A Pilot Scale Study on the Treatment of Sanitary Landfill Le

A Pilot Scale Study
on the Treatment of Sanitary Landfill Leachate

Li Jun1* Wang Baozhen2 Nie Meisheng3 WangShuYing1 ZhaoHongjing2

1. College of Civil Engineering & Architecture, Beijing Polytechnic University, Beijing, 100022, PRChina;
2. College of Municipal & Environmental Engineering, Harbin University of Technology , Harbin, 150008, PRChina;
3. Housing Industrialization Office，Ministry of Construction, Beijing, 100044, PRChina

ABSTRACT

A system was studied and developed for the treatment of a high-strength sanitary landfill leachate, which mainly consisted of (a) an upflow anaerobic sludge blanket/anaerobic filter(UASB-AF)hybrid anaerobic reactor,(b) an air stripper for NH3 removal ,and(c)an anoxic zone/aerobic zone(A/O) submerged biofilm aeration basin. these three units were used to treat high-strength leachate, with high removal efficiencies for COD, BOD5, and NH4+-N. The research results have already been applied in the design if the treatment station of sanitary landfill leachate at Xiaping Sanitary Landfill, Shenzhen City. The system is also available for the treatment of other kinds of organic waste water.
Keywords: landfill leachate，hybrid anaerobic reactor，a/o submerged biofilm aeration basin，ammonia stripper

INTRODUCTION

Sanitary landfill is the most widely applied method for refuse disposal．This may cause problems because of the landfill leachate it produces，which could cause secondary pollution of the groundwater or the nearby surface water[1-3]．To prevent the pollution by the leachate，the quantity of landfill leachate must be reduced as far as possible and must be treated by available methods，which vary depending on the quantity and quality of the landfill and on the requirements its effluent discharge must meet．The characteristics of the leachate from the landfills at Shenzhen City are quite different from those of many other cites both in China and abroad．The concentrations of chemical oxygen demand(COD)，biochemical oxygen demand(BOD5) and NH4+-N in the leachate from Shenzhen City are much higher，see Table 1．The treated sanitary landfill leachate from the landfill Xiaping in Shenzhen City will be drained into the municipal sewer and then into the Binhe Sewage Treatment Plant for final treatment．The local authority，the Sewerage Department，required that the pre-treated effluent from the landfill must reach COD<600 mg／L and NH4+-N <25 mg／L．A treatment system and a flowchart were developed，considering the characteristics of the landfill leachate and the local discharge standard for pre-treated effluent．

TREATMENTTECHNOLOGY AND EXPERIMENTAL EQUIPMENT

An analysis showed that the BOD5／CODCr radio of the leachate from the landfill Yulongkeng in Shenzhen City was between 0.4 and 0.7，which means that this landfill leachate was an easily biodegradable，organic wastewater．However，as the raw leachate contained extremely high organic contents，with a typical COD of 25，000 mg／L，the COD removal efficiency had to reach 96％ to reduce the effluent COD to<600 mg／L. Consequently the anaerobic and aerobic treatment system must be based on a technical and economical feasibility study．The concentration of NH4+-N in the leachate was between 400 and 1500 mg／L，which was to be high to be completely nitrified and denitrified，in particular due to lack of organic carbon．This problem was solved by the addition of an ammonia stripper behind the hybrid anaerobic reactor．
The operating procedures were as follows．The pH was raised to 9 or higher by the addition of lime to the effluent from the hybrid anaerobic reactor，thus converting a part of the NH4+ ions into free NH3，which was then pumped into the stripper．Here the alkaline leachate was rinsed by sprayers from the top and contacted with the blown up flow air counter currently，thus removing a major part of the ammonia nitrogen．This in turn increased the carbon／nitrogen (C：N)ratio of the influent of the anoxic zone／aerobic zone(A／O) in the submerged biofilm aeration basin to a reasonable value，which could provide the returned nitrified effluent of the A／O basin with sufficient carbon for denitrification and hence TN removal．In order to make the effluent COD less than 600 mg／L，the simultaneous chemical precipitation was conducted in the final settling basin．Therefore，the treatment flow sheet was as follows：raw leachate——>hybrid anaerobic reactor——>ammonia stripper——>A／O submerged biofilm aeration basin ——>final settling basin——> municipal sewer．

