Utilization of water treatment plant sludge in structural ceramics

lable at ScienceDirect

Journal of Cleaner Production xxx (2014) 1e8

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Utilization of water treatment plant sludge in structural ceramics

Eliane Wolff a, Wilfrid Keller Schwabe a, Samuel Vieira Conceiç~ao b, *

a Department of Sanitary and Environmental Engineering, School of Engineering, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais 31270-901, Brazilb Department of Industrial Engineering, School of Engineering, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais 31270-901, Brazil

a r t i c l e i n f o

Article history:Received 27 September 2013Received in revised form30 May 2014Accepted 6 June 2014Available online xxx

Keywords:Cleaner productionWater treatment plant sludgeStructural ceramicsBuilding bricksPulp mill

* Corresponding author.E-mail addresses: [email protected]

(S.V. Conceiç~ao).

http://dx.doi.org/10.1016/j.jclepro.2014.06.0180959-6526/© 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Wolff, EProduction (2014), http://dx.doi.org/10.1016

a b s t r a c t

Industrial solid waste generated by a water treatment plant (WTP) at a pulp mill was used formanufacturing construction bricks. The sludge from the WTP was mixed with three types of wasteobtained via recovery of chemicals generated by the same pulp mill and with waste obtained by crushingand grinding granite rock. The formulation of these mixtures was based on the grain size distribution ofthe residues and the proportions of calcium, sodium, and potassium oxides in the mixtures. Specimens ofthese mixtures were then fired at 850, 950, and 1050 �C in order to obtain crystalline phases (anorthite,albite, gehlenite, and mullite) that would confer good mechanical properties. The technological prop-erties of the specimens were evaluated after drying and firing. The statistical analysis of technologicalproperties of the proposed mixtures suggests that sludge can be used as a substitute for clay in theformulation of clay masses and the mixtures B, C, D, and F at 850 and 950 �C, should be tested in theceramic industry on a pilot scale in order to evaluate their suitability for the production of interiorcoatings or acoustic bricks. The utilization of WTP sludge in brickmaking eliminates an environmentalproblem, several economies related to the replacement of a natural raw material are generated. Finally,by using WTP sludge in the manufacturing of acoustic bricks and other products, it is shown that cleanerproduction practices promote innovation in the pulp mill and the red ceramic industry, leading toenvironmentally friendly practices.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

To maintain a high environmental performance, the pulp andpaper industry has made important investments in more efficientproduction processes and technologies (Ghose and Chinga-Carrasco, 2013), and the adoption of CP methodologies and thepotential reduction in the negative environmental impact havebeen major concerns. Cleaner production is an integrated approachto handling waste and pollutants in industries (Visvanathan andKumar, 1999). This approach entails an economic environmentallyfriendly production system that encompasses improvements inenergy use, raw material use, and a reduction in emissions andwaste. In addition, the importance of CP has motivated leaders ofdifferent countries at diverse levels of industrial development toestablish national strategies or programs to accelerate the imple-mentation of CP (Ghazinoory and Huisingh, 2006).

om, [email protected]

., et al., Utilization of wate/j.jclepro.2014.06.018

The CP principle involves the continuous use of industrial pro-cesses and products to increase efficiency and to reduce theirimpact on humans and the environment. CP represents viablepreventive environmental approaches for the reduction of pollu-tion at the source. In addition, the ability of cleaner productionpractices to improve production systems and generate economicand social benefits has been demonstrated (Zarkovic et al., 2011;Giannetti et al., 2008).

In a pulp mill, cleaner production can be developed by using thefollowing two approaches (Huang et al., 2013): (1) developing acomplete cleaner production technology and applying an innova-tive process route for the whole production process; and (2)improving production efficiency, waste reduction, and recycling bypromoting cleaner production practices in a key production pro-cess. The former is convenient for upgrading the entire industry;however, the long research cycle and large investment requirementmake it a high-risk approach in the new process route development(Huang et al., 2013). As we are focusing on solid waste manage-ment, i.e., the utilization of water treatment plant sludge in struc-tural ceramics, which represents one portion of the entireproduction process in the pulp mill, the second approach is moresuitable for our research.

r treatment plant sludge in structural ceramics, Journal of Cleaner

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1 Tons of air-dried cellulose pulp.

