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Mass Flow Analyses of Mercury 2001

3 Circulation in waste products

3.1 Recycling of mercury
3.2 Miscellaneous circulation in solid waste
   3.2.1 Total quantities of solid waste
   3.2.2 Thermal treatment of waste
   3.2.3 Sources of mercury in waste destined for incineration and
   deposition in landfills
   3.2.4 Treatment of biological waste
3.3 Circulation in hazardous waste
3.4 Circulation in wastewater and wastewater sludge
   3.4.1 Wastewater
   3.4.2 Wastewater sludge
3.5 Summary of mercury losses in waste treatment

3.1 Recycling of mercury

It has not been possible to find firms that purchase metallic mercury in the recycling industry. The firms that did this when the last mass flow analysis was conducted quote prices, general inconvenience and small quantities as the reason for terminating such collection.

Instead, mercury is collected through municipal collection schemes, from which the main part is sent to Kommunekemi.

Metallic mercury comes from broken thermometers, monitoring equipment, laboratory and school chemicals and from district heating power plants. As far as can be ascertained, mercury stand pipes are no longer in use in district heating power plants but AVV, in Hjørring, received about 230 kg of pure mercury from such a plant as recently as 2000. This probably comes from a stand pipe that was not disposed of until 2000. Such individual cases completely change the total collected quantity of pure mercury, which for AVV in 1999-2001 amounted to 3-10 kg/year.

With the exception of Elektromiljø, which handles straight fluorescent tubes, there are as far as is known no firms in Denmark today that separate mercury-containing products. Sorting is the only activity carried out in Denmark. However, it is common to remove button cells and mercury-containing relays from printed circuit boards during sorting.

There is reason to believe that the quantity of mercury in fragmentation plants has dropped significantly.

This is partly due to a lower consumption of mercury in products, partly to the removal of mercury-containing units by car breakers, by TDC and by the recycling firms that process freezers, and partly to the collection of electronic scrap.

During the last few years, it has been mandatory for the car industry to remove mercury-containing switches from cars before scrapping. Removal is first carried out by the car breaker, followed by checks prior to fragmentation. For this purpose, there is a list of the car models that can contain such switches.

During the past three years, many of the whole telephones collected have been exported whole, instead of being cut up in Denmark.

Worn-out refrigerators and freezers are drained of CFC prior to scrapping. Any mercury-containing switches are also removed at the same time.

Mercury-containing parts are removed from electronic waste before apparatuses are scrapped.

In other words, it is normal for mercury-containing parts to be removed prior to the compression, division or shredding of metallic scrap. Nevertheless, some mercury-containing parts that will break and release mercury when compressed, divided or shredded can be overlooked. The temperature during shredding can reach about 300°C, at which a large part of the mercury will evaporate. In Denmark, there are five firms that operate fragmentation plant (or shredders) and about 30 firms that operate other mechanical divider plant (Danish EPA, 1998). 1,089,000 tonnes of Danish iron and other metallic scrap destined for recycling was collected in 2000 (Danish EPA, 2001a).

At the time of the previous mass flow analysis, the total emission to air from fragmentation plants was estimated at <50 kg of mercury/year. No improvement can be found in today's emission figures.

In the fragmentation and division processes, about 8% (87,000 tonnes) of the input scrap becomes waste destined for landfills, about 0.2% (about 2,000 tonnes) becomes waste destined for incineration and a very small fraction (wet-wash sludge) is sent to Kommunekemi as hazardous waste (Danish EPA, 1998). This quantity was estimated at less than 50 tonnes/year in the previous mass flow analysis.

Based on individual measurements in the waste, the previous mass flow analysis estimated the quantity of mercury in waste from fragmentation plants at about 200 kg. There is no improvement in today's release figures. Of this quantity, about 5 kg is incinerated, whereas the remainder is landfilled.

Re-smelting of iron and steel
Any mercury suspended in metallic scrap will be emitted to air in connection with re-smelting. It is known that the re-smelting of iron and steel in Denmark causes the emission to air of about 0.5 kg of mercury/year (year 2000). This is significantly lower than the 70 kg of mercury/year found in the previous mass flow analysis (Maag et al., 1996). This could be due to improved flue gas cleaning, as about 360 kg of mercury is found in residual products, of which about 310 kg is included in products that are recycled. In addition, there is roughly 14 kg of mercury in steel products.

Collected together with metallic scrap
Based on the foregoing, the mercury collected in metallic scrap amounts to about 625 kg/year.

Miscellaneous recycling activities
In all recycling activities, the mercury that occurs as an impurity in the recycled materials will also be recycled. With a total recycled quantity of approximately 5,544,000 tonnes (see Table 3.1), and a mercury content of 0.01-0.2 g/tonne (see Section 2.4.6), the quantity of mercury can be estimated as being approximately 55-1,100 kg/year.

