Highlights

• Physicochemical characteristics of wastewater produced in the UCG process of Polish bituminous coal.
• For chemical coagulation, the highest measured removal of cyanide ions reached 24%, while electrocoagulation removed close to 90%.
• High doses of commercial coagulants can lead to cross-contamination with heavy metals.
• Electrocoagulation proved to be an effective method for the removal of cyanide, sulphide, organics, and trace elements.

Introduction

Materials and Methods

UCG Wastewater Characteristic

Wastewater from oxygen blown UCG of bituminous coal from the "Wesoła" Coal Mine was used in this study. Table 2 presented below collates basic physicochemical properties of the wastewater. Further details regarding the process and results of UCG experiments can be found in [38,39,40].

Table 1. Average physical and chemical characteristics of the UCG wastewater.

Table 1. Average physical and chemical characteristics of the UCG wastewater.

Parameter	Value
pH, [-]	8.193
Conductivity, [mS/cm]	1600.5
Redox, [mV]	-113.3
COD, [mg O2/l]	189.03
CNfree, [mg/dm3]	0.971
S2-, [mg/dm3]	0.215
Trace elements
Al [mg/kg]	2.520
As [mg/kg]	<0.02
Cd [mg/kg]	<0.02
Co [mg/kg]	<0.05
Cr [mg/kg]	<0.025
Cu [mg/kg]	<0.025
Fe [mg/kg]	0.179
Mn [mg/kg]	0.333
Mo [mg/kg]	<0.05
Ni [mg/kg]	0.492
Pb [mg/kg]	<0.05
Sb [mg/kg]	0.080
Ti [mg/kg]	<0.02
Zn [mg/kg]	0.213
Sum [mg/kg]	3.817
BTX
Benzene [mg/dm3]	0.210
Toluene [mg/dm3]	0.080
Ethylbenzene [mg/dm3]	0.004
m-xylene [mg/dm3]	0.004
p-xlene [mg/dm3]	0.010
Isopropylbenzene [mg/dm3]	0.002
o-xylene [mg/dm3]	0.010
Sum [mg/dm3]	0.320

Mean values of 3 measurements.

Herein investigated coagulation process is based on the use of iron ions, so the concentration of iron in UCG wastewater equal to 0.179 mg Fe/kg must be treated as a reference. Among trace elements, only the concentrations of Al, Ni, Mn, Fe, Zn, and Sb (listed in decreasing order of detected concentration) were determined above the lower limit of detection (LDL). Also, the wastewater was low in BTX and indicated concentrations of PAH <LDL. This influences high measurement error of testing applicability of coagulation methods for removal of organic pollutants.

Feedstocks

Coagulants

Four main types of iron-based coagulants were used in this study, namely acidic solutions of iron (II) and iron (III) sulphates and chlorides.

The table below presents a comparison of the basic characteristics of herein utilized coagulants. The last column represents the total Fe concentration in the coagulants, as the sum of all valences.

Table 4. Table summarizing the basic properties of the coagulants used.

Table 4. Table summarizing the basic properties of the coagulants used.

Trade name	Chemical specie	Density, kg/dm3	Total Fe content, % w/w
PIX-100	FeCl2	1.2659	9.76
PIX-100 COP	FeSO4	1.0552	1.75
PIX-113	Fe2(SO4)3	1.5262	11.39
PIX-116	FeCl3	1.3143	10.26

Chemical Coagulation Setup

Sample notation:

Samples taken prior or after treatments, as well as characteristic parameters collected during the study, are described using a coding method presented in Table 5.

Test procedure

Jar tests were conducted during experiments on the chemical coagulation process of UCG wastewater. The main goal of coagulation was to maximise the removal of organic compounds, metalloids, and cyanides. The setup consisted of four identical sets of beakers with a capacity of 2 litres each. The beakers were placed on magnetic stirrers, and the type and length of the stir bars used were selected to enable both high turbulence and slow, gentle mixing required for proper coagulation during the different stages of the procedure. As a result, 40 mm rods with a diameter of 5 mm were found to perform best. No additional flocculant (e.g., polymeric) was added to enhance clarification.

