phosphorus recycling
PHOSPHORUS RECYCLING – A WAY TO REPLACE THE CONVENTIONAL PFERTILIZERS BY P-PRODUCTS DERIVED FROM SEWAGE SLUDGE S. VACLAVKOVA*, M. ŠYC*, M. POHOŘELÝ*, B. ZACH*, J. MOŠKO*, K. SVOBODA*, M. PUNČOCHÁŘ * Institute of Chemical Process Fundamentals of the CAS, v.v.i., Environmental Process Engineering Laboratory, Rozvojova 135, 165 02 Praha 6 – Suchdol, Czech Republic
SUMMARY: Paper is discussing possibilities and limitations of phosphorus recovery from sewage sludge ash, a secondary raw material derived from wastewater. The crucial factors determining the value of this secondary source for possible P-fertilizer production are pointed out. Presented data point the influence of industrial sewage sludge incineration on the bioavailability of sewage sludge ash phosphorus in different soil types. Additionally, the limits given by heavy metal contamination of sewage sludge ash are shown.
1. INTRODUCTION Phosphorus (P), the 11th most abundant element in Earth, is an essential nutrient for plants as well as animals. P is incorporated in tissue structures such as bones or teeth, P as part of DNA and RNA molecules contributes to genetic information transfer and at the same time plays a key role in mammalian energy metabolism as part of ATP and ADP molecules. P is therefore non-replaceable part of human diet. In order to secure well nutrient balanced diet, mineral nutrient fertilizers are used in crop production. The main source of P for mineral fertilizer production is phosphate rock, the apatite (with general formula Ca10(PO4)6X2 (X = F-, OH-, Cl-, CO32-), which consist of about 5-40% of P2O5. About 85 to 90% of world's remaining reserves of phosphate rock are controlled by five countries only, i.e. Morocco, China, Algeria, Syria and Jordan. With the increasing world population, rising of its living standards and changes in diet (higher contribution of meat) the demand for P fertilizers increases. Consequently, the European Commission classifies phosphate rock as a critical raw material. Recycling secondary and waste materials for fertilizer use is therefore becoming an important part of circular economy, reducing resource depletion and supply risk. Studies on secondary sources potential show that P obtained by recycling from local secondary sources might cover about half of the P used in mineral fertilizers in Europe yearly and thus significantly reduce the dependence of European countries on imports of phosphate rock. The most promising secondary sources of P are, due to their annual production, municipal wastewater and related waste streams, especially sewage sludge.
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2. WASTEWATER AND SEWAGE SLUDGE AS NUTRIENT P SOURCES 2.1 Properties and handling of wastewater and sewage sludge as potential P sources In principle, it is possible to obtain P directly from wastewater, or from different flows during its treatment. A common practice of obtaining P in a form of struvite precipitate directly from wastewater stream shows low efficiency. Moreover, most of P from the incoming wastewater flow is concentrated in the sewage sludge with the common practice of P precipitation by Al or Fe salts at wastewater treatment plants. Typically, sewage sludge contains about 10-20% of P2O5, which is comparable with phosphate rock. Direct agricultural application of wastewater and sewage sludge, as the simplest way of P recycling, is decreasing in many European countries due to the potential health and environmental risk associated with wastewater contamination by organic compounds (including hormones, antibiotics, endocrine disruptors and persistent organic pollutants), heavy metal compounds and pathogens. Complete removal of organic pollutants from sewage sludge is possible by incineration. P and heavy metals remain mostly in the resulting sewage sludge ash, which typically contains between 15 and 25% of P2O5, when sewage sludge is incinerated separately. With this P content and annual sewage sludge ash production in developed countries, reaching to over 30 million tons annually, sewage sludge ash must be considered as promising secondary raw material for P fertilizer production. However, the practice of incinerating sewage sludge differs country by country: while in Switzerland and the Netherlands, the entire amount of sewage sludge is incinerated. In the Czech Republic and Slovakia incineration doesn’t play an important role in sewage sludge treatment yet. 2.2 Determinants of the nutrient P source quality There are two main factors limiting the potential use of P from the sewage sludge ash for agricultural and feed production purposes: the purity i.e. the presence and the amount of contaminants - particularly heavy metals and the efficiency, i.e. the bioavailability of P provided, as a measure of crops ability to use provided