3.2 Miscellaneous circulation in solid waste

3.2.1 Total quantities of solid waste

The total net waste production in Denmark - minus waste/residual products from secondary sources (processing plants, incinerator plants, composting plants/biogas plants and landfills) - amounted to about 9.5m tonnes in 2000 (cf. Table 3.1). The quantity of waste has been increasing, and the corresponding quantity in 1993 was about 6.8m tonnes.

3.2.2 Thermal treatment of waste

In 2001 (cf. Table 3.1), incinerator plants received about 2.8m tonnes of waste, about 1.4m tonnes of which was refuse (80% of all refuse) and about 0.35m tonnes was from bulky waste (48% of all bulky waste).

Table 3.1 Waste production in Denmark in 2000, broken down by source and type of treatment 1) (Danish EPA, 2001a)

1) The table shows the quantity of each type of waste entering a particular treatment stream from the primary sources. The primary sources are: households, trade and offices, manufacturing firms, building and demolition, roads and construction works, treatment plants and containers/waste transfer stations. This means, for instance, that all refuse is recorded under the waste type, "refuse," regardless of whether its source is households or, e.g., trade and offices. The table does not include waste/residual products from secondary sources (processing plants, incinerator plants, composting plants/biogas plants and landfills.

There are no up-to-date studies of the mercury content of refuse or bulky waste, but the total quantity disposed of by thermal waste treatment can be estimated from knowledge of the mercury content of the residual products from the incineration process and from the emissions in flue gas.

Questionnaires were sent to all Danish waste incinerator plants in conjunction with the preparation of this mass flow analysis. The waste incinerator plants were requested to provide information on the quantity of waste incinerated, the type of acidic flue gas cleaning (wet, dry or semi-dry), types of filter, flue gas emission and concentrations of mercury in slag, fly ash and products of flue gas cleaning.

Based on the responses to the questionnaires, it has been possible to estimate the emission of mercury and its circulation in slag, fly ash and products of flue gas cleaning.

The available knowledge on the mercury content of residual products from waste-incinerator plants is shown in Table 3.2, whereas the emission to air in cleaned flue gas from waste- incinerator plants is estimated in Table 3.3. Based on the current practice with respect to the disposal of waste products, there are no requirements regarding on-going assays of residual products, for which reason it has not been possible to obtain up-to-date values for all waste incinerator plants. The available knowledge on residual products is therefore chiefly concerned with data for the period 1998-2001.

This report distinguishes between the following solid residual products of waste incineration:

• slag
• fly ash
• products of flue gas cleaning

Slag
Slag is the solid residual product that is removed from the bottom of the combustion chamber. "Slag" includes grate riddlings and boiler ash, which are typically mixed with the true slag. Slag contains iron and other metals to a varying extent, possibly with small quantities of unburned material. If the slag is to be kept for recovery, it will be processed by screening (possibly after crushing) and magnetic separation, from which three fractions are obtained:

• screened slag (screened and magnetically separated)
• scrap iron
• residuals, i.e., scrap slag (large unburned pieces and slag that has melted into large lumps)

Fly ash
Fly ash is the solid residue of combustion that can be extracted from flue gas without the need for any form of chemical separation. Fly ash is conventionally separated by means of electrostatic precipitators.

Products of flue gas cleaning
The products of flue gas cleaning result from the purging of acidic gasses from flue gas. Flue gas cleaning methods are normally divided into "dry," "semi-dry" and "wet" methods. In dry and semi-dry flue gas cleaning, the flue gas is passed through a lime suspension. The resulting sludge product is often handled separately from the fly ash. It has not been possible to obtain information on the quantities of residual products produced in 2001, for which reason the available residual product quantity data for 2000 has been used.

68,000 tonnes of flue gas cleaning products were removed from incinerator plants in 2000. By far the greatest part was exported for depositing in Norway or Germany. Thus, 85,700 tonnes was exported in 2000 (Danish EPA, 2001a). The difference between the removed and exported quantities is due to stock fluctuations. These exports are handled by two firms, i.e., Dansk Restprodukt Håndtering and Special Waste Systems.

The concentrations of mercury in the residual products, which are shown in Table 3.2, have been taken from responses to the questionnaires, from green accounts for the incinerator plants and from Dansk Restprodukt Håndtering, which handles a very large proportion of Danish residual products. It has not been possible to obtain sufficient data for 2001 and therefore data from a sizeable number of years has been used - however most data stem from the period 1998-2001. The data for dry and semi-dry residual products has been rather sparse, i.e., 3 and 4 assays, respectively. Much better data is available for wet residual products, a total of 14 assays, although two of them have not been included as they are considered to be highly unusual/incorrect (one sample was shown as being below the detection threshold of 20 mg Hg/kg dry matter and the other sample showed 1,400 mg Hg/kg dry matter). For this reason, 12 assays were used when determining the mercury content of the wet residual products. The mercury content of fly ash is considered to have been determined quite satisfactorily, as it was based on 16 assays. The mercury content of slag was determined on the basis of 10 assays.

