ISSN 1068-364X, Coke and Chemistry, 2017, Vol. 60, No. 8, pp. 326–331. © Allerton Press, Inc., 2017. Original Russian Text © T.M. Sabirova, N.A. Ryazantseva, 2017, published in Koks i Khimiya, 2017, No. 8, pp. 35–40.
INDUSTRIAL SAFETY
Treatment of Coke-Plant Wastewater at Bhilai Steel Plant T. M. Sabirovaa, * and N. A. Ryazantsevab, ** aYeltsin
Ural Federal University, Chemical Technology Institute, Yekaterinburg, Russia b AO VUKhIN, Yekaterinburg, Russia *e-mail:
[email protected] **e-mail:
[email protected] Received June 20, 2017
Abstract⎯The existing biochemical system in the coke plant at Bhilai Steel Plant is inspected and assessed, with a view to introducing nitrification and denitrification, without reconstruction. It is shown that the existing biochemical-treatment facility will accommodate a nitrification and denitrification system capable of processing coke at a rate of 3.0–3.5 million t/yr. With reconstruction of the aeration system, the throughput may be increased to 4.8 million t/yr. That entails continuous dosing of alkaline reagent in the treated water. Recommendations are made for improved operation of the biochemical system. Keywords: coke-plant wastewater, biochemical treatment, nitrogen removal, nitrification and denitrification, integrated biotechnology, reactor, aeration tank, final purification DOI: 10.3103/S1068364X17080051
Biochemical treatment of coke-plant wastewater is widely used in India, as elsewhere. By this means, almost all the pollutants—including phenols, thiocyanates, cyanides, ammoniacal nitrogen, hydrogen sulfide, pyridine, and naphthalene—may be efficiently oxidized to mineral compounds (CO2, H2O, N2,
SO 24 − , etc.). The existing biochemical system in the coke plant at Bhilai Steel Plant was designed by S.K. Chakravarty
& Company in 1994 to process wastewater at a rate of 200 m3/h. Table 1 summarizes its parameters. The biochemical system went into operation in 1995. The design was based on a system developed by United States scientists. Note that, despite some design differences, the biochemical system in the coke plant at Bhilai Steel Plant has all the basic consecutive components characteristic of Russian systems: a pumping station, the wastewater intake tank, settling
Table 1. Design parameters of the biochemical system in the coke plant at Bhilai Steel Plant Design parameters Characteristic
at the system output (treated wastewater) at the system input 1995 standards
current standards
рН Phenols, mg/dm3 Thiocyanates, mg/dm3 Cyanides, mg/dm3 Ammoniacal nitrogen, mg/dm3 COD, mg O2/dm3
7–9 825 395 48 370 7500
6–8 1 – 0.2 100 250
6–8 1 – 0.2 50* 250
Suspended particles, mg/dm3 Tars and oils, mg/dm3 Nitrite nitrogen, mg/dm3 Nitrate nitrogen, mg/dm3
750 275 – –
100 10 – –
100 10 – 10
* Specified for the adjustment period.
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tanks for tar, averaging tanks, air-cooling units, a reagent vessel, flotation units, incubators, first-stage reactors, secondary settling tanks, intermediate collectors, a second stage (two trickling biofilters), secondary settling tanks, treated-water collectors, and the active-sludge area. Advantages of the biochemical-system design employed are as follows. (1) The volume of the two-section averaging tank permits residence of the wastewater for more than two days at a flow rate of 200 m3/h. That facilitates natural cooling of the phenolic water. (2) The use of aluminum-sulfate coagulant to remove oils from the wastewater in the flotation units. (3) The use of surface mechanical aerators that not only saturate the wastewater with oxygen but also cool it to standard temperatures of 30–35°C. (4) The creation of fine water sprays from jets around the perimeter of the aeration tanks to reduce atmospheric emissions. (5) The use of trickling biofilters, which form a biofilm on the charge and improve the separation of the treated water from suspended active sludge in the secondary settling tanks. In turn, disadvantages of the biochemical-system design are as follows.. (1) The lack of a unit for continuous dosing of alkaline reagent in the treated water. (2) The absence of denitrification (reduction of nitrite and nitrate nitrogen to gaseous nitrogen), which not only removes nitrites and nitrates but also restores almost 50% of the alkalinity lost in nitrification. (3) Large open surfaces in the aeration tanks and surface aeration units, which facilitate the emission of toxic pollutants into the atmosphere, despite the water sprays. In the new standard, the residual ammoniacalnitrogen content required in the treated coke-plant wastewater has been reduced from 100 to 50 mg/dm3 (Table 1). Accordingly, Russian specialists must inspect existing biochemical systems and assess their potential for purifying wastewater to that standard. As we see in Table 2, the ammoniacal-nitrogen content at the beginning of the adjustment period is above the design standard, despite the dilution of the wastewater and the preliminary removal of most of the volatile and fixed ammonia in the ammonia columns. The mean total ammoniacal-nitrogen content is ~190 mg/dm3 at the input to the biochemical system and between 130 and 165 mg/dm3 at the output of the biochemical system. Only part of the wastewater (~35 m3/h) is sent to the biochemical system, since that is the limit of its capacity. The main problem with ammoniacal-nitrogen removal in the coke plant at Bhilai Steel Plant is that COKE AND CHEMISTRY
