An Evaluation into the Potential of Biological Processing for the Removal of Metals from Sewage Sludges

Copyright © 1999, CRC Press LLC — Files may be downloaded for personal use only. Reproduction of this material

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275

Critical Reviews in Microbiology, 25(4):275–288 (1999)

An Evaluation into the Potential of BiologicalProcessing for the Removal of Metals fromSewage Sludges

Ana T. Lombardi and Oswaldo Garcia, Jr.

Dept. Bioquímica e Tecnologia Química, UNESP, Campus Araraquara, C.P. 355,Araraquara — SP CEP: 14801-970, Brazil

ABSTRACT: The use of sewage sludge in agricultural land as a means of sludge disposal andrecycling has been shown to be economical and suitable because of the presence of nutrients suchas nitrogen and phosphorus. However, municipal sludges often contain high quantities of toxicmetals and other compounds that must be removed for its safe use in agricultural soils. Thebiological leaching of metals from sewage sludges has been shown to be a promising techniquefor metal detoxifying in such complex matrix. The process efficiency is dependent on severalphysico-chemical parameters, such as total solids concentration, metal forms, pH-ORP, andtemperature. Scale-up of the process has not yet been defined and is still pursuing the correctoperational design. Current research involving the bioleaching of metals from sewage sludge andits application to land, which affects soil physical properties, are presented and discussed.

KEY WORDS: bioleaching, Thiobacillus, sewage sludge, metals.

TABLE OF CONTENTS

I. Introduction .................................................................................................................... 276

II. Mechanisms of Metal Removal from Sludges ............................................................. 277

A. Chemical Processes .................................................................................................. 277B. Biological Processes and Microorganisms ............................................................ 277

III. Factors Affecting the Bioleaching Process .................................................................. 279

A. Solid Content ............................................................................................................ 279B. pH–ORP .................................................................................................................... 280C. Temperature ............................................................................................................. 280D. Metal Forms ............................................................................................................. 280

IV. Disposal of Sludge to Land ........................................................................................... 282

A. Effects of Sludge on Soil Physical Properties ....................................................... 282B. Nutrients and Toxic Elements ................................................................................ 283

V. Process Scale-Up .............................................................................................................283

VI. Concluding Remarks ...................................................................................................... 284

References........................................................................................................................ 284

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I. INTRODUCTION

Wastewater treatment results in highamounts of sludges. The disposal of suchproduct, either of industrial or municipalorigin poses economical and ecological prob-lems. An alternative would be to recycle thematerial. However, to safely use sludges, itis necessary to have it without toxic ele-ments.

The principal methods of sludge disposalare landfill, incineration, agricultural landapplication, and disposal at sea. Employinglandfill, heavy metals may be leached out,contaminating soil, streams, and undergroundwater. Incineration is costly due to the highenergy consumption. Disposal at sea mayproduce toxic effects on aquatic life due tothe metal-laden sludge. Agricultural landapplication seems to be the most economicalmethod of sludge disposal due to the pres-ence of fertilizers such as nitrogen and phos-phorus (and in lesser amounts potassium) inmost municipal sludges.1–3 The organic mat-ter content of sludges may improve the struc-ture and water-holding capacity of poor soilsand the microbiological cycling of elementsby organic matter degradation. However, dueto its origin, municipal sludges contain highquantities of contaminants, such as toxicmetals, pathogenic organisms, and hazard-ous organic compounds.4–7 To reduce to aminimum the possibilities of undergroundwater contamination and food web accumu-lation,8,9 the undesirable elements must beremoved before the use of sewage sludge asagricultural fertilizer.

The mechanisms of sewage treatmentare responsible for the biosolids generatedthat contain high amounts of metals. Thereis a large variation of biosolids, as well asmetals, per unit volume of wastewater pro-duced by primary treatment (removal of sub-stances from wastewater by mechanicalscreening or sedimentation), secondary bio-logical treatment (oxidation by organic ma-

terial by microorganisms), and chemicaltreatment (addition of salts to remove sus-pended solids and phosphorus). These stepsinclude precipitation and flocculation of or-ganic matter.10,11 Altogether, these mecha-nisms are responsible for most of the metalpresent in sludges. Tijero et al.12 showed that90% of the metals in sewage are present inthe solid phase (sludge).