P. Both purity as well as bioavailability are dependent not only on the total content of contaminants and P, but also on their solubility, thus mobility and bioavailability. In general, a compound is mobile or bioavailable when soluble in soil solution. When considering the use of certain nutrient fertilizers, soil pH is the most informative single soil characteristic, as soil properties including plant availability of nutrients, microbial activity, base saturation or soil structure depend on pH. Soil pH affects the solubility and specific adsorption of anions such as phosphate, oxalate and heavy metals plus it influences the solubility of Al3+ and Mn2+ ions. Soil may be divided into six basic groups according to its pH: Table 1. Basic soil types by pH Soil type Calcareous soil Neutral soil Slightly acidic soil Moderately acidic soil Strongly acidic soil Very strongly acidic soil Extremely acidic soil
Typical pH over 8 7 7-6 6-5 5-4 4-3 below 3
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2.2.1 Purity - contaminants in sewage sludge ash Heavy metals in sewage sludge ash are related to various industries, households, ground surface activities, and corrosion of different pipe and construction metallic systems. Among toxic metals As, Cd, Co, Cr, Cu, Hg, Pb, Mn, Ni, Sb, Se, Sn, Tl, V, and Zn are often identified in sewage sludge, where Zn is typically the most abundant. Pb, Cu, Cr, Ni, Mn, Cd and As may also be found in significant concentrations, whereas Hg typically occurs in low concentrations. To be fair, it is necessary to mention, that a majority of phosphate rock resources is contaminated by heavy metals like U, Cd or Th. 2.2.2 Estimating the P bioavailability in sewage sludge ash The bioavailability is referred to as the state of compound in which the crops or (micro)organisms are able to utilize the compound. P is bioavailable when dissolved, where P solubility is highly related to soil pH. Nearly all of the P absorbed by plants is taken up in a form of two inorganic phosphate ions (HPO42- or H2PO4-). HPO42- is the most abundant in calcareous soils and the form of P absorbed when crops are grown on these soils. H2PO4- form is the most abundant in acidic soils and the dominant form of P absorbed by plants when the soil pH is less than 7. Commonly used methods to determine P-bioavailability, i.e. the content of bioavailable species, are single or sequential chemical extractions, a reaction based techniques. State of the art extraction tests for predicting the P bioavailability in current P fertilizer include extraction with water, citric acid, neutral/alkaline ammonium citrate, mineral acids or weak basis, depending on soil pH desired. With the same logic, secondary raw materials, such as sewage sludge ash should be analysed for P bioavailability.
3. QUALITY OF SEWAGE SLUDGE ASH AS NUTRIENT P RESOURCE 3.1 P content in sewage sludge ash and its bioavailability in different soil types The total P content and its potential bioavailability in different soil types were estimated in well described representative sewage sludge ash samples from fluidized bed combustion at industrial scale. It is expected that in accordance with the conventional P fertilizers, the three most important parameters determining the secondary raw material quality will be: the total P content, the solubility in neutral ammonium citrate and the water solubility. The neutral ammonium solubility is understood as an indication of long term bioavailability (slow solubility in neutral soils), while the water solubility is indicating immediate bioavailability. However, as there is a wide range of soil types in different countries, wide range of methods for estimation of P fertilizer quality is standardized among them. In the Czech Republic fertilizers are tested for P availability in weak citric acid (for use at slightly to moderately acidic soils) and by Olsen’s method (for use at calcareous soils, with slightly alkaline pH). 3.1.1 Methods used The total content of P was determined after acid digestion in aqua regia solution according to ISO 11466 methodology. The neutral ammonium soluble P content was estimated in accordance with the Regulation (EC) No 2003/2003, the Olsen’s extraction was performed in accordance with the ISO 11263, the water solubility extraction as well as citric acid extraction were performed in accordance with method by Pierzynski. Extracted P was then determined by ICP-OES (inductively coupled plasma optical emission spectroscopy).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.1.2 Results
