Treatment of rinsing water from electroless nickel plating with a biologically active moving-bed sand filter

Ž .Hydrometallurgy 59 2001 383–393www.elsevier.nlrlocaterhydromet

Treatment of rinsing water from electroless nickel plating with abiologically active moving-bed sand filter

T. Pumpel a,), C. Ebner a, B. Pernfuß a, F. Schinner a, L. Diels b, Z. Keszthelyi c,¨A. Stankovic c, J.A. Finlay d, L.E. Macaskie d, M. Tsezos e, H. Wouters f

a Institut fur Mikrobiologie, UniÕersitat Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria¨ ¨b ( )Vlaamse Instelling Õoor Technologisch Onderzoek VITO , Boeretang 200, B-2400 Mol, Belgium

c Collini-Skolnik Oberflachentechnik AG, Lohnergasse 4, A-1210 Vienna, Austria¨d School of Biological Sciences, UniÕersity of Birmingham, Birmingham B15 2TT, UK

e National Technical UniÕersity of Athens, Heroon Polytechniou 9, GR-15780 Zografou, Greecef Astraco Water Engineering BV, T. de Boerstraat 24, NL-8561 EL Balk, Netherlands

Received 28 September 1999; accepted 24 March 2000

Abstract

Ž .The MERESAFIN MEtal REmoval by SAnd Filter INoculation process presented here was designed to combine theoptimum conditions for more than one of the well-known processes of biological metal immobilisation like biosorption andbioprecipitation in a treatment system for industrial waste water. The approach makes use of a continuously operatedmoving-bed Astrasandw filter which has been inoculated with a mixed population of selected metal biosorbing, bioprecipi-

Ž .tating and biodegrading bacteria. One of four pilot plants has been erected at a metal plating company in Vienna A to treatwaste water from an electroless nickel plating line. In addition to several mgrL of nickel the rinsing water also containsorganic acids and inorganic phosphates, which make conventional treatment difficult.

Metal laden biomass is continuously removed from the sand grains in the filter and settled in a lamella separator. TheŽ .thickened biosludge contained 2% of Ni at only 2–5 mgrL in the feed water , which could be recycled in a shaft furnace.

Regeneration of biofilms on the sand is achieved by dosing a cheap carbon source; all other nutrients are available from thatspecific waste water. For the removal of 0.8 mgrL of nickel the biofilms consumed 8 mg carbonrL and, in addition to 8mgrL of dissolved oxygen, 3.4 mg NO –NrL as additional electron acceptor.3

The process was shown to be economically favourable over comparable conventional techniques of metal removal. AŽfurther advantage of the biological system is its ability to cleave organo-metal complexes e.g. nickel lactate in the presented

.case , to degrade organic molecules like organic acids, surface active substances, etc., often present in industrial wastewaters, or to reduce ammonium, nitrite and nitrate. Proposed areas of application comprise the final polishing of industrialand mining water, but also the full treatment of contaminated ground water or drainage water. q 2001 Elsevier Science B.V.All rights reserved.

Keywords: Sand filter; Nickel recovery; Biosorption; Bioprecipitation; Bacteria; Galvanic waste water; Bioremediation

) Corresponding author. Tel.: q43-512-507-6007; fax: q43-512-507-2929.Ž .E-mail address: [email protected] T. Pumpel .¨

0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0304-386X 00 00168-7

( )T. Pumpel et al.rHydrometallurgy 59 2001 383–393¨384

1. Introduction

European legislation on waste management in theŽ .1990s is based on the principles of 1 minimisation,

Ž . Ž .2 recycling and valorisation, 3 incineration withŽ . Ž .energy valorisation, 4 incineration, and 5 landfill

in that order. Many attempts have been made toimprove points one and two in the plating businesstoo, including regeneration of baths, recycling ofmetals and acids from waste baths, and introductionof water-saving rinsing technologies. With all thoseefforts a significant reduction in waste water produc-tion has been achieved, but waste water cannot beavoided completely within the next years. The needto develop more effective and cheaper technologiestherefore remains.

