Abstract

This study investigated the effect of different inorganic salts on the treatment of simulated secondary effluent (SE) by ozone (O3) in a rotating packed bed (RPB) – (O3-RPB process), with Escherichia coli (E. coli) and UV254 of fulvic acid as the indicators. The inactivation efficiency of E. coli and removal percentage of UV254 were studied under different rotation speeds of the RPB and varying concentrations of inorganic salts such as NaHCO3, Na2SO4 and CaCl2. Results indicate that both the inactivation efficiency of E. coli and removal percentage of UV254 increased with an increasing rotation speed of the RPB but decreased with an increase in concentrations of the inorganic salts. Analyses on the mechanism of the treatment process reveal that the inorganic salts consume O3 and ·OH to generate products with lower oxidation ability, and thus result in a poor treatment effect on the effluent. This work provides fundamentals for the O3-RPB process in the treatment of SE from urban wastewater treatment plants.

NOMENCLATURE

     
  • A0

    UV254 value of the SE before ozonation in RPB

  •  
  • At

    UV254 value of the SE after ozonation in RPB

  •  
  • C(CaCl2)

    CaCl2 concentration, mg/L

  •  
  • C(NaHCO3)

    NaHCO3 concentration, mg/L

  •  
  • C(Na2SO4)

    Na2SO4 concentration, mg/L

  •  
  • C(Ozone)

    gaseous ozone concentration, mg/L

  •  
  • N0

    number of E. coli colonies in the SE before ozonation in RPB, cfu/mL

  •  
  • N

    number of E. coli colonies in the SE after ozonation in RPB, cfu/mL

  •  
  • T

    temperature, °C

  •  
  • βB

    removal percentage of UV254, %

  •  
  • ηE

    inactivation efficiency of E. coli

INTRODUCTION

Water from urban wastewater treatment plants (UWTPs) is usually used for landscaping, industrial cooling and even agricultural activities in an effort to mitigate water crisis (Ferro et al. 2015). Nevertheless, the secondary effluent (SE) from the UWTPs contains a variety of organic and inorganic compounds as well as microorganisms that limit its utilization. Soluble organic compounds in the effluent are mainly humic acids, which are refractory natural organic substances. Fulvic acid is a typical humic acid that accounts for a majority of the total soluble organic compounds in the SE. The effluent also contains a large number of microorganisms such as Escherichia coli (E. coli), which has a considerable survival rate and easily spreads infectious diseases (Rizzo et al. 2013). E. coli has thus been used as an indicator microorganism to represent the environmental bacteria (Paruch & Mæhlum 2012).

Most of the microorganisms in the SE are pathogenic and should therefore be inactivated. Existing inactivation methods include ultraviolet sterilization, chlorine disinfection, chlorine dioxide disinfection and ozone disinfection. Ultraviolet sterilization is not a thorough disinfection method since the inactivated microorganisms can easily revive their physiological activity under natural light (Wang et al. 2015). Chlorine disinfection can cause byproducts such as trihalomethanes and haloacetic acids, which are recognized as carcinogens (Watson et al. 2012). Ozone has a higher oxidation potential (2.07 V) compared with chlorine dioxide (1.15 V) and as a result exhibits a better oxidation ability. Furthermore, hydroxyl radicals (·OH) with much stronger oxidation ability (2.80 V) than ozone can be produced during ozonation. Although ozonation can produce toxic byproducts such as aldehydes and bromate ions, there are few problems with residual ozone, ozonation byproducts and its toxicity when ozonation is performed at O3 concentrations less than 20 mg/L (Hatatatsu & Suzuki 2000). Ozonation thus demonstrates outstanding advantages in inactivation of microorganisms and removal of organic substances.

High-gravity technology is an efficient process-intensification technique that is achieved through multiphase reactors such as a rotating packed bed (RPB), in which liquid is split into thin films, fine filaments and tiny droplets by a shear force created from the rotation of the rotor in the RPB, leading to a huge and violently renewed gas–liquid interface and thus a significant intensification of mass transfer and micromixing in multiphase systems. The technology has found wide applications in wastewater treatment (Arowo et al. 2015), nanomaterials preparation (Zhao et al. 2015) and gas absorption (Zhao et al. 2016).