Figure 1. Total P content in sewage sludge ash and its solubility in different soil types with indicated ratio of solubility above (0-100%) 3.2 Contaminant content in the sewage sludge ash and its mobility As organic substances are expected to be completely decomposed during sewage sludge incineration, non-easily volatile heavy metals are expected to be the main contaminants present in sewage sludge ash. According to current fertilizer regulations (EC Regulation No. 2003/2003 on fertilizers and Czech Decree No. 437/2016 on the conditions for the use of treated sludge on agricultural land), it is mandatory to determine the total fertilizer contamination by As, Cd, Cr, Ni and by Pb. The quality of sewage sludge ash is determined by the quality of sewage sludge incinerated. As mentioned above, P is in the sewage sludge often present in the form of Al or Fe salts, depending on the precipitation agent used during wastewater treatment. Therefore, total contents of Al and Fe was determined as well. Total contents of Ca and Mg, as naturally phosphate forming elements, and total contents of Cu and Zn, as the most common additives in metallic pipe and construction systems were determined additionally. Immediate release of heavy metals to the environment may not be expected unless acidic conditions occurs, but after mid- to long-term exposure to even neutral soil solution conditions, release is more expectable. To describe this possible effect named metallic elements solubility in neutral ammonium citrate was determined. 3.2.1 Methods used The same methods as described in chapter 3.1.1 were used.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.2.2 Results
Figure 2. Total metallic content in sewage sludge ash and its mid- to long-term solubility in neutral soil 3.3 Shown limitations of using P fertilizers on different soils Despite the fact, that the total content of P in sewage sludge ash is comparable with phosphate rock (about 90 g.kg-1), plants are able to use it as nutrient only in acidic soils. As soon as pH of the soil increases, P bioavailability sharply decreases and apparently is dependent also on the composition of soil solution. While the immediate bioavailability (solubility in water) is shown to be very low (less than 1%), long term bioavailability is only about 35% of P total content. In calcareous soils P is almost unavailable for plants and organisms. Similar results were shown in German studies, where the P bioavailability (in slightly acidic conditions) was shown to vary between 18 and 56% of total P content. About one third of Czech soil is neutral or calcareous and therefore additional treatment of sewage sludge ash might be desired to increase the P solubility (bioavailability). The total content of some monitored heavy metals would not meet Czech limits given for conventional fertilizers. Concentration of As is occurring around the limit, Cr and Pb concentrations exceed set limits. Ni and Cd concentration are under the limits for conventional Czech fertilizers. When considering heavy metals possible release to neutral soil solution after mid- to long-term exposure only Pb shows solubility exceeding the allowed content. Due to shown total concentrations of monitored heavy metals and relatively low bioavailability of P in neutral and calcareous soils, direct agricultural application of sewage sludge ash produced in current industrial technologies might not be advised. More than 50 technologies of post incineration treatment of sewage sludge are according to some studies currently under different stage of development, but none of them is at a full industrial scale, due to their extremely high technological and economical demands.
4. CONCLUSION In presented contribution, the P bioavailability in different soil types and level of heavy metal contamination in the representative sample of industrially incinerated sewage sludge is shown. Although different methods for its post-treatment increasing P bioavailability and separating heavy metals are under development with the current P prices, none of the processes for P
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
recycling from secondary raw materials (including SSA) seems to be economically competitive with P production from the phosphate rock. Moreover, rules for the use of sewage sludge derived fertilizers are currently missing limiting the possibilities of their use. Some countries, i.e. Germany, Austria and Switzerland, lately opt for mandatory P recovery from sewage sludge produced at large wastewater treatment plants, opening the possibilities for recovered P sources on the market. Authors of this contribution believe, that the way of handling sewage sludge in order to recover its nutrient, especially P value, should be diversified. Depending on the used technology of wastewater treatment and characteristic of source area, sewage sludge might not need organic contaminants treatment thus incineration.
AKNOWLEDGEMENTS This work was conducted within the Waste-to-Energy Competence Centre (project no. TE02000236) with support from the Technology Agency of the Czech Republic.