Conventional physico-chemical treatment ofmetal-bearing waste waters may not always provesuccessful due to the high costs of processing efflu-ents of high volume and low contamination or be-cause the treated water does not meet certain legalstandards, e.g. it contains complexing organic matter.Biotechnological approaches can succeed in thoseareas and are designed to cover such niches. Certainmicro-organisms are able to remove heavy metals

Žfrom diluted solutions by a variety of biological e.g.. Ž .uptake , biologically mediated e.g. bioprecipitation

Ž .and physico-chemical e.g. biosorption mechanisms.The objective of the present work was the devel-

opment of an efficient and cheap, biotechnologicalsystem for the removal of nickel from a rinsingwater of an electroless nickel plating line.

The biosorptive potential of microbial biomass forthe element nickel was investigated in the past and

w xcompared with other metals 1–4 . It turned out thatin comparison to other elements present in industrialwaste waters only a low loading of the microbialbiomass can be achieved in the case of nickel.AAmong heavy metals nickel is particularly recalci-trant to bioremediation using the standard ap-proaches of biosorption and bioaccumulation. Thereason for this is still not clear but may relate to theco-ordination number and stereochemistry of this

w xmetalB 5 .Due to this fact it seems to be impossible to

develop a continuous system only based on biosorp-tive removal of nickel with microbial biomass.

The MERESAFIN system for microbial heavymetal removal aims at the optimal combination ofdifferent processes of biological immobilisation of

Žheavy metals uptake, bioprecipitation and biosorp-.tion for a new, efficient polishing technology for

metal containing waste waters. The MERESAFINsystem has been developed through the synergisticeffort of 10 partners from industry and research,comprising engineers, chemists and biologists, sup-ported by the Brite-Euram program of the CEC.

2. Experimental

2.1. Laboratory scale experiments

A series of laboratory scale experiments was per-formed in order to select efficient bacterial strainsand to identify suitable nutrition and microenviron-mental conditions for a good metal removal.

Bacteria with the potential to sorb and precipitateheavy metals or to degrade organic compounds typi-cally found in industrial waste waters were suppliedby four microbiological laboratories. The collectionof micro-organisms was screened for the followingfeatures:

v w xrisk classification 6v Žgrowth on economically acceptable nutrients Sec-

.tion 2.1.1v tolerance to the metals present and to the waste

Ž .waters Section 2.1.2v biosorptionr bioprecipitation of the metals of in-

Ž .terest Sections 2.1.3 and 2.1.4v Ždegradation of organic substances present Section

.2.1.4 .

The finally selected strains were adapted to theirtarget area by cultivation on waste water agars,supplemented with the ingredients of a minimal

w xmedium 7 .

2.1.1. Screening for feasible nutrientsThe application of the BIOLOGe system for

Ž .identification of bacteria BIOLOG, USA yielded an

( )T. Pumpel et al.rHydrometallurgy 59 2001 383–393¨ 385

overview of the ability of individual bacteria toŽoxidise 95 defined carbohydrates. The system makes

use of the reduction of tetrazolium violet to a redform by the electron transfer chain. The reactionsoccur in the 96 inoculated wells of a microtiter plate,each well containing a single carbon compound andone blank, respectively. The intensity of colouring isa measure of the oxidation of the specific substrate

.and is read with a microtiter-plate photometer. Theprocedure was carried out according to the manufac-turer’s instructions.

The growth of all the preselected strains wasassessed on agars prepared from defined substrates,selected with the BIOLOG test, as well as fromvarious complex nutrients like molasses, green syrup,sweet whey, waste waters from paper mill, brewery,dairy, fruit juice and cigarette production, as well assupplementary mixtures thereof.

2.1.2. Tolerance of micro-organisms for waste waterWaste water was supplemented with the compo-

nents of a minimal medium, enabling growth of allselected strains. The pH was adjusted to 7, and the

Ž .solution filtered 0.2 mm and diluted 1:2 with sterileminimal medium in four subsequent steps. Thesesolutions and a blank without any waste water weretransferred to glass tubes and inoculated with sus-pensions of the individual strains. Growth was deter-mined by measuring the optical density at 660 nmafter 24 h of incubation.

2.1.3. Biosorption capacity of biomassIndividual bacteria were cultivated in 500-mL

conical flasks on a gyratory shaker. After sufficientgrowth the biomass was harvested by centrifugation,washed with 0.6% NaNO solution, and resuspended3

in fresh NaNO solution. The dry weight content of3

the bacterial suspension was determined gravimetri-cally.

