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Review

Legislation for the Reuse of Biosolids on Agricultural Land in Europe: Overview

1
Department of Civil and Architectural Engineering, University of Pavia, via Ferrata 1, 27100 Pavia, Italy
2
Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, via Branze 43, 25123 Brescia, Italy
3
ARPA Lombardia, Pavia Department, via Nino Bixio 13, 27100 Pavia, Italy
4
Department of Chemistry, Laboratory of Chemical and Environmental Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
5
Department of Theoretical and Applied Sciences, University of Insubria, via G.B. Vico 46, 21100 Varese, Italy
*
Author to whom correspondence should be addressed.
Sustainability 2019, 11(21), 6015; https://doi.org/10.3390/su11216015
Received: 17 September 2019 / Revised: 22 October 2019 / Accepted: 25 October 2019 / Published: 29 October 2019

Abstract

The issues concerning the management of sewage sludge produced in wastewater treatment plants are becoming more important in Europe due to: (i) the modification of sludge quality (biological and chemical sludge are often mixed with negative impacts on sludge management, especially for land application); (ii) the evolution of legislation (landfill disposal is banned in many European countries); and (iii) the technologies for energy and material recovery from sludge not being fully applied in all European Member States. Furthermore, Directive 2018/851/EC introduced the waste hierarchy that involved a new strategy with the prevention in waste production and the minimization of landfill disposal. In this context, biological sewage sludge can be treated in order to produce more stabilized residues: the biosolids. In some European countries, the reuse of biosolids as soil improver/fertilizer in arable crops represents the most used option. In order to control the quality of biosolids used for land application, every Member State has issued a national regulation based on the European directive. The aim of this work is to compare the different approaches provided by European Member States for the reuse of biosolids in agricultural soils. A focus on the regulation of countries that reuse significant amount of biosolids for land application was performed. Finally, a detailed study on Italian legislation both at national and regional levels is reported.
Keywords: biosolids; agricultural reuse; European legislation; sludge management; Italian legislation; policy choice biosolids; agricultural reuse; European legislation; sludge management; Italian legislation; policy choice

1. Introduction

During the last 20 years in EU-15, the sewage sludge produced by urban wastewater treatment plants (WWTPs) has increased from 6.5 million tDM (dry matter) up to 9.5 million tDM [1]; in EU-28, more than 10 million of tDM of sewage sludge have been produced [2]. The contemporary presence of a low wastewater quality [3,4,5] and more stringent requirements for WWTPs effluents quality [6,7] increases the production, and worsens the quality, of the sewage sludge [8]. Therefore, the issues related to sewage sludge management are increasing. The introduction of waste hierarchy, with Directive 2018/851/EC [9], and the opposition of citizens (who perceived sewage sludge as dangerous for human health and for the environment) [10] forced the technicians to re-think completely the sewage sludge management strategy: landfilling was banned, land application was limited, and other routes of recovery are underdeveloped [11,12,13].
Directive 2018/851/EC [9] provided a waste hierarchy that shall apply as a priority order in waste prevention and management legislation and policy: prevention (e.g., minimization techniques), preparing for reuse (e.g., chemical or biological stabilization), recycling (e.g., matter recovery), other recovery (e.g., energy recovery), and disposal (e.g., landfilling). Usually, the terms “sewage sludge” and “biosolids” are used interchangeably, but “sludge” refers to a liquid produced by WWTPs that is not submitted to further treatments, while “biosolids” indicates a sludge that had received one or more treatments [14,15,16], which can be: aerobic or anaerobic digestion, thermal drying, alkaline stabilization, composting, acid oxidation/disinfection, etc. [16]. This work focuses on the matter recovery, in particular on the reuse of biosolids in agricultural soils.
The biosolids application on agricultural land can represent an interesting strategy to improve crops productivity by increasing soil organic matter (SOM) content, fertility, and nutrient presence; moreover, biosolids can also improve soil physical properties, especially in cases of heavy textured and poorly structured soils [17,18,19,20]. Furthermore, the spreading of biosolids on agricultural land reduces the effect of organic matter loss in the soil, especially in southern Europe, where the depletion of SOM is one of the most serious processes of soil degradation [21,22].
In addition, developing a sustainable and integrated circular system to reuse biosolids in land application can be entirely inserted in the concept of circular economy [23,24,25]. In recent years, the European Commission adopted an ambitious Circular Economy Package to promote the reuse, recycling, and recovery of wastes [26]. The reuse of biosolids on land application enhanced the recovery of resources (e.g., nutrients [27,28]) and, therefore allows changing the classic view of WWTP in a more sustainable water resources recovery facility (WRRF) [16,29,30,31].
The main problems related with the reuse of biosolids concern the presence of heavy metals, organic contaminants, and/or pathogens in the sludge [32,33]. In the scientific literature, no agreement can be found about the adverse effects caused by the land application of biosolids. According to the literature [34,35,36], the following aspects can be reported: (i) raising of the levels of persistent toxins in soil, vegetation, and wild life, (ii) potentially slow and long-termed biodiversity reduction through the fertilizing nutrient pollution operating on the vegetation, (iii) greenhouse gas emissions (e.g., CH4 and N2O), and (iv) the release of odorous compounds.
In some European countries, the reuse of biosolids as soil improver/fertilizer in arable crops represents the most used disposal option [1]. This trend led to restricting the use of biosolids based on quality—Directive 86/278/EEC [37] (and subsequent amendments [38,39]) introduced limit values in order to protect human health by applying biosolids with good qualities. Based on the European directive, every Member State has issued a national regulation, which, in some cases, provided more stringent limit values and introduce more restrictions, such as limit values for pathogen and organic micropollutants.
This work reports the management options in every Member State; moreover, the regulations of European countries for the reuse of biosolids in agricultural land are compared considering the limit values in the biosolids and in the soils. Furthermore, this work focuses on the regulations provided in the Member States that spread on agricultural soil a significant amount of biosolids (more than 300,000 tDM year−1 or more than 70% with respect the sludge produced). Moreover, the comparison between the different requirements in the European countries, based on political aspects, are carried out in order to investigate the choices and the prospective on the reuse of biosolids in agriculture. Finally, a focus on the Italian situation, with a comparison of national and regional legislation, is reported.
It is important to note that the land application of biosolids includes both agricultural use and compost production, but not the reuse for silviculture, forest, land reclamation, and green areas.

