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1 PROPERTIES OF BIOSOLIDS FROM SLUDGE TREATMENT WETLANDS FOR 2 LAND APPLICATION 3 4 Enrica Uggetti1, Ivet Ferrer1, Esther Llorens1, David Güell2, Joan García1,* 5 6 1 7 Environmental Engineering, Technical University of Catalonia (UPC) 8 c/ Jordi Girona 1-3, Building D1, E-08034 Barcelona, Spain. 9 E-mail 10 Environmental Engineering Division, addresses: [email protected], Department of Hydraulic, Maritime and [email protected], [email protected], [email protected] 11 Depuradores d’Osona, S.L. c/Historiador Ramon d’Abadal i de Vinyals 5, 3r, E-08500 Vic, 12 2 13 Spain 14 15 * Corresponding author: 16 Tel: +34 934016464 17 Fax: +34 934017357 18 E-mail address: [email protected] 19 1 20 Abstract 21 22 Sludge treatment wetlands consist of constructed wetlands which have been upgraded for 23 sludge treatment over the last decades. Sludge dewatering and stabilisation are the main 24 features of this technology, leading to a final product which may be recycled as an organic 25 fertiliser or soil conditioner. In this study, biosolids from full-scale treatment wetlands were 26 characterised in order to evaluate the quality of the final product for land application, even 27 without further post-treatment such as composting. Samples of influent and treated sludge 28 were analysed for pH, Electrical Conductivity, Total Solids (TS), Volatile Solids (VS), 29 Chemical Oxygen Demand (COD), Dynamic Respiration Index (DRI), nutrients (Total 30 Kjeldahl Nitrogen (TKN), Total Phosphorus (TP) and Potasium (K)), heavy metals and faecal 31 bacteria indicators (E. coli and Salmonella spp.). According to the results, sludge water 32 content and therefore sludge volume are reduced by 25 %. Organic matter biodegradation 33 leads to VS around 43-44 %TS and COD around 500 g·kgTS-1. The values of DRI24h (1000- 34 1500 mgO2∙kgTS-1∙h-1) indicate that treated sludge is almost stabilised final product. Besides, 35 the concentration of nutrients is quite low (TKN~4 %TS, TP~0.3 %TS and K~0.2-0.6 %TS). 36 Both heavy metals and faecal bacteria indicators meet current legal limits for land application 37 of the sludge. Our results suggest that biosolids from the studied treatment wetlands could be 38 valorised in agriculture, especially as soil conditioners. 39 40 Keywords: Compost, Reed Beds, Organic Waste, Wastewater. 41 2 42 1. Introduction 43 44 Sewage sludge is the organic waste generated by wastewater treatment processes, after solid 45 and liquid separation units. The amount of sludge produced and its composition depend on the 46 influent’s characteristics and wastewater treatment used. Sludge production in conventional 47 activated sludge processes ranges from 60 to 80 g of total solids per person per day . In 48 Europe, the Urban Wastewater Treatment Directive 91/271/EEC (Council of the European 49 Union, 1991) promoted the implementation of wastewater treatment plants (WWTP) with 50 secondary wastewater treatment in municipalities above 2000 Persons Equivalent (PE); and 51 the Water Framework Directive (Council of the European Union, 2000) encouraged 52 wastewater treatment even in municipalities below 500 PE. As a result sludge production has 53 increased in the European Union by 50 % since 2005 (Fytili and Zabaniotou, 2008). For 54 instance, in Catalonia (Spain) around 50 % of the WWTP (170) were constructed between 55 2000 and 2006 (Agencia Catalana del Agua, 2007). 56 57 In Spain, in order to manage the increasing amount of sludge produced the following 58 hierarchy was proposed (Consejo de Ministros, 2001): 1) valorisation in agriculture, 2) 59 energetic valorisation, and 3) landfilling. Agricultural valorisation is nowadays preferred to 60 landfilling, since sludge recycling ensures the return of organic constituents, nutrients and 61 microelements to crop fields which eases the substitution of chemical fertilizers 62 (Oleszkiewicz and Mavinic, 2002). Sludge disposal onto agricultural land is regulated by the 63 European Sludge Directive, which sets up land application of sewage sludge based on 64 maximum heavy metals concentrations (Council of the European Union, 1986). Recent 3 65 regulation proposals are more restrictive in terms of heavy metals, and also consider emerging 66 pollutants and microbial faecal indicators (Environment DG, EU, 2000). 