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Table 2. Characteristics of coke-plant wastewater before and after biochemical treatment in the existing system At the At the system treatment- output (treated system input wastewater)
Characteristic рН Phenols, mg/dm
8.0 372
6.0 0.23
Thiocyanates, mg/dm3
180
–
3
mg/dm3
Cyanides, Ammoniacal nitrogen, mg/dm3
3.1
0.16
190
130–165
COD, mg O2/dm3
1800
–
–
–
Suspended particles, mg/dm3
–
–
3
–
–
mg/dm3
–
–
Tars and oils, mg/dm3 Nitrite nitrogen, mg/dm Nitrate nitrogen,
the original developers failed to take account of the additional ammoniacal-nitrogen production due to biological oxidation of thiocyanates, pyridine, and other nitrogen-bearing pollutants in the wastewater [1]. They also disregarded the conversion of ammonium sulfide (NH4)2S to fixed ammonia. Accordingly, the design consumption of alkaline reagent in wastewater treatment was significantly too low. As is evident from Table 2, the pH of around 6 for the treated wastewater (less than the initial value of 8) indicates nitrification in the treatment system but also a lack of alkalinity, since the residual ammoniacalnitrogen content is more than 130 mg/dm3. Compensation of the lack of alkalinity in the biochemical system is impossible, not only because of the underestimates of the soda required but also because of technical constraints. The volume of the collector provided for this purpose is only able to ensure the standard pH for an actual wastewater flow rate of 3–5 m3/h, in contrast to the design flow rate of 200 m3/h. Thus, the original designers incorrectly calculated what was required to meet the standard for the removal of ammoniacal nitrogen in the ammonia columns. What is needed today is not only the introduction of the nitrification and denitrification process but also increase the wastewater flow rate in the biochemical system to match the rate at which it is formed in the coke plant. As we have established, the main requirements in adopting the nitrification and denitrification process when the initial ammoniacal-nitrogen content is 250– 600 mg/dm3 and further ammoniacal nitrogen is created by the oxidation of other nitrogen-bearing pollutants are as follows.
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(1) Residence of the wastewater in the aeration tanks for no less than 84 h. That is necessary so that the slowly growing nitrification bacteria become attached to the treatment system and are not entrained with the treated water. (2) The creation of conditions for adaptation and accumulation of the necessary dose of nitrification bacteria in the treatment system, with allowance for their growth, maturation, and clumping. (3) Saturation of the treated wastewater with compressed air so that the content of dissolved oxygen is no less than 2 mg/dm3 during the accumulation of nitrification bacteria. The content of dissolved oxygen in the treatment system must be adjustable as required. (4) Continuous dosing of alkaline reagent to maintain pH ≥ 6.5 for the treated wastewater. REQUIREMENTS FOR NITRIFICATION AND DENITRIFICATION Assuming the design wastewater throughput of 200 m3/h, we find that the existing volume of the aeration tanks in the coke plant at Bhilai Steel Plant ensures a residence time of around 50 h for the wastewater in the treatment zone. This is insufficient for the introduction of nitrification and denitrification. However, as established On inspection of the biochemical system, its actual wastewater throughput is about half of the design value. In other words, the wastewater production in the coke plant at Bhilai Steel Plant is 0.25–0.3 m3/t of coke. This is typical of the climatic conditions for plants in hot countries, where significantly less steam is required for flushing and heating of the equipment and piping than in northern countries, and hence the condensates that are the source of wastewater are formed in much smaller quantities. We know that the mechanical surface aerators saturate the treated wastewater to dissolved-oxygen concentrations greater than 2 mg/dm3 if the residence time of the wastewater in the treatment zone is at least two days, as is the case. Thus, preliminary estimates suggest that all of the requirements may be met, except for the dosing of alkaline reagent in the first stage. To that end, we recommend the additional supply of caustic soda in the ammonia columns, rather than in the first stage. Note that the alkaline reagent is necessary not only to neutralize the nitric and nitrous acids formed in nitrification but also for the conversion of fixed ammonia to volatile form prior to biological oxidation [2]. Otherwise, nitrification would stop. To ensure the physicochemical conversion of the fixed ammonia in the ammonia columns, it is expedient to use commercial caustic soda in liquid form, so as to rule out the additional operations required by the dry product. In turn, for biological nitrification, it is preferable to use magnesium hydroxide or soda ash;