Ozonation is generally limited by absorption efficiency of ozone into water. Inorganic compounds also have a significant effect on ozonation (Xing et al. 2016). This work therefore employed high-gravity technology via an RPB in an attempt to improve the mass transfer efficiency of ozone for inactivation of E. coli and removal of UV254 of fulvic acid in a simulated SE. The effect of inorganic salts such as NaHCO3, Na2SO4 and CaCl2 on the inactivation efficiency of E. coli and removal percentage of fulvic acid was investigated. UV254 of fulvic acid and the total number of bacteria colonies were used as the water quality indicators to analyze the effect of the O3-RPB treatment process of the SE.

MATERIALS AND METHODS

Materials

Fulvic acid (85%) was purchased from Shanghai Macklin Biochemical Co., Ltd, China. Beef extract (BR) and peptone (BR) were provided by Beijing Aobox Bio-technology Co., Ltd, China, and nutrient agar (BR) by Qingdao Hope Bio-technology Co., Ltd, China. E. coli was purchased from China Center of Industrial Culture Collection. Sodium chloride (AR), sodium bicarbonate (AR), sodium sulfate (AR) and calcium chloride (AR) were obtained from Beijing Chemical Works, China.

Water sample preparation

The simulated SE was prepared with fulvic acid, E. coli and sterile saline (0.9 wt% of NaCl) as described herein. The standard strains of E. coli were stored at −20 °C prior to activation in a broth medium at 37 °C for 5 hours to obtain the bacterial suspension, which was subsequently cultured at 37 °C for 12 hours with a shaking frequency of 120 r/min. The broth medium consisted of 0.5 wt% of beef extract, 1 wt% of peptone and 0.5 wt% of NaCl. The cultured E. coli was centrifuged at a rotation speed of 4,000 r/min for 3 min to obtain the sediment, which was then washed three times by sterile saline. The prepared E. coli was added into sterile saline to maintain the bacterial osmotic pressure prior to mixing with the fulvic acid solution. The apparatus and chemicals were disinfected at 121 °C for 20 min before use in the experiments.

Experimental setup and procedure

Figure 1 shows the experimental setup. It comprises mainly an RPB (a 3D view of the RPB is given in Figure S1 in the Supplementary Materials), an ozone generator (Tonglin Sci. and Tech. Ltd, China), and an ozone analyzer (Double UV Light Ozone Meter, Limicen Ozone R&D Center, China). Ozone was produced from air in the ozone generator. The ozone analyzer was used to monitor and measure the concentration of ozone in the gas-stream. The ozone-containing gas was subsequently introduced into the RPB via the gas inlet and flowed from the outer edge of the packing to the inner edge, while the SE was fed into the RPB via the liquid inlet and flowed from the inner edge of the packing to the outer edge where it contacted with the ozone-containing gas in a counter-current manner to realize ozone absorption and the subsequent treatment of E. coli and fulvic acid. Both the gas and liquid flow rates were maintained at 0.5 L/min in all the experiments.

Figure 1

Schematic diagram of the experimental setup.

Figure 1

Schematic diagram of the experimental setup.

UV254 analysis

Fulvic acid was added into the simulated SE to adjust the UV254 value to about 0.200. The removal percentage of UV254 (βB) is defined by the UV254 value before and after ozonation and calculated by the following Equation (1): 
formula
(1)

Bacteriological analysis

The flat counting method was employed to calculate the number of E. coli colonies on a Petri dish with a diameter of 90 mm. The SE sample was added into the nutrient agar medium on the Petri dish and incubated at 37 °C for 24 h before the number of E. coli colonies was counted. The number of colonies should range from 30 to 300. The inactivation efficiency based on the logarithmic value (ηE) is defined by Equation (2): 
formula
(2)

RESULTS AND DISCUSSION

Effect of rotation speed of RPB in the presence of different inorganic salts

Figure 2 shows the effect of rotation speed of the RPB on inactivation of E. coli and removal percentage of UV254. It is evident that when the rotation speed increased from 200 to 1,000 rpm, βB increased from 9.10% to 18.65% (Figure 2(a)), 6.57% to 12.63% (Figure 2(b)), 8.54% to 15.58% (Figure 2(c)) and 0.95% to 8.53% (Figure 2(d)) in the systems with NaHCO3, Na2SO4, CaCl2 and a combination of the above three compounds, respectively. Similarly, ηE increased from 0.13 to 4.48 (Figure 2(a)), 0.20 to 2.26 (Figure 2(b)), 0.40 to 4.41 (Figure 2(c)) and 0.06 to 3.62 (Figure 2(d)) in all of the four systems.

