Nowadays, the special development of catalysts used in oil refining processes results in a great economic benefit. Among these catalysts, Hollow Titanium Silicate (HTS) zeolite is a new catalyst (Xia et al., 2016), which is widely used in various oil refining operations. However, the production and the use of these catalysts generate a large amount (thousands tons/year) of toxic and harmful wastewaters (Cao et al., 2019; Boethling & Lynch, 1992; Xiang et al., 2016). These wastewaters generally consist of high salt, low (or sometimes high) ammonia-nitrogen and high COD concentrations. Their structures contain tetrapropylammonium hydroxide (referred to as TPAOH) which are similar to that of quaternary ammonium salt compounds (QACs) (Tezel et al., 2006; Xia et al., 2017). Moreover, they are also full of organic pollutants which are not easy to biodegrade (Cao et al., 2019; Boethling & Lynch, 1992). In fact, the persistence of high organic pollutant concentrations may be due to HTS zeolite’s physicochemical structure and proprieties which for example do not favor the accessibility of bulk organic molecules to inner tetrahedral framework Ti sites (Boethling & Lynch, 1992; Xia et al., 2017). Thus, the external drainage from HTS molecular sieves production device wastewaters should contain a large number of TPAOH and a very high COD concentration in the effluent which are not environmental-friendly (Friedel, 1994; Shon et al., 2011).
Since wastewaters coming from the production of catalysts have been subjected to strict regulations, several technologies for organic matter removal have been proven to be relatively effective (Herney-Ramirez et al., 2010) . Among them are the following methods: physical (adsorption), chemical (micro-electrolysis, photoelectric Catalytic oxidation), photocatalytic degradation, Fenton reagent (Fongsatitkul et al., 2004; Pan et al., 2001), potassium tetraoxo ferrate (II) reagent, and cationic surfactant method, biochemical (aerobic, anaerobic, and aerobic-anaerobic combination process) (Scott & Ollis, 1995; Zheng et al., 2013). However, because of their high energy consumption, their affinity for certain dangerous reagents and therefore their high pollution and complex functioning, most of these methods are sometimes considered as pre-treatment methods. It is also estimated that these methods have high overall costs as much as the quality of their effluents does not always meet discharge standards. In comparison, and as they provide less costly and environmental-friendly solutions, biological treatment methods seem to be more appropriate (Kargi & Dinçer, 1997; Morsyleide et al., 1998; Ren & Zhang, 2010). Moreover, such HTS-sieve wastewaters have low and sometimes high concentrations of ammonia species, and therefore special attention should be paid on them. In fact, SBR nitrification and denitrification, biofilms and other biological treatment processes used to treat this type of ammonia species has shown the difficulty of their effective removal (Morsyleide et al., 1998; Ren & Zhang, 2010). Despite a number of studies on wastewater containing QACs materials (Xiang et al., 2016; Li, 2011; Zhu & Xiao, 2000), it should be noted that very little attention has so far been paid to the biological disposal of HTS type catalysts used by petroleum refineries.
The main objectives of this study are to investigate: 1) whether the HTS molecular sieves wastewater COD can be biodegraded; 2) whether the ammonia-nitrogen (NH3-N) and the total nitrogen (TN) removals are efficient; and 3) whether the HTS molecular sieves wastewater COD can be used as a carbon source for denitrification.
2. Materials and Methods
2.1.1. Quality of HTS Molecular Sieve Wastewaters
Quality and biochemical properties of HTS molecular sieve wastewaters have been measured using China National Environmental Protection Agency’s methods from factory as given by Table 1.
Effects of sample softness and biodegradation are particularly pronounced for the No. 2 wastewater such as BOD/COD ratio is less significant (0.06) in No. 1 wastewater than the No. 2 (BOD/COD ratio = 0.30).
2.1.2. Chemical Parameters and Analytical Methods
During the whole process, the wastewaters’ chemical parameters characterized and determined were COD (Chemical oxygen demand), NH3-N (Ammonia-nitrogen), TN (Total nitrogen), Cl− (Chlorine ions), and (Sulfate ions). Technical details of analyzed parameters and analytical methods are presented in Table 2.
2.1.3. SBR Reactor
1) Experimental apparatus
The main parts of SBR bioreactor (Figure 1) are made with transparent plexiglass. During this study, the SBR processes involved 3 L and 9 L total volumes reactors both made with transparent plexiglass.
Table 1. Quality of HTS molecular sieve wastewaters.
Table 2. Analytical projects and methods.
All these parameters are measured at the beginning and the end of each SBR cycle.