2. Sewage Sludge Production and Management Options: European Situation

In EU-28, the urban sewage sludge production is more than 10 million tDM [2]. The amounts are very different in the European countries, due to the percentage of the resident population connected to WWTPs and the technologies used. Germany, the UK, Spain, France, and Italy account more than 55–65% of the total amount produced in EU-28 with a decrease in the last years [1,2] due to the implementation of procedures for evaluating the performance of the WWTPs (as already suggested for drinking water treatment plants [40,41]) and for minimizing sewage sludge production with integrated technologies, both in wastewater and sludge-handling units [8,42,43,44,45,46,47]. On other side, there are countries with a low sewage sludge production due to small population (e.g., Malta, Latvia, Estonia, and Luxembourg) or due to the low percentage of resident population connected to WWTPs. For instance, in 2017 in Bulgaria, almost 13% of population was not served by any treatment plant [48].
Nowadays, in EU-28, land application is the main route for sewage sludge recovery: 50% of sewage sludge is spread on agriculture soils, 28% is incinerated, and 18% are still disposed in landfills [2]. The remaining fraction is disposed through other methods such as pyrolysis, storage (e.g., Greece, Italy, and Poland), reuse in green areas and forestry (e.g., Ireland, Latvia, and Slovakia), and landfill cover (e.g., Sweden) [1]. Figure 1 shows the sewage sludge recovery route for every Member State. The political choices on the alternative routes of sludge recovery/disposal, in particular for the land spreading, are strongly influenced by the population density and the availability of agricultural lands. The low availability of soils for the spreading of biosolids led the northern European countries (such as Netherlands and Germany) to choose incineration as the principle recovery route. Furthermore, even though all of the sludge produced could be applied in less than 5% of the agricultural area (in most Member States), the limited use of biosolids in agriculture is due to the low level of acceptance by farmers and the public [49]. This aspect influences policy decisions on sludge management, too; therefore, every Member State has issued a national regulation.
However, some inconsistencies in the EU terminology related to the management and recovery of sludge can be highlighted. For example, analyzing in more detail the trend of the data from 2012 to 2016 provided by Eurostat [2], the fraction reused in agriculture in some countries (e.g., Portugal) has decreased significantly unlike “other uses”. However, it is legitimate to think that these “other uses” also include applications in agriculture. Moreover, sludge directed to grow plants for further compost production in Poland is classified in other countries as a sludge dewatering technique. Sometimes, in fact, the different national regulations lead to classifying the recoveries in different ways. This can sometimes create interpretation problems when the data is collected and made available by Eurostat.

3. The Reuse of Biosolids in Agriculture: Comparison Between Legislations in the European Countries

According to the data shown above, the yearly amount of biosolids recovered in agricultural soils is about 6 million tDM. As reported in Figure 1, the scenarios (both in terms of quantities and percentages) are very different in the Member States: there are countries with high amounts of biosolids recovered in agriculture (e.g., France), and other states where percentage is close to zero (e.g., Malta).
Moreover, the legislation of the Member States is different: the national regulations, which have been established based on EU Directive 86/278/EEC [37], have often introduced provisions that go beyond the requirements of the directive. Every Member State issued own normative, which could have different limit values for biosolids and soils.
Furthermore, in order to reduce possible health risks, the regulations of some countries (e.g., France and Italy) include limit values for pathogens and in a larger number of cases for organic compounds, both of which are not included in Directive 86/278/CEE [37,50]. A new directive that provided limits for organic micropollutants and pathogens was proposed in 2000, but it was withdrawn [51].
Directive 86/278/EEC [37] allows Member States to choose between limit values in the soil or in the sludge: every country has chosen to establish limit values for biosolids and soil except the UK, which imposed limit values only for the soils. The countries can be classified into two different categories in relation to their comparison with Directive 86/278/EEC [37]: (i) with national requirements (in some cases even much) more stringent than the European directive, and (ii) with national requirements similar to those in the European directive [50]. The countries are classified as follows:
  • Czech Republic, Denmark, Finland, Luxembourg, the Netherlands, Sweden, Austria, Belgium, Malta, Croatia, France, Germany, Hungary, Lithuania, Poland, Slovenia, and Romania
  • Bulgaria, Cyprus, Estonia, Latvia, Greece, Ireland, Italy, Portugal, Slovakia, Spain, and the United Kingdom

