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Abstract. Leachate from a municipal landfill was combined with domestic wastewater and treated in batch, semi-continuously fed-batch (SCFB) and ...
Biotechnology Letters 23: 1607–1611, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

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Nitrification inhibition in landfill leachate treatment and impact of activated carbon addition Özgür Aktas & Ferhan Çeçen∗ Boˇgaziçi University, Institute of Environmental Sciences, 80815 Bebek, Istanbul, Turkey ∗ Author for correspondence (Fax: +90-212-257 50 33; E-mail: [email protected]) Received 10 May 2001; Revisions requested 31 May 2001; Revisions received 24 July 2001; Accepted 24 July 2001

Key words: activated carbon, inhibition, leachate, nitrification, nitrite build-up

Abstract Leachate from a municipal landfill was combined with domestic wastewater and treated in batch, semi-continuously fed-batch (SCFB) and continuous-flow (CF) activated sludge systems with and without powdered activated carbon (PAC) addition. In the absence of PAC, nitrification was severely inhibited and nitrite accumulated to about 85– 100% of the total NOx -N. Addition of PAC to activated sludge reactors enhanced nitrification. In continuous-flow operation, nitrite accumulation could be completely prevented by PAC addition.

Introduction Sanitary landfill leachate is usually a very high strength wastewater containing many organic and inorganic constituents. The high strength of leachate may have adverse effects on the activated sludge process and combined treatment of leachate and domestic wastewater may prevent those effects (Çeçen & Çakiroˇglu 2001, Diamadopoulos et al. 1997). Severe nitrification inhibition was observed in the treatment of high-strength leachates due to the presence of high free ammonia and other inhibitors (Çeçen & Çakiroˇglu 2001). Nitrification involves two different bacteria. Nitrobacter that oxidize nitrite to nitrate are much more sensitive to inhibitors than Nitrosomonas that convert ammonia to nitrite. Therefore, nitrite accumulation is observed in many cases where nitrification is inhibited (Rhee et al. 1997, Hwang et al. 2000). Activated carbon addition to activated sludge in the form of PAC is known for its ability to remove refractory organic compounds and to enhance nitrification (Ng et al. 1987). The objective of this study was to investigate the impact of PAC on the achievement of nitrification in the co-treatment of sanitary landfill leachate and domestic wastewater. The removal of organic matter from leachate was extensively reported

in other studies (Çeçen & Aktas 2001, Aktas & Çeçen 2001). Materials and methods Landfill leachate and domestic wastewater The leachate sample was taken from a sanitary landfill site in Istanbul and had the following characteristics: pH: 8.2, total Chemical Oxygen Demand: 10750 mg l−1 , Soluble Chemical Oxygen Demand: 9070 mg l−1 , BOD5 : 6380 mg l−1 , total Kjeldahl Nitrogen (TKN): 2031 mg l−1 , NH4 -N: 2002 mg l−1 , NOx -N: 128 mg l−1 , total P: 6.8 mg l−1 , alkalinity:10600 mg CaCO3 l−1 . In this study, leachate and domestic wastewater were mixed such that the volumetric ratio of leachate in the total wastewater varied from 5 to 20%. Domestic wastewater was prepared as a stock solution having COD 10000 mg l−1 , TKN 1060 mg l−1 , total P 812 mg l−1 and pH 7.1 and then diluted to 500 mg COD l−1 . Activated sludge studies Nitrification performance of the leachate and domestic wastewater mixture was studied in batch, semicontinuously fed batch (SCFB) and continuous-flow

