Treatment of coke wastewater in a sequential batch reactor (SBR) at pilot plant scale

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Bioresource Technology 99 (2008) 4192–4198

Treatment of coke wastewater in a sequential batch reactor (SBR) at pilot plant scale E. Maran˜o´n *, I. Va´zquez, J. Rodrı´guez, L. Castrillo´n, Y. Ferna´ndez, H. Lo´pez Department of Chemical and Environmental Engineering, Higher Polytechnic School of Engineering, University of Oviedo, 33204 Gijo´n, Spain Received 19 October 2006; received in revised form 24 August 2007; accepted 29 August 2007 Available online 24 October 2007

Abstract Coke wastewater is a highly toxic industrial effluent which is usually treated by a combination of physico-chemical and biological treatments. With the aim of completing prior studies carried out in CSTR, in this work we studied the treatment of coke wastewater in a pilot plant equipped with a 400 L stripping tank, a 350 L neutralization/homogenization tank and a 6 m high 1500 L sequential batch reactor (SBR), controlled by a PLC. Ammonia stripping efficiencies of 96% were obtained for HRT of 66 h. The biological treatment in the SBR led to removal efficiencies of 85% COD, 98% thiocyanate and 99% phenols for HRT of 115 h. Final concentrations in the effluent of 1.8 mg phenols/L, 5.4 mg SCN/L, 206 mg COD/L and 78 mg N–NHþ 4 =L were obtained. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Coke wastewater; Phenols; Thiocyanate; Aerobic treatment; SBR

1. Introduction In the coke-making process, ammonium is removed from the exhaust gas by adsorption onto water in order to reduce its concentrations to acceptable levels in the gas outlet of the plant, i.e. to around 0.1 g/m3. The generated aqueous effluent contains ammonium concentrations ranging between 5 and 10 g/L (Rancan˜o, 2000). Besides this pollutant, coke wastewater also contains substantial amounts of certain toxic compounds such as phenols, thiocyanates, cyanides, sulphides and chlorides, as well as small amounts of polyaromatic hydrocarbons and heterocyclic nitrogenous compounds, the presence of which in water sources is severely limited by current legislation, and which result in dark brown colouring of this wastewater (Li et al., 2003; Staib, 1998; Va´zquez et al., 2006). Table 1 shows the concentrations of the main pollutants in typical effluents from coke ovens. The concentration of each component varies as a function of the types *

Corresponding author. Tel.: +34 985182027; fax: +34 985182337. E-mail address: [email protected] (E. Maran˜o´n).

0960-8524/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.08.081

of coal used and the different modifications of the process employed to manufacture the coke. The presence of the aforementioned pollutants makes it necessary to treat the wastewater before disposal. Such treatment usually consists of a biological process preceded by a series of physico-chemical treatments to reduce the concentrations of solids, greases and ammonium (Ghose, 2002). In general, the ammonium concentration should be reduced by means of a stripping process. Using this process, the ammonium concentration can be reduced to 50 mg/L, although values of around 100 and 300 mg/L are more common. Phenol is the pollutant which contributes the most to the total COD in coke wastewater. Despite being a readily biodegradable substrate, phenol is also an inhibitor of the biological process. Therefore, the biomass employed for this wastewater should be especially acclimated. Under the appropriate operational working conditions, the acclimated biomass can oxidize the biodegradable organic compounds, phenols included, present in the wastewater and biological processes may be used for coke wastewater (Ashmore et al., 1967, 1968; Catchpole and Cooper, 1972;

E. Maran˜o´n et al. / Bioresource Technology 99 (2008) 4192–4198 Table 1 Typical pollutant concentrations in the coke wastewater Parameter

Coke wastewater Australia

Germany

China

Spain

BOD5 (mg L1) COD (mg L1) TSS (mg L1) TKN (mg L1) 1 NHþ 4 –N (mg L ) 1 Total-P (mg L ) Phenols (mg L1) SCN (mg L1) CN (mg L1)

