In 2014, 25% of the sludge produced at Swedish municipal wastewater treatment plants was applied to agricultural land. Even though the Swedish heavy metal limits for sludge to be used in agriculture are amongst the most stringent in the EU, more stringent heavy metal limits are proposed. Most sludge applied to agricultural land is recycled within a certification system, Revaq. Revaq has targets for control at source management and improvement of sludge quality. Statistics based on data collected within the Revaq system was used to differentiate between local and general sources of heavy metals and assess the need to improve sludge quality. The analysis indicates that proposed future national limits on the quality of the sludge can be met by most of the sludge. The improvement needed for about 20% of the sludge is feasible through local control at source management. The levels of cadmium, copper and mercury need to be reduced if these metals are not to limit the amount of sludge that may be applied per unit area of arable land. Finally, the long term Revaq targets for cadmium and silver will be difficult to meet.

INTRODUCTION

Water and wastewater services must be managed to fulfil environmental goals set by the Swedish Parliament and the Government. The overall goal of Swedish environmental policy is to pass on to the next generation a society in which the major environmental problems in Sweden have been solved, without increasing environmental and health problems outside Sweden's borders. There are 16 Swedish environmental quality objectives, decreed by the Swedish Parliament. One of these objectives specifies targets on recycling phosphorus and nitrogen from wastewater to agricultural land. The use of sludge in agriculture has been debated in Sweden in recent decades, mainly due to concerns about the heavy metal content of the sludge. However, four long term field trials in Sweden involving sludge application during periods spanning 10–40 years show very low uptake of heavy metals in crops (Börjesson et al. 2014). The quality of the sludge applied in the field tests has improved over time, due to the improved quality of sludge in general in Sweden. The current low level of heavy metals in Swedish sludge is due to active local control at source management as well as national and international trends and legislation (Balmér & Davidsson 2009; Mattsson et al. 2012; Cantinho et al. 2016). There is a general decrease of the stocks and flows of heavy metals in the urban environment, as described by Sörme et al. (2003). Control at source management, or upstream management as it is known in Sweden, is where the operator of the wastewater treatment plant (WWTP), by applying different measures, targets pollutants in industrial wastewater or other local sources to the WWTP. According to the European Union's hierarchy of waste and recycling management, re-use of sludge is preferred to recycling only phosphorous, or only using the energy value in the sludge (i.e. incineration without extraction of phosphorus) (EC 2009a).

In order to allow recycling of sludge to agriculture without causing environmental damage due to contaminants in the sludge, thorough investigations have been performed by the Swedish EPA. This has resulted in proposed new limits in sludge for eight heavy metals (Table 1 and Table 2). A specified concentration limit in relation to dry solids (DS) (mg Me+/kg DS) or a specified ratio to phosphorus (mg Me+/kg P) for each metal must be met. This means that improved dewatering is not penalized by the regulator due to higher concentrations of all particle bound substances, including heavy metals and phosphorus. It is also a way of considering the fertilizer value of the sludge and making it easier to compare the quality of sludge with other sources of phosphorus. Thus sludges with either a lower concentration than the concentration limit, or lower ratios than the ratio limit, may be permitted for use in agriculture in the future. All proposed limits will be incrementally implemented and are increasingly stringent in 2015, 2023 and 2030 (although 2015 has already passed without the legislation being finalized). The amount of sludge that may be applied is further regulated in terms of mass of heavy metals and phosphorus that may be applied per hectare per year. The amount of sludge (and thus phosphorus) applied may be reduced in order to meet the heavy metal limits. The proposed legislation (see Table 2) will result in the following, rather complex, decision steps:

  • Is the quality of the sludge good enough? Are either the heavy metal concentration (mg Me+/kg DS) or the ratio to phosphorus (mg Me+/kg P) below the set limit for all the regulated heavy metals?

  • How much sludge can be applied per hectare per year in order to avoid spreading more than the maximum allowable 22 kg P/ha yr. and limit the risk of eutrophication in local water bodies?

  • How much sludge can thus be applied per hectare per year without exceeding the maximum allowable heavy metal loading? Calculate how much the application per hectare per year needs to be further reduced in order to avoid exceeding the maximum application rates for any of the regulated heavy metals.

This procedure also applies to the present Swedish legislation, although without the alternative quality regulation of the ratio of heavy metals to phosphorus. The limits of the present EU legislation (EEC 1986) on heavy metals are in general much higher than the present Swedish national limits (EC 2009b). National regulations of different countries can be grouped in relation to the European Directive 86/278/EEC:

  • Much more stringent: Denmark, Finland, Sweden, The Netherlands.