REFERENCES Aydin F., Saydut A., Aydin I., Hamamci C. (2011). Phosphorus speciation in coal bottom ash. At. Spectrosc., vol. 32, 194–199. Elberling B., Borggaard O.K. (2007). Pedological Biogeochemistry. Copenhagen, Denmark, 2007. Cordell D., Drangert J.O., White, S. (2009). The story of phosphorus: Global food security and food for thought. Glob. Environ. Chang., vol. 19, 292–305. Egle L., Rechberger H., Zessner M. (2015). Overview and description of technologies for recovering phosphorus from municipal wastewater. Resour. Conserv. Recycl. vol. 105, 325– 346. Egle L., Rechberger H., Krampe J., Zessner M. (2016). Phosphorus recovery from municipal wastewater: An integrated comparative technological, environmental and economic assessment of P recovery technologies. Sci. Total Environ., vol. 571, 522–542. Herzel H., Krüger O., Hermann L., Adam C. (2016). Sewage sludge ash - A promising secondary phosphorus source for fertilizer production. Sci. Total Environ., vol. 542, 1136–1143. Kürger O., Adam C. (2017). Phosphorus in recycling fertilizers - analytical challenges. Environ. Res., vol. 155, 353–358. Krüger O. (2016). Recycled fertilizers: Do we need new regulations and analytical methods? Waste Manag., vol. 50, 1–2. Metson G.S., Bennett E.M., Elser J.J. (2012). The role of diet in phosphorus demand. Environ. Res. Lett., vol. 7, 44043. Nanzer S., Oberson A., Huthwelker T., Eggenberger U., Frossard E. (2014). J. Environ. Qual., vol. 43, 1050–1060. Pagliari P. H., Kaiser D. E., Rosen C. J., Lamb J. A. (2016). FO-0792-F Understanding Phosphorus in Minnesota Soils, University of Minesota Extension, 2016. Pierzynski G.M., Sims J.T., Vance G.F. (2000). Soils and Environmental Quality. CRC Press. Boca Raton, Florida. 459 p. Schröder J.J., Cordell D., Smit A.L., Rosemarin A. (2010). Sustainable Use of Phosphorus,
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Report 357. Wageningen, Netherlands, 2010 Zoboli O., Zessner M., Rechberger H. (2016). Supporting phosphorus management in Austria: Potential, priorities and limitations. Sci. Total Environ., vol. 565, 313–323.
SUMMARY: Paper is discussing possibilities and limitations of phosphorus recovery from sewage sludge ash, a secondary raw material derived from wastewater. The crucial factors determining the value of this secondary source for possible P-fertilizer production are pointed out. Presented data point the influence of industrial sewage sludge incineration on the bioavailability of sewage sludge ash phosphorus in different soil types. Additionally, the limits given by heavy metal contamination of sewage sludge ash are shown.
1. INTRODUCTION Phosphorus (P), the 11th most abundant element in Earth, is an essential nutrient for plants as well as animals. P is incorporated in tissue structures such as bones or teeth, P as part of DNA and RNA molecules contributes to genetic information transfer and at the same time plays a key role in mammalian energy metabolism as part of ATP and ADP molecules. P is therefore non-replaceable part of human diet. In order to secure well nutrient balanced diet, mineral nutrient fertilizers are used in crop production. The main source of P for mineral fertilizer production is phosphate rock, the apatite (with general formula Ca10(PO4)6X2 (X = F-, OH-, Cl-, CO32-), which consist of about 5-40% of P2O5. About 85 to 90% of world's remaining reserves of phosphate rock are controlled by five countries only, i.e. Morocco, China, Algeria, Syria and Jordan. With the increasing world population, rising of its living standards and changes in diet (higher contribution of meat) the demand for P fertilizers increases. Consequently, the European Commission classifies phosphate rock as a critical raw material. Recycling secondary and waste materials for fertilizer use is therefore becoming an important part of circular economy, reducing resource depletion and supply risk. Studies on secondary sources potential show that P obtained by recycling from local secondary sources might cover about half of the P used in mineral fertilizers in Europe yearly and thus significantly reduce the dependence of European countries on imports of phosphate rock. The most promising secondary sources of P are, due to their annual production, municipal wastewater and related waste streams, especially sewage sludge.