450–720 1800–2200 40–50 200–330 200–275 <1 60–330 180–200 70–95

1600–2600 4000–6500 2–10 300–500 50–150 <1 400–1200 200–500 4–15

200–380 630–860 – 305–390 220–280 – 50–80 – –

500–1000 800–3000 25–50 250–550 200– 400 <1 180– 300 180–450 15–40

Cooper and Catchpole, 1973; Park et al., 2007; Li et al., 2003; Luthy and Tailor, 1980; Papadimitriou et al., 2006; Qi et al., 2007). With the aim of completing previous studies carried out at laboratory scale employing an activated sludge process in one, two and three steps (Va´zquez et al., 2006a,b; Macho´n et al., 2007), in the present research work the authors studied the biotreatment of coke wastewater in a pilot plant equipped with a sequencing batch reactor (SBR) located at the Arcelor facilities in Avile´s (Spain). The SBR is widely used for the removal of organic and nitrogenous compounds in industrial wastewaters (Villaverde et al., 2000; ¨ ztu¨rk, 2001). The wastewater underwent a Yalmaz and O stripping process prior to biological treatment in order to reduce the concentration of ammonia. 2. Methods The pilot plant employed, a flow chart of which is shown in Fig. 1, consisted of a 400 L volume steel cooling tank in which a stripping process using NaOH was performed to decrease the concentration of NHþ 4 –N in the wastewater.

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The air required to facilitate ammonia removal was furnished by the company’s compressed air supply system and was introduced via a pipe with orifices placed at the bottom of the tank. Ammonia gas was incorporated into the effluent of the industrial scale stripping column and transferred into ammonium sulphate. The effluent from the first tank flowed into a 350 L volume homogenization tank in which the mix was maintained by means of aeration. The wastewater was neutralised in this tank before entering the 6 m high 1500 L biological sequencing batch reactor controlled by means of a PLC. With the aim of monitoring the stripping and biodegradation processes taking place at the plant, the influent and the effluents were analysed using standard methods (APHA, 1998). In the case of not being able to carry out immediate analyses, samples were always kept under refrigeration at 4 °C. Phenol, COD and nitrates were analysed by colorimetric methods using a HACH DR/2010 Spectrophotometer. NHþ 4 –N concentration was measured by potentiometry using an ORION 95-12 BN ion selective electrode. SCN was analysed via a colorimetric method based on the formation at an acid pH of an intense red complex between Fe3+ and SCN. To start up the biological reactor, sludge from the leachate treatment plant at the Central Landfill for Municipal Solid Waste of Asturias was introduced in order to obtain a concentration of total suspended solids of around 5 g/L in the reactor. The effectiveness of this sludge as an inoculum has been verified in a previous laboratory-scale study (Va´zquez et al., 2006a,b). During the start-up of the reactor, cycles of 48 h and Hydraulic Residence Times (HRTs) of 240 h were employed. This stage, which was completed when good removal efficiencies of the pollutants were achieved, lasted 70 days. After this starting period, different operating working conditions were studied, as it is shown in Table 2.

O

Effluent

Coke wastewater NaOH pH

H 2 SO4

pH

Air HOMOGENIZATION TANK STRIPPING AND COOLING TANK

Fig. 1. Pilot plant flow chart.

Sludge purge

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Table 2 Characteristics of the operating cycles employed in the biological treatment of coke wastewater at pilot plant scale in a SBR HRTstr (h)

HRTSBR (h)

Operating days

Cycle length (h)

Filling (h)

Aeration (h)

Settling (h)

Draining (h)

66 40 34 17

225 137 115 58

1–60 61–120 120–180 180–220

48 48 24 24

1 1 1.5 1.5

43 43 21 21

3.5 3.5 1 1

0.5 0.5 0.5 0.5

Automatic control of the aeration process was employed during the whole process. The equipment installed consisted of a regulating valve (Schubert & Salzer GS2 8043), dissolved oxygen sensor (Foxboro 871DO-Constitution), provided with an electrochemical analyser (873DO), controller (Eurotherm 2216e) and a graphical recorder (Eurotherm 5100 V). The arrangement of these devices follows a control scheme of the standard feedback type. Control is performed by measuring the concentration of dissolved oxygen, comparing the value with the fixed value, and the intensity of aeration is modified by varying the valve opening in order to always maintain the oxygen concentration close to the fixed value of 6 mg/L, as a smaller value was very difficult to control with the regulating valve. From Day 130 on, two dose regulation units were installed with the aim of improving operational control and functioning of the pilot plant. The first unit is made up of a Dosapro Milton Roy GM 25S pump with a 316 L stainless steel body, suitable for working with strongly basic media. This pump is equipped with a Stegmann ER 20 electric actuator governed by a PID Eurotherm 2216e controller, which uses an input signal supplied by the Foxboro 871A pHmeter of the stripping unit. The aim of this dose regulation unit consists in maintaining the pH of the stripping reactor constant at a value of 11.7 by means of the regulated addition of 15% NaOH. The second dose regulation unit is made up of a Dosapro Milton Roy GM 25S pump

