The Danish Environmental Protection Agency has carried out a series of experimental projects for clean up of council-owned gas works. The Environmental Protection Agency and the Local Authorities have jointly funded these projects. The aim of the experimental projects was to obtain information on new and improved remedial treatments for use in connection with future clean up of gas works sites. The experimental projects were termed collectively as "The Gas works Pack" or in Danish as " Gasværkspakken".

The five experimental projects were carried out at the following gas works:

The projects were primarily concerned with testing of in-situ and on-site techniques, since, at that time, few large-scale projects had been carried out with these types of techniques.

The clean up techniques used for the five projects in the Gas works Pack are as follows:

Valby:
Clean up of tar contaminated soils by an on-site two stage thermal treatment

Hjørring:
In-situ forced leaching of the water soluble components and recirculation of groundwater to enhance biological degradation of the water soluble components in the soil and groundwater at the source and prevent spreading to the environment.

Frederiksberg:
Clean up of tar contaminated soil by on-site biological degradation and leaching aided by detergents, oxygen addition, nutrients, structural materials and compost.

Esbjerg:
Clean up of tar contaminated soils by on-site biological degradation and leaching aided by detergents.

Mørkhøj:
Clean up of tar contaminated soil by in-situ biological degradation and leaching aided by detergents.

Contamination at gas works is associated with the different processes in the various plants at the gas works. Close to these sources of pollution, high levels of contamination are often found in the soil or in the perched groundwater tables. Chemical wastes such as coal tars or spent bog ore are often found. These very contaminated areas are called hotspots.

Excavation of hotspots

Hotspots and the very contaminated building materials and installations at gas works can not be treated in-situ, and therefore all remedial projects require excavation of these hotspots.

Removal of installations

Redevelopment of a gas works site often necessitates a complete or partial removal of the installations.

Free tar

Many remedial projects have experienced problems with the handling of free tars. Mixing the tars with sand or gravel or crushed cement has often solved these problems, so that diggers can excavate the tars. This often results in the need for clean up of considerable larger quantities of contaminated materials than calculated in the project plan.

Contaminated water

Basins or tanks that are intact often contain large amounts of contaminated water in the bottom of the tank. This water requires treatment before discharge or needs to be collected and transported to a treatment plant. Similar problems can arise during lowering of the ground water table during excavation of contaminated soils or installations. The most usual water treatment technique is mechanical filtration and filtering with activated carbon before discharge to the sewers.

All methods for the treatment of contaminated soils require an extensive pre-treatment in the form of sorting and removal of particles over a certain size such as; cement rumble, stones, metallic items, asphalt and tar balls as well as washing of the building materials before recycling.

Pre-treatment

Pre-treatment must almost invariably be carried out on-site and requires space. Space is required for the equipment used to sort large stones, rumble and tar balls, for the washing facilities, and for the cement crushing equipment as well as for short-term placement of the sorted materials.

Recycling

Pre-treatment gives a lot of possibilities to optimise and carry out for recycling of materials such as iron, cement, asphalt (bitumen). Coal tar can not usually be recycled.

Offensive odours

During excavation and pre-treatment, there is the possibility for offensive odours and health risks for the remedial project personnel and for the local residents.

Noise

Pre-treatment often generates noise during crushing and cutting of metal bars, wires etc.

No soil product is present after incineration at the coal-fired power stations and at Kommunekemi.

Thermal treatment

After thermal treatment, a clean material is obtained which is completely black, but has a structure that allows use as fill soil.

Clean up effectiveness

The thermal treatment is very effective for the very contaminated materials and can clean to 10 - 100 mg tar/kg soil. The treatment can be repeated if a cleaner product is required, but involves, of course, greater expense.

The prototype on-site thermal plant was designed for a capacity of 2 tons/hour, but due to production stops, maintenance etc. the time-averaged capacity was about 1 ton/hour, which is a restricting parameter for on-site clean up.

Biological degradation of tar

Biological degradation of tar compounds such as BTEX, phenols, NSO compounds and PAH compounds are well documented. The PAH compounds are often critical components during biological treatment of soils in that they bind to soils and degrade very slowly.

