Use of contact tank to enhance denitrification in oxidation ditches

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Pergamon

War. Sci. Tech. Vol. 34, No. 1-2. pp. 195-202. 1996. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 S15·OO + 0-00

PH: S0273-l223(96)005l0-0

USE OF CONTACT TANK TO ENHANCE DENITRIFICATION IN OXIDATION DITCHES X. Hao*, H. J. Doddema** and J. W. van Groenestijn** * Department of Environmental Management, Shanxi Institute ofEconomic Management, Taiyuan, Shanxi 030006. P.R.C. ** Department of Environmental Biotechnology, TNO Institute of Environmental Sciences, P.O. Box 6011, 2600 JA Delft, The Netherlands ABSTRACT Poor denitrification in a Pasveeer oxidation ditch is attributed to a lack of carbon sources available in the anoxic zone as it is essential to maintain a high CIN ratio for denitrification. Influent of sewage directly into the anoxic zone is not useful to maintain a high C/N ratio. The adsorptive capacity of activated sludge can rapidly increase the CIN ratio. Similar to a contact-stabilization process, a contact tank can be combined with the Pasveer ditch; it provides contact time (zone) between raw sewage and return sludge before entering the ditch. In principle, insoluble organic substrate can be easily adsorbed onto the floc surfaces and enmeshed in the floc structure at a short retention time. After the contact. mixed influent is introduced into the anoxic zone. As a result. a high CIN ratio is obtained which enhances denitrification. Using this set up, the Pasveer ditch was operated. The experimental results show that the efficiency of denitrification has been enhanced from 45 to 83% for NO-3·N removal. The corresponding denitrification capacity of the sludge is increased by 240%. The contact tank has also the same principle as a 'selector' to control bulking sludge caused by filamentous bacteria. The SVI data and microscopic examination indicated improved settleability of the sludge. Further enhancement of denitrification needs an exact control of the dissolved oxygen level in the ditch and/or a concentration increase of denitrifying microorganisms. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS Adsorption; anoxic zone; CIN ratio; contact tank; denitrification; oxidation ditch; selector; sludge. INTRODUCTION Under international agreements from the Rhine Action Programme (RAP) and the North Sea Action Programme (NSAP), the Dutch authorities have taken actions to reduce the quantity of phosphate and l1itrogen discharged onto surface waters by 50% in 1995 and by 70% in 2000. In practice, the effluent ;;tandard for total nitrogen will be no higher than 10-15 mg I-I, depending on the capacity of treatment plants. The stricter rules have resulted in new developments for nitrogen removal. [n the Netherlands, nearly 40% of wastewater treatment plants is based on the principle of oxidation ditches :with extremely low loaded completely mixed systems, such as Pasveer ditches, Carrousels, Schreiber ,lants, etc.). Furthermore, the oxidation ditch processes have been or will be applicable for such developing 195

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countries as China because of their simplicity and economy to construct and operate. Therefore. enhancement of denitrification in oxidation ditches will, to a large extent, be useful for both existing and new treatment plants. On the basis of an extensive investigation on denitrification in a Pasveer ditch. some technical possibilities to enhance denitrification have been raised (Hao et al., 1995). One of the possibilities is to incorporate a contact tank to the Pasveer ditch to enmesh organic substances for denitrification ahead of carbonaceous oxidation. Similar contact tanks are presently used as 'selector' to control bulking sludge (Eikelboom, 1991). In this paper, the profile of denitrification during the investigation is briefly described first. The emphasis is put on the principle and operational mode of the contact tank. Finally, the paper presents the results of experimental research on site. PROFILE OF DENITRIFICATION IN THE PASVEER DITCH Original flow sheet The original flow sheet of the Pasveer ditch is shown in Figure 1. The design volume of the ditch is 150 m3 with an average recirculation length of 68.5 m and a depth of 1 m. The cross section of the ditch is trapezoid with the top and bottom widths of 3.6 m and 1.2 m respectively. The aerating brush is a typically horizontal steel-angle rotor (Spaans) with a diameter and length of 0.7 m and 1.5 m respectively. The originally designed capacity of the ditch was 500 p.e.; the practical capacity is about 400 p.e.