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2. WASTEWATER AND SEWAGE SLUDGE AS NUTRIENT P SOURCES 2.1 Properties and handling of wastewater and sewage sludge as potential P sources In principle, it is possible to obtain P directly from wastewater, or from different flows during its treatment. A common practice of obtaining P in a form of struvite precipitate directly from wastewater stream shows low efficiency. Moreover, most of P from the incoming wastewater flow is concentrated in the sewage sludge with the common practice of P precipitation by Al or Fe salts at wastewater treatment plants. Typically, sewage sludge contains about 10-20% of P2O5, which is comparable with phosphate rock. Direct agricultural application of wastewater and sewage sludge, as the simplest way of P recycling, is decreasing in many European countries due to the potential health and environmental risk associated with wastewater contamination by organic compounds (including hormones, antibiotics, endocrine disruptors and persistent organic pollutants), heavy metal compounds and pathogens. Complete removal of organic pollutants from sewage sludge is possible by incineration. P and heavy metals remain mostly in the resulting sewage sludge ash, which typically contains between 15 and 25% of P2O5, when sewage sludge is incinerated separately. With this P content and annual sewage sludge ash production in developed countries, reaching to over 30 million tons annually, sewage sludge ash must be considered as promising secondary raw material for P fertilizer production. However, the practice of incinerating sewage sludge differs country by country: while in Switzerland and the Netherlands, the entire amount of sewage sludge is incinerated. In the Czech Republic and Slovakia incineration doesn’t play an important role in sewage sludge treatment yet. 2.2 Determinants of the nutrient P source quality There are two main factors limiting the potential use of P from the sewage sludge ash for agricultural and feed production purposes: the purity i.e. the presence and the amount of contaminants - particularly heavy metals and the efficiency, i.e. the bioavailability of P provided, as a measure of crops ability to use provided P. Both purity as well as bioavailability are dependent not only on the total content of contaminants and P, but also on their solubility, thus mobility and bioavailability. In general, a compound is mobile or bioavailable when soluble in soil solution. When considering the use of certain nutrient fertilizers, soil pH is the most informative single soil characteristic, as soil properties including plant availability of nutrients, microbial activity, base saturation or soil structure depend on pH. Soil pH affects the solubility and specific adsorption of anions such as phosphate, oxalate and heavy metals plus it influences the solubility of Al3+ and Mn2+ ions. Soil may be divided into six basic groups according to its pH: Table 1. Basic soil types by pH Soil type Calcareous soil Neutral soil Slightly acidic soil Moderately acidic soil Strongly acidic soil Very strongly acidic soil Extremely acidic soil
Typical pH over 8 7 7-6 6-5 5-4 4-3 below 3
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
2.2.1 Purity - contaminants in sewage sludge ash Heavy metals in sewage sludge ash are related to various industries, households, ground surface activities, and corrosion of different pipe and construction metallic systems. Among toxic metals As, Cd, Co, Cr, Cu, Hg, Pb, Mn, Ni, Sb, Se, Sn, Tl, V, and Zn are often identified in sewage sludge, where Zn is typically the most abundant. Pb, Cu, Cr, Ni, Mn, Cd and As may also be found in significant concentrations, whereas Hg typically occurs in low concentrations. To be fair, it is necessary to mention, that a majority of phosphate rock resources is contaminated by heavy metals like U, Cd or Th. 2.2.2 Estimating the P bioavailability in sewage sludge ash The bioavailability is referred to as the state of compound in which the crops or (micro)organisms are able to utilize the compound. P is bioavailable when dissolved, where P solubility is highly related to soil pH. Nearly all of the P absorbed by plants is taken up in a form of two inorganic phosphate ions (HPO42- or H2PO4-). HPO42- is the most abundant in calcareous soils and the form of P absorbed when crops are grown on these soils. H2PO4- form is the most abundant in acidic soils and the dominant form of P absorbed by plants when the soil pH is less than 7. Commonly used methods to determine P-bioavailability, i.e. the content of bioavailable species, are single or sequential chemical extractions, a reaction based techniques. State of the art extraction tests for predicting the P bioavailability in current P fertilizer include extraction with water, citric acid, neutral/alkaline ammonium citrate, mineral acids or weak basis, depending on soil pH desired. With the same logic, secondary raw materials, such as sewage sludge ash should be analysed for P bioavailability.