Coagulation procedure

• pH correction
• Measurement of basic parameters of raw wastewater: pH, conductivity, redox
• Introducing 1,000 cm³ of wastewater into each of the beakers
• Measurement of preset doses of coagulant and their introduction into syringes
• Start of the mixing and coagulation program
• Measurement of basic parameters in the effluent after sedimentation is finished
• Sampling and lab analyses of the supernatant of the treated wastewater

Mixing program

• Initiation of mixing; 1200 rpm for 1 min; very intensive mixing
• Coagulant injection
• 700 rpm for 5 min; medium intensity mixing
• 400 rpm for 5 min
• 200 rpm for 10 min; minimal intensity mixing, minimal agitation of the liquid
• Cessation of mixing
• Sedimentation – 120 min

Preliminary tests on the effect of coagulant dosage on the change in solution pH

pH corrections necessary to maintain the optimal working range for each specific coagulant were determined experimentally for each object (type of coagulant and dosage). In general, it was observed that the optimal pH range in which iron sulphates and chlorides achieved their maximum effectiveness fell within the range of 4–9. pH corrections were made only when the resulting pH of the solution fell outside the optimal range (when no signs of coagulation-flocculation were observed). For none of the samples was an external source of alkalinity added.

Electrocoagulation setup

In this study, the batch electrocoagulation tests were performed using an adapted version of the Jar Tests presented above. The experiments were carried out in a system consisting of two beakers of 2 dm³ each, an electrode support stand, and a Twintex TP-2305 dual-channel power supply (30 V DC, 2x5 A). Both electrodes had an area of 20 cm² each and were spaced 2 cm apart during all experiments. Since the currents used in this study were very small (<0.5 A), to increase the resolution of the measurement, an additional measurement system consisting of a set of multimeters was used in addition to the measurement indicated by the power source. The measurements thus had a resolution of 1 mA/mV and a precision of 0.03%. The beakers were thermostated in a water bath at (19.7 ± 0.3°C).

The characteristics of Fe dissolution were investigated at both constant and variable current. It was found that in the constant-current mode, higher control precision and the ability to more closely maintain the controlled parameters were observed. This mode was also easier to control. On average, a 7.6% lower loss of electrode mass was observed than was calculated using the Faraday equation. Working in a closed system, a steady increase in the pH of the wastewater was observed as the dose of Fe introduced increased.

Sample notation

Samples taken prior or after treatments, as well as characteristic parameters collected during the study, were described using a coding method presented in

Table 6. Coding of samples – electrocoagulation tests.

Table 6. Coding of samples – electrocoagulation tests.

test and sample notation	el_dose [mg Fe/dm3]_current [mA]_time [min]_pH before the process
example	el_d60_i58_t60_pH8,5

Calculation of removal efficiency / reduction of the value of component i,

$${\mathsf{\eta }}_i=1-{\displaystyle \frac{c_{i\_outlet}}{c_{i\_inlet}}}\left[\mathrm{\%}\right]$$

(1)

where:

c_{i_ inlet} – concentration of component i at the inlet to the reactor [mg/dm³]

c_{i_outlet} – concentration of component i at the outlet from the reactor [mg/dm³]

The effectiveness of treatments was determined here based on the removal efficiency, while cyanides, heavy metals, and organic pollutants (BTX) were the main analysed variables. Additionally, such parameters as redox or COD were used here as secondary evaluation measures.

Results and Discussion

Chemical Coagulation of UCG Wastewater

FeCl₂ as a source of Fe ions – PIX-100

Table 7 collates the results of using FeCl₂ as a source of Fe ions. None of the tested coagulant doses (123–309 mg Fe/dm³) resulted in a reduction in the concentration of cyanide ions greater than 24%. With increasing doses of the coagulant, an increase in the values of redox potential, COD, and conductivity of the wastewater was observed. In chemical coagulation, this observation is attributed to the introduction of an excess of iron, chloride, and sulphate ions, as well as secondary contaminants brought into the wastewater with the coagulant itself. This notion will be further explored in the following paragraphs.

FeSO₄ as a source of Fe ions – PIX-100 COP

In the case of PIX-100 COP, much lower doses of coagulant were introduced into the treated UCG wastewater stream (37–92 mg Fe/dm³). This action was aimed to indicate the effectiveness of cyanide reduction using very low doses of divalent iron.

Table 8 indicates that already a dose of approximately 74 mg Fe/dm³ led to a 19.6% reduction of cyanide. Similarly to PIX-100, increasing the dosage resulted in an increase in the redox value and the conductivity of the wastewater. However, contrary to PIX-100, lower doses of PIX100 COP caused a slight (5–6%) decrease in the COD.