E. Wolff et al. / Journal of Cleaner Production xxx (2014) 1e82

Cleaner production has allowed industrial production to find aplace in this vision by recasting negative impacts of polluting in-dustrial processes and products into positive images of newstechnologies that are materials-conserving, energy- efficient, non-polluting and low-waste, and that produce ecologically friendlyproducts, like which are responsibly managed throughout theirlifecycle (Geiser, 2001).

Solid waste management is very important from the publichealth, as well as from the socio-environmental performance andindustrial perspectives because continuously increasing quantitiesof materials, hazardous or non-hazardous, must be discarded in asafe and economical manner or, preferably, recycled.

Like any other industrial segment, drinking water treatmentplants produce residues from the processes of decantation andfilter washing. A large quantity of water treatment residues isgenerated each year from fresh water treatment plants (Huanget al., 2005). The usual practice is to discharge these residues intorivers without any treatment. This procedure is not in accordancewith cleaner production practices since its degrades not only thequality of the rivers increasing the concentration of solids, silting,causing color changes and turbidity, inhibiting biological activity,and increasing the concentrations of aluminum, iron, and someother elements; but it also poses a danger to the lives of current andfuture human generations.

In Brazil, procedures for adequate disposal of sludge generatedby treatment of natural surface waters are already known. How-ever, problems arising from inadequate disposal of residues seemfar from being resolved and significant amounts of these residuesare still being discharged in rivers.

In Brazil, NBR 10.004 (ABNT, 2004), in which sludge generatedby WTPs are classified as solid waste that must be processed forfinal disposal, Law 9433 (Brasil, 1997), which established the Na-tional Policy on Water Resources, and Law 9605 (Brasil, 1998), theEnvironmental Crimes Law, have already been established.Considering these laws and the need for sanitation companies toshow good environmental performance in order to obtain nationaland international funding, a new procedure is required for safedisposal of these residues in a way that minimizes the environ-mental impact.

The sludge produced by WTPs can be a potential substitute forbrick clay because its chemical composition is very close to that ofbrick clay (Hegazy et al., 2012). In addition, the use of sludge in theconstruction industry is considered to be an economic and envi-ronmentally sound option (Ramadan et al., 2008).

The concentration of sludge that can be incorporated into claysin order to produce bricks depends partly on the sludge properties(grain-size distribution and chemical andmineral composition) buteven more so on the properties of the raw materials used (Teixeiraet al., 2011). Using bench-scale experimentation, Alleman andBerman (1984) showed that conventional clay and shale in-gredients for bricks could be partially supplemented with sludge.They called this clay product “biobrick”.

Bricks manufactured from dried sludge collected from an in-dustrial wastewater treatment plant were investigated by Lin andWeng (2001) and Weng et al. (2003). These reports showed thatthe sludge proportion and the firing temperature were the two keyfactors determining brick quality. In accordance with a previousstudy, bricks produced from sewage sludge of different composi-tionswere investigated by Liewet al. (2004). Results of the tests alsoindicated that the sludge proportion is a key factor in determiningthe brick quality. Increasing the sludge proportion resulted in adecrease in brick shrinkage, bulk density, and compressive strength.Brick weight loss on ignition was mainly due to the containedorganic matter from the sludge being burnt off during the firingprocess and the inorganic substances found in both clay and sludge.

Please cite this article in press as: Wolff, E., et al., Utilization of wateProduction (2014), http://dx.doi.org/10.1016/j.jclepro.2014.06.018

The literature also discusses the use of WTP sludge inconjunction with other alternatives. For instance, Huang et al.(2005) and Chiang et al. (2010) discussed the use of scrap glass inconjunction with WTP sludge for manufacturing building bricks.Due to the similar mineralogical composition of clay and WTPsludge, Hegazy et al. (2012) proposed the complete substitution ofbrick clay by sludge incorporated with some agricultural and in-dustrial wastes, such as rice husk ash and silica fume. Toya et al.(2007) proposed the use of waste generated during the beneficia-tion of silica sand and plastic clay in manufacturing glass ceramics.

The search for adequate solutions for solving environmentalproblems is an important condition for cleaner production in thepulp industry. Such solutions would eliminate or minimize theenvironmental impact of these residual materials, promotecontinuous improvement of environmental management in pro-duction processes, take into account technical, economic, andenvironmental perspectives, and promote sustainability with astrategic outlook toward future generations.

In this study, industrial solid waste generated by aWTP at a pulpmill was used to manufacture construction bricks. The recycling ofsuch waste to fabricate structural ceramics can be technologically,economically, and environmentally attractive because it producesmaterials with greater flexural strengths and provides for adequatetreatment of the WTP sludge. This article does not include the lifecycle assessment methodology analysis.