3.1. Limit Values for Biosolids

The presence of heavy metals in the biosolids is an important parameter to check before its spreading on land. Their presence in biosolids can be due to the presence of industrial wastewater treated by WWTPs [52,53]. Some heavy metals are considered essential micronutrients for plant growth, but elevated concentrations of these compounds are toxic to food crops, domestic animals, and humans [54,55]. It is also known that heavy metals are not biodegradable, and their persistence in soil is much longer than any other reactive components of the terrestrial ecosystems [33].
Table 1 shows a comparison of limit values for heavy metals in the biosolids provided in Directive 86/278/EEC [37] and the legislations in different Member States.
It can be observed that the limit values provided for the European countries are very different; moreover, almost all countries provide limit values for total chromium, and in some cases for arsenic, too. Furthermore, Austria, Italy, Hungary, Romania, and the United Kingdom introduced limit values for additional heavy metals in the biosolids: Styria and Lower Austria (Austrian Landers) provide limits for molybdenum (20 mg kgDM−1) and cobalt (10–100 mg kgDM−1); Italy for beryllium (20 mg kgDM−1) and selenium (10 mg kgDM−1); Hungary for molybdenum (20 mg kgDM−1), cobalt (50 mg kgDM−1) and selenium (100 mg kgDM−1); Romania for cobalt (50 mg kgDM−1); and the United Kingdom for molybdenum (3 mg kgDM−1), selenium (2 mg kgDM−1) and fluoride (200 mg kgDM−1).
Despite Directive 86/278/EEC [37] not providing any limit values or requirements for organic micropollutants in biosolids, several national regulations on the use of biosolids have added specifications, as shown in Table 1. It can be noted that the organic compounds most checked by different Member States are PCB (polychlorinated biphenyls), AOX (absorbable organic halogens), and PAH (polycyclic aromatic hydrocarbons).
In order to reduce possible health risks related to pathogens, different Member States (e.g., Austria and Bulgaria) have added specific requirements for biosolids spread on land. Despite Directive 86/278/EEC [37] not including limit values for the pathogens content in biosolids, most countries’ national legislation checks the presence of salmonella (with the exception of Lithuania, Luxembourg, and Slovakia) and, in many cases, other pathogens are included. Types of pathogens and limit values are quite different (see Table 2).
The regulatory requirements on pathogens content in biosolids still remains quite limited in most countries’ national legislation. This can be partially explained by the fact that national codes of practice are considered to sufficiently cover this issue, by providing recommendations on sludge treatment (mainly with biological, chemical, and heat treatments) and biosolids land spreading (i.e., the UK).
Finally, in order to have a good fertilizer, biosolids must contain a significant amount of agronomic parameters such as organic matter, nitrogen, and phosphorus. Most of the Member States (e.g., France and Spain) prescribe the analyses of agronomic parameters, but only Italy sets minimum values for organic matter (>20% of dry matter (DM)), total nitrogen (>1.5% of DM), and total phosphorus (>0.4% of DM). Instead, other countries (such as Sweden and Latvia) provide a maximum annual rate of nitrogen and/or phosphorus that can be spread on land.

3.2. Limit Values for Soil

In addition to restriction for heavy metals in biosolids, the European legislation provides limit values for heavy metals in soils. In all Member States, regulations on the use of biosolids specify limit values, which are in most cases similar or lower than the requirements set in Directive 86/278/EEC [37] (Table 3). The limit values have been defined by three different criteria as follows:
  • Soil pH: Bulgaria, Slovenia, Romania, Croatia, Latvia, Malta, Portugal, the United Kingdom, and Spain defined limit values according to the soil pH, which vary between 5 and 7.5. In some cases, these values can change in relation to the different region/lander (e.g., in Austria and in Belgium).
  • Granulometric content: Czech Republic, Lithuania, and Latvia based their prescription on soil granulometric content (sand or clay). Poland adopted another classification (heavy, medium, or light soils) which is not yet defined by the decree [94].
  • Italy uses cation exchange capacity (CEC) in order to characterize soil for land spreading of biosolids.
Greece and Cyprus have minimum values, which are recommended, and threshold values. In addition, the legislation in several Member States (Austria, Belgium, Cyprus, Estonia, Greece, Finland, France, Hungary, Latvia, Lithuania, Luxembourg, Netherlands, Portugal, Slovakia, Slovenia, Spain, Sweden, Romania, and the UK) includes restrictions in terms of the maximum annual load of heavy metals on agricultural land. Moreover, some countries (Austria, Hungary, and the UK) introduce limit values for additional heavy metals in the soil—Styria (Austria Lander) provides limits for molybdenum (10 mg kgDM−1) and cobalt (50 mg kgDM−1); Hungary for molybdenum (7 mg kgDM−1), cobalt (30 mg kgDM−1), and selenium (1 mg kgDM−1); and the United Kingdom for molybdenum (4 mg kgDM−1), selenium (3 mg kgDM−1) and fluoride (500 mg kgDM−1).
Studies on the heavy metals content in the soils due to the application of biosolids were carried out, but in some cases, the results are conflicting. For instance, Collivignarelli et al. [53] analyzed the heavy metal presence in the soils where the spreading of biosolids occurs during 2012 to 2016 in Lombardy (Italy). In the analyzed area, the soils presented mainly silty–sandy components [95]. They found that heavy metal average concentrations were largely below the national and regional normative limits in the whole period of the survey [53]. Only sporadic overruns of the Zn, Ni, and Cd have been reported [53]. However, in a study conducted in the USA, Islam et al. [33] studied the effects of the application of biosolids on well-drained silt soil, and they found that the heavy metal concentration increased significantly. Moreover, they highlighted that the extractable fractions of Pb, As, Zn, and Cu were significantly higher at soil depth from 0 to 15 cm. Consequently, they noted that accumulated heavy metals may mobilize from the soils to groundwater and surface water bodies [33]. Several studies try to understand the effect of heavy metals content on soil, due to biosolids application, on crops [96,97].