In addition to the mercury that accompanies residual products, some mercury will be emitted with flue gas through the chimney. Flue gas is the waste incinerator plant's airborne emission.

It should be noted that today all waste incinerator plants are equipped with acidic flue gas cleaning as has been the case since 1995. Acidic flue gas cleaning was introduced for about 86% of all incinerated waste in 1992 (Maag et al., 1996).

In Tables 3.2 and 3.3, allowance has been made for the quantities of waste incinerated with the different types of acidic flue gas cleaning. The variations that can be seen in the mercury content of slag and products of flue gas cleaning could be a consequence of the fact that mercury distributes itself differently in different plants, as this distribution is affected by many chemical and technical factors. This circumstance has not been studied in more detail.

Table 3.2 Estimated quantities of mercury in residual products from incinerator plants in Denmark during 2001 1)

The following conditions were applied when calculating quantities of residual products:

1) In 2001, about 3,014,000 tonnes of waste was incinerated, with the following distribution: about 259,000 tonnes (8.5%) in plant with a dry process, about 755,000 tonnes (25%) in plant with a semi-dry process and about 2,001,000 (66.5%) in plant with a wet process. It has not been possible to obtain information on the quantities of residual products produced in 2001 for which reason, the residual product quantity data for 2000 has been used. In 2000, 494,000 tonnes of slag, 5,500 tonnes of dry flue gas residual products, 25,000 tonnes of semi-dry flue gas residual products, 9,900 tonnes of wet flue gas residual products (sludge from flue gas cleaning) and 27,600 tonnes of fly ash (Nørby, 2003). The quantities for 2001 are expected to be basically similarly to those of 2000.

2) The slag production is shown as 494,000 tonnes in Waste Statistics 2000, which corresponds to a slag production of 16% of the incinerated waste. Based on the results of the questionnaire survey, the content of dry matter in slag was found to be about 85%.

3) Based on the results of the questionnaire survey, the content of dry matter in the composite product from the dry process was found to be 100%. In 2000, the production of the composite product from the dry process amounted to 21 kg of dry matter/tonne of waste. This value is at the low end of the interval given by Flyvbjerg & Hjelmar (1997), which shows a production of residual products from the dry process of 24-50 kg dry matter/tonne of waste.

4) Based on the results of the questionnaire survey, the content of dry matter in the composite product from the dry process was found to be about 99%. In 2000 the production of the composite product from the semi-dry process amounted to 33 kg of dry matter/tonne of waste. This agrees with the findings of Flyvbjerg & Hjelmar (1997), which give a production for the semi-dry process of 16-36 kg dry matter/tonne of waste incinerated.

5) Based on the results of the questionnaire survey, the content of dry matter in fly ash was found to be about 99.6%, which corresponds to a fly ash production of 14 kg of dry matter/tonne of waste incinerated. This agrees with the findings of Flyvbjerg & Hjelmar (1997), which give values of 10-30 kg of fly ash/tonne of waste.

6) Based on the results of the questionnaire survey, the content of dry matter in the flue gas cleaning product from the wet process was found to be about 49%, which corresponds to a production of wet flue gas cleaning products of 2.4 kg of dry matter/tonne of waste incinerated. This agrees with the findings of Flyvbjerg & Hjelmar (1997), which give an interval of 0.5-5 kg of dry matter/tonne incinerated waste.

7) An 80% confidence interval was used for setting the interval limits.

According to the latest mass flow analysis - data from this analysis is from 1992-93 - the total mercury circulation in residual products from waste incinerator plants amounted to 790-2,100 kg (Maag et al., 1996). As has been mentioned above, it has not been possible to use data from 2001, for which reason quantity data for 2000 has been used. For 2000, this quantity was determined as being 2,000-2,900 kg of mercury. Thus, there was an increase in the circulation of mercury in 2000 of 800-1,200 kg of mercury/year as compared to 1992-93. This increase is due to increases in the quantity of waste incinerated. In 1992, about 1.8m tonnes of waste was incinerated, as opposed to 2.8m tonnes in 2000, corresponding to a 50% increase. However, the increased quantity of incinerated waste cannot in itself explain why a greater quantity of mercury is retained in the residual products. The reason why more mercury is retained lies in the improved efficiency of flue gas cleaning.

