Partial nitritation treating nitrogen in old landfill leachate

In this study, a lab-scale Partial Nitritation Sequencing Batch Reactor (PNSBR) was implemented for treating high-ammonium old landfill leachate to yield an appropriate NO2 — N/ NH4 + -N ratio from 1/1 to 1.32/1 mixture as a pretreatment for subsequent Anammox. The objective of this study was to determine the optimal hydraulic retention time (HRT) at different influent ammonia concentrations for 210 days. The experimental results showed that with the influent ammonia concentrations of 500, 1000, 1500 and 2000 mg/L, HRT is 12 h, 21 h, 30 h and 48 h, respectively. The range of free ammonia (FA) concentration from 17 to 44 mg/L completely inhibited nitrite oxidizing bacteria (NOB) for long time operation. The COD removal efficiency was very low (6±2) %.


INTRODUCTION
In Vietnam, sanitary landfilling is the most common way to treat municipal solid wastes.One of the main environmental problems generated from landfill is leachate, which is containing high ammonia concentration and refractory organics [1].The conventional nitrogen removal process requires high oxygen supplied for nitrification and external carbon source for denitrification that result in high treatment costs.In recent years, a partial nitritation coupled with anammox process was proven as a advanced technology for nitrogen removal as its low demand for oxygen and no external carbon added ([2];[3]; [4]).The process involve two stages: Partial Nitritation oxidizing a part ammonium to nitrite until NH 4 + -N/NO 2 --N ratio is about 1-1.32, ideal for the next stageanammox process (Strous et al., 1997).Application of different operational strategies for the partial nitritation has been found to enhance ammonia oxidizing bacteria (AOB) and to inhibit nitrite oxidizing bacteria (NOB) activity.
The inhibition of FA and/or free nitrous acid (FNA) to AOB and NOB is different levels.AOB and NOB are inhibited at higher than 10 mgFA/L, 0.1-1.0mgFA/L, respectively [7].FNA higher than 2.8 mg/L inhibits all nitrification bacteria [8].The growth rate of AOB is faster than NOB that is basis of selection of the suitable HRT for partial nitritation.

Lab-scale PN-SBR
The reactor is shown in Figure 1.It is cylindrical tank with total height of 0.6 m and internal diameter of 0.42 m, corresponding to working volume of 66.5 liters.The operating minimum volume was 26.5 liters, which is equivalent to the volume exchange ratio (VER) of 60%.Air was supplied from the bottom of the reactor through air distributors, and the air flow was adjusted by using a manual valve.The feed leachate stored in 300L tank, was pumped to the reactor.Completed mixture was achieved by a mechanical stirrer at 5-10 rpm.

Wastewater and sludge characteristics
Landfill leachate used for the study was collected from Go Cat municipal solid waste landfill in Ho Chi Minh city, Vietnam.This landfill was closed 6 years ago.The characteristics of leachate were as shown in The feed leachate was diluted with tap water in order to obtain the influent ammonium concentrations of 500, 1000, 1500 and 2000 mg/L.The seed sludge was activated sludge from activated sludge tank of the Go Cat leachate treatment plant.95 gVSS sludge was seeded to obtain 1500 mgMLVSS/L the reactor with ratio of MLVSS to MLSS of 0.25.This sludge was washed by tap water in order to eliminate the residue prior to the enrichment.

Operating conditions
All experiments were operated using a fedbatch mode.Each cycle included 10 minutes of feed, 45 minutes of settle and 5 minutes of decantation.The aerobic reaction time was determined by HRT, total cycle time and volume exchange ratio [9].The HRT of the reactor was adjusted depending on the effluent NO 2 --N to NH 4 + -N ratio.According to Ganigué et al ( 2007), nitrogen loading rate (NLR) of PN-SBR fed-batch is from 0.5 to 1.5 kgN/m 3 .d[17].
Choosing NLR of 0.5 kgN/m 3 .d,this research determine HRT for enrichment phase is 21 h.pH of the influent was adjusted at 7.5±0.2by adding HCl 20% solution into storage tank and pH in the reactor was not controlled.The reactor was run in the following operating conditions as presented in Table .2  , 1995).Samples were filtered using 45 µm membrane filters from Whatman, India.

Enrichment of AOB
One of the studies indicated that NOB growth is more inhibited than AOB under the low DO concentration [6].Thus, in phase 1, the reactor was operated at DO of 1.5-2 mg/L.The experimental result show that the ammonia oxidation took place slowly and the NH 4 + -N conversion efficiency was about (17±5) %.The concentration of nitrite in the effluent was low and not stable (35±33) mg/L (as shown in Fig. 2).This demonstrates that activity AOB and NOB was both low in this phase.At the 15 th day, DO increased (phase 2).The results indicated that the conversion efficiency of NH 4 + -N to NO 2 --N gradually rose.Higher level of DO did not promote the conversion nitrite to nitrate (the average effluent nitrate concentration is 15 mg/L).It means that the high DO concentration enhanced AOB activity while NOB activity was still inhibited.The strong inhibition of NOB may have caused by the relatively high average FA concentration.This result was similar to the previous studies on partial nitritation ([10]; [11]).On 45 th day, the enriched AOB sludge was completed, 92% of NH 4 + -N converted to NO 2 --N (as shown

The influent ammonia concentration of 1500 mg/L
In this phase, the reactor was run at HRT of 30 h.From 111 st to 120 th day, the conversion of ammonia to nitrite was low because AOB was not adapted to increased ammonia concentration.The ammonia conversion efficiency during these days was about 14-48%, effluent NO 2 --N/NH 4 + -N ratio was (0.72±0.2).After 120 th day, AOB was gradually adapted to high ammonia concentration.The conversion of NH The influent ammonia concentration of 2000 mg/L Figure.6 illustrates that the conversion of ammonia to nitrite gradually increase when HRT was extended.Efficiency of conversion ammonia to nitrite is very low (30±2)% with the effluent NO 2 --N/NH 4 + -N ratio of (0.3±0.04) at HRT of 38 h for 30 days.When HRT was raised to 41 h, partial nitritation did not achieve yet.HRT continued to rise to 48 h.Partial nitritation was achieved.The effluent NH 4 + -N/NO 2 --N ratio was about 1 and the average effluent ammonia and nitrite concentration was 1006 mg/L, 1004 mg/L.The effluent nitrate concentration is still low (20±1) mg/L.The results showed that the increase of HRT up to 48h did not cause noteworthy formation of nitrate.However, findings of Hellinga et al [12] and Akio Ota et al [11] showed that HRT 48 h existed accumulation of NOB.In the end days of each phase, when partial nitritation was achieved, the result shows that FA concentration was the range of (17-44 mg/L).According to Anthonisen et al., FA higher

COD removal
COD removals of PN are shown in Figure .7. The influent COD of leachate ranges from 567 to 2189 mg/L.Fig. 7 shows that the COD removal efficiency was low (6±2)%, due to the low ratio of BOD 5 /COD about 0.1.Ganigué et al [17] presented, the leachate had BOD 5 /COD ratio of 0.15, COD removal ranged from 11 to 14%.It shows that the organic matter of this old leachate is mainly refractory.So, influence of COD concentration on partial nitritation did not exist in this study.-N from 1/1 to 1.32/1 which is suitable for anammox process at the influent ammonia concentration of 500 mg/L with HRT of 12 h, of 1000 mg/L with HRT of 21 h, of 1500 mg/L with HRT of 30 h, of 2000 mg/L with HRT of 48 h.
Although DO is not controlled and considered high compare to other similar researches, NOB was inhibited for old landfill leachate.
Organic removal was not significant due to low BOD 5 /COD ratio of old landfill leachate.
In this study, the old landfill leachate was diluted.Therefore, further studies need to increase the influent concentration of ammonia or nitrogen loading rate to evaluate the nitrogen removing ability of the PN-SBR model.
,Liang, Z. & Liu, J. (2007) claimed that pH, ammonia and alkalinity are not limiting factors for nitritation of landfill leachate treatment because of high strength of ammonia, alkalinity in old leachate and acclimation of AOB to FA. DO range of 0.8-2.3mg/L, the steady partial nitritation was achieved,

Figure
Figure.2), effluent nitrite concentration increased from 74 to 487 mg/L at HRT of 21h.

Figure 5 .
Figure 5.Time course of nitrogen transformation in the PN-SBR at the influent ammonia concentration of 1500 mg/L

Figure 6 .
Figure 6.Time course of nitrogen transformation in the PN-SBR at the influent ammonia concentration of 2000 mg/L

Figure 7 .
Figure 7. Time course of COD removal

Table 1 .
Characteristics of feed old landfill leachate

Table 2 .
The operating conditions of PN-SBR than 10 mg/L inhibited AOB[7].It indicated that the AOB population used in this experiment adapted to high FA concentrations.