BIOLOGICAL NITROGEN REMOVAL FOR LONG-TERM LANDFILL LEACHATE BY USING MLE PROCES

This study was conducted to investigate the effects of hydraulic loading rate and recycle ratio on biological denitrification and nitrification for leachate containing NH4 +-N with high concentration about 1,500~2,000 mg/L discharged from SUDOKWON landfill site. Pilot-scale MLE(modified ludzack ettinger) process was employed in this study. As a result of this examination, we found out that about 2.3 days in denitrification tank and 5.7 days in nitrification tank are the optimal HRT for obtaining the removal efficiency of about 80 % for T-N and 99% for NH4 +-N at the conditions of recycle ratio of about 600 % and BOD/NH4 +-N ratio of about 3.0. In addition, optimal recycle ratio for obtaining the maximum nitrogen removal efficiency while keeping proper microbes concentration in nitrification and denitrification tank was 200 % for external recycle and about 400% for internal recycle. The maximum removal rates for each load of T-N and NH4 +-N were 0.055kgT-N/kgVSS/d and 0.07kgNH4 +-N/kgVSS/d, respectively. The ratio of alkalinity consumed per T-N removed in this process (△alkalinity/△T-N) was about 5.0.

Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 155 - BIOLOGICAL NITROGEN REMOVAL FOR LONG-TERM LANDFILL LEACHATE BY USING MLE PROCESS 1 Cho-Hee Yoon † , Seung-Hyun Kim and Jong-Choul Won* Kyungnam University, #449 Wolyoung-dong, Masan, Kyungnam, 631-701 Korea *The SUDOKWON Landfill Site Management Corporation, Seo-gu, Incheon, 404-140 Korea † Corresponding author, chyoon@kyungnam.ac.kr, Tel,+82-55-249-2663 2 3-line space Summary This study was conducted to investigate the effects of hydraulic loading rate and recycle ratio on biological denitrification and nitrification for leachate containing NH 4 + -N with high concentration about 1,500~2,000 mg/L discharged from SUDOKWON landfill site. Pilot-scale MLE(modified ludzack ettinger) process was employed in this study. As a result of this examination, we found out that about 2.3 days in denitrification tank and 5.7 days in nitrification tank are the optimal HRT for obtaining the removal efficiency of about 80 % for T-N and 99% for NH 4 + -N at the conditions of recycle ratio of about 600 % and BOD/NH 4 + -N ratio of about 3.0. In addition, optimal recycle ratio for obtaining the maximum nitrogen removal efficiency while keeping proper microbes concentration in nitrification and denitrification tank was 200 % for external recycle and about 400% for internal recycle. The maximum removal rates for each load of T-N and NH 4 + -N were 0.055kgT-N/kgVSS/d and 0.07kgNH 4 + -N/kgVSS/d, respectively. The ratio of alkalinity consumed per T-N removed in this process (△alkalinity/△T-N) was about 5.0. Keywords : leachate, biological denitrification, nitrification, hydraulic loading rate, recycle ratio Ⅰ. Introduction Wastes to be landfill generate leachate containing organic materials with high concentration at the initial period of landfill through complex decomposition processes such as biological decomposition by microbes in the waste layer and soil layer, as well as physical and chemical process such as hydrolysis, dissolution, settlement and adsorption. As the time has passed since landfill, the organic materials decreased but nitrogen component gradually increased 1~3) . SUDOKWON landfill site also showed similar trend as total nitrogen(T-N) contained in the leachate was about 300~500 mg/L in 1992 at the 1st year of landfill, but increased to about 2,200 mg/L, about 5 ~ 7 times increase after 7 years 4) . 90% of nitrogen contained in the leachate with landfill period over 7 years existed in NH 4 + -N form. There are physicochemical treatment processes such as ammonia stripping and MAP to remove NH 4 + -N. However, these process is rather complex, requires excessive cost for maintenance and has low treatment efficiency compared to biological treatment process. Therefore, biological treatment process can be regarded as more effective 5~8) . However, biological nitrogen removal process for leachate is still incomplete since nitrite nitrogen(NO 2 - -N) accumulate in the nitrification tank due to free-ammonia generated from high strength NH 4 + -N, or causes various kinds of problems such as decline in denitrification rate due to lack of organic matter, decline in sludge settlement and increase in suspended solids in the effluent. 