these are milder reagents than caustic soda in terms of their action on the bacteria. Accordingly, the system may operate at pH 6.5–9.0, without inhibition of the active sludge, whereas excess alkalinity is practically impermissible when working with caustic soda. PREPARATION OF THE BIOCHEMICAL SYSTEM FOR NITRIFICATION AND DENITRIFICATION After several years of operation, the concrete structure and auxiliary equipment used in the biochemical system have suffered considerable wear. Structures have begun to disintegrate, and leaks are apparent in the walls of the aeration tanks, the tar separators, and the flotation units. Many measuring instruments and electrical components, including the pumps, are also out of operation. Accordingly, the structures must be systematically repaired and some of the equipment and instruments must be replaced. Analytical monitoring of the composition of the incoming and treated water is also inadequate. For most of the water flows, there is no analysis of the COD or the content of volatile ammonia, phosphorus, oils, nitrites, nitrates, suspended particles, dissolved oxygen, and thiocyanates. That creates further difficulties for the introduction of nitrification. Gradual elimination of these problems is included in the plan for coke-plant operation. ACCUMULATION OF NITRIFYING BACTERIA IN THE AERATION TANKS In integrated wastewater treatment with nitrification and denitrification (simultaneous oxidation of all the pollutants, including nitrification and denitrification), the consumption of alkaline reagent is stoichiometric and is calculated on the basis that 0.5 mole of soda ash (Na2CO3) or 1 mole of caustic soda (NaOH) is required for 1 mole of fixed ammonia (N–NH4). In the nitrification and denitrification of volatile ammonia, except for (NH4)2S, no alkaline reagent is required. Note that, in the biochemical system, (NH4)2S is rapidly converted to ammonium sulfate— that is, to fixed ammonia. In the absence of denitrification, which restores the alkalinity, the consumption of alkaline reagent in the oxidation of fixed ammonia increases and is calculated on the basis that 1 mole of soda ash (Na2CO3) or 2 moles of caustic soda (NaOH) is required for 1 mole of N–NH4. By contrast, 0.5 mole of soda ash (Na2CO3) or 1 mole of caustic soda (NaOH) is required for the oxidation of 1 mole of volatile ammonia [2]. As already noted, before adjustment, the wastewater throughput at the biochemical system is ~35 m3/h, or around 45–50 m3/h if we take account of dilution with industrial-grade water. Such flow ensures a resiCOKE AND CHEMISTRY
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Table 3. Wastewater composition before and after biochemical treatment during the adjustment period
Input to first stage Characteristic
First phase of adjustments
Second phase of adjustments
With Stabilization integrated period, after nitrification after first after second after first after second second stage and denitstage stage stage stage rification
рН
9.3–9.7
6.5–7.0
6.3
6.9–7.2
6.7
6.0–6.7
6.5–8.0
Phenols, mg/dm3
480–702
0.3
0.25
–
0.23
0.2–0.23
0.02–0.05
Thiocyanates, mg/dm3
160–250
27.9
12
9.1
5.5
5–7
0.04–2.00
Cyanides, mg/dm3
4.0–4.5
0.31
0.24
–
0.2
0.19–0.20
0.1–0.2
Ammoniacal nitrogen, mg/dm3
200–320 150–280
54–100 45–85
45–84 37–70
15–49 12–49
3–27 3–23
24–27 –
0.5–10 –
1800–2900
732
238–350
456
135–462
450
250–450
–
181
110–180
180
60–200
150
100–150
35–158
–
–
–
–
5–7
3–5
Nitrite nitrogen, mg/dm3
0
15–65
10–45
2.5–35.0
1.8–45.0
1.4–2.0
0.5–2.0
Nitrate nitrogen, mg/dm3
0
14–96
15–100
8–45
9–50
8.4–10.0
7–15
COD, mg O2/dm3 Suspended particles, mg/dm3 Tars and oils, mg/dm3
dence time of about nine days in the first stage. That corresponds to adaptation of the nitrifying bacteria and oxidation of 10–15% of the ammoniacal nitrogen in the first stage, with the oxidation of 17–20% of the ammoniacal nitrogen in the second stage, if alkalinity is still present in the water after the first stage. Accordingly, the supply of nitrifying bacteria to the treatment system is not required, and work to increase the throughput of the biochemical system began so as to improve the removal of ammoniacal nitrogen from the wastewater. The first step is to stabilize the composition of the wastewater in terms of all the pollutants. That includes reduction in the ammoniacal-nitrogen content to the standard level. To that end, the wastewater flow rate at one of the two reactors (reactor 1) is gradually decreased, by increasing the flow rate at the other. The load is reduced until stable decrease in pH begins on account of nitrification in reactor 1. The redistribution of the load over the reactors promoted nitrification and increase in the dose of nitrifying sludge in both reactors, on account of its recirculation. As already noted, the supply of additional promoted introduction of alkaline reagent to the ammonia columns in ammonium-sulfate shops 1 and 2 is organized to neutralize the pH. In addition, measures are taken to normalize the operation of the ammonia columns. The supply of alkaline reagent is gradually increased to maintain a pH of about 7. That promotes COKE AND CHEMISTRY