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Investigating method and results The investigation was based on daily analyses and measurements on influent, effluent and sludge characteristics, such as COD, KfN, NH+ 4 -N, NO-rN, NO- 3-N, SS, pH, MLSSIMLVSS, SVI, OUR and DO. There were generally an aerobic zone and an anoxic zone over the length of the Pasveer ditch. In general, the first half-loop of the ditch (A~B~C) was aerobic, and the second half-loop (Location C 0 A) was anoxic. Carbonaceous oxidation, nitrification and denitrification took place respectively in the aerobic and anoxic zones. Total nitrogen data in influent and effluent are plotted Figure 2. The corresponding efficiency of nitrogen removal is shown in Figure 3. Nitrogen removal was unstable and poor; the average efficiency of nitrogen removal was about 45%.

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According to the original flow sheet and working conditions of the Pasveer ditch, one of the major reasons for poor denitrification is the lack of carbon sources available in the anoxic zone. At the original influent location, it is supposed that readily biodegradable COD can be oxidized at a high rate with molecular oxygen (02) when it passes the brush. Slowly biodegradable COD adsorbed by floes might be the only carbon sources available to denitrification. It was concluded that the availability of organic substances might be a key factor for denitrification (Hao et al., 1995). 120 110 100 90 r. tTl

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PRINCIPLE AND EXPERIMENT OF A CONTACT TANK INCORPORATED Principle The change of the influent location into anoxic zones seems to be conducive to denitrification. In oxidation ditches, however, influent substrate is easily diluted with the total content of mixed liquor and high recirculation flow rate, that is, there is a dilution factor. For example, the recirculation flow rate in the Pasveer ditch is usually 1,000 times higher than the influent flow rate, which means that a high CIN ratio for denitrification can not be maintained even though the influent location is at the anoxic zone. The adsorptive capacity of activated sludge may weaken the effect of the dilution on the CIN ratio. If raw sewage and return sludge are mixed in a tank/zone before entering the ditch, a certain percentage of organic substrate can be almost instantaneously adsorbed onto the floc surfaces and enmeshed in the floc structure. This is a physico-chemical process, independent of oxygen concentration in mixed liquor, and so aeration is unnecessary for the contact tank/zone and should be avoided. It is expected that a great extent of insoluble/particulate organic substances can be adsorbed onto floes in the contact tank/zone in a short time. When the mixed influent from the contact tank/zone enters the ditch, the adsorbed substrate is no longer available to the total content of mixed liquor. As a result, the effect of the dilution factor may be reduced. Similar to the contact-stabilization process, the contact tank/zone can be either established separately (tank) or combined within the ditch (zone). The contact tank/zone provides contact between biomass (return sludge from the settler) and substrate. It operates at a short retention time, sufficient only for the transfer of substrate from the liquid to the solid phase. The admixed influent from the contact tank is introduced directly into anoxic zones. In this way, soluble organic substrate can be used for denitrification in the contact tank/zone at a relatively constant rate if any nitrate/nitrite exists in return sludge and/or raw sewage. Furthermore, denitrification with readily biodegradable substrate is a fast process and will be completed within 15 to 30 minutes (Haandel et al., 1981). For a short contact time, organic substrate has been either transferred into the cells of bacteria (soluble substrate) or adsorbed onto the floc surfaces and enmeshed in the floc structure (insoluble substrate). When the mixed influent enters the anoxic zones, denitrification is supposed to occur effectively at a relatively high CIN ratio. The incorporation of the contact tank/zone to oxidation ditches is also quite similar to an anoxic/anaerobic 'selector' to control bulking sludge caused by filamentous bacteria. During the last decades, various biological remedial/preventive strategies to control bulking sludge have been developed. They are based on the following principle: create conditions which allow the flocforming bacteria to take up a great percentage of the available substrate. One effective way to accomplish this is to maintain anoxic or anaerobic conditions until the bulk of the substrate is taken up by biomass. It has been ascertained that the competition between flocforming and filamentous bacteria is strongly affected by the substrate concentration during admixing of raw sewage and return sludge (Houtmeyers et al., 1980; Chambers and Tomlinson, 1982; van den Eynde et al., 1983; Jenkins et al., 1984; van Niekerk, 1985; Chudoba, 1985). In such completely mixed systems as oxidation ditches, influent is diluted with the total content of mixed liquor. The substrate concentration is low as well, i.e., equals that in the final effluent. Such conditions are beneficial to microorganisms which still grow relatively well at carbon-limited conditions. Therefore, bulking frequently occurs in completely mixed systems. Under the circumstance, the operational mode with a separate 'selector' should be selective for floc forming bacteria. It has been concluded that the rapid adsorption of insoluble/particulate compounds to flocs is sometimes sufficient already for controlling so-called low FIM microorganisms (Jenkins et al., 1984).

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New operational mode Based on the above principle, a new operational mode of the Pasveer ditch is shown in Figure 4. A circle tank is used as the contact tank. Influent (2) and return sludge (5) are introduced simultaneously in the contact tank. With a submerged mixer in the contact tank, influent and return sludge are fully mixed. The mixed influent (7) is drawn into the anoxic zone (near location C). 7

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Figure 4. New operational mode of the Pasveer ditch.

In our experiment, the hydraulic retention time (HRT) in the contact tank was designed to be 30 minutes. Influent flow rate and return sludge rate were 2.5 m 3/h and 3.75 m 3/h respectively; the return ratio was 1: 1.5. Experiment The experiment under the new operational mode was conducted from June to October 1994. Because carbonaceous oxidation and nitrification were excellent in the Pasveer ditch (Hao et al., 1995), the emphasis during the experiment was put on denitrification. KfN, NH+ 4- N, NO- 3-N and NO-TN were analyzed frequently. As a judgement to bulking sludge, MLSSIMLVSS and SVI were also measured. Besides that, microscopic examination was regularly done to determine filamentous index (FI). 120 110

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RESULTS AND DISCUSSION Enhanced denitrification The experimental results on denitrification are shown in Figures 5 and 6. The experiment began on day 28, Before day 28, there are four sets of experimental data under the original operational mode. Roughly, the efficiency of nitrogen removal has been almost doubled. An average efficiency of nitrogen removal is about 83%. As shown in Figure 5, the influent concentration of total nitrogen was high, but the efficiency of nitrogen removal was always kept at a high level. This means that the capacity of denitrification has been enhanced to a large extent. 1.1

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The rate of denitrification is shown in Figure 7. As compared with a corresponding value of 0.18 mg N Ig MLSS·h under the original operational mode, an average rate of denitrification of 0.61 mg N Ig MLSS.h was obtained in the new operational mode. In other words, the capacity for denitrification had increased by about 240%. Nevertheless, it could also be observed (Figure 5) that the concentration of total nitrogen in effluent was still high. An average value of total nitrogen in effluent was 16 mg I-I under the new operational mode. Though lower than the corresponding value of 36 mg 1-1 under the originally operational mode, this value is still higher than the new effluent standard of 10 or 15 mg I-I. This is attributed to daily variations of DO level in the Pasveer ditch. Because of the lack of control for DO, the size of the anoxic zone was variable. Occasionally, no anoxic zone existed. For this reason, a stable anoxic zone is essential for further enhancement of denitrification. Improved settleability The settling characteristics of sludge found under the original operational mode were not good/stable. During an investigation period of 10 months, bulking sludge caused by filamentous bacteria occurred three times. When bulking sludge appeared, SVI data (sludge volume index) were often over 200 mllg and even exceeded 300 mllg (just before day 28 in Figure 8). An average SVI of 160 mllg during the investigation means that the settling characteristics of sludge under the original operational mode were not good. Improvements in settleability came about when the Pasveer ditch operated under the new mode. As shown in Figure 8, SVI data from the beginning of the experiment (day 28) tend to stability. An average SVI of 120 mllg in an experimental period of almost 5 months shows a better settleability. This also shows that a relatively high FIM ratio in the early stage of mixing minimizes the chance of developing a poorly-settling sludge. Microscopic examination revealed only small numbers of filamentous bacteria. The average PI (filament index) was about 1.5. 350

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The mixing of raw sewage with return sludge before entering the ditch increased the substrate gradient and efficiently imposed a greater plug-flow character on the system. So improvements in settleability of sludge came about as expected.