  • More stringent: Austria, Belgium, France, Germany, Poland.

  • Similar: Greece, Ireland, Italy, Luxembourg, Portugal, Spain, United Kingdom.

For copper, nickel and zinc the suggested EU limits are equal to, or slightly more stringent than the present Swedish limits (Table 1). Swedish sludge contains low levels of most heavy metals in a European context (EC 2009b; Cantinho et al. 2016). The exception is copper, where the levels in Swedish sludge are amongst the highest.
Table 1

Criteria for agricultural use of sludge in EU, Denmark, France and Sweden

  Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper 
Present limits, mg/kg DS 
 EU Directive 86/278/EEC a na 20–40 2,500–4,000 750–1,200 16–25 300–400 na 1,000–1,750 
 Denmark na 0.8 4,000 120 0.8 30 100 1,000 
 France na 20 3,000 800 10 200 1,000 1,000 
 Sweden na 800 100 2.5 50 100 600 
Proposed limits, mg/kg DS 
 EU leg.(opt. 2)b na 10 2,500 750 10 300 1,000 1,000 
 EU leg.(opt. 3)b na 600 250 50 150 400 
 Sweden: proposed legislation 2030 0.8 700 25 0.6 30 35 475 
  Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper 
Present limits, mg/kg DS 
 EU Directive 86/278/EEC a na 20–40 2,500–4,000 750–1,200 16–25 300–400 na 1,000–1,750 
 Denmark na 0.8 4,000 120 0.8 30 100 1,000 
 France na 20 3,000 800 10 200 1,000 1,000 
 Sweden na 800 100 2.5 50 100 600 
Proposed limits, mg/kg DS 
 EU leg.(opt. 2)b na 10 2,500 750 10 300 1,000 1,000 
 EU leg.(opt. 3)b na 600 250 50 150 400 
 Sweden: proposed legislation 2030 0.8 700 25 0.6 30 35 475 

For EU also option 2 and 3 (EC 2009b) and Swedish proposed limits for 2030.

Table 2

Criteria for use of sludge in agriculture in Sweden at present and in year 2030

  Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper Phosphorus 
Concentration mg/kg DS 
 Present legislation na 800 100 2.5 50 100 600 na 
 Proposed legislation 2030 0.8 700 25 0.6 30 35 475 na 
Ratio metal/phosphorus, mg/kg P 
 Proposed legislation 2030 100 30 25,000 900 20 1,000 1,200 17,000 na 
Application on agricultural land, g/ha yr 
 Present legislation na 0.8 600 25 1.5 25 40 300 22,000 
 Proposed legislation 2030 2.5 0.35 550 20 0.3 25 35 250 22,000 
 Revaqs target 2025 0.56 0.37 na 25 0.23 na na na na 
  Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper Phosphorus 
Concentration mg/kg DS 
 Present legislation na 800 100 2.5 50 100 600 na 
 Proposed legislation 2030 0.8 700 25 0.6 30 35 475 na 
Ratio metal/phosphorus, mg/kg P 
 Proposed legislation 2030 100 30 25,000 900 20 1,000 1,200 17,000 na 
Application on agricultural land, g/ha yr 
 Present legislation na 0.8 600 25 1.5 25 40 300 22,000 
 Proposed legislation 2030 2.5 0.35 550 20 0.3 25 35 250 22,000 
 Revaqs target 2025 0.56 0.37 na 25 0.23 na na na na 
Table 3

Concentration of heavy metals and phosphorus in sludge from Revaq certified WWTPs in 2014, mg/kg DS

  Phosphorus Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper 
Average 27,200 1.6 0.7 498 16 0.5 15 30 314 
Weighted average 30,200 2.1 0.7 566 20 0.4 17 30 375 
50 percentile 29,500 1.8 0.7 590 21 0.4 18 30 388 
90 percentile 36,000 3.0 0.9 653 27 0.7 20 41 455 
Maximum 37,500 3.8 1.1 682 32 1.1 27 65 511 
Present Swedish limit na na 800 100 2.5 50 100 600 
  Phosphorus Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper 
Average 27,200 1.6 0.7 498 16 0.5 15 30 314 
Weighted average 30,200 2.1 0.7 566 20 0.4 17 30 375 
50 percentile 29,500 1.8 0.7 590 21 0.4 18 30 388 
90 percentile 36,000 3.0 0.9 653 27 0.7 20 41 455 
Maximum 37,500 3.8 1.1 682 32 1.1 27 65 511 
Present Swedish limit na na 800 100 2.5 50 100 600 

