Arbejdsrapport fra Miljøstyrelsen 17/2006 – Kulbrinte i betonslam

Kulbrinte i betonslam

Summary and conclusions

Cleaning equipment in the concrete industry generates a waste product in the form of a concrete slurry or sludge. The waste typically contains hydrocarbons, and often in concentrations that the authorities cannot accept if the waste is to be used and deposited without any restrictions. The authorities often compare concrete sludge to soil and therefore use the same threshold values for concrete sludge as for soil when the concrete sludge is landfilled. The typical threshold value for hydrocarbon in soil is 100 mg/kg dry matter.

Sources

Hydrocarbon is a chemical substance containing only carbon and hydrogen, and the sources of hydrocarbon in concrete sludge are:

Form-release oils
Lubricants and hydraulic oils
Fuel from vehicles
Road dirt

Typically, form-release oils are either mineral-oil based and therefore contain large amounts of hydrocarbons, or they are based on vegetable oils and then contain no hydrocarbons.

The vegetable-based form oils typically consist of vegetable cooking oil mixed with a fatty acid ester and/or a free fatty acid.

A number of organic chemical additives are used in modern concrete. These compounds are typically substances with a high molecular weight, which do not leach from the concrete or concrete sludge.

Chemical analysis of a number of selected additives typically used for concrete verifies that these chemicals do not show up as hydrocarbon in the analysis for hydrocarbon.

In none of the chemical analyses carried out on samples of concrete sludge has it been possible to detect any traces of the organic additives in the concrete

Consumption

On the basis of interviews with suppliers of form-release oils, consumption in Denmark is estimated as:

Total amount of form-release oil per year: 1000 ton

Mineral-oil based: 70 %

Vegetable-oil based: 30 %

(The emulsified oils, which very often are of the vegetable-based type, are included in the above 30 %.)

Very small amounts of form-release oils are incinerated or treated in other types of waste processing plants, therefore approx. 700 tonnes of mineral oils must be expected to be lost to the environment in Denmark every year.

Chemical analysis

Chemical analysis for hydrocarbons in concrete sludge is typically carried out by means of gas chromatography with flame ionisation detector (GC/FID). The method often mistakenly includes free fatty acids and fatty acid esters in the total amount of hydrocarbons. As a consequence of this, many analyses of concrete sludge samples have shown too high hydrocarbon content. This is particularly the case if the sludge originates from plants where vegetable-based form-release oils are used. It is therefore recommended to use gaschromatography with mass spectroscopy (GC/MS) to ensure the correct identification of hydrocarbons, fatty acid esters and free fatty acids.

Solutions

Form-release oils are used on mixers and transport equipment as well as on moulds. In the present project it has, without any technical problems, been possible to substitute mineral-oil-based form-release oils with vegetable-based form-release oils on the mixers and transport equipment. This substitution has in every case resulted in extensive reductions in the hydrocarbon concentrations in the sludge from the plants.

At a couple of plants, total substitution of mineral-oil-based form-release oils with vegetable-based oils has been performed without any technical difficulties. The substitution has resulted in hydrocarbon concentration in the concrete sludge of less than 100 mg/kg dry matter.

Based on the experience in the present project, it must be expected that an extensive substitution of mineral-based form-release oil with the vegetable-based oils will extensively reduce the hydrocarbon concentrations in the sludge from the concrete producers. Beside the substitution it is also necessary to be very careful to reduce spills of fuel, lubricating oils and grease, and perhaps also include substitution of mineral-based hydraulic oils with vegetable-based hydraulic oils.

Experience from this project shows that it is difficult to keep different kinds of oils separated in the same factory. If only a part of the mineral-oil-based form-release oil is substituted with vegetable-based oil, there is a great risk that in the end the mineral-based oil will end up in the sludge, resulting in high concentrations of hydrocarbons in the sludge. It is therefore recommended to substitute all mineral-oil-based form-release oils with vegetable-based oils in the plant.

Substitution of the mineral-based form-release oils is in most cases carried out without any technical difficulties. However there are some examples from industry that substitution is rather difficult. A few cases of local retardation (delay of the hardening process) of the concrete have been observed and also some problems with water-repellent concrete surfaces have been observed. The manufactures of form-release oils are undertaking great efforts to solve these problems and in several cases the problems have been solved in cooperation between the producer of concrete and the supplier of the form-release oil.

Vegetable-based form-release oils are more expensive than the traditional mineral-based oils. Of course the actual prices depend on a number of parameters such as amount purchased. However with to-day’s prices it must be expected that the direct costs related to the purchase of form-release oils could grow by approx. 25% when the mineral-based oils are substituted with the vegetable-based oils. The additional cost is normally more or less balanced by the savings related to the management of waste and savings related to after-treatment of concrete surfaces.

Leaching and limit values

It is difficult to measure the rate of leaching of hydrocarbon from concrete and from concrete sludge. However experiments suggest that rate of leaching of hydrocarbons is slower from regular concrete than from concrete sludge.

The practical experience with testing leaching of hydrocarbon from inorganic waste materials is limited, and these test cannot justify putting forward threshold limit values for hydrocarbon in concrete sludge.

If sludge of concrete is deposited as polluted soil it is therefore difficult to justify different threshold limit values than are typically applied by the authorities for polluted soil.

Recommendations

On the basis of the results from this project it is recommended to:

Substitute all mineral-based form-release oils with vegetable-based form-release oils as much as possible
Carry out chemical analysis of sludge and concrete by GC/MS. This makes it possible to distinguish between hydrocarbon and fatty acid/fatty acid ester.