67 68 In practice, sludge treatment systems have to provide a final product suitable for land 69 application (fulfilling legislation requirements), with reasonable investment as well as 70 operational and maintenance costs. In this sense, sludge treatment wetlands might be regarded 71 as a recent technology for sludge management, which is particularly appropriate for small 72 communities from both an economical and environmental point of view. 73 74 Treatment wetlands (TW) reproduce self-cleaning processes occurring in natural wetlands 75 and are being used for wastewater treatment in many regions of the world (Caselles-Osorio et 76 al., 2007). Since the late 1980s, TW have been adapted for sludge treatment developing a 77 technology that is nowadays used in most European countries and in North America (Uggetti 78 et al., 2010). 79 80 Sludge treatment wetlands consist of shallow tanks filled with a gravel layer and planted with 81 emergent rooted wetland plants such as Phragmites australis (common reed) (Cole, 1998). 82 Thickened secondary sludge is pumped and spread on the wetland’s surface. Here, part of the 83 sludge water content is rapidly drained by gravity through the gravel layer; while another part 84 is evapotranspirated by the plants. In this way, a concentrated sludge residue remains on the 85 surface of the bed where, after the resting time, thickened sludge is anew spread, starting the 86 following feeding cycle. 87 4 88 The roots of the plants ease oxygen transfer to the gravel and sludge layers, creating aerobic 89 microsites that promote sludge mineralization and stabilization (Reed et al., 1988). 90 Furthermore, the complex root system maintains pores and small channels within sludge layer 91 that preserve the drainage efficiency through the bed (Nielsen, 2003b). When the sludge is 92 dry, the movement of plant stems by the wind prompts the cracking of the surface, improving 93 the aeration of the sludge layer. 94 95 During feeding periods, the sludge layer height increases at a certain rate (around 10 cm·year- 96 1 97 period (from 1–2 months to 1 year), aimed at improving sludge dryness and mineralisation. 98 The final product is subsequently withdrawn, starting the following operating cycle. ). When the layer approaches the top of the tank, feeding is stopped during a final resting 99 100 The quality of this product is the result of both dewatering processes (draining and 101 evapotranspiration) and organic matter biodegradation (Nielsen, 2003b). According to 102 Nielsen and Willoughby (2005) it is suitable for land application; although further post- 103 treatments might be required to improve sludge hygienisation (Zwara and Obarska- 104 Pempkowiak, 2000). Nevertheless, detailed studies on the properties of biosolids from 105 treatment wetlands are still lacking in the literature. 106 107 In this study, full-scale treatment wetlands were evaluated with the aim of studying the 108 efficiency of the process in terms of sludge dewatering, mineralization and hygienisation; 109 while assessing the quality of the final product for land application. To this end, physico- 110 chemical and microbiological parameters, together with stability indexes, were considered as 111 proposed in the European Sludge Directive (Council of the European Union (1986)), the 3rd 5 112 Draft EU Working Document on Sludge (Environment DG, EU, 2000) and the 2nd Draft EU 113 Working Document on Biological Treatment of Biowaste (Environment DG, EU, 2001). 