The costs of a sludge treatment and finaldisposition may reach the high amount of50% of the total spent in a treatment plant.13

This is an indication of the importance ofsearching for an economic alternative for thesludge produced.

Several countries already use sewagesludge in agricultural areas; however, litera-ture data show that more than 50% of thesludges are inadequate for such use due to itsmetal content.14 In fact, metal contaminationin soils amended with untreated sewagesludge have been reported by several au-thors.15–17

The ultimate cause for concern aboutheavy metals in the environment is theirextreme toxicity toward man, causing carci-nogenic, mutagenic, and teratogenic effects.18

Injuries caused by chronic exposure to cad-mium (Itai-Itai Disease) and mercury(Minimata Disease) are well documented.19,20

The toxic action of heavy metals is primarilya result of their affinity for complexing orchelating groups, such as thiol groups inbiological molecules.21

The preliminary mode of action by whichheavy metals exert toxicity in man and ani-mals is by reacting with sulfur-donor atomsof proteins, resulting most commonly inenzyme deactivation. In addition, by replac-ing other essential elements such as calciumand magnesium, heavy metals can destabi-lize the structure of biomolecules, resultingin genotoxic and mutagenic effects, produc-ing heritable genetic disorders and cancers.Moreover, the chemical nature of some met-als and metal species permits transplacental

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movement inducing to embryo toxicity andteratogenicity.

One of the fundamental factors thatheighten the concern over the presence ofpotentially toxic heavy metals in the envi-ronment is their nonbiodegradability andconsequent persistence, which is furthermagnified by the tendency to become con-centrated in food chains through bioaccu-mulation. Therefore, the removal of metalsfrom sewage sludge before its use in agricul-tural soils is of fundamental importance.

In this article we synthesize current re-search involving the most important aspectsof the bioleaching of metals from sewagesludge, a complex matrix.

II. MECHANISMS OF METALREMOVAL FROM SLUDGES

There are two basic ways for diminish-ing metals in sewage sludge: reduction in thesource or metal extraction. While the secondoption may be achieved by lowering the pHeither by acid addition or acid production viamicrobial action, reduction in the source isdifficult to control.

The removal of metals from sewagesludge may be accomplished by chemical orbiological methods. Once they have beensolubilized, the separation from the solidmatrix may be performed by conventionalmethods such as centrifugation or othermeans of solid-liquid separation. Metals maybe further precipitated from the leachate bypH adjustment to 10 with lime, for example.After metal recovery, the solution can bedischarged into receiving waters or submit-ted to further treatment.22

Biological methods comprise the use ofspecific microorganisms, which under aero-bic conditions mediate several chemical re-actions, reducing the local pH to 1.0. Suchreactions are a cause of concern becausethey may produce acid corrosion in equip-

ment, material and installations of sewagetreatment plants. However, if adequate con-ditions are given, these same reactions maysolubilize metals in sewage sludge.

A. Chemical Processes

Chemical processes that have been at-tempted in metal removal from sludges in-clude H2SO4, HCl, HNO3, and EDTA (ethyl-enediamine tetraacetic acid) extraction orwashing.23–25 Some authors26,27 have reportedthat metal solubilization may be improvedby heating with acid treatment. Using a com-bination of nitric acid/water treatment,Abrego28 obtained 86% solubilization effi-ciency for copper and 100% for arsenic andnickel when chemically removing heavymetals from sludges. The author also dis-cusses the competitive effect between met-als when these are mixed together, conclud-ing that the solubility of some metals isreduced in that circumstance. However, fac-tors such as high cost and operational diffi-culties have made a practical application ofchemical extraction techniques unattractive,yet good metal extraction rates have beenobtained under laboratory conditions.

B. Biological Processes andMicroorganisms

Of all aspects of microbial technology,probably the most underrated and least pub-licized is the application of microorganismsto the solubilization of metals in sludges.

