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Continuous regeneration of degreasing solutions from electroplating operations using a membrane bioreactor
DESALINATION ELSEVIER
Desalination 162 (2004) 315-326
Continuous regeneration of degreasing solutions from electroplating operations using a membrane bioreactor Christoph B16cher"*, Ulrike Bunse b, Berthold SeBler~, Horst Chmiel aob, H a n s Dieter Janke a ~Institute for Environmentally Compatible Process Technology mbH (upO, Im Stadtwald 47, D-66123 Saarbriicken, Germany Tel. +49 (681) 9345-212; Fax +49 (681) 9345-380," email: [email protected] bDepartment of Process Technology, Saarland University, Saarbriicken, Germany CSessler Galvanotechnik mbH, Wiirzburg, Germany Received 25 July 2003; accepted 5 September 2003
Abstract
To extend the service life and to improve the quality of degreasing solutions from surface refining processes in the metal working industry a process based on a membrane bioreactor (MBR) with submerged multi-channel fiat sheet ceramic membranes was developed. This MBR-based regeneration process combines the retention by membranes and the biodegradation of oils/grease. The objective is to retain the biomass as well as the hydrocarbons in the bioreactor by the microfiltration membranes to enhance biodegradation. Simultaneously, low retention is aimed at for the surfactants since these substances have to be returned to the degreasing bath. The process was tested with a mobile pilot plant over a three-month period on site in an enterprise which carries out electroplating on commission. The multi-channel flat sheet ceramic membranes, which had an average pore diameter of 0.3 gin, fulfilled the objective of retaining the hydrocarbons to a high extent while allowing relatively high permeation of the surfactants (in order to minimise additional dosing). Permeate was free of solid matter and hydrocarbon concentration was reduced by 85-90% (compared to the feed). The reduction in non-ionic surfactants was only 2 5 4 0 % . Compared to conventional ("open") biological regeneration, a five fold increase in volumetric biodegradation rate was achieved due to the higher biomass concentration. A preliminary economic analysis for this case study showed, that there was a reduction in costs of roughly one third for the process under study in contrast to conventional hot alkaline degreasing. The main reason for this saving is the lower cost of wastewater treatment.
Keywords: Electroplating industry; Degreasing; Regeneration of degreasing solutions; Membrane bioreactor *Corresponding author.
Presented at PERMEA 2003, Membrane Science and Technology Conference of l,~segradCountries (Czech Republic, Hungary, Poland and Slovakia), September 7-11, 2003, TatranskAMatliare, Slovakia. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved pII: S0011-9164(04)00065-7
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1. I n t r o d u c t i o n
If surface treatment of metal products in the electroplating industry is to meet quality requirements, it is essential to ensure that the workpieces are subject to an effective degreasing and subsequent pickling process. To this end, metal workpieces pass through different process baths (degreasing, pickling, various rinsing stages) prior to actual surface refining involving zinc plating, chromate coating, electropolishing, copperplating. Hot alkaline degreasing (T-~ 60°C) is usually applied to free the workpieces from grease. Oils or hydrocarbons are removed from the workpieces and then accumulate in the degreasing solutions, making it necessary to discard these after a certain period of time. The disposal of the wastewater and sludge generated in this process is problematic from both an ecological and economic viewpoint. State-of-the-art technology involves two regeneration processes, applied to minimise wastewater quantities and material loss. However, both processes have specific shortcomings. • Cross-flowultm-/microfiltration for separating the hydrocarbons whereby the service life of the degreasing solution can be extended [ 1,2]. Disadvantages are: - High energy consumption - For incineration purposes, the hydrocarbons have to be concentrated to a great extent which has adverse effects on membrane performance (fouling) • "Open" biological regeneration (e.g. the ENREP TM Bioclean process). This process, in operation for some time now, involves degreasing treatment using special surfactants at a temperature of 40°C while the hydrocarbons are degraded in an integrated bioreactor. Hence, there results a considerable reduction in sludge quantities and a distinct extension to the service life of the degreasing solutions. Sometimes even the degreasing bath itself is used as a bioreactor [3]. However the disadvantages are:
- The biomass is carried into the downstream process baths because the bioreactor and the degreasing bath are not separated adequately or not at all, which also results in a high biomass concentration in the degreasing bath itself (-> adverse effects on operating efficiency) - Presence of potentially pathogenic germs (Pseudomonas aeruginosa and others) in the degreasing and downstream process units (-> low operational reliability) - Efficiency is solely dependent on the biodegradability of the oils (-> low process reliab~ty) In view of these constraints, a technical concept was developed, which, while combining both of the above-mentioned processes, makes use of the advantages but minimises the disadvantages. The process entails a bioreactor with submerged ceramic membranes (MBR) in which the retention and biodegradation of the oils/grease concur. The degreasing solution is routed continuously to the bioreactor where the hydrocarbons are biodegraded while the biomass is completely retained by the membranes. Thus, no biomass is carried into the downstream stages and considerably higher volumetric biodegradation rates (in terms of g hydrocarbons biodegraded per hour and m 3 of bioreactor volume) are achieved due to the increase in the dry matter content (biomass). Since the hydrocarbons are retained to a great extent, difficult-to-degrade oils are also removed from the degresasing solution. A simplified flow diagram is shown in Fig. 1. 2. E x p e r i m e n t a l
2.1. Membranes
Ceramic membranes (0~-A1203) were considered most suitable for the tests conducted because of the expected low fouling tendency for oil/water emulsions owing to their hydrophilic surface properties [1,3]. Multi-channel flat sheet
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metal work pieces . . . . . . . . . . .
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further treatment (surface refining)
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Fig. 1. Schematic representation of the concept for process-integrated regeneration of degreasing solutions from surface refining processes in the metal-working industry.
-'---]-4,5mm g
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Fig. 2. Schematic representation of the multi-channel flat sheet ceramic membrane. membranes (Fig. 2) [4], supplied by the Hermsdorf Institute for Technical Ceramics (HITK), Thiiringen, Germany and produced according to the solgel process, were used. The separating layer in these membranes is on the outside while permeate is withdrawn via the permeate channels on the inside. A permeate collector was attached to one side of each membrane which was fitted vertically into the reactor.