Figure 2

Effect of rotation speed of RPB on βB and ηE in the presence of different inorganic salts: (a) C(NaHCO3) = 100 mg/L; T = 20.8 °C; C(Ozone) = 12.4 mg/L; A0 = 0.210; N0 = 0.84 × 106 cfu/mL; pH = 7.46. (b) C(Na2SO4) = 200 mg/L; T = 28.5 °C; C(Ozone) = 10.0 mg/L; A0 = 0.198; N0 = 0.87 × 106 cfu/mL; pH = 5.36. (c) C(CaCl2) = 100 mg/L; T = 30.0 °C; C(Ozone) = 13.4 mg/L; A0 = 0.200; N0 = 0.79 × 106 cfu/mL; pH = 5.54. (d) C(NaHCO3) = 100 mg/L; C(Na2SO4) = 100 mg/L; C(CaCl2) = 50 mg/L; T = 15.0 °C; C(Ozone) = 11.7 mg/L; A0 = 0.210; N0 = 0.76 × 106 cfu/mL; pH = 7.42.

Figure 2

Effect of rotation speed of RPB on βB and ηE in the presence of different inorganic salts: (a) C(NaHCO3) = 100 mg/L; T = 20.8 °C; C(Ozone) = 12.4 mg/L; A0 = 0.210; N0 = 0.84 × 106 cfu/mL; pH = 7.46. (b) C(Na2SO4) = 200 mg/L; T = 28.5 °C; C(Ozone) = 10.0 mg/L; A0 = 0.198; N0 = 0.87 × 106 cfu/mL; pH = 5.36. (c) C(CaCl2) = 100 mg/L; T = 30.0 °C; C(Ozone) = 13.4 mg/L; A0 = 0.200; N0 = 0.79 × 106 cfu/mL; pH = 5.54. (d) C(NaHCO3) = 100 mg/L; C(Na2SO4) = 100 mg/L; C(CaCl2) = 50 mg/L; T = 15.0 °C; C(Ozone) = 11.7 mg/L; A0 = 0.210; N0 = 0.76 × 106 cfu/mL; pH = 7.42.

A higher rotation speed gives rise to a larger centrifugal force that shears the liquid into thinner films, threads and smaller droplets, leading to a larger gas–liquid interfacial area and faster surface renewal rate. Consequently, the absorption of ozone into the SE improved significantly as a result of an enhanced gas–liquid mass transfer effect, resulting in increased inactivation of E. coli and removal percentage of UV254.

Effect of NaHCO3 concentration

Figure 3 illustrates the effect of NaHCO3 concentration on inactivation of E. coli and removal percentage of UV254. It is evident that both βB and ηE decreased from 11.11% to 4.83% and 2.13 to 0.14 respectively when the concentration of NaHCO3 increased from 0 mg/L to 350 mg/L, indicating that NaHCO3 has a negative effect on the inactivation of E. coli and removal percentage UV254. Direct ozonation by ozone is the main pathway for inactivation of E. coli, while ·OH only plays a minor role (von Gunten 2003a, 2003b). Ozone attacks the cell membrane of the E. coli and changes its permeability. Ozone can therefore enter the cell to destroy nucleic acids, proteins and DNA, hinder the synthesis of enzymes and eventually cause the death of the bacteria (Verma et al. 2016).

Figure 3

Effect of NaHCO3 concentration on βB and ηE (T = 14.0 °C; C(Ozone) = 7.0 mg/L; A0 = 0.207; rotation speed = 1,000 rpm; N0 = 0.90 × 106 cfu/mL).

Figure 3

Effect of NaHCO3 concentration on βB and ηE (T = 14.0 °C; C(Ozone) = 7.0 mg/L; A0 = 0.207; rotation speed = 1,000 rpm; N0 = 0.90 × 106 cfu/mL).

Higher NaHCO3 concentration results in a higher pH. When NaHCO3 concentration increased from 0 mg/L to 350 mg/L, the initial pH of the SE increased from 5.41 to 8.17, leading to generation of ·OH with consumption of ozone. However, NaHCO3 can further react with ·OH as shown in the following Equations (3)–(5) (Gottschalk et al. 2000). NaHCO3 therefore consumed both ozone and ·OH, resulting in a lower efficiency of the inactivation of E. coli and removal of fulvic acid. Thus, the treatment effect of the SE declined with an increase in NaHCO3 concentration. 
formula
(3)
 
formula
(4)
 
formula
(5)

Effect of Na2SO4 concentration

The variation of βB and ηE with Na2SO4 concentration is presented in Figure 4. It is evident that both βB and ηE reduced from 10.73% to 7.81% and 5.11 to 3.75 respectively with an increase in Na2SO4 concentration from 0 mg/L to 350 mg/L.

