Bioresource Technology 174 (2014) 167–175

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Bioresource Technology

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Supercritical water gasification of sewage sludge: Gas productionand phosphorus recovery

http://dx.doi.org/10.1016/j.biortech.2014.10.0030960-8524/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: s.r.a.kersten@utwente.nl (S.R.A. Kersten).

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, Colombiab 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 2014Received in revised form 29 September2014Accepted 1 October 2014Available online 8 October 2014

Keywords:Sewage sludgeSupercritical water gasificationPhosphorusLeachingOxalic 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 inves-tigated. SCWG temperature (400 �C, 500 �C, 600 �C) and residence time (15 min, 30 min, 60 min) werevaried to investigate their effects on gas production and the P recovery by acid leaching. The results showthat the dry gas composition for this uncatalyzed gasification of sewage sludge in SCW mainly comprisedof CO2, CO, CH4, H2, and some C2–C3 compounds. Higher temperatures and longer residence timesfavored the production of H2 and CH4. After SCWG, more than 95% of the P could be recovered fromthe solid residue by leaching with acids. SCWG combined with acid leaching seems an effective methodfor both energy recovery and high P recovery from sewage sludge.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

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

of the phosphate ends up in a concentrated form in the sewagesludge (Blöcher et al., 2012). Secondly, wastewater treatment facil-ities usually operate at an industrial scale, resulting in largeamounts of the phosphorus-rich sludge. Technologies that reducemass and volume of sewage sludge and at the same time producea usable sludge product have gained more and more interest inrecent years. Sludge combustion is widely practiced on a full-scalebasis 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 ashesproduced in thermal processes, such as pyrolysis, combustion orgasification (Azuara et al., 2013; Petzet et al., 2012; Yanagidaet al., 2009). However, sewage sludge contains a high percentageof water, which causes high drying costs when these thermal pro-cesses are used. Therefore, supercritical water gasification (SCWG)has been identified as a promising technology for the conversion ofwet biomass streams with a low overall heating value to high

168 N.Y. Acelas et al. / Bioresource Technology 174 (2014) 167–175

heating value product gases (Chakinala et al., 2013), while still hav-ing the possibility of phosphorus recovery.

In supercritical water (SCW), the pressure and temperaturehave been increased to or over the critical point (22.1 MPa and374 �C). As a reaction medium, supercritical water is featured ashaving a high diffusivity, a low viscosity, and a high solvatingability for organic compounds (Youssef et al., 2010). In addition,water is an active reactant in steam reforming and water-gas shiftreactions in the supercritical region. Therefore, supercritical wateris able to transform sewage sludge into fuel gases such as H2, CO,and CH4 with reduced tar and coke formation compared to the tra-ditional thermal processes (Zhang et al., 2010). The selectivity ofthe gas production toward H2, syngas (H2 + CO), or CH4 can besteered by tuning the process conditions and by the use of catalysts(Chakinala et al., 2009).

From the perspective of cost-effective disposal of bio-basedwastes, the costs associated with SCWG of sewage sludge may behigh, but when the solid residue generated during SCWG is usedto recover and recycle phosphorus, this makes the process moreeconomical. In most SCWG studies however, most of the attentionhas been paid on energy recovery (Azadi et al., 2013; Wilkinsonet al., 2012; Xu et al., 2013) and on catalysts to promote productionyield of hydrogen from different types of biomass, among whichsewage sludge (Fiori et al., 2012; Gasafi et al., 2008; Zhang et al.,2010), manure (Youssef et al., 2010) and wood (Yong andMatsumura, 2012).

To date, very few studies have reported on phosphorus recoveryfrom solid residues generated during SCWG of sewage sludge. In arelated study, (Zhu et al., 2011) showed that during SCWG ofsewage sludge the majority of phosphorus ends up into the solidresidue (20 mg P/g). This is in agreement with (Yanagida et al.,2008), who evaluated the behavior of some inorganic elements inpoultry manure after SCWG and found that the liquid phase con-tained nearly all the N, K and Cl, while nearly all Ca, P and Si endedup in the solid phase. S was divided more or less evenly betweenboth phases. Clearly, the solid residue after SCWG is a promisingsource of P.

The research aim of this work is to study the feasibility of theSCWG of sewage sludge in combination with P-recovery from thesolid residue generated in this process. The effects of SCWG tem-perature and residence time are evaluated using a high-pressureautoclave and a heated fluidized bed. Acid leaching of phosphatefrom SCWG residue is studied with oxalic and sulfuric acid, andcompared to ash resulting from combustion.

2. Methods

2.1. Raw material

The dewatered sewage sludge (DSS) used as feedstock forsupercritical water gasification was collected from the wastewatertreatment plant of the Water Board Regge and Dinkel located inHengelo, the Netherlands (lat. 52.27957�, long. 6.77086�). Thesludge was dried (105 �C, until constant weight), pulverized in amortar and then sieved into two fractions of different particle size(the first: 600 lm 6U 6 1 mm, the second: U 6 600 lm), andstored in airtight containers. The material with particlesize 6 600 lm was used in the SCWG experiments, because itwas the largest fraction (by weight). The water content, as wellas the proximate and the ultimate analysis of the dried dewateredsewage sludge (DDSS) are shown in Table 1. The C, H, and Ncontent of the DDSS (dry ash free, daf) were measured in triplicateusing an Elemental Analyzer (Flash 2000-Interscience), with oxy-gen calculated by difference. The higher heating value (HHV) wascalculated according to the Dulong equation (Xu et al., 2012), using

the results of the elemental analysis. The mineral composition ofthe DDSS was determined by X-ray fluorescence spectrometry(XRF) and is shown in Table 2. For the XRF measurements sampleswere dissolved in lithiumtetraborate (Li2B4O7, flux) to make fusedbeads. These were measured together with standards with knowncomposition containing the same elements in the same range ofconcentration on a P analytical PW 1480 using the SuperQ soft-ware. 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 reac-tor (Inconel alloy: �72% Ni, 15% Cr and �8% Fe) with an internalvolume of 45 mL. A diagram of the set-up is shown in Fig. 1. Forsafety reasons, the reactor set-up was placed in a safety chamberand was controlled from outside the chamber during the experi-ments. The inner temperature of the reactor was measured by athermocouple inserted through an orifice in the bottom cap, andthe pressure was measured via a gas connection from the top lidto a pressure transmitter. The pressure and the temperature ofthe reactor were monitored and recorded using Pico Log software.With the help of a pneumatic arm, the autoclave was immersed inand removed from the fluidized sand bed. This sand bed washeated by an electric oven and preheated air which was also usedfor the fluidization of the bed.

In a typical run, the autoclave was loaded with 10 g of a mixtureof 15 wt% of DDSS and 85 wt% of deionized water, after which theautoclave was tightly closed and connected to the pneumatic armof the set-up. Prior to each experiment, a leakage test was per-formed by pressurizing the autoclave with 100 bar of nitrogenand monitoring pressure after closing the gate valve. This alsoserved to remove any air from the reactor. Finally, after restoringambient pressure in the reactor, the rest of the experiment wascontrolled from outside the safety chamber via the control panel.Using the pneumatic arm, the reaction was initiated by immersingthe autoclave into the preheated fluidized sand bed. After thedetermined residence time, starting from when the set tempera-ture was reached, the reactor was lifted and quenched in the coldwater bath. Triplicate runs were conducted for each combination oftemperature and reaction time.

2.3. Separation and analysis of the reaction products

The procedure for separating gasification products is shown inFig. 2. Once the reactor was cooled down to room temperatureand before depressurizing the system, gas samples were takenwith 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 athermal conductivity detector to determine H2, CO2, CO, CH4 andother light organic gases (C2H4, C2H6, etc).

The remaining product (liquid and solids in suspension; Product1) was then collected in a polypropylene bottle. Then, the reactorwas rinsed with de-ionized water to remove any solid depositsfrom the autoclave. The resulting suspension (Product 2) was alsocollected. The suspensions were separated into solid and liquidphases by vacuum filtration using a 1.0 lm pore size membranefilter (Glass microfiber filters. Whatman. GF/B). The filtrate fromProduct 1 was collected in a sampling vial to determine phospho-rus concentration whereas the solid residue on the filter was col-lected and named Solid 1. The solid residue after the filtration ofProduct 2 was also collected and named Solid 2. Both solids wererinsed with demineralized water while on the filter, then the filterwas dried at 105 �C until constant weight and then the solids werecollected and mixed together in a sampling tube for analysis.

Table 1Properties of the dewatered sewage sludge.

Water content (%) Proximate analysis (wt%)a Ultimate analysis (wt%)a HHV (MJ/kg)c

VM FC Ash C N H Ob

78.0 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 2Mineral 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).

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2.4. Leaching of phosphorus from the residue after SCWG of dewateredsewage sludge

All residue leaching experiments were carried out at room tem-perature, with the pH continuously measured after being set at pH2.0 to ensure that no acid limitations occurred and all availablephosphorus would be leached. It has been found in literature(Petzet et al., 2012) and in a study done in our lab that at this pHthe phosphorus dissolution process is efficient (Kootstra et al.,2015). Each experiment was started with 0.1 g of residue, 100 mLof demineralized water, and the necessary amount of acid to reachthe 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 thetime of maximal leaching, samples were collected at differenttimes (15, 30, and 45 min; 1, 2, 4, 8, and 24 h). The samples werefiltered using a 0.45 lm pore size membrane filter (Filtereinheit/Filter unit Whatman) and stored in a refrigerator at 4 �C for thedetermination of phosphorus (orthophosphate and total phospho-rus). For the phosphorus measurements Hach Lange kit LCK-349was used on appropriate dilutions, with an LT-200 heater andDR5000 spectrophotometer. All experiments were conducted intriplicate. In order to determine directly which crystalline com-pounds were dissolved or newly formed during the acid leaching,untreated and treated SCWG residues were analyzed by X-ray

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

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diffraction (XRD). The measurements were performed on aSiem5005. Furthermore, the appearance and the elemental distri-bution in SCWG residues before and after the acid treatment wereexamined using scanning electron microscopy (SEM), includingenergy dispersive spectroscopy (EDS) using a Philips XL 30 SFEG.

2.5. Terms and definitions

In discussing the results, the following definitions are used. Theyield (Yi) of product gas is defined as:

Yimolkg

� �¼ Ni

Nfeedð1Þ

where Ni is the gas produced during the SCWG, expressed in moles(mol) and, Nfeed is the initial amount of DDSS, expressed inkilograms (kg).

The carbon gasification efficiency (CGE) is defined as the degreeof conversion of carbon in the feed to permanent gases:

Carbon gasification efficiency;CGE ð%Þ ¼

Xi

NC;i

NC;feed� 100 ð2Þ

where Nc;feed is the amount of carbon in the feed, and Nc;i is theamount 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 ofsewage sludge are steam reforming (Eq. (3)), water-gas shift (Eq.(4)) and methanation (Eq. (5)) (Chakinala et al., 2009; Withaget al., 2012). However, many other side reactions like cracking,

chain rearrangements, condensation, and polymerization reactionswill occur.

CxHyOz þ ð2x� zÞH2O! xCO2 þy2þ 2x� z

� �H2 ð3Þ

COþH2O() CO2 þH2 ð4ÞCOþ 3H2 () CH4 þH2O ð5Þ

3.1.1. Effects of temperatureThe product gas composition obtained at various reaction tem-

peratures (400–600 �C) is presented in Fig. 3A. The correspondingpressures 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 thewater was in supercritical state. The gaseous products were mainlycomposed of CO2, H2, CO, CH4, and small quantities of light C2 andC3 compounds, such as ethylene (C2H4), ethane (C2H6), propane(C3H8), and propylene (C3H6). As expected, temperature has a sig-nificant influence on the product gas composition. At lower tem-peratures, decarboxylation reactions are more dominant and, as aresult, CO2 is the main product obtained at 400 �C. At higher tem-peratures (500 �C and 600 �C), supercritical water becomes a morepowerful oxidant, and free radical reactions prevail. Water acts as asolvent and promotes solute-solvent reactions such as the decom-position of the organic compounds in the feedstock.

The decrease of CO formation at increasing temperaturesimplies that the water-gas shift reaction occurs and increasedwater-gas shift activity may be a cause for the increase in H2

production. At higher temperatures endothermic reactions arefavored while at lower temperatures the equilibrium shifts toexothermic reactions. Steam reforming (Eq. (3)) is endothermicand therefore an increased temperature would favor the formationof H2 gas. Another possible way of H2 formation is the thermaldecomposition of the intermediate compounds. On the other hand,the increase of temperature (from 400 to 600 �C) shifts the

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 gasificationefficiency. (C) effect of temperature and reaction time on gas yield from the SCWG of dried dewatered sewage sludge. Average values, triplicate experiments, single analysisper experiment. Error bars represent standard deviation.

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

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

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Fig. 3B shows the carbon gasification efficiency (CGE) of sewagesludge in supercritical water at different temperatures. The CGEincreased from 16% to 52% with increasing the temperature from400 �C to 600 �C.