The biological leaching of metals fromsludges occur via acidophilic micro-organisms. The most widely used microor-ganisms are the iron- and sulfur-oxidizingThiobacillus ferrooxidans and the sulfur-oxidizing Thiobacillus thiooxidans.4,22,29,30–32

Both species are capable of oxidizing elemen-tal sulfur and partially reduced sulfur com-

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pounds to sulfuric acid. Even the presence oflarge quantities of organic matter in sewagesludges does not affect or modify theirmetabolism.22 More recently, the less acido-philic thiobacilli, Thiobacillus thioparus,have been tested in this process and someuse have been made with success.33,34 As anatural process of succession in sludges,Thiobacillus thioparus brings the sludge tothe pH required for the two acidophilic mi-croorganisms (T. thiooxidans and T. ferro-oxidans) so preparing the substrate for theirgrowth.35,36

T. ferrooxidans, T. thiooxidans andT. thioparus are chemolithotrophs, for ex-ample, organisms that obtain their energyfrom the oxidation of inorganic substances.They belong to a diverse group of bacteriathat vary in their morphology, habitat, andenergy source. Differentiated on the basis ofwhat inorganic compounds they use as en-ergy, the chemolithotrophs include the sul-fur oxidizers (Thiobacillus sp., Thiomicro-spira sp., Beggiatoa sp., and others), and thenitrifying bacteria (Nitrosomas sp., Nitro-sococcus sp., Nitrospira sp., Nitrobacter sp.,and others).

The sulfur oxidizers are important mi-croorganisms in metal solubilization fromminerals, and the most studied are the mem-bers of the genus Thiobacillus. Nutrition-ally, most thiobacilli are strict autotrophs;however, some may be facultative autotro-phs. Thiobacilli occur widely in terrestrialand aquatic habitats wherever inorganic sul-fur and iron are present, such as in sewage,estuaries, and tidal muds acid mine drain-age. The production of sulfates from theoxidation of sulfur compounds creates highlyacid environments, thus some species haveevolved to tolerate low pH conditions.

The presence of sulfur-oxidizing micro-organisms in 23 different sewage sludgesand from different municipal waste watertreatment plants has been reported by Tyagiet al.22 This renders the “bioleaching” a suit-

able process for metal solubilization in slud-ges. However, to have an effective process,eventually some substrate should be fur-nished to the bacteria, so it can execute thework. Among the substrates, elemental sul-fur (S0 ) has been shown to be the mostefficient.37,38

The biochemical activity for a specificsubstrate and environmental conditions isthe result of a series of complex chemicalreactions, that pass through intermediate stepsand side reactions up to the end product. Thefollowing reactions show how these micro-organisms may solubilize metals.1,35

MS O M SO+ → ++2 22

42T. ferrooxidans – (1)

Where MS = insoluble metal sulfide andM2+ = soluble-free metal ion and MSO4 =soluble metal sulfate.

Without direct bacterial metabolism, themetal sulfide may be oxidized by ferric ionas shown in the following reaction (2), apurely chemical proceeding reaction:

4 2 4 22 4 3 2 2Fe SO MS H O O( ) + + +

→ + +2 4 84 2 4 4MSO H SO FeSO (2)

Then the newly formed ferrous ions arereoxidized by T. ferrooxidans as shown inreaction 3

2 0 5 24 2 2 4

2 4 3 2

FeSO O H SO

Fe SO H O

+ +

→ ( ) +

.

T. ferrooxidans

(3)

A cyclic process is generated involvingreactions 2 and 3, so more and more metalsolubilization occurs, increasing the overallmetal solubilization efficiency. Moreover,in the presence of H SO2 4 and O2 , thefollowing chemical reaction, without the in-volvement of microorganisms, occurs:

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MS O H SO MSO S H O+ + → + +0 5 2 2 4 40

2.

(4)

The free S0 as a result of the abovereaction is oxidized to H SO2 4 by T. ferro-oxidans and/or T. thiooxidans according tothe following reaction:

2 3 2

2

02 2

2 4

S O H O

H SO

+ +

→T. thiooxidans & T. ferrooxidans

(5)

It has been shown that the rates of mi-crobial leaching of metal sulfides are di-rectly related to their respective solubilityproducts, with the following order of metalleaching: NiS > CoS > ZnS > CdS > CuS >Cu2S.39 Nevertheless, these results were ob-tained from mineral bioleaching, not sewagesludge.

The microbial activity may result in theproduction and accumulation of undesirableintermediate compounds such as poly-thionates, which could cause soil acidifica-tion. However, it has been shown that themicrobial oxidation of sulfur and thiosulfatecan occur without the production of interme-diate compounds (S3O6

2–, S3O62–).22,37,38 So,

reduced risk of acidification of agriculturalsoil, as well as of the leachate disposed off inthe receiving waters, is achieved.22

III. FACTORS AFFECTING THEBIOLEACHING PROCESS

Several environmental factors, as wellas the sludge type (primary, activated, di-gested), affect the bioleaching of metals fromthe sludge, presenting different metal solu-bilization efficiencies. Factors such as or-ganic content, sulfur concentration, oxida-tion-reduction potential (ORP), temperature,the metal ‘per se’, metal forms, pH, and

solid content are key components in the op-timization of the bioleaching process.

