Biological removal of EDTA in conventional activated-sludge plants operated under alkaline conditions

Bioresource Technology 59 (1997) 151-155 © 1997 Elsevier Science Limited All rights reserved. Printed in Great Britain 0960-8524/97 $17.00 ELSEVIER

PII:S0960-8524(96)O0158-7

BIOLOGICAL REMOVAL OF EDTA IN CONVENTIONAL ACTIVATED-SLUDGE PLANTS OPERATED U N D E R ALKALINE CONDITIONS C. G. van Ginkel, K. L. Vandenbroucke & C. A. Stroo Akzo Nobel Central Research Arnhem, GeneralAnalytical and Environmental Chemistry Department, Velperweg 76, 6800 SB Arnhem, The Netherlands

(Received 26 September 1996; revised version received 6 November 1996; accepted 16 November 1996) As EDTA is water soluble and not volatile, it is released with wastewater streams after use. EDTA pollution has become a cause of concern in recent years, so that both chemical, physical and biological techniques for reducing EDTA in industrial wastewater have attracted a great deal of attention. So far attempts to improve chemical and physical methods have not resulted in an economically feasible process. In conventional biological wastewater-treatment plants EDTA is neither degraded by microorganisms nor adsorbed onto activated sludge (Gardiner, 1976; Alder et al., 1990; Pavlostathis & Morrison, 1994; Saunam/iki, 1995; Kari & Giger, 1996). Similarly, no biodegradation has been detected in OECD tests (van Ginkel & Stroo, 1992; Wolf & Gilbert, 1992). However, N6rtemann (1992) obtained a pure culture which could quantitatively utilize EDTA as a sole source of carbon and energy for growth. EDTA removal has also been demonstrated in a bioreactor with immobilized microorganisms maintained at very high pH values (Gschwind, 1992). These findings point to the possibility of removing EDTA biologically in conventional activated-sludge plants. This paper describes the invention of an activated-sludge process operated under moderate alkaline conditions in which EDTA is removed. Removal of EDTA was investigated in closed-bottle tests, semicontinuous activated-sludge units and a full-scale activated-sludge plant. Although this study focused on EDTA, additional parameters, such as nonpurgeable organic carbon (NPOC), COD, BOD and nitrate, were also evaluated.

Abstract

Semicontinuous activated-sludge (SCAS) units inoculated with activated sludge from a plant treating predominantly domestic wastewater were used to study EDTA removal from wastewater at different pH values. Almost complete removal of EDTA was found in units maintained at pH>8.0, but high EDTA removal was obtained only in SCAS units run with sludge retention times >_12 days. A maximum EDTA removal rate of 0"2 kg EDTA/m3/day was achieved in the SCAS units. A full-scale activated-sludge system operating under alkaline conditions provided high removal percentages, confirming the removal of EDTA in activated-sludge systems as established in the SCAS tests. The results in both laboratory and full-scale activated-sludge units showed that a reduction of more than 90% of the incoming EDTA was attainable. Ratios of biological oxygen demand to theoretical oxygen demand of > 0.6 in closed-bottle tests, inoculated with either sludge from a full-scale treatment plant or an SCAS unit, proved that EDTA was removed by biodegradation. These results clearly demonstrated that EDTA-containing wastewater is amenable to activated-sludge treatment in conventional plants operated under alkaline conditions. © 1997 Elsevier Science Ltd. Key words: EDTA, SCAS test, closed-bottle test, biological oxidation, nitrate formation, activated sludge, full-scale plant.

INTRODUCTION Ethylenediaminetetraacetic acid (EDTA) is an important chelating agent. This compound is used in the pulp and paper industry to stabilize the action of hydrogen peroxide on pulp by complexing with metals that catalyze the decomposition of hydrogen peroxide. In the photographic industry EDTA is used to prevent precipitation of, for instance, carbonates onto the photosensitive layer, and in industrial and institutional cleaners it prevents precipitation of calcium and magnesium.

METHODS Chemicals

Ethylenediaminetetraacetic acid (Dissolvine®) was obtained from Al~zo Nobel Chemicals, BU Functional Chemicals, Amersfoort, The Netherlands. Potassium hydrogen phthalate, batch No. B18099, 151

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used as standard in the nonpurgeable organic carbon (NPOC) determination was obtained from J.T. Baker BV, Deventer, The Netherlands. All other chemicals used were of reagent-grade quality. Inoculum and settled sewage Secondary activated sludge and primary settled sewage water were collected from the wastewater treatment plant (WWTP) Nieuwgraaf in Duiven, The Netherlands. WWTP Nieuwgraaf is an activated-sludge plant predominantly treating domestic sewage. The primary settled sewage was collected weekly and stored at -20°C until required, then 150ml of secondary activated-sludge, containing approximately 2 g of suspended solids (SS) per litre, was used as an inoculum for each unit. Analyses EDTA was determined by isocratic high-performance liquid chromatography (HPLC). The HPLC system consisted of a pump model 660E (Waters Associates, Etten Leur, The Netherlands) a LiChrosphere 60 RP select B 5/2 (250 x 4.0 mm) column (Merck, Darmstadt, Germany), and a UV detector model 9945 (Waters Associates, Etten Leur, The Netherlands). The concentration of EDTA was measured at a wavelength of 260 nm. The mobile phase was 10% methanol (v/v) in demineralized water containing 25 mr,4 sodium acetate and 10 mM tetrabutylammonium bromide. This solution was filtered and degassed prior to use. The flow rate was 1 ml/min. Samples (50/21) were injected after sludge was removed by filtration through a 1.2 pm filter. The nonpurgeable organic carbon (NPOC) contents were determined using a TOC analyzer (Shimadzu Corporation, Kyoto, Japan). Samples from the influent and effluent streams were passed through a membrane filter (8/am pore diameter) prior to analysis. Samples were acidified prior to injection in the TOC apparatus. NPOC was determined in triplicate. The standard deviation of NPOC analyses was always less than 5%. Dissolved oxygen was monitored with a WTW OXI 530 meter with a WTW Trioxmatic EO 200 probe (Retch BV, Ochten, The Netherlands). Nitrate was measured colorimetrically as 4-nitro2,6-dimethylphenol using the method described by Hartley and Asai (1963). Chemical oxygen demand (COD) was determined using the method of Jirka and Carter (1975). The pH of the supernatant liquors was determined with a microcomputer pH meter Consort P207 (Consort, Turnhout, Belgium). Suspended solids (SS) concentrations in the SCAS units were measured by filtering samples through a preweighed 12/am pore size (cellulose nitrate) membrane filter (Schleicher and Schiill, Darmstadt, Germany), drying the filter at 105°C overnight and measuring the weight increase.

Closed-bottle test The closed-bottle test was performed in accordance with Organization for Economic Co-operation and Development (1992) with some minor modifications. Activated sludge diluted with mineral salts medium to a concentration of 2 mg SS/I in the bottles was used as inoculum. Ammonium chloride was omitted from the medium to prevent nitrification. The biochemical oxygen demand (BOD) was determined in triplicate in control and test bottles using a special funnel to prevent spillage of the medium during the determination of the oxygen concentration (van Ginkel & Stroo, 1992). The triplicate analyses did not vary by more than 10%. Biodegradation was calculated as the ratio of biological oxygen demand (BOD) to theoretical oxygen demand (ThOD) of EDTA. SCAS test In the first phase of the study a series of SCAS tests was performed in accordance with the Organization for Economic Co-operation and Development (1981). In general SCAS units were operated in a periodic cycle with two discrete time periods. These consisted of an aeration period followed by a settling period without aeration. The SCAS units were operated in a 24 h cycle with 23 h aeration time and 1 h settling time. The units were operated under varying pH by adding sodium hydroxide to the wastewater. The influence of the pH on EDTA removal was further evaluated by operating two separate units. Once these SCAS units reached steady-state kinetics for EDTA degradation, one unit was subjected to a pH shock. For these experiments, units were run at an EDTA influent concentration of 65 mg/l so that differences in performance could be ascertained. A maximum removal rate was assessed by feeding 200 instead of 65 mg I/EDTA. Knowledge about the effect of sludge retention time (SRT) was obtained by operating SCAS units at different SRT. The SCAS units were operated at SRT of 6, 12 and 30 days by wasting sludge once a day. Glucose (500 mg/1) and ammonium chloride (50 mg/l) were added to the primary, settled, domestic wastewater to simulate highly loaded treatment plants. The units were brought to a steady-state condition for the desired SRT and, over a period of five consecutive days, NPOC, COD, EDTA and NO3-N were measured. Full-scale plant In the second phase EDTA removal in a full-scale plant was monitored. Operating characteristics of the system were also monitored with respect to pH and COD. The full-scale plant consisted of an equalization tank, a plug-flow aeration tank and a settler. From the settler, sludge was continuously recycled to the aeration tank. The hydraulic retention time and the sludge retention time of this plant were 24 h and 20 days, respectively.