Fe₂(SO₄)₃ as a source of Fe ions – PIX-113

PIX-113 was the first of the two coagulants used that was based on trivalent iron ions. Theoretically, an increase in ion charge should result in significantly higher effectiveness in neutralising negatively charged colloids. However, an undesirable effect is that when using equivalent doses of Fe, it is necessary to perform corrections to maintain the process within the optimal pH range. Corrections were applied to all tested samples where the dose exceeded 190 mg Fe/dm³.

Similar to the coagulants based on Fe²⁺, no improvement in cyanide removal efficiency was achieved. The maximum reduction value of 21% was recorded for a dose of 260 mg Fe/dm³. Increasing the dose of PIX-113 resulted in an increase in the conductivity of the wastewater; however, the observed impact of the dose on the redox value was ambiguous. Similar to PIX-100 COP, a 5–6% decrease in the COD value of the wastewater was observed.

Table 9. Chemical coagulation of UCG wastewater using Fe₂(SO₄)₃.

Table 9. Chemical coagulation of UCG wastewater using Fe₂(SO₄)₃.

	PIX113_0,7_8,3	PIX113_1,1_10,0	PIX113_1,5_10,0	PIX113_1,9_10,0
Fe dose, mg Fe/dm3	121.68	191.22	260.75	330.28
pH after introduction of the coagulant, -	5.46	8.88	7.71	6.28
Conductivity, mS/cm	1.7646 (-10.25%)	2.383 (-48.89%)	2.696 (-68.45%)	2.847 (-77.88%)
Redox, mV	50 (-144.12%)	-149 (-31.47%)	-76 (-32.94%)	2 (-101.76%)
CN-, mg/dm3	12.1 (16.55%)	12.9 (18.35%)	12.5 (20.89%)	12.6 (20.25%)
COD, mg/dm3	177.2 (7.32%)	175.2 (3.31%)	169.2 (6.62%)	169.2 (6.62%)

The concentration of cyanide measured in the effluent prior to treatment was equal to 15.8 mg/dm³.

FeCl₃ as a source of Fe ions – PIX-116

The last of the tested coagulants was ferric chloride, which was expected to exhibit the highest coagulation activity. Corrections were performed on all tested samples.

For all assessed categories, the impact of using PIX-116 did not differ from the observations made for the other coagulants. At the lowest dose, a 5–7% lower efficiency in cyanide removal was observed, while an insignificant 1–2% higher efficiency in COD removal was also noted.

Table 10. Chemical coagulation of UCG wastewater using FeCl₃.

Table 10. Chemical coagulation of UCG wastewater using FeCl₃.

	PIX116_0,9_10	PIX116_1,3_10	PIX116_1,7_10	PIX116_2,1_10
Fe dose, mg Fe/dm3	121.36	175.30	229.24	283.18
pH after introduction of the coagulant, -	8.97	8.59	7.30	5.84
Conductivity, mS/cm	2.381 (-48.77%)	2.623 (-63.89%)	2.93 (-83.07%)	3.068 (-91.69%)
Redox, mV	-154 (-35.88%)	-132 (-16.47%)	-57 (-49.71%)	27 (-123.82%)
CN-, mg/dm3	13.3 (12.5%)	12.6 (17.11%)	12.2 (19.74%)	12.2 (19.74%)
COD, mg/dm3	177.2 (6.83%)	173.2 (8.94%)	175.2 (7.89%)	174.2 (8.41%)

During this measurement series, the cyanide concentration in the effluent before coagulation was 15.2 mg/dm³.

Effect of coagulant on the removal of cyanide ions

For all tested doses and forms of Fe used in the chemical coagulation experiments, only a slight decrease in cyanide concentration was measured. The maximum reduction of 24% was achieved using the coagulant PIX-100 (FeCl₂) at a dose of 185 mg Fe/dm³.

The collected experimental data indicate that the upper limit of cyanide removal efficiency from UCG process wastewater using the tested chemical coagulants is confined to the range of 22–24%. Importantly, none of the tested configurations succeeded in reducing the cyanide concentration in the solution below the limit of 10 mg/dm³.

Figure 1. Effect of Fe dose on removal of free cyanide from UCG wastewater – chemical coagulation.

Figure 1. Effect of Fe dose on removal of free cyanide from UCG wastewater – chemical coagulation.

