evaluate the ability and reaction mechanism of ammonium in synthetic water samples with the • OH radical. The research results show that at pH ≤ 7.0, meaning that over 99.5% of ammonium exists in the form of NH 4 + ions , after 5 hours of UV irradiation, the ammonium removal efficiency is negligible (only 1.23% of ammonium is converted to NO 3 - and NO 2 - ). When the pH is raised to 9.3, after 4 hours of UV irradiation, about 26.4% of ammonium is decomposed. Also in this study, Huang and his colleagues demonstrated that the main product of the NH 3aq decomposition process by the • OH free radical is nitrate NO 3 - , as well as a very small amount of nitrite NO 2 - and N 2 is almost not produced, so the total N before and after the UV/H 2 O 2 process has almost no difference. He and colleagues [166] when studying the treatment of ammonium by a catazone process (O 3 /CeO 2 -MnO 2 ) stated that the free radical • OH attacked ammonium, mainly producing nitrate NO 3 - and a small amount of N 2 . Thus, in the study of this thesis, the total N treatment efficiency by EF only reached about 4.41%, which is reasonable, at that time a small amount of ammonium may have been decomposed by the radical • OH to form NO 3 - (mainly), NO 2 - and N 2 ; N 2 escaped from the solution, causing a slight decrease in total N. In addition, a small part of NH 4 + may have been removed by direct oxidation on the anode or coagulation caused by Fe(OH) 3 [69].
Low ammonium removal efficiency in wastewater by free radicals in general, and the EF process in particular, has also been reported by several other research groups. Indeed, in Atmaca's study, the EF system using a pair of cast iron electrodes supplemented with H 2 O 2 was studied to treat leachate (COD concentration = 2350 mg/L, NH 4 -N concentration = 310 mg/L) taken from the solid waste treatment area of Sivas city, Turkey. Under optimal conditions: current density 100 mA/cm 2 , pH = 3, H 2 O 2 concentration = 2000 mg/L, distance between the two electrodes is 1.8 cm; after 45 minutes of electrolysis, the ammonium removal efficiency was only 25.8%, much lower than the COD removal efficiency (72%) [69]. Compared with the thesis research, Atmaca's research has a shorter treatment time but the ammonia treatment efficiency is higher because it used a much larger current density in this thesis (100 mA/cm 2compared to 8.333 mA/cm 2 ), Lin and Chang used an EF system with a pair of cast iron electrodes (area 22.6 cm 2 , distance between 2 electrodes is 1.5 cm) to treat leachate after chemical coagulation (COD concentration = 951 mg/L; NH 4 -N concentration = 33.5 mg/L). At the optimal condition of pH = 4, the amount of H 2 O 2 added is 750 mg/L, after 30 minutes of electrolysis with a current density of 110 mA/cm 2 , only 15.8% of ammonium was removed [160]. Although the electrolysis time is shorter, only 30 minutes, but with the use of a higher current density in the study of the thesis
multiple times (110 mA/cm 2compared to 8.333 mA/cm 2 ) so the ammonium removal efficiency is slightly higher than the results of the thesis, which is also reasonable. Huang et al. studied the use of anodic oxidation process using a pair of Ti (cathode)/BDD (anode) electrodes to treat pig farming wastewater with initial COD of about 1930 mg/L and NH 4 -N of about 65 mg/L. The results showed that under optimal conditions: current density of 250 mA/cm 2 , additional electrolyte Na 2 SO 4 = 0.05M, pH = 6.7; after 60 minutes of electrolysis, only 23% of ammonium was removed [167]. This treatment efficiency is also slightly higher than the results of the thesis because the author used a current density that was too high compared to the research of the thesis (250 mA/cm 2 compared to 8.33 mA/cm 2 ). Even in the study by Lau et al., a chemical Fenton process did not seem to be effective in removing ammonium from leachate. Indeed, in this study, the leachate after UASB reactor with COD = 1910 ± 140 mg/L and NH 4 - N = 1753 ± 130 mg/L was treated by a chemical Fenton process and the results showed that the effluent ammonium concentration was approximately equal to the influent ammonium concentration [161].
For total P, the results in Table 3.3 show that the influent wastewater has a total P content of 22.7 mg/L. The EF process can remove 59.34% of total P after 60 minutes of electrolysis and this treatment efficiency is lower than some other studies. For example, in Atmaca's study, the EF system using a pair of cast iron electrodes with added H 2 O 2 was studied to treat leachate (COD concentration = 2350 mg/L, P concentration = 310 mg/L) taken from the solid waste treatment area of Sivas city, Turkey. Under optimal conditions: J = 100 mA/cm 2 , pH = 3, H 2 O 2 concentration 2000 mg/L, distance between the 2 electrodes is 1.8 cm, after 45 minutes of electrolysis, the P removal efficiency reached 87% [69].