Since 1995, legislation has required all waste incinerator plants to be equipped with acidic flue gas cleaning, whereas in 1992 about 14% of waste was incinerated in plants that lacked such flue gas cleaning. Moreover, relatively more waste was incinerated in plants with wet flue gas cleaning, which (cf. Table 3.3) exhibits significantly better retention of mercury in the product of flue gas cleaning than the dry and semi-dry processes. Thus, in 1992, about 33% of the waste was incinerated in plants equipped with wet flue gas cleaning, whereas in 2001 67% of waste was incinerated at plants equipped with wet flue gas cleaning.

Table 3.3 Estimated emission of mercury to air from waste incinerator plants in Denmark in 2001 1)

1) Based on the results of the questionnaire survey, there is an airborne quantity of 6,100-6,500 Nm³/tonne of incinerated waste.

2) The measurements shown apply to dry flue gas at 0°C, 1,013 mbar and 11% O₂. Information on the mercury content in flue gas was gathered from the responses to the questionnaires sent to Danish waste incinerator plant in connection with this project.

There are significant uncertainties in the mercury content of flue gas emissions, which could be due to the method selected for quantifying the mercury in flue gas. Current legislation prescribes that the emission of mercury be measured in samples taken six times a year as the average of at least two samples taken over a period of one hour (Danish EPA, 1993). However, this method of sampling can only yield samples that represent a given moment and which make no allowance for possible operating disturbances or variations in the composition of the incinerated waste.

In comparison to the 1992 situation, when about 690-2,070 kg of mercury was emitted (Maag et al., 1996), mercury emissions from waste incinerator plants have dropped. This drop in emissions, which has occurred despite a 50% increase in the quantity of waste incinerated, is partly due to the fact that today all waste incinerator plants are equipped with acidic flue gas cleaning. It is also due to the fact that today's waste is more often incinerated in plants equipped with wet flue gas cleaning, which removes mercury more efficiently than plants equipped with dry or semi-dry flue gas cleaning.

3.2.3 Sources of mercury in waste destined for incineration and deposition in landfills

Balance, waste incineration
As can be seen from Tables 3.2 and 3.3, the total content of mercury in the waste incinerated in waste incinerator plants in Denmark in 2001 is estimated at (2,000 to 2,900) + (270 to 1,000) = 2,300 to 3,900 kg/year.

For the sake of comparison, Table 3.4 shows that, based on the estimates given for the mercury contribution to combustible waste from each of the individual areas of use, it is possible to account for about 270-1,800 kg/year. This could be related to the fact that the mercury content of products is dropping sharply. This makes the estimation of the quantities disposed of or lost very sensitive to the "box-room" effect, which is taken into account in the case of mercury-containing products. The box-room effect is an expression of the fact that, when products are worn out, they are stored by consumers for some period before they are disposed of. The available information does not make it possible to determine whether the streams have been underestimated or whether the results are due to general uncertainties in the accounts.

Table 3.4 Sources of mercury in waste destined for incineration and deposition in landfills

1) Used as filling or deposited.

2) Sludge from flue gas cleaning and from refinery tanks.

3) Includes the content of mercury as an impurity in various goods. Calculated for waste quantities of 2,825,000 tonnes and 1,109,000 tonnes, respectively, which are destined for incineration or deposition in landfills (see Table 3.1), and a mercury content of 0.01-0.2 g/tonne.

4) In addition, there is a small quantity of mercury in sludge incinerated at waste incinerator plants, see Section 3.4.2.

Deposition activities
No overall information is available on the quantity of mercury in waste deposited as landfill. Based on available knowledge, Table 3.4 estimates the quantity of mercury that can be assumed to be deposited at controlled landfills in Denmark. There is no information that provides a check on this estimate. Apart from the quantities shown in Table 3.4, there is additional mercury in polluted soil which is landfilled and mercury in residual products from waste incineration, scrap management and wastewater sludge, see Table 3.11. Deposited mercury will gradually evaporate or be leached from landfills.

Leaching from landfills
The previous mass flow analysis estimated the quantity of mercury that is leached in percolate from landfills as being about 2.5 kg of mercury/year (Maag et al., 1996). No attempt has been made to find a better estimate for 2001. This percolate is sent to sewage treatment plants.

3.2.4 Treatment of biological waste

Figures for 2000 have been used here as the data for 2001 is still not available. In 2000, a total of about 0.45m tonnes of compost was produced, of which about 80% consisted of pure garden and park waste, 9% consisted of compost produced from household waste and 11%, of compost produced from wastewater sludge (Petersen, 2002). The statistics are based on reports from 133 composting plants and five biogas plants. Only biogas plants that treat organic refuse are included. The waste from plants that receive and chip garden and park waste without composting it are not included in the total quantities of compost (Petersen, 2002). Apart from compost, about 19,000 tonnes of screening residue was removed.