Figure 1. Schematic diagram of SBR. 1—Water inlet tank; 2—pH-meter; 3—DO-meter; 4—Automatic control device; 5—Heater; 6—Stirrer; 7—Air stone diffuser; 8—Water outlet tank; 9—Air rotameter; 10—Air pump.
Air stone diffuser is installed in the reactor, and the aeration amount (dissolved oxygen DO) is controlled by adjusting the intensity of the aerator (DO-meter). The bioreactor is also equipped with related temperature and pH control equipment, and continuous online dissolved oxygen analyzer. An automatic control device, allows the regular schedule of water outlet, water inlet, aeration and idle processes.
2) SBR flow chart
The SBR cycle (Figure 2) consists of five steps, which are carried out in sequence as: 1) fill, 2) react (On/Off aeration), 3) settle, 4) draw, and 5) idle.
Figure 2. Schematic diagram of a five steps SBR system cycle.
2.2.1. Domestication of Municipal Sewage Plant’s Sludge
The activated sludge from one municipal sewage plant was selected for SBR experiments. First, the sludge was recovered after precipitation and supernatant removal. Thereafter, water with a small amount of glucose and ammonia-nitrogen was injected into the reactor for stuffy exposure of 2 days. After supernatant removal, the remaining sludge was added to 9 L total volume SBR reactor.
In the early stage of domestication, only glucose water was used in order to facilitate sludge domestication. Then, for each cycle the feed (or intake) water was composed by reducing gradually the glucose concentration while increasing the HTS molecular sieves wastewater concentration. A solution of trace elements nutrient was added, and sodium bicarbonate or phosphoric acid was used to maintain the pH neutral around 7. Each cycle was composed of 4 L feed water and the operation consisted of 8 h aeration phase and 2 hours anoxic phase. The Hydraulic Residence Time (HRT) and the settle steps were both of 2 hours. At the end of each cycle, the supernatant was discharged in respect with a liquid exchange ratio of 44%. For each cycle, feed water and effluent characteristics of COD as well as sludge’s SV30, MLSS and SVI were tested.
2.2.2. Treatment of HTS-Sieves Wastewater by SBR Domesticated Sludge
At the end of domestication processes, processes of COD removal from the HTS molecular sieves wastewater were tested during three months. The feed water was entirely composed of HTS molecular sieves wastewater, pH was maintained between 6 and 8. Feed water’s COD concentrations were changed by cycle and were between 400 and 2900 mg/L. Operational conditions were those adapted to sludge domestication. For each cycle, feed water and effluent characteristics of COD as well as sludge’s SV30, MLSS and SVI were measured.
2.2.3. Nitrification and Denitrification Properties of HTS-Sieves Wastewater by SBR Processes
To perform nitrification and denitrification processes, and contrary to the previous water quality data, another HTS molecular sieves wastewater with high ammonia-nitrogen concentration was studied by SBR system.
1) Nitrification process
The process involved 9 L effective volume SBR reactor. At the beginning, 3 L of activated sludge and 2 L of HTS molecular sieves wastewater were poured into the SBR reactor. When the sludge’s MLSS concentration and the MLVSS/MLSS ratio were 9.9 mg/L and 0.9, respectively. The aeration condition was maintained to provide dissolved oxygen (DO) of around 2 mg/L. Operational temperature of first case was room temperature (winter period with temperature around 10˚C), and second case at 40˚C. Wastewater’s characteristics of COD, NH3-N, NO3-N and TN concentrations were analyzed.
2) Denitrification process
It is well established that the microorganisms cannot degrade the HTS molecular sieves wastewater when used as carbon source. Hence, two SBR devices were used in this experiment: HTS molecular sieves wastewater, sludge and glucose as carbon source were added into the first one while into the second only HTS molecular sieves wastewater and sludge were added.
At the beginning of the experiment, 1.5 L of sludge and 1.5 L of HTS molecular sieves raw wastewater were added into the two devices. After adding 500 mg/L of glucose in the second device, sludge’s properties test has showed MLSS of 4.1 mg/L and MLVSS/MLSS ratio of 0.9. The operational conditions involved stirring of wastewater, the temperature kept at 24˚C and the DO below 0.5 mg/L. Wastewater’s characteristics of COD, NO3-N and TN concentrations were analyzed.
3. Results and Discussion
3.1. Municipal Sludge Domestication by SBR Process
SBR process summary of COD removal rate, sludge properties and the specific parameters are summarized in subsequent Table 3 while Figure 3 and Figure 4 illustrate the results of COD removal efficiency and the sludge performance, respectively.