9~12) . In addition, the leachate showed remarkable difference in flowrate due to seasonal features, in particular, between dry seasons and rainy seasons when the rainfalls are concentrated. SUDOKWON landfill site is found to have more leachate in rainy seasons by about 40 ~ 60% than in dry seasons 4) . Since the leachate that temporarily increased in the rainy seasons exceeds the treatment capacity in general, it is required to install separate reservoir and to additionally install and operate the facility to remove malodor generated from the leachate, causing many difficulties in maintenance. Accordingly, this study carried out to determine the optimal hydraulic loading rate when treating the leachate containing 1,500~2,000 mg/L NH 4 + -N with high concentration discharged from SUDOKWON landfill site, using pilot-scale MLE process, and also to examine the effect of recycle ratio on biological denitrification and nitrification rate based on the determined hydraulic loading rate. Ⅱ. Materials and Methods 1. Experimental Equipment Pilot scale MLE process consists of denitrification tank, nitrification tank and settlement tank with internal and external recycle systems as shown in Fig. 1. Vinyl film was installed to protect the experimental facility such as blower, pump and to prevent the penetration of foreign substance from outside. Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 156 - Fig. 1. Schematic diagram of pilot-scale MLE process 2. Experiment Method 2.1 Operation Conditions Pilot scale plant was operated for the test period based on the affecting factors. For the experiment on changes of hydraulic loading rate, we increased the flow rate from 3 to 12 m 3 /d at the interval of 3 m 3 /d. In addition, for the experiment on changes in recycle ratio, we increased it up to 400~800%, compared to influent flow rate(Q). Operation conditions for each experiment stage are summarized in Table 1. 2.2 External Carbon Source Methanol was used as external carbon source to provided organic carbon source which is not sufficient in leachate. Table 2 shows properties of the methanol. Table 1. Operating conditions in this study Conditions Applied values flow rate(m 3 /day) 3 6 9 12 recycle ratio(%) 600 400 600 800 dinitrification 14 Tank volume (m 3 ) nitrification 34 denitrification 4.7 2.3 1.6 1.2 nitrification 11.3 5.7 3.8 2.8 HRT (day) Total 16 8.0 5.4 4.0 DO(mg/L) 0.05~0.2 Temp.(℃) 15~28 DT* MLSS(mg/L) 7,000~9,000 DO(mg/L) 2.8~3.9 Temp.(℃) 13~27 NT** MLSS(mg/L) 6,000~8,000 * DT : denitrification tank, **NT : nitrification tank Table 2. Properties of the domestic methanol Items Values purity(%) 97.5 Specific gravity(mg/L) 0.79 COD Cr (mg/L) 1,455,000 BOD 5 (mg/L) 833,000 BOD 5 / COD Cr 0.573 2.3 Target Leachate Table 3 shows raw leachate characteristics used during the pilot scale test period(before adding external carbon source). 2.4 Analysis Method The influent, effluent from denitrification tank and nitrification tank were sampled and analyzed more than 2 times a week. COD Cr and alkalinity were analyzed by standard methods 13) . BOD, SS, T-N, NH 4 + -N, MLSS(VSS) are analyzed according to “Analytical Methods for Environmental Pollutants(Water)". 14) . NO 2 - -N, NO 3 - -N are analyzed by using IC(Dionex, DX-300), pH using pH meter(Orion-720A), and DO using DO meter(YSI-58). Table 3. Characteristics of raw leachate component concentration[mg/L] component a average ratio [ - ] BOD 618~3,514(1,530)* BOD/ COD Cr 0.3~0.7 COD Cr 2,480~4,720(3,688) COD Cr /T-N 1.9~2.6 T-N 938~2,423(2,068) BOD/ NH 4 + -N 1.2~1.5 NH 4 + -N 515~2,340(2,068) COD Cr / NH 4 + -N 2.0~4.8 Alkalinity 2,117~10,586(9,312) NH 4 + -N/T-N 0.56~0.95 TSS 100~1,420(690) PH 7.8~8.5 * numbers in parentheses indicate average values Ⅲ. Results and Discussion 1. Start-up Since about 90% of T-N contained in leachate are NH 4 + -N, it is difficult to induce normal nitrification due to adversely effects of nitrifier growth by free- ammonia. 