4. Land Spreading as Main Route: Detailed Survey of European Legislations

4.1. Comparison Between Regulations in EU-15 Member States with Land Spreading as Main Route

This section concerns the detailed study (and comparison) of regulations in EU-15 Member States where the recovery on land represents the main routes for sludge management. The countries considered in this section are the Member States where the percentage of biosolids recycled to land is more than 70%, or the amount of biosolids spread on soil is higher than 300,000 tDM year−1 [2]—France, Finland, Germany, Ireland, Italy, Spain, and the United Kingdom. In addition, Denmark has been considered due to its very detailed regulation on the reuse of biosolids [73,74]. At a first analysis, in 2016, Portugal seemed to show a very low value of reuse in agriculture. However, as already discussed in Section 2, analyzing in greater detail the trend of data from 2012 to 2016 provided by Eurostat [2], the fraction reused in agriculture decreased significantly (from 90% to 12%) in spite of “other uses” (from 0% to 84%). However, it is legitimate to think that these other uses also include applications in agriculture, such as compost. For this reason, Portugal was still included in the analysis. The selected countries, the percentage of biosolids recycled to land, and the annual amount of biosolids spread on soil are reported in Figure 2.
Considering that the selected countries belong to different climatic areas (Mediterranean, continental, oceanic, and subarctic), with different types of soils, this allows developing a complete view of the situation in Europe concerning the spreading of biosolids on the soil.
The legislation of the countries analyzed presents significant differences, in addition to differences among the heavy metals, pathogen, and organic micropollutants (see Table 1, Table 2 and Table 3), as concerns, there is also: (i) the maximum amount of biosolids spread on land; (ii) the soils on which the use of biosolids is prohibited; and (iii) the treatment requirements.
The maximum amount of biosolids that can be spread on land is not prescribed in Directive 86/278/EEC, but the regulation states that it is necessary to limit the amount of heavy metals added to cultivated soil [37]. Thus, five countries have set the maximum amount of biosolids that can be spread on land per year (Table 4). The maximum amount of biosolids differs as well as their quality—for example, Denmark allows spreading 10 tDM ha−1 year−1 on land, but it prescribes requirements more stringent than the European directive. In contrast, Ireland set a maximum amount of biosolids that is significantly lower than that of Denmark (2 tDM ha−1 year−1), but in this case, the limit values for pollutants in biosolids are similar to Directive 86/278/EEC [37]. Therefore, the maximum load of pollutants (heavy metals, pathogens, organic micropollutants) permitted on agricultural land in different Member States is comparable. Moreover, different amounts of biosolids are associated with SOM declines in different countries: the content of SOM tends to decrease when moving from a warmer (e.g., Italy, Spain) to cooler climate (e.g., Denmark, Sweden). This is due to the overall trend in the decomposition of SOM that is accelerated in warm climates, while a lower rate of decomposition is the case for cool regions [21].
Nitrates Directive 91/676/EEC [98] (and subsequent amendments [99,100]) set out the maximum permissible addition of total nitrogen which for most purposes is 250 kgN ha−1 year−1 and, generally, it represents the limiting factor determining the rate of application of biosolids to the land. This value will be reduced in Nitrate Vulnerable Zones to 175 kgN ha−1 year−1. In some cases, it may be permissible to apply 500 kgN ha−1 every two years if the nitrogen availability of the material is low (e.g., sludge compost, dewatered sludge cake) [49]. Currently, soils in different areas of Europe, particularly in the Mediterranean area, suffer from nutrient depletion [101,102]. Therefore, the application of biosolids can represent a solution to this problem [16,103].
Concerning the soils on which the use of biosolids is prohibited, Directive 86/278/EEC [37] (article 7) provides restrictions concerning the spreading of biosolids on grazing and pastureland, and on land on which vegetables and fruits are grown. These dispositions have been transposed by Member States, which have introduced additional requirements (Table 4) for land spreading.
Concerning the requirement for sludge treatment, Directive 86/278/EEC specifies that sludge need to be treated before being spread on land in order to reduce its fermentability and the health hazards [37]. However, the European directive allows the use of untreated sludge in the case of injection or if it is worked into the soil [37]. Most countries prohibit the use of untreated sludge, while some Member States (France, Ireland, and the UK) permit the use of untreated sludge [50].

4.2. Political Choices in Land Spreading Regulations

As reported in the Section 3, the countries mentioned above could be classified into two different categories depending on the severity of existing legislation. The differences are connected to different aspects (e.g., the economic impact, the public perception, etc.), but in particular to the extension of agricultural land and the stakeholders’ (farmers, landowners, communities, etc.) positions that influence the policy decisions.
In the Member States with national requirements (even much) more stringent than the European directive, farmers and landowners’ associations have expressed their growing hostility toward the agricultural use of biosolids [56]. In Germany and France, the normative is more stringent than the European directive, and the debate is open: the stakeholders have divergent opinions [56,104]. These conflicting positions in France and Germany have led to changing the regulation despite different stakeholders highlighting the problem connected with high limit values [56]. In Denmark and Finland, instead, in addition to the hostility of stakeholders, there is less land suitable for the land spreading of urban biosolids [56]. For this reason, the government introduced new legislation (e.g., in Denmark [74]) but, despite the improvements in biosolids quality, the perception on land spreading remains negative [105].
Finally, for the Member States that have requirements similar to the European directive (Ireland, Italy, Portugal, Spain, and the United Kingdom), the scenarios vary widely. In most cases (e.g., Ireland, Italy, Portugal, and Spain), there is not a real debate on the agricultural recycling of biosolids [105], while in other cases (e.g., the UK), the debate is over [56]. Despite the absence of a real debate leading these countries to follow the values imposed by the European directive, stakeholders’ opinions change in different countries [56]. For instance, in Ireland, the majority of farmers and national authorities are reportedly very positive about land, although some concerns remain regarding environmental problems [105,106]. In Portugal, someone highlighted the worse quality of the biosolids [56,107].