A. Solid Content

The solid content of sewage sludges af-fect the bioleaching of metals in several ways.Increased concentration of solids increasesthe buffering capacity, nutrient concentra-tion and availability as well as the organicmatter content of the sludge. Henry et al.32

report that total solid concentration also af-fects the survival of pathogenic microorgan-isms present in the sludge. Higher the solidconcentration, higher the microorganismsurvival rate.

Several studies35,36,40 show that metalsolubilization rates may vary according tothe metal per se, for example, the effect ofsolid content on copper solubilization is morepronounced than on zinc.35

An increase in the buffering capacity ofthe sludge will reduce metal solubilization,as this is better accomplished at low pHvalues.41 However, Tyagi et al.42 showed thatthe oxidation of sulfur (or acid production)by sulfur-oxidizing thiobacilli is not inhib-ited by increased solid concentration in anaerobically digested sludge. Nutrient con-centration (micro-nutrients and trace met-als), essential for bacterial growth are sup-plied by the sludge itself and increases withincreasing solid content of the sludge.

Investigating the effects of sludgesolids concentration on sulfate production,Sreekrishnan et al.41 obtained a higher sul-fate production rate (or acid production) athigher solid concentration (up to 70 g/L–1).The authors explain such effect as the resultof increased nutrient concentration.

The organic matter content of sewagesludges will affect metal solubilization effi-ciency via three different pathways: metalspecies, pH control, and nutrient availabil-ity. Besides other categories of organic mat-

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ter, the presence of humic and fulvic mate-rials in sludge solids function as both metalcomplexants and buffer for pH control. Someauthors43,44 showed that toxic metals associ-ated to organic matter are less available forsolubilization than those associated to theinorganic fraction.

Recently, Tyagi et al.36 showed a de-crease in metal solubilization efficiency withincreased sludge solids, the effect being metaldependent: Cd, Cr, and Pb were the lesssolubilized. Sreekrishnan et al.41 showed thatmore of a given metal was solubilized forthe same amount of acid production in slud-ges with lower solids concentration, sug-gesting that metal solubilization dependsmore on the resulting pH of the medium thanon acid production.

A consideration we can make is thatsludge solid content affect the bioleachingprocess in sewage sludges mainly by actingon the pH.

B. pH-ORP

Values of pH on the bioleaching of met-als in sewage sludges may affect both bacte-ria growth and metal solubilization. Theleaching bacteria have optimum environmen-tal pH requirements, hence maximum growthrates may be achieved. To optimize bacterialgrowth, sludge pH must obey bacterial re-quirements. This means that in most cases, alowering of initial pH of the sludge is neces-sary.45

Although metal solubility has beenshown to be primarily governed by pH,46,47

as happens with solids, it is dependent on theeluted metal, for example, copper is affecteddifferently than zinc.35 However, for metalsolubilization in sludges, pH reduction aloneis not sufficient.46,47 A rise in oxidation-re-duction potential (ORP) has been shown tobe also necessary.35

We may conclude that a metal solubili-zation strategy in sludges should decrease

the pH and increase the ORP, so insolublemetal sulfides will tend toward metallic ionformation.

C. Temperature

Temperature is an important parameterfor the growth of microorganisms, and assuch it affects metal solubilization rate re-sulting from a biological process. As such,the growth of leaching bacteria representsthe rate-limiting step for metal solubiliza-tion in sludge. Blais et al.48 showed a reduc-tion of incubation time (from 14 to 5 days)required to acidify sludges from pH 7.0 topH 2.0 with an increase in temperature from7°C to 28°C (or 35°C). At a temperature of42°C no microorganism growth occurred,thus no pH reduction and ORP rise tookplace; consequently, no metal solubilizationwas detected. Similar results were obtainedby Tyagi et al.22,34

D. Metal Forms

For a safe disposal of sewage sludge inagricultural soils as well as a prediction ofthe fate of each metal during the bioleachingprocess, considerations of the influence oftotal metal concentration and sludge charac-teristics on metal speciation is necessary.33