Effect of coagulant on the removal of metals and BTX

Analyses of heavy metal and BTX content in coagulated wastewater were carried out using the supernatant obtained after two hours of sedimentation. The samples were not subjected to filtration before analysis. Generally, the coagulation and flocculation processes were rapid and efficient, as evidenced by the iron content not exceeding the detection level in 3 of the 4 samples compared in Table 11. The table collates the results from all four tested coagulants at doses that resulted in the highest removal efficiency.

When analysing metals and metalloids, all tested coagulants reduced Al and Sb concentrations to below the detection limit (<LDL). Coagulants with sulphate as the counter ion demonstrated higher efficiency in removing trace elements. However, these coagulants also caused cross-contamination of the treated wastewater with Ni and Mn. Specifically, PIX-100 introduced Cu into the solution, and PIX-116 introduced Zn. These findings indicate that while pure reagents can achieve high efficiency in heavy metal removal, industrial or technical grade coagulants may produce effects contrary to the intended reduction of trace elements.

The coagulants were also analysed for their effect on the content of BTX and PAH in the UCG wastewater. Table 11 presents only the results for BTX, as all tested PAHs were measured to be <LDL. For BTX, it should be noted that the use of chemical coagulants led to inconclusive results. The observed small increases in the content of individual BTX compounds can be attributed to both the very low levels of measured values and the heterogeneity of the effluent. Overall, it appears that the coagulants had no positive effect on the removal of BTX.

Electrocoagulation of UCG wastewater

As previously introduced, general considerations lead to the conclusion that the processes of chemical and electrocoagulation, when performed using the same cation, differ mainly in their effect on pH (during electrocoagulation, pH increases). Consequently, electrocoagulation results in a significantly lower amount of cross-contaminants introduced. A drawback, particularly important from a technological perspective, is that compared to chemical methods, electrocoagulation leads to the formation of finer flocs and thus, even with longer sedimentation times, results in less effective spontaneous sedimentation. Similarly to chemical coagulation, the efficiency of batch electrocoagulation experiments was also assessed based on the removal efficiency of cyanides, sulphides, heavy metals, and organic compounds (BTX, PAH).

Effect the dose of Fe

The first set of electrocoagulation experiments was designed to determine the effect of the Fe dose. During these tests, the electrode dissolution time was kept constant at 60 minutes and the pH of the effluent was not adjusted.

The first two tests in this series were conducted using constant-voltage mode (where the current varied due to changes in the conductivity of the effluent), while the subsequent two tests were carried out in constant-current mode (with the voltage of the source adjusted to maintain a constant current flow). As no significant differences were observed between the modes, and the constant-current mode was easier to control, the remaining electrocoagulation experiments were conducted in constant-current mode. collates all data characterising the conditions of the tests as well as the measured parameters.

Due to the increasing pH of the sample during electrocoagulation, both cyanides and sulphides were introduced to the UCG effluent prior to treatment. For cyanides, a clear effect of the coagulant dose on the reduction efficiency was observed. The highest cyanide removal efficiency achieved during this series of tests was 89%, indicating that a dose of 225 mg Fe/dm³ reduced the concentration to less than 1 mg/dm³. Even more substantial results were obtained for sulphide ions, where more than 98% reduction was observed for all tested process variables. The only slight difference occurred when electrocoagulation was performed at higher pH levels. Further details regarding these experiments are presented in subsequent sections of this work.

Table 12. Results of a study on the effect of Fe dose at constant anode dissolution time.

Table 12. Results of a study on the effect of Fe dose at constant anode dissolution time.

	el_d60_i58_t60_pH8,5	el_d120_i115_t60_pH8,5	el_d180_i172_t60_pH8,5	el_d240_i230_t60_pH8,5
Current [mA]	58	115	172	230
Voltage min–max [V]	2.79	4.51	6.46–7.25	7.82–8.97
Time [s]	3600	3600	3600	3600
Measured Fe dose /difference from Faraday's law, mg Fe/dm3 / %	56.8 / 6.25%	110.6 / 7.93%	165.2 / 8.05%	226.2 / 5.85%
Parameters of the effluent after electrocoagulation
pH, -	8.74	8.91	9.06	9.18
Conductivity, mS/cm	1.8339 (-14.58%)	1.7657 (-10.32%)	1.6886 (-5.5%)	1.5938 (0.42%)
Redox, mV	-142 (-25.29%)	-151 (-33.24%)	-160 (-41.18%)	-166 (-46.47%)
CN-, mg/dm3	3.74 (60.59%)	2.51 (73.55%)	1.48 (84.4%)	1.03 (89.15%)
S2-, mg/dm3	0.395 (98.96%)	0.405 (98.93%)	0.306 (99.19%)	0.217 (99.43%)
COD, mg/dm3	247.2 (13.02%)	238.2 (16.19%)	227.2 (20.06%)	222.2 (21.82%)

During this series of experiments, the cyanide in the effluent before coagulation was 9.49 mg/dm³ while as the baseline for the calculation of sulphide removal the value of 37.9 mg/dm³ was used (Table 3).