5. Italian Scenario: Comparison of Legislation at National and Regional Level

In Italy, the reuse of biosolids in agriculture in 2012 was about 30% [53]. Agricultural reuse is the most prevalent route in the regions of northern Italy. In fact, it can be observed that the total amounts of biosolids reused in agricultural land was more than 50% in Lombardy, Emilia Romagna, and Veneto [108].
This context, in which the reuse of biosolids on soil represents a significant route, has led to issuing regional legislations in order to regulate the reuse operation in these territories; in particular, Lombardy, Veneto, and Emilia Romagna have their own regional regulation derived from the Italian legislation.
These regions prescribe the same limit values of Legislative Decree n.99/1992 [64] for heavy metals in soil (Table 3). Instead, for the biosolids that can be spread on agricultural land, the regional regulations, especially for Lombardy, initially show significantly different issues (Table 5). As a consequence of a new Italian regulation [63], Italy, Lombardy, Veneto, and Emilia Romagna imposed the same limits on heavy metals and organic micropollutants [109,110,111]. Furthermore, legislation at the regional level has maintained the same limit of Italy regulation for salmonella and for agronomic parameters (organic carbon, total nitrogen, and phosphorus).
An interesting approach was introduced in the Lombardy regulation that concerns the classification of biosolids in two different classes [112]:
  • the “biosolids suitable for spreading”, which must respect the limit values imposed by the current Italian normative;
  • the “high quality biosolids”, which requires more stringent limit values (similar to the limits prescribed in Denmark).
Moreover, for both kinds of biosolids, the volatile suspended solids (VSS) to total suspended solids (TSS)−1 ratio and organic micropollutants (also for the regulation of Emilia Romagna and Veneto) are prescribed. In particular, the VSS TSS−1 are provided in order to minimize the problems concerning the odor emissions (one of the main critical issues of biosolids land spreading).
It should be noted that exceeding the limit values for “biosolids suitable for spreading” involves choosing another route (such as incineration).
Concerning the maximum amount of biosolids that can be spread on land, the regulations at the regional level are different from those at the national one. The Italian legislation provides three soil categories based on pH and CEC, while Emilia Romagna, Veneto, and Lombardy provide five to six categories with different maximum amounts of biosolids.
Moreover, the regional regulations provide a more detailed list of soil on which the use of biosolids is prohibited, with respect to Legislative Decree n.99/1992 [64]. The use of biosolids on grazing, pastureland, and land on which vegetables and fruits are grown (prescribed by Directive 86/278/EEC [37]) is prohibited both at national and regional levels. As shown in Table 5, while Italian regulation prohibits only the use of biosolids on sloping land and wetland, for Lombardy, Veneto, and Emilia Romagna, the application of biosolids on other soil is prohibited.
Concerning the treatment of sludge, Legislative Decree n.99/1992 [64] provides the requirement for treatment (e.g., stabilization) for biosolids before the use on agricultural soil. This aspect is also prescribed in regional regulations.
Finally, an important aspect introduced in Italian and regional regulation is the limit for hydrocarbon C10–C40—the concentration must be equal or less than 1000 mg kgWW−1, and only for the Lombardy Region, it must be less than 10,000 mg kgDM−1. The introduction of a hydrocarbon limit is becoming important in Europe, too, and some countries have imposed limits in their regulations such as Hungary (4000 mg kgDM−1 for C5–C40) and Belgium (5600 mg kgDM−1 for C20–C40 in the Flemish region) [57].
Another possibility for the reuse of biosolids is through the production and spreading of defecation gypsum on land. In this way, a fertilizer is produced that does not fall under the legislation of Legislative Decree 99/1992 [64] and which, on suitable land, can replace the biosolids regulated by the aforementioned decree. It should be noted that in more than one case, the spreading of defecation gypsums can rise to a significant malodor, without it being clarified whether it is due to the material or to the spreading methods. The defecation gypsums are classified by the Legislative Decree 75/2010 [116] (and subsequent amendments [117]). The qualitative characteristics of defecation gypsums must comply with the limits shown in Table 6. As can be seen, the concentration of heavy metals allowed in sludge defecation plasters is significantly lower than that imposed for the spreading of biosolids in agriculture.

6. Discussion

Analysing and comparing the different regulations of EU countries, there is a strong heterogeneity in the monitored parameters and in the limits imposed. While some countries have imposed only the limits reported in EU Directive 86/278/EEC, others have imposed more stringent limits and/or additional parameters. As regards the limits of heavy metals in soils, this difference is partially justifiable by the marked pedoclimatic differences that involve the European continent, and which have therefore led countries to practice different choices. It would be interesting to foresee in the European directive different ranges of limits according to the prevailing pedoclimatic zone in each nation. Although not easy to implement, this method would allow countries to better adapt their legislation to the different needs of the territories.
Different types of soil react differently to the same contribution of pollutants. To date, this aspect is little considered by the legislation. However, in some countries such as in Italy, national legislation already differentiates the maximum amounts of biosolids to be spread according to the pH and the CEC of soils. One of the key points derived from the analysis of the current regulatory overview could be to differentiate soils, even in countries where this approach is not currently followed, by distinguishing the maximum amounts of biosolids spread on the basis of the maximum value of the pollutant load tolerable by soils. In fact, providing a limit to the pollutant load and not to the concentration would make it possible to operate more efficiently considering the effective response capacity of the soils.
As for the actions on the quality of the reused biosolids in the agricultural land, the introduction of specific legislation that regulates the acceptability limits for the sludge inlet to sludge treatment plants (STPs) would improve the selection of sludge and the quality of biosolids recovered. In fact, STPs are often not equipped to remove specific pollutants such as heavy metals, organic contaminants, etc.
It is considered of fundamental importance that biosolids reused for agricultural purposes must be those of better quality. Denmark has already legislated in this direction by placing a greater maximum amount for recovery in agriculture when the quality of biosolids is better. In addition, in Italy, more precisely in Lombardy, the legislation clearly distinguishes between “suitable” biosolids recovered in agriculture (which must comply with the limits of the current Italian law) and “high quality” biosolids that instead must meet more stringent limits. Furthermore, the adoption of regulations concerning the biosolids spreading methodology would make it possible to legislate in a sector that to date is not completely regulated and controlled.

7. Conclusions

In the last years, the main route for the reuse of biosolids is the application on agricultural land. This practice is controlled in different ways at the European level, due to the implementation in the Member States of Directive 86/278/CEE, which allows the reuse on land only for biosolids with a good quality. However, every country provides different requirements (with respect to Directive 86/278/CEE) for heavy metals, pathogens, and organic micropollutants both in biosolids and soils. Significant differences are also clear in other aspects of regulation (the maximum amount of biosolids spread on land, the soil where the use of biosolids is prohibited, the treatment requirements) in Member States where the reuse on land represents the main route of sludge management.
In order to comply with the European trend shown in this report, some actions, addressed to the stakeholders of biosolids land spreading activity (the WWTPs managers that produce biosolids and the farmers that recover the treated sludge), could improve the policy decisions making:
  • In order to follow the steps of Directive 2008/98/EC, the minimization from the producers (WWTPs managers) is the first step to control the quantitative of sludge.
  • In addition to the enhancement of minimization, the reuse of biosolids must be improved in order to cope with nutrient depletion in soils (especially phosphorous).
  • Monitoring all discharges is necessary. In fact, the lack of knowledge about the industrial discharges into the public sewer can significantly affect the wastewater treated and the sludge quality.
  • Suitable characterization, especially in terms of heavy metals content and stabilization degree, of sludge deriving from WWTPs must be carried out, with the aim of sending the best quality sludge to agricultural land and reducing the environmental impacts.
  • Correct planning is very useful to give structure to the policy decisions at different levels (national, regional, etc.) in order to guarantee proper sludge management that provides for the safeguard of environment and public health.