According to this variability scheme (Table 3, and Figure 3 and Figure 4), the rate of COD removal is in gradual adaptation of microorganisms to the increase of feed water concentration. In fact, on the 1st and 2nd days, feed water’s COD concentrations of HTS molecular sieves wastewater (HTS-COD) was 25 mg/L while glucose concentration (glucose-COD) was 600 mg/L corresponding to HTS-COD/glucose-COD ratio of 0.04:1. The COD removal rates were 69% and 70%, respectively. Thereafter, the sludge domestication continued, and from the 3rd to the 7th day, when the ratios were increased gradually by 0.08:1, 0.08:1, 0.15:1, and 0.25:1, respectively, COD removal rate remained stable at 69%. This indicates that sludge domestication started. On the 9th and 10th days, the ratio was increased by 0.35:1 and 0.45:1 with appropriate increase of HTS-COD.
The COD removal rate records from mixed wastewater were 85% and 89%, respectively, and the sludge domestication was relatively smooth. The sludge domestication continued and from the 11th to 15th day the HTS-COD concentrations were increased with related gradual ratios of 0.5:1, 0.6:1, 0.7:1, 0.8:1 until 1:1 and then the average removal rate of COD in wastewater became 86% which means that the biochemical properties of sludge are good. From 16th day to 23rd day,
Table 3. Feed water and domestication results of sludge from municipal wastewater treatment plant.
Figure 3. Variation of COD removal rate.
Figure 4. Variation of sludge characteristics.
the HTS-COD concentrations were gradually elevated than that of glucose-COD with the ratios of 1.2:1, 1.6:1, 2:1, 3:1, 5:1, 5.5:1, 8.3:1 until 16:1, respectively. At the end of reactions, the recorded average of COD removal rate was 83%. Thereafter, the HTS-COD concentrations were increased again and the average removal rate of COD was between 83% up to 92% which indicates that the sludge domestication is effective. This stage achieves the adaptation period of sludge’s microbial community to the high organic matter of HTS molecular sieves wastewater.
From above Figure 3, it is also noted that on the 23rd day, the rate of removal of COD was 92%; on the 24th day, glucose was no longer added, and 614 mg/L HTS-COD was supplemented. At the end of this step, effluent showed 80 mg/L COD concentration corresponding to 87% COD removal rate. Following some early evidence based on better sedimentation index of the domesticated sludge, we show here (Figure 4) that the degradation of COD is better biochemical treatment performance. This implies enhanced the COD removal rate in HTS molecular sieves wastewater is significantly improved, and an increased contribution of microorganisms’ biodiversity living in the sludge of municipal sewage plant.
3.2. HTS Molecular Sieves Wastewater Treatment by Domesticated Sludge
Some of the specific parameters of feed water, the treatment efficiency and the sludge’s properties are summarized in Table 4. The capability of the SBR has been evaluated by following the process of COD removal from HTS molecular sieves wastewater and conducting experiments during three months overall to acquire data.
While the effluent’s COD concentration is greater than that of feed water’s COD, the initial treatment effect is abnormal in the first few days (Table 4 and Figure 5). In fact, the previous stage of water shortage has caused the sludge to self-decompose which was resulted to an increase of effluent’s COD. Thereafter, on 11.05, the COD removal rate began to increase. At the 25th day, the COD removal efficiency remained more stable reaching about 50%. From the 25th day to the end of operations, the COD removal rate fluctuated between 47% to 67%. In addition, unlike the case where glucose was a carbon source, and because of the low C/N ratio of wastewater, the COD removal rate can be considered as moderate justifying the difficult biodegradation of COD of this kind of wastewaters. In fact, this situation is particularly related to the proprieties of high salinity and refractory to treatment of high salt organic wastewater (Cao et al., 2019).
Moreover, the persistence of high organic pollutants concentrations is also favored by HTS molecular zeolite cavities which do not promote the accessibility of bulk organic molecules to inner tetrahedral framework Ti sites (Xia et al., 2017). In addition, HTS molecular sieves like QACs species have high surface activity and moderate-high activity which are proved to be harmful to microorganisms even at low concentration (Xiang et al., 2016). Furthermore, such species have great influence on the bioavailability and mobility of the other coexisting pollutants (Xiang et al., 2016). Hence, the COD removal rate is an average value which can be considered acceptable knowing that such organic matters, even after pretreatment by physicochemical methods, are very difficult to manage (Herney-Ramirez et al., 2010 ).
These results could be useful for the petroleum and petrochemical industries in the design of further COD treatment processes of high COD and high salinity wastewaters. In fact, they could be useful to determine additional procedures which can consider the fact to decompose or to reduce HTS surfactants from such wastewaters prior to any COD removal.