Physical conditions and inhomogeneity

It is probable that the physical size and distribution of tar particles in the soil determine the availability of tar components during degradation. It is unlikely that degradation occurs within the tar particles. Furthermore, it is very likely that the tar contamination is randomly distributed within the soil.

Effectiveness

Composting or landfarming is time consuming and the length of treatment as well as the contaminant composition in the contaminated soil determines the effectiveness of treatment. Clean up efficiency is often low compared to other treatment methods, although these usually required much greater energy consumption. There are many investigations, which have documented clean up from about 2,000 down to about 200 mg total PAH/kg with time scales of 6 months to several years. Generally, it is difficult to calculate the time scale for biological techniques.

Degradation of PAH

Based on the results achieved by biological degradation for different types of plants in other lands and the results for the Gas Works Pack, it is assessed that it is mainly the 2 - 4 ringed PAH, which degrade.

Generally, it is noted that degradation times are much lower in the field than in the laboratory and that results from laboratory experiments can not be directly applied to field or full-scale experiments. The half lives for PAH´s in the degradation project at Frederiksberg gas works were 400 - 800 days.

Stimulation of degradation

Stimulation of degradation by addition of soil improvements agents, nutrients or by other treatments can be carried out, but has seldom given convincing results. The two gas works projects have not documented that the additions of detergents or organic materials such as spruce chips or compost enhance degradation. To the contrary, addition of wood chips apparently had an inhibitory effect on biological degradation.

Clear guidelines, which can ensure or predict degradation rates, have not been found in the literature or in the experimental projects.

The project in Esbjerg has indicated that leaching of tar components during composting is limited and that discharge of drainage water to a traditional wastewater treatment plant does not create problems.

Documentation of degradation

The half-lives for biological degradation for PAH´s are long and sampling variation is large. Therefore, there are no benefits in analysing at time intervals that are significantly shorter than the half-lives, especially if only a few samples are analysed at each time interval. Due to inhomogeneity, there is a need to analyse many samples to demonstrate a statistically significant reduction in soil concentrations. It is an advantage if the monitoring program is based on intensive and detailed sampling during the initial and final stages of the clean up with only few control measurements in the intervening period.

Prerequisites

A prerequisite for biological degradation is that the hotspots are removed, i.e. that soil with concentrations greater than 1% tar are excluded.

At very high concentrations (more than 1% tar in the soil), degradation will be slow with the risk that degradation stops altogether. The optimal treatment area is estimated to be 0.1 - 0.3% tar. It is always an advantage to sort the soil with respect to contamination level and soil type.

Composting

Composting or landfarming can be used for non-critical contamination, where there is a need to stabilise the contamination. Biological treatment can reduce smell in the treated product and reduce leaching potential, i.e. the removal of the high volatile and soluble compounds.

Composting or landfarming can also be used as an environmentally correct and cheap method in situations where no requirements as to the time of completion of clean up, the clean up criteria or to the final usage of the soil are made.

These conditions will often be acceptable when the soil is excavated and treated at a commercial biological treatment plant, but are seldom acceptable during treatment on-site.

In-situ forced leaching

At Hjørring gas works, the remaining soil and groundwater contamination after removal of hotspots is considered as the collective treatment media. No attempt is made to document the remaining contamination levels in the soil after removal of the hotspots as the contamination is spread over a greater area and to greater depths. The objective for the remediation is to remove the lighter volatile and soluble contaminants. Any remaining immobile contamination is not critical for the future usage of the site.

During forced leaching, water-soluble components are leached to the groundwater where the contamination can be quantified and the degradation processes can be measured. The infiltration plant supplies nitrate and water to the unsaturated zone. Especially for the areas previously covered by buildings or roadways, this treatment can enhance biological degradation in the unsaturated zone. Furthermore, the infiltration plant can function as a capillary break zone and prevent upward transport of contamination in the soil water.