Removal of EDTA by activated sludge 100

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Fig. 1. Removal by biodegradation of EDTA (65 mg/l) added to domestic wastewater in SCAS units run at pH 6.5 (u) and 8.5 ([])

RESULTS AND DISCUSSION

An SCAS test is an assay for biodegradability that also allows prediction of the behaviour of chemicals in wastewater-treatment plants. Activated sludge in an SCAS test operated at pH 7.0 failed to degrade EDTA (Fig. 1), even after an incubation of 6 months, which is in accordance with literature on the biodegradability of EDTA (van Ginkel & Stroo, 1992; Wolf & Gilbert, 1992). Because of the low biodegradability found in these experiments and because EDTA does not adsorb onto sludge, no removal of EDTA would be expected in activatedsludge plants. Indeed EDTA has been reported to pass through treatment plants without notable degradation (Alder et al., 1990; Pavlostathis & Morrisen, 1994; Saunam/iki, 1995; Kari & Giger, 1996). In contrast, extensive removal of nonpurgeable organic carbon (NPOC) from EDTA was achieved with SCAS units fed with 65 mg 1/EDTA operated at pH 8.0 to 9.0. Three weeks after start-up, the NPOC values dropped and reached the NPOC values of the control unit without EDTA additions. From 4 weeks onwards, up to 100% of the EDTA carbon was removed in this SCAS unit (Fig. 1). After 4 weeks the effluent EDTA concentration was below 2 mg/1, while the effluent BeD7 was consistently below 1 mg/1 from the start. Concurrently with the removal of EDTA, additional formation of 5.6 mg/1 NO3-N was detected. The mass balance on nitrogen showed a discrepancy of approximately 10% between the theoretical and measured values, with the theoretical evolution of nitrate being higher than that measured. The discrepancy is attributed to the use of nitrogen for the synthesis of biomass. The formation of nitrate in the SCAS unit fed with EDTA is additional evidence of mineralization of EDTA. In order to confirm the biodegradation of EDTA, closed-bottle tests were carried out with sludge originating from this SCAS unit. The closed-bottle tests were conducted at a pH which ranged from 8.0 to 8.5.

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These alkaline conditions were achieved by adding NaOH to the closed-bottle test medium. The lag period was only a few days. The time to reach a ratio 0"6 was 3 weeks after initiation of detectable biodegradation. EDTA was not degraded at pH 7 in a closed-bottle test inoculated with pre-exposed sludge from the SCAS unit operated under alkaline conditions. Once EDTA removal in the SCAS unit receiving domestic wastewater fed with 65 mg/l EDTA was >90%, EDTA loadings were increased by raising EDTA concentration up to 200 mg/1. In this way maximum degradation rates of 0.2 kg EDTA/m3/day were achieved at the end of the test period. The wastewaters treated in the SCAS units covered a wide range of EDTA concentrations from 5 to 200 mg/l. An additional experiment was set up to evaluate the influence of the pH on the removal of EDTA in SCAS units which already degraded EDTA. On day 0 the pH was lowered to 6.5, while all other operating conditions remained unchanged. The influence of pH on EDTA removal was straightforward. The degradative ability was not retained at pH 6.5 (Fig.