The process is innovative because it uses only waste from pulp(WTP sludge, dregs, grits, lime mud) and from crushing andgrinding granite rock (granite fines) to produce structural bricks.Therefore this technological innovation for manufacturing newsproducts able to minimize the impacts of WTP residues can be seenas an environmental performance of industrial solid waste fromWTP.

In this context, clay extraction is reduced, thereby natural re-sources are preserved, spending on the rehabilitation of exploitedclay sites is reduced; and the amount of waste coming from thepulp industry and released in industrial landfills is reduced.Therefore, the proposed process can promote the safe disposition ofWTP sludge.

2. Materials and methods

2.1. Waste characteristics

The sludge used in this study was generated by the WTP at apulp mill that performs kraft pulping, located at Belo Oriente in thestate of Minas Gerais (Brazil). The nominal installed capacity of thismill is 1,190,000 ADT1/year [100% elemental chlorine-free (ECF)bleaching agent]. Also, this mill employs a conventional watertreatment method, that involves coagulation with aluminum sul-fate, flocculation, sedimentation, filtration, and, finally, pHcorrection.

WTP sludge, three types of solid waste (dregs, grits, and limemud) generated by the same plant from the recovery of chemicalreagents, and granite fines from Te�ofilo Otoni in the state of MinasGerais (Brazil) were used as raw materials for the fabrication ofbuilding bricks classified in Brazil as structural ceramics or redceramics.

For the production of ceramic masses, the Winkler diagramwasused to define the grain size composition by weight percentage:25% clay, 40% silt, and 35% sand (Winkler, 1954). Noting that over-sized grains of granite and grits could damage the rolling mill, theparticle size of the materials was reduced to a maximum of 1.2 mm


E. Wolff et al. / Journal of Cleaner Production xxx (2014) 1e8 3

by sieving and grinding the oversized grains in a rod mill for 5 min.The final homogenization of all raw materials was performedmanually.

For mixture H (85% sludge þ 15% granite powder), themaximum size of the granite fines was set at 2.4 mm in order toinvestigate the influence of coarse grains on the plasticity andmechanical characteristics of the specimens.

In addition to controlling the particle size, further criteria suchas the proportions of calcium, sodium, and potassium oxides in thefinal mixture were established in order to obtain a material withhigher mechanical strength after firing.

The variables combination method was used to optimize themixture design and FORTRAN was used to implement the method(Wolff, 2008). The concentrations of fluxing agents and calciumoxide in each mixture were calculated, and those with the highestpercentage of these oxides were chosen for the preparation ofspecimens. Thus, eight compositions were prepared (A, B, C, D, E, F,G, H) and the dry powder components were manually homoge-nized in a plastic bag. WTP sludge without any added componentswas used as a control (I).

Table 1Chemical composition of raw materials (aggregate %).

Sample Al2O3 CaO Fe2O3 K2O MgO MnO Na2O SiO2 TiO2 IL

2.2. Physical and chemical analyses of the waste

To determine the size distribution of the particles constitutingthe sludge samples, a low-angle laser scattering method (lasergranulometer) was employed. The equipment used was manufac-tured by CILAS, model 1064, which can cover particle sizes from0.04 to 500 mm. Using ultrasound, the samples were dispersed witha solution containing 0.05% sodium hexametaphosphate. For otherwastes, the Brazilian standard NBR 7181 (ABNT, 1984a) was used.

The plasticity index (PI) and Atterberg limits were evaluatedaccording to the Brazilian standards NBR 6459 (ABNT, 1984b) andNBR 7180 (ABNT, 1984c).

The concentrations of the different constituents of oxides,sludge, dregs, grits, lime mud, and granite fines were determinedvia quantitative analysis performed using a Philips spectrometer(model PW 1480) equipped with a 3 kW Rh X-ray tube. For thisanalysis, the samples were prepared by fusion with lithium tetra-borate. The concentrations of Fe2O3, SiO2, Al2O3, CaO, MnO, TiO2,K2O, Na2O, and MgO were analyzed.

The qualitative determination of the crystal structures in theresidues and mixtures was conducted using a Philips X-Raydiffractometer (XRD), model PW 3710, using monochromatic Cu Karadiation with the following attributes: l ¼ 1.54184 Ǻ,voltage¼ 40 kV, current¼ 20mA, scan step size of 0.06�, count rateper step of 1 s, and calibrated for 2q using a silicon standard. Sludgespecimens were dried in an incubator at 110 �C for 2 h, and theresulting residues were ground using an agate mortar.

The crystalline phases shown in the diffraction patterns wereidentified by comparison with the diffraction patterns of variousminerals obtained from the International Center for DiffractionData (ICDD) database.

Thermal analyses of the mixtures were conducted using athermogravimetric/derivative thermogravimetric/differential ther-mal (TG/DTG/DTA) Netzsch simultaneous analyzer, model STA 409.These analyses were conducted in atmospheric air at temperaturesranging from 30 to 1200 �C and a heating rate of 10 �C/min.

WTP sludge 30.1 0.2 12.3 0.9 0.4 0.3 0.2 37.5 1.0 17.1Dregsa 1.2 40.8 0.9 0.5 4.9 0.6 3.1 2.2 0.1 45.6Grits <0.1 55.4 0.1 0.04 0.5 <0.01 0.5 0.5 0.03 42.7Lime mud 0.2 55.8 0.2 0.03 0.6 0.01 0.4 1.0 0.1 41.1Granite finesa 12.7 1.2 2.1 3.7 0.3 0.1 2.8 76.2 0.2 0.6

IL ¼ ignition loss.a Dregs and granite fines contain significant quantities of other elements that

were not analyzed.

2.3. Preparation of specimens

The amount of water to be added was determined using theplasticity limit (PL) of each mixture (ABNT, 1984c). Solid/liquidmixtures were prepared using a kneader (PAVITEST), model 8071A,operating at 1680 rpm (28 s�1). After mixing, the mixtures were


allowed to stand for 30 days, wrapped in plastic, and stored in amoist chamber.

The specimens were extruded to obtain dimensions of70 mm � 25 mm � 15 mm. The process was performed using avacuum extruder (050 C) at a pressure of 91 kPa. After extrusion, 70specimens of each mixture were coded, numbered, and dried in airfor a minimum period of 72 h and then in an oven at 110 �C for 24 h.

Subsequently, the specimens were fired in a muffle furnace(Termolab MLR) at 850, 950, and 1050 �C with heating and coolingrates of 2.5 �C/min. In this process, the maximum temperature wasmaintained for 7 h.

The mechanical properties of the specimens were evaluatedafter drying (linear shrinkage and flexural strength) and firing(ignition loss, linear shrinkage, flexural strength, water absorption,apparent porosity, and density (ABNT, 2005a,b; ABNT, 1997)) as thecrystalline phases (anorthite, albite, gehlenite, and mullite) wouldhave formed in the final product. In this paper, the technologicalproperties of the fired specimens are given; the properties ofspecimens dried at 110 �C can be found in Wolff (2008).

2.4. Statistical tests

The differences between the ceramic specimens obtained usingeight different mixtures and control sample (I) were determinedaccording to the nonparametric tests of KruskaleWallis and themedian test, followed by a multiple comparison test of classes forall the mixtures. The test of multiple comparisons is a complementto KruskaleWallis and it points out the mixtures that differ. Thesignificance level in these tests was 5%.

All tests were conducted using the software Statistica 6.1®. Theresults are presented as box and whisker graphs in order to visu-alize the behavior and variability of the data in relation to thetemperatures tested.

3. Results and discussion

The chemical compositions of the raw materials are shown inTable 1. As we were mainly interested in the amount of alkali(which act as fluxes) and alkaline earth elements (calcium andmagnesium are components of mineral phases that confer goodmechanical properties to structural ceramic materials), other ele-ments that were not essential for this study were not analyzed.

Transformation of WTP sludge into adequate ceramic mass forbricks requires a few changes in its chemical composition andplastic properties. The calcium oxide content of the WTP sludgewas 0.2%, which is the lowest value among all the residuesconsidered in this study. One of the requirements for a good finishof a structural ceramic material is the presence of mineral phasesthat are typical of ternary systems such as CaOeSiO2eAl2O3(Newman, 1979). Examples of these phases are anorthite (CaAl2-Si2O8) and gehlenite (Ca2Al2Si2O7); these require about 10% and20%, respectively, of CaO (Brownell, 1976). The plasticity of the


Table 2Mixture composition (wt. % wastes).

Mixture Composition

Sludge Dregs Grits Lime mud Granite fines

A 55 15 e 30 e

B 50 e e 30 20C 50 8 e 30 12D 70 30 e e e

E 50 e 10 20 20F 65 20 e e 15G 75 e 10 e 15H 85 e e e 15I 100 e e e e

Fig. 1. TG/DTG/DTA curves of mixture B.


specimens was controlled according to variations in their gran-ulometric composition.

Some material parameters of the mixtures, Table 2, that areimportant for the production of structural ceramics are listed inTable 3.

The PL values of the mixtures containing granite powder,namely, B, C, E, F, and H, were almost equal to those of the mixturesthat did not contain granite powder, namely, A, D, and G. The PL of amixture is used to determine the amount of water to be added to it.However, in practice, the amount of water used is always lowerthan the calculated value. Somemixtures had zero PI; however, thisdid not prevent their extrusion.

The IL of all mixtures shows that the addition of carbonatewaste(dregs, grits, and lime mud) increased this loss regarding the con-trol sample (I); it was higher for mixtures with carbonate residuesabove 30%. The addition of 10% carbonate material and granitepowder slightly reduced the IL.

The alkali flux content (Na2O, K2O) of all mixtures shows thatgranite powder and carbonatewaste increases this content. The highcontent of sodium oxide (Na2O) in the dregs results from sodiumcarbonate (Na2CO3) and sodium sulfide (Na2S), which are present ingreen liquor. This is not shown by the diffraction patterns of thismaterial because it was not characterized as a crystalline phase.

Since thermal analyses did not show any significant differencesbetween the mixtures owing to the use of the same residues fordifferent formulations, Fig. 1 shows the thermogram (TG/DTG/DTA)of just mixture B and Fig. 2 shows the diffraction patterns of itscrystalline phases.

In the overall analysis of the results formixture B shown in Fig.1,removal of free water and physically adsorbed water at 80.0 �C wasobserved, which resulted in a mass loss of 2.9%. Further, disap-pearance of gibbsite due to its dehydroxylation was observed at284.1 �C; moreover, dehydroxylation and loss of kaolinite crystal-linity due to transformation in the metakaolinite amorphous phaseat 506.9 �C, which disappeared from the diffraction pattern at850 �C, was also observed (Fig. 2). The decomposition of calcite

Table 3Material parameters of the mixtures and sludge (wt. %).

Mixture Material parameters

Fraction <2 mm SiO2/Al2O3 Na2O þ K2O

A 14 1.0 1.3B 5 1.3 1.4C 12 1.3 1.6D 11 1.0 2.0E 23 1.5 1.7F 21 1.2 2.3G 21 1.1 1.2H 29 1.0 0.8I 36 1.0 <0.7

PL ¼ plasticity limit; LL ¼ liquid limit; PI ¼ plasticity index.


resulted in a maximum peak of mass loss at 813.8 �C, but the calcitecompletely disappeared at 950 �C, which is associated with a massloss of 11.8%. Muscovite dehydroxylation and dissolution of finequartz particles were observed at 850 �C, accompanied by theformation of anorthoclase. Further, hematite was also observed bythat time, remaining in the mass until 1050 �C. At 950 �C gehleniteand anorthite were formed as a result of the reaction between CaO,SiO2, and Al2O3 in the mixture. The quantities of quartz andmuscovite in the sample decreased further and the muscovitedisappeared at 1050 �C. Formation of a high amount of gehleniteand a decrease in the amount of anorthite were observed at1050 �C. Mixture B showed a total mass loss of 24% during heating.The results for all mixtures can be found in Wolff (2008).

The statistical test results for the fired specimens are shown inFigs. 3e5. Specimens of mixtures E and G completely crumbled at850 �C and partially crumbled at 950 and 1050 �C; thus,we analyzedthese mixtures only with regard to IL at 850 �C. For the other twotemperatures, the mixtures were analyzed for all parameters eval-uated. However, the E and G specimens were also used in the waterabsorption test. This fact is explainedby the formation of portlandite[Ca(OH)2] from the reaction between the CaO present in the massand water from atmospheric moisture. The portlandite wasresponsible for the expansion and crumbling of the specimens.

When the ceramic specimens intended for absorption, porosity,and bulk density tests were passed through a water shower justafter taking them out of the furnace no crumbling was observedowing to portlandite dissolution in water. The flexural strengthshowed by these specimens at 950 and 1050 �C was in compliancewith the reference values for the production of roof tiles owing tothe formation of albite, anorthite, anorthoclase, diopside, gehlenite,andmullite phases in the specimens from 950 �C, as shown by XRD.