Fig. 3. Module unit (top view). The module unit consisted of three levels each containing five membranes (Fig. 3). Air was introduced below the modules to create certain shearrates across the membrane surface (fouling control) and to simultaneously supply oxygen to the biodegradation process. With respect to the separation properties of the membranes under study, the following objectives were aimed at: • Total retention of the biomass • Highest possible retention of the hydrocarbons ° Lowest possible retention of the surfactants Five different membrane types, the properties of which are shown in Table 1, were investigated in membrane screening. For that purpose, the
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Table 1 Parameters of the multi-channel fiat sheet ceramic membranes tested Membrane type
K4
Size, mm Support material Active layer material Mean pore size acc. to pore flow method, gm Pure water permeability, L/(rn2 h bar)
100 (width) x 6 (thickness) x 100 (length in lab-scale) Korund (A1203)
H9
H20
H21
(A1203) 1.0
<0.1
0.35
0.45
0.28
5,000
1,300
4,500
4,500
3,000
mobile MBR pilot plant (described below) was used and operated in recirculation mode.
2. 2. On-site operation of the MBR pilot plant The process concept described above was tested on-site in a medium-sized enterprise which carries out electroplating on commission (Sessler Galvanotechnik GmbH). In this case, metal surface refinement includes zinc plating, chromate coating, electropolishing, copper plating. Aprevious study in this company [5] has shown that by switching the treatment in the degreasingbath from hot alkaline degreasing to process-integrated "open" biological regeneration (ENPREP TM
Bioclean process) together with the implementation of a cascade pickling process, a considerable reduction in costs and wastewater quantities could be achieved. However, the afore-mentioned disadvantages of"open" biological regeneration were very pronounced. A mobile pilot plant (Fig. 4), consisting of a bioreactor with loop configuration and a working volume of 45 L was used in the tests. The contents of the reactor were heated to 40°C. The pilot plant provided the possibility ofautomatically interrupting permeate withdrawal within certain intervals as well as chemically cleaning the membranes from the permeate side (CIP- cleaning in place). Transmembrane pressure was applied by pressurizing
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the bioreactor with the air which was introduced for aeration. It was controlled by a valve which regulated the air outlet. By this method hydrostatic operation of the membranes was simulated. However, the results can also be applied to operation with a vacuum pump. The pilot plant was run in by-pass operation to the "open" biological regeneration (Fig. 5) which means that it was directly affected by the properties of this process solution, in particular dry matter content (biomass) as well as hydrocarbon and surfactant concentration, entering via the feed pipe. The tests were conducted for a total period of three months. The surfactant mixture ENPREP TM Bioclean BN 22 012 0 produced by Enthone Ltd. and consisting of a combination of non-ionic and cationactive surfactants, was used to emulsify the oils and grease. Since the solution is made up mostly of non-ionic surfactants, which are responsible for the actual degreasing effect, the focus of the study centred on these. Mainly, surfactants were added in the form of so-called Bioclean-Starter solution (target concentration 1.0%), which contains builder substances in addition to the surfactants. The pH in the bioreactor was left unchanged as it resulted naturally from the feed and the biodegradation processes. It ranged from 8.8 to " ........
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9.2 during the on-site test operation. No nutrients in addition to those in the feed were added because C:N and C :P ratios in the feed were 14:1 and 12:1, respectively, indicating sufficient nutrient supply. 2.3. Analysis
The dry matter (suspended solids) content was determined as dry filtrate residue according to the German standard method DIN 38 409-H 1-2 prior to which centrifugation was performed over a period often minutes at 16,000 rpm (26,045 x g). The latter was conducted to separate the deemulsified oil phase which had developed during cooling of the sample. Analysis of the total hydrocarbons was conducted in line with the German standard DIN H53. Non-ionic surfactants were analysed using the cuvette test LCK 333 (Dr. Lange). Membrane pore size distribution was determined in line with the American standard method E1294-89 with a PMI capillary flow porometer (Porous Materials Inc.). 3. Results and discussion 3.1. Membrane screening
As already mentioned, the goal for these ceramic membranes was to retain the hydrocarbons P"
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CIP (membrane cleaning)
Fig. 5. Integration of the MBR pilot plant into the surface refining process with "open" biological regenerationof the degreasing solution.
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as far as possible while allowing the greatest possible quantity of non-ionic surfactants to pass through the membrane. The five membrane types characterised above (Table 1) were assessed with respect to this goal in membrane screening. As a medium to be filtrated, the degreasing solution containing biomass from a degreasing bath with "open" biological regeneration was used. The permeate generated was re-routed to the bioreactor. The average hydrocarbon and suffactant concentrations in the degreasing solution (i.e. in the bioreactor) was 350 and 500 mg/L respectively, while dry matter content was roughly 0.6 g/L. The permeate generated by all membrane types was free of turbidity, i.e. of biomass and other particulate matter. Retention of hydrocarbons and non-ionic surfactants as a function of operating time is shown in Fig. 6. In general, retention for both parameters increased for all the membranes throughout the operating time. Hydrocarbon retention was mostly above 90%, while surfactant retention was initially about 25% for all membranes but then rose to 50% on average. Neither hydrocarbon nor surfactant retention depended on the average pore size of the membranes. On the contra]% the membrane with the largest nominal pore size (K 4) showed the highest
retention for the surfactants. It can be concluded, that retention is controlled to a great degree by the coating layer which forms during operation, namely by membrane fouling (i.e. adsorption in the pores). This is in line with the observation that flux was also similar for all membranes investigated. Therefore the choice of a membrane type for the on-site operation could be based on other criteria. For reasons of availability and production quality the membrane type H22 was used in the subsequent on-site tests.
