Waste Management 33 (2013) 2328–2340

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Waste Management

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Review

Recycling and recovery routes for incinerated sewage sludge ash (ISSA):A review

0956-053X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.wasman.2013.05.024

⇑ Corresponding author. Tel.: +34 913 020 440; fax: +34 913 020 700.E-mail address: shanedonatello@ietcc.csic.es (S. Donatello).

Shane Donatello a,⇑, Christopher R. Cheeseman b

a Department of Cement and Material Recycling, Eduardo Torroja Institute of Construction Sciences (CSIC), C/Serrano Galvache 4, 28033 Madrid, Spainb Department of Civil and Environmental Engineering, Imperial College London, South Kensington SW7 2AZ, London, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 December 2012Accepted 29 May 2013Available online 29 June 2013

Keywords:Sewage sludge incinerationAsh characteristicsPozzolanic cementsPhosphate recoveryCeramicsSintered brick and tile

The drivers for increasing incineration of sewage sludge and the characteristics of the resulting inciner-ated sewage sludge ash (ISSA) are reviewed. It is estimated that approximately 1.7 million tonnes of ISSAare produced annually world-wide and is likely to increase in the future. Although most ISSA is currentlylandfilled, various options have been investigated that allow recycling and beneficial resource recovery.These include the use of ISSA as a substitute for clay in sintered bricks, tiles and pavers, and as a rawmaterial for the manufacture of lightweight aggregate. ISSA has also been used to form high densityglass–ceramics. Significant research has investigated the potential use of ISSA in blended cements foruse in mortars and concrete, and as a raw material for the production of Portland cement. However,all these applications represent a loss of the valuable phosphate content in ISSA, which is typically com-parable to that of a low grade phosphate ore. ISSA has significant potential to be used as a secondarysource of phosphate for the production of fertilisers and phosphoric acid. Resource efficient approachesto recycling will increasingly require phosphate recovery from ISSA, with the remaining residual fractionalso considered a useful material, and therefore further research is required in this area.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2329

1.1. Sewage sludge disposal practices in the EU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23291.2. Mono-combustion of sewage sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2329

2. Incinerated sewage sludge ash (ISSA) characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23313. Recycling and recovery options for ISSA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2331

3.1. Sintered materials containing ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2331

3.1.1. Sintering studies using ISSA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23313.1.2. Bricks, tiles and pavers containing ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23323.1.3. Manufacture of lightweight aggregates from ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2332

3.2. Glass–ceramics containing ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23333.3. Lightweight aerated cementitious materials containing ISSA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23333.4. Use of ISSA in cementitious materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2333

3.4.1. Use of ISSA in the Portland cement manufacturing process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23333.4.2. Use of ISSA as an additive to Portland cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23343.4.3. Pozzolanic activity of ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2334

3.5. Phosphate recovery from ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2335

3.5.1. Recovery of P by acid leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23363.5.2. Recycling of acid insoluble ISSA residue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23373.5.3. Thermal methods of P recovery from ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2337

3.6. Other recycling and recovery options for ISSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2338

4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2338

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2338

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1. Introduction

1.1. Sewage sludge disposal practices in the EU

The application of sewage sludge to agricultural land is gener-ally considered to be the ‘‘Best Practical Environmental Option’’ be-cause the N, P and K content of sludge provides high fertiliser valueand the organic matter acts as a useful soil conditioner. However, anumber of factors are making land-spreading of sewage sludgeincreasingly difficult. For example, the transport time and dis-tances between utilities producing sludge and suitable agriculturalland are generally increasing and this is increasing costs. Whilesludge disposal to land is regulated by the EU Sludge Directive(86/278/EC), many countries have applied tighter limits becauseof public concerns associated with pathogen transfer to cropsand the accumulation of heavy metals in agricultural soils. Forexample, in the UK a voluntary code of conduct known as the ‘‘SafeSludge Matrix’’ has been introduced and this only permits limitedapplication of pre-treated sewage sludge under specific conditions(ADAS, 2001). However, it should also be noted that sewage sludgeis exempt from controls and charges for the land disposal of otherwastes levied via the UK Environmental Permitting scheme. In theNetherlands, the Flemish region of Belgium and regions of Ger-many that have sandy soils, land-spreading has effectively beenbanned due to the adoption of prohibitively restrictive heavy metallimits for sewage sludge and sludge treated soils (Milieu et al.,2010). In other countries such as Greece, Italy, Malta and Iceland,landfill remains the major disposal route for sewage sludge. Thiswill become difficult to justify in the EU as the EU Landfill Directive(99/31/EC) places increasing restrictions on the quantities of bio-degradable waste that can be landfilled due to concerns over meth-ane generation under anaerobic conditions. An alternative to theseoptions was sea disposal of sewage sludge but this has beenbanned in EU countries since 1999 following the implementationof the EU Urban Wastewater Treatment Directive (1991). The dif-ferences in current sewage sludge disposal practices in EU coun-tries from data available via Eurostat are shown in Fig. 1.

The major alternative to land-spreading and landfill are thermaltreatment processes. It can clearly be seen from Fig. 1 that thecountries with low levels of land-spreading have invested signifi-cantly in incineration. An important advantage of incineration isthe degree of control this provides to sewage sludge managers.

Fig. 1. Sewage sludge disposal management practices in EU countries in 2009 or inthe year of latest available data on Eurostat. Data expressed as % of total sludgemass produced in each country. Note that data for Portugal and Denmark was notavailable.

Poor weather, changes in landowner attitudes and unexpectedoccurrences such as the foot and mouth disease outbreak in theUK in 2001 can have a dramatic effect on land disposal capacity.Such impacts do not normally affect sewage sludge disposal usingthermal treatment technologies. For an excellent review of thermaltreatment options of sewage sludge the reader is directed to workby Werther and Ogada (1999) and Fytili and Zabaniotou (2008).

Outside of the EU, there is a long history of sewage sludge incin-eration in the USA and Japan. Densely populated regions such asthose in Japan have the double problem of high quantities of sludgeproduction and low land availability. The largest sewage sludgeincineration plant in the world is currently under construction inHong Kong and is expected to produce around 240,000 tonnes ofISSA per year from 2013 onwards.

1.2. Mono-combustion of sewage sludge

During incineration, organic matter is combusted to CO2 andother trace gases, with water removed as vapour. The process can-not be considered as a complete disposal option because signifi-cant quantities of inorganic incinerated sewage sludge ash (ISSA)remain. This is removed from flue gases and requires furthermanagement.

This paper focuses on the ISSA generated by conventionalmono-combustion of sewage sludge. Although there are someexamples of co-combustion of sewage sludge with coal (Irelandet al., 2004; Leckner et al., 2004; Wolski et al., 2004), there areimportant legal issues that need to be overcome involving boththe definition of sewage sludge as a waste or fuel and standardsfor the use of subsequent co-combustion ashes (Cenni et al.,2001; EN 197-1). These issues also apply to ISSA despite the factthat mono-combustion of sewage sludge has been widely practisedat an industrial scale in many dedicated plants across the worldover several decades (Werther and Ogada, 1999).