Author Contributions

A.F. contributed to the drafting of the Section 1, Section 2, Section 3, while M.C.M. and S.P. contributed to the drafting of Section 4 and Section 5, respectively. A.A. contributed to the drafting of the discussion and the conclusions. M.C.C. and V.T. and I.K. supervised the study and the paper drafting. All authors have read and approved the final manuscript.

Funding

This research received no external funding.

Acknowledgments

This work is dedicated to the Memory of Vincenzo Riganti.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

AOXAbsorbable organic halogens
CECCation exchange capacity
CFU Colony-forming unit
DEHPDi(2-ethylhexyl) phthalates
DMDry matter
GWGroundwater
LASLinear alkylbenzene sulfonates
NPNonylphenols
NPE Nonylphenol ethoxylates
MPCNMost probable cytopathic number
MPNMost probable number
PAHPolycyclic aromatic hydrocarbons
PCBPolychlorinated biphenyls
PCDDPolychlorinated dibenzo-p-dioxins
PCDFPolychlorinated dibenzofuran
P.E.Population equivalent
SOMSoil organic matter
STPSludge treatment plant
TEToxic equivalency
TSSTotal suspended solids
UKUnited Kingdom
VSSVolatile suspended solids
WWWet weight
WRRFWater resources recovery facility
WWTPWastewater treatment plant