with a PVDF body, suitable for working with strongly acid media. This pump is also equipped with a Stegmann ER 20 electric actuator governed by a PID Eurotherm 2216e controller, which adjusts the necessary dose flow on the basis of the values supplied by the Foxboro 871A pHmeter of the homogenization unit, thus closing the control loop. The aim of this dose regulation unit consists in maintaining the pH of the homogenization reactor constant at a value of 6.0–6.5 by means of the regulated addition of 10% H2SO4, since this value was found to be the optimum for the biodegradation of SCN in a previous study at laboratory scale (Va´zquez et al., 2006a). The reagents consumed in the process were: a small amount of anti-foaming agent, NALCO 71D; 130 g Na2HPO4/m3 as phosphorus source for biodegradation; 15% NaOH (12–16 L/m3) to remove NHþ 4 –N in the stripping tank, and 10% H2SO4 (11–15 L/m3) to neutralize the influent to the SBR in the homogenisation tank. 3. Results and discussion The NHþ 4 –N concentration of the coke wastewater ranged between 401 and 750 mg/L, as can be seen in Fig. 2. Usual concentrations at the steel works are lower (Table 1) as a great part of ammonia is recovered by stripping, but due to some operational problems in the industrial stripping columns during this research, a stripping step

2000

100

1800

90 80

1600 Coke wastewater Stripping effluent

70 HRT= 34 h

Removal

1200

60

+

mgNH4 -N/L

1400

1000

50

800

40

600

30 HRT= 40 h

400

HRT= 17 h

HRT= 66 h

20 10

200

0

0 0

50

100

150

200

Time (days) Fig. 2. NHþ 4 –N evolution in coke wastewater and in the stripping effluent.

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prior to the biological treatment in the SBR was performed in order to reduce possible toxic effects for the microorganisms that may decrease the removal efficiencies of the pollutants (Kwon et al., 2002).

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period coincides with the installation of the automatic dosification unit (on Day 130). As can be seen in Fig. 2, after Day 155 the removal increased to values higher than 80%. Apart from this exception the removal efficiency tends to increase with HRT.

3.1. Ammonia removal by stripping 3.2. Biological treatment of coke wastewater in an SBR

COD (mg O2 /L)

The stripping process consists of fizzing air through the wastewater to remove ammonia, which would pass from the liquid to the gaseous phase in an alkaline medium. As the stripping tank was smaller in volume than the SBR, the HRTs employed were lower than those of the biological reactor, being 66, 40, 34 and 17 h. The liquid in the tank was always saturated with oxygen due to strong aeration. As no heating element was used, the operating temperature in the tank was slightly higher than that of the environment, since the wastewater entered at temperatures of around 35 °C, and ranged between 11.4 and 18.6 °C. The NHþ 4 –N loading rate decreased with increasing 3 HRT and varied between 2.5 kg NHþ 4 –N=m day for an þ 3 HRT of 66 h and 9.8 kg NH4 –N=m day for 17 h, variations being observed for each HRT due to the varying ammonium concentrations of the wastewater. The pH of the wastewater was kept at high alkaline values by adding NaOH, and ranged between 10 and 12.5 in the first part of the study. The use of the automatic dosage system from Day 130 on allowed both a saving in NaOH consumption and better control of the pH, which was kept constant at around 11.7 during the rest of the operational period. As can be observed in Fig. 2, removal efficiencies of between 37% (HRT = 34 h) and 96% (HRT = 66 h) were achieved. The low removal obtained at the beginning of the operation at HRT 34 h may be due to some adjustment problems in the automatic addition of NaOH, since this

