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Mass Flow Analysis of Chromium and Chromium Compounds

5 Circulation with waste

Disposal of chromium and chromium compounds mainly takes place in connection with disposal of products and waste products which contain chromium. Chromium is an element and so will not disappear. This means that we cannot dispose of chromium by disposing of products which contain chromium – we merely move it.

Once an object, a product, has been used for the last time, it can either be disposed of actively or simply "perish" where it was last used. In both cases, the contents of such products may enter the environment. This includes chromium. Active disposal allows some control over emissions into nature.

When we dispose of chromium, we often do so in the form of relatively small concentrations in other products: tapes, chromium plated objects, impregnated wood, painted objects, glass, etc.

The only known way to release chromium from the product it occurs in is to burn or melt it.

As metallic chromium occurs as part of other metals, it is also recycled along with those metals – primarily stainless steel, steel, iron, aluminium and copper. When products made from these metals are worn out, they are collected for recycling, and of course the chromium in such metals will be included in this recycling.

5.1.1 Circulation of chromium with iron and steel

The quantities of metal collected in Denmark can be described as being equal to the net export of scrap metal plus metal recycled in Denmark plus losses from reprocessing.

In a life cycle analysis of steel, it is estimated that the scrap industry collects 97% of all steel scrap in Denmark (Kjeldahl, 1991, p. 103). In a new study prepared by DEMEX and others, it is estimated that 90% of all stainless steel in the building sector is reused, while the remaining 10% is disposed of by means of landfilling (5%) and incineration (5%). The study also estimates that 95% of all painted surfaces are disposed of by means of incineration, while the remaining 5% are sent to landfills (Lauritzen et al., 2002). Stainless steel is sent for re-melting in Sweden (Kjeldahl, 1991, p. 106).

Table 5.1
Recycling of iron and scrap metal in 1999 (The Danish EPA, 2000).

According to Statistics Denmark, a total of 965,000 tonnes scrap metal is recycled in Denmark each year. In 1999, a total of 987,000 tonnes scrap metal was collected in Denmark. If we assumed that this corresponds to 97% of the total amount of scrap metal, this means that 3% – or 31,000 tonnes scrap metal – ends up in the environment. It is estimated that much of this scrap metal is iron with an average chromium content of approximately 0.05–0.1%. This means that 15.5–31 tonnes metallic chromium ends up in the environment, primarily in the soil.

Stålvalseværket and other Danish foundries do not accept stainless steel. As a result, we can safely assume that the 570,000 tonnes sent for export contain large amounts of stainless steel. As the exact figures are not known, it is estimated that stainless steel accounts for approximately 25%. This corresponds to 142,500 tonnes stainless steel or 24,000–26,000 tonnes chromium. It is assumed that the percentages stated for recycling of stainless steel within the building industry also apply to Denmark as a whole.

This means that approximately 2,700–2,900 tonnes chromium end up at landfills. Half of this chromium has passed through an incineration plant first.

5.1.2 Circulation with aluminium

By consulting the updated mass flow analysis for aluminium (see section 2.2), we can see that 18,600–22,300 tonnes of aluminium are recycled in Denmark each year. This corresponds to 4–22 tonnes chromium. Aluminium will be reprocessed by means of re-melting. The chromium content of the reprocessed material will not be any different from that of the source material. The total net export of scrap aluminium is 8,000–17,000 tonnes, corresponding to 2–18 tonnes of chromium.

5.1.3 Circulation with copper

By consulting the updated mass flow analysis for copper (see section 2.3), we can see that 9,000–10,000 tonnes of copper are recycled in Denmark each year. This corresponds to 2–3 tonnes chromium. Copper is reprocessed by means of re-melting. The chromium content of the reprocessed material will not be any different from that of the source material. The total net export of scrap copper is 15,000–24,000 tonnes, corresponding to 4–6 tonnes of chromium. The exported scrap is often reprocessed by means of electrolytic refining, which reduces or removes the chromium in the copper.

5.2 Waste disposal

5.2.1 Total annual quantities of solid waste

Various types of waste are actively disposed of. This concerns waste from households, industry, the building sector, etc. The waste statistics include figures on disposal of solid waste.

Table 5.2
Waste Statistics 1999 (The Danish EPA, 2000).

In 1999, Denmark exported significant amounts of waste.

Table 5.3
Export of waste from Denmark in 1999 (The Danish EPA, 2000).

In 1999, Denmark imported waste corresponding to 4% of the domestic waste production. This imported waste was, however, mainly "green" waste or metal and was exclusively intended for recycling or incineration. Except for the chromium content in the 252,000 tonnes scrap iron and metal imported for recycling, it is estimated that no significant quantities of chromium are imported with waste.

5.2.2 Thermal waste treatment

Table 5.4
Quantities of residual products caused by waste incineration (Hjelmar & Hansen, 2002).