UASB-AF hybrid anaerobic reactor

In this experiment，a hybrid anaerobic reactor was used，which was a combination of a UASB(up flow anaerobic sludge blanket) in the lower part and an AF (anaerobic filter)，packed with fibrous carrier for attached growth of submerged biofilm，in the upper part，and thus had two advantages．

• l Firstly，in the upper zone，biofilm carriers packed in the form of synthetic fibres，fixed in circular plastic plates that were evenly distributed，were capable of entrapping the up flow sludge particles effectively through filtration and adsorption，while separating the gas from the liquid phase．Thus，the complicated three-phase separator was omitted．Instead，only gas collection space and an effluent overflow weir were set．

• l Secondly，the biomass was markedly greater and more multiple in micro-organic species than those either in UASB or in AF, which made the hybrid anaerobic reactor operate at a higher loading rate．The configuration of this type of reactor is simple and efficient in organics removal．

Table 1．Comparison between Shenzhen City and other cities concerning the quality of sanitary landfill leachate

Paramete

Shangha

Hangzhou

Guangzhou

Shenzhe

Taipei

Bryn Posteg, UK[4]

Barcelona,
Spain[1]

COD

1500-8000

1000-5000

1400-5000

15000-60000

4000-37000

5518

86,000

BOD

200-4000

400-2500

400-2000

5000-36,000

6000-28,000

3670

73,000

TN

100-700

80-800

150-900

650-2000

200-2000

157

2750

SS

30-500

60-650

200-600

1000-6000

500-2000

184

1500

NH4+-N

60-450

50-500

160-500

400-1500

100-1000

130

1750

PH

5-6.5

6-6.5

6.5-8.0

6.2-8.0

5.6-7.5

5.0-8.0

6.2

　

Fig.1．Schematic diagram of the pilot scale experimental system

1, influent high level tank; 2，rotameter；3，hybrid (UASB-AF) anaerobic reactor；4, pump；5，ammonia stripper；
6, recycle water tank;7,pump；8, high level tank for influent to a/o submerged biofilm aeration tank;
9. constant level tank for influent to A/O tank; 10，rotameter；11, rotameter；12, A/O submerged biofilm aeration tank;
13, pump；14，high level tank for return flow from A/O tank; 15, settling tank; 16, compressor；
17，compressed air holder；18,air flow meter；19，blowers．

The UASB-AF hybrid anaerobic reactor，applied in the study，had a cylindrical form and was 0.8 min diameter，with a bottom part with a truncated circular cone form and a top part with an effluent overflow weir with a circular ring form and a gas collector, having a total height of 2.5 m．The main part of the reactor was divided into two parts，i．e．the upper part and the lower part，which were anaerobic submerged biofilm zones of 0.9 m in height. and an up flow anaerobic sludge bed zone of l.1 min height，of which the bottom conical part occupied 0.5 m．Above the submerged biofilm was a 0.25 m high clarification zone．Clarified water flew over the peripheral weir into an intermediate colletion sump．The total volume of the hybrid anaerobic reactor was 1.16 m3，which consisted of a sludge bed zone occupying 1.4m3，and a submerged biofilm zone of 0.43m3．Under stable operating conditions，the influent flow rate，q，was maintained at 20L／h or 0.48 m3／day，and the corresponding HRT(hydraulic retention time)was 2 days．The temperature in the anaerobic reactor was between 20℃and 35℃ from March to July，without heating.