The average mercury content of compost based on refuse has been quantified at 0.11 mg/kg dry matter (five samples), whereas the compost from pure garden and park waste and garden/park waste mixed with sludge has been quantified at 0.08 (average of 10 samples) and 0.4 mg of mercury/kg dry matter (1 sample).

Based on the available information and with allowance for uncertainty in the data, we estimate the circulation of mercury in compost at about 29-44 kg/year, the greater part of which (more than 90%) occurs in compost produced from refuse. It is assumed that this quantity is released into the soil.

Screening residue mainly consisting of branches that do not degrade easily results from the composting process. Such screening residue has not been tested. As it primarily consists of branches, its average mercury content is probably lower than that of compost and we estimate the average mercury concentration as being roughly 0.02-0.09 mg/kg. The total circulation in screening residue is thus estimated at 0.32-1.5 kg of mercury/year.

In 2000, the quantity of liquid fertiliser produced that was obtained from treatment in biogas plants is estimated as corresponding to 0.2m m³ (i.e., the production figure for 1999), with a dry-matter content of 3.5-5% (Petersen, 2001). It has not been possible to obtain any measurements of mercury in liquid fertiliser. Assuming that mercury is primarily bound to the dry matter, and that this material has a concentration corresponding to that of the dry matter in compost, we estimate the total mercury content of liquid fertiliser at about 0.9-3.9 kg of mercury/year. This liquid fertiliser is used for agricultural purposes.

We estimate an overall circulation of mercury in residual products from biological waste treatment of 30-49 kg/year, which is released to soil.

3.3 Circulation in hazardous waste

In Denmark, collected mercury-containing waste ends at Kommunekemi, Elektromiljø (in Vejle), at the producers of dentists' amalgam filters, at the collectors of silver oxide batteries from watchmakers, at Renoflex, Nicha Miljøteknik or in more or less temporary local depots.

The latter applies, e.g., to alkaline batteries, which are deposited regionally (see the section on batteries). This also applies to mercury-containing waste collected by AVV, which covers nine municipalities in Vendsyssel. AVV stores the collected mercury-containing waste until it can find a recipient that does not recover mercury for recirculation, but that stores it so that it is removed from the global consumption of mercury (Nørregaard, 2002). In 2001, AVV collected about 20 kg of mercury (2.4 tonnes of mercury-containing waste) in thermometers, switches, light sources and as pure mercury. In 2000, AVV received about 250 kg of mercury, of which 233 kg came from individual district-heating power plants. This is stored locally.

The mercury-containing waste collected at Kommunekemi and Elektromiljø is exported to Germany or Belgium, respectively, where it is processed for reuse. However, Kommunekemi did not send mercury-containing waste to Germany in 2000 or 2001, but is temporarily storing it. Amalgam and amalgam filters from dentists are collected by the filter manufacturers and either sent to The Netherlands or Sweden for recovery. Part of the mercury-containing waste treated by Nicha Miljøteknik is received by Elektromiljø. The mercury-containing waste that is directly exported from Renoflex and Nicha Miljøteknik constitutes only small quantities and is included in Table 3.6, whereas Table 3.5 only deals with waste delivered to the largest recipient of mercury-containing waste.

Table 3.5 shows the quantities of mercury-containing waste collected by Kommunekemi, Elektromiljø, and AVV in 2001.

Table 3.5 Quantities of mercury-containing waste collected by Kommunekemi, Elektromiljø, and AVV

Table 3.6 shows the estimated quantities of mercury delivered as hazardous waste in Denmark.

Table 3.6 Estimated quantities of mercury delivered as hazardous waste in Denmark

The mercury-containing waste delivered to Kommunekemi is disposed of as follows: manganese dioxide and alkaline batteries are cast in concrete and deposited at Kommunekemi's landfill. Metallic mercury is collected and exported for processing/reuse. Filter sludge and various electrical contacts and switches, monitoring equipment, and light sources are also exported by Kommunekemi (periodically stored by Kommunekemi) to a mercury recovery plant. Other waste is exported for deposition in Germany.

Mercury-containing waste delivered to Elektromiljø is disposed of as follows: straight fluorescent tubes are processed in a closed system in Vejle, where the ends are cut off and the mercury-containing fluorescent powder is blown out. About 70% of the powder can immediately be reused by light source manufacturers, while 30% is sent to Belgium for processing together with other light sources and other mercury-containing waste. Carbon filters from plants that process straight fluorescent tubes are also sent to Belgium.

Mercury-containing waste delivered to AVV is stored by AVV in Vendsyssel, with a view to safe final deposition in the future.