2). In the SCAS test performed according to the OECD Guidelines sludge is not deliberately discarded so that very high sludge retention times (SRT) are maintained. To investigate the effect of SRT on removal efficiency, various amounts of sludge were removed from the units. The steadystate data for the influent, effluent and reactor performances are summarized in Table 1. EDTA removal percentages in excess of 90 were consistently achieved for SRT>12days (Table 1; Fig. 3). Additional analysis of the effluent of the unit maintained at an SRT of 6 days demonstrated that the amount of NPOC removed from the system was comparable to EDTA removal calculated with HPLC data, confirming that partial degradation of 75

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Fig. 2. EDTA concentrations in the effluent of SCAS units removing 65 mg/l EDTA upon lowering the pH to 6.5 on day 0 (-). The pH was maintained at 8.1 in a control unit (D) throughout the test.

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Table 1. Process parameters for SCAS units maintained at various SRT and concentrations of EDTA, NPOC, COD and NOa-N present in the influent and effluent of the units. The SCAS units were operated under alkaline conditions (pH 8.0-9.0). The concentrations of EDTA, NPOC, COD and NOa-N present in the influent were 65, 220, 710 and < I mg/I, respectively

Sludge retention time

24 days

Suspended solids (SS) (g/l) Organic load (g COD/g SS day) Effluent COD (mg/l) Removal COD (%) Effluent EDTA (mg/1) Removal EDTA (%) Effluent NPOC (mg/l) Effluent NO3-N (mg/l)

2.7

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0.24 23.5 96 2.8 95 12.1 10.4

0.39 84.5 88 44.6 31 34.8 2.4

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Fig. 3. Steady-state removal of EDTA in SCAS units

maintained at SRT of 24 days (t~), 12 days (I) and 6 days

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0.18 25.3 96 5.8 91 11.3 9.8

E D T A occurred. In the units maintained at S R T > 12 days, the nitrate levels increased compared to the unit maintained at an SRT of 6 days. The data also show that nitrate was released into the solution, as EDTA was consumed by the microorganisms. Finally, Table 1 demonstrates that the sludge degraded other constituents of domestic wastewater at alkaline pH, which is a prerequisite for practical application in wastewater-treatment plants. Based upon the success of the SCAS tests, the performance of a full-scale treatment plant treating wastewater from a dairy plant was monitored. The dairy wastewater contained approximately 30 mg/l EDTA. Measured influent concentrations of EDTA in the treatment plant corresponded well with the projected concentration based on usage and accounting for the flows. The dairy wastewater was treated at a hydraulic residence time of 1 day and a sludge retention time of 20 days. The temperature varied between 25 and 31°C. The pH of the dairy plant effluent was 8.5-9.0 but fell in the aeration tank. In this tank the pH varied between 7.5 and 8.0. A removal of > 90% was found at the start of the monitoring program. The removal decreased to approximately 80% during the

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12 days

Table 2. Concentrations of EDTA in mg/I in the influent and effluent of a full-scale activated-sludge plant. The pH in the aeration tank of the treatment plant varied between 7.5 and 8.0

Date 28 August 1995 29 August 1995 30 August 1995 31 August 1995 1 September 1995 2-3 March 1995 Mean

Influent, mg/l

Effluent, mg/l

Removal, %

25 34 36 35 34 32 33

2 2 2 5 8 8 4

94 95 95 87 82 78 89

monitoring study (Table 2). The reduction in the removal efficiency probably reflects the E D T A use, which had increased during the test period. In the full-scale treatment, a large fraction of the incoming COD was removed. The COD reduction of the unfiltered samples was 90%. It is necessary in an activated-sludge plant to verify not only that EDTA is removed from the dairy wastewater but also that that removal is brought about by mineralization. To confirm that E D T A removal in the full-scale activated-sludge plant was due to mineralization, closed-bottle tests inoculated with sludge from the full-scale plant were performed. Owing to the presence of acclimatized microorganisms in the inoculum, a short lag period was detected at pH 8.1 (Fig. 4). At pH 6.7 no oxygen consumption was measured. These closed-bottle tests clearly illustrate that microorganisms utilize E D T A as a carbon and energy source under alkaline conditions. The fact that E D T A can act as the sole source of carbon and energy indicates that no additional measures, such as the addition of co-substrates or growth factors, are necessary to enable EDTA degradation in activated-sludge systems. The biodegradation observed in the SCAS and closed-bottle tests demonstrates that mixed microbial cultures will degrade E D T A at alkaline pH. The reason for this phenomenon is unknown. The observed effect of the pH on E D T A biodegradation may be explained by the presence of different