Ammonia stripper

The ammonia nitrogen in the effluent of the hybrid anaerobic reactor was as mach as 1000mg/L，but the ratio of COD／NH4+-N was only about 4，which was insufficient for denitrification in the A／O basin，and thus an ammonia stripper was installed in front of the A／O submerged biofilm aeration basin for the partial removal of ammonia nitrogen．
The ammonia stripper used in the experiment had a cylindrical shape，with a diameter of 0.8m and a height of 2.0 m．In it synthetic，multi-surface，hollow ball-shaped stuffing was packed with a height of 1．2 m．The effluent of the anaerobic reactor flew into a recycling water basin，from which it was pumped into the top of the ammonia stripper. Here the wastewater was distributed by sprayers in the form of fine water drops，which flow evenly over the packing and counter currently contacted the enforced air flow，supplied by two blowers installed at the bottom of the stripper．As a result，a part of the ammonia nitrogen in the form of free ammonia，was removed from the wastewater during the air stripping process．This wastewater thereupon flew into the recycling basin, in which a large part of the flow(500L/h) was recycled to the top of the stripper，and 20 L/h into a submerged biofilm aeration basin for further treatment．Thus the recycling ratio was 25：1．The flow rate of enforced air supplied by blowers was 70 m3/h．set，the total flow rate of the two air blowers was 140 m3/h，and the air to water volume ratio was 280：1．
The effective water volume in the recycling water basin was 102L，with a corresponding HRT of 5 h．

A／O submerged biofilm aeration basin

The advantage of the A／O submerged biofilm process is that the packed carriers can entrap and adsorb suspended solids and organic substrates，which promote the attached growth of biofilm, which contains different dominant species of bacteria in its different zones under different conditions．In the anoxic zone，the dominant bacterial species are heterotroph species capable of denitrification and organic degradation．In the aerobic zone the dominant bacteria species are heterotroph in the front part，for further organic degradation，and autotrophic nitrifiers in the rear part for nitrification[5]．
In the A／O submerged biofilm aeration basin，nitrification and denitrification take place in the respective zones by means of the fixed growth of nitrifiers and denitrifiers in the respective biofilms．These are more stable and active than those in activated sludge，because of the frequent changes of their living conditions between aerobic and anoxic zones．Both nitrifiers and denitrifiers are excluded from the changes in the activated sludge process．Therefore，the time necessary to complete the nitrification and denitrification in a A／O submerged biofilm aeration basin is usually much shorter than that in an activated sludge system，which is about 1／3 of extended aeration．Another advantage of the submerged biofilm process is that there are more species of organisms，which form various longer food chains，and the developed biomass in the form of biofilm is consumed by protozoa and mesozoa，which results in much less surplus biomass，being only about 1/10-1/5 surplus activated sludge[6,7]．

Cultivation of anaerobic sludge and submerged biofilm

An adequate quantity of activated sludge taken from the Binhe Sewage Treatment Plant was put into the anaerobic reactor to culture anaerobic sludge for a period of 2 months．Then COD removal reached 70％ while generating CH4 and C02 gases．Meanwhile，the anaerobic submerged biofilm attached to the packed fibrous carriers also matured．The surplus sludge of a secondary settling basin in the sewage treatment plant was also put into the A/O submerged biofilm aeration basin for the cultivation of biofilm on the packed fibrous carriers．After a month in the A/O basin，the biofilm gradually developed and matured．Microscopic observation showed that the biofilm had a large quantity of paramecia, rotifer, lorticella and nematode．In the anoxic zone，the colour of the biofilm was black-brown，with a high density，and there were some little gas bubbles adsorbed on the biofilm surface，which illustrated that the denitrifying bacteria had already existed in the biofilm and reduced a part of the nitrate into nitrogen gas．

EXPERIMENTAL RESULTS AND DISCUSSION

In order to determine suitable operating parameters for each treatment unit, an experimental study was conducted under different operating conditions for each treatment unit.

Hybrid anaerobic reactor
The variations of the influent and effluent COD and BOD of the hybrid anaerobic reactor at two different operating temperatures are shown in Fig.2，which shows that the removal efficiency of COD in the anaerobic reactor was greatly affected by temperature．At 20 ℃the average removal efficiencies of COD and BOD5 were 70.9％ and 77.3％，and at 34℃ these increased to 83.0％and 88.4％，respectively．The average influent concentrations of COD and BOD5 were 23，888 mg/L and 15，250 mg/L，and the average effluent concentrations of COD and BOD5 4060 mg/L and l771 mg/L，respectively．Therefore，the suitable operating temperature was 34℃，the hydraulic retention time(HRT) 2 days，and the average COD volumetic loading rate 9.5 kg COD／m3 per day．The variation of influent and effluent total volatile acids(TVA) and alkalinity in the anaerobic reactor are shown in Fig.3．It was found that the variation tendency of TVA was similar to that of COD．The effluent TVA was reduced with the reduction of effluent COD，when the effluent TVA was 700 mg/L，the effluent COD was less than 4500 mg/L．The experiment illustrated that with more accumulation of TVA，there was less activity of methanobacteria．Therefore，TVA can be regarded as a control parameter that is able to judge whether an anaerobic reactor operates normally or not．

Fig.2. Variations of influent and effluent COD and BOD5 of anaerobic reactor at different temperature.

Fig.3. Variations of influent and effluent TVA and alkalinity in thehybrid anaerobic reactor．

Fig. 4. Ammonia nitrogen removal
as a function of pH．

The effluent alkalinity and pH were higher than those of the influent in the hybrid anaerobic reactors, which was attributed to the production of ammonia and bicarbonates by the ammonification of organic nitrogen and the conversion of volatile fatty acids into CH4 by methanogenous fermentation．The effluent alkalinity was between 4500 mg/L and 5000 mg/L，which provided the leachate with a certain buffering capacity．
It should be noted that the performance of the hybrid anaerobic reactor was not optimal because of a variation of the temperature from 20℃ to 35℃，thus causing unstable operation，which resulted in lower COD and BOD5 removal efficiencies than those achieved at the automatically controlled temperature of 35℃ in abench scale unit with COD and BOD5 removals of 88.1％(mean value)and 93.8％，respectively [8]．

Ammonia stripper
Fig.4 shows the variation of the NH3-N concentration，depending on the operating time，at two different pH and the increase with operating time of the ammonia removal．For example，under the conditions of an operating time of 5h and pH 8，the ammonia removal was 35.3％．When the pH was raised to 9.1 by lime，the ammonia removal increased to 67.8％．A1though NH3-N can be removed with much higher efficiency at pH>11，the chemicals consumption is then too high and too expensive．To achieve the required removal efficiency and to save chemicals，the operating parameters were determined as follows：HRT 5 h，pH 9.1 and air to water volumetric ratio 280：1．
With the removal of ammonia nitrogen，COD was also removed partially，but with mach less efficiency of 19.7％ under the conditions of the ratio of effluent COD to NH3-N of 7.9 and dissolved oxygen(DO)0.2 mg/L，which was available for denitrificaion in the anoxic zone of the A／O submerged biofilm aeration basin．
The effect of ammonia nitrogen removal can be explained by the distribution ratio of ionic ammonia(NH4+) and free ammonia(NH3)，whch was varied with temperature and pH．At a temperature of 20 and pH 8.0，NH3 accounts for 8％ of the total ammonia，while at pH 9.1 this is 37.4％．Therefore，at a same HRT the ammonia nitrogen removal was much higher at pH 9.1 than that at pH 8.0 in the Stripper．In addition，the biofilm which developed on the synthetic packing surface in the stripper was analyzed by microscope，from which it was found that there were various species of micro-organisms，such as Vorlicolla, Epistylis,Paramecium，Colpedium and Nematode．Therefore，with the removal of ammonia nitrogen，COD and BOD5 were also removed partially by biological degradation．

A／O submerged biofilm aeration basin
The ammonia nitrogen contents were quite high in the landfill leachate of Shenzhen City．Therefore，the leachate was first treated to remove ammonia in the ammonia stripper，whose effluent contained NH4+-N 300 mg/L．The ratio of COD to NH4+-N increased during the treatment to 7.9．This effluent was denitrified in the following A／O submerged biofilm aeration basin．In its anoxic zone the influent，i.e．the effluent of the ammonia stripper，provided the returned nitrified effluent from the oxic zone of the basin with organic carbon to be consumed by heterotrophic denitrifing bacteria，and to produce energy for denitrification．As a result，the COD and BOD5 were removed partially in the course of denitrification，with removal efficiencies of 50-60％．In the oxic zone，the ammonia nitrogen was removed by nitrifying bacteria developed in the biofilm with a NH4+-N removal efficiency of 90-95％at DO 3.0 mg/L or more．
The total COD and BOD5 removal in the A／O submerged biofilm basin was 75-80％ and 80-85％，respectively，under suitable operation conditions；the HRTs in the anoxic and oxic zones were 6.5 h and 15.6 h，DO in anoxic and oxic zones were<0.5 and 3.0 mg/L，respectively，and the effluent return ratio was R=300％，The removal of COD，BOD5 and NH4+-N could be further improved if the A／O submerged biofilm aeration basin was designed more reasonably，with a much deeper effective water column depth，say 3-4 m instead of the 0.35 m in the exisdng basin．In theory the relationship between removal efficiency of TN and effluent return ratio was
ηTN=R/(1+R)
in which R is the effluent return ratio，and ηTN is the TN removal efficiency．
The equation shows that the greater the R value，the higher the TN removal efficiency. However, in fact the removal efficiency of nitrate nitrogen was found to be affected by the amounts of both denitrifying bacteria and organic substrates in terms of COD and BOD5，as well as by the concentration of DO in the anoxic zone，which was in turn dependent on the effluent return ratio．When the ratio was higher than 4，the removal efficiency of nitrate nitrogan removal was reduced significantly．In the anoxic zone the loading rate of nitrate nitrogen and return ratio were affected by each other，the relationship between them is shown in the following equation：

where[NO3--N]ML is the NO3--N concentration of the mixed liquid in anoxic zone；[NO3--N]i is the influent NO3--N concentration in anoxic zone，which was equal or close to 0；and [NO3--N]e is the effluent NO3--N concentration．
With the increase of the recycling ratio，the concentration of organic substances in the mixed 1iquid was markedly reduced，while the loading rate of nitrate nitrogen increased in the anoxic zone．Also，the hydraulic retention time was also decreased and DO increased．Therefore，the nitrogen removal efficiency did not increase with the increase of the return ratio．
As shown in Fig.5，the organic nitrogen was transformed into ammonia nitrogen through ammonification in the anoxic zone，and nitrate and nitrite were transformed into nitrogen gas through denitrification，thus increasing NH4+-N and alkalinity．The NO3--N concentration in mixed liquid was 29.87 mg/L，which after denitrification in the anoxic zone decreased to 3.22 mg/L，with a removal efficiency of 89.0％．On the contrary，in the oxic zone，the NH4+-N concentration and alkalinity were reduced significantly with the increase of NO3--N concentration，because of nitrification. The removal efficiency for TN in A／O submerged biofilm aeration basin was 64.8％．
The variation of NO3--N concentration with HRT in the A／O submerged biofilm aeration basin at different effluent return ratios is shown in Fig.6，from which it was found that at R=3 the removal efficiency both for TN and COD were the highest，with a COD removal of 68.6％．Therefore，the optimal return rate was R=3，at which the removal efficiencies at that different HRT were determined to be the optimum HRT in the A／O submerged biofilm aeration basin．It was found that when the HRT was reduced from77.5 h to 22.1 h in the A／O basin，the NH4+-N concentration in the effluent was less than 21 mg/L．When it was further reduced to 17.6h，the effluent NH4+-N concentration sharply rose to over 49 mg/L．In order to let the effluent NH4+-N concentration meet the discharge standard，the effluent NH4+-N concentration must be less than 25 mg/L，so the HRT shoud be greater than 22.1 h．The variation of NO3--N concentration of mixed liquid with HRT in A／O basin at different recycling ratios are shown in Fig.6．The variation of NO3--N concentration in mixed 1iquid with HRT in A／O basin under conditions of R=3 and total HRT=22.1 is shown in Fig.7．The variation of influent and effluent COD are shown in Fig.8，from which it is known that the effluent COD was about 700 mg/L．

