Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery

Bioresource Technology 174 (2014) 167–175

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Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery Nancy Y. Acelas a, Diana P. López a, D.W.F. (Wim) Brilman b, Sascha R.A. Kersten b,⇑, A. Maarten J. Kootstra b a Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia, UdeA – Colombia, Calle 70 No. 52-21, Medellín, Colombia b Sustainable Process Technology Group, Faculty of Science and Technology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands

h i g h l i g h t s  SCWG can be used for converting sewage sludge into energy and P-recovery.  The efficiency of uncatalyzed gasification of sewage sludge was 52% at 600 °C.  Phosphorus content went up from 3% in sewage sludge to 9% in the solid after SCWG.  The highest release of phosphate from SCWG residue was 95.5%.  Oxalic acid had better performance than sulfuric acid in P leaching (95%).

a r t i c l e

i n f o

Article history: Received 21 August 2014 Received in revised form 29 September 2014 Accepted 1 October 2014 Available online 8 October 2014 Keywords: Sewage sludge Supercritical water gasification Phosphorus Leaching Oxalic acid

a b s t r a c t In this study, the feasibility of the gasification of dewatered sewage sludge in supercritical water (SCW) for energy recovery combined with P-recovery from the solid residue generated in this process was investigated. SCWG temperature (400 °C, 500 °C, 600 °C) and residence time (15 min, 30 min, 60 min) were varied to investigate their effects on gas production and the P recovery by acid leaching. The results show that the dry gas composition for this uncatalyzed gasification of sewage sludge in SCW mainly comprised of CO2, CO, CH4, H2, and some C2–C3 compounds. Higher temperatures and longer residence times favored the production of H2 and CH4. After SCWG, more than 95% of the P could be recovered from the solid residue by leaching with acids. SCWG combined with acid leaching seems an effective method for both energy recovery and high P recovery from sewage sludge. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Phosphorus is an important nutrient for all living organisms. It is part of a large number of biological systems and is a main constituent of fertilizers used in agriculture (Acelas et al., 2014). Almost all phosphorus used in fertilizers is derived from phosphate rock, and due to the decreasing availability and quality of mineral phosphate resources and the increase of the phosphate rock price, alternative resources for the production of phosphate fertilizers for agriculture must be found (Acelas et al., 2013; Cordell et al., 2009). Sewage sludge represents an important secondary phosphorus source for two main reasons. Firstly, during wastewater treatment with chemical and/or enhanced biological P-removal (EBPR) most

⇑ Corresponding author. E-mail address: [email protected] (S.R.A. Kersten). http://dx.doi.org/10.1016/j.biortech.2014.10.003 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

of the phosphate ends up in a concentrated form in the sewage sludge (Blöcher et al., 2012). Secondly, wastewater treatment facilities usually operate at an industrial scale, resulting in large amounts of the phosphorus-rich sludge. Technologies that reduce mass and volume of sewage sludge and at the same time produce a usable sludge product have gained more and more interest in recent years. Sludge combustion is widely practiced on a full-scale basis in many highly populated urban areas, e.g. in Japan, Germany, and the Netherlands (Lederer and Rechberger, 2010). One way to recover phosphorus is from sewage sludge ashes produced in thermal processes, such as pyrolysis, combustion or gasification (Azuara et al., 2013; Petzet et al., 2012; Yanagida et al., 2009). However, sewage sludge contains a high percentage of water, which causes high drying costs when these thermal processes are used. Therefore, supercritical water gasification (SCWG) has been identified as a promising technology for the conversion of wet biomass streams with a low overall heating value to high