Biosorption tests were carried out in acid-leachedŽ .centrifugation tubes Polyallomer using five differ-

Žent concentrations of biomass in waste water Table.1 . The suspensions were thoroughly mixed on a

Circo-Mix and allowed to equilibrate at 208C. After30 min the metal laden bacteria were sedimented by

Ž .centrifugation 10 min, 10,000 rpm , the super-natants decanted and acidified prior to residual metalanalysis.

Table 1Experimental pattern of biosorption tests

Tube a Bacterial 0.6% NaNO Waste3Ž . Ž .suspension mL solution mL water or

syntheticmetal

Ž .solution mL

1 0 2 62 0.5 1.5 63 1 1 64 1.5 0.5 65 2 0 6

ŽThe five resulting data pairs dry weightrresidual.metal concentration are subjected to mathematical

Ž .fitting first-order exponential decay . The calculatedŽ .slope first derivative of the regression line at zero

dry weight gives a good estimation of the equilib-rium biosorption capacity at the original metal con-

Ž w x.centration Fig. 1; 8 .

2.1.4. Integrated eÕaluation of microbial waste watertreatment

Experiments with real waste water samples werecarried out under non-sterile conditions. In 100-mlconical flasks, water samples were supplementedwith the various nutrients under investigation. Afteradjustment of the pH value either individual bacterialstrains or mixtures thereof were inoculated fromadapted cultures on waste water agar. In each test,the non-inoculated control was used to assess thepotential of autochthonous micro-organisms from thewater samples. The flasks were incubated on a gyra-tory shaker at 258C. After 24 to 48 h, the following

Žanalyses were performed: pH, optical density 660.nm ; residual concentrations of metals and organic

Žacids in supernatants after centrifugation Section.2.3 .

2.1.5. Characteristics of the waste water selected forpilot plant studies

Based on toxicity tests for a range of waste watersof interest and on analyses of their constituents, therinsing water of an electroless nickel plating plantŽ‘Skol6’; Collini-Skolnik Oberfachentechnik, Vi-¨

.enna was selected for process development. BesidesŽ .a few 2–10 mgrL of Ni, the water mainly contains


Fig. 1. Example of the calculation of biosorption capacity.

Ž . Žinorganic chloride -100 mgrL , sulphate ;100. Ž .mgrL , phosphate ;300 mgrL and ammonium,

Žtraces of base metals, organic acids lactic, malic,.acetic , and some unknown surface active com-

pounds in varying concentrations. The pH-value fluc-tuates around the neutral point.

2.2. The pilot plant

2.2.1. Technical principle of the Astrasandw moÕ-ing-bed filter

w ŽIn the moving-bed Astrasand filter Astraco Wa-.ter Engineering; Fig. 2 , the water to be treated

Ž . Ž .enters the device via feed pipe 1 , supply pipe 2 ,Ž .and distributor 3 . It flows in an upward direction

Ž .through the sand bed 4 in a cylindrical vessel. Thefiltrate is discharged in the upper part of the filterŽ .5 .

The filter bed is continuously moving downward.Ž .In the bottom of the filter dirty sand 6 is abstracted

from the sand bed and washed, after which it isŽ .released back on the top of the sand bed 7 . The

sand circulation is based on the airlift principle,forcing a mixture of dirty sand and water upward Fig. 2. Construction of the Astrasandw moving-bed sand filter.


Ž .through a central pipeline 8 . The intensive scouringmovements separate the impurities from the sandparticles. At the top of the pipeline the air is re-

Ž .leased, the dirty water is discharged 9 to a settlingdevice, e.g. a lamella separator.

Ž .The sand settles in the washer 10 with its well-designed hydraulic features. The grains fall throughthe washer, where they are finally washed by a smallamount of clean filtrate, flowing through the washercountercurrently. That filtrate flow is generated by adifference in discharge levels between the filtrateŽ . Ž . w x11 and the wash water 9 9 .

