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Filtrasorb® 400, aktiv kul til rensning af MTBE-forurenet grundvand - detailundersøgelse

Summary

The purpose of the present project is to perform a series of detailed technical and economic evaluations for granular activated carbon of the type Filtrasorb^® 400, in order to evaluate the suitability of the product in relationship to on-site remediation of MTBE contaminated groundwater.

Batch experiments have been performed in order to determine the MTBE sorption capacity for Filtrasorb^® 400 under a series of conditions with relation to price estimates for on-site remediation of MTBE contaminated groundwater. Experiments have been carried out for the following variables: Temperature, crushed carbon, demineralised water, pH, and presence of BTX in the influent water.

The results show that at 10°C, between 10% higher and 15% lower capacity can be expected for Filtrasorb^® 400 than predicted from batch experiments performed at a temperature of 20-23°C, depending on the influent concentration (0.5 and 10 mg/L, respectively).

In the same concentration range, approximately 45-50% lower sorption capacity was observed at 30°C than at 20-23°C.

The results indicate that the sorption capacity is approximately 15% lower for granulated carbon than for crushed carbon.

According to the results, the sorption capacity is approximately 15-30% lower when determined in Aalborg tap water than in demineralised water.

At pH 5 and influent concentrations of 0.5 and 10 mg/l, a tendency towards 5% higher, and 10% lower sorption capacities has been observed than at pH 7. The tendencies have not been found statistically significant.

The sorption capacity at pH 9 has been observed at approximately 20% higher at 0.5 mg/l and 25% lower at 10 mg/l than at pH 7.

A tendency towards unchanged MTBE sorption capacity at 0.5 mg/l and approximately 55% higher capacity at 10 mg/l has been observed with the presence of benzene and MTBE. This tendency has not been found statistically significant.

The results for the experiments with competitive sorption with the presence of toluene and m-xylene indicate that the MTBE sorption capacity at 0.5 mg/l is approximately 10 and 25% higher with the presence of m-xylene and toluene, respectively. At 10 mg/l the MTBE sorption capacity is approximately 20 and 35% higher with the presence of m-xylene and toluene, respectively.

The results for the experiments with competitive sorption with the presence of both benzene, toluene and m-xylene indicate that the combined presence of these three compounds at 0.5 mg/l increases MTBE sorption capacity by approximately 20%, while increasing the capacity by approximately at 10 mg/l.

Column experiments have been carried out in order to determine MTBE sorption capacity for Filtrasorb^® 400 under dynamic conditions that correspond more closely to the conditions during actual on-site remediation of MTBE-contaminated groundwater. Two parallel column experiments have been carried out; one with only MTBE present in the influent water, and one with MTBE and benzene present in the influent.

The columns have been constructed with a volume of approximately 2.5 litres, a height/diameter-ratio of 7.5-8 and an inlet flow of approximately 5 l/h, corresponding to an empty bed hydraulic contact time of approx. 30 min. An inlet concentration of 10 mg/l was used as a target value and a benzene concentration of approx. 3.8 mg/l was reached. Two replicates of each experiment have been performed and measurements have been performed using two different analytical procedures.

Good agreement was observed between the results using the two different analytical procedures and good agreement was observed between the replicate experiments.

The column experiments indicate that a hydraulic contact time of 30 min is on the safe side in relation to a full usage of the sorption capacity in a filter construction with two columns in series, even though there might be considerable variations in the influent conditions.

The results for the column experiments with only MTBE in the influent water indicate that the actual/dynamic sorption capacity is approximately 15-20% lower than estimated from the sorption isotherm.

The results for the column experiments with MTBE and benzene in the influent water indicate that the actual sorption capacity is approximately 2.1 – 2.4 times higher with the presence of benzene compared to only having MTBE in the influent water. This tendency is in disagreement with the expected results, although in agreement with the results from the batch experiments, and two possible explanations are considered: 1) at a low benzene/MTBE-ratio and/or high concentration levels, MTBE has access to sorption sites that are otherwise inaccessible. The phenomenon possibly depends on the water type in question. 2) MTBE might sorb onto benzene degraders in the columns.

Assuming a regulatory limit on the effluent concentration of 5 m g/l it is estimated that carbon consumption is approximately 2 – 3.3 times higher with one-filter operation than with two filters in series.

Twelve scenarios for on-site remediation of MTBE-contaminated groundwater have been suggested, and for each scenario unit treatment prices have been calculated (DKK/m³). The scenarios span treatment times of 3 months to 10 yrs., flows between 2 and 20 m³/h, and inlet concentrations of 0.2 to 10 mg/l. It is assumed that the effluent concentration is below a regulatory limit of 5m g/l at all times, and that standard available adsorbers are used in the filter construction.

It has been observed that the operating expenses for activated carbon comprise up to 60-85% of the combined unit treatment price. Since single adsorbers have an estimated carbon usage rate that is approximately 2 – 3.3 times higher than using two or more filters in series, it is concluded that two or more filters in series operation should be used in treating MTBE-contaminated groundwater.

In remediation of MTBE-contaminated groundwater, unit prices ranging between 4.3 and 32.6 DKK/m³ have been estimated. The lowest prices are obtained at high flow rates, low concentrations, and long time periods.

By assuming a positive effect of benzene presence on MTBE sorption capacity, unit prices ranging between 2.3 and 19.5 DKK/m³ have been estimated. The average price level is approximately 45% lower than the price level for remediation of groundwater contaminated with MTBE only.

By assuming a negative effect of benzene presence on MTBE sorption capacity, unit prices ranging between 6.0 and 39.2 DKK/m³ have been estimated. The average price level is approximately 35% higher than the price level for remediation groundwater contaminated with MTBE only.

In remediation of benzene-contaminated groundwater (an inlet concentration of 1 mg/L), unit prices ranging between 0.9 and 3.3 DKK/m³ have been estimated. The average price level for treating MTBE contaminated groundwater is approximately 7.3 times higher than for treating benzene contaminated groundwater.

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