Fig. 3C shows the gas yield obtained at the reaction tempera-tures. It can be observed that as the temperature increased from400 �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 athigher temperatures. From a thermodynamic perspective, a hightemperature favors gas formation, since the overall SCWG processis endothermic (Guo et al., 2007; Lu et al., 2006).

Compared to temperature, reaction time has a smaller influenceon the individual gaseous species produced (see Fig. S1 in thesupplementary material). The H2 and CH4 content in the gas phaseincrease slightly with reaction time while CO content has a ten-dency to decrease with increasing reaction time. This behaviorcan be attributed to hydrogen production via the water-gas shiftand methanation reactions.

The influence of reaction time is small enough to suggest shortreaction times are sufficient in the SCWG of sewage sludge, aslonger reaction times increase process costs per treated amountof raw material. The present results were generally consistent withprevious work of SCWG of biomass (Chakinala et al., 2009; Lu et al.,2006; Youssef et al., 2010). These results suggest that sewagesludge can be converted into high heating values product gasesusing the SCWG, and that at 600 �C the production of H2 and CH4

is increased compared to lower temperatures. The calculatedHHV of the gas mixtures were 27 MJ/kg, 39 MJ/kg, and 48 MJ/kgfor SCWG at 400 �C, 500 �C, and 600 �C, respectively.

3.2. Release of phosphorus from the residue after SCWG of dewateredsewage sludge

During SCWG, not only gases but also solids and liquids are pro-duced (Fig. 2). In our study, we found that during SCWG of sewagesludge the majority of the P migrated into the solid residue. Thephosphorus content went up from 3.4% in the dried sewage sludgeto 8.3%, 8.7% and, 9.4% in the solid residue generated during SCWGat 400, 500 and, 600 �C, respectively. The total mass balance wasclosed for 97.8–98.6%, with no apparent losses other than thoseoccurring during normal lab scale processing, such as, feeding,sampling and collecting. After SCWG, the liquid phase containedaround 2% of all phosphorus originally present in the DDSSindependent from the experimental conditions. The increasedphosphorus concentration in the SCWG residue and the fact thatbuffering organics, e.g., proteins, are no longer present in that res-idue are positive for the leaching process. A more concentratedstream facilitates the extraction of phosphorus, if only by reducingthe equipment scale, and the reduction of buffering organicsmeans that less acid is needed to reach the desired pH during sol-ubilizing/leaching of the phosphorus.

3.2.1. Effect of leaching timeFig. 4A shows the increase in leached phosphorus yield with

time, using sulfuric and oxalic acid, while maintaining pH 2. Theexperiment was conducted using the residue obtained after SCWGof 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% and11% more solubilized P after 2 and 4 h, respectively. Furthermore,after 4 h oxalic acid reached its maximum yield, while with sulfuricacid more time was needed. After 8 h, oxalic and sulfuric acid havereached their maximum yield of 92% and 95%, respectively. Follow-ing these results, leaching time for the other residues was set at8 h.

3.2.2. Effects of temperatureThe 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 acidand from 80% to 82% and 87% for sulfuric acid at 400 �C, 500 �C and600 �C, respectively.

To help understand the observed temperature effect in leachingphosphorus it should be considered that different organic and inor-ganic phosphorus compounds may exist in the solid residue. Thehigher the SCWG temperature, the more organic P is converted intoinorganic P. The destruction of organic compounds in SCWG takesplace primarily through free radical pathways rather than the ionicpathways that dominate in liquid water. This is because supercrit-ical water acts as a nonpolar solvent. At supercritical operatingconditions, gasification proceeds quickly and converts the H–C–Ncompounds to hydrogen, carbon monoxide, carbon dioxide, andmolecular nitrogen as the main products. For compounds contain-ing heteroatoms such as chlorine, sulfur, and phosphorus, reactionalso produces the corresponding inorganic acids: HCl, H2SO4, andH3PO4, which can be neutralized to their corresponding salts andseparated as concentrated liquid brine or precipitated solid.Sullivan and Tester (2004) studied the supercritical water oxida-tion (SCWO) of methylphosphonic acid as a model compound oforganic P, and they found that it could be destroyed completelywithin a short period of reaction time and produced phosphatein the final residue. The P–C bond destruction during SCWG isimportant for leaching process because the P–C bond is inert toacidic and basic hydrolysis. However, the organic P contained insewage sludge is composed collection of phospholipids, phos-phonic compounds, nucleic acids, phosphoproteins etcetera, andthese are more complex than the model compound. Therefore,future work is needed to clarify the conversion of organic P duringSCWG of sewage sludge. Conversion of organic P to phosphoric acidwhich is neutralized to apatite during SCWG of poultry manuremay be complete at high reaction temperatures, according to(Yanagida et al., 2008) (Eq. (6)). As apatite is readily dissolved inacidic aqueous conditions (Eq. (7), using sulfuric acid)(Kuligowski and Poulsen, 2010), this may explain the observedhigher P leaching yields at higher SCWG temperatures.