An overview of a typical modern fluidised bed sewage sludgemono-combustion process is given in Fig. 2. Primary and secondarysewage sludge typically consists of 1–4 wt.% solids and this ispumped to tanks for further treatment. Fig. 2 shows a thickeningstage where sludge settles and the supernatant is removed. Thisraises the solids content to 3–8 wt.% solids. Thickened sludge isthen dewatered typically using plate or belt presses. At this stageorganic or inorganic additives can be employed to improve dewa-tering. For incineration there is an obvious incentive to optimisedewatering using organic additives as there are dual advantagesof improving sludge calorific value and reducing inorganic ash con-tent. The solids content of dewatered sludge typically varies from18 to 35 wt.%.

Although the calorific value of sewage sludge is often regardedas similar to that of brown coal, this is somewhat misleading. Thecalorific value of the solid organic matter present in sewage sludgedoes have similar calorific value to brown coal, but when sewagesludge is considered as a potential fuel, consideration has to be gi-ven to the accompanying inorganic solids, which have no calorificvalue. In addition, the water content consumes heat as it is vapour-ised. Sewage sludge typically has to be at least 28–33 wt.% solids toburn auto-thermically, with no requirement for external fuel tomaintain the incineration process. Some researchers have exam-ined the combustion of sewage sludge with significantly highersolids content, with the aim of minimising supplementary fuelrequirements (Sanger et al., 2001). However, any gain in energyoutput must be balanced against the energy input required for dry-ing the feed sludge to higher solids content.

Sludge and hot compressed air (ca. 500–600 �C) are fed to thecombustion chamber. The sand bed temperature is typically750 �C and the overhead freeboard zone at 800–900 �C. Guidanceon good operation is provided by technical documents (PD CEN,

Fig. 2. Overview of the sewage sludge incineration process (adapted from Arundel (2000)).

Fig. 3. A breakdown of likely physicochemical processes occurring to a sludge floc upon entering a sewage sludge incinerator and heat exchanger (from Donatello (2009)).

2330 S. Donatello, C.R. Cheeseman / Waste Management 33 (2013) 2328–2340

2004; USEPA, 2003). Temperatures can be finely controlled by theinjection of water or liquefied gas oil. The sand bed acts as a ‘‘ther-mal fly wheel’’ and helps stabilise temperature fluctuations in theincinerator. Particle residence times in the combustion chamberare typically only 1–2 s (Arundel, 2000) and during this time wateris evaporated, volatile metals vapourise and organic compoundsare combusted completely to gases, either directly or via the for-mation of an intermediate char. The remaining inorganic materialis carried out of the chamber as fine particulates with the exhaustgases. The ash is generally removed by bag filters, electrostatic pre-

cipitators or cyclones after passing through a heat exchanger. Theflue gas is then treated using a wet scrubber with acid, alkali andpossibly activated carbon dosing to comply with emission limits,as required by EU Waste Incineration Directive (2000/76/EC). Thescrubbing process produces an additional waste sludge, which isdewatered and normally disposed of in hazardous waste landfill.

Sewage sludge combustion is different from normal fuel com-bustion due to the high quantity of water present (Solimeneet al., 2010; Urciuolo et al., 2012). The characteristics of the ashesresulting from sewage sludge combustion differ significantly from

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those of, for example, coal fly ash. Coal fly ashes contain differentamorphous and crystalline aluminosilicate phases due to the muchhigher temperatures involved in coal combustion (1500–1700 �Cversus 800–900 �C) and coal is generally poor in nutrients suchas P, which are concentrated in sewage sludge ash.

2. Incinerated sewage sludge ash (ISSA) characteristics

A schematic diagram is given in Fig. 3 that highlights the rapidphysical and chemical processes that occur to sewage sludge par-ticles in the combustion chamber and the flue gas heat exchangersof a sludge incinerator. Approximately one third of the solids con-tent of sewage sludge consists of inorganic matter which formsISSA particles during combustion. Typical removal efficiencies ofash particles suspended in the flue gas are 95–99%. This translatesto an estimated global ISSA production of 1.7 mt per year, mainlyfrom the USA, the EU and Japan, which are the main regions oper-ating sewage sludge incinerators (Cyr et al., 2007; Murakami et al.,2009).

The general characteristics of ISSA have been reported in the lit-erature (Cyr et al., 2007) and this shows that the major elements inISSA are Si, Al, Ca, Fe and P. Crystalline forms of these elements areinvariably quartz (SiO2), whitlockite (Ca3(PO4)2) and hematite(Fe2O3). Aluminium is typically present in feldspar and XRD amor-phous glassy phases (Mahieux et al., 2010). These authors reportthat the amorphous glassy phase content can vary considerably be-tween ISSA samples. This is an important characteristic when con-sidering ISSA as a potential pozzolanic additive in blendedcements. The ISSA particle size distribution is also important. Aliterature review by Cyr et al., (2007) and data from samples fromdifferent UK incinerators reported by Donatello (2009) showedthat mean particle diameters can range from 8 to 263 lm withparticle sizes ranging from submicron to around 700 lm. The exactcontents of the major elements depend on the sludge treatmentprocesses applied at wastewater plants and other factors, such asthe level of industrial activity in the catchment area and whetheror not the sewerage system is combined (collecting storm-water)(Wiebusch and Seyfried, 1997). The choice of sludge dewateringaid will affect the ISSA composition. If tertiary sludge is includedin the feed, then the precipitation salt used to remove P (typicallyFeCl3 or Al2(SO4)3 are used) will increase the Fe or Al content inISSA respectively. Even in a plant operating under steady condi-tions, the major elements in ISSA can vary (Anderson and Skerratt,2003; Wiebusch and Seyfried, 1997). Loss on ignition values forISSA, which is essentially a measure of unburned carbon content,are typically below 3 wt.%, which is indicative of efficient combus-tion of the feed sludge.

Minor elements present in ISSA are much more variable and canbe strongly influenced by the degree and nature of industrial activ-ity within the catchment area of the wastewater treatment plant.Metals such as Hg, Cd, Sb, As and Pb are expected to be volatilisedduring combustion (Elled et al., 2007). However, in an industrialscale study run over 1 year, it was found that 20% of Hg, 93% ofAs and almost 100% of Cd and Pb were retained in the ISSA (Vande Velden et al., 2008). These findings support the theory that vol-atile trace metals condense on ash surfaces as temperatures fallduring flue gas heat recovery, as illustrated in Fig. 3. A laboratoryscale study by Corella and Toledo (2000) revealed that thermody-namic predictions of the fate of heavy metal during fluidised bedsewage sludge combustion were not accurate because metalbehaviour is controlled by fluid dynamics and kinetic factors asso-ciated with the limited residence time in the combustion chamber.

Ecotoxicity is a relatively new hazardous property used to clas-sify wastes (EU Hazardous Waste Directive, 91/689/EC). As far asthe authors are aware, only two papers have been published to

date regarding the ecotoxicity of ISSA from the mono-combustionof sewage sludge. A study by Lapa et al. (2007) focused on bothecotoxicological and chemical methods to analyse leachates, pre-pared according to EN 12457-3. Results were inconsistent andthere was poor correlation between the methods used. In workpublished by Donatello et al. (2010a), the chemical method wasfollowed according to UK Environment Agency guidance (Environ-ment Agency, 2011), and revealed that the major element of con-cern was Zn. Several compounds that contain Zn have beenassigned ecotoxicological risk phrases (R50-53). If these com-pounds can be assumed to exceed a combined concentration of2500 mg/kg, in the EU at least, the ash would be classified as haz-ardous waste via the H14 ecotoxic category. If all Zn is assumed toexist as ZnO, total Zn concentrations of 2009 mg/kg would reachthe H14 threshold. If assuming all Zn to be present as other com-pounds where Zn constitutes a smaller% mass, such as ZnCl2 orZn3(PO4)2, the total Zn level permitted would be even lower,around 1200 mg/kg (Donatello et al., 2010a). There is a need to bet-ter understand to real Zn speciation in ISSA. Geochemical model-ling could be particularly beneficial although the very shortresidence times in modern incinerators mean that kinetic factorsare likely to be more important than thermodynamic ones. Whendisposing ISSA to landfill in the EU, it is the level of soluble heavymetals that is considered important rather than total metal con-tent. Comparing soluble metal levels from EN 12457-3 leachingtests with ISSA, it was found that levels of Sb, Mo and Se were ofmost concern when comparing results to landfill waste acceptancecriteria (Donatello et al., 2010a).

In Japan, the use of temperature resistant ceramic filters to re-move ISSA prior to heat recovery has been investigated (Kataokaet al., 2006). By removing the ash at high temperature, when thevolatile metals are present as vapours (see Fig. 3), a greater degreeof separation of ash from volatile species can be achieved, with vol-atile metals later being recovered during gas scrubbing. Retrofit-ting of ceramic filters on flue gas treatment systems couldpotentially prevent ISSA from retaining relatively high levels of sol-uble Mo, Sb and Se, and this would reduce the cost of landfilldisposal.

Further research is needed to determine the most likely specia-tion of metals in ISSA to support a robust waste classificationexercise.

3. Recycling and recovery options for ISSA

A wide variety of potential reuse applications have been re-ported in the literature for ISSA and each of these is discussed inthe following sections.

3.1. Sintered materials containing ISSA

Sintering is a process in which a relatively weak compactedmaterial consisting of discrete particles is consolidated into astrong material. Sintering occurs when particles bond togetherafter exposure to sufficiently high temperatures to promote atomicdiffusion between neighbouring particles. The driving force caus-ing this effect is the reduction in particle surface energy by thereduction in vapour–solid surface area. Sintering is essential tothe ceramics industry involved in the production of bricks, tilesand lightweight aggregate.

3.1.1. Sintering studies using ISSAThe elemental composition of ISSA favours the formation of a li-

quid phase during sintering, which greatly reduces the tempera-ture and time required to form sintered products. Sinteringcauses shrinkage of the as-formed ‘‘green’’ sample along with den-

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sification due to the elimination of porosity. This is also associatedwith significant improvements in physical properties includingstrength and hardness. Decomposition reactions that involve gas-eous products, such as the reduction of Fe3+ in Fe2O3, can createa high degree of porosity in sintered materials. Results reportedby Cheeseman et al. (2003) showed that for samples of ISSApressed into cylindrical specimens and heated in the range of980–1080 �C, maximum sample density, maximum shrinkage andminimum water absorption were achieved after treatment at1000–1020 �C for 1 h. Heating above this temperature resulted ina decrease of sample density associated with the formation ofspherical pores from the decomposition of trace inorganic compo-nents in the ISSA matrix. Heating above 1080 �C caused the ISSA tosoften significantly and samples were deformed. Although heattreatment generally reduced metal solubility, significant leachingwas still demonstrated under acidic (pH 3.6) conditions. Theauthors reported no changes in the major crystalline phases pres-ent (quartz (SiO2), whitlockite (Ca9(MgFe)(PO4)6PO3OH) and hema-tite (Fe2O3)) as a result of sintering.

Similar studies by Lin et al. (2006) using pressed ISSA cylindersconcluded that significant sintering of ISSA occurred between 900and 1000 �C. Merino et al. (2005) reported large increases in thedensity of ISSA specimens between 1100 and 1200 �C. The sameauthors also highlighted the great stability of calcium-magnesiumphosphate minerals in ISSA even up to 1300 �C. When blendingISSA with 25% glass powder, Merino et al. (2007) showed that max-imum sample density, maximum compressive strength and mini-mum water absorption resulted from heating to 1125 �C.Different optimum temperatures were found when the type ofadditive (powdered glass, kaolin, illite and montmorillonite) andthe percentage addition (12.5–75 wt.%) were varied.

3.1.2. Bricks, tiles and pavers containing ISSAISSA consists predominantly of Si, Al, Ca and Fe and therefore

one of the earliest uses investigated was as raw material for themanufacture of bricks and tiles (Anderson, 2002; Anderson et al.,1996; Chen and Lin, 2009a; Liew et al., 2004; Lin et al., 2008; Linand Weng, 2001; Schirmer et al., 1999; Tay, 1987a,b: Tay andShow, 1992; Trauner, 1991; Wiesbusch et al., 1998; Wiebuschand Seyfried, 1997).

When manufacturing glazed tiles at laboratory scale, the substi-tution of 30 wt.% of clay by ISSA was shown to increase tile waterabsorption and decrease bending strength, irrespective of the glazeused after firing tiles at 1050 �C (Lin et al., 2008). These authorsshowed that tile warp increased progressively as ISSA content in-creased. An assessment of tile properties made from two differenttypes of clay substituted by 0–50 wt.% ISSA and fired at either 1000or 1100 �C showed that while bending strengths slightly decreasedas ISSA content increased, the firing temperature ultimately deter-mined tile properties such as shrinkage, water absorption, abrasionresistance and bending strength (Chen and Lin, 2009a).

The substitution of clay with ISSA by Tay (1987a,b) caused aslight but continual reduction in the compressive strength of bricksas the weight percentage of ISSA increased. Generally satisfactoryresults of 71 MPa for 50 wt.% ISSA bricks were reported, whichcompared well with those of 87 MPa for 100% clay bricks. How-ever, disappointing results were reported by Trauner (1991), whereadding 30 wt.% ISSA to the raw brick mix caused compressivestrengths to reduce from around 46 to 20 MPa. Other effects of par-tial clay replacement by ISSA reported are an increase in ‘‘gaugingwater’’ content, an increase in brick water absorption, a decrease inbulk density and a decrease in sintering temperature (Wiebuschand Seyfried, 1997). Whether or not the overall effects of ISSA addi-tion are beneficial or detrimental depend on the replacement leveland the specific Si, Ca, P and Fe content in the ISSA.

Laboratory scale manufacture of bricks containing 5 wt.% of claysubstituted by ISSA revealed a number of positive results, despitethe fact that ‘‘gauging water’’ content was increased from 4 to9 wt.% (Anderson, 2002). In 5 wt.% ash containing bricks, unfiredstrengths were increased by up to 29%, fired strengths increasedby 51% or 54% and water absorption of fired bricks decreased by25% or 78% depending on whether the bricks were fired at a max-imum temperature of 1050 or 1070 �C.

