Biotechnology Letters 23: 821–826, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.
821
Impact of landfill leachate on the co-treatment of domestic wastewater Ferhan Çeçen∗ & Didem Çakıroˇglu Boˇgaziçi University, Institute of Environmental Sciences, 80815 Bebek, Istanbul, Turkey ∗ Author for correspondence (Fax: +90-212-257 50 33; E-mail:
[email protected]) Received 19 January 2001; Revisions requested 6 February 2001; Revisions received 14 March 2001; Accepted 16 March 2001
Key words: activated sludge, co-treatment, leachate, nitrogen, organic carbon
Abstract Landfill leachate and domestic wastewater were co-treated in batch activated sludge reactors and the ratio of leachate varied from 5 to 20% (v/v). The leachates had a non-biodegradable COD fraction of at least 20%. An increase in leachate adversely affected the co-treatment and it was concluded that the leachate ratio should never exceed 20% of the total wastewater or 50% of the initial COD. The initial Total Kjeldahl Nitrogen (TKN) and the free ammonia level were identified as factors influencing the completion of nitrification.
Introduction
Biological treatability
Solid waste landfill sites generate leachates with high concentrations of organic and inorganic contaminants. However, there is relatively limited information on the co-treatment of landfill leachate and domestic wastewater in a publicly owned treatment work and the kinetic aspects are usually disregarded (Quasim & Chiang 1994, Diamadopoulos et al. 1997, Booth et al. 1996). The objective of this study was the investigation of this co-treatment alternative in a laboratory scale activated sludge system, with emphasis on the evaluation of COD and nitrogen removal and the determination of non-biodegradable fraction.
All experiments were conducted in 2 l batch activated sludge reactors aerated by air diffusers and at a room temperature of 22 ± 2 ◦ C (Çakıroˇglu 1998). First, domestic wastewater was treated alone in Reactor A serving as a reference study for co-treatment. The stock domestic wastewater solution in Table 2 was diluted to an initial COD of 500 mg l−1 . In total, 26 successive batch runs were conducted, each lasting from 24 to 240 h. The initial Mixed Liquor Suspended Solid (MLSS) and Mixed Liquor Volatile Suspended Solid (MLVSS) concentrations were in the ranges of 865–1970 mg l−1 and 690–1480 mg l−1 , respectively. The typical MLVSS/MLSS ratio was about 0.8. Then, raw leachate and domestic wastewater were combined and treated in Reactors B, C and D as summarised in Table 2.
Materials and methods Leachate characterisation The leachate originated from the Gaziantep landfill in Turkey which receives primarily domestic and some industrial wastes. The sampling and characterisation of all leachate samples have been reported in another study (Çeçen & Gürsoy 2000). The characteristics of the leachates A and B used in the present study (Table 1) reflected a relatively young landfill.
Reactor B. The initial seed sludge stemmed from Reactor A. In total 22 successive experiments were conducted using Leachate A and the leachate ratio was gradually increased in order to determine the optimum ratio at which no adverse effect was observed. The initial MLSS and MLVSS in these runs were in the ranges of 1180–1555 mg l−1 and 990–1240 mg l−1 , respectively.
822 Table 1. Characterisation of landfill leachate samples used in biological experiments. Sample No.
A
B
COD (mg l−1 ) BOD5 (mg l−1 ) TKNa (mg l−1 ) NH4 -N (mg l−1 ) NOX -N (mg l−1 ) pH Cl− (mg l−1 ) Alkalinity (mg CaCO3 l−1 ) Hardness (mg CaCO3 l−1 ) T-Pb (mg P l−1 ) Cu (mg l−1 ) Pb (mg l−1 ) Fe (mg l−1 ) Mn (mg l−1 ) Zn (mg l−1 ) Ni (mg l−1 ) Cr (mg l−1 ) Cd (mg l−1 )
2431 500 1602 1379 60.7 7.90 5725 12897 1634 7.7 0.26 0.67 2.66 0.20 0.47 1.23 0.00 0.12
37024 15625 2730 2430 285 7.30 9702 18150 3556 14.4 1.45 1.91 25.2 0.85 2.20 5.80 2.24 0.25
a Total Kjeldahl nitrogen. b Total phosphorous (inorganic and organic).
