To incinerate or not? : Effects of incineration on the concentrations of heavy metals and leaching efficiency of post-precipitated sewage sludge (RAVITA™)

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-ND 4.0 https://creativecommons.org/licenses/by-nc-nd/4.0/ To incinerate or not? : Effects of incineration on the concentrations of heavy metals and leaching efficiency of post-precipitated sewage sludge (RAVITA™) © 2020 Elsevier Ltd. All rights reserved. Accepted version (Final draft) Reuna, Sini; Väisänen, Ari Reuna, S., & Väisänen, A. (2020). To incinerate or not? : Effects of incineration on the concentrations of heavy metals and leaching efficiency of post-precipitated sewage sludge (RAVITA™). Waste Management, 118, 241-246. https://doi.org/10.1016/j.wasman.2020.08.022 2020 1 To incinerate or not? –Effects of incineration on leaching and 1 heavy metal concentrations of post-precipitated sewage sludge 2 (RAVITA™) 3 Sini Reunaa* and Ari Väisänena 4 5 aDepartment of Chemistry, Renewable Natural Resources, and Chemistry of Living 6 Environment, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland 7 *(corresponding author: [email protected]) 8 Keywords: phosphorus recovery; sewage sludge ash; characterization, thermal treatment 9 10 Abstract 11 The major element and heavy metal concentrations of post-precipitated sewage sludge (PPS) 12 and its ash residue (PPA) were determined using microwave digestion followed by 13 inductively coupled plasma optical emission spectrometry (ICP-OES) and mass spectrometry 14 (ICP-MS). To the best of our knowledge, this has not been previously done. Compared to 15 average heavy metal concentrations in sewage sludge in Europe the obtained concentrations 16 resulted in notably lower in both PPS and PPA. The leaching efficiency of the metal (Al/ Fe) 17 used as a precipitation agent from post-precipitated sludge and its ash residue with 18 phosphoric acid was also investigated. Tests resulted in leaching efficiencies for Al of 85 ± 1 19 % and 99.5 ± 0.7 % for PPS and PPA, respectively which were produced with aluminum as 20 precipitation agent for phosphorus. Sludge, which was produced using iron as a precipitation 21 agent, had a leaching efficiency of Fe 36.6 ± 0.9 % and 68.0 ± 1.1 %, for PPS and PPA, 22 respectively. The leaching efficiency for P was 94 ± 3 % and 96 ± 5 % for Al-PPA and Fe23 PPA, respectively. 24 25 2 1 Introduction 26 Since the European Commission listed phosphate rock as a critical raw material in 2014, the 27 development of methods to recover phosphorus (P) from secondary sources has been 28 increasing steadily (European Commission, 2014; Scopus, 2019). One of the main secondary 29 sources can be the sewage sludge produced in wastewater treatment plants. It is estimated 30 that 90 % of the P in wastewater end up in sewage sludge (SS) (Liang et al., 2019). 31 Currently, the three main applications for SS are landfilling, use in agriculture, and 32 incineration (Kacprzak et al., 2017). In 2016, 35 percent of sewage sludge was incinerated in 33 the European countries producing thousands of tonnes of ash (EUROSTAT, 2019). For that 34 reason, many of the wet chemical methods developed for phosphorus recovery utilize the 35 incinerated sewage sludge ash (ISSA). The problem is that incineration concentrates the 36 harmful heavy metals in ash causing challenges in the development of the phosphorus 37 recovery processes (Franz, 2008). 38 The usual first step in P recovery is the leaching of ISSA with either inorganic or organic 39 acids. This extracts P from the ash along with metals and metalloids (Fang et al., 2018). The 40 purification process for heavy metal separation can be done with solvent extraction, ion 41 exchange resins or membranes before the leachate is utilized for the recovery of P (Biswas et 42 al., 2009; Donatello et al., 2010; Guedes et al., 2014; Paltrinieri et al., 2019; Shiba and Ntuli, 43 2017). Another method for separation of P from heavy metals is the pretreatment of ISSA. 44 For instance, a chelating agent EDTA (ethylenediaminetetraacetic acid) has been successfully 45 tested as a pre-leaching agent for reducing the metals before P leaching (Fang et al., 2018). 