9~12) Accordingly, we seeded return activated sludge(MLSS 13,000 mg/L) assimilated in aerated lagoon tank to the denitrification tank and nitrification tank about 50%(48 ㎥) to adapt the them on the leachate and filled the leachate about 50%. After diluting the concentration of NH 4 + -N to 750 mg/L, about 1/2, we inputted leachate in the system. At this time, the operation condition of denitrification tank and Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 157 - nitrification tank was about 15~20 ℃ , pH of 8.0~8.2, MLSS concentration of 6,500~7,500 mg/L, and DO of nitrification tank of about 3~4 mg/L. NH 4 + -N oxidation in nitrification tank started in about 5 days after aeration, showed nitrification efficiency of about 99 % and showed no accumulation of NO 2 - -N in 20 days. And pH of nitrification tank was the range of 7.5~7.7 Raw leachate was influent after the aeration time of about 15 days when over 90% of NH 4 + -N was oxidized. We also added methanol to maintain BOD/NH 4 + -N ratio of about 3 in consideration of C/N ratio for complete biological denitrification upon addition of raw leachate. 9) 2. Effects of Hydraulic Loading Rate 2.1 Nitrogen For the experiment on hydraulic loading rate, we increased leachate containing the nitrogen of 1,700~2,200 mg/L(Fig. 2) from 3 to 12 m 3 /day. Recycle ratios were kept at 600%. In terms of effluent nitrogen concentration in each flow rate, T-N showed stable at 400 mg/L and NH 4 + -N below 10 mg/L up to 6 m 3 /day as shown in Fig. 3(a). But when the flow rate increased over 9 m 3 /day, T-N increased to about 600~1,100 mg/L and NH 4 + -N suddenly increased to 100~850 mg/L. However, in the treatment efficiency, T- N showed high treatment efficiency over about 80% and NH 4 + -N over 99 % up to 6 m 3 /day as shown in Fig. 3(b), but gradually decreased from 9 m 3 /day. At 12 m 3 /day, T-N was found to suddenly decrease to about 40 % and NH 4 + -N about 50 %. Fig. 2. Change of influent T-N concentration Fig. 3. Variation of effluent nitrogen concentration and removal efficiency (a)eff. nitrogen concentration, (b)nitrogen removal efficiency The removal rates of T-N and NH 4 + -N according to increase in flow rate showed linear proportion up to about 9 m 3 /day as shown in Fig. 4(a). At this time, removal rate of T-N was about 0.055 kgN/kgVSS/day, and NH 4 + -N about 0.07 kgN/kgVSS/day. When flow rate increased to 12 m 3 /day, removal rate of T-N decreased to about 0.05 kgN/ kgVSS/day and NH 4 + -N to about 0.06 kgN/kgVSS/day. The trend of removal rates according to hydraulic nitrogen loading rate showed similar trend as flow rates. The rates were about 0.08 kgN/kgVSS/day for T-N and 0.07 kgN/kgVSS/day for NH 4 + -N, but the removal rate decreased after that loading rate as shown in Fig.4(b). Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 158 - Fig. 4. Variation of nitrogen removal rate to flow rate and loading rate (a) flow rate, (b) loading rate. As shown in Fig. 5(a), the form of nitrogen in the effluent was mainly NO 3 - -N up to 6 m 3 /day and was NO 3 - -N and NH 4 + -N at the ratio of 1:1 at 9 m 3 /day. It was mostly NH 4 + -N at 12 m 3 /day, showing that nitrification rarely happened. From this result, as shown in Fig. 5(b), it was found to rapidly increase at the alkalinity of the effluent at 12 m 3 /day. Microbes concentration in the thank was found to decrease as the flow rate increased, which is attributable to the wash- out of microbes due to increase in overflow rates of settlement tank. Accordingly, when treating the leachate contained total nitrogen of about 1,700~2,200 mg/L using this process, proper flow rate for obtaining removal efficiency of 80% for T-N and about 99% for NH 4 + -N, at the recycle ratio of 600% and C/N ratio of about 3 was about 6 m 3 /day, which means that about 2.3 days is proper for denitrification tank HRT and about 5.7 days for nitrification