3.2. Performance of the membrane bioreactor during the on-site test operation To assess the performance of the MBR during the on-site test operation the relevant parameters were determined in the feed, in the bioreactor solution and in the permeate. In Fig. 7 the timedependent concentrations of hydrocarbons, nonionic surfactants and suspended solids are presented. The feed (that is the process solution from the "open" biological regeneration) was characterised by variations in hydrocarbon as well as surfactant concentration• These ranged from 150 to 600 mg/L and from 150 to 700 rag/L, respectively. Suspended solids content in the feed was mostly
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C. Bldcher et al. /Desalination 162 (2004) 315-326
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around 0.5 g/L, although values as low as 0.2 g/L were observed. As all suspended solids were retained by the membranes, concentration in the bioreactor increased continuously during the test operation up to 16 g/L. On the 71st day of operation, the concentrated biomass solution was withdrawn and the bioreactor was refilled with feed, which resulted again in the afore-mentioned concentration. In general, also hydrocarbon concentration increased during the test run, resulting in a maximum of 4000 mg/L. This means that in this period not all hydrocarbons, that were retained by the mem-
solids
Fig. 7. Composition of feed water, bioreactor solution and permeate during the on-site test operation of the MBR pilot plant.
branes, were biodegraded. However, also decreasing hydrocarbon concentrations were observed at certain times, meaning that the accumulated hydrocarbons were finally biodegraded. As plant throughput was kept nearly constant (1-2 L/h), this behaviour is either due to (i) an increase in feed concentration that was not detected in the samples of the feed, or (ii) varying hydrocarbon composition including difficult-to-degrade fractions. Also, surfactant concentration in the bioreactor increased during the test rtm. However, it was less pronounced, with the maximum concentration being 1000 mg/L. Naturally, hydrocarbon
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and surfactant concentrations started again at the values of the "open" biological regeneration when the bioreactor was refilled. Despite of the high variations in hydrocarbon and surfactant concentration in the bioreactor, permeate quality was stable during the three months' test operation. Even in the period with the highest hydrocarbon concentration in the bioreactor, the hydrocarbon level in the permeate was below 50 mg/L. To summarise the performance of the process under study, in Table 2 the mean values for the entire process are listed. The reduction rates specified for the hydrocarbons, the non-ionic surfactants and the suspended solids are based on the respective concentrations determined in the feed. Considering the process as a whole, it can be stated that the objective of high hydrocarbon retention as well as low surfactant reduction and total biomass retention was fulfilled. Noteworthy is however, that the hydrocarbon concentration determined in the permeate during the test operation is not identical to the value which would become established as the steady state concentration in the degreasing solution in the process bath, if the process were implemented in full-scale. This is principally determined by the recirculation volume flow in addition to the level of hydrocarbons
entering the system. The higher the flow through the membrane bioreactor, the lower the concentration in the degreasing bath. Although the overall treatment quality was very good, it is necessary to study the two concurring processes (biodegradation as well as the retention by the ceramic membranes used) more closely in order to evaluate the process. 3.2.1. Biodegradation processes in the MBR pilot plant
As was indicated above, for the process design it is necessary for the hydrocarbon load to be equal to the biodegradation rate. To determine the respective biodegradation rates for the hydrocarbons and the non-ionic surfactants in the MBR-based regeneration process, a balance was drawn up for the loads contained in the feed and permeate, taking into consideration also the respective substance concentration in the reactor. In this context, a difference is made between the volumetric biodegradation rate (in terms o f g hydrocarbons biodegraded per hour and m 3 of bioreactor volume) and the specific biodegradation rate (in terms ofg hydrocarbons biodegraded per hour and kg of suspended solids/biomass). Table 3 shows these average biodegradation rates for hydrocarbons and surfactants.
Table 2 Mean treatment efficiency of the MBR during the on-site test operation
Feed Permeate Reduction
Q Hydrocarbons 352 mg/L 42 mg/L 86%
Q Surfactants 455 mg/L 350 rng/L 24%
Q Suspendedsolids (SS) 0.4 g/L Below detection limit >99%
Table 3 Biodegradation rates for hydrocarbons and surfactants in the MBR as determined during the on-site test operation (values calculated as median) Specific biodegradationrate, gbiodegr./(kgSS'd) Volumetricbiodegradationrate, gbio~g/(m3d)
Hydrocarbons 52 226
Surfactants (non-ionic) 27 88
C. Bl6cher et al. /Desalination I62 (2004) 315-326
The specific biodegradation rates were similar to those of the "open" biological regeneration process run in parallel. However, the considerably higher suspended solids content of~3 4.3 g/L compared to the ~ 0.6 g/L in the feed (SS content of the "open" biological regeneration process) led to a higher volumetric biodegradation rate. In view of the fact that the biodegradation rates for the surfactants, in general, were clearly lower than for the hydrocarbons, the treatment objectives (as outlined in the beginning) were fulfilled in this case.
323
increased level of retention. As was shown in Fig. 7, the concentration in the bioreactor increased throughout the operating time. The fact that hydrocarbon concentration in the permeate was, at 45 mg/L on average, very low and did not depend on the bioreactor concentration to any great degree indicates that the bioreactor allows, in terms of process stability, a certain buffering effect for the concentration peaks. However, the increase in surfactant retention has to be judged as a shortcoming. The resulting increase in surfactant concentration in the bioreactor results in higher biodegradation of the surfactants which should be minimised.