( )Technical data of the pilot filter DST-06-D3Sand filling 1.7 m2Filter bed area 0.6 m

Effective bed height 2 mTotal height 4.5 mDesigned surface loading rate 1.7–13 mrh

3Feed flow rate 1–8 m rh

2.2.2. Biological operation of the Astrasandw filterThe five selected bacteria were grown in acetate-

supplemented waste water up to the 50 L stage inairlift reactors. The sand of the pilot filter was theninoculated with the final mixed population. For aero-

bic microbial growth, the chosen waste water had tobe amended with a carbon source only. To test alsoanaerobic conditions, a dosing equipment for nitrate

Ž .was installed too Fig. 4 .Inoculated bacteria grow on the sand grains form-

ing a biofilm, which traps dissolved and particulateŽ .metals from the feed water Fig. 3 . As described

above, the laden sand is moved upwards by theinternal airlift, cleaned by attrition and returns to thesand bed. The sand grains retain a residual biofilmŽ .base film and fall to the top of the bed, wherebacteria start to grow again and to immobilise themetals. Particles of metal containing biofilm areextracted from the system with some wash water.

2.2.3. Experimental set-up of the pilot plantThe sand filter forms the core of a water treatment

Ž . Ž .system Fig. 4 which consists of 1 a buffer tank toŽ .smooth concentration and flow spikes, 2 the sand

Ž .filter itself, 3 a lamella separator to separate theŽ .metal laden biomass flocs from the wash water, 4Ž .bag filters to dehydrate the settled biosludge, 5

dosing equipment to supply nutrients, and on-linemeasuring devices for pressure difference, pH, feedand wash water flow, temperature, electrical conduc-

Ž .tivity and dissolved oxygen not shown .

Fig. 3. MERESAFIN concept: schematic presentation of metal binding and precipitation to biofilms on sand grains, and biofilmregeneration, in an Astrasandw moving-bed sand filter.


Ž .Fig. 4. Simplified flow diagram of the pilot plant explanation in the text .

2.3. Analytical methods

Sample preparation: centrifugation in PolyallomerŽ .tubes 10 min, 10,000 rpm . DissolÕed metals: flame

Žatomic absorption spectrometry in acidified to 1%.HNO supernatants, standard conditions of manu-3

facturer.Ž .DissolÕed organic acids acetic, lactic, malic :

HPLC; Column: Biorad’s Aminex HPX87H, 408C;eluent: 2 mM H SO , 0.6 mlrmin; detection: UV2 4

210 nm.Nitrate and nitrite: HPLC; Column: LiChrospher

100RP18 5 m, 258C; eluent: 0.1 M NaCl with 1.3mlrL n-octylamine, pH 3.5 with o-phosphoric acid,

w x1 mLrmin; detection: UV 210 nm 10 .

3. Results and discussion

3.1. Selection and characteristics of bacterial strains

Ž .According to the selection criteria Section 2.1five bacterial strains were chosen to form the consor-tium for detailed laboratory investigations as well as

Žfor the inoculation of the pilot plant the ‘5-Mix’.consortium :

Pseudomonas mendocina AS302: high biosorptioncapacity, grows on lactic and acetic acid, metal

Ž . w xresistant Sb, Hg, As, Tl 1,11 ;Arthrobacter sp. BP7r26: high biosorption and

Ž .probably bioprecipitation, metal resistant Ni, Pbw x1,12,13 ;

ŽRalstonia eutropha CH34 former Alcaligenes eu-. Ž .trophus : good bioprecipitation metal carbonates ,

grows on lactic and acetic acid, cleaves somemetal–lactate complexes, alkalises the mediumŽ .promotion of hydroxidic precipitation , also growsunder micro-aerophilic conditions, metal resistantŽ . w xCd, Zn, Ni, Co, Cr, Tl, Pb 1,14,15 ;P. fluorescens K1r8a: good bioprecipitation,grows on lactic acid;Methylobacillus sp. MB127: high biosorption ca-

Ž .pacity, metal resistance U , grows on acetic acidw x1,16,17 .

All strains belong to risk class 1, and each strainprovides important features to the genetic pool of thesand bed. Due to practical reasons the decision wasto work with all the bacterial strains as a mixedpopulation, and to follow the surveillance and fre-quency of each strain during the operation of theplant.

3.2. Biosorption of nickel by selected strains

As already known from previous work, the sorp-tion capacity of bacterial biomass for nickel cationsgenerally is rather low, compared with many othertransition elements. The reported values range arounda few milligrams of Ni per gram dry weight, at anequilibrium Ni-concentration of 50 to 100 mgrLw x1–4 . In addition to the low biosorption capacityexpected, the presence of complexing organic acids,phosphates, and other unknown organic compounds


Žin the waste water rinsing water of chemical nickel.plating was thought to further decrease the biosorp-

tive potential. With regard to the nickel and organicacids only, the PHREEQE-code for modelling solu-tion equilibria indicated the nickel to be present as

Ž . Žthe lactate complex Ni lac at neutral pH in the2

calculation other waste water constituents were not.considered .

Experiments with a waste water sample contain-Ž .ing 10 mgrL of Ni ‘Skol6’ revealed that the

measured biosorption capacities deviated onlyw xmarginally from the isotherm predictions 1 deter-

Ž .mined in pure Ni-sulphate solutions Fig. 5 . Therewas practically no negative effect due to the wastewater matrix. The best biosorbents were MB127,AS302, and BP7r26 with 3.5, 2.9, and 2.7 mg Nirgdry wt., respectively.

Waste water from a nickel sulphate productionŽ .plant ‘MB5’ was also investigated. Although it

does not contain organic matter, nickel biosorptionŽ .was much lower than predicted Fig. 5 . The concen-

tration of inorganic ions is much higher than in theNi rinsing water, but the full chemical composition

of the water matrix could not yet be analysed indetail.

The low biosorptive capacity of bacteria for nickelshowed that a technology based on simple biosorp-tion with dead bacterial biomass would never lead toan economically working system. Moreover, the nec-essary diminution of the COD content of the wastewaters requires an additional water treatment stage.

3.3. The integrated biological approach to wastewater treatment

In contrast to conventional chemo-physical andbiosorptive methods, active micro-organisms openup the possibility to develop a single-stage processfor the cleavage of organo-metallic complexes,degradation of organic compounds, immobilisationof dissolved and fine-dispersed metallic and metal-loid elements and, to some extent, also to the re-moval or degradation of inorganic ions like ammo-nium, nitrate and phosphate. All these classes of

Ž w x. ŽFig. 5. Biosorption capacities of selected bacteria for nickel in synthetic solution isotherms 1 and real waste water samples single.points .


substances contribute to the matrix of a typical wastewater from the plating industry, like the selectedrinsing water ‘Skol6’.

Concentrations of organic acids in ‘Skol6’ weretoo low, on average, for sufficient biomass produc-tion by the selected bacteria. So an additional carbonsource had to be identified. Making use of the ‘BIO-LOG’ system only a few substrates were found to be

Žoxidised by most of the bacteria in principle thesystem aims at producing a very distinct pattern for

.each organism . Also, a range of complex substrateswas tested on normal agar plates.

Finally, acetate was selected as the substrate ofchoice for further process development. Acetate sup-ports good growth of most of the selected bacteria, itdoes not complex metal cations, it is economicallyacceptable, and it can be accurately traced in theprocess with usual analytical methods. The complexsources are much cheaper, but they are never de-graded completely and therefore contribute to CODin the effluent. This fact is of special importance iftreated water is to be re-used, discharge limits are tobe met, or ecotaxes to be paid. With regard to boththe pilot-scale and future full-scale operations, safeand risk-free storage and handling of a substratemust also be considered.

With the chosen waste water the only supplemen-tation required is carbohydrate as the growth andenergy source. Nitrogen is available from ammonia,added for pH control. Phosphorous is present ininorganic phosphates, and trace elements come from

Ž .the water Ca, Mg as well as from the platingŽprocess etched base metals and all their alloying

.constituents .With regard to the prerequisites mentioned above,

laboratory experiments were performed in order tooptimise supplementation and the experimental set-

Ž .up. Under aerobic conditions agitated conical flasks ,the addition of 50 to 200 mgrL of acetate-C resultedin sufficient growth of the mixed bacterial popula-tion ‘5-Mix’ to reduce the nickel concentration from10 to below 1 mgrL.

ŽThe estimated Ni-load of the biomass approx. 50.mg Nirg dry wt. by far exceeded the potential of

simple biosorption, and raised hopes for successfulprocess development. These results clearly indicatethe predominance of biologically mediated processesof nickel immobilisation over passive biosorption

and will be discussed with the results of the pilotplant.

3.4. The pilot plant

3.4.1. Basic considerationsAnalysis of the pros and cons of the various

techniques applied in biological waste water treat-Žment e.g. fermentor, packed and fluidised bed reac-

.tor, rotating disc reactor, trickling reactor revealedthat the concept of the Astrasandw moving-bed sandfilter promised to fulfil most requirements. Sand is areliable carrier for micro-organisms, and the continu-ous regeneration of the sand allows for steady re-moval of metal-bearing biomass. It was further evi-

Ž .dent that a biofilm reactor like a sand filter wouldoffer some advantages over reactors with suspended

Ž .biomass: i biofilms are known to withstand ex-treme conditions which may arise in industrial waste

Žwater e.g. spikes of extreme pH or high metal. Ž .concentration . ii Biofilms provide a micro-en-

vironment which could be very beneficial for metalŽprecipitation e.g. alkaline pH, high concentration of

. Ž .CO . iii Biofilms contain a lot of microbial ex-2

opolymers which may also help to entrap dispersedsolids, as well as to biosorb dissolved metals.

The only problem of the sand filter concept,which was recognised before starting the first plant,is the limited availability of oxygen. With the origi-nal concept, designed for physical filtration, oxygeninput is limited to its solubility in the feed water.With the maximum of 8 to 10 mgrL of dissolvedoxygen, only 9 to 11 mgrL of acetate-C can be

Žoxidised determined in laboratory experiments with.the bacterial culture ‘5-Mix’ . As around 100 mgrL

of acetate-C was shown to be necessary for theremoval of 10 mgrL of Ni, an additional electronacceptor had to be identified. Nitrate was tested andfinally chosen due to its widespread microbial utili-sation and its non-complexing properties.

3.4.2. Operation strategyA clear strategy was followed to start-up and

operate the pilot installation.

3.4.2.1. Phase 1. In order to gain experience with thesystem and to analyse the parameters of the selected


feed flow, the filter was run without inoculation andnutrient supplementation and without any modifica-tion of the water. This phase was completed withintensive flushing of the plant in order to remove thebulk of spontaneously emerged biomass.

Ž .3.4.2.2. Phase 2. 2a After inoculation of the sandŽ .bed with the selected bacteria ‘5-Mix’ dosing of

Ž .the additional carbon source was initiated. 2b Inorder to reach stable aerobic conditions, the feedwater had to be aerated in the buffer tank. In foursub-phases, the dosing of carbon and nitrate wasincreased step by step.

( )3.4.3. Results of aerobic operation phases 1, 2a, 2bThe pH of the rinsing water varied around neutral

and needed no correction for optimal growth of theselected bacteria. The concentration of dissolvedoxygen fluctuated greatly and had to be stabilised ata high level by aerating the buffer tank. So a ratherconstant oxygen concentration in the feed flow of7–8 mgrL could be achieved. Without any inocula-tion and dosing of extra carbon some autochthonousmicro-organisms emerged in the filter in phase 1.They were able to use the low concentrations of

Ž .organic acids acetic, lactic and malic present in therinsing water, but the established micro-consortiumdid not efficiently remove nickel.

After inoculation with the selected bacteria, thedosage of carbon has been increased step by step inphase 2b, up to the maximum concentration whichcould be used aerobically, with the available oxygen,

Ž .by the micro-organisms ;5 mg CrL . The removalof nickel from the rinsing water could be increased

Ž .to 0.53 mgrL Table 2 , which is the same as shownwith suspended biomass in 100 ml conical flasks.

The production of biomass and the activity ofmicro-organisms had to be increased in order toincrease nickel removal. Due to the physical limita-tion of available oxygen in the feed flow, nitrate wasdosed as a second electron acceptor. With 3.4 mgrLnitrate-N reduced, 3.4 mgrL carbon was additionally

Ž .oxidised total s 8.4 mgrL , and the nickel re-Ž .moval raised to 0.8 mgrL Table 2, Fig. 6 .