Total concentration of metals in sludgesindicate the extent of contamination, but givelittle insight into the forms in which themetals are present or their potential for mo-bility and bioavailability after dispersal inthe environment. The toxicity of a metal isdependent on the form or chemical speciesto which an organism is exposed, on its con-centration, and on the organism being af-fected.49,50 Complexation generally lowersthe biological availability of a given metal,because free metal ions are considered to bethe most biologically available forms.49 Cer-tain elements, such as arsenic, may be ben-

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eficial at low doses but become hazardouswhen in excess of physiological require-ments.51 Moreover, when present in excess,some of the essential elements such as sele-nium and vanadium can exert greater toxic-ity than nonessential elements such as mer-cury and thallium.52

Free metal ions are rare in sludges.53

Normally, they exist in the form of sulfides,oxides, hydroxides, silicates, insoluble salts,or linked with sludge organic matter.54,55 Thedistribution of metals between the specificforms varies according to the chemicalproperties of the individual metal and thecharacteristics of the sludge, which includeparameters such as pH, temperature, oxida-tion-reduction potential, and the presence ofcomplexing agents and minerals.56

The forms of heavy metals present insludges vary widely according to the natureof the individual metal, the characteristics ofthe wastewater treated, and the sludge treat-ment employed.53,57 Under reducing condi-tions, which prevail in anaerobic digestion,highly insoluble metal sulfides are formed.58

According to these authors, 70% of the met-als in sludge exist as insoluble sulfides, and30% is adsorbed into cellular material. Otherforms may also exist: carbonates, phosphates,and organically bound.33

Organically complexed metals incorpo-rate those forms that are bound to insolubleorganic matter, in addition to components ofliving cells, their exudates, and a spectrumof degradation products.59,60 Metal complexedto organic fractions is less available for solu-bilization than those associated with the in-organic fraction.

Adsorption, defined as the adhesion ofdissolved elements to the surface of solidswith which they are in contact, may occurthrough physical, chemical, or ion-exchangeprocesses.61,62

Precipitated metal forms are defined asinsoluble substances formed in solution asthe result of a chemical reaction and includemetal hydroxides, carbonates, phosphates,

and sulfides. Residual metals may occur asions inertly bound in crystal lattices of highlystable minerals.56

An empirical approach to the speciationof toxic elements in soils and sludges has notyet been developed, and existing speciationschemes have several limitations. Most meth-ods use extraction schemes as attempts tocorrelate the extractability of trace elementswith plant uptake. However, it is importantto consider the long-term availability andchanges in the relative importance of soilfactors as a result of successive transforma-tions.

Sequential extraction techniques is nor-mally used. It is derived from schemesoriginally applied to sediment matrices.63–65

Although varying in manipulative complex-ity, such techniques generally fractionatemetals in sludges (or soils) into the follow-ing sequence of operationally definedphases:

1. Soluble exchangeable phase: extractionwith NH4OAc, BaCl2, MgCl2 or KNO3.

2. Adsorbed phase: extraction with KF orion exchange.

3. Organically bound: NaOH or Na4P2O7.

4. Carbonate: EDTA or NaOAc.5. Sulfide: HNO3 6 M.

6. Oxide bound: NH2OH.HCL or HOAc.7. Residual: HNO3, HF-HCLO4 (acid di-

gestion).

Problems associated with the use of se-quential extraction techniques in wastewatermatrices and soils include the diversity ofreagents used to extract specific metal forms,as well as the variety of extraction proce-dures employed. This renders difficult re-sults inter-comparison and variations in theinterpretation of the chemical forms of tracemetals extracted by particular reagents.

Attempts to relate plant uptake param-eters to soil factors or speciation provideuseful information regarding the availabilityof toxic elements to plants.

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As sludges are applied to soils the formsof trace elements as well as toxic metalspresent will change as the sludge becomesmineralized and incorporated as part of thesoil. In addition, the application of othermaterials in sludges or soils, that is, fertiliz-ers and lime, may influence the changes inthe forms of trace elements. Literature datahave shown that although only a variableproportion of the metal present in sludgemay be readly leached by water, the prob-ability is that all of the metal present willeventually be solubilized and become avail-able for plant.66 For this reason, besides aspeciation scheme to access the various metalforms, we suggest the determination of totalmetal with complete acid digestion beforeapplying sludges to land.

