Abstract
The public water supply in Germany is mainly based on groundwater, and great care is taken to protect these water resources. A major challenge is, however, the remediation of polluted aquifers. Such is the case at a former gas works site in Karlsruhe, Germany. The "Gaswerk Ost" of the local gas and water supply company, the Stadtwerke Karlsruhe, was in operation for nearly 80 years until it was closed down in 1965. Unwanted by-products from the gas production still contaminate the soil and groundwater of this site. The main contaminants are benzene and polynuclear aromatic hydrocarbons (PAH) such as acenaph-thene, acenaphthylene, fluorene and fluoranthene. For remediation a novel passive methodology was planned. It was decided to install a funnel and gate system to purify the contaminated groundwater in situ by letting it pass through subterranean activated carbon reactors located downstream of the polluted site. During the construction of the remediation system a further pollutant, vinyl chloride (VC), was detected in the groundwater, a substance which could not be removed adequately by the technology employed. The objective of this research project was to find out whether the PAH and vinyl chloride could be removed from the groundwater by UV irradiation prior to the activated carbon filtration. Investigations consisted of two parts: laboratory experiments were conducted to prove the general degradability of the pollutants and field experiments were earned out to confirm these results on a pilot scale. In addition to sole UV irradiation, ozonation and advanced oxidation processes (AOPs) such as UV/aeration, UV/hydrogen peroxide and UV/ozone were performed in the laboratory to generate highly reactive hydroxyl radicals. For the contaminants present at the gas works site, the individual molar absorption coefficients were determined at 254 nm to estimate the degradation performance by direct photolysis at the main emission line of the UV lamps used for the irradiation experiments. It could be shown that all investigated substances were degradable in model test solutions prepared with reverse osmosis water, the degradation of PAH being significantly better than that of benzene and VC depending on the absorption of UV light of the individual substances. During the irradiation of acenaphthene the detection of by-products with an aromatic character showed that no complete mineralisation could, however, be achieved in an acceptable period of time. Degradation experiments performed in tap water as well as in groundwater showed slower degradation behaviour due to the high carbonate and bicarbonate concentrations and a high organic carbon content, mainly resulting from humic substances in the case of groundwater. The high iron concentration of the reduced groundwater led to an increase in turbidity during irradiation, since iron II was oxidised to iron III. In contrast to UV irradiation only, better results were achieved with combined treatment methods and with ozonation. For all methods tested in the laboratory the "Electrical Energy per Order of Magnitude" (EEO) was calculated from the degradation data, ozonation being most efficient for all investigated substances with regard to the energy consumption. On the basis of the laboratory findings it was decided to perform not only UV experiments at the gas works site but to extend the in-situ investigation programme by including ozonation. With UV irradiation the concentrations of the PAH acenaphthylene, fluorene, fluoranthene and pyrene could be reduced by nearly 90%. The total efficiency of UV irradiation in this specific groundwater was, however, unsatisfactory because the main contaminant acenaph-thene was removed by only 50% to a concentration of approximately 35 g/l (remediation target value = 0.2 g/l) although 1.33 kWh per m3 were applied with four lamp modules, each containing six lamps. In contrast to this, ozonation resulted in a complete elimination of VC and PAH as well as nearly 90% removal of benzene, thus confirming the laboratory findings. As almost all contaminants could be removed by ozonation only, combined UV/ozone treatment was not applied in situ. Ozonation technology was found to be the most favourable method for the removal of contaminants present at the former gas works site in Karlsrahe based on its practicability, economic advantages and high efficiency.