46 This however produces a waste faction containing EDTA and leached metals, which must be 47 processed. Several studies have also investigated the possibilities to directly transform P from 48 the acidic leachate into a plant-available form. Biochars derived from waste materials (peanut 49 3 shells, sewage sludge) have been successfully used for P-adsorption and then used as 50 fertilizer (Fang et al., 2020a, 2020b). Direct precipitation with calcium silicate hydrates has 51 also been implemented resulting in a leaching efficiency of 55 % for P (Lee et al., 2018). 52 One way to prevent heavy metals from contaminating the phosphorus-rich sludge, without 53 extra process steps, is so-called post-precipitation (PP) (Eklund et al., 1991). In PP the 54 precipitation agent, typically aluminum or iron salt, is added into effluent wastewater after 55 other wastewater treatment procedures, such as primary treatment and biological treatment. 56 Post-precipitation of P is utilized by RAVITA™ -process. It is a process developed and 57 patented by the Helsinki Region Environmental Services Authority to recover phosphorus 58 and nitrogen from municipal wastewater (Fred et al., 2019, 2018; Rossi et al., 2018). In 59 RAVITA™ the PP produces a chemical sludge that mainly consists of aluminum or iron 60 phosphate depending on the used precipitation agent. The chemical sludge is separated from 61 effluent wastewater by disc filtration. This results in an extremely low total P concentration 62 of 0.1 mg L-1 in effluent wastewater (Rossi, 2014). Formed chemical sludge is leached with 63 dilute phosphoric acid. Next, the leach solution is processed to separate the precipitation 64 metal and phosphorus from each other. Currently, the purification of phosphoric acid with 65 solvent extraction is researched. The purified phosphoric acid solution is partly used in 66 nitrogen recovery to produce ammonium phosphate and the excess phosphoric acid can be 67 utilized in the fertilizer industry. The separated metal is recycled back to the wastewater 68 treatment process to be used as a precipitation agent again. The recycling of the precipitation 69 agent is not utilized in any other P recovery process. 70 RAVITA™ will utilize only the chemical sludge formed in PP. The biosludge that is formed 71 during biological treatment will contain an estimated 30-35 % of the P that comes with 72 incoming wastewater and it is digested (FCG Suunnittelu ja Tekniikka, 2015). Because of the 73 4 lower P content, the biological sludge has a better nutrient ratio and greater amounts can be 74 used in agriculture. Also, when P is not chemically bound with iron or aluminum its 75 bioavailability for plants increases. The heavy metal concentrations of the biological sludge 76 depend on industries that produce wastewater (Persson et al., 2015). However, the 77 concentration levels of heavy metals in biological sludge are controlled at the EU level by 78 The Sewage Sludge Directive 86/278/EEC (The Council of the European Communities, 79 1986). The final treatment for biological sludge varies between the EU member states 80 (Raheem et al., 2018). After digestion and composting the sludge can be utilized in green 81 building or it can be incinerated. 82 Previously for RAVITATM -process, we have optimized the leaching procedure for 83 aluminum-based post-precipitated sewage sludge (Al-PPS) with dilute phosphoric acid 84 (Reuna and Väisänen, 2018). The optimized leaching conditions for iron-based post85 precipitated sewage sludge (Fe-PPS) have not been previously published. Phosphoric acid 86 was chosen as a leaching solution instead of sulfuric acid or hydrochloric acid in order to 87 avoid the removal of sulfate or chloride ions from the leachate. (Ottosen et al., 2013). In this 88 study, we have studied if the incineration of PPS enhances the leaching process and 89 determined the concentrations of heavy metals (HMs) in the sludge (PPS) and ash residue 90 (PPA). To our knowledge, the HM concentrations have not been previously determined from 91 PPS and PPA. Nor has the leaching of PPA from the post-precipitation of P with dilute 92 phosphoric acid previously been tested. 93 94 5 2 Experimental 95 2.1 Materials and chemicals 96 The standard stock solutions of the elements (1000 mg L-1, analytical grade) were supplied by 97 PerkinElmer. Boric acid (H3BO3, 99.99 %, AlfaAesar) and phosphoric acid (85 wt. %) were 98 obtained from VWR International. Analytical grade nitric acid (65-68 wt. %) and 99 hydrochloric acid (37-39 wt. %) were purchased from Merck. Analytical grade hydrofluoric 100 acid (40 %) was procured from Merck. Ultra-pure hydrochloric and nitric acid were 101 purchased from ANALYTIKA, spol. s.r.o. (34-37 %, Prague, Czech Republic). Certified 102 reference materials CRM029 Heavy Metals – Sewage Sludge 2 (Lot LRAB1332) and SRM 103 1663c Heavy Metals in Coal Fly Ash were used for method validation. All the chemicals 104 were used as obtained without further purification. High-purity water produced by the Elga 105 Purelab Ultra water purification system was used throughout the experiments. 