114 115 2. Materials and methods 116 117 2.1. System’s description 118 119 The studied full-scale treatment wetlands are located at the WWTP of Seva (1500 PE), in the 120 province of Barcelona (Catalonia, Spain). The main characteristics of the facility are 121 summarised in Table 1. The sludge treatment wetlands were set-up in 2000 by transforming 122 existing conventional drying beds. They are planted with Phragmites australis. The total 123 surface area is 175 m2 and the sludge loading rate around 125 kg TS/m2·year, much higher 124 than the recommended value of 50-60 kgTS·m-2·year-1 (Burgoon et al., 1997; Edwards et al., 125 2001; Nielsen, 2003a). Other details on the design and operation of the wetlands may be 126 found in Uggetti et al. (2009). 127 128 The first operating cycle lasted about 5 years; the sludge was then removed and the process 129 re-started between 2004 and 2005. The second operating cycle was finished by the end of 130 2008 in two of the wetlands (named 1 and 2). After a resting period of 4 months, the wetlands 131 were emptied with a power shovel and the final product was thereafter transported to a 132 composting plant. 133 134 2.2. Sludge sampling and characterization 135 6 136 When the wetlands were emptied (in 2008), the layer of dry sludge was about 60 cm high. 137 Two composite samples were prepared by mixing subsamples from each wetland; while an 138 integrated influent sample was obtained from subsamples collected during a whole week. 139 140 As recommended in the literature (Mujeriego and Carbó, 1994; Obarska-Pempkowiak et al., 141 2003b; Soliva, 2001), the sludge quality was characterised in terms of: pH, Electrical 142 Conductivity (EC), Total and Volatile Solids (TS and VS), Chemical Oxygen Demand 143 (COD), Total Kjehldahl Nitrogen (TKN), Potassium (K), Total Phosphorous (TP), heavy 144 metals and faecal bacteria indicators (Salmonella spp. and Escherichia coli). Additionally, the 145 Dynamic Respiration Index (DRI) was determined according to Adani et al. (2000) and 146 Barrena et al. (2009). 147 148 Sludge samples were analysed following the Standard Methods (APHA-AWWA-WPCF, 149 2001). Samples for COD, TKN, TP, K and heavy metals’ analyses were previously air dried 150 at room temperature (until constant weight); hence the results are expressed on a dry matter 151 basis (per kg or %TS). Air dried samples were subsequently diluted in distilled water (1:5) for 152 pH and EC measurements. 153 154 3. Results and discussion 155 156 The efficiency of the process in terms of sludge dewatering and mineralisation is usually 157 evaluated by the increase in dry matter (TS) and the decrease in organic matter (VS and COD) 158 contents, respectively. However, total organic matter content is not sufficient to assess the 159 stability of the product; indeed information on the amount of readily biodegradable organic 7 160 matter fraction is also needed. For instance, it can be deduced from the DRI. Besides, the 161 concentration of nutrients, heavy metals and faecal bacteria indicators are used to determine 162 the quality of biosolids for its application on land as organic fertilizers. 163 164 Tables 2 and 3 show the main characteristics of the influent and treated sludge from the full- 165 scale treatment wetlands. The results are here examined and discussed. A comparison with 166 average compost characteristics is proposed to assess the requirement of additional 167 composting post-treatment. 168 169 3.1 Sludge dryness 170 171 Secondary sludge produced by the contact-stabilization process is spread on the beds with 172 very high water content (typically around 99 %) (Table 2). Sludge moisture is significantly 173 reduced down to about 75 % during the treatment and after a resting period of 4 months. The 174 sludge volume is consequently reduced by 25 %. This is the main goal of dewatering 175 processes. 