In 2008 a certification system for applying sludge to agriculture, called Revaq, was established in Sweden (IEA Bioenergy Task 37 2015). Revaq is administered by the Swedish Water & Wastewater Association, the Federation of Swedish Farmers (LRF), the Swedish Food Federation and Swedish Food Retailers Federation, in close cooperation with the Swedish Environmental Protection Agency. The system is regularly audited by a third party certification body. The number of WWTPs within the system has doubled from 20 in 2009 to 41 in 2014. Currently, about 50% of the Swedish population are connected to a municipal WWTP certified according to Revaq. The Revaq plants supply the majority of the sludge used in agriculture in Sweden (Figure 1).
Figure 1

Production and agricultural use of municipal WWTP sludge in Sweden. Expressed as tons DS per year (yr).

Figure 1

Production and agricultural use of municipal WWTP sludge in Sweden. Expressed as tons DS per year (yr).

The purpose of Revaq is to ensure a systematic control at source management for the WWTP so that the production of sludge for use in agriculture maintains a high quality, and that the sludge quality improves over time. The Revaq certified WWTPs follow a number of clearly defined rigorous rules concerning sludge production and traceability. The WWTPs must have source control systems targeting wastewater from industrial and other sources which may contain more contaminants. They must also have plans for reducing point sources as well as diffuse sources of pollutants in the collection system. The long term target of Revaq is to reduce harmful substances in the sludge to a level at which the continued use of sludge in agriculture does not cause accumulation of these substances in the agricultural soil. The definition of ‘no increase’ is that sludge can be used at the maximum application rate of phosphorus (22 kg P/ha yr) without the concentration of the contaminants doubling in 500 years (assuming no removal via crop or ground water). This corresponds to a theoretical increase of the concentration in the soil by 0.2% per year. This level of sludge quality is to be reached by 2025.

One aim of this paper is to compare the current Swedish sewage sludge quality with proposed Swedish legislation as well as the Revaq targets. A further aim is to apply simple statistical methods to separate the impact of local sources from the general or national levels in heavy metal data. This can indicate how much improvement can be achieved by local control at source management, and how much the general level needs to decrease due to national measures. Finally, the information is used to estimate which levels of heavy metal content are reasonable to reach within the timeframe set by the proposed limits, and targets applying control at source management focused on local sources.

METHODS

Annual data on heavy metals, phosphorus and DS content of sludge were collected from the 41 WWTPs within the Revaq certification system for the year 2014 (for silver 2012–2015). These data are collected in the Swedish Water and Wastewater Association database (VASS). The metal content of the sludge from the WWTPs is evaluated statistically and compared with Revaqs target values and the various limits of the proposed national legislation.

For each WWTP, all the sludge is assumed to have the same heavy metal concentration throughout 2014. Thus the median concentration or ratio actually means that about half of the produced sludge comes from WWTPs where the annual average was lower than the stated concentration or ratio. In reality of course there is a variation within the sludge produced during a year at each of the WWTPs. A large difference between the 90 percentile and 50 percentile may be used in order to illustrate a significant variation indicating local sources at WWTPs at the high end. Where the term average is used, it refers to the numerical average from all the WWTPs giving each WWTP the same weight, even though the amount of sludge varies. When the weighted average is used, the actual sludge production from each WWTP is taken into account. A difference between these values could indicate that sludge from large WWTPs systematically has higher or lower concentrations than the small WWTPs.

In order to differentiate between local and general sources of heavy metals, cumulative probability of exceedance graphs are used. In these graphs, the values on the y-axis are organized from left to right in ascending order. The x-axis scale here is either equal for each WWTP or in proportion to the amount of sludge produced at the WWTP.

RESULTS AND DISCUSSION

All the sludge from the 41 WWTPs within Revaq met the present Swedish limits (and thus EU limits) on heavy metal concentration on an annual basis (Table 3). Due to variation during the year, individual batches of sludge may have exceeded the limits and were not applied to agricultural land. Only the general quality of the sludge will be discussed here.