Figure 4

Effect of Na2SO4 concentration on βB and ηE (T = 13.2 °C; C(Ozone) = 7.5 mg/L; A0 = 0.205; rotation speed = 1,000 rpm; N0 = 1.43 × 106 cfu/mL; pH = 5.40).

Figure 4

Effect of Na2SO4 concentration on βB and ηE (T = 13.2 °C; C(Ozone) = 7.5 mg/L; A0 = 0.205; rotation speed = 1,000 rpm; N0 = 1.43 × 106 cfu/mL; pH = 5.40).

It was noted that the pH value of the SE was maintained at 5.40 with varying Na2SO4 concentration. Equations (6) and (7) indicate that SO42− can react with ·OH to produce S2O82− under acidic conditions (Muthukumar & Selvakumar 2004): 
formula
(6)
 
formula
(7)
·OH exhibits a stronger oxidation ability than S2O82− owing to its higher potential (2.80 V) compared with that of S2O82− (2.01 V). Thus, the consumption of ·OH by Na2SO4 resulted in reduction of both βB and ηE.

Effect of CaCl2 concentration

Figure 5 shows the effect of CaCl2 concentration on βB and ηE. It is evident that when CaCl2 concentration increased from 0 mg/L to 200 mg/L, both βB and ηE decreased from 8.87% to 6.90% and 3.07 to 1.27, respectively.

Figure 5

Effect of CaCl2 concentration on βB and ηE (T = 11.8 °C; C(Ozone) = 6.8 mg/L; A0 = 0.203; rotation speed = 1,000 rpm; N0 = 1.07 × 106 cfu/mL; pH = 5.34).

Figure 5

Effect of CaCl2 concentration on βB and ηE (T = 11.8 °C; C(Ozone) = 6.8 mg/L; A0 = 0.203; rotation speed = 1,000 rpm; N0 = 1.07 × 106 cfu/mL; pH = 5.34).

Intermediates formed by the reaction of ozone and organic substances can combine with Ca2+ to form precipitates which can increase the efficiency of degradation (Hsu et al. 2007). However, the negative impact of Cl on the removal of fulvic acid and inactivation of E. coli overrode the positive influence of Ca2+ in this work. The reaction mechanism between ozone and Cl at different pH has been proposed by Levanov et al. (2008) and Razumovskii et al. (2010). Under alkaline conditions, the reactions of Cl with ozone can be expressed according to the following Equations (8)–(10): 
formula
(8)
 
formula
(9)
 
formula
(10)

Both ozone and ·OH can react with Cl to produce ClO, which as compared with ozone and ·OH, has an inferior ability for inactivation of E. coli and oxidation of fulvic acid.

When the pH is less than 3, another reaction mechanism can be established according to Equations (11)–(13). Cl reacts with ozone to generate Cl2 and O2, which have a lower oxidation ability than ozone. 
formula
(11)
 
formula
(12)
 
formula
(13)
Since the pH of the SE with CaCl2 is 5.34, it is assumed that both of the above mechanisms played a role in reduction of the inactivation of E. coli and removal of fulvic acid in the presence of CaCl2.

CONCLUSION

This work investigated the inactivation of E. coli and the removal percentage of UV254 by ozonation in an RPB in the presence of different kinds of inorganic salts. Results indicate that NaHCO3, Na2SO4 and CaCl2 have negative influence on inactivation of E. coli and removal percentage of UV254. The reaction mechanisms of the three inorganic compounds were analyzed, and it was noted that the compounds can consume ozone and ·OH to generate products with lower oxidation ability and consequently lead to a poor treatment effect on the simulated SE. It was observed that a higher rotation speed of the RPB enhances mass transfer efficiency, leading to improved absorption of ozone and hence increased inactivation efficiency of E. coli and removal percentage of UV254. This work lays the foundation for actual application of the O3-RPB process in the treatment of SE in UWTPs. It also demonstrates that the O3-RPB process may be an efficient route to achieve the reuse of SE.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (No. 21676008).

SUPPLEMENTARY DATA

The Supplementary Data for this paper are available online at http://dx.doi.org/10.2166/ws.2019.107.

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Supplementary data