Arguably the most ambitious and advanced application of ISSAin brick manufacture was presented by Okuno and Takahashi(1997). These authors reported commercial scale manufacture ofbricks consisting of 100 wt.% ISSA. They stated that importantparameters for the starting ash were an average particle size<30 lm, loss on ignition <1 wt.% and CaO content <15 wt.%. Theserequirements could potentially impact on process decisions madeby water utilities. Fluidised bed incinerators tend to give finerash compared to multiple heath type furnaces and the use of Feor Al salts or organic polymers may be preferred to Ca salts duringsludge dewatering prior to incineration. The authors also describedthe optimisation of the dry pressing and firing process, which hasan optimum maximum temperature of 1070–1080 �C but whichcan vary depending on the P2O5 content of the ISSA. The brickscomplied with all relevant Japanese standards. However, duringservice life, problems with moss growth, efflorescence and ice for-mation were observed that were linked to water absorption. Theauthors applied a silicon-resin coating that eliminated these per-formance problems. Unfortunately the coating resulted in a signif-icant increase in manufacturing costs compared to conventionalclay bricks. It is not clear the extent to which the ‘‘avoided costs’’of ISSA disposal to landfill in Japan would positively contributeto making the ISSA brick product economically feasible. As landfillcosts continue to increase, this will become an ever more impor-tant factor in determining the economic viability of ISSA basedproducts.

Leaching tests on ISSA containing bricks revealed concerns overthe leaching of Cl�, SO2�

4 and certain heavy metals. This led authorsto increasing the brick firing temperature from 1000–1060 �C to1100–1200 �C (Wiesbusch et al., 1998).

3.1.3. Manufacture of lightweight aggregates from ISSAResearch investigating sintering of ISSA pellets to form light-

weight aggregates has been reported by a number of authors(Bhatty and Reid, 1989a; Cheeseman and Virdi, 2005; Chiouet al., 2006; Wainwright and Cresswell, 2001; Yip and Tay, 1999).Lightweight aggregates (LWA) are relatively high value due tothe scarcity of suitable natural lightweight aggregates in manycountries and the beneficial impact LWA can have on reducing con-crete density and improving thermal insulating properties. UsingISSA combined with 1–16% clay as a pelletising aid, Cheesemanand Virdi (2005) showed that the optimum sintering temperatureranged between 1050 and 1070 �C when considering water absorp-tion, density and aggregate strength. The ISSA LWA formed com-pared well to commercially available LWA based on sintered coalfly ash. Work carried out by Chiou et al. (2006) showed that com-bining ISSA with limited amounts of sewage sludge (<30 wt.%) fa-voured formation of lower density aggregates after sintering inthe temperature range 1050–1150 �C. The authors attributed thiseffect to bloating caused by decomposition of the organic matterpresent in the blended sewage sludge. Work carried out by Wain-wright and Cresswell (2001) showed that ternary mixtures of ISSA,clay and sewage sludge containing 64 wt.% ISSA produced low den-sity aggregate with properties comparable to commercially avail-able Lytag. These authors emphasised the importance ofincorporating a preliminary ‘‘burnout’’ stage when using mixeswith high organic content. The aim was to reduce the organic mat-ter content to <4 wt.%. This involved preliminary firing of pellets to

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700–800 �C to reduce the organic matter while avoiding sintering.This research also investigated the performance of ISSA based LWAin lightweight concrete (LWC) and found that 28 day strength com-pared well to control concretes, but that LWA performance was im-proved by increasing the clay content in the blend by 10 wt.% at theexpense of the sewage sludge component.

The key to successful lightweight aggregate manufacture is theformation of gaseous products from the decomposition of compo-nents at temperatures where the gas bubbles are trapped in a vis-cous pyro-plastic mass and remain as isolated pores upon coolingto room temperature. Obvious factors affecting this process are thesintering temperature and exposure time. However, the SiO2–Al2O3

ratio is also important (Tsai et al., 2006). These authors varied theSiO2–Al2O3 ratio of ISSA pellets by blending with glass cullet, alu-mina or municipal solid waste fly ash and found that addingAl2O3 greatly improved the strength and density of pellets and thatadding glass cullet decreased aggregate density and also resultedin higher water absorption. This was attributed to the influenceof glass cullet on the ‘‘bloating effect’’.

3.2. Glass–ceramics containing ISSA

Glass–ceramics are a formed by controlled crystallisation of aglass. They can exhibit very useful combinations of properties suchas high strength, high chemical durability and high temperatureresistance. The manufacture of glass–ceramics from ISSA was re-ported by Suzuki et al. (1997) who combined ISSA with an optimalquantity of limestone (40–60%) and melted the mixture at 1450 �C.A nucleation pre-treatment at 800 �C for 1 h was then applied tothe melt before heating to 1100 �C for 2 h to initiate anorthite crys-tallisation. The Fe and S in the ISSA formed FeS during the nucle-ation treatment under reducing conditions (crucible lid closed),but under oxidising conditions (crucible lid open) FeS nucleationwas inhibited and this adversely affected anorthite crystallisationand limited the physical properties of the glass–ceramics formed.

The conversion of ISSA samples involving pre-treatment at1500 �C prior to a controlled heat treatment to form glass–ceram-ics was reported by Park et al. (2003). These authors showed thatcombining ISSA with 10 wt.% CaO prior to melting produced diop-side or anorthite based glass–ceramics depending on the heattreatment applied. The excellent physical properties of the result-ing glass–ceramics were attributed to the interlocking diopsidecrystals formed.

Interesting results have been obtained from fusion analysis ofthree different ISSA samples (Wang et al., 2012). This work identi-fied the initial deformation temperature (IDT), the softening tem-perature (ST), the hemisphere temperature (HT) and the flowingtemperature (FT) of ISSA. The results showed clear differences inthe thermal behaviour of different ISSA samples which correlatedstrongly with the relative Al2O3 and Fe2O3 contents. Samples richin Al2O3 were poor in Fe2O3 and required much higher tempera-tures to undergo sintering and eventual melting. The sample richin Fe2O3 began to sinter and later melted at much lower tempera-tures, and this was attributed to the formation of Fe-silicates andFe-aluminosilicates. The relative Fe and Al contents of ISSA arestrongly influenced by the choice of dewatering aids and, wheretertiary treatment is applied to wastewater, the choice of phos-phate precipitation agent.