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heating value product gases (Chakinala et al., 2013), while still having the possibility of phosphorus recovery. In supercritical water (SCW), the pressure and temperature have been increased to or over the critical point (22.1 MPa and 374 °C). As a reaction medium, supercritical water is featured as having a high diffusivity, a low viscosity, and a high solvating ability for organic compounds (Youssef et al., 2010). In addition, water is an active reactant in steam reforming and water-gas shift reactions in the supercritical region. Therefore, supercritical water is able to transform sewage sludge into fuel gases such as H2, CO, and CH4 with reduced tar and coke formation compared to the traditional thermal processes (Zhang et al., 2010). The selectivity of the gas production toward H2, syngas (H2 + CO), or CH4 can be steered by tuning the process conditions and by the use of catalysts (Chakinala et al., 2009). From the perspective of cost-effective disposal of bio-based wastes, the costs associated with SCWG of sewage sludge may be high, but when the solid residue generated during SCWG is used to recover and recycle phosphorus, this makes the process more economical. In most SCWG studies however, most of the attention has been paid on energy recovery (Azadi et al., 2013; Wilkinson et al., 2012; Xu et al., 2013) and on catalysts to promote production yield of hydrogen from different types of biomass, among which sewage sludge (Fiori et al., 2012; Gasafi et al., 2008; Zhang et al., 2010), manure (Youssef et al., 2010) and wood (Yong and Matsumura, 2012). To date, very few studies have reported on phosphorus recovery from solid residues generated during SCWG of sewage sludge. In a related study, (Zhu et al., 2011) showed that during SCWG of sewage sludge the majority of phosphorus ends up into the solid residue (20 mg P/g). This is in agreement with (Yanagida et al., 2008), who evaluated the behavior of some inorganic elements in poultry manure after SCWG and found that the liquid phase contained nearly all the N, K and Cl, while nearly all Ca, P and Si ended up in the solid phase. S was divided more or less evenly between both phases. Clearly, the solid residue after SCWG is a promising source of P. The research aim of this work is to study the feasibility of the SCWG of sewage sludge in combination with P-recovery from the solid residue generated in this process. The effects of SCWG temperature and residence time are evaluated using a high-pressure autoclave and a heated fluidized bed. Acid leaching of phosphate from SCWG residue is studied with oxalic and sulfuric acid, and compared to ash resulting from combustion.

2. Methods 2.1. Raw material The dewatered sewage sludge (DSS) used as feedstock for supercritical water gasification was collected from the wastewater treatment plant of the Water Board Regge and Dinkel located in Hengelo, the Netherlands (lat. 52.27957°, long. 6.77086°). The sludge was dried (105 °C, until constant weight), pulverized in a mortar and then sieved into two fractions of different particle size (the first: 600 lm 6 U 6 1 mm, the second: U 6 600 lm), and stored in airtight containers. The material with particle size 6 600 lm was used in the SCWG experiments, because it was the largest fraction (by weight). The water content, as well as the proximate and the ultimate analysis of the dried dewatered sewage sludge (DDSS) are shown in Table 1. The C, H, and N content of the DDSS (dry ash free, daf) were measured in triplicate using an Elemental Analyzer (Flash 2000-Interscience), with oxygen calculated by difference. The higher heating value (HHV) was calculated according to the Dulong equation (Xu et al., 2012), using

the results of the elemental analysis. The mineral composition of the DDSS was determined by X-ray fluorescence spectrometry (XRF) and is shown in Table 2. For the XRF measurements samples were dissolved in lithiumtetraborate (Li2B4O7, flux) to make fused beads. These were measured together with standards with known composition containing the same elements in the same range of concentration on a P analytical PW 1480 using the SuperQ software. Results were not normalized for loss on ignition. 2.2. Super-critical water gasification (SCWG) of sewage sludge SCWG experiments were carried out in a batch autoclave reactor (Inconel alloy: 72% Ni, 15% Cr and 8% Fe) with an internal volume of 45 mL. A diagram of the set-up is shown in Fig. 1. For safety reasons, the reactor set-up was placed in a safety chamber and was controlled from outside the chamber during the experiments. The inner temperature of the reactor was measured by a thermocouple inserted through an orifice in the bottom cap, and the pressure was measured via a gas connection from the top lid to a pressure transmitter. The pressure and the temperature of the reactor were monitored and recorded using Pico Log software. With the help of a pneumatic arm, the autoclave was immersed in and removed from the fluidized sand bed. This sand bed was heated by an electric oven and preheated air which was also used for the fluidization of the bed. In a typical run, the autoclave was loaded with 10 g of a mixture of 15 wt% of DDSS and 85 wt% of deionized water, after which the autoclave was tightly closed and connected to the pneumatic arm of the set-up. Prior to each experiment, a leakage test was performed by pressurizing the autoclave with 100 bar of nitrogen and monitoring pressure after closing the gate valve. This also served to remove any air from the reactor. Finally, after restoring ambient pressure in the reactor, the rest of the experiment was controlled from outside the safety chamber via the control panel. Using the pneumatic arm, the reaction was initiated by immersing the autoclave into the preheated fluidized sand bed. After the determined residence time, starting from when the set temperature was reached, the reactor was lifted and quenched in the cold water bath. Triplicate runs were conducted for each combination of temperature and reaction time. 2.3. Separation and analysis of the reaction products The procedure for separating gasification products is shown in Fig. 2. Once the reactor was cooled down to room temperature and before depressurizing the system, gas samples were taken with a syringe and analyzed with an off-line gas chromatograph (Varian Micro GC CP-4900) containing two analytical columns (Molsieve 5A Ar, 10 m and PPQ He, 10 m) and equipped with a thermal conductivity detector to determine H2, CO2, CO, CH4 and other light organic gases (C2H4, C2H6, etc). The remaining product (liquid and solids in suspension; Product 1) was then collected in a polypropylene bottle. Then, the reactor was rinsed with de-ionized water to remove any solid deposits from the autoclave. The resulting suspension (Product 2) was also collected. The suspensions were separated into solid and liquid phases by vacuum filtration using a 1.0 lm pore size membrane filter (Glass microfiber filters. Whatman. GF/B). The filtrate from Product 1 was collected in a sampling vial to determine phosphorus concentration whereas the solid residue on the filter was collected and named Solid 1. The solid residue after the filtration of Product 2 was also collected and named Solid 2. Both solids were rinsed with demineralized water while on the filter, then the filter was dried at 105 °C until constant weight and then the solids were collected and mixed together in a sampling tube for analysis.

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78.0

Proximate analysis (wt%)a

Ultimate analysis (wt%)a

HHV (MJ/kg)c b

VM

FC

Ash

C

N

H

O

51.1 ± 0.3

13.2 ± 0.0

35.7 ± 0.3

32.9 ± 0.1

6.13 ± 0.3

4.72 ± 0.1

56.3 ± 0.5

7.78

VM: volatile matter; FC: fixed carbon; C: carbon; N: nitrogen; H: hydrogen; O: oxygen. Average values with their standard deviation. 3 analyzed samples for each parameter. a On dry basis. b By difference (O% = 100%  C%  N%  H%). c Higher heating value (HHV) calculated by the Dulong Formula, i.e., HHV (MJ/kg) = 0.3383 C + 1.443 (H  O/8).

Table 2 Mineral composition of the dried dewatered sewage sludge. Element

P

Ca

Al

Fe

Si

Mg

K

Na

(%)

3.42 ± 0.08

2.39 ± 0.06

1.77 ± 0.04

2.85 ± 0.08

3.93 ± 0.10

0.68 ± 0.01

0.37 ± 0.01

0.17 ± 0.01

Average values with their standard deviation. 3 analyzed samples for each parameter.

Fig. 1. Diagram of the experimental batch autoclave reactor set-up. Adapted from (Chakinala et al., 2013).

2.4. Leaching of phosphorus from the residue after SCWG of dewatered sewage sludge All residue leaching experiments were carried out at room temperature, with the pH continuously measured after being set at pH 2.0 to ensure that no acid limitations occurred and all available phosphorus would be leached. It has been found in literature (Petzet et al., 2012) and in a study done in our lab that at this pH the phosphorus dissolution process is efficient (Kootstra et al., 2015). Each experiment was started with 0.1 g of residue, 100 mL of demineralized water, and the necessary amount of acid to reach the pH 2.0, i.e. a mass ratio of solid to liquid approximately of

1:1000. The mixture was stirred at 200 rpm and, to identify the time of maximal leaching, samples were collected at different times (15, 30, and 45 min; 1, 2, 4, 8, and 24 h). The samples were filtered using a 0.45 lm pore size membrane filter (Filtereinheit/ Filter unit Whatman) and stored in a refrigerator at 4 °C for the determination of phosphorus (orthophosphate and total phosphorus). For the phosphorus measurements Hach Lange kit LCK-349 was used on appropriate dilutions, with an LT-200 heater and DR5000 spectrophotometer. All experiments were conducted in triplicate. In order to determine directly which crystalline compounds were dissolved or newly formed during the acid leaching, untreated and treated SCWG residues were analyzed by X-ray

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Fig. 2. Scheme for SCWG, product separation, phosphorus leaching, and analyses.

diffraction (XRD). The measurements were performed on a Siem5005. Furthermore, the appearance and the elemental distribution in SCWG residues before and after the acid treatment were examined using scanning electron microscopy (SEM), including energy dispersive spectroscopy (EDS) using a Philips XL 30 SFEG. 2.5. Terms and definitions In discussing the results, the following definitions are used. The yield (Y i ) of product gas is defined as:

  mol Ni ¼ Yi kg Nfeed

ð1Þ

where N i is the gas produced during the SCWG, expressed in moles (mol) and, Nfeed is the initial amount of DDSS, expressed in kilograms (kg). The carbon gasification efficiency (CGE) is defined as the degree of conversion of carbon in the feed to permanent gases:

X Carbon gasification efficiency; CGE ð%Þ ¼

NC;i

i

NC;feed

 100

ð2Þ

where Nc;feed is the amount of carbon in the feed, and N c;i is the amount of carbon in product gas (i.e. CO, CO2, CH4, C2H4, C2H6, C3H6, C3H8). 3. Results and discussions 3.1. Gas production The major chemical reactions that take place during SCWG of sewage sludge are steam reforming (Eq. (3)), water-gas shift (Eq. (4)) and methanation (Eq. (5)) (Chakinala et al., 2009; Withag et al., 2012). However, many other side reactions like cracking,

chain rearrangements, condensation, and polymerization reactions will occur.

Cx Hy Oz þ ð2x  zÞH2 O ! xCO2 þ

y 2

 þ 2x  z H2

ð3Þ

CO þ H2 O () CO2 þ H2

ð4Þ

CO þ 3H2 () CH4 þ H2 O

ð5Þ

3.1.1. Effects of temperature The product gas composition obtained at various reaction temperatures (400–600 °C) is presented in Fig. 3A. The corresponding pressures inside the reactor were 24.6 MPa (400 °C), 37.0 MPa (500 °C), and 49.6 MPa (600 °C), indicating that in all cases the water was in supercritical state. The gaseous products were mainly composed of CO2, H2, CO, CH4, and small quantities of light C2 and C3 compounds, such as ethylene (C2H4), ethane (C2H6), propane (C3H8), and propylene (C3H6). As expected, temperature has a significant influence on the product gas composition. At lower temperatures, decarboxylation reactions are more dominant and, as a result, CO2 is the main product obtained at 400 °C. At higher temperatures (500 °C and 600 °C), supercritical water becomes a more powerful oxidant, and free radical reactions prevail. Water acts as a solvent and promotes solute-solvent reactions such as the decomposition of the organic compounds in the feedstock. The decrease of CO formation at increasing temperatures implies that the water-gas shift reaction occurs and increased water-gas shift activity may be a cause for the increase in H2 production. At higher temperatures endothermic reactions are favored while at lower temperatures the equilibrium shifts to exothermic reactions. Steam reforming (Eq. (3)) is endothermic and therefore an increased temperature would favor the formation of H2 gas. Another possible way of H2 formation is the thermal decomposition of the intermediate compounds. On the other hand, the increase of temperature (from 400 to 600 °C) shifts the

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171

Fig. 3. Dried dewatered sewage sludge gasification in supercritical water, with (A) effect of temperature on dry gas composition, reaction time 60 min; (B) carbon gasification efficiency. (C) effect of temperature and reaction time on gas yield from the SCWG of dried dewatered sewage sludge. Average values, triplicate experiments, single analysis per experiment. Error bars represent standard deviation.

equilibrium in Eq. (5) to the left hand side, whereas the amount of methane produced increased, going from 11.3% to 41.8% for 400 °C and 600 °C, respectively. Methane is believed to be a very stable compound under SCWG conditions (Lee et al., 2002). The product

trends from SCWG of sewage sludge in this study are in agreement with those reported in the literature on similar materials (Letellier et al., 2010; Matsumura et al., 2013; Wilkinson et al., 2012; Withag et al., 2012).

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Fig. 3B shows the carbon gasification efficiency (CGE) of sewage sludge in supercritical water at different temperatures. The CGE increased from 16% to 52% with increasing the temperature from 400 °C to 600 °C. Fig. 3C shows the gas yield obtained at the reaction temperatures. It can be observed that as the temperature increased from 400 °C to 600 °C, the yield of gas products improved considerably, indicating cracking of the heavier oil-type products into gases, most likely via free-radical reactions, which become dominant at higher temperatures. From a thermodynamic perspective, a high temperature favors gas formation, since the overall SCWG process is endothermic (Guo et al., 2007; Lu et al., 2006). Compared to temperature, reaction time has a smaller influence on the individual gaseous species produced (see Fig. S1 in the supplementary material). The H2 and CH4 content in the gas phase increase slightly with reaction time while CO content has a tendency to decrease with increasing reaction time. This behavior can be attributed to hydrogen production via the water-gas shift and methanation reactions. The influence of reaction time is small enough to suggest short reaction times are sufficient in the SCWG of sewage sludge, as longer reaction times increase process costs per treated amount of raw material. The present results were generally consistent with previous work of SCWG of biomass (Chakinala et al., 2009; Lu et al., 2006; Youssef et al., 2010). These results suggest that sewage sludge can be converted into high heating values product gases using the SCWG, and that at 600 °C the production of H2 and CH4 is increased compared to lower temperatures. The calculated HHV of the gas mixtures were 27 MJ/kg, 39 MJ/kg, and 48 MJ/kg for SCWG at 400 °C, 500 °C, and 600 °C, respectively. 3.2. Release of phosphorus from the residue after SCWG of dewatered sewage sludge During SCWG, not only gases but also solids and liquids are produced (Fig. 2). In our study, we found that during SCWG of sewage sludge the majority of the P migrated into the solid residue. The phosphorus content went up from 3.4% in the dried sewage sludge to 8.3%, 8.7% and, 9.4% in the solid residue generated during SCWG at 400, 500 and, 600 °C, respectively. The total mass balance was closed for 97.8–98.6%, with no apparent losses other than those occurring during normal lab scale processing, such as, feeding, sampling and collecting. After SCWG, the liquid phase contained around 2% of all phosphorus originally present in the DDSS independent from the experimental conditions. The increased phosphorus concentration in the SCWG residue and the fact that buffering organics, e.g., proteins, are no longer present in that residue are positive for the leaching process. A more concentrated stream facilitates the extraction of phosphorus, if only by reducing the equipment scale, and the reduction of buffering organics means that less acid is needed to reach the desired pH during solubilizing/leaching of the phosphorus. 3.2.1. Effect of leaching time Fig. 4A shows the increase in leached phosphorus yield with time, using sulfuric and oxalic acid, while maintaining pH 2. The experiment was conducted using the residue obtained after SCWG of DDSS at 600 °C and 60 min. In the first few hours of the reaction, oxalic acid seems to outperform sulfuric acid, with a clear 6% and 11% more solubilized P after 2 and 4 h, respectively. Furthermore, after 4 h oxalic acid reached its maximum yield, while with sulfuric acid more time was needed. After 8 h, oxalic and sulfuric acid have reached their maximum yield of 92% and 95%, respectively. Following these results, leaching time for the other residues was set at 8 h.

3.2.2. Effects of temperature The phosphorus dissolution yield from the SCWG residue of sewage sludge increases with increasing reaction temperature (Fig. 4B). P yields increase from 84% to 86% and 95% for oxalic acid and from 80% to 82% and 87% for sulfuric acid at 400 °C, 500 °C and 600 °C, respectively. To help understand the observed temperature effect in leaching phosphorus it should be considered that different organic and inorganic phosphorus compounds may exist in the solid residue. The higher the SCWG temperature, the more organic P is converted into inorganic P. The destruction of organic compounds in SCWG takes place primarily through free radical pathways rather than the ionic pathways that dominate in liquid water. This is because supercritical water acts as a nonpolar solvent. At supercritical operating conditions, gasification proceeds quickly and converts the H–C–N compounds to hydrogen, carbon monoxide, carbon dioxide, and molecular nitrogen as the main products. For compounds containing heteroatoms such as chlorine, sulfur, and phosphorus, reaction also produces the corresponding inorganic acids: HCl, H2SO4, and H3PO4, which can be neutralized to their corresponding salts and separated as concentrated liquid brine or precipitated solid. Sullivan and Tester (2004) studied the supercritical water oxidation (SCWO) of methylphosphonic acid as a model compound of organic P, and they found that it could be destroyed completely within a short period of reaction time and produced phosphate in the final residue. The P–C bond destruction during SCWG is important for leaching process because the P–C bond is inert to acidic and basic hydrolysis. However, the organic P contained in sewage sludge is composed collection of phospholipids, phosphonic compounds, nucleic acids, phosphoproteins etcetera, and these are more complex than the model compound. Therefore, future work is needed to clarify the conversion of organic P during SCWG of sewage sludge. Conversion of organic P to phosphoric acid which is neutralized to apatite during SCWG of poultry manure may be complete at high reaction temperatures, according to (Yanagida et al., 2008) (Eq. (6)). As apatite is readily dissolved in acidic aqueous conditions (Eq. (7), using sulfuric acid) (Kuligowski and Poulsen, 2010), this may explain the observed higher P leaching yields at higher SCWG temperatures.

2ðHm PO4 Þ3m þ ð3  mÞCa2þ ! Ca3m ðHm PO4 Þ2

m ¼ 0; 1; 2

ð6Þ

Ca3 ðPO4 Þ2 þ 3H2 SO4 þ 6ðxÞðH2 OÞ ! 3CaSO4  2ðxÞH2 O þ 2H3 PO4 þ heat

ð7Þ

where x = 0, 0.5, or 1. 3.2.3. Effect of reaction time Fig. 4C illustrates the variation of P leached in different reaction time at 600 °C. P yields were only slightly – if at all – influenced by SCWG reaction time, with 87%, 88% and 92% yield for sulfuric acid at 15, 30 and 60 min, respectively, while yields stayed constant at around 95% for oxalic acid. Similar behavior was observed with residues produced at 400 °C and 500 °C (results not shown). It appears that SCWG for 15 min already makes most of the P available for acid leaching, by the same proposed reaction mechanism (Eq. (6)) as discussed earlier. The advantage of shorter reaction times while sacrificing neither gas production (Fig. S1 in the supplementary material) or P-leaching performance is obvious: the final process becomes more economical due to lower energy requirements and less needed equipment and capital per amount of residue. 3.2.4. Effect of acid type The use of oxalic acid results in a higher phosphate yield than sulfuric acid for all solid residues, under all tested SCWG

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173

Fig. 4. Leaching performance of phosphorus, (A) effect of time using sulfuric and oxalic acid [SCWG conditions: 15 wt% DDSS, 60 min of reaction time and 600 °C of temperature reaction. Leaching conditions: pH 2 and solid/liquid ratio: 1:1000]. (B) Effect of SCWG temperature with sulfuric and oxalic acid. [SCWG conditions: 15 wt% DDSS, 15 min. reaction time. Leaching conditions: 8 h at pH 2 and solid/liquid ratio: 1:1000]. (C) Effect of reaction time with sulfuric and oxalic acid. [SCWG conditions: 600 °C, 15 wt% DDSS. Leaching conditions: 8 h at pH 2 and solid/liquid ratio: 1:1000]. Average values, triplicate experiments, single analysis per experiment. Error bars represent standard deviation.

conditions, after 8 h of leaching (Fig. 4B and C). The performance of oxalic acid may be attributed to its ability to form complexes with metal ions, which will improve the phosphate removal (Azuara et al., 2013; Kootstra et al., 2015). In comparison with other organic acids, oxalic acid has the largest ability to chelate many metal ions (K+, Mn2+, Zn2+, and Cu2+). In fact, the chelating stability constant for oxalate-K+ complexes of five-ring structures is 1.61 times that of citrate-K+ complexes and 2.25 times that of malate-K+ complexes (Jones, 1998). In a related study, it was reported that after treatment with oxalic acid, the crystallinity of fluorapatite decreased (Jiang et al., 2012), which is an indication of increasing P release. Oxalic acid in contact with the solid residue eliminates

the adsorption sites and reduces the adsorption of some anions, such as phosphate. Additionally, it has been reported that oxalic acid can efficiently solubilize phosphate from calcium phosphate in calcareous soils and from iron phosphate and aluminum phosphate in acidic solutions (Jones, 1998; Tu et al., 2007), and deduced from the mineral composition of sewage sludge (Table 2), these will be the phosphorus containing compounds present in the residue and therefore its leaching is improved by using oxalic acid. Thus, oxalic acid can even dissolve iron and aluminum phosphates. The choice for sulfuric acid as an inorganic extraction medium is based on the fact that this is the standard acid used for many processes that solubilize P, such as artificial fertilizer production.

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Other acid such as, hydrochloride acid would add chloride ions to the leachate and these could form soluble complexes with most heavy metals, thus increasing the amount of metals in solution (Pettersson et al., 2008). 3.2.5. Comparison between leaching from combustion ash and SCWG residue It seems to be easier to release phosphate from SCWG residue of DDSS than from combustion ash of the same material using sulfuric acid at pH 2. At all three tested temperatures, leaching SCWG residue resulted in higher yields (see Fig. S2 in the supplementary material). The maximum P yield from sewage sludge combustion ash and SCWG residue are 85% and 92%, respectively. This difference may be attributed to the different chemical compounds formed during these two thermal treatments. For example, under combustion conditions of 400 °C could be possible to find organic P compounds with P–C bonds which are inert to acid treatment while in the SCWG residue of sewage sludge, phosphorus is present mostly in the form of calcium, aluminum, and iron phosphate (Yanagida et al., 2008). Acid leaching dissolves these compounds and phosphoric acid is obtained. The chemical reactions involved in phosphorus dissolution from SCWG residue at pH 2 are likely as follows:

Ca5 ðPO4 Þ3 OH þ 10Hþ ! 5Ca2þ þ 3H3 PO4 þ H2 O 3þ

þ

XPO4 þ 3H ! X þ

þ H3 PO4

Fe3 ðPO4 Þ2 þ 6H ! 3Fe

2þ

X ¼ Al or Fe

þ 2H3 PO4

ð8Þ

raw sewage sludge to pH 2, around 29.9 kg of sulfuric and 36.7 kg of oxalic acid were needed per kg of dissolved phosphorus, corresponding to 85% and 94% of the total phosphorus present, respectively. When leaching phosphorus at pH 2 from SCWG residue, 12.2 kg of sulfuric and 16.0 kg of oxalic acid per kg of phosphorus resulted in 92% and 96% leached phosphorus, respectively. A reduction of around 60% in the amount of needed acid per amount of phosphorus was reached, while also resulting in more solubilized phosphorus per amount of sewage sludge. The economic feasibility of phosphorus dissolution from SCWG residue with sulfuric acid can be evaluated by comparison with the acid consumption needed for phosphoric acid production from apatite. For P extraction from phosphate rock, sulfuric acid consumption is in the range of 4–7 kg H2SO4 per kg P (Azuara et al., 2013; Kuligowski and Poulsen, 2010). In a related study (Cohen, 2009), it was found that the amount of acid necessary to release more than 85% P from sewage sludge incineration ash ranged from 4.6 to 18.1 kg H2SO4 per kg P, with the extent of the range probably due to ash composition, the presence of cations in solution, and the presence of other phosphate compounds requiring less or more acid for dissolution. In our investigation, 1.1 kg H2SO4 per kg residue was necessary to release 92% of P in comparison to 1.0 kg H2SO4 per kg ash to release 96% of P. It allows us to infer that optimizing the amount of acid needed for P leaching will show the SCWG process as a cost-effective method for P-recovery.

ð9Þ ð10Þ

3.2.6. XRD-analyses of SCWG residue; before and after leaching Fig. S3 in the supplementary material shows the diffraction patterns for SCWG residue before and after the oxalic acid leaching process. It can be observed that after leaching process, the signals caused by compounds such as P-Fe (whitlockite), P-Al (AlPO4), and P-Ca (hydroxyapatite) have disappeared. This indicates decomposition of phosphate compounds, resulting in more dissolution of P. After the acid treatment certain stable compounds are visible, in this case, the signals for calcium and iron compounds such as, calcite and hematite appear during leaching process. Quartz (SiO2) is presented in the residue before and after the acid treatment which means that this is stable to acid attack. These three species are the main components of the solid residue after acid leaching. The SEM-EDS results support the findings using XDR analysis. The elements such as Ca, Fe and Si stay in the solid part after the leaching procedure (Fig. S4, in the supplementary material). SEM-EDS results of the SCWG residue before and after leaching indicated both physical and chemical transformation during leaching. Fig. S4A and S4C show a slight difference in morphology of the particles of the SCWG residue before and after acid treatment. Before leaching, the SCWG residue consists of defined, individual particles, whereas the leached residue contains agglomerated clusters of smaller particles. The elemental analysis of the SCWG residue surface before leaching is shown in Fig. S4C. It can be seen that Fe, Al, Ca, P and Si are the most common elements for this type of sample. The acid leaching efficiency can be corroborated by the absence of the phosphorus peak in the surface elemental analysis of the SCWG residue after the acid treatment, while the other elements seem to stay in the solid part (Fig. S4D). 3.2.7. Acid consumption for phosphorus recovery from SCWG residue of sewage sludge The economics of phosphorus leaching from SCWG residue are mostly dependent on the amount of acid required per amount of phosphorus extracted. In preliminary experiments (results not shown) for this study, it was determined that, for acidification of

4. Conclusions The work demonstrates a sustainable approach to convert sewage sludge into fuel gases and to recover phosphorus from SCWG process, making it possible for the wastewater treatment industry to make a significant contribution to the recycling of this vital and finite resource. For the uncatalyzed conversion, it was found that the gasification efficiency of sewage sludge increases with higher temperature, 52% at 600 °C for residence times of 15 min and more. The highest release of phosphate, 95.5% of what was present in the SCWG residue, was obtained from the residue after SCWG at 600 °C and 60 min reaction time. Acknowledgements The authors would like to acknowledge to the BE2.O (BioEnergy to Overijssel) program for the financial support of this study, to people from waste water treatment plant in Hengelo for kindly supplying material. N.A is grateful to EPM/CIIEN and COLCIENCIAS for financing the project 1115-4547-21979, to the University of Antioquia for the financial support of the ‘‘Programa Sostenibilidad 2013-2014’’ and, to COLCIENCIAS for the PhD scholarship. High pressure lab technicians (Johan, Benno and Karst) are acknowledged for their assistance in the experimental work and Louise Vrielink (UTwente) for the XRF analyses. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2014. 10.003. References Acelas, N.Y., Martin, B.D., López, D., Jefferson, B., 2014. Selective removal of phosphate from wastewater using hydrated metal oxides dispersed within anionic exchange media. Chemosphere. http://dx.doi.org/10.1016/ j.chemosphere.2014.02.024. Acelas, N.Y., Mejia, S.M., Mondragón, F., Flórez, E., 2013. Density functional theory characterization of phosphate and sulfate adsorption on Fe-(hydr)oxide:

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