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Figure 1. Sewage sludge recovery routes in Europe [2].
Figure 1. Sewage sludge recovery routes in Europe [2].
Sustainability 11 06015 g001
Figure 2. The selected countries, the percentage of biosolids recycled to land, and the total annual amount of biosolids spread on soil [2].
Figure 2. The selected countries, the percentage of biosolids recycled to land, and the total annual amount of biosolids spread on soil [2].
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Table 1. Limit values for heavy metals and organic compounds in the biosolids—all values are reported in mg kgDM−1 [13,50,56,57].
Table 1. Limit values for heavy metals and organic compounds in the biosolids—all values are reported in mg kgDM−1 [13,50,56,57].
LegislationHeavy MetalsOrganic CompoundsReferences
CdCrCuHgNiPbZnAsPCBAOXLASDEHPNP/NPEPAHPCDD/F g
Directive 86/278/EEC20–40-1000–175016–25300–400750–12002500–4000--------[37]
EU-15
Germany1090080082009004000-0.1 o400----100[58,59]
UK-------2-------[60]
Spain a20–4015001000–175016–25300–400750–12002500–4000--------[61]
France2010001000102008003000-0.8 l----2–5 p-[62]
Italy202001000103007502500200.8----625[63,64]
Netherlands1.2575750.753010030015-------[65]
Austria b2–1050–50070–5000.4–1025–10045–500200–2000200.2–1500---650–100[66,67,68,69]
Sweden c0.75403001.52525600-0.4 l---503 f-[70]
Portugal2010001000163007502500-0.8-5000-4506100[71]
Finland1.53006001100100150025-------[72]
Denmark0.810010000.8301204000250.2 l-130050103 f-[73,74]
Ireland20-1000163007502500--------[75]
Greece d20–405001000–175016–25300–400750–12002500–4000--------[76]
Belgium b6–10100–150600–8001–1.6100300–5001500–200020–1500.6–0.8 l----3–2020[77,78]
Luxembourg d2.51007001.6802003000-0.2 m----20 i20[79]
EU-13
Poland205001000163007502500--------[80]
Hungary1010001000102007502500751 l----10 i-[81]
Czech Republic520050041002002500300.6 l500---10 j-[82,83]
Romania1050050051003002000100.8 l500---5 k-[84]
Lithuania e1.5–20140–40075–10001–850–300140–750300–2500--------[85]
Slovakia1010001000103007502500200.8 l500---6 h-[86]
Bulgaria30500160016350800300025-------[87]
Estonia2010001000163007502500--------[88]
Cyprus d20–40-1000–175016–25300–400750–12002500–4000--------[89]
Latvia f2–10100–600400–8003–1050–200150–500800–2500--------[90]
Slovenia1.52003001.5752501200--------[91]
Malta580080052005002000--------[92]
Croatia55006005805002000-0.2 n-----100[93]
a Different values for different soil pH; b Different values for different regions/Landers; c Value expressed as g ha−1 year−1; d Lower values = recommended values, higher values = maximum limits (the same limits of Directive 86/278/EEC); e Different values for different sludge categories; f Different values for capacity of wastewater treatment plants (WWTPs) (expressed in P.E.—population equivalent); g PCDD/F are expressed in ng TE (Toxic Equivalency) kgDM−1; h Sum of acenaphthene, fluorene, phenanthrene, fluoranthene, pyrene, benzo(b+j+k)fluoranthene, benzo(a)pyrene, benzo(ghi)perylene, indeno(1,2,3-c,d)pyrene; i Sum of 16 United States Environmental Protection Agency (US EPA) PAH (naphthalene, acenaphthylene, acenaphthene, fluorene, fenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a,h)anthracene, indeno(1,2,3-c,d)pyrene, and benzo(ghi)perylene); j Sum of anthracene, benzo(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, benzo(ghi)perylene, fenanthrene, fluoranthene, chrysene, indeno(1,2,3–cd)pyrene, naphthalene, and pyrene; k Sum of anthracene, benzoanthracene, benzofluoranthene, benzoperylene, benzopyrene, chrysene, fluoranthene, indeno (1,2,3)pyrene, naphthalene, fenanthrene, and pyrene; l Sum of seven congeners: PCB 28, 52, 101, 118, 138, 153, and 180; m Sum of six congeners: PCB 28, 52, 101, 138, 153, and 180; n For each of these congeners: PCB 28, 52, 101, 141, and 180; o For each congener; p Different values for different compounds (fluoranthene-5, benzo(b)fluoranthene-2.5, benzo(a)pyrene-2); PCB: polychlorinated biphenyls; AOX: absorbable organic halogens; LAS: linear alkylbenzene sulfonates; DEHP: di(2-ethylhexyl) phthalates; NP: nonylphenols; NPE: nonylphenol ethoxylates; PAH: polycyclic aromatic hydrocarbons; PCDD/F: polychlorinated dibenzo-p-dioxins and furan; UK: United Kingdom.
Table 2. Limit values for pathogens in the biosolids [13,50,56,57].
Table 2. Limit values for pathogens in the biosolids [13,50,56,57].
LegislationTypes of PathogensLimitUnits of Measure (u.m.)References
Directive 86/278/EEC---[37]
Austria 1
Enterococci<103CFU gDM−1[66,67,68,69]
Escherichia Coli100CFU gDM−1
Helminths eggsAbsenteggs kgDM−1
SalmonellaNo occurrence in 1 g
Bulgaria
SalmonellaAbsentMPN 20−1 gWW−1[87]
Escherichia Coli100MPN gWW−1
Clostridium perfringens300MPN gWW−1
Viable eggs of helminths1eggs kgDM−1
Czech Republic
SalmonellaAbsentCFU gDM−1[82,83]
Thermotolerant coliforms<103CFU gDM−1
Enterococci<103CFU gDM−1
Denmark 2
SalmonellaNo occurrence[73,74]
Faecal streptococci<100 g−1
Finland
SalmonellaNot detected in 25 g[72]
Escherichia Coli1000CFU g−1
France
Salmonella8MPN 10−1gDM−1[62]
Enterovirus3MPCN 10−1gDM−1
Helminths eggs3eggs 10−1gDM−1
Italy
Salmonella1000MPN gDM−1[63,64]
Lithuania
Escherichia Coli1000CFU g−1[85]
Helminths eggs0Units kg−1
Enterobacteria0CFU g−1
Clostridium perfringens100,000CFU g−1
Luxembourg
Enterobacteria<100 g−1[79]
Helminths eggsNo eggs of worm likely to be contagious
Malta
SalmonellaAbsentCFU 50−1 gWW−1[92]
Poland
SalmonellaBiosolids cannot be used in agriculture if it contains salmonella in 100 gDM[80]
Helminths eggs0eggs kgDM−1
Portugal
SalmonellaNo occurrence in 50 g[71]
Escherichia Coli1000CFU g−1
Slovakia
Thermotolerant coliforms2 × 106CFU gDM−1[86]
Fecal streptococci2 × 106CFU gDM−1
1 Only for three Lander: Carinthia, Lower Austria, and Styria; 2 For advanced treated sludge only; u.m.: units of measure; MPN: most probable number; MPCN: most probable cytopathic number; CFU: colony-forming unit; WW: wet weight; DM: dry matter.
Table 3. Limit values for heavy metals in the soil. All values are reported in mg kgDM−1 [13,50,56,57].
Table 3. Limit values for heavy metals in the soil. All values are reported in mg kgDM−1 [13,50,56,57].
LegislationCdCrCuHgNiPbZnAsReferences
Directive 86/278/EEC1–3-50–1401–1.530–7550–300150–300-[37]
EU-15
Germany0.4–1.530–10020–600.1–115–7040–10060–200-[58,59]
United Kingdom340080–200150–110300200–30050[60]
Spain 11–3100–15050–2101–1.530–11250–300150–450-[61]
France2150100150100300-[62]
Italy1.5-100175100300-[63,64]
Netherlands0.810360.33035140-[65]