CaO Fe2O3 LL PL PI IL

31.8 5.8 47 47 0 33.518.7 8.3 44 36 8 25.721.5 7.6 43 43 0 27.612.5 10.3 62 60 2 27.317.3 7.9 43 34 9 23.810.3 9.9 59 53 6 24.14.8 12.4 52 50 2 20.30.2 14.1 59 45 14 19.70.2 14.6 73 61 12 20.4


0 10 20 30 40 50 60 70 80 90

CCCC CC

KKCCCC

K

KCM

KA

A

CQ

KMK

M

KI

QC A

M

110ºC

1050ºC

950ºC

850ºCC

C

CCCCC

CCH

C

MQ

M

MQ C

AT

H

G

GG

GGGGQ

GGGG

QG

QGGGG

G

G

N

Q

GG

NGGN GQ

H

GGGGGGQ

GGGGGGGGQG

G

GNQGGGGN G

Q N H

2 Θ (degree)

Inte

nsity

(pul

ses/

seco

nd)

Fig. 2. Diffraction pattern of mixture B. Legend: C, calcite; K, kaolinite; I, gibbsite; M,muscovite; Q, quartz; H, hematite; A, albite; N, anorthite; G, gehlenite; AT,anorthoclase.


Control sample (I), containing only WTP sludge, showedcracking and bending at 950 and 1050 �C, only was analyzed forignition loss (IL), water absorption (WA), porosity (P), and bulkdensity (BD).

Results for mixtures fired at 850 �C are shown in Fig. 3.The box and whisker graphics show that all mixtures were in

compliance with reference values for linear shrinkage (LS � 6%);however, mixtures A and D showed higher than desirable IL values(IL � 10%). Overall, the different behaviors of mixtures wereconfirmed by statistical tests.

With regard to flexural strength (FS), it was found that mixturesA, B, and C can be used for the production of solid bricks(FS � 20 kgf/cm2) and mixtures D and F can be used for the pro-duction of roof tiles (FS � 30 kgf/cm2). Statistical tests confirmedsignificant differences between the control sludge (I) and mixturesA, C, D, and F.

Ignition loss

A B C D E F G H IMixture

5

6

7

8

9

10

11

12

13

Ignition loss (%)

Linear shrin

A B C DMix

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

Linear shrinkage (%)

Water absorption

A B C D F H IMixture

3638404244464850525456

Water absorption (%

)

Porosit

A B C DMix

48

50

52

54

56

58

60

62

Porosity (%)

75%25%

MaMi

Fig. 3. Comparative evaluation of mixtures fired at 850 �C for evaluating


In the water absorption, porosity, and bulk density tests, none ofthe mixtures showed compliance with reference values. Statisticaltests indicated that mixtures F and H showed the lowest waterabsorption, mixtures D, F, and H showed the lowest porosity, andmixtures A, D, and sludge (I) showed the lowest bulk density.

Thus, despite being lightweight and porous, thus promotingwater absorption, mixtures B, C, and F showed compliance withreference values with regard to IL, LS, and FS when processed at850 �C.

In Fig. 4, the results for mixtures fired at 950 �C are shown.Similar to the results found at 850 �C, the 950 �C specimens wereporous, lightweight, and showed a high water absorption rate. Interms of IL, only mixtures G and H showed compliance withreference values (IL� 10%), which was confirmed by statistical teststhat showed significant differences between mixture H and mix-tures A, B, C, D, and E. However, significant differences betweensludge (I) and mixtures D, F, G, and H were not observed.

In terms of linear shrinkage, all mixtures were in compliancewith the reference value (LS � 6%). In general, statistical testsshowed that different mixtures showed different behaviors.Although mixtures A-G did not comply with the reference valuesfor ignition loss, water absorption, porosity, and bulk density, theydid show good flexural strength. Thus, in terms of linear shrinkageandwith the exception of mixtureH, all mixtures can be used in theproduction of solid bricks (FS � 20 kgf/cm2). Further, mixtures C-Gcan be used in the production of roof tiles because they showedhigher flexural strength (FS � 30 kgf/cm2). Statistical testsconfirmed significant differences between mixture H and mixturesA, D, E, F, and G.

Large variations were observed in the flexural strength test formixture A; this observation, along with its non-compliance withreference values in the remaining tests (with the exception of thelinear shrinkage test), confirmed that mixture A at 950 �C was notthe most adequate mixture for preparing red ceramic masses.