2ðHmPO4Þ3�m þ ð3�mÞCa2þ ! Ca3�mðHmPO4Þ2 m ¼ 0;1;2 ð6Þ

Ca3ðPO4Þ2 þ 3H2SO4 þ 6ðxÞðH2OÞ! 3CaSO4 � 2ðxÞH2Oþ 2H3PO4 þ heat ð7Þ

where x = 0, 0.5, or 1.

3.2.3. Effect of reaction timeFig. 4C illustrates the variation of P leached in different reaction

time at 600 �C. P yields were only slightly – if at all – influenced bySCWG reaction time, with 87%, 88% and 92% yield for sulfuric acidat 15, 30 and 60 min, respectively, while yields stayed constant ataround 95% for oxalic acid. Similar behavior was observed withresidues produced at 400 �C and 500 �C (results not shown). Itappears that SCWG for 15 min already makes most of the P avail-able for acid leaching, by the same proposed reaction mechanism(Eq. (6)) as discussed earlier. The advantage of shorter reactiontimes while sacrificing neither gas production (Fig. S1 in the sup-plementary material) or P-leaching performance is obvious: thefinal process becomes more economical due to lower energyrequirements and less needed equipment and capital per amountof residue.

3.2.4. Effect of acid typeThe use of oxalic acid results in a higher phosphate yield

than sulfuric acid for all solid residues, under all tested SCWG

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 oftemperature 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 barsrepresent standard deviation.

N.Y. Acelas et al. / Bioresource Technology 174 (2014) 167–175 173

conditions, after 8 h of leaching (Fig. 4B and C). The performance ofoxalic acid may be attributed to its ability to form complexes withmetal ions, which will improve the phosphate removal (Azuaraet al., 2013; Kootstra et al., 2015). In comparison with other organicacids, oxalic acid has the largest ability to chelate many metal ions(K+, Mn2+, Zn2+, and Cu2+). In fact, the chelating stability constantfor oxalate-K+ complexes of five-ring structures is 1.61 times thatof citrate-K+ complexes and 2.25 times that of malate-K+ com-plexes (Jones, 1998). In a related study, it was reported that aftertreatment with oxalic acid, the crystallinity of fluorapatitedecreased (Jiang et al., 2012), which is an indication of increasingP 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 oxalicacid can efficiently solubilize phosphate from calcium phosphatein calcareous soils and from iron phosphate and aluminum phos-phate in acidic solutions (Jones, 1998; Tu et al., 2007), and deducedfrom the mineral composition of sewage sludge (Table 2), thesewill be the phosphorus containing compounds present in the resi-due 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 mediumis based on the fact that this is the standard acid used for manyprocesses that solubilize P, such as artificial fertilizer production.

174 N.Y. Acelas et al. / Bioresource Technology 174 (2014) 167–175

Other acid such as, hydrochloride acid would add chloride ions tothe leachate and these could form soluble complexes with mostheavy metals, thus increasing the amount of metals in solution(Pettersson et al., 2008).

3.2.5. Comparison between leaching from combustion ash and SCWGresidue

It seems to be easier to release phosphate from SCWG residue ofDDSS than from combustion ash of the same material using sulfu-ric acid at pH 2. At all three tested temperatures, leaching SCWGresidue resulted in higher yields (see Fig. S2 in the supplementarymaterial).

The maximum P yield from sewage sludge combustion ash andSCWG residue are 85% and 92%, respectively. This difference maybe attributed to the different chemical compounds formed duringthese two thermal treatments. For example, under combustionconditions of 400 �C could be possible to find organic P compoundswith P–C bonds which are inert to acid treatment while in theSCWG residue of sewage sludge, phosphorus is present mostly inthe form of calcium, aluminum, and iron phosphate (Yanagidaet al., 2008). Acid leaching dissolves these compounds andphosphoric acid is obtained. The chemical reactions involved inphosphorus dissolution from SCWG residue at pH 2 are likely asfollows:

Ca5ðPO4Þ3OHþ 10Hþ ! 5Ca2þ þ 3H3PO4 þH2O ð8ÞXPO4 þ 3Hþ ! X3þ þH3PO4 X ¼ Al or Fe ð9ÞFe3ðPO4Þ2 þ 6Hþ ! 3Fe2þ þ 2H3PO4 ð10Þ

3.2.6. XRD-analyses of SCWG residue; before and after leachingFig. S3 in the supplementary material shows the diffraction

patterns for SCWG residue before and after the oxalic acid leachingprocess. It can be observed that after leaching process, the signalscaused by compounds such as P-Fe (whitlockite), P-Al (AlPO4), andP-Ca (hydroxyapatite) have disappeared. This indicates decomposi-tion of phosphate compounds, resulting in more dissolution of P.After the acid treatment certain stable compounds are visible, inthis case, the signals for calcium and iron compounds such as, cal-cite and hematite appear during leaching process. Quartz (SiO2) ispresented in the residue before and after the acid treatment whichmeans that this is stable to acid attack. These three species are themain 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 theleaching procedure (Fig. S4, in the supplementary material).

SEM-EDS results of the SCWG residue before and after leachingindicated both physical and chemical transformation during leach-ing. Fig. S4A and S4C show a slight difference in morphology of theparticles of the SCWG residue before and after acid treatment.Before leaching, the SCWG residue consists of defined, individualparticles, whereas the leached residue contains agglomeratedclusters of smaller particles. The elemental analysis of the SCWGresidue surface before leaching is shown in Fig. S4C. It can be seenthat Fe, Al, Ca, P and Si are the most common elements for this typeof sample. The acid leaching efficiency can be corroborated by theabsence of the phosphorus peak in the surface elemental analysisof the SCWG residue after the acid treatment, while the other ele-ments seem to stay in the solid part (Fig. S4D).

3.2.7. Acid consumption for phosphorus recovery from SCWG residueof sewage sludge

The economics of phosphorus leaching from SCWG residue aremostly dependent on the amount of acid required per amount ofphosphorus extracted. In preliminary experiments (results notshown) for this study, it was determined that, for acidification of

raw sewage sludge to pH 2, around 29.9 kg of sulfuric and36.7 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 SCWGresidue, 12.2 kg of sulfuric and 16.0 kg of oxalic acid per kg of phos-phorus resulted in 92% and 96% leached phosphorus, respectively.A reduction of around 60% in the amount of needed acid peramount of phosphorus was reached, while also resulting in moresolubilized phosphorus per amount of sewage sludge. The eco-nomic feasibility of phosphorus dissolution from SCWG residuewith sulfuric acid can be evaluated by comparison with the acidconsumption needed for phosphoric acid production from apatite.For P extraction from phosphate rock, sulfuric acid consumption isin 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 morethan 85% P from sewage sludge incineration ash ranged from 4.6to 18.1 kg H2SO4 per kg P, with the extent of the range probablydue to ash composition, the presence of cations in solution, andthe presence of other phosphate compounds requiring less or moreacid for dissolution.

In our investigation, 1.1 kg H2SO4 per kg residue was necessaryto release 92% of P in comparison to 1.0 kg H2SO4 per kg ash torelease 96% of P. It allows us to infer that optimizing the amountof acid needed for P leaching will show the SCWG process as acost-effective method for P-recovery.

4. Conclusions

The work demonstrates a sustainable approach to convert sew-age sludge into fuel gases and to recover phosphorus from SCWGprocess, making it possible for the wastewater treatment industryto make a significant contribution to the recycling of this vital andfinite resource. For the uncatalyzed conversion, it was found thatthe gasification efficiency of sewage sludge increases with highertemperature, 52% at 600 �C for residence times of 15 min and more.The highest release of phosphate, 95.5% of what was present in theSCWG residue, was obtained from the residue after SCWG at 600 �Cand 60 min reaction time.

Acknowledgements

The authors would like to acknowledge to the BE2.O (BioEnergyto Overijssel) program for the financial support of this study, topeople from waste water treatment plant in Hengelo for kindlysupplying material. N.A is grateful to EPM/CIIEN and COLCIENCIASfor financing the project 1115-4547-21979, to the University ofAntioquia for the financial support of the ‘‘Programa Sostenibilidad2013-2014’’ and, to COLCIENCIAS for the PhD scholarship. Highpressure lab technicians (Johan, Benno and Karst) are acknowl-edged for their assistance in the experimental work and LouiseVrielink (UTwente) for the XRF analyses.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.biortech.2014.10.003.

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