176 177 The dryness of the final product (TS~25 %) is lower than that observed in other facilities after 178 a resting period of approximately one year (TS around 30-40 %) (Nielsen, 2003b). In this 179 sense, a previous study indicated a poor dewatering efficiency in Seva’s system compared to 180 other Catalan facilities (Uggetti et al., 2009). One possible reason for this is that the sludge 181 loading rate (125 kgTS·m-2·year-1) is over twice the recommended value (50-60 kgTS·m- 182 2 ·year-1) (Burgoon et al., 1997; Edwards et al., 2001; Nielsen, 2003a). This fact suggests that 8 183 the dryness of the final product could be further increased with a better system management 184 (i.e. reducing the sludge loading rate). 185 186 3.2 Organic matter content and stability 187 188 Table 2 shows organic matter contents expressed as VS and COD; as well as the DRI value. 189 The concentration of VS in the influent is quite low (~51 %TS) as a result of the high solids 190 retention time in the contact-stabilisation process.. Final values (43-45 %TS) are within the 191 range obtained after conventional sludge stabilisation techniques, such as anaerobic digestion 192 (Ferrer, 2008; Ferrer et al., 2008). Therefore, VS removal (7-8 %) is remarkably lower than 193 that usually observed in these type of processes with higher VS in the influents (Ferrer, 2008; 194 Ferrer et al., 2008). Again, the results are in accordance with a previous study on this system 195 (Uggetti el al., 2009). COD values indicate total organic matter reduction from 700 to some 196 500g·kgTS-1 (Table 2). 197 198 Comparisons with other systems are not straightforward, since the biodegradability of the 199 sludge depends on a number of parameters, including its nature and composition, amongst 200 others. For instance, VS were reduced to 20 %TS in pilot scale treatment wetlands in China 201 (Yubo et al., 2008). On the other hand, in compost samples organic contents are usually much 202 higher, around 60 % for compost of sewage sludge mixed with vegetable wastes (Bertan et 203 al., 2004), due to humic-like substances produced during composting process. 204 205 Organic matter in soil amendments can improve the properties and quality of soils, which is 206 essential to guarantee long-term soil fertility (Draeger et al.,1999). In particular, an increase in 9 207 organic matter content can improve physical properties (water retention, soil structure, water 208 infiltration, bulk density, porosity), chemical properties (cation exchange capacity, pH) and, 209 in some cases, biological properties (Moss et al., 2002, Andreoli et al., 2007). Such a 210 response depends on the sludge:soil ratio (Singh and Agrawal, 2008). 211 212 On the other hand, higher biological stability implies lower environmental impacts (like odour 213 generation, biogas production, leaching and pathogen’s re-growth) during land application of 214 the product (Mullet et al., 1998). Lasaridi et al. (1998) define biological stability as a 215 characteristic that determinates the extent to which readily biodegradable organic matter has 216 been decomposed. Referred to compost, the stability is a quality parameter related to the 217 microbial decomposition or microbial respiration activity of the composted matter (Komilis et 218 al., 2009). 219 220 The DRI is based on the rate of oxygen consumption and is a useful indicator of the biological 221 stability of a sample. In this study, the DRI24h from wetland samples ranged between 1100 222 and 1400 mgO2∙kgTS-1∙h-1. Such a stability degree is much higher than the values reported in 223 the literature for a mixture of primary and activated sludge (6680 mgO2∙kgTS-1∙h-1) and for 224 anaerobically digested sludge (3740 mgO2∙kgTS-1∙h-1) (Pagans et al., 2006). 225 226 In a recent study, Ponsá et al. (2008) analysed the DRI of the organic fraction of municipal 227 solid wastes at different stages of a mechanical biological treatment. These authors observed 228 DRI values above 7000 mgO2∙kgTS-1∙h-1 for the input material, a decrease to around 1500 229 mgO2∙kgTS-1∙h-1 for digested material and near 1000 mgO2∙kgTS-1∙h-1 for composted material, 230 with a value of 1000 mgO2∙kgTS-1∙h-1 for the output material. Similarly, Scaglia and Adani 10 231 (2008) found values around 2500 mgO2∙kgTS-1∙h-1 for input samples around 1100 232 mgO2∙kgTS-1∙h-1 for intermediate samples and between 300-600 mgO2∙kgTS-1∙h-1 for the final 233 product of the stabilisation process. 