1608 (CF) activated sludge reactors. Each reactor was initially seeded with a typical municipal sludge. Aeration and mixing in reactors were provided by a compressor using bubble diffusers. In PAC assisted reactors the PAC used was Norit SA 4 in powdered form. Many batch runs were performed using leachate and domestic wastewater as outlined in detail in another study (Aktas & Çeçen 2001). In this study the results of four runs are discussed in which nitrification was extensively studied. The volume of wastewater in each batch reactor was 2 l and the volumetric ratio of leachate in the feed was 5%. In each case, batch reactors were operated in parallel without PAC addition (control AS reactors) and with PAC addition (AS+PAC reactors). The PAC concentration in the AS+PAC reactors ranged between 250–3500 mg l−1 . Three separate semi-continuously fed batch (SCFB) activated sludge runs were performed in the same AS and AS+PAC configurations as explained in batch experiments. The reactors had again a liquid volume of 2 l. To these reactors 1.5 l wastewater was discontinuously fed once a day and 1.5 l was withdrawn on the next day. Consequently, the hydraulic residence time was adjusted to 32 h and a continuous flow operation was simulated. The sludge inside the reactors was discontinuously wasted once a day to keep the mixed liquor suspended solids (MLSS) at a constant level. The sludge age was adjusted to 20 days in SCFB runs 1 and 2 and to 10 days in SCFB run 3. PAC was daily added in small amounts to the AS+PAC reactors in order to compensate the PAC loss with sludge wastage. Experimental conditions for the three SCFB runs are shown below in Table 1. The volumetric ratio of leachate in the feed was 6.7%, 13.3% and 20%, respectively. Experiments were also performed in a continuousflow (CF) activated sludge reactor with sludge recycle. The set-up consisted of aeration and settling tanks having volumes of 3.6 l and 2.5 l, respectively. The hydraulic residence time was adjusted to about 32 h. Experimental conditions for the CF operation are shown below in Table 2. Details about SCFB and CF operations are outlined in another study (Çeçen & Aktas 2001). All analyses were conducted in accordance with Standard Methods (APHA et al.1989) except the NO2 N analysis which was done according to the high range nitrite method using Nitriver 2 test kits (Hach 1985).

Fig. 1. Effect of PAC addition on the NOx -N formation in batch operation (AS: activated sludge reactor, AS+PAC: PAC assisted activated sludge reactor).

Results and discussion Batch experiments The measure of nitrification was the production of NOx -N (NO2 -N + NO3 -N). Nitrification was almost completely inhibited in the AS reactors as seen in Figure 1. The stimulating effect of PAC on NOx -N production was best observed in the AS+PAC configuration when the PAC dose was about 500 and 1000 mg l−1 . Data were analysed by the paired ttest and the differences in the absence and presence of PAC were statistically significant at the level of 95% confidence. NO2 -N (nitrite nitrogen) concentrations were also measured in order to assess the extent of complete nitrification. NO2 -N constituted about 80–90% of the final NOx -N in those AS+PAC reactors in which nitrification took place. This showed that Nitrobacter inhibition could not be alleviated with PAC addition in batch reactors; only the activity of Nitrosomonas was enhanced and nitritification (conversion of ammonia to nitrite) was achieved. Free ammonia (FA) and free nitrous acid (FNA) are well known inhibitors for nitrifiers. Theoretically, the threshold FA concentration for Nitrosomonas inhibition is 10–150 mg l−1 whereas it is about 0.1–1 mg l−1 for Nitrobacter. The FNA concentration inhibitory to Nitrobacter activity starts between 0.22 and 2.8 mg l−1 (Anthonisen et al. 1976). Throughout batch experiments, pH was in the range of 8–9, and at the present high bulk NH4 -N concentrations this resulted in high bulk FA concentrations up to 17.7 mg l−1 . Therefore, inhibition of nitrification due to FA was quite plausible. Data points in Figure 2 represent the nitrification rates (NOx -N formation rates) at the corresponding bulk free ammonia concentrations at various times of aeration in

1609 Table 1. Nitrification performance in semi-continuously fed batch (SCFB) operation. Run

1 (pH adjust.) 2 3

Reactor configuration

AS AS+PAC AS AS+PAC AS AS+PAC AS AS+PAC

Leachate ratio in the feed (% v/v) 6.7 6.7 6.7 6.7 13.3 13.3 20 20

PAC conc. (mg l−1 )

Average MLVSS (mg l−1 )

0 1000 0 1000 0 2000 0 2000

2082 2082 2083 1738

Feed NH4 -N (mg l−1 )

NOx -N production rate (g NOx -N g MLVSS−1 d−1 )

197 197 197 197 345 345 484 484

0.019 0.020 0.038 0.040 0 0.072 0.002 0.002

AS: Activated sludge reactor. AS+PAC: Powdered activated carbon (PAC) assisted activated sludge reactor. MLVSS: Mixed liquor volatile suspended solids.

Semi-continuously fed batch (SCFB) activated sludge operation

Fig. 2. Dependence of the nitrification rate on the bulk free ammonia (FA) concentration in batch activated sludge (AS) and PAC assisted activated sludge (AS+PAC) reactors in various batch runs (leachate ratio in the feed: 5% v/v).

several batch runs. At high bulk FA concentrations lower NOx -N production rates were observed. When the FA concentration was above 2 mg l−1 , no nitrification took place in both AS and AS+PAC reactors indicating that the activities of Nitrosomonas and Nitrobacter were inhibited. On the other hand, nitrification proceeded to the stage of nitrite when the FA concentrations were below 1 mg l−1 in both AS and AS+PAC reactors. As seen in Figure 2, at similar FA levels below 1–2 mg l−1 , significantly higher NOx -N formation rates were measured in AS+PAC reactors compared to AS reactors. This suggested that besides free ammonia other constituents in leachate contributed to the inhibition of nitrification. In PAC assisted cases, these constituents were probably adsorbed onto PAC and a higher nitrification rate could be reached.

The feed NH4 -N concentrations and the nitrification rates (as NOx -N production rates) obtained are shown in Table 1. At a leachate ratio of 6.7%, no significant differences were observed between the AS and AS+PAC (1000 mg PAC l−1 ) reactors in terms of nitrification. At relatively low pH values between 5–6.5, the steady state free nitrous acid (FNA) concentration rose up to 3.26 mg l−1 and the nitrification rates were 0.019 mg NOx -N mg MLVSS−1 d−1 and 0.020 mg NOx -N mg MLVSS−1 d−1 in AS and AS+PAC reactors, respectively. When the pH was adjusted to 6.5–7.3, FNA concentrations decreased to 0.6 mg l−1 which was not inhibitory to Nitrosomonas but was to Nitrobacter. Nitrification rates doubled in both AS (0.038 mg NOx -N mg MLVSS−1 d−1 ) and AS+PAC (0.040 mg NOx -N mg MLVSS−1 d−1 ) reactors after this pH adjustment, indicating the sensitivity of nitrifiers to pH changes. However, nitrification was seen to proceed to the stage of NO2 -N only and the major portion (almost 100%) of NOx -N consisted of nitrite. The difference in nitrification between the AS and AS+PAC (2000 mg l−1 PAC) reactors was best observed when the volumetric leachate ratio in the feed was 13.3% as seen in Figure 3. Increasing the leachate amount in the feed increased the FA concentrations and other inhibitory leachate constituents. When the FA concentrations were in the range of 38–69 mg l−1 , Nitrosomonas was completely inhibited and no nitrification could be achieved in the AS reactor. On the other hand, in the AS+PAC reactor nitrification took place at a relatively high rate of 0.072 mg NOx -N mg

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Fig. 3. NOx -N formation in activated sludge (AS) and PAC assisted activated sludge (AS+PAC) reactors after the start-up of a semi-continuously fed batch operation (SCFB 2).

MLVSS−1 d−1 (Table 1). This increased nitrification was accompanied by a decrease in pH and free ammonia (down to 0.34 mg l−1 ). However, also in this run nitrite build-up was extensive and constituted about 100% of the NOx -N. When the leachate ratio in the feed was increased to 20%, no recordable nitrification was observed both in AS and AS+PAC reactors (Table 1). Although the feed pH was about 7.5, as a result of aeration and CO2 stripping, inside the reactor pH values exceeded 9. Correspondingly, in both AS and AS+PAC reactors the free ammonia (FA) level was as high as 44–86 mg l−1 resulting in inhibition of Nitrosomonas. Thus, an increase in leachate ratio caused a severe nitrification inhibition which could not be relieved by PAC addition. Continuous-flow (CF) activated sludge operation In the first period of continuous-flow operation between 0–28 d, the leachate ratio in the feed was 6.7% (Table 2). FA concentrations were in the range of 0.10–3.27 mg l−1 and might have contributed to nitrification inhibition both before and after PAC addition. Before PAC addition, nitrification rate was calculated as 0.032 mg NOx -N mg MLVSS−1 d−1 and 80–95% of the effluent NOx -N existed in the form of NO2 -N as in former batch and SCFB operations. After the PAC addition to yield 2000 mg PAC l−1 , nitrification rate increased to 0.089 mg NOx -N mg MLVSS−1 d−1 (Table 2). As seen in Figure 4, PAC addition also led to the observation that a smaller ratio (50–55%) of the NOx -N consisted of NO2 -N. Thus, the inhibition of Nitrobacter was prevented to some extent. This was not observed in any of the former batch and SCFB operations. In the second period between 28–44 d, the leachate ratio in the feed was increased to 13.3%. In this period