tank HRT. 2.2 Organic Materials Organic loading rates gradually increased as flow rates increased. However, as shown in Fig. 6(a), the removal efficiencies of BOD and COD Cr were found to be kept constant at about 99 % and 85 %. The removal rates according to organic loading rate showed linear proportion up to BOD of about 0.35kgBOD/kgVSS/day and COD Cr of about 0.65kgCOD Cr /kgVSS/day as shown in Fig. 6(b). However, when the flow rate increased to 12 ㎥/day, effluent COD Cr increased to over 1,400 mg/L as shown in Fig.7. This result indicates that it is to decrease in consumption rate of organics due to reduce in denitrification rate. Fig. 5. Variation of effluent NOx-N and MLSS(MLVSS), alkalinity (a) NOx-N, (b)MLSS(MLVSS), alkalinity 0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 loading rate (kg/kgVSS/day) removal rate (kg/kgVSS/day ) BOD CODcr (b) Fig. 6. Variation of removal efficiency and removal rate (a) BOD and COD Cr removal efficiency (b) removal rate Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 159 - Fig. 7. Change of effluent COD Cr concentration 3. Effects of Recycle Ratio 3.1 Nitrogen This test was performed for the effects of recycle ratio after about 50 days of stabilization period for sufficient adaptation of methanol to be inserted as external carbon source into leachate. Influent nitrogen concentration was almost constant at about 2,200 mg/L for T-N and 2,000 mg/L for NH 4 + -N. Effluent T-N under steady state condition was about 400 mg/L for the recycle ratio 400% and 330 mg/L for 600 % and declined to about 260 mg/L at 800 %. Fig. 8. Variation of effluent nitrogen concentration and removal efficiency (a) T-N, NH 4 + -N concentration , (b) removal efficiency In the case of increasing recycle ratio to 800 %(internal recycle of 600 %, external recycle of 200 %), T-N removal efficiency on 70 days in initial period was rapidly decreased to about 60%, which might be to decline in microbes concentration in the tank due to their wash-out according to rapid increase in recycle ratio. These results indicate that SS in the effluent was gradually increased but MLSS in the bulk solution was decrease as shown in Fig. 9 Fig. 10 shows the relationship between nitrogen loading rates and removal rate in the course of experiment on change in recycle ratio. This relation was linear proportion to about 0.08 kgN/kgVSS/day for T-N and 0.07 kgN/kgVSS/day for NH 4 + -N. However, as the load increased over that, the removal rate rather decreased, showing the similar result as that of experiment on the effects of hydraulic loading rate. Fig. 9. Variation of MLSS and effluent TSS concentration Fig. 10. Variation of nitrogen removal rate to loading rate Nitrogen-oxide(NO 2 - -N, NO 3 - -N) concentration in effluent was gradually decreased as the recycle ratio increased as shown in Fig. 11. But as the recycle rate increased up to 800 %, NO 2 - -N, which doesn’t appear at the recycle ratio of 400~600 %, appeared up to about 200 mg/L and NO 3 - -N was also unstable. Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 160 - 3.2 Organic Materials As shown in Fig. 12, the average removal efficiencies of BOD and COD Cr in recycle ratio tests were about 99% and 90% at the recycle ratio of 400~600 %, respectively. However, as the recycle ratio increased to 800 %, BOD showed removal efficiency of about 99% without any change, but COD Cr removal efficiency was rapidly decreased from 90% to 80%. This mean that microbes concentration in the tank were rapidly declined due to wash-out as the above mentioned and decrease HRT in reaction tank as shown in Table 4. Fig. 11. Variation of effluent NO 2 - -N, NO 3 — N concentration Fig. 12. Variation of BOD and CODCr removal efficiency Table 4. HRT of denitrification and nitrification tank recycle ratio denitrification tank nitrification tank 400 600 normal HRT (day) 800 2.3 5.7 400 0.47 (11.2hr) 1.13 (27.2 hr) 600 0.33 (8 hr) 0.81 (19.4 hr) actual HRT (day) 800 0.26 (6.2 hr) 0.63 (15.1 hr) 3.3. pH and alkalinity/T-N Ratio pH of bulk solution was average 8.7 for denitrification tank and average 