IV. DISPOSAL OF SLUDGE TOLAND

The majority of sludges used in agricul-ture are applied to general arable and graz-ing land, with very little being used in for-estry or horticulture. Although additionaltreatment such as digestion, conditioning,and dewatering may increase total opera-tional costs of sludge disposal to land, it isstill the most economically satisfactorymethod of sludge disposal. Besides the ben-eficial effects on soil physical properties,municipal sludges are a potential source ofnitrogen and phosphorus.67

The effects of sludge application to soilsis greatly dependent on the application rate.The recommended sludge application ratesin several countries oscillates from 1 to 4 tonha–1 yr–1, depending on whether sludge isapplied to pasture or arable land, or elseplant nitrogen demand is considered. Thepresence of toxic elements in sludges maylimit the maximum annual rate of sludgeapplication because of metal loading allow-ances. The maximum annual loading for

cadmium is 15 g ha–1 in Sweden and Den-mark, 20 g ha–1 in Holland and Finland, and30 g ha–1 in Norway. In the U.K. and theU.S., annual rates of application are deter-mined from considerations of crop nitrogenrequirements and limited by the cation-ex-change capacity of the soil.68

Excessive rates of application of sewagesludges to soils can result in the develop-ment of anaerobic conditions in the soil, lead-ing to increased mineralization of the or-ganic nitrogen present in the sludge organicmatter. For this reason, it has been suggestedthat both the inorganic and organic nitrogencontent of sludge should be considered inapplying sludge to land.

A. Effects of Sludge on SoilPhysical Properties

Soil physical properties may be summa-rized as a balance between texture, structure,porosity, clay, and organic matter content.Clay and organic matter are important be-cause they affect the water holding capacityand bind the soil into structural aggregates,thus holding nutrients. A good soil structureis considered to be about 50% solid, 30%water, and 20% air at field capacity.69 Ex-periments involving sludge application tosoils have shown a 75% increase in organicmatter content after 9 years of application ata rate of 75 and 150 t ha–1 every 2 years. Theexperiment was conducted during 18 years;however, no further increase in organic mat-ter content was observed, indicating that therewas an upper limit for the increase in soilorganic matter. Due to the content of organicmatter in sludges, its application to soils willaffect the cation-exchange capacity, whichis associated to the ability of a soil to retainnutrients such as calcium, magnesium, andpotassium. Beneficial effects on the cation-exchange capacity of a silt loam soil hasbeen reported by Epstein.70

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B. Nutrients and Toxic Elements

The main nutrient value of sludges liesin its nitrogen and phosphorus contents;however, the concentration of nutrients in asludge produced at a particular sewage treat-ment plant varies according to the opera-tional conditions the sewage has been sub-mitted. Generally, nitrogen concentrationvary from 1.6 to 10.7%. Phosphorus is presentat about 4.5%, whereas potassium is presentin very low amounts. If applied to soils asfertilizer, potassium would have to be addedto the sludge or soils directly.

The background concentrations of toxictrace elements in soils are much lower thanin sewage sludges. Unless the toxic elementsare removed, any addition of sludge to landwould almost certainly result in increasedconcentrations of several toxic elements inthe soil. The concentrations of toxic ele-ments in sludges varies according to the typeof sewage, that is, industrial or domestic,entering the treatment plant.71 Even if theaverage concentration of certain elements insludges are similar to their concentration insoils, repeated applications of sludge to landcould increase their concentrations in soil assludge organic matter is degraded and min-eralization occurs.

The usefulness of sewage sludge as afertilizer is affected by the rate at which itdecomposes when added to soil. Most of thedecomposition has been found to occur within1 month of sludge addition.72 About 20% ofthe organic matter in digested sewage sludgeis more readly decomposed than the remain-ing 80%.

The organic matter remaining in the soilmay immobilize toxic elements,73,74 althoughno conclusive evidence as to the chemicalnature of the organic complexing agentspresent in soils and sludges is present inliterature. It has been demonstrated thatsoluble fulvic acid-metal complexes may beformed at alkaline pH;75 nevertheless, most

of the work on soil organic matter-metalcomplexes has been done acid pH values.76

At near neutral pH, De Oliveira et al.77 andLombardi and Jardim73 showed copper-fulvicacid complexation and Lombardi andJardim74 copper-humic acid complexation.