Fig. 4. Effect of PAC addition on the effluent nitrogen profiles in Period I of continuous-flow (CF) operation (leachate ratio in the feed: 6.7% v/v).

Fig. 5. Effect of PAC addition on the effluent nitrogen profiles in Period II of continuous-flow (CF) operation (leachate ratio in the feed: 13.3% v/v).

the nitrification rate was calculated as 0.136 mg NOx N mg MLVSS−1 d−1 before PAC addition and after the new PAC addition at 2000 mg l−1 (total PAC conc.: 4000 mg l−1 ) it was increased to 0.160 mg NOx -N mg MLVSS−1 d−1 (Table 2). The most striking effect of PAC in this period was the achievement of complete nitrification. The fraction of NO2 -N in the effluent NOx -N decreased to about 8% indicating that inhibition of Nitrobacter had been almost completely prevented (Figure 5). The FA concentrations ranging from 0.10 to 2.79 mg l−1 during this period may be inhibitory to Nitrobacter. However, after PAC addition almost complete nitrification was achieved although the FA was still in the range of 0.1–1 mg l−1 . Hence, the main factor in nitrification inhibition seemed to be the presence of inhibitory matter in leachate which was removed by adsorption onto PAC. The effect of PAC was in any case more pronounced in continuous-flow operation than in batch and SCFB operations. In a continuous-flow reactor at steady-state, PAC was always exposed to much lower effluent concentrations than the influent. On the other hand, in batch and SCFB operations, PAC was initially exposed to high substrate levels and became probably

1611 Table 2. Nitrification performance in continuous-flow (CF) operation.

Period I Period II

Reactor operation

Leachate ratio in the feed (% v/v)

AS AS+PAC AS+PAC AS+PAC

6.7 6.7 13.3 13.3

PAC conc. (mg l−1 )

– 2000 2000 4000

Average MLVSS (mg l−1 )

Feed NH4 -N (mg l−1 )

NOx -N production rate (g NOx -N g MLVSS−1 d−1 )

2018 1080 1065 1200

205 205 345 345

0.032 0.089 0.136 0.160

AS: Activated sludge reactor. AS+PAC: Powdered activated carbon (PAC) assisted activated sludge reactor. MLVSS: Mixed liquor volatile suspended solids.

saturated with leachate constituents which decreased its effectiveness. Overall, the results showed that PAC addition to conventional activated sludge systems could effectively increase nitrification rates and lead to complete nitrification by adsorbing compounds inhibitory to Nitrosomonas and Nitrobacter. Besides the leachate constituents, the free ammonia in highly nitrogenous leachates was also a decisive factor in nitrification inhibition and nitrite build-up. Free ammonia concentrations as low as 0.1–2 mg l−1 contributed to nitrite build-up while higher concentrations usually led to complete nitrification inhibition. PAC addition to activated sludge reactors certainly increases the operating costs by about 1.12 USD per kg of activated carbon used. However it brings additional advantages such as better sludge settling and increased sludge dewaterability as shown in another study (Çeçen et al. 2001). Therefore, separation of PAC and sludge handling will not be a problem.

Acknowledgement The support of the Research Fund of Boˇgaziçi University (Project No. 98HY02) is gratefully acknowledged.

References Aktas Ö, Çeçen F (2001) Addition of activated carbon to batch activated sludge reactors in the treatment of landfill leachate and domestic wastewater. J. Chem. Technol. Biotechnol. 76: 793–802.

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