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growth of the nitrifying bacteria. In the oxidation of ammonia, nitrites are primarily formed. Their oxidation to nitrates and the further removal of ammoniacal nitrogen occurs in the second stage: in the trickling biofilters (Table 3). In this period, the level of denitrification is about 20%. Therefore, the consumption of alkaline reagent exceeds the standard. The second phase of the work, with increase in the load, begins when regular supplies of caustic soda are available at the coke plant, in quantities required for oxidation of the ammonia in the biochemical system. After stable decrease in the ammoniacal-nitrogen content in reactor 2 below 50 mg/dm3, the load at the reactors is gradually increased, in accordance with the growth and maturity of the nitrification bacteria. As nitrification develops, the necessary alkalinity and pH are ensured by increasing the excess of alkaline reagent in the ammonia columns. When the wastewater throughput at the biochemical system is 70 m3/h, the composition of the treated wastewater is actually improved, as seen in Table 3. The only exception is the COD. The increase in the COD of the treated wastewater is due to proportional increase in the metabolite concentration produced by the active sludge, because the dilution of the incoming wastewater with industrial water is discontinued [3]. Note that further increase in the load is impossible, both because of the limits on the soda supplies to the plant and because of the high tar and oil content in the
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remaining two flows of the tar-processing shops after the failure of the tar settling tanks. After stabilization of nitrification in the first stage, with 80–85% oxidation of the ammonia, the dilution of the incoming wastewater with industrial water is discontinued. The next step is to organize integrated nitrification and denitrification [4, 5]. That entails reducing the content of dissolved oxygen in the treated water to <2 mg/dm3. When using surface aeration units, the only possibility of reducing the excess dissolved oxygen in the water is to shut down one or two surface aeration units in the reactor. That approach is adopted. As is evident from Table 3, shutting down the aeration units does not harm nitrification, whereas the degree of denitrification is increased from 20 to 85– 94%. That creates alkalinity, and the caustic-soda supply to the ammonia columns for nitrification in the biochemical system may be reduced practically to stoichiometric levels [2]. The trickling biofilters in the second stage further reduce the content of thiocyanates, cyanides, and ammonia in the wastewater and also ensure oxidation of the nitrites and nitrates. However, the ammonia removal is unstable, since the wastewater supplied to the filters does not always contain sufficient alkalinity for its oxidation. In addition, the formation of biofilms and aggregates of active sludge in the trickling biofilters facilitates the removal of suspended particles from the water in the secondary settling tanks. The design standards for ammonia cannot expediently be met by increasing the dose of expensive caustic soda so as to improve ammonia removal. The actual soda consumption exceeds the design standard, which takes no account of the growth in ammoniacal-nitrogen content due to the biological oxidation of nitrogenbearing pollutants, as already noted. Accordingly, we require new standards for caustic soda or additional standards for soda ash. Note that, thanks to stable operation of the ammonia columns, fluctuation in the total ammoniacalnitrogen content in the range 150–200 mg/dm3 in the water supplied to the biochemical system is favorable for the adjustment of the technology and the introduction of nitrification and denitrification. Thus, to ensure the necessary parameters of the treated wastewater during the adjustment of the biochemical system at Bhilai Steel Plant, the following measures are taken. (1) Stabilization of the wastewater composition at the input to the biochemical system. Dilution of the wastewater with industrial-grade water is discontinued, and the wastewater supply to the biochemical system is increased from 35 to 70 m3/h. (2) Optimization of sampling and analysis of wastewater samples beyond the ammonia columns and improved monitoring of their operation.
(3) Restoration of the inoperative tar settling tanks, oil separators, and averaging tanks. (4) Installation of a stand-by recirculation pump for the active sludge in the first stage and repair of the surface aeration units. (5) Installation of additional measuring instruments and restoration of the ~20% of the existing instruments that are inoperative. (6) Correction of the operation of the reagent units, including the supply of phosphoric acid. (7) Development of a new system for analytical monitoring of all the wastewater flows entering and leaving the biochemical system. (8) Development of recommendations for stable operation of the biochemical system with nitrification and denitrification. On the basis of monitoring and assessment of the system, the following steps are recommended to further improve the operation of the biochemical system at Bhilai Steel Plant. (1) Complete replacement of the unreliable aeration tanks in one or both reactors by airlift aerators, with the supply of compressed air from an air line [6]. That permits regulation of the compressed-air flow for integrated nitrification and denitrification. (2) Construction of a soda unit ensuring continuous supply of alkaline reagent (preferably soda ash) to the first stage. The flow rate of the alkaline reagent is increased with increase in the supply of wastewater. (3) Recirculation of the active sludge from the secondary settling tanks of the second stage to the first stage, since the trickling biofilters facilitate the formation of slowly growing bacteria. (4) Introduction of monitoring instruments for all the main operations, including measurements of the pH and the concentration of nitrites, nitrates, ammoniacal-nitrogen, and phosphorus. If all the existing structures and equipment in the biochemical system are maintained in working order, these recommendations will ensure compliance with the standards and more complete removal of pollutants from wastewater flows of 100–125 m3/h—in other words, from all the wastewater produced in the coke plant at Bhilai Steel Plant. If increase in throughput by 50 m3/h—from 125 to 175 m3/h—is required, compliance with the standards entails increasing the removal of ammoniacal-nitrogen in the ammonia columns by 50%, so that its residual content is less than 100 mg/dm3. CONCLUSIONS The introduction of integrated nitrification and denitrification ensures that the treated wastewater in the coke plant at Bhilai Steel Plant complies with the applicable standards. COKE AND CHEMISTRY
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That reduces the environmental impact of the plant, thanks to decrease in water pollution and decrease in the toxic emissions to the atmosphere when coke is quenched with the biochemically treated water. REFERENCES 1. Sabirova, T.M, Prospects for biotechnology in wastewater processing at coke plants, Coke Chem., 2016, vol. 59, no. 3, pp. 111–116. 2. Sabirova, T.M., Biological denitrification of wastewaters at byproduct coke plants, Coke Chem., 1999, no. 11, pp. 39–43. 3. Sabirova, T.M., Nevolina, I.V., Stukov, R.M., and Kopeliovich, B.L, Influence of endogenous respiration
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on the coloration and chemical oxygen demand of biologically purified wastewater, Coke Chem., 2011, vol. 54, no. 3, pp. 99–101. 4. Sabirova, T.M., Study and development of single-phase nitrification and denitrification of sewage waters, Coke Chem., 2000, no. 9, pp. 46–50. 5. Ozerskii, Y.G., Kovalev, A.V., and Volokh, V.M., Recommended best available techniques in wastewater purification at coke plants, Coke Chem., 2011, vol. 54, no. 6, pp. 215–223. 6. Sabirova, T.M., Nevolina, I.V., and Milenina, E.A., Coke-chemical production wastewater treatment, Khim. Tekhn., 2008, no. 4, pp. 32–36.
Translated by Bernard Gilbert