Thus, it can be seen that when applying the EF process with the appropriate experimental conditions found above to real wastewater samples, due to the high content of pollutants, especially organic substances, in real wastewater samples compared to synthetic water samples, the treatment efficiency of glyphosate and other organic substances is reduced compared to the above study. In order to improve the treatment efficiency of pollutants to meet the discharge standards according to QCVN 40:2011/BTNMT column B, the thesis tested extending the electrolysis time of the above wastewater sample to 150 minutes. The results are shown in Figure 3.18. It can be seen that increasing the treatment time can increase the efficiency of glyphosate decomposition, removing other organic substances as well as ammonium, but the treatment efficiency of the above subjects does not increase significantly. Specifically, when increasing the treatment time from 60 to 150 minutes, the decomposition efficiency
glyphosate removal increased by only 5.93% (from 86.47% to 92.4%), COD removal efficiency increased by 4.72% (from 72.13% to 76.85%), while BOD 5 removal efficiency increased by a small 4.85% (from 51.41% to 56.26%). Ammonium and total N changed little when increasing treatment time from 60 min to 150 min: for total N, removal efficiency increased by only 0.56% (from 4.41 to 4.97%) while total N remained almost unchanged. The reason why prolonging the treatment time only slightly increases the efficiency of treating pollutants as observed above may be because when the treatment time is prolonged, the decomposition of pollutants may have formed some stable intermediate products or due to the slow decomposition of organic substances adsorbed on iron hydroxide Fe(OH) n [69]. In the study of Atmaca [69], the efficiency of COD and ammonium removal of leachate by EF process using cast iron electrode pair almost reached the maximum after 30 minutes of treatment and prolonging the electrolysis time beyond 30 minutes did not increase the efficiency but even reduced the efficiency of COD removal of leachate. Similarly, the study of Lin and Chang [160] used EF system with cast iron electrode pair to treat leachate after chemical coagulation and the efficiency of COD and ammonium removal reached the maximum after 30 minutes of electrolysis. Prolonging the electrolysis time beyond 30 min even slightly reduced the COD and ammonium removal efficiency.
COD
BOD 5
NH 4 +
TN
TP
100
80
Efficiency (%)
60
40
20
0
60 minutes
90 minutes
120 minutes
150 minutes
Time (minutes)
Figure 3.19. Treatment efficiency of COD, BOD 5 , NH 4 + , TN, TP of wastewater of Viet Thang Company Ltd. after EF process at different times
As shown in Figure 3.19, prolonging the treatment time does not bring high efficiency in treating pollutants, while increasing energy costs due to higher electricity consumption. However, after only 60 minutes of electrolysis, the BOD5/COD ratio or the biodegradability index (BI) of the wastewater increased significantly from 0.23 to 0.4. According to the study of Soloman et al. in 2009 when pre-treating paper industry wastewater by an electrochemical process to enhance biodegradability, the results showed that in order to completely decompose wastewater by a biological process, the BI index of the wastewater must be at least 0.4 [168]. Thus, in the study of the thesis, to thoroughly treat pollutants, meeting the discharge standards of QCVN 40:2011/BTNMT column B, applying a secondary biological treatment process after the EF process is a feasible solution. In this thesis, the secondary treatment of glyphosate as well as other organic components in wastewater will be studied by a membrane biological process (MBR). The overall efficiency of the combined EF and MBR process will be much more effective than each separate treatment process.
Based on the assessment and research of the impact in the EF process, the thesis studies the next secondary processing stage including the following specific contents:
Research to find suitable conditions affecting the MBR process to treat secondary products of glyphosate after decomposition by EF process.
Research on the application of the optimized MBR process above to treat wastewater of Viet Thang Company Limited and propose a technological process for treating wastewater containing plant protection chemicals.
3.2. Study on factors affecting MBR for secondary treatment of glyphosate
3.2.1. Study of factors affecting the ability to decompose glyphosate and other organic substances by MBR
In the MBR process, the efficiency of pollutant decomposition depends on many factors such as dissolved oxygen concentration in the system, hydraulic retention time, sludge retention time, etc. Therefore, it is necessary to evaluate these experimental conditions to achieve maximum treatment efficiency. Because the wastewater of Viet Thang Company Limited has relatively fluctuating pollutant content, depending on the production time of the company, to ensure the uniformity of input parameters of the experiments on the MBR system, the thesis uses synthetic water samples with similar composition to the wastewater sample after pretreatment with EF.