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REFERENCES

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Alder, A. G., Siegrist, H., Gujer, W. & Giger, W. (1990). Behaviour of NTA and EDTA in biological wastewater treatment. WaterRes., 24, 733-742. Gardiner, J. (1992). Complexation of trace metals by EDTA in natural waters. Water Res., 10, 507-514. Ginkel, C. G. van & Stroo, C. A. (1992). Simple method to prolong the closed-bottle test for the determination of the inherent biodegradability. Ecotoxicol Environ. Saf., 24, 319-327. Gschwind, N. (1992). Biologischer Abbau von EDTA in einem Modellabwasser. Wasser Abwasser, 10 (133), 546-549. Hartley, A. M. & Asai, R. J. (1963). Spectrophotometric determination of nitrate with 2,6-xylenol reagent. Analyt. Chem., 35, 1207-1213. Henneken, L., N6rtemann, B. & Hempel, D. C. (1995). Influence of physiological conditions on EDTA degradation. Appl. Microbiol Biotechnol., 44, 190-197. Jirka, A. M. & Carter, M. J. (1975). Micro semi-automated analysis of surface and wastewaters for chemical oxygen demand. Anal Chem., 47, 1397-1402. Karl, F. G. & Giger, W. (1996). Specification and fate of ethylenediamine tetraacetate (EDTA) in municipal wastewater treatment. Water Res., 30, 122-134. Madsen, E. L. & Alexander, M. (1985). Effects of chemical speciation on the mineralization of organic compounds by microorganisms. Appl. Environ. Microbiol., 50, 342-349. N6rtemann, B. (1992). Total degradation of EDTA by mixed cultures and a bacterial isolate. Appl. Environ. Microbiol., 58, 671-676. Organization for Economic Co-operation and Development (1992). Guidelines for testing of chemicals, degradation and accumulation, ready biodegradability, Guideline 301. Organization for Economic Co-operation and Development, Paris, France. Organization for Economic Co-operation and Development (1981). Guidelines for testing chemicals, degradation and accumulation, inherent biodegradability; modified SCAS test Guideline 302 A. Organization for Economic Co-operation and Development, Paris, France. Pavlostatis, S. A. & Morrison, D. (1994). Aerobic biodegradation potential of photoprocessing wastewaters. Water Environ. Res., 66, 211-219. Saunam/iki, R. (1995). Treatability of wastewaters from totally chlorine-free bleaching. Tappi J., 78, 185-192. Wolf, K. & Gilbert, P. A. (1992). EDTA-ethylenediaminetetraacetic acid. In The Handbook of Environmental Chemistry, Volume 3, Part F. Anthropogenic Compounds, Detergents, ed. N. de Oude. Springer, Berlin, pp. 243-259.

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Fig. 4. Biodegradation of EDTA in closed-bottle tests inoculated with sludge from an activated sludge plant treating dairy wastewater containing approximately 30 mg/ 1 EDTA. Biodegradation is expressed as the ratio of the biological oxygen demand to the theoretical oxygen demand of EDTA. The pH in the closed-bottle tests was 6"7 (n) and 8"1 (B).

E D T A species. E D T A present in a solution can have various species, depending on the concentrations and types of metal ions present and the pH. Effects of metal ions on the decomposition of E D T A and NTA, another aminocarboxylate, have been described (Madsen & Alexander, 1985; Henneken et al., 1995). In conclusion, E D T A removal occurs only in SCAS units and full-scale plants maintained at p H > 8 . 0 . EDTA-containing wastewater can therefore be treated in a conventional activated-sludge system operated under alkaline conditions. From the results reported here it is apparent that the observed E D T A removal can be explained by biological oxidation.

ACKNOWLEDGEMENTS The research described in this article was funded by Akzo Nobel's Business Unit Functional Chemicals. The monitoring study was performed by IMd Micon, Barneveld, The Netherlands. We also acknowledge S. Hartmans.