Mixture G, when fired at 950 �C, showed compliance with the ILreference values, and after undergoing the water shower for theWA, P, and BD tests, it showed compliance with the LS and FSreference values. Mixtures B, C, D, and F, on the other hand, showed

kage

F H Iture

Flexural strength


-10

0

10

20

30

40

50

60

Flexural strength (kgf/cm)

massive bricks

rooftiles

hollow bricks

y

F H Iture

Density


1,101,121,141,161,181,201,221,241,261,281,301,321,341,361,38

Density (g/cm)

xn Median

compliance with reference values applicable to structural ceramics.


Ignition loss


6789

10111213141516

Ignition loss (%)

Linear shrinkage

A B C D E F G HMixture

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

6,5


Flexural strength


10

20

30

40

50

60

70

80


hollow bricks

roof tiles

massive bricks

Water absorption


283032343638404244464850

Water absorption (%

)

Porosity


44

46

48

50

52

54

56

58

60

62

Porosity (%)

Density


1,15

1,20

1,25

1,30

1,35

1,40

1,45

1,50

1,55

1,60

Density (g/cm)

75%25%

MaxMin Median

Fig. 4. Comparative evaluation of mixtures fired at 950 �C in terms of compliance with reference values applicable to structural ceramics.


compliance with LS and FS reference values, as was the case withmixture E after it was subjected to the water shower for the WA, P,and BD tests.

Results for mixtures fired at 1050 �C are shown in Fig. 5.From the results, it can be concluded that mixtures G, H, and

sludge (I) showed compliance with reference values for IL � 10%and mixtures A, B, C, E, and F showed compliance with the refer-ence values for LS � 6%. The difference in the behaviors of themixtures was confirmed by statistical tests. In the FS test, it wasobserved that only mixture B cannot be used for the production of

Ignition loss


6789

10111213141516

Ignition loss (%)

Linear shri

A B C DMixt

3

4

5

6

7

8

9


Water absorption


15

20

25

30

35

40

45

50

55

Water absorption (%

)

hollow bricks

roof tiles

Porosit

A B C D EMix

323436384042444648505254565860

Porosity (%)

75%25%

MM

Fig. 5. Comparative evaluation of mixtures fired at 1050 �C in terms of


solid bricks whereas only mixture C should be employed for thatpurpose (FS � 20 kgf/cm2). However, mixtures A, D, E, F, G, and Hcan be used for the production of roof tiles (FS� 30 kgf/cm2). Linearshrinkage and flexural strength tests were not conducted on con-trol sample (I) specimens because they showed cracking andbending. Statistical tests confirmed significant differences (a ¼ 5%)between mixtures B and C and mixtures D, F, G, and H.

Mixture H showed the lowest absorption and porosity, followedby control sample (I), which showed increased bulk density. Ac-cording to the reference values for WA < 20%, this mixture may be

nkage

E F G Hure

Flexural strength


0

10

20

30

40

50

60

70

80

90


hollow bricks

roof tiles

massive bricks

y

F G H Iture

Density


1,1

1,2

1,3

1,4

1,5

1,6

1,7

1,8

1,9

2,0

Density (g/cm)

axin Median

compliance with reference values applicable to structural ceramics.


Table 4Avoided cost* resulting from use of WTP sludge in the ceramic industry.

Item Estimated cost(US$ per ton)

Waste treatment 0Waste transportation cost to landfill 8Cost of disposal of industrial waste in landfill 250Energy savings in ceramic industry 2Quality improvement of ceramic material 4Cost of studying the economic and technical feasibility of

the clay mining operation35

Cost of clay extraction 3Clay transportation from the clay mining to the ceramic

industry10

Cost of recovering the degraded area at the end of the claymining

2

Depletion of natural resource 2Environmental licensing of mining area 2Total 318

*Source: Based on http://www.meioambiente.mg.gov.br/regularizacao-ambiental/custos-de-analise/; http://mineraldata.cetem.gov.br/mineraldata/app/ and http://www.cenibra.com.br/cenibra/MeioAmbiente/MeioAmbienteFabril/ResiduosSolidos.aspx.


used for the production of roof tiles or, with regard to bulk density(BD � 1.7 g/cm3), for the production of bricks (massive and hollowbricks) or roof tiles. Further, the porosity values of mixture H werefound to be very close to the reference values (17% � P � 35%).Significant differences between mixture H and mixtures A, B, C, D,E, and F were confirmed by statistical tests with regard to WA andBD.