Based on the results of section 3.1.8 above (wastewater sample after treatment by EF has parameters as in Table 3.3), the thesis uses the input solution of the MBR system as synthetic water samples with parameters as in Table 3.4.
Table 3.4. Input water parameters of MBR reactor
Order
Parameter | Unit | Result | |
1 | pH | – | 7.4 ± 0.8 |
2 | DO | mg/L | 4 ÷ 5 |
3 | COD | mg/L | 800 ÷ 1000 |
4 | NH 4 + | mg/L | 25 ÷ 35 |
5 | Total N | mg/L | 35 ÷ 50 |
6 | Total P | mg/L | 5 ÷ 15 |
7 | Glyphosate | mg/L | 2.5 ÷ 3.0 |
8 | Glycine | mg/L | 1.0 ÷ 1.5 |
9 | Temperature | o C | 20 o C ÷ 35 o C |
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3.2.1.1. Effects of different aeration modes on the treatment capacity of COD, glyphosate, NH 4 + ; total N; total P of the MBR process
In biological treatment systems (decomposing organic matter by aerobic, anaerobic or anoxic microorganisms), aeration plays an important role in the treatment of pollutants. Indeed, aeration aims to provide dissolved oxygen (DO) to the microorganisms, so increasing the aeration process will help the aerobic microorganisms develop, conversely, stopping the aeration will help the anaerobic microorganisms develop, while weak aeration will create conditions for the anoxic microorganisms to develop. Many previous studies have shown that increasing DO increases the ability to treat COD and ammonium but affects the ability to treat nitrate, indirectly affecting the ability to treat total N. In fact, intermittent aeration can help to simultaneously remove nitrogen and phosphorus by nitrification and denitrification, P absorption and P release in the same biological reactor with suitable aeration and non-aeration cycles [169, 170].
In addition, the ability to degrade pesticides also depends on the aeration conditions. Indeed, according to Shawaqfeh [24], the interleaving of anaerobic and aerobic biological processes will increase the biomass of active bacteria in the bioreactors and improve the efficiency of pesticide treatment compared to aerobic processes.
or anaerobic separately, leading to a reduction in retention time. The study of Navaratna et al. [171] also showed that the anaerobic MBR system has a fairly high capacity to treat pesticides, with a retention time of 15 hours, the removal rate of the broad-spectrum herbicide ametryn (S-triazine) is 65% at initial concentrations from 1 to 4 mg/L. Another study by Monsasalvo et al. was conducted to remove the pesticides atrazine and linuron using an anaerobic MBR system (AnMBR). The results showed that pesticides are very difficult to treat under anaerobic conditions with very low removal rates of atrazine (6.8%) and linuron (10.5%) [172]. Therefore, to increase the efficiency of treating pesticides, people often combine aerobic and anaerobic decomposition processes.
On the other hand, in reality, aeration can consume up to 60% ÷ 70% of the total energy used by the entire water or wastewater treatment system [131]. Therefore, it is necessary to conduct alternating aeration (repeated S/D cycle) to save energy and enhance the glyphosate treatment process and nitrification process. Therefore, the experiment to study the effect of aeration mode on the treatment efficiency of the MBR system was conducted. The thesis conducted experiments on the MBR system in three S/D modes: 50 min/70 min; 60 min/60 min; 70 min/50 min. The aeration rate is in the range of 1.5 ÷ 10 L/min to maintain DO when aerating at 4 - 5 mg/L, the input pH of wastewater is 7.4 ± 0.8; Room temperature 25 o C ÷ 32 o C, input load according to Table 2.5, input flow is continuously loaded with a flow rate of 96 L/day corresponding to a hydraulic retention time HRT of 9 h; MLSS ranges from 7,500 ÷ 9,000 mg/L. Each aeration mode is carried out for 10 days and the COD, glyphosate, NH 4 + ; TN; TP are analyzed daily.
Study on the effect of S/D mode on COD treatment capacity
The effect of S/D mode on COD treatment capacity is shown in Figure 3.20.