3.3. Lightweight aerated cementitious materials containing ISSA

Low density, aerated cementitious materials can be formedusing air entraining agents such as Zn or Al during mixing (Duand Folliard, 2005). The proposed reaction for in situ gas formationassociated with Al addition is given in the following equation (Peraet al., 1997):

2AlðsÞ þ CaðOHÞ2ðaqÞ þ 2H2OðlÞ ! CaðAlO2Þ2ðsÞ þ 3H2ðgÞ ð1Þ

Work in Taiwan has focused on using this reaction to producelightweight foamed materials using an ISSA–Portland cement mix(Chen et al., 2006; Wang et al., 2005a,b). Initial results showed thatvisible foaming occurred within 5 min of mixing ISSA–cement–Almetal blends with water, and that visible signs of reaction onlylasted for 10–20 min (Wang et al., 2005a). Optimum conditionsof 70–80 wt.% ISSA, 0.5–0.7 water–solids ratio and 0.1–0.2 wt.%fine Al metal powder were found based on the strength, waterabsorption, density and thermal conductivity of the aerated pastesformed. Further work with similar samples exposed to tempera-tures up to 1000 �C showed that foamed pastes containing 70 or80 wt.% ISSA by dry mass underwent gradual sintering above600 �C (Chen et al., 2006). The improvement in sample densityand compressive strength of high temperature exposed foamedISSA pastes was in stark contrast to results for control Portland ce-ment pastes, which were greatly deteriorated.

3.4. Use of ISSA in cementitious materials

The cement industry has three main options for using wastematerials. These are the beneficial recycling of wastes as alterna-tive raw materials to form clinker, use of wastes as alternative fuelsand use of wastes as supplementary materials in blended cements,effectively substituting for Portland cement. Given that ISSA pos-sesses no calorific value, use as an alternative fuel is not appropri-ate. The major elements present in Portland cement are Ca, Si, Aland Fe. These compare reasonably well to the major elements inISSA, with the notable exception of P. Thus ISSA could be used toa limited extent as an alternative raw material for cement manu-facture. It is worth noting that the use of dried or even dewateredsewage sludge has received considerable attention in the litera-ture, as this can simultaneously use the calorific value of sewagesludge to reduce fuel requirements and the inorganic content ofthe sludge to reduce cement raw material requirements (Husilloset al., 2013; Stasta et al., 2006; Zabaniotou and Theofilou, 2008).

3.4.1. Use of ISSA in the Portland cement manufacturing processCement is manufactured by firing a combination of limestone

(�80 wt.%) and clay (�20 wt.%). Small amounts of quartz sand,bauxite and/or hematite may be added to optimise the Si, Al andFe contents. In the cement kiln, all organic material is combustedand inorganic compounds, including those from any ISSA used asan alternative raw material, fuse into molten clinker phases ataround 1450 �C, with flame temperatures reaching 1800–2000 �C.In cement raw meal blended with ISSA it was shown that whenthe P2O5 content increased above 0.46 wt.%, the belite content ofclinkers increased at the expense of alite and this caused longersetting times and lower strength development in cement pastes(Lin et al., 2009, 2005). Lam et al. (2010) showed that clinkers pro-duced containing 2 wt.% ISSA were satisfactory but that when theISSA content increased to 8 wt.%, a significant reduction in alitecontent and increase in free lime content was observed. This wasattributed to the elevated phosphate and possibly sulphate con-tents of ISSA inhibiting alite formation. Pre-treatment of ISSA to re-move phosphates prior to use as a raw material in the productionof cement clinker was suggested.

From a practical point of view, significant quantities of ISSAcould in theory be diverted from landfill by use in cement kilnswithout reaching the problematic P levels reported by variousauthors (Halicz et al., 1984; Nastac et al., 2007). Global cement pro-duction was estimated at 3.6 billion tonnes in 2011 (CEMBUREAUwebsite, 2012) whereas a reasonable estimate of global ISSAproduction is approximately 1.7 million tonnes, over 2000 timesless. However, where sewage sludge is not incinerated, the direct

Fig. 4. Representation of trends reported by various authors upon increasingcement replacement rates with ISSA, of using milled ISSA and of acid washing andmilling ISSA. (Numbers denoted above data points represent the % cement replacedby ISSA).

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use of dried sewage sludge in the cement industry is generally pre-ferred because this avoids the need for investment in a dedicatedmono-incineration plant for sewage sludge, although investmentwould still be required in a thermal drying facility. The use of driedsewage sludge instead of ISSA allows the calorific value of the or-ganic matter to be exploited, reducing kiln fuel requirements(Husillos et al., 2013; Lin et al., 2012).

3.4.2. Use of ISSA as an additive to Portland cementWastes can be recycled in cement based materials as either ac-

tive pozzolanic materials, partially replacing cement, or as inert fil-ler, replacing sand and/or aggregates. A considerable number ofpapers have reported the use of ISSA as a partial replacement forPortland cement and the effect on the workability and strengthdevelopment of pastes, mortars and/or concretes (Bhatty and Reid;1989b; Cyr et al., 2007; Donatello et al., 2010b,c; Garces et al.,2008; Lisk, 1989; Luo et al., 2004; Monzo et al., 2003, 1999,1997, 1996; Pan et al., 2003a, 2002; Pan and Tseng, 2001; Pinarliand Kaymal, 1994; Tay and Show, 1994, 1992; Tay, 1987a,b, 1986).

The data up to 2007 has been summarised by Cyr et al. (2007),and a selection of the data available in the literature is presented inFig. 4. Two clear trends are evident within any given data set:increasing ISSA contents causes a decrease in compressive strengthand milling of ISSA generally improves strengths at a given per-centage of ISSA addition. When attempting to compare resultsfrom different authors using the same cement replacement rate,it is clear that large differences in relative strengths exist. Forexample, when replacing 20% of Portland cement with ISSA, reduc-tions in mortar compressive strengths of 5% (Pinarli and Kaymal,1994), 24% (Donatello et al., 2010b), 32% (Tay, 1986), 51% (Lisk,1989) or 52% (Pan et al., 2003a,b) have been reported. While obvi-ous differences will arise due to factors such as mortar specimendimensions and water/binder ratio used, an important factor notgenerally considered has been the processes used to produce theISSA. For example, the pioneering work of Tay (1986) was carriedout using a dewatered digested sludge fired in a laboratory ovenat 550 �C for an unspecified period. Lisk (1989) used ash producedby a multiple hearth furnace. Pan et al. (2003a) used dewateredprimary sludge fired at 700 �C for 3 h and the ISSA used by Dona-tello et al. (2010a–c) was sourced from industrial scale fluidisedbed incinerators where ash residence times in the combustionzone were of the order of seconds at 800–900 �C. Each of the afore-mentioned methods of ash preparation will impart a specific ther-

mal history to the ash and may affect the physical and chemicalproperties of the ISSA formed.

Regardless of the combustion history of ISSA, the irregular par-ticle morphology causes a decrease in workability when replacingcement, even at low percentage additions. To some extent, poorworkability can be overcome by milling (Pan et al., 2003a), addi-tion of plasticising agents (Monzo et al., 2003) or by incorporationof coal fly ash into the mortar mix (Paya et al., 2002). The results ofMonzo et al. (1999, 1997 and 1996) stand out due to the moderateincrease in compressive strength shown in ISSA mortars whencompared to control samples. They showed increases in averagestrengths of 8.3–15.3% when replacing 15 wt.% of Portland cementwith ISSA in 3:1 mortars. These samples were cured by waterimmersion at 40 �C and the moderately elevated curing tempera-ture may well explain the unusual results reported by theseauthors.