Figure 2 clearly indicates that, through electrocoagulation, a dose of approximately 55–60 mg Fe/dm³ can reduce the cyanide concentration by over 60%. This result alone is nearly three times better than what is observed in the case of chemical coagulation. Further increasing the dose to 230 mg Fe/dm³ results in an increase in cyanide reduction to 90%.

An additional parameter demonstrating the beneficial effect of electrocoagulation on wastewater was the decrease in COD. However, in none of the tests did the COD decrease by more than 28%.

Similarly to chemical coagulation, the redox potential increased with low doses of Fe, while the conductivity of the solution increased across the full range of Fe doses tested. It is worth noting that, due to the mechanistic effect of the formation of very fine flocs, the change in conductivity is not straightforward.

Effect of the electrode dissolution time at a constant dose of Fe

Subsequently, a series of experiments was conducted to determine the effect of electrocoagulation time. This series of tests indicated that as the time of electrode dissolution increased, the resulting concentration of cyanides in the treated effluent also increased, leading to a decrease in removal efficiency. However, for cyanides, sulphides, and COD, clear and overlapping minima were observed, suggesting that for a dose of 104–115 mg Fe/dm³, a process time of 60 minutes is optimal.

Table 13. Results of the anode dissolution time study at a constant dose of Fe.

Table 13. Results of the anode dissolution time study at a constant dose of Fe.

	el_d120_i230_t30_pH8,7	el_d120_i153_t45_pH8,5	el_d120_i115_t60_pH8,5	el_d120_i77_t90_pH8,6	el_d120_i57_t120_pH8,7
Current [mA]	230	153	115	77	57
Voltage min–max [V]	8.50–9.86	5.50–5.93	4.51	3.48–3.69	2.63–2.81
Time [s]	1800	2700	3600	5400	7200
Measured Fe dose / difference from Faraday's law, mg Fe/dm3	104.2 / 13.26%	109 / 9.06%	110.6 / 7.93%	113.7 / 5.76%	114.9 / 3.51%
Parameters of the effluent after electrocoagulation
pH, -	8.99	9.04	8.91	9.02	9.03
Conductivity, mS/cm	1.7945 (-12.12%)	1.7643(-10.23%)	1.7657(-10.32%)	1.7537(-9.57%)	1.7891 (-11.78%)
Redox, mV	-156 (-37.65%)	-157 (-38.53%)	-151 (-33.24%)	-157 (-38.53%)	-158 (-39.41%)
CN-, mg/dm3	2.93 (69.13%)	3.38 (64.38%)	2.51 (73.55%)	3.62 (61.85%)	4.14 (56.38%)
S2-, mg/dm3	0.771 (97.97%)	0.563 (98.52%)	0.405 (98.93%)	0.595 (98.43%)	0.996 (97.37%)
COD, mg/dm3	256.2 (26%)	254.2 (16.44%)	238.2 (16.19%)	261.2 (19.43%)	266.2 (23.11%)

Determination of main effects and interactions between time and dose

The electrocoagulation studies performed were further analysed to determine the existence of main effects as well as interactions between the controlled variables. For the case of time and dose, a positive interaction effect was found for cyanide removal from UCG wastewater. As expected, the dose had the dominant effect in this process. For COD, a negative effect of time and a positive effect of dose were observed. Notably, in this system, the interaction effect of dose and time had the most significant impact on COD reduction.

The following Figure 3, Figure 4 and Table 14 illustrate graphically and numerically the results of these analyses.

Effect of effluent pH

Furthermore, a series of studies was conducted to determine the effect of changing the pH of the treated wastewater. At higher pH levels, an increase in the amount of iron hydroxide flocs produced is expected; however, at the same time, the amount of free Fe ions available in solution decreases.

From the data presented in Table 15 it can be seen that at a final process pH of 11, the efficiency of sulphide and cyanide removal decreased from previously observed levels of approximately 89% and 99% to 48.5% and 89%, respectively. Thus, although producing more flocs can be crucial for separating suspended solids or grease from other effluents, in the case of UCG wastewater, this approach proves to be less effective.

Efficiency of electrocoagulation on removal of metals and BTX from wastewater

During the batch electrocoagulation tests, the removal of elements such as aluminium, manganese, and zinc was primarily observed (reduction to below the limit of detection). However, electrode dissolution, along with the excess Fe introduced, was also associated with a slight increase in Ni and Sn content in the treated effluent. The maximum recorded concentrations of these elements were 0.77 mg/kg and 0.096 mg/kg, respectively.

Using the data as an example, it can be determined that doses of 225–325 mg Fe/dm³ introduced into UCG wastewater resulted in a residual concentration of Fe in the treated effluent in the range of 2–3 mg Fe/kg. This is expected, as under basic conditions Fe primarily forms very poorly soluble hydroxides.

In the case of BTX, a 2–50% decrease in concentration was observed for benzene, toluene, and ethylbenzene. Compared to chemical coagulation, these results are also more promising and conclusive.

Table 16. Results on the efficiency of removal of trace elements and organic compounds – electrocoagulation.

Table 16. Results on the efficiency of removal of trace elements and organic compounds – electrocoagulation.

	el_d240_i230_t60_pH8,5	el_d240_i306_t45_pH8,5	el_d180_i345_t30_pH8,7	-
Current [mA]	230	306	345	-
Voltage min–max [V]	7.82–8.97	10.12–11.97	11.64–12.65	-
Time [s]	3600	2700	1800	-
Measured Fe dose / difference from Faraday's law, mg Fe/dm3 / %	226.2 / 5.85%	227 / 5.31%	162.2 / 9.98%	-
Parameters of the effluent after electrocoagulation
pH, -	9.18	9.22	9.16	-
Conductivity, mS/cm	1.5938 (0.42%)	1.6005 (0%)	1.7334 (-8.3%)	-
Redox, mV	-166 (-46.47%)	-168 (-48.24%)	-166 (-46.47%)	-
CN-, mg/dm3	1.03 (89.15%)	2.43 (74.39%)	2.25 (76.29%)	-
S2-, mg/dm3	0.217 (99.43%)	0.103 (99.73%)	0.647 (98.29%)	-
COD, mg/dm3	222.2 (21.82%)	229.2 (24.65%)	250.2 (27.73%)	-
Trace elements
Al, mg/kg	<0.125 (>95.04%)	<0.125 (>95.04%)	<0.125 (>95.04%)	-
Fe, mg/kg	1.94 (-983.8%)	2.55 (-1324.58%)	2.99 (-1570.39%)	-
Mn, mg/kg	<0.2 (>39.94%)	<0.2 (>39.94%)	<0.2 (>39.94%)	-
Ni, mg/kg	0.748 (-52.03%)	0.77 (-56.5%)	0.686 (-39.43%)	-
Sb, mg/kg	0.085 (-6.25%)	0.084 (-5%)	0.096 (-20%)	-
Zn, mg/kg	<0.02 (>90.61%)	<0.02 (>90.61%)	<0.02 (>90.61%)	-
Sum of metals, mg/kg	3.118 (18.31%)	3.749 (1.78%)	4.117 (-7.86%)	-
BTX
Benzene, mg/dm3	0.204 (2.86%)	0.1 (52.38%)	0.2 (4.76%)	-
Toluene, mg/dm3	0.269 (-236.25%)	0.07 (12.5%)	0.04 (50%)	-
Ethylobenzene, mg/dm3	0.01 (-150%)	0.002 (50%)	-	-
m-xylene, mg/dm3	0.02 (-400%)	0.03 (-650%)	-	-
p-xsylene, mg/dm3	0.02 (-100%)	0.01 (0%)	0.02 (-100%)	-
Isopropylbenzene, mg/dm3	0.01 (-400%)	-	0.009 (-350%)	-
o-xylene, mg/dm3	0.03 (-200%)	0.09 (-800%)	0.04 (-300%)	-
Sum of BTX, mg/dm3	0.563 (-411.82%)	0.302 (-174.55%)	0.309 (-180.91%)	-

Conclusions

Abbreviations

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