234 235 If DRI values are expressed with respect to the organic matter content (VS) of the sample, the 236 values from wetlands’ biosolids correspond to 490 and 610 mgO2∙kgVS-1∙h-1. According to 237 Scaglia et al. (2000) and Adani et al. (2004), stability values of 1000 mgO2∙VS-1∙h-1 are 238 representative of medium compost. This is in accordance with compost classes I and II 239 proposed by the American Society for Testing and Materials (1996), while compost classes III 240 and IV have higher biological stabilities (<500 mgO2∙VS-1∙h-1). 241 242 From the comparison of the obtained results whith those of other systems, it seems that 243 biosolids from the studied treatment wetlands may be considered a partially (and almost) 244 stabilised material. Therefore it can be speculated that, with sufficient resting time, the final 245 product may be valorised in agriculture even without further post-treatment in a composting 246 plant. Consequently, this would result in additional reduction of sludge treatment costs. 247 248 3.3 Nutrients 249 250 Sewage sludge may provide essential nutrients for plant growth. Biosolids are able to restore 251 nitrogen, phosphorus, sulphur and other nutrients in soils. The concentration of nutrients in 252 biosolids depends on sewage composition and treatment used, and on subsequent sludge 253 treatment processes. 254 11 255 Although they are essential for plant growth, nutrients (particularly nitrogen and phosphorus) 256 can be harmful when excessively applied. Different works have proved nitrogen accumulation 257 in soil (Walter et al., 2000, Hernandez et al., 1990); as well as phosphorus increase in sludge- 258 amended soils (Hernandez et al., 1990). It is well known that over application of nitrogen can 259 lead to nitrate contamination of groundwater; although such a risk is reduced if nutrients are 260 applied at agronomic rates (Moss et al., 2002). The great solubility of nitrate poses a high 261 contamination hazard to groundwater and is the main reason why biosolids application in 262 agricultural land is usually limited by the nitrogen uptake crop capability. In this sense, the 263 application rate must not lead to nitrogen inputs greater than the crop nitrogen requirements, 264 in order to avoid leaching to occur (Andreoli et al., 2007). 265 266 The concentration of the main nutrients (nitrogen, phosphate and potassium) in wetland 267 biosolids is shown Table 2. The results are consistent with a previous study carried out at the 268 same facility (Uggetti et al., 2009). 269 270 Sludge’s nitrogen comes from microbial biomass present in sludge and from wastewater 271 residues. In this study, TNK values (Table 2) decrease about a 50% from the influent to the 272 biosolids (~4 %TS). The values of the final sludge are therefore within the range of activated 273 sludge (Andreoli et al., 2007). For compost of sewage sludge, Bertran et al. (2004) give 274 slightly lower TNK values (2.53 %TS). Even lower TNK values (1.5 %TS) are given for 275 aerobically digested sewage sludge used on land (Gascó and Lobo, 2007). 276 277 Phosphorus in sludge comes from biomass formed during wastewater treatment, residues and 278 phosphate-containing detergents and soaps. Biosolids can be seen as phosphorus source 12 279 assuring a slow and continued release to plants (Andreoli et al., 2007). In this study, TP 280 values show a clear decrease from the influent to treated sludge (0.08-0.28 %TS). The values 281 of the final product are within the range of digested sludge (Gascó and Lobo, 2007) and quite 282 lower than in composted sewage sludge (Bertran et al., 2004). 283 284 The concentration of potasium does not seem to vary along the treatment, with values ranging 285 between 0.18 and 0.62 %TS. These values are in accordance with sludge compost (Bertran et 286 al., 2004); but lower than in digested sludge (Gascó and Lobo, 2007). 