The above results when increasing the aeration time (while reducing the aeration stop time) from 50 minutes to 70 minutes, the COD treatment capacity increased from 89.70% to 96.96% after 30 days. Specifically, at the S/D mode of 50/70 (minutes), it means that the dissolved oxygen level is low, the operating time of anaerobic microorganisms increases, anaerobic microorganisms will be inhibited, leading to low COD treatment efficiency, so after treatment, the COD concentration of this mode is 90.01 mg/L, the result in this mode is that the COD concentration exceeds the threshold of QCVN 40:2011/BTNMT column A. On the contrary, at the S/D mode of 60/60 (minutes) and 70/50 (minutes), when the aeration time increases, the amount of DO supplied
The high treatment system creates a favorable environment for the development of aerobic microorganisms while inhibiting the development of anaerobic microorganisms. As a result, the COD treatment capacity of the MBR system increased from 95.48% to 96.96% corresponding to each S/D mode: 60/60 minutes and 70/50 minutes and the COD concentration after treatment was 39.82 mg/L and 26.79 mg/L, respectively. In these two S/D modes, the treatment efficiency is not significantly different and the COD concentration meets QCVN 40:2011/BTNMT column A for the corresponding COD index.
1200
COD in COD out
Efficiency (%)
100
1000
80
COD concentration (mg/L)
800
Efficiency (%)
60
600
400
200
HRT: 9 h
OLR: 2.35kg COD/m3 day
MLSS: 7500 - 9000 (mg/L)
S/D: 50/70 (shot)
HRT: 9 h
OLR: 2.35 kg COD/m3 day
MLSS: 7500 - 9000 (mg/L)
S/D: 60/60 (fps)
HRT: 9 h40
OLR: 2.35 kg COD/m3 day
MLSS: 7500 - 9000 (mg/L)
S/D: 70/50 (shot)
20
0 0
5 10 15 20 25 30
Time (day)
Figure 3.20. Effect of S/D mode on COD treatment capacity
In addition, it can be seen that the ability to treat organic substances by the MBR process is very high, over 95%. This proves the advantage of the MBR process compared to the traditional biological process because the MBR system has a long sludge retention time, little loss during operation, high biomass concentration, leading to higher efficiency in treating organic substances. The results of this study can be relatively compared with the results of the study by Jin et al.: the system using activated sludge was studied to treat wastewater from a factory specializing in the production of plant protection chemicals, the COD removal efficiency only reached 85% and when increasing the pressure on the activated sludge system to 0.3 MPa, the COD removal efficiency increased to 92.5% [173]. The COD removal efficiency in the thesis study is high, reaching over 95%, partly due to the role of the filter membrane in the MBR system, partly due to the concentration of
The low concentration of glyphosate in the influent water has little effect on the microorganisms present in the MBR tank, so the COD treatment capacity is less affected. Indeed, according to Vasque et al., the presence of micropollutants such as pesticides at low concentrations does not significantly affect the treatment efficiency of other organic substances in wastewater [174].
The results of the thesis are similar to some other studies: for example, in the study of Vasque et al., an MBR system using MF membrane made of PVDF material (pore size 0.4 µm), filter surface area 1 m 2 , SRT: 30 - 60 days, HRT = 20h, O 2 concentration 5 mg/L; can remove 96% of COD in synthetic wastewater (initial COD about 3000 mg/L supplemented with some organochlorine pesticides such as lindane, alachlor, heptachlor) [174]. In another study by the same group of authors, the above MBR system was studied to treat synthetic wastewater with COD load = 0.23 Kg COD/Kg SSV/day and containing some pesticides such as terbuthylazine, simazine, atrazine. With SRT = 50 days, HRT = 20h, the COD treatment capacity reached 96% [175]. Almeida Lopes' research group established an MBR system with HRT = 25h that could remove over 99% of COD in industrial wastewater (with very high initial COD, up to 2630 mg/L) and added a concentration of 2,4–D 200 mg/L, and an atrazine concentration of 15 mg/L. However, in this study, the authors used a UF membrane with very small pore size, below 0.1 µm [176]. These results demonstrate that the MBR system has the ability to treat organic substances in wastewater well and the presence of low concentrations of pesticides does not greatly affect the ability to remove other organic substances.
Study on the effect of S/D regime on glyphosate treatment capacity
Before and after treatment with each different S/D regime above, water samples were taken with a sampling cycle of 24 hours/time and analyzed for glyphosate concentration.
From the results in Figure 3.21, it can be seen that the aeration mode affects the glyphosate removal efficiency: when the aeration time increases, which means the aeration stop time decreases, the glyphostae removal efficiency gradually increases. Specifically, in Figure 3.21, the average glyphosate removal efficiency increases from 92.19% to 95.88% when the aeration time increases from 50 minutes to 70 minutes. This proves that the MBR system effectively treats glyphosate, the efficiency