Reactor C. The sludge in Reactor B had been contaminated with leachate for a long time. For the purpose of testing the reproducibility of results, a fresh sludge was taken from Reactor A again and four runs were conducted with the same sludge using Leachate A, in parallel to the last B-runs (Table 2). Reactor D. The highly concentrated Leachate B was pretreated with 4000 mg l−1 FeSO4 and an anionic polyelectrolyte of type SF-380 before mixing with domestic wastewater (Çakıroˇglu, 1998). Thus, the total COD was decreased from 37 000 to around 25 000 mg l−1 . Pretreatment is a common measure to decrease the load of concentrated leachates (Çeçen & Gürsoy 2000) and may have a positive effect on organic carbon removal and nitrification. In total, four successive runs were conducted, the last being without any pretreatment. The average MLVSS concentrations in Runs D1, D3, D4 and D5 were about 2245, 2175, 1840 and 2490 mg l−1 , respectively.
subsequent titration. TKN analysis was done by the same procedure after digestion of samples. The NOX N (NO2 -N + NO3 -N) concentration was determined by the Devarda’s Alloy Reduction Method. NO2 -N analysis was done according to the high range nitrite method using Nitriver 2 test kits (Hach 1985). MLSS analyses were carried out by drying the filter (Millipore 0.45 µm) residue for 1 h at 103 ◦ C. MLVSS analyses were carried out by the ignition of the residue for 30 min at 600 ◦ C. For pH measurement an Orion SAS20 pH meter was used. Substrate concentration decreases and microbial activity were also followed by Oxygen Uptake Rate (OUR) measurements with the membrane electrode of the portable Hach DO apparatus.
Results and discussion Removal of COD in co-treatment experiments In the treatment of domestic wastewater alone, COD depletion took place rapidly and almost no residual COD was left over (data not shown). However, co-treatment of landfill leachate and domestic wastewater led to different results. Figure 1a illustrates the experimental COD decreases and the model curves belonging to B-runs as will be explained in a further section. The similarity of results between Band C-runs (Figure 1b) indicated that the new sludge in Reactor C rapidly acclimatised to leachate and that the experiments were reproducible. Leachate A had a low BOD5 /COD ratio of 0.2 and the inert COD was estimated roughly as 52–80% of the initial leachate-COD (Table 2). Figure 1c illustrates total COD (TCOD) decreases in Reactor D whereas Figure 2a illustrates the depletion of both total and soluble COD (SCOD). The SCOD/TCOD ratio of about 0.8– 0.9 did not change much during aeration. Despite the dominance of the leachate (Table 2), the residual COD was lower in this case since leachate biodegradability was higher as seen from the higher BOD5 /COD ratio (Table 1). In Reactor D as in others, the OUR of the sludge paralleled substrate consumption (Figure 2b) and was regarded as a rapid tool for assessing substrate changes.
Analytical techniques Analyses were carried out in accordance with Standard Methods (1989). COD analyses were performed by the dichromate closed reflux method. NH4 -N analysis was carried out by ammonia distillation and
COD removal kinetics Leachate contained a high non-biodegradable fraction and a first-order model presented by Braha (1988) was used which incorporated this fraction:
823 Table 2. Operational conditions in batch activated sludge reactors used for the co-treatment of leachate and domestic wastewatera . Run
Reactor B B1–B2 B3–B11 B12–B14 B15–B17 B18 B19 B20 B21 B22 Reactor C C1 C2 C3 C4 Reactor D D1 D2 D3 D4 D5
Leachate in total wastewater (L/TW) (%)
Initial COD (mg l−1 )
Initial leachate COD (mg l−1 )
Initial TKNb (mg l−1 )
Total residual COD (mg l−1 )
Inert COD in leachate COD (%)
% removal in total COD
Initial COD/TKN ratio
Domestic only 5 7.5 10 10 10 12.5 12.5 20
800 500 500 500 500 500 500 500 500
0 125 188 250 250 250 262 262 420
– – – – 189 210 – 203 381
–c –c –c –c 130 139 168 195 305
–c –c –c –c 52 56 64 74 73
–c –c –c –c 74 72 66 61 16
15.2d 4.2d 3.2d 2.6d 2.6 2.6 2.5 2.5 1.3
10 12.5 12.5 Diluted leachate
500 500 500 500
250 262 262 500
205 202 202 441
186 173 209 403
74 66 80 81
63 65 58 19
2.4 2.5 2.5 1.1
882 889 1569 2650 2077
450 450 1125 2250 1593
83 87 136 240 163
162 104 242 375 276
36 23 22 17 17
82 88 85 86 87
10.6 10.2 11.5 11.0 12.7
2e 5e 10e 5e 5
a Stock domestic wastewater (COD = 10000 mg l−1 , pH 7.2, composition: 3000 mg sodium acetate l−1 , 2800 mg glucose l−1 , 1000 mg peptone l−1 , 2500 mg (NH4 )2 SO4 l−1 , 250 mg KH2 PO4 l−1 , 500 mg K2 HPO4 l−1 , 1000 mg MgSO4 · 7H2 O l−1 , 300 mg CaCl2 · 2 H2 O l−1 , 100 mg FeCl3 · 6H2 O l−1 ). b Total Kjeldahl nitrogen. c Uncertain COD data due to nitrite interference was not reported. d Estimated values. e Pretreated leachate.