3.4.3. Pozzolanic activity of ISSADespite the generally negative effects on compressive strength

of ISSA in blended cements, many authors have attributed a certaindegree of ‘‘pozzolanic activity’’ to ISSA. The definition of a ‘‘pozzo-lanic’’ material as given in (ASTM C618, 2008) is:

A siliceous and aluminous material which, in itself, possesses littleor no cementitious value but which will, in finely divided form inthe presence of moisture, react chemically with calcium hydroxideat ordinary temperature to form compounds possessing cementi-tious properties.

The requirement of significant SiO2 and Al2O3 content suggestthat ISSA may have potential as a pozzolan. Another importantconsideration is that any clay present in sludge fed to the inciner-ator may be thermally activated and this can contribute pozzolanicproperties to ISSA. Such a phenomenon is well known in papersludge ash in which kaolin may be converted to pozzolanic meta-koalinite (Fernandez et al., 2010; Frias et al., 2010, 2008).

Many methods are available to determine the pozzolanic activ-ity of a material. These can be broadly classified as either direct orindirect methods. The pozzolanic reaction involves Ca(OH)2 react-ing with silicate or aluminosilicate phases to form amorphous C–S–H or C–A–S–H type gel products. Thus direct methods measurethe change in Ca(OH)2 concentration as the pozzolanic reactionproceeds. Examples of direct methods are the Frattini test (EN196-5), the saturated lime test (Fernández et al., 2010) and TG-DTA analysis of pastes with hydration reactions being inhibitedafter specific times. Indirect methods measure a physical propertyof pastes that is linked to the pozzolanic reaction. Examples of indi-rect methods are the strength activity index (ASTM C311, 2007)and electrical conductivity methods (McCarter and Tran, 1996;Paya et al., 2001).

The majority of work involving the assessment of ISSA pozzola-nic activity has used indirect methods, monitoring the effect ofreplacing cement with ISSA on compressive strength of pastesand mortars. Effects were generally negative but this could bestrongly influenced by the increased water demand caused bythe irregular particle morphology of ISSA. In a comprehensiveassessment of ISSA pozzolanic activity by Donatello et al. (2010b)and 2009), it was found that ISSA gave highly positive results inthe saturated lime test, negative results in the strength activity in-dex test and positive or negative results depending on the percent-age cement replacement in the Frattini test. The authors concludedthat ISSA was not pozzolanic but instead possessed a limited affin-ity for Ca2þ

ðaqÞ ions via an ion exchange mechanism. They also con-cluded that the saturated lime test method was biased in favourof positive results with ISSA due to the lower total quantity of

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Ca2þðaqÞ present in this system compared to the Frattini or strength

activity index tests.Increasing the fineness of ISSA has also been shown to increase

setting times, which was attributed by Pan et al. (2003a) to the im-proved ability of finer ISSA to adsorb Ca2+ ions from the liquidphase during cement hydration, inhibiting the massive precipita-tion of C–S–H gel responsible for setting of Portland cement (Chenand Odler, 1992).

Given the general lack of pozzolanic activity of ISSA, another op-tion for recycling in cement-based materials is as a fine aggregate.Very little work has been presented on this aspect. In one set ofexperiments, concretes containing up to 30% replacement of sandby ISSA showed a 22% reduction in 28 day compressive strength(Khanbilvardi and Afshari, 1995). The increased water requirementdue to the porous nature of ISSA relative to sand is likely to limitsand replacement rates to <5–10 wt.%.

3.5. Phosphate recovery from ISSA

A major disadvantage of using ISSA in construction materials ingeneral is that it represents a loss of potentially valuable P. Phos-phate is essentially a finite material in the sense that all phosphateore is mined from a limited number of geological reserves and onceP enters rivers, lakes or seas it can no longer be economicallyrecovered. The major demand for phosphate is in the manufactureof agricultural fertilisers (ca. 80%), animal feed (ca. 12%) and deter-gents (ca. 5%) (Smil, 2000; C.E.E.P, 2012). No alternative to phos-phate is feasible in fertilisers or animal feeds since phosphate isan essential element in the structures of DNA, bone, cell mem-branes and energy carrying molecules. At current rates of con-sumption, it is estimated that only 50–100 years of economicallyviable phosphate reserves remain (Cordell et al., 2009; Franz,2008; Steen, 1998).

The 2010 output from mining of phosphate rock reserves is de-picted in Fig. 5. It is apparent that no economically viable phos-phate reserves exist within the EU. Consequently, the use ofrecovered phosphates is a priority for the European phosphateindustry (Levlin et al., 2002). One of the most promising opportu-nities to recover phosphate is from sewage sludge collected atlarge centralised wastewater treatment plants. There are variousoptions for recovering P from sewage sludge but the disadvantagesare the relatively high water and organic matter contents, which

Fig. 5. Global phosphate rock output per country in millions of tonnes. The data

increase the processing capacities required. One well known exam-ple of phosphate recovery from sewage is controlled struvite pre-cipitation but this is a complex process dependent on manyfactors such as pH, Mg2+ and NH4

+ concentrations (Doyle and Par-sons, 2002). Phosphate is thermally stable and does not volatiliseduring sludge drying or incineration at 800–900 �C. Instead phos-phate is concentrated in the ISSA as whitlockite type, tri-calciumphosphates (Ca3(PO4)2). Some evidence has suggested that in ISSA,Ca2+ in whitlockite may be partially substituted by Mg2+, Fe3+ orAl3+ (Adam et al., 2009; Biswas et al., 2009; Donatello, 2009; Petzetet al., 2012; Wzorek et al., 2006). Phosphate contents in ISSA aretypically 10–25 wt.% as P2O5, while phosphate rock ore can consistof 5–40 wt.% P2O5 (Steen, 1998). The fact that ISSA is a dry and freeflowing powder greatly simplifies processing operations for subse-quent phosphate extraction when compared to either phosphaterock or liquid and dilute sewage sludge.

The conversion of phosphate ore to phosphorus at an industrialscale uses thermal methods in which Ca3(PO4)2 is reacted withcoke and quartz at 1200–1500 �C in an electric arc furnace. Thereaction is given in the following equation:

2Ca3ðPO4Þ2 þ 6SiO2 þ 10C! 6CaSiO3 þ 4Pþ 10CO ð2Þ

Schipper et al. (2001) examined the feasibility of ISSA as analternative source of Ca3(PO4)2 for the thermal process and it wasconcluded that the use of ISSA is limited due to the need to main-tain maximum Fe content of the phosphate feed at less than10,000 mg/kg to minimise the formation of unwanted FeP by-products.

The other major phosphate ore processing technique is knownas the ‘‘wet process’’ and involves dissolution of phosphate rockin concentrated sulphuric acid to form phosphoric acid. Gypsumis formed as a by-product. The general chemical reactions in thecontinuous process are given in the following equations:

2Ca3ðPO4Þ2ðsÞ þ 4H3PO4ðaqÞ ! 3CaðH2PO4Þ2ðaqÞ ð3Þ

CaðH2PO4Þ2ðaqÞ þ 3H2SO4ðaqÞ ! 3CaSO4ðsÞ þ 6H3PO4ðaqÞ ð4Þ

The above reactions form slurry which is filtered to separate thephosphoric acid product from the calcium sulphate crystals.Whether the precipitate is anhydrite (CaSO4), hemihydrate (CaSO4-

�0.5H2O) or dehydrate (CaSO4�2H2O) will depend on the reaction

is for 2010 and totalled around 181 million tonnes (data from USGS, 2012).