In-situ vs. on-site

Generally, an in-situ method will be more environmentally friendly than an on-site method in that the contaminated soil is not excavated. However, in-situ methods are usually less predicable and slower than on-site methods.

On-site vs. ex-site

The environmental advantages for treating contaminated soil on-site are greatest if the soil can be used as fill soil on-site rather than if the soil is to be transport away from the site after treatment.

Time scales for on-site treatment

A prerequisite for on-site treatment is that the clean up method must clean the contaminated soil, so that soil criteria for end use can be met, and that the project can be completed within a realistic time frame.

Economy for on-site treatment

On-site plants are most suitable for treatment of large quantities of contaminated soil and for sites placed at a distance from housing areas. On-site plants are not suitable for areas where there are noise limits, shortage of space or tight time schedules for the reestablishment of end usage. The treatment price per ton contaminated soil for treatment of smaller quantities of contaminated soil is generally considerably higher than for larger quantities of soil.

Energy consumption for ex-site treatment

Treatment ex-site requires energy in the form of transport of the contaminated soil from the site to the treatment plant. The transport creates noise and dust problems for the surroundings both around the actual site, during transport and unloading of the soil at the treatment plant.

The contaminated soil is treated ex-site at plants, which are in constant operation or where the treatment can be optimised by collection or sorting of soil in larger quantities from other remedial projects. The treatment ex-site is therefore less sensitive to delays or interruptions in the excavation project or by shortage of time to complete the project within the time schedule for the remediation project.

Biological treatment ex-site

With biological treatment ex-site at commercial biological plants, it is possible to achieve an optimisation of the biological processes by application of an efficient production system. Commercial plants have a working organisation with the necessary resources and experience to follow the degradation at suitable time intervals. If it is necessary, the commercial plants can stimulate the biological processes by addition of bacteria, nutrients, water or structural materials, to turn and ventilate the soil or to initiate other treatments.

At these plants, there is time to wait for the soil to be cleaned to an acceptable level. Furthermore, the end use of the soil after treatment can be evaluated according to circumstances.

Mixed contamination

Contaminated soil can comprise several types of contamination (mixed contamination). Often, there can be mixtures of oils, heavy metals, cyanides as well as tars. Oil pollution and to some extent cyanide compounds can be treated together with the tars, but a high content of heavy metals can be decisive for the choice of treatment.

Traditional waste water treatment plants

The treatment of contaminated drainage water from a gas works in a traditional wastewater treatment plant gave no problems. The contaminated water was diluted with wastewater before flow into the waste water plant and the tar components could not be detected in the treated water.

Water treatment

Treatment with stripping and cleaning of the air phase with active carbon filter is a well-proven technique to clean up groundwater contaminated with benzene and other volatile aromatics. The method is not so suitable for the very soluble tar compounds such as the phenols and NSO-hetereocycles, which usually require direct filtration on active carbon or destructive treatments.

In-situ water treatment

The objective for the infiltration plant at Hjørring Gas works was to combine an increased leaching of the water-soluble components with an enhanced biological degradation. The project comprises a low technological treatment to stimulate the natural biological degradation processes, in the saturated and unsaturated zones. These processes are under natural conditions often inhibited by shortage of oxidation agents and nutrients.

The infiltration plant creates the opportunity for a reduction of the remaining soil pollution (after removal of the hotspots) at source within a shorter time frame.

Natural degradation

In the long term, it is possibly that natural degradation alone will be able to reduce the remaining contamination and prevent unacceptable contamination of a groundwater resource.

Natural degradation of tar contamination in groundwater is documented in the literature and observed in the Gas Works Pack projects. During evaluation of natural degradation, a suitable monitoring program is required.

Monitoring programs

The monitoring program for groundwater can comprise both field measurements, total parameters such as NVOC/VOC (non-volatile organic carbon volatile organic carbon), specific organic parameters (benzenes, phenols, NSO, etc) and inorganic parameters including redox parameters such as oxygen, reduction potential, nitrate, iron, manganese, sulphate and methane.

Conclusions

The main conclusions for the five projects can be summarised as follows: