Intensification of the leachate treatment process of nitrocellulose production

Purification of cellulose is one of the most important steps in the production of nitrocellulose for explosives. However, it generates highly polluting wastewater. In this study, nitrocellulose industry wastewater (leachate) was characterized and treated chemically and biologically. Untreated leachate had a pH of 12.4 ± 0.5, color of 27,065 ± 879 units, chemical oxygen demand (COD) of 7,615 ± 252 mg/L, biological oxygen demand (BOD) of 4,413 ± 194 mg/L, total organic carbon (TOC) content of 2,455 ± 158 mg/L, total solids of 8,613 ± 232 mg/L, fixed solids of 3,845 ± 103 mg/L, and volatile solids of 4,768 ± 129 mg/L and was toxic to Escherichia coli and Artemia salina. Industrial-scale chemical treatment followed by pilot-scale biological treatment reduced COD by 97%, BOD by 99%, and TOC by 97% and eliminated toxicity. Keywords— Delignification, Explosives, Nitration, Treatment, Wastewater.


INTRODUCTION
The discharge of untreated or inadequately treated wastewater into water bodies can cause serious damage to aquatic ecosystems. Wastewater may contain high levels of phosphorus, nitrogen, antibiotics, herbicides, pesticides, heavy metals, and organic matter [1][2][3][4]. Such contaminants have been associated with acute and chronic toxicity, endocrine-disrupting effects, and antibiotic resistance [4].
Several biological, chemical, and physical wastewater treatments have been proposed [5][6][7][8][9][10], but their implementation in industries is not always feasible from operational and economic points of view. Prior to biological treatments, wastewater may need to be treated chemically to reduce the negative effects of recalcitrant contaminants on biological agents [10][11][12][13].Chemical treatments remove or convert contaminants through chemical reactions. Typical chemical processes include coagulation, precipitation, and chemical oxidation [5]. Chemical precipitation is achieved by using reagents capable of reacting and forming stable precipitates with contaminants. The precipitate can then be removed. Organic matter is transformed into carbon dioxide, water, and inorganic ions via degradation reactions involving oxidizing species, particularly hydroxyl radicals [11][12][13].
In biological treatments, contaminant removal is achieved by the action of microorganisms. The process is based on the self-regeneration of water bodies, whereby organic material is transformed into inert substances [12]. Activated sludge processes are the most commonly used biological treatments. Aerobic microorganisms digest organic matter and form flocculated particles (active sludge) and a liquid practically free of suspended solids and organic material. The organic matter is broken down via biological oxidation, resulting in CO2, H2O, NH3, energy, and other products [12,13]. Activated sludge treatment can be combined with other processes to improve the quality of the final effluent [10][11][12][13].
Textile, paper, and explosives wastewaters are opaque and have a high color because of the presence of lignin and its derivatives. In water bodies, these industrial wastewaters prevent light penetration and, consequently, photosynthesis. Furthermore, they contain high amounts of organic matter and toxic chemicals [14,18,32].
This study aimed to characterize and intensify of the treatment of the leachate to reduce its color, organic load, and toxicity by integration of processes: chemical followed by biological treatment.

Acute toxicity assay
E .coli was cultured in medium containing K2HPO4, KH2PO4, trisodium citrate, (NH4)2SO4, and MgSO4diluted in 800 mL of deionized water to the concentrations shown in Table 1. The culture medium was placed in a microwave oven and boiled for 10 min. A 200 mL solution of 10% (w/v) glucose was prepared and boiled for 5 min. The two solutions were cooled to 90 °C and mixed, and the pH was adjusted to 7.0 ± 0.2 using 4 mol/L NaOH.
A 100 mL stock solution containing 100 mmol/L Na2CO3 (previously oven dried at 120°C for 1 h) was prepared and diluted to obtain 0.25, 0.50, 1.0, 2.0, and 3.0 mmol/L solutions. Aliquots of 135Lwere used to construct a calibration curveusing a conductometry system [29]. The culture medium was inoculated with E. coli,and CO2 concentration was monitoreduntil reaching 0.5 mmol/L. The initial pH of the samples was adjusted to 7.0 ± 0.2 using1 mol/L NaOH or 1 mol/L H2SO4. Leachatewas added to culture flasks atconcentrations of 2, 6, and 10%, and CO2measurements were taken at 30 min intervals. The experiment lasted for 3 h.

Chronic toxicity assay
A. salina cysts (eggs) were incubated in 3.8% (w/v) NaCl in deionized water at 28-30 °C under a60 W fluorescent lamp for 24 h. After hatching, larvae were separated and placed in 5 mL vials containing 1 mL of saline solution. Vials contained 10 larvae each and received the addition of 1.5 or 3.0 mL of leachate, corresponding to 30 and 60% (v/v), respectively. Vials were then filled to 5 mL with saline solution and incubated for 24 h at 28-30 °C. A controlvial was prepared and subjected to the same conditions but without the addition of leachate. Dead and live larvae were counted in each vial, and results were expressed as the percentage of dead larvae [30].

Industrial-scale chemical treatment of leachate
Leachate is transported from the nitrocellulose production plant to the treatment plant via a 4-inch PVC pipe. The liquid is sieved (SS) and is discharged, by gravity, into the reservoir tank (RT). Then, it is pumped into RST 1 and RST 2, which are operated alternatively in batch. In RSTs, leachate is acidified to pH <1.5 using the acid wastewater from the nitration step or, when not available, sulfuric acid. Solutions are mixed using air diffusers. After 2 h, the supernatant follows to the compartmentalized tank for coagulation (CT), pH adjustment (AT), flocculation (FT), and decantation (ST). Solids retained in the decanter are sent to a filter press (FP). The filtered liquid returns to the treatment system, and the sludge is directed to the final treatment step at the outlet of the decanter.The chemically treated leachate is then subjected to biological treatment using an activated sludge process in a sequencing batch reactor. designed taking into account the decanting time. Flow, hydraulic retention time, and other process parameters were taken into account in the design of the coagulation, alkalization, flocculation, and settling tanks, according to literature data [11][12][13].

Fig.1: Industrial chemical system for treatment of nitrocellulose leachate
Source: Authors, 2020.

Pilot-scale biological treatment
Biological reactions were carried out in a 500 L stainless steel reactor (Fig. 2) equipped with three valves and operated in sequencing batch mode with 6 h cycles consisting of fill, react, settle, and draw periods. The time for sludge sedimentation ranged from 20 to 30 min [12]. Air was supplied to the system using an air compressor and diffusers.
The airflow was adjusted to provide a minimum oxygen concentration of 3 mg/L and ensure that the microbial biomass remained in suspension during the entire reaction period. Prior to the reaction, the pH of leachate was adjusted to 7.0 ± 0.3 with 10% (w/v) NaOH solution. pH (B-374 pH-meter, Micronal, São Paulo, Brazil), dissolved oxygen (TO 401 analyzer, Digimed, São Paulo, Brazil), and temperature were monitored throughout the process [13].   Untreated leachate had a very high color intensity (27,065 ± 879 color units), COD (7,615 ± 252 mg/L), BOD5,20 (4,413 ± 194 mg/L), and TOC content (2,455 ± 158 mg/L). The COD/BOD ratio was 1.73, indicating that leachate is susceptible to biological degradation [11]. Nevertheless, an industrial chemical process was used before biological treatment.

III. RESULTS AND DISCUSSION
Chemical treatment decreased color intensity by 93%, COD by 88%, BOD5,20by 92%, and TOC by 94%. After biological treatment, COD was reduced by 97%, BOD5,20 by 99%, and TOC by 97% compared with untreated leachate. No changes in color intensity were observed, suggesting that activated sludge is not effective in removing or degrading color compounds present in nitrocellulose leachate. Chemical processes, such as coagulation, are widely used as tertiary treatment for the removal of suspended solids, organic matter, and phosphorus [31,32].
However, in this study, the chemical process was used to intensify the biological process for the treatment of leachate from nitrocellulose production. Fig. 3 and 4 show the effects of integrated treatment. Source: Authors, 2020.

International Journal of Advanced Engineering Research and Science (IJAERS)
[ Vol-7, Issue-3, Mar-2020]  https://dx.doi.org/10.22161/ijaers.73.53  ISSN: 2349-6495(P) | 2456-1908(O) All model parameters were significant at P < 0.05. A P-value of 0.27 was obtained for the Anderson-Darling statistic, ruling out the hypothesis of normality of data. This result indicates that the model can adequately predict BOD values after biological treatment. With an R 2 = 0.49, the model shows that the characteristics of untreated and chemically treated leachate influence the performance of biological treatment.
We calculated the 95% confidence intervals for experimental results and model predictions (Fig.5). The model indicates that the combined treatment can reduce BOD to less than 60 mg/L. Source: Authors, 2020.
The acute toxicity of untreated leachate was assessed using E. coli. Black pulp at 2% immobilized 69% of microorganisms. At 6 and 10%, leachate completely immobilized E. coli. After chemical treatment, bacteria mobility was not affected by 2% leachate. At 6 and 10%, chemically treated leachate decreased bacterial mobility by 16 and 50%, respectively. Biologically treated leachate did not affect E. coli mobility.
Untreated leachate at concentrations of 30 and 60% showed chronic toxicity to A. salina, killing all microorganisms. Toxicity was not fully eliminated by chemical treatment. At 30 and 60%, chemically treated leachate killed 36 and 62% of micro crustaceans, respectively. Wastewater treatment should eliminate as many toxic compounds as possible, as contaminants are generally carcinogenic (35,36). The complexity of wastewater composition can further increase its recalcitrance to degradation [35]. Chemical treatment, despite reducing the organic load of leachate by more than 90%, was not sufficient to eliminate toxicity. Therefore, a second treatment was necessary. After treatment with activated sludge, leachate was not toxic to A. salinaat the tested concentrations.
The toxicity of leachate to the two microorganisms was probably due to the high concentration of organic matter and the presence of high molecular weight compounds (>kDa) derived from lignin. The recalcitrance to activated sludge treatment may be related to the limited ability of microorganisms to metabolize high molecular weight compounds [19,20].

IV.
CONCLUSION Leachate had high levels of lignin and organic matter, as evidenced by the high COD (7,615± 252 mg/L), color intensity (27,065 ± 879 units), and toxicity to E. coli and A. salina, which indicates that this wastewater can cause serious environmental contamination if released untreated.
Industrial chemical treatment reduced color intensity by 93%, COD by 88%, BOD by 92%, and TOC content by 94%. However, these reductions were not sufficient to meet regulatory requirements for wastewater discharge. Therefore, chemically treated leachate was subjected to a biological treatment with activated sludge, which reduced COD (198 ± 17 mg/L), BOD (43 ± 7 mg/L), and TOC content (83 ± 7 mg/L) to levels below regulatory limits. Chemical treatment did not eliminate toxicity but increased the susceptibility of leachate to biological treatment. Overall, the results show that integrated chemical and biological processes are promising for the remediation of leachate.
All wastewater generated by the pulp nitration step was reused for pH correction in the leachate treatment process. The reuse of the effluent from the nitration step made it possible to reduce the expenses with reagents for pH correction of this effluent before its release into the environment.