Austria 20.5–250–10040–1000.2–1.530–7050–100100–300-[66,67,68,69]
Sweden0.460400.33040100-[70]
Portugal 11–450–30050–2001–230–11050–450150–450-[71]
Finland0.52001000.26060150-[72]
Denmark0.530400.51540100-[73,74]
Ireland1-5013050150-[75]
Greece 31–3-50–1401–1.530–7550–300150–300-[76]
Belgium 21.2–1.591–10050–721–1.520–5650–12020022[77,78]
Luxembourg 321501001.575200300-[79]
EU-13
Poland 41–350–10025–750.8–1.520–5040–8080–180-[80]
Hungary175 (CrVI)750.54010020015[81]
Czech Republic 40.4–0.555–9045–600.345–5055–60105–12015–20[82,83]
Romania310010015050300-[84]
Lithuania 41–1.550–8050–800.6–150–6050–80160–260-[85]
Slovakia160500.5507015025[86]
Bulgaria 11.5–320080–2001.575–11060–120200–30025[87]
Estonia3100501.550100300-[88]
Cyprus 31–3-50–1401–1.530–7550–300150–300-[89]
Latvia 1,40.5–0.940–9015–700.1–0.515–7020–4050–100-[90]
Slovenia1100600.85085200-[91]
Malta 10.5–1.530–10020–1000.1–115–7070–10060–200-[92]
Croatia 10.5–1.550–10040–1000.2–130–7050–100100–200-[93]
1 Different values for different soil pH values; 2 Different values for different regions/Lander; 3 Lower values = recommended values, Higher values = maximum limits (the same limits as those of Directive 86/278/EEC); 4 Different values for different types of soil.
Table 4. Maximum amount and soils on which the use of biosolids is prohibited [50,56].
Table 4. Maximum amount and soils on which the use of biosolids is prohibited [50,56].
Directive 86/278/EECDenmarkFinlandFranceGermanyIrelandItalyPortugalSpainUK
Maximum quantitative (tDM ha−1 year−1)-10--1.7256--
Obligations for treatmentPermit use of untreated sludge under certain conditionsStabilization, composting or pasteurizationDigestion or lime stabilizationPermit use of untreated sludge under certain conditionsProhibition of use of untreated sludgePermit use of untreated sludge under certain conditionsProhibition of use of untreated sludgeProhibition of use of untreated sludgeProhibition of use of untreated sludgePermit use of untreated sludge under certain conditions
Soil
Grazing and pasturelandPPPPPPPPPP
Vegetables and fruits cropsPPPPPPPPPP
Frozen or snow-covered groundNPNPNPPNPNPNPNPNPNP
Sloping landNPNPNPPNPNPPNPNPNP
Wet land, or after heavy rainNPNPNPPNPNPPPNPNP
GW protection areasNPPNPPPNPNPPNPNP
Near surface waterNPPNPPPNPNPPNPNP
Forest SoilNPPNPPPNPNPNPNPNP
Additional restriction On soil where sludge is likely to cause significant nuisances or unsanitary conditions Not regularly worked out land in areas close to human settlements and public buildings Soils of pH <5, and CEC <8 meq 100−1 g−1In areas close to individual houses and human settlements Soils of pH <5
P: Prohibited; NP: Not prohibited; GW: Groundwater; UK: United Kingdom; meq: milliequivalents.
Table 5. Limit values, maximum quantitative values, and soil on which the use of biosolids is prohibited in Italy and in some regions [63,64,109,110,111,113,114,115].
Table 5. Limit values, maximum quantitative values, and soil on which the use of biosolids is prohibited in Italy and in some regions [63,64,109,110,111,113,114,115].
Parameter.Unit of Measure (u.m.)ItalyLombardyVenetoEmilia Romagna
Legislative Decree n.99/1992 and n. 130/2018DGR n. X/2031/2014 and d.d.u.o. 6665/2019DGR n. 2241/2005 and n. 130/2018DGR n. 285/2005 and n. 326/2019
SuitableHigh Quality
Heavy metalsAsmg kgDM−1<20≤10≤20<20<20
Bemg kgDM−1≤2≤2≤2≤2
Cdmg kgDM−1<20≤20≤5<20≤20
Crmg kgDM−1<200≤200≤150<200≤200
Cr (VI)mg kgDM−1<2<2<2<2
Cumg kgDM−1<1000≤1000≤400<1000≤1000
Hgmg kgDM−1<10≤10≤5<10≤10
Nimg kgDM−1<300≤300≤50<300≤300
Pbmg kgDM−1<750≤750≤250<750≤750
Semg kgDM−1≤10≤10≤10≤10
Znmg kgDM−1<2500≤2500≤600<2500≤2500
Chemical/physical parametersVSS TSS−1%-<65<60--
pH--5.5 < pH ≤ 11->5.5
Agronomic parametersOrganic carbon% DM>20>20>20>20
Total nitrogen% DM>1.5>1.5>1.5>1.5
Total phosphorus% DM>0.4>0.4>0.4>0.4
Organic micropollutantsPAHmg kgDM−1≤6<6≤6≤6
PCBmg kgDM−1≤0.8<0.8≤0.8≤0.8
AOXmg kgDM−1-<500-<500
LASmg kgDM−1---<2600
Toluenemg kgDM−1≤100≤100≤100≤100
DEHPmg kgDM−1-<100-≤100
NPEmg kgDM−1-<50-≤50
PCDD/FngTE kgDM−1≤25≤25≤25≤25
Hydrocarbon (C10-C40)mg kgDM−1-<10,000--
Hydrocarbon (C10-C40)mg kgWW−1<1000<1000≤1000≤1000
Microbiological parametersSalmonellaMPN gDM−1<1000<100<1000≤1000
Faecal ColiformsMPN gDM−1-<10,000--
Maximum quantitative (based on pH and CEC values)tDM ha−1 year−1 for maximum quantitative meg 100−1 g−1 for CEC2.5pH < 6;
CEC < 15
2.5pH < 6;
CEC ≤ 15
2.55 < pH < 7.5;
CEC < 15
2.55 < pH < 6;
CEC ≤ 15
3.75 < pH < 6;
CEC > 15
2.55 < pH < 6;
CEC > 15
3.75 < pH < 6;
CEC > 15
56 < pH < 7.5;
CEC > 15
3.76 < pH < 7.5;
CEC ≤ 15
56 < pH < 7.5;
CEC > 15
3.76 < pH < 7;
CEC ≤ 15
56 < pH < 7.5;
CEC > 15
5pH > 7.5;
CEC < 15
56 < pH < 7.5;
CEC > 15
7.5pH > 7.57.5pH > 7.57.5pH > 7.5;
CEC > 15
5pH > 7.5;
CEC ≤ 15
7.5pH > 7.5;
CEC > 15
SoilGrazing and pastureland PPPP
Vegetables and fruits crops PPPP
Frozen or snow-covered ground NPPPP
Sloping land PPPP
Wet land, or after heavy rain PPPP
Groundwater protection areas NPPPP
Near surface water NPPPP
Forest soil NPPPNP
u.m.: unit of measure; meq: milliequivalents; TE: toxic equivalency; PCB: polychlorinated biphenyls; AOX: absorbable organic halogens; LAS: linear alkylbenzene sulfonates; DEHP: di(2-ethylhexyl) phthalates; NP: nonylphenols; NPE: nonylphenol ethoxylates; PAH: polycyclic aromatic hydrocarbons; PCDD/F: polychlorinated dibenzo-p-dioxins and furan; CEC: cation exchange capacity; DM: dry matter; VSS: volatile suspended solids; TSS: total suspended solids; WW: wet weight; P: prohibited; NP: not prohibited.
Table 6. Limiting concentrations of heavy metals and microbiological parameters for defecation gypsum from sludge and biosolids [109,113,116,117].
Table 6. Limiting concentrations of heavy metals and microbiological parameters for defecation gypsum from sludge and biosolids [109,113,116,117].
ParameterUnit of Measure (u.m.)D.Lgs. 75/2010 and M.D. 28 June 2016DGR n. X/2031/2014 and d.d.u.o. 6665/2019
SuitableHigh Quality
Heavy metalsPbmg kgDM−1140750250
Cdmg kgDM−11.5205
Nimg kgDM−110030050
Znmg kgDM−15002500600
Cumg kgDM−12301000400
Hgmg kgDM−11.5105
CrVImg kgDM−10.5<2<2
Microbiological parametersSalmonellaMPN gDM−1Absence in 25 g of sample as it is; n (1) = 5; c (2) = 0; m (3) = 0; m (4) = 0100100
E. ColiMPN gDM−1In 1 g of sample as it is.; n (1) = 5; c (2) = 1; m (3) = 1000 CFU g−1; m (4) = 5000 CFU g−1--
Total ColiformsMPN gDM−1-<10,000<10,000
Minimum titulusCaO-15% on DM--
SO3-10% on DM--
u.m.: unit of measure; MPN: most probable number; CFU: colony-forming unit; DM: dry matter.
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