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.
Trang 1BIOLOGICAL 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
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
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 increased1~3)
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 years4) 90% of nitrogen contained in
the leachate with landfill period over 7 years existed in
NH4+-N form There are physicochemical treatment processes such as ammonia stripping and MAP to remove NH4+-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 effective5~8) However, biological nitrogen removal process for leachate is still incomplete since nitrite nitrogen(NO2--N) accumulate
in the nitrification tank due to free-ammonia generated from high strength NH4+-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 seasons4) 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 NH4+-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
Trang 2Fig 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 m3/d at the interval of 3 m3/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
recycle ratio(%) 600 400 600
800 dinitrification 14
Tank
volume
denitrification 4.7 2.3 1.6 1.2
nitrification 11.3 5.7 3.8 2.8
HRT
(day)
DO(mg/L) 0.05~0.2
DT*
MLSS(mg/L) 7,000~9,000
DO(mg/L) 2.8~3.9
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
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 CODCr and alkalinity were analyzed by standard methods13) BOD, SS, T-N, NH4+-N, MLSS(VSS) are analyzed according to “Analytical Methods for Environmental Pollutants(Water)".14) NO2
N, NO3--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
NH4+-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 NH4+-N to 750 mg/L, about 1/2, we inputted leachate in the system At this time, the operation condition of denitrification tank and
Trang 3nitrification 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 NH4+-N oxidation
in nitrification tank started in about 5 days after
aeration, showed nitrification efficiency of about 99 %
and showed no accumulation of NO2--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 NH4+-N was oxidized
We also added methanol to maintain BOD/NH4+-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 m3/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 NH4+-N below 10 mg/L up to 6
m3/day as shown in Fig 3(a) But when the flow rate
increased over 9 m3/day, T-N increased to about
600~1,100 mg/L and NH4+-N suddenly increased to
100~850 mg/L However, in the treatment efficiency,
T-N showed high treatment efficiency over about 80%
and NH4+-N over 99 % up to 6 m3/day as shown in Fig
3(b), but gradually decreased from 9 m3/day At 12
m3/day, T-N was found to suddenly decrease to about
40 % and NH4+-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 NH4+-N according to increase in flow rate showed linear proportion up to about 9 m3/day as shown in Fig 4(a) At this time, removal rate of T-N was about 0.055 kgN/kgVSS/day, and NH4+-N about 0.07 kgN/kgVSS/day When flow rate increased to 12 m3/day, removal rate of T-N decreased to about 0.05 kgN/ kgVSS/day and NH4+-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 NH4+-N, but the removal rate decreased after that loading rate as shown in Fig.4(b)
Trang 4Fig 4 Variation of nitrogen removal rate to flow rate
As shown in Fig 5(a), the form of nitrogen in the
effluent was mainly NO3--N up to 6 m3/day and was
NO3--N and NH4+-N at the ratio of 1:1 at 9 m3/day It
was mostly NH4+-N at 12 m3/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 m3/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
NH4+-N, at the recycle ratio of 600% and C/N ratio of
about 3 was about 6 m3/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 CODCr 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 CODCr of about 0.65kgCODCr/kgVSS/day as
shown in Fig 6(b) However, when the flow rate
increased to 12 ㎥/day, effluent CODCr 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),
0 0.1 0.2 0.3 0.4 0.5 0.6
loading rate (kg/kgVSS/day)
(b)
Fig 6 Variation of removal efficiency and removal rate
Trang 5Fig 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 NH4+-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
(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 NH4+-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
Nitrogen-oxide(NO2--N, NO3--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 %, NO2--N, which doesn’t appear at the recycle ratio of 400~600 %, appeared up to about
200 mg/L and NO3--N was also unstable
Trang 63.2 Organic Materials
As shown in Fig 12, the average removal efficiencies
of BOD and CODCr 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 CODCr 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 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 NH4+-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/mgNH4+-N9)
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 NH4+-N were about 80 % and 99 % up to the flow rate of 6 m3/day, respectively, but removal efficiencies of T-N and NH4+
-N rapidly decreased to 40 % and 50 % over the flow rate 9 to 12 m3/day Optimal HRT was about 2.3 days
Trang 7for denitrification tank and about 5.7 days at about
nitrification tank at about 6 m3/day
2) Maximum removal rates for T-N and NH4+-N were
about 0.055 kgT-N/kgVSS/d, 0.07 kgNH4+-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 kgCODCr/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 CODCr 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
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