradation yields (COD, BOD5 TSS, TKN, TP and OFL were 88%, 89%, 82%, 95%, 79%, and 97%, respectively) which were obtained with 2 g/L Nano-GO/M concentration after 60 min irradiation time under 300 W UV power. At this optimum Nano-GO/M dose, the diameter of the nanoparticle increased, leading to an increase in the production of reactive species such as OH. A high dosage of the catalyst (5 and 10 g/L) led to an increase in the suspension turbidity which can mask the penetration of photons. Increasing the UV power from 100 W to 300 W also increased the removal efficiencies of all pollutants in the HWW. With the increase of UV power to the optimum, the catalyst absorbs more photons, producing more electron-hole pairs in the catalyst surface, and this increases the concentration of hydroxyl radicals. Photodegradation of the HWW depended on pH levels. The best results were obtained at the original pH of the HWW (pH: 7.8). The photodegradation yields decreased under acidic and basic conditions. The total cost of the photodegradation process was 80.65 €. The Nano-GO/M composite was able be synthesized easily and reused twice. Furthermore, after the second treatment of the HWW, the water quality obtained was first class according to the Turkish Water Pollution Control Regulations criteria (SWQMR, 2012). This treated water could be used for the irrigation of green places, cooling water or process water in different industries.

ACKNOWLEDGEMENTS

The authors would like to express appreciation for the support of the sponsors Dokuz Eylul University Scientific Research Project (KB.FEN.020) and TUBITAK 2210-C National grant for Master Thesis relevant to priority subjects.

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