8.3 for nitrification tank as shown in Fig. 13(a), showing almost no change in recycle rate. Ratio of alkalinity consumed to nitrogen removed (△alkalinity/△N) increased up to about 6.0 due to decrease in denitrification rate at the initial period when the recycle ratio increased to 800 %, but showed almost constant level at about 5.0 on the average for the recycle ratio from 400 to 600 % as shown in Fig. 13(b). In addition, the consumed alkalinity per mg of NH 4 + -N in nitrification tank was about 7.1 mg on the average, almost demonstrating similar result as the theoretical consumption quantity of 7.14 mgAlk/mgNH 4 + -N 9) . Fig. 13. Variation of pH and △Alkalinity/△N ratios (a) pH : DNR(denitrification), NR(nitrificaton) (b) △Alkalinity/△N ratios. Ⅳ. Conclusions This study results could be summarized as follows. 1) The removal efficiencies of T-N and NH 4 + -N were about 80 % and 99 % up to the flow rate of 6 m 3 /day, respectively, but removal efficiencies of T-N and NH 4 + - N rapidly decreased to 40 % and 50 % over the flow rate 9 to 12 m 3 /day. Optimal HRT was about 2.3 days Journal of Water and Environment Technology, Vol.1, No.2, 2003 - 161 - for denitrification tank and about 5.7 days at about nitrification tank at about 6 m 3 /day. 2) Maximum removal rates for T-N and NH 4 + -N were about 0.055 kgT-N/kgVSS/d, 0.07 kgNH 4 + -N /kgVSS/d at the test of hydraulic loading rates, respectively. 3) Removal rates showed linear proportion up to 0.35 kgBOD/kgVSS/d and 0.65 kgCOD Cr /kgVSS/d of hydraulic loading rate, respectively. 4) The optimal internal recycle ratio based on external recycle ratio of 200 % was about 400%. 5) The removal efficiency of COD Cr at the over recycle ratio(800% in the case of this study) was decreased up to 90% or 80% due to decline in the microbes concentration in the tank. 6) The ratio of alkalinity consumed per T-N removed in this process( △ alkalinity/ △ T-N) was about 5.0. Acknowledgements This study was supported financially by Kyungnam university research fund, 2003. Reference 1.Robinson, H. D., (1982) "Generation and Treatment of Leachate from Domestic Wastes in Landfills," Wat. Pollut. Control, 465~478 2.Robinson, H. D., Maris, P. J., (1985) "The Treatment df Leachate from Domestic Waste in Landfill Sites," JWPCF. 57(1), 30~38 3.Krug, M. N., Ham, R. K., (1997) "Analysis of Long- term Leachate Characteristics," SARDINIA 97, Sixth International Landfill Symposium, 117~131 4.Environmental Management Corporation., (1997) "Report on Operation Status and Proper Treatment Method for SUDOKWON leachate treatment facility.," 5.Keenan, J. D., Steiner, R. L., (1984) "Landfill Leachate Treatment," JWPCF. 56(1), 27~33 6.Iwami, N., (1992) "Treatment of a Landfill Leachate Containing Refractory Organics and Ammonium Nitrogen by the Microorganism-Attached Activated Carbon Fluidized Bed Process," Wat. Sci. Tech, 26(9-11), 1999~2002 7.Boyle, W. C., Ham, R. K., (1974) "Biological Treatment of Landfill Leachate," JWPCF, 46(5), 860~872 8.Christensen T. H., (1989) "Sanitary Landfilling Process, Technology and Environmental Impact, Academic Press, 186~187 9.U.S. EPA., (1993) "Manual : Nitrogen Control," EPA/625/R-93/010 10.Siegrist, H (1996): Nitrogen Removal from Digester Supernatant - Comparison of Chemical and Biological Methods, Wat. Sci. Tech. 34(1-2), 399~406 11.Sharma, B., Ahlert, R. C., (1977) "Nitrification and Nitrogen Removal, Water Research," 11, 897~925 12.Anthonissen, A. C., Loehr, R. C., (1976) "Inhibition of Nitrification by Ammonia and Nitrous Acid," JWPCF, 48(5), 835~852 13.APHA-AWWA-WPCF, (1993) "Standard Methods for Examination of Water and Wastewater," 18th ed. 14.The Ministry of Environment in Korea., (1992) "Analytical Methods for Environmental Pollutants (Water)." . (day) 800 2.3 5.7 400 0.47 (11 .2hr) 1. 13 (27.2 hr) 600 0.33 (8 hr) 0. 81 (19 .4 hr) actual HRT (day) 800 0.26 (6.2 hr) 0.63 (15 .1 hr) 3.3. pH and alkalinity/T-N. 2 ,11 7 ~10 ,586(9, 312 ) NH 4 + -N/T-N 0.56~0.95 TSS 10 0 ~1, 420(690) PH 7.8~8.5 * numbers in parentheses indicate average values Ⅲ. Results and Discussion 1.