Table 5.5 shows the chromium content of the most important residual products. Assuming that a total of 2,929,000 tonnes waste is incinerated each year (The Danish EPA, 2000), we can estimate the total amount of chromium in residual products from waste incineration.

Table 5.4
Chromium content in residual products from waste incineration (Hjelmar & Thomassen, 1992; Hjelmar & Hansen, 2002).

For 1995–1996, the emission factors for waste incineration plants equipped with various clean-up technologies have been set at 0.1–1.7 g Cr/tonne waste (Illerup et al., 1999). This makes for an emission of chromium to air of 0.3–5.0 tonnes. Only small amounts of waste (approximately 2%) were treated at the plant with the high emission factor in 1996/1996, so a more realistic estimate would be emissions to air of 0.3–1.5 tonnes chromium.

5.2.3 Landfilling activities

Some waste cannot be incinerated and is deposited along with non-recyclable residual products from incineration of waste and combustion of fossil fuels (coal). Table 5.6 shows the quantities deposited at landfills in 1999. Some dangerous waste is also deposited at landfills. For example, sludge from the chromium plating industry – which contains a great deal of chromium – is deposited with Kommunekemi. In 1999, approximately 106 tonnes of chromium were deposited with Kommunekemi. It should be noted that a very significant amount (99%) of all fly ash from coal incineration was recycled in 1999. Larger quantities than this have been deposited at landfills in other years.

Table 5.6
Waste deposited at landfills (The Danish EPA, 2000).

If we use the Cr concentrations for slag and fly ash given in Table 5.5 and combine them with estimated contents in residual products from coal combustion (Hjelmar & Thomasen, 1992), we can calculate the quantities of chromium deposited at landfills. This figure does not, however, include the quantities deposited with standard non-flammable waste. No information on the chromium contents of standard non-flammable waste deposited at landfills is available from either Waste Centre Denmark or the Household Waste department within the Danish EPA.

Table 5.7
Chromium deposited at landfills

Some chromium may enter the environment from landfills and in connection with leaching percolate. This issue is not addressed here.

All in all, it is estimated that approximately 175 tonnes chromium is deposited at landfills each year. This figure does not include all the deposits made in the form of standard non-flammable waste.

5.2.4 Biological waste treatment

Parts of the organic fraction of refuse and green waste from gardens, parks, etc. undergo biological treatment. Such waste ends up as compost or as residual products from biogas generation. Due to their nutrient content, these residual products are applied to agricultural soil. In addition to nutrients, however, they also contain various heavy metals, including chromium. The table below shows the chromium content in the residual products and the annual chromium input from residual products to agricultural soil.

Table 5.8
Average chromium contents and annual total chromium quantities in compost and residual products from biogas generation (The Danish Forest and Nature Agency, 1998).

5.3 Circulation with chemical waste

Chemical waste/dangerous waste is disposed of through approved specialists, including Kommunekemi. Table 5.9 illustrates the disposal of fractions of dangerous waste which may contain chromium. No chromium analyses of the individual waste fractions are available.

Table 5.9
Disposal of dangerous waste in 1999; fractions which may contain chromium (The Danish EPA, 2000).

In 1999, a total of 3.782 kg chromium(VI) was leached with percolate. This figure is above average due to the unusually high rainfalls in 1999. The corresponding figures for 2000 and 2001 are 0.492 kg and 1.478 kg. The landfill is located close to the coastline, which ensures that the leached chromium(VI) cannot reach the groundwater (Kommunekemi, 2002b).

5.4 Circulation with wastewater and wastewater sludge

During a period in 1996, studies were made in the towns Bagsværd and Skovlunde of xenobiotic substances in the run-off from built-up areas. Here, concentrations of 10.6–18.5 µg Cr/L were measured (Kjølholt et al., 1997). These levels correspond to those found in the input to a number of treatment plants.

Table 5.10
Inflow to specific treatment plants 1997 (Jepsen & Grüttner, 1997)

In 1999, the average concentration of chromium in inflow to the "Lynetten" treatment plant was 7.8 µg/L, while the corresponding figure for "Damhusåen" was 8.9 µg/L. On average, the total input was 1.4 kg Cr/day to "Lynetten" and 0.7 kg/day to "Damhusåen" (Lynettefællesskabet I/S, 2000). The total amount of water treated in 1999 was in the region of 825 million m³. If we take this information as the basis for assuming an average content of 5–10 µg Cr/L, the average inflow to treatment plants is 4.1–8.3 tonnes Cr/year. As we can see, these figures correspond well to the quantities found in sludge. In previous studies, it has been estimated that 80% of the chromium is transferred to the sludge at treatment plants (Grüttner & Jacobsen, 1994). If we assume that 80% is held back at the plants, we arrive at a discharge from treatment plants into the aquatic environment of 1.2 tonnes Cr/year. The Danish EPA states that discharges into the aquatic environment from specific industrial contributors is 0.219 tonnes Cr/year, possibly more (The Danish EPA, 1999b).