106 The batch of post-precipitated aluminum phosphate (Al-PPS) and iron phosphate sludge (Fe107 PPS) were received from the RAVITATM pilot plant located in the Viikinmäki wastewater 108 treatment plant, Helsinki. Post-precipitated sewage sludge (PPS) batches were dried in a fume 109 cupboard for 72 h and ground manually before sample treatment. Parts of the batches were 110 incinerated (550 °C, 2 h) to produce post-precipitated sewage sludge ash (PPA). 111 112 2.2 The procedure of sample treatment 113 2.2.1 Dry matter determination, incineration, and particle size determination 114 The dry matter content of sludge samples and reference materials were determined according 115 to the Finnish Standards Association's standard SFS 3008 (SYKE, 2011). Samples of 500 mg 116 were weighed and kept 16 hours at 120°C. After cooling the weight was measured and dry 117 6 weight and moisture content were calculated. The particle size range was determined with 118 the Retch AS200 sample sieve. The results are presented in supplementary data Table S1. 119 2.2.2 Microwave-assisted digestion 120 Sewage sludge samples (200 mg, dry weight content 81.8 % for aluminum-based sludge and 121 70.1 % for iron-based sludge) and ash residue samples (200 mg) were weighed in digestion 122 vessels and 9 mL nitric acid (HNO3, 65 %), 3 mL hydrochloric acid (HCl, 35 %) and 1 mL 123 hydrofluoric acid (HF, 40 %) was added. Vessels were closed and a digestion program based 124 on EPA 3052 method was performed with CEM Mars6 –microwave oven. After cool down 125 10 milliliters of boric acid (H3BO3, 5 wt.-%) was added and the HF neutralization program 126 was executed. Temperature profiles of digestion programs are presented in supplementary 127 data (Table S2). Digested samples were filtered (filter paper Whatman 41) and diluted to a 128 volume of 40 milliliters with high-purity water. Also, samples from certified reference 129 materials Heavy metals-Sewage sludge CRM029-50G (250 mg, dry weight content 89.8 %), 130 SRM 1663c Heavy Metals in Coal Fly Ash (200 mg, dry weight content 99.73 %) were 131 digested similarly. 132 133 2.2.3 Leaching 134 Leaching of Al-PPS and Al-PPA was done according to optimized leaching conditions 135 (Reuna and Väisänen, 2018): solid to liquid ratio S/L of 15.9 g dry weight (d.w) L-1, the 136 phosphoric acid concentration of 0.5 M and leaching time of 360 minutes. The Fe-PPS and 137 Fe-PPA were leached in the following matter: S/L 121 g (d.w.) L-1, the phosphoric acid 138 concentration of 2 M and leaching time of 60 minutes. The PPS was used in the leaching test 139 as received. The properties of PPS and PPA are listed in Table S1 in the supplementary data. 140 7 The specific compounds of metals in the solution after leaching with phosphoric acid is 141 discussed in the supplementary data. 142 143 2.3 ICP-OES measurements 144 The concentrations of the major elements Al, Ca, Fe, and Mg were determined with 145 PerkinElmer ICP-OES Avio 500 –spectrometer for Fe-PPS and Fe-PPS and with 146 PerkinElmer Optima 8300 for Al-PPS and Al-PPA. The GemCone low flow –nebulizer with 147 Tracey spray chamber (HF resistant) was used for sample introduction. The parameters for all 148 measurements are presented in supplementary data (Table S3). The wavelengths, calibration 149 ranges, and the limits of quantification (LOQ) are presented for each element in 150 supplementary data (Table S4). 