3.2. 2. Membrane separation properties Hydrocarbon and surfactant retention throughout the test period is shown in Fig. 8. It is important to keep in mind the difference between the retention (calculation based on the concentration in the bioreactor) and the reduction as defined in Table 2 (calculation based on the feed concentration). As was observed during membrane screening, retention of hydrocarbons as well as surfactants rose during the on-site tests as operating time increased. This confirms the finding that retention is controlled by the coating layer, which means that an increase in membrane fouling concurs with a rise in retention. On the other hand, however, it was already explained that the permeate concentration did not depend on the concentrations in the bioreactor to any great extent. Thus a higher concentration level in the bioreactor results in an
3.2.3. Hydraulic performance of the submerged membranes Besides retention characteristics, any membrane process has to be considered in terms of the flux that is achieved. Fig. 9 shows the hydraulic performance of the membranes during the test period. At times, the net permeate flux stabilised at 9 L/(m2.h) but decreased to approx. 7 L/(m2.h) towards the end of the test operation. Once the reactor had been refilled with the feed, flux started again at 15 L/(mah). Various chemical combinations were tested for CIR Only by combining an oxidising cleaning agent (NaOC1) followed by the Bioclean starter solution (for emulsifying purposes and thus for
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C. Bl&cher et aL /Desalination 162 (2004) 315-326
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removing the hydrocarbons adsorbed in and on the membrane) could a stable membrane flux be achieved (days of operation 19-32). However, this same cleaning procedure did not prevent a decrease in permeate flux after the reactor had been refilled at the end of the test period, although it must be stated that not enough cleaning agent entered the membranes during the cleaning processes at that time. The different permeate fluxes before and after refilling highlight the fact, which is a known characteristic of the filtration of activated sludge from sewage treatment, that the properties of the process solution (in this case the oil/water emulsion containing biomass) have a crucial influence on membrane fouling [6,7]. Compared to biological wastewater treatment, the type of oil/hydrocarbon entering the system, the condition of the emulsions and the surfactants as well as the respective interaction are of significance in addition to the properties of the biomass. Thus the difference in flux cannot be attributed solely to the difference in biomass concentration.
3.3. E c o n o m i c analysis
To evaluate economic efficiency, a treatment unit for the degreasing bath (hydrocarbon input rate 349 g/d) in the electroplating enterprise under
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Fig. 9. Hydraulic performance of the membranes during on-site test operation; operating cycle: 10 min permeation - 1 min interruption of permeate withdrawal, CIP: every 12 h, TMP = transmernbrane pressure.
study was designed. This was based on the following data (a conservative assumption): N e t p e r m e a t e flux
of the membranes Target hydrocarbon concentration in the degreasing bath Hydrocarbon concentration in the permeate Specific biodegradationrate for hydrocarbons (HC) Suspended solids content
7 L/(m2h) 350 mg/L 50 mg/L 30 ~I-IC~oev./(kgSS-d)
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The required recirculation volume flow to be treated was calculated by drawing up a balance of the hydrocarbons for the entire system (degreasing bath-membrane bioreactor). The result is a recirculation volume flow of 43.8 L/h for which a membrane area of 6.3 m 2 is needed. The volume required for the bioreactor results from the abovementioned specific degradation rates and the suspended solids content to 1.3 m 3. Of particular importance for designing the membrane bioreactor is the target hydrocarbon concentration in the degreasing bath. Frequently, industrial enterprises are not able to specify substantiated limit concentrations. However, it is generally valid that, as target hydrocarbon concentration decreases, the recirculation volume flow and also the required membrane area increase
C. Bldcher et aI. /Desalination 162 (2004) 315-326
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200 400 600 800 1000 target concentration ofhydrocarbonsinthe degreasing solution[mg/L]
exponentially (Fig. 10). From an economic viewpoint, the hydrocarbon concentration aimed at for the degreasing process should therefore be considered very carefully. An estimate of the investment and operating costs for treating the degreasing solution of an electroplating company using a membrane bioreactor could be drawn up on the basis of this data. Specific treatment costs ore 0.34/m2of metal work-pieces were calculated for the degreasing/ pickling system with regeneration by a membrane bioreactor compared to ~ 0.47/m 2 for hot alkaline degreasing. Depending on the annual throughput of workpieces, payback times will be between 1.8 and 2.8 years.
4. Conclusions
In the process under study, the quality of the treated process solution was determined by the complex interaction of the membrane separation properties and the biodegradation processes. Owing to the relatively short test period and the specified constellation of the test setup (bypass operation to the existing "open" biological regeneration process), the process could not yet be fmally evaluated. Nevertheless, the results obtained in this work are very promising and show that the
Fig. 10. Membrane area required for a MBR-based regeneration of the degreasing solution in dependence on target hydrocarbon concentration and permeate concentration.
regeneration ofdegreasing solutions via membrane bioreactors has significant potential. In contrast to an "open" biological regeneration, also difficult-to-degrade hydrocarbons are separated from the degreasing solution. Furthermore, no particulate impurities are carried into the degreasing and downstream stages, thus resulting in considerable advantages for the entire system: • Since the process solutions contain less impurities, disposal is less frequent and the electrolyres have a longer service life. • The process line components are soiled to a lesser degree and cleaning is therefore less frequent. Considering the fact that the process proved feasible, from both a technical and economic viewpoint, in an electroplating enterprise working on commission where the degreasing baths contain a variety of oils with different compositions often unknown to the company itself, it can be assumed that the process can be implemented in industrial sectors with defined operating conditions (such as the car industry). The tests described above will be continued in a test operation at industrial level which should provide information on the growth behaviour of biomass, the long-term performance of the process as well as a detailed economic analysis.
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Acknowledgements We would like to express our gratitude to the G e r m a n y Federal Ministry for Education and Research (BMBF) for funding this work (FKZ 01 RW0189) as well as the Enthone G m b H for their co-operation.
References [1] J.M. Benito, G Rios, C. Pazos and J. Coca, Methods for the separation of emulsified oil from water: a state-of the-art review. Trends in Chem. Eng., 4 (1998) 203-231. [2] M. Cheryan and N. Rajagopalan, Membrane processing of oily streams. Wastewater treatment and waste reduction. J. Membr. Sci., 151 (1998) 13-28. [3] B. G6pfert, Biologisches Entfettungsspfilbad. Metalloberflgche, 51(10) (1997) 762-765. [4] H. Chmiel, Druckstabile anorganische Membran.
Deutsche Patentschriff DE 4329473.1, 1994. [5] SesslerGmbH Galvanotechnik, Verfahrensvergleich zur biologischen Regeneration yon Entfettungs16sungen in einer Lohngalvanik. Schlussbericht zum Teilvorhaben FKZ 01 RW0189 des BMBF-Verbundvorhabens ,Umstellung bestehender galvanotechnischer Anlagen auf eine stoffverlustminimierte Prozesstechnik bei gleichzeitiger Kostensenkqmg", 2003. [6] T. 8tephenson, S. Judd, B. Jeffersen and K. Brindle, Membrane Bioreactors for Wastewater Treatment. 1WA Publishing, London, 2000. [7] C. B16cher, T. Britz, H.D. Janke and H. Chmiel, Biological treatment of wastewater from fruit-juice production using a membrane bioreactor: Parameters limiting membrane performance. 5th conference on Membranes in Drinking and Industrial Water Production, 22-26.09.2002, Mfilheim/Ruhr, Proc. (Berichte ans dem IWW Bd. 37a), 2002, pp. 235243.
Desalination 162 (2004) 315-326
Continuous regeneration of degreasing solutions from electroplating operations using a membrane bioreactor Christoph B16cher"*, Ulrike Bunse b, Berthold SeBler~, Horst Chmiel aob, H a n s Dieter Janke a ~Institute for Environmentally Compatible Process Technology mbH (upO, Im Stadtwald 47, D-66123 Saarbriicken, Germany Tel. +49 (681) 9345-212; Fax +49 (681) 9345-380," email: [email protected] bDepartment of Process Technology, Saarland University, Saarbriicken, Germany CSessler Galvanotechnik mbH, Wiirzburg, Germany Received 25 July 2003; accepted 5 September 2003
Abstract
To extend the service life and to improve the quality of degreasing solutions from surface refining processes in the metal working industry a process based on a membrane bioreactor (MBR) with submerged multi-channel fiat sheet ceramic membranes was developed. This MBR-based regeneration process combines the retention by membranes and the biodegradation of oils/grease. The objective is to retain the biomass as well as the hydrocarbons in the bioreactor by the microfiltration membranes to enhance biodegradation. Simultaneously, low retention is aimed at for the surfactants since these substances have to be returned to the degreasing bath. The process was tested with a mobile pilot plant over a three-month period on site in an enterprise which carries out electroplating on commission. The multi-channel flat sheet ceramic membranes, which had an average pore diameter of 0.3 gin, fulfilled the objective of retaining the hydrocarbons to a high extent while allowing relatively high permeation of the surfactants (in order to minimise additional dosing). Permeate was free of solid matter and hydrocarbon concentration was reduced by 85-90% (compared to the feed). The reduction in non-ionic surfactants was only 2 5 4 0 % . Compared to conventional ("open") biological regeneration, a five fold increase in volumetric biodegradation rate was achieved due to the higher biomass concentration. A preliminary economic analysis for this case study showed, that there was a reduction in costs of roughly one third for the process under study in contrast to conventional hot alkaline degreasing. The main reason for this saving is the lower cost of wastewater treatment.
Keywords: Electroplating industry; Degreasing; Regeneration of degreasing solutions; Membrane bioreactor *Corresponding author.
Presented at PERMEA 2003, Membrane Science and Technology Conference of l,~segradCountries (Czech Republic, Hungary, Poland and Slovakia), September 7-11, 2003, TatranskAMatliare, Slovakia. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved pII: S0011-9164(04)00065-7
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1. I n t r o d u c t i o n
If surface treatment of metal products in the electroplating industry is to meet quality requirements, it is essential to ensure that the workpieces are subject to an effective degreasing and subsequent pickling process. To this end, metal workpieces pass through different process baths (degreasing, pickling, various rinsing stages) prior to actual surface refining involving zinc plating, chromate coating, electropolishing, copperplating. Hot alkaline degreasing (T-~ 60°C) is usually applied to free the workpieces from grease. Oils or hydrocarbons are removed from the workpieces and then accumulate in the degreasing solutions, making it necessary to discard these after a certain period of time. The disposal of the wastewater and sludge generated in this process is problematic from both an ecological and economic viewpoint. State-of-the-art technology involves two regeneration processes, applied to minimise wastewater quantities and material loss. However, both processes have specific shortcomings. • Cross-flowultm-/microfiltration for separating the hydrocarbons whereby the service life of the degreasing solution can be extended [ 1,2]. Disadvantages are: - High energy consumption - For incineration purposes, the hydrocarbons have to be concentrated to a great extent which has adverse effects on membrane performance (fouling) • "Open" biological regeneration (e.g. the ENREP TM Bioclean process). This process, in operation for some time now, involves degreasing treatment using special surfactants at a temperature of 40°C while the hydrocarbons are degraded in an integrated bioreactor. Hence, there results a considerable reduction in sludge quantities and a distinct extension to the service life of the degreasing solutions. Sometimes even the degreasing bath itself is used as a bioreactor [3]. However the disadvantages are:
- The biomass is carried into the downstream process baths because the bioreactor and the degreasing bath are not separated adequately or not at all, which also results in a high biomass concentration in the degreasing bath itself (-> adverse effects on operating efficiency) - Presence of potentially pathogenic germs (Pseudomonas aeruginosa and others) in the degreasing and downstream process units (-> low operational reliability) - Efficiency is solely dependent on the biodegradability of the oils (-> low process reliab~ty) In view of these constraints, a technical concept was developed, which, while combining both of the above-mentioned processes, makes use of the advantages but minimises the disadvantages. The process entails a bioreactor with submerged ceramic membranes (MBR) in which the retention and biodegradation of the oils/grease concur. The degreasing solution is routed continuously to the bioreactor where the hydrocarbons are biodegraded while the biomass is completely retained by the membranes. Thus, no biomass is carried into the downstream stages and considerably higher volumetric biodegradation rates (in terms of g hydrocarbons biodegraded per hour and m 3 of bioreactor volume) are achieved due to the increase in the dry matter content (biomass). Since the hydrocarbons are retained to a great extent, difficult-to-degrade oils are also removed from the degresasing solution. A simplified flow diagram is shown in Fig. 1. 2. E x p e r i m e n t a l
2.1. Membranes
Ceramic membranes (0~-A1203) were considered most suitable for the tests conducted because of the expected low fouling tendency for oil/water emulsions owing to their hydrophilic surface properties [1,3]. Multi-channel flat sheet
C. BlUcher et aL /Desalination 162 (2004) 315-326
metal work pieces . . . . . . . . . . .
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further treatment (surface refining)
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Fig. 1. Schematic representation of the concept for process-integrated regeneration of degreasing solutions from surface refining processes in the metal-working industry.
-'---]-4,5mm g
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Fig. 2. Schematic representation of the multi-channel flat sheet ceramic membrane. membranes (Fig. 2) [4], supplied by the Hermsdorf Institute for Technical Ceramics (HITK), Thiiringen, Germany and produced according to the solgel process, were used. The separating layer in these membranes is on the outside while permeate is withdrawn via the permeate channels on the inside. A permeate collector was attached to one side of each membrane which was fitted vertically into the reactor.
Fig. 3. Module unit (top view). The module unit consisted of three levels each containing five membranes (Fig. 3). Air was introduced below the modules to create certain shearrates across the membrane surface (fouling control) and to simultaneously supply oxygen to the biodegradation process. With respect to the separation properties of the membranes under study, the following objectives were aimed at: • Total retention of the biomass • Highest possible retention of the hydrocarbons ° Lowest possible retention of the surfactants Five different membrane types, the properties of which are shown in Table 1, were investigated in membrane screening. For that purpose, the
318
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Table 1 Parameters of the multi-channel fiat sheet ceramic membranes tested Membrane type
K4
Size, mm Support material Active layer material Mean pore size acc. to pore flow method, gm Pure water permeability, L/(rn2 h bar)
100 (width) x 6 (thickness) x 100 (length in lab-scale) Korund (A1203)
H9
H20
H21
(A1203) 1.0
<0.1
0.35
0.45
0.28
5,000
1,300
4,500
4,500
3,000
mobile MBR pilot plant (described below) was used and operated in recirculation mode.
2. 2. On-site operation of the MBR pilot plant The process concept described above was tested on-site in a medium-sized enterprise which carries out electroplating on commission (Sessler Galvanotechnik GmbH). In this case, metal surface refinement includes zinc plating, chromate coating, electropolishing, copper plating. Aprevious study in this company [5] has shown that by switching the treatment in the degreasingbath from hot alkaline degreasing to process-integrated "open" biological regeneration (ENPREP TM
Bioclean process) together with the implementation of a cascade pickling process, a considerable reduction in costs and wastewater quantities could be achieved. However, the afore-mentioned disadvantages of"open" biological regeneration were very pronounced. A mobile pilot plant (Fig. 4), consisting of a bioreactor with loop configuration and a working volume of 45 L was used in the tests. The contents of the reactor were heated to 40°C. The pilot plant provided the possibility ofautomatically interrupting permeate withdrawal within certain intervals as well as chemically cleaning the membranes from the permeate side (CIP- cleaning in place). Transmembrane pressure was applied by pressurizing
)ressurised air
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i~)
C. Bldcher et al. /Desalination 162 (2004) 315-326
the bioreactor with the air which was introduced for aeration. It was controlled by a valve which regulated the air outlet. By this method hydrostatic operation of the membranes was simulated. However, the results can also be applied to operation with a vacuum pump. The pilot plant was run in by-pass operation to the "open" biological regeneration (Fig. 5) which means that it was directly affected by the properties of this process solution, in particular dry matter content (biomass) as well as hydrocarbon and surfactant concentration, entering via the feed pipe. The tests were conducted for a total period of three months. The surfactant mixture ENPREP TM Bioclean BN 22 012 0 produced by Enthone Ltd. and consisting of a combination of non-ionic and cationactive surfactants, was used to emulsify the oils and grease. Since the solution is made up mostly of non-ionic surfactants, which are responsible for the actual degreasing effect, the focus of the study centred on these. Mainly, surfactants were added in the form of so-called Bioclean-Starter solution (target concentration 1.0%), which contains builder substances in addition to the surfactants. The pH in the bioreactor was left unchanged as it resulted naturally from the feed and the biodegradation processes. It ranged from 8.8 to " ........
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319
9.2 during the on-site test operation. No nutrients in addition to those in the feed were added because C:N and C :P ratios in the feed were 14:1 and 12:1, respectively, indicating sufficient nutrient supply. 2.3. Analysis
The dry matter (suspended solids) content was determined as dry filtrate residue according to the German standard method DIN 38 409-H 1-2 prior to which centrifugation was performed over a period often minutes at 16,000 rpm (26,045 x g). The latter was conducted to separate the deemulsified oil phase which had developed during cooling of the sample. Analysis of the total hydrocarbons was conducted in line with the German standard DIN H53. Non-ionic surfactants were analysed using the cuvette test LCK 333 (Dr. Lange). Membrane pore size distribution was determined in line with the American standard method E1294-89 with a PMI capillary flow porometer (Porous Materials Inc.). 3. Results and discussion 3.1. Membrane screening
As already mentioned, the goal for these ceramic membranes was to retain the hydrocarbons P"
'
CIP (membrane cleaning)
Fig. 5. Integration of the MBR pilot plant into the surface refining process with "open" biological regenerationof the degreasing solution.
320
C. BlOeher et al. /Desalination 162 (2004) 315-326
as far as possible while allowing the greatest possible quantity of non-ionic surfactants to pass through the membrane. The five membrane types characterised above (Table 1) were assessed with respect to this goal in membrane screening. As a medium to be filtrated, the degreasing solution containing biomass from a degreasing bath with "open" biological regeneration was used. The permeate generated was re-routed to the bioreactor. The average hydrocarbon and suffactant concentrations in the degreasing solution (i.e. in the bioreactor) was 350 and 500 mg/L respectively, while dry matter content was roughly 0.6 g/L. The permeate generated by all membrane types was free of turbidity, i.e. of biomass and other particulate matter. Retention of hydrocarbons and non-ionic surfactants as a function of operating time is shown in Fig. 6. In general, retention for both parameters increased for all the membranes throughout the operating time. Hydrocarbon retention was mostly above 90%, while surfactant retention was initially about 25% for all membranes but then rose to 50% on average. Neither hydrocarbon nor surfactant retention depended on the average pore size of the membranes. On the contra]% the membrane with the largest nominal pore size (K 4) showed the highest
retention for the surfactants. It can be concluded, that retention is controlled to a great degree by the coating layer which forms during operation, namely by membrane fouling (i.e. adsorption in the pores). This is in line with the observation that flux was also similar for all membranes investigated. Therefore the choice of a membrane type for the on-site operation could be based on other criteria. For reasons of availability and production quality the membrane type H22 was used in the subsequent on-site tests.