The proposed future concentration limits are more stringent; however, the limit definition also, as mentioned above, considers the fertilizer value of the sludge by including the metal to phosphorus ratio. When compared with the limits for 2030, nearly all the sludge from the certified WWTPs in 2014 met either the concentration or the ratio limits with respect to zinc, nickel and copper (Table 4). For the remaining heavy metals, up to around 20% of the sludge exceeds the proposed 2030 limits. If the sludge meets either the concentration limit or the ratio limit, the sludge may be used as a fertilizer in agriculture. As mentioned earlier, the purpose of this construction is to avoid disqualifying technology that gives higher concentrations due to improved dewatering or digestion of sludge. The next step is to determine the amount of sludge that can be applied. The proposed legislation contains limits of application per unit area of arable land for phosphorus as well as for heavy metals (Table 2). If sludge is to be an attractive fertilizer, however, it is preferable that the application rate is limited by the amount of phosphorus applied rather than the heavy metal. The limits set for silver, zinc, nickel and chromium will seldom limit application, however lead and mercury will limit application rates for a substantial fraction of the sludge, and cadmium and copper for more than 80% of the sludge. The amount of sludge, and thus phosphorus, applied can be reduced in order to meet the application limits of heavy metals. If an application rate of 15 kg P/ha yr is assumed to be the minimum practical rate for the farmers instead of the maximum permitted application rate of 22 kg P/ha yr, then 37% of the sludge will not be practical to use due to its cadmium content; 20% due to its mercury content, and 5% due to its copper content. In practice, even less sludge (and thus phosphorus) could be applied per hectare in order to reduce the metal application rate, but this is not assumed to be economically attractive for the farmer. Thus cadmium, mercury and copper need to be further reduced before 2030 in Swedish sludge if these metals are not to reduce the value of sludge as a source of phosphorus to agriculture.

Table 4

Proportion of sludge produced by WWTPs within the Revaq system in 2014 that will meet future limits and targets, and where the metal content does not limit sludge application in agriculture

  Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper 
Percentage of sludge that meets proposed 2030 legislation 
 Concentration limit (mg/kg DS) 76 86 100 70 80 100 78 90 
 Ratio metal to phosphorus (mg/kg P) 84 86 100 72 79 99 80 95 
 Either concentration or ratio or both 84 87 100 76 82 100 86 99.9 
Percentage of sludge 
 …where metal will not limit application rate in 2030a 100 15 99.6 72 58 100 97 19 
 …will meet 2025 Revaq target 15 100 100 37 na na na 
  Silver Cadmium Zinc Lead Mercury Nickel Chromium Copper 
Percentage of sludge that meets proposed 2030 legislation 
 Concentration limit (mg/kg DS) 76 86 100 70 80 100 78 90 
 Ratio metal to phosphorus (mg/kg P) 84 86 100 72 79 99 80 95 
 Either concentration or ratio or both 84 87 100 76 82 100 86 99.9 
Percentage of sludge 
 …where metal will not limit application rate in 2030a 100 15 99.6 72 58 100 97 19 
 …will meet 2025 Revaq target 15 100 100 37 na na na 

Decimals are only used for values between 99 and 100%.

aThe maximum phosphorus application rate of 22 kg/ha.year can be used without exceeding the maximum application rate of the metal.

The Revaq rules also have an application rate limit for non-essential elements, including the legislated heavy metals. Note that the Revaq target for application rate differs from the proposed national limit (Table 2). This is a long term target to be reached by 2025, and the application rate of 22 kg P/ha yr must be used for the calculation. This is in contrast to the proposed national regulation, which will allow reducing the application rate of phosphorus in order to meet the maximum application rates of metals, as discussed above. Thus, with the observed sludge quality from 2014, all the sludge from the WWTPs within the Revaq system would be disqualified by 2025 due to the content of silver, whilst most of it would be disqualified due to cadmium and mercury concentrations (Table 4).

Thus, comparing the sludge quality of 2014 with future legislation and Revaq targets, the following conclusions can be drawn. Firstly, silver, lead and mercury will each need to be further reduced for around 20% of the sludge in order to meet the legislation limits, and chromium and cadmium for 10–15% of the sludge. Secondly, for most of the sludge, cadmium and/or copper will need to be reduced in order to be able to apply the maximum amount of phosphorus allowed, and for a substantial part of the sludge, lead and mercury need further reduction. Thirdly, all of the sludge will need reduced amounts of silver, and most of the sludge will need reduced amounts of cadmium and lead if the targets of Revaq are to be met. This leaves us with all of the heavy metals with the exception of nickel and zinc needing further reduction. An important question when addressing these challenges is whether heavy metals in the sludge come from local sources that can be reduced through local control at source management from the WWTP. However, if the heavy metal levels reflect the general level in society, coming from food, construction material, atmospheric downfall or other diffuse and general sources, a substantial reduction of the metal input will require changes to national legislation or changed use of materials in society.

