Summary and discussion
The aim of this report has been to demonstrate the existing knowledge regardng the harmful effects of heavy metals, the principal routes for the metals via waste management systems to the environment and the strength and weaknesses of recycling and substitution as options for risk reduction.
Mainly related to the heavy metals lead, mercury and cadmium but covering other heavy metals like chromium as well extensive documentation of the potential harmful effects to humans and the environment is available. Generally heavy metals have each their story. Common to lead, mercury and cadmium is that neither has any known useful function in biological organisms.
Lead is causing concern in particular due to the possible impacts on children. Lead influences the nervous system, slowing down nerval response. This influences learning abilities and behaviour. Children are exposed to lead right from their birth, as children in the embryonic stage receive lead from their mothers through the blood. Children is, furthermore, exposed to lead via dust and soil contaminated by deposition from air and other sources. In the environment lead is known to be toxic to plants, animals and microorganisms. Effects are generally limited to specially contaminated areas.
Concerning mercury the primary focus is on methyl mercury originating from the diet in particular though the consumption of fish and fish products. In humans methyl mercury affects among other organs also the brain, and it is documented that (as for lead) children in the embryonic stage receive mercury via the placenta causing persistent effects on children.s mental development. In the environment animals placed highly in the food chain and in particular the marine food chain are assumed exposed to mercury poisoning due to the ability of methyl mercury to concentrate via the food chain. However, notable effects on microorganisms are believed to take in large parts of Europe in forest soils dominated by organic material.
Cadmium accumulates especially in the kidneys leading to dysfunction of the kidney with increased secretion of e.g. proteins in urine (proteinuri) and other effects. Intake of cadmium is generally based on the diet, in particular vegetables and corn products. The concern of this pathway is based on the knowledge that an increase in the content of cadmium in agricultural soil will result in an increased uptake of cadmium by plants. However, for smokers also the use of tobacco is of concern. In the environment cadmium is reported toxic to especially animals and microorganisms. Also for animals kidney damage is the dominating effect. Concerning microorganisms cadmium is known to significantly influence leaf litter decomposition.
Chromium differs from the 3 other heavy metals discussed here by being essential in form of Cr(III) to humans and animals. The most widespread human effect is chromium allergy caused by exposure to chromium (especially Cr(VI) compounds) in the working environment. Chromium compounds are also assumed to cause cancer. Environmentally Cr(VI)-compounds are generally considered the most toxic.
Sources to waste
The content of heavy metals in waste is primarily a consequence of the intended use of heavy metals in industrial products. At the end of their useful life all products will end up in waste to the extent they are not attractive for re cycling. Heavy metals may, however, also be lost to waste during production and use phases. Losses in the manufacturing process are often disposed of as manufacturing waste, while products may be exposed to wear and tear inclusive corrosion during the use phase.
Identification of the actual sources for heavy metals observed in different waste types and waste products may only be done with some uncertainty. E.g. a lead atom in flue gas cannot tell whether it originates from lead pigments in plastic or a lead battery been disposed of as combustible waste. A useful tool in this context is Substance Flow Analysis, which based on a knowledge on applications for the substance in question and flow patterns for relevant products allows for the development of a realistic picture of significant sources for different waste types. The analysis of sources to waste has focused on solid waste.
It is characteristic for lead, that many different products containing lead will end up in waste management systems and be a source of lead to incineration plants and/or landfills. Important sources include (reference is made to table 3.3): Plastics, fishing tools, lead crystal glass inclusive cathode ray tubes, ceramics, solders, pieces of lead flashing and many other minor products. To these waste types must be added residues from metal shredding, steel reclamation and cable reclamation.
Important sources for mercury to waste include (reference is made to table 3.10, 3.12 and 3.13): Dental amalgam, measurement and control devices inclusive thermometers, batteries, tubes and lamps etc. It is interesting to note the significant differences between countries, which may be explained by differences in regulation as well as tradition.
For cadmium the picture is somewhat simpler, as the use of cadmium has been restricted for some years and NiCad batteries today is the all-dominating product. However, to understand the picture of sources to incineration plants, it is necessary to remember uses as pigments and stabilisers in plastic as well as plating on steel, which have been significant uses 1-2 decades ago. Many of the relevant cadmium products were quality goods with an expected lifetime of 10-20 years or even more (e.g. PVC-window frames). Such goods are only slowly released to waste. Concerning landfills table 3.20 indicates that manufacturing waste is a source of the same magnitude as industrial products in municipal waste.
The environmental concerns related to chromium are focused on applications like tanning, wood preservation and pigments and dyes for plastics, paint and textiles. Chromium alloys and in particular stainless steel are by far the dominating field of application for chromium, but normally not regarded as a serious waste problem due to the high value of chromium alloys, which motivates collection for recycling. One may, however, pay attention to that many types of stainless steel are not magnetic and cannot be separated from waste streams by magnetic separation.
Fate by waste treatment and disposal
Modern waste treatment technology have developed to ensure that the immediate release of heavy metals to the environment (air, water) from waste treatment facilities inclusive incineration plants in general today are small. An exemption may be mercury, which due to its very volatile nature is extremely difficult to retain. However, also for mercury the dominating part of the content in combustible waste will be collected with residues, slag and flue gas cleaning residues. The basic problems related to these residues can be summarised to:
- Lack of space for landfills are forcing some countries to utilise the residues for civil works and similar purposes, but the content of heavy metals in residues constrains this kind of material recycling.
- No matter whether the residues are utilised for civil works or placed in landfills the overall consequence is a continued creation of heavy metal stocks in the European society.
This process cannot be considered sustainble taking into account the potential for future release to environment eiher by leaching or by more drastic geological events as e.g. a new Ice Age. The issues related to landfilling in several ways resemble the issues of disposal of incineration residues. Although the mobility of heavy metals inside landfills is low, and a complete wash-out of a specific metals may require a time of hundreds to thousands of years and in special cases even more, no evidence exist that landfills can be regarded as a permanent containment of heavy metals.
Thus the basic question to be considered is: For how long into the future are we responsible for the consequences of our actions of today?
Recycling may be a way to significantly reduce the loss of heavy metal to the environment and at the same time avoid that new metal enter into circulation.
However, the concept of recycling covers an array of very different activities. Dedicated recycling of heavy metals may be carried out rather efficiently with very small losses to the environment and residues, assuming that proper technology are applied. The main problem here is to ensure an effective collection of item made of heavy metals. The fact is that effective collection only will work for items present in such a quantity and condition that collection is feasible.
Reality is that significant quantities of heavy metals will never be collected for recycling by the present waste management systems. Thus recycling will not prevent a continued release to the environment of heavy metals in circulation in the technosphere.
Regarding recycling of other metals, heavy metals are present as contaminants requiring special pollution prevention operations and special disposal of residues. To some extent the heavy metals will be integrated in the metals in question, e.g. chromium and nickel will be integrated in steel. Continued recycling of plastics only exist for few products like boxes for beer ad soft water bottles. Apart from such product recycling will typically only delay the disposal of the heavy metals in question for a single product generation.
Systems are being developed for recycling of cathode ray tubes. Apart from this lead crystal glass will mainly be source for lead contamination of ordinary secondary glass and may be a source for lead emission from glass manufacturing as well as from landfills at the time of final degradation of the glass matrix.
Recycling of organic materials to be utilised as soil improvement media will lead to dispersal of heavy metals present in other waste types, e.g. plastics, contaminating the organic material.
Substitution is the option left, when recycling cannot solve the problem. By substitution pollution is prevented at the source, as it is avoided that heavy metals are taken into the manmade circulation.
Alternatives have been developed for many of the applications of lead and nearly all applications of mercury and cadmium, while the development regarding chromium so far has focused in particular on tanning and wood preservation. The knowledge available with respect to alternatives has been presented in table 5.1-5.4.
The costs of substitution vary between applications and may range from below zero to a much higher price level. It is, however, always possible to discuss, what should be included in such cost estimates. Attention is e.g. drawn to the fact, that in order to restrict emissions of heavy metals to the environment by waste treatment and disposal processes, significant costs are paid by local governments or other parties to control operations and special treatment and disposal activities. Such costs are generally not included in estimates for costs of substitution.