3.3. SBR Nitrification Process
There are some fluctuations of COD concentrations while the changes of ammonia-nitrogen, nitrate-nitrogen and total nitrogen concentrations are fairly small with less 16% TN removal efficiency (Table 5). The slow nitrification reaction can be probably attributed to low temperature. In fact, the registered high sludge concentration can be explained by the death of certain microorganisms due to low temperature conditions. Thereafter, 3 L of wastewater (1.5 L HTS molecular sieves raw water +1.5 L deionized water) and heating rods were added into the reactor. The following analysis of sludge proprieties gave MLSS of 4.6 mg/l and MLVSS/MLSS ratio of 0.9. The aeration was continued at room temperature with dissolved oxygen (DO) of above 2 mg/L and the results of COD, ammonia-nitrogen, nitrate-nitrogen and total nitrogen tests are shown in the following Table 6.
Table 4. Feed water and experimental results.
Figure 5. Variation of COD removal rate vs Feed water COD concentrations.
Table 5. Analysis of water quality of HTS molecular sieves wastewater.
Table 6. Quality analysis of HTS molecular sieves wastewater.
According to Table 6, during the 25 hours of operations some fluctuations are observed for all parameters. This situation can be explained by the process of microorganisms’ reconstitution and adaptation to new operational conditions. Thereafter, COD decreases gradually up to 70% elimination at the end of operation showing the presence of microorganisms suitable for organic matter removal along with favorable operational conditions. Unfortunately, any improvement of the other parameters has been registered at the end of experiments. Thus, less than 35% nitrate-nitrogen was removed while ammonia-nitrogen concentration increased and TN removal remained completely low. This may be due, on the one hand to high operational temperature (about 40˚C) and, on the other hand to high concentration of HTS molecular sieves wastewater which after being adsorbed by the sludge’s surface can hinder the nitrification reaction. In fact, high temperature values have been proved to influence many biological and chemical processes (Chapman, 1996) and this has probably an impact on dissolved oxygen (DO) concentration which becomes low. In addition, saline wastewaters have also an influence on the solubility of DO. Besides, the decrease of DO solubility in water may impacts nitrifying bacteria activities so that ammonia-nitrogen may not be easily degraded. Likewise, the persistence of high salt organic pollutants concentrations (high COD and high salt concentrations) may be harmful to microorganisms which death can increase nitrogen species concentration. In fact, the activated sludge’s microorganisms should be disturbed by the high salt content (about 3%) of this type of wastewater. Their activities could also be affected by the difficult access to nitrogenous materials since these wastewaters contain surfactants in the form of HTS’ micelles.
3.4. Denitrification Process
All over the experiment, the results of COD, nitrate-nitrogen and total nitrogen for denitrification were determined as shown in Table 7.
In Table 7, the left data are the experimental results without glucose, and the right data are the experimental results with the initial addition of 500 mg/L glucose. For both cases, the COD concentrations decrease somewhat at the beginning from 0 h to 3.5 h, and then increase gradually. The removal rates of ammonia-nitrogen (ammonia-N), nitrate-nitrogen (nitrate-N) and total nitrogen (total N) concentrations were fairly small. The possible reason is that microorganisms in the presence of high concentrations pollutants are hibernated, and eventually some of them are disintegrated so that pollutants and nitrogen species concentrations remain high in the system. Furthermore, the persistence of high organic pollutants concentrations is also favored to HTS molecular zeolite cavities which do not promote the accessibility of bulk organic molecules to inner tetrahedral framework Ti sites (Xia et al., 2017). According to the data of the two situations (i.e. with and without glucose addition), neither case allows the promotion of the denitrification reaction. Therefore, the specific characters of the HTS molecular sieves wastewaters have a great effect on the denitrification efficiency.
Table 7. Quality analysis of the HTS molecular sieves wastewater.
SBR intermittent domestication tests of sludge were successfully performed with wastewater from one municipal sewage treatment plant giving 87% COD removal rate. The results of HTS molecular sieves wastewater’s COD treatment showed partial efficiency with a removal rate fluctuating around 47% to 67%. In addition, unlike the case where glucose is a carbon source, the COD removal rate can be considered as passable.
The nitrification process showed no changes in NH3-N and TN concentrations. Under temperature below 10˚C (winter condition), less than 16% TN was removed. When temperature was increased to 40˚C, TN removal efficiency remained worse. These observations mean that the change in temperature had no effect on the TN elimination. Problems still exist in the choice of suitable operational conditions along with the impact of HTS molecular sieves on activated sludge’s surface which hinders the nitrification reaction. Suitable operational conditions such as pH, temperature, DO and low C/N ratio should be redefined after reducing the salinity of this wastewater which probably disturbed activated sludge’s efficiency. The renewal of activated sludge capable of oxidizing TN in salty conditions should be an option.
