Cadmium og DEHP i kompost og bioafgasset materiale

English Summary

The objective of the study has been to investigate whether Danish compost and residuals from biogas production based completely or partially on organic domestic waste, comply with the existing and future threshold values for heavy metals and organic micropollutants. An additional objective has been to identify possible contamination sources and to suggest actions to reduce the problems, if relevant. The study, which was carried out from August 1997 to March 1998, has comprised nine composting facilities and one biogas plant.

The first activity in the study programme was to review existing data from the selected plants regarding the levels of contaminants, and to conduct a small supplementary analytical programme in order to confirm the previous findings and identify the critical substances. Based on the findings of this reconnoitering, it was decided to limit the objects of the main investigation to the heavy metal cadmium and the plasticizer DEHP.

The basic element of the main investigation, was a programme comprising two rounds of compost sampling and analysis at all the selected plants, and a third round only covering the four largest plants in Vejle, Frederikssund (AFAV), Århus and Audebo (Noveren) characterised by a dominant fraction of organic domestic waste in the raw material. All samples in the main programme have been analysed independently by two laboratories.

Supplementary to the main programme, samples from a selected windrow at AFAV were taken over a period of 10 weeks in order to study the trend in DEHP contents with time. Finally, some samples of non-organic products often observed in the organic waste fraction (plastic bags, wrappings, diapers, magazines etc.) at the plants were analysed for their contents of DEHP.

The overall tendency in the results for cadmium is that the levels are lowest, approximately 0.25-0.30 mg/kg dw, at the two plants having the highest fraction of organic domestic waste in their raw material (Vejle and AFAV). When green waste is used in significant amounts for structural purposes, the cadmium level typically increases to about 0.40-0.50 mg/kg dw. The origin of the green waste is important; the cadmium level is significantly higher in green waste from large urban areas than from rural residential areas.

Generally speaking, the composting plants are complying with the current threshold value for cadmium (0.8 mg/kg dw), whereas a number of them will experience difficulties in complying with the tightened value as per 1 July 2000 (0.4 mg/kg dw), if the present level of contamination or impurities in the raw material is not reduced.

As regards DEHP, the tendency in the results is largely the opposite of that of cadmium; the plants with the highest shares of organic domestic waste also have the highest levels of DEHP in the produced compost. Thus, the DEHP levels at the four plants with more than 75% of organic domestic waste in their raw material (Vejle, AFAV, Århus and Noveren), were in the range 20-139 mg/kg dw, while the levels at the facilities with 50% or more green waste generally was about 1 mg/kg dw or even lower. At one facility with 50% green waste the concentrations were, however, in the range 4.2-48 mg/kg dw. At that facility the organic domestic waste contained a substantial fraction of impurities.

The threshold value for DEHP is presently 100 mg/kg dw but will be tightened as per 1 July 2000 to a value of 50 mg/kg dw. One of the large plants is already today occasionally experiencing difficulties in complying with the present threshold value for DEHP, while two other plants will be in the critical zone when the tightened value comes into force in the year 2000.

With regard to the possible sources of cadmium contamination, it should first be realised that today a background level exists for this metal in products of both vegetable and animal origin, which sets a minimum for the achievable contents in compost from organic domestic waste of approximately 0.15-0.25 mg/kg dw. If green waste is added in significant quantities, this level will increase with about 0.1 mg/kg or even more, depending on the origin of this material.

All uses of cadmium which realistically can be expected to influence the quality of compost, seem to have ceased in Denmark several years ago. The exception is batteries which still, by occasional mistakes, can end up in the organic fraction of the domestic waste. Other sources must be sought among certain old plastic or metal products with long durability or contaminated soil, though these materials ought not to occur in the waste.

The case of DEHP is different as there is a wide range of current applications and products which make use of this or related substances. E.g. a wide range of plasticized PVC products, certain glues and paints, and printing ink for colour printing of plastic bags, magazines, advertising brochures etc. At some of the composting plants, newspapers, magazines and brochures are deliberately added to the raw material for structural purposes, in others they just occur regularly due to mis-sorting in the households. An investigation of the DEHP content in 17 categories of materials frequently observed as impurities in organic waste from households revealed that no category can be singled out as the main responsible of the observed contamination level. Rather, this is a result of the cumulated contributions from a considerable number of smaller sources.

In the case of DEHP certain processes or operational conditions in either the collection or the processing stage may enhance the migration of DEHP from the original material to the waste or raw compost matrix, or reduce the rate of biotic degradation of the substance (e.g. anaerobic conditions).

It seems that the greatest possibilities (at the municipal or composting plant level) for significantly reducing the levels of cadmium, DEHP and other undesired contaminants, lie the collection stage. The existing guidelines for waste sorting in households should, in many cases, be more precise than today and furthermore recurrent campaigns of information and motivation of the citizens are probably indispensable to maintain an acceptable standard of sorting.