Once the removal of a major part of the NHþ 4 –N in the coke wastewater was achieved, the wastewater had to be neutralised before entering into the biological reactor owing to the high pH values employed in the stripping process. Consequently, a homogenization tank was placed between the stripping tank and the SBR in order to add the necessary reagents. H2SO4 was used to neutralise the wastewater, since this reagent was available in the company facilities as a by-product. As no heating element was used, the reactor temperature was approximately the same as that of the environment, possibly being slightly higher due to microbial activity, and ranged between 14.7 °C for an HRT of 115 h and 21.7 °C for HRTs of 225 and 137 h. The oxygen dissolved in the mixed liquor was kept around 4.5 mg/L by the oxygen sensor and the automatic regulation valve. The pH inside the reactor was fixed at 6.5, optimum value for the biodegradation of thiocyanate, pollutant that needs longer time to biodegrade than phenols or other organic compounds in coke wastewater (Va´zquez et al., 2006a,b). The food/microorganisms (F/M) ratio increased with decreasing HRT, ranging between 0.06 and 0.24 kg COD/ kgVSS.day. The organic loading rate (OLR) varied between average values of 0.14 and 0.56 kg COD/m3 day, also increasing with decreasing HRT. Due to variations in the composition of the coke wastewater, it was very dif-

2000

100

1800

90

1600

80

1400

70

1200

60

1000

50 Coke wastewater Influent Effluent Removal

800 600

HRT= 58 h

30

HRT= 225 h HRT= 137 h

400

40

HRT= 115 h

20

200

10

0

0

0

50

100

150

200

Time (days) Fig. 3. COD evolution in the different process streams and removal obtained.

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ficult to maintain a fixed value for the OLR, and so the chosen operating parameter was the HRT. The concentration of volatile suspended solids (VSS) ranged between 1.5 and 2.9 g/L, with an average value of 2.2 mg/L, representing 79% of TSS. The sludge volumetric index (SVI) presented values of between 47 and 80 cm3/g, the adding of a coagulant not being necessary.

the biological treatment ranged between 1.7 and 5 mg/L, the removal efficiency being always higher than 97%. No relation between removal efficiencies and HRT employed was observed, since the HRTs were always high enough to achieve almost complete biodegradation of phenols.

3.3. COD removal

Fig. 5 shows the evolution of SCN concentrations in the wastewater, the influent and effluent of the SBR and removal efficiencies for different operating conditions. In the influent to the SBR, the concentration of thiocyanate ranged between 210 and 487 mg/L and after the treatment concentrations decreased between 1 and 20 mg/ L. Very good removal efficiencies were reached, around 95% or higher when operating at HRT P 115 h. The efficiency decreased to 90% for HRT of 58 h.

3.5. Thiocyanate removal

Fig. 3 shows the evolution of COD concentration in the wastewater, influent and effluent of the biological reactor as well as the removal efficiencies for the different operating conditions. The COD of the wastewater ranged between 1100 and 1700 mgO2/L, decreasing slightly after stripping and homogenization, as can be observed from the SBR influent values. This could be due to oxidation by air of polyhydroxyphenols such as catechol during the stripping process, according to bibliography (Luthy and Tailor, 1980; Rancan˜o, 2000). A change in colour after the stripping process from pale yellow to dark brown was observed. The effluent COD was always lower than that of the influent, varying between 155 and 560 mg/L. Removal efficiencies ranged between 80% and 90%, except for the shortest HRT employed, decreasing to 62%.

3.6. Variation in ammonium concentration As mentioned above, the coke wastewater underwent a stripping process prior to biological treatment in order to reduce the concentration of NHþ 4 –N. Although the goal of the SBR experiments was not to remove NHþ 4 –N, monitoring was carried out in order to observe possible variations in its concentration. Fig. 6 shows the evolution of the NHþ 4 –N concentration in the different process streams. The removal of ammonia was obtained in the stripping process (see also Fig. 2). An increase in the concentrations was observed throughout the biological process. This is due to the presence of organic nitrogen and thiocyanate in the influent, since the former is transformed into NHþ 4 –N during the biodegrada2 tion process and the latter into NHþ 4 –N, CO2 and SO4

3.4. Phenols removal The evolution of phenols concentration in the different process streams of the plant is represented in Fig. 4, along with the removal efficiencies for the different operating conditions. Phenols concentration in the wastewater varied between 185 and 253 mg/L, generally decreasing very slightly after the stripping process. The concentration after

100

450 400 350

Phenols (mg O2 /L)

90

Coke wastewater Influent Effluent Removal

80 70

300

60 250 50 200 40 150

30

HRT= 225 h

100

HRT= 137 h

HRT= 115 h

HRT= 58 h

50

20 10 0

0 0

50

100

150

200

Time (days) Fig. 4. Phenols evolution in the different process streams and removal obtained.