dS = −q X (S − Si ). dt
(1)
Replacing q X with k , and integrating between the limits S = S0 and S = S, and t = 0 and t = t led to the following expression:
S = (S0 − Si ) e−k t + Si ,
(2)
where in these equations S: COD concentration at time t, S0 : initial COD concentration, Si : inert (nonbiodegradable) COD concentration at the end of each run, X : average MLVSS concentration in the run, q : specific substrate utilisation rate, k : COD removal rate constant, 1/time. Equation (2) was applied and by iteration the most appropriate k was found. Figures 1a–c show that this model was satisfactory in description of data. When
84% of the initial COD was attributable to leachate as in Run B 22 (Figure 1a), the COD removal rate constant k decreased to a very low value of 0.05 h−1 . Similarly, in Run C4 a very low k was found (0.018 h−1 ) when the feed consisted of leachate alone (Figure 1b). In that case also OUR profiles were impaired (data not shown). In Run D4 the same pattern was seen at high leachate inputs. Based on all results, it was recommended that the leachate should never exceed 20% of the total wastewater, even in the case of low-strength leachates. Generally, when leachate-COD approached 50% of the total initial COD, a significant decrease was observed in reaction rates indicating the inhibitory effect of leachate. In practice, an increase in leachate content implies that a high amount of non-biodegradable COD will
824
Fig. 2. Change in (a) total COD and soluble COD (b) oxygen uptake rate (OUR) with operation time in Reactor D (Feed: leachate and synthetic domestic wastewater).
be observed in the effluent. In that respect, this study brought a novel approach by demonstrating the impact of leachate. Nitrogen removal
Fig. 1. Experimentally determined COD decreases and model COD curves showing substrate removal from the leachate-synthetic domestic wastewater mixture in (a) Reactor B, (b) Reactor C and (c) Reactor D.
Extensive nitrification was observed as exemplified by the TKN and NH4 -N decreases (Figures 3a–c). Mass balance revealed that also ammonia stripping had occurred at pHs above 8.5. TKN and NH4 -N removal and NOx -N production rates followed zero-order kinetics as also stated in literature (Çeçen et al. 1995, EPA 1993). The NOx -N production rate was regarded as a true indication of nitrification. In B- and C-runs the initial COD/TKN ratio was rather low (Table 2). It is a fact that the nitrifier fraction in increases with decreasing COD/TKN ratio (EPA 1993). Below the ratio of 3, a high NH4 -N removal rate and consequently a high NOx -N production rate was observed. With acclimatisation of the sludges in B- and C-runs, the NOx -N production rate increased to about 11.8 mg NOx -N l−1 h−1 or per biomass to
825 Consequently, the conversion of nitrite into nitrate was inhibited and nitrite concentrations rose to 328 mg NO2 -N l−1 (Çakıroˇglu 1998). The importance of free ammonia for nitrite accumulation was thoroughly discussed in other studies (Çeçen et al. 1995, Hwang et al. 2000, Liu & Capdeville 1994). On the other hand, in Reactor D the initial COD/TKN ratio was much higher (Table 2). The maximum NOx -N production rate was estimated as 5.15 mg NOx -N l−1 h−1 or per biomass as 0.0024 mg NOx -N mg−1 VSS−1 h−1 . Although these rates were lower than in other runs, nitrite build-up did not usually exceed 30 mg NO2 N l−1 since the initial NH4 -N concentrations were as 77–202 mg l−1 and the initial free ammonia concentrations were only about 0.51–1.61 mg NH3 -N l−1 at pHs 7–7.5. If leachate contribution increases the initial TKN and NH4 -N concentrations and the free ammonia level, a serious nitrite build-up may be expected in co-treatment.
Acknowledgement The financial support of this study by the Research Fund of Boˇgaziçi University (Project No. 96 HY0029) is gratefully acknowledged.
References
Fig. 3. Change in Total Kjeldahl nitrogen, ammonia nitrogen, NOx nitrogen and nitrite nitrogen during the runs: (a) RUN B18, (b) RUN C1 and (c) RUN D1 carried out with a leachate-synthetic domestic wastewater mixture.
0.012 mg NOx -N mg−1 VSS−1 h−1 . However, the NOx -N was mainly in the form of nitrite. Since in domestic wastewater nitrite concentrations remained below 1 mg NO2 -N l−1 , this nitrite build-up was mainly attributable to leachate. In these runs, the initial NH4 -N concentrations were as 171–418 mg l−1 and the initial free ammonia (FA) concentrations ranged from 1.78 to 12.72 mg NH3 -N l−1 at pHs 7.4–8.1.
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