3.2. Performance of the membrane bioreactor during the on-site test operation To assess the performance of the MBR during the on-site test operation the relevant parameters were determined in the feed, in the bioreactor solution and in the permeate. In Fig. 7 the timedependent concentrations of hydrocarbons, nonionic surfactants and suspended solids are presented. The feed (that is the process solution from the "open" biological regeneration) was characterised by variations in hydrocarbon as well as surfactant concentration• These ranged from 150 to 600 mg/L and from 150 to 700 rag/L, respectively. Suspended solids content in the feed was mostly
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C. Bldcher et al. /Desalination 162 (2004) 315-326
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around 0.5 g/L, although values as low as 0.2 g/L were observed. As all suspended solids were retained by the membranes, concentration in the bioreactor increased continuously during the test operation up to 16 g/L. On the 71st day of operation, the concentrated biomass solution was withdrawn and the bioreactor was refilled with feed, which resulted again in the afore-mentioned concentration. In general, also hydrocarbon concentration increased during the test run, resulting in a maximum of 4000 mg/L. This means that in this period not all hydrocarbons, that were retained by the mem-
solids
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branes, were biodegraded. However, also decreasing hydrocarbon concentrations were observed at certain times, meaning that the accumulated hydrocarbons were finally biodegraded. As plant throughput was kept nearly constant (1-2 L/h), this behaviour is either due to (i) an increase in feed concentration that was not detected in the samples of the feed, or (ii) varying hydrocarbon composition including difficult-to-degrade fractions. Also, surfactant concentration in the bioreactor increased during the test rtm. However, it was less pronounced, with the maximum concentration being 1000 mg/L. Naturally, hydrocarbon
322
C. Bldcher et aL /Desalination 162 (2004) 315-326
and surfactant concentrations started again at the values of the "open" biological regeneration when the bioreactor was refilled. Despite of the high variations in hydrocarbon and surfactant concentration in the bioreactor, permeate quality was stable during the three months' test operation. Even in the period with the highest hydrocarbon concentration in the bioreactor, the hydrocarbon level in the permeate was below 50 mg/L. To summarise the performance of the process under study, in Table 2 the mean values for the entire process are listed. The reduction rates specified for the hydrocarbons, the non-ionic surfactants and the suspended solids are based on the respective concentrations determined in the feed. Considering the process as a whole, it can be stated that the objective of high hydrocarbon retention as well as low surfactant reduction and total biomass retention was fulfilled. Noteworthy is however, that the hydrocarbon concentration determined in the permeate during the test operation is not identical to the value which would become established as the steady state concentration in the degreasing solution in the process bath, if the process were implemented in full-scale. This is principally determined by the recirculation volume flow in addition to the level of hydrocarbons
entering the system. The higher the flow through the membrane bioreactor, the lower the concentration in the degreasing bath. Although the overall treatment quality was very good, it is necessary to study the two concurring processes (biodegradation as well as the retention by the ceramic membranes used) more closely in order to evaluate the process. 3.2.1. Biodegradation processes in the MBR pilot plant
As was indicated above, for the process design it is necessary for the hydrocarbon load to be equal to the biodegradation rate. To determine the respective biodegradation rates for the hydrocarbons and the non-ionic surfactants in the MBR-based regeneration process, a balance was drawn up for the loads contained in the feed and permeate, taking into consideration also the respective substance concentration in the reactor. In this context, a difference is made between the volumetric biodegradation rate (in terms o f g hydrocarbons biodegraded per hour and m 3 of bioreactor volume) and the specific biodegradation rate (in terms ofg hydrocarbons biodegraded per hour and kg of suspended solids/biomass). Table 3 shows these average biodegradation rates for hydrocarbons and surfactants.
Table 2 Mean treatment efficiency of the MBR during the on-site test operation
Feed Permeate Reduction
Q Hydrocarbons 352 mg/L 42 mg/L 86%
Q Surfactants 455 mg/L 350 rng/L 24%
Q Suspendedsolids (SS) 0.4 g/L Below detection limit >99%
Table 3 Biodegradation rates for hydrocarbons and surfactants in the MBR as determined during the on-site test operation (values calculated as median) Specific biodegradationrate, gbiodegr./(kgSS'd) Volumetricbiodegradationrate, gbio~g/(m3d)
Hydrocarbons 52 226
Surfactants (non-ionic) 27 88
C. Bl6cher et al. /Desalination I62 (2004) 315-326
The specific biodegradation rates were similar to those of the "open" biological regeneration process run in parallel. However, the considerably higher suspended solids content of~3 4.3 g/L compared to the ~ 0.6 g/L in the feed (SS content of the "open" biological regeneration process) led to a higher volumetric biodegradation rate. In view of the fact that the biodegradation rates for the surfactants, in general, were clearly lower than for the hydrocarbons, the treatment objectives (as outlined in the beginning) were fulfilled in this case.
323
increased level of retention. As was shown in Fig. 7, the concentration in the bioreactor increased throughout the operating time. The fact that hydrocarbon concentration in the permeate was, at 45 mg/L on average, very low and did not depend on the bioreactor concentration to any great degree indicates that the bioreactor allows, in terms of process stability, a certain buffering effect for the concentration peaks. However, the increase in surfactant retention has to be judged as a shortcoming. The resulting increase in surfactant concentration in the bioreactor results in higher biodegradation of the surfactants which should be minimised.