Fig. 6. Summary of some experimental processes for P recovery from ISSA reported in the literature; (a) Takahashi et al. (2001), (b) Franz (2008) and (c) Petzet et al. (2011).

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temperature (typically in the range 70–105 �C). The phosphoricacid filtrate can then be used to manufacture fertiliser or be furtherpurified for technical grade applications.

3.5.1. Recovery of P by acid leachingNaturally occurring phosphate ores are of relatively similar

composition and phosphate is extracted on an industrial scale byacid washing. The acid of choice is invariably H2SO4 due to lowcost, wide availability and ability to easily remove unwantedCa2+ from mixtures by controlled precipitation of gypsum (CaSO4-

�2H2O). Calculations to identify the minimum quantity of acid re-quired for maximum phosphate extraction by acid leaching havebeen presented in the literature (Franz, 2008; Petzet et al., 2012).

A number of papers have been published on the recovery ofphosphate from ISSA using a wet acid leaching process (Biswaset al., 2009; Donatello et al., 2010d; Franz, 2008; Oliver and Carey,1976; Petzet et al., 2012; Stark et al., 2006; Takahashi et al., 2001;Wzorek et al., 2006) and selected experimental approaches arecompared in Fig. 6.

Early work by Oliver and Carey (1976) showed that averages of76 and 61 wt.% of total P present in ISSA were recovered from eightash samples from different plants when washed with H2SO4 or HClrespectively (both acids at pH 1.5). They concluded that recoverywas not economically feasible based on market prices for acidsand phosphate in 1976. Phosphate prices have increased consider-ably since then and there is added environmental awarenessamongst governments and companies which is a significant driverfor phosphate recovery (Cordell et al., 2009; Driver et al., 1999;Levlin et al., 2002; Schipper et al., 2001).

A problem with acid leaching of P from ISSA is that a number ofother metals simultaneously dissolve. An interesting solution,demonstrated by Takahashi et al. (2001) was to break up the disso-lution process into three separate stages. In the first stage, sulphu-ric acid (pH 2) was added and the soluble P and heavy metalsseparated from the insoluble ISSA residue. In the second stage,the pH was raised to around 4 by the addition of sodium bicarbon-ate. At this pH, and with a very specific amount of Al2(SO4)3 added,phosphate was selectively precipitated as AlPO4 and separatedfrom the solution by filtration. The third and final stage involved

adjustment of the pH of the remaining heavy metal rich liquidphase to 10 by the addition of NaOH or Ca(OH)2, causing the pre-cipitation of many heavy metals as insoluble hydroxides. One po-tential limitation of the process used by Takahashi is themarketability of the AlPO4 product. High purity applications tendto work with concentrated H3PO4 and the release of soluble Al3+

would be a concern in any lower grade fertiliser application.The SESAL process presented by Petzet et al. (2012) is an inter-

esting alternative to the process used by Takahashi, as this ulti-mately produces a solid Ca-phosphate precipitate and solubleAlCl3 solution. The latter by-product can be recycled in wastewatertreatment plants for tertiary treatments involving chemical precip-itation of P from sewage effluent, potentially representing a closedloop for Al cycling. In the SESAL process, a pH of 3 is carefully main-tained with HCl, under which conditions the authors claim thatCa–P compounds dissolve and Al–P compounds simultaneouslyprecipitate (Petzet et al., 2011). Existing and newly formed Al-phosphates are retained on the filter along with acid insoluble ISSAresidues, while soluble heavy metals and Ca2+ pass to the filtrate.The solid fraction is then treated with NaOH at pH 13, where theAl-phosphate is dissolved and separated from the insoluble silicate,aluminosilicate and hematite components of ISSA. Finally the Al-phosphate filtrate is treated with CaCl2 to precipitate P as Ca-phos-phate and the soluble Al passes to the filtrate as AlCl3(aq).

The effect of laboratory scale incineration of small sewagesludge samples at different temperatures on P recovery from theresulting ISSA by leaching with nitric acid has been reported byWzorek et al. (2006). The authors suggested an optimum incinera-tion temperature of 950 �C, although this is perhaps too close tothe sintering point of ISSA to be advisable in an industrial scaleincinerator, which is generally operated at 780–880 �C. Resultspresented by Stark et al. (2006) showed that over 80% of P couldbe extracted by shaking with 1 M HCl for 2 h. The authors alsoshowed that extraction with 1 M NaOH was significant (ca. 70%),but less than with HCl. The very high liquid to solid ratios used(ca. 50 g acid: 1 g ISSA) are unlikely to be economically viable atan industrial scale. These authors also reported significant co-dis-solution of Ca2+. This highlights a clear advantage of H2SO4 overHCl to remove Ca2+ via the precipitation of CaSO4�2H2O.

Fig. 8. The effect of milling or acid washing and milling of ISSA on Frattini testCa(OH)2 removal results after 8 d at 40 �C. Paste mixtures were 20% ISSA–80% OPCand results were averages of duplicate analyses. Data taken from Donatello (2009).

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An alternative approach to converting P from ISSA into plantfertiliser was reported by Franz (2008). The process involved dis-solving P in H2SO4. As with other researchers, co-dissolution ofheavy metals occurred. The paper by Franz investigated the abilityof ion exchange and sulphide treatment to selectively remove hea-vy metals and reported that both processes were suitable for thispurpose. Phosphate was then precipitated out of the purified solu-tion following lime addition and the precipitate dried, ground andused in plant trials along with a commercial phosphate fertiliser.The ISSA derived P fertiliser was found to perform satisfactorilyover a 6 week fertiliser trial.

The paper by Biswas et al. (2009) reported on the recovery of Pfrom ISSA via leaching with NaOH HCl or H2SO4 and then recoveryof P via adsorption onto an unusual ‘‘saponified orange waste’’(SOW) gel loaded with Zr(IV). Using a 0.1 M concentrated leachant,the authors found that alkali leaching of P from ISSA was poor com-pared to acid leaching under the same conditions. The dissolutionof P from ISSA was relatively unaffected by increasing the acidtemperature from 30 to 70 �C. Recovery of P from acidic leachatewas found to be around 100% when adding 100 mg SOW adsorbentper 10 ml of leachate. The authors reported the successful elutionof most of the adsorbed P from the SOW gel by rinsing with0.2 M NaOH, implying that the gel could be reused. The authorsdid not elaborate on potential uses for the P rich alkaline eluate.

Given that the total P potentially recoverable from ISSA is rela-tively small compared to the global phosphate market, Donatelloet al. (2010d) investigated the potential to produce a high valuetechnical grade phosphoric acid from ISSA via an optimised sul-phuric acid leaching process, followed by cation exchange to re-move impurities and evaporation of excess water to produce aca. 80% H3PO4 product. The final product had acceptable levels ofheavy metals but needed more turbulent mixing conditions to re-duce the liquid to solid ratio and minimise evaporation energycosts. The need to remove SO2�

4 from the leachate was also identi-fied by the authors.