Amalgam filters and mercury-containing waste collected by the manufacturers of amalgam filters are sent to Sweden and The Netherlands. Some of the filters are emptied in Denmark before their content is sent to Sweden and the filters are cleaned for reuse. There are no measurements of the emissions resulting from this emptying and cleaning. We estimate that about 1-2% of the mercury is emitted.

Kommunekemi has also informed us that the incinerator plant emitted <1.8 kg of mercury to air, 0.103 kg of mercury to water (direct discharge), 0.04 kg of mercury to wastewater, while 7.6 kg was deposited in Kommunekemi's landfill together with ash and filter cakes.

In summary, the following applies:
about 5-11 kg of mercury is emitted to air;
<1kg is discharged to water;
about 23-33 kg of mercury is deposited annually (including deposition at AVV);
about 2,000-3,900 kg is exported annually (in practice, exports do not occur every year, but the waste is temporarily stored and only exported when a suitable quantity has accumulated).

Waste oil
As there is no data for 2001, data for 2000 has been used. In Denmark, many waste oil fractions are collected for treatment. According to Waste Statistics 2000 (Danish EPA, 2001), about 19,500 tonnes of recoverable oil was collected, which consisted of engine oil, gear oil, hydraulic oil, lubricating oil, etc. Of this, about 12,500 tonnes was burned in district heating power plants and about 6,912 tonnes was recovered.

In addition, about 14,000 tonnes of other oil-based products, including the output of petrol interception traps, oil separators, oil emulsions and other oil-based products (Danish EPA, 2001a), was collected. These products generally have a high content of water, so the actual quantity of oil is considerably lower.

Based on information from the industry, we estimate the average mercury content of waste oil at 20-52 mg/tonne, from this, the total circulation of mercury in waste oil can be roughly estimated at 0.68-1.8 kg/year.

The residues from the refining of waste oil, as well as from waste containing fuel oil, cutting oil and coolants, are used in cement production and are therefore included in the mass balance for cement, see Section 2.4.3.

An estimated 4,000 tonnes of waste oil, which corresponds to <0.2 kg of mercury/year, is disposed of by Kommunekemi.

3.4 Circulation in wastewater and wastewater sludge

3.4.1 Wastewater

Mercury and wastewater will be calculated from the following point sources:

• sewage treatment plant
• storm water outflows, from overflow structures and areas with separate sewer systems for wastewater and rain water
• industries with special discharges
• rural areas

The total quantity of wastewater discharged through municipal sewage treatment plants amounted to about 720m m³ in 2001, whereas storm-water discharges diverted around sewage treatment plants amounted to about 200m m³ (Danish EPA, 2002).

The quantification of xenobiotic substances in wastewater is carried out under the National Programme for Monitoring of the Aquatic Environment (NOVA 2003), and was most recently reported in the publication Punktkilder 2001 ("Point Sources", Danish EPA, 2002).

Sewage treatment plants
Measurements of xenobiotic substances and of heavy metals are carried out at selected sewage treatment plants, which the Danish EPA considers to be reasonably typical from the viewpoint of the management and composition of wastewater in Denmark (Danish EPA, 2002).

Current data is available for 37 plants, for the period 1998 to 2001. Wastewater from these plants represented about 45% of the total quantity of wastewater. The industrial contribution in the influent to the 19 plants at which xenobiotic substances and heavy metals were measured, averaged about 35 % which corresponds reasonably well to the national average. 33 of the 37 plants for which xenobiotic substances and heavy metals were measured are designed for the removal of nitrogen and phosphorus and they generally purify the water better than is specified by the current requirements. Four of the 37 plants are small and use mechanical and mechanical-biological treatment processes.

The averages of the measured quantities, as well as the 5th and 95th percentiles of the inflow and discharge are shown in Table 3.7. It can be seen that there is significant variation in the measured values.

Table 3.7 Averages and percentiles for heavy metals in inflows to, and outflows from, sewage treatment plants, 1998-2001 (Danish EPA, 2002)

It should be noted that, although the percentiles give information on the scatter in the data, they say nothing about the precision with which the average is determined, as this depends on the number of samples taken.

Based on the concentrations shown in Table 3.7 and the quantities mentioned above, the total discharge from sewage treatment plants is roughly estimated at 14-280 kg of mercury/year, with an average value of 120 kg of mercury/year.

Similar calculations based on the average concentration of mercury in the influent, i.e., of 0.50 µg/l, show that the sewage treatment plants should receive about 360 kg of mercury. If the average values are considered, about 120 kg of mercury is discharged to recipients and it should therefore be possible to find the remainder, about 240 kg, in the sludge. When the uncertainties are taken into consideration, this quantity agrees with the calculated quantity in wastewater sludge from municipal sewage treatment plants, which is discussed later in this section. Based on a very large number of assays conducted in sludge, we estimate an annual total of about 150-230 kg of mercury in the sludge. The total quantity of mercury in sludge is assumed to have been determined with considerable certainty because of the large number of sludge samples. On average, 53% of the mercury content of wastewater is retained in sludge.