287 288 In general, sludge is characterized by a considerable variability in nutrient’s content, 289 depending on the wastewater source and treatment process (Moss et al., 2002). The 290 concentration of nutrients is needed to ensure appropriate dosages of the sludge prior to land 291 application. The required agricultural doses are frequently dependent on the fertilizer and soil 292 characteristics (Pomares and Canet, 2001; Andreoli et al., 2007). Since nitrogen concentration 293 in biosolids usually meets the crop needs, application rates are generally calculated based on 294 the nitrogen requirements of each crop, whereas phosphorus and potasium can be 295 supplemented with chemical fertilisers (Andreoli et al., 2007). 296 297 3.4 Heavy metals and faecal bacteria indicators 298 299 The main hazard associated to sludge application on agricultural soils is the potential long 300 term accumulation of toxic elements (Singh and Agrawal, 2008), which may then be uptaken 301 by crops. Such elements include both inorganic pollutants, like heavy metals, and organic 302 micropollutants. Currently, however, only heavy metals concentrations are regulated for land 13 303 application of sewage sludge (Council of the European Union, 1986). Since treated sludge 304 may have considerable amounts of pathogens, depending on the treatment processes used, 305 limit values for faecal bacteria indicators have also been proposed (Environment DG, EU, 306 2000). According to this proposal, conventionally treated sludge has to contain ≤3 log10 307 E.coli·gTS-1, and Salmonella spp. has to be absent in 2 gTS. 308 309 Table 3 summarises the concentration of heavy metals and faecal bacteria indicators in the 310 treatment wetlands samples, together with the limits proposed in the3rd Draft EU Working 311 Document on Sludge (Environment DG, EU, 2000). There are only little differences between 312 influent sludge and the final product with regards to heavy metals, suggesting that heavy 313 metals accumulation is negligible. Furthermore, in all cases the concentrations are clearly 314 below the limits proposed, which are more restrictive than current legislation (Council of the 315 European Union, 1986). 316 317 Heavy metals bioavailability in soil and plants depends on the following parameters: soil pH, 318 plant species and their cultivars, growth stage, biosolids source, soil condition and the 319 chemistry of the element (Warman and Termeer, 2005). According to these authors, it is 320 important to monitor Cu and Zn contents of plant tissues after a few years of sludge 321 applications to verify the tolerance levels for animals and feed, or human food. 322 323 With regards to pathogens, it can be seen that Salmonella spp. was not detected (Table 3). On 324 the other hand, E. coli was present but in all cases in small quantities. Both faecal bacteria 325 indicators are well below the limits proposed. 326 14 327 4. Conclusions 328 329 This study looked at the properties of biosolids from sludge treatment wetlands and 330 compared them with other stabilised products, such as anaerobic digested sludge and 331 compost. Focus was put on the quality of biosolids for its use on land as organic fertilisers and 332 soil conditioners. From this work, the following conclusions can be drawn. 333 334 In the full-scale wetlands studied, sludge water content (and volume) is reduced by 25 %, 335 from 99 to 75 %. Apparently, these results would be further improved by adjusting the sludge 336 loading rate to recommended values, which may reduce the moisture content to 60-70 %. 337 338 Organic matter biodegradation leads to VS around 45 %TS and COD around 500 g·kgTS-1 in 339 the final product, within the range of digested sludge but lower than in sludge compost. 