In 1999, a total of 1,409 treatment plants were registered in Denmark. These plants treat wastewater corresponding to 8.09 million person equivalents (PE). In 1999, the plants produced 155, 621 tonnes sludge (dry matter). That year, the limit value for chromium in sludge was 200 mg/kg total phosphorus (P) or 100 mg/kg sludge (dry matter). In 1999, the weighted average concentrations of chromium in sludge were 33.2 g Cr/tonnes sludge (dry matter) for all sludge, and 24.6 g Cr/tonne sludge (dry matter) for sludge applied to agricultural soil (The Danish EPA, 2001). This corresponds to a total content of 4.9 tonnes Cr in the sludge, of which 2.1 tonnes were applied to agricultural soil. The chromium which was not applied to agricultural soil was sent for incineration (approximately 45%) or to landfills (approximately 51%) (The Danish EPA, 2001b). This means that 1.3 tonnes chromium from sludge is sent for incineration each year, and that 1.4 tonnes chromium from sludge is deposited at landfills each year.

5.5 Emissions to air

As has been demonstrated elsewhere, large amounts of chromium are emitted to air in connection with combustion of fossil fuels and incineration of waste.

Table 5.11
Emissions of heavy metals to air in 1999
(National Environmental Research Institute, Denmark, 2002).

Some of these emissions are caused by chromium contents in fuel.

5.6 Area deposition

Based on statistics from 7 stations, it has been ascertained that the bulk deposition of chromium onto Danish soil is 125 µg Cr/m² each year. This does not include aerosol deposition, which may correspond to 5–20% of the bulk deposition.

The total annual contribution to Danish soil (43,000 km²) from atmospheric deposition is estimated to be 10 tonnes Cr (average for the last 10 years).

As regards Danish waters, approximately 167 µg Cr/m² falls on the Danish inland waterways (40,000 km²) each year. This corresponds to a total of 7 tonnes per year (Hovmand et al., 2000).

5.7 Summary

Chromium emissions associated with waste treatment can be summarised as shown in Table 5.12.

Table 5.12
Chromium emissions associated with waste treatment in Denmark, 1999.

5.7.1 Recycling

Chromium is not recycled in pure form in Denmark. Instead, it is recycled as part of iron, steel, aluminium and copper. The quantities of recycled chromium can be calculated as follows: 2,600–3,600 tonnes chromium with iron and steel, 7–22 tonnes chromium with aluminium, and 2–3 tonnes chromium with copper. Recycling of chromium as part of residual products (fly ash and clinker) from coal-fired CHP plants accounts for 62–185 tonnes Cr/year. Chromium compounds are not recycled as part of products. They can, however, be extracted from wastewater from relevant processes, e.g. chromating.

5.7.2 Waste treatment

Chromium enters the waste treatment system with household waste as well as commercial waste. No statistics on chromium in inflows to the various plant types are available, but more information is available on the outputs. For example, residual products from waste incineration plants (fly ash, residual products containing fly ash from flue gas cleaning, sludge, and slags) represent chromium quantities of 158–418 tonnes per year, while the emission to air can be estimated at 0.3–1.5 tonnes chromium. In 1999, a total of 77% of the residual products (corresponding to 122–322 tonnes chromium) was recycled, while the remaining quantities (corresponding to 36–96 tonnes chromium) were deposited at landfills.

5.7.3 Chemical waste/dangerous waste

Several waste fractions containing chromium are classified as dangerous waste. This means that they must be disposed of through approved specialists such as Kommunekemi. No analyses of inputs to Kommunekemi are available, but chromium contents have been determined for a number of residual products (e.g. clinker, filter cakes, etc.) deposited. A total of 100–110 tonnes residual products containing chromium is deposited.

5.7.4 Wastewater and wastewater sludge

Chromium contents have been identified in samples of inflow to three treatment plants. On the basis of concentrations of 19–47 µg Cr/L, the total input of chromium to wastewater treatment plants can be estimated to be 4.1–8.3 tonnes Cr/year. Of this amount, approximately 80% is held back. The emissions/discharges into the aquatic environment are estimated to be 1.2 tonnes Cr/year. The remaining quantities are caught in sludge. With a chromium concentration in sludge of 33.2/tonne dry matter (average for all sludge) and 24.6 g/tonne dry matter in sludge for application to agricultural soil, we arrive at an output of 4.9 tonnes chromium. Of this amount, 2.1 tonnes were applied to agricultural soil, 1.3 tonnes were sent for incineration, and 1.4 tonnes were deposited at landfills.