Regarding the denitrification process, changes in NH3-N, NO3-N and (TN) concentrations were fairly small. This can be the result on one hand of the hibernation of microorganisms because of high concentrations of contaminants which lead to their disintegration and an increase of nitrogen, and one the other hand due to the lack of specific activated sludge which could be capable to oxidize nitrogen species. It can be also concluded that this HTS molecular sieves wastewater’s salinity had a great effect on the nitrification and denitrification reactions.
Further studies should be directed towards the choice of specialized activated sludge along with suitable operational parameters which could improve TN removal. Experimental conditions should be changed and additional physicochemical pretreatment on HTS molecular sieves wastewater could help reduce their impact on activated sludge.
Special thanks to East China University of Science and Technology (ECUST) and all the members of administrative office of international students for their support during my last internship stay at their university in late 2019. I want also to acknowledge the cooperation between my country and China which gave me the chance to perform this training. I would like to thank the administration of our University Dan Dicko Dankoulodo of Maradi (UDDM) for financial support.
 Boethling, R. S., & Lynch, D. G. (1992). Quaternary Ammonium Surfactants. In N. T. de Oude (Ed.), Detergents. Anthropogenic Compounds (Vol. 3/3F, pp. 144-177). Berlin, Heidelberg: Springer.
 Cao, M. L., Li, H., Liu, F. C., Li, D., & Zhong, C. M. (2019). Recent Development in the Process for High Salt Organic Wastewater. Nonferrous Metals Science and Engineering, 10, 92-98. (In Chinese)
 Fongsatitkul, P., Elefsiniostis, P., Yamasmit, A., & Yamasmit, N. (2004). Use of Sequencing Batch Reactors and Fenton’s Reagent to Treat a Wastewater from a Textile Industry. Biochemical Engineering Journal, 21, 213-220.
 Herney-Ramirez, J., Vicente, M. A., & Madeira, L. M. (2010). Heterogenous Photo-Fenton Oxidation with Pillared Clay-Based Catalysts for Wastewater Treatment. A Review. Applied Catalysis B, 98, 10-26.
 Kargi, F., & Dinçer A. R. (1997). Biological Treatment of Saline Wastewater by Fed-Batch Operation. Journal of Chemical Technology and Biotechnology, 69, 167-172.
 Pan, L. T., Xiao, J., & Zhu, Y. (2001). Treatment of High Strength Quaternary Ammonium Salt-Organic Wastewater with Biochemical-Oxidation and Flocculation Process. Research of Environmental Sciences, 4, 46-50. (In Chinese)
 Rosa, M. F., Furtado, A. A. L., Albuquerque, R. T., Leite, S. G. F., & Medronho, R. A. (1998). Biofilm Development and Ammonia Removal in the Nitrification of a Saline Wastewater. Bioresource Technology, 65, 135-138.
 Scott, J. P., & Ollis, D. F. (1995). Integration of Chemical and Biological Oxidation Processes for Water Treatment: Review and Recommendations. Environmental Progress, 14, 88-10.
 Shon, H. K., Phuntsho, S., Vigneswaran, S., Kandasamy, J., Cho, J., & Kim, J. H. (2011). Physico-Chemical Processes for Organic Removal from Wastewater Effluent. Water and Wastewater Treatment Technologies.
 Tezel, U., Pierson, J., & Pavlostathis, S. G. (2006). Fate and Effect of Quaternary Ammonium Compounds on a Mixed Methanogenic Culture. Water Research, 40, 3660-3668.
 Xia, C., Lin, M., Zhu, B., Luo, Y., & Shu, X. (2016). Hollow Titanium Silicate Zeolites: From Fundamental Research to Commercial Application in Environmental-friendly Catalytic Oxidation Processes. In Zeolites-Useful Minerals. Croatia: InTech.
 Xia, C., Peng, X., Zhang, Y., Wang, B. R., Lin, M., Zhu, B., Luo, Y. B., & Shu, X. T. (2017). Environmental-Friendly Catalytic Oxidation Processes Based on Hierarchical Titanium Silicate Zeolites at SINOPEC (pp. 119-150). London: Intech.
 Xiang, L., Sun, T. F., Mo, C. H., Li, Y. W., Cai, Q. Y., & Li, H. (2016). Related Environmental Problems and Research Progresses of Quaternary Ammonium Compounds (QACs). Progress in Chemistry, 28, 727-736. (In Chinese)