340 Besides, DRI values (1000-1400 mgO2∙kgTS-1∙h-1) indicate a partly stabilised product, close 341 to a high stabilisation degree corresponding to the final product of a composting process. This 342 suggests that composting post-treatments would not be needed, if sufficient resting time 343 would be left at the end of the wetland’s cycle. Monitoring the stabilisation degree during the 344 final resting period would be advisable to minimise the duration of such a period. 345 346 The concentration of nutrients, heavy metals and faecal bacteria indicators suggest that the 347 final product would be suitable as organic fertiliser and soil conditioner. 348 349 On the whole, the studied system demonstrates the efficiency of sludge treatment wetlands for 350 sludge dewatering and stabilisation, with low treatment costs; and leading to a final product 15 351 which could be used on land without further post-treatment, reducing sludge management 352 costs. Characterization of similar systems would be advisable to corroborate these results. 353 354 Acknowledgements 355 356 This work was financed by the Catalan Water Agency (ACA) and the Spanish Ministry of 357 Environment (MMARM, Projects A335/2007 and 087/PC08). Technicians of Depuradores 358 d’Osona S.L. are greatly acknowledged for their support to this study. Enrica Uggetti kindly 359 acknowledges the Technical University of Catalonia and Esther Llorens the Juan de la Cierva 360 Programme of the Spanish Ministry of Education and Science (MEC). Joan García is grateful 361 to the School of Civil Engineering of Barcelona (ETSECCPB). 362 363 References 364 365 366 Adani, F., Lozzi, P., Genevini, P.L. (2000). Determination of biological stability by oxygen uptake on municipal solid waste and derived products. Compost Sci.Util. 9, 163-178. 367 Adani, F., Confalonieri, R., Tambone, F. (2004). Dynamic Respiration Index as descriptor of 368 the biological stability of organic wastes, Journal of Environmental Quality 33, pp. 369 1866–1876. 370 Agència Catalana de l’Aigua (2007) La gestió de fangs de depuració d’aigües residuals 371 municipals a Catalunya. Evolució històrica i estratègia de futur (nou programa de 372 fangs). In: III Jornades Tècniques de Gestió de Sistemes de Sanejament d’Aigües 373 Residuals, Barcelona, Spain. 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Journal of Environmental Sciences 20, 385-389. 21 490 491 Zwara, W. and Obarska-Pempkowiak, H. (2000). Polish experience with sewage sludge utilization in reed beds. Water Science and Technology 41 (1), 65-68. 492 22 493 Table 1. Characteristics of the WWTP in Seva (Catalonia, Spain). Treated population equivalent Type of treatment 1500 Contact-stabilisation Wastewater flow rate (m3/d) 180 (summer) / 400 (winter) Sludge production (kg TS/d) 60 Number of treatment wetlands (beds) 7 Total surface area (m2) 175 Nominal height for sludge accumulation (m) ~0.8 Sludge loading rate (kg TS/m2·year) 125 494 495 23 496 Table 2. Physico-chemical properties of influent sludge and biosolids from treatment wetlands. Parameter Influent Wetland 1 Wetland 2 pH 6.75 6.21 6.27 EC 1:5 (dS/m) 0.3 1.51 1.88 1.1 ± 0.0 24.2 ± 0.6 25.8 ± 2.1 VS (TS%) 51.5 ± 0.8 42.9 ± 1.8 44.6 ± 3.0 COD (g·kgTS-1) 709 ± 11 554 ± 32 494 ± 55 - 1400 ± 300 1100 ± 200 TKN (%TS) 9.76 4.02 4.87 TP (%TS) 2.68 0.13 0.39 K (%TS) 0.27 0.18 0.62 Physical properties TS (%) Organic matter DRI24h (mgO2∙kgTS-1∙h-1) Nutrients 497 Note: TS, VS and COD were analysed in triplicate; DRI was analysed in duplicate. 498 24 499 Table 3. Concentration of heavy metals and faecal bacteria indicators in influent sludge and biosolids from treatment wetlands. Parameter Influent Wetland 1 Wetland 2 Limit values Heavy metals Cr (ppm) 51 55 59 800 Ni (ppm) 39 30 32 200 Cu (ppm) 252 318 213 800 Zn (ppm) 719 588 641 2000 Cd (ppm) 1.7 0.8 0.8 5 Hg (ppm) <1.5 <1.5 <1.5 5 Pb (ppm) 53 73 76 500 Absence Absence Absence Absence in 50g <3 <3 <3 <500 MPN·g-1 Faecal bacteria indicators Salmonella spp. (presence/absence in 25g) E. coli (MPN·g-1) 500 Note: Limit values proposed in the 3rd Draft EU Working Document on Sludge (Environment 501 DG, EU, 2000). 502 25