HPLC-analysis of organic acids showed theircomplete degradation, including the part of the lac-tate which was calculated to be complexed withnickel. Thereby nickel was set free and could precip-itate at the pH of 8. The bio-sludge produced duringthat period contained about 20 mgrg Ni in the drymatter. With the Ni-concentration typically rangingbetween 2 and 5 mgrL in the rinsing water, theconcentration factor therefore reaches 10,000. Theanalysed nickel content in the sludge exceeds thebiosorptive capacity of the biomass by a factor of 10Ž .Fig. 5 , which also is clear evidence of additionalbiological or biologically mediated processes. Be-sides the already mentioned precipitation of Ni-hy-droxides, the formation of Ni-phosphate may alsooccur.

R. eutropha CH34 formed the bulk of the biofilmeight months after inoculation of the filter, as shown

w xby Pernfuß et al. 18 . None of the other addedstrains could be found with certainty, but some of there-isolates were related to the genus PseudomonasŽlike P. mendocina AS302 and P. fluorescens

. ŽK1r8a and to the family Nocardiaceae like.Arthrobacter BP7r26 . One of the most frequent

autochthonous bacteria was related to Acinetobacterw xcalcoaceticus 18 .

Results from two other pilot filters operated at thesame time within the same project, but with mixed

Table 2Average values of selected parameters for the operation phases 2b-1, 2b-2, and 2b-4

3Ž . Ž . Ž . Ž . Ž .Phase Flow m rh O mgrL Acetate mg CrL NO mg NrL pH Ni mgrL2 3

a a a afeed wash feed rem feed rem feed rem feed rem

2b-1 1.00 0.83 7.2 5.7 2.5 2.9 0 7.7 0.282b-2 0.94 0.68 7.1 7.4 5.1 5.0 0 0.5 7.8 0.532b-4 1.02 0.67 6.7 6.7 10.4 8.4 3.7 3.4 8.2 0.76

aRemovedsdifference between feed and filtrate.


Fig. 6. Box and whisker plot of nickel removal in the pilot filter during different phases of operation.

waste waters containing nickel, confirmed our resultsw x19 .

4. Conclusions

The MERESAFIN concept, metal removal with amicrobiologically active moving-bed Astrasandw fil-ter, was shown to work with a complex waste waterfrom electroless nickel plating. The carefully se-lected bacteria formed a continuously regeneratingbiofilm on the sand grains in the filter, and even aftereight months of operation inoculated bacteria builtthe major part of the biomass. With the addition of acarbon source and some nitrate as a second electronacceptor to oxygen, the bacteria cleave the presentcomplexes of nickel with organic acids, which arecompletely degraded. The released nickel isbiosorbed by the biomass and, to a much higherextent, precipitated within the biofilms.

The biosludge separated from the wash water wasthickened in the bag filters and contained a 2% ofnickel in the dry matter. The nickel could be returnedto the metal cycle via pyrometallurgical treatment in

a shaft furnace. There was no problem burningbiomass as a substituent of coal.

Pure nickel sorption to dead biomass or naturalminerals only reaches a loading of 0.05% to 0.3%, atthe same equilibrium concentration of 2–5 mg NirLw x1–4,20 . The sludge volume therefore would be6–40 times greater. The often claimed reusability ofbiosorbing biomass after desorption of the metalmust be seen with scepticism. For desorption, onlyrelatively weak chemicals can be used in order not todestroy the biomass. This results in eluents which arelower concentrated than eluents from ion exchangers.Further, the number of sorption-desorption cyclesmay reach only 5 to 10, before biomass has to beexchanged.

With the present knowledge, a biotechnologicalapproach to remove small concentrations of metalsfrom water can overcome the economical barrieronly when using active micro-organisms, living fromwaste water ingredients and cheap nutrients. Costcalculations comprising all investment and opera-tional costs demonstrated that comparable conven-tional water treatment techniques like precipitationwith filtration, reversed osmosis or ion exchange are

w x50% to 100% more expensive than the patented 21MERESAFIN process.


Acknowledgements

The authors are grateful for financial support fromthe Brite-Euram program of the European Commu-

Ž .nity contract BRPR-CT96-0172 .

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Documents

Treatment of rinsing water from electroless nickel plating with a biologically active moving-bed sand filter