Numerous studies have shown that thesolubility and availability of trace elementsas well as toxic metals in soils and sludgesare affected by pH.78 Although sludge addi-tions to soil may initially increase soil pH,79

it is further reduced due to nitrification andformation of organic acids.80 Thus, cautionshould be taken as metal availability for plantuptake is greater at low than at high pHvalues.

V. PROCESS SCALE-UP

Several studies have shown (referencesherein) that bioleaching of metals can beoptimized in order to reduce metal content insludges. Based in laboratory experimenta-tion, these studies employ controlled condi-tions and low total solids concentration (5 to50 g.l–1). However, an estimate of thebiosolids (sludge) production levels in theU. S. reported by Bastian81 amounts approxi-mately to 6.856.168 dt/yr. Of the total amountproduced, 54% is used as organic fertilizeror soil amendment with land applicationpractices. This means that a viable bio-leaching operation system would need to beefficient with high total solids concentration(70 to 100 g/l) in order to at least partiallysatisfy, the demand. However, besides metalcontamination and toxic organic compounds,the presence of pathogens is another prob-lem sludge-recycling technologies must lookat. At high solid content, pathogen elimina-tion is hard to accomplish.32 A scale-up of abioleaching process would need to be pro-portional to the sludge producer facility,however, accounting for a 15-day residencetime necessary for metal solubilization to

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occur. Besides this, before land application,a means of sludge dewatering other thendrying is necessary in order to remove inter-sticial metal-containing water. After thesesteps, an evaluation of the major pathwaysof human exposure (sludge —› soil —› plant—› human) associated with land applicationpractices must be established for a safe landapplication of sewage sludge.

The biosolids disposal fees have in-creased dramatically in the U.S., with somerecent short-term private contracts to imple-ment land-based alternatives reaching thevalue of US $800/dry ton, as reported byBastian.81 This means that about 6 billions/yr(or more) would be necessary for alterna-tives to the sludge produced, while the pub-lic spending for wastewater treatment in theU.S. increased from 4.3 billions in 1960 toapproximately 12 billions in 1984, as re-ported by Bastian.81 With these values, wewould like to reinforce that no one treatmentfor sludge or other waste is likely to becostless. Bacterial leaching may be the leastexpensive among other methods known tosolubilize metals from sewage sludge, butthe best, however, seems to be the reductionof metals in the source.

VI. CONCLUDING REMARKS

The benefits of applying sewage sludgeto agricultural land as a fertilizer are some-times accompanied by risks due to the con-centrations of toxic elements that they maycontain. Phytotoxicity, which is related tothe forms of the toxic elements, may occuras a result of their accumulation in soils.

Toxic organic compounds, metals, andpathogenic microorganisms are constituentsof municipal sludge, introducing seriousproblems to its use in agricultural land. Thebioleaching process aims especially at theremoval of metals; however, it has also ob-tained a reduction of pathogenic organisms.

This is the result of highly drastic conditionsintroduced by thiobacilli during the bio-leaching process operation, at least underlaboratory conditions. Thus, ideally thesludge comming out from a bioleachingoperating system would present reducedcontents of both toxic metals and pathogenicorganisms. If it gets in the bioleaching oper-ating system already free (or with reducedcontent) of toxic organics, the three mostserious problems related to its use in agricul-tural land would be solved. From what hasbeen discussed in the present text, we mayconclude the following four points.

• Under controlled conditions, the bio-leaching of metals is a viable methodol-ogy for metal removal from sewage slud-ges, and it would reduce metal load insoils treated with sewage sludge.

• The fertilizer value of sewage sludge (pres-ence of nitrogen and phosphorus) makesthis material of considerable value to ag-riculture. However, the proportion andrates of sludge application to land must bepreviously defined so to reduce contami-nation risks.

• Although several investigations on thebioleaching of metals from sewage sludgepoint to its suitability as a means of metalremoval, more results are necessary inrelation to the production of intermediatecompounds.

• Knowledge of metal forms of those met-als that remain in the sludge after thebioleaching process. This is necessary forsafe use of the sludge, as toxicity is betterrelated to the metal form, and not to itstotal concentration.

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Documents

An Evaluation into the Potential of Biological Processing for the Removal of Metals from Sewage Sludges