Fig.5.Variationof nitrate and ammonia nitrogen concentrations and alkaline with HRT in the A／O submerged biofilm aeration basin．

Fig.6.Variation of NO3--N oncentration with HRT in A／O basin at different return ratios and total HRT=77.5h．
Operating time

Fig.7.Variation of NH4+-N and The variation of NO3--N concentrations with HRT in basin under the conditions of R=3 and total HRT=22.1h．

To sum up，the suitable operating conditions of submerged biofilm basin were HRT=22.1 h，of which in the anoxic zone 6.5 h．and in the oxic zone 15.6 h，and R=3. Under these conditions, when the average influent concentrations of COD were 2890.7 mg/L，the average effluent concentration of COD was 662 mg/L，with an average removal efficiency of 71.7％．The average effluent NH4+-N concentration was 20 mg/L，with an average removal efficiency of 90.8％．

Chemical precipitation
The comparative experiment was conducted by two kinds of coagulants in common use，i．e．PAC and FeCl3．At the same dosage，the COD removal by using PAC was higher than that by using FeCl3，and the suitable dosage of PAC was 200-300 mg/L，at which the COD removal efficiency was 32.6％，and the final effluent COD was 600 mg/L，or less．

Operation experiment of the whole treatment system
For the determination of suitable operating parameters，the pilot scale treatment system was operated in series as a whole．The results are shown in Fig.9 and Fig.10，in which the data shown in curves are the average values of eight times of samplings in a continuous test，from which it is evident that this treatment system can let the final effluent meet the discharge standard for municipal sewerage．

Fig.8. Variation of influent and effluent COD in A/O submerged biofilm basin．

Fig.9. Variation of COD along the flowchart in the pilot treatment system．

Fig.10. Variation of NH4+-N along the flowchart in the pilot treatment system．

CHARACTERISTICS OF FIBROUS PACKING BIOFILM

The biofilm growing on the fibrous carriers in anaerobic reactor consisted of tiny black flocs with an offensive smell．
In the ammonia stripper, the biofilm growing on the multi-surface hollow ball-shaped carriers were of brown colour，and the amount of biofilm was the highest at the bottom and gradually decreased upflow．There was almost no biofilm on the carrier surface at the top．The biofilm washed out of the carriers had the form of large flocs，in which some species of protozoa and metozoa，such as Epistylis, Paramecium, Colpedium and Nematode，were observed by microscope．
The growth rate of biofilms in the anoxic zone was slower than that in the oxic zone in the A／O basin．The biofilm in the anoxic zone was less in both quantity and density．The surface of the biofilm in this zone looked thick，and had a black colour and an offensive odour．The biofilm washed out of fibrous carriers had the form of loose flocs and exhibited similar characteristics along the water flow direction．On the contrary，the biofilm in the oxic zone in A／O basin was abundant and dense，had a brown colour and was without smell．The biofilm in the front part of the oxic zone in the A／O basin had the form of loose and large flocs，while that in the rear part had the form of small granules．The biofilm growing on the surface of fibrous carriers in the anoxic zone in the A／O basin was observed by microscope，by which only zooglea were found，without any protozoa．In the oxic zone，under stable operating conditions，there were lots of Epistylis, Colpedium and Nematode，and less Paramecium.