3. QUALITY OF SEWAGE SLUDGE ASH AS NUTRIENT P RESOURCE 3.1 P content in sewage sludge ash and its bioavailability in different soil types The total P content and its potential bioavailability in different soil types were estimated in well described representative sewage sludge ash samples from fluidized bed combustion at industrial scale. It is expected that in accordance with the conventional P fertilizers, the three most important parameters determining the secondary raw material quality will be: the total P content, the solubility in neutral ammonium citrate and the water solubility. The neutral ammonium solubility is understood as an indication of long term bioavailability (slow solubility in neutral soils), while the water solubility is indicating immediate bioavailability. However, as there is a wide range of soil types in different countries, wide range of methods for estimation of P fertilizer quality is standardized among them. In the Czech Republic fertilizers are tested for P availability in weak citric acid (for use at slightly to moderately acidic soils) and by Olsen’s method (for use at calcareous soils, with slightly alkaline pH). 3.1.1 Methods used The total content of P was determined after acid digestion in aqua regia solution according to ISO 11466 methodology. The neutral ammonium soluble P content was estimated in accordance with the Regulation (EC) No 2003/2003, the Olsen’s extraction was performed in accordance with the ISO 11263, the water solubility extraction as well as citric acid extraction were performed in accordance with method by Pierzynski. Extracted P was then determined by ICP-OES (inductively coupled plasma optical emission spectroscopy).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.1.2 Results
Figure 1. Total P content in sewage sludge ash and its solubility in different soil types with indicated ratio of solubility above (0-100%) 3.2 Contaminant content in the sewage sludge ash and its mobility As organic substances are expected to be completely decomposed during sewage sludge incineration, non-easily volatile heavy metals are expected to be the main contaminants present in sewage sludge ash. According to current fertilizer regulations (EC Regulation No. 2003/2003 on fertilizers and Czech Decree No. 437/2016 on the conditions for the use of treated sludge on agricultural land), it is mandatory to determine the total fertilizer contamination by As, Cd, Cr, Ni and by Pb. The quality of sewage sludge ash is determined by the quality of sewage sludge incinerated. As mentioned above, P is in the sewage sludge often present in the form of Al or Fe salts, depending on the precipitation agent used during wastewater treatment. Therefore, total contents of Al and Fe was determined as well. Total contents of Ca and Mg, as naturally phosphate forming elements, and total contents of Cu and Zn, as the most common additives in metallic pipe and construction systems were determined additionally. Immediate release of heavy metals to the environment may not be expected unless acidic conditions occurs, but after mid- to long-term exposure to even neutral soil solution conditions, release is more expectable. To describe this possible effect named metallic elements solubility in neutral ammonium citrate was determined. 3.2.1 Methods used The same methods as described in chapter 3.1.1 were used.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
3.2.2 Results
Figure 2. Total metallic content in sewage sludge ash and its mid- to long-term solubility in neutral soil 3.3 Shown limitations of using P fertilizers on different soils Despite the fact, that the total content of P in sewage sludge ash is comparable with phosphate rock (about 90 g.kg-1), plants are able to use it as nutrient only in acidic soils. As soon as pH of the soil increases, P bioavailability sharply decreases and apparently is dependent also on the composition of soil solution. While the immediate bioavailability (solubility in water) is shown to be very low (less than 1%), long term bioavailability is only about 35% of P total content. In calcareous soils P is almost unavailable for plants and organisms. Similar results were shown in German studies, where the P bioavailability (in slightly acidic conditions) was shown to vary between 18 and 56% of total P content. About one third of Czech soil is neutral or calcareous and therefore additional treatment of sewage sludge ash might be desired to increase the P solubility (bioavailability). The total content of some monitored heavy metals would not meet Czech limits given for conventional fertilizers. Concentration of As is occurring around the limit, Cr and Pb concentrations exceed set limits. Ni and Cd concentration are under the limits for conventional Czech fertilizers. When considering heavy metals possible release to neutral soil solution after mid- to long-term exposure only Pb shows solubility exceeding the allowed content. Due to shown total concentrations of monitored heavy metals and relatively low bioavailability of P in neutral and calcareous soils, direct agricultural application of sewage sludge ash produced in current industrial technologies might not be advised. More than 50 technologies of post incineration treatment of sewage sludge are according to some studies currently under different stage of development, but none of them is at a full industrial scale, due to their extremely high technological and economical demands.