3.5.2. Recycling of acid insoluble ISSA residueOf the two general methods investigated for recovering P from

ISSA, the acid washing process is the simplest and cheapest option.However, one major consideration that has not been adequatelyaddressed in the literature to date concerns the remaining acid

Fig. 7. The effect of milling or acid washing in 0.19 M H2SO4 and milling on the28 day SAI value of mortars. Note that all mortars used a water/binder ratio of 0.5and a standard flowability due to super-plasticiser addition where necessary. In allcases 20% of cement was substituted for ISSA. The control mortar (0% ISSA) had anaverage 28 d strength of 42.8 MPa (SAI = 1.00). Results are averages of 3 measure-ments ± 1 standard deviation. Data obtained from Donatello (2009).

insoluble ISSA residue generated. This acid treated ISSA will con-tain low concentrations of P. The concentrations of other major ele-ments will also be altered. If H2SO4 is used in the acid treatment,gypsum crystals may be present. The only work that has investi-gated the potential recycling of this acid insoluble ISSA residuewas published by Donatello (2009) and Donatello et al. (2010c).

When blended with Portland cement, it was shown that aftermilling, the acid insoluble ISSA residue produced considerable in-creases in mortar compressive strengths when compared to bothuntreated ISSA and milled but not acid washed ISSA, as is shownin Fig. 7.

To investigate whether or not the improved strengths were dueto improved pozzolanic activity in acid washed ISSA, Frattini testswere applied to untreated, milled or acid washed and milled ISSAsamples. The results in Fig. 8 clearly demonstrates that the acidwashing process was not associated with any increase in pozzola-nic activity and so the improvements in compressive strengths ofacid washed ISSA mortars must be related to other factors suchas the gypsum content. Further work with acid-washed ISSA resi-dues is required to improve understanding of this material andwhy it shows particularly promising effects in blended cements.

3.5.3. Thermal methods of P recovery from ISSAThe main alternative for P recovery from ISSA is via thermal

methods. As with acid leaching, one issue is how to separate thevaluable P from problematic heavy metals. From an industrial per-spective considering the production of white phosphorus, the po-tential of ISSA is considerable as long as Fe contents are low. Thiscan be controlled to an extent by avoiding the use of Fe-salts dur-ing sludge processing at wastewater treatment plants. However,levels of Cu and Zn are also considered to be of concern. Regardingremoval of heavy metals from ISSA, thermochemical treatmentwith 5–15% of KCl or MgCl2 and heating at 900–1000 �C resultedin high percentage removals of Pb, Cd, Cu and Zn (Mattenbergeret al., 2008). However, as much as 30% of P could also be lost in fineashes that were carried out of the rotary kiln with exhaust gases,and removal percentages for Ni and Cr were unsatisfactory. To re-duce the problem of P loss in fine ashes, the authors modified theprocess to work with granulated ISSA pellets instead of ash (Mat-tenberger et al., 2010).

The effect of thermochemical treatment of ISSA on the bio-availability of P has been reported by Adam et al. (2009). This workshowed that untreated ISSA exceeded German and Austrian limitsfor heavy metals and particularly Zn and Cu in fertilisers. Treat-

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ment with 15% MgCl2 at 1000 �C for 60 min was shown to removewell over 90% of Cu and Zn from ISSA by volatilisation as CuCl2 orZnCl2. As the ISSA treatment temperature was increased, so did thebio-available P content. Treatment at 800 �C resulted in similaravailable-P levels to a commercial fertiliser. The changes in P avail-ability were attributed to the conversion of whitlockite (Ca3(PO4)2)to chlorapatite (Ca5(PO4)3Cl1�x(OH)x) via an intermediary chlor-spodiosite (Ca2PO4Cl) species, with the formation of new Mg-phos-phates (farringtonite Mg3(PO4)2) or Mg–Ca-phosphates. The sameresearch group report results extending earlier research on P-fertil-isers based on thermo-chemically treated ISSA to NPK fertilisersfollowing treatment with NH4NO3 and K2SO4 (Vogel et al., 2010).

3.6. Other recycling and recovery options for ISSA

Although the majority of research on ISSA recycling has fo-cussed on sintered materials, cements and phosphate recovery, itis worthwhile to mention other, more unusual recycling applica-tions reported in the literature.

The concentration of many trace elements and minor nutrientsin ISSA has led to untreated ISSA being considered as a potentialsoil amendment (Zhang et al., 2002a). A study by Escudey et al.(2007) showed that most of the important plant nutrients in ISSAwere only slightly soluble and provided slow release of nutrientsto volcanic soils. Direct application of ISSA to soil in Japan was re-stricted to a maximum application of 40 tonne/ha due to high hea-vy metal content relative to Japanese soils (Zhang et al., 2002b).

The potential recycling of ISSA in combination with Ca(OH)2 orcement has been reported for soil stabilisation applications (Linet al., 2007; Chen and Lin, 2009b). ISSA has also been used to re-place limestone as mineral filler in asphalt (Al Sayed et al., 1995).

The use of ISSA as a Cu adsorbent has been investigated by Panet al. (2003b) and Bouzid et al. (2008). Up to 98% removal of Cu wasreported by Pan et al. (2003b) with estimated maximum Cuadsorption capacities of 3.2–4.1 mg Cu2+/g ISSA. Bouzid et al.(2008) used ISSA from the combined incineration of sewage sludgecake and an olive mill waste and found that almost all Cu adsorp-tion occurred within the first 30 min of contact between the ashand the Cu bearing solution. These authors reported a Cu2+ adsorp-tion capacity of 5.7 mg Cu2+/g ash, which is significantly higherthan coal fly ash, but much lower than commercial activated car-bon adsorbents or other sewage sludge based adsorbents (Smithet al., 2009). In both studies, Cu adsorption efficiency was very sen-sitive to the system pH.

4. Conclusions

The majority of research on recycling and recovery of ISSA hasbeen completed during the last 15 years, due to the increasingattention given by water utilities to mono-incineration of sewagesludge as traditional disposal routes become increasingly re-stricted. Research has focused on the use of ISSA as a clay substi-tute in bricks and as a partial cement replacement material. Inboth applications, small additions of ISSA can be made withoutdetrimental effects to the final product. Larger additions requireprocess adjustments and may affect product performance. Thepozzolanic activity of ISSA is at best limited. These recycling appli-cations fail to consider the potentially valuable P content of ISSA.Recent research has recognised the potential for recovery of phos-phate from ISSA using both acid leaching and thermochemicalmethods. Both approaches allow ISSA to be converted into fertiliseror phosphate rich products, with acceptably low heavy metal con-tamination. It is likely that these processes will become moreattractive as both phosphate prices and ISSA disposal costs con-tinue to increase. Acid washing to recover P requires the recycling

potential of the acid-insoluble ISSA to be considered. When milled,this acid-insoluble residue has promise as a partial cement replace-ment. ISSA can be successfully recycled via a number of differentroutes and the main reason why this material is sent to landfill islack of industrial scale examples of the recycling applications al-ready demonstrated at the laboratory scale.

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