Setting characteristics of stripped biofilm and the determination of biomass

Four sampling points were set along the water flow direction，with sampling point No.1 in the anoxic zone and point Nos.2，3 and 4 in the front，middle and rear parts of the oxic zone．The samples taken from the anaerobic reactor were marked as No.5，and those from carriers of the ammonia stripper as No.6．
The sampled carriers with biofilm were put into different containers and washed with clean water．Then the water containing the washed out settled biofilm was put into a 1000 mL graduated cylinder．This operation was repeated 3-5 times，till the biofilm was washed out of the fibrous packing completely and the packing became white. Then the water was put into the graduated cvlinder that contained the washed-out biofilm mixed liquid，until the water reached 1000 mL．The added water was mixed with the biofilm containing washed water，and then static settling was conducted．The settling time and the volume of settling sludge were noted；the experimental results are shown in Table 2．

Table 2．The SVI(sludge volume index) of washed-out biofilm

simple

No.1

No.2

No.3

No.4

No.5

No.6

SVI（g/ml）

206

200

159

126

2506

131

As shown in Table 2，in the biofilm system on the surface of the carrier packing，the washed-out biofilm in the front part was not so good as that in the rear part in terms of settling characteristics in the A／O basin．However，it was more active concerning the degradation of organic substances compared with the biofilm in the rear part of the A／O basin．It was found that the surplus biomass in the form of biofilm was much less than that from the activated sludge process．as the biofilm developed in the ammonia stripper for a longer time，the stripper biofilm took on the form of larger flocs and exhibited rapidly settling characteristics．
The biofilm quantity，i.e. biomass concentration in different parts of the treatment system，were determined as ollows(MLSS=mixed1iquor suspended solids)：
In the anaerobic reactor, MLSS=3389 mg/L; In anoxic zone of A／O basin, MLSS=2828 mg/L;
In oxic zone of A／O basin, MLSS=4699 mg/L; In ammonia stripper, MLSS=2157 mg/L.

CONCLUSION

The landfill leachate in Shenzhen City was characterised by extremely high concentrations of COD，BOD and NH4+-N，a suitable treatment system was developed in the study，which consisted of the following treatment units in series：UASB-AF hybrid anaerobic reactor，ammonia stripper，A／O submerged biofilm aeration basin，and settling basin with simultaneous chemical precipitation．
By means of the above mentioned treatment system and flow sheet，the high strength leachate was treated effectively and efficiently，with a final effluent COD less than 600 mg/L and NH4+-N less than 25 mg/L，which well met the local discharge standards for the municipal sewerage system．
The hybrid anaerobic reactor was operated under the conditions of HRT 2d，T=20-34℃ and COD volumetric loading rate 9.5 kgCOD／m3 per day，with the removal efficiencies for COD and BOD of 83.0％and 88.4％，respectively，which made the post positioned treatment units operate normally. Its performance could be further improved if the operating temperature was automatically controlled at 35土l℃．
Ammonia nitrogen was removed efficiently in the ammonia stripper，with a removal efficiency of 69.8％ at pH 9.1，HRT 5 h，water recycling ratio of 25：1 and an air to water ratio of 280：1．COD was also removed by some 20％ simultaneously，thus increasing the C：N ratio from 4 to 8，which was adequate for denitrification in the A／O basin．
The suitable operating conditions of A／O submerged biofilm were determined as follows： HRT=22.1 h，of which 6.5 h in the anoxic zone，and 15.6h in the oxic zone；effluent return ratio，R=3，under which the removal efficiency of COD=71.7％ and that of NH4+-N =90．8％．
By means of simultaneous chemical precipitation in the final settling basin，the final effluent met the local discharge standard for municipal sewerage．

REFERENCES
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[4] H.D. Robinson and G. Grantham. The treatment of landfill leachate in on site aerated lagoon plant: Experience in Britain and Ireland. Water Res., 1988, 22(6):733-747.
[5] B.Wang,G.Li and R. Liu. Nitrogen removal by a submerged biofilm process with fibrous carriers. Water Sci. Tech., 1992, 26(9-11):2039-2042.
[6] B.Wang, Q. Tang, R. Liu, J. Uuan, F. Ma,J. He and G. Li. A study on simultaneous organics and nitrogen removal byextended aeration submerged biofilm process. WaterSci. Tech., 1991, 24(5): 197-214.
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*Corresponding author
Tel： （010）65958512（H） 67391648（O） E-mail : lj21c@263.net

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