4. CONCLUSION In presented contribution, the P bioavailability in different soil types and level of heavy metal contamination in the representative sample of industrially incinerated sewage sludge is shown. Although different methods for its post-treatment increasing P bioavailability and separating heavy metals are under development with the current P prices, none of the processes for P
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
recycling from secondary raw materials (including SSA) seems to be economically competitive with P production from the phosphate rock. Moreover, rules for the use of sewage sludge derived fertilizers are currently missing limiting the possibilities of their use. Some countries, i.e. Germany, Austria and Switzerland, lately opt for mandatory P recovery from sewage sludge produced at large wastewater treatment plants, opening the possibilities for recovered P sources on the market. Authors of this contribution believe, that the way of handling sewage sludge in order to recover its nutrient, especially P value, should be diversified. Depending on the used technology of wastewater treatment and characteristic of source area, sewage sludge might not need organic contaminants treatment thus incineration.
AKNOWLEDGEMENTS This work was conducted within the Waste-to-Energy Competence Centre (project no. TE02000236) with support from the Technology Agency of the Czech Republic.
REFERENCES Aydin F., Saydut A., Aydin I., Hamamci C. (2011). Phosphorus speciation in coal bottom ash. At. Spectrosc., vol. 32, 194–199. Elberling B., Borggaard O.K. (2007). Pedological Biogeochemistry. Copenhagen, Denmark, 2007. Cordell D., Drangert J.O., White, S. (2009). The story of phosphorus: Global food security and food for thought. Glob. Environ. Chang., vol. 19, 292–305. Egle L., Rechberger H., Zessner M. (2015). Overview and description of technologies for recovering phosphorus from municipal wastewater. Resour. Conserv. Recycl. vol. 105, 325– 346. Egle L., Rechberger H., Krampe J., Zessner M. (2016). Phosphorus recovery from municipal wastewater: An integrated comparative technological, environmental and economic assessment of P recovery technologies. Sci. Total Environ., vol. 571, 522–542. Herzel H., Krüger O., Hermann L., Adam C. (2016). Sewage sludge ash - A promising secondary phosphorus source for fertilizer production. Sci. Total Environ., vol. 542, 1136–1143. Kürger O., Adam C. (2017). Phosphorus in recycling fertilizers - analytical challenges. Environ. Res., vol. 155, 353–358. Krüger O. (2016). Recycled fertilizers: Do we need new regulations and analytical methods? Waste Manag., vol. 50, 1–2. Metson G.S., Bennett E.M., Elser J.J. (2012). The role of diet in phosphorus demand. Environ. Res. Lett., vol. 7, 44043. Nanzer S., Oberson A., Huthwelker T., Eggenberger U., Frossard E. (2014). J. Environ. Qual., vol. 43, 1050–1060. Pagliari P. H., Kaiser D. E., Rosen C. J., Lamb J. A. (2016). FO-0792-F Understanding Phosphorus in Minnesota Soils, University of Minesota Extension, 2016. Pierzynski G.M., Sims J.T., Vance G.F. (2000). Soils and Environmental Quality. CRC Press. Boca Raton, Florida. 459 p. Schröder J.J., Cordell D., Smit A.L., Rosemarin A. (2010). Sustainable Use of Phosphorus,
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Report 357. Wageningen, Netherlands, 2010 Zoboli O., Zessner M., Rechberger H. (2016). Supporting phosphorus management in Austria: Potential, priorities and limitations. Sci. Total Environ., vol. 565, 313–323.