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CONCLUSIONS With a contact tank incorporated to a Pasveer oxidation ditch, the average efficiency of denitrification can be enhanced from 45% to 83%. The capacity of denitrification has been also increased by about 240%. Improvements on settleability of sludge were obvious under the new operational mode. The a~erage SVI ha! been reduced from 160 ml/g to 120 ml/g. The average FI was 1.5 during an experimental penod of almost 5 months, and bulking sludge did not occur in the experiment. The technical idea of the contact tank is based on the adsorptive ability of activated sludge. In the contact tank, insoluble organic substrate can be adsorbed onto the floc surfaces and enmeshed in the floc structure at a short retention time with biomass before entering the ditch. The contact tank has the same principle as a 'selector' to control bulking sludge caused by filamentous bacteria. The improved settlement of sludge is due to the maintenance of a gradient in substrate concentration which is detrimental to the growth of filamentous bacteria. The contact tank incorporated in oxidation ditches can be used to enhance denitrification and to control bulking sludge. Further enhancement of denitrification needs a control of DO level in the ditch. ACKNOWLEDGEMENT The experiment is related in a project of joint research between Dutch and Chinese institutions, which is financially supported by the Royal Dutch Academy of Sciences (KNAW) and the Netherlands Organization for Applied Scientific Research (TNO). The authors thank Dr B. Etty (KNAW, The Netherlands), Prof. W. Harder (TNO, The Netherlands), Prof. B. Wang (HUA, China) and Prof. Z. Feng (SIEM, China) for their concern and support. Mr K. Oskam and Mr H. Monfils kindly gave their technical assistance in the adjustment of the original operational mode at site. The authors also wish to thank them for their contributions to the experiment. REFERENCES Chambers, B. and Tomlinson, E. J. (1982). Bulking of activated sludge. Preventative and remedial methods. Ellis Horwood Publ., Chichester. Chudoba,1. (1985). Control of activated sludge filamentous bulking - VI. Formulation of basic principles. Water Res., 19, 1017• 1022. Eikelboom, D. H. (1991). The role of competition between flocforming and filamentous bacteria in bulking of activated sludge. Proc. Int. Biological Approach to Sewage treatmem Process: Current Status and Perspectives, Perugia. Italy, 143-149. Eynde. E. van den. Geerts, J.. Maes. B. and Verachtert, H. (1983). Influent of the feeding pattern on the glucose metabolism of Arthrobacter sp. and Sphaerotilus natans; Growth in chemostat culture, simulating activated sludge bulking. European J. Appl. Microbiol., 17,35-43. Haandel. A. C. van, Ekama, G. A. and Marais, G. v. R. (1981). The activated sludge process-3: single sludge denitrification. Water Res., 15. 1135-1152. Hao, X.. Doddema, H. J. and Groenestijn, J. W. van (1995). Investigation of simultaneous denitrification and nitrification in a Pasveer oxidation ditch. Environm. Tech., accepted for publication. Houtmeyers. J., Eynde. E. van den. Poffe, R. and Verachtert, H. (1980). Relations between substrate feeding pattern and development of filamentous bacteria in activated sludge processes. Part I. Influence of process parameters. Eur. 1. Microb. Biotechnol. 9,63-77. Jenkins. D., Richards, M. G. and Neethling, J. B. (1984). Causes and control of activated sludge bulking. J. Water Pollut. Contr., 83.455-472. Niekerk. A. van (1985). Competitive growth of flocculant and filamentous microorganisms in activated sludge systems. Thesis, Univ. of California, Berkeley.