It is apparent from these results that the specimens of mixturesG andH at 1050 �C showed compliance with the reference values ofIL and FS, whereas mixtures A, E, and F showed compliance withthe reference values of LS and FS.

Table 4 shows the economic environmental performanceresulting from the use of WTP sludge in the ceramic industry, i.e.,Table 4 presents an estimate of the cost reduction resulting fromthe use of WTP sludge for manufacturing structural bricks.

The cost of treating the waste, either in a landfill or by preparingit for reuse, has a value of zero in this table, since the waste willneed to be dewatered for recycling and also before final disposal.

Based on Table 4, we can infer that by introducing the sludge inthe production cycle of ceramic bricks, several economies related tothe replacement of a natural raw material are generated. Therecycling to ceramic industry implies cost reduction related todisposal of the industrial waste in landfill. The substitution ofnatural clay avoids also the necessity of economic and technicalfeasibility studies of clay mining operation, environmentallicensing, clay extraction, clay transportation frommine to ceramicindustry, eventual fees to pay for depletion of a natural resourceand recovering the degraded area at the end of the clay mining.

But the main advantage is the possibility to obtain a betterproduct by facilitating the formation of ceramic phases, mainlyanorthite and gehlenite, a form of optimization which saves energyin ceramic processing. These results can be seen as a bridge con-necting a new cleaner production process and sustainabilitydimension.

Therefore, an estimated saving of 318.00 US$/t of clay is gener-ated by introducing the sludge in the brick production cycle.

4. Conclusion

This research was aimed at studying the use of waste materials(WTP sludge, dregs, grits, lime mud) from the pulp industry andfrom crushing and grinding granite rock (granite fines) for theproduction of structural bricks.


The paper discusses the environmental performance of indus-trial solid waste from WTP as alternatives considering the socio-environmental aspects, the technological innovation and the eco-nomic benefits related to the use of WTP sludge for manufacturingstructural bricks.

The obtained optimum formulation was based on the particlesize distribution of the waste materials and the proportion of theoxides of calcium, sodium, and potassium in the mixtures in orderto obtain a material with higher mechanical strength after firing.

The recycling of such waste into structural ceramics can betechnologically, economically, and environmentally attractivebecause it produces materials with higher mechanical resistance,enables the reuse of water treatment sludge, reduces explorationcosts for clay and thus natural resources are preserved, reduces thecost of treatment, provides for environmentally sound disposal ofwaste, and improves the company's imagewith stakeholder, amongothers, by providing for the sustainability of the pulp industry, witha strategic outlook toward future generations. In addition, new rawmaterials for ceramic products can be generated and products ofquality can be manufactured.

The statistical analysis and technological response of the pro-posed mixture suggest that sludge can be used as a substitute forclay in the formulation of clay masses. According to the conditionsused in this study, none of the mixtures can be used for productionof massive or hollow bricks. However, in compliancewith referencevalues for massive and hollow bricks regarding ignition loss,shrinkage upon drying, and bending rupture tension, and the for-mation of albite, gehlenite, and anorthite phases (which conferhigher mechanical resistance to structural ceramic materials) themixtures B, C, D, and F at 850 and 950 �C, should be tested in theceramic industry on a pilot scale in order to evaluate their suit-ability for the production of interior coatings or acoustic bricks.

The recovery of waste from one ormore industries located in thesame geographic region is one of the basic management principlesof industrial solid waste. Its purpose should be reuse and recycling,closing the raw material cycles with minimal expenditure of en-ergy, and other valuable resources.

The recovery of waste, therefore, constitutes integration be-tween the pulp mill and the red ceramics industry, which promotesCP practices between them. The utilization of WTP sludge inbrickmaking eliminates a socio-environmental problem, becausethe industrial residue ceases to exist avoiding danger to the envi-ronment and preserving lives of current and future humangenerations.

Furthermore, this integration increases the sustainable devel-opment of both industries and the region in which they operate bytransforming a water treatment process waste in a raw materialwith savings of energy and natural resources.

Finally, by using WTP sludge in the manufacturing of acousticbricks and other products, it is shown that cleaner productionpractices promote innovation in the pulp mill and the red ceramicindustry, leading to environmentally friendly practices.

Acknowledgements

The authors would like to thank CENIBRA (Celulose Nipo-Brasileira S/A) and UFMG (Universidade Federal de Minas Gerais)for their financial support.

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Utilization of water treatment plant sludge in structural ceramics