One way of differentiating between local and general sources is through cumulative probability of exceedance graphs (Figure 2). The general level of silver can be assumed to be represented by the plateau in the graph at an application of about 1–1.5 g/ha yr when applying the maximum phosphorus load of 22 kg/ha yr (Figure 2(a)). The steeper slope on the right hand side of the diagram can be assumed to be due to local sources. Thus, with reduction of local sources, it should be possible to reduce the silver content to a level where reduction of the phosphorus application due to silver is avoided. However, the Revaq target will not be reached without a substantial reduction of the general level as well as local sources. For zinc (Figure 2(c)), lead (Figure 2(d)), nickel (Figure 2(f)) and chromium (Figure 2(g)), the future limits can be reached with little or no reduction of local sources. Assuming the general level of cadmium (Figure 2(b)) to correspond to an application rate of around 0.4–0.6 g/ha yr, a substantial reduction of the general level will be needed if cadmium is not to limit application in 2023 and 2030, and the Revaq target is to be met. For mercury (Figure 2(e)), more than 50% of the sludge is at or below the future limits and Revaq targets, whereas the remainder exceeds these limits by a varying amount. The steeper increase within the top 40% of the sludge indicates the presence of local sources that may be reduced through local control at source management. Finally, for copper (Figure 2(h)) the reduction of local sources in combination with a modest reduction of the general level should be enough to avoid copper limiting application in 2030.
Figure 2

Amount of heavy metal applied in sludge when applying 22 kg P/ha yr. Horizontal lines indicate present limits and future proposed Swedish limits. Dashed horizontal lines represent quality target of Revaq for year 2025.

Figure 2

Amount of heavy metal applied in sludge when applying 22 kg P/ha yr. Horizontal lines indicate present limits and future proposed Swedish limits. Dashed horizontal lines represent quality target of Revaq for year 2025.

Thus the levels of zinc, lead, mercury, nickel and chromium either already meet future limits and targets, or may reach these levels for most of the sludge through successful control at source management or a modest reduction in the general levels. This is not the case for silver and cadmium, where substantial reduction for most of the sludge will be necessary in order to reach the Revaq targets (Figure 2(a) and 2(b)). It is of interest to assess whether the necessary reductions are likely to be achieved. One way of tackling this question is by analysing the historic change with time. However, data regarding the amount of DS generated over time were not available for all of the WWTPs. Therefore, a comparison of how many of the WWTPs would exceed a certain application rate is used in Figure 3(b), instead of the previously used base of the amount of sludge, to illustrate the change with time. In Figure 3(a), the data from 2014 are organized both in relation to the amount of DS and to the number of WWTPs, indicating that both bases can be used to differentiate between local and general sources. A reduction of the general level with time is indicated (Figure 3(b)). Although the total reduction between 2012 and 2015 may seem substantial, the reduction is not evenly distributed between the years and the annual reduction would appear to decrease with time. Although the reduction during the previous three years is substantial, it is not sufficiently large to guarantee meeting the Revaq target by 2025, especially if it is assumed that the remaining potential for improvement decreases over time. Similar comparisons for cadmium indicate an even lower reduction rate.
Figure 3

Cumulative probability of exceedance graphs for application rate of silver if applying 22 kg P/ha yr with sludge.

Figure 3

Cumulative probability of exceedance graphs for application rate of silver if applying 22 kg P/ha yr with sludge.

CONCLUSIONS

Cumulative probability of exceedance graphs and percentiles are useful in order to estimate general levels as well as for pinpointing the presence of significant local sources of metals.

According to the proposed legislation for 2030, the heavy metal content in the sludge (2014) from the Revaq WWTPs was low enough to allow use in agriculture. The heavy metal content of most of the sludge either meets the proposed concentration limit or the ratio of heavy metals to phosphorus. For cadmium, chromium and mercury, some WWTPs need further reduction in order to comply with suggested national 2030 limits. This reduction may be feasible through local control at source management.

The Swedish national application limits proposed for 2030 imply a necessary improvement of sludge quality with respect to cadmium, mercury and copper if these metals are not to limit the amount of phosphorus that may be applied per unit area of arable land with sludge in 2030. For mercury, and perhaps copper, the reduction can be achieved through successful local control at source management. For cadmium, a reduction of the general level is needed.

The long Revaq targets for silver, cadmium and mercury demand substantial improvements before 2025. For mercury, the removal of local sources may be sufficient. Revaqs targets for cadmium and silver will be difficult to reach without reductions of the general levels.

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