3.2. 2. Membrane separation properties Hydrocarbon and surfactant retention throughout the test period is shown in Fig. 8. It is important to keep in mind the difference between the retention (calculation based on the concentration in the bioreactor) and the reduction as defined in Table 2 (calculation based on the feed concentration). As was observed during membrane screening, retention of hydrocarbons as well as surfactants rose during the on-site tests as operating time increased. This confirms the finding that retention is controlled by the coating layer, which means that an increase in membrane fouling concurs with a rise in retention. On the other hand, however, it was already explained that the permeate concentration did not depend on the concentrations in the bioreactor to any great extent. Thus a higher concentration level in the bioreactor results in an
3.2.3. Hydraulic performance of the submerged membranes Besides retention characteristics, any membrane process has to be considered in terms of the flux that is achieved. Fig. 9 shows the hydraulic performance of the membranes during the test period. At times, the net permeate flux stabilised at 9 L/(m2.h) but decreased to approx. 7 L/(m2.h) towards the end of the test operation. Once the reactor had been refilled with the feed, flux started again at 15 L/(mah). Various chemical combinations were tested for CIR Only by combining an oxidising cleaning agent (NaOC1) followed by the Bioclean starter solution (for emulsifying purposes and thus for
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time of membrane operation [d]
56
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Fig. 8. Retention of the ceramic membrane H22 for hydrocarbons and surfactants as a function of operating time during the on-site test operation. (Note that the time scale is different from that in Fig. 7 since only those periods in which the membranes were actually in operation are considered. Periods with no permeate withdrawal, the objective of which was to study biodegradation rates independently, were deducted.)
324
C. Bl&cher et aL /Desalination 162 (2004) 315-326
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removing the hydrocarbons adsorbed in and on the membrane) could a stable membrane flux be achieved (days of operation 19-32). However, this same cleaning procedure did not prevent a decrease in permeate flux after the reactor had been refilled at the end of the test period, although it must be stated that not enough cleaning agent entered the membranes during the cleaning processes at that time. The different permeate fluxes before and after refilling highlight the fact, which is a known characteristic of the filtration of activated sludge from sewage treatment, that the properties of the process solution (in this case the oil/water emulsion containing biomass) have a crucial influence on membrane fouling [6,7]. Compared to biological wastewater treatment, the type of oil/hydrocarbon entering the system, the condition of the emulsions and the surfactants as well as the respective interaction are of significance in addition to the properties of the biomass. Thus the difference in flux cannot be attributed solely to the difference in biomass concentration.
3.3. E c o n o m i c analysis
To evaluate economic efficiency, a treatment unit for the degreasing bath (hydrocarbon input rate 349 g/d) in the electroplating enterprise under
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Fig. 9. Hydraulic performance of the membranes during on-site test operation; operating cycle: 10 min permeation - 1 min interruption of permeate withdrawal, CIP: every 12 h, TMP = transmernbrane pressure.
study was designed. This was based on the following data (a conservative assumption): N e t p e r m e a t e flux
of the membranes Target hydrocarbon concentration in the degreasing bath Hydrocarbon concentration in the permeate Specific biodegradationrate for hydrocarbons (HC) Suspended solids content
7 L/(m2h) 350 mg/L 50 mg/L 30 ~I-IC~oev./(kgSS-d)
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The required recirculation volume flow to be treated was calculated by drawing up a balance of the hydrocarbons for the entire system (degreasing bath-membrane bioreactor). The result is a recirculation volume flow of 43.8 L/h for which a membrane area of 6.3 m 2 is needed. The volume required for the bioreactor results from the abovementioned specific degradation rates and the suspended solids content to 1.3 m 3. Of particular importance for designing the membrane bioreactor is the target hydrocarbon concentration in the degreasing bath. Frequently, industrial enterprises are not able to specify substantiated limit concentrations. However, it is generally valid that, as target hydrocarbon concentration decreases, the recirculation volume flow and also the required membrane area increase
C. Bldcher et aI. /Desalination 162 (2004) 315-326
325
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200 400 600 800 1000 target concentration ofhydrocarbonsinthe degreasing solution[mg/L]
exponentially (Fig. 10). From an economic viewpoint, the hydrocarbon concentration aimed at for the degreasing process should therefore be considered very carefully. An estimate of the investment and operating costs for treating the degreasing solution of an electroplating company using a membrane bioreactor could be drawn up on the basis of this data. Specific treatment costs ore 0.34/m2of metal work-pieces were calculated for the degreasing/ pickling system with regeneration by a membrane bioreactor compared to ~ 0.47/m 2 for hot alkaline degreasing. Depending on the annual throughput of workpieces, payback times will be between 1.8 and 2.8 years.
4. Conclusions
In the process under study, the quality of the treated process solution was determined by the complex interaction of the membrane separation properties and the biodegradation processes. Owing to the relatively short test period and the specified constellation of the test setup (bypass operation to the existing "open" biological regeneration process), the process could not yet be fmally evaluated. Nevertheless, the results obtained in this work are very promising and show that the
Fig. 10. Membrane area required for a MBR-based regeneration of the degreasing solution in dependence on target hydrocarbon concentration and permeate concentration.
regeneration ofdegreasing solutions via membrane bioreactors has significant potential. In contrast to an "open" biological regeneration, also difficult-to-degrade hydrocarbons are separated from the degreasing solution. Furthermore, no particulate impurities are carried into the degreasing and downstream stages, thus resulting in considerable advantages for the entire system: • Since the process solutions contain less impurities, disposal is less frequent and the electrolyres have a longer service life. • The process line components are soiled to a lesser degree and cleaning is therefore less frequent. Considering the fact that the process proved feasible, from both a technical and economic viewpoint, in an electroplating enterprise working on commission where the degreasing baths contain a variety of oils with different compositions often unknown to the company itself, it can be assumed that the process can be implemented in industrial sectors with defined operating conditions (such as the car industry). The tests described above will be continued in a test operation at industrial level which should provide information on the growth behaviour of biomass, the long-term performance of the process as well as a detailed economic analysis.
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Acknowledgements We would like to express our gratitude to the G e r m a n y Federal Ministry for Education and Research (BMBF) for funding this work (FKZ 01 RW0189) as well as the Enthone G m b H for their co-operation.
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