151 152 2.4 ICP-MS measurements 153 The heavy metals analyzed were As, Cd, Cr, Cu, Ni, Pb, Sb, Se, Sn, Te, and Zn. The element 154 concentrations were determined with PerkinElmer NexION 350D inductively coupled plasma 155 mass spectrometer (ICP-MS). The ICP-MS operating conditions are shown in supplementary 156 data (Table S5). Before measurement, the samples were diluted by a factor of 200 or 50 with 157 ESI Prep-Fast. Method detection limits (MDL) were determined from method blanks (n=9) 158 for each element according to US EPA Method 200.7 (U. S. Environmental Protection 159 Agency, 2001) and are presented along with calibration ranges and internal standards used for 160 analytes in supplementary data (Table S6). Validation of ICP -OES and ICP-MS 161 measurements is presented in the supplementary data. 162 163 8 3 Results and discussion 164 3.1 Determination of element concentrations of PPS and PPA 165 Table 1 presents the major and heavy metal concentrations by weight percentages (w-%) for 166 main elements and mg kg-1 for heavy metals in PPS and PPA. Concentrations for As, Se, and 167 Cr are not presented since they resulted in concentrations lower than MDLs in all sample 168 types. To see if the heavy metal concentrations are lower in the PPS than in traditionally 169 produced sewage sludge the values were compared to existing literature. Liang et al., (2019) 170 determined the elemental concentrations of raw waste activated sludge and ISSA and the 171 results are presented in Table 1 along with data from this study. Liang et al, (2019) 172 determined that the concentrations of P in raw sludge and ISSA were 29.4 g kg-1 and 52.1 g 173 kg-1, respectively. From Table 1 we can see that PPS and PPA contain higher concentrations 174 of phosphorus regardless of the metal used in post-precipitation. The same trend applies to all 175 concentrations of the main elements. For instance, aluminum is present almost thrice as high 176 in concentration in Al-PPA than in ISSA (109.7 g kg-1) that Liang et al., (2019) have 177 researched. This is expected since the precipitation in PP happens after the removal of 178 biological solids, hence increasing the concentrations of main elements. 179 In both sludge types, the heavy metal concentration is increased by a factor of 1.3 after 180 incineration. However, even after incineration, most heavy metal concentrations are 181 considerably below the average heavy metal content of the sewage sludge in the EU and 182 Finland (Helsinki Region Environmental Services Authority, 2018; Inglezakis et al., 2014). 183 Likewise, Finland’s limit values for heavy metals in sludge for use in agriculture are 184 straightforwardly passed (Ministry of the Environment, 1994). This is illustrated in Figure 1a 185 for Al-PPS/PPA and Figure 1b for Fe-PPS/PPA. The only exception is cadmium which 186 average concentration in Fe-PPS is the same as Finland’s limit value for Cd in sludge for use 187 in agriculture. 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Elemental concentrations of waste activated sludge (WAS) and ISSA incinerated at 600 °C determined by Liang et al. (2019). 428 Al Fe Liang et al. 2019 PPS PPA PPS PPA WAS ISSA Unit n 6 9 6 5 Al (w-%) 18.3 ± 0.3 29.1 ± 0.3 0.221 ± 0.01 0.351 ± 0.009 5.43 ± 0.03 10.97 ± 0.02 Ca (w-%) 1.14 ± 0.01 1.53 ± 0.03 1.97 ± 0.02 2.39 ± 0.02 1.69 ± 0.05 3.19 ± 0.05 Fe (w-%) 1.03 ± 0.01 1.62 ± 0.02 36.8 ± 0.2 43.5 ± 0.4 2.64 ± 0.09 5.14 ± 0.06 Mg (w-%) 0.075 ± 0.002 0.114 ± 0.01 — 0.144 ± 0.002 0.67 ± 0.03 1.3 ± 0.02 P (w-%) 10.0 ± 0.2 14.85 ± 0.15 8.1 ± 0.08 9.52 ± 0.12 2.84 ± 0.09 5.5 ± 0.07 Cu (mg kg-1) 52.6 ± 0.8 85 ± 3 37.7 ± 0.9 49.8 ± 0.5a 90 ± 1 423 ± 10 Zn (mg kg-1) 219 ± 4 360 ± 7 470 ± 13 587 ± 7a 225 ± 12 895 ± 49 Sn (mg kg-1) 1.87 ± 0.09 3.1 ± 0.2 1.512 ± 0.014 1.79 ± 0.03a ndb nd Pb (mg kg-1) 0.66 ± 0.04 1.01 ± 0.05 — 0.481 ± 0.015a nd 460 ± 5 Ni (mg kg-1) 10.6 ± 0.6 18 ± 3 — — nd 209 ± 1 Sb (mg kg-1) — — 1.09 ± 0.04 1.31 ± 0.02a nd nd Cd (mg kg-1) — — 3 ± 0.4 — nd 126 ± 3 — Below LOQ/MDL; a n= 6; bnot detected 429 430 431 23 Table 2. The determined concentrations of Al, Fe, and P (mean ± s.d, n= 3.) from the leachate, when the 432 leaching solution is 0.5 M H3PO4 for Al-PPS/PPA and 2 M H3PO4 for Fe-PPS/PPA. 433 Al Fe PPS PPA PPS PPA Al (g L-1) 2.46 ± 0.05 4.60 ± 0.03 0.08 ± 0.01 0.40 ± 0.05 Fe (g L-1) 0.151 ± 0.008 0.25 ± 0.1 16.4 ± 0.3 36 ± 2 Pa (g L-1) — 2.24 ± 0.03 — 11.1 ± 0.6 pHafter leaching 1.5 1.9 1.6 1.2 a P concentration determined by subtracting the theoretical P concentration of H3PO4 from the measured value. 434 435 436 Table 3. Student’s t-test values for Fe-PPS/PPA and Al-PPS/PPA to determine if there is a statistically 437 significant difference between the metal concentration in leach solution after leaching with either PPA or PPS 438 Fe Al PPA PPS PPA PPS Mean 67.963 36.634 99.547 84.653 Variance 9.588 0.807 0.460 1.128 Observations 3 3 4 3 Pooled Variance 5.197 0.73 Hypothesized Mean Difference 0 0 df 4 5 t Stat 16.831 22.873 P one-tail < 0.0001 < 0.0001 t Critical one-tail 2.132 2.015 439 440