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100

700

500

SCN (mg O2 /L)

90

Coke wastewater Influent Effluent Removal

600

80 70 60

400

-

50 300

40 30

200 HRT= 225 h

HRT= 115 h

HRT= 137 h

HRT= 58 h

20

100 10 0

0 0

50

150

100

200

Time (days) 

Fig. 5. SCN evolution in the different process streams and removal obtained.

1000 Coke wastewater Influent Effluent

HRT= 137 h

HRT= 115 h

HRT= 58 h

600

+

mgNH4 -N/L

800

400 HRT= 225 h

200

0 0

50

100

150

200

Time (days) Fig. 6.

NHþ 4 –N

evolution in the different process streams and total removal obtained.

Table 3 Average concentration of the different pollutants in the coke wastewater and the effluent and removal efficiencies obtained in a by biological treatment in a SBR with prior ammonia stripping HRTstr (h)

HRTSBR (h)

66 40 34 17

225 137 115 58

COD (mgO2/L)

SCN (mg/L)

Phenols (mg/L)

1 NHþ 4 –N (mg L )

Wastewater

Effluent

(%)

Wastewater

Effluent

(%)

Wastewater

Effluent

(%)

Wastewater

Effluent

(%)str

%tot

1285 1345 1303 1536

241 206 206 472

81.2 84.5 84.3 69.0

229 216 207 221

4.5 5.9 1.8 3.9

98.0 97.2 99.1 98.2

215 318 244 285

11.1 10.1 5.4 26.4

94.6 96.6 97.8 90.7

532 548 489 567

78 154 275 111

89.9 75.5 54.7 81.9

85.1 68.7 41.7 79.6

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(Kwon et al., 2002; Staib and Lant, 2007; Stratford et al., 1979). As a result of this increase, the total NHþ 4 –N removal efficiency was lower than that obtained by means of the stripping process, ranging between 42% and 85%, as can be observed in Table 3, which shows the average values of the concentrations of the different pollutants in the coke wastewater and the final effluent, as well as the removal efficiencies obtained under the different working conditions. Although HRTs of 80 h had been found as the minimum for the biodegradation of pollutants in coke wastewater in previous studies (Rancan˜o, 2000), the results obtained in the present research study show that an HRT of 58 h is long enough to obtain very high removal percentages of COD, phenols and SCN. The concentration of ammonium in the effluent can be kept around 40– 100 mg/L operating at HRT of 66 h in the stripping process. To obtain lower concentrations of ammonium in the effluent, a possible option would be to insert a nitrification/denitrification step in the treatment scheme. This step could be performed in the SBR, optimizing the operating times of the different stages (Keller et al., 2001; Chakraborty and Veeramani, 2002). 4. Conclusions The treatment of coke wastewater was studied using a pilot plant composed of a stripping unit, a homogenization tank and a biological reactor operated in sequencing batch mode. NHþ 4 –N removal by stripping is influenced by the HRT employed, efficiencies of 90% being obtained for HRT of 66 h. After stripping and subsequent neutralization with H2SO4, the biodegradation of pollutants in an SBR led to removal efficiencies higher than 69%, 98% and 90% for COD, phenols and SCN, respectively, even for the lower HRT (58 h). Increasing this time, higher removals were achieved, especially in COD. Acknowledgements The authors gratefully acknowledge the funding received from the European Union for the project ‘‘Advanced Process Control for Biological Water Treatment Plants in Steelworks’’, Contract No. ECSC-7210-PR-235, and the Arcelor Group for their collaboration. They also wish to thank Mr. Paul Barnes for proof reading the English version of the manuscript. References APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, DC.

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