Table 3.8 Sources of mercury in municipal wastewater

1) The atmospheric deposition is calculated on the basis of a sewerage area of 1,718,000 m³, on which there is an annual mercury precipitation of about 7 mg/m³ (Kjølholt et al., 1998).

2) There may still be a considerable quantity of mercury in Danish sewer systems, from which it is only released slowly by continuous erosion, or quickly in connection with flushing of the sewers. Thus, an examination of the sources of wastewater sludge with a high mercury content reveals that accumulated mercury in sewers, which results from historical discharges from district heating power plant, is often the cause of the high concentrations of mercury (Markmann et al., 2001).

For the sake of comparison, there is an estimated 360 kg of mercury/year, calculated on the basis of the concentrations in the influent to sewage treatment plants. In other words, the results agree quite well. Part of the explanation of the difference could be the slow release of mercury accumulated in sewers from earlier discharges, see also Table 3.8, Note 2. The figures could also indicate that the current discharges from dental clinics are in truth at the high end of the interval studied.

Storm water outflows
Storm water outflows can be subdivided into separate discharges of surface water and overflow from areas with combined sewer systems for wastewater and rain water (especially old urban areas), which consist of a mixture of surface water and wastewater. Discharges from areas with combined sewer systems for wastewater and rain water contain not only surface water but are a mixture of municipal wastewater, re-suspended sewer sediment and biofilm, as well as surface run-off. Effluent from areas with separate sewer systems for wastewater and rain water chiefly contains surface water from built-up areas (roofs, roads, etc.,) and re-suspended material from sewer pipes. The actual process of quantifying the volume and content of a storm water outflow demands advanced test equipment. As such outflows only occur as a result of heavy precipitation, it can be very difficult to plan quantification programmes. The volumes shown below are based on model calculations; considerable uncertainty is associated with the calculation of storm water outflows, which becomes apparent when these calculations are compared to actual measurements (Danish EPA, 1997).

In 2001, 149m m³ of water was discharged in separate storm water outflows and 36m m³, through overflow structures, both from areas with combined and separate sewer systems for wastewater and rain water (Danish EPA, 2002). A large study of Danish and international discharges through overflow structures has ascertained that the information available on the content of xenobiotic substances in sediment and biofilm is particularly limited (Arnbjerg-Nielsen et al., 2002). According to the study, there is reason for assuming that the heavy metal content of biofilm and sediment in sewer systems from catchment areas with combined sewer systems for wastewater and rain water is comparable to - and in some cases higher than - the corresponding quantity in the particles from wastewater and urban surfaces. The estimated concentration interval for mercury in overflow water is given as 0.05-0.2 µg/l (Arnbjerg-Nielsen et al., 2002). This interval appears reasonable in relation to the concentration found in inflows to sewage treatment plants, i.e., an average of 0.5 µg of mercury/l. Assuming that this interval applies to the total quantities from storm water outflows and overflow structures, the total discharge in conjunction with rain events in 2001 is estimated at 10-41 kg of mercury.

However, it should be noted that some mercury is retained in basins and removed in conjunction with the cleaning of the basins. There is a clear tendency towards more areas being drained through discharges over basins, both in areas with combined and separate sewer systems for wastewater and rain water. Thus, within the areas with combined sewer systems for wastewater and rain water, there was a 58% increase in the built-up area from which water is discharged via basins, while there has been a 19% drop in the areas from which discharges occur without basins (Danish EPA, 2001b). No studies are available of the magnitudes of the quantities of heavy metals retained in the basins.

Industries with direct discharge
Sampling for heavy metals was only carried out at seven selected firms in 2001, and an account of the quantities discharged is, thus, not applicable to Denmark as a whole (Danish EPA, 2002). About 10% of all the wastewater samples taken for mercury showed concentrations of more than 10 times the stated quality requirements on aquatic areas. In these cases, it is to be expected that the requirements have generally not been satisfied, and that the concentrations must be considered critical (Danish EPA, 2002).

Information was collected for the year 2000 from 162 firms, 82 of which discharge heavy metals and/or xenobiotic substances (Danish EPA, 2001b). In addition, there is information on 14 firms, which are assumed to discharge heavy metals and/or xenobiotic substances and which are not included in the account (Danish EPA, 2001b). The total discharge for 2000 is estimated at 1.9 kg of mercury, and a maximum of 3.3 kg of mercury (Danish EPA, 2001b).

In 2000, about 74m m³ of wastewater was discharged by special industrial dischargers, which - in comparison to the quantities mentioned above - corresponds to a mercury content in wastewater in excess of 26 µg/m³. In 2001, about 65m m³ was discharged by special industrial dischargers and the total discharge for 2001 is, thus, estimated at 0.42-2.9 kg of mercury.

Rural areas
Properties in rural areas, villages and summer cottage areas are often not connected to combined sewer systems for wastewater and rain water but can have water treatment facilities, such as septic tanks, which release to soil or field drainage systems. Wastewater drainage from all municipalities is used in estimating water consumption and the total discharge of mercury (Danish EPA, 2002). No information on the quantities of mercury discharged from rural areas is available for 2001. The figures must be expected to be similar to 2000 figures, when the discharge was estimated at 25 kg of mercury/year. Because of uncertainties, this study estimates the total discharge in 2001 13-38 kg of mercury.

The total discharges of mercury from point sources to the aquatic environment are summarised in Table 3.9.

Table 3.9 Discharges of mercury from point sources to the aquatic environment in 2001 - based on information for 2000 and 2001 (Danish EPA, 2001b) and (Danish EPA, 2002).

3.4.2 Wastewater sludge

Sludge must be disposed of after the completion of processing by waste treatment plants. Until 1995-96, there were in principle three main options for the final disposal of wastewater sludge, i.e., application to agricultural soil, incineration in external or internal plants and deposition at controlled landfills.

Developments in wastewater sludge disposal have meant that, during the last few years, other options for the disposal of this sludge have been used which do not fit in with the above main options.

The number of sludge mineralisation plants has increased noticeably over the past few years. Municipalities consider sludge mineralisation to be an alternative method of sludge disposal. Mineralisation plants are expected to be able to store sludge for up to 10 years, before it is necessary to decide whether it should be applied to agricultural soil, incinerated, or deposited at controlled landfills (Danish EPA, 2001c).

During the course of 1997-98, a number of private firms have also worked to establish alternative sludge-disposal methods in which the inorganic fraction (the ash) is reused in such products as cement and sand-blasting agents.

Samples representative of about 94.5% of the dry matter content of sludge were studied in 1999, with a view to determining their content of mercury and other heavy metals (Danish EPA, 2001c). The weighted average for mercury was about 1,200 mg/tonne and, on the assumption that the studied sludge was representative of the total quantity of collected sludge, the total quantity of mercury in sludge can be estimated at 150-230 kg/year. The distribution of this over the various types of disposal method is shown in Table 3.10.

In comparison to 1994, when the circulation of mercury in sludge was estimated at about 250 kg in 170,000 tonnes of dry matter, it can be seen that there is a possible trend towards a drop in the total circulation of mercury.

Agricultural soil, forests, and market gardens received a total of 62-94 kg of mercury in sludge. A small part of this sludge was treated in biogas plants or composting plants before its application to soil.

Part of wastewater sludge is disposed of by incineration - either externally or internally, at the individual sewage treatment plant. The incinerator plant is fully equipped with flue gas cleaning equipment. The emission factor for mercury at Danish incinerator plants is roughly estimated at 50-70%. Against this background, the total emission resulting from the incineration of sludge can be estimated at 25-50 kg of mercury/year, whereas any other mercury content of the sludge is deposited together with the residual products.

Table 3.10

Mercury in wastewater sludge, distributed by final disposal (based on Danish EPA, 2001c)

Notes:

1) The average mercury content of sludge applied to agricultural soil or in forests, etc., is stated to be 910 mg/tonne of dry matter. This average value is considered to have been determined with considerable certainty because of the large number of samples taken.

2) The average mercury content of sludge that is not applied to agricultural soil can be estimated at 1,530 tonnes/year on the basis of Danish EPA 2001c. This value is considered to have been determined with relatively high certainty. No specific values are quoted for sludge deposited in landfills, incinerated, etc. We assume that the average mercury concentration has remained roughly the same for each of these types of disposal.

3) Of this, about 21.5-46 kg of mercury/year is emitted to air.

3.5 Summary of mercury losses in waste treatment

The available information on the loss of mercury in connection with the circulation of waste products is summarised in Table 3.11.

Table 3.11 Losses of mercury in the management of waste products in Denmark, 2001 (kg/yr of mercury)

1) Percolate from landfills is piped to municipal waste treatment plants.

2) Iron and steel for recovery.

3) Mercury-containing waste, which is exported.

4) Wastewater sludge, which is not exported but is temporarily stored.

5) By far the greater part of this is exported for deposition in Norway